Separation with Surface Diffusion and Capillary Condensation

3 Separation with Surface Diffusion and Capillary Condensation 

The permeation of gases in membranes due to surface diffusion and capillary condensation has been discussed in Section 9.2.3.S. together with some illustrative data. The total flux of a single gas is usually calculated as the sum of the flux by surface diffusion and the flux through the gas phase. As shown the surface flux can contribute considerably to the total flux (increased by factor 2-3 of gas diffusional flux), especially with smaller and uniform pore sizes (compare Eqs. (9.9a) and (9.15). With decreasing pore size the flux through the bulk gas decreases while the surface diffusional flux increases. With very small pore diameter ( 2-3 nm) the effective diameter for bulk gas transport is less than the geometric pore diameter due to the thickness of the absorbed layer which 

With gas mixtures, enhancement of the separation factor can be obtained by preferential sorption of mobile species of one of the components of the gas mixture. Adsorption does not always lead to enhanced separation. In a mixture of light non-adsorbing molecules and heavy molecules, the heavy molecules move slower than the lighter ones but in many cases are preferentially adsorbed. Consequently the flux of the heavier molecules is better enhanced by surface diffusion and the separation factor decreases. This occurs, e.g., in CH4/CO2 

9 — TRANSPORT AND SEPARATION PROPERTIES OF MEMBRANES WITH GASES AND VAPOURS 

Uhlhorn et al [28] reported for a H2/N2 mixture a separation factor of about 9 compared to the Knudsen value of 3.74. As shown in Fig. 9.17 the ratio of the H2 flux over that of the N2 flux decreases from 9 at a pressure of 50 kPa to 5 at 200 kPa. This result is obtained on 7-AI2O3 membranes (thickness 100 pm, pore diameter 2.5-4.0 nm) impregnated with 17 wt% (finely dispersed) Ag. The increase of the H2 flux is obtained by the Ag impregnation. Probably the decrease of the separation factor is caused by a decreasing contribution of the surface diffusion to the total flux with increasing pressure due to saturation of the adsorption. 

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