天大化工原理课件各种版本全英文版bbs

1、124.4 EQUILIBRIUM MOISTURE CONTENT（平衡湿含量） OF MATERIALSMoisture content in solid2. Equilibrium moisture X* and free moisture of a substance2equilibrium moisture X* free moisture2. Equilibrium moisture X* and free moisture of a substancedepends on the material and the conditions of hot air33. Bound an

2、d Unbound Water in Solidsbound water（结合水）:The water in the solid exerts a vapor pressure less than that of liquid water at the same temperature.unbound water（非结合水）:The water in the solid exert only a vapor pressure as high as that of ordinary water at the same temperature4bound water（结合水）: Moisture

3、in cell or fiber walls. Liquid water in capillaries. Water in natural organic materials. unbound water（非结合水）: water on the surface of material.5Properties of water in solidToal waterEquilibrium moistureFree moistureUnbound waterBound water6The amount of bound water or unbound water in a solid depend

4、s only on the properties of material.724.5 RATE OF DRYING1. Drying curves for constant-drying conditions （恒定干燥条件） 2. Experimental determination of rate of drying.3. Drying in the constant-rate period4. Drying in the falling-rate period5. Critical moisture content81. Drying Curves for Constant-Drying

5、 Conditions （恒定干燥条件）Assume that the temperature, humidity, and velocity and direction of flow of the air across the drying surface are constant. This is called drying under constant drying conditions. 91. Experimental determination of rate of drying. The process of experiment Note: the condition of

6、batch-drying experiments should be similar to that of the large-scale operation. bed depth, ratio of drying to nondrying surface (insulated surface)，velocity, humidity, temperature, and direction of the air.10Flow sheet of drying experiment 1洞道干燥室 2离心鼓风机 3孔板流量计 4温度计 5干燥物料 6重量传感器 7加热器 8湿球温度计 9干球温度计10

7、重量显示仪11温度显示仪12湿球温度显 示仪13电加热控制 仪表11Experiment datatime Temperature of material Moisture content of material X12Drying curve （X ）Preheat periodConstant rate periodFalling rate period13Drying curve（ ）Preheat periodConstant rate periodFalling rate period14Conversion of data to rate-of-drying curve. Conv

8、ert the data of X t to the data of rate-of-drying U t15The rate of drying R(干燥速度) is defined asKg water/(m2) :water to be vaporized，kg :is exposed surface area for drying in m2. :is kg of dry solid used, kg16Drying curve（X）Rate-of-drying curve （ U X）Rate-of-drying curve 17Rate-of-drying curvePreheat

9、ing periodConstant rate periodFalling rate periodConstant drying rateCritical moisture content18airFrom inside of solid to surfaceFrom surface to the airWet materialMechanism of drying193. Drying in the Constant-Rate PeriodA continuous water film exists on the drying surface.Heat transferThe surface

10、 temperature = the wet bulb temperature Controlled by the rate of heat transfer to the surface.Drying rate is independent of the solidThe water evaporated is entirely unbound water表面汽化控制阶段20空气传给湿物料的显热水分汽化所需的汽化热=Heat transferMass transferrearranging214. Drying in the Falling-Rate PeriodThe entire sur

11、face is no longer wetted.Heat transferThe surface temperature increaseWater to be evaporated is unbound water and bound water. The surface is completely dry at point D.内部迁移控制阶段225. Critical moisture content XcThe point at which the constant-rate period ends.The properties of the material being dried

12、Thickness of the materialRate of dryingThe resistances of heat and mass transfer within the material2324.6 CALCULATION OF DRYING TIME UNDER CONSTANT DRYING CONDITIONS1. Drying time in constant-rate period2. Drying time of falling-rate period241. Drying time in constant-rate perioda. Method Using Exp

13、erimental Drying Curveb. Method Using Predicted Transfer Coefficients for Constant-Rate Periodc. Effect of Process Variables on Constant-Rate Period25a. Method Using Experimental Drying Curve-drying time of constant-rate period26Preheat periodConstant rate periodFalling rate period2728b. Method Usin

14、g Predicted Transfer Coefficients for Constant-Rate PeriodAssuming only heat transfer to the solid surface by convection from the hot gas to the surface of the solid and mass transfer from the surface to the hot gas 29Therefore To predict Uc , the heat-transfer coefficient must be known. For the cas

15、e where the air is flowing parallel to the drying surface, equation (24.11)For the case where the air is flowing perpendicular to the drying surface, equation (24.12)30c. Effect of Process Variables on Constant-Rate Period(1) Effect of air velocity. (2) Effect of gas humidity(3). Effect of gas tempe

16、rature.(4). Effect of thickness of solid being dried.312. Drying time of falling-rate perioda. Method using graphical integration or numerical integrationb. Rate is a linear function of X.c. Rate is a linear function through origin.32a. Method Using Graphical Integration or Numerical Integration The

17、 time for drying for any region between Xc and X2 has been given, -drying time of falling-rate period33Ifb. Rate is a linear function of X. Pass two points Therefore 34integrating35c. Rate is a linear function through origin. Total drying time3624.7 DRYING EQUIPMENTDryer for solids and pastes37Tray dryer(箱式干燥器）Chamber dryer 9-0238Screen-conveyor dryer(带式干燥器）394041Turbodryer( 转盘式干燥器）42Fluid-bed dryer(流化床干燥器）Fluidized bed dryer 9-0443Rotary