Transmembrane pressure cycle

FIGURE 16.35 Transmembrane flux as a function of the number of emulsification cycles at different transmembrane pressures. (From Vladisavljevic, G.T., Shimizu, M., and Nakashima, T., J. Membr. Set, 244 (1-2), 97-106, 2004. With permission.)... 

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. 

Dead end filtration operation mode was used, and the installation was continuously running 24 h daily. The membrane flux was 60 L/(m h) and filtration cycle was 30 min, and the backwash time was set up at 1 min. To inhibit the growth of bacteria on the surface, membrane was immersed with 10 mg/L of NaClO solution for 5 min every 8 h. When the membrane flux reduced or transmembrane pressure (TMP) increased significantly, the backwash flow was increased to enhance washing. The quality of membrane inflow and effluent were examined and monitored during the experiment. 

Figure 33.9 Membrane strain and transmembrane pressure difference during a compaction (at a transmembrane pressure difference of 2.8 MPa) and recovery cycle for a commercial asymmetric cellulose-acetate membrane with nitrogen as the feed gas.

In an elegant modeling study of lipid bilayers, Yagisawa et al. showed that oscillations can be induced by a transmembrane pH and salt gradient, with no electrical stimulation or pressure gradients [51]. Briefly, the pH difference leads to a transmembrane dipole and electrical stress on the nonpolar interior of the bilayer, triggering a gel/liquid crystal transition. Following this transition, permeability to salt increases and there is a relaxation of electrical stress, followed by reversal of the lipid transition, restoration of membrane potential, and reinitiation of the cycle. 

Compaction of polymeric membranes also occurs during gas separation. Reinsch et al. (2000) described the use of UTDR to measure compaction of 175-p.m-thick (with backing) asymmetric cellulose-acetate gas separation membranes provided by Grace Davison (Littleton, CO). Figure 33.8 shows a schematic of the membrane cell used in these characterization smdies of membrane compaction during gas separation and the primary reflections of acoustic waves A and B, which correspond to the cell top-plate-gas interface and the gas-membrane interface, respectively. Compaction was studied as a function of feed gas pressure and composition. Figure 33.9 shows a plot of the membrane strain as a function of time for compaction at a transmembrane nitrogen gas pressure difference 2.8 MPa followed by a recovery cycle at atmospheric pressure for a commercial asymmetric cellulose-acetate membrane. An instantaneous strain of approximately 13% is observed followed by a small time-dependent strain. 

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