第8章氣體吸收-81概述課件

Aconcentrationgradienttendstomovethecomponentinsuchadirectionastoequalizeconcentrationsanddestroythegradient.濃度梯度驅(qū)使擴(kuò)散組分朝著使?jié)舛染鶆蚧姆较驍U(kuò)散，從而使?jié)舛忍荻融呌谙?。Whenthegradientismaintainedbyconstantlysupplyingthediffusingcomponenttothehigh-concentrationendofthegradientandremovingitatthelowconcentrationend,thereisasteady-statefluxofthediffusingcomponent.Thisischaracteristicofmanymass-transferoperations.

當(dāng)不斷地向高濃度一端提供擴(kuò)散組分，從低濃度一端除去擴(kuò)散組分時，擴(kuò)散組分便具有穩(wěn)定的傳質(zhì)通量。這就是許多傳質(zhì)操作的特點(diǎn)。例如，當(dāng)在填料塔中用水吸收氣體中的NH3時，在塔內(nèi)的任意一點(diǎn)上，在氣相濃度梯度的驅(qū)使下，NH3由氣相主體擴(kuò)散至氣～液界面，在界面上溶解，然后，在液相濃度梯度驅(qū)使下，擴(kuò)散至液相主體。

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

Instrippingasolutefromaliquidthegradientsarereversed;herediffusionbringssolutefromthebulkliquidtotheinterfaceandfromthereintothegasphase.

當(dāng)從吸收液中解吸溶質(zhì)時，濃度梯度與吸收過程相反。在此，擴(kuò)散作用使溶質(zhì)從液相主體移至界面，再從界面移至氣相。

Insomeothermass-transferoperationssuchasleachingandadsorption,unsteady-statediffusiontakesplace,andthegradientsandfluxesdecreasewithtimeasequilibriumisapproached.

在其它一些諸如浸取和吸咐操作中發(fā)生的是不穩(wěn)定擴(kuò)散，隨著過程趨于平衡，擴(kuò)散組分的濃度梯度和通量隨時間而降低。

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

雖然擴(kuò)散的一般原因是濃度梯度，但能動梯度也能引起擴(kuò)散如反滲透過程，壓力梯度、溫度梯度和外場力都可引起擴(kuò)散，如離心分離。Moleculardiffusioninducedbytemperatureisthermaldiffusion,andthatfromanexternalfieldisforceddiffusion.Bothareuncommoninchemicalengineering.Onlydiffusionunderaconcentrationgradientisconsideredinthischapter.由溫度引起的分子擴(kuò)散稱為熱擴(kuò)散，由外場力引起的擴(kuò)散稱為強(qiáng)制擴(kuò)散。這兩種擴(kuò)散方式在化學(xué)工程中并不常見。本章僅討論由濃度梯度引起的質(zhì)量擴(kuò)散。Diffusionisnotrestrictedtomoleculartransferthroughstagnantlayersofsolidorfluid.Italsotakesplacewhenfluidsofdifferentcompositionsaremixed.

擴(kuò)散并不局限于分子通過固體或液體停滯膜的傳遞。當(dāng)不同組成的流體相混合時也會發(fā)生擴(kuò)散。

Thefirststepinmixingisoftenmasstransfercausedbytheeddymotioncharacteristicofturbulentflow.Thisiscallededdydiffusion.Thesecondstepismoleculardiffusionbetweenandinsidetheverysmalleddies.

混合過程首先是因湍流的渦流特性引起的質(zhì)量傳遞。這一過程被稱為渦流擴(kuò)散。其次是在微小的漩渦間及其內(nèi)部的分子擴(kuò)散。

Sometimesthediffusionprocessisaccompaniedbybulkflowofthemixtureinadirectionparalleltothedirectionofdiffusion.

有時擴(kuò)散過程還伴隨著與擴(kuò)散方向相平行的混合物主體流動。Inallthemass-transferoperations,diffusionoccursinatleastonephaseandofteninbothphases.Indistillation,thelowboilerdiffusesthroughtheliquidphasetotheinterfaceandawayfromtheinterfaceintothevapor.

在所有的質(zhì)量傳遞操作中，擴(kuò)散至少在一相中發(fā)生，更多則是在兩相中發(fā)生。蒸餾時，低沸物由液相擴(kuò)散至界面，離開界面進(jìn)入汽相。Roleofdiffusioninmasstransfer擴(kuò)散在質(zhì)量傳遞中的角色

Thehighboilerdiffusesinthereversedirectionandpassesthroughthevaporintotheliquid.Inleaching,diffusionofsolutethroughthesolidphaseisfollowedbydiffusionintotheliquid.

高沸物逆向擴(kuò)散從汽相進(jìn)入液相。浸取時，溶質(zhì)擴(kuò)散穿過固相，然后再擴(kuò)散進(jìn)入液相。

Inliquidextraction,thesolutediffusesthroughtheraffinatephasetotheinterfaceandthenintotheextractphase.Incrystallization,solutediffusesthroughthemotherliquortothecrystalsanddepositsonthesolidsurfaces.

液～液萃取時，溶質(zhì)從萃余相擴(kuò)散至液～液界面，然后進(jìn)入萃取相。結(jié)晶時，溶質(zhì)由母液擴(kuò)散到晶體，然后積淀于晶體表面。

Inhumidificationordehumidificationthereisnodiffusionthroughtheliquidphasebecausetheliquidphaseispureandnoconcentrationgradientthroughitcanexist;butthevapordiffusestoorfromtheliquid-gasinterfaceintooroutofthegasphase.

增濕和減濕時，因液相是純液體，無濃度梯度存在。故液相無擴(kuò)散。但增濕時，蒸汽離開氣液界面，擴(kuò)散進(jìn)入氣相；而減濕時，則為蒸汽朝向氣液界面，擴(kuò)散離開氣相。

Inmembraneseparationsdiffusionoccursinalltherephases:inthefluidsoneithersideofthemembraneandinthemembraneitself.

膜分離時，所有相中即膜兩側(cè)的流體中和膜自身內(nèi)均會發(fā)生擴(kuò)散現(xiàn)象。Chap.8GasabsorptionThischapterdealswiththemass-transferoperationsknownasgasabsorptionandstripping

ordesorption.⒈氣體吸收:利用氣體混合物各組份在吸收劑中的溶解度不同，而使氣體混合物得以分離的單元操作。Ingasabsorptionasolublegasisabsorbedfromitsmixturewithaninertgasbymeansofliquidinwhichthesolutegasismoreorlesssoluble.§8-1Introduction

氣體混合物：MixtureorRichGas～A+B溶質(zhì)：Solute～A

惰性氣體：AnInertGas～B吸收劑：Solvent～S溶解度：Solubility分離：Separation可溶的：Soluble單元操作：UnitOperation吸收液：AbsorbingliquidorThestrongliquid尾氣：DilutegasorLeangasKEYWORDS§8-1Introduction2規(guī)定3Examples（P546）⑴AmajorapplicationofabsorptiontechnologyistheremovalofCO2andH2Sfromnaturalgas（天然氣）orsynthesisgas（合成氣）byabsorptioninsolutionsofamines（有機(jī)胺）oralkalinesalts（堿式鹽）.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原料氣凈化

CO2

andH2Sareremovedfromnaturalgasorsynthesisgasbyabsorptioninsolutionsofaminessalts.

在合成NH3工藝中，來自變換工段的變換氣約含28％CO2。因C02可使catalyst中毒。所以，變換氣進(jìn)合成塔前，必須經(jīng)過水洗和銅洗兩個工段，將CO2的含量減小到0.01％以下。

§8-1Introduction⑵Recoveringofusingmaterials⑶Preparationofsomesolution

Forexamples:NO2isabsorbedbyliquidwater.→HNO3

～化學(xué)吸收HClisabsorbedbyliquidwater.→HCl～物理吸收⑷Purificationofwastegas8Classification

⑴Absorptionandabsorptionwithchemicalreaction

物理吸收與化學(xué)吸收⑵Isothermalabsorptionandno-isothermalabsorption