Membranes in ultrafiltration

Flat membranes from these polymers were tested for desalination and found to be of low salt rejecting type. Hov/ever, the copolymer was found to possess more than 90 per cent rejection for 1 per cent dextran solution with 10.0 gfd water flux at 200 psi thus indicating the possibility of application of these membranes in ultrafiltration and hemodialysis. 

The theory of permeation through microporous membranes in ultrafiltration and microfiltration is much less developed and it is difficult to see a clear path forward. Permeation through these membranes is affected by a variety of hard-to-compute effects and is also very much a function of membrane structure and composition. Measurements of permeation through ideal uniform-pore-diameter membranes made by the nucleation track method are in good agreement with theory. Unfortunately, industrially useful membranes have nonuniform tortuous pores and are often anisotropic as well. Current theories cannot predict the permeation properties of these membranes. 

G. Pozniak, 1. Gancarz, and W. Tylus. Modified poly(phenylene oxide) membranes in ultrafiltration and micellar-enhanced ultrafiltration of organic compounds. Desalination, 198(l-3) 215-224, October 2006. 

Appropriate membrane technologies in the context of solid-liquid separation are microfiltration and the use of the more open membranes in ultrafiltration. Other membrane processes, including the pressure-driven processes of hyperfihration and reverse osmosis, are concerned primarily with the removal of dissolved species fi om a solvent and shall not be considered. The boundary between the finer end of microfiltration and the coarser end of ultrafiltration is not sharp, and ultrafiltration is used for fine colloid-liquid separation. The start of the regions of ndcrofiltratian, ultrafihration and hyperfiltration occurs, approximately, with the fihration of particles of diameter 10, 0.1 and 0.005 rm, respectively. 

Macromolecules are retained by the membrane in ultrafiltration whereas tow molecular weight components permeate through freely. Because the main contribution to the osmotic pressure of a solution arises from the low molecular weight solutes (the concentration of these being the same in the feed and permeate), the osmotic pressure of the retained macromolecules is often neglected. 

For example, among the collodion gels used as membranes in ultrafiltration, those with average pore diameters (determined by measuring the rate of flow of water through the membranes under pressure Elford, 1937 Ferry, 1936) below 400 A. are transparent, from 400 to 1200 A. opalescent, and above 1200 A. opaque. 

Wet spinning of this type of hoUow fiber is a weU-developed technology, especiaUy in the preparation of dialysis membranes for use in artificial kidneys. Systems that spin more than 100 fibers simultaneously on an around-the-clock basis are in operation. Wet-spun fibers are also used widely in ultrafiltration appUcations, in which the feed solution is forced down the bore of the fiber. Nitto, Asahi, Microgon, and Romicon aU produce this type of fiber, generaUy with diameters of 1—3 mm. 

A second factor determining module selection is resistance to fouling. Membrane fouling is a particularly important problem in Hquid separations such as reverse osmosis and ultrafiltration. In gas separation appHcations, fouling is more easily controlled. Hollow-fine fibers are notoriously prone to fouling and can only be used in reverse osmosis appHcations if extensive, costiy feed-solution pretreatment is used to remove ah. particulates. These fibers caimot be used in ultrafiltration appHcations at ah. 

A key factor determining the performance of ultrafiltration membranes is concentration polarization due to macromolecules retained at the membrane surface. In ultrafiltration, both solvent and macromolecules are carried to the membrane surface by the solution permeating the membrane. Because only the solvent and small solutes permeate the membrane, macromolecular solutes accumulate at the membrane surface. The rate at which the rejected macromolecules can diffuse away from the membrane surface into the bulk solution is relatively low. This means that the concentration of macromolecules at the surface can increase to the point that a gel layer of rejected macromolecules forms on the membrane surface, becoming a secondary barrier to flow through the membrane. In most ultrafiltration appHcations this secondary barrier is the principal resistance to flow through the membrane and dominates the membrane performance. 

One unique appHcation area for PSF is in membrane separation uses. Asymmetric PSF membranes are used in ultrafiltration, reverse osmosis, and ambulatory hemodialysis (artificial kidney) units. Gas-separation membrane technology was developed in the 1970s based on a polysulfone coating appHed to a hoUow-fiber support. The PRISM (Monsanto) gas-separation system based on this concept has been a significant breakthrough in gas-separation... 

P. Dejmek, "PermeabiHty of the Concentration Polarization Layer in Ultrafiltration of Macro Molecules," Proceedings of the International Symposium, Separation Processes by Membranes, Paris, Mar. 13—14,1975. 

Cellulose acetate, the earhest reverse osmosis membrane, is still widely used. Asymmetric polyamide and thin-film composites of polyamide and several other polymers have also made gains in recent years whereas polysulfone is the most practical membrane material in ultrafiltration appHcations. 

Crossflow Filters - These are usually membrane-type filters used for ultrafiltration. In the field of biotechnology these types of filters are used in ultrafiltration devices used in concentrating solutions, and performing buffer exchanges. 

The statistical properties of polymer chains in a quenched random medium have been the subject of intensive investigations during the last decades, both theoretically [79-89] and experimentally [90-96], because diffusion in such media is of great relevance for chromatography, membrane separation, ultrafiltration, etc. 

The most important application of semi-permeable membranes is in separations based on reverse osmosis. These membranes generally have pores smaller than 1 nm. The pressure across the semi-permeable membranes for reverse osmosis is generally much larger than those for ultrafiltration, for example. This is because reverse osmosis is usually used for small molecules which have a much higher osmotic pressure, because of the higher number density, than the colloids separated in ultrafiltration. As a result reverse osmosis membranes have to be much more robust than ultrafiltration membranes. Since the focus of our discussion in this chapter will be on reverse osmosis based separations, we will describe these membranes in greater detail. 

Ultrafiltration of micellar solutions combines the high permeate flows commonly found in ultrafiltration systems with the possibility of removing molecules independent of their size, since micelles can specifically solubilize or bind low molecular weight components. Characteristics of this separation technique, known as micellar-enhanced ultrafiltration (MEUF), are that micelles bind specific compounds and subsequent ultrafiltration separates the surrounding aqueous phase from the micelles [70]. The pore size of the UF membrane must be chosen such, that the micelles are retained but the unbound components can pass the membrane freely. Alternatively, proteins such as BSA have been used in stead of micelles to obtain similar enan-tioselective aggregates [71]. 

The discussion so far implies that membrane materials are organic polymers, and in fact most membranes used commercially are polymer-based. However, in recent years, interest in membranes made of less conventional materials has increased. Ceramic membranes, a special class of microporous membranes, are being used in ultrafiltration and microfiltration applications for which solvent resistance and thermal stability are required. Dense, metal membranes, particularly palladium membranes, are being considered for the separation of hydrogen from gas mixtures, and supported liquid films are being developed for carrier-facilitated transport processes. 

Early ultrafiltration membranes had thin surface retentive layers with an open structure underneath, as shown in Fig. 20-62. These membranes were prone to defects and showed poor retention and consistency. In part, retention by these membranes would rely on large retained components in the feed that polarize or form a cake layer that plugs defects. Composite membranes have a thin retentive layer cast on top of a microfiltration membrane in one piece. These composites demonstrate consistently high retention and can be integrity-tested by using air diffusion in water. 

Until this point, the sample preparation techniques under discussion have relied upon differences in polarity to separate the analyte and the sample matrix in contrast, ultraflltration and on-line dialysis rely upon differences in molecular size between the analyte and matrix components to effect a separation. In ultrafiltration, a centrifugal force is applied across a membrane filter which has a molecular weight cut-off intended to isolate the analyte from larger matrix components. Furusawa incorporated an ultrafiltration step into his separation of sulfadimethoxine from chicken tissue extracts. Some cleanup of the sample extract may be necessary prior to ultrafiltration, or the ultrafiltration membranes can become clogged and ineffective. Also, one must ensure that the choice of membrane filter for ultrafiltration is appropriate in terms of both the molecular weight cut-off and compatibility with the extraction solvent used. 

Cools and Janssen [545] studied the effect of background salt on the permeability of warfarin through octanol-impregnated membranes (Millipore ultrafiltration filters, VSWP, 0.025-pm pores). At a pH where warfarin was in its ionized form, it was found that increasing background salt increased permeability (Fig. 7.7). This... 

In ultrafiltration, the effluent is passed across a semiper-meable membrane (see Chapter 10). Water passes through the membrane, while submicron particles and large molecules are rejected from the membrane and concentrated. The membrane is supported on a porous medium for strength, as discussed in Chapter 10. Ultrafiltration is used to separate very fine particles (typically in the range 0.001 to 0.02 xm), microorganisms and organic components with molar mass down to 1000 kg kmol. Pressure drops are usually in the range 1.5 to 10 bar. 

The third membrane process that has been used successfully in water purification is ultrafiltration. As with reverse osmosis, the driving force is pressure. However, in ultrafiltration the separation is merely based on the size of the molecules. Here the passage of molecules having molecular weights above 100 can be deterred. The pressure differences are usually between 20 and 50 psi (1.4-3.5 kg/cm2). 

Figure 6.6 ULtrafiLtration separates molecules based on size and shape, (a) Diagrammatic representation of a typical laboratory-scale ultrafiltration system. The sample (e.g. crude protein solution) is placed in the ultrafiltration chamber, where it sits directly above the ultrafilter membrane. The membrane, in turn, sits on a macroporous support to provide it with mechanical strength. Pressure is then applied (usually in the form of an inert gas), as shown. Molecules larger than the pore diameter (e.g. large proteins) are retained on the upstream side of the ultrafilter membrane. However, smaller molecules (particularly water molecules) are easily forced through the pores, thus effectively concentrating the protein solution (see also (b)). Membranes that display different pore sizes, i.e. have different molecular mass cut-off points, can be manufactured, (c) Photographic representation of an industrial-scale ultrafiltration system (photograph courtesy of Elga Ltd, UK)...

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