Membranes, phase interfaces, and separations: Novel techniques and membranes-an overview

Membranes and bulk phase interfaces share a close relation in separations. The bulk phase interface may be a fluid-fluid, fluid-solid, or solid-solid interface; it may be immobilized or mobile. In conventional membrane separations, two-phase interfaces exist on two sides of the membrane, two solid-fluid interfaces for solid membranes. Conventional equilibrium separations, however, have only one bulk phase interface. Such an interface in gas-liquid, vapor-liquid, liquid-liquid, and subcriticalZsupercritical fluid-liquid separation systems may be created by phase-interface immobilization at a membrane pore mouth as in membrane contactor-based nondispersive equilibrium separations. Membrane emulsification/sparging/degassing leads to dispersion-based creation of mobile phase interfaces and phase contacting. Convective transport through membrane pores provides efficient solid-liquid contacting as in membrane chromatography. Antisolvent crystallization via dispersionZmixing through membrane pores creates new mobile solid-liquid interfaces. Solid-solid phase interfaces developed via a minor phase consisting of inorganic flakes, inactive fillers, active fillers, or a second polymer phase facilitate the development of a variety of polymeric membranes, porous as well as nonporous. By having two fluid-fluid phase interfaces immobilized on two sides of a porous membrane, liquid membranes and gas membranes have been developed successfully. A composite membrane having a nonporous/nanoporous coating can lead to two fluid-solid phase interfaces or tiiree fluid-fluid phase interfaces for novel separations. The presence of two or more contiguous porousZnonporous membranes in one separation device leads to the existence of multiple bulk phase interfaces or membrane-fluid phase interfaces creating novel liquid membrane structures and separation capabilities in addition to internal staging for separation. The critical role played by the interaction of phase interfaces and membranes vis-à-vis many new separation techniques and membranes developed, commercialized, or being developed will be illustrated.