Process Economics Program Report 190
Membranes for Gas Separation
Published: February 1991
Nonporous polymeric membranes are used for the separation of gases on an industrial scale. The separation is based on the observation that gases pass through thin membranes at different rates because their solubilities in the membrane and diffusivity through the membrane differ.
Polymers are selected for their permeability and selectivity for the gases to be processed. The polymers must also be processable into practical membranes. Leading polymers include polysulfones, polyimides, silicones, cellulosics, polycarbonates and others. In this report, we review the development of modified polymers that have superior separation characteristics. Expensive polymers can be used because the cost of the polymer is a small part of the cost of the separation equipment.
Separating membranes generally comprise a very thin nonporous separating layer on a nonselective porous membrane. Membranes were first made in the form of flat films, which are now largely supplanted by hollow fibers. Thin hollow fibers--less than one mm diameter--offer greater burst strength (which permits a higher pressure to be applied as the separation driving force) and higher surface-to-volume ratio (which permits high flow in small equipment). In this report, we review methods of forming thin films and hollow fibers for gas separations.
Hollow fibers are made from polymer melt or polymer solutions by several techniques adapted from textile spinning technology. Fibers may be treated or coated to improve their permeability and/or selectivity. (Modifications of the polymer or the fiber that increase permeability generally decrease selectivity and vice versa.)
Flat films are spirally wound into modules: hollow fibers are gathered into compact modules of 1,000 square feet or more. In this report, we examine module-making techniques and include rough economic analyses of the costs involved in fiber production and in module making.