Diffusion in Zeolites and Carbon Molecular Sieves

MICROPORE DIFFUSION IN ZEOLITES AND CARBON MOLECULAR SIEVES 

Micropore diffusion in zeolites and carbon molecular sieves has been widely studied. The subject has been reviewed by Barrer, Karger et al.,  

The relationship between sorbate activity and concentration for zeolitic systems is highly nonlinear so, except at very low concentrations approaching the Henry s law region, the thermodynamic correlation factor d np/d nc [Eq. (5.6)] is large, approaching infinity in the saturation region of the isotherm. In analyzing the dependence of diffusivity on concentration and temperature, it is therefore important to consider the corrected diffusivity or the self-diffusivity rather than the transport diffusivity. 

FIGURE 5.8. Diffusivities calculated from uptake curves measured with different size fractions of 4A zeolite crystals. Error bars show 15% which was the scatter of individual time constants for each size fraction. (From ref. 48, with permission.) 

FIGURE 5.10. Variation of (a) diffusivity and (6) corrected diffusivity Dq [Eq. (5.6)] with sorbate concentration for -heptane in Linde 5A zeolite crystals (r = 1,8 /tm). (Data from ref. I (a) reproduced by permission of the National Research Council of Canada from the Canadian 

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Micropore Diffusion in Zeolites and Carbon Molecular Sieves 143... 

The main focus of this volume is on imderstanding the transport of molecules in microporous solids such as zeolites and carbon molecular sieves, and the kinetics of adsorption/desorption. This subject is of both practical and theoretical interest, since the performance of zeohte-based catalysts and adsorbents is strongly influenced by resistances to mass transfer and intracrystalline diffusion. However, at an even more basic level, the performance of microporous catalysts and adsorbents depends on favorable adsorption equilibria for the relevant species, so a general imderstanding of the fundamentals of adsorption equilibrium is a necessary prerequisite for understanding kinetic behavior. This chapter is intended to provide a concise summary of the general principles of adsorption equiHbriiun and of the main features of sorption kinetics in microporous solids, which generally depend on a combination of both equilibriiun and kinetic properties. 

The primary requirement for an economic adsorption separation process is an adsorbent with sufficient selectivity, capacity, and life. Adsorption selectivity may depend either on a difference in adsorption equilibrium or, less commonly, on a difference in kinetics. Kinetic selectivity is generally possible only with microporous adsorbents such as zeolites or carbon molecular sieves. One can consider processes such as the separation of linear from branched hydrocarbons on a 5A zeolite sieve to be an extreme example of a kinetic separation. The critical molecular diameter of a branched or cyclic hydrocarbon is too large to allow penetration of the 5A zeolite crystal, whereas the linear species are just small enough to enter. The ratio of intracrystalline diffusivities is therefore effectively infinite, and a very clean separation is possible. 

Fig. 8. Variation of activation energy with kinetic molecular diameter for diffusion in 4A zeolite ( ), 5A zeolite (Q), and carbon molecular sieve (MSC-5A) (A). Kinetic diameters are estimated from the van der Waals co-volumes. From ref. 7. To convert kj to kcal divide by 4.184.

FIGURE 5.12. Variation of diffusional activation energy with van der Waals diameter for diffusion in 4A and 5A zeolites and 5A molecular sieve carbon. Van der Waals diameters are estimated according to Eq. (2.5) from values of the van der Waals co-volume b) given in the Handbook of Physics and Chemistry, 55lh ed. C.R.C Press 1974, (Diffusivity data are from refs. 1-3, 48, 49,51-53, and 81.)... 

Fig. 11 Variation of diffusional activation energy with van der Waals diameter for diffusion in 4A and 5A zeolites and molecular sieve carbon [36]...

Equilibrium separation factors depend upon the nature of the adsorbate -adsorbent interactions, that is, on whether the surface is polar, non-polar, hydrophilic, hydrophobic, etc. and on the process conditions such as temperature, pressure and concentration. Kinetic separations are generally, but not exclusively, possible only with molecular sieve adsorbents such as zeolites and carbon sieves. The kinetic selectivity in this case is largely determined by the ratio of micropore diffusivities of the components being separated. For a useful separation to be based on kinetics the size of the adsorbent micropores must be comparable with the dimensions of the diffusing adsorbate molecules. 

The standard adsorbent contactor is a randomly packed bed of pelletized or particulate adsorbents. The commonly used adsorbents include activated carbons, which separate mostly based on dispersive interactions zeolites that separate based on polarity and size carbon molecular sieves, which use the relative differences in intra-particle diffusion rates or silica gel and alumina, generally hydrophilic. The particle size and shape should provide a suitable compromise between pressure drop (AP) and mass transfer resistance. Note that AP is rarely a dominant economic problem, except for the largest systems. In order to minimize it, the cross-sectional area must be increased, leading to a small length-to-diameter ratio LID). In small systems, pressure drop is relatively less important than performance in terms of separation efficiency. 

This cost differential can be tolerated only in applications in which polymeric membranes completely fail in the separation [78]. Demanding separation applications, where zeolite membranes could be justified, due to the high temperatures involved or the added value of the components, and have been tested at laboratory scale, are the following separation of isomers (i.e., butane isomers, xylene isomers), organic vapor separations, carbon dioxide from methane, LNG (liquefied natural gas) removal, olefines/paraffins and H2 from mixtures. In most cases, the separation is based on selective diffusion, selective adsorption, pore-blocking effects, molecular sieving, or combinations thereof. The performance or efficiency of a membrane in a mixture is determined by two parameters the separation selectivity and the permeation flux through the membrane. 

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