Ultrafiltration and Microfiltration

Common polymers currently used to prepare MF and UF membranes include  

Microfiltration and UF membranes are available in tubular, spiral wound, and hollow fiber membrane module configurations. Tubular and spiral MF and UF modules are similar to RO tubular and spiral wound membrane modules described in Chapters 4.3.2 and 4.3.3. However, while the thickest feed spacer in a spiral RO module is 34-mil, UF and MF modules nominally have up to a 45-mil spacer due to the relatively high concentration of suspended solids these membranes are called upon to treat (TriSep Corporation offers a special 65-mil spacer for dairy applications). 

Improvements made over the last few years in MF and UF membranes and modules, including the development of a new generation of hollow-fiber (HF) membranes and modules for industrial applications has led to wider application of these membrane separation technologies. The new generation HF membranes are characterized by high porosity, strength, and flexibility, all important characteristics for MF and UF applications. 

Microfiltration and UF hollow fiber membranes are different than the hollow fine fibers discussed in Chapter 4.3.4. The MF and UF membranes are thicker and not quite as flexible, resembling fine-diameter straws rather than human hair. Diameter of fibers ranges 

Hollow fibers can be created with the dense side on the inside or lumen of the fiber or on the outside of the fiber, or they can be doubleskinned, where both the lumen and the outside of the fiber are dense. Location of the denser side of the membrane determines whether the service flow is outside-in or inside-out. Outside-in systems are typically used in a dead-end mode (or some variation thereof), while 

Microporous membranes are used to effect the separation by MF and UF processes. These microporous membranes differ from polyamide composite RO membranes in that they are not composites of two different polymeric materials they are usually constructed using a single membrane polymeric material. In simple terms, both UF and MF technologies rely on size as the primary factor determining which 

1900 s. Bechhlold developed the first synthetic UF membranes made from nitrocellulose in 1907. He is also credited with coining the term ultrafilter. By the 1920 s and 1930 s both MF and UF nitrocellulose membranes were commercially available for laboratory use. The first industrial applications of MF and UF came in the 1960 s and 1970 s. Microfiltration membranes became viable for industrial application in the 1970 s when Gelman introduced the pleated MF cartridge. Ultrafiltration membranes became industrially viable in the 1960 s when Amicon began preparing UF membranes using a modified Loeb-Sourirajan method (see Chapter 4.2.1).  

Polypropylene (PP) a hydrophobic membrane with good chemical resistance and tolerance of moderately high temperatures sensitive to chlorine. 

Polytetrafluoroethylene (PTFE) (MF only) an extremely hydrophobic membrane, with high tolerance of acids, alkalis, and solvents can be used at temperatures up to 260 C. Polyvinylidene fluoride (PVDF) a hydrophobic membrane (can be surface-modified to become more hydrophilic) with good resistance to chlorine. Stable pH range up to 10. Polysulfone (PS) good resistance to chlorine and aliphatic hydrocarbons (not compatible with aromatic hydrocarbons, ketones, ethers, and esters), stable pH range from 1-13, and tolerance of up to 125 C. 

Polyethylene (PE) poor resistant to chemical attack and relatively low strength. 

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Polymer Membranes These are used in filtration applications for fine-particle separations such as microfiltration and ultrafiltration (clarification involving the removal of l- Im and smaller particles). The membranes are made from a variety of materials, the commonest being cellulose acetates and polyamides. Membrane filtration, discussed in Sec. 22, has been well covered by Porter (in Schweitzer, op. cit., sec. 2.1). 

Microfiltration and ultrafiltration have recently been introduced for the removal of particles down to any desired size. Their capital cost is relatively high. Experience with them is limited, and a short trial with a small-scale pilot element is advisable. Prediction of full-scale performance from such trials is normally quite reliable. 

A significant recent advance has been the development of microfiltration and ultrafiltration membranes composed of inorganic oxide materials. These are presently produced by two main techniques (a) deposition of colloidal metal oxide on to a supporting material such as carbon, and (b) as purely ceramic materials by high temperature sintering of spray-dried oxide microspheres. Other innovative production techniques lead to the... 

ZEMAN, L. J. and ZYDNEY, A. L., Microfiltration and Ultrafiltration. Principles and Applications (Marcel Dekker, New York, 1996). 

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. 

Cot, L., C. Guizard and A. Larbot. 1988. Novel ceramic material for liquid separation process Present and prospective applications in microfiltration and ultrafiltration. Industrial Ceramics 8(3) 143-48. 

Gillot, J. and D. Garccra. 1984. New ceramic filter media for cross-flow microfiltration and ultrafiltration. Paper presented at Filtra 84 Conference, 2-4 October 1984, Paris. 

Separation or clarification techniques, grit separation, sedimentation, air flotation, filtration, microfiltration and ultrafiltration, and oil-water separation. 

Zeman, I.J. and Zydney, A.E. (1996) Microfiltration and Ultrafiltration, Marcel Dekker. 

Brandhuber and Amy (1998) and Shih (2005) provide concise summaries of arsenic removal with different types of membranes. Microfiltration and ultrafiltration are used to remove particles that contain sorbed, coprecipitated, or precipitated arsenic (US EPA, 2002b, 35 Shih, 2005, 93, 94). Both reverse osmosis and nanofiltration can effectively remove As(V) oxyanions from water and, in some cases, H3ASO3 can also be treated without preoxidation (Brandhuber and Amy, 1998, 9 Uddin et al., 2007a Xia et al., 2007 Uddin et al., 2007b). 

Nanofiltration Compared to microfiltration and ultrafiltration, nanofiltration and reverse osmosis are more expensive and susceptible to fouling (Shih, 2005, 95). Most of the expenses result from the high densities of the membranes, which require high pressures (0.34-6.9 MPa) and a considerable... 

W. Eykamp, Microfiltration and Ultrafiltration, in Membrane Separation Technology Principles and Applications, R.D. Noble and S.A. Stem (eds), Elsevier Science, Amsterdam, pp. 1-40 (1995). 

For relatively porous nanofiltration membranes, simple pore flow models based on convective flow will be adapted to incorporate the influence of the parameters mentioned above. The Hagen-Poiseuille model and the Jonsson and Boesen model, which are commonly used for aqueous systems permeating through porous media, such as microfiltration and ultrafiltration membranes, take no interaction parameters into account, and the viscosity as the only solvent parameter. It is expected that these equations will be insufficient to describe the performance of solvent resistant nanofiltration membranes. Machado et al. [62] developed a resistance-in-series model based on convective transport of the solvent for the permeation of pure solvents and solvent mixtures ... 

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. 

Catalytic reactions can be combined in membrane-assisted integrated catalytic processes with practically all the membrane unit operations available today. Many examples of integration of membrane contactors, pervaporation, gas separation, nanofiltration, microfiltration, and ultrafiltration operations together with catalytic reactions, have been proposed in the literature. 

Qualitatively similar polarization responses are apparent for both microfiltration and ultrafiltration processes, but for microfiltration, additional more complex can occur as is discussed below (Belfort, Davis, and Zydney, 1994 Segre and Silberberg, 1962). The rejection parameter mentioned earlier can be written as either an observed, R0, or an intrinsic, R , value ... 

Mobile units for photocatalytic treatment have been constructed (126,127). The European Joint Research Center laboratory pilot plant, placed on a truck, includes Ti02 loaded on membranes in UV-irradiated tubular reactors behind microfiltration and ultrafiltration modules. The waste water flow rate for this unit was typically 40 L hr-1, and hydrogen peroxide was added to the photocatalytic process (134). 

L.J. Zeman and A.L. Zydney, Microfiltration and Ultrafiltration Principles and Applications, CRC Press, Boca Raton, FL, 1996. 

The aim of the present chapter is to provide an overview of recent work on the development of quantitative predictive methods for membrane processes at the Centre for Complex Fluids Processing, University of Wales Swansea. The main aim of such work is the development of predictive methods that require no adjustable parameters—that is methods that are truely ab initio. Three theoretical aspects will be illustrated the prediction of rates of ultrafiltration, the prediction of rejection in microfiltration and ultrafiltration, and the prediction of the rejection in nanofiltration. These examples show the importance of bridging the gap between physical chemistry and process engineering in the development of new process technologies. Finally, the use of AFM in the quantification of the adhesion of... 

W.R. Bowen and A.O. Sharif, Hydrodynamic and colloidal interaction effects on the rejection of a particle larger than a pore in microfiltration and ultrafiltration membranes, Chem. Eng. Sci. 53 (1998) 879-890. 

Table 16.3 Advantages and limitation of various module configurations for microfiltration and ultrafiltration membranes.

Many models have been published to calculate the microfiltration process. One important factor is the concentration polarization, which represents the most important limiting physical obstacle. At high particle concentration and with time, a layer is formed on the membrane. This layer is responsible for the flux reduction. A comprehensive overview on this technique is given by Ripperger52 and Staude.53 Often similar or identical module types are used in microfiltration and ultrafiltration. 

Membrane Porosity Separation membranes run a gamut of porosity (see Fig. 22-48). Polymeric and metallic gas separation membranes, electrodialysis membranes, pervaporation membranes, and reverse osmosis membranes are nonporous, although there is lingering controversy over the nonporosity of the latter. Porous membranes are used for microfiltration and ultrafiltration. Nanofiltration membranes are probably charged porous structures. 

Other The cassette (Fig. 22-54), a modification of a plate-and-frame device that is favored because of the ease of scale-up from laboratory to small plants is widely used in pharmaceutical microfiltration and ultrafiltration. An entirely different module also called a cassette is used in the MF of water. There are a host of other clever module designs in use, and new ones appear frequently. 

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