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 of concentration of the most condensable component this is of importance for petrochemistry applications.

The procedures rely on determining an effective Lennard-Jones {6–12} potential force constant and molecular diameter for each gas which hold in all polymers above T_g. Slightly smaller diameters are needed for multiatomic 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.