Microfiltration suspension

Cross-flow-elec trofiltratiou (CF-EF) is the multifunctional separation process which combines the electrophoretic migration present in elec trofiltration with the particle diffusion and radial-migration forces present in cross-flow filtration (CFF) (microfiltration includes cross-flow filtration as one mode of operation in Membrane Separation Processes which appears later in this section) in order to reduce further the formation of filter cake. Cross-flow-electrofiltratiou can even eliminate the formation of filter cake entirely. This process should find application in the filtration of suspensions when there are charged particles as well as a relatively low conduc tivity in the continuous phase. Low conductivity in the continuous phase is necessary in order to minimize the amount of elec trical power necessaiy to sustain the elec tric field. Low-ionic-strength aqueous media and nonaqueous suspending media fulfill this requirement. 

The cross-flow filtration method is applied mainly to hyper- and ultrafiltration as well as to some microfiltration.8 In cross-flow filtration the slurry solution or suspension fed to the filter flows parallel to the filter medium or membrane. The filtration product (permeate or filtrate) leaves the filtration module at right angles to the filter medium (the membrane). The traditional perpendicular flow filtration (where the flow of the suspension is directed at right angles to the filter medium and the permeate leaves the filter medium in the same direction) entails filter cake buildup, whereas cross-flow filtration is intended to prevent such filter... 

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

Microfiltration (MF) pressure difference filtration of cell suspensions blood plasma recovery... 

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). 

Figure 7.16 An illustration of the efficiency of back-pulsing in removing fouling materials from the surface of microfiltration membranes. Direct microscopic observations of Mores and Davis [9] of cellulose acetate membranes fouled with a 0.1 wt% yeast suspension. The membrane was backflushed with permeate solution at 3 psi for various times. Reprinted from J. Membr. Sci. 189, W.D. Mores and R.H. Davis, Direct Visual Observation of Yeast Deposition and Removal During Microfiltration, p. 217, Copyright 2001, with permission from Elsevier...

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. 

The principle of microfiltration is the application of hydrostatic pressure on a microporous filter membrane, so that the pressure difference forces solutes, water molecules, and particles smaller than the membrane pore size to flow across the pores, retaining and concentrating the larger particles in the suspension. 

In microfiltration, the permeate flux increases inversely with the suspension viscosity and proportionally to the applied pressure, provided that there is no membrane fouling (Belford, 1988 Ho and Zydney, 2000). To accelerate the process, it is possible to decrease the solution viscosity by increasing the temperature, although not so much as to denature the protein. 

Microfiltration is widely used for the removal of cells and fragments from suspension. It is also used as a method of sterilization of solutions, and has the advantage of high efficiency, simplicity, compactness, and reliability. 

Cross-flow filtration is also referred to as tangential flow filtration or microfiltration, but all three terms refer to a process by which membranes are used to separate components in a liquid solution (or suspension) on the basis of their size. The development of robust membranes in polymeric and ceramic materials has provided a powerful new technology for bioseparations, which is already widespread in the process industries as well as for water treatment processes. 

In filtration unit operation, especially in microfiltration, one usually differentiates between dead-end filtration (with cake formation) and cross-flow filtration [25] (Fig. 5). The cross-flow filter can have different geometries (Fig. 6) phase membranes, tubular membranes, or pleated membranes, of which the tubular and pleated ones are already accepted as cross-flow geometries in reactor technology, as mentioned above. In filtration engineering the cross-flow term means that the filtrate flows perpendicularly to the suspension stream. Cross-flow may not be considered a sufficiently illustrative term here [25]. A better term would be parallel filtration, but the term cross-flow filtration has been accepted generally and may be difficult to change at present. 

The most common sort of embodiment involving a liquid phase is the membrane separation of suspended solids from liquids, denoted variously by the terms filtration, microfiltration, and ultrafiltration, depending on the particle size, and which may include colloidal suspensions and emulsions. The solid particulates, for the most part, are deposited in the interstices or pores of a membrane barrier, and accordingly will require an intermittent backflushing operation. 

Later, Papet [58] presented an alternative process for preparing mbular ceramic cross-fiow filtration membranes. Papet s method consists of the casting of mbular mineral microfiltration membranes with titanium dioxide suspensions. The deposited particles on the porous support were then compressed and finally, the layer was consolidated by firing. 

Brou A, Ding L, Boulnois P, and Jaffrin M, Dynamic microfiltration of yeast suspensions using rotating disks equipped with vanes, J. Membr. Sci. 2002 197 269-282. 

Recent research efforts brought about new and exciting developments in membrane technology, some with direct implications for the membrane filtration of beer. For example, Stopka et al. [21] reported flux enhancement in the microfiltration of a beer yeast suspension when using a ceramic membrane with a helically stamped surface. A relatively simple modification of the ceramic membrane surface resulted in modified hydrodynamic conditions and disturbance of the fouling layer. As compared with a regular, smooth ceramic membrane of the same nominal pore size, the stamped membrane leads to higher flux and lower power consumption per unit volume of permeate at the same velocity of the feed. 

Stopka J, Bugan SG, Broussous L, Schlosser S, and Larbot A. Microfiltration of beer yeast suspensions through stamped ceramic membranes. Separat. Purif. TechnoL, 2001 25(l-3) 535-543. 

Hwang, K.-J., Chou, F.-Y., and Tung, K.-L., Effects of operating conditions on performance of cross-flow microfiltration of fine particle/protein binary suspension, J. Membr. Sci., 274, 183, 2006. 

Reverse osmosis enables complete retention of aU dissolved compounds, even small monovalent ions. To avoid the membrane blocking and scaling before reverse osmosis, microfiltration, or ultrafiltration pretreatment can be applied. Apart from preliminary treatment, ultrafiltration can be used for separation of suspensions or colloids, which are often formed by actinides or ions such as " Mg, Fe, °Co, and Sb. Microfiltration found the application for waste dewatering after precipitation. Nanofiltration (NF) that uses lower pressures than reverse osmosis is applied for separation of bivalent from monovalent ions. The most common application of NF process in nuclear industry is boric acid separation from the reactor coolant. 

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