The performance of molecular sieve adsorption columns: systems with micropore diffusion control

Garg, Deepak; Ruthven, Douglas M.

doi:10.1016/0009-2509(74)80068-0

Cited by 42 publications

(19 citation statements)

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“…The homogeneous model was evaluated for application to the Ionsiv IE-96 particles since this model had been previously reported to accurately predict ion exchange in zeolite particles as well as zeolite crystals (Rosen, 1952;Prazniak and Byers, 1987;Garg and Ruthven, 1974;Liapis and Crosser, 1982;Ruthven and Doetsch, 1979;Hsu and Hayes, 1981;Weber and Smith, 1987;Dominguez et al, 1991). The values for the micropore diffusivities obtained for the Ionsiv IE-90 zeolite crystals (Table 1) were used in the homogeneous diffusion model for Ionsiv IE-96 system.…”

Section: Homogeneous Diffusion Model Results For Ionsiv Ie-96 Zeolitementioning

confidence: 99%

“…The homogeneous diffusion model (Rosen, 1952;Prazniak and Byers, 1987;Garg and Ruthven, 1974;Liapis and Crosser, 1982;Ruthven and Doetsch, 1979;Hsu and Hayes, 1981;Weber and Smith, 1987;Dominguez et al, 1991) assumes that the particle is a homogeneous solid through which diffusion can be modeled by a solid-phase diffusivity. The heterogeneous diffusion model assumes that diffusion in the particle occurs by macropore diffusion through the voids of the porous binder and micropore diffusion in the zeolite crystals.…”

Section: Introductionmentioning

confidence: 98%

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Mass‐transfer mechanisms for zeolite ion exchange in wastewater treatment

1994

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Section: Homogeneous Diffusion Model Results For Ionsiv Ie-96 Zeolitementioning

confidence: 99%

Section: Introductionmentioning

confidence: 98%

Mass‐transfer mechanisms for zeolite ion exchange in wastewater treatment

1994

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“…From the simulation results, it is seen that the crystal radius of 2 pm is in a region where larger sizes will result in substantially worse separations, and a further decrease of the size does not improve the separation appreciably. However, if an ultrahigh-purity H2 is the desired product, the equilibrium model can give a good prediction.A semiempirical criterion has been proposed (Ruckenstein et al, 1971;Garg and Ruthven, 1974) for evaluating the relative importance of micropore and macropore diffusion for constantpressure adsorber dynamics. The criterion is based on the magnitude of the following dimensionless group:…”

mentioning

confidence: 99%

Bidisperse pore diffusion model for zeolite pressure swing adsorption

Doong

Yang

1987

AIChE Journal

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The published theoretical models for pressure swing adsorption (PSA) are of either the equilibrium type, i.e., instantaneous equilibrium is assumed between the gas and adsorbed phases, or the diffusion type considering only a monodisperse pore structure (Yang and Doong, 1985; Doong and Yang, 1987). There is reason for doubt that either type of model is applicable to adsorption processes using zeolite sorbent, which has a bidisperse pore structure. Commercial zeolite sorbents contain crystals of the size 1-9 microns that are pelletized with a binder. Sorption is entirely within the crystals, which contain micropores, whereas the binder contains macropores with a negligible sorption capacity. This paper presents a general PSA model for zeolite sorbents. Both micropore and macropore diffusion are considered. The mathematical complexity of the pore diffusion equations for the two types of pores is reduced by assuming parabolic concentration profiles in both crystals and pellets. Thus the two partial differential equations are converted into ordinary differential equations containing only time derivatives, and the burden of integration along the radial distance is completely eliminated. The model is general enough to be applied to bulk, multicomponent separations using any PSA cycle. The specific separation discussed in this work is the bulk separation of a hydrogenmethane mixture using 5A zeolite. The boundary conditions in the model depend on the PSA cycle. A wide variety of PSA cycles has been commercialized (Yang, 1987). Under consideration here is the most widely used five-step cycle in which each adsorber undergoes the following:I. Repressurization with the light product 11. High-pressure feed 111. Cocurrent depressurization IV. Countercurrent blowdown V. Low-pressure purge. The model is formulated for an n-component mixture. TheIdeal gas behavior 0 Negligible pressure drop across the bed assumptions made in the model are:Thermal equilibrium between the gas flow and the solid sorNo radial temperature and concentration gradients in bed Spherical pellets and crystals The dimensionless mass balance equations for the gas flow in the bed are, respectively, for the individual component and for the mixture (Doong, 1986): If molecular diffusion is assumed to be dominant in the binder phase of zeolite, the mass balance equations for the macropores will be similar to those for the monodisperse pore structure in Yang and Doong (1985). The resulting dimensionless equations are: ( 1 Ob) Since the crystals account for all of the adsorbed amount, the P b 910 6 I 621 --p c * &---first overbar on Q denotes the volume-averaged amount adsorbed over the crystal, and the second overbar denotes that averaged over the pellet. The mole fraction on the surface of the pellet Yip is the same as that in the bulk flow, V,, since the film resistance is negligible. is required:The local adsorption rate, s, through which the bulk flowThe heat capacity of the wall is not negligible in the experimental unit. Therefore, an energy balance equation...

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“…Ruckenstein et al (1971) and others (Garg and Ruthven, 1974;Doong and Yang, 1987) have modeled the sorption behavior of solids with bidisperse pore structures, formally similar to the packed bed of NaY crystallites considered here, and established the criteria necessary for identifying rate-limiting features of guest transport in these systems. The distribution of adsorbed guest species in a zeolite packed bed depends on the adsorption properties of the system, together with the relative transport rates of the guest species in the interstitial void spaces between the micron-sized crystallites ( N a y macropores) and in the supercages and windows within individual zeolite crystallites ( N a y micropores).…”

Section: Sharp Adsorption Frontsmentioning

confidence: 99%

Transport of aromatic molecules in NaY Zeolite powders

et al. 1990

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Xenon-I29 NMR is used to probe macroscopic distributions of aromatic molecules adsorbed in a packed bed of 1-pm NaY zeolite particles. Relative rates of guest transport through the intracrystalline (micro) and intercrystalline (macro) pores play a unique role in the axial distribution of sorbate molecules, such as hexamethylbenzene, in a zeolite powder. Xenon-129 NMR spectra show that a sharp HMB B. F. Chmelkaadsorption front advances through a bed of dehydrated NaY crystallites at 523 K. However, at 573 K or in the presence of coadsorbed water, J. V. GillisHMB species disperse through the bed without forming a sharp E. E. Petersenboundary between adsorption zones. C. J. RadkeWhen guest transport is controlled by pseudosteady-state diffusion in Department of Chemical Engineering the macropores, axial penetration of the bed by vapor-phase guest University of California species occurs in a sharp adsorption front. A shrinking-core transport Berkeley, CA 94720 model then quantitatively estimates the intracrystalline diffusivities of HMB in dehydrated and partially hydrated NaY zeolite of lo-" and m2/s, respectively, at 523 K. Xenon-129 NMR proves to be a powerful tool for probing adsorbed guest distribution in zeolites, allowing relative time scales to be established for transport of molecular guests in NaY powders.Correspondence mncerning this papcr should he addressed to C. J. Radke. 1562October 1990 sponsible for the macroscopic adsorbate concentration profiles observed. Adsorbate Distributions by '"Xe NMRAs a sensitive, nonreactive probe of supercage cavities within the crystalline zeolite matrix, room-temperature 129Xe N M R spectroscopy provides unique information on interparticle adsorbate heterogeneities in NaY powders. By monitoring the chemical shift of physisorbed 12'Xe, distinct regions containing different concentrations of adsorbed guests can be identified within NaY zeolite samples.All zeolite samples were prepared in 10-mm N M R tubes connected to a vacuum apparatus through a Kontes highvacuum stopcock. NaY zeolite was synthesized according to the procedure of Breck (1982), with crystallinity confirmed by X-ray diffraction studies. Separate scanning electron microscopy experiments establish NaY crystallite grain diameters of 1-2 Mm (Chmelka, 1990). Before adding the adsorbate species, the NaY samples were dehydrated in the sample tubes by heating a t 723 K under vacuum (ca. lo-' torr or 1.3 x Pa)

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The performance of molecular sieve adsorption columns: systems with micropore diffusion control

Cited by 42 publications

References 10 publications

Mass‐transfer mechanisms for zeolite ion exchange in wastewater treatment

Mass‐transfer mechanisms for zeolite ion exchange in wastewater treatment

Bidisperse pore diffusion model for zeolite pressure swing adsorption

Transport of aromatic molecules in NaY Zeolite powders

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