Depth filters, cartridge filtration

Filtration can be distinguished in two types (1) absolute and (2) depth. In absolute filtration a steel or plastic screen, cartridge, or paper filter removes particles from the oil. These filters have certain porosity and most of them retain particles down to 5 pm, when used for passive filtration [72]. If diato-maceous earth or other absorbents are used, they are mixed with the oil in the fryer at the end of the day and/or shift and the suspension is pumped through the filter and circulated through the filter and back into the fryer until a filter bed is established and the fryer is free of fines and particulates. Filter media suppliers recommend a dose between 0.1% and 2.0%, by weight on the frying medium [8]. 

Clarification of liquids often calls for the use of depth filtration equqnnent. The process techniques used and the filtration mechanisms involved in depth filters are outlined in Ch t 6, which includes information on sand filters and cartridges. 

Depth Filters. All the mechanisms for particle collection provided in Section 6.1, except surfece straining, are relevant in cartridge depth filtration. Metal fibers are preferred to... 

Filtration processes may be classified as either depth filtration or surface filtration. Depth filtration relies on a layer of porous media in which suspended particles in the beer are trapped within the media. Examples in brewing include filter aid filtration, sheet filtration and some forms of filter cartridge. Surface filtration normally refers to membrane technology. A thin layer of membrane has pores throughout the structure. This means that it is possible to achieve very exact filtration, perhaps enabling sterilisation, but typically the quantity of suspended beer particles that may be removed is less than for depth filtration. 

At the time of writing the most important technique to achieve sterile filtration in a brewery is through the use of cartridge filters. These comprise a membrane or depth filter that is a relatively thin layer and is pleated within the snpport stmctnre to provide a high filtration area in a compact volume (Figure 11.3). 

The essential method to obtain sterile air, whether packed-bed or cartridge filters are used, is to reduce the humidity of the air after compression so that the filter material always remains dry. The unsterilized compressed air must never reach 100% relative humidity. Larger plants install instrumentation with alarms set at about 85% relative humidity. Careful selection of the cartridge design or the design of packed-bed filters will result in units that can operate in excess of three years without replacement of filter media. If a fiber material is used in a packed-bed type filter, the finer the fiber diameter the shallower the bed depth needs to be for efficient filtration. Other filter media are less common and tend to have special problems and/or shorter life. The bed depth of filters is only 10 to 18 inches for fibers of less than 10 microns. These filters run clean for 2 weeks or longer before being resterilized. 

Filter media behaviour is also rq>orted in other sections of the text, e.g. Cbapto 10 deals with the cruda% important area of mooihrane sqiarations. Agahi, Ch ter 6 deals with depth filtration sterns such as deq> sand fih and cartridges, and describes the media used in sudi equpment. 

Surface Filters. The mechanism of filtration of a tme surface filter is sur ce straining only. Most commerdal suifiice fiber cartridges also fiber by means of depth filtration... 

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