Microfilters

Microfilters differ from ultrafilters primarily in the pore size range and construction of the membranes used to achieve a separation. The polymeric, flat sheet types are usually of a symmetric construction and exhibit either microporous or track-etched forms to facilitate either depth or surface filtration. These membranes are manufactured with pore ratings of 0.02-10 pm. 

A microfilter typically operates with a flux of less than 1 m h at a 

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Smaller pore size microfilters in single-pass systems which have pore sizes small enough to remove some vimses by size exclusion have been examined (26,37,38). Minimum levels of vims removal can be estabhshed for these systems if fluid and process conditions are employed which minimize removal of viral particles by mechanisms other than size selection. 

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. 

Ceramics Ceramic microfilters for commercial applications are almost always employed as tube-side feed multitube monoliths. They are also available as flat sheet, single tubes, discs, and other forms primarily suited to lab use. They are used for a few high-temperature applications, in contact with solvents, and particularly at very high pH. 

Chemical MF is used in several apphcations to recover caustic values from cleaning or processing streams. An example is the caustic solution used to clean dairy evaporators, which may be cleaned for reuse by passing it through a microfilter. Significant savings in caustic purchase and disposal costs provide the incentive. Acids are also recovered and reused. Ceramic microfilters are most commonly used in these apphcations. 

Hygiene and Regulation Almost unique to MF is the influence of regulatoiy concerns in selec tion and implementation of a suitable microfilter. Since MF is heavily involved with industries regulated by the Food and Drug Administration, concerns about process stabihty, consistency of manufacture, virus reduc tion, pathogen control, and material safety loom far larger than is usually found in other membrane separations. 

Mesh strainers finer than 100 mesh/inch (<150 /rm) should be treated as microfilters. Coarser strainers up to 50 mesh/inch (300 /rm) may generate significant static when fouled with accumulated debris, so should be treated as microfilters except in cases where fouling is not expected or may be rapidly identified by either periodic inspection or monitored pressure drop. Clean strainers should nevertheless be placed as far upstream as practical for nonconductive liquid service. A theoretical model for the charging process in strainers (screens) is given in [119-120]. Viscous nonconductive liquids (5-2.5.4) may produce unusually high charging currents in strainers. 

TABLE (a). Experimental Data for Charge Density Q Downstream of a Microfilter... 

What reasons would a water treatment operation employ microfilters ... 

Partial List of Commercial Crossflow Microfilter Media-materials and Geometries... 

Partial List of Crossflow Microfilter Media in Chemical Service Applications... 

The ability of a feed solution to plug a membrane is determined by the sit density index (SDl) employing a 0.45-pm PVDF microfilter at 30-psi differential pressure (ASTM Standard D-4189-82, 1987). 

A continuous stirred tank reactor has been reported for the hydrolysis of the triglycerides existing in vegetable oil in the presence of the aqueous phase and for synthesis reactions (Table 5). A microfilter can be used to prevent the immobilized enzyme from leaving the reactor. Kawano et al. [115] investigated the hydrolysis of olive oil in octane with Candida cylindracea lipase in aqueous solution in a Vibro Mixer reactor containing vibration plates connected to the crankshaft of a motor and oscillated with fixed rates. 

Efforts to overcome the limitations of the fragile membranes (as delicate as soap bubbles) have evolved with the use of membrane supports, such as polycarbonate filters (straight-through pores) [543] or other more porous microfilters (sponge-like pore structure) [545-548]. 

Figure 7.7 Permeation of anionic warfarin (pH 11) through octanol-soaked (impregnated) microfilter as a function of sodium ion concentration.

Figure 7.8 Permeation of drugs through oil-soaked microfilters comparisons to Caco-2 permeabilities (dashed curves) [546] (a) oil-free (untreated hydrophilic) filters (b) unstirred water layer permeability versus log MW (c) octanol-soaked (impregnated) filters (d) isopropylmyristate-soaked filters.

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