Separation membrane processes

Reverse osmosis is a high-pressure membrane separation process (20 to 100 bar) which can be used to reject dissolved inorganic salt or heavy metals. The concentrated waste material produced by membrane process should be recycled if possible but might require further treatment or disposal. 

The phenomenon of concentration polarization, which is observed frequently in membrane separation processes, can be described in mathematical terms, as shown in Figure 30 (71). The usual model, which is weU founded in fluid hydrodynamics, assumes the bulk solution to be turbulent, but adjacent to the membrane surface there exists a stagnant laminar boundary layer of thickness (5) typically 50—200 p.m, in which there is no turbulent mixing. The concentration of the macromolecules in the bulk solution concentration is c,. and the concentration of macromolecules at the membrane surface is c. 

Cross-flow-elec trofiltratiou (CF-EF) is the multifunctional separation process which combines the electrophoretic migration present in elec trofiltration with the particle diffusion and radial-migration forces present in cross-flow filtration (CFF) (microfiltration includes cross-flow filtration as one mode of operation in Membrane Separation Processes which appears later in this section) in order to reduce further the formation of filter cake. Cross-flow-electrofiltratiou can even eliminate the formation of filter cake entirely. This process should find application in the filtration of suspensions when there are charged particles as well as a relatively low conduc tivity in the continuous phase. Low conductivity in the continuous phase is necessary in order to minimize the amount of elec trical power necessaiy to sustain the elec tric field. Low-ionic-strength aqueous media and nonaqueous suspending media fulfill this requirement. 

Process Description lectrodialysls (ED) is a membrane separation process in which ionic species are separated from water, macrosolutes, and all uncharged solutes. Ions are induced to move by an electrical potential, and separation is facilitated by ion-exchange membranes. Membranes are highly selective, passing either anions or cations andveiy little else. The principle of ED is shown in Fig. 22-56. 

Filtration Cross-flow filtration (microfiltration includes cross-flow filtration as one mode of operation in Membrane Separation Processes which appears earlier in this section) relies on the retention of particles by a membrane. The driving force for separation is pressure across a semipermeable membrane, while a tangential flow of the feed stream parallel to the membrane surface inhibits solids settling on and within the membrane matrix (Datar and Rosen, loc. cit.). 

Although ED is more complex than other membrane separation processes, the characteristic performance of a cell is, in principle, possible to calculate from a knowledge of ED cell geometry and the electrochemical properties of the membranes and the electrolyte solution. 

Bitter, J. G. A. (1991) Transport Mechanisms in Membrane Separation Processes, Plenum Press, New York. 

Matsuura, T. (1994). Synthetic Membranes and Membrane Separation Processes. CRC Press, Boca Raton, FL. 

A. K. Jain, R. K. Srivastava, M. K. Gupta, S. K. Das. A novel technique in membrane separation processes Electroosmotic separation of benzene in ethanol solution. J Memb Sci 75 53, 1993. 

Table 16.1. Classification of membrane separation processes for liquid systems...

Industrial membrane processes may be classified according to the size range of materials that they are to separate and the driving force used in separation. There is always a degree of arbitrariness about such classifications, and the distinctions that are typically drawn. Table 16.1 presents classification of membrane separation processes for liquid systems. 

Ultrafiltration is one of the most widely used of the pressure-driven membrane separation processes. The solute retained or rejected by ultrafiltration membranes are those with... 

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