Theoretical aspects of (1) hydraulic deliquoring, (2) rotary drum filtration and (3) disc filtration



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This work deals with theoretical aspects of compressible cake filtration and continuous rotary vacuum filters. Reduction of moisture in filter cakes is important when heat requirements for drying are excessive. During filtration, the porosity of the cake is lowest at the points of maximum accumulative drag or lowest hydraulic pressure. By reversing the flow through the cake, it is possible to reduce the average porosity and hence the moisture content of the cake. An equation is developed for calculating filtration rates in a rotary vacuum drum filter, taking into account finite partitioning of the drum, variable hydrostatic head and medium resistance. Constant average specific resistance of the cake is assumed. Formula based on sectioning gives rates which vary as much as 15% from those presently in use. The emerging cake varies in thickness in a periodic manner because portions of a section are subjected to vacuum for different cake formation times. This variation in thickness of the emerging cake can be as much as 20% and therefore accounts for the difference in the conventional and the proposed formula. Analytical formulas for the overall filtration rate through a continuous rotary disc filter are developed. Previously theoretical equations have not been available for design purposes. In this derivation the following are taken into account: 1. Division of the filtration surface into N equal radial sections with inner and outer radii of R[subscript 1] and R[subscript 2] 2. Separation of sections by a blank strip representing a dead area for flow 3. Variable hydrostatic head 4. Medium resistance. It is shown that the inner radius R[subscript 1] can be optimized to produce a maximum rate of filtration. A simple rule is presented for calculating the inner radius. The flow rates of disc and rotary drum filters are compared.