884化工原理的电子版本课件双语chapter5

1、1Chapter 5 Separation of particle and fluid2precipitation 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, th

2、e 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） 3gravitational precipitationut represent the speed of separation. ut is proport

3、ional 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 reaso

4、n is: the collision between the particles changes their momentum and velocity. Only when the particle concentration is lower than uc. Critical point when zone C disappears (at t = t5)，there are only zones A and D with clear interface.Deposit compression the distance between deposited particles reduc

5、es and the liquid between particles rises to the clean liquid zone. It will take a long time. Wall effect If dp /D 0.01，the wall effect will reduce the precipitation velocity.6dust settling chamberUsed for removing dust from gas.The gas (horizontal) velocity is u, and dust terminal velocity is ut .

6、Let the particle horizontal velocity is also u，then it takes time, t = L/u to pass the whole length L. On the other side, a particle of ut takes time, t = H/ut to settle through a distance of H . Therefore, the condition to settle all particles of diameter, dp is t t，or gas ingas outHBLuttVuV7dust s

7、ettling chamberThe maximum flow rate for this chamber The capacity of a chamber is proportional to a) terminal velocity of particles. b) area on settling direction. it is nothing to do with the height of the chamber.Commercial equipment are low in height or divided into several rooms on vertical dir

8、ection.Partitions, or baffles improve the separation efficiency, because they reduce the precipitation distance.dirty gasclear gas dust1234dirty gasclear gas dust123dust58dust settling chamberFor homogeneous distributed inlet particles, the separation efficiency in the chamber, d can be For a certai

9、n structure and a certain qv, the size of smallest particle which can be collected in the chamber isFollowing this Eq., separation efficiency curves for particles with different sizes can be drawnIf a particle of diameter dp and terminal velocity ut drops a height y in time t , then9Settling tank Ba

10、tch or continuous. work in disturbance precipitation.In this settling tank, the liquid height can be divided into 4 zones of disturbance precipitation.The important thing is that to distribute uniform the inlet and outlet liquid.For a continuous tank, data from batch experiment should be corrected b

11、efore use.10Centrifugal precipitationGravitational acceleration is small and leads to large precipitation equipments. If put the same precipitation system in a centrifugal field of acceleration, r2，the terminal velocity (for small particle，Rep2.0，Stokes Law) will be when r and uT ，then a . A high ef

12、ficiency can be achieved for low density, small size particles.Note direction of ur is radius，but the particle moving direction is not the same. It is along a expanding spiral orbit. Centrifugal separation equipments：gas or liquid cyclone fluid rotates in a static equipment.centrifugal subsider flui

13、d rotates with rotating machine.11cyclone Gas cyclone for gas solid systemLiquid cyclone for liquidsolid systemApplication for separating particles larger than 5 m with concentration of 0.1500g/m3.Structure dirty gas flows into cyclone along tangential direction at a high velocity, forms an outer cy

14、clone field towards bottom between the column wall and the exhaust pipe. The gas at the bottom then flows up along a inner cyclone direction, and finally flows out through a exhaust pipe. The dust particles are pushed toward the wall by the centrifugal field, drop down to the bottom, and leave the c

15、yclone from a dust opening.dirty gasclear gasdustBB12Separation efficiency of cyclonesTotal efficiency the ratio of separated particle mass to total mass of inlet particlesc1, c2 particle mass concentration in inlet and outlet gas (kg/m3) ci1, ci2 particle mass concentration in inlet and outlet gas

16、for size diGrain-size efficiency the efficiency to separate particles of a certain size, dixi mass fraction of size di particles in inlet gasIt is widely used in industry.13Separation efficiency of cyclonesGrain-size efficiency is used to study the detail performance of the cyclone. For a certain cy

17、clone, the total efficiency is higher than its grain-size efficiency for small dust particles.Let the precipitation distance is the width of gas inlet pipe, B, then the needed time to separate a particle of size dc isWhere for tangential inletdc (or d100) critical size, the minimum diameter which ca

18、n be separated (i=100%) in a cyclone of inlet velocity ui .14Separation efficiency of cyclonesBased on the condition of and omitted the gas density, the critical diameter for a cyclone will beAssume the gas rotates in the cyclone for N turns (for standard, N=5) with average radius r，then the average

19、 residence time is This Eq. gives the effects of cyclone structure and operation conditions to its efficiency. It can be seen that for a small r and a high ui , a high separation efficiency will be achieved.Grain-size efficiency can be expressed as a function of di/dc 15Separation efficiency of cycl

20、onesIdeal all particles larger than dc will be separated.Practical some of those particles will not be separated. The reason is some of the particles of ddc are taken out before they reach to the wall or raised again after the reaching. Some of the particles of ddc can be separated because of agglom

21、eration effect. Practical critical size is lager than the calculated value. In engineering, a cutting size d50（grain-size efficiency= 50% ）is more often applied. Meaning of d50 assume that on those particles the centrifugal and resistance forces are balances and their position is at the half of the

22、precipitation distance. Therefore, they have 50% chance to be separated. d50 is common used as a basic parameter to give grain-size efficiency16Separation efficiency of cyclonesSince particles of size d50 positioned at the half of the precipitation distance, B/2, we havegrain-size efficiency curve f

23、rom experiment0.81.023456789f1fh()50cddddor17Pressure drop of cyclonesFeatures of cyclone large flow rate, low head. the pressure drop comes from(1) gas expansion and contraction(2) gas acceleration(3) friction and local resistances An empirical equatio

24、n is used to calculate the pressure dropresistance factor decided by structure. is almost constant for cyclones of different size in a same series. The value of is 5.08.0 . If ui，but pressure drop，not considerable. The pressure drop is controlled in the range of 0.52 kPa (corresponding ui =1520m/s).

25、18Types of cyclones different structure：standard, long cone, diffusion and by-passIn China，the common types are CLT, CLT/A, XLP/A, XLP/B and CLK, etc .The No of the cyclone represents its size, and their details can be found in handbooks.19Types of cyclonesXLP型：带旁路，有 A、B 两种形式。特点为进气口上沿梢低于筒体顶部，含尘气体进入筒

26、体后随之分为两路，较大的颗粒随向下旋转的主流气体运动，沉到筒壁落下；细微粒则随一小部分气体在顶部旋转聚集形成灰环，再随气流经旁路分离室旋转向下并沿壁面落下。 这种结构能促进细粉尘的聚结，对细粉的分离效率较高。阻力系数一般为 6-7。 20Types of cyclonesCLK 型：扩散式旋风分离器，带倒锥体，并在锥的底部装有反射屏，反射屏可使已被分离的粉尘沿着锥体与反射屏之间的环缝落入灰斗，有效防止了上升的净化气体重新把粉尘卷起带出，从而提高了除尘效率。 适用于捕集粒度在 5-10m 以下的干燥的非纤维颗粒粉尘。阻力系数在 7.5-9 之间。 21Types of cyclones新型高效低

27、阻旋风分离器用于石油、化工、肥料、冶金、煤炭及环保的尾气除尘和高炉烟气净化、颗粒回收等。已成功应用于氮肥厂造气炉的除尘及煤粉回收，也可用于石油化工装置中如丙烯腈、苯酐、百菌清、苯胺等作为流化床内旋风分离器用以回收昂贵的细颗粒催化剂，以及钙镁磷肥回转窑烘干、球磨系统的尾气除尘和高炉烟气净化等。 在正常工作状态下，气固分离效率不小于99%，阻力不大于1000Pa。22Types of cyclones为长岭炼油化工有限公司制造的旋风分离器在预组装为九江石油化工厂制造的旋风分离器在预组装为上海氯碱股份有限公司制造的氧氯化反应旋风分离器在预组装23Types of cyclones多管旋风分离器 用高

28、效分离单管（可多达90根）组装成多管并联的大型旋风分离器，用于处理 600700 的含有微小催化剂颗粒的高温烟气，其临界粒径 dc8.0mm，负荷可达 1500Nm3/min，分离总效率092%。 此种型号的旋风分离器已成为 FCC装置的关键设备。 24Centrifugal subsider Suitable for suspensions, especially the system with small particle size and small density difference. It can be used for separating protein and other v

29、ery fine particles with liquid. The separation factor can be up to 50,000. principle of centrifugal subsider RR 0slurryclear liquiddrum25Capacity of centrifugal subsiderAssume the suspension flow in the subsider is a piston type，i. e., flow from one end to another in an uniform velocity and distribu

30、tes on the whole ring surface of the drum wall. The particles move to the wal l at velocity u and stop on the solid surface. The liquid overflows from the final end . The terminal velocity can be calculated by the Stokes Law The maximum distance in radius direction is from r=R0 to r=R，so the precipi

31、tation time needed is 26Capacity of centrifugal subsider If the working length is L and the flow rate is V(m3/s), then the average residence time of the particles is If the capacity is given, the critical size to be separated islet ，then the theoretical capacity for critical particle size dp is27Typ

32、es of centrifugal subsiderAccording to their structure, the subsiders can be divided as 无孔转鼓离心机 间歇操作。悬浮液由转鼓底部加入，随转鼓高速旋转，在离心力作用下颗粒向转鼓壁沉降，清液从内层溢流。随着鼓壁上沉渣增厚，液体有效流道面积减小，轴向流速增大，临界粒径增大，溢流液澄清度降低，到一定程度时则停止加料，降速后用机械刮刀或停机后人工卸出沉渣。常用于处理粒度为 540m、固液密度差大于 50kg/m3、固含量小于 10% 的悬浮液分离。 28types of centrifugal subsider螺旋

33、卸料沉降离心机 有卧式和立式两种。连续操作，悬浮液经加料管由螺旋内筒进料孔进入，随同转鼓高速旋转，固体沉降到鼓壁，由与转鼓有一定转速差的螺旋向小端输送并排出，清液则由转鼓大端溢流而出。分离因素可达 6000。可处理粒度 2mm5mm，固含量小于 10%50%，固液密度差大于 0.05g/cm3 的悬浮液。 horizontal with screw discharger1螺旋送料器；2机壳；3转鼓；4行星差速器 29types of centrifugal subsider碟式离心机(薄层分离沉降离心机) 转鼓内装有一叠随转鼓旋转的倒锥形碟片，碟片间隙为0.51.5mm，分离因数可达300010000。悬浮液由中心管引入转鼓，分配在碟片之间形成薄层流动。在离心力作用下，颗粒沉降到碟片内侧表面并向外滑动