Transmembrane pressure constant flux operation

Constant Flux, Constant Pressure Membrane systems operate in one of two possible ways constant transmembrane water flux (flow rate per unit membrane area) with variable pressure or constant transmembrane pressure with variable water flux (Fig. 6.9). An increase in transmembrane pressure is required as the membrane fouls to maintain a particular water flux (constant-flux operation). In constant-pressure systems. 

Microfiltration cross-flow systems are often operated at a constant applied transmembrane pressure in the same way as the reverse osmosis and ultrafiltration systems described in Chapters 5 and 6. However, microfiltration membranes tend to foul and lose flux much more quickly than ultrafiltration and reverse osmosis membranes. The rapid decline in flux makes it difficult to control system operation. For this reason, microfiltration systems are often operated as constant flux systems, and the transmembrane pressure across the membrane is slowly increased to maintain the flow as the membrane fouls. Most commonly the feed pressure is fixed at some high value and the permeate pressure... 

If a transmembrane pressure difference is imposed at a given constant temperature, the reaction zone will be shifted toward the lower pressure side. The mole fraction of the reactant entering the lower pressure side of the membrane surface drops to a level lower than that in the absence of a pressure difference. It has been shown [Sloot et al., 1990] that the molar fluxes of, say, hydrogen sulfide increases as the pressure on its side increases, thus potentially reducing the membrane area required. A serious drawback with this mode of operation, however, is the amount of inert gas introduced. 

In recent years, a new mode of operation has been suggested in which permeate flow is controlled at a constant rate, while transmembrane pressure is kept low. This approach was developed and tested for mammalian cells (23), where concern about cell damage due to compression on the membrane surface prompted an interest in controlling TMP and flux rate. The data reported in this paper demonstrate that the controlled permeate flow operating scheme also works well for bacterial systems, yielding more stable and improved flux as well as high protein passage. 

Figure 4. Concentration of Cells (AG Techn. 500K MWCO Hollow Fibers) Flux and Average Transmembrane Pressure Profiles for Constant Permeate Flow-Controlled Operation...

Figure 9. Influence of average transmembrane pressure on membrane flux (0.45 pirn microporous). Recirculation rate (40 psi) is kept constant as TMP is varied. Operation is in the total recycle mode.

Nonuniform TMP values over the filtration surface area may cause substantial (up to 50%) reduction in the product recovery in the permeate. A novel approach to improving the flux and/or product recovery utilizes the concept of a uniform transmembrane pressure. lf This is achieved by varying the permeate side pressure with an independent recirculation pump to adjust the TMP to a constant value. A schematic of the UTP and conventional cross-flow configuration is shown in Figs. 11 and 12, respectively. The TMP profiles for the two operational modes are shown in Fig. 13. Flux improvements up to 500% have been achieved compared with the conventional cross-flow mode in many important food, beverage and biotechnology applications. 

Membrane fouling may result in a significant increase in filtration resistance, leading to unstable filtration behavior. The pressure-driven membrane processes can be operated either with constant feed pressure or in constant flux mode. For constant pressure operation where the transmembrane pressure (TMP) is maintained at a constant value during the filtration, the flux will decline with time due to the... 

As shown in Figure 16.21, the oil flux increases with time at the constant transmembrane pressure, reflecting the fact that more and more pores are simultaneously active in droplet formation during the operation. The same type of behavior was found in ME with a-alumina and zirconia membranes [50,51]. It is reasonable to suggest that when t Q, only the largest pores are partially... 

Compared to the systems described above, the membrane reactor system has the advantage of continued operation. However, the decrease in the enzymatic reaction and an increase in the transmembrane pressure was detected after five cycles (Jeon and Kim, 2000a Kou et al., 2004). These researches have found that the membrane reactor could be operated continuously for at least 15 h, maintaining a constant permeate flux and product output rate (Kou et al., 2004). The continued production of ultraflltration membrane reactor system gets obstructed due to membrane fouling after several cycles. Therefore, scientists interest has moved toward the development of a new system that can be helpful in the efficient production of COS continuously. 

Fouling propensity is commonly assessed by monitoring permeate flux and transmembrane pressure (TMP). Since membrane processes are generally operated either under constant TMP or constant permeate flux, a decrease in permeate flow rate or an increase of TMP is observed, respectively, once the fouling forms on the membrane. From these two parameters, calculation of hydraulic resistance (m ), membrane permeability (Lm h bar ) or specific cake resistance (mkg ) is also possible and allows further assessment of fouling conditions. The hydraulic resistance of the filtration system can be quantified by correlating TMP and permeate flux during a clean water test. This correlation is described as the Darcy s law ... 

Membrane Fouling Fouling is the reduction in membrane permeability [= flux divided by pressure expressed in L/(m - h - bar)]. Typically, the transmembrane pressure (TMP) has to be increased to keep the flux at a constant level (Fig. 9.8). Thus, membrane fouling reduces productivity by increasing TMP, which in turn increases maintenance and operational costs. 

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