化工四大化学(专业)解吸(课堂讲义)

1、1CHAPTER 7&29 Mechanical Separations2n1. Diffusional operations: nseparate homogeneous mixturesndistillation, gas absorption, extractionn2. Mechanical separations:nseparate heterogeneous mixturesnsedimentation, filtrationSeparations are divided into two classes.Mechanical methods are based on th

2、e physical difference between particles, such as size, shape or density, is the subject of this chapter.3Application examples of mechanical separationsnHeterogeneous mixtures nSolids from gases, nLiquid drops from gases, nSolids from solids, nSolids from liquids 4ContentnChapter 29nScreening(p986-99

3、1)n29.6. Filtration(p991-1017)n29.3. Gravity sedimentation process (p1035-8)n29.4 Centrifugal sedimentation process (p1045-8)nChapter 7n29.1. Drag and drag coefficient(p150-p157)n29.5. Flow through beds of solids(p157-161)n29.2. Motion of particles through fluid(p162-171)n29.7. Fluidization(p171-182

4、)5Topics in this chaptern1. Gravity(centrifugal) sedimentation process. 重力（离心）沉降过程n2. Filtration 过滤n3. Fluidization 流态化629.1. Drag and drag coefficient (p150-p157)n1. Drag coefficientsn2. Drag coefficients of typical shapesn 3. Form drag and streamliningn 4. Stagnation pressure71. Drag coefficientsF

5、IGURE 7.2 Flow past immersed sphere.81. Drag coefficientsp cos a dAFigure 7.1 Wall drag and form drag on immersed bodypressureshear forceswall dragform drag9nTotal drag = wall drag + form dragnFD=w sin a dA + p cos a dA. (1). Drag FD The force in the direction of flow exerted by the fluid on the sol

6、id . 10n(2) drag coefficients nJust as friction factor, drag coefficient is defined:2/20uAFCPDDWhere, FD: is the total drag, N. Ap projected area, m2. u0 is the velocity of the approaching stream (by assumption u0 is constant over the projected area), m/s. (7.1)11nFrom dimensional analysis, the drag

7、 coefficient of a smooth solid in an incompressible fluid depends upon a Reynolds number and the necessary shape factor.n CD= (Rep, shape factor)nThe Reynolds number for a particle in a fluid ppDG0Re 00uG where Dp = characteristic length of the particle12sphericity s球形度nThe surface-volume ratio for

8、a sphere of diameter Dp divided by the surface-volume ratio for the particle whose nominal size is Dp . nThis surface-volume ratio is 6/Dp for a sphere, since Sp =Dp2 and vp=(1/6)Dp3 , Thus pppsvsD/6psppDvs6(7.10)(P158) Table 7.1 Sphericity of several materials( / )(/)sppps vssvs 132. Drag coefficie

9、nts of typical shapes14For a spherenWhen Re particle will go down p= particle will float p particle will go up18Three forces act on a particle moving through a fluid:Only one-dimensional motion is consider here.FeFbFD(3) the drag force,FD.(1) the external force, gravitational or centrifugal, Fe;(2)

10、the buoyant force, Fb;192. Equations for one-dimensional motion of particle through fluid Newtons 2nd law of motionDbeFFFdtdum(7.25)u is the velocity of particle relative to the fluid.20nTherefore,eemaF pebamF22pDDAuCFmAuCamAuCaadtdupDppepDpee2222The external forceThe buoyant forceThe drag force(7.2

11、6)(7.27)(7.28)(7.1)(7.29)21Motion from gravitational force nae=gmAuCgdtdupDpp22(7.30)Motion in a centrifugal field. 2raemAuCrdtdupDpp222(7.32)where r = radius of path of particle, m; = angular velocity, rad/s; u = the velocity of the particle relative to the fluid and is directed outwardly along a r

12、adius, m.223. Terminal velocitynterminal velocity utmAuCamAuCaadtdupDppepDpee2222(7.29)the time for acceleration to the terminal velocity is still quite small and is often ignored in analysis of the process.23For centrifugal sedimentationDppptCAmgu)(2DppptCAmru)(2For gravitational sedimentation(7.33

13、)(7.34)244. Drag coefficient nFigure 7.6 Drag coefficients for spheres and irregular particles. 25Figure 7.326free settling(自由沉降） : the particles fall is not affected by the boundaries of the container and other particles.hindered settling（干扰沉降） : the motion of the particle is impeded by other parti

14、cles.275. Motion of spherical particles nIf the particles are spheres of diameter Dp, ppDm361241ppDASubstituting DppptCAmgu)(2DpptCDgu3)(4(7.33)(7.37)28nFor Rep1pDCRe24ptDDuF318)(2pptgDu -Stokes law For 1,000 Rep 200,000 44. 0DC22055. 0tpDuDF )(75. 1pptgDu-Newtons law (7.40)(7.43)29For 1 Rep 1000 ,

15、intermediate range6 . 0Re/5 .18pDC6 . 0Re27. 0psptgDu43. 029. 071. 014. 171. 0153. 0sptDguor30Calculation the terminal velocity of spheresn1. the terminal velocity （or diameter of particle）can be found by trial and error.Choose equation to calculate utCheck flow regimeAssume flow regimeCalculate Ret

16、NOYESut312. Find a criterion which does not contain ut.326. Criterion for settling regimeLet 18)(2pptgDu2318)(RepptppgDuDStokes law regimeTherefore3/12)(ppgDKWhen Rep 1, 18Re3Kp6 . 2K33Therefore, for 1,000 Rep 200,000 )(75. 1pptgDu2)(75. 1ReppppDgDSame for Newtons law regimeUse 3/12)(ppgDK23609 .68

17、K5 . 175. 1ReKp34If K less than 2.6, Stokes law applies. If K greater than 2.6 but less than 68.9, intermediate range applies.If K is greater than 68.9 but less than 2,360, Newtons law applies. EXAMPLE 7.1 p1673/12)(ppgDKCriterion 35中文教材 is used as criterion of settling regime when calculating Dp.32

18、13)(4RetppDugC2323)(4RepppDgDC is used as criterion of settling regime when calculating ut.DC362ReRett1ReRett3224()Re3stdg 2RettRetReduttRe1RettRe1234 ()Re3sttgu tRettudReRet377. Hindered settling干扰沉降 n1. each particle are affected by the presence of nearby particlesn2. the particles in settling dis

19、place liquid, which flows upward and makes the particle velocity relative to the fluid greater than the absolute settling velocity n3. Comparing with free settling, the drag force is much greater, and terminal velocity is much small for hindered settling.3829.3. Gravity sedimentation process (p1035-

20、1038)n1. Removal of solids from gasesn2. Sorting classifiers 39nClarifier(澄清器): A settler that removes virtually all the particles from a liquid. nClassifier(分级器): a device that separates the solids into two fractions.nThe same principles of sedimentation apply to both kinds of equipment.401. Remova

21、l of solids from gasesnGravity settling chamber is shown in the following figure.Settling box or settling tank41nChamber: length-ln width-bn height-Hnu-velocity of the gasnut-velocity of the particle settling towards floor, terminal velocity.n-the time of residence for the gas in the chamber, =l/u.n

22、t -the time of particle settling from top to bottom of the chamber, t =H/ut.42nWhere Vs is the volumetric flow rate of gas.nTherefore,tHbVusttsAubluVIt should be satisfactory,ul /ttuH /Therefore, Vs is independent of the height of the chamber, and t can be reduced by decrease H.43nMultilayer chamber

23、 多层降尘室 n Vs=(n+1)(bl)utnBut on the other hand, u=Vs/bH must smaller than a certain value to prevent from particle being blowing away again. 44Figure for the multilayer settling box452. Sorting classifiers nDevices that separate particles of differing densities are known as sorting classifiers. nTher

24、e are two method:n(1). sink-and-float n(2). differential settling 46(1). Sink-and-float method(heavy-fluid separation)nA particle + B particle + a liquid sorting mediumn A B mn A(or B)mB(or A)nSeparation depends only on the difference in the densities of the two substance and is independent of the p

25、article size.47(2). Differential-settling methodnA particle + B particle + a liquid sorting mediumn A B mn A(or B) B(or A) mnSeparation of the particles into several size fractions based upon the difference in terminal velocity.nThe density of the medium is less than that of either substance. 48Clas

26、sifier 水力分级器Water velocity can be adjusted by the position of M.49n Consider particles of two materials A and B settling through a medium with density and viscosity . 18)(2pApAtAgDu18)(2pBpBtBgDu(29.66)(29.67)A is galena方铅矿(diameter DpA, density pA)B is quartz石英(diameter DpB, density pB). For settli

27、ng in the Stokes law regime. 50The relationship of DpA and DpB for equal-settling particles is,tBtAuupApBpBpADDequal-settling particles: particles which have equal settling velocity 18)(2pApAtAgDu18)(2pBpBtBgDuFrom: (29.68)51For settling in the intermediate range, the relation of diameters of equal-

28、settling particles is, 625. 0pApBpBpADDFor settling in the Newtons law regime, the relation of diameters of equal-settling particles is, pApBpBpADD(29.69)52Figure 29.27 equal-settling particles53nEXERCISE A mixture of galena(A) and silica(B) sphere particles has a size range of 0.14mm to 1.0mm in di

29、ameter and is to be separated by rising stream of water at 293.2K. The density of A is 7500kg/m3 and that of B is 2650kg/m3. What is the size range of pure product? And what velocity of water flow is needed to get pure product?54classifier5529.4 CENTRIFUGAL SEDIMENTATION PROCESSES (p1045-1048) Princ

30、iples of centrifugal sedimentation Separation of solids from gases; cyclones Liquid-solid separations; hydroclones Centrifuges56Settling velocityFor a sphere particleDppptCAmru)(2DpptCDru3)(42For the settling in stokes law regime18)(22pptDru(7.34)1. Principles of centrifugal sedimentation57Separatio

31、n Factor : 分离因数The ratio of the centrifugal force to the gravity force.rgumgrmuFFgc2tan2tan/(29.72) For a cyclone 1 ft (0.3 m) in diameter with a tangential velocity of 50 ft/s (15 m/s) near the wall, the separation factor, is 2,500/(0.5 x 32.2) = 55. Separation efficiency is much greater in centrifugal separator.582. Separation of solids from gases; cyclones 旋风分离器an outlet for dustFigure 29.33 cyclonea vertical cylindera conica