Trans-membrane flux

In general, concentration-polarization effects are much more noticeable on the feed side of the membrane than on the product side. As discussed earlier, the trans-membrane flux is relatively insensitive to the metal ion concentration in the product solution but varies substantially with feed solution metal ion concentration, particularly with low concentration feed solutions. Concentration polarization effects can be a serious problem when treating these solutions with practical membrane systems. Figure 9.25 shows the effect of concentration polarization with a hollow fiber supported membrane system described in the following section. In this system, a feed solution velocity of greater than 20 cm/sec is required to overcome polarization. 

In pervaporation, as the feed fluid is a liquid, a thin, stagnant boundary layer always exists over the membrane surface in which the solute transport is diffusive (Fig. 3.6-11). The thickness of this boundary layer (stagnant liquid film) can be calculated from well-established boundary layer equations (for critical reviews on the use of the most common correlations see, for example, Gekas and Hallstrom, 1987 and Cussler, 1997). If the flux of a solute i across the concentration boundary layer toward the membrane is lower than the maximum (for the respective solute bulk feed concentration) attainable solute flux across the membrane, then solute i will be depleted in the boundary layer over the membrane upstream surface. As a consequence, the concentration of i in the membrane upstream surface will also be lower (assuming a constant sorption coefficient), the concentration gradient over the membrane will decrease and hence so will the trans-membrane flux. 

Figure 13.7. Comparison of the trans-membrane fluxes obtained with the different membranes at 20°C (a) and 40°C (b). Feed distilled water Reynolds number = 1700 (adapted from Criscuoli et al, 2013).

Figure 13.7 shows the effect of the porosity (e), pore size (rfp) and thickness (S) ofthe different membranes on the distillate flux the trans-membrane flux inereased with the sdpS ratio, due to the lower membrane resistance. The highest fluxes were obtained with the membrane M4 (around 3 kg m h at 20°C and 12.5 kg m hat 40°C), while the lowest values occurred with the Ml (2.3 and 9 kg m h at 20°C and 40°C, respectively). 

The biochemical similarity between the A-M and the FO-Fl systems, as well as other soluble and membrane-bound ATPases, is revealed in their utilization of the quaternary MgATP complex within the P-loop catalytic site [49]. This evolutionary conserved framework is probably required for a stereospecific interaction that ensures an appropriate channeling of the ballistic protons. Thus, in reconstitution experiments in the absence of Mg ions, ATP hydrolysis by A-M or FO-Fl is carried out as intensively as by M or Fl alone, but producing no myofibril contraction or outward trans-membrane flux of protons, respectively. In such reconstitution experiments, the y-axis rotation on the Fl head is still present under hydrolysis of CaATP [50]. This demonstration is consistent with the ballistic proton mechanism, where the electrostatic rotational drive is not the main energy consumer. 

The key process parameters for filtration scale-up are trans-membrane pressure, filtration area, shear rate, operating time, temperature, flux rate, protein concentration, and solution viscosity (5). 

If operation above design flux is required for short periods of time, pressurized systems are preferred, as they operate under a positive trans-membrane pressure positive pressure can always be increased, while vaccum is limited. 

Moncorge and Pascal [61] and Bauer et al. [42] describe the use of the carbon/carbon composite membranes of Le Carbone Lorraine in the filtration of drinking water. With 0.2 pm membranes the fluxes range between 1000 and 2000 1/m h at trans-membrane pressures from 1 to 2 bars. The use of Kerasep membranes [65] (Rhone-Poulenc s alumina/alumina membranes, 0.2 pm pore size) leads to fluxes of 600-12001/m h at 2 bar transmembrane pressure. 

The very serious problem of fouling by proteins is corroborated by many publications [41,70,71]. Various parameters influencing the fouling behaviour have been studied. Clark et al. [70] discuss the influence of protein concentration, trans-membrane pressure, cross flow velocity and pH. For pore sizes of 0.1 pm (Membralox membranes), filtering bovine serum albumin, the flux has a minimum at the pH of the protein isoelectric point. Dumon and Barnier [71] show that the amount of protein adsorption depends on previous adsorption. Contacting with citrate or phosphate lowers a subsequent protein adsorption contacting with nitrate increases the protein adsorption. 

Rim Control Scheme Average Trans- Membrane Pressure (psi) Recirc. Rate (1pm) Time Averaged Flux (l/m -hr)... 

Operating conditions and the corresponding performance of a membrane filter system have been examined for recovering proteins from a bacterial lysate. Specifically, the dependence of membrane flux on average trans membrane pressure and recirculation rate has been investigated for both whole cells and lysate. The recovery of an intracellular protein was simulated by adding a marker protein, IgG, to the lysate after cell lysis. 

Figures 13 and 14 respectively show the relationship of average trans membrane pressure and circulation rate on flux for both whole cells and lysate suspensions. The lysate was produced via the lysozyme procedure. Qualitatively, the behaviors are quite similar to those seen in Figures 9 and 10 where the cells are lysed by sonication. Beyond minimum circulation flows, little extra flux is attained as the circulation rate increases. The average TMP is the dominant factor in producing flux, however, if Pin - Pout less than about 20 psi, debris can accumulate at the membrane surface and inhibit flow. It should be emphasized that these results pertain to 0.45 micron pore size microporous membrane only.

In terms of optimizing system performance, the flux for both cells and lysate suspensions seem to be most strongly influenced by the average trans membrane pressure, although maintaining a minimum circulation flow is critical also. Flux rates on microporous membranes for lysates are typically less than for whole cell suspensions as would be expected because of the dispersed cell debris present. Filtrates from lysate processing are typically clear, but do depend on the membrane used and the method of lysing the cells. The ultra-... 

Membrane filters are Memcor CMF. Trans-membrane pressure (TMP) and flux are maintained by regular - every 30 min - air-scour backwash. Eventually, TMP increases over time (normally about 30-35 days) due to particulate and biological fouling. When the TMP reaches 1.5 bar, the membranes are cleaned with chemicals using solutions of sodium hypochlorite and/or citric acid sequentially. The membrane flux is 44 Imh and the MF filtrate has a SDI value of 3 to 4 and turbidity 0.1 to 0.2. 

Membrane systems operate in either constant fltrx (variable feed pressure) or constant pressure (variable water flux) mode. During constant flux operation, trans-membrane pressure (TMP) is increased to maintain the desired or des u flux that otherwise... 

It has been shown that UF PSU membranes treated for 20 sec with oxygen plasma showed increased hydrophilicity. X-ray photoelectron spectroscopy (XPS) analysis proved that this improvement was caused by the presence of the hydroxyl, carbonyl, and carboxyl groups on the surface. For such modified membranes, the flow rate of pure water and gelatin increased and the membranes showed fewer fouling properties (Kim et al. 2002a). By using O2 plasma treatment, the UF property of the PAN (Tran et al. 2007) and PET (Touflk et al. 2002) track membranes could be improved with the enhancement of the membrane flux. Meanwhile, their rejection of albumin and dextrans was almost maintained. 

Figure 4.12 Hydrogen fluxes dependency from trans-membrane pressure drop (Gallucci et al., 2008).

High-Pe devices are dominated by transmembrane water transport, and detailed discussion must be left to the above cited references. However, it is important to recognize their primary function is to remove water and undesired solutes while retaining one solute which is desired. Rejection of the desired product increases toward an asymptote as water flux increases, and one should operate near this asymptote if at all possible. The relation between water flux and rejection is perhaps best determined experimentally. However, as water flux increases the rejected solute concentration at the interface between the feed stream and the membrane also increases. This process, usually known as concentration polarization, typically produces a significant increase in osmotic pressure which acts to reduce the flow. Polarization is a complex process, but to a good approximation the trans-membrane water velocity is given by... 

Trans-membrane water flux q" Heat flux T. Temperature... 

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