Liquid separation membranes cross flow filtration

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 a general way, most of ceramic membrane modules operate in a cross-flow filtration mode [28] as shown in Figure 6.18. However, as discussed hereafter, a dead-end filtration mode may be used in some specific applications. Membrane modules constitute basic units from which all sorts of filtration plants can be designed not only for current liquid applications but also for gas and vapor separation, membrane reactors, and contactors, which represent the future applications of ceramic membranes. In liquid filtration, hydrodynamics in each module can be described as one incoming flow on the feed side gf, which results in two... 

Liquid Separation. Ceramic ultrafilters and nanofilters for liquid mass separation are now produced at an industrial scale and most often are synthesized by the sol-gel process. These membranes operate according to a cross-flow filtration process in which the feeding fluid flows over the surface of the membrane and the permeate flow is recovered across the membrane in a perpendicular direction to the feed flow. The driving force for liquid flow... 

Cross-flow filtration allows the olein fraction to be recovered continuously. However, this type of filtration only concentrates the crystal slurry and does not separate the crystals. There is a tendency for the smaller, needle-like crystals to pass through the filter medium, contaminating the liquid fraction. Complete separation of the fractions still requires conventional filtration such as with a membrane filter. 

Process Description Microfiltration (MF) separates particles from true solutions, be they liquid or gas phase. Alone among the membrane processes, microfiltration may be accomplished without the use of a membrane. The usual materi s retained by a microfiltra-tion membrane range in size from several [Lm down to 0.2 [Lm. At the low end of this spectrum, very large soluble macromolecules are retained by a microfilter. Bacteria and other microorganisms are a particularly important class of particles retained by MF membranes. Among membrane processes, dead-end filtration is uniquely common to MF, but cross-flow configurations are often used. 

The separations feasible by filtration have expanded enormously over the last generation. The developments this symposium has commemorated, and the individuals it has honored, have been largely responsible. The removal of dissolved solutes or other low-molecular-weight substances from water by hyperfiltration or reverse osmosis, which the Loeb-Sourlrljan membrane made technically and economically feasible, has become an industrial-scale operation. Ultrafiltration of colloids and filtration of coarser materials from liquids have become much more efficient with the use of cross flow of liquid to slow the buildup of flltercake appreciation of the benefits from shear at the Interface has become much more general from the necessity of controlling concentration polarization and fouling in salt filtration. 

In the context of PFC removal, the improvement of PAC filtration is of particular interest since sorption of PFC on GAC may be inefficient (see Sect. 2.1). The filtration effect for PAC can be achieved by its immobilization on an appropriate supporting material such as polystyrene balls (Haberer process) [66, 67] or porous polyurethane cylinders [68]. A relatively new process in drinking water and pool water treatment is the combination of PAC with membranes (MFAJF). A full-scale application of PAC/UF for water treatment is the CRIST AL process (Combined Reactors /ntegrating a Separation by membranes and Treatment by Adsorption in Liquid), which is applied in Slovenia, France, and Switzerland for drinking water treatment [20]. Thereby, PAC is added to the raw water prior to filtration via UF-membranes operated in cross-flow mode [20]. The PAC-membrane process can be... 

The driving force for filtration in pressure filters is usually the liquid pressure developed by pumping or by the force of gas pressure in the suspension feed vessel. Alternatively, or in addition, the liquid may be squeezed through and out of the cake by the mechanical action of an inflatable membrane, a piston or a porous medium pressed on top of the cake. Pressure filtration is, therefore, defined here as any means of surface filtration where the liquid is driven through the medium by either hydraulic or mechanical pressure, greater than atmospheric. The solids are deposited on top of the filter medium (as in all surface filters), with the possible exception of some cartridge filters which also use a certain amount of depth filtration. In this chapter, the suspension is assumed to approach the medium at 90° and this excludes the so-called dynamic fUter/thickeners or cross-flow filters (also driven by pressure) which are dealt with in a separate chapter (11). 

Belfor G. Fluid mechanics and cross-flow membrane filtration. In Muralidhara HS, ed. Advanees in Solid Liquid Separation. Columbus Battelle Press, 1986, pp 182 183. 

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