第8章气体吸收-81概述课件

Aconcentrationgradienttendstomovethecomponentinsuchadirectionastoequalizeconcentrationsanddestroythegradient.浓度梯度驱使扩散组分朝着使浓度均匀化的方向扩散，从而使浓度梯度趋于消失。Whenthegradientismaintainedbyconstantlysupplyingthediffusingcomponenttothehigh-concentrationendofthegradientandremovingitatthelowconcentrationend,thereisasteady-statefluxofthediffusingcomponent.Thisischaracteristicofmanymass-transferoperations.

当不断地向高浓度一端提供扩散组分，从低浓度一端除去扩散组分时，扩散组分便具有稳定的传质通量。这就是许多传质操作的特点。例如，当在填料塔中用水吸收气体中的NH3时，在塔内的任意一点上，在气相浓度梯度的驱使下，NH3由气相主体扩散至气～液界面，在界面上溶解，然后，在液相浓度梯度驱使下，扩散至液相主体。

Forexample,whenammoniaisremovedfromagasbyabsorptioninwaterinapackedcolumn,ateachpointinthecolumnaconcentrationgradientinthegasphasecausesammoniatodiffusetothegas-liquidinterface,whereitdissolves,andagradientintheliquidphasecausesittodiffuseintothebulkliquid.

Instrippingasolutefromaliquidthegradientsarereversed;herediffusionbringssolutefromthebulkliquidtotheinterfaceandfromthereintothegasphase.

当从吸收液中解吸溶质时，浓度梯度与吸收过程相反。在此，扩散作用使溶质从液相主体移至界面，再从界面移至气相。

Insomeothermass-transferoperationssuchasleachingandadsorption,unsteady-statediffusiontakesplace,andthegradientsandfluxesdecreasewithtimeasequilibriumisapproached.

在其它一些诸如浸取和吸咐操作中发生的是不稳定扩散，随着过程趋于平衡，扩散组分的浓度梯度和通量随时间而降低。

Althoughtheusualcauseofdiffusionisaconcentrationgradient,diffusioncanalsobecausedbyanactivitygradient,asinreverseosmosis,byapressuregradient,byatemperaturegradient,orbytheapplicationofanexternalforcefield,asinacentrifuge.

虽然扩散的一般原因是浓度梯度，但能动梯度也能引起扩散如反渗透过程，压力梯度、温度梯度和外场力都可引起扩散，如离心分离。Moleculardiffusioninducedbytemperatureisthermaldiffusion,andthatfromanexternalfieldisforceddiffusion.Bothareuncommoninchemicalengineering.Onlydiffusionunderaconcentrationgradientisconsideredinthischapter.由温度引起的分子扩散称为热扩散，由外场力引起的扩散称为强制扩散。这两种扩散方式在化学工程中并不常见。本章仅讨论由浓度梯度引起的质量扩散。Diffusionisnotrestrictedtomoleculartransferthroughstagnantlayersofsolidorfluid.Italsotakesplacewhenfluidsofdifferentcompositionsaremixed.

扩散并不局限于分子通过固体或液体停滞膜的传递。当不同组成的流体相混合时也会发生扩散。

Thefirststepinmixingisoftenmasstransfercausedbytheeddymotioncharacteristicofturbulentflow.Thisiscallededdydiffusion.Thesecondstepismoleculardiffusionbetweenandinsidetheverysmalleddies.

混合过程首先是因湍流的涡流特性引起的质量传递。这一过程被称为涡流扩散。其次是在微小的漩涡间及其内部的分子扩散。

Sometimesthediffusionprocessisaccompaniedbybulkflowofthemixtureinadirectionparalleltothedirectionofdiffusion.

有时扩散过程还伴随着与扩散方向相平行的混合物主体流动。Inallthemass-transferoperations,diffusionoccursinatleastonephaseandofteninbothphases.Indistillation,thelowboilerdiffusesthroughtheliquidphasetotheinterfaceandawayfromtheinterfaceintothevapor.

在所有的质量传递操作中，扩散至少在一相中发生，更多则是在两相中发生。蒸馏时，低沸物由液相扩散至界面，离开界面进入汽相。Roleofdiffusioninmasstransfer扩散在质量传递中的角色

Thehighboilerdiffusesinthereversedirectionandpassesthroughthevaporintotheliquid.Inleaching,diffusionofsolutethroughthesolidphaseisfollowedbydiffusionintotheliquid.

高沸物逆向扩散从汽相进入液相。浸取时，溶质扩散穿过固相，然后再扩散进入液相。

Inliquidextraction,thesolutediffusesthroughtheraffinatephasetotheinterfaceandthenintotheextractphase.Incrystallization,solutediffusesthroughthemotherliquortothecrystalsanddepositsonthesolidsurfaces.

液～液萃取时，溶质从萃余相扩散至液～液界面，然后进入萃取相。结晶时，溶质由母液扩散到晶体，然后积淀于晶体表面。

Inhumidificationordehumidificationthereisnodiffusionthroughtheliquidphasebecausetheliquidphaseispureandnoconcentrationgradientthroughitcanexist;butthevapordiffusestoorfromtheliquid-gasinterfaceintooroutofthegasphase.

增湿和减湿时，因液相是纯液体，无浓度梯度存在。故液相无扩散。但增湿时，蒸汽离开气液界面，扩散进入气相；而减湿时，则为蒸汽朝向气液界面，扩散离开气相。

Inmembraneseparationsdiffusionoccursinalltherephases:inthefluidsoneithersideofthemembraneandinthemembraneitself.

膜分离时，所有相中即膜两侧的流体中和膜自身内均会发生扩散现象。Chap.8GasabsorptionThischapterdealswiththemass-transferoperationsknownasgasabsorptionandstripping

ordesorption.⒈气体吸收: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离的单元操作。Ingasabsorptionasolublegasisabsorbedfromitsmixturewithaninertgasbymeansofliquidinwhichthesolutegasismoreorlesssoluble.§8-1Introduction

气体混合物：MixtureorRichGas～A+B溶质：Solute～A

惰性气体：AnInertGas～B吸收剂：Solvent～S溶解度：Solubility分离：Separation可溶的：Soluble单元操作：UnitOperation吸收液：AbsorbingliquidorThestrongliquid尾气：DilutegasorLeangasKEYWORDS§8-1Introduction2规定3Examples（P546）⑴AmajorapplicationofabsorptiontechnologyistheremovalofCO2andH2Sfromnaturalgas（天然气）orsynthesisgas（合成气）byabsorptioninsolutionsofamines（有机胺）oralkalinesalts（碱式盐）.⑵Anotherexampleisthewashingofammoniafromamixtureofammoniaandairbymeansofliquidwater.§8-1Introduction4Thesolute&theabsorbingliquidThesoluteissubsequentlyrecoveredfromtheliquidbydistillation,andtheabsorbingliquidcanbeeitherdiscardedorreused.5Adsorptionsometimesasoluteisremovedfromaliquidbybringingtheliquidintocontactwithaninertgas;suchanoperation,thereverseofgasabsorption,isdesorptionorgasstripping.§8-1Introduction

Acommonapparatususedingasabsorptionandcertainotheroperationsisthepackedtower.Therichgasentersthedistributingspacebelowthepackingandflowsupwardthroughtheinterstices(空隙)inthepackingcountercurrenttotheflowoftheliquid.

填料：Packing

填料塔：Packedtower6Theprocessofgasabsorption§8-1Introduction⑴

Thepackingprovidesalargeareaofcontactbetweentheliquidandgasandencouragesintimatecontactbetweenthephases.⑵Thesoluteintherichgasisabsorbedbythefreshliquidenteringthetower,anddilute,orlean,gasleavesthetop.⑶Theliquidis

enrichedinsoluteasitflowsdownthetower,andconcentratedliquid,calledthestrongliquid,leavesthebottomofthetowerthroughtheliquidoutlet.§8-1Introduction7Majorapplicationsforgasabsorptionincommercialindustry⑴Purificationofthematerialgases原料气净化

CO2

andH2Sareremovedfromnaturalgasorsynthesisgasbyabsorptioninsolutionsofaminessalts.

在合成NH3工艺中，来自变换工段的变换气约含28％CO2。因C02可使catalyst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，将CO2的含量减小到0.01％以下。

§8-1Introduction⑵Recoveringofusingmaterials⑶Preparationofsomesolution

Forexamples:NO2isabsorbedbyliquidwater.→HNO3

～化学吸收HClisabsorbedbyliquidwater.→HCl～物理吸收⑷Purificationofwastegas8Classification

⑴Absorptionandabsorptionwithchemicalreaction

物理吸收与化学吸收⑵Isothermalabsorptionandno-isothermalabsorption