Petr Uchytil

An experimental study of combined gas phase and surface diffusion in porous glass

Abstract The diffusion of inert and adsorbable gases and binary gas mixtures through porous glass has been studied experimentally. A modified Wicke–Kallenbach cell consisting of two gas compartments separated by a tubular mesoporous membrane was used. The scope of this paper is to quantify the contributions of gas phase and surface diffusion. Adopting the dusty gas model (DGM) for the description of gas phase mass transfer and a generalized Stefan–Maxwell (GSM) theory to quantify surface diffusion a combined transport model has been applied. The DGM was found to be well suited for the description of transport through the pores. Surface diffusion of adsorbable gases was analyzed experimentally for different loadings. The obtained Fickian surface diffusivities were found to be strongly concentration dependent. Multicomponent surface diffusion is mainly affected by adsorption equilibrium. Reliable predictions require an accurate knowledge of the competitive adsorption isotherms.

Influence of the transport direction on gas permeation in two-layer ceramic membranes

Abstract Experimental and theoretical results of studying gas permeation through porous membranes are presented. In order to mimic an asymmetric membrane two porous ceramic disks with different pore radii were arranged in series. Besides the possibility to perform conventional permeation measurements, the applied experimental setup permits the determination of the pressure at the interface between the two discs. To predict the performance of the asymmetric structure, in preliminary experiments structure parameters were determined for both membranes separately. For the same total pressure difference across the two-disk arrangement, different interlayer pressures and fluxes were predicted and detected experimentally depending on the flow direction.

Liquid expulsion permporometry for characterization of porous membranes

The processes in a liquid pre-wettted porous membrane are described for the situation when a gradually increasing pressure difference of an inert gas is imposed across the membrane (liquid expulsion permporometry). The description takes into account the compressible nature of the gas and the distribution of the pore sizes. By comparing the experimentally obtained dependence of the gas flowrate through the membrane versus the imposed pressure difference (wet curve) it is possible to evaluate the distribution of the fractional membrane porosity (pore-volume distribution) with the pore size. At the same time the ratio of porosity and tortuosity of transport pores (geometric membrane parameter, ψ) is determined. This parameter can be used together with the mean integral pore radius and the mean integral squate of pore radii (determined by analysis of the gas flowrate dependence versus Δp for a dry membrane - dry curve) for the prediction of diffusion and/or viscous flow for any gas or gas mixture under any temperature and pressure condition. The stimulated pore-size distributions are compared with the results of the traditional method; it appears that the traditional approach leads to systematic deviations in the obtained pore-size distribution.

Gas permeation in ceramic membranes Part I. Theory and testing of ceramic membranes

Abstract A mathematical model for the gas permeation in ceramic membranes (support and supported separation layer) is proposed. The model is based on the relationship between the structure parameters of the porous material and the gas permeability. This procedure makes it possible to determine the mean pore radius, r , and geometric factor (ratio of porosity and tortuosity), Ψ, of the support and the supported layer. The model was verified with the membrane supports made of α-aluminum oxide. The results were compared with the mercury porosimetry measurements.

Influence of capillary condensation effects on mass transport through porous membranes

The importance of capillary condensation effects in connection with properly quantifying the rate of mass transport through porous media has been already described (J. Coll. Interf. Sci. 110 (1986) 544; J. Membr. Sci. 66 (1992) 259; J. Membr. Sci. 182 (2001) 91). However, there is still not enough work done in order to understand, quantify and possibly exploit this phenomenon. In this work permeation and binary diffusion experiments have been performed at ambient temperature in combination with measurements of the adsorption isotherms using porous Vycor glass possessing a mean pore radius of approximately 4 nm. Main focus was set on a comparison of the transport of (a) permanent gases (helium, nitrogen, methane and hydrogen) and (b) condensable gases (butane, propane and freon 112) as a function of the pressure conditions in the membrane. The adsorption isotherms of the condensable gases were measured using a volumetric method. The results indicate the occurrence of capillary condensation at a reduced pressure P /P0 of approximately 0.8 for all three gases. There is a good agreement between the volume of condensate at saturation and the pore volume determined in independent porosimetric studies. Results of a large set of experiments with butane revealed that the permeation is only a function of the mean gas pressure in the membrane. This contradicts the available models suggested in (J. Coll. Interf. Sci. 110 (1986) 544; J. Membr. Sci. 66 (1992) 259) predicting that the transport mechanism in pores changes with different pressure conditions. Finally, in binary isobaric diffusion experiments with pure helium on one side of the membrane and pure butane on the other, a remarkable increase in selectivity of the transport process with increasing pressure in the membrane was observed. # 2003 Elsevier B.V. All rights reserved.

Gas permeation in ceramic membranes Part II. Modeling of gas permeation through ceramic membrane with one supported layer

Abstract Results of mathematical modeling of gas permeation and pressure profiles in a support, and microfiltration and ultrafiltration layers of ceramic membranes are presented. Two orientations of gas flow through membranes were chosen: the ordinary one oriented from the separation layer to the support, and the opposite one known as back-flushing. The results obtained are applicable for testing ceramic membranes by the gas permeation method and for a description of backflushing.

Evaluation of selected methods for the characteristics of ceramic membranes

Abstract Defect-free surface layers with narrow pore size distributions and optimal porosity of support and supporting layers are desirable properties of ceramic membranes. When fouling, concentration polarisation and interactions of molecules with pore walls are negligible, the permeability in ceramic membranes and the efficiency of the separation process are determined mainly by the membrane pore structure. That is why the porous structure of ceramic membranes must be reliably tested. Ceramic membranes are often tested by mercury porosimetry. Other suitable methods — Coulter porosimetry and permeation — are relative new. A comparison of the possibilities and limitations of these methods and an explanation of the differences in the obtained results are discussed. Problems connected with the preparation of membrane samples for measurment are also embraced.

Possibility of pore size determination in separation layer of ceramic membrane using permeation method

Utilization of the mathematical model of gas permeation through a ceramic membrane consisting of a support and one separation layer is presented. The influence of errors in measured variables upon reliability of determination of structure parameters of the separation layer is discussed. The usability of the permeation method for the pore size determination was verified by fitting computer generated pseudoexperimental data with Gaussian distribution and preselected variance. All calculations were performed for hydrogen but results for other gases will be similar.

Measurements on the flow of vapors near saturation through porous Vycor glass membranes

We present experimental data of the flow of butane and isobutane vapors through porous Vycor glass membranes. The pressure driven flow of vapors near and far from saturation through membranes with pore diameters of 20 and 33 nm is investigated. The upstream pressures lie between the saturation pressure at the upstream temperature to approximately half that value. The pressure differences are between a few kPa to about 100 kPa. From an adiabatic description of the flow process, we expect condensation of a vapor close enough to saturation and hence, due to the action of capillary forces, an increase in mass flux with respect to the mass flux of a vapor that remains in a gaseous state. According to the adiabatic description, a vapor that flows through a porous membrane may condense for two reason: One reason is capillary condensation in the pores of the membrane, the second reason is heat conduction from the upstream to the downstream side of the membrane due to the Joule-Thomson effect. If the flux of heat ...