was the pivotal invention that has made most of modern membrane technology possible. A variety of composite membrane concepts were developed later that have the advantage that the skin and porous support are not integral and can, in fact, be made of different materials. This is especially useful when the active skin material is very expensive. Reverse osmosis and gas separation membranes of both types are in current use.

There is growth in almost all sectors of the membrane industry; however, the opportunities for future impact by new polymer technology appear somewhat uneven. For example, one of the major limitations to the use of ultrafiltration-type processes in the growing biotechnology arena is the tendency for surface fouling by protein and related macromolecules. The discovery of new membrane materials or surface treatments that solve this problem would be of major importance.

Intense polymer research related to reverse osmosis during the 1960s and 1970s led to commercial installation of desalinization plants around the world. Membranes in use are made of cellulose acetate and polyamides. Future demands for fresh water from the sea could stimulate renewed research interest in this area. Currently most of the efforts are devoted to developing reverse osmosis membranes and processes for removal of organic pollutants, rather than salt, from water.

Gas separation is clearly one of the most active and promising areas of membrane technology for polymer science and engineering (Figure 3.8). The first commercial membranes introduced in the late 1970s were hollow fibers formed from polysulfone by using a unique technology to remove minute surface defects. Since then, other polymers have been introduced in the United States, including cellulose acetate, polydimethylsiloxane (PDMS), ethyl cellulose, brominated polycarbonate, and polyimides. The first materials selected for this purpose were simply available commercial polymers that had adequate properties. New generations of materials especially tailored for gas separation are being sought to open new business opportunities. The key issues involve certain trade-offs. The polymer must be soluble enough to be fabricated into a membrane, but it needs resistance to chemicals that may be in the feed streams to be separated. The membrane should have a high intrinsic permeability to gases in order to achieve high productivity, but the permeation should be selective; that is, one gas, for example, O2, must permeate much faster than another, for example, N2. New polymers whose permeability and selectivity are higher than those of current membrane materials are being developed via synthesis of novel structures that prevent dense molecular packing, thus yielding high permeability, while restraining chain motions that decrease selectivity.

Pervaporation is a process in which a liquid is fed to a membrane process and a vapor is removed. The difference in composition between the two streams is governed by permeation kinetics rather than by vapor-liquid equilibrium as in simple evaporation. Thus, pervaporation is useful for breaking azeotropes and is



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