Applied transmembrane pressure

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... 

The NF/LPRO pilot plant was supplied by Sepratech (Separation Technoloy, INC, US), and consisted of a feed tank, a pump and planar module, as detailed in Fig. 5. All studies were done using a low conversion rate (5%) and a high tangential flow rate ( 4 ms-1) in order to minimize the polarization concentration effects. The applied transmembrane pressures were in the range of 0-25 bar. The temperature was maintained at 25°C. 

Normalized permeate flow (NPF) is a function of the average applied transmembrane pressure, the osmotic pressures of the feed and permeate, and temperature, as shown in Equation 11.1. Factors that cause an increase or decrease in the NPF are discussed below. 

In the case of the osmotic-pressure model, the rigorous theory allowed the conclusion that at high applied transmembrane pressure, the permeate flux increased as a cube root of the pressure, so that the limiting flux was never reached ... 

Applied Transmembrane Pressure and Velocity along the Membrane Surface. These process parameters may be varied extensively at the laboratory scale to achieve the ophmal combinahon of process parameters for a given beer or application. Variations in industrial installahons are normally possible, but limited due to given sizes of the connechng piping, pump capacity etc., which might not allow the process to be conducted under desired process condihons for each individual application. 

Generally, the pure solvent transporting through porous UF membranes is directly proportional to the applied transmembrane pressure (AP). The Kozeny-Carman and Hagen-Poiseuille equations describe the convection flow (J ) as follows (49) ... 

Highly efficient enzyme membrane reactors can be also produced by immobilizing enzymes in membranes or in hollow fibers. For example, enzymes can be confined in the porous support matrix of an asymmetric capillary membrane, while substrate-containing solution flows through the fiber lumen. The dense skin layer at the lumen wall should be impermeable to the enzyme molecules. The latter diffuse through the inner wall of the fiber to the enzyme into the spongy part, where the conversion takes place. Applied transmembrane pressure and axial flow rate are parameters that contribute to control of the reactor performance. 

Physical sieving is believed to be the major rejection mechanism for MF with water convecting through the membrane due to an applied transmembrane pressure. The deposit or cake on the membrane can act as a self-rejecting layer, and retain even smaller particles or solutes than would be expected to be removed given the pore size of the membrane ( dynamic membrane ). Thus a fouled MF membrane may have UF rejection characterisncs and flux may decline significantly due to the build-up of this deposit. 

In RO, the osmotic pressure of a solution has to be overcome by an applied transmembrane pressure to achieve solvent flux and separation. Recovery (ratio of product/feed) has a high impact on flux and rejection, and both decrease with increasing recovery. 

In a key finding, they showed that the adsorption resistance was of the same order of magnitude as the membrane resistance. Surprisingly, the osmotic pressure was negligible in comparison to the applied transmembrane pressure. 

PT is the applied transmembrane pressure p is the viscosity of the fluid permeating the membrane Ax is the length of the channel (the membrane skin thickness)... 

Figure 8.12 Degradation study of 2,2 -dichloiobiphenyl (DCB) vs. residence time under pressure-induced operation. Top right shows the applied transmembrane pressure vs. residence time. Initial DCB concentration = 15 mg/L, solvent =1/1 (v/v) of ethanol and water, solvent permeability = 0.50 X 10 cm s cm bar. ...

Figure 12.11 Water flux vs. applied transmembrane pressure, AP, for PVDF (GVHP and HVHP) and PTFE (Gore, 0.2-p.m mean pore size) membranes at 25°C. (Adapted from Garcfa-Payo et al., 2000a.)...

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