Membrane separations, combined

Liquid membrane separation combines the solvent extraction and stripping processes (re-extraction) in a single step. The great potential for energy saving, low capital and operating cost, and the possibility to use expensive extractants, due to the small amounts of the membrane phase, make SLMs an area deserving special attention. 

FIG. 15-25 Combination coalescer, settler, and membrane separator. (Courtesy of Selas Corporation of Ametica. )... 

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. 

The combination of diafiltration and batch concentration can be used to fractionate two macrosolutes whose retentions differ by as little as 0.2. It is possible in principle to achieve separations that are competitive with chromatography. When tanks and other equipment are considered, as well as the floor space they occupy, the economics of membrane separation of proteins may be attractive [R. van Reis, U.S. Patent 5,256,294 (1993)]. 

Catalytic Membrane Reactors Membrane reactors combine reaction and separation in a single vessel. By removing one of the... 

Fig. 5-6. Chiral separation by MIP membranes a combination of sieving and selective transport [44],...

Brief Examples Microfiltration is the oldest and largest membrane field. It was important economically when other disciphnes were struggling for acceptance, yet because of its incredible diversity and lack oT large apphcations, it is the most difficult to categorize. Nonetheless, it has had greater membrane sales than all other membrane apphcations combined throughout most of its history. The early success of microfiltration was hnked to an ability to separate microorganisms from water, both as a way to detect their presence, and as a means to remove them. Both of these apphcations remain important. 

Y.V. Plekhanova, A.N. Reshetilov, E.V. Yazynina, A.V. Zherdev, and B.B. Dzantiev, A new assay format for electrochemical immunosensors polyelectrolyte-based separation on membrane carriers combined with detection of peroxidase activity by pH-sensitive field-effect transistor. Biosens. Bioelectron. 19, 109-114(2003). 

The combination of CO shift with membrane separation of H2 and in situ C02 capture is mentioned as an advanced process to optimize the hydrogen yield LHV-based cold hydrogen efficiencies of 72% are reported in such configurations.93... 

Membrane-Enclosed Enzymatic Catalysis (MEEC) has been developed as a useful, practical new method for the manipulation of enzymes in organic synthesis. The enzyme in soluble form is enclosed in commercially available dialysis membranes. It combines the simplicity of use of soluble enzymes with certain of the advantages of immobilized enzymes. Containment permits separation of the enzyme from the reaction medium, straightforward separation of the product, and recovery of the enzyme for reuse [53],... 

The idea of using membranes to filter molecules on the basis of size is not without precedent. Dialysis is used routinely to separate low molecular weight species from macromolecules [105]. In addition, nanofiltration membranes are known for certain small molecule separations (such as water purification), but such membranes typically combine both size and chemical transport selectivity and are particularly designed for the separation involved. Hence, in spite of the importance of the concept, synthetic membranes that contain a collection of monodisperse, molecule-sized pores that can be used as molecular filters to separate small molecules on the basis of size are currently not available. 

Another participant in the French nuclear program, Le Carbone-Lorraine, developed inorganic membranes by combining their know-how in the field of membranes with their expertise in carbon. They developed tubular UF and MF membranes using a tubular carbon support (inner diameter 6 mm, outer diameter 10 mm). The carbon support is made of carbon fibers coated with and bonded by CVD carbon, the separating layers also being made of carbon. These membranes have been marketed since 1988. 

Gavalas, G. R., C. Megiris and S. W. Nam. 1989. A novel composite inorganic membrane for combined catalytic reaction and product separation. Chem. Eng. Sci. 44(9) 1825L-35. 

Microporous membranes (pore radius less than 10 A) are ideal materials to be used as separators in membrane reactor processes. Microporous membranes also combine the high selectivities to certain components with high permeation rates. The high selectivities mean that maximum conversions (and thus equilibria shifts) higher than those achieved by porous membranes can be attained, while the high permeation rates allow for high reaction rates... 

The wall of the small intestine is permeable to water and to small molecules such as the amino acids produced by protein breakdown and sugars produced by carbohydrate breakdown so this system is a reactor-separator combination, a membrane reactor. Finally the undigested food passes into the large intestine, where more water is removed through the permeable wall before exiting the reactor. 

Naturally, there exist a variety of membrane separation processes depending on the particular separation task [1]. The successful introduction of a membrane process into the production line therefore relies on understanding the basic separation principles as well as on the knowledge of the application limits. As is the case with any other unit operation, the optimum configuration needs to be found in view of the overall production process, and combination with other separation techniques (hybrid processes) often proves advantageous for large-scale applications. 

A few years ago, a new class of ligands namely the sulfonated phosphites (for examples see Table 7, 132, 133) was described.283 287 They show remarkable stabilities in water compared to conventional phosphites such as P(OPh)3 and rhodium catalysts modified with 132 exhibited much higher catalytic activities in the hydroformylation of 1-tetradecene than conventional Rh/P(OPh)3 or Ph/PPh3 catalysts even at lower reaction temperatures.285,286 Sulfonated phosphite ligands may play a role in the emerging field of biphasic catalysis in ionic liquids15 22 or in combination with membrane separation of the metal complexes of these bulky ligands. 

Because most industrial effluent streams contain a multitude of contaminants in many different forms, the greatest potential for membrane separation technologies is in combination with such other technologies as ion exchange, activated carbon adsorption, anaerobic digestion, etc. 

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