Membrane separation operation modes

Diagram of membrane separation operation modes (a) standard batch configuration and (b) topped-off batch system with recirculation loop. 

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. 

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.). 

It has been mentioned earlier that using porous membranes for product separation during the course of an equilibrium reaction, maximum attainable conversions are limited because of reactant permeation. This is the case where the membrane forms the wall of the reactor in which a catalyst is packed. It has also been mentioned that in this mode equilibrium conversions for some slow reactions could be increased by factors ranging between 1.3 and 2.3. Another important operation mode arises when the membrane is inherently catalytic or when the catalytically active species are placed within the membrane pores (catalytically active membrane as shown in Figure 7.2b and 7.2c). In this case, reaction and separation take place simultaneously and are combined in parallel rather than in series as was the case in the previous mode. 

FIGURE 44 Closed-loop cascade HPTFF system operating in diafiltration mode with buffer regeneration by a conventional ultrafiltration unit. [Adapted from Zydney, A. L. and van Reis, R. (2001). In Membrane Separation in Biotechnology (W. H. Wong, ed.), Marcel Dekker, New York.]... 

Membrane processes can be operated in two major modes according to the direction of the feed stream relative to the orientation of the membrane surface dead-end filtration and crossflow filtration (Figure 1.1). The majority of the membrane separation applications use the concept of crossflow where the feed flows parallel to and past the membrane surface while the permeate penetrates through the membrane overall in a... 

As most of the membrane separation processes arc operated in the crossflow mode for the reasons discussed earlier (Section 5.5.1 Crossflow Configuration), the crossflow velocity has marked effects on the permeate flux. A higher crossflow velocity typically results in a higher flux. The rate of flux improvement as a function of the crossflow velocity usually can be described by the following equation with specific units ... 

In a general way, most of ceramic membrane modules operate in a cross-flow filtration mode [28] as shown in Figure 6.18. However, as discussed hereafter, a dead-end filtration mode may be used in some specific applications. Membrane modules constitute basic units from which all sorts of filtration plants can be designed not only for current liquid applications but also for gas and vapor separation, membrane reactors, and contactors, which represent the future applications of ceramic membranes. In liquid filtration, hydrodynamics in each module can be described as one incoming flow on the feed side gf, which results in two... 

As is clear solid oxide electrolytes are not useful for applications as oxygen separation membrane, unless operated with external circuitry (oxygen pump) or as a constituent phase of a dual-phase membrane. Both modes of operation, classified in this paper as electrochemical oxygen separation, are briefly discussed in Section 10.4.3. But we first start with a discussion of the models that have been developed to describe the oxygen semi-permeability of solid oxide... 

The idea of separating homogeneous TMCs with solvent-resistant NF(SRNF) membranes has only emerged over the last decade with the advent of commercial SRNF membranes. A nondestructive, energy efficient separation and concentration of reusable catalysts from reaction products can thus be realized. The SRNF-coupled catalysis can be run continuously, or alternatively in a semibatch operation mode to lower reactor occupancy or when reaction conditions are too harsh to be membrane compatible. Precipitating by-products could be simply removed from the reactor after filtration before refilling it. 

Consider the air separation problem of Example 9.2. In this case, the separation will be performed in a hollow-fiber module using the same PEMA membrane and operating in a crossflow mode. For the operating pressures considered in Example 9.2, calculate the cut, composited permeate concentration, and membrane area required to produce a retentate oxygen concentration of 7%. Compare these results to those obtained for perfect mixing in Example 9.2. 

Process intensifying methods, such as the integration of reaction and separation steps in multifunctional reactors (examples reactive distillation, membrane reactors, fuel cells), hybrid separations (example membrane distillation), alternative energy sources, and new operation modes (example periodic operations). 

---