Cross-flow ultrafiltration system

Also included are sections on how to analyze mechanisms that affect flux feature models for prediction of micro- and ultrafiltration flux that help you minimize flux decline. Descriptions of cross-flow membrane filtration and common operating configurations clarify tf e influence of important operating parameters on system performance. Parameters irdlucnc irxj solute retention properties during ultrafiltration arc identified and discussed or treated in detail. 

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 difference between conventional dead-end filtration and cross-flow filtration is the configuration of the system. For large-scale operations, only cross-flow filtration will be used. The membranes for miocrofiltration as well as ultrafiltration are commonly utilized in a variety of filtration devices. There are three basic types of tangential flow filtration devices plate and frame, hollow fiber, and spiral wound membranes. 

Asymmetrical flow-FFFA (A-Fl-FFF) was introduced by Wahlund and Giddings in 1987 [47]. The same system was independently suggested slightly earlier by Granger et al. [237,248], but their application of the technique suffered from a lack of a primary relaxation step preceding separation [47]. A-Fl-FFF is notable for a channel which has only one permeable wall so that the solvent can leave the channel only via the accumulation wall and thus generates a cross-flow. The permeable wall is usually a sintered metal plate or ceramic frit covered by an ultrafiltration membrane (see Fig. 20). 

Karthik V, DasGupta S, and De S. Modeling and simulation of osmotic pressure controlled electro-ultrafiltration in a cross-flow system. J Membr Sci 2002 199 29 0. 

Pleated ultrafiltration module. The axial filter is convenient for experiments, in that volumes small relative to ordinary ultrafiltration systems can be studied and in that pumping of viscous solutions is limited to that necessary to replace filtrate or concentrate bled from the chamber, rather than that necessary to maintain desired cross flow velocities. There is no obvious reason it could not be scaled up to moderate sizes for practical separations, but so far as we know, no large-volume axial filters are available. For the operations of interest, any of the commercial ultrafiltration systems would be candidates. We have tested one module, recently developed by Gelman, which incorporates a pleated membrane (Figure 5), with somewhat more open feed passages than those of spiral-wound membranes, and which allows backwashing. Other applications of the module were discussed at this symposium by A. Korin in a paper coauthored by G. B. Tanny, and a written account is presumably in these proceedings. 

Further optimization of the polishing step is clearly necessary. The mediocre fluxes in cross-flow and axial filtration suggest that systems allowing backwash at frequent intervals may be necessary. Many commercial ultrafiltration systems, including the one used here, have this capability. 

Gel polarized ultrafiltration was recently analyzed for cross flow and unstirred batch cell systems by Trettin and Doshi (1980 a,b). We have shown in these papers that the widely used film theory does not predict the limiting flux accurately. The objective of this paper is to derive an expression for the permeate flux when the pressure independent ultrafiltration of macro-molecular solutions is osmotic pressure limited. We will also attempt to distinguish between gel and osmotic pressure limited ultrafiltration of macromolecular solutions. 

An early work considering osmotic pressure in the ultrafiltration of macromolecular solutions was done by Blatt, et al,. (1970), who employed a theory which had been developed for cross flow reverse osmosis systems. They essentially suggested that the film theory relationship given by Eq. (2) could be solved simultaneously with Eq. (1) to predict permeate rates, where the... 

With the ultrafiltration of macromolecular solutions in cross flow systems such as thin channel or tubular systems it is usually the procedure to measure average flux rates at steady state. Therefore, Eq. (81) may be integrated to give... 

In macromolecular ultrafiltration, 2is pressure is increased, permeate flux first Increases and then In a large number of cases levels out and remains more or less pressure Independent. This could be due to the increase In solute concentration at the membrane surface such that either gel formation occurs or the corresponding osmotic pressure approaches the applied pressure. Limiting flux for the gel polarized case was recently analyzed for cross flow and unstirred batch cell systems by Trettln and Doshi (1980,a, b). In this paper we have analyzed the osmotic pressure limited ultrafiltration for the two systems. Our unstirred batch cell data and the literature cross flow data agree quite well with the theory. We have further shown that an unstirred batch cell system can be used to determine whether pressure Independent ultrafiltration of macromolecular solution is gel or osmotic pressure limited. Other causes for the observed pressure Independence may be present but are not considered in this paper. 

Flow FFF. The three flow FFF systems (Flow I, II, and III) are each constructed of two Lucite blocks with inset ceramic frits, a membrane, and a spacer. The frits provide a homogeneous distribution of cross-flow over the entire channel area. A membrane stretched over one frit surface serves as the accumulation wall and retains sample inside the channel. Systems Flow I and II were assembled with the YM-30 ultrafiltration membrane (Amicon, Danvers, MA) and Flow III with the Celgard 2400 polypropylene membrane (Hoechst-Celanese, Separations Products Division, Charlotte, NC). The frit of the second Lucite block defines the opposite (depletion) wall. The spacers, consisting of Teflon (Flow I) or Mylar (Flow II, Flow III), determine the channel thickness. 

Both the membrane and the housing in such UF units are made of polysulfone material. Epoxy-based adhesives are commonly used in such units. Such materials have enough heat resistance to be processed by hot water sterilization. External pressurized UF is used to avoid particle contamination in the filtered water. However, higher cross flow inside the hollow fibers cannot be created in the external pressurized-type UF units. Higher cross flow inside the fibers minimizes the number of particles adhering to the inside surface of the fibers. Fortunately, clogging of fibers due to the presence of particles in external pressurized ultrafiltration units is not a serious problem with the level of particle contamination found in conventional makeup systems used in the industry. 

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... 

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