Arlette Zikánová

Molecular transport through assemblages of microporous particles

Steady-state diffusion studies and the NMR pulsed field gradient technique are introduced as equivalent methods for determining long-range diffusivities. Theoretical methods for mass transfer in heterogeneous media are reconsidered and adapted to adsorbate-adsorbent systems. Comparison with the experimental data for n-butane diffusion in NaX zeolites shows that long-range diffusivities are predominantly determined by intercrystalline transport. Estimates of the absolute value of the temperature dependence and of the influence of the pressure of compaction are in agreement with this conclusion.

Comparison of nuclear magnetic resonance tracer exchange and molecular uptake of benzene on pentasils

Conventional NMR spectroscopy is applied to study tracer exchange in adsorbate–adsorbent systems. In the proposed experimental procedure, heat effects are completely excluded and possible transport resistances due to gas-phase diffusion and intercrystalline diffusion are reduced to a minimum. The influence of deviations from the spherical shape of the adsorbent particles as well as possible anisotropy of intracrystalline diffusion on tracer exchange curves is estimated. The obtained relations are likewise applicable to uptake studies. For benzene on different ZSM-5 specimens, the tracer exchange data are compared with the results of uptake measurements. Though carried out independently from each other with different techniques and different specimens, the experiments provide coinciding results. In particular, it appears that the ratio between the self-diffusion coefficient obtained in the tracer experiments and the diffusion coefficient resulting from uptake studies is indeed given by the logarithmic derivative of the adsorbate concentration versus the adsorbate pressure as conventionally assumed on comparing self-diffusion and uptake measurements.

Molecular mobility of methane adsorbed on ZSM-5 containing co-adsorbed benzene, and the location of the benzene molecules

The molecular mobility of methane adsorbed on ZSM-5 as investigated by both the n.m.r. pulsed field gradient technique and proton magnetic relaxation studies is found to decrease significantly with increasing amounts of coadsorbed benzene molecules. Comparison with a computer simulation of the random-walk process within the ZSM-5 network suggests that the channel intersections, rather than the channel segments between them, should form the adsorption sites of the benzene molecules. Both the second moments of the proton magnetic resonance lines of methane, as determined from the transverse relaxation times, and the laser-Raman spectra of benzene adsorbed on ZSM-5 are in agreement with this model.

Statistical moments theory of gas-solid chromatography : Diffusion controlled kinetics

By means of the system CO2-activated carbon the applicability of the statistical moments theory to gas-solid chromatography was verified under the assumption that the adsorption rate is controlled by interparticle diffusion. The experimental results are in very good agreement with theoretical predictions. The theory shows that all the gas chromatographic characteristics may be influenced by the column and solid porosity and by the eddy diffusion constant. This fact has to be respected in presenting the gas chromatography data.

The Statistical Moments Method for the Calculation of the Coefficients of Radial Diffusion from Kinetic Measurements

The submitted paper presents a suggested method for the calculation of the coefficients of radial diffusion from the kinetic measurements carried out with a cylindrical sorbent granule by means of a gravimetric apparatus in the flow of a carrier gas. The evaluation of the diffusion coefficient is based on the calculation of the statistical moments of kinetic curves and the inlet concentration profile, for which comparatively simple relations were derived. The concentration profile, put on the surface of the adsorbent particle, can have a quite general course. Zusammenfassung Die vorliegende Arbeit betrifft eine Methode für die Berechnung von radialen Diffusionskoeffizienten aus kinetischen Messungen, die man im Strom eines inerten Gases in einer gravimetrischen Apparatur mit einem zylindrischen Teilchen des Adsorbens durchführen kann. Die Diffusionskoeffizienten werden mit Hilfe von statistischen Momenten der kinetischen Kurven und des zeitlichen Verlaufes der Adsorbatkonzentration an der Oberfläche des Teilchens ausgewertet. Dieser zeitliche Verlauf an der Oberfläche des Teilchens kann ziemlich allgemein sein. Introduction

Intercrystalline molecular transport in zeolites studied by uptake experiments and by nuclear magnetic resonance pulsed field gradient techniques

A comparison of n.m.r. self-diffusion and uptake measurements was performed with the systems cyclohexane (n-hexane)+NaX-zeolite. For intracrystalline adsorption kinetics a significant difference in the values of the corresponding rate constants was found. Intercrystalline self-diffusion coefficients and corresponding uptake coefficients, however, were found to be in good agreement. In accordance with this result, in both n.m.r. and uptake experiments the variation of pressure of compaction, sorbate concentration and temperature led to the same changes in the effective coefficients of intercystalline transport.

Long-term stability of composite zeolite MFI membranes

Zeolite MFI layers were grown on asymmetric α-alumina supports developed in cooperation with an industrial partner. The layers were prepared by direct hydrothermal synthesis procedure using aged synthesis solutions composed of silica source, structure directing agent and deionized water. Selected zeolite MFI composite membranes were tested during long-term permeation and separation experiments to determine flux and separation factor as a function of total molar amount fed to zeolite membrane surface area. The flux of permeating components (n-butane, isobutane) was reduced by an average factor of 1.6 and 2 for n-butane and isobutane, respectively after 30 mol of butane isomers passed the selected membranes. The separation factor towards n-butane showed the maximum or decreased with increasing feeding amount of butane isomers.

Intracrystalline Diffusion of Benzene in Ga-Silicate

Abstract The sorption kinetics of benzene in large Ga-MFI crystals was investigated under constant volume- variable pressure conditions. A complete analysis of the uptake curves has been performed using solution of a nonlinear Volterra equation which describes the interaction of uptake process with the apparatus. Within the time scale of uptake measurements (10 3 -10 4 s) the uptake curves were found to be consistent with the solution of the second Ficks law. Corrected diffusion coefficients were found to be essentially independent of loading within the loading range investigated and in contrast to the system benzene-HNaZSM-5 [1,2] their temperature dependence is much stronger.

Mathematical modelling of multicomponent transport in composite all-ceramic membranes containing a zeolitic phase

Abstract The membrane model describes (i) mass transport in macroporous support comprising diffusion in transition region between Knudsen and bulk diffusion superimposed on convective flow and (ii) mass transport in zeolitic phase accompanied by a parallel transport in defects of zeolitic layer. The mass transport in the support is described by Dusty Gas Model (DGM) and that in the zeolitic phase by Generalized Maxwell Stefan (GMS) approach. Parallel transport in defects of zeolitic layer is also described by DGM. Modelling is exemplified on dynamics of a semi-open Wicke-Kallenbach (W-K) cell operated with a flat silicalite-1-α-Al2O3 membrane using a CH4/N2 mixture. It appeared that measurement of dynamic responses of the semi-open W-K cell represents a rapid and precise method to characterize composite membrane quality.

Intracrystalline Diffusion of Benzene in ZSM-5 Type Zeolites

Case (i) allows for the application of a modified free volume theory [2] as done to describe the self-diffusion pattern of hydrocarbons in X type zeolites [3]. A weak temperature dependence of diffusivity is characteristic of this model. Case (ii) describes the molecular mobility as proceeding via structure-related jumps of nearly constant length between adjacent sorption sites. A corresponding model has been applied to diffusion of hydrocarbons in both chabazite and analcite [4] as well as in ZSM-5 zeolites [5]. In contrast to the principles of the free volume theory, this model is appropriate for describing activated diffusional processes. In the present paper, the extent to which experimental intracrystalline molecular mobility data satisfy the model of case (ii) will be proved. Prerequisites of such examination are given as follows: