相际扩散与传质原ppt课件

8.1 Introduction 8.2 Equilibrium between liquid&gas 8.3 Diffusion&mass transfer within single phase 8.4 Between g-phase&l-phase for mass transfer 8.5 Gas absorption at low concentration 8.6 Absorption from rich gases 8.7 absorption plus reaction 2020学时学时第第1717章章 相际分散相际分散与传质原理与传质原理 分散是一种在物理驱动下单组分经过混合物的运动。分散最通常的缘由是分散组分浓度梯度的存在。Diffusion is the movement,under the influence of a physical stimulus,of an individual component through a mixture.The most common cause of diffusion is a concentration gradient of the diffusing component.Chapter 17 Principle of Diffusion and Mass Transfer Between Phase A concentration gradient tends to move the component in such a direction as to equalize concentrations and destroy the gradient.浓度梯度驱使分浓度梯度驱使分散组分朝着使浓散组分朝着使浓度均匀化的方向度均匀化的方向挪动，使浓度梯挪动，使浓度梯度消逝。度消逝。When the gradient is maintained by constantly supplying the diffusing component to the high-concentration end of the gradient and removing it at the low concentration end,there is a steady-state flux of the diffusing component.This is characteristic of many mass-transfer operations.当不断地向当不断地向高浓度一端提供高浓度一端提供分散组分，从低分散组分，从低浓度一端移去分浓度一端移去分散组分，分散组散组分，分散组分便具有稳定的分便具有稳定的传质通量。这就传质通量。这就是许多传质操作是许多传质操作的特点。的特点。例如，当在填料塔中例如，当在填料塔中用水吸收气体中的用水吸收气体中的NH3NH3时，在塔内的恣时，在塔内的恣意一点上，在气相浓意一点上，在气相浓度梯度的驱使下，度梯度的驱使下，NH3NH3由气相主体分散由气相主体分散至气液界面，在界至气液界面，在界面上溶解。然后，在面上溶解。然后，在液相浓度梯度驱使下，液相浓度梯度驱使下，分散至液相主体。分散至液相主体。For example,when ammonia is removed from a gas by absorption in water in a packed column,at each point in the column a concentration gradient in the gas phase causes ammonia to diffuse to the gas-liquid interface,where it dissolves,and a gradient in the liquid phase causes it to diffuse into the bulk liquid.In stripping a solute from a liquid the gradients are reversed;here diffusion brings solute from the bulk liquid to the interface and from there into the gas phase.当从吸收液中当从吸收液中解吸溶质时，解吸溶质时，浓度梯度与吸浓度梯度与吸收过程相反。收过程相反。此时，分散作此时，分散作用使溶质从液用使溶质从液相主体移至界相主体移至界面，再从界面面，再从界面移至气相。移至气相。In some other mass-transfer operations such as leaching and adsorption,unsteady-state diffusion takes place,and the gradients and fluxes decrease with time as equilibrium is approached.在其它一些诸如浸在其它一些诸如浸取和吸咐操作中发取和吸咐操作中发生的是不稳定分散，生的是不稳定分散，随着过程趋于平衡，随着过程趋于平衡，分散组分的浓度梯分散组分的浓度梯度和通量随时间而度和通量随时间而降低。降低。Although the usual cause of diffusion is a concentration gradient,diffusion can also be caused by an activity gradient,as in reverse osmosis,by a pressure gradient,by a temperature gradient,or by the application of an external force field,as in a centrifuge.虽然分散的普通缘由是浓度梯度，但能动梯度也能引起分散如反浸透过程，压力梯度、温度梯度和外场力都可引起分散，如离心分别。Molecular diffusion induced by temperature is thermal diffusion,and that from an external field is forced diffusion.Both are uncommon in chemical engineering.Only diffusion under a concentration gradient is considered in this chapter.由温度引起的分子由温度引起的分子分散称为热分散，分散称为热分散，由外场力引起的分由外场力引起的分散称为强迫分散。散称为强迫分散。这两种分散方式在这两种分散方式在化学工程中并不常化学工程中并不常见。本章仅讨论由见。本章仅讨论由浓度梯度引起的质浓度梯度引起的质量分散。量分散。Diffusion is not restricted to molecular transfer through stagnant layers of solid or fluid.It also takes place when fluids of different compositions are mixed.分散并不局限于分子经过固体或液体停滞膜的传送。当不同组成的流体相混合时也会发生分散。The first step in mixing is often mass transfer caused by the eddy motion characteristic of turbulent flow.This is called eddy diffusion.The second step is molecular diffusion between and inside the very small eddies.混合过程首先是因湍流的涡流特性引起的质量传送。这一过程被称为涡流分散。其次是在微小的漩涡间及其内部的分子分散。Sometimes the diffusion process is accompanied by bulk flow of the mixture in a direction parallel to the direction of diffusion.有时分散过程还伴随着与分散方向相平行的混合物主体流动。In all the mass-transfer operations,diffusion occurs in at least one phase and often in both phases.In distillation,the low boiler diffuses through the liquid phase to the interface and away from the interface into the vapor.在一切的质量传在一切的质量传送操作中，分散送操作中，分散至少在一相中发至少在一相中发生，更多那么是生，更多那么是在两相中发生。在两相中发生。蒸馏时，低沸物蒸馏时，低沸物由液相分散至界由液相分散至界面，分开界面进面，分开界面进入汽相。入汽相。Role of diffusion in mass transfer分散在质量传送中的角色分散在质量传送中的角色 The high boiler diffuses in the reverse direction and passes through the vapor into the liquid.In leaching,diffusion of solute through the solid phase is followed by diffusion into the liquid.高沸物逆向分散高沸物逆向分散从汽相进入液相。从汽相进入液相。浸取时，溶质分散浸取时，溶质分散穿过固相，然后再穿过固相，然后再分散进入液相。分散进入液相。In liquid extraction,the solute diffuses through the raffinate phase to the interface and then into the extract phase.In crystallization,solute diffuses through the mother liquor to the crystals and deposits on the solid surfaces.液液萃取液液萃取时，溶质从萃余时，溶质从萃余相分散至液液相分散至液液界面，然后进入界面，然后进入萃取相。结晶时，萃取相。结晶时，溶质由母液分散溶质由母液分散到晶体，然后堆到晶体，然后堆积于晶体外表。积于晶体外表。In humidification or dehumidification there is no diffusion through the liquid phase because the liquid phase is pure and no concentration gradient through it can exist;but the vapor diffuses to or from the liquid-gas interface into or out of the gas phase.增湿和减湿增湿和减湿时，因液相是纯时，因液相是纯液体，无浓度梯液体，无浓度梯度存在。故液相度存在。故液相无分散。但增湿无分散。但增湿时，蒸汽分开汽时，蒸汽分开汽液界面，分散进液界面，分散进入汽相；而减湿入汽相；而减湿时，那么为蒸汽时，那么为蒸汽朝向汽液界面，朝向汽液界面，分散分开汽相。分散分开汽相。In membrane separations diffusion occurs in all there phases:in the fluids on either side of the membrane and in the membrane itself.膜分别时，膜分别时，一切相中即膜一切相中即膜两侧的流体中两侧的流体中和膜本身内均和膜本身内均会发生分散景会发生分散景象。象。Chap.8 Gas absorptionThis chapter deals with the mass-transfer operations known as gas absorption and stripping or desorption.气体吸收气体吸收:利用气体混合物各组份在吸收剂中的溶解利用气体混合物各组份在吸收剂中的溶解度不同，而使气体混合物得以分别的单元操作。度不同，而使气体混合物得以分别的单元操作。In gas absorption a soluble gas is absorbed from its mixture with an inert gas by means of liquid in which the solute gas is more or less soluble.8-1 Introduction 气体混合物：气体混合物：Mixture or Rich Gas Mixture or Rich Gas A+B A+B 溶质：溶质：Solute Solute A A 惰性气体：惰性气体：An Inert Gas An Inert Gas B B 吸收剂：吸收剂：Solvent Solvent S S 溶解度：溶解度：SolubilitySolubility 分别：分别：SeparationSeparation 可溶的：可溶的：SolubleSoluble 单元操作：单元操作：Unit OperationUnit Operation 吸收液：吸收液：Absorbing liquid or The strong liquidAbsorbing liquid or The strong liquid 尾气：尾气：Dilute gas or Lean gasDilute gas or Lean gasKEY WORDS8-1 Introduction2 2 规定规定3 ExamplesP546 A major application of absorption technology is the removal of CO2 and H2S from natural gas 天天然气然气or synthesis gas 合成气合成气by absorption in solutions of amines 有机胺有机胺or alkaline salts碱碱式盐式盐.Another example is the washing of ammonia from a mixture of ammonia and air by means of liquid water.8-1 Introduction4 The solute&the absorbing liquid The solute is subsequently recovered from the liquid by distillation,and the absorbing liquid can be either discarded or reused.5 Adsorption Sometimes a solute is removed from a liquid by bringing the liquid into contact with an inert gas;such an operation,the reverse of gas absorption,is desorption or gas stripping.8-1 Introduction A common apparatus used in gas absorption and certain other operations is the packed tower.填料：Packing 填料塔：Packed tower6 The process of gas absorption8-1 IntroductionThe rich gas enters the distributing space below the packing and flows upward through the interstices(空空隙隙)in the packing countercurrent to the flow of the liquid.8-1 Introduction The packing provides a large area of contact between the liquid and gas and encourages intimate contact between the phases.The solute in the rich gas is absorbed by the fresh liquid entering the tower,and dilute,or lean,gas leaves the top.The liquid is enriched in solute as it flows down the tower,and concentrated liquid,called the strong liquid,leaves the bottom of the tower through the liquid outlet.8-1 Introduction7 Major applications for gas absorption in commercial industry Purification of the material gases 原料气净化 CO2 and H2S are removed from natural gas or synthesis gas by absorption in solutions of amines salts.在合成NH3工艺中，来自变换工段的变换气约含28CO2。因C02可使catalyst中毒。所以，变换气进合成塔前，必需经过水洗和铜洗两个工段，将CO2的含量减小到0.01以下。8-1 Introduction7 Major applications for gas absorption in commercial industry Recovering of using materialsPreparation of some solution For examples:NO2 is absorbed by liquid water.HNO3 HCl is absorbed by liquid water.HClPurification of waste gas 8-1 Introduction化学吸化学吸收收物理吸物理吸收收等温吸收与等温吸收与非等温吸收非等温吸收8 Classification Absorption and absorption with chemical reactionIsothermal absorption and no-isothermal absorption In general，当气体溶解于液体时，经常伴随有溶，当气体溶解于液体时，经常伴随有溶解热。假设有化学反响存在，还会有反响热。这些要解热。假设有化学反响存在，还会有反响热。这些要素最终导致溶液温度逐渐升高。但假设热效应较小，素最终导致溶液温度逐渐升高。但假设热效应较小，或气相中或气相中A的浓度很低，或吸收剂用量很大时，温度的浓度很低，或吸收剂用量很大时，温度升高不显著，便可以以为是升高不显著，便可以以为是Isothermal absorption。8-1 Introduction物理吸收与物理吸收与化学吸收化学吸收8 Classification Single and multiple component absorption Single component:混合气中只需一个组份进入混合气中只需一个组份进入液相。液相。Multicomponent:混合气中有两或更多组份进入混合气中有两或更多组份进入液相。液相。This chapter deals with the isothermal single absorption.8-1 Introduction9 This chapter will discuss some questions:吸收过程进展的限制和影响要素吸收过程进展的限制和影响要素 Phase-equilibrium 相平衡相平衡 吸收剂用量的计算操作型问题吸收剂用量的计算操作型问题 Calculation of amount of the liquid 吸收塔高度的计算设计型问题吸收塔高度的计算设计型问题 Calculation of tower height 提高吸收塔消费才干的措施提高吸收塔消费才干的措施 Methods to enhance the ability of absorption tower8-1 Introduction10 10 吸收设备吸收设备 吸收设备吸收设备有多种构造，有多种构造，但以塔设备最但以塔设备最为常用。为常用。板式塔板式塔plate plate towertower8-1 Introduction 填料塔填料塔 packed towerpacked tower一、一、The solubility of gas in the liquid At T and P,S+mixture of gases of A+B,intimate contact。随着两者的不断接触，。随着两者的不断接触，solute A from gas phase transfer to liquid phase,until液相中液相中solute到到达达un-saturation。8.2.1 Solubility at equilibrium 平衡溶解度平衡溶解度8.2 Equilibrium between liquid and gas limitation of gas absorption一、一、The solubility of gas in the liquid It is dynamic balance at the moment which concentrations of A are constant within L&V,that is,the amount of molecules of A entering in L phase =the amount of molecules of A escaping from L phase.从宏观上看，吸收过程终止，即气液两相到达相平衡。从宏观上看，吸收过程终止，即气液两相到达相平衡。显然，此时显然，此时A在液相中的浓度最大。此浓度称为饱和浓在液相中的浓度最大。此浓度称为饱和浓度，或溶质度，或溶质A的溶解度的溶解度CA.Definition solubility:8.2.1Solubility at equilibrium 平衡溶解度平衡溶解度3kgA*=or kg AkgSolutiAM SolutioonnC1.Solubility curve 溶解度曲线溶解度曲线 Assume:that only one component is transferred between phases.According to the phase rule:F=c p+2 p=2 and there are three components:c=3 F=3 2+2=38.2.1 Solubility at equilibrium 平衡溶解度平衡溶解度1.Solubility curve 溶解度曲线溶解度曲线 There are four variables:Pressure,Temperature,and the concentrations of component A in liquid and gas.The temperature and pressure may be fixed.One concentration may be chosen as the remaining independent variable.The other concentration is then determined.8.2.1 Solubility at equilibrium 平衡溶解度平衡溶解度1.Solubility curve 溶解度曲线溶解度曲线溶解度曲线的溶解度曲线的3种表示种表示,即气液平衡相图即气液平衡相图(简称简称相图相图)8.2.1 Solubility at equilibrium 平衡溶解度平衡溶解度8.2.1 Solubility at equilibrium 平衡溶解度平衡溶解度1.Solubility curve 溶解度曲线溶解度曲线溶解度曲线的溶解度曲线的3种表示种表示,即气液平衡相图即气液平衡相图(简称简称相图相图)For example Fig.8-4，p=101.3kpa，t=40,all points of(ye,xe)on the curve pertain to(关于关于the same temperature and pressure.8.2.1 Solubility at equilibrium 平衡溶解度平衡溶解度1.Solubility curve 溶解度曲线溶解度曲线2 Factors affecting on solubility According to the phase rule,F=3 In the four variables of T,P,pA and cA,there are three independent variables only.That is:where:pA partial pressure of A in gas phase.8.2.1 Solubility at equilibrium 平衡溶解度平衡溶解度A*AC=f T,P,p Affection of temperature In Fig.8-3,at pA=20 kPa，T=30,cA*=0.12;T=60,cA*=0.045.T cA*温度愈低，愈有利于吸收。温度愈低，愈有利于吸收。But，在实践吸收操作时，思索某，在实践吸收操作时，思索某一适宜的温度下的溶解度曲线。一适宜的温度下的溶解度曲线。That is:*AACfP,p8.2.1 Solubility at equilibrium 平衡溶解度平衡溶解度Affection of pressure 实验证明实验证明:When P is less than 5 atm,the affection may be neglected by pressure on the equilibria for system.Commonly the pressure in the system of absorption operation 5 atm.Then：xA*=f(pA)or pA*=f(xA)8.2.1 Solubility at equilibrium 平衡溶解度平衡溶解度Affection of pressureFor example:in Fig.8-3 at T=30,pA=20 kPa,xA*=0.12；pA=40 kPa,xA*=0.19.即：即：pA愈高，愈有利于吸愈高，愈有利于吸收。收。8.2.1 Solubility at equilibrium 平衡溶解度平衡溶解度 因气液平衡实际研讨尚不完善，气液相平衡数据普通因气液平衡实际研讨尚不完善，气液相平衡数据普通都是对特定物系进展实验测定的。都是对特定物系进展实验测定的。测定的结果或用相图表示，或用表格表示。测定的结果或用相图表示，或用表格表示。8.2.1 Solubility at equilibrium 平衡溶解度平衡溶解度 在进展详细计算时，需求相关的平衡数据。假设手在进展详细计算时，需求相关的平衡数据。假设手算，人工查图或查表；假设机算，那么可将图或表上的算，人工查图或查表；假设机算，那么可将图或表上的相关数据进展阅历回归，得到阅历回归方程相关数据进展阅历回归，得到阅历回归方程xAxA*=f =f(pA)or pA(pA)or pA*=f(xA)=f(xA)提供于计算机运用。提供于计算机运用。Conclusions:Conclusions:a.a.系统压力不太高时的等温吸收，可以为气体在液系统压力不太高时的等温吸收，可以为气体在液体体 中的溶解度只取决于该气体的分压；中的溶解度只取决于该气体的分压；b.b.吸收操作的适宜条件：低温高压。吸收操作的适宜条件：低温高压。8.2.1 Solubility at equilibrium 平衡溶解度平衡溶解度3 Differences of solubility of gases in liquid O2 :Oxygen CO2:Carbon Dioxide SO2:Sulphur Dioxide NH3:Ammonia 如图如图,由实验测得由实验测得T=293 K下下 SO2,CO2,NH3 and O2 在水中的溶解度曲线。显然，在水中的溶解度曲线。显然，在一定温度和分压下，不同气在一定温度和分压下，不同气体得溶解度相差很大。体得溶解度相差很大。8.2.1 Solubility at equilibrium For example:at Pi=20kpa CO2*=0.00028 kmol/m3,CCO2*=0.0078 kmol/m3,CSO2*=0.34 kmol/m3,CNH3*8.9 kmol/m3.8.2.1 Solubility at equilibrium 平衡溶解度平衡溶解度3 Differences of solubility of gases in liquid3 Differences of solubility of gases in liquid CCO2*/CO2*=28倍，倍，CNH3*/CSO2*26倍倍 根据气体在水中溶解度的大小，可把气根据气体在水中溶解度的大小，可把气体大体大 致分为如下四类：致分为如下四类：难溶气体难溶气体 O2难溶于水难溶于水,非常难以吸非常难以吸收；收；较难溶气体较难溶气体 CO2居中居中，较难吸收；较难吸收；较易溶气体较易溶气体 SO2居中居中,较易吸收；较易吸收；易溶气体易溶气体 NH3易溶于水，非常容易吸易溶于水，非常容易吸收。收。8.2.1 Solubility at equilibrium 平衡溶解度平衡溶解度Straight line and pass point of 0.二、二、Henrys law亨利定律亨利定律 1 稀溶液下的溶解度稀溶液下的溶解度 For dilute solution,namely,在原点附近在原点附近,由图由图可知可知If 用用H作为比例系数作为比例系数,那么那么:Definition:H is efficient of solubility.kmol/m3pa or kmol/m3atm or kg/m3atm H*AAAAC=pC=p8.2.1 Solubility at equilibrium 平衡溶解度平衡溶解度*AAAAC=f pC=f p2 Expressing Henrys law There are three expressions for Henrys law:pe=E x (8-1)，pe=H CA (8-2)，ye=m x (8-3)Where:pe=Equilibrium partial press of solute A in gas phase，Pa or atm x =Mole fraction f component A in liquid phase E=Henry efficient，Pa or atm H=Efficient of solubility，kmol/m3pa or kmol/m3atm or kg/m3atm m=phase equilibrium constant.dimensionless,determined by experiment 8.2.1 Solubility at equilibrium 平衡溶解度平衡溶解度2 Expressing Henrys law pe=E x (8-1)pe=H CA (8-2)ye=m x (8-3)8.2.1 Solubility at equilibrium 平衡溶解度平衡溶解度这些常数的值越小，这些常数的值越小，A组分的溶解度越大。组分的溶解度越大。3 Relationships among E,H and m E=HCM,m=E/P4 Features of E,H and m T E,H and m (See p268 Table 2)在同一溶剂中，在同一溶剂中，E愈难溶。愈难溶。by E可定量判别吸收过程的难易程可定量判别吸收过程的难易程度。度。8.2.1 Solubility at equilibrium 平衡溶解度平衡溶解度AMMAMe eH HC CH HC CH HC CC C=x xC Cp pTotal mole concentration kmol(A+S)/m3溶液溶液 5 Henrys law suits for some conditions 适宜常压或接近常压下的难溶气体适宜常压或接近常压下的难溶气体;对易溶气体只适宜于低浓度对易溶气体只适宜于低浓度;气相组份分压气相组份分压常压。常压。8.2.1 Solubility at equilibrium 平衡溶解度平衡溶解度6 6 用比摩尔分率表示的亨利定律用比摩尔分率表示的亨利定律 比摩尔分率：比摩尔分率：L-phase:X=CA/Cs L-phase:X=CA/Cs V-phase:V-phase:Y=CA/CBY=CA/CB Obviously:x=X/Obviously:x=X/1+X1+X,y=Y/(1+Y)y=Y/(1+Y)Substitute them into y=mx:Substitute them into y=mx:溶液很稀时溶液很稀时,X very very small,X very very small,1+(1-m)X11+(1-m)X1 Ye=mX Ye=mX 在吸收计算中在吸收计算中运用最为广泛。运用最为广泛。8.2.1 Solubility at equilibrium 平衡溶解度平衡溶解度 eeeXmX=m =1+1+X1+1-m XYYY一、判别过程的方向一、判别过程的方向 例例:at P=101.3kpa,T=20,ye0.94x,x=0.05 dilute solution ammonia 与与 y=0.1 mixture of NH3+air intimate contact。Absorption or De-absorption?How to determine?According to phase diagram.或根据气相传质推进力：或根据气相传质推进力：y ye 或根据液相传质推进力：或根据液相传质推进力：x xe ye0.940.05=0.047 0.1=0.094 y=0.05 y=0.05 Stripping Stripping 8.2.28.2.2相平衡与吸收过程的关系相平衡与吸收过程的关系吸收吸收or 解吸解吸?一、判别过程的方向一、判别过程的方向Conclusion:Conclusion:If y ye or x ye or x xe,that is,point 1 locating in that is,point 1 locating in the region of up-left the the region of up-left the equilibrium,then equilibrium,then absorption;absorption;If y xe,If y xe,that is,point 2 locating in that is,point 2 locating in the region of down-right the the region of down-right the equilibrium,then equilibrium,then stripping.stripping.8.2.2相平衡与吸收过程的关系相平衡与吸收过程的关系吸收吸收or 解吸解吸?二二.指明吸收过程的极限指明吸收过程的极限 x1,max=x1e=x1,max=x1e=y1/m y1/m 溶剂分开塔底溶剂分开塔底时时 的最高含量的最高含量 y2,min=y2e=y2,min=y2e=mx2 mx2 气体混合物分气体混合物分开开 塔顶时的最低含塔顶时的最低含量量8.2.28.2.2相平衡与吸收过程的关系相平衡与吸收过程的关系三三.Calculation of driving.Calculation of driving force for absorption processforce for absorption processEquilibrium is limitation of Equilibrium is limitation of the processthe process。只需不平衡的两相互相接触才会只需不平衡的两相互相接触才会发生气体的吸收或解析。发生气体的吸收或解析。8.2.28.2.2相平衡与吸收过程的关系相平衡与吸收过程的关系 吸吸收收推推动动力力吸吸收收速速率率 传传质质系系数数 传传质质推推动动力力吸吸收收阻阻力力吸收推进力：吸收推进力：假设以气相浓度计算，那么：假设以气相浓度计算，那么：yyy-y-yeye假设以液相浓度计算，那么：假设以液相浓度计算，那么：x=xe-x=xe-x x8.2.28.2.2相平衡与吸收过程的关系相平衡与吸收过程的关系一、一、DiffusionP511Features:1.Whats diffusion？Diffusion is the movement,under the influence of a physical stimulus物理驱动物理驱动力力,of an individual component单独的组单独的组份份 through a mixture.8.3.1 Molecular diffusion in binary mixtures8.3 Diffusion and mass transfer within single phase分散和单向分散和单向传质传质一、一、DiffusionP5112.Why reason?The most cause of diffusion is a concentration gradient of the diffusing component.3.Purpose?A concentration gradient浓度梯度浓度梯度 tends to move the component in such a direction as to equalize concentration and destroy the gradient.8.3.1 Molecular diffusion in binary mixtures一、一、DiffusionP5114.Steady-state flux When the gradient is maintained by constantly supplying the diffusing component to the high-concentration end of the gradient and removing it at the low-concentration end,there is a steady-state flux of the diffusing component.This is characteristic of many mass-transfer operations.8.3.1 Molecular diffusion in binary mixtures一、一、DiffusionP5115.Other reasons Although the usual cause of diffusion is a concentration gradient,diffusion can also be caused by an activity gradient能动梯度能动梯度,as in reverse osmosis反浸透反浸透,by a pressure gradient,by a temperature gradient,or by the application of an external force field外场力外场力,as in a centrifuge离离心力场心力场.8.3.1 Molecular diffusion in binary mixtures一、一、DiffusionP5116.Whats thermal diffusion?热分散热分散 Molecular diffusion induced by temperature gradient is thermal diffusion.7.Whats the force diffusion?强迫分散强迫分散 Molecular diffusion induced by the application of a force from an external filed is forced diffusion.8.3.1 Molecular diffusion in binary mixtures一、一、DiffusionP5118.Diffusion in a direction 分散方向分散方向 Diffusion in a direction is perpendicular to the interface between the phases and at a definite location in the equipment.9.Steady state Steady state is assumed,and the concentrations at any point do not change with time.8.3.1 Molecular diffusion in binary mixtures一、一、Ficks first law of diffusion for a binary mixture (P514)双组份分散时的费克第一定律双组份分散时的费克第一定律1.Whats Ficks first law?分子分散的本质是分子的微观随机运动。分子分散的本质是分子的微观随机运动。对于一定温度和压力下的一维定态分散，其统计规对于一定温度和压力下的一维定态分散，其统计规律可用宏观的方式表达。这就是费克第一定律：律可用宏观的方式表达。这就是费克第一定律：8.3.1 Molecular diffusion in binary mixtures AABAdC=-8-9dZJD 一、一、Ficks first law of diffusion for a binary mixture1.Whats Ficks first law?Where:JA=the diffusion flux of component A;A moles/per unit area per unit time A moles/m2s dCA/dz=the concentration gradient of component A diffusing in a direction，(mol/m3)/m DAB=Diffusivity of component A in component B，m2/s 8.3.1 Molecular diffusion in binary mixtures AABAdC=-8-9dZJD一、一、Ficks first law of diffusion for a binary mixture 1.Whats Ficks first law?Reference to8-9,a similar equation for component B:For binary mixture DAB=DBA=D JA=JB 8-13 BBABdC=-8-11dZJ D8.3.1 Molecular diffusion in binary mixtures2.“2.“景象定律景象定律 Thats phenomena Thats phenomena law.law.流膂力学和传热学要学两个根本定律：流膂力学和传热学要学两个根本定律：牛顿粘性定律牛顿粘性定律:Newtons law of Newtons law of viscosityviscosity Molecular momentum transportMolecular momentum transport傅立叶定律傅立叶定律:Fouriers law of heat Fouriers law of heat conduction conduction Molecular energy transportMolecular energy transportComparison to those three formulas:Comparison to those three formulas:传送的物理量：动量，热量，质量传送的物理量：动量，热量，质量 momentum,energy,and massmomentum,energy,and mass8.3.1 Molecular diffusion in binary mixtures d ud y d tq=kd y 均为传送通量：传送的物理量均为传送通量：传送的物理量/m2s 大小大小(Magnitudes):与对应强度要素梯与对应强度要素梯 度成正比。度成正比。方向方向Directions):沿着梯度减小的方向传送。沿着梯度减小的方向传送。各式中的系数仅为形状函数，即是各式中的系数仅为形状函数，即是T,P和组成的函和组成的函 数，而于流动无关。数，而于流动无关。又一致称这三个定律为又一致称这三个定律为 “景象定律景象定律 Thats phenomena law.大家在学习质量传送时，完全可以与动量、热量传大家在学习质量传送时，完全可以与动量、热量传 递进展类比。递进展类比。8.3.1 Molecular diffusion in binary mixtures二、二、Molecular diffusion and bulk flowMolecular diffusion and bulk flow 分子分散与主体流动分子分散与主体流动1.Molecular diffusion1.Molecular diffusion 如下图：当提起挡板后，如下图：当提起挡板后，在浓度梯度的驱动下，必有在浓度梯度的驱动下，必有A A分子分子向右挪动，向右挪动，B B 分子向左挪动分子向左挪动 。8.3.1 Molecular diffusion in binary mixtures