CHAPTER 8 Evaporation(蒸发)[1].ppt

CHAPTER8Evaporation（蒸发）,Thereasonsforevaporation(concentration):Increasingmicrobiologicalstabilityandshelflife,Reductionintransportandstoragecosts,Increasetheconcentrationpriortotheremovalofremainingwaterbydrying,particularlyspraydrying.Theprincipleisverysimpletheliquidisboiledinasuitablevesselandthevaporisremovedleavingbehindaresidueofconcentratedliquorintheevaporator.,heatingsteam(freshsteam)secondarysteamsingleeffectmultipleeffectdifferentpressures,Introduction,Evaporationheatingandconcentratingadilutesolutiontoobtainasolidsoluteorrecoverasolvent.Itcanbelookedasaseparationprocessbetweenaninvolatilesoluteandavolatilesolvent.,BasicConcepts,dilutesolution,Steamandvapor,vapor一般作为蒸汽的一种现象使用steamn.Thevaporphaseofwater.水的蒸汽形式Amistofcoolingwatervapor.水气水蒸汽冷却形成的薄雾Pressurizedwatervaporusedforheating,cooking,ortoprovidemechanicalpower.steam一般作为蒸汽的动力使用.Steam压力蒸汽用于加热、烹调或提供机械动力的加压水蒸汽Thepowerproducedbyamachineusingpressurizedwatervapor.蒸汽动力使用加压水蒸汽机器产生的动力Steamheating.蒸汽加热Power;energy.电力；能源steam.,Single-effectandmultipleeffectevaporation,8.1Boiling-PointElevation（沸点升高）,Boilingpointelevationofasolution(liquidfood)isdefinedastheincreaseinboilingpointoverthatofpurewater,atagivenpressure.AsimplemethodtoestimateboilingpointelevationistheuseofDuringsrule,Alinearrelationshipexistsbetweentheboilingpointtemperatureofthesolutionandtheboilingpointtemperatureofwateratthesamepressure.,Example8.1,8.2TypesofEvaporators（蒸发器的类型）,Batch-TypePanEvaporator（间歇式锅式蒸发器）NaturalCirculationevaporators（自然循环式蒸发器）Rising-FilmEvaporator（升膜式蒸发器）Falling-filmEvaporator（降膜式蒸发器）,Rising/Falling-Filmevaporator（升/降膜式蒸发器）ForcedCirculationEvaporator（强制循环蒸发器）AgitatedThin-FilmEvaporator（搅拌薄膜式蒸发器）,8.2.1Batch-TypePanEvaporator（间歇式锅式蒸发器）,将溶液加热，使其中的部份水份（或溶剂）气化并不断去除，以提高溶液中的溶质浓度的过程叫蒸发操作。蒸发按压力分类，分为常压、加压和减压蒸发。按二次蒸汽是否利用，分为单效蒸发和多效蒸发。,外加热式蒸发器动画,8.2.2NaturalCirculationevaporators（自然循环式蒸发器）,升膜式蒸发器动画,8.2.3Rising-FilmEvaporator（升膜式蒸发器）,Single-effectEvaporator,降膜式蒸发器动画,8.2.4Falling-filmEvaporator（降膜式蒸发器）,8.2.5Rising/Falling-Filmevaporator（升/降膜式蒸发器）,强制循环蒸发器动画,8.2.6ForcedCirculationEvaporator（强制循环蒸发器）,withscratchplates,8.2.7AgitatedThin-FilmEvaporator（搅拌薄膜式蒸发器）,8.3DesignofaSingle-EffectEvaporator（单效蒸发器的设计）,MassBalance-CalculationofwaterremovalperunittimeEnergyBalance-CalculationofSteamRequirement,MassBalance,Normally，mvandmPisrequired,xpisaknown.,EnergyBalance-requirems,SubstitutingEqs(8-4)and(8-5)into(8-3),itobtains:,Latentheatofsecondsteam,Latentheatofsteam,Thus,IfsetCppCpfandthetemperatureoffeedisequalstoTf=T1，沸点进料theequationbecomesfollowing,Example8.2,8.4ConceptsforMultiple-EffectEvaporation（多效蒸的概念）,FlowchartsettingConcurrentflowprocessCountercurrentflowprocessParallelcurrentflowprocess,Multiple-effectevaporation,Forsingle-effectevaporation：1kgH2O/1.1kgfreshsteamConditiontousesecondaryvaporasheatingsteaminsecondeffect:pressureinsecondeffectislowerthanthefirsteffect.Vacuumornegativeconditionwillreducethepressureandtheboilingpoint.Multiple-effectevaporationcanreducetheconsumptionoffreshsteam,andincreasetheenergeticefficiency.,Advantages:Becausethepressureinlatereffectislow,thesolutioncanbetransferbythepressuredifferencewithoutpump.Alsobythepressuredifference,thesolutioninlatereffectcanbeevaporateditself(flashevaporation)sinceboilingpointreducing.,Disadvantage:Inlatereffect,becausetheconcentrationishighandthetemperatureislow,thesolutionviscosityissohighthattheheattransfercoefficientislowhere.,LayoutoftheflowforMultiple-effectevaporation,CocurrentFeedProcess,Countercurrentfeedprocess,advantagesThetemperatureishighwherethesolutionconcentrationishigh.Thustheheattransferrateisalmostthesameineacheffect.,disadvantagesPumpsareneededbetweeneffectsandmoreenergyareconsumed.Suitableforsolutionsthatitsviscosityvarieslargelywithtemperatureandconcentration.Notsuitableforheatsensitivesolutions.,Multiple-effectevaporation,parallelfeedprocess,dilutesolutionfeedsintoeveryeffectevaporatorsanddischargesfromeveryeffect,respectively.Theflowsinalleffectsareparallel.,Suitableforsolutioninwhichcrystallizationeasilyoccurs.e.g.,saltsolutioninlowtemperature.Itcanavoidtotransfersolutionwithmanycrystals.,Multiple-effectevaporation,Example8.3,8.5V