Gaseous Diffusion in a Natural Zeolite in Relation to Crystalline Disorder

Brandt, Werner; Rudloff, Winfried K.

doi:10.1524/zpch.1964.42.3_4.201

Cited by 7 publications

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“…The effect of molecular size upon the diffusion rate as observed for counterdiffusion is very similar to the effect observed for single component unidirectional diffusion of permanent gases and hydrocarbons in zeolites (Habgood, 1958;Nelson and Walker, 1961;Barrer, 1949;Brandt and c. , Rudloff, 1964;Satterfield and Cheng, 1971). Figure 2 shows that previously reported data is also correlated well as In (Deff) VS. (dpore -d,,,oi).…”

Section: Effect Of M O L E C U L a R Sizesupporting

confidence: 67%

Counterdiffusion of liquid hydrocarbons in type Y zeolite: Effect of molecular size, molecular type, and direction of diffusion

Moore

Katzer

1972

AIChE Journal

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This paper presents experimental results on the counterdiflusion of liquid hydrocarbons in the Y zeolite as determined by an unsteady state technique using the zeolite powder. The critical diameter of aromatic molecules counterdiffusing with respect to cyclohexane into NaY zeolite has a marked effect upon the counterdiffusion rate. For a given molecular type, the s a l e parameter of critical molecular diameter correlates the size effect well. Molecular type, for example, toluene vs. phenol, also has a pronounced effect on the counterdiffusion rate. Those molecules showing stronger interactions with the zeolite have lower counterdiffusion rates. In both NaY and SK-500, the diffusion coefficients for adsorptive counterdiff usion are an order of magnitude greater than the diffusion coefficients for desorptive counterdiffusion indicating that factors other than simple intracrystalline diffusion as such are also important in zeolite counterdiffusion. Simple diffusion models do not account for the differences observed indicating that a more sophisticated approach is necessary. SCOPEZeolites play a very important role in the chemical and petroleum industry as both adsorbents and catalysts (Collins, 1968). Adsorbent applications utilize the strong selective adsorption which zeolites often show toward a compound or a class of compounds or the very fine, uniform pore structure which permits the adsorption of molecules smaller than a given critical diameter. Adsorbent applications involve the diffusion of at least one species and often the counterdiffusion of different species within the pore structure, and thus design of adsorption units requires information on diffusion and/or counterdiffusion rates (Roberts and York, 1967; Gehrhardt and Kyle, 1967; Fukunaga et al., 1968). Such information is not generally available, and thus the design of such units remains mainly an empirical art. In catalytic applications Correspondence concerning this paper should be addressed to J. R. Katzer. R. M. Moore is with the University of Technology, Loughborougb, England.since the locus of catalytic activity for many reactions is within the pore structure, the simultaneous counterdiffusion of reactants and products must occur within the structure. The presence of diffusion limitations in zeolite catalysts would reduce the activity observed and for multiple-step reactions could also alter the selectivity. Thus a quantitative understanding of counterdiff usion in zeolites is important to the engineering design of zeolite catalysts and of processes utilizing them.Very little information is available on counterdiff usion in zeolites. Furthermore, counterdiffusion rates are grossly different from single componcnt unidirectional rates, cannot be estimated from single component rates, and thus must be determined independently (Satterfield and Katzer, 1971). Studies of the counterdiffusion of hydrocarbons in large pore zeolites under practical conditions are necessary to provide an understanding of the important variables affecting it. Such work wi...

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Section: Effect Of M O L E C U L a R Sizesupporting

confidence: 67%

Counterdiffusion of liquid hydrocarbons in type Y zeolite: Effect of molecular size, molecular type, and direction of diffusion

Moore

Katzer

1972

AIChE Journal

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Sorption and diffusion of gaseous hydrocarbons in synthetic mordenite

Satterfield

Frabetti

1967

AIChE Journal

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0 = time, min. p = viscosity p = mass density, lb./cu.in. o = mass fraction, dimensionless = velocity outside the boundary layer, in./min. = aluminum plate weight, lb. = coordinate parallel to the plate surface = variable defined by Equation (14), dimensionless = constant defined by Equation ( 6 ) Subscripts A, B, C = component, A, B, C, e L = evaluation at x = L s = evaluation outside the boundary layer = evaluation at the plate surface Superscripts i = initial f = final -= averageDiffusion coefficients for the Ci to C4 paraffin hydrocarbon gases in single crystals of the synthetic zeolite (molecular sieve) sodium mordenite were determined from transient sorption rate and desorption rate measurements over the temperature range 25" to 140°C. and pressure range 0 to 20 cm. Hg, and found to be of the order of 10-9 to sq. cm./sec. Diffusion coefficients for desorption were from three to sixty times smaller than those for sorption.Activation energies for methane and ethane were 1.7 and 1.9 kcal./mole, respectively.Equilibrium sorption capacity and diffusion coefficients are markedly affected by the conditions of synthesis and by mechanical treatments such as grinding. For propane a t 29°C. and at 2.0 cm. Hg pressure, the sorption capacity in 2.5 + crystals was only 50% of that in 21 by 21 by 33 p crystals and the diffusion coefficient was only 1/50 of that in the larger crystals. Light mechanical grinding of the larger crystalms lowered the sorption capacity 30% and the diffusion coefficient by a factor of 35.strain and a more plausible explanation is that the pores of this and other zeolites may be artly obstructed in various places by cations or by ot B er substances. The Vol. 13, No. 4AlChE Journal

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Desorption of Kr (or Kr85) and SF6 from Vitreous and Crystalline SiO2, and of Kr from B2O3 and GeO2 Glasses

Brandt

1977

Reactivity of Solids

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Gaseous Diffusion in a Natural Zeolite in Relation to Crystalline Disorder

Cited by 7 publications

References 0 publications

Counterdiffusion of liquid hydrocarbons in type Y zeolite: Effect of molecular size, molecular type, and direction of diffusion

Counterdiffusion of liquid hydrocarbons in type Y zeolite: Effect of molecular size, molecular type, and direction of diffusion

Sorption and diffusion of gaseous hydrocarbons in synthetic mordenite

Desorption of Kr (or Kr85) and SF6 from Vitreous and Crystalline SiO2, and of Kr from B2O3 and GeO2 Glasses

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