Ultrafiltration diafiltration mode

FIGURE 44 Closed-loop cascade HPTFF system operating in diafiltration mode with buffer regeneration by a conventional ultrafiltration unit. [Adapted from Zydney, A. L. and van Reis, R. (2001). In Membrane Separation in Biotechnology (W. H. Wong, ed.), Marcel Dekker, New York.]... 

At the end of the fermentation, protein is separated from cell mass by filtration, typically with a rotary vacuum filter. The crude enzyme concentration is often lower than suitable for commercial applications, so the concentration of enzyme is increased by ultrafiltration. Most cellulase enzymes have a molecular weight of 25,000 to 75,000 and are retained by ultrafiltration membranes of 5000 molecular weight cutoff. The membranes permit the passage of low molecular weight salts, sugars and other impurities, and are sometimes operated in a diafiltration mode to increase the purity of the enzymes. The crude broth at this point is dark brown. 

An ultrafiltration setup can also be used to wash out lower molecular impurities by diafiltration. In this mode of operation, water (tap water or deionized water) is added to the ultrafiltration concentrate and the liquid is concentrated to compensate for the dilution. With this approach, the content of low molecular impurities can be reduced. 

There are two other aspects of importance in practical ultrafiltration processing time and the membrane area. A brief treatment of processing time with a view to minimizing it will be considered now. The processing time depends on a variety of factors the total volume to be processed the concentration of retained solids, especially if it leads to gel polarized operation (equation (6.3.143)) the mode of operation (continuous, batch, etc.) (Cherytm, 1986). One approach suggested by Ng et al. (1976) employs the gel-polarized condition and perfect rejection of the protein i for continuous diafiltration operation ... 

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