Pore diameter ultrafiltration

MGmbranG Applications. Cellulose esters are effective in membrane applications. Ultrafiltration membranes based on cellulose nitrate were first described by Collander in 1924 (41). Cellulose nitrate films cast from mixtures of methyl acetate or acetone with glycerol and mixtures of ether and ethanol produce microfiltration and ultrafiltration membranes, respectively (42). Cellulose nitrate microfiltration membranes typically have 0.02-10 /um diameter pores and 4 X 10 - 15 cm /cm s atm permeability. Cellulose nitrate ultrafiltration membranes t5 ically have 0.003-0.03 )um diameter pores and (1-100) x 10 cm /cm s atm permeability (42). Nitrocellulose membranes are used in numerous biochemical and diagnostic applications and is discussed later. 

In reverse osmosis membranes, the pores are so smaH, in the range 0.5— 2 nm in diameter, that they ate within the range of the thermal motion of the polymer chains. The most widely accepted theory of reverse osmosis transport considers the membrane to have no permanent pores at aH. Reverse osmosis membranes are used to separate dissolved microsolutes, such as salt, from water. The principal appHcation of reverse osmosis is the production of drinking water from brackish groundwater or seawater. Figure 25 shows the range of appHcabHity of reverse osmosis, ultrafiltration, microfiltration, and conventional filtration. 

Fig. 25. Reverse osmosis, ultrafiltration, microfiltration, and conventional filtration are related processes differing principally in the average pore diameter of the membrane filter. Reverse osmosis membranes are so dense that discrete pores do not exist transport occurs via statistically distributed free volume areas. The relative size of different solutes removed by each class of membrane is illustrated in this schematic.

Both xerogels and aerogels are characteristically high surface area materials (surface areas normally exceed 500 m2/g). Unlike wet gels, many uses exist for dried gels due to their high surface areas and small pore sizes (typically, < 20 nm diameters). Examples include catalyst supports (12.). ultrafiltration media (18), antireflective coatings (19-20), and ultra-low dielectric constant films. (Lenahan, P. M. and Brinker, C. J., unpublished results.)... 

As discussed previously, the technique of microfiltration is effectively utilized to remove whole cells or cell debris from solution. Membrane filters employed in the microfiltration process generally have pore diameters ranging from 0.1 to 10 pm. Such pores, while retaining whole cells and large particulate matter, fail to retain most macromolecular components, such as proteins. In the case of ultrafiltration membranes, pore diameters normally range from 1 to 20 nm. These pores are sufficiently small to retain proteins of low molecular mass. Ultrafiltration membranes with molecular mass cut-off points ranging from 1 to 300 kDa are commercially available. Membranes with molecular mass cut-off points of 3,10, 30, 50, and 100 kDa are most commonly used. 

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)...

Ceraver s entry into the microfiltration and ultrafiltration field followed a completely different approach. In 1980, it became apparent that the type of product made by Ceraver for uranium enrichment, which was a tubular support and an intermediate layer with a pore diameter in the microfiltration range, might be declassified. Ceraver therefore developed a range of a-AljOj microfiltration membranes on an a-AljOs support with two key features first, the multichannel support and second, the possibility to backflush the filtrate in order to slow down fouling. 

Ultrafiltration hollow-fiber modules are usually made with a shell and tube configuration. The fibers are potted at both ends of the module with the fiber lumen open for recirculation of the process stream (Figure 21). Naturally, strainers or prefilters must be utilized to eliminate plugging of the fibers. At Nude-pore, it has been shown that larger diameter hollow fibers, 1.5 to 3mm in i.d., are much less prone to fouling. Fortunately, all UF hollow fiber systems can be back-washed and are amenable to a number of cleaning techniques. 

Since both aerogels and xerogels have high surface areas and small pore diameters they are used as ultrafiltration media, antireflective coatings, and catalysts supports. Final densi-fication is carried out by viscous sintering. 

Filtration can remove fine suspended solids and microorganisms, and microfiltration membranes of cellulose acetate or polyamides are available that have pores 0.1-20 /xm in diameter. Clogging of such fine filters is an ever-present problem, and it is usual to pass the water through a coarser conventional filter first. Ultrafiltration with membranes having pores smaller than 0.1 fim requires application of pressures of a few bars to keep the membrane surface free of deposits, water flows parallel to the membrane surfaces, with only a small fraction passing through the membrane. The membranes typically consist of bundles of hollow cellulose acetate or polyamide fibers set in a plastic matrix. Ultrafiltration bears some resemblance to reverse osmosis technology, described in Section 14.4, with the major difference that reverse osmosis can remove dissolved matter, whereas ultrafiltration cannot. 

Electron micrograph of an ultrafiltration membrane showing the two layers. Particles greater than 0.1 /xM in diameter are retained on the surface or within pores. Courtesy of the Millipore Corporation. 

The range of application of the three pressure-driven membrane water separation processes—reverse osmosis, ultrafiltration and microfiltration—is illustrated in Figure 1.2. Ultrafiltration (Chapter 6) and microfiltration (Chapter 7) are basically similar in that the mode of separation is molecular sieving through increasingly fine pores. Microfiltration membranes filter colloidal particles and bacteria from 0.1 to 10 pm in diameter. Ultrafiltration membranes can be used to filter dissolved macromolecules, such as proteins, from solutions. The mechanism of separation by reverse osmosis membranes is quite different. In reverse osmosis membranes (Chapter 5), the membrane pores are so small, from 3 to 5 A in diameter, that they are within the range of thermal motion of the polymer... 

Although reverse osmosis, ultrafiltration and microfiltration are conceptually similar processes, the difference in pore diameter (or apparent pore diameter) produces dramatic differences in the way the membranes are used. A simple model of liquid flow through these membranes is to describe the membranes as a series of cylindrical capillary pores of diameter d. The liquid flow through a pore (q) is given by Poiseuille s law as ... 

Membranes in the third group contain pores with diameters between 5 A and 10 A and are intermediate between truly microporous and truly solution-diffusion membranes. For example, nanofiltration membranes are intermediate between ultrafiltration membranes and reverse osmosis membranes. These membranes have high rejections for the di- and trisaccharides sucrose and raffi-nose with molecular diameters of 10-13 A, but freely pass the monosaccharide fructose with a molecular diameter of about 5-6 A. 

The most important property characterizing a microporous membrane is the pore diameter (d). Some of the methods of measuring pore diameters are described in Chapter 7. Although microporous membranes are usually characterized by a single pore diameter value, most membranes actually contain a range of pore sizes. In ultrafiltration, the pore diameter quoted is usually an average value, but to confuse the issue, the pore diameter in microfiltration is usually defined in terms of the largest particle able to penetrate the membrane. This nominal pore diameter can be 5 to 10 times smaller than the apparent pore diameter based on direct microscopic examination of the membrane. 

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. 

Figure 7.4 Membrane pore diameter from bubble point measurements versus Bacillus prodigiosus concentration [1], Reprinted from W.J. Elford, The Principles of Ultrafiltration as Applied in Biological Studies, Proc. R. Soc. London, Ser. B 112, 384 (1933) with permission from The Royal Society, London, UK...

In contrast to hemodialysis that uses ultrafiltration membranes, plasma separation (also called plasmapheresis) requires microfiltration membranes with a pore size from 0.2 to 0.6 pm, in order to transmit all proteins and lipids, including LDL cholesterol (2000kDa) and retain completely platelets (2 pm diameter), red blood cells (8 pm diameter) and white blood cells. Thus, membrane plasmapheresis can yield high-quality platelet-free plasma and red cells can be either continuously returned to the donor or saved in another bag for blood transfusion. But it is important, in the case of plasma collection from donors, to minimize the membrane area, in order to reduce the cost of disposable hollow-fiber filters and to avoid the risk of hemolysis (free hemoglobin release) due to RBC damage by contact at the membrane if the pressure difference across the membrane is too high. 

In the absence of suspended solutes or colloids, the pure solvent flux through an ultrafiltration membrane is directly proportional to the applied pressure difference and inversely proportional to the viscosity of the solvent and the membrane thickness. Transport within the pores occurs in the creeping flow regime, since kinematic viscosities of liquids are sufficient to make Re < C 1 for practical pore sizes. In the simplest case, the membrane can be considered to be a packed array of straight, equal diameter nonintersecting capillary tubes. The observed volumetric flux, nAvA (cc/sec cm2), equals the product of the mass flux of solvent based on the total membrane area, nA... 

In ultrafiltration, water and other low molar mass molecules are forced through a semi-permeable membrane by the application of high pressures (1-7 bar) or of a centrifugal field. This technique involves membranes with pore diameters in the range of 1.0-20 nm, which are most commonly characterized and selected based on their nominal molar mass cut-off... 

Hydrodynamic. For a pressure driven process such as ultrafiltration the flow of solvent towards the membrane results in a drag which carries the solute in the same direction. This drag is a function of the distance of the solute from the pore entrance. At large distances it is equal to the isolated solute value (Stokes limit), but as the solute approaches and begins to enter the pore, the drag, for a constant filtration velocity, increases due to the restriction of solvent flow. This increase depends on the ratio of solute diameter to pore diameter. 

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