Ultrafiltration Membrane Processes

Membrane-retained components are collectively called concentrate or retentate. Materials permeating the membrane are called filtrate, ultrafiltrate, or permeate. It is the objective of ultrafiltration to recover or concentrate particular species in the retentate (eg, latex concentration, pigment recovery, protein recovery from cheese and casein wheys, and concentration of proteins for biopharmaceuticals) or to produce a purified permeate (eg, sewage treatment, production of sterile water or antibiotics, etc). Diafiltration is a specific ultrafiltration process in which the retentate is further purified or the permeable sohds are extracted further by the addition of water or, in the case of proteins, buffer to the retentate. 

Fouling is controlled by selection of proper membrane materials, pretreatment of feed and membrane, and operating conditions. Control and removal of fouling films is essential for industrial ultrafiltration processes. 

Ultra filtration. This process removes macromolecules, microorganisms, particulate matter, and pyrogens using a thin, selectively permeable membrane. Ultrafiltration caimot remove ions from water and is generally employed as a polishing process. 

Ultrafiltration is one of the most widely used of the pressure-driven membrane separation processes. The solute retained or rejected by ultrafiltration membranes are those with... 

A limitation to the more widespread use of membrane separation processes is membrane fouling, as would be expected in the industrial application of such finely porous materials. Fouling results in a continuous decline in membrane penneation rate, an increased rejection of low molecular weight solutes and eventually blocking of flow channels. On start-up of a process, a reduction in membrane permeation rate to 30-10% of the pure water permeation rate after a few minutes of operation is common for ultrafiltration. Such a rapid decrease may be even more extreme for microfiltration. This is often followed by a more gradual... 

Ultrafiltration processes (commonly UF or UF/DF) employ pressure driving forces of 0.2 to 1.0 MPa to drive liquid solvents (primarily water) and small solutes through membranes while retaining solutes of 10 to 1000 A diameter (roughly 300 to 1000 kDa). Commercial operation is almost exclusively run as TFF with water treatment applications run as NFF. Virus-retaining filters are on the most open end of UF and can be run as NFF or TFF. Small-scale sample preparation in dilute solutions can be run as NFF in centrifuge tubes. 

Membrane Ultrafiltration Membrane ultrafiltration is often one of the favored unit operations used for the isolation and concentration of biomolecules because they can be easily scaled up to process large feed volumes at low costs. Toward the end of an ultrafiltration operation, additional water or buffer is added to facilitate the passage of... 

In 1966, Cadotte developed a method for casting mlcroporous support film from polysulfone, polycarbonate, and polyphenylene oxide plastics ( ). Of these, polysulfone (Union Carbide Corporation, Udel P-3500) proved to have the best combination of compaction resistance and surface microporosity. Use of the mlcroporous sheet as a support for ultrathin cellulose acetate membranes produced fluxes of 10 to 15 gfd, an increase of about five-fold over that of the original mlcroporous asymmetric cellulose acetate support. Since that time, mlcroporous polysulfone has been widely adopted as the material of choice for the support film in composite membranes, while finding use itself in many ultrafiltration processes. 

In addition to equipment used in the actual fractionation processes, a variety of other items are needed. In particular it should be possible to change buffers quickly and to concentrate protein solutions with ease. These operations require such things as dialysis membranes, ultrafiltration cells, and gel-exclusion columns of various sizes. 

We can use the same filtration principle for the separation of small particles down to small size of the molecular level by using polymeric membranes. Depending upon the size range of the particles separated, membrane separation processes can be classified into three categories microfiltration, ultrafiltration, and reverse osmosis, the major differences of which are summarized in Table 10.2. 

The four developed industrial membrane separation processes are microfiltration, ultrafiltration, reverse osmosis, and electrodialysis. These processes are all well established, and the market is served by a number of experienced companies. 

B. Baum, W. Holley, Jr and R.A. White, Hollow Fibres in Reverse Osmosis, Dialysis, and Ultrafiltration, in Membrane Separation Processes, P. Meares (ed.), Elsevier, Amsterdam, pp. 187-228 (1976). 

The layer of solution immediately adjacent to the membrane surface becomes depleted in the permeating solute on the feed side of the membrane and enriched in this component on the permeate side. Equivalent gradients also form for the other component. This concentration polarization reduces the permeating component s concentration difference across the membrane, thereby lowering its flux and the membrane selectivity. The importance of concentration polarization depends on the membrane separation process. Concentration polarization can significantly affect membrane performance in reverse osmosis, but it is usually well controlled in industrial systems. On the other hand, membrane performance in ultrafiltration, electrodialysis, and some pervaporation processes is seriously affected by concentration polarization. 

Figure 6.1 Schematic illustrating in vivo membrane separation processes. Left microdialysis sampling probe with curved lines indicating analyte tortuous diffusion through the tissue into and out of the probe. Right ultrafiltration representing interstitial fluid flow into the membrane device. Tissue is represented with cells and hlood vessels (dark circles in light circles).

Sometimes the surfactants used for cleaning in manufacturing processes create undesirable O/W emulsions. This can be difficult to deal with since emulsification is usually an important aspect of a cleaning process. If the process or the detergent formulation cannot be adjusted to prevent the undesirable emulsion formation then a separate demulsification/separation step may be needed. In some cases these emulsions can be broken by separating out and concentrating the dispersed phase, such as by membrane ultrafiltration [454],... 

Mass-transport limitations are common to all processes involving mass transfer at interfaces, and membranes are not an exception. This problem can be extremely important both for situations where the transport of solvent through the membrane is faster and preferential when compared with the transport of solute(s) - which happens with membrane filtration processes such as microfiltration and ultrafiltration - as well as with processes where the flux of solute(s) is preferential, as happens in organophilic pervaporation. In the first case, the concentration of solute builds up near the membrane interface, while in the second case a depletion of solute occurs. In both situations the performance of the system is affected negatively (1) solute accumulation leads, ultimately, to a loss of selectivity for solute rejection, promotes conditions for membrane fouling and local increase of osmotic pressure difference, which impacts on solvent flux (2) solute depletion at the membrane surface diminishes the driving force for solute transport, which impacts on solute flux and, ultimately, on the overall process selectivity towards the transport of that specific solute. 

The excellent review by van Reis and Zydney [1] provides a comprehensive discussion of all major uses of membranes for processing of large molecules. This chapter will be essentially focused on the use of ultrafiltration for the concentration and fractionation of proteins, and the development of membrane chromatography systems. 

Ultrafiltration, which uses selective membranes to separate materials on the basis of different molecular sizes, has become a valuable separation tool for a wide variety of industrial processes, particularly in the separation of dispersed colloids or suspended solids. In many cases where a high degree of separation is desired, a batch ultrafiltration process is used because it is the most economical in terms of membrane area. 

Then, water is added continuously while the filtration continues at nearly constant flux. This latter filtration stage, when water is added to maintain a constant flux, is referred to as diafiltration. Proper choice of the diafiltration starting time can minimize the required membrane area, which is often the major part of the capital cost in an ultrafiltration process. 

In this paper, complete mathematical formulations for correlating the time cycles with other operating conditions are presented. The optimum diafiltration cycle (in terms of volume fraction), and the total cycle time are solved as functions of membrane area, flux, initial volume and recovery. Convenient charts, which can be used as a guide in designing or modifying an ultrafiltration process, are provided. 

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