Membranes, membrane processes, and their applications: Needs, unsolved problems, and challenges of the 1990's

Abstract While membrane separation processes have, over the past two decades, achieved impressive industrial importance for the resolution of gaseous and aqueous liquid mixtures, and for the purification of chemical and biological products, their potential for even more extensive industrial application in such fields as food/beverage processing, waste water reclamation, gaseous waste detoxification, large-scale air-gas separation, hydro-metallurgical processing, and production of gaseous and liquid fuels and petrochemicals, remains unexploited. This is partly traceable to limitations imposed by inherent membrane properties-deficiencies, and partly to deficiencies of membrane module design or configuration, or of fluid-management strategy. These deficiencies render membrane processes either technically impractical, or (more often) uneconomic, for such applications. The recent development of (1) novel polymeric materials of unique functionality or microstructure; (2) inorganic (ceramic) semipermeable materials; (3) novel ultrathin-barrier laminate structures comprised of both organic and refractory components; and (4) interpenetrating multiphase structures with anomalous transport characteristics, promises to yield membranes with superior chemical/thermal stability, fouling resistance, organic solvent resistance, and unusually high permselectivities and permeabilities. Such membranes may well circumvent many of these limitations. Similarly, recent developments in membrane module design, including rotational membrane devices and pulsed-flow fluid management for polarization control, use of low-cost refractory monoliths as membrane supports, and use of electric potentials to minimize macrosolute polarization and fouling, may permit practical and economic application of membrane processes to liquid and gaseous streams which today are untreatable by such methods. Of growing industrial importance are a family of new applications for membranes which make use of membrane structures not as intrinsic separation barriers, but as substrates for immobilization of catalysts (e.g., enzymes) or of specific complexing agents (e.g., affinity ligands). These developments should lead to important new chemical synthesis processes, and to novel and efficient strategies for industrial-scale purification of complex biological products. Exciting opportunities also exist for the imaginative marriage of membrane separation techniques with other physical or chemical/biochemical transformation/ separation procedures to yield integrated processes which circumvent the limitations of the individual steps, and achieve far greater selectivity, efficiency, or economic utility than either process element alone. Examples include membrane solvent-extraction, affinity-complexation ultrafiltration, selective precipitation/membrane filtration, “cryo-filtration”, and extractive, membrane-moderated immobilized-cell biotransformation. Lastly, the overlooked interface between electrochemistry and membrane technology merits very serious attention as a route to even more dramatic solutions to problems in industrial separations and chemical synthesis. The production of ultrapure gases, the removal of trace-concentrations of toxicants or high-value substances from liquid or gaseous streams, the development of novel chemical and biochemical sensors, and the synthesis of high-value chemical intermediates via membrane-immobilized catalysts in an electrochemical cell, are among the many opportunity areas for ongoing electrochemical and membrane process research and development.