川大884化工原理电子版本课件双语chapter53f.pptx

Chapter 5 Separation of particle and fluidprecipitation Definition Solid particles in fluid move under force field to separate with the fluid. The force field can be gravitational, centrifugal or electric. gravitational precipitation Because of the density difference between particle and fluid, the gravity on them are different and the particles will be moved on the gravity direction.In addition to the gravity, there is also a draft force here. According to Stokes Eq., the terminal velocityfor laminar flow（Rep 2） gravitational precipitationut represent the speed of separation. ut is proportional to the driving force (?p-?)g and the square of particle diameter dp2. free precipitation for single particle or very dilute concentration of suspensions.disturbance precipitation the particles affect each other in precipitation. Often found in liquid-solid system of high concentration. The reason is: the collision between the particles changes their momentum and velocity. Only when the particle concentration is lower than 0.2%，or the distance between the particles is larger than 10～20 times of their diameters，the process can be considered as free. gravitational precipitation100clean liquidA75Ahomogeneous precipitationAAA AB50BconcentratingBinterface heightCC25CdepositCDDDDDD0t0tttttt0123456timePut a mixed suspension liquid in a vertical column for a period of time, the liquid will show 4 zones.A： clean liquid zone, no particle.B：homogeneous precipitation with particle concentration almost the same of original liquid (c=c0)； C： concentrating zone, the particle concentration and size increase from top to bottom in this zone.D：very dense of deposited particles. gravitational precipitationFollowing the precipitation，A and D grow，and B narrows. the interface of A and B is called muddy liquid surface. The surface will go down to the interface of B and C, when zone B disappears (at t = t3).The falling velocity of muddy liquid surface, uc is the also the apparent precipitation v