Note: Molecular diffusivity in a small pore zeolite measured by a variable pressure (piezometric) uptake method (original) (raw)
Related papers
Aiche Journal, 1981
The gas chromatography technique was applied to measurements of diffusion of n-butane in zeolite Nay. The linear chromatography theory failed to explain these results quantitatively, and a significant system nonlinearity was demonstrated. This nonlinearity is likely associated with non-Fickian diffusion. Order of magnitude estimates of the diffusivity could still be obtained, however. Over the range of temperatures 105 to 240°C, the n-butane diffusivities were in the range cm*/s. Similar results were obtained with n-hexane and calculated diffusivities were about an order of magnitude smaller than the corresponding n-butane values. In contrast, limited experiments with cyclohexane, 2, to
The Canadian Journal of Chemical Engineering, 1984
Intracrystalline diffusion is usually calculated from sorption kinetic data in bi-disperse systems (such as beds of porous solids), by applying, depending on the sorption time, two linearizable equations which constitute the simplified forms of the general integral of the non-stationary diffusion equation in a spherical particle. Objections to this procedure are noted, and two empirical equations are proposed on the basis of accurate interpolation of the data of the general integral of the diffusion equation. The method is successfully applied to eight sorption kinetic experiments employing 13X and ZSM zeolites as sorbents and xylenes as sorbed molecules. On calcule d'ordinaire la diffusion intracristalline ?i partir de donnkes de vitesse d'adsorption dans des systtmes bidispersks (tels que les lits de solides poreux), en appliquant, selon le temps de sorption, deux kquations linkarisables qui constituent des formes simplifites de l'intkgrale gknkrale de I'kquation de diffusion non stationnaire dans une particule sphkrique. On indique des objections ?i cette prockdure et I'on propose deux kquations empiriques, baskes sur une interpolation prkcise des donnkes de I'intkgrale gtnkrale de I'kquation de diffusion. On a applique avec succts la mkthode ?i huit expknences de cinktique d'adsorption portant sur les dolites 13X et ZSM comme sorbants et sur les xyltnes comme molkcules adsorb&. n bi-disperse systems, such as beds of porous zeolite I crystals, intracrystalline (or intraparticle) diffusion coefficients of a diffusing single component, at constant pressure, are often calculated from sorption kinetics which usually present plots (Q,-Qo)/(Qm-Qo) vs. t or d (Eberly, 1976; Barrer, 1978, 1979). If intercrystalline diffusion is not rate controlling (Barrer, 1979) the solution of the diffusion equation ac/dt = div (D grad c) is: for a spherical particle of radius ro, replacing D with Di, independent on c (Crank, 1975). The boundary conditions are: c = cm at r = ro for t > 0 c = co for 0 5 r < ro at t = 0 Equation (1) is neither linearizable with respect to the
Separation and Purification Technology, 2010
The recently introduced relevant site model (RSM) ( Van den Bergh et al., J. Phys. Chem. C, 113 (2009), 17840) to describe the loading dependency of diffusion in zeolite DDR is successfully extended to a variety of light gases (CH 4 , CO 2 , Ar and Ne) and binary mixtures thereof in other zeolite topologies, DDR, CHA, MFI and FAU, utilizing the extensive diffusivity dataset published by Krishna and van Baten for this variety of zeolite-guest systems (e.g. Chem. Eng. Sci., 63 (2008), 3120 (supplementary material)).
Adsorption and diffusion properties of zeolite membranes by transient permeation
Desalination, 2002
Adsorption isotherms and diffusion coefficients for light gases and butane isomers were measured for the transport pathways involved in gas permeation through H-ZSM-5 membranes by a transient permeation technique. The permeate responses to step changes in the feed were measured, and the transport was modeled as Maxwell-Stefan diffusion with single-site Langmuir adsorption in the zeolite. Isotherms measured for N 2 , CO 2 , and CH 4 at 295 K were nearly identical to those measured by calorimetry on H-ZSM-5 powders. Isotherms for butane isomers were also similar to isotherms for MFI powders and heats of adsorption and diffusion activation energies were in the ranges reported in the literature. Maxwell-Stefan diffusion coefficients for all gases studied increased slightly with feed partial pressure and were similar to those measured by other macroscopic methods for zeolite membranes and crystals. Effective membrane thicknesses were also determined non-destructively for tubular zeolite membranes by the transient permeation technique.
The Journal of Physical Chemistry B, 2000
A method for estimating gas permeability through a zeolite membrane, using a molecular simulation technique and a theoretical permeation model, is presented. The estimate of permeability is derived from a combination of an adsorption isotherm and self-diffusion coefficient based on the adsorption-diffusion model. The adsorption isotherm and self-diffusion coefficients needed for the estimation were calculated using conventional Monte Carlo and molecular dynamics simulations. The calculated self-diffusion coefficient was converted to the mutual diffusion coefficient and the permeability estimated using the Fickian equation. The method was applied to the prediction of permeabilities of methane and ethylene in silicalite at 301 K. Calculated permeabilities were larger than the experimental values by more than an order of magnitude. However, the anisotropic permeability was consistent with the experimental data and the results obtained using a grand canonical ensemble molecular dynamics technique (Pohl et al.
On diffusion in zeolites : a simulation study
One of the problems one encounters when studying diffusion, is that it can be expressed in a variety of ways. In macroscopic experiments, such as gravimetric measurements of the uptake and permeation rate, the diffusion measured is the
The effect of the concentration dependence of diffusivity on zeolitic sorption curves
Chemical Engineering Science, 1972
Solutions of the transient diffusion equation are presented for sorption in a system of spherical particles in which the diffusivity of the sorbate varies with concentration. The functional forms of the concentration dependence of diffusivity considered are commonly observed for the sorption of gases in zeolites. It is shown that, for such systems, a difference between the rates of adsorption and desorption is to be expected. Effective diffusivities are calculated by comparing the theoretical sorption curves for the concentration dependent diffusivity with the standard solution of the diffusion equation for a constant diffusivitv. The validity of the analysis is confirmed by comparison with experimental data.
2021
A set of three commercial zeolites (13X, 5A, and 4A) of two distinct shapes have been characterized: (i) pure zeolite powders and (ii) extruded spherical beads composed of pure zeolite powders and an unknown amount of binder used during their preparation process. The coupling of gas porosimetry experiments using argon at 87 K and CO2 at 273 K allowed determining both the amount of the binder and its effect on adsorption properties. It was evidenced that the beads contain approximately 25 wt% of binder. Moreover, from CO2 adsorption experiments at 273 K, it could be inferred that the binder present in both 13X and 5A zeolites does not interact with the probe molecule. However, for the 4A zeolite, pore filling pressures were shifted and strong interaction with CO2 was observed leading to irreversible adsorption of the probe. These results have been compared to XRD, IR spectroscopy, and ICP-AES analysis. The effect of the binder in shaped zeolite bodies can thus have a crucial impact o...
Chemical Engineering Science, 2005
Simulation of air separation in zeolites requires an accurate mass transfer model particularly under rapid cycling conditions. An earlier study [Todd et al., 2005. Adsorption 11, 427-432] examined the application of the dusty gas model to the simulation of the adsorption of O 2 /N 2 system in lithium-exchanged low silica-to-alumina ratio x-type zeolite (LiLSX) pellets. In that study we characterised Knudsen diffusion (C K ) and viscous flow (C v ) structural parameters, two of three parameters that constitute the viscous flow plus dusty gas model (VF + DGM) intrapellet flux equation for a macroporous pellet under adsorbing conditions. This paper quantifies the third structural parameter (C m ), related to molecular diffusion that arises when a multicomponent gas mixture is introduced into the pore network of a sorbent pellet. With these three parameters defined, the mass transfer behaviour for the O 2 /N 2 system in our adsorbent is completely characterised (under the assumption of no surface diffusion). To experimentally characterise C m , a series of breakthrough experiments were performed with a packed bed of LiLSX pellets. Six independent breakthrough runs over a range of conditions were performed with one of these runs used to determine C m . The remaining five runs were used for validation. Our adsorption simulator previously described [Todd et al., 2003. Computers and Chemical Engineering 27, 883-899] was enhanced with the addition of a rigorous wall model and was used to perform all breakthrough simulations. A value of C m = 0.122 was obtained giving a molecular diffusion tortuosity of 2.5. This is typical of tortuosities for porous adsorbent pellets. Experiments and simulations revealed operating conditions were very close to adiabatic during each experimental breakthrough run while a sensitivity analysis on the intrapellet structural coefficients revealed the sensitivity coefficients of C m and C K are similar in magnitude suggesting both mechanisms of diffusion are important. Viscous flow was several orders of magnitude smaller and hence is of minor importance in breakthrough simulations. Simulations also suggest the level of radial discretisation within a sorbent pellet is an important aspect of a discretised pellet model to consider when simulating a non-isothermal and non-isobaric breakthrough experiment. Relatively high pellet discretisations are needed to obtain physically meaningful structural coefficients.