![]() In contrast to this procedure, this paper describes a way to model such phenomena comprising internal diffusion in microporous particles coupled with any first-order rate process inherent both in the physical system, i.e. Transport and reaction rates are usually determined independently of each other. ![]() Often complex phenomena govern the overall rate of processes of both physical adsorption and catalytic reaction. ![]() Microporous solids with pore diameters comparable to effective molecular cross-sections are mainly used as both stereo-selective absorbents and shape-selective catalysts. This behaviour pattern depends on the relative affinities and saturation capacities of the transport channels and the capacity channels and also on the ratio of the mass exchange rate at the channel intersection and the diffusion rate in the transport channels. We have shown in this analysis that the apparent intracrystalline diffusivity obtained under differential conditions and defined in accordance with the traditional Fickian equation may exhibit diverse trends with increasing loading. A general model is proposed and degenerate models are also identified when one of the mass transfer resistances is negligible. There is assumed to be no diffusional resistance within the capacity channels. The interchange of mass between the sorbates in the transport channels and the capacity channels is assumed to follow Langmuir type kinetics at the intersection between the two channel systems. The transport of sorbate molecules in the transport channels is assumed to be driven by the gradient of chemical potential. The sorbate molecules in these channels are treated in this analysis as two distinct sorbed species or phases. The transport channels not only allow sorbate molecules an access into the crystal interior but also provide a capacity sink for these sorbate molecules, while the capacity channels intersecting with the transport channels act only as a capacity sink. In this paper we present a theoretical analysis of sorption kinetics in zeolite crystals which are assumed to consist of transport channels and capacity channels. Therefore, it is possible that the surface barrier effect observed in zeolites and carbon molecular sieves is governed by highly localized (single atomic layer) structural details. Perhaps the most important observation is that when the pore size approaches the kinetic diameter of the gas molecules, changes of a few percent in the pore size cause order-of-magnitude changes in the resistance. Thermal vibrations of the adsorbent atoms have little effect on the adsorption/surface diffusion mechanisms but cause fluctuations in the effective pore mouth area which can significantly affect transport rates. Adsorption followed by surface diffusion to the pore mouth makes a significant contribution to the mass transfer when the temperature is low or, equivalently, when the adsorptive potential is strong. The effects of temperature, pore size, and thermal motion of the adsorbent atoms are studied for a slit pore mouth model. The mass transfer resistance associated with penetrating the mouth of a very small pore is evaluted using classical molecular dynamics simulation techniques. The equilibrium concentration of o-xylene at P/P0 = 0.1 was only 0.38-0.68 mol/unit cell, while that of p-xylene was 4.40-5.98 mol/unit cell. Values for the effective diffusivities decreased from 2.6 × 10-12 m2/s for the H-form to 2.0-2.3 × 10-12 m2/s for the modified forms. The sorption isotherms for o-xylene showed some unusual steps. 1.7 × 10-11 m2/s, irrespective of whether the H-form or the modified forms were investigated. ![]() The rate of sorption of p-xylene on all the samples investigated followed Fick's second law of diffusion, leading to an effective diffusivity of ca. The BET specific surface area decreased from 358 m2/g to 35-90 m2/g mainly because of a decrease in micropore volume as determined by the t-plot method. The diffusivities of the samples were measured using a new flow gravimetric method involving the sorption of p- and o-xylenes. ZDENEK BOBOK PLUSA candidate with a seat obtained based on preferential votes is marked by the plus sign (+) in front of his/her number.The sorption properties of samples of H-ZSM-5 zeolite and their forms modified with P, B and Mg were determined by the physical adsorption of nitrogen and evaluated by BET isotherm and t-plot methods. A candidate with a seat obtained in the 1st scrutiny is marked by the asterisk sign (*) in the Seat column and a candidate with a seat obtained in the 2nd scrutiny is marked by the double asterisk sign (**). The table displays preferential votes for candidates. Preferential votes for candidates – Party OtCe – a part 1 ![]()
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