Membrane Gas Separation in Petrochemistry: Problems of the Polymeric Membrane Selection

Abstract

Although the solubility and diffusivity of each gas and vapour in each polymer are temperature and in some systems pressure (concentration) dependent, regular trends are noted when either many gases are studied in a single polymer or a single gas is studied in many polymers. Several empirical though scientifically based correlations of such data have been proposed which are at best semi-quantitative. In this paper improved correlations are described to correlate data on diffusivities D and solubilities S of rare gases, multiatomic gasesand lower hydrocarbons in polymers. The use of these, demonstrated by modelling of the variation of the membrane permselectivity to hydrocarbon containing gas mixtures with variation ofconcentration of the most condensable component this is of importance for petrochemistry applications.The procedures rely on determining an effective Lennard-Jones {6-12} potentialforce constant and molecular diameter for each gas which hold in all polymers above T g • Slightly smaller diameters are needed for mul tiatomic gases below T g . Each polymer is characterized by four temperature and in some systems concentration dependent parameters. two for diffusivity and two for solubility, Which hold with all gases.The procedure described may be used to predict D and S, and hence permeabilities, in cases where data do not already exist. The values and ratios of the predicted permeabilities are a valuable guide when seeking polymers for separating gas mixtures by membrane processing and also for modelling of the permselectivity profile under concentration-dependent conditions. INTRODUCTIONIn the last few years membrane separation of different liquid and gaseous mixtures took on great si~ificance as highly effective technological processes realized in dlfferent branches of industry. Prospects of use membrane technology development in petrochemistry include the following (Durgaryan 1983):1. The extraction of valuable components from petrochemical streams either for returning into the process or for application in other processes.2. The opportunities of process operation by membrane control of compositions of the circulating streams that increases the productivity of existing technology.3. The solution of the problems of the gas mixture separation in cases where criogenic and other traditional techniques are not effective enough.4. Removing of deleterious components from gaseous and liquid streams for solution ot the ecology problems.Considering the advantageous economic, ergonomic and