流体力学与 及传热课件filtration fundamentals.pptVIP

3.4 filtration fundamentals;A filtration system can be shown schematically ;There are four types of driving force: Gravity, Vacuum, Pressure, Centrifugal.;There are basically two types of filtration used in practice: Surface filtration: The surface filters are used for cake filtration in which the solids are deposited in the form of a cake on the up-stream side of a relatively thin filter medium.;Depth filtration: The depth filter are used for deep bed filtration in which particle deposition takes place inside the medium and cake deposition on the surface is undesirable. ; Surface filters are usually used for suspensions with higher concentrations of solids, say above 1% by volume, because of the blinding of the medium that occurs in the filtration of dilute suspensions. ;discontinuous pressure filters ;Filter press ;The plates have channels cut in them so that clear filtrate liquid can drain down along each plate. ;If the cake is to be washed, the wash water then flows through the filter cloth, through the entire cake (not half the cake as in filtration), through the filter cloth at the other side of the frames, and out the discharge channel. ; Filter presses are used in batch processes but cannot be employed for high-throughput processes. ;They are simple to operate, very versatile and flexible in operation, and can be used at high pressures, when necessary, if viscous solutions are being used or the filter cake has a high resistance. ;shell-and-leaf filters ; A number of leaves are hung in parallel in a closed tank.; The filtrate flows inside the hollow framework and out a header. The wash liquid follows the same path as the slurry. ;Continuous pressure filters ;This filter filters, washes, and discharges the cake in a continuous, repeating sequence. The drum is covered with a suitable filtering medium. ; The drum rotates and an automatic valve in the center serves to ‘activate the filtering, drying, washing, and cake-discharge functions in the cy