Permeability and adsorbed flow of carbon dioxide and propene through porous silica.

It has been known for over a century that gases diffuse through some porous barriers at a rate inversely proportional to the square root of the molecular weight. The difference in diffusion rate can obviously be used to separate gases w1th different molecular weight. Since other methods for separat1onhke d1st1llat1on and adsorption usually are simpler and cheaper, separation by gaseous diffusion has found few practical applicat1ons. One notable exception 1s the separation of the gaseous hexafluor1des of u238 and u235. Later Investigations on the permeation of gases through porous materials have shown that when gases diffuse through porous solids with large surface area there is an additional flow caused by adsorbed gas. Although there has been an increasing amount or research on adsorbed flow the last fifteen years, the phenomenon is not yet known well enough for general relationships to have been accepted. Qualitatively it can be said that conditions favoring adsorption also favor adsorbed flow. Since adsorption does not depend directly upon the molecular weight as does gaseous diffusion, adsorbed flow possibly offers a way to overcome some of the limitations of pure gaseous diffusion. So far, the majority of the investigations on adsorbed flow have been on adsorbed flow of single gases. The most successful model for adsorbed flow seems to be one which treats the adsorbed flow as a hydrodynamic flow. There has only been found one instance 1n the literature where the adsorbed flow of a gas mixture has been investigated. Letting mixtures of carbon dioxide and propane permeate porous glass an enrichment in propane was obtained in the permeated stream. Since the two gases have equal molecular weight, the separation must be attributed to adsorbed flow. There 1s consequently a great need for further investigation before it is possible to evaluate the potential use of adsorbed flow in gas separation. The purpose of this investigation was to study the permeability of carbon dioxide, propene and a mixture of carbon dioxide and propene through a plug of silica, to determine the amount of adsorbed flow and to determine any separation obtained with the mixture. It was also the purpose to measure separately the adsorption of the same gases on the silica plug and if possible, correlate the adsorption and the adsorbed flow - Introduction, pp. 1-