传质与分离工程1.ppt

1 16 UseofMurphreeefficiency默弗里板效率 Determiningthenumberofactualplates实际板 pp 692 693 WhentheMurphreeefficiencyisknown itcanreadilybeusedintheMcCabe Thielediagram Fig 21 33 p 693 2 Fig 21 33 3 Triangleacdrepresentstheidealplateandtriangleabetheactualplate Theactualplate insteadofenrichingthevaporfromyn 1toy n accomplishedalesserenrichmentyn yn 1 4 ToapplyaknownMurphreeefficiencytoanentirecolumn itisnecessaryonlytoreplacethetrueequilibriumcurveyeversusxebyaneffectiveequilibriumcurvey eversusxe whoseordinatesarecalculatedfromtheequation 21 70 5 Howtoplottheeffectiveequilibriumcurve Givingxn findy nfromequilibriumcurveandyn 1fromoperatingline calculateynfromequation 21 70 thenplotpoint xn yn Howtodeterminethenumberofactualplates Thereboilerisnotsubjecttoadiscountforplateefficiency andthetrueequilibriumcurveisusedforthelaststepinthestrippingsection 6 Howmanyactualplatesareneededintheleftconstruction 7 Howmanyactualplatesareneededintheleftconstruction Nofractionalactualplate then 8 Murphreeefficiencycanbedifferentfromindividualplates andthisismoreaccuratetocalculatethenumberofactualplatesthanbyoverallefficiencyofacolumn全塔效率 9 10 Therearemanycaseswherebatchdistillation间歇精馏ispreferred particularlyinthefoodandpharmaceuticalindustries Batchdistillationisusedwhensmallamountsofproductaremadeinapilotplanttoprovidesamplesforproductsamplingortesting Features 1 Unsteadystate 2 Withoutreflux 3 One stagepartialvaporizationprocess Thevaporleavingthestillatanytimeisinequilibriumwiththeliquidinthestill yisinequilibriumwithx 11 12 2 Rayleighequation 瑞利方程 Heater Eq 21 83 isknownastheRayleighequation dx y x canbeintegratedgraphicallyornumericallyusingequilibriumdata y xequilibriumrelationship 13 3 Foranidealmixture Thatis Because or 14 Afterintegrationbetweenlimits 15 Numberofmolesofproduct D EXAMPLE21 9 Averageconcentrationofproduct 16 Advantages Theadvantagesofbatchdistillationarethatseveralproductscanbemadefromasingleunit andcaneffectivelyhandlesludgesandsolids Disadvantages Foragivenproductrate theequipmentislarger Itrequiresmoreoperatorattention usesmoreenergy andbecauseitisadynamicprocess ishardertocontrolandmodel 17 2 BatchDistillationwithreflux间歇精馏 有回流的间歇蒸馏 pp 702 703 Inmanycases arectifyingcolumnwithrefluxisusedtoimprovetheperformanceofthebatchstill Flowdiagram Features Unsteadystate Onlyrectifyingsectioninthecolumn Productconcentrationdiffersfromdifferentoperations 18 Twobasicoperationmethods a Keepthetopcomposition xD constantbyincreasingtherefluxratioasthecompositionoftheliquidinthereboilerchanges b Fixtherefluxratioandlettheoverheadproductpurityvarywithtime stoppingthedistillationwhentheamountofproductortheaverageconcentrationinthetotalproductreachesacertain 某个 value 19 Calculationandanalysisfortheoperationofadistillationcolumn Theplatecolumnhasbeenestablished andthefollowingdataaregiven Equilibriumrelationship xF q Nand M Nactual InordertoobtainoverheadandbottomproductswithrequiredmolefractionsofxDandxB therefluxratioRDandthefeedplatepositionmustbedetermined Calculationmethod Trialanderror 反复试验 20 AttentionsofOperationanalysisofdistillationcolumn a Suitablefeedplateposition b Keepmaterialbalance IfDandBchanged xDandxBwillbechangedtoo c Refluxratioisthekeymeasureofcontrollingproductquality Forexample ifNisgiven RD xD xB d IfxFchanged feedplatepositionshouldbechangedtoo 21 4 AZEOTROPICANDEXTRACTIVEDISTILLATION pp 131 恒沸精馏和萃取精馏 Whenrelativevolatility 相对挥发度 nearorequaltounity theseparationofcomponentsisdifficultbyconventionaldistillation orimpossiblebecauseofazeotrope恒沸混合物formation Basicprinciplesofazeotropicandextractivedistillation Addingathirdcomponent solvent toincreasetherelativevolatilityoftheoriginalcomponents sothatthemixtureswithrelativevolatilitynearorequaltounitycanbeseparatedbyconventionaldistillation 22 1 Azeotropicdistillation恒沸精馏Materialscalledentrainers 夹带剂 thirdcomponent areaddedtotheoriginalmixture andusuallyanewazeotropewithlowest boilingpoints有最低恒沸点的恒沸物isformed Thenewazeotropewillofcoursecontainoneormoreofthefeedcomponents andleavingbehindcomponent s whichmayberecoveredinthepurestate Example Recoveryandpurificationofethanolandwater Flowdiagram Selectionofentrainers 23 2 Extractivedistillation萃取精馏Asolventisaddedtothedistillationtowertoincreaserelativevolatilityofkeycomponentsinthefeedmixture Theeffectiveextractivesolvent萃取剂willsel