Tangential-flow filtration operation modes

Membrane operations are conducted either in a direct flow filtration (also called dead end) mode or in a tangential flow filtration (TFF) mode. Direct flow filtration is simple and easy to implement but has limited capacity for applications with high-solid mass. TFF is capable of processing large-solid masses but is more complex and capital intensive. 

Tangential-flow filtration is typically operated in one of two modes concentration or diafiltration (see Fig. 20.2). For microfiltration, in the concentration mode, the product of interest usually passes through the pores of the membrane and is collected in the permeate. However, since MF membranes are nonselective, flie concentration of the product of interest will be the same in the retentate and permeate streams. 

Figure 20.2 Modes of operation of tangential-flow filtration (a) concentration and (b) diafiltration.

Tangential-flow filtration could also be used to validate virus clearance. Today most virus clearance filters are operated in normal flow mode. However, when the size of the model virus particle for which clearance is being validated, and the desired product are within an order of magnimde of each other, tangential-flow filtration may be the preferred mode of operation. 

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.). 

Having distinguished between MF, UF and RO, and identified the two prevalent kinds of UF membranes, we will now discuss modes of operation cross-flow (also tangential-flow and/or split-stream) filtration versus dead-end filtration. The numbers of particles per unit volume generally diminish in the order RO (ions) > UF (molecules) > MF (bacteria, etc.) when the retained particles are comparatively small in number, as is usually the case in MF filtration, de -end filtration is suitable (Table 10). At the other... 

A major development in filtration, which came with the first, reverse osmosis membrane process, was the fact that the flow of the fluid at the membrane snrface is tangential to it, rather than perpendicular to that surface. This has become known as cross-flow filtration (see Figure 2.21), and almost all membrane processes now operate in crossflow rather than through flow mode. Further scouring action is achieved by having the membrane medium move relative to the hquid flow, either rotating close to a stator, or vibrating. 

In this situation, if the pressure filtration stays unchanged, the filtrate rate will decrease with time. When unacceptable values of the filtrate rate are reached, the process must be stopped and the membrane cleaned or replaced. This mode of operation is uneconomical. One solution to this problem is to increase the transmembrane pressure in order to maintain the flow rate but, in this case, the pumping flow rate has to be reduced because pumps generally present a pre-established and characteristic flow rate-pressure relation which is, a priori, unchangeable. Consequently, when the pressure is continuously increased, the clogging rate will increase faster than when a high tangential velocity is used in the unit. 

A.2 Cross-Flow, Dead-End Configurations Microfiltration and UF systems are operated in two possible filtration modes. Figure 6.10 shows the cross-flow configuration in which the feed water is pumped tangential to the membrane. Clean water passes the membrane while the water that does not permeate is recirculated as concentrate and combined with additional feed water. To control the concentration of the sohds in the recirculation loop, a portion of the concentrate is discharged at a specific rate. In dead-end or direct filtration, all the feed water passes through the membrane. Therefore, the recovery is 100%, and a small fraction is used periodically for backwash in the system (5-15%). 

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