Microfiltration depth filters

Depth filters are usually preferred for the most common type of microfiltration system, illustrated schematically in Figure 28. In this process design, called "dead-end" or "in-line" filtration, the entire fluid flow is forced through the membrane under pressure. As particulates accumulate on the membrane surface or in its interior, the pressure required to maintain the required flow increases until, at some point, the membrane must be replaced. The useful life of the membrane is proportional to the particulate loading of the feed solution. In-line microfiltration of solutions as a final polishing step prior to use is a typical apphcation (66,67). 

The second category of microporous membranes is the depth filter (b), which captures the particles to be removed in the interior of the membrane. The average pore diameter of a depth filter is often 10 times the diameter of the smallest particle able to permeate the membrane. Some particles are captured at small constrictions within the membrane, others by adsorption as they permeate the membrane by a tortuous path. Depth filters are usually isotropic, with a similar pore structure throughout the membrane. Most microfiltration membranes are depth filters. 

The two principal types of microfiltration membrane filter in use—depth filters and screen filters—are illustrated in Figure 7.3. Screen filters have small pores in their top surface that collect particles larger than the pore diameter on the surface of the membrane. Depth filters have relatively large pores on the top... 

Cold sterilization of beer using microfiltration was introduced on a commercial scale in 1963. The process was not generally accepted at that time, but has recently become more common. Sterilization of beer and wine is much less stringent than pharmaceutical sterilization. The main objective is to remove yeast cells, which are quite large, so the product is clear and bright. Bacterial removal is also desirable a 106 reduction in bacteria is equivalent to the best depth filters. The industry has found that 1-p.m filters can remove essentially all the yeast as well as provide a 106 reduction in the common bacteria found in beer and wine. Because the cost structure of beer and wine production is very different from that... 

Separation takes place in microfiltration primarily between solids and liquids, and many established applications are simply extensions of conventional filtration into a lower particle size range. (See Section I.A.) A homogeneous porous membrane used as a conventional depth filter traps particles on its surface and inside the tortuous pores. The membrane can become clogged... 

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