Elevated temperature application of polymer hollow-fiber membranes

Three asymmetric hollow-fiber polymer membrane systems were studied for application in elevated temperature, low feed pressure systems: (1) a single component polyaramide, (2) a single component polyimide, and (3) a composite polyimide on a polyimide/polyetherimide blend support. Permeation driving force was increased for the 2.2 psig feed pressure by sweeping an inert gas along the downstream side of the membrane. Both cocurrent and countercurrent sweep flow patterns were examined with only minimal differences found. The polyaramide membrane was stable in the entire range of temperatures tested (23–220°C). After utilizing a silicone rubber post-treatment, the membrane exhibited a hydrogen permeance of approximately 300 GPU at 175°C with a hydrogen to n-butane selectivity of 700. The polyimide-containing membranes had superior room-temperature properties; however, the thin skins aged at elevated temperatures. This aging effect decreased the permeance of the membranes approximately 40% at 175°C and slightly increased the permselectivity; however, the effects of aging leveled out over 200–250 h at 175°C and the membrane properties became constant. At this level, the polyimide membranes exhibited approximately 400 GPU of hydrogen permeance with a 660 selectivity to n-butane.