1、 A concentration gradient tends to move the component in such a direction as to equalize concentrations and destroy the gradient.浓度梯度驱使扩浓度梯度驱使扩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扩散，从而使浓扩散，从而使浓度梯度趋于消失。度梯度趋于消失。vWhen the gradient is maintained by constantly supplying the diffusing component to the high-concen
2、tration 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扩散组分，从低扩散组分，从低浓度一端除去扩浓度一端除去扩散组分时，扩散散组分时，扩散组分便具有稳定组分便具有稳定的传质通量。这的传质通量。这就是许多传质操就是许多传质操作的特点。作的
3、特点。例如，当在填料塔例如，当在填料塔中用水吸收气体中中用水吸收气体中的的NHNH3 3时，在塔内的时，在塔内的任意一点上，在气任意一点上，在气相浓度梯度的驱使相浓度梯度的驱使下，下，NHNH3 3由气相主体由气相主体扩散至气液界面，扩散至气液界面，在界面上溶解，然在界面上溶解，然后，在液相浓度梯后，在液相浓度梯度驱使下，扩散至度驱使下，扩散至液相主体。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
4、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 li
5、quid 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。在此，扩散作在此，扩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 d
6、ecrease with time as equilibrium is approached.在其它一些诸如在其它一些诸如浸取和吸咐操作浸取和吸咐操作中发生的是不稳中发生的是不稳定扩散，随着过定扩散，随着过程趋于平衡，扩程趋于平衡，扩散组分的浓度梯散组分的浓度梯度和通量随时间度和通量随时间而降低。而降低。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 p
7、ressure gradient,by a temperature gradient,or by the application of an external force field,as in a centrifuge.虽然扩散的一般虽然扩散的一般原因是浓度梯度，但原因是浓度梯度，但能动梯度也能引起扩能动梯度也能引起扩散如反渗透过程，压散如反渗透过程，压力梯度、温度梯度和力梯度、温度梯度和外场力都可引起扩散，外场力都可引起扩散，如离心分离。如离心分离。Molecular diffusion induced by temperature is thermal diffusion,and tha
8、t 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扩散称为热扩散，扩散称为热扩散，由外场力引起的扩由外场力引起的扩散称为强制扩散。散称为强制扩散。这两种扩散方式在这两种扩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扩散。量扩散。D
9、iffusion is not restricted to molecular transfer through stagnant layers of solid or fluid.It also takes place when fluids of different compositions are mixed.扩散并不局扩散并不局限于分子通过固限于分子通过固体或液体停滞膜体或液体停滞膜的传递。当不同的传递。当不同组成的流体相混组成的流体相混合时也会发生扩合时也会发生扩散。散。The first step in mixing is often mass transfer caused by
10、 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.混合过程首先混合过程首先是因湍流的涡流特是因湍流的涡流特性引起的质量传递。性引起的质量传递。这一过程被称为涡这一过程被称为涡流扩散。其次是在流扩散。其次是在微小的漩涡间及其微小的漩涡间及其内部的分子扩散。内部的分子扩散。Sometimes the diffusion proces
11、s is accompanied by bulk flow of the mixture in a direction parallel to the direction of diffusion.有时扩散过程有时扩散过程还伴随着与扩散还伴随着与扩散方向相平行的混方向相平行的混合物主体流动。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
12、 the liquid phase to the interface and away from the interface into the vapor.在所有的质量传在所有的质量传递操作中，扩散递操作中，扩散至少在一相中发至少在一相中发生，更多则是在生，更多则是在两相中发生。蒸两相中发生。蒸馏时，低沸物由馏时，低沸物由液相扩散至界面，液相扩散至界面，离开界面进入汽离开界面进入汽相。相。Role of diffusion in mass transfer扩散在质量传递中的角色扩散在质量传递中的角色 The high boiler diffuses in the reverse directi
13、on and passes through the vapor into the liquid.In leaching,diffusion of solute through the solid phase is followed by diffusion into the liquid.高沸物逆向扩高沸物逆向扩散从汽相进入液散从汽相进入液相。浸取时，溶相。浸取时，溶质扩散穿过固相，质扩散穿过固相，然后再扩散进入然后再扩散进入液相。液相。In liquid extraction,the solute diffuses through the raffinate phase to the int
14、erface 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，溶质从萃余相扩散至液液相扩散至液液界面，然后进入界面，然后进入萃取相。结晶时，萃取相。结晶时，溶质由母液扩散溶质由母液扩散到晶体，然后积到晶体，然后积淀于晶体表面。淀于晶体表面。In humidification or dehumidification there
15、 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，因液相是纯液时，因液相是纯液体，无浓度梯度存体，无浓度梯度存在。故液相无扩散。在。故液相无扩散。但增湿时，蒸汽离但增湿时，蒸汽离开气液界面，扩散开气
16、液界面，扩散进入气相；而减湿进入气相；而减湿时，则为蒸汽朝向时，则为蒸汽朝向气液界面，扩散离气液界面，扩散离开气相。开气相。In membrane separations diffusion occurs in all there phases:in the fluids on either side of the membrane and in the membrane itself.膜分离时，膜分离时，所有相中即膜所有相中即膜两侧的流体中两侧的流体中和膜自身内均和膜自身内均会发生扩散现会发生扩散现象。象。Chap.8 Gas absorptionThis chapter deals wit
17、h the mass-transfer operations known as gas absorption and stripping or desorption.气体吸收气体吸收:利用利用气体混合物气体混合物各组份在各组份在吸收剂吸收剂中的中的溶溶解度解度不同，而使气体混合物得以不同，而使气体混合物得以分离分离的单元操作。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
18、or less soluble.8-1 Introduction 气体混合物：气体混合物：Mixture or Rich Gas A+B 溶质：溶质：Solute A 惰性气体：惰性气体：An Inert Gas B 吸收剂：吸收剂：Solvent S 溶解度：溶解度：Solubility 分离：分离：Separation 可溶的：可溶的：Soluble 单元操作：单元操作：Unit Operation 吸收液：吸收液：Absorbing liquid or The strong liquid 尾气：尾气：Dilute gas or Lean gasKEY WORDS8-1 Introduct
19、ion2 2 规定规定3 Examples（P546）A major application of absorption technology is the removal of CO2 and H2S from natural gas（天然气）（天然气）or synthesis gas（合成气）（合成气）by absorption in solutions of amines（有机胺）（有机胺）or alkaline salts（碱式盐）（碱式盐）.Another example is the washing of ammonia from a mixture of ammonia and
20、air by means of liquid water.8-1 Introduction4 The solute&the absorbing liquidThe 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 w
21、ith 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.The rich gas enters the distributing space below the packing and flows upward through the inters
22、tices(空隙空隙)in the packing countercurrent to the flow of the liquid.填料：填料：Packing 填料塔：填料塔：Packed tower6 The process of gas absorption8-1 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 i
23、s 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 a
24、bsorption in commercial industry Purification of the material gases 原料气净化原料气净化 CO2 and H2S are removed from natural gas or synthesis gas by absorption in solutions of amines salts.在合成在合成NHNH3 3工艺中，来自变换工段的变换气工艺中，来自变换工段的变换气约含约含2828COCO2 2。因因C0C02 2可使可使catalyst中毒。所以，变中毒。所以，变换气进合成塔前，必须经过水洗和铜洗两个工段，换气进合成塔
25、前，必须经过水洗和铜洗两个工段，将将COCO2 2的含量减小到的含量减小到0.010.01以下。以下。8-1 IntroductionRecovering of using materialsPreparation of some solution For examples:NO2 is absorbed by liquid water.HNO3 化学吸收化学吸收HCl is absorbed by liquid water.HCl 物理吸收物理吸收Purification of waste gas 8 Classification Absorption and absorption with
26、 chemical reaction 物理吸收与化学吸收物理吸收与化学吸收Isothermal absorption and no-isothermal absorption 等温吸收与非等温吸收等温吸收与非等温吸收8-1 Introduction In general，当气体溶解于液体时，常常伴当气体溶解于液体时，常常伴随有溶解热。若有化学反应存在，还会有反应随有溶解热。若有化学反应存在，还会有反应热。这些因素最终导致溶液温度逐渐升高。但热。这些因素最终导致溶液温度逐渐升高。但若若热效应较小热效应较小，或，或气相中气相中A A的浓度很低，的浓度很低，或或吸收吸收剂用量很大剂用量很大时，温度升
27、高不显著，便可以认为时，温度升高不显著，便可以认为是是Isothermal absorption。Single and multiple component absorption 单组份吸收与多组份吸收单组份吸收与多组份吸收Single component:混合气中混合气中只有一个组份只有一个组份进入液相。进入液相。Multicomponent:混合气中有混合气中有两或更多组份两或更多组份进入液相。进入液相。This chapter deals with the isothermal single absorption。8-1 Introduction精品课件精品课件！精品课件精品课件！9 T
28、his chapter will discuss some questions:吸收过程进行的限度和影响因素吸收过程进行的限度和影响因素 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吸收塔高度的计算吸收塔高度的计算（设计型问题）（设计型问题）Calculation of tower height提高吸收塔生产能力的措施提高吸收塔生产能力的措施 Methods to enhance the ability of absorption tower10 10 吸收设备吸收设备吸收设备有多种结构。吸收设备有多种结构。但以但以塔设备塔设备最为常用。最为常用。填料塔填料塔packed tower 板式塔板式塔plate tower8-1 Introduction
