Cross-flow microfiltration

The idea of ultrafiltration has been extended ia recent years to the filtration of particles ia the micrometer and submicrometer range ia porous pipes, usiag the same cross-flow principle. In order to prevent blocking, thicker flow channels are necessary, almost exclusively ia the form of tubes. The process is often called cross-flow microfiltration but the term cross-flow filtration is used here. 

The concept of cross-flow microfiltration is shown in Figure 16.11, which represents a cross-section through a rectangular or tubular membrane module. The particle-containing fluid to be filtered is pumped at a velocity in the range 1-8 m/s parallel to the face of the membrane and with a pressure difference of 0.1-0.5 MN/m2 (MPa) across the membrane. The liquid penneates through the membrane and the feed emerges in a more concentrated form at the exit of the module.1617 All of the membrane processes are listed in Table 16.2. Membrane processes are operated with such a cross-flow of the process feed. 

Ideally, cross-flow microfiltration would be the pressure-driven removal of the process liquid through a porous medium without the deposition of particulate material. The flux decrease occurring during cross-flow microfiltration shows that this is not the case. If the... 

The early stages of cross-flow microfiltration often follow such a pattern. However, the growth of the cake is limited by the cross-flow of the process liquid. There are several ways of accounting for the control of cake growth. A useful method is to rewrite (16.6.1) as ... 

Film and gel-polarisation models are developed for ultrafiltration. These models are also widely applied to cross-flow microfiltration. 

Cross-flow is the usual case where cake compressibility is a problem. Cross-flow microfiltration is much the same as cross-flow ultrafiltration in principle. In practice, the devices are often different. As with UF, spiral-wound membranes provide the most economical configuration for many large-scale installations. However, capillary devices and cassettes are widely employed, especially at smaller scale. A detailed description of cross-flow microfiltration had been given by Murkes and Carlsson [Crossflow Filtration, Wiley, New York (1988)]. 

Pharmaceutical Removal of suspended matter is a frequent application for MF. Processes may be either clarification, in which the main product is a clarified liquid, or solids recovery. Separating cells or their fragments from broth is the most common application. Clarification of the broth in preparation for product recovery is the usual objective, but the primary goal may be recovery of cells. Cross-flow microfiltration competes w l with centrifugation, conventional filtration by rotary vacuum filter or filter press and decantation. MF delivers a cleaner permeate, an uncontaminated, concentrated cell product... 

Cakl, J., Bauer, I., Dolecek, P., and Mikulasek, P. 2000. Effects of backflushing conditions on permeate flux in membrane cross-flow microfiltration of oil emulsion. Desalination 127 189-98. 

Ideally, cross-flow microfiltration would be the pressure-driven removal of the process liquid through a porous medium without the deposition of particulate material. The flux decrease occurring during cross-flow microfiltration shows that this is not the case. If the decrease is due to particle deposition resulting from incomplete removal by the cross-flow liquid, then a description analogous to that of generalised cake filtration theory, discussed in Chapter 7, should apply. Equation 8.2 may then be written as ... 

The layers deposited on the membrane during cross-flow microfiltration are sometimes thought to constitute dynamically formed membranes with their own rejection and permeation characteristics. 

In the following section, film and gel-polarisation models are developed for ultrafiltration. These models are also widely applied to cross-flow microfiltration, although even these cannot be simply applied, and there is at present no generally accepted mathematical description of the process. 

Schneider, K. and Klein, W. Desalination, 41 (1983) 271. The concentration of suspensions by means of cross flow microfiltration. 

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. 

After extraction, the solute-laden CLAs need to be separated from the mother liquor so that they can be back stripped. Hence attempts were made to filter the solute-rich CLAs from the aqueous phase using cross-flow microfiltration [70]. The filtration characteristics of the CLAs as indicated by the flux, CLA size, and concentration showed that they are completely retained by the membrane and do not foul the membrane surface. Using this system, the CLAs could easily be concentrated up to 30% w/v at low pressures, and the permeate stream remained totally clear. The CLAs appear to maintain their structural integrity because only 3 mg dm of SDS was... 

Environmental Research and Development, Inc., offers the neutral process, which reduces hex-avalent chromium using sulfide catalyzed by ferrous iron, while precipitating heavy metals at pH ranges from 7.4 to 8.4. The vendor has combined this technology with cross-flow microfiltration to remove heavy metals from contaminated groundwater and wastewater without the need for large clarifiers. The technology has been used at U.S. Department of Defense (DOD) sites and is commercially available. 

The membrane processes of cross-flow microfiltration, ultrafiltration, and reverse osmosis offer excellent potential for continuous removal of these contaminants. The selection of the optimum process is a function of the form of the contaminants present as well as several other factors. 

Figure Three (3) illustrates the mechanism of cross flow microfiltration. Microfiltration involves the removal of insoluble particulate materials ranging in size from 0.1 to 10.0 microns (1000 to 100,000 angstroms). Microfiltration membrane polymers include ...

M.G.E. Goemans, F. Tiller, L. Li, E.F. Gloyna, The Separation of Inorganic Salts from Supercritical Water by Cross-flow Microfiltration, J. Membrane Science, 124, (1997) 129. 

G. Belfort, R.H. Davis and A.L. Zydney, The Behavior of Suspensions and Macro-molecular Solutions in Cross Flow Microfiltration, J. Membr. Sci. 1, 96 (1994). 

Depth membrane filters are usually preferred for in-line filtration. As particles are trapped within the membrane, the permeability falls, and the pressure required to maintain a useful filtrate flow increases until, at some point, the membrane must be replaced. The useful life of the membrane is proportional to the particle loading of the feed solution. A typical application of in-line depth microfiltration membranes is final polishing of ultrapure water just prior to use. Screen membrane filters are preferred for the cross-flow microfiltration systems shown in Figure 7.1(b). Because screen filters collect the retained particles on the surface of the membrane, the recirculating fluid helps to keep the filter clean. 

The key innovation that has led to the increased use of cross-flow microfiltration membrane modules in the last few years has been the development of back-pulsing or backflushing to control membrane fouling [9-11]. In this procedure, the water flux through the membrane is reversed to remove any particulate and fouling material that may have formed on the membrane surface. In microfiltration several types of backflushing can be used. Short, relatively frequent flow reversal lasting a few seconds and applied once every few minutes is called... 

Table 7.2 Comparison of advantages and disadvantages of in-line and cross-flow microfiltration...

Wen, X., Bu, Q. and Huang, X. (7-10 June 2004) Study on fouling characteristic of axial hollow fibers cross-flow microfiltration under different flux operations, in Proc of the IWA Specialty Conference -WEMT 2004, Seoul, Korea. 

Belfort G, Davis RH, Zydney AL (1994), The behavior of suspensions and macro-molecular solutions in cross-flow microfiltration, J. Membr. Sci. 96 1-58. 

Microfiltration is a unit operation for the separation of small particles. The separation limits are between 0.02 and 10 (jum particle dimensions. Microfiltration can be carried out in a dead-end mode and a cross-flow mode. In downstream processing, the cross-flow filtration is carried out continuously or discontinuously. The most important parameters that determine the productivity of cross-flow microfiltration are transmembrane pressure, velocity, particle size and surface, viscosity of the liquid and additives such as surfactants, and changing the surface and surface tension. 

Boissier, B., Lutin, F., Moutounet, M., Vernhet, A. (2008). Particles deposition during the cross-flow microfiltration of red wines -incidence of the hydrodynamic conditions and of the yeast to fines ratio. Chem. Fngin. Process., 47(3) 276-286... 

Van der Horst, H.C., M. Timmer and I. Piersma, 1994, Cross-flow microfiltration of skim milk, presented at 3rd Int. Conf. Inorg. Membr., Worcester, MA, USA. 

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