2028
Ind. Eng. Chem. Res. 1990,29, 2028-2032
MATERIALS AND INTERFACES Ultrathin Multicomponent Poly(ether sulfone) Membranes for Gas Separation Made by Dry/Wet Phase Inversion Ingo Pinnau,+Jan Wind, and Klaus-Viktor Peinemann* GKSS-Forschungszentrum, Institut fuer Chemie, 2054 Geesthacht, West Germany
Asymmetric gas separation membranes were prepared by a dry/wet phase inversion process using poly (ether sulfone) as membrane-forming material. Variations in the composition of multicomponent casting solutions and choice of the coagulation medium resulted in membranes with drastically different gas separation properties. Optimized integral-asymmetric poly(ether sulfone) membranes cm3 (STP)/(cm2.s.cmHg) coagulated in methanol had pressure-normalized CO:, fluxes of 7.2 X and a C02/N2selectivity of 40. However, asymmetric membranes coated with a thin layer of silicone rubber showed substantially improved gas separation performance. Poly(ether sulfone)/silicone multicomponent membranes showed pressure-normalized C 0 2 fluxes of u p to 6.2 X cm3 (STP)/(cm2.s.cmHg) combined with a C02/N2selectivity of 60, essentially equal t o the intrinsic permselectivity of poly(ether sulfone). The estimated, effective skin thickness of these membranes is on the order of 500 A. Pressure-normalized fluxes of poly(ether sulfone) membranes made by dry/wet phase inversion are equal to or higher than those of graded density skin membranes made by a traditional wet phase inversion process using Lewis acid-Lewis base complex solvent systems. Background Asymmetric membranes are generally prepared by a wet phase inversion process (Kesting, 1985). A homogeneous polymer solution is cast as a flat film onto a suitable substrate or spun as a fiber, exposed to air for a specific time, and, thereafter, immersed in a coagulation medium which is a nonsolvent for the membrane-forming polymer. Phase inversion occurs by bringing the initially thermodynamically stable polymer solution to an unstable state by solvent/nonsolvent exchange during the coagulation step. The resulting asymmetric membrane structure generally consists of a thin, selective skin layer supported by a porous substructure. Ideal asymmetric membranes for gas separation must meet the following requirements: (1) The skin layer must be defect-free to assure that permeation is exclusively controlled by a solution/diffusion mechanism (Koros and Chern, 1987) to achieve maximum permselectivity. Pores or defects on the order of 5-10 A over an area fraction of only a few parts per million reduce the permselectivity of membranes substantially (Henis and Tripodi, 1981). (2) The skin layer should be as thin as possible to maximize the membrane productivity. (3) The substructure should provide sufficient mechanical strength to support the delicate skin layer during high-pressure operation. The combination of an ultrathin and defect-free skin layer is extremely difficult to obtain for integralasymmetric membranes (Henis and Tripodi, 1981; Murphy et al., 1989; van't Hof, 1988). On the other hand, multicomponent membranes can combine high selectivity with high productivity (Henis and Tripodi, 1980; Kesting et al., 1987). Multicomponent membranes are based on asymmetric membrane structures
* To whom correspondence should be addressed.
Present address: Department of Chemical Engineering, The University of Texas at Austin, Austin, TX 78712.
with a low degree of skin porosity coated with a thin layer of a highly permeable, but rather nonselective, polymer. In most cases, silicone rubber has been used as a preferred coating material. The silicone coating serves to plug pores in the skin layer of the initially defective asymmetric membrane. The resistance to gas transport through these plugged pores is orders of magnitude higher than that of unplugged pores (Henis and Tripodi, 1981). Therefore, gases predominantly permeate through the glassy regions of the skin layer of the asymmetric membrane. Optimized multicomponent membranes show selectivities that are essentially equal to the intrinsic selectivity of the asymmetric membrane material. Poly(ether sulfone) has previously been investigated as a material for ultrafiltration (Tweddle et al., 1983) and gas separation applications (Ellig et al., 1980; Paulson et al., 1983). Studies on thick, isotropic poly(ether sulfone) films indicated that this material showed superior CO2/CH4 selectivity as well as better stability of transport properties compared to standard membrane-forming materials like cellulose acetate or bisphenol-A polysulfone. A C02/CH4 selectivity of 40 has been reported for gas mixtures at 25 "C (Ellig et al., 1980; Paulson et al., 1983). However, a different study showed that poly(ether sulfone) has only a moderate COz permeability of 3 X cm3 (STP). cm/(cm2.s-cmHg)a t 25 "C and 5 atm upstream pressure (Sanders, 1986). Therefore, poly(ether sulfone) membranes have to be made extremely thin (
