1 Chapter 10 Drying of Solids

2 Introduction

3 Introduction 1.Methods for removing liquid from solid materials (1)Mechanically: By presses or centrifuges, etc. (2)By physical-chemical process: Only small amounts of liquid removed. (3)Thermally: Liquid is heated and vaporized, and then removed---Drying.It is generally cheaper to remove liquid mechanically than thermally, and thus it is advisable to reduce the liquid content as much as practicable before feeding the material to a heated dryer.

4 Introduction 2.Purposes and applications of dryingPurposes: Drying makes materials more convenient in packaging, transporting, preserving, fabricating, and applying; and improves quality of products.Applications: ….

5 Introduction 3.Classification of drying processes (1)Atmospheric and vacuum drying processes. Question: why no drying process with pressure greater than 1 atm? (2)continuous and batchwise drying processes. (3)According to the heat transfer manners: 1)Indirect dryers/dryers in which heat is transferred to the solid from an external medium(condensing steam, etc) 2)Convective/direct dryers/Dryers in which the solid is directly exposed to a hot gas

6 Introduction 3)Drying by radiant energy 4)Drying by dielectric/microwave energy 5)Freeze-drying 6)Combining drying Most common used: Convective/direct dryersMost common used: Convective/direct dryers Common used drying medium: air;Common used drying medium: air; Common liquid existing in solids: Water.Common liquid existing in solids: Water.

7 4.Drying conditions for convective dryers Question: Is the drying process a combination of heat transfer and mass transfer?Why? What is the effect of drying medium? Question: Is the drying process a combination of heat transfer and mass transfer?Why? What is the effect of drying medium? Heat transfer is the means, and mass transfer is the end. Heat transfer is the means, and mass transfer is the end.

8 Schematic diagram of convective drying process 4.Drying conditions for convective dryers4.Drying conditions for convective dryers Schematic diagram of convective drying process Schematic diagram of convective drying process

9 5.Problems discussed in this chapter (1)Properties of Moist air; (2)Material balances——Air flow rate; (3)Heat balances——Flow rate of heating steam, heat transfer area of heater; (4)Calculation of drying rates and drying time; (5)Equilibrium relationship; (6)Selection of drying equipment; (7)Operation and enhancement of drying equipment.

10 (1)Properties of Moist air; (2)Phase equilibria of drying process; (3)Drying curve and drying rate curve under constant drying conditions; (4) Material and heat balances, calculation of drying rates and drying time; (5)Principles and structures of typical drying equipment. Principal subject matter of this chapter

12 1 Properties of Moist Air and Humidity-enthalpy Chart （ H-I ） or temperature-humidity(t-H) chart [Reading Materials: ] Moist air=Dry air+water vaporMoist air=Dry air+water vapor

13 (1)Moist air can be considered as ideal gas, I.e., mixtures of gas and vapor follow the ideal gas laws. (2)The usual basis for engineering calculations: (a)A unit mass of vapor-free air/dry air[ 计算基准 ：单位质量的绝干空气 ], where vapor means the gaseous form of the component that is also present as liquid and gas is the component present only in gaseous form; (b)A unit mass of bone-dry material[ 单位质量的 绝干物料.]

14 1. Moist air properties (1. a).Humidity H [kg water vapor/kg dry air] p=p v —partial pressure of water vapor in moist air, N/m 2 ; p g —partial pressure of dry air in moist air; P—total pressure of moist air, N/m 2. H=f （ P ， p ）

11 Difficult points: 1) Understanding adiabatic saturation temperature tas and wet-bulb temperature tw; 2)Heat balances of non-ideal drying process.

15 Where p=ps=f(t), ps=vapor pressure of water, Hs=f(P, t) (1. b).Saturation humidity Hs: [kg water vapor/kg dry air] or[ kg 水气 /kg 干空气 ], when p=p s

16 (2)Specific volume of moist air : Specific volume of dry air : （ 5-6 ） Specific volume of water vapor: Specific volume of water vapor:

18 (3) Relative humidity  p and p s are under the same temperature. Saturated air is air in which  =100%. air capacity of absorbing water vapor  When p=0,  =0 ，湿空气不含水，为绝干空气。 When p= p s,  =100%, 空气中水气已饱和，无干燥 能力 。

18 (5)Total enthalpy of moist air ： I 焓 [kJ/kg dry air] 。 (4)Humid Heat C H ： [KJ/ （ kg dry air· ℃） ]

19 Assignment: Problem5-1 Attention: Enthalpy is a relative value. Datum temperature: 0 ℃. At 0 ℃, enthalpy of dry air or liquid water is zero.

20 Problem5-1. The total pressure of moist air at 60 0 C and 40% relative humidity is 50kPa. Calculate, (1)Partial pressure of water vapor in the moist air; (2)Humidity; (3)Density of the moist air. Hint:

21 Example5-1. The humidity of moist air at 1 atm and 20 ℃ is kg/kg dry air, and p s (at 20 ℃ ) is kPa. Calculate: (1) Relative humidity of the moist air; (2)Humid specific volume; (3)Humid heat; (4)Total enthalpy. If the moist air is heated to the temperature of 50 ℃, calculate the above items respectively.

22. (6)Dry-bulb temperature t and wet-bulb temperature t w t Under steady- state, t w <t. Why ？ Make-up water twtw u>5m/s (Turbulent flow) Dry-bulb temperature t: the actual gas temperature. Dry-bulb temperature t: the actual gas temperature.

23 The liquid temperature when steady state is reached is the wet bulb temperature. Meanings of the symbols refer to p If air is not saturated, some liquid evaporates, cooling the remaining liquid until the rate of heat transfer to the liquid just balances the heat needed for evaporation.If air is not saturated, some liquid evaporates, cooling the remaining liquid until the rate of heat transfer to the liquid just balances the heat needed for evaporation.

24 When steady state is reached, Combining above equations gives, Mass transfer coefficient based on

25 Implications of Eq.(5-12):

26 When measuring tw: In order to ensure  /k H =Const ， air flow must be kept in turbulent state. Experiments prove that when u>5m/s, the radiant and conduction effects can be omitted. In order to ensure  /k H =Const ， air flow must be kept in turbulent state. Experiments prove that when u>5m/s, the radiant and conduction effects can be omitted. 3)When  <1, t w <t; When  =1, t w =t. Measurement of t as of air (t w ) 2)Measurement of humidity H: A very common method of measuring H is to determine simultaneously the t w and t.