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 effectandmultiple effectevaporation 8 1Boiling PointElevation 沸点升高 Boilingpointelevationofasolution liquidfood isdefinedastheincreaseinboilingpointoverthatofpurewater atagivenpressure AsimplemethodtoestimateboilingpointelevationistheuseofDuring srule Alinearrelationshipexistsbetweentheboiling pointtemperatureofthesolutionandtheboilingpointtemperatureofwateratthesamepressure 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 IfsetCpp CpfandthetemperatureoffeedisequalstoTf 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