Poly(dimethyl siloxane) thin film composite membranes for propylene separation from nitrogen

Poly(dimethyl siloxane) (PDMS) thin film composite membranes originally developed for pervaporation applications were investigated for the separation of propylene from nitrogen, and the feasibility of adapting such membranes to olefin recovery from polyolefin purge gas was demonstrated. Considering that the pure gas permeation data may not be sufficient to evaluate the actual separation properties of the membrane (particularly in the case of solubility selective membranes due to strong penetrant–membrane interactions), the membrane permselectivity was extensively evaluated with propylene/nitrogen mixtures at various operating conditions (i.e., temperature, pressure, feed composition). It was shown that the membrane could be adapted to propylene/nitrogen separation, and the membrane selectivity increased with a decrease in temperature and an increase in the feed pressure. The actual permeability ratio of propylene over nitrogen in the gas mixture was lower than the permeability ratio of pure gases at the same operating conditions, but it was still sufficiently high for practical propylene/nitrogen separation. At −20 °C and 500 kPa, the membrane exhibited a propylene/nitrogen permeability ratio of as high as 125 for feed mixtures containing 7–40 mol% propylene.