Ultrafiltration operating modes

In cases where a dilute solution containing small quantities of solids which tend to blind the filter cloth is to be filtered, cross-flow filtration is extensively used. This is the normal mode of operation for ultrafiltration using membranes, a topic which is discussed in Chapter 8. 

Cabral and coworkers [253] have investigated the batch mode synthesis of a dipeptide acetyl phenylalanine leucinamide (AcPhe-Leu-NH2) catalyzed by a-chymotrypsin in a ceramic ultrafiltration membrane reactor using a TTAB/oc-tanol/heptane reverse micellar system. Separation of the dipeptide was achieved by selective precipitation. Later on the same group successfully synthesized the same dipeptide in the same reactor system in a continuous mode [254] with high yields (70-80%) and recovery (75-90%). The volumetric production was as high as 4.3 mmol peptide/l/day with a purity of 92%. The reactor was operated for seven days continuously without any loss of enzyme activity. Hakoda et al. [255] proposed an electro-ultrafiltration bioreactor for separation of RMs containing enzyme from the product stream. A ceramic membrane module was used to separate AOT-RMs containing lipase from isooctane. Application of an electric field enhanced the ultrafiltration efficiency (flux) and it further improved when the anode and cathode were placed in the permeate and the reten-tate side respectively. 

The cross-, co- and counter-flow schemes are illustrated in Figure 4.17, together with the concentration gradient across a median section of the membrane. It follows from Figure 4.17 that system performance can be improved by operating a module in an appropriate flow mode (generally counter-flow). However, such improvements require that the concentration at the membrane permeate surface equals the bulk concentration of the permeate at that point. This condition cannot be met with processes such as ultrafiltration or reverse osmosis in which the permeate is a liquid. In these processes, the selective side of the membrane faces the... 

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.]... 

The enzymatic system used for the continuous production of Mn3+-malonate is presented in Fig. 10.3. It is composed by a stirred tank reactor (200-mL working volume) operated in continuous mode coupled to a 10 kDa cutoff ultrafiltration membrane (Prep/Scale-TFF Millipore), which permits the recycling of the enzyme to the reaction vessel. The enzyme was recycled in a recycling feed flow ratio of 12 1. 

The ultrafiltration process is operated in a batch mode at a temperature of about 50 C. Ceramic membranes with 0.1 or 0.2 pm pore diameter enable processing of this highly viscous and concentrated raw or pasteurized whole milk due to their inherent mechanical strength. The viscosity of the concentrate has been found to increase exponentially with the rise of protein content in the precheese. Polymeric membranes have also been considered not suitable for this process in view of their structural compaction under pressure and their difficulty of cleaning. 

Membrane modules can be operated in the dead-end or cross-flow modes (see Figure 18.8). Dead-end ultrafiltration is used mostly for laboratory-scale applications and industrial ultrafiltration processes are usually carried out in the cross-flow mode. The main advantage of cross-flow ultrafiltration is the lower extent of concentration polarization. The cross-flow mode also allows recirculation of the retentate stream to the feed tank followed by its mixing with fresh feed that leads to several operational advantages. 

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. 

The concentration polarization effects for hollow fibers is often quite small because of the low solvent flux. Hence, Eq. (13.11-1) describes the flux. In order to increase the ultrafiltration solvent flux, cross-flow of fluid past the membrane can be used to sweep away part of the polarized layer, thereby increasing in Eq. (13.11-4). Higher velocities and other methods are used to increase turbulence, and hence, k. In most cases the solvent flux is too small to operate in a single-pass mode. It is necessary to recirculate the feed by the membrane with recirculation rates of 10/1 to 100/1 often used. 

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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