流体力学与传热英文课件：3-4filtration fundamentals

1、3.4 filtration fundamentals3.4.1 IntroductionFiltration may be defined as the separation of solids from liquids by passing a suspension through a permeable medium which retains the particles. A filtration system can be shown schematically suspensionmediumfiltrateIn order to obtain fluid flow through

2、 the filter medium, a pressure drop p has to be applied across the medium. 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 sol

3、ids 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. In a surface filter, part

4、icles of the same size as or larger than the openings wedge into the openings and create smaller passages which remove even smaller particles from the fluid. A filter cake is thus formed, which in turn functions as a medium for the filtration of subsequent input suspension. Surface filters are usual

5、ly 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 Pressure filters can apply a large pressure differential across the septum to give economica

6、lly rapid filtration with viscous liquid or fine solids. Filter press A filter press contains a set of frame designed to provide a series of chambers or compartments in which solids may collect. The frames are covered with a filter medium such as canvas. Slurry is admitted to each compartment under

7、pressure; liquor passes through the canvas and out a discharge pipe, leaving a wet cake of solids behind.The plates have channels cut in them so that clear filtrate liquid can drain down along each plate. The filtrate flows between the filter cloth and the face of the plate through the channels to t

8、he outlet. When the frames are completely full, the frames and plates are separated and the cake removed. Then the filter is reassembled and the cycle is repeated. 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 filtra

9、tion), 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, wh

10、en necessary, if viscous solutions are being used or the filter cake has a high resistance. shell-and-leaf filters The filter press is useful for many purposes but is not economical for handling large quantities of sludge or for efficient washing with a small amount of wash water. A number of leaves

11、 are hung in parallel in a closed tank.The leaf filter shown in figure. Each leaf is a hollow wire framework covered by a sack of filter cloth. The filtrate flows inside the hollow framework and out a header. The wash liquid follows the same path as the slurry. The slurry enters the tank and is forc

12、ed under pressure through the filter cloth, where the cake deposits on the outside of the leaf. Continuous pressure filters Batch filters often require considerable operating labor, which in large-scale processing may be prohibitively expensive. The continuous vacuum filters described below were dev

13、eloped to reduce the labor required for filtration. This filter filters, washes, and discharges the cake in a continuous, repeating sequence. The drum is covered with a suitable filtering medium. Continuous rotary vacuum-drum filter The drum rotates and an automatic valve in the center serves to act

14、ivate the filtering, drying, washing, and cake-discharge functions in the cycle. The filtrate leaves through the axle of the filter.Automatic valve septum cake washingslurry. Connections from the periphery to an automatic filter valve Filter operates with approximately 30%-40% of the drum submerged

15、in the tank. In figure, horizontal drum with a slotted face turns at 0.1 to 2 r/min in an agitated slurry trough. The performance of drum filters is controlled by adjustments of in three main variables: drum speed, vacuum, and submergence. Deep bed filtration The principles of deep bed filtration ar

16、e quite well-known. It is a clarification process using a deep bed of granular media, usually sand. As a sewage tertiary treatment process it can frequently produce filtrates containing only 5mg/l or less of suspend matter. Rates of filtration are normally in the range of 5 to 15 m/h overall depths

17、are usually about 0.6 to 1m dilute suspension, say less than 0.1% by volume In a depth filter, particles are smaller than the medium openings and hence they proceed through relatively long and tortuous pores where they are collected by a number of mechanisms ( gravity, diffusion and inertia) and att

18、ached to the medium by molecular and electrostatic forces. The initial pressure drop across the depth filter is generally small than that across a surface filter of comparable efficiency but the build-up of pressure drop as particles are collected is more gradual for a depth filter. Open gravity filters usually operate to a head loss limit of 2.5m water gauge, but pressure filters may use up to 2 or 3times this value. Backwashing is normally employed