Diffusion in synthetic faujasite powder and pellets

H., Y.; Ho, S. Y.

doi:10.1002/aic.690200211

Cited by 38 publications

(21 citation statements)

References 7 publications

Supporting

Mentioning

Contrasting

Order By: Relevance

“…(This assumption is not important since later results will show that the separation does not depend on H2 diffusivity.) The macropore (1,700 A or 170 nm) diffusivities are calculated using a tortuosity of the 4 (Ma and Ho, 1974). The void fraction in the pellet is 0.615.…”

Section: (7)mentioning

confidence: 99%

Bidisperse pore diffusion model for zeolite pressure swing adsorption

Doong

Yang

1987

AIChE Journal

View full text Add to dashboard Cite

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...

show abstract

Section: (7)mentioning

confidence: 99%

Bidisperse pore diffusion model for zeolite pressure swing adsorption

Doong

Yang

1987

AIChE Journal

View full text Add to dashboard Cite

show abstract

“…The macropore diffusivity was calculated from molecular diffusivity (pore size = 1700 A) using a tortuosity of 4 (Ma and Ho, 1974). The diffusivity in the mixture was calculated by the Wilke-Lee method.…”

Section: Experimental and Psa Cyclesmentioning

confidence: 99%

Zeolite Psa Cycles for Producing a High-Purity Hydrogen From a Hydrogen-Lean Mixture.

Cen

Yang

1989

Chemical Engineering Communications

View full text Add to dashboard Cite

“…Recent studies on the diffusion in solids with a bipore distribution include the fheoretical and experimental investigation of diffusion in ion exchanged resins by Ruckenstein et al (1971), the experimental study of diffusion of carbon dioxide in commercial 5A pellets by Sargent and Whitford (1971), and mathematical and experimental investigation of diffusion in Linde 13X by Ma and Ho (1974). All the above-mentioned studies were performed under constant pressure conditions.…”

Section: Scopementioning

confidence: 99%

“…A single effective diffusion coefficient is sometimes used to interpret experimental results due to the lack of adequate mathematical solutions, but such an interpretation may lead to erroneous results (Furusawa and Smith, 1973;Ma and Ho, 1974).…”

Section: Page 148mentioning

confidence: 99%

See 1 more Smart Citation

Transient diffusion in solids with a bipore distribution

Lee

1976

AIChE Journal

Self Cite

View full text Add to dashboard Cite

Transient diffusion of n-butane, iso-butane, and 1-butene in synthetic CaX(Na) spherical pellets is studied in a constant volume, well-stirred and T. Y. LEE system. The intracrystalline diffusion coefficients are in the order of 10-14 cm2/s, and the activation energy is about 7.0 Kcal/g-mole. Experimental data agree well with a mathematical model developed to describe Department of Chemical Engineering transient diffusion. SCOPEThe diffusion in porous pellets formed from zeolite crystals usually involves two main mechanisms: diffusion through the micropore structure within the crystals (intracrystalline diffusion) and diffusion through macropore structure between crystals (intercrystalline diffusion). The determination of each individual diffusion coefficient can be of importance in data interpretation and improving engineering design in operations such as purification, separation, and catalytic conversions.Previous work on the study of diffusion in zeolites concentrated mostly in micropore diffusion with powder forms. Recent studies on the diffusion in solids with a bipore distribution include the fheoretical and experimental investigation of diffusion in ion exchanged resins by Ruckenstein et al. (1971), the experimental study of diffusion of carbon dioxide in commercial 5A pellets by Sargent and Whitford (1971), and mathematical and experimental investigation of diffusion in Linde 13X by Ma and Ho (1974). All the above-mentioned studies were performed under constant pressure conditions. The present work employed a constant volume technique to examine quantitatively the diffusion of C4-hydrocarbons in micropores. Diffusion coefficients were evaluated based on a mathematical model taking into account intercrystalline and intracrystalline diffusion and adsorption under constant volume conditions. Rates of sorption of n-butane, iso-butane, and 1-butene in synthetic CaX(Na) were measured at three temperatures, and the activation energies for diffusion were determined from an Arrhenius plot. CONCLUSIONS AND SIGNIFICANCEMicropore diffusion coefficients were determined by assuming that the transport processes in macropores and micropores obey Fick's equation. The activation energy for diffusion was found to be about 6 to 8 Kcal/g-mole for the C4-hydrocarbons used in the study. The relatively low activation energy may, in part, be due to the large openings of the zeolite X.A mathematical model was developed to describe the diffusion in a solid with a bipore distribution. The present model differs from previous studies in that it deals with a well-stirred system where the total quantity of the diffusing species is finite. Agreement between experimental data and theoretical results was good. It should be noted that the present model can be reduced to those developed January 1976

show abstract

Diffusion in synthetic faujasite powder and pellets

Cited by 38 publications

References 7 publications

Bidisperse pore diffusion model for zeolite pressure swing adsorption

Bidisperse pore diffusion model for zeolite pressure swing adsorption

Zeolite Psa Cycles for Producing a High-Purity Hydrogen From a Hydrogen-Lean Mixture.

Transient diffusion in solids with a bipore distribution

Contact Info

Product

Resources

About