[ { "id": "authors:ce0sk-ejw66", "collection": "authors", "collection_id": "ce0sk-ejw66", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20170929-120230481", "type": "article", "title": "Comparison of effective conductivities calculated by the effective medium approximation and the self consistent approximation for core-shell particulate composites", "author": [ { "family_name": "Gavalas", "given_name": "George R.", "orcid": "0000-0003-1468-6835", "clpid": "Gavalas-G-R" } ], "abstract": "The effective conductivity of composites containing simple or core-shell particles has been estimated in the literature using the Mean Field Approximation (MFA) and the Self-Consistent Approximation (SCA) among other techniques. It is shown here that for both simple and core-shell particles the two approximations agree to first order in the particle volume fraction but differ at the second order term. For simple particles the coefficient of the second order term calculated by SCA is at much better agreement with previous exact results than the coefficient calculated by MFA. For core-shell particles the results of the two approximations are almost identical up to particle volume fraction 0.20 but diverge with increasing volume fraction and particle-to-matrix conductivity ratio.", "doi": "10.1063/1.4999331", "issn": "2158-3226", "publisher": "American Institute of Physics", "publication": "AIP Advances", "publication_date": "2017-09", "series_number": "9", "volume": "7", "issue": "9", "pages": "Art. No. 095222" }, { "id": "authors:g96p6-j0k24", "collection": "authors", "collection_id": "g96p6-j0k24", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20140725-075706960", "type": "article", "title": "Extension of Rayleigh\u2013Ritz method for eigenvalue problems with discontinuous boundary conditions applied to vibration of rectangular plates", "author": [ { "family_name": "Gavalas", "given_name": "George R.", "orcid": "0000-0003-1468-6835", "clpid": "Gavalas-G-R" }, { "family_name": "El-Raheb", "given_name": "Michael", "clpid": "El-Raheb-M" } ], "abstract": "The Rayleigh\u2013Ritz (R\u2013R) method is extended to eigenvalue problems of rectangular plates with discontinuous boundary conditions (DBC). Coordinate functions are defined as sums of products of orthogonal polynomials and consist of two parts, each satisfying the BC in its respective region. These parts are matched by minimizing the mean square error of functions and their x-derivatives at the interface between regions. Matching defines a positive definite 2N^2 \u00d7 2N^2 matrix Q whose eigenvectors form the orthogonal coordinate functions. The corresponding eigenvalues measure the matching error of the two parts at the interface. When applying the R\u2013R method, the total error is the sum of the matching error and that arising from the finite number of coordinate functions. Although most of the coordinate functions correspond to the zero eigenvalue, these do not suffice and additional functions corresponding to small but finite eigenvalues must be included. In three examples with discontinuous BC of the clamped, simply supported and free kind, the calculated frequencies match closely those from a finely discretized finite element method.", "doi": "10.1016/j.jsv.2014.03.030", "issn": "0022-460X", "publisher": "Elsevier", "publication": "Journal of Sound and Vibration", "publication_date": "2014-08-18", "series_number": "17", "volume": "333", "issue": "17", "pages": "4007-4016" }, { "id": "authors:9sm23-bmy97", "collection": "authors", "collection_id": "9sm23-bmy97", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20151006-150209770", "type": "article", "title": "Diffusion in Microporous Membranes: Measurements and Modeling", "author": [ { "family_name": "Gavalas", "given_name": "George R.", "orcid": "0000-0003-1468-6835", "clpid": "Gavalas-G-R" } ], "abstract": "Permeation of gases and vapors through microporous membranes, principally zeolite membranes, is reviewed, focusing on macroscopic diffusion models, measurement techniques, and applications of models to experimental data. Brief reference is made to the choice of adsorption isotherms for single components or mixtures. Concerning diffusion, the Maxwell\u2212Stefan (M-S) model is reviewed in some detail, as it is the one universally adopted in recent literature. Emphasis is placed on the coverage dependence of the diffusion coefficients and on the proper handling of the cross terms. In the experimental technique section, the key distinctions are between steady state and transient measurements, and between the use or nonuse of a sweep gas. A few special techniques are also briefly reviewed. Interpretation of transient measurements using the time lag method is reviewed in some detail, especially for coverage-dependent diffusion coefficients. Several of the studies reviewed focus on the ability of the M-S model to match the measurements, and specifically on the suitability of various simplifying approximations. Two common approximations are (i) to treat the M-S diffusion coefficients as coverage-independent and (ii) to neglect the cross terms.", "doi": "10.1021/ie800420z", "issn": "0888-5885", "publisher": "American Chemical Society", "publication": "Industrial & Engineering Chemistry Research", "publication_date": "2008-08-20", "series_number": "16", "volume": "47", "issue": "16", "pages": "5797-5811" }, { "id": "authors:jfv83-2h923", "collection": "authors", "collection_id": "jfv83-2h923", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20151006-150210067", "type": "article", "title": "Measurement of concentration-dependent gas diffusion coefficients in membranes from a psuedo-steady state permeation run", "author": [ { "family_name": "Villet", "given_name": "Michael C.", "clpid": "Villet-M-C" }, { "family_name": "Gavalas", "given_name": "George R.", "orcid": "0000-0003-1468-6835", "clpid": "Gavalas-G-R" } ], "abstract": "Transient permeation measurements were conducted for carbon dioxide through a PDMS membrane and for carbon dioxide and methane through a ZSM-5 zeolite membrane. After pressurization the feed tank was shut off from the supply and pressure and flux measurements were conducted while the feed-side pressure gradually declined over the course of each run. A series of steady state runs were also conducted for the same range of feed-side pressures. In all runs the transient fluxes were close to the steady state fluxes at corresponding feed-side pressures indicating that the transient evolved at a pseudo-steady state. A dimensionless parameter depending on the feed tank volume, membrane geometry, and adsorption properties was defined to characterize the deviation from steady state. \n\nConcentration-dependent diffusion coefficients were estimated from (i) steady state fluxes from a sequence of runs, (ii) transient fluxes from a single run, and (iii) transient pressure measurements from a single run. The fluxes and the diffusion coefficients obtained from a single transient run, and from a sequence of steady state runs are compared and the observed differences are discussed.", "doi": "10.1016/j.memsci.2007.03.045", "issn": "0376-7388", "publisher": "Elsevier", "publication": "Journal of Membrane Science", "publication_date": "2007-07-05", "series_number": "1-2", "volume": "297", "issue": "1-2", "pages": "199-205" }, { "id": "authors:zpe73-qt795", "collection": "authors", "collection_id": "zpe73-qt795", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20151006-150210337", "type": "article", "title": "Preparation of Oxygen Ion Conducting Ceramic Hollow-Fiber Membranes", "author": [ { "family_name": "Liu", "given_name": "Shaomin", "clpid": "Liu-Shaomin" }, { "family_name": "Gavalas", "given_name": "George R.", "orcid": "0000-0003-1468-6835", "clpid": "Gavalas-G-R" } ], "abstract": "Hollow-fiber membranes of mixed conducting perovskite Ba_(0.5)Sr_(0.5)Co_(0.8)Fe_(0.2)O_(3-\u03b4) were prepared using a sequence of extrusion, gelation, and sintering steps. For this purpose, a finely divided mixture of the component oxides was prepared by the modified Pechini technique and suspended in a polymer solution. The suspension was extruded through a spinneret and gelled in water. The resulting fiber was first heated at 800 \u00b0C to remove the polymer and then at 1100 \u00b0C to form the perovskite and simultaneously sinter the particles to a gastight membrane. The fibers were characterized by scanning electron microscopy and tested for air separation at ambient pressure and temperature between 700 and 950 \u00b0C. The maximum oxygen flux measured was 3.9 mL/min/cm^2 at 950 \u00b0C and 0.022 atm of average permeate oxygen partial pressure.", "doi": "10.1021/ie040279i", "issn": "0888-5885", "publisher": "American Chemical Society", "publication": "Industrial & Engineering Chemistry Research", "publication_date": "2005-09-28", "series_number": "20", "volume": "44", "issue": "20", "pages": "7633-7637" }, { "id": "authors:m9x4d-smt55", "collection": "authors", "collection_id": "m9x4d-smt55", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20151006-150210599", "type": "article", "title": "Oxygen selective ceramic hollow fiber membranes", "author": [ { "family_name": "Liu", "given_name": "Shaomin", "clpid": "Liu-Shaomin" }, { "family_name": "Gavalas", "given_name": "George R.", "orcid": "0000-0003-1468-6835", "clpid": "Gavalas-G-R" } ], "abstract": "Oxygen ion conducting Ba_(0.5)Sr_(0.5)Co_(0.8)Fe_(0.2)O_(3\u2212\u03b4) hollow fiber membranes with o.d. 1.15 mm and i.d. 0.71 mm were fabricated using a sequence of extrusion, gelation, coating and sintering steps. The starting ceramic powder was synthesized by combined EDTA\u2013citrate complexing followed by thermal treatment at 900 \u00b0C. The powder was then dispersed in a polymer solution, and extruded through a spinerette. After gelation, an additional thin coating of the ceramic powder was applied on the fiber, and sintering was carried out at 1190 \u00b0C to obtain the final ceramic membrane. The fibers were characterized by SEM, and tested for air separation at ambient pressure and at temperatures between 700 and 950 \u00b0C. The maximum oxygen flux measured was 5.1 mL/min/cm^2 at 950 \u00b0C.", "doi": "10.1016/j.memsci.2004.09.028", "issn": "0376-7388", "publisher": "Elsevier", "publication": "Journal of Membrane Science", "publication_date": "2005-01-01", "series_number": "1", "volume": "246", "issue": "1", "pages": "103-108" }, { "id": "authors:h7b4t-fvx38", "collection": "authors", "collection_id": "h7b4t-fvx38", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20151006-150210837", "type": "article", "title": "Glass\u2212Carbon Composite Hollow Fibers", "author": [ { "family_name": "Liu", "given_name": "Shaomin", "clpid": "Liu-Shaomin" }, { "family_name": "Gavalas", "given_name": "George R.", "orcid": "0000-0003-1468-6835", "clpid": "Gavalas-G-R" } ], "abstract": "Glass\u2212carbon composite hollow fibers were prepared by extruding a suspension of glass particles in an N-methylpyrrolidone solution of poly(ether sulfone). The fibers were gelled in water and pyrolyzed for 0.5\u221230 min in a furnace preheated to 1100\u22121200 \u00b0C. The resulting composite carbon\u2212glass fibers had about 2.1 mm o.d. and 1.3 mm i.d. Scanning electron microscopy was used to examine fiber morphology, while the nitrogen permeance was about 7 \u00d7 10^(-5) mol/m^2\u00b7Pa\u00b7s, indicating pores of micron size. Compared to pure glass fibers, which are easily deformed at 900 \u00b0C, the composite fibers could withstand temperatures up to 1200 \u00b0C without suffering deformation.", "doi": "10.1021/ie0308644", "issn": "0888-5885", "publisher": "American Chemical Society", "publication": "Industrial & Engineering Chemistry Research", "publication_date": "2004-06-09", "series_number": "12", "volume": "43", "issue": "12", "pages": "3137-3140" }, { "id": "authors:2a9e2-bss65", "collection": "authors", "collection_id": "2a9e2-bss65", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20151006-150211096", "type": "article", "title": "Intrapore Synthesis of Silicalite Membranes at Temperatures below 100 \u00b0C", "author": [ { "family_name": "Kang", "given_name": "Beom Seok", "clpid": "Kang-Beom-Seok" }, { "family_name": "Gavalas", "given_name": "George R.", "orcid": "0000-0003-1468-6835", "clpid": "Gavalas-G-R" } ], "abstract": "Growth of silicalite membranes inside the pores of \u03b1-Al_2O_3 support tubes was carried out at 95 \u00b0C. The support tubes had a 1-\u03bcm mean pore diameter and were seeded by impregnation in a suspension of 0.1-\u03bcm silicalite particles prior to membrane growth. The membranes were weighed to determine the total reaction product and examined by SEM and EDS to estimate the spatial distribution. Reaction product was found at a depth of 100 \u03bcm or more inside the pores of the support. The membranes were tested in single gas permeations of hydrogen, nitrogen, n-butane, and isobutane and in the separation of n-butane/isobutane mixtures. After three growth periods, mixture measurements gave n-butane permeances of 10\u221217 MPU [1 MPU = 10^(-8) mol/(m^2 s Pa)] and n-butane/isobutane selectivities of 30\u221240 at room temperature. The selectivity results are discussed in terms of gaps between the intrapore crystals.", "doi": "10.1021/ie010918e", "issn": "0888-5885", "publisher": "American Chemical Society", "publication": "Industrial & Engineering Chemistry Research", "publication_date": "2002-06-26", "series_number": "13", "volume": "41", "issue": "13", "pages": "3145-3150" }, { "id": "authors:4wp13-hna15", "collection": "authors", "collection_id": "4wp13-hna15", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20151006-150211344", "type": "article", "title": "A multiple microreactor system for parallel catalyst preparation and testing", "author": [ { "family_name": "Pantu", "given_name": "Piboon", "clpid": "Pantu-P" }, { "family_name": "Gavalas", "given_name": "George R.", "orcid": "0000-0003-1468-6835", "clpid": "Gavalas-G-R" } ], "abstract": "A system containing nine catalytic microreactors for parallel preparation and testing of heterogeneous catalysts is described. The catalyst samples are prepared in the form of thin films coated on thin quartz rods by dip-coating in solutions of different composition. Catalyst-coated rods are placed within thin tubes housed inside a wider tube heated in an electrical furnace. A multiport valve serves to sequentially conduct the reaction products from each microreactor to a mass spectrometer for analysis. The system was tested with the reaction of methane reforming with carbon dioxide over Pt/Ce_(1\u2212x)Gd_xO_(2\u22120.5x) and Pt/Ce_(1\u2212x)Sm_xO_(2\u22120.5x) at 650 and 700\u00b0C. Individual catalysts were characterized by X-ray analysis, and induction-coupled plasma mass spectrometry for catalyst mass and elemental composition. The measurements showed that Pt/CeO_2 had the highest activity among the series of catalysts tested and generally the activity increased with the cerium oxide content. After exposure to the feedstream for 2\u20133 h at 700\u00b0C, most catalysts suffered significant deactivation except the mixed oxides with 25\u201385% samarium oxide that maintained relatively stable activity.", "doi": "10.1002/aic.690480415", "issn": "0001-1541", "publisher": "Wiley", "publication": "AIChE Journal", "publication_date": "2002-04", "series_number": "4", "volume": "48", "issue": "4", "pages": "815-819" }, { "id": "authors:btjn3-41721", "collection": "authors", "collection_id": "btjn3-41721", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20151006-150211577", "type": "article", "title": "Methane partial oxidation on Pt/CeO_2 and Pt/Al_2O_3 catalysts", "author": [ { "family_name": "Pantu", "given_name": "Piboon", "clpid": "Pantu-P" }, { "family_name": "Gavalas", "given_name": "George R.", "orcid": "0000-0003-1468-6835", "clpid": "Gavalas-G-R" } ], "abstract": "Partial oxidation of methane to synthesis gas over 0.5 wt.% Pt/Al_2O_3 and 0.5 wt.% Pt/CeO_2 catalysts was studied in a packed-bed reactor and supplementary runs of methane reforming with carbon dioxide were carried out. Fresh and used catalysts were characterized by nitrogen adsorption (BET method) for total surface area, and by H_2 and CO chemisorption or by the rate of propene hydrogenation for metal surface area. At temperatures up to 650\u00b0C, the Pt/CeO_2 catalyst gave considerably higher methane conversion and higher selectivity to CO and H_2 but above 700\u00b0C, the activities and selectivities of both catalysts were comparable. The Pt/CeO_2 catalyst maintained high selectivity to CO and H_2 when the CH_4:O_2 feed ratio varied from 1.7 to 2.3 while the Pt/Al_2O_3 catalyst had lower activity and selectivity under methane-rich conditions. The Pt/CeO_2 catalyst was also more active for methane reforming by carbon dioxide.", "doi": "10.1016/S0926-860X(01)00761-X", "issn": "0926-860X", "publisher": "Elsevier", "publication": "Applied Catalysis A: General", "publication_date": "2002-01-10", "series_number": "1-2", "volume": "223", "issue": "1-2", "pages": "253-260" }, { "id": "authors:q9527-91t64", "collection": "authors", "collection_id": "q9527-91t64", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20151006-150211841", "type": "article", "title": "Parallel Synthesis of ZSM-5 Zeolite Films from Clear Organic-Free Solutions", "author": [ { "family_name": "Lai", "given_name": "Re", "clpid": "Lai-Re" }, { "family_name": "Kang", "given_name": "Beom Seok", "clpid": "Kang-Beom-Seok" }, { "family_name": "Gavalas", "given_name": "George R.", "orcid": "0000-0003-1468-6835", "clpid": "Gavalas-G-R" } ], "abstract": "Twenty-one zeolite films were grown on a 2.5 cm alumina disk using a multiwell reactor (see picture). This parallel synthesis from clear organic-free solutions offers an interesting approach to the preparation of catalytic libraries. The morphologies of the films produced were found to be similar to those synthesized under conventional conditions but their X-ray diffraction patterns indicated a lower degree of orientation of crystallites.", "doi": "10.1002/1521-3773(20010119)40:2<408::AID-ANIE408>3.0.CO;2-V", "issn": "1433-7851", "publisher": "Wiley", "publication": "Angewandte Chemie International Edition", "publication_date": "2001-01-19", "series_number": "2", "volume": "40", "issue": "2", "pages": "408-411" }, { "id": "authors:9n2cm-pq016", "collection": "authors", "collection_id": "9n2cm-pq016", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20151106-072040903", "type": "article", "title": "Intrapore Synthesis of Silicalite Membranes", "author": [ { "family_name": "Kang", "given_name": "Beom Seok", "clpid": "Kang-Beom-Seok" }, { "family_name": "Gavalas", "given_name": "George R.", "orcid": "0000-0003-1468-6835", "clpid": "Gavalas-G-R" } ], "abstract": "The zeolitic layer of most supported zeolite membranes lies\npartially outside of the pores and partially inside the pores of the support. Coronas t al prepared ZSM-5 membranes by two techniques, one resulting in an external layer with little pore infiltration, the other resulting in a patchy external layer and a deep internal layer, the fatter structure having better separation properties. Generally, it has proven difficult to determine which of the two layers is responsible for the separation properties. Intrapore separation layers have the advantage that they can be grown on supports of lesser quality and are not\nsusceptible to crack formation during calcination.\nIn this study intrapore membrane growth was achieved by means of seeding inside the pores of the support. Subsequent growth of the seeds led to pore filling by the zeolite up to a certain depth from the external surface. In a departure from previous ZSM-5 membrane studies, growth of the zeolite layer was carried out at temperatures below 100\u00b0C that dispenses with pressure vessels and allows facile\nstirring and midsynthesis change of solution.", "issn": "1550-6703", "publisher": "American Chemical Society", "publication": "PMSE Preprints", "publication_date": "2001", "volume": "85", "pages": "290-291" }, { "id": "authors:26jnp-axz65", "collection": "authors", "collection_id": "26jnp-axz65", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20151006-150212176", "type": "article", "title": "Spontaneous Formation of Periodically Patterned Deposits by Chemical Vapor Deposition", "author": [ { "family_name": "Tsapatsis", "given_name": "Michael", "orcid": "0000-0001-5610-3525", "clpid": "Tsapatsis-M" }, { "family_name": "Vlachos", "given_name": "Dionisios G.", "clpid": "Vlachos-D-G" }, { "family_name": "Kim", "given_name": "Soojin", "clpid": "Kim-Soojin" }, { "family_name": "Ramanan", "given_name": "Harikrishnan", "clpid": "Ramanan-H" }, { "family_name": "Gavalas", "given_name": "George R.", "orcid": "0000-0003-1468-6835", "clpid": "Gavalas-G-R" } ], "abstract": "Patterns of colloidal origin that are formed in ionic precipitation-diffusion systems and are known as Liesegang Rings (LR) have fascinated researchers since the beginning of the century and have been the subject of numerous theoretical, numerical, and experimental studies. The terms recurrent, rhythmic, or periodic precipitation are often used to describe the temporal pattern evolution in such systems. Pattern formation in the vapor phase reaction of NH_3 and HCl has also been reported since 1930. It has been suggested that any reaction giving an insoluble product is capable of LR pattern formation. However, the potential of such self-organized deposition for the fabrication of materials (other than NH_4Cl) from vapor precursors has not been demonstrated. Moreover, reproducible LR patterns are usually characterized by uneven spacing (Matalon-Packter law) and band thickness, although nearly periodic patterns have been reported as well. Here we propose, for the first time, a connection of LR phenomena, with the spontaneous (i.e., involving no external concentration, temperature, or other parameter forcing) formation of micrometer size periodic stripes of nanocrystalline titania deposited in mesoporous glass by counter diffusion chemical vapor deposition (CVD) using titanium chloride and water.", "doi": "10.1021/ja002228s", "issn": "0002-7863", "publisher": "American Chemical Society", "publication": "Journal of the American Chemical Society", "publication_date": "2000-12-27", "series_number": "51", "volume": "122", "issue": "51", "pages": "12864-12865" }, { "id": "authors:gppcb-zvm14", "collection": "authors", "collection_id": "gppcb-zvm14", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20151006-150212434", "type": "article", "title": "Preparation of supported carbon membranes from furfuryl alcohol by vapor deposition polymerization", "author": [ { "family_name": "Wang", "given_name": "Huanting", "clpid": "Wang-Huanting" }, { "family_name": "Zhang", "given_name": "Lixiong", "clpid": "Zhang-Lixiong" }, { "family_name": "Gavalas", "given_name": "George R.", "orcid": "0000-0003-1468-6835", "clpid": "Gavalas-G-R" } ], "abstract": "Supported carbon membranes were prepared from furfuryl alcohol (FA) precursor by vapor deposition polymerization (VDP). For this purpose \u03b3-Al_2O_3/\u03b1-Al_2O_3 or glass/\u03b1-Al_2O_3 support tubes were pretreated with an acid catalyst and exposed to FA vapors at 90\u00b0C. The tubes were then heated at 200\u00b0C to crosslink the poly(furfuryl alcohol) (PFFA) polymer and carbonized slowly to 600\u00b0C. The polymerization and carbonization cycle was repeated once to improve the permeation properties. The membranes were examined by scanning electron microscopy (SEM) and tested in a permeation cell with single gases (H_2, N_2, O_2, CO_2, CH_4) and with the mixture CO_2\u2013CH_4. After the first polymerization/carbonization cycle the membranes had little selectivity for gas separations. After the second polymerization/carbonization cycle the membranes had ideal selectivities 10\u201313 for O_2:N_2, 80\u201390 for CO_2:CH_4, and 90\u2013350 for H_2:N_2 at room temperature. The permeance was 0.6\u20132.5 for H_2, 0.27\u20130.58 for CO_2 and 0.08 for O_2, all in MPU (1 MPU=10^(\u22128) mol/m^2 Pa s). The permeances were sharply higher at 150\u00b0C but the selectivities were lower, e.g. one of the membranes had H2 permeance 10.6 MPU and H_2:N_2 ideal selectivity 30.", "doi": "10.1016/S0376-7388(00)00444-0", "issn": "0376-7388", "publisher": "Elsevier", "publication": "Journal of Membrane Science", "publication_date": "2000-08-30", "series_number": "1-2", "volume": "177", "issue": "1-2", "pages": "25-31" }, { "id": "authors:w0t8c-xe490", "collection": "authors", "collection_id": "w0t8c-xe490", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20151006-150212970", "type": "article", "title": "Mesoporous glass films supported on \u03b1-Al_2O_3", "author": [ { "family_name": "Wang", "given_name": "Huanting", "clpid": "Wang-Huanting" }, { "family_name": "Gavalas", "given_name": "George R.", "orcid": "0000-0003-1468-6835", "clpid": "Gavalas-G-R" } ], "abstract": "Glass\u2013alumina composites were developed for possible use as membrane supports. Preparation involved dip-coating of \u03b1-Al_2O_3 tubes with a suspension of borosilicate glass particles (9.1% Na_2O\u201329.7% B_2O_3\u201361.2% SiO_2), sintering to convert the particle layer into a nonporous layer of phase separated glass, and leaching the glass with a strong acid to remove the soluble phase and obtain the final porous layer of about 10 \u03bcm thickness. In some preparations a 10% \u03b1-Al_2O_3 powder was added to the initial suspension. The composite supports were characterized by SEM, EDAX and EPMA for elemental composition, XRD for crystalline phase content, nitrogen adsorption for surface area and pore size distribution, and by permeation measurements with single gases and mixtures. After leaching the glass layers had pore size 1\u20134 nm, and contained varying amounts of boron, sodium, and aluminum oxides, in addition to silica. The nitrogen permeance of the composite supports was 10\u2013100 times higher than that of standard porous Vycor tubing.", "doi": "10.1016/S0376-7388(00)00432-4", "issn": "0376-7388", "publisher": "Elsevier", "publication": "Journal of Membrane Science", "publication_date": "2000-08-15", "series_number": "1", "volume": "176", "issue": "1", "pages": "75-85" }, { "id": "authors:dm0s2-j5c11", "collection": "authors", "collection_id": "dm0s2-j5c11", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20151006-150213210", "type": "article", "title": "ZSM-5 membrane synthesis with organic-free mixtures", "author": [ { "family_name": "Lai", "given_name": "Re", "clpid": "Lai-Re" }, { "family_name": "Gavalas", "given_name": "George R.", "orcid": "0000-0003-1468-6835", "clpid": "Gavalas-G-R" } ], "abstract": "Preparation of supported ZSM-5 membranes using a TPA-free synthesis gel was investigated. After exploring the effect of reaction mixture the composition SiO_2:0.0125Al_2O_3:0.2675Na_2O:46H_2O was selected for membrane synthesis. Membranes were prepared by hydrothermal reaction on asymmetric \u03b1-Al_2O_3 tubular supports. The membranes were characterized by scanning electron microscopy, EDS, X-ray diffraction and Ar and N_2 adsorption. Permeation measurements with single gas and mixtures yielded selectivities for H_2 over n-butane above 104 and those for O2 over N_2 were 9\u201310. Permeation was strongly activated with the activation energies increasing sharply with molecular sizes.", "doi": "10.1016/S1387-1811(00)00143-8", "issn": "1387-1811", "publisher": "Elsevier", "publication": "Microporous and Mesoporous Materials", "publication_date": "2000-08-01", "series_number": "2-3", "volume": "38", "issue": "2-3", "pages": "239-245" }, { "id": "authors:tr636-vj447", "collection": "authors", "collection_id": "tr636-vj447", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20151006-150213451", "type": "article", "title": "Growth of ZSM-5 films on alumina and other surfaces", "author": [ { "family_name": "Lai", "given_name": "Re", "clpid": "Lai-Re" }, { "family_name": "Yan", "given_name": "Yushan", "clpid": "Yan-Yushan" }, { "family_name": "Gavalas", "given_name": "George R.", "orcid": "0000-0003-1468-6835", "clpid": "Gavalas-G-R" } ], "abstract": "Growth of ZSM-5 films on porous \u03b1-Al2O3 and other substrates was studied with the main focus on the effect of the substrate on surface gel formation and zeolitization. The films formed on the substrate and the powders that settled at the bottom of the autoclave were characterized by scanning electron microscopy (SEM) and X-ray diffraction (XRD). Adsorption of Ar and iso-butane was used to estimate the crystalline fraction of the product formed on and in the pores of the substrate. The presence of alumina substrates accelerated bulk zeolitization for certain solution compositions. Aluminum added to the solution had dual effects, to induce gel layer formation on the substrate surface and to retard zeolitization of that layer as well as retard zeolitization in the bulk of the solution.", "doi": "10.1016/S1387-1811(99)00188-2", "issn": "1387-1811", "publisher": "Elsevier", "publication": "Microporous and Mesoporous Materials", "publication_date": "2000-05", "series_number": "1-2", "volume": "37", "issue": "1-2", "pages": "9-19" }, { "id": "authors:36qhh-0he37", "collection": "authors", "collection_id": "36qhh-0he37", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20151006-150213697", "type": "article", "title": "Methane Pyrolysis in a Hot Filament Reactor", "author": [ { "family_name": "Sun", "given_name": "Qi", "clpid": "Sun-Qi" }, { "family_name": "Tang", "given_name": "Yongchun", "clpid": "Tang-Yongchun" }, { "family_name": "Gavalas", "given_name": "George R.", "orcid": "0000-0003-1468-6835", "clpid": "Gavalas-G-R" } ], "abstract": "An electrically heated platinum filament mounted inside a quartz tube was used as a high temperature flow reactor for methane pyrolysis. The reaction products included carbon and a mixture of hydrocarbons, mainly C1\u2212C5 alkanes, alkenes, and benzene. The hydrocarbons were measured by gas chromatography and the carbon gravimetrically. At 1275 \u00b0C filament temperature and 100ms residence time in the hot region of the reactor, methane conversion was 19.7% and selectivity of hydrocarbon products 68%. Lowering the flow rate increased the conversion but lowered sharply the selectivity of hydrocarbon products. Coking of the filament surface gradually lowered the filament temperature and the conversion of methane. This deactivation process could be slowed by adding a few percent of oxygen to the methane feed.", "doi": "10.1021/ef9901995", "issn": "1520-5029", "publisher": "American Chemical Society", "publication": "Energy and Fuels", "publication_date": "2000-03-20", "series_number": "2", "volume": "14", "issue": "2", "pages": "490-494" }, { "id": "authors:0nd1d-zgv96", "collection": "authors", "collection_id": "0nd1d-zgv96", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20151007-090423328", "type": "article", "title": "Methane partial oxidation on Pt/CeO_2\u2013ZrO_2 in the absence of gaseous oxygen", "author": [ { "family_name": "Pantu", "given_name": "Piboon", "clpid": "Pantu-P" }, { "family_name": "Kim", "given_name": "Kiseok", "clpid": "Kim-Kiseok" }, { "family_name": "Gavalas", "given_name": "George R.", "orcid": "0000-0003-1468-6835", "clpid": "Gavalas-G-R" } ], "abstract": "Partial oxidation of methane to synthesis gas over platinum or ruthenium supported on Ce_(1\u2212x)Zr_xO_2 (x = 0, 0.2 and 0.5) was studied at 550\u2013700\u00b0C in the absence of gaseous oxygen. The reaction was carried out in a packed-bed reactor under continuous or pulsed flows of methane. Oxidation utilized oxide oxygen and was initially very fast but slowed down as the oxide support became progressively reduced. Addition of ZrO_2 into CeO_2 considerably increased the rate of methane oxidation and enhanced the reducibility of CeO_2 but decreased the selectivity to carbon monoxide and hydrogen. Specifically it was found that significant production of carbon dioxide and water occurred on the freshly oxidized solid until a certain degree of reduction was reached beyond which the selectivity to carbon monoxide and hydrogen rose to over 90%. This critical degree of reduction was 10%, 40% and 65% for the solid compositions x = 0, 0.2 and 0.5, respectively. Additional experiments carried out using carbon monoxide pulses showed that carbon monoxide oxidation declines sharply and becomes negligible beyond this degree of reduction while oxidation of methane continues much further. Comparison of the two metals showed that platinum is more active but the reaction rate did not change much in the range of platinum loadings of 0.25\u20131 wt.%.", "doi": "10.1016/S0926-860X(99)00429-9", "issn": "0926-860X", "publisher": "Elsevier", "publication": "Applied Catalysis A: General", "publication_date": "2000-02-28", "series_number": "1-2", "volume": "193", "issue": "1-2", "pages": "203-214" }, { "id": "authors:jre6r-k1s25", "collection": "authors", "collection_id": "jre6r-k1s25", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20151006-150212735", "type": "book_section", "title": "Chemical vapor deposition membranes", "book_title": "Recent Advances in Gas Separation by Microporous Ceramic Membranes", "author": [ { "family_name": "Tsapatsis", "given_name": "M.", "orcid": "0000-0001-5610-3525", "clpid": "Tsapatsis-M" }, { "family_name": "Gavalas", "given_name": "G. R.", "orcid": "0000-0003-1468-6835", "clpid": "Gavalas-G-R" }, { "family_name": "Xomeritakis", "given_name": "G.", "clpid": "Xomeritakis-G" } ], "contributor": [ { "family_name": "Kanellopoulos", "given_name": "N. K.", "clpid": "Kanellopoulos-N-K" } ], "abstract": "This chapter is divided into four sections the first of which treats issues of general relevance to Chemical Vapor Deposition (CVD) of membranes, the second reviews work on dense silica membranes, the third is devoted to Y_2O_3-stabilized ZrO_2 (YSZ) membranes, and the fourth treats CVD of Pd membranes.", "doi": "10.1016/S0927-5193(00)80017-0", "isbn": "978-0-444-50272-8", "publisher": "Elsevier", "place_of_publication": "New York, NY", "publication_date": "2000", "pages": "397-416" }, { "id": "authors:mz3k8-env28", "collection": "authors", "collection_id": "mz3k8-env28", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20151105-083817088", "type": "article", "title": "Synthesis of Porous Inorganic Membranes", "author": [ { "family_name": "Tsapatsis", "given_name": "Michael", "orcid": "0000-0001-5610-3525", "clpid": "Tsapatsis-M" }, { "family_name": "Gavalas", "given_name": "George R.", "orcid": "0000-0003-1468-6835", "clpid": "Gavalas-G-R" } ], "abstract": "Here we will attempt a brief overview of recent synthetic efforts for micropore and lower-end mesopore membranes. We will not address the very important classes of nonporous membranes, such as dense metals and solid electrolytes with applications in H_2 and O_2 separations, or meso- and macroporous membranes, which find applications in food processing and water treatment. Microporous materials provide high permselectivities for molecules encountered in the chemical-processing industry but suffer from low intrinsic permeabilities. Therefore, in order to bring microporous membrane materials to commercial applications, functional composites with small effective thicknesses (in the micron or submicron range) must be developed. For example, to achieve economical membrane-reactor sizes, fluxes as high as 0.1 mol/(m^2 s) are desirable. Approaches to microporous membranes include modification of mesoporous membranes by sol-gel and chemical-vapor-deposition (CVD) techniques, carbonization of polymers to form molecular-sieve carbon, and polycrystalline-film growth of zeolites and other molecular sieves.", "doi": "10.1557/S0883769400051885", "issn": "0883-7694", "publisher": "Cambridge University Press", "publication": "MRS Bulletin", "publication_date": "1999-03", "series_number": "3", "volume": "24", "issue": "3", "pages": "30-35" }, { "id": "authors:5q6jv-x7b58", "collection": "authors", "collection_id": "5q6jv-x7b58", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20151007-090423856", "type": "article", "title": "Molecular Dynamics Simulations of Diffusion in Mesoporous Glass", "author": [ { "family_name": "Fernandes", "given_name": "Neil E.", "clpid": "Fernandes-N-E" }, { "family_name": "Gavalas", "given_name": "George R.", "orcid": "0000-0003-1468-6835", "clpid": "Gavalas-G-R" } ], "abstract": "The effect of gas\u2212solid intrapore potential and surface roughness on diffusion in a single capillary was investigated by molecular dynamics simulations. Calculations were carried out for nitrogen and isobutane under free molecular flow conditions in pores of diameter 4\u221214 \u00c5 at temperatures of 200\u2212800 K. The gases were treated as Lennard-Jones atoms and the pore surface was taken as cylindrical, exerting a 9\u22123 potential. No energy transfer was considered between the gas and solid, but interaction with the roughened pore wall provided the scattering required for diffusive transport. Two effects of the gas\u2212solid potential were examined in some detail. One is the enhanced intrapore gas concentration which increases the flux, and the other is the bending of the molecular trajectories which decreases the flux. In pores of radius 20 \u00c5, both effects were significant for temperatures as high as 500 K and were enhanced as the temperature decreased. For nitrogen, the two effects partially canceled each other over the temperature range examined, resulting in a temperature dependence similar to that of Knudsen diffusion. For isobutane, the partitioning effect dominated the path curvature effect at temperatures as high as 500 K.", "doi": "10.1021/ie9801150", "issn": "0888-5885", "publisher": "American Chemical Society", "publication": "Industrial & Engineering Chemistry Research", "publication_date": "1999-03", "series_number": "3", "volume": "38", "issue": "3", "pages": "723-730" }, { "id": "authors:00a52-47990", "collection": "authors", "collection_id": "00a52-47990", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:ZHAcc99", "type": "article", "title": "Vapor-phase transport synthesis of ZnAPO-34 molecular sieve", "author": [ { "family_name": "Zhang", "given_name": "Lixiong", "clpid": "Zhang-Lixiong" }, { "family_name": "Gavalas", "given_name": "George R.", "orcid": "0000-0003-1468-6835", "clpid": "Gavalas-G-R" } ], "abstract": "ZnAPO-34 molecular sieve can be synthesized by the vapor-phase transport technique using triethylamine as a structure-directing agent.", "doi": "10.1039/a809399g", "issn": "1359-7345", "publisher": "Chemical Communications", "publication": "Chemical Communications", "publication_date": "1999-01-07", "series_number": "1", "volume": "1999", "issue": "1", "pages": "97-98" }, { "id": "authors:8xyx9-syf69", "collection": "authors", "collection_id": "8xyx9-syf69", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20151007-090424153", "type": "article", "title": "Surface Seeding in ZSM-5 Membrane Preparation", "author": [ { "family_name": "Lai", "given_name": "Re", "clpid": "Lai-Re" }, { "family_name": "Gavalas", "given_name": "George R.", "orcid": "0000-0003-1468-6835", "clpid": "Gavalas-G-R" } ], "abstract": "ZSM-5 zeolite membranes were prepared by in situ hydrothermal synthesis on macroporous \u03b1-alumina tubes coated with silicalite seeds. Seed particles were prepared by hydrothermal synthesis at 95 \u00b0C to mean size 0.4 or 2 \u03bcm, purified of unconverted reactants, and suspended in a buffered aqueous solution. The support tubes were seeded by immersion in the particle suspension under ultrasonication. Membrane growth on seeded supports was carried out at temperatures 110\u2212150 \u00b0C and different reaction times. An external polycrystalline layer gradually grows on top of the support, with siliceous deposits accumulating as deep as 100 \u03bcm inside the pores of the support. Thermogravimetric analysis and nitrogen and argon adsorption measurements indicate graduate conversion of deposits from amorphous to crystalline. Pure gas permeation results are presented for membranes prepared using 0.4 and 2 \u03bcm seeds under different compositions and temperatures.", "doi": "10.1021/ie980265a", "issn": "0888-5885", "publisher": "American Chemical Society", "publication": "Industrial & Engineering Chemistry Research", "publication_date": "1998-11", "series_number": "11", "volume": "37", "issue": "11", "pages": "4275-4283" }, { "id": "authors:t5vp0-6mr76", "collection": "authors", "collection_id": "t5vp0-6mr76", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20151007-090424422", "type": "article", "title": "Gas transport in porous Vycor glass subjected to gradual pore narrowing", "author": [ { "family_name": "Fernandes", "given_name": "Neil E.", "clpid": "Fernandes-N-E" }, { "family_name": "Gavalas", "given_name": "George R.", "orcid": "0000-0003-1468-6835", "clpid": "Gavalas-G-R" } ], "abstract": "Porous Vycor\u00ae glass was modified by deposition of silica on the internal pore surface using consecutive cycles of liquid-phase silylation with silicon tetrachloride, and hydrolysis. Macroscopically uniform deposition was achieved by exploiting the self-limiting nature of the reaction and the extent of deposition was monitored by the weight change of the samples. Weight increases as high as 24% were recorded and the average pore diameter was estimated to decrease from \u223c44 \u00c5 to \u223c20 \u00c5. Permeation measurements were conducted in the Henry's law region at various levels of deposition for hydrogen, methane, isobutane and nitrogen, at temperatures between 60\u00b0C and 180\u00b0C. The measurements were compared to values calculated with a model using the effective medium approximation to treat network effects and Clausing's correction to account for conductances in pores of finite aspect ratio. The calculated values proved to be inaccurate for hydrogen, overestimating the permeance by a factor of 2 at high levels of deposition possibly because of non-ideal pore shapes accentuated by the deposition. For nitrogen and methane the agreement between calculations and measurements was better due to a fortuitous cancellation of deviations caused by the enhanced potential energy well within the pores and the non ideal pore shape. The intrapore potential energy effect was especially strong for isobutane and as a result the calculated flux was always less than the experimental.", "doi": "10.1016/S0009-2509(97)00399-0", "issn": "0009-2509", "publisher": "Pergamon", "publication": "Chemical Engineering Science", "publication_date": "1998-02-06", "series_number": "5", "volume": "53", "issue": "5", "pages": "1049-1058" }, { "id": "authors:swscw-bpv84", "collection": "authors", "collection_id": "swscw-bpv84", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20151007-090424728", "type": "article", "title": "Modeling of SiO_2 deposition in porous Vycor: Effects of pore network connectivity", "author": [ { "family_name": "Tsapatsis", "given_name": "Michael", "orcid": "0000-0001-5610-3525", "clpid": "Tsapatsis-M" }, { "family_name": "Gavalas", "given_name": "George R.", "orcid": "0000-0003-1468-6835", "clpid": "Gavalas-G-R" } ], "abstract": "A mathematical model developed for SiO_2 deposition in porous Vycor glass using SiCl_4 hydrolysis describes reaction, diffusion and evolution of the pore structure due to accumulation of the solid product. The deposition reaction is described by transient heterogeneous kinetics in terms of the concentrations of silanol and chloride groups in the product layer as well as those of the gaseous reactants. For typical deposition conditions the pseudo steady-state approximation for surface species could lead to erroneous predictions. Pore structure evolution is modeled by incorporating results of percolation theory. For this purpose the porous glass is represented by a Bethe lattice with coordination number 3 and alternatively by a decorated Bethe lattice in which each bond is replaced by a composite bond consisting of two bonds in series. The second network can capture the effect of pore radius variation along a single pore. For the decorated lattice, pore connectivity interruption at a higher void fraction leads to thinner deposits and shorter deposition times for pore plugging compared to the corresponding ones for the simple lattice.", "doi": "10.1002/aic.690430720", "issn": "0001-1541", "publisher": "Wiley", "publication": "AIChE Journal", "publication_date": "1997-07", "series_number": "7", "volume": "43", "issue": "7", "pages": "1849-1860" }, { "id": "authors:gp5n7-bg883", "collection": "authors", "collection_id": "gp5n7-bg883", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20151007-090424989", "type": "article", "title": "Use of diffusion barriers in the preparation of supported zeolite ZSM-5 membranes", "author": [ { "family_name": "Yan", "given_name": "Yushan", "clpid": "Yan-Yushan" }, { "family_name": "Davis", "given_name": "Mark E.", "orcid": "0000-0001-8294-1477", "clpid": "Davis-M-E" }, { "family_name": "Gavalas", "given_name": "George R.", "orcid": "0000-0003-1468-6835", "clpid": "Gavalas-G-R" } ], "abstract": "Zeolite ZSM-5 membranes were prepared by in situ crystallization on porous \u03b1-Al_2O_3 disks that contained a diffusion barrier to limit excessive penetration of siliceous species into the alumina pores. The barrier was introduced into the alumina pores by impregnating the porous disk with a 1:1 molar mixture of furfuryl alcohol (FA) and tetraethylorthosilicate (TEOS), polymerizing the mixture retained in the disk, and carbonizing the resulting polymer at 600\u00b0C in N_2. Following carbonization, a partial carbon burn-off was carried out by catalyzed oxidation in 2% O_2\ue5f8N_2 at 600\u00b0C to generate a carbon-free region near the surface of the support. After zeolite crystallization, the remaining carbon and the organic structure directing agent were removed by calcination in air, at 500\u00b0C. It was found that pure carbon does not support zeolite growth while the solid obtained from a mixture of FA and TEOS does, due to the presence of dispersed silica. Membranes synthesized using barriers have n-butane flux and n-butane/isobutane selectivity 2.7 \u00d7 10^(\u22123) mol m^(\u22122) s^(\u22121) and 45, respectively, at 185\u00b0C, which are, respectively, ca. 1.6 and 4 times as large as those of membranes prepared without the use of barriers. Electron probe microanalysis (EPMA) and X-ray diffraction (XRD) revealed that the internal layer of the barrier-pretreated membrane has smaller thickness and higher crystallinity accounting for the increased flux and selectivity.", "doi": "10.1016/S0376-7388(96)00263-3", "issn": "0376-7388", "publisher": "Elsevier", "publication": "Journal of Membrane Science", "publication_date": "1997-04-02", "series_number": "1", "volume": "126", "issue": "1", "pages": "53-65" }, { "id": "authors:enz5d-5wk89", "collection": "authors", "collection_id": "enz5d-5wk89", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20151007-090425546", "type": "article", "title": "Preparation of highly selective zeolite ZSM-5 membranes by a post-synthetic coking treatment", "author": [ { "family_name": "Yan", "given_name": "Yushan", "clpid": "Yan-Yushan" }, { "family_name": "Davis", "given_name": "Mark E.", "orcid": "0000-0001-8294-1477", "clpid": "Davis-M-E" }, { "family_name": "Gavalas", "given_name": "George R.", "orcid": "0000-0003-1468-6835", "clpid": "Gavalas-G-R" } ], "abstract": "Zeolite ZSM-5 membranes with high n-butane:isobutane selectivities, e.g., 322 at 185\u00b0C, are obtained by a selective deposition of coke into non-zeolitic pores. The zeolite membranes are prepared by in situ crystallization on either bare porous \u03b1-Al_2O_3 support disks or disks that are pretreated to include a diffusion barrier. The post-synthetic coking treatment is accomplished by impregnating these membranes with liquid 1,3,5-triisopropylbenzene (TIPB) for 24 h at room temperature and then calcining them in air at 500\u00b0C for 2 h. Calcination at 500\u00b0C for up to 30 h does not destroy the high n-butane:isobutane selectivity. Thermogravimetric analysis (TGA) experiments on two model pore systems ZSM-5 (5.5 \u00c5) and Vycor glass (40\u201350 \u00c5) suggest that micro-defects are selectively eliminated by the TIPB coking treatment while the intracrystalline pore space of the ZSM-5 is not affected. The elimination of non-zeolitic pores results in a large increase of n-butane:isobutane pure gas flux ratio (45 vs. 320 at 185\u00b0C) accompanied by a fourfold reduction of the n-butane flux. The permeation experiments reveal that the n-butane flux increases nonlinearly with the partial pressure in the feed while the n-butane:isobutane pure gas flux ratio remains relatively unchanged.", "doi": "10.1016/S0376-7388(96)00206-2", "issn": "0376-7388", "publisher": "Elsevier", "publication": "Journal of Membrane Science", "publication_date": "1997-01-08", "series_number": "1", "volume": "123", "issue": "1", "pages": "95-103" }, { "id": "authors:sschj-hzz56", "collection": "authors", "collection_id": "sschj-hzz56", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20151007-090425260", "type": "article", "title": "Permeation studies on oriented single-crystal ferrierite membranes", "author": [ { "family_name": "Lewis", "given_name": "John E., Jr.", "clpid": "Lewis-J-E-Jr" }, { "family_name": "Gavalas", "given_name": "George R.", "orcid": "0000-0003-1468-6835", "clpid": "Gavalas-G-R" }, { "family_name": "Davis", "given_name": "Mark E.", "orcid": "0000-0001-8294-1477", "clpid": "Davis-M-E" } ], "abstract": "Large, high-quality, single crystals of pure-silica ferrierite are synthesized, and the structure is described. Selected individual crystals (approximately 600 \u03bcm X 500 \u03bcm X 20 \u03bcm) are mounted in a membrane configuration so that only the 10-membered-ring channels (5.4 \u00c5 X 5.4 \u00c5 X 4.2 \u00c5) or the 8-membered-ring channels (4.6 \u00c5 X 3.7 \u00c5 X 3.0 \u00c5) are accessible for gas-molecule permeation. The first examples of transport exclusively through 8- or 10-membered-ring channel systems are reported and obtained through crystal orientation in the membrane. A series. of adsorption experiments are conducted to help select suitable probe molecules and evaluate the role of adsorption in the permeation process for single-crystal membranes. Methane, n-butane, isobutane and nitrogen probe molecules are used to study intracrystalline sorption and transport effects for different crystal orientations, pressures and temperatures. Both pure-gas selectivities and mixed-gas separation factors are reported. A mixed-gas separation factor of n-butane/isobutane = 116 for the 10-membered-ring orientation of the crystal at 383 K and a transmembrane pressure difference of 1.01 X 10^5 Pa are found using this technique. In addition, molecular sieving is observed for the 8-membered-ring orientation of the crystal since methane, but not butane, transport is observed for this crystal orientation.", "doi": "10.1002/aic.690430111", "issn": "0001-1541", "publisher": "Wiley", "publication": "AIChE Journal", "publication_date": "1997-01", "series_number": "1", "volume": "43", "issue": "1", "pages": "83-90" }, { "id": "authors:4hwm3-swb88", "collection": "authors", "collection_id": "4hwm3-swb88", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20151007-090425847", "type": "article", "title": "Solution of equations describing fluid\u2013soil reactions in packed columns", "author": [ { "family_name": "Fernandes", "given_name": "Neil", "clpid": "Fernandes-N-E" }, { "family_name": "Gavalas", "given_name": "George R.", "orcid": "0000-0003-1468-6835", "clpid": "Gavalas-G-R" } ], "abstract": "A semianalytical technique is presented frr solving the equations describing an isothermal, irreversible reaction of a trace component in a packed bed of a solid reactant or adsorbent The reaction-rate expression is assumed to have an arbitrary dependence on the solid reactant concentration, but a first-order dependence on the trace gas component. The technique relies on an integral transformation that reduces the set of partial differential equations to a set of two coupled ordinary differential equations in the spatial variables. Solving these two equations is simpler than solving the original equations by finite differences or finite elements, especially in the presence of steep concentration gradients. Two examples presented illustrate the technique: a progressive conversion model with reaction occurring at dispersed sites within the adsorbent pellet and a reaction proceeding in the shrinking core mode.", "doi": "10.1002/aic.690411205", "issn": "0001-1541", "publisher": "Wiley", "publication": "AIChE Journal", "publication_date": "1995-12", "series_number": "12", "volume": "41", "issue": "12", "pages": "2549-2555" }, { "id": "authors:7118z-mnf54", "collection": "authors", "collection_id": "7118z-mnf54", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20151007-090426154", "type": "article", "title": "Temporary carbon barriers in the preparation of H_2-permselective silica membranes", "author": [ { "family_name": "Jiang", "given_name": "S.", "clpid": "Jiang-S" }, { "family_name": "Yan", "given_name": "Y.", "clpid": "Yan-Yushan" }, { "family_name": "Gavalas", "given_name": "G. R.", "orcid": "0000-0003-1468-6835", "clpid": "Gavalas-G-R" } ], "abstract": "Temporary carbon barriers were used to prepare hydrogen-permselective membranes by chemical vapor deposition (CVD) of silica on porous Vycor support tubes. The carbon barriers were introduced into the pores of the support by vapor deposition polymerization of furfuryl alcohol, catalyzed by p-toluene sulfonic acid. After polymerization and crosslinking the polymer was carbonized by slow heating to 600\u00b0C and CVD of silica was carried out at 600\u00b0C by alternating flows of SiCl_4 and H_2O. During CVD the carbon residue inside the pores limits the penetration of SiCl_4 resulting in a thinner deposit layer. After CVD the carbon was removed by oxidation. The hydrogen permeance of the deposit layer obtained using the carbon barriers was 2\u20134 times higher than the permeance achieved without the use of carbon barriers. Thermogravimetric experiments were carried out to obtain information about the amount and spatial distribution of the carbon within the porous support.", "doi": "10.1016/0376-7388(95)00004-V", "issn": "0376-7388", "publisher": "Elsevier", "publication": "Journal of Membrane Science", "publication_date": "1995-07-31", "series_number": "3", "volume": "103", "issue": "3", "pages": "211-218" }, { "id": "authors:0jkbk-71356", "collection": "authors", "collection_id": "0jkbk-71356", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20151007-090426721", "type": "article", "title": "Laser Ignition of Levitated Char Particles", "author": [ { "family_name": "Wong", "given_name": "Brian A.", "clpid": "Wong-Brian-A" }, { "family_name": "Gavalas", "given_name": "George R.", "orcid": "0000-0003-1468-6835", "clpid": "Gavalas-G-R" }, { "family_name": "Flagan", "given_name": "Richard C.", "orcid": "0000-0001-5690-770X", "clpid": "Flagan-R-C" } ], "abstract": "The temperature history of a char particle that was heated by a carbon dioxide laser to the ignition point was studied in the electrodynamic balance. The ignition characteristics of char particles are an important part of the overall coal combustion process, and the aim of research in this area is to improve efficiency and minimize waste products and pollution in current combustors. The experiments were conducted using 158-210 \u00b5m diameter Spherocarb particles, and 200-300 \u00b5m diameter char particles from a bituminous coal. Individual charged char particles were suspended in the electrodynamic balance and heated by a 420 ms pulse of radiation from a CO_2 laser at a heating rate of approximately 4.5 x 10^3 K/s. The temperature of the particle was measured with an optical pyrometer. The atmosphere in the electrodynamic balance was initially pure nitrogen. The oxygen concentration was slowly increased as successive laser pulses were applied, until the particle ignited. Upon ignition, the particle lost charge and dropped from the center of the balance. The particle temperature profile preceding ignition shows the ignition delay predicted by the general ignition condition of Semenov's thermal explosion theory. The measured temperature traces could be described well by calculations treating the particle as a sphere and using rate parameters given in the literature.", "doi": "10.1021/ef00051a013", "issn": "0887-0624", "publisher": "American Chemical Society", "publication": "Energy and Fuels", "publication_date": "1995-05", "series_number": "3", "volume": "9", "issue": "3", "pages": "484-492" }, { "id": "authors:8r5av-sae62", "collection": "authors", "collection_id": "8r5av-sae62", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20151007-090426421", "type": "article", "title": "Effect of Char Formation Temperature on the Densification of a Bituminous Coal Char during Gasification", "author": [ { "family_name": "Wong", "given_name": "Brian A.", "clpid": "Wong-Brian-A" }, { "family_name": "Gavalas", "given_name": "George R.", "orcid": "0000-0003-1468-6835", "clpid": "Gavalas-G-R" }, { "family_name": "Flagan", "given_name": "Richard C.", "orcid": "0000-0001-5690-770X", "clpid": "Flagan-R-C" } ], "abstract": "It has been observed that some porous carbon particles shrink when oxidized under kinetically limited conditions, instead of reacting at constant diameter, as expected. This study was performed to determine whether or not char from PSOC 1451 coal prepared at different temperatures would also shrink during low-temperature gasification. Coal chars, formed at 1200 and 1600 K, were levitated in the electrodynamic balance and heated with a carbon dioxide laser. The 1200 K char did shrink, as observed in the previous study, while the 1600 K char reacted at constant diameter, as expected under the kinetically limited conditions. These results demonstrate that the time and temperature history of a char influence its behavior during subsequent oxidation.", "doi": "10.1021/ef00051a014", "issn": "1520-5029", "publisher": "American Chemical Society", "publication": "Energy and Fuels", "publication_date": "1995-05", "series_number": "3", "volume": "9", "issue": "3", "pages": "493-499" }, { "id": "authors:f5kd4-f4678", "collection": "authors", "collection_id": "f5kd4-f4678", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20151007-090427013", "type": "article", "title": "Preparation of Zeolite ZSM-5 Membranes by In-Situ Crystallization on Porous \u03b1-Al_2O_3", "author": [ { "family_name": "Yan", "given_name": "Yushan", "clpid": "Yan-Yushan" }, { "family_name": "Davis", "given_name": "Mark E.", "orcid": "0000-0001-8294-1477", "clpid": "Davis-M-E" }, { "family_name": "Gavalas", "given_name": "George R.", "orcid": "0000-0003-1468-6835", "clpid": "Gavalas-G-R" } ], "abstract": "Zeolite ZSM-5 membranes were grown on porous \u03b1-Al_2O_3 disks by in-situ hydrothermal synthesis at 175 \"C. The zeolite layers were formed on the bottom face of disks placed horizontally near the air-liquid interface of clear synthesis solutions. Extensive experimentation was carried out to find compositions that gave continuous polycrystalline films. The films grown with the optimized composition were about 10 \u00b5m thick and consisted of well-intergrown crystals of about 2 \u00b5m size. Pure gas permeation measurements of the most successful preparation yielded hydrogen:isobutane and n-butane:isobutane ratios of 151 and 18 at room temperature and 54 and 31 at 185 \u00b0C, respectively.", "doi": "10.1021/ie00044a018", "issn": "0888-5885", "publisher": "American Chemical Society", "publication": "Industrial & Engineering Chemistry Research", "publication_date": "1995-05", "series_number": "5", "volume": "34", "issue": "5", "pages": "1652-1661" }, { "id": "authors:ynp5g-cs518", "collection": "authors", "collection_id": "ynp5g-cs518", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20120215-105850717", "type": "article", "title": "Zeolite ZSM-5 membranes grown on porous \u03b1-Al_2O_3", "author": [ { "family_name": "Yan", "given_name": "Yushan", "clpid": "Yan-Yushan" }, { "family_name": "Tsapatsis", "given_name": "Michael", "orcid": "0000-0001-5610-3525", "clpid": "Tsapatsis-M" }, { "family_name": "Gavalas", "given_name": "George R.", "orcid": "0000-0003-1468-6835", "clpid": "Gavalas-G-R" }, { "family_name": "Davis", "given_name": "Mark E.", "orcid": "0000-0001-8294-1477", "clpid": "Davis-M-E" } ], "abstract": "ZSM-5 membranes with good permeation selectivity for n-butane\u2013isobutane are grown hydrothermally from a clear solution on horizontally-held porous \u03b1-Al_2O_3 disks.", "doi": "10.1039/c39950000227", "issn": "0022-4936", "publisher": "Royal Society of Chemistry", "publication": "Journal of the Chemical Society. Chemical Communications", "publication_date": "1995-01-21", "series_number": "2", "volume": "1995", "issue": "2", "pages": "227-228" }, { "id": "authors:qkv7h-xtg60", "collection": "authors", "collection_id": "qkv7h-xtg60", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20151007-090427301", "type": "article", "title": "Preparation of H_2 Permselective Silica Membranes by Alternating Reactant Vapor Deposition", "author": [ { "family_name": "Kim", "given_name": "Soojin", "clpid": "Kim-Soojin" }, { "family_name": "Gavalas", "given_name": "George R.", "orcid": "0000-0003-1468-6835", "clpid": "Gavalas-G-R" } ], "abstract": "Thin SiO_2 layers were deposited on the surface of porous Vycor glass by alternating vapor phase reaction with SiCl_4 and H_2O. The membranes prepared by this technique had a H_2 permeance of 0.3-0.4 cm^3(STP)/(min\u2022cm^2\u2022atm) and a H_2:N_2 selectivity of 500-1000 at 600 \u00b0C. The SiCl_4 dosage at each silylation cycle, the concentration of initial surface OH groups, and the reaction temperature significantly influence the deposit layer thickness. After two weeks of heating at 550 \u00b0C under 3 atm of water vapor, the membrane H_2 permeance decreased by about 20%, and the selectivity increased to more than 2000. The membrane properties after this hydrothermal treatment are superior to those of membranes prepared earlier by one-sided (steady flow) deposition. A simple model incorporating diffusion and surface reaction was used to study the effect of various parameters on the formation of the deposit layer.", "doi": "10.1021/ie00040a016", "issn": "0888-5885", "publisher": "American Chemical Society", "publication": "Industrial & Engineering Chemistry Research", "publication_date": "1995-01", "series_number": "1", "volume": "34", "issue": "1", "pages": "168-176" }, { "id": "authors:jfsjk-5z444", "collection": "authors", "collection_id": "jfsjk-5z444", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20151210-154115769", "type": "book_section", "title": "New Development in Hydrogen Perm Selective Membranes", "author": [ { "family_name": "Gavalas", "given_name": "George R.", "orcid": "0000-0003-1468-6835", "clpid": "Gavalas-G-R" }, { "family_name": "Johnson", "given_name": "Richard A.", "clpid": "Johnson-R-A" }, { "family_name": "Venkataraman", "given_name": "Ventak K.", "clpid": "Venkataraman-V-K" } ], "contributor": [ { "family_name": "McDaniel", "given_name": "Heather M.", "clpid": "McDaniel-H-M" }, { "family_name": "Staubly", "given_name": "Ronald K.", "clpid": "Staubly-R-K" }, { "family_name": "Venkataraman", "given_name": "Ventak K.", "clpid": "Venkataraman-V-K" } ], "abstract": "The objectives of the original project was to\ndevelop silica hydrogen permselective membranes\nand evaluate the economic feasibility of these\nmembranes in hydrogen production from coal gas.\nThe objectives of the work reported here were to\nincrease the membrane permeance by developing\nnew precursors or deposition conditions, and to\ncarry out fundamental permeability measurements\nof the membrane at different stages of pore\nnarrowing.", "publisher": "U.S. Dept. of Energy, Office of Fossil Energy, Morgantown Energy Technology Center", "publication_date": "1994-10-01" }, { "id": "authors:f5wfc-2d042", "collection": "authors", "collection_id": "f5wfc-2d042", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20151007-090427609", "type": "article", "title": "Structure and aging characteristics of H_2-permselective SiO_2-Vycor membranes", "author": [ { "family_name": "Tsapatsis", "given_name": "Michael", "orcid": "0000-0001-5610-3525", "clpid": "Tsapatsis-M" }, { "family_name": "Gavalas", "given_name": "George", "orcid": "0000-0003-1468-6835", "clpid": "Gavalas-G-R" } ], "abstract": "Hydrogen-permselective membranes were prepared by atmospheric pressure chemical vapor deposition (APCVD) of SiO_2 layers in porous Vycor tubes. The deposition was carried out by passing the reactants SiCl_4 and H_2O through the bore of the support tubes at temperatures ranging from 600 to 800\u00b0C. The deposit layers were examined by TEM, SEM, and EPMA. When the deposit was confined inside the pores of the Vycor substrate, the membranes were mechanically stable but when it extended substantially outside of the porous matrix the stresses induced by thermal cycling led to crack formation and propagation. Electron microscopy revealed that the SiO_2 deposit density is maximum in a region \u22480.5 \u03bcm thick adjacent to the bore surface and gradually declines to zero within a depth of \u224810 \u03bcm from the surface. The thin region of maximum deposit density is responsible for permselectivity, for it essentially blocks the permeation of nitrogen and larger molecules while allowing substantial permeation of hydrogen. This region contains \u224810% by volume trapped voids and as a result has relatively high permeability as suggested by the percolation theory. Annealing at high temperatures causes densification of the deposited material as evidenced by increased activation energy for H_2 permeation and correspondingly reduced permeance. The presence of H_2O vapor accelerates the densification process. The densified membranes had a H_2 permeance as high as 0.1 cm^3(STP)/min-atm-cm^2 at 500\u00b0C and a H_2/N_2 permeance ratio above 500.", "doi": "10.1016/0376-7388(94)87034-9", "issn": "0376-7388", "publisher": "Elsevier", "publication": "Journal of Membrane Science", "publication_date": "1994-03-02", "series_number": "3", "volume": "87", "issue": "3", "pages": "281-296" }, { "id": "authors:k6qvt-7qn13", "collection": "authors", "collection_id": "k6qvt-7qn13", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20151007-090428227", "type": "article", "title": "Kinetic Study of the Reactions of Chlorosilanes with Porous Vycor Glass", "author": [ { "family_name": "Kim", "given_name": "Soojin", "clpid": "Kim-Soojin" }, { "family_name": "Gavalas", "given_name": "George R.", "orcid": "0000-0003-1468-6835", "clpid": "Gavalas-G-R" } ], "abstract": "The alternating reactions of SiCl_4 and H_2O with the surface of Vycor glass have been studied by thermogravimetric analysis at 400-600\u00b0C. The results are interpreted by postulating two types of silanol groups (Si-OH) on the Vycor surface, paired and isolated. Reaction of SiCl_4 with paired silanols proceeds by competing steps which control the total extent of silylation. Upon reaction with H_2O, all surface Si-Cl groups are hydrolyzed, reproducing Si-OH. Simultaneous silanol-silanol and silanol-surface chloride condensations eliminate H_2O or HCl to form new Si-O-Si bridges with final product silica. A kinetic model incorporating the silylation, hydrolysis, and condensation steps describes well the TGA data.", "doi": "10.1006/jcis.1993.1433", "issn": "0021-9797", "publisher": "Elsevier", "publication": "Journal of Colloid and Interface Science", "publication_date": "1993-11", "series_number": "1", "volume": "161", "issue": "1", "pages": "6-18" }, { "id": "authors:19par-c1v16", "collection": "authors", "collection_id": "19par-c1v16", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20151007-090428811", "type": "article", "title": "Catalytic isobutane dehydrogenation in a dense silica membrane reactor", "author": [ { "family_name": "Ioannides", "given_name": "T.", "clpid": "Ioannides-T" }, { "family_name": "Gavalas", "given_name": "G. R.", "orcid": "0000-0003-1468-6835", "clpid": "Gavalas-G-R" } ], "abstract": "The dehydrogenation of isobutane on a commercial chromia-alumina catalyst was studied using a conventional and a membrane reactor. A series of differential runs were conducted varying the temperature and feed composition. Extensive loss of catalyst activity was observed within the first two or three hours on stream and this deactivation intensified with temperature and olefin content but was inhibited by hydrogen. The membrane reactor utilized a dense silica membrane prepared by chemical vapor deposition on a porous Vycor substrate. The membrane permeance as 0.3 cm^3/cm^2-min-atm and the hydrogen to hydrocarbon permeance ratio was 80\u2013300. Integral runs were conducted to compare the performance of the membrane reactor with that of a conventional reactor. At all temperatures and space times the membrane reactor gave higher isobutene yield and selectivity, the difference increasing with the space time. The membrane exhibited stable permeance and permselectivity during several days of operation.", "doi": "10.1016/0376-7388(93)85070-D", "issn": "0376-7388", "publisher": "Elsevier", "publication": "Journal of Membrane Science", "publication_date": "1993-03-04", "series_number": "2-3", "volume": "77", "issue": "2-3", "pages": "207-220" }, { "id": "authors:se5jc-azq76", "collection": "authors", "collection_id": "se5jc-azq76", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20151007-090429100", "type": "article", "title": "Reduction, sulfidation, and regeneration of mixed iron-aluminum oxide sorbents", "author": [ { "family_name": "Patrick", "given_name": "Valerie", "clpid": "Patrick-V" }, { "family_name": "Gavalas", "given_name": "George R.", "orcid": "0000-0003-1468-6835", "clpid": "Gavalas-G-R" }, { "family_name": "Sharma", "given_name": "Pramod K.", "clpid": "Sharma-P-K" } ], "abstract": "The reduction and sulfidation of Fe_2O_3 and Fe_2O_3-Al_2O_3 sorbents were investigated by thermogravimetric analysis (TGA). Mixed iron-aluminum oxides were reduced more slowly and by a more complex mechanism than pure iron oxide. Several phases that were responsible for this difference were identified by temperature-programmed reduction (TPR), X-ray diffraction (XRD), BET surface area (BET), and scanning electron microscopy (SEM). Sulfidation of reduced sorbents in the TGA at 600 \"C produced two crystalline phases: high-temperature, hexagonal pyrrhotite (Fe_(1-x)S), and unreacted FeAl_2O_4. The reaction of the pure and mixed oxide with a mixture of H_2S-H_2-H_2O-N_2 was studied in a tubular microreactor to evaluate the performance of these materials as high-temperature H2S sorbents. At 650 \u00b0C the mixed oxide yielded considerably lower prebreakthrough outlet H_2S levels than the pure iron oxide. Regeneration in pure SO_2 and SO_2-air mixtures of sulfided samples resulted in complete conversion of iron sulfide to iron oxide and quantitative recovery of elemental sulfur.", "doi": "10.1021/ie00015a015", "issn": "0888-5885", "publisher": "American Chemical Society", "publication": "Industrial & Engineering Chemistry Research", "publication_date": "1993-03", "series_number": "3", "volume": "32", "issue": "3", "pages": "519-532" }, { "id": "authors:q221w-wme32", "collection": "authors", "collection_id": "q221w-wme32", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20151105-115204740", "type": "monograph", "title": "Silica membranes for hydrogen separation in coal gas processing. Final report, January 1993", "author": [ { "family_name": "Gavalas", "given_name": "G. R.", "orcid": "0000-0003-1468-6835", "clpid": "Gavalas-G-R" } ], "abstract": "The general objective of this project was to synthesize permselective membranes suitable for hydrogen separation from coal gas. The specific objectives were: (i) to synthesize membranes by chemical vapor deposition (CVD) of SiO(sub 2) or other oxides on porous support tubes, (ii) characterize the membranes by permeation measurements of various gases and by electron microscopy, and (iii) obtain information about the mechanism and kinetics Of SiO(sub 2) deposition, and model the process of membrane formation. Silica glass and certain other glasses, in dense (nonporous) form, are highly selective to hydrogen permeation. Since this high selectivity is accompanied by low permeability, however, a practical membrane must have a composite structure consisting of a thin layer of the active oxide supported on a porous tube or plate providing mechanical support. In this project the membranes were synthesized by chemical vapor deposition (CVD) of SiO(sub 2), TiO(sub 2), Al(sub 2)O(sub 3) and B(sub 2)O(sub 3) layers inside the walls of porous Vycor tubes (5 mm ID, 7 mm OD, 40 (Angstrom) mean pore diameter). Deposition of the oxide layer was carried out using the reaction of SiCl(sub 4) (or TiCl(sub 4), AlCl(sub 3), BCl(sub 3)) and water vapor at elevated temperatures. The porous support tube was inserted concentrically into a larger quartz tube and fitted with flow lines and pressure gauges. The flow of the two reactant streams was regulated by mass flow controllers, while the temperature was controlled by placing the reactor into a split-tube electric furnace.", "publisher": "Caltech Library", "publication_date": "1993-01" }, { "id": "authors:mj58r-njm12", "collection": "authors", "collection_id": "mj58r-njm12", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20151007-090428510", "type": "article", "title": "A Theoretical Study of Combustion of Nonspherical Particles", "author": [ { "family_name": "Choi", "given_name": "Moon Kyu", "clpid": "Choi-Moon-Kyu" }, { "family_name": "Gavalas", "given_name": "George R.", "orcid": "0000-0003-1468-6835", "clpid": "Gavalas-G-R" } ], "abstract": "A theoretical study is carried out of the combustion of nonspherical carbonaceous particles in the regime of shrinking core reaction. The first problem addressed is the calculation of the pseudosteady temperature and oxidation rate for a particle of given shape. This problem involves the solution of the external diffusion and heat conduction equations with the reaction entering as a boundary condition over the particle surface. Using the boundary integral method, the problem is reformulated as a system of two coupled integral equations which are solved numerically by suitable discretization. The complete transient problem addressing the evolution of particle shape and particle temperature during burnout is similarly formulated by the boundary integral method and solved numerically. Over a broad range of parameters, the pseudosteady particle temperature and rate of oxidation are very nearly equal to those of spherical particles of equal volume and surface area respectively. The transient solutions obtained for parameters typical of pulverized combustion show that during burnout the particle becomes increasingly nonspherical. As expected, nonspherical particles burn faster than spherical particles of the same initial volume, but the difference in burnout times is less than 20% for initial aspect ratios between one and three.", "doi": "10.1080/00102209308924099", "issn": "0010-2202", "publisher": "Taylor & Francis", "publication": "Combustion Science and Technology", "publication_date": "1993", "series_number": "1-4", "volume": "89", "issue": "1-4", "pages": "9-26" }, { "id": "authors:3v3js-1tv97", "collection": "authors", "collection_id": "3v3js-1tv97", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20151104-095847610", "type": "article", "title": "A Kinetic Model of Membrane Formation by CVD of SiO_2 and Al_2O_3", "author": [ { "family_name": "Tsapatsis", "given_name": "Michael", "orcid": "0000-0001-5610-3525", "clpid": "Tsapatsis-M" }, { "family_name": "Gavalas", "given_name": "George R.", "orcid": "0000-0003-1468-6835", "clpid": "Gavalas-G-R" } ], "abstract": "Silica and alumina layers deposited onto the walls of porous Vycor tubes by chloride hydrolysis in an opposing reactants geometry have been characterized by scanning electron microscopy and electron microprobe analysis. The layers are asymmetric, having a long tail toward the side of the chloride flow and a sharp boundary at the other side. The deposit thickness is several tenths of microns, while the totally plugged region is of order of 1 micron. \n\nA model has been developed describing reaction, diffusion and evolution of the porous structure in the Vycor substrate due to the accumulation of the solid product. The deposition reaction is described by transient kinetics in terms if the concentrations of silanol and chloride groups in the product layer, as well as the concentrations of the gaseous reactants. The model is capable of generating deposit profiles in good agreement with those measured by electron microprobe analysis.", "doi": "10.1002/aic.690380606", "issn": "0001-1541", "publisher": "Wiley", "publication": "AIChE Journal", "publication_date": "1992-06", "series_number": "6", "volume": "38", "issue": "6", "pages": "847-856" }, { "id": "authors:fzp7x-gr893", "collection": "authors", "collection_id": "fzp7x-gr893", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20151105-120142265", "type": "monograph", "title": "Thermally induced structural changes in coal combustion. Final report", "author": [ { "family_name": "Flagan", "given_name": "R. C.", "orcid": "0000-0001-5690-770X", "clpid": "Flagan-R-C" }, { "family_name": "Gavalas", "given_name": "G. R.", "orcid": "0000-0003-1468-6835", "clpid": "Gavalas-G-R" } ], "abstract": "The effects of the temperature-time history during coal devolitization and oxidation on the physical properties and the reactivity of resulting char were studied experimentally for temperatures and residence times typical of pulverized combustion. Experiments were also carried out at somewhat lower temperatures and correspondingly longer residence times. An electrically heated laminar flow reactor was used to generate char and measure the rates of oxidation at gas temperatures about 1600K. Partially oxidized chars were extracted and characterized by gas adsorption and mercury porosimetry, optical and scanning electron microscopy, and oxidation in a thermogravimetric analysis system (TGA). A different series of experiments was conducted using a quadrople electrodynamic balance. Single particles were suspended electrodynamically and heated by an infrared laser in an inert or oxygen-containing atmosphere. During the laser heating, measurements were taken of particle mass, size/shape, and temperature.", "doi": "10.2172/10159795", "publication_date": "1992-01-01" }, { "id": "authors:nsq39-4xx23", "collection": "authors", "collection_id": "nsq39-4xx23", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20151104-095847943", "type": "article", "title": "Optimal temperature of fixed-bed reactor for high temperature removal of hydrogen sulfide", "author": [ { "family_name": "Lee", "given_name": "Young-Soo", "clpid": "Lee-Young-Soo" }, { "family_name": "Yoo", "given_name": "Kyong-Ok", "clpid": "Yoo-Kyong-Ok" }, { "family_name": "Gavalas", "given_name": "G. R.", "orcid": "0000-0003-1468-6835", "clpid": "Gavalas-G-R" } ], "abstract": "To find optimal temperature of the reaction between H_2S gas and ZnO-5 at% Fe_2O_3 sorbent, the effluent gas from a fixed-bed reactor was analyzed by gas chromatography. The experimental results showed that H_2S removal efficiency of sorbent was maximum at 650\u00b0C and EDX data were in accordance with this feature. XRD analysis exhibited intriguing phenomenon in that different mechanisms were observed at different temperatures. Chemisorption and chemical reaction was considered to be the main mechanism of H_2S removal at 600 C and 650\u00b0C, respectively. SEM photographs supported this interesting phenomenon, but unfortunately TGA and DTA results could not distinguish it. To investigate the effect of sorbent deactivation on the reaction rate, deactivating factor was considered.", "doi": "10.1007/BF02707186", "issn": "0256-1115", "publisher": "Springer", "publication": "Korean Journal of Chemical Engineering", "publication_date": "1991-10", "series_number": "4", "volume": "8", "issue": "4", "pages": "214-219" }, { "id": "authors:6ejyx-gga57", "collection": "authors", "collection_id": "6ejyx-gga57", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20151008-112329165", "type": "article", "title": "Synthesis of hydrogen permselective silicon dioxide, titanium dioxide, aluminum oxide, and boron oxide membranes from the chloride precursors", "author": [ { "family_name": "Tsapatsis", "given_name": "Michael", "orcid": "0000-0001-5610-3525", "clpid": "Tsapatsis-M" }, { "family_name": "Kim", "given_name": "Soojin", "clpid": "Kim-Soojin" }, { "family_name": "Nam", "given_name": "Suk Woo", "clpid": "Nam-Suk-Woo" }, { "family_name": "Gavalas", "given_name": "George", "orcid": "0000-0003-1468-6835", "clpid": "Gavalas-G-R" } ], "abstract": "Hydrogen-permselective membranes were synthesized by chemical vapor deposition of SiO_2, TiO_2, Al_2O_3, and B_2O_3 layers within the pores of Vycor tubes. The deposition involved reaction of SiCl_4, etc., with water at 100-800 \u00b0C depending on the chloride and the reaction geometry. Permselective SiO_2 layers could be formed in either the one-sided or the two-sided (opposing reactants) geometry, while deposition of TiO_2 and Al_2O_3 layers was achieved only in the two-sided geometry. The permeation coefficients at 450 \u00b0C were 0.3 and 0.1 cm^3/(cm^2\u2022min\u2022atm) for SiO_2 membranes produced in the one-sided and two-sided geometries, respectively. The H_2:N_2 permeation ratios were 1000-5000. The TiO_2 and Al_2O_3 membranes had somewhat lower permeation coefficients and H_2:N_2 ratios. The membranes were characterized by scanning electron microscopy (SEM) and electron microprobe analysis (EMA). A reaction mechanism is suggested involving heterogeneous reactions of chloride and water molecules with hydroxyl and chloride groups in the growing deposit layer.", "doi": "10.1021/ie00057a015", "issn": "0888-5885", "publisher": "American Chemical Society", "publication": "Industrial & Engineering Chemistry Research", "publication_date": "1991-09", "series_number": "9", "volume": "30", "issue": "9", "pages": "2152-2159" }, { "id": "authors:6bt6d-70t06", "collection": "authors", "collection_id": "6bt6d-70t06", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20151008-112329475", "type": "article", "title": "Combustion characteristics of carbonaceous residues from heavy oil fired boilers", "author": [ { "family_name": "Northrop", "given_name": "P. Scott", "clpid": "Northrop-P-S" }, { "family_name": "Levendis", "given_name": "Yiannis A.", "clpid": "Levendis-Y-A" }, { "family_name": "Gavalas", "given_name": "George R.", "orcid": "0000-0003-1468-6835", "clpid": "Gavalas-G-R" } ], "abstract": "The combustion of solid carbonaceous particles generated in residual oil-fired furnaces has been investigated. These particles are large ash-rich cenospheres that are not completely burned in the furnace and end up in the effluent gases, creating problems associated with both emissions and reduced operating efficiency. To study the reactivity of these chars, single-particle combustion experiments were conducted where the particle temperature and burnout times were measured by near-infrared two-color pyrometry. The results show that the particle temperature-burn time traces exhibited higher particle-to-particle variability than those of coal-derived char particles, but the overall burnout times were of the same order of magnitude as coal chars of similar size. Taking into account random particle-to-particle property variations, and using a particle combustion model, reaction rate parameters were estimated. Two limiting forms of the model were used for this estimation. The first assumed the particles to burn as shrinking cores, with all reaction concentrated very near the external particle surface. The second assumed the particles to burn at constant radius, with increasing void fraction. Using first-order kinetics, the shrinking core model gave an average apparent activation energy of about 18 kcal/(g mol), with a corresponding average preexponential factor of approximately 37 g/(cm^2 s atm of O_2. These values are similar to the respective parameters estimated for lignite and bituminous chars burned under similar conditions, indicating that these particles can be burned at temperatures and residence times similar to those existing in coal-fired furnaces. The constant radius model gave an intrinsic activation energy of 40 kcal/(g mol), with a preexponential factor of 550 g/(cm^2 s atm of O_2). The constant radius model gave a slightly better fit to the experimental data.", "doi": "10.1021/ef00028a010", "issn": "1520-5029", "publisher": "American Chemical Society", "publication": "Energy and Fuels", "publication_date": "1991-07", "series_number": "4", "volume": "5", "issue": "4", "pages": "587-594" }, { "id": "authors:26fhq-6c606", "collection": "authors", "collection_id": "26fhq-6c606", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20151104-095848238", "type": "article", "title": "Reduction of sulfur dioxide and surface sulfate on \u03b3-alumina", "author": [ { "family_name": "Nam", "given_name": "Suk Woo", "clpid": "Nam-Suk-Woo" }, { "family_name": "Gavalas", "given_name": "George R.", "orcid": "0000-0003-1468-6835", "clpid": "Gavalas-G-R" } ], "abstract": "The desorption and reduction of SO_2 adsorbed on \u03b3-Al_2O_3 and the reduction of surface sulfate on \u03b3-Al_2O_3 have been investigated by microreactor experiments and thermogravimetric analysis (TGA). Temperature-programmed desorption (TPD) of adsorbed SO_2 showed one broad desorption peak. Temperature-programmed reduction (TPR) resulted in the desorption of SO_2 and the production of H_2S and elemental sulfur, the latter two products increasing with increasing pressure of hydrogen. The reduction of surface sulfate is very slow below 550\u00baC with the main reduction product being SO_2. A small amount of H_2S was produced at higher temperatures, probably from the reduction of elemental sulfur by hydrogen. The reduction experiments showed that surface sulfate formed on strong surface sites yielded H_2S or S (el) upon reduction, while sulfate formed on weak sites released SO_2 upon reduction.", "doi": "10.1016/0166-9834(91)90008-V", "issn": "0166-9834", "publisher": "Elsevier", "publication": "Applied Catalysis", "publication_date": "1991-06-27", "series_number": "1", "volume": "74", "issue": "1", "pages": "53-64" }, { "id": "authors:gq5zj-tz335", "collection": "authors", "collection_id": "gq5zj-tz335", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20151106-081103425", "type": "book_section", "title": "Combustion Behavior of Carbon Cenospheres Derived from Pyrolysis of Residual Oil in Furnaces", "author": [ { "family_name": "Northrop", "given_name": "P. S.", "clpid": "Northrop-P-S" }, { "family_name": "Levendis", "given_name": "Y. A.", "clpid": "Levendis-Y-A" }, { "family_name": "Gavalas", "given_name": "G. R.", "orcid": "0000-0003-1468-6835", "clpid": "Gavalas-G-R" } ], "contributor": [ { "family_name": "Gouldin", "given_name": "F. C.", "clpid": "Gouldin-F-C" } ], "abstract": "The combustion behavior of solid carbonaceous particles derived from the pyrolysis of heavy petroleum fuel has been investigated. These particles are generated in residual oil-fired furnaces from pyrolysis of petroleum fuels in the cooler and fuel-rich regions of the furnace. They are large, ash-rich cenospheres that remain unburned in the effluent gases, degrading performance and causing emission problems in utility and industrial furnaces. In order to eliminate the problems related to these residual particles, current research is aimed at (a) the prevention of their formation by improving atomization and evaporation processes[l-3] and (b) oxidative destruction of any formed cenospheres in the combustion chamber by increasing turbulence and mixing, increasing residence time, etc. The results of the investigation described herein are pertinent to the second approach.", "publisher": "Combustion Institute", "publication_date": "1990-12" }, { "id": "authors:7nph2-s2j96", "collection": "authors", "collection_id": "7nph2-s2j96", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20151104-143220828", "type": "article", "title": "Structure and Reduction and Mixed Copper-Aluminum Oxide", "author": [ { "family_name": "Patrick", "given_name": "Valerle", "clpid": "Patrick-V" }, { "family_name": "Gavalas", "given_name": "George", "orcid": "0000-0003-1468-6835", "clpid": "Gavalas-G-R" } ], "abstract": "Mixed oxides of copper and aluminum were prepared in porous form by the citrate process under various calcination conditions. The oxide samples were characterized by several techniques to determine chemical structure and texture. Atomic absorption spectroscopy provided the fractions of copper soluble and insoluble in hot nitric acid, which closely corresponded to CuO and CuAl_2O_4, respectively. X-ray diffraction (XRD) provided complementary information about the content of the pure and compound oxides. X-ray line broadening, scanning electron microscopy, and transmission electron microscopy were used to estimate the size of crystallites or phase domains. The reduction by hydrogen of samples containing different amounts of CuO and CuAl_2O_4 was studied by thermogravimetric analysis and XRD. The compound oxide CuAl_2O_4 and part of CuO closely associated with Al_2O_3 were reduced much more slowly than bulk CuO. Oxides of 1+ oxidation state, Cu_2O and CuAlO_2, were identified as reduction intermediates.", "doi": "10.1111/j.1151-2916.1990.tb06519.x", "issn": "0002-7820", "publisher": "Wiley-Blackwell", "publication": "Journal of the American Ceramic Society", "publication_date": "1990-02", "series_number": "2", "volume": "73", "issue": "2", "pages": "358-369" }, { "id": "authors:qzcbd-35f69", "collection": "authors", "collection_id": "qzcbd-35f69", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20151104-095848622", "type": "article", "title": "Statistical and continuum models of fluid-solid reactions in porous media", "author": [ { "family_name": "Sahimi", "given_name": "Muhammad", "orcid": "0000-0002-8009-542X", "clpid": "Sahimi-M" }, { "family_name": "Gavalas", "given_name": "George R.", "orcid": "0000-0003-1468-6835", "clpid": "Gavalas-G-R" }, { "family_name": "Tsotsis", "given_name": "Theodore T.", "clpid": "Tsotsis-T-T" } ], "abstract": "In this review, we discuss past theoretical works on fluid-solid reactions in a porous medium. Such reactions are often accompanied by a continuous alteration of the pore structure of the medium, and at high conversions they exhibit percolation-type behavior, i.e. the solid matrix of the medium and/or the fluid phase lose their macroscopic connectivity. These phenomena are, therefore, characterized by a percolation threshold which is the volume or area fraction of a phase (solid or fluid) below which that phase exists only in isolated clusters or islands. Important classes of such processes are acid dissolution of a porous medium and gas\u2014solid reactions with pore volume growth, e.g. coal gasification, and with pore closure, e.g. lime sulfation, and catalyst deactivation. These processes are characterized by continuous changes in the pore space as a result of a chemical reaction. We also consider here other processes such as the flow of fines, stable emulsions and solid particles in a porous medium which also alter the structure of the pore space, but by physical interaction of the particles and the solid surface of the pores. In this review we compare two different modelling approaches to reactions accompanied by structural changes. First we review the continuum approach, which is based on the classical equations of transport and reaction supplemented with constitutive equations describing the effect of structural changes on reaction and transport parameters. We then outline the relevant concepts, ideas and techniques of percolation theory and the statistical physics of disordered media, and review their application to the phenomena mentioned above. In particular, we emphasize the fundamental role of connectivity of the porous medium in such phenomena. Since in both approaches one needs to estimate the effective transport properties of the porous medium that is undergoing continuous change, we also review continuum and statistical methods of estimating the effective transport properties of disordered porous media.", "doi": "10.1016/0009-2509(90)80001-U", "issn": "0009-2509", "publisher": "Pergamon", "publication": "Chemical Engineering Science", "publication_date": "1990", "series_number": "6", "volume": "45", "issue": "6", "pages": "1443-1502" }, { "id": "authors:w6cej-71766", "collection": "authors", "collection_id": "w6cej-71766", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20151104-143220301", "type": "article", "title": "Adsorption and Oxidative Adsorption of Sulfur Dioxide on \u03b3-Alumina", "author": [ { "family_name": "Nam", "given_name": "Suk Woo", "clpid": "Nam-Suk-Woo" }, { "family_name": "Gavalas", "given_name": "George R.", "orcid": "0000-0003-1468-6835", "clpid": "Gavalas-G-R" } ], "abstract": "The adsorption and oxidative adsorption of SO_2 on \u03b3-Al_2O_3 have been investigated at temperatures of 500\u2013700\u00b0C by Fourier transform infrared spectroscopy (FTIR) and thermogravimetric analysis (TGA). At temperatures above 500\u00b0C the adsorbed SO_2 species was characterized by an IR band at 1065 cm^(\u22121). The equilibrium coverage and initial rate of adsorption decreased with temperature, suggesting a two-step adsorption. When \u03b3-Al_2O_3 was contacted with a mixture of SO_2 and O_2, adsorption of SO_2 and oxidation of the adsorbed SO_2 to a surface sulfate characterized by broad IR bands at 1070 cm^(\u22121) and 1390 cm^(\u22121) took place. Sulfate formation was found to be reversible, but the reverse reaction took several hours even at 700\u00b0C. The results of a series of TGA experiments under different atmospheres (SO_2-O_2-N_2;SO_2-N_2;O_2-N_2;N_2) strongly suggests that surface SO_2 and surface sulfate involve the same active sites such that SO_2 adsorption is inhibited by already formed sulfate. The results also indicate a broad range of site strengths.", "doi": "10.1016/S0166-9834(00)82328-3", "issn": "0166-9834", "publisher": "Elsevier", "publication": "Applied Catalysis", "publication_date": "1989-11-01", "series_number": "1", "volume": "55", "issue": "1", "pages": "193-213" }, { "id": "authors:ebndv-jn254", "collection": "authors", "collection_id": "ebndv-jn254", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20150805-110229393", "type": "article", "title": "Discrete Simulation of Cenospheric Coal-Char Combustion", "author": [ { "family_name": "Sahu", "given_name": "Ranajit", "clpid": "Sahu-Ranajit" }, { "family_name": "Flagan", "given_name": "Richard C.", "orcid": "0000-0001-5690-770X", "clpid": "Flagan-R-C" }, { "family_name": "Gavalas", "given_name": "George R.", "orcid": "0000-0003-1468-6835", "clpid": "Gavalas-G-R" } ], "abstract": "A phenomenological model of structural transformations during pulverized coal char combustion is presented. The random geometry of the voids is modeled realistically by means of a random assemblage of spheres in a spherical particle. Pore connectedness and growth are accounted for as the combustion proceeds. The percolation behavior of the void space is dramatically demonstrated. The effects of different initial void fractions and of the random nature of the structure itself, at a given initial void fraction, are shown. This sheds some light on our ability to realistically model the solid. The limitation of shrinking core models of combustion of cenospheric chars are demonstrated. In this initial application, the model is limited to isothermal combustion with no diffusional limitations.", "doi": "10.1016/0010-2180(89)90139-9", "issn": "0010-2180", "publisher": "Elsevier", "publication": "Combustion and Flame", "publication_date": "1989-09", "series_number": "3-4", "volume": "77", "issue": "3-4", "pages": "337-346" }, { "id": "authors:ppydc-51q11", "collection": "authors", "collection_id": "ppydc-51q11", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20151019-094243231", "type": "article", "title": "Post-ignition transients in the combustion of single char particles", "author": [ { "family_name": "Levendis", "given_name": "Yiannis A.", "clpid": "Levendis-Yiannis-A" }, { "family_name": "Sahu", "given_name": "Ranajit", "clpid": "Sahu-Ranajit" }, { "family_name": "Flagan", "given_name": "Richard C.", "orcid": "0000-0001-5690-770X", "clpid": "Flagan-R-C" }, { "family_name": "Gavalas", "given_name": "George R.", "orcid": "0000-0003-1468-6835", "clpid": "Gavalas-G-R" } ], "abstract": "The temperature-time histories of single char particles burning at temperatures above 1500 K have been measured by two-colour near infrared pyrometry techniques. Two coal chars and a synthetic char consisting of spherical glassy carbon particles of uniform size were used in the experiments. The results indicate that with lean combustion and low-to-medium temperatures, the char particles do not ignite over their whole external surface, but exhibit preferential ignition at specific sites. These 'hot-spots' probably involve regions in which the material exhibits strong inhomogeneities, either on or below the surface. Following ignition, the reaction may propagate over the particle surface. At elevated combustor temperatures and/or high oxygen partial pressures, ignition spreads quickly over the entire particle surface and the importance of the hot-spots diminishes. A simple model is used to estimate the rate of growth of a hot-spot under various experimental conditions.", "doi": "10.1016/0016-2361(89)90119-1", "issn": "0016-2361", "publisher": "Elsevier", "publication": "Fuel", "publication_date": "1989-07", "series_number": "7", "volume": "68", "issue": "7", "pages": "849-855" }, { "id": "authors:pyav9-06g06", "collection": "authors", "collection_id": "pyav9-06g06", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20151008-112329792", "type": "article", "title": "High-temperature sulfidation-regeneration of CO-Al_2O_3 sorbents", "author": [ { "family_name": "Patrick", "given_name": "V.", "clpid": "Patrick-V" }, { "family_name": "Gavalas", "given_name": "G. R.", "orcid": "0000-0003-1468-6835", "clpid": "Gavalas-G-R" }, { "family_name": "Flytzani-Stephanopoulos", "given_name": "M.", "clpid": "Flytzani-Stephanopoulos-M" }, { "family_name": "Jothimurugesan", "given_name": "K.", "clpid": "Jothimurugesan-K" } ], "abstract": "Thermogravimetric analysis (TGA) and flow-reactor experiments were used to study sulfidation-regeneration of highly porous CuO-Al_2O_3 sorbents. In TGA studies, sulfidation of reduced sorbents produced a high-temperature form of digenite (Cu_(9+x)S_5) as the major crystalline product. When a platinum pan was used in the TGA, significant sulfur chemisorption on alumina occurred. Sulfur chemisorption on alumina was eliminated by use of a quartz sample pan. Reaction of the mixed-oxide sorbents with a mixture of H_2S, H_2, H_2O, and N_2 in a packed-bed microreactor, at temperatures between 550 and 800 \u00b0C, yielded prebreakthrough outlet-H_2S levels considerably lower than those predicted by the sulfidation equilibrium of metallic copper. Independent reduction experiments confirmed that alumina stabilizes CuO against complete reduction to Cu. With this mechanism at work, low prebreakthrough H_2S levels are attributed to sulfidation reactions of copper at oxidation states of +1 or +2. The sulfided sorbents were completely regenerable in air/N_2 mixtures with no deterioration of subsequent sulfidation performance.", "doi": "10.1021/ie00091a008", "issn": "0888-5885", "publisher": "American Chemical Society", "publication": "Industrial & Engineering Chemistry Research", "publication_date": "1989-07", "series_number": "7", "volume": "28", "issue": "7", "pages": "931-940" }, { "id": "authors:c2bv6-q4e25", "collection": "authors", "collection_id": "c2bv6-q4e25", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20150805-110050653", "type": "article", "title": "Oxidation Kinetics of Monodisperse Spherical Carbonaceous Particles of Variable Properties", "author": [ { "family_name": "Levendis", "given_name": "Yiannis A.", "clpid": "Levendis-Y-A" }, { "family_name": "Flagan", "given_name": "Richard C.", "orcid": "0000-0001-5690-770X", "clpid": "Flagan-R-C" }, { "family_name": "Gavalas", "given_name": "George R.", "orcid": "0000-0003-1468-6835", "clpid": "Gavalas-G-R" } ], "abstract": "Synthetic chars of variable physical and chemical properties have been developed to study char oxidation mechanisms and rates. The char particles were spherical and monodisperse, with sizes ranging from a few microns to several tens of microns. The particles were made from a carbon-yielding polymer and pore-forming additives. The surface areas of the chars made from different additives varied by more than two orders of magnitude and the porosities varied by a factor of five. The pore size distributions included both micro and transitional pores. X-ray studies revealed that all chars were amorphous when heat treated to temperatures up to 1600 K in an inert atmosphere. However, upon oxidation at 1600 K, the carbon matrix underwent partial graphitization. This transformation was particularly pronounced for some of the polymer pore-former chars. Combustion experiments showed that the total surface area of the chars increased dramatically with conversion, revealing the existence of a vast network of micropores. Apparent oxidation rates were higher for the chars that contained transitional pores in a microporous matrix. When compared with the rates reported in the literature for coal derived chars, the calculated intrinsic rates were lower at intermediate particle temperatures (800-1600 K) but comparable at elevated temperatures (1800-2300 K). As the temperature was increased further, the intrinsic rates decreased consistent with the Nagle and Strickland-Constable kinetic mechanism [l].", "doi": "10.1016/0010-2180(89)90108-9", "issn": "0010-2180", "publisher": "Elsevier", "publication": "Combustion and Flame", "publication_date": "1989-06", "series_number": "3-4", "volume": "76", "issue": "3-4", "pages": "221-241" }, { "id": "authors:xvwm7-7h943", "collection": "authors", "collection_id": "xvwm7-7h943", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20150805-110546824", "type": "article", "title": "Catalysis of the combustion of synthetic char particles by various forms of calcium additives", "author": [ { "family_name": "Levendis", "given_name": "Y. A.", "clpid": "Levendis-Yiannis-A" }, { "family_name": "Nam", "given_name": "S. W.", "clpid": "Nam-S-W" }, { "family_name": "Lowenberg", "given_name": "M.", "clpid": "Lowenberg-M" }, { "family_name": "Flagan", "given_name": "R. C.", "orcid": "0000-0001-5690-770X", "clpid": "Flagan-R-C" }, { "family_name": "Gavalas", "given_name": "G. R.", "orcid": "0000-0003-1468-6835", "clpid": "Gavalas-G-R" } ], "abstract": "The reactivity of calcium-laden carbonaceous particles to oxygen was investigated in the range 670-3000 K. The char particles were prepared from poly(furfural alcohol) (PFA) and were spherical and of uniform size. Three different methods were used to introduce the calcium additive: precipitation of calcium carbonate, impregnation with calcium acetate, and calcium ion exchange. Electron microscopy showed that the distribution of calcium was remarkably uniform in particles containing a bimodal distribution of micropores and transitional pores, whereas for particles with micropores only the Ca concentration was high at the surface and low at the center. X-ray analysis indicated that the conversion of the carbonate to the oxide at low temperatures (below 900 K) takes place only after all carbon has been consumed. Combustion studies showed that the calcium catalyzed the oxidation reaction at all temperatures investigated by up to 2 orders of magnitude. The effectiveness of the catalyst introduced by the different methods was comparable, with the calcium ion exchanged chars being, in general, the most reactive.", "doi": "10.1021/ef00013a006", "issn": "1520-5029", "publisher": "American Chemical Society", "publication": "Energy and Fuels", "publication_date": "1989-01-01", "series_number": "1", "volume": "3", "issue": "1", "pages": "28-37" }, { "id": "authors:ha9tw-hrm14", "collection": "authors", "collection_id": "ha9tw-hrm14", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:LOEjcp89", "type": "article", "title": "Time-dependent, diffusion-controlled reactions: The influence of boundaries", "author": [ { "family_name": "Loewenberg", "given_name": "M.", "clpid": "Loewenberg-M" }, { "family_name": "Gavalas", "given_name": "G. R.", "orcid": "0000-0003-1468-6835", "clpid": "Gavalas-G-R" } ], "abstract": "A configurational averaging procedure is presented for the analysis of diffusion-controlled reactions in a medium containing a hard-sphere distribution of spherical sinks. The hierarchy of equations produced from the averaging procedure is truncated assuming low concentration of sinks. The method is shown to reproduce previous results for the decay of an initially uniform distribution of reactant in an unbounded medium and is subsequently applied to determine the reactant flux on the boundary of a semiinfinite domain. In the latter problem, the boundary has a significant effect which cannot be accounted for by the usual rate coefficient.", "doi": "10.1063/1.456511", "issn": "0021-9606", "publisher": "American Institute of Physics", "publication": "Journal of Chemical Physics", "publication_date": "1989-01-01", "series_number": "1", "volume": "90", "issue": "1", "pages": "177-182" }, { "id": "authors:a7b9n-be266", "collection": "authors", "collection_id": "a7b9n-be266", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20151105-153703909", "type": "book_section", "title": "Stability of H_2-Permselective SiO_2 Films Formed by Chemical Vapor Deposition", "book_title": "Membrane reactor technology", "author": [ { "family_name": "Nam", "given_name": "S. W.", "clpid": "Nam-S-W" }, { "family_name": "Gavalas", "given_name": "G. R.", "orcid": "0000-0003-1468-6835", "clpid": "Gavalas-G-R" } ], "contributor": [ { "family_name": "Govind", "given_name": "Rakesh", "clpid": "Govind-R" }, { "family_name": "Naotsugu", "given_name": "Itoh", "clpid": "Naotsugu-Itoh" } ], "abstract": "Thin SiO_2 films were heat treated in different gas mixtures to determine their stability in functioning as high-temperature hydrogen permselective membranes. The films were formed within the walls of porous Vycor tubes by SiH_4 oxidation in an opposing reactants geometry. Film deposition was carried out at 450 \u00b0C in the presence and absence of water vapor. Immediately after formation, the films were highly selective to hydrogen permeation having a H_2:N_2 permeability ratio of about 3000 : 1. The films were subsequently heat treated at 450-700\u00b0C in dry N_2, dry O_2, N_2- H_2O, and O_2-H_2O mixtures. The permeation rates of H_2 and N_2 changed depending on the original conditions of film formation as well as on the heat treatment. Heating in dry N_2 reduced slowly the permeation rates of both H_2 and N_2. Heating in N_2-H_2O atmosphere led to a steeper decline of H_2 permeability. But the permeation rate of N_2 increased or decreased according to whether the film deposition had been carried out in the absence or presence of H_2O vapor, respectively. Thermal treatment in O_2 caused rapid decline of the permeation rates of H_2 and N_2 in films that were deposited under dry conditions. The decline was moderate in films deposited under wet conditions.", "isbn": "9780816904648", "publisher": "American Institute of Chemical Engineers", "place_of_publication": "New York, NY", "publication_date": "1989", "pages": "68-74" }, { "id": "authors:jym73-wd824", "collection": "authors", "collection_id": "jym73-wd824", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20151104-112518407", "type": "article", "title": "Deposition of H_2-permselective SiO_2 films", "author": [ { "family_name": "Gavalas", "given_name": "G. R.", "orcid": "0000-0003-1468-6835", "clpid": "Gavalas-G-R" }, { "family_name": "Megiris", "given_name": "C. E.", "clpid": "Megiris-C-E" }, { "family_name": "Nam", "given_name": "S. W.", "clpid": "Nam-S-W" } ], "abstract": "Films of amorphous SiO_2 were deposited within the walls of porous Vycor tubes by SiH_4 oxidation in an opposing-reactants geometry: SiH_4 was passed inside the tube while O_2 was passed outside the tube. The two reactants diffused opposite to each other and reacted within a narrow front inside the tube wall to form a thin SiO_2 film. Once the pores were plugged the reactants could not reach each other and the reaction stopped. At 450\u00b0C and 0.1 and 0.33 atm of SiH_4 and O_2, the reaction was complete within 15 min. The thickness of the SiO_2 film was estimated to be about 0.1 \u03bc. Measurements of H_2 and N_2 permeation rates showed that the SiO_2 film was highly selective to H_2 permeation. The H_2: N_2 flux at 450\u00b0C varied between 2000 and 3000. Thermal annealing at 600\u00b0C reduced somewhat that selectivity. Thermal annealing in the presence of H_2O vapor decreased further the flux of H_2 and increased the flux of H_2. Permeation of H_2 is believed to occur through an activated diffusion mechanism. Applications of such H_2-permeable films to membrane reactors for equilibrium-limited reactions are discussed.", "doi": "10.1016/0009-2509(89)85125-5", "issn": "0009-2509", "publisher": "Pergamon", "publication": "Chemical Engineering Science", "publication_date": "1989", "series_number": "9", "volume": "44", "issue": "9", "pages": "1829-1835" }, { "id": "authors:2v89n-pz313", "collection": "authors", "collection_id": "2v89n-pz313", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20151104-112517859", "type": "article", "title": "Chemical engineering", "author": [ { "family_name": "Fredrickson", "given_name": "A. G.", "clpid": "Fredrickson-A-G" }, { "family_name": "Gavalas", "given_name": "George R.", "orcid": "0000-0003-1468-6835", "clpid": "Gavalas-G-R" }, { "family_name": "Ray", "given_name": "W. Harmon", "clpid": "Ray-W-H" }, { "family_name": "Varma", "given_name": "Arvind", "clpid": "Varma-A" } ], "abstract": "Chemical engineering, like everything else in the world, is changing. One of the signs of its change is the recent appearance of several Festschrift publications commemorating the birthdays of a few exceptional people who, as everyone sees and acknowledges, have done so much to lead our profession forward to the advanced position it enjoys today. Chemical Engineering Science has produced festival issues for P. V. Danckwerts and N. R. Amundson, and now, on the occasion of his 60th birthday, it is time for us to honor another giant of the profession: Rutherford Aris.", "doi": "10.1016/0009-2509(89)85116-4", "issn": "0009-2509", "publisher": "Pergamon", "publication": "Chemical Engineering Science", "publication_date": "1989", "series_number": "9", "volume": "44", "issue": "9", "pages": "1751-1752" }, { "id": "authors:r0dqw-t6285", "collection": "authors", "collection_id": "r0dqw-t6285", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20151008-112330062", "type": "article", "title": "Hydrogen sulfide removal by supported vanadium oxide", "author": [ { "family_name": "Bagajewicz", "given_name": "Miguel J.", "clpid": "Bagajewicz-M-J" }, { "family_name": "Tamhankar", "given_name": "Satish S.", "clpid": "Tamhankar-S-S" }, { "family_name": "Stephanopoulos", "given_name": "Maria F.", "clpid": "Stephanopoulos-M-F" }, { "family_name": "Gavalas", "given_name": "George R.", "orcid": "0000-0003-1468-6835", "clpid": "Gavalas-G-R" } ], "abstract": "The retention of hydrogen sulfide by alumina-supported vanadium oxide at 650-700 \u00b0C is studied with flow reactor experiments. The effects of sorbent prereduction and gas-phase composition (H_2 and H_2O content) are discussed. It is found that hydrogen sulfide is chemisorbed reversibly on a nonstoichiometric vanadium oxide. Bulk sulfide is not formed.", "doi": "10.1021/es00169a017", "issn": "0013-936X", "publisher": "American Chemical Society", "publication": "Environmental Science and Technology", "publication_date": "1988-04", "series_number": "4", "volume": "22", "issue": "4", "pages": "467-470" }, { "id": "authors:jqjzr-wcw85", "collection": "authors", "collection_id": "jqjzr-wcw85", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20151104-143221128", "type": "article", "title": "Char Combustion: Measurement and Analysis of Particle Temperature Histories", "author": [ { "family_name": "Sahu", "given_name": "Ranajit", "clpid": "Sahu-Ranajit" }, { "family_name": "Northrop", "given_name": "Paul S.", "clpid": "Northrop-Paul-S" }, { "family_name": "Flagan", "given_name": "Richard C.", "orcid": "0000-0001-5690-770X", "clpid": "Flagan-R-C" }, { "family_name": "Gavalas", "given_name": "George R.", "orcid": "0000-0003-1468-6835", "clpid": "Gavalas-G-R" } ], "abstract": "Single particle combustion experiments were carried out with 50-100 \u03bcm char particles derived from one lignite and one HVA bituminous coal. Reactor wall temperatures in the range 1050 K to 1450 K and oxygen partial pressures of 0.21 and 0.5 were used. The complete temperature-time histories of individual burning particles were measured using two-color optical pyrometry techniques. Apparent combustion rate parameters of the chars were then estimated using a model of single particle combustion and taking into account the random particle-to-particle property variation.", "doi": "10.1080/00102208808923985", "issn": "0010-2202", "publisher": "Taylor & Francis", "publication": "Combustion Science and Technology", "publication_date": "1988", "series_number": "1-3", "volume": "60", "issue": "1-3", "pages": "215-230" }, { "id": "authors:r7qhr-1zn64", "collection": "authors", "collection_id": "r7qhr-1zn64", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20151104-112517607", "type": "article", "title": "Steady-state reactant flux into a medium containing spherical sinks", "author": [ { "family_name": "Loewenberg", "given_name": "M.", "clpid": "Loewenberg-M" }, { "family_name": "Gavalas", "given_name": "G. R.", "orcid": "0000-0003-1468-6835", "clpid": "Gavalas-G-R" } ], "abstract": "A detailed analysis is presented for determining the reactant flux into a composite material containing randomly-located, reactive spherical centers. The governing equations are configurationally averaged assuming the spheres are distributed according to a hard-sphere potential. The resulting infinite hierarchy of coupled equations is truncated using physical consideratiosn of screening and diluteness of the particulate phase. A general asymptotic solution is obtained valid to order c for all bounded and semi-infinite domains. The solution reproduces the mean-field result and includes the previously-neglected effects of sphere-boundary interactions. These interactions dominate the effect of two-sphere interactions which affect the solution at order c^(3/2) ln c. Explicit consideration is given tot he case in which the domain is spherical or semi-infinite and a solution obtained for each. The solutions obtained generally predict an enhanced reactant flux relative to the mean-field result. The solution is illustrated by application to the problem of coal ash vaporization.", "doi": "10.1016/0009-2509(88)85177-7", "issn": "0009-2509", "publisher": "Pergamon", "publication": "Chemical Engineering Science", "publication_date": "1988", "series_number": "9", "volume": "43", "issue": "9", "pages": "2431-2444" }, { "id": "authors:y5g68-vfy56", "collection": "authors", "collection_id": "y5g68-vfy56", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20150805-104411710", "type": "article", "title": "Measurement of gas adsorption isotherms by continuous adsorbate addition", "author": [ { "family_name": "Northrop", "given_name": "P. S.", "clpid": "Northrop-Paul-S" }, { "family_name": "Flagan", "given_name": "R. C.", "orcid": "0000-0001-5690-770X", "clpid": "Flagan-R-C" }, { "family_name": "Gavalas", "given_name": "G. R.", "orcid": "0000-0003-1468-6835", "clpid": "Gavalas-G-R" } ], "abstract": "A technique of measuring gas adsorption isotherms by continuous addition of adsorbate gas to the sample is described. Flow rates in the range of 0.01-1 mL (STP)/min can be obtained by means of orifices with diameters of 1-10 \u03bcm. The continuous-flow technique is compared with the customary incremental addition of the adsorbate gas for two sorbents, a \u03b3-alumina and a high volatile bituminous coal.", "doi": "10.1021/la00074a027", "issn": "0743-7463", "publisher": "American Chemical Society", "publication": "Langmuir", "publication_date": "1987-03", "series_number": "2", "volume": "3", "issue": "2", "pages": "300-302" }, { "id": "authors:s4byt-67t26", "collection": "authors", "collection_id": "s4byt-67t26", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20151105-124153112", "type": "article", "title": "Changes in coal composition during air oxidation at 200\u2013250\u00b0C", "author": [ { "family_name": "Kalema", "given_name": "W. S.", "clpid": "Kalema-W-S" }, { "family_name": "Gavalas", "given_name": "G. R.", "orcid": "0000-0003-1468-6835", "clpid": "Gavalas-G-R" } ], "abstract": "Air oxidation of two bituminous coals and a lignite was carried out in a fluidized bed reactor at 200\u2013250 \u00b0C. The gaseous products were analysed by gas chromatography and solid samples were titrated to determine carboxylic and total acidity and were subjected to ^(13)C n.m.r. spectrometry to determine the fraction of aromatic carbon. Elemental composition, heating value and ash content were also determined. During the first few hours oxidation-induced devolatilization proceeded simultaneously with oxidation. Oxidation of the high volatile bituminous coal for 12 h at 200 \u00b0C raised the carboxylic groups from 0.37 to 2.1 meq g^(\u22121) and the total acidity from 1.5 to 5.3 meq g^(\u22121). The acidic groups seem to be largely distinct from oxygenated groups that are precursors to CO and CO_2 products. Oxidation of the same coal at 200 \u00b0C consumed aromatic as well as aliphatic carbon at relative amounts 1:3 to 1:4.", "doi": "10.1016/0016-2361(87)90234-1", "issn": "0016-2361", "publisher": "Elsevier", "publication": "Fuel", "publication_date": "1987-02", "series_number": "2", "volume": "66", "issue": "2", "pages": "158-164" }, { "id": "authors:6qbtn-7rq94", "collection": "authors", "collection_id": "6qbtn-7rq94", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20151105-124152829", "type": "article", "title": "Calcium pretreatment of coal for sulphur emissions control in combustion", "author": [ { "family_name": "Sharma", "given_name": "P. K.", "clpid": "Sharma-P-K" }, { "family_name": "Gavalas", "given_name": "G. R.", "orcid": "0000-0003-1468-6835", "clpid": "Gavalas-G-R" }, { "family_name": "Flagan", "given_name": "R. C.", "orcid": "0000-0001-5690-770X", "clpid": "Flagan-R-C" } ], "abstract": "A new technique is described for precipitating CaCO\u2083 within the porous space of coal with the objective of reducing sulphur emissions during combustion/gasification. Results are reported from two combustion experiments conducted under fuel lean conditions in a laminar flow reactor.", "doi": "10.1016/0016-2361(87)90242-0", "issn": "0016-2361", "publisher": "Elsevier", "publication": "Fuel", "publication_date": "1987-02", "series_number": "2", "volume": "66", "issue": "2", "pages": "207-209" }, { "id": "authors:z3j8m-7av81", "collection": "authors", "collection_id": "z3j8m-7av81", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20151105-124152487", "type": "article", "title": "Alkali-Alumina Sorbents for High-Temperature Removal of SO_2", "author": [ { "family_name": "Gavalas", "given_name": "G. R.", "orcid": "0000-0003-1468-6835", "clpid": "Gavalas-G-R" }, { "family_name": "Edelstein", "given_name": "Sergio", "clpid": "Edelstein-S" }, { "family_name": "Flytzani-Stephanopoulos", "given_name": "M.", "clpid": "Flytzani-Stephanopoulos-M" }, { "family_name": "Weston", "given_name": "T. A.", "clpid": "Weston-T-A" } ], "abstract": "The high-temperature removal of SO_2 by sorbents consisting of sodium and lithium salts supported on \u03b1-Al_2O_3 has been investigated with emphasis on the chemistry of regeneration. The sulfated sorbents were regenerated by reduction with CO at 700\u2013800\u00b0C in a thermogravimetric analyzer and a packed-bed microreactor. Sulfur removal from the sorbent and distribution of gaseous products were measured at different alkali loadings, temperatures, and CO concentrations. The results are interpreted in terms of a network of reactions wherein alumina is important as a catalyst and as a reactant. During regeneration sulfate is converted to aluminate and sulfide, the fraction of aluminate defining the extent of regeneration. The rate and extent of sulfur removal increase with the ratio of alumina to alkali and are higher in the presence of lithium. The product gas consists of SO_2, COS, and elemental sulfur, the latter compound constituting up to 35% of the sulfur removed.", "doi": "10.1002/aic.690330212", "issn": "0001-1541", "publisher": "Wiley", "publication": "AIChE Journal", "publication_date": "1987-02", "series_number": "2", "volume": "33", "issue": "2", "pages": "258-266" }, { "id": "authors:jkshd-rbe94", "collection": "authors", "collection_id": "jkshd-rbe94", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20151105-105158121", "type": "article", "title": "A Simplified Description of Char Combustion", "author": [ { "family_name": "Loewenberg", "given_name": "M.", "clpid": "Loewenberg-M" }, { "family_name": "Bellan", "given_name": "J.", "orcid": "0000-0001-9218-7017", "clpid": "Bellan-J" }, { "family_name": "Gavalas", "given_name": "G. R.", "orcid": "0000-0003-1468-6835", "clpid": "Gavalas-G-R" } ], "abstract": "A simplified analysis of carbonaceous particle combustion is presented that includes the effects of pore diffusion and growth as well as gas-phase heat and mass transfer. The combustion dynamics are described by time-dependent equations for particle temperature, radius and a number of intraparticle conversion variables. These are coupled to pseudosteady equations for gas-phase transport and internal reaction and diffusion. The differential equations for gas-phase transport are reduced by quadrature to a nonlinear boundary condition to the intraparticle boundary value problem. Numerical calculations are performed for conditions relevant to pulverized coal combustion. An analytical solution of the intraparticle problem, pertinent to the regime of strong diffusional limitations, reduces the intraparticle solution into a set of two quadratures which drastically simplifies the numerical calculations. The simplified intraparticle solution is in excellent agreement with the full solution at 1800 K free stream temperature and fair agreement at 1500 K.", "doi": "10.1080/00986448708911961", "issn": "0098-6445", "publisher": "Gordon and Breach", "publication": "Chemical Engineering Communications", "publication_date": "1987", "series_number": "1-6", "volume": "58", "issue": "1-6", "pages": "89-103" }, { "id": "authors:xwnzg-9dn20", "collection": "authors", "collection_id": "xwnzg-9dn20", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20151008-112330339", "type": "article", "title": "Mixed-oxide sorbents for high-temperature removal of hydrogen sulfide", "author": [ { "family_name": "Tamhankar", "given_name": "S. S.", "clpid": "Tamhankar-S-S" }, { "family_name": "Bagajewicz", "given_name": "M.", "clpid": "Bagajewicz-M" }, { "family_name": "Gavalas", "given_name": "G. R.", "orcid": "0000-0003-1468-6835", "clpid": "Gavalas-G-R" }, { "family_name": "Sharma", "given_name": "P. K.", "clpid": "Sharma-P-K" }, { "family_name": "Flytzani-Stephanopoulos", "given_name": "M.", "clpid": "Flytzani-Stephanopoulos-M" } ], "abstract": "Several pure and mixed oxides (ZnO, CuO, ZnO-Fe_2O_3, CuO-Fe_2O_3, CuO-AI_O_3, and CuO-Fe_2O_3-Al_2O_3) were investigated as regenerable sorbents for the removal of H_2S at high temperatures. A special technique was used to prepare the sorbents in highly porous form. The sorbents were subjected to successive sulfidation-regeneration cycles in a packed-bed microreactor. Sulfidation was carried out at 538-650 \u00b0C with H_2S-H_2O-H_2-N_2 mixtures, regeneration with O_2-N_2-H_2O mixtures. Fresh, sulfided, and regenerated sorbents were characterized by XRD, SEM-EDS, and BET surface area analysis. Solid conversion and the prebreakthrough exit concentration of H_2S are discussed in terms of physical structure and thermodynamic properties of the sorbents. The performance of the sorbents CuO-Fe_2O_3, CuO-Al_2O_3, and CuO-Fe_2O_3-AI_2O_3 is discussed in terms of changes in the oxidation state of copper during sulfidation.", "doi": "10.1021/i200033a014", "issn": "0196-4305", "publisher": "American Chemical Society", "publication": "Industrial & Engineering Chemistry Process Design and Development", "publication_date": "1986-04", "series_number": "2", "volume": "25", "issue": "2", "pages": "429-437" }, { "id": "authors:gz5zz-d6774", "collection": "authors", "collection_id": "gz5zz-d6774", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20151105-124153381", "type": "article", "title": "Combustion of calcium-exchanged coals", "author": [ { "family_name": "Chang", "given_name": "K. K.", "clpid": "Chang-Karl-Kho-Chung" }, { "family_name": "Flagan", "given_name": "R. C.", "orcid": "0000-0001-5690-770X", "clpid": "Flagan-R-C" }, { "family_name": "Gavalas", "given_name": "G. R.", "orcid": "0000-0003-1468-6835", "clpid": "Gavalas-G-R" }, { "family_name": "Sharma", "given_name": "P. K.", "clpid": "Sharma-P-K" } ], "abstract": "The combustion of calcium-exchanged coals was studied in a laminar flow reactor under strongly fuel-lean conditions. The aim was to study the nature of release and capture of sulphur oxides in the presence of finely dispersed calcium oxide and explore the effectiveness of calcium-exchange pretreatment for sulphur emission control in coal combustion. The experimental variables were residence time, oxygen concentration, reactor temperature and amount of calcium additive. A Texas lignite and an Indiana high volatile B bituminous coal were used. Analysis of the experimental data indicated that sulphur capture increased with residence time and oxygen concentration. The results also showed that sulphur capture was approximately proportional to the Ca/S ratio of the coals.", "doi": "10.1016/0016-2361(86)90145-6", "issn": "0016-2361", "publisher": "Elsevier", "publication": "Fuel", "publication_date": "1986-01", "series_number": "1", "volume": "65", "issue": "1", "pages": "75-80" }, { "id": "authors:76sjt-z6y30", "collection": "authors", "collection_id": "76sjt-z6y30", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20151008-112330650", "type": "article", "title": "Pyrolysis of a wood-derived material. Effects of moisture and ash content", "author": [ { "family_name": "Gray", "given_name": "Murray R.", "clpid": "Gray-M-R" }, { "family_name": "Corcoran", "given_name": "William H.", "clpid": "Corcoran-W-H" }, { "family_name": "Gavalas", "given_name": "George R.", "orcid": "0000-0003-1468-6835", "clpid": "Gavalas-G-R" } ], "abstract": "The influence of moisture on the thermal degradation of wood waste was investigated by pyrolyzing ground samples in a batch fluid-bed reactor at between 320 and 470 \u00b0C in helium at 101-104 kPa. The wood samples were heated at about 300 \u00b0C/min, so that drying and pyrolysis were simultaneous. The presence of moisture increased the yield of char by as much as 5 yield % , relative to dry samples. Moisture suppressed tar formation from demineralized samples at 470 \u00b0C but enhanced the formation of tar above and below 390 \u00b0C from samples containing ash. Ash and added calcium catalyzed the conversion of tar into aqueous product, while moisture interacted with tar formation or trapping reactions to increase the yield of char. Moisture also modulated the activity of the ash components for the Conversion of tar Into aqueous product. The native mineral components in the wood waste gave effects equivalent to ion-exchanged calcium.", "doi": "10.1021/i200030a020", "issn": "0196-4305", "publisher": "American Chemical Society", "publication": "Industrial & Engineering Chemistry Process Design and Development", "publication_date": "1985-07", "series_number": "3", "volume": "24", "issue": "3", "pages": "646-651" }, { "id": "authors:xwwgq-x1237", "collection": "authors", "collection_id": "xwwgq-x1237", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20151106-090106075", "type": "book_section", "title": "Alkali-Alumina Sorbents for Regenerable SO_2 Removal in Fluidized-Coal Combustion", "author": [ { "family_name": "Gavalas", "given_name": "George R.", "orcid": "0000-0003-1468-6835", "clpid": "Gavalas-G-R" }, { "family_name": "Weston", "given_name": "Theresa A.", "clpid": "Weston-T-A" }, { "family_name": "Stephanopoulos", "given_name": "Maria F.", "clpid": "Stephanopoulos-M-F" } ], "abstract": "Sodium and sodium-lithium oxides supported on porous alumina have been investigated as regenerable SO_2 sorbents for fluidized coal combustion. In adsorption the oxides are converted to sulfates. In regeneration, carried out by reduction with CO, the sulfates are converted back to oxides while sulfur is removed in elemental form (S_2), SO_2 and COS. The transient composition of sorbent and gaseous products was measured in a thermogravimetric analyzer and a packed-bed microreactor in order to delineate the basic chemistry of regeneration and determine conditions that maximize the yield of elemental sulfur.", "publisher": "Department of Energy", "publication_date": "1985" }, { "id": "authors:hvxk0-dnp32", "collection": "authors", "collection_id": "hvxk0-dnp32", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20230417-377880000.30", "type": "article", "title": "Identifiability of Estimates of Two-Phase Reservoir Properties in History Matching", "author": [ { "family_name": "Watson", "given_name": "A. T.", "clpid": "Watson-A-T" }, { "family_name": "Gavalas", "given_name": "G. R.", "orcid": "0000-0003-1468-6835", "clpid": "Gavalas-G-R" }, { "family_name": "Seinfeld", "given_name": "J. H.", "orcid": "0000-0003-1344-4068", "clpid": "Seinfeld-J-H" } ], "abstract": "Since the number of parameters to be estimated in a reservoir history match is potentially quite large, it is important to determine which parameters can be estimated with reasonable accuracy from the available data. This aspect can be called determining the identifiability of the parameters. The identifiability of porosity and absolute and relative permeabilities on the basis of flow and pressure data in a two-phase (oil/water) reservoir is considered. The question posed is: How accurately can one expect to estimate spatially variable porosity and absolute permeability and relative permeabilities given typical production and pressure data? To gain insight into this question, analytical solutions for pressure and saturation in a one-dimensional (ID) waterflood are used. The following conclusions are obtained.\n\n1. Only the average value of the porosity can be determined on the basis of water/oil flow measurements. \n\n2. The permeability distribution can be determined from pressure drop data with an accuracy depending on the mobility ratio. \n\n3. Exponents in a power function representation of the relative permeabilities can be determined from WOR data alone but not nearly so accurately as when pressure drop and flow data are used simultaneously.", "doi": "10.2118/12579-pa", "issn": "0197-7520", "publisher": "Society of Petroleum Engineers", "publication": "Society of Petroleum Engineers Journal", "publication_date": "1984-12", "series_number": "6", "volume": "24", "issue": "6", "pages": "697-706" }, { "id": "authors:k3pr4-g5e43", "collection": "authors", "collection_id": "k3pr4-g5e43", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20151104-143219422", "type": "article", "title": "Structure and Activity of NiO/\u0251-Al_2O_3 and NiO/ZrO_2 Calcined at High Temperatures II. Activity in the Fuel-Rich Oxidation of Methane", "author": [ { "family_name": "Gavalas", "given_name": "G. R.", "orcid": "0000-0003-1468-6835", "clpid": "Gavalas-G-R" }, { "family_name": "Phichitkul", "given_name": "C.", "clpid": "Phichitkul-C" }, { "family_name": "Voecks", "given_name": "G. E.", "clpid": "Voecks-G-E" } ], "abstract": "The activity of NiO/\u03b1-Al_2O_3 and NiO/ZrO_2 in the fuel-rich oxidation of methane was measured as a function of time-on-stream for different initial calcination temperatures and for different reduction-reoxidation treatments. The activity was found to decrease substantially with increasing temperature of calcination. However, the activity of the calcined catalysts increased sharply following reduction in H_2 prior to reaction. The changes in total activity are discussed in terms of changes in surface area and specific activity, the latter being the dominant factor. The change in specific activity is discussed in terms of changes in the content of excess oxygen.", "doi": "10.1016/0021-9517(84)90050-2", "issn": "0021-9517", "publisher": "Elsevier", "publication": "Journal of Catalysis", "publication_date": "1984-07", "series_number": "1", "volume": "88", "issue": "1", "pages": "65-72" }, { "id": "authors:eg25b-xef98", "collection": "authors", "collection_id": "eg25b-xef98", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20151104-143219052", "type": "article", "title": "Structure and Activity of NiO/\u03b1-Al_2O_3 and NiO/ZrO_2 Calcined at High Temperatures I. Structure", "author": [ { "family_name": "Gavalas", "given_name": "G. R.", "orcid": "0000-0003-1468-6835", "clpid": "Gavalas-G-R" }, { "family_name": "Phichitkul", "given_name": "C.", "clpid": "Phichitkul-C" }, { "family_name": "Voecks", "given_name": "G. E.", "clpid": "Voecks-G-E" } ], "abstract": "A series of NiO/\u03b1-Al_2O_3 and NiO/ZrO_2 catalysts were prepared by impregnation and calcination in air between 750 and 1050 \u00b0C. Examination of the catalysts by means of nitrogen adsorption, H_2 chemisorption, X-ray diffraction, scanning electron microscopy, and X-ray photoelectron spectroscopy showed marked variations in physical and chemical properties as a function of calcination temperature. The results indicate that NiO does not interact with ZrO_2 whereas NiO-Al_2O_3 interaction is extensive at the interface between NiO and Al_2O_3 when the calcination temperature exceeds 850 \u00b0C. When the calcined samples of NiO/ZrO_2 were reduced in hydrogen, extensive redispersion of the metal was observed.", "doi": "10.1016/0021-9517(84)90049-6", "issn": "0021-9517", "publisher": "Elsevier", "publication": "Journal of Catalysis", "publication_date": "1984-07", "series_number": "1", "volume": "88", "issue": "1", "pages": "54-64" }, { "id": "authors:a87t5-fbf23", "collection": "authors", "collection_id": "a87t5-fbf23", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20151105-110243121", "type": "article", "title": "Determination of functional groups of coal-derived liquids by n.m.r. and elemental analysis", "author": [ { "family_name": "Alien", "given_name": "David T.", "clpid": "Alien-D-T" }, { "family_name": "Petrakis", "given_name": "Leonidas", "clpid": "Petrakis-L" }, { "family_name": "Grandy", "given_name": "David W.", "clpid": "Grandy-D-W" }, { "family_name": "Gavalas", "given_name": "George R.", "orcid": "0000-0003-1468-6835", "clpid": "Gavalas-G-R" }, { "family_name": "Gates", "given_name": "Bruce C.", "orcid": "0000-0003-0274-4882", "clpid": "Gates-B-C" } ], "abstract": "A new method of structural analysis was applied to a group of hydroliquefied coal samples. The method uses elemental analysis and n.m.r. data to estimate the concentrations of functional groups in the samples. The samples included oil and asphaltene fractions obtained in a series of hydroliquefaction experiments, and a set of nine fractions separated from a coal-derived oil. The structural characterization of these samples demonstrates that estimates of functional group concentrations can be used to provide detailed structural profiles of complex mixtures and to obtain limited information about reaction pathways.", "doi": "10.1016/0016-2361(84)90071-1", "issn": "0016-2361", "publisher": "Elsevier", "publication": "Fuel", "publication_date": "1984-06", "series_number": "6", "volume": "63", "issue": "6", "pages": "803-809" }, { "id": "authors:2z88d-bqh62", "collection": "authors", "collection_id": "2z88d-bqh62", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20151105-105157818", "type": "article", "title": "Reactions of methylene and ether bridges", "author": [ { "family_name": "Allen", "given_name": "David T.", "clpid": "Allen-D-T" }, { "family_name": "Gavalas", "given_name": "George R.", "orcid": "0000-0003-1468-6835", "clpid": "Gavalas-G-R" } ], "abstract": "The reactions of model compounds containing methylene and ether bridges were examined, in the temperature range 350\u2013400 \u00b0C. Aromatic displacement reactions, in which atomic hydrogen displaced the bridges, and homolysis via a keto-enol intermediate were the dominant modes of bridge decomposition. The aromatic displacement reactions of methylene-bridge compounds occurred with activation energies of \u224810 kcal mol^(\u22121) and with A factors of 10^(10.5\u201312) Is^(\u22121) m^(\u22121). The A factors vary with the size of the rings and the substituents on the rings connected by the methylene bridges. The activation energy is relatively independent of these effects. When phenolic substituents are present, methylene bridges can unimolecularly decompose via a keto-enol tautomerism, and the rate of product formation is solvent-dependent. Reactions for ether bridge decomposition were more complex and the rate parameters encompassed a much larger range of values.", "doi": "10.1016/0016-2361(84)90150-9", "issn": "0016-2361", "publisher": "Elsevier", "publication": "Fuel", "publication_date": "1984-05", "series_number": "5", "volume": "63", "issue": "5", "pages": "586-592" }, { "id": "authors:08c4f-xez92", "collection": "authors", "collection_id": "08c4f-xez92", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20151104-143219929", "type": "article", "title": "A novel approach to the calculations of mass transfer rates from the linearized equations", "author": [ { "family_name": "Vickery", "given_name": "David J.", "clpid": "Vickery-D-J" }, { "family_name": "Vickery", "given_name": "Ross", "clpid": "Vickery-R" }, { "family_name": "Gavalas", "given_name": "George R.", "orcid": "0000-0003-1468-6835", "clpid": "Gavalas-G-R" } ], "abstract": "The linearized equations describing binary and multicomponent mass transfer for a general class of problems are transformed to a \"one-dimensional\" root finding problem in the total molar flux. An algorithm is presented capable of solving this problem for a variety of mass transfer models. Applications to the film and penetration models are considered in detail. Numerical results are given which show that the new method is much faster than other currently available methods.", "doi": "10.1016/0098-1354(84)87005-2", "issn": "0098-1354", "publisher": "Elsevier", "publication": "Computers & Chemical Engineering", "publication_date": "1984", "series_number": "3-4", "volume": "8", "issue": "3-4", "pages": "179-183" }, { "id": "authors:c0x5j-kwc16", "collection": "authors", "collection_id": "c0x5j-kwc16", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20151104-112515809", "type": "book_section", "title": "Functional Group Analysis of Coal Liquids", "book_title": "Magnetic Resonance : Introduction, Advanced Topics and Applications to Fossil Energy", "author": [ { "family_name": "Allen", "given_name": "D.", "clpid": "Allen-D" }, { "family_name": "Petrakis", "given_name": "L.", "clpid": "Petrakis-L" }, { "family_name": "Grandy", "given_name": "D.", "clpid": "Grandy-D" }, { "family_name": "Gavalas", "given_name": "G.", "orcid": "0000-0003-1468-6835", "clpid": "Gavalas-G-R" }, { "family_name": "Gates", "given_name": "B.", "clpid": "Gates-B" } ], "contributor": [ { "family_name": "Petrakis", "given_name": "Leonidas", "clpid": "Petrakis-L" }, { "family_name": "Fraissard", "given_name": "Jacques P.", "clpid": "Fraissard-J-P" } ], "abstract": "A new method of structural analysis is applied to a group of hydroliquefied coal samples. The method uses elemental analysis and NMR data to estimate the concentrations of functional groups in the samples. The structural characterization of these samples demonstrates that estimates of functional group concentrations can be used to provide detailed structural profiles of complex mixtures and to obtain limited information about reaction pathways.", "doi": "10.1007/978-94-009-6378-8_37", "isbn": "9789027717528", "publisher": "Kluwer Academic Publishers", "place_of_publication": "Dordrecht, Netherlands", "publication_date": "1984", "pages": "699-707" }, { "id": "authors:f5e9x-fe237", "collection": "authors", "collection_id": "f5e9x-fe237", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20151008-112330946", "type": "article", "title": "Analysis of synthetic fuels for functional group determination", "author": [ { "family_name": "Petrakis", "given_name": "Leonidas", "clpid": "Petrakis-L" }, { "family_name": "Allen", "given_name": "David T.", "clpid": "Allen-D-T" }, { "family_name": "Gavalas", "given_name": "George R.", "orcid": "0000-0003-1468-6835", "clpid": "Gavalas-G-R" }, { "family_name": "Gates", "given_name": "Bruce C.", "orcid": "0000-0003-0274-4882", "clpid": "Gates-B-C" } ], "abstract": "A method for the structural characterization of complex mixtures is developed which utilizes elemental analysis, NMR, and other analytical data to determine functional group concentrations. The method is tested with mixtures of known composition and is applied to a set of hydrodesulfurlzed coal liquids. The results are compared with mass spectrometric data and with the results of established methods of structural analysis.", "doi": "10.1021/ac00260a025", "issn": "0003-2700", "publisher": "American Chemical Society", "publication": "Analytical Chemistry", "publication_date": "1983-08", "series_number": "9", "volume": "55", "issue": "9", "pages": "1557-1564" }, { "id": "authors:hs65x-2rg27", "collection": "authors", "collection_id": "hs65x-2rg27", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20151105-105158432", "type": "article", "title": "Kinetics of Dialin Thermolysis", "author": [ { "family_name": "Allen", "given_name": "D. T.", "clpid": "Allen-D-T" }, { "family_name": "Gavalas", "given_name": "G. R.", "orcid": "0000-0003-1468-6835", "clpid": "Gavalas-G-R" } ], "abstract": "The liquid-phase thermolysis of 1,2-dihydronaphthalene was studied in a batch reactor in the range of 350\u2013400\u00b0C. The measured product distributions were in good agreement with calculations based on a free-radical scheme with rate constants estimated by thermochemical methods. The kinetic calculations were carried out by numerical integration and by the long-chain approximation (LCA), which yielded a closed-form solution.", "doi": "10.1002/kin.550150303", "issn": "0538-8066", "publisher": "Wiley", "publication": "International Journal of Chemical Kinetics", "publication_date": "1983-03", "series_number": "3", "volume": "15", "issue": "3", "pages": "219-233" }, { "id": "authors:b4c3z-w2133", "collection": "authors", "collection_id": "b4c3z-w2133", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20151105-083611305", "type": "article", "title": "Heat transfer ahead of moving condensation fronts in thermal oil recovery processes", "author": [ { "family_name": "Yortsos", "given_name": "Y. C.", "clpid": "Yortsos-Y-C" }, { "family_name": "Gavalas", "given_name": "G. R.", "orcid": "0000-0003-1468-6835", "clpid": "Gavalas-G-R" } ], "abstract": "This is a study of heat transfer in the liquid zone preceding an advancing condensation front in a 1-dim oil reservoir undergoing a thermal recovery process such as hot waterflood, steam injection or in situ combustion. A model is developed that allows for heat transfer by horizontal conduction and convection in the reservoir and by vertical (conjugate) conduction in the surrounding the reservoir formations to account for the lateral heat losses. The model is formulated in terms of an integro-differential equation involving an integral representation of the lateral heat losses for a region with a moving boundary. According to the magnitude of the Peclet number, and the velocity of the advancing front, various analytical expressions that describe the temperature distribution in the hot liquid zone are derived. The discussion emphasizes the cases Pe \u2aa2 1 (high injection rates) and Pe = 1 (low injection rates). The case of arbitrary Pe is treated by a quasisteady state approximation.", "doi": "10.1016/0017-9310(82)90183-1", "issn": "0017-9310", "publisher": "Elsevier", "publication": "International Journal of Heat and Mass Transfer", "publication_date": "1982-03", "series_number": "3", "volume": "25", "issue": "3", "pages": "305-316" }, { "id": "authors:280wm-fv746", "collection": "authors", "collection_id": "280wm-fv746", "cite_using_url": "https://resolver.caltech.edu/CaltechBOOK:1982.001", "type": "book", "title": "Coal Pyrolysis", "author": [ { "family_name": "Gavalas", "given_name": "George R.", "orcid": "0000-0003-1468-6835", "clpid": "Gavalas-G-R" } ], "abstract": "Prompted by the need of non-petroleum-based fuels, coal research has reemerged to center stage after a lengthy dormant period. Pyrolysis research, in particular, has gained considerable momentum because of its close connection to combustion, hydropyrolysis and liquefaction. Spectroscopic and other instrumental techniques are currently producing prodigious information about coal structure and pyrolysis mechanisms, while modeling efforts are breaking new ground in sorting out chemical and physical phenomena to provide a fundamental although simplified description. \n\nThe continuing generation of experimental data will lead to revisions, in some cases drastic, of current structural and kinetic precepts. Yet, the postulates and assumptions of current work provide a meaningful starting point in elaborating theoretical descriptions of greater validity and applicability. \n\nThis monograph was written to organize recent results of pyrolysis research. Experimental and theoretical aspects, given approximately equal weight, are discussed in the light of basic chemical and physical mechanisms. With this orientation the monograph should be useful to chemists, engineers and graduate students with interests in coal research. \n\nI would like to express my appreciation to the copyright holders for permission to reproduce various figures: to the American Institute of Chemical Engineers for Figs. 4.21, 5.7, 5.14; to the Combustion Institute for Figs. 4.3, 4.14, 4.15; to IPC Science and Technology Press, Ltd. for Figs. 4.16-4.18, 4.20, 5.1-5.6, 5.8, 5.9, 6.1, 6.6, 7.10-7.13; to Dr. W. R. Ladner for Figs. 7.6-7.9; to Dr. P. R. Solomon for Fig. 4.1; to Mr. M. Steinberg for Figs. 7.14-7.16 and to Dr. E. M. Suuberg for Figs. 5.10, 5.11, 6.2-6.5, 7.1-7.4. \n\nI would also like to thank Lenore Kerner and Majda Andlovec for typing the manuscript and Heather Marr for drawing the figures.\n\nG. R. GAVALAS\nPasadena, California\nApril, 1982", "isbn": "0-444-42107-6", "publisher": "Elsevier", "place_of_publication": "New York, NY", "publication_date": "1982" }, { "id": "authors:gcxam-fhd97", "collection": "authors", "collection_id": "gcxam-fhd97", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20151008-112328574", "type": "article", "title": "Model of coal pyrolysis. 2. Quantitative formulation and results", "author": [ { "family_name": "Gavalas", "given_name": "George R.", "orcid": "0000-0003-1468-6835", "clpid": "Gavalas-G-R" }, { "family_name": "Jain", "given_name": "Ravi", "clpid": "Jain-R" }, { "family_name": "Cheong", "given_name": "Paul How-Kei", "clpid": "Cheong-Paul-How-Kei" } ], "abstract": "The reaction rates are expressed in terms of concentrations of reactive configurations which are calculated by randomly distributing the functional groups among the a carbons. The probability of formation of a free unit (tar molecule) following a bridge dissociation is calculated by a similar random placement technique. The rates of change of the state variables and the rates of product formation are expressed in terms of the various reaction rates and the rate of loss of free units. Computer simulations are carried out for an hvc bituminous coal and a subbituminous coal and the results are compared with limited experimental data. A sensitivity analysis is carried out to study the importance of various structural and kinetic parameters relative to the yield of various products.", "doi": "10.1021/i100002a002", "issn": "0196-4313", "publisher": "American Chemical Society", "publication": "Industrial & Engineering Chemistry Fundamentals", "publication_date": "1981-05", "series_number": "2", "volume": "20", "issue": "2", "pages": "122-132" }, { "id": "authors:586y8-1q618", "collection": "authors", "collection_id": "586y8-1q618", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20151008-112328912", "type": "article", "title": "Model of coal pyrolysis. 1. Qualitative development", "author": [ { "family_name": "Gavalas", "given_name": "George R.", "orcid": "0000-0003-1468-6835", "clpid": "Gavalas-G-R" }, { "family_name": "Cheong", "given_name": "Paul How-Kei", "clpid": "Cheong-Paul-How-Kei" }, { "family_name": "Jain", "given_name": "Ravi", "clpid": "Jain-R" } ], "abstract": "Coal is represented as a collection of 14 functional groups including aromatic rings, aliphatic chains and bridges, and oxygen-carrying groups. A procedure is developed for determining the concentrations of these groups from elemental analysis and nuclear magnetic resonance data. The chemical changes taking place during pyrolysis are described by a set of elementary reactions selected on the bask of chemical theory and information from model compound studies. The kinetic parameters of these reactions are estimated using the methods of thermochemical kinetics. Product formation is discussed qualitatively in terms of the elementary reactions.", "doi": "10.1021/i100002a001", "issn": "0196-4313", "publisher": "American Chemical Society", "publication": "Industrial & Engineering Chemistry Fundamentals", "publication_date": "1981-05", "series_number": "2", "volume": "20", "issue": "2", "pages": "113-122" }, { "id": "authors:rk3a7-gs635", "collection": "authors", "collection_id": "rk3a7-gs635", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20151105-101252302", "type": "article", "title": "Comment on \"Analysis of Char Combustion Including the Effect of Pore Enlargement\"", "author": [ { "family_name": "Simons", "given_name": "Girard A.", "clpid": "Simons-G-A" }, { "family_name": "Gavalas", "given_name": "George R.", "orcid": "0000-0003-1468-6835", "clpid": "Gavalas-G-R" } ], "abstract": "In a current article, Gavalas (1980) considers the effect of pore shape change during the combustion of porous carbon char. The pore enlargement is most rapid near the outer edge of the char particle. The pore radius increases until the mouths of adjacent pores coalesce and cause the recession of the exterior surface. The net influence of this effect is to increase the char reactivity above that predicted by traditional theories (all other variables being equal). When the theory of Gavalas is used to infer an intrinsic rate constant from the data of Smith (1971), it is shown that the inferred rate decreases by a factor of 150 for a 2 percent porous particle and by a factor of 3 at 10 percent porosity.", "doi": "10.1080/00102208008952439", "issn": "0010-2202", "publisher": "Taylor & Francis", "publication": "Combustion Science and Technology", "publication_date": "1981", "series_number": "5-6", "volume": "24", "issue": "5-6", "pages": "211-213" }, { "id": "authors:bywj0-1kx15", "collection": "authors", "collection_id": "bywj0-1kx15", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20151104-112517343", "type": "article", "title": "Periodic capillary models of diffusion in porous solids", "author": [ { "family_name": "Gavalas", "given_name": "G. R.", "orcid": "0000-0003-1468-6835", "clpid": "Gavalas-G-R" }, { "family_name": "Kim", "given_name": "S.", "clpid": "Kim-S" } ], "abstract": "Periodic capillary networks are proposed as models for describing diffusion in porous solids. The capillary network is analyzed by means of the nodal concentration equations for a single unit cell and expressions are derived for the diffusional flux in terms of the macroscopic concentration gradient. Conditions for isotropy are derived and the validity of the smooth field assumption is discussed. The theory is illustrated with a macropore-micropore model of porous catalyst pellets and comparisons are made with an earlier capillary model.", "doi": "10.1016/0009-2509(81)85058-0", "issn": "0009-2509", "publisher": "Pergamon", "publication": "Chemical Engineering Science", "publication_date": "1981", "series_number": "7", "volume": "36", "issue": "7", "pages": "1111-1122" }, { "id": "authors:ryb9h-bt284", "collection": "authors", "collection_id": "ryb9h-bt284", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20230413-768778000.22", "type": "article", "title": "History Matching in Two-Phase Petroleum Reservoirs", "author": [ { "family_name": "Watson", "given_name": "A. T.", "clpid": "Watson-Albert-T" }, { "family_name": "Seinfeld", "given_name": "J. H.", "orcid": "0000-0003-1344-4068", "clpid": "Seinfeld-J-H" }, { "family_name": "Gavalas", "given_name": "G. R.", "orcid": "0000-0003-1468-6835", "clpid": "Gavalas-G-R" }, { "family_name": "Woo", "given_name": "P. T.", "clpid": "Woo-P-T" } ], "abstract": "An automatic history-matching algorithm based on an optimal control approach has been formulated for joint estimation of spatially varying permeability and porosity and coefficients of relative permeability functions in two-phase reservoirs. The algorithm uses pressure and production rate data simultaneously. The performance of the algorithm for the water-flooding of one- and two-dimensional hypothetical reservoirs is examined, and properties associated with the parameter estimation problem are discussed.", "doi": "10.2118/8250-pa", "issn": "0197-7520", "publisher": "Society of Petroleum Engineers", "publication": "Society of Petroleum Engineers Journal", "publication_date": "1980-12", "series_number": "6", "volume": "20", "issue": "6", "pages": "521-532" }, { "id": "authors:fbr8h-48f28", "collection": "authors", "collection_id": "fbr8h-48f28", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20151105-083517511", "type": "article", "title": "A Random Capillary Model with Application to Char Gasification at Chemically Controlled Rates", "author": [ { "family_name": "Gavalas", "given_name": "George R.", "orcid": "0000-0003-1468-6835", "clpid": "Gavalas-G-R" } ], "abstract": "Porous particles are described by a random capillary model predicting the frequency of pore intersections and the evolution of pore volume and surface area during reaction. The model is applied to char gasification at chemically controlled rates, and the results are compared with data from the literature.", "doi": "10.1002/aic.690260408", "issn": "0001-1541", "publisher": "Wiley", "publication": "AIChE Journal", "publication_date": "1980-07", "series_number": "4", "volume": "26", "issue": "4", "pages": "577-585" }, { "id": "authors:aevs0-8e673", "collection": "authors", "collection_id": "aevs0-8e673", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20151105-090442442", "type": "article", "title": "Intra particle Mass Transfer in Coal Pyrolysis", "author": [ { "family_name": "Gavalas", "given_name": "George R.", "orcid": "0000-0003-1468-6835", "clpid": "Gavalas-G-R" }, { "family_name": "Wilks", "given_name": "Karl A.", "clpid": "Wilks-K-A" } ], "abstract": "Intraparticle mass transfer in coal pyrolysis is described by ternary diffusion and viscous flow, in conjunction with a simple pore model to predict concentration profiles for gases and tar. At low pressures, product yields depend on particle size only, while at high pressures they depend on pressure and particle size. Limited experimental data from a subbituminous coal confirm these trends. Data from a bituminous coal show different trends, as expected from the drastic changes the pore structure undergoes during pyrolysis.", "doi": "10.1002/aic.690190203", "issn": "0001-1541", "publisher": "Wiley", "publication": "AIChE Journal", "publication_date": "1980-03", "series_number": "2", "volume": "26", "issue": "2", "pages": "201-212" }, { "id": "authors:248xh-7js88", "collection": "authors", "collection_id": "248xh-7js88", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20151105-083517788", "type": "article", "title": "Analysis of Char Combustion Including the Effect of Pore Enlargement", "author": [ { "family_name": "Gavalas", "given_name": "George R.", "orcid": "0000-0003-1468-6835", "clpid": "Gavalas-G-R" } ], "abstract": "The combustion of char particles including the effects of external mass transfer, surface reaction, pore diffusion, and pore growth is analyzed theoretically. A random pore model is used to relate pore surface area, pore volume and effective diffusivity to the local burnoff. The equations for oxygen concentration and burnoff as functions of time and position are solved analytically for the limiting case of strong diffusional limitations (high temperature or particle size) and numerically for the general case. The results include the variation of particle size and density with burnoff under conditions pertinent to pulverized and fluidized combustion. The analysis is compared with previous analyses from the literature.", "doi": "10.1080/00102208008952438", "issn": "0010-2202", "publisher": "Taylor & Francis", "publication": "Combustion Science and Technology", "publication_date": "1980", "series_number": "5-6", "volume": "24", "issue": "5-6", "pages": "197-210" }, { "id": "authors:0j8ex-t5q58", "collection": "authors", "collection_id": "0j8ex-t5q58", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20151008-112331288", "type": "article", "title": "Pyrolysis of a Precipitated Kraft Lignin", "author": [ { "family_name": "Iatridis", "given_name": "Basil", "clpid": "Iatridis-B" }, { "family_name": "Gavalas", "given_name": "George R.", "orcid": "0000-0003-1468-6835", "clpid": "Gavalas-G-R" } ], "abstract": "A precipitated kraft lignin was pyrolyzed in a \"captive sample\" reactor at 400-700 \u00b0C for 10-120 s. Weight loss and tar were determined gravimetrically and oxides of carbon, hydrocarbons, methanol, acetone and single-ring phenols were determined by gas chromatography. The single-ring phenols (phenol, guaiacol, etc.), which are the most valuable among the products, were obtained at yields up to 3% of the original dry lignin.", "doi": "10.1021/i360070a010", "issn": "0196-4321", "publisher": "American Chemical Society", "publication": "Industrial & Engineering Chemistry Product Research and Development", "publication_date": "1979-06", "series_number": "2", "volume": "18", "issue": "2", "pages": "127-130" }, { "id": "authors:mvjs6-5cs07", "collection": "authors", "collection_id": "mvjs6-5cs07", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20230414-532588000.3", "type": "article", "title": "Error Analysis in History Matching: The Optimum Level of Parameterization", "author": [ { "family_name": "Shah", "given_name": "P. C.", "clpid": "Shah-P-C" }, { "family_name": "Gavalas", "given_name": "G. R.", "orcid": "0000-0003-1468-6835", "clpid": "Gavalas-G-R" }, { "family_name": "Seinfeld", "given_name": "J. H.", "orcid": "0000-0003-1344-4068", "clpid": "Seinfeld-J-H" } ], "abstract": "The accuracy of the porosity and permeability estimates obtained in reservoir history matching is investigated using covariance analysis. The estimate covariance matrix is obtained for the following cases: (1) estimation of all individual grid properties, (2) parametrization using sensitivity vectors, (3) parametrization by zonation, and (4) Bayesian estimation. The trace of the covariance matrix used as a measure of the over-all accuracy is studied as a function of the number of unknown parameters, and a procedure for selecting the parameters, and a procedure for selecting the optimum parametrization is developed. Numerical calculations with a one-dimensional reservoir are used to illustrate the theory.", "doi": "10.2118/6508-pa", "issn": "0197-7520", "publisher": "Society of Petroleum Engineers", "publication": "Society of Petroleum Engineers Journal", "publication_date": "1978-06", "series_number": "3", "volume": "18", "issue": "3", "pages": "219-228" }, { "id": "authors:7atx1-04k41", "collection": "authors", "collection_id": "7atx1-04k41", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20151105-083250561", "type": "article", "title": "Characterization of the heavy products of coal pyrolysis", "author": [ { "family_name": "Gavalas", "given_name": "George R.", "orcid": "0000-0003-1468-6835", "clpid": "Gavalas-G-R" }, { "family_name": "Oka", "given_name": "Mitsuo", "clpid": "Oka-Mitsuo" } ], "abstract": "In order to obtain information about the production mechanism of large molecules in coal pyrolysis, a subbituminous and two bituminous coals were heated at 500\u00b0 C for 30s under vacuum. The heavy products were dissolved in tetrahydrofuran (THF) and analysed by ^1H n.m.r. spectroscopy and gel-permeation chromatography in an atmospheric-pressure column. Parallel GPC, ^1H n.m.r. and ^(13)C n.m.r. studies were conducted with THF extracts of the same coals. The results are combined with elemental analysis data to obtain structural parameters of the extracts and pyrolysates.", "doi": "10.1016/0016-2361(78)90005-4", "issn": "0016-2361", "publisher": "Elsevier", "publication": "Fuel", "publication_date": "1978-05", "series_number": "5", "volume": "57", "issue": "5", "pages": "285-288" }, { "id": "authors:d574q-1hs54", "collection": "authors", "collection_id": "d574q-1hs54", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20151008-112331561", "type": "book_section", "title": "Coal Desulfurization by Low-Temperature Chlorinolysis", "book_title": "Coal Desulfurization: Chemical and Physical Methods", "author": [ { "family_name": "Hsu", "given_name": "George C.", "clpid": "Hsu-George-C" }, { "family_name": "Kalvinskas", "given_name": "John J.", "clpid": "Kalvinskas-J-J" }, { "family_name": "Ganguli", "given_name": "Partha S.", "clpid": "Ganguli-P-S" }, { "family_name": "Gavalas", "given_name": "George R.", "orcid": "0000-0003-1468-6835", "clpid": "Gavalas-G-R" } ], "contributor": [ { "family_name": "Wheelock", "given_name": "Thomas D.", "clpid": "Wheelock-T-D" } ], "abstract": "Since most of the coals in this country, particularly the eastern and mid-western coals, have a high sulfur content (>2%), there is a need for an economical process of converting high-sulfur coals to clean fuel (<1.2 lbs of SO_2 emission per million Btu by EPA standards) so that coal can be used as a source of energy without causing serious air pollution. \n\nAmong the three principal methods for precombustion desulfurization of coal - physical depyriting, chemical desulfurization, and coal conversion to low-sulfur liquid and gaseous fuels, the potential of chemical methods looks promising in terms of both total sulfur removal and processing cost. The principal chemical methods for coal desulfurization involve treatment with either oxidizing agents or basic media at elevated temperature and pressure. For example, the method of oxidation by contacting an aqueous slurry of coal with air at pressures up to 1000 psi and temperatures.", "doi": "10.1021/bk-1977-0064.ch017", "isbn": "9780841204003", "publisher": "American Chemical Society", "place_of_publication": "Washington, DC", "publication_date": "1977-06-01", "pages": "206-217" }, { "id": "authors:q0tac-db356", "collection": "authors", "collection_id": "q0tac-db356", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20151105-083250247", "type": "article", "title": "Computer-Assisted Molecular Structure Construction for coal-derived compounds", "author": [ { "family_name": "Oka", "given_name": "Mitsuo", "clpid": "Oka-Mitsuo" }, { "family_name": "Chang", "given_name": "Hsueh-Chia", "clpid": "Chang-Hsueh-Chia" }, { "family_name": "Gavalas", "given_name": "George R.", "orcid": "0000-0003-1468-6835", "clpid": "Gavalas-G-R" } ], "abstract": "A new technique, Computer-Assisted Molecular Structure Construction (CAMSC), has been developed for the structural elucidation of coal-derived compounds. It utilizes elemental analysis, n.m.r. spectra and molecular weight data to determine the allowable combinations of functional groups constituting the structure. CAMSC is applied to four cases of experimental data from the literature. The results compare well with those obtained by other methods and in some cases provide additional information.", "doi": "10.1016/0016-2361(77)90033-3", "issn": "0016-2361", "publisher": "Elsevier", "publication": "Fuel", "publication_date": "1977-01", "series_number": "1", "volume": "56", "issue": "1", "pages": "3-8" }, { "id": "authors:e4816-dvp22", "collection": "authors", "collection_id": "e4816-dvp22", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20230413-139427000.9", "type": "article", "title": "Reservoir History Matching by Bayesian Estimation", "author": [ { "family_name": "Gavalas", "given_name": "G. R.", "orcid": "0000-0003-1468-6835", "clpid": "Gavalas-G-R" }, { "family_name": "Shah", "given_name": "P. C.", "clpid": "Shah-P-C" }, { "family_name": "Seinfeld", "given_name": "J. H.", "orcid": "0000-0003-1344-4068", "clpid": "Seinfeld-J-H" } ], "abstract": "The estimation of reservoir properties is inherently an underdetermined problem (one having a nonunique solution) because of the large number of unknown parameters relative to the available data. parameters relative to the available data. The common zonation approach to reducing the number of parameters introduces considerable modeling error by insisting that reservoir properties are uniform within each zone and by assigning the boundaries of these zones more or less arbitrarily. In this paper, Bayesian estimation theory is applied to history matching as an alternative to zonation. By using a priori statistical information on the unknown parameters, the problem becomes statistically better determined. Bayesian estimation and zonation are applied to the problem of porosity and permeability estimation in a one-dimensional, permeability estimation in a one-dimensional, one-phase reservoir.", "doi": "10.2118/5740-pa", "issn": "0197-7520", "publisher": "Society of Petroleum Engineers", "publication": "Society of Petroleum Engineers Journal", "publication_date": "1976-12", "series_number": "6", "volume": "16", "issue": "6", "pages": "337-350" }, { "id": "authors:b195b-s7r06", "collection": "authors", "collection_id": "b195b-s7r06", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20151104-143218784", "type": "article", "title": "Characterization of Acid-Base Catalysts by Calorimetric Titration I. Correlation with Alcohol Dehydration Activity", "author": [ { "family_name": "Bakshi", "given_name": "K. R.", "clpid": "Bakshi-K-R" }, { "family_name": "Gavalas", "given_name": "G. R.", "orcid": "0000-0003-1468-6835", "clpid": "Gavalas-G-R" } ], "abstract": "Several commercial aluminas, silica-aluminas and clays are characterized by calorimetric titration with n-butylamine and trichloroacetic acid. The heat of adsorption distributions obtained by titration are found to be sufficient measures of surface acidity and basicity in\ncorrelating catalyst activity towards alcohol dehydration. A correlation is obtained by dividing the distributions into groups of suitable acidic and basic site pairs, and assigning to each group a specific reaction rate by least-squares fitting with the observed rates of dehydration. The correlation describes well both olefin and ether formation and provides support for a reaction mechanism proposed in the literature.", "doi": "10.1016/0021-9517(75)90093-7", "issn": "0021-9517", "publisher": "Elsevier", "publication": "Journal of Catalysis", "publication_date": "1975-06-23", "series_number": "1-3", "volume": "38", "issue": "1-3", "pages": "312-325" }, { "id": "authors:w758x-cvn92", "collection": "authors", "collection_id": "w758x-cvn92", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20151104-143218498", "type": "article", "title": "Characterization of Acid-Base Catalysts by Calorimetric Titration II. Effect of Poisoning on Titration Curves and Alcohol Dehydration Activity", "author": [ { "family_name": "Bakshi", "given_name": "K. R.", "clpid": "Bakshi-K-R" }, { "family_name": "Gavalas", "given_name": "G. R.", "orcid": "0000-0003-1468-6835", "clpid": "Gavalas-G-R" } ], "abstract": "Several commercial aluminas, silica-aluminas and clays have been subjected to poisoning by ammonia and organic bases and subsequently characterized (1) by calorimetry, yielding the heat of adsorption of bases and acids as a function of coverage (2) activity in ethanol and methanol dehydration reactions. A correlation developed in the previous paper [Bakshi, K. R., and Gavalas, G. R., J. Catal. 38, 312 (1975)] describing the activity of fresh catalysts in terms of their acidity and basicity distributions has been used to describe the activity of the poisoned catalysts. Certain rather subtle selectivity changes caused by poisoning have been explained by the corresponding changes in the acidity and basicity distributions.", "doi": "10.1016/0021-9517(75)90094-9", "issn": "0021-9517", "publisher": "Elsevier", "publication": "Journal of Catalysis", "publication_date": "1975-06-23", "series_number": "1-3", "volume": "38", "issue": "1-3", "pages": "326-334" }, { "id": "authors:hrxdf-ydz10", "collection": "authors", "collection_id": "hrxdf-ydz10", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20151105-083145510", "type": "article", "title": "Effects of Nonseparable Kinetics in Alcohol Dehydration over Poisoned Silica-Alumina", "author": [ { "family_name": "Bakshi", "given_name": "K. R.", "clpid": "Bakshi-K-R" }, { "family_name": "Gavalas", "given_name": "G. R.", "orcid": "0000-0003-1468-6835", "clpid": "Gavalas-G-R" } ], "abstract": "Methanol and ethanol dehydration over fresh and poisoned silica-alumina have been used as a model system to demonstrate experimentally certain effects of nonseparable kinetics including the change in product distribution upon flow reversal in a reactor with a poisoning gradient. The rate expression r = kK_A^C_A^(1/2)/(1 + K_A^C_A^(1/2) + K_W^C_W) describes the epxerimental data for all reactions although the constants for ether and olefin formation are entirely different and vary between fresh and poisoned catalysts. The nonseparability of kinetics and the concomitant variation of K_A, K_W with poisoning are briefly discussed in terms of interactions between poison molecules and surface acid-base sites.", "doi": "10.1002/aic.690210310", "issn": "0001-1541", "publisher": "Wiley", "publication": "AIChE Journal", "publication_date": "1975-05", "series_number": "3", "volume": "21", "issue": "3", "pages": "494-500" }, { "id": "authors:e969q-ezt22", "collection": "authors", "collection_id": "e969q-ezt22", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:CHEspej74", "type": "article", "title": "A New Algorithm for Automatic History Matching", "author": [ { "family_name": "Chen", "given_name": "Wen H.", "clpid": "Chen-Wen-H" }, { "family_name": "Gavalas", "given_name": "George R.", "orcid": "0000-0003-1468-6835", "clpid": "Gavalas-G-R" }, { "family_name": "Seinfeld", "given_name": "John H.", "orcid": "0000-0003-1344-4068", "clpid": "Seinfeld-J-H" }, { "family_name": "Wasserman", "given_name": "M. L.", "clpid": "Wasserman-M-L" } ], "abstract": "History-matching problems, in which reservoir parameters arc to be estimated from well pressure data, are formulated as optimal control problems. The necessary conditions for optimality lead naturally to gradient optimization methods for determining the optimal parameter estimates. The key feature of the approach is that reservoir properties are considered as continuous functions\nof position rather than as uniform in a certain number of zones. The optimal control approach is illustrated on a hypothetical reservoir and on an actual Saudi Arabian reservoir, both characterized by single-phase flow. A significant saving in computing time over conventional constant-zone gradient optimization methods is demonstrated.", "issn": "0197-7520", "publisher": "Society of Petroleum Engineers", "publication": "Society of Petroleum Engineers Journal", "publication_date": "1974", "series_number": "6", "volume": "14", "issue": "6", "pages": "593-608" }, { "id": "authors:ew197-gp872", "collection": "authors", "collection_id": "ew197-gp872", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20230413-139792000.41", "type": "article", "title": "Reservoirs With Spatially Varying Properties: Estimation of Volume From Late Transient Pressure Data", "author": [ { "family_name": "Gavalas", "given_name": "G. R.", "orcid": "0000-0003-1468-6835", "clpid": "Gavalas-G-R" }, { "family_name": "Seinfeld", "given_name": "J. H.", "orcid": "0000-0003-1344-4068", "clpid": "Seinfeld-J-H" } ], "abstract": "The transient pressure in a reservoir of arbitrary shape and spatially varying properties is expressed in terms of the eigenvalues and eigenfunctions of the region. Although these eigenvalues and eigenfunctions cannot be obtained analytically, except in homogeneous reservoirs of regular shape, the structure of the solution allows the estimation of the pore volume in the general case using data in the pseudosteady or the late-transient period. A least-squares analysis of late transient data is presented which yields, in addition to the pore presented which yields, in addition to the pore volume, the first eigenvalue, which can often be related to reservoir properties. Numerical examples are given using simulated data from a rectangular 4 x 1 reservoir with constant permeability and with spatially varying permeability.", "doi": "10.2118/4169-pa", "issn": "0197-7520", "publisher": "Society of Petroleum Engineers", "publication": "Society of Petroleum Engineers Journal", "publication_date": "1973-12-01", "series_number": "6", "volume": "13", "issue": "6", "pages": "335-342" }, { "id": "authors:gkvjb-8xq72", "collection": "authors", "collection_id": "gkvjb-8xq72", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20151105-083145260", "type": "article", "title": "A New Method of Parameter Estimation in Linear Systems", "author": [ { "family_name": "Gavalas", "given_name": "George R.", "orcid": "0000-0003-1468-6835", "clpid": "Gavalas-G-R" } ], "abstract": "A time domain method is given for estimating the matrix of related parameters in linear systems with constant coefficients and real eigenvalues. The method consists of a one-dimensional search for the local minima of a scalar function \u03bc(\u03bb), which provide the eigenvalues of the system matrix and the matrix itself when observable. Applications are given to the determination of a transfer function and the estimation of the rate matrix of a monomolecular reaction system. Questions of accuracy, number, and type of measurements required are discussed.", "doi": "10.1002/aic.690190203", "issn": "0001-1541", "publisher": "Wiley", "publication": "AIChE Journal", "publication_date": "1973-03", "series_number": "2", "volume": "19", "issue": "2", "pages": "214-222" }, { "id": "authors:fnq7g-kw123", "collection": "authors", "collection_id": "fnq7g-kw123", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20151104-143219672", "type": "article", "title": "Estimation of Catalyst Deactivation Parameters from Operating Reactor Data", "author": [ { "family_name": "Gavalas", "given_name": "G. R.", "orcid": "0000-0003-1468-6835", "clpid": "Gavalas-G-R" }, { "family_name": "Hsu", "given_name": "G. C.", "clpid": "Hsu-G-C" }, { "family_name": "Seinfeld", "given_name": "J. H.", "orcid": "0000-0003-1344-4068", "clpid": "Seinfeld-J-H" } ], "abstract": "Techniques are presented for estimating the parameters of catalyst deactivation models from the operating data of industrial reactors. In the case of a first-order deactivation model, the partial differential equations describing the main reaction and deactivation are transformed to an ordinary differential equation for which estimation techniques are well established. The general power deactivation model can only be treated approximately and the case of a bifunctional catalyst must be treated by special methods. A numerical example is presented for the oxidation of butene to butadiene.", "doi": "10.1016/0300-9467(72)80056-X", "issn": "1385-8947", "publisher": "Elsevier", "publication": "Chemical Engineering Journal", "publication_date": "1972-10", "series_number": "1", "volume": "4", "issue": "1", "pages": "77-84" }, { "id": "authors:7hp1m-sxq18", "collection": "authors", "collection_id": "7hp1m-sxq18", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20151008-112331825", "type": "article", "title": "On the Estimation of Kinetic Rate Constants", "author": [ { "family_name": "Churchill", "given_name": "Stuart W.", "clpid": "Churchill-S-W" }, { "family_name": "Seinfeld", "given_name": "John H.", "orcid": "0000-0003-1344-4068", "clpid": "Seinfeld-J-H" }, { "family_name": "Gavalas", "given_name": "George R.", "orcid": "0000-0003-1468-6835", "clpid": "Gavalas-G-R" }, { "family_name": "Tanner", "given_name": "Robert D.", "clpid": "Tanner-R-D" } ], "abstract": "[No abstract]", "doi": "10.1021/i160043a031", "issn": "0196-4313", "publisher": "American Chemical Society", "publication": "Industrial & Engineering Chemistry Fundamentals", "publication_date": "1972-08", "series_number": "3", "volume": "11", "issue": "3", "pages": "429-429" }, { "id": "authors:9vb1m-n3204", "collection": "authors", "collection_id": "9vb1m-n3204", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20151104-112517076", "type": "article", "title": "Estimation of catalyst activity profiles in fixed bed reactors", "author": [ { "family_name": "Gavalas", "given_name": "G. R.", "orcid": "0000-0003-1468-6835", "clpid": "Gavalas-G-R" }, { "family_name": "Hsu", "given_name": "G. C.", "clpid": "Hsu-G-C" }, { "family_name": "Seinfeld", "given_name": "J. H.", "orcid": "0000-0003-1344-4068", "clpid": "Seinfeld-J-H" } ], "abstract": "The description of the steady state operation of a fixed bed catalytic reactor requires a certain amount of information about the catalyst activity distribution. For a monofunctional catalyst with no transport limitations the total catalyst activity is sufficient for describing the steady reactor operation. The total activity can be easily estimated from measurements at the reactor exit. In the presence of diffusional limitations, or in the case of bifunctional catalysts, the reactor description requires knowledge of the detailed activity profile. This can be approximately determined by parametrization and estimation of the parameters from a few measurements along the reactor. An application of the parametrization technique is made to a simplified model of naphtha reforming.", "doi": "10.1016/0009-2509(72)85069-3", "issn": "0009-2509", "publisher": "Pergamon", "publication": "Chemical Engineering Science", "publication_date": "1972-02", "series_number": "2", "volume": "27", "issue": "2", "pages": "329-338" }, { "id": "authors:2eacw-6t740", "collection": "authors", "collection_id": "2eacw-6t740", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20151008-112332113", "type": "article", "title": "Properties of Partially Deactivated Reactors with Applications in Bifunctional Catalysis", "author": [ { "family_name": "Gavalas", "given_name": "George R.", "orcid": "0000-0003-1468-6835", "clpid": "Gavalas-G-R" } ], "abstract": "It is shown theoretically that the reaction paths and the product distribution in partially deactivated fixed-bed reactors depend on the catalyst activity profile, the direction of flow, and the space velocity. These \"directional effects,\" which are more pronounced in bifunctional catalysis, suggest new degrees of freedom in reactor design and operation and may be useful in the interpretation of kinetic experiments. The theory is applied to pentane isomerization and naphtha reforming, indicating that in pentane isomerization the effects under consideration are very small. In naphtha reforming on the other hand, the effects are significant. Numerical calculations show that conversion and selectivity can change by as much as 25% when the direction of flow is reversed.", "doi": "10.1021/i160040a014", "issn": "0196-4313", "publisher": "American Chemical Society", "publication": "Industrial & Engineering Chemistry Fundamentals", "publication_date": "1971-11", "series_number": "4", "volume": "10", "issue": "4", "pages": "621-626" }, { "id": "authors:v1qtg-spq43", "collection": "authors", "collection_id": "v1qtg-spq43", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20151105-083144987", "type": "article", "title": "Optimal Operation of Regeneratively Cooled Fixed Bed Reactors", "author": [ { "family_name": "Gavalas", "given_name": "George R.", "orcid": "0000-0003-1468-6835", "clpid": "Gavalas-G-R" } ], "abstract": "A regeneratively cooled fixed bed reactor, shown in Figure\n1, consists of two sections alternating roles as reactor\nand heat regenerator by periodic flow reversals. The reactor\ncan have any desired diameter because the cooling \"Occurs strictly by regeneration. The system possesses four control variables that can be adjusted for optimal operation. The first is the temperature profile in the reacting section at the beginning of each cycle and is implemented during regeneration. The second is the temperature of the feed to the reacting section as a function of time. The third\nis the catalyst activity profile and can be implemented by\nmixing the catalyst with an inert solid. The last is the\nperiod of cycling, that is, the time interval between flow\nreversals. These four variables are not available in steady\nstate designs.", "doi": "10.1002/aic.690170406", "issn": "0001-1541", "publisher": "Wiley", "publication": "AIChE Journal", "publication_date": "1971-07", "series_number": "4", "volume": "17", "issue": "4", "pages": "787-792" }, { "id": "authors:q3f29-twt39", "collection": "authors", "collection_id": "q3f29-twt39", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20151008-112332393", "type": "article", "title": "Fixed-Bed Reactors with Regenerative Cooling", "author": [ { "family_name": "Gavalas", "given_name": "George R.", "orcid": "0000-0003-1468-6835", "clpid": "Gavalas-G-R" } ], "abstract": "A regeneratively cooled fixed-bed reactor system was studied theoretically. It consists of two sections which alternate roles as reactor and heat regenerator by periodic flow reversals. Calculations were performed for a single exothermic reaction using a forward cell model. The temperature profiles can be maintained within preassigned limits by a suitable choice of operating conditions and by diluting the catalyst with an inert solid. Because of the facility of heat removal and the favorable temperature profiles, the regenerative system can be used for strongly exothermic reactions such as partial hydrocarbon oxidations.", "doi": "10.1021/i160037a013", "issn": "0196-4313", "publisher": "American Chemical Society", "publication": "Industrial & Engineering Chemistry Fundamentals", "publication_date": "1971-02", "series_number": "1", "volume": "10", "issue": "1", "pages": "71-74" }, { "id": "authors:jdx41-a5j26", "collection": "authors", "collection_id": "jdx41-a5j26", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20151008-112332705", "type": "article", "title": "Control of Plug-Flow Tubular Reactors by Variation of Flow Rate", "author": [ { "family_name": "Seinfeld", "given_name": "John H.", "orcid": "0000-0003-1344-4068", "clpid": "Seinfeld-J-H" }, { "family_name": "Gavalas", "given_name": "George R.", "orcid": "0000-0003-1468-6835", "clpid": "Gavalas-G-R" }, { "family_name": "Hwang", "given_name": "Myungkyu", "clpid": "Hwang-Myungkyu" } ], "abstract": "The control of isothermal and adiabatic plug-flow tubular reactors by variation of flow rate was studied. Proportional feedback, feedforward, and optimal control responses were compared for the regulation of reactor conversion in the presence of inlet disturbances. The optimal control, consisting of a singular solution in each case, produces a considerably improved response over both feedforward and proportional feedback control.", "doi": "10.1021/i160036a021", "issn": "0196-4313", "publisher": "American Chemical Society", "publication": "Industrial & Engineering Chemistry Fundamentals", "publication_date": "1970-11", "series_number": "4", "volume": "9", "issue": "4", "pages": "651-655" }, { "id": "authors:h57wg-y0s93", "collection": "authors", "collection_id": "h57wg-y0s93", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20151105-083031329", "type": "article", "title": "Analysis of Kinetic Parameters from Batch and Integral Reaction Experiments", "author": [ { "family_name": "Seinfeld", "given_name": "John H.", "orcid": "0000-0003-1344-4068", "clpid": "Seinfeld-J-H" }, { "family_name": "Gavalas", "given_name": "George R.", "orcid": "0000-0003-1468-6835", "clpid": "Gavalas-G-R" } ], "abstract": "The estimation of chemical reaction rate constants from experimental batch or integral reactor data is examined. Consideration is given to the effect on the accuracy of the parameter estimates of the number of concentrations measured, extending the range of measurements to higher conversions, and experimental errors. The study is based on estimates of two rate constants obtained by quasilinearization in the pyrolytic dehydrogenation of benzene. It is found that if fewer than the number of independent components are to be measured, certain components produce better parameter estimates than others. The level of experimental errors is by far the most significant factor in obtaining reliable parameter estimates.", "doi": "10.1002/aic.690160423", "issn": "0001-1541", "publisher": "Wiley", "publication": "AIChE Journal", "publication_date": "1970-07", "series_number": "4", "volume": "16", "issue": "4", "pages": "644-647" }, { "id": "authors:r9jp0-ggq73", "collection": "authors", "collection_id": "r9jp0-ggq73", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20151016-124526265", "type": "article", "title": "Theory of the Positive Column Including Electron-Ion Recombination", "author": [ { "family_name": "Lindgren", "given_name": "R. G.", "clpid": "Lindgren-R-G" }, { "family_name": "Gavalas", "given_name": "G. R.", "orcid": "0000-0003-1468-6835", "clpid": "Gavalas-G-R" } ], "abstract": "A theoretical study of the effect of electron\u2010ion recombination on the voltage\u2010current characteristic of the positive column of a glow discharge is presented. In the absence of recombination, as the current increases the voltage decreases and reaches a constant value. In the presence of recombination and at sufficiently high currents, the voltage increases with increasing current.", "doi": "10.1063/1.1693109", "issn": "0031-9171", "publisher": "American Institute of Physics", "publication": "Physics of Fluids", "publication_date": "1970-06", "series_number": "6", "volume": "13", "issue": "6", "pages": "1499-1503" }, { "id": "authors:5g94e-2hm20", "collection": "authors", "collection_id": "5g94e-2hm20", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20151008-112333015", "type": "article", "title": "Control of Nonlinear Stochastic Systems", "author": [ { "family_name": "Seinfeld", "given_name": "John H.", "orcid": "0000-0003-1344-4068", "clpid": "Seinfeld-J-H" }, { "family_name": "Gavalas", "given_name": "George R.", "orcid": "0000-0003-1468-6835", "clpid": "Gavalas-G-R" }, { "family_name": "Hwang", "given_name": "Myung", "clpid": "Hwang-Myung" } ], "abstract": "The control of nonlinear lumped-parameter dynamical systems subject to random inputs and measurement errors is considered. A scheme is developed whereby a nonlinear filter is included in the control loop to improve system performance. The case of pure time delays occurring in the control loop is also treated. Computations are presented for the proportional control on temperature of a CSTR subject to random disturbances.", "doi": "10.1021/i160030a013", "issn": "0196-4313", "publisher": "American Chemical Society", "publication": "Industrial & Engineering Chemistry Fundamentals", "publication_date": "1969-05", "series_number": "2", "volume": "8", "issue": "2", "pages": "257-262" }, { "id": "authors:fsd9v-mqq86", "collection": "authors", "collection_id": "fsd9v-mqq86", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20151104-112516816", "type": "article", "title": "Sequential estimation of states and kinetic parameters in tubular reactors with catalyst decay", "author": [ { "family_name": "Gavalas", "given_name": "G. R.", "orcid": "0000-0003-1468-6835", "clpid": "Gavalas-G-R" }, { "family_name": "Seinfeld", "given_name": "J. H.", "orcid": "0000-0003-1344-4068", "clpid": "Seinfeld-J-H" } ], "abstract": "A method is presented for the on-line sequential estimation (filtering) of state variables and kinetic parameters in plug flow reactors with slow catalyst decay. The essential element of this method is the approximation, for purposes of estimation only, of the distributed parameter system by a lumped parameter system. This approximation is made possible by the slow catalyst decay The resulting sequential estimation algorithms, or filters, are computationally simple, especially the discrete filters, i e those based on a difference equation description. An application of two alternative discrete filters to a plug flow reactor with a first order irreversible reaction and catalyst decay has given good results The paper includes a brief general discussion of sequential estimation and its possible applications.", "doi": "10.1016/0009-2509(69)80056-4", "issn": "0009-2509", "publisher": "Pergamon", "publication": "Chemical Engineering Science", "publication_date": "1969-04", "series_number": "4", "volume": "24", "issue": "4", "pages": "625-636" }, { "id": "authors:zypfz-xay50", "collection": "authors", "collection_id": "zypfz-xay50", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20151008-112333322", "type": "article", "title": "Diffusion Coefficients in Hydrocarbon Systems. Homogeneous Phases at Elevated Pressures", "author": [ { "family_name": "Gavalas", "given_name": "G. R.", "orcid": "0000-0003-1468-6835", "clpid": "Gavalas-G-R" }, { "family_name": "Reamer", "given_name": "H. H.", "clpid": "Reamer-H-H" }, { "family_name": "Sage", "given_name": "B. H.", "clpid": "Sage-B-H" } ], "abstract": "The phenomenon of change in pressure accompanying gaseous diffusion in a closed isochoric, isothermal system has been studied experimentally and theoretically. Special equipment has been developed for measuring pressure differences with an accuracy better than 0.01 p.s.i. at total pressure levels of hundreds of pounds per square inch. The theoretical analysis shows that as the system approaches equilibrium, the pressure as a function of time assumes the simple exponential form p - p_(eq) = -ae^(-bD_(12)t, where p_(eq) is the equilibrium pressure, D_(12) is the binary diffusion coefficient, a is a thermodynamic quantity, and b is a constant depending on the size and shape of the diffusion cell. This expression can be used for the determination of D_(12) from pressure-time measurements. Measurements upon the system CH_4-C_3H_8 at T = 160\u00b0 F. and p_(eq) = 570 p.s.i. have shown the applicability of this method, but more experimental work is required for ascertaining its accuracy.", "doi": "10.1021/i160026a020", "issn": "0196-4313", "publisher": "American Chemical Society", "publication": "Industrial & Engineering Chemistry Fundamentals", "publication_date": "1968-05", "series_number": "2", "volume": "7", "issue": "2", "pages": "306-312" }, { "id": "authors:zxv4s-6vv22", "collection": "authors", "collection_id": "zxv4s-6vv22", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20151104-112516570", "type": "article", "title": "The behavior of distributed systems with complex reactions and diffusion in the regime of transport limitation", "author": [ { "family_name": "Gavalas", "given_name": "George R.", "orcid": "0000-0003-1468-6835", "clpid": "Gavalas-G-R" } ], "abstract": "The be havior of distributed systems with complex chemical reactions and diffusive transport is investigated in the regime of transport limitation. It is shown that, asymptotically with increasing size, the reaction rates and concentration gradients are negligible except in a surface layer of constant thickness. Due to a lack of symmetry in the constraints, the equilibrium state in distributed systems is not related to the extremization of a thermodynamic potential function. It is closely related, however, to the entropy production and flow. Upper bounds for the surface fluxes and effectiveness factors are derived for general complex reactions. These have theoretical interest but are not accurate enough for predictive purposes. A less general but more accurate estimating technique is also developed. Examples of two simultaneous second-order exothermic irreversible reactions and of two simultaneous isothermal, isomerization reactions with nonlinear kinetics are treated in detail.", "doi": "10.1016/0009-2509(67)80163-5", "issn": "0009-2509", "publisher": "Pergamon", "publication": "Chemical Engineering Science", "publication_date": "1967-07", "series_number": "7", "volume": "22", "issue": "7", "pages": "997-1011" }, { "id": "authors:atjqv-zn019", "collection": "authors", "collection_id": "atjqv-zn019", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20151026-152850063", "type": "article", "title": "On the Theory of Reactions in Continuous Mixtures", "author": [ { "family_name": "Aris", "given_name": "R.", "clpid": "Aris-R" }, { "family_name": "Gavalas", "given_name": "G. R.", "orcid": "0000-0003-1468-6835", "clpid": "Gavalas-G-R" } ], "abstract": "A mixture with a very large number of components approaches the condition of a continuous mixture in which the components are not distinguished by a discrete index but by a continuous variable. Such a mixture can be described by distributions of concentration and is capable of sustaining an infinite number of reactions. Polymerization and cracking reactions can be treated in this way and there may be applications to the very complex processes of biology. The aim of this paper is to lay the foundations for the stoicheiometry, thermodynamics and kinetics of such reactions and to outline several techniques for solving the resulting integro-differential equations. Attention is also paid to the problem of fitting the parameters of such a model to experimental data.", "doi": "10.1098/rsta.1966.0054", "issn": "1364-503X", "publisher": "Royal Society", "publication": "Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences", "publication_date": "1966-09", "series_number": "1112", "volume": "260", "issue": "1112", "pages": "351-393" }, { "id": "authors:6b1kj-h9396", "collection": "authors", "collection_id": "6b1kj-h9396", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20151104-112516273", "type": "article", "title": "The long chain approximation in free radical reaction systems", "author": [ { "family_name": "Gavalas", "given_name": "G. R.", "orcid": "0000-0003-1468-6835", "clpid": "Gavalas-G-R" } ], "abstract": "A long chain approximation is presented for the analysis of free radical mechanisms. This permits the calculation of relative concentrations on the basis of the chain propagation reactions alone and is valid for large chain lengths. An application is given to the calculation of product distributions in the high-pressure thermal cracking n-hexadecane and comparisons are made with existing experimental data. Due to the meager information on rate constants and to the limited accuracy of the data these comparisons indicate the plausibility of the approximation but do not provide a measure of its accuracy.", "doi": "10.1016/0009-2509(66)85004-2", "issn": "0009-2509", "publisher": "Pergamon", "publication": "Chemical Engineering Science", "publication_date": "1966-02", "series_number": "2", "volume": "21", "issue": "2", "pages": "133-142" }, { "id": "authors:jd6dx-8wn24", "collection": "authors", "collection_id": "jd6dx-8wn24", "cite_using_url": "https://resolver.caltech.edu/CaltechAUTHORS:20151104-112515516", "type": "article", "title": "On the steady states of distributed parameter systems with chemical reactions, heat, and mass transfer", "author": [ { "family_name": "Gavalas", "given_name": "G. R.", "orcid": "0000-0003-1468-6835", "clpid": "Gavalas-G-R" } ], "abstract": "The number of steady states for a distributed system with chemical reactions, heat, and mass transfer is investigated by means of fixed point methods. The main step in the analysis is the establishment of a priori bounds for the steady state concentrations and temperature. Such bounds can be derived from the conservation of atomic species and energy, independently of the reaction rates and the size of the medium. It is shown in great generality that steady states exist and they are odd in number. Improved bounds obtained by considering the reaction rates and the size of the medium are used in developing criteria for the uniqueness of the steady state. It is shown that if the medium is sufficiently small there is one only steady state. An example of two second order simultaneous exothermic chemical reactions in a catalyst particle is treated in detail and numerical results are obtained.", "doi": "10.1016/0009-2509(66)85062-5", "issn": "0009-2509", "publisher": "Pergamon", "publication": "Chemical Engineering Science", "publication_date": "1966", "series_number": "6-7", "volume": "21", "issue": "6-7", "pages": "477-492" } ]