Fluxes and Driving Forces in Membrane Separation Processes

Fluxes and Driving Forces in Membrane Separation Processes 

Separations in membrane processes are the result of differences in the transport rates of chemical species through the membrane interphase. The transport rate is determined by the driving force or forces acting on the individual components and their mobility and concentration within the interphase. The mobility is primarily determined by the solute s molecular size and the physical structure of the interphase material, while the concentration of the solute in the interphase is determined by chemical compatibility of the solute and the interphase material, the solute s size, and the membrane structure. The mobility and concentration of the solute within the interphase determine how large a flux is produced by a given driving force. 

For membrane separation processes, only driving forces which induce a significant flux of matter are of practical importance. These driving forces are hydrostatic pressure, concentration, and electrical potential differences. These driving forces can also lead to the separation of chemical species. 

The permeabilities of different components in a membrane depend on the mechanism by which the components are transported. For example, in homogeneous polymer membranes, the various chemical species are transported under a concentration or pressure gradient by diffusion. The permeability of these membranes is determined by the diffusivities and concentrations of the various components in the membrane matrix and the transport rates are, in general, relatively slow. In porous membrane structures, however, mass is transported under the driving force of a hydrostatic pressure difference via viscous flow and, in gen- 

For a given driving force, the flux through a unit membrane area is always inversely proportional to the thickness of the selective barrier. For economic reasons, membranes should in general be as thin as possible. 

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