Membrane filtration concentration polarization boundary layer

FIGURE 8.1 Schematics of the concentration polarization boundary layer for membrane filtration. 

Prevention or minimization of fouling and concentration polarization represents one of the main challenges that confronts membrane processing in general and membrane filtration of beer in particular. Various approaches have been developed to control membrane fouhng and increase the permeate flux in CMF, including membrane selection and modification, boundary layer control, use of turbulence inducers, or pretreatment of the feed. The two main strategies that are currently used in beer CMF are proper membrane selection and boundary layer control. 

During TFF, species retained by the membrane form a concentrated boundary layer, called concentration polarization layer, on the surface of the membrane that creates a resistance to the filtrate flow and reduces flux [11], The effect of polarization is a flux—usually called limiting flux—that does not change with TMP above a critical pressure as shown in Figure 14.2. 

In a membrane filtration process, the retentate in suspension may build up a high concentration adjacent to the membrane surface forming a dyamic boundary layer (a gel layer). This concentration gradient becomes a driving force to pull the retentate from the boundary layer back to the bulk flow. This phenomenon is referred to as concentration polarization. The accumulation of retentate at the membrane surface will result in a hydraulic resistance that may reduce the permeability of the membrane. This phenomenon is called fouling. Membrane fouling is a common phenomenon observed in the operation of any membrane filtration process, which leads to a reduction in permeate flux and selectivity. 

With reverse osmosis and other filtration processes, a basic problem is concentration polarization. On the feed side of the membrane the solute can be enriched as water permeates through the membrane leaving a higher concentration of solute (salt) at the membrane surface. This is a problem particularly for static systems, but can also be a problem for dynamic systems where the flow rate past the membrane does not prevent the bormdary layer from forming. A similar polarization can occur on the permeate side of the membrane, but is generally less of a concern for high-reject dynamic membrane systems. This situation is similar to boundary layer heat and mass transfer problems well covered in the literature. 

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