884化工原理的電子版本課件雙語chapter12

1、1Chapter 12 Drying2calculation of drying processbasic rules of convective dryingdrying speed and ratemass balanceheat balance3basic rules of convective dryingBecause drying processes are complicated, its rate is often measured in experiment in addition to predicted with theoretical model.For simplif

2、ying the effect factors, the experiment is done usually under constant condition. i. e., constant gas T, H, u and certain gas-solid contacting form. For keeping constant gas T, H and u, large flow rate of gas and small quantities of material are used in the experiment.The experiment is in batch oper

3、ation. Therefore, the moisture content and temperature of the material are changed with the time.drying experiment4Constant-rate period keeps at tw，X changes with time linearly. Here the heat transferred from gas is totally used for moisture evaporation.Preheat period the moisture content and temper

4、ature change from its initial values to XC and tw.。In this period, raises, but the X changes very little.Drying curve: the relationship of X vs. drying time, .drying and drying-rate curvesAXXctwDCBADCBtX*surface T,drying time, preheatconstantratefallingrateFalling-rate period increases with time, X

5、change rate slow down. This means that, part of the transferred heat is used for heating the material rather than for evaporating the moisture. This period ends at X = X* where t.5Drying curve The shape of falling rate period depends on the structure and moisture adsorb nature of materialPorous mate

6、rial moisture moves to surface mainly in capillarity. Such as in actived carbon and catalysts.Nonporous material moisture moves in gas or liquid diffusion. Such as in soap, wood and leather. For hygroscopic material with strong link with water, the internal water is difficult to dry. But with non-hy

7、groscopic materials, there is little water in it, the falling rate is not so typical. 6drying speed and rateDrying rate, U evaporated moisture mass per unit time (kg/s). where U drying rate in the dryer，kg/s W evaporated moisture mass，kg Gc mass of absolute dry material，kg X moisture content in dry

8、base，kg/kg drying time，s。 Drying speed or flux, N evaporated moisture mass per unit material surface area per unit time (kg/(m2s)where N drying speed，kg/(m2s) A material surface area or transfer area，m2If the material surface area is easily found. This is convenient.If the surface area can not be fo

9、und, this is often used.7 Let the initial and end moisture contents areX1，and X2, respectively. if X1Xc and X2Xc , the process has two periods. if X2Xc , the process has only one period, the constant rate.Since the preheat period is very short, it is often merged into constant rate period. Thus, the

10、 process is divided into constant and falling rate periods in boundary of the critical moisture content, Xc . The relationship of U or N vs. X. This is straightforward to show the details of the drying process.drying rate curveABCDU or NABCDtwXcX*XIIIC8the reason of the rate change How to explain th

11、e shape of the curve ?Drying is a mass-heat transfer process and the transfer rates can be expressed aswhere q heat transfer flux，W/m2； N drying or mass transfer flux，kg/(m2s)； Q heat transferred，J； W moisture mass in material，kg； h convective heat transfer coefficient，W/(m2K)； kp convective mass tr

12、ansfer coefficient，kg/(m2ps)； t gas temperature，； surface temperature of material，； pi moisture partial pressure on surface，kN/m2； p moisture partial pressure in gas，kN/m2。 9 In constant rate period, the surface is wet，X Xc，the evaporated is bined water. On the other hand, in falling rate period, X

13、XcThe real evaporation area reduces ( dry zone appears), (the first period)；The evaporation surface moves in, (the second period)；Vapor partial pressure reduces (combined water)；Water diffusion is very slow in solid.constant condition = tw，p = psh and kp constant Here, the moved moisture from depth

14、of the material to surface is enough to keep there fully wet, the drying rate is controlled by the surface evaporation ( or outer drying conditions).q and N constantthe reason of the rate change Here the drying rate depends on the material structure and the water/solid combine form, not the outer co

15、nditions.10critical moisture content Xc is the boundary of two periods and is very important for drying method planning and optimization. material air conditions Xcmaterialthickness, mmu, m/sT,%kg/ kgclay6.41.0370.100.11clay15.91.0320.150.13clay25.410.6250.400.17kaoline302.1400.400.181leather101.549

16、-1.25sand 0.044mm252.0540.170.21 0.0440.074mm253.4530.140.10 0.0740.177mm253.5530.150.053 0.2080.295mm253.5550.170.053 news paper-0190.351.00hemlock wood254.0220.341.28Wool cloths-25-0.31Following table shows typical Xc data. Xc depends on the thickness and other sizes, drying rate, not a property o

17、f material.11effect factors to drying ratematerial size Reducing size will increase surface area and accelerate drying process. Following is an example, for 1kg of clay,gas/solid contact typeclay dp，m0.12410-210-310-410-510-6area A，m20.0240.33303003000（c）gas passes through the material from bottom t

18、o form fluidized bed (good)（b）gas passes through the material from top to form fixed bed（a）gas passes the material surface parallel (poor)12Gas condition Material nature Enhance gas conditions (T ，H ，u ) can decrease gas resistance. Thus to increase drying rate in constant rate period. However, it i

19、s almost no use to falling rate period.Enhance gas condition will increase Xc , which means that more moisture will be evaporated in falling rate period.T is limited by heat source and material sensibility. Low H means more gas and more energy consumption .No effect to drying rate in constant rate p

20、eriod. Its structure and link form with water affect drying rate in falling rate period strongly.Product properties should be considered in enhancing drying condition to avoid product damage or burning. 13Mass balanceG1 flow rate of feed，kg/sG2 flow rate of product, kg/sGc flow rate of absolute dry

21、material，kg/s； w1 initial moisture content w2 product moisture content L flow rate of absolute dry gas，kg/s； H1 humidity of entering gas H2 humidity of leaving gasW evaporated moisture per unit time，kg/swet feedG1 , w1productG2 , w2hot gasL , H1flue gasL , H2absolute dry gasconsumption for 1kg evapo

22、ration14Heat balancewhere Qp heat from preheater to gas，kW； Qd heat supplied in the dryer，kW； Ql heat loss of dryer，kW。 wet feedG1 , w1 , 1, cm1productG2 , w2 , 2, cm2hot gasL, H1, t1, i1wet flue gasL, H2, t2, i2cold gasL, H0, t0, i0QpQdQlpreheaterdryer15heat balance for whole drying system Under st

23、eady condition, in 1 second timefeedG1 , w1 , 1, cm1productG2 , w2 , 2, cm2hot gasL, H1, t1, i1flue gasL, H2, t2, i2cold gasL, H0, t0, i0QpQdQlgas enthalpychangematerial enthalpychangematerial enthalpygas enthalpy changeSince 16heat for evaporating moisture material enthalpy changeheat for heating p

24、roduct heat loss to flue gastotal heat balance heat balance for whole drying system 17Air preheater can be divided asDirect heated use liquid, gas or solid burned flue gas directly as drying agent. Indirect heated use fresh air which is heated indirectly by other heat source as drying agentThe heat supplied from the preheater isIf gas