Medium resistance, centrifugal filtration

FIG. 18-179 Centrifugal filtration rate as a function of both cake and medium resistance. 

Filtration is a mechanical means of separating solids from a liquid suspension via a porous medium or screen that permits the liquid to pass while retaining the solids. Centrifugal filtration is driven by the pressure exerted by a liquid medium and is opposed by the combined resistance of the porous filter and filter cake. 

Figure 6.6 Representation of a centrifugal pump curve. Point (1) corresponds to the start of filtration and denotes the point where the filter medium resistance is overcome such that equation (6.18) is satisfied. Point (2) represents a point close to the maximum pressure at which variable pressure filtration could occur any further filtration with the pump would generally be performed at constant pressure.

A batch centrifugal filter (bowl diameter of 0.762 m, height of 0.4572 m, speed of 2000 rpm) is used to process a suspension (60 g/liter concentration, dry solid density of 2000 kg/m, liquid is water). Cake parameters are porosity of 0.435, cake thickness of 0.1524 m, specific cake resistance of 1.316 x 10 m/kg, filter medium resistance of 8.53 x 10 ° m . Final cake is washed such that inner surface of liquid is 0.2032 m. Find wash water rate if it equals final filtrate rate of flow. 

Cakes will be more compacted in a filtering centrifuge as compared to a sedimentation centrifuge. As solids build at the filter medium surface, a pressure drop is induced by this cake resistance, similar to that in pressure filtration. Sedimentation therefore has an inherent advantage, particularly with difficult filtering, compactible materials. 

Pollen Screening for Disease Resistance Using Phytotoxins. A liquid shake culture was made using a 4 1 (V-8 juice glass distilled water) medium. The pH was adjusted to 5.8, and the medium sterilized at 121°C for 15 min. Two hundred and fifty ml of medium in 500 ml Erlenmeyer flasks was inoculated with Fusarium and Verticillium races supplied by Dr. Ken Kimball, Moran Seed Co., Davis, CA, and shaken for 7 days at 24°C. The medium was then centrifuged at 1000 rpm to separate out the mass of spores and filaments. The filtrate was run through a... 

There is a resistance to flow of liquid passing through a filter medium which causes a pressure drop across the filter. This resistance increases as the solid cake builds up on the filter medium and it is necessary, therefore, to apply a force to maintain the filtration at a steady rate. The nature of this force provides a convenient means of classifying the many types of filter. Thus, we have gravity filters, vacuum filters, pressure filters, and centrifugal filters (see Chapter 3). 

Depending on the task of the separation process, a large number of different types of equipment for the solid-liquid separation are available. Criteria for the selection are, inter alia, the filtration properties of the crop and the size of the particles, the suspension density, and the mode of operation, batchwise or continuous. Two main types of solid-liquid separations can be distinguished, cake forming filtration under pressure and centrifugation. In both cases, the crystals form a filter cake on a medium through which the mother liquor has to pass. The resistance is the rate-limiting step for the filtration and the characteristic property of the suspension. 

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