Ultrafiltration membranes retention ratings

Figure 6.3 Ultrafiltration membranes are rated on the basis of nominal molecular weight cut-off, but the shape of the molecule to be retained has a major effect on retentivity. Linear molecules pass through a membrane, whereas globular molecules of the same molecular weight may be retained. The table shows typical results obtained with globular protein molecules and linear polydextran for the same polysulfone membrane [8]...

The same nanofiltration experiments were performed with a 50-A ultrafiltration membrane (available from US Filter/Membralox, Warrendale, PA,USA), this time with a monodentate phosphite ligand (24) used for comparison and toluene as the solvent (Table V). Both higher retentions and flux rates for the dendrimers were obtained relative to what was observed with the reverse osmosis membranes. Dendrophite G4 was used in three subsequent reactions carried out with this procedure. 

Rhodium Retention and Flux Rate Measurements using a 50 A Ultrafiltration Membrane... 

NADH may be very high, but then the reaction should be started with a very small amount of NADH and the reaction rate would be very low. A sufficiently high rate is obtained if the concentration of NAD(H) is around its Kj but at those concentrations it will be necessary to recover it after the reaction. A solution to this problem is the enzyme membrane reactor (Fig. 7.4). This is a kind of CSTR (continuous stirred tank reactor) that retains the enzyme and the cofactor using an ultrafiltration membrane. This membrane has a molecular weight cut-off of about 10,000. Enzymes usually have a molecular mass of 25,000-250,000, but the molecular mass of NAD(H) is much too low for retention. Hence, it is first derivatized with polyethylene glycol (PEG 20,000). The reactivity of NAD(H) is usually not greatly affected by the derivatization with this soluble polymer. As a result, alanine can now be produced continuously by high concentrations of both enzymes and of NAD(H) in this reactor. 

The increase in filtration rate allows the use of microporous or ultrafiltration membranes for retention of micrometre sized particulates thus increasing the degree of separation of solid and liquid. 

Membrane extraction offers attractive alternatives to conventional solvent extraction through the use of dialysis or ultrafiltration procedures (41). The choice of the right membrane depends on a number of parameters such as tlie degree of retention of the analyte, flow rate, some environmental characteristics, and tlie analyte recovery. Many early methods used flat, supported membranes, but recent membrane technology has focused on the use of hollow fibers (42-45). Although most membranes are made of inert polymers, undesired adsorption of analytes onto the membrane surface may be observed, especially in dilute solutions and when certain buffer systems are applied. 

Polymer-Assisted Ultrafiltration of Boric Acid. The Quickstand (AGT, Needham, MA) filtration apparatus is pictured schematically in Figure 3. The hollow fiber membrane module contained approximately 30 fibers with 0.5 mm internal diameter and had a nominal molecular weight cut-off of 10,000 and a surface area of 0.015 m2. A pinch clamp in the retentate recycle line was used to supply back pressure to the system. In a typical run, the transmembrane pressure was maintained at 25 psig and the retentate and permeate flow rates were 25 ml/min and 3 ml/min, respectively. Permeate flux remained constant throughout the experiments. 

Diafiltration is a variation of ultrafiltration, in which fresh solvent is added to the feed solution to replenish the volume ultrafiltered, and in the process washes small molecules such as salts away from the retained macromolecules. Using appropriate replenishing solutions, diafiltration is a common procedure to perform buffer exchange of proteins. Alternatively, a dilute solution may be first ultrafiltered to concentrate the feed material, then diafiltered to purify the retentate. It is sometimes possible to fractionate a mixture of macrosolutes by sequential diafiltration with a series of membranes of progressively lower molecular weight cutoff ratings. 

Several factors can affect the retention properties of the membrane and some of these will be discussed here. During ultrafiltration the transport of solute to the surface of the filter Is faster than the rate at which permeation through the membrane occurs. This is further complicated, as ultrafiltration progresses, by an increase in the concentration of retained molecules at the membrane. Both events contribute to the phenomenon called concentration polarization. This effectively introduces a second layer of membrane , and as a consequence the retention characteristics of the system are altered. The build-up of solute can be reduced by introducing some form of agitation at the filter surface. However. this procedure does not seem to be effective against the gel-type layers formed by proteins. Various procedures have been suggested to slow down this build-up of solute the solution can be diluted with an appropriate solvent the ultrafiltration process can be interrupted and the flow reversed momentarily a low operation pressure could be used. 

Figure 2 A semiquantitative representation of the change in retention of a membrane filter with flow rate due to concentration polarization (high flow-rate domain) and aggregation in the filtration cell (low flow-rate domain). Only an intermediate flow-rate window is usable for size fractionation with a minimal artifact. (Reprinted with permission from Buffle J, Perret D, and Newman M (1992) The use of filtration and ultrafiltration for size fractionation of aquatic particles, colloids and macromolecules. In Buffle J and van Leeuwen HP (eds.) Environmental Particles. lUPAC Environmental Chemistry Series, vol. 1, pp. 171-230. Chelsea, Ml Lewis Publishers Lewis Publishers, an imprint of CRC Press, Boca Raton, FL.)...

The above analysis/description of solvent flux and macrosolute rejection/retention/ttansmission far an ultra-flllration membreme was carried out in the context of a pseudo steady state analysis in a batch cell (Figure 6.3.26 (a)). Back diffusion of the macrosolute from the feed solution-membrane interface to the bulk solution takes place by simple difflision against the small bulk flow parallel to the force direction. The resulting mass-transfer coefficients for macrosolutes will be quite small the solvent flux levels achievable will be quite low. For practically useful ultrafiltration rates, the mass-transfer coefficient is increased via different flow configurations with respect to the force. 

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