化工过程设计讲义

RobinSmith,ChemicalProcessDesign,McGraw-Hill

BookCo.世界图书出版公司，1995.

双语教学：bilingualeducation

目的：以专业知识的教学为载体，为学生创造一种学习专业英语知识的环境，

使学生能用英语进行专业方面的交流，逐步过渡到用全英语进行专业思维。

以学习英语知识为主的初级阶段；

以运用英语知识进行专业思维为主，以英语知识的教学为辅的中级阶段；

自觉熟练运用英语进行专业思维的高级阶段。

IntroductionoftheCourse(2学时)

Thestepsofchemicalprocessdesign:

1Selectaseriesofprocessingsteps(operationunits)andconnectthem(little

attention)

Reactor

Separator(distillationcolumn,absorption,evaporator,dryer,adsorption,etc.)

Heatexchanger

Compressor

Pump

Etc.

2Analysisorsimulationflosheet(moreattention)

Duringthesynthesisofflowsheet,selectionofoperationsisveryimportant—may

seriouslyaffectthecapitalcost,operationcost,safety,andenvironment.

Inthiscourse,wewillintroducethesystematicdesignofreactorandseparatorwith

theemphasisoncosts,safetyandenvironment.

第一章化工过程设计的层次(Thehierarchyofchemicalprocess

design)

1.1Introduction

Synthesisofflowsheet

•Usually,wecan'ttransformrawmaterialintodesiredproductsinonestep.

•Generally,achemicalprocessisbrokendownintoanumberofsteps,whichare

carriedoutthroughreaction,separationetc.

•Interconnectionofindividualstepstoformtheflowsheet

1

1

Feed1►Products

streamsIProcess?__________bstreams

।

►

।

Fig.1.1a

Synthesisofachemicalprocessincludingtwosteps:

Selectionofindividualsteps

Interconnectionofindividualsteps

Simulation

A

►

►Products

Feed_A

streamsProcess________Astreams

►A

Fig.1.1b

Optimization

1.2OverallProcessDesign

Isitsimpletodesignachemicalprocess?Let'stakeasimpleexample:transformfeed

orrawmaterialintodesiredproducts.

Wecanperformthistransformationbyreactor,asshownbyFig.1.2(a).However,not

allfeedcanbetransformedintoproducts,andtherearesomebyproducts.Therefore,

togetdesiredproductswithcertainpurity,wehavetoseparateproductsfromthe

unreactedfeedandbyproducts,lbuserawmaterialefficiently,theunreactedfeedcan

berecycledtothereactor.Basedonthis,thewholeflowsheetcanbeobtained.

Considertheheatinteeration,nowflowsheetcanbedesignedandisbetterthenthe

originalone(Fig.1.3).

Retrofitthereactor(Fig.1.4),andcorrespondinglyretrofittheseparationorder.

Again,heatintegration(Fig.1.5)

Consideralltheseflowsheets,whichoneisthebest?Optimizeandevaluatethese

flowsheetstogettheanswer.

Fromthisprocess,itcanbeseenthecomplexityofchemicalprocess.Itistwofold:

Identifyallpossiblestructureorflowsheet:manypossibleoptions,connections

Optimizeeachflowsheet:manystructureoperatingconditions

1.3Thehierarchyofprocessdesignandtheonionmodel

Forachemicalprocess,ifitneedareactor,itsdesigngenerallystartsfromhere.

过程设计中最重要的任务就是:

synthesiscorrectstructure

optimization(parametersinreactionandseparationsystems)

1.4Approachestoprocessdesign

Twomethods:

1.buildinganirreduciblestructure(构建不可缩减的结构)

基于onionmodel

Ateachlayer,makebestlocaldecisionsbasedonincompleteinformation

设计出的流程的机构是固定的，在以后的optimization过程中不可改变。

缺点:

1.在设计的每一步都有不同的选择，但又不能评价他们。只能等设计完成

后通过优化每一设计才能评价他们。这就使优化工作量很大。

2.不能保证设计出最优的结构（或流程）。

2.创建并优化可缩减的结构

Buildthesuperstructure:

Allfeasibleprocessoperations

Allfeasibleinterconnections

Initially,includingredundantfeatures

Formulationoftheproblem

Designequations(units,constraints)

Designvariables

Continuousvariables(flowrate,composition,temperature,pressure,etc.)

Integervariables(describethestructureofflowsheet)

Optimizetheproblem

Maximizetheprofit

Minimizethecapitalcost

Thestructureofthesuperstructureisreduced

困难：

1.如果初始结构不包括最优结构，优化后也得不到最优结构

2.在如果每•个单元操作都精确的描述，目标的优化问题将非常大而且

不规则。

局部最优(localoptimum)、全局最优(globaloptimum)

3.设计者不能参与到优化过程，以至于许多难以明了的不能用数学模型

表示的量，例如安全性等，不能包括在优化模型中。

优点：

1.可同时考虑许多设计变量

2.整个设计过程可以用计算机程序描述，设计快速高效

1.5小结

在化工过程设计中，有两个基本问题：

1.是否能识别所有的结构

2.是不是每一个结构都可以优化，并且所有的流程结构能够有效地

比较

洋葱模型：反应、分离和回流、换热和公用工程的关系

常用的两种设计方法：

建立不可缩减的模型

建立并优化可缩减的模型

Chapter2ChoiceofReactor

化工过程设计的第一步就是从反应器开始的。反应器的选择非常重要，其性

能对全过程的经济性以及过程对环境的影响极为重要。

大多数反应器都使用催化剂，这种情况下，催化剂的选择也很重要。在反应

器的设计中需要确定的主要变量有：

反应器类型（平推流反应器、全混流反应器）

浓度

压力

温度

相（液相、汽液、液固、汽固、汽液固、固相、汽相）

催化剂

在选择反应器时，为保证反应的进行，应使其尽可能接近设计值。但是，在

设计的初始阶段，反应器的具体设计不能确定。本章主要介绍反应器的选取，并

不介绍其具体的设计。

2.1反应路径

通过不同的反映路径，可以得到相同的目标产品。用最便宜的原料生产出最

少的副产品的反应路径是最好的。应当尽量避免选择生产大量副产品的反应路

径。这类反应路径产率低，所生产的副产品也会对环境造成影响。

在选择反应路径时也需要考虑许多其他的因素。

商业性的commercial：对将来原材料和副产品的价格不确定

技术性的technical：安全性、能量的消耗等

缺乏合适的催化剂catalyst是阻止新的反映途径开发的重要原因。在设计的

第…步，不能确定选择一个反应路径对总体设计的影响。但有些比较明显。

ChemicalprocessDesign====OnionmodelReactor

Reactorperformance===ReactorProductcompositions(desiredproductsand

byproducts)====separation/heatexchangernetworkandutilities

Onthewhole,reactorperformancewillaffectthetotalcostoftheprocessandtheir

effecttotheenvironment(byproducts)

Todesignorselectareactor,weshoulddecidetheir:

Type

Concentration

Temperature

Pressure

Phase

Catalyst

Whentheseinformationareobtained,areactorcanbeselected.However,atthe

preliminarydesignstage,wejustneedtoselectthetypeofreactor.Wedon'tneedto

decidethedetailedsizeofreactor,thereasonisthat:

Reactorinteractwiththerestofchemicalprocess

Recycles:feedsdidnotreactcanberecycledtothereactor

Heatintegration

ReactionPath:

Givenmaterialscanbeproducedintodesiredproductsthroughdifferentreaction

paths.

ReactionpathRawmaterialproductsbyproducts

PreferredcheapestLargequantitySmallestquantity

NotpreferexpensiveSmallquantityLargequantity

Exceptthat,manyotherfactorsshouldbeconsidered,including:

Commercial:priceofbyproducts,futurepriceofrawmaterialandbyproducts,

Technical:safety,energyconsumptionetal.

Generally,therearenovelreactionpaths(知道由某种原料可以生产所需产品)，but

lackcatalyst(原料的转化率较低)。

Atthefirststageofdesign,therearemanypossiblereactionpaths,also,itis

impossibletodesignthechemicalprocesscorrespondingtoeachreactionpaths,then

whatcanwedo?

Forsomecase,wecanmakechoicebysimplecalculation:

Example2.1:

Example2.2

TypesofReactionSystems

在选定了反应路径后，我们就可以选择反应器的类型以及其操作条件，从而可

以进一步评价反应器的优劣。

在确定这些之前，我们先介绍一下反应系统的分类，共分为五种：

1.Singlereaction(单一反应)

Insuchasystem,mayincludesseveralreactions,excepttheprimaryreaction,other

reactionscanbeneglected.

Rawmaterialsdirectedtransferredintoproductsandbyproducts,whichcannotbe

reactedagain.

FEED=====PRODUCT(2.1)

FEED=====PRODUCT+BYPRODUCT(2.2)

FEEDl+FEED2=====PR0DUCT(2.3)

Anexamplewithoutproducebyproduct:

环氧丙稀===烯丙醇

anexamplewithbyproduct:

异丙醇===丙酮

2.Multiplereactionsinparallelproducingbyproducts(平行反应，并生产副产品)

Insuchasystem,excepttheprimaryreaction,therearesecondaryreactions,which

cannotbeneglectedandtranfertherawmaterialintobyproducts.

FEED=====PRODUCT(2.4)

FEED=====PRODUCT+BYPRODUCT(2.4)

FEED=====PRODUCT+BYPRODUCT1(2.5)

FEED=====BYPRODUCT2+BYPRODUCT2(2.5)

FEED1+FEED2=====PR0DUCT(2.6)

FEED1+FEED2=====BYPRODUCT(2.6)

EXAMPLE:

主反应：乙烯+氧气===环氧乙烯

平行反应：乙烯+氧气===二氧化碳+水

3.Multiplereactionsinseriesproducingbyproducts链反应

Theprimaryreactionandthesecondaryreactionsarenotinparallel,butinseries:

FEED=====PRODUCT(2.7)

PRODUCT=====BYPRODUCT(2.7)

FEED=====PRODUCT+BYPRODUCT1(2.8)

PRODUCT=====BYPRODUCT2+BYPRODUCT3(2.8)

FEED1+FEED2=====PRODUCT(2.9)

PRODUCT=====BYPRODUCT1+BYPRODUCT1(2.9)

Example:

甲醇(Methanol)+1/2氧气(oxygen)=甲醛(formaldehyde)+水

甲醛(formaldehyde)==一氧化碳(Carbonmonoxide)+氢气Hydrogen

4.MIXEDparallelandseriesreactionsproducingbyproducts混合反应

FEED====PRODUCT

FEED====BYPRODUCT

PRODUCT===BYPRODUCT（2.10）

FEED====PRODUCT

2FEED====BYPRODUCT1

PRODUCT===BYPRODUCT2（2.11）

FEED1+FEED2====PRODUCT

FEED1+FEED2====BYPRODUCT1

PRODUCT===BYPRODUCT2+BYPRODUCT3（2.12）

Example:

Ethyleneoxide环氧乙烯+ammonia氨===monoethanolamine（乙醇胺）

monoethanolamine（乙醇胺）+Ethyleneoxide环氧乙烯=二乙醇氨diethanolamine

二乙醇氨diethanolamine++Ethyleneoxide环氧乙烯=三乙醇氨triethanolamine

5.Polymerizationreactions聚合反映

twotypes:oneincludingterminationstep,theotheroesn'tincludethat.

Anexampleincludingterminationstepisfree-radical（自由基）polynerizationofan

alkenemolecular（烯燃分子）。

Examplesseepage21,22.

ReactorPerformance

Theperformanceofreactorsgenerallycannotbeevaluateddirectly.Toquantitatively

analyzetheperformanceofreactors,weneedsomeparameters.

Forpolymerizationreactors:thedistributionofmolecularweightsdictatesthe

mechanicalpropertiesofthepolymer,isanimportantmeasureofthereactor's

performance.

Forotherreactors:threeimportantparameterscanbeused:

Conversion=reactantconsumedinthereactor/reactantfedtothereoctor二组分反应掉

的摩尔数/进入反应器的该组分的摩尔数2.13

若原始物料中各个组份含量比不同于其计量系数之比时，各个组份的转化率是不相同。

Selectivity=desiredproductproduced/reactantconsumedinthereactor*stoichiometric

factor（化学计量系数）2.14

stoichiometricfactor:thestoichiometricmolesofthereactantrequiredpermoleof

product

Reactoryield=desiredproductproduced/reactantfedtothereactor*stoichiometric

factor（化学计量系数）2.15

Example2.3:

Forareversiblereaction,thereisamaximumconversion,theequilibriumconversion,

whichislessthan1.0.Themaximumconversioncanbefixedwiththefollowingdata

determined:moleratioofreactants,temperature,pressure

Here,wedefinedthreeparametersforreactors,thennowthequestionishowtoselect

reactors?

Indesigningthereactorperformance,selectivityisusuallymoremeaningfulthan

reactoryield.Thereasonishat:

Theformerisbasedontheconsumedreactant,accordingtothisvalue,wecan

knowproductquantity=====consumedreactantquantity

Thelatterisbasedonthereactantfedinthereactor,whichincludingthoseinthe

freshfeedandtherecycle.Wecan'tclearlyknowthequantitativereactionship

betweentheproductandthereactant.

Forareactor,ifthereisnorecyclefeedtoit,thereactoryieldisanimportant

parameter.Thefollowingequationisright:

Reactoryield=conversion*selectivity

Conversion:theconsumedreactant/thereactantfedtothereactor:

Unconvertedmaterialcanbeseparatedandrecycledtothereactor,however,at

theprilimarystage,wedon'tknowthequantityofthetoberecycledrawmaterial,

therefore,wecan'tpreciselycalculatetheconversion.

Butweneedtosetthevalueoftheconversionsothatthedesigncanproceed.So,we

needtoguesstheconversion.But,weshouldrememberthattheconversionmight

changewhenmoredetailedinformationisavailable.

Forareaction,unwantedbyproductswillconsumerawmaterialandalsomightcause

problemtoenvironment.

Selectivityreflecttheratioofreactanttransferringintoproducts,alsoreflectstheratio

ofreactanttransferringintounwantedbyproducts.

Themaximumselectivityisneed,sothatthereactorcanproduceasmoreproductsas

possible.Fordifferentreactions,thereactorconversionandreactorselectivitycan

besetasfollowing:

1.Singlereactions

Byproductswillnotaffectthemoleofproducts.Therefore,selectivitywillnotbe

affectedbyproducts.

Inthissituation,ouraimwillbereducingthecapitalcostforagivenreactor

conversion.Increasingreactorconversionwillincreasethecapitalcost,butwill

reducethecostofmanyotherparts.

Becauseofthis:theinitialsettingofconversionforsingleirreversiblereactionsis

around95%,forasinglereversiblereaction,itwillb95%oftheequilibrium

conversion.

2.Multiplereactionsinparallelproducingbyproducts

Forsuchreactions,rawmaterialcostsgenerallydominatestheeconomicsofprocess

(thereasonisthatalotofmaterialwillreactintobyproducts).

Becauseofthis,whendealingwithmultiplereactions,whetherparallel,seriesor

mixed,thegoalisusuallytominimizebyproductformation(maximizethe

selectivity)forafixedreactorconversion.

Toimproveselectivity,weshouldselectreactorconditionsexploitingdifferences

betweenthekineticsandequilibriumeffectsintheprimaryandsecondaryreactions.

这种情况下，影响转化率的因素也可能影响选择性，估计反应转化率的初值比单

一反应更困难。

例如：

FEED======PRODUCT心=k.C1''

FEED======BYPRODUCT

Z2_>2042-3

-二丁°FEED

Maximumselectivitycorrespondingtominimumr2/rl.

Forconversion:highconversiontendstodecreaseCfeed

A2>al:Cfeeddecrease,r2/rldecrease,selectivityincreasesasconversionincreases

Initialsettingofconversion:95%forirreversiblereactions

95%ofequilibriumconversionforreversiblereactions

A2<al:Cfeeddecrease,r2/rlincrease,selectivitydecreasesasconversionincreases

Initialsettingofconversionisquitedifficult:50%forirreversiblereactions

50%ofequilibriumconversionforreversiblereactions

3.Multiplereactionsinserriedproducingbyproducts

FEED======PRODUCTr尸匕/以

PRODUCT======BYPRODUCTr2=k.C^onuCT

Itcanbeseenthat,toincreaseselectivity,wehavetoreducethereactionrateofthe

secondaryreaction,itmeanswehavetoreducetheconcentrationofproductinthe

reactor,inotherwords,thereactorshouldbeoperatedatlowproductconcentration.

Fromtheaboveequations,italsocanbeseenhat:astheselectivitydecrease,the

conversionwillincrease.Then,itisdifficulttoselecttheinitialconversion.Generally:

weselectas50%forirreversiblereactions,50%ofequilibriumconversionfor

reversiblereactions.

Idealizedreactormodel

(a)Idealbatchmodel(Fig.2.1a)

Reactantsarechangedatthebeginningoftheoperation

Theystayinthereactorforacertainperiod.Duringthisperiod,thecontentsare

alwayswell-stirred.

Productsaredischarged

Inthereactor,theconcentrationofreactants,productswillchangealongthe

proceedofreactor,ortime.Aslongasthecontentsinsidethereactorareperfectly

mixed,thetemperatureandconcentrationthroughoutthereactorareuniform.

(b)Continuouswell-stirredmodel(fig.2.1b)

Feedarechargedcontinuously,alsoproductsaretakenoffcontinuously.

Reactorcontentsareassumedtobeperfectlymixed.

Compositionandtemperatureareuniformthroughoutthereactor,alsothroughout

thenormaloperationperiod.

Theresidencetimeofindividualfluidelementsvariesinthereactor.

Someareveryshort:leavesattheinstantitentersthereactor

Somearelonger.

Meanresidencetime:volumeofthereactor/volumeflowrateoffeed

(c)Theplug-flowmodel(Fig.2.1c)

Thereactantsareassumedtomovesteadilyanduniformly.

Nomixingareinducedalongthedirectionoflow.

Theconcentrationchangealongthedirectionofflow.

Sametoidealbatchreactor:theresidencetimeofallreactantsaresame.

Plug-flowmodelcanbeapproachedbyaseriesofcontinuouswell-mixedreactors.

AsshownonFig.2.Id.Thegreaterthenumberofwell-mixedreactors,the

closeristheapproachtoplug-flowoperation.

Wealreadyintroducedfivekindsofreactionsystems:

Singlereaction

Multiplereactionsinparallelproducingbyproducts

Multiplereactionsinseriesproducingbyproducts

Mixedparallelandseriesreactionsproducingbyproducts

Polymerizationreactions

Now,weneedtomatchreactormodelswithreactionsystems,weneedtoknowuse

whichkindofreactors,thereactionhavethehighestreactionrare,orconversion,or

selectivity,orreactoryield.

1.Singlereactions

Forsinglereaction:Feed====product

a

Wehave:r-kCfeed（a:orderofreaction）

Fromit,wecanseethat:

•Atthehighestconcentrationofthefeed,thereactionhavethehighestreaction

rate

•Thewell-stirredreactor:feedisdilutedinstantlyatitentersthereactor,therefore,

itsreactionrateissmallerthanthatofidealbatchreactorandplug-flowreactor.

•Togetthesamereactionresults,theresidencetimeshouldbeincreased,therefore

thecontinuouswell-mixedreactorneedsagreatervolumethantheidealmatch

andplug-flowreactors.

Therefore,forsinglereactions,theidealbatchorplug-flowreactorsarepreferred.

2.Multiplereactionsinparallelproducingbyproducts

Considertheparallelreactionsystem:

FEED=====PRODUCTr,=kxC^EED

（

FEED=====BYPRODUCTr2=k.C^ED

Batchorplug-flowreactor:higheraverageconcentrationoffeedthancontinuous

well-mixedreactor

Ifal>a2,theprimaryreactiontoproductisfavoredbyahighconcentrationof

feed（高浓度的原料更有利于主反应）

a2>al:theprimaryreactiontoproductisfavoredbyalowconcentrationoffeed（低

浓度的原料更有利于主反应）

THEREFORE:

•A2<aluseabatchorplug-flowreactor

•A2>aluseacontinuouswell-mixedreactor

Ingeneral,ifthereactiontothedesiredproducthasahigherorderthanthebyproduct

reaction,useabatchorplug-flowreactor.

Iftheprimaryreactionhasalowerorderthanthebyproductreaction,useacontinuous

well-mixedreactor.

Weanalyzedreactionswithonlyonefeed.Howaboutreactionswithmorethanone

feed?Let'sanalyzethereactionsystemnow:

b

FEED1+FEED2======PRODUCTr.1=k.IC^IEL.EILDUI.CriZ'LUZ.

FEED1+FEED2=====BYPRODUCTr2=k2C^EDlC^ED2

Wewanttominimize:

r2_^2pa2-a\pb2-h\

一=丁CFEEDICFEED2

rl匕

Fig.2.2givestheargumenttochoosearectorforsuchkindofreactionsystems.

•A2>al,B2>bl,keepbothCfeediandCfeed2low,useacontinuouswell-mixed

reactor

•A2<al,B2<bl,keepbothCfeediandCfeed2high,useaidealbatchorplug-flow

reactor

•A2>al,B2<bl,keepCfeedilow,andCfeed2high,useabatchorplugflowreactor

withFEED1chargedasthereactionprogresses,(canweusecontinuous

well-mixedreactorhere?)

A2<al,B2>bl,keepCfeedihigh,andC饱ed2low,useabatchorplugflowreactorwith

FEED2chargedasthereactionprogresses.

3.Multiplereactionsinseriesproducingbyproducts

FEED======PRODUCTr、=k\C?EED

PRODUCT======BYPRODUCTr2=k2c丸DUCT

Foracertainconversion,thefeedshouldhaveacorrespondingresidencetime.

However,inthecontinuouswell-mixedreactor,feedcanleavetheinstantitentersor

remainforacertaintime.Similarlyistheproduct.

Substantialfractionsofthefeedandproductleavebeforeandaftertheresidencetime

foragivenconversion.Thus,thiskindofreactorwouldbeexpectedtogiveapoorer

selectivitythanabatchorplug-flowreactorforagivenconversion.

Abatchorplug-flowreactorshouldbeusedformultiplereactionsinseries.

4.Mixedparallelandseriesreactionsproducingbyproducts

Considerreactionsystem:

FEED======PRODUCTry=kxC^EED

FEED======BYPRODUCTr2=k2C^ED

PRODUCT======BYPRODUCTr3=k,C^ODVCT

FORparallelreaction:

•A2<aluseabatchorplug-flowreactor

•A2>aluseacontinuouswell-mixedreactor

Forseriesreaction:itrequireaplug-flowreactororbatchreactor.Thus,forthe

mixedparallelandseriessystemabove,if

•A2<aluseabatchorplug-flowreactor

Then,thereisthequestion:whatkindofreactorshouldbeselectedwhena2>al?

Theparallelreactioncallfor:continuouswell-mixedreactor

Thebyproductseriescallfor:batchorplug-flowreactor.

Inthissituation,somelevelofmixingbetweenaplug-flowandcontinuous

well-mixedreactorwillgivethebestresults.Thiscouldbeobtainedby:

•Aseriesofcontinuouswell-mixedreactors(Fig.2.3a)

•Aplug-flowreactorwitharecycle(Fig.2.3b)

•Aseriesofcombinationofplug-flowandcontinuouswell-mixedreactors(Fig.

2.3c,2.3d)

Moredetailedcalculationisneededtodeterminethebestarrangement.（这里我们只

做了定性分析，对于具体的问题，我们还需要具体的计算来确定最好的安排）

5.Polymerizationreactions

OneimportantcharacteristicsofPolymersisthat:thedistributionofmolecularweight.

Thebreadthofthedistributionofthemolecularweightdeterminesthepolymer's

mechanicalandotherproperties.

Accordingtothis,wecanselectreactor.

Therearetwobroadclassesofpolymerizationreactions:

a.Noterminationreactionsortheactivepolymerlifeislongcomparedwiththe

averageresidencetimeinthereactor

Inabatchorplug-flowreactor:

allmoleculeshavesameresidencetime

withouttermination(reactionhavenotermination)

therefore,moleculewillgrowtoapproximatelysamemolecularweights.

While,acontinuouswell-mixedreactorwill:causeawiderdistributionofthe

molecularweight.Thereasonisthedistributionofresidencetimesinthereactor.

Therefore,forthiskindofreaction,batchorplug-flowreactorispreferred.

b.Activepolymerlifeisshortcomparedwithaverageresidencetimeinthereactor

•Thiskindofreactionisgenerallywiththeexistenceoffreeradicals.

•Whentwofreeradicalsunion,thepolymerizationwillbeterminated.

•Theterminationprocessesareinfluencedbytheconcentrationoffreeradicals

•Inbatchorplug-flowreactors,thefreeradicalconcentrationdecreasealong

theresidencetime.====increasingchainlengthwithincreasingresidence

time.Themolecularweightdistributeinabroadrange.

•Incontinuouswellmixedreactor:theconcentrationofthefreeradicalare

constant,andthus,aconstantchain-terminationrate.Thisresultsinanarrow

distributionofmolecularweight.

•Sincetheactivelifeofthepolymerizationisshort,thevariationofresidence

timedoesnothaveasignificanteffect.

ReactorConcentration

Intheprecedingsection,thechoiceofreactorwasaccordingtowhichreactorcan

givemostappropriateconcentrationprofile,forminimizethevolumeofthereactorof

maximizetheselectivity,foragivenconversion.

Asweintroducedearlier,themaximumconversionofreversiblereactionscanbe

fixedwiththefollowingdatadetermined:moleratioofreactants(orconcentrationof

reactants),temperature,pressure.

Althoughforirreversiblereactions,thereisnosuchamaximumconversion,butthese

threevaluesalsoaffecttheperformanceofthereactors.

Therefore,weneedtoanalyzethethereeifectstotheperformanceofreactors.

Wefirstanalyzethereactorconcentration.

Whenmorethanreactantisused,usuallytheratiobetweenthemolefractionsofthese

reactantsinthefeedarenotequaltotheratioofthestoichiometricfactor,itisoften

desirabletouseanexcessofoneofthereactants.

Sometimes,itisdesirabletofeedaninertmaterialtothereactor.Whatistheaim?To

reducetheconcentrationoftheproductsorthefeed.Disadvantage:increasethesize

ofreactor.

Also,wecanseparatetheproductawayafterthereactionproceedforsomeextend,

butbeforeitisfinished.

Anotherway,torecycleunwantedbyproductstothereactor.

Now,letusanalyzethesecases.

1.Singleirreversiblereactions

Nosecondaryreactions.

Allreactantscantransferintoproducts.

Selectivityalwaysequalsto1.

Anexcessofonefeedcomponentcanmakeanothercomponentcompletelytransferre

intoproducts.

Usually,wemakethereactantexcesstoeliminatethecomponent,whichismore

difficulttoseparateinthedownstreamseparationsystem.

Alternatively,ifacomponentishazardous(危险的)，wewillalsotrytoreduceit.

Forexample:

C2H4(ethylene)+C12(chlorine)======C2H4C12(Dichloroethane)

Anexcessofethylenecanmakechlorinecompletelyconvertintoproducts.Asitis

dangeroustotheprocess,itisdesirabletodothis.

2.Singlereversiblereactions

Insuchasystem,thereismaximumconversion,whichequalstoequilibrium

conversion.Generally,weneedtoselectthereactorconditionstoincreaseconversion.

LeChatelier^pinciple(法国著名化学家)say:

Ifanychangeintheconditionofasysteminequilibriumcausestheequilibriumtobe

displaced,thedisplacementwillbeinsuchadirectionastoopposetheeffectofthe

change.(对于一个处于化学平衡的系统，如果其系统的条件发生变化，平衡也将

移动，平衡的移动是朝着减小系统的变化的方向)。

Now,letusseetheeffectofthereactantconcentration.

a.Feedratio.

Toasysteminequilibrium,ifoneexcessofreactantisadded,theequilibriumwill

movetowardsthedirectionofreducingtheconcentrationofthisreactant.Thisway

theequilibriumconversionisincreased(对于其他反应成分，对于所加入的反应成

分，其转化率反而降低)。

Forexample,

C2H5OH(ethyl