Membranes retentate-recycle mode

A bench top polysulfone hollow fiber membrane (0.0325m ) with molecular weight cutoff (MWCO) of 30K (A/G Technology Corp., Needham MA) was used (24). UF was run in a total recycle mode at a rate of 1.2 L/min (flow speed of 0.73 m/sec), cross membrane pressure of 25 PSIG and 10 + 1°C. PE permeability is expressed as the fraction of PEU/mL in the permeate to PEU/mL in the retentate. Data presented are representative of at least duplicate replications. 

The top product recycle mode in Figure 11.12 brings part of the permeate stream at a lower pressure to join the feed suream at a higher pressure. Thus, additional energy external to the membrane reactor will be required to recompress the recycled permeate. On the contrary, in the bottom product recycle, also shown in Figure 11.12, only the transmembrane pressure difference and the longitudinal pressure drop need to be overcome between the recycled portion of the bottom product (or retentate) and the feed. Therefore, the required pressure recompression is expected to be small compared to the top product recycle mode. 

Most membrane processes operate by means of cross-flow filtration, in which only part of the fluid passes through the membrane as filtrate (or, more correctly, permeate, since some membrane processes operate by permeation rather than filtration) the retained part, the concentrate or retentate, conseqnently becomes more concentrated in particulate or solute species. Membrane systems are frequently operated in a closed loop, with the retentate recycled, and final concentrate is taken from the loop in proportion to the added feed suspension. Whereas microfiltration utilizes both through-flow and cross-flow filtration, cross-flow is the nsnal mode for the other membrane filtration processes, and has thereby grown to its present level of importance. 

There are two modes that can be used to operate a membrane filter, concentration or diafiltration. In concentration operations, the permeate passes through the membrane and is collected as a filtrate while the retentate (the unfiltered part) is recycled back to the feed vessel. As more and more permeate is removed, the solid concentration in the retentate increases until the desired concentration is reached or the cross flow becomes inefficient due to the high viscosity. The retentate concentration can be calculate as (Cheryan 1998)... 

Another parameter affecting the treatment efficiency in PMRs is the operational mode (Table 6.3).TTie PMRs can work either in batch or continuous modes. Sometimes, semi-batch systems are also applied. In batch mode, the feed tank is filled with the treated solution at the beginning of the process and no refilling of the tank before the end of the operation is done. The permeate is collected in a permeate tank and the retentate (concentrate) is recycled back to the feed tank. At the end of the batch process, a small volume of concentrate remains in the feed tank. Then the system is drained, the membranes can be cleaned if necessary, and the tank is refilled with a new batch. A modification of the batch mode is the semi-batch system. In this case the feed tank is refilled with fresh feed solution as the permeate is removed, in order to keep a constant volume in the feed tank. After a defined time, or once a predetermined concentration or flux is reached, the supply of fresh feed is stopped and the remaining solution in the tank is concentrated, as in batch mode. In continuous mode the feed solution is continuously supplied to the feed... 

In Fig. 21.7 a laboratory scale PMR coupling photocatalysis with MF is shown. The PMR was applied for the removal of trichloroethylene (TCE) from water (Choo et al., 2008). The system was composed of a photocatalytic reactor (volume of 700 cm ) and a hollow fiber MF module (effective membrane surface area of 20.7 cm ). A UV-A light source was placed in the inner chamber of the photoreactor, whereas in the outer chamber the solution undergoing the photocatalytic reaction was flowing. Feed from the feed tank was pumped through the photoreactor to the membrane module. The PMR was operated either in batch or in continuous mode. In batch operations, the permeate and retentate were recycled to the photoreactor. In continuous mode, the permeate was discharged and the same volume of the solution was fed into the reactor. Thus the working volume of the photoreactor was maintained at a constant level. 

A common altemative used where possible is the diafUtration mode with a crossflow UF membrane unit and concentrate recycle (Figure 7.2.5(e)). Here the solution concentration and viscosity are not allowed to increase due to the continuous addition of buffer replacing the permeate volume lost Equations developed in Section 6.4.2.1 for well-stirred UF cells having continuous diafUtration may be used here with appropriate care since we can treat the crossflow UF device as a blackbox far the purpose of an overall process mass balance and solute selectivity analysis. Similarly, the equations developed in Section 6.4.2.1 for a batch concentration process may be utilized here to determine various quantities, such as the yield of macrosolute, retentate concentration, etc. 

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