过程装备与控制工程专业英语翻译14

Readingmaterial14Evaporation1.IntroductionTheobjectiveofevaporationistoconcentrateasolutionconsistingofanonvolatilesoluteandavolatilesolvent.Intheoverwhelmingmajorityofevaporationsthesolventiswater.Evaporationisconductedbyvaporizingaportionofthesolventtoproduceaconcentratedsolutionofthickliquor.Evaporationdiffersfromdryinginthattheresidueisaliquid-sometimesishighlyviscousone-ratherthanasolid;itdiffersfromdistillationinthatthevaporusuallyisasinglecomponent,andevenwhenthevaporisamixture,noattemptismadeintheevaporationsteptoseparatethevaporintofractions;itdiffersfromcrystallizationinthatemphasisisplacedonconcentratingasolutionratherthanformingandbuildingcrystals.Incertainsituations,e.g.,intheevaporationofbrinetoproducecommonsalt,thelinebetweenevaporationandcrystallizationisfarfromsharp.Evaporationsometimesproducesaslurryofcrystalinasaturatedmotherliquor.Normally,inevaporationthethickliquoristhevaluableproductandthevaporiscondensedanddiscarded.Inonespecificsituation,however,thereverseistrue.Mineral-bearingwateroftenisevaporatedtogiveasolid-freeproductforboilerfeed,forspecialprocessrequirements,orforhumanconsumption.Thistechniqueisoftencalledwaterdistillation,buttechnicallyitisevaporation.Large-scaleevaporationprocesseshavebeendevelopedandusedforrecoveringpotablewaterfromseawater.Herethecondensedwateristhedesiredproduct.Onlyafractionofthetotalwaterinthefeedisrecovered,andtheremainderisreturnedtothesea.2.LiquidCharacteristicsThepracticalsolutionofanevaporationproblemisprofoundlyaffectedbythecharacteroftheliquortobeconcentrated.Itisthewidevariationinliquorcharacteristics(whichdemandsjudgmentandexperienceindesigningandoperatingevaporatorsthatbroadensthisoperationfromsimpleheattransfertoaseparateart.Someofthemostimportantpropertiesofevaporatingliquidsareasfollows.ConcentrationAlthoughthethinliquorfedtoanevaporatormaybesufficientlydilutetohavemanyofthephysicalofwater,astheconcentrationincreases,thesolutionbecomesmoreandmoreindividualistic.Thedensityandviscosityincreasewithsolidcontentuntileitherthesolutionbecomessaturatedortheliquorbecomestooviscousforadequateheattransfer.Continuedboilingofasaturatedsolutioncausescrystalstoform;thesemustberemovedorthetubesclog.Theboilingpointofthesolutionmayalsoriseconsiderablyasthesolidcontentincreases,sothattheboilingtemperatureofaconcentratedsolutionmaybemuchhigherthanthatofwateratthesamepressure.FoamingSomematerials,especiallyorganicsubstances,foamduringvaporization.Astablefoamaccompaniesthevaporoutoftheevaporator,causingheavyentertainment.Intheextremecases,theentiremassofliquidmayboiloverintothevaporoutletandbelost.TemperaturesensitivityManyfinechemicalspharmaceuticalproducts,andfoodsaredamagedwhenheatedtomoderatetemperaturesforrelativelyshorttimes.Inconcentratingsuchmaterialsspecialtechniquesareneededtoreduceboththetemperatureoftheliquidandthetimeofheating.ScaleSomesolutionsdepositscaleontheheatingsurface.Theoverallcoefficientthensteadilydiminished,untiltheevaporatormustbeshutdownandthetubescleaned.Whenthescaleishardandinsoluble,thecleaningisdifficultandexpensive.MaterialsofconstructionWheneverpossible,evaporatoraremadeofsomekindofsteel.Manysolutions,however,attackferrousmetalsorarecontaminatedbythem.Specialmaterialssuchascopper,nickel,stainlesssteel,aluminum,imperiousgraphite,andleadarethenused.Sincethesematerialsareexpensive,highheattransferratesbecomeespeciallydesiredtominimizethefirstcostoftheequipment.Manyotherliquidcharacteristicsmustbeconsideredbythedesignerofanevaporator.Someofthesearespecificheat,heatofconcentration,freezingpoint,gasliberationonboiling,toxicity,explosionhazards,radioactivity,andnecessityforsterileoperation.Becauseofthevariationinliquorproperties,manydifferentevaporatordesignershavebeendeveloped.Thechoiceforanyspecificproblemdependsprimarilyonthecharacteristicsoftheliquid.3.Singleandmultiple-effectoperationMostevaporatorsareheatedbysteamcondensingonthemetaltubes.Nearlyalwaysthematerialtobeevaporatedflowsinsidethetubes.Usuallythesteamisatlowpressure,below3atmabs;oftentheboilingliquidisundermoderatevacuum,atpressuredowntoabout0.05atmabs.Reducingtheboilingtemperatureoftheliquidincreasethetemperaturedifferencebetweenthesteamandtheboilingliquidandthusincreasetheheattransferrateintheevaporator.Whenasingleevaporatorisused,thevaporfromtheboilingliquidiscondensedanddiscarded.Thismethodiscalledsingle-effectevaporation,andalthoughitissimple,itutilizessteamineffectively.Toevaporate1kgwaterfromasolutioncallforfrom1to1.3kgofsteam.Ifthevaporfromoneevaporatorisfedintosteamchestofasecondevaporatorandthevaporfromsecondisthensenttoacondenser,theoperationbecomesdouble-effect.Theheatintheoriginalsteamisreusedinthesecondeffect,andtheevaporationachievedbyaunitmassofsteamfedtothefirsteffectisapproximatelydoubled.Additionaleffectscanbeaddedinthesamemanner.Thegeneralmethodofincreasingtheevaporationperkilogramofsteambyusingaseriesofevaporatorbetweenthesteamsupplyandthecondenseriscalledmultiple-effectevaporation.4.GeneraltypesofevaporatorHorizontal-tubenaturalcirculationevaporatorthehorizontalbundleofheatingtubesissimilartothebundleoftubesinaheatexchanger.Thesteamentersintothetubes,whereitcondenses.Thesteamcondensateleavesattheotherendofthetubes.Theboilingliquidsolutioncoversthetubes.Thevaporleavestheliquidsurface,oftengoesthroughsomedeentrainingdevicesuchasabaffletopreventcarryoverofliquiddroplets,andleavesoutthetop.Thistypeisrelativelycheapandisusedfornoviscousliquidhavinghighheattransfercoefficientsandliquidsthatdonotdepositscale.Sinceliquidcirculationispoor,theyareunsuitableforviscousliquid.Inalmostallcases,thisevaporatorandthetypesdiscussedbelowareoperatingcontinuously,wherethefeedentersataconstantrateandtheconcentrateleavesataconcentraterate.Vertical-typenaturalcirculationevaporatorinthistypeofevaporator,verticalratherthanhorizontaltubesareused,andtheliquidisinsidethetubesandthesteamcondensesoutsidethetubes.Becauseofboilinganddecreasesindensity,theliquidrisesinthetubesbynaturalcirculationandflowsdownwardthroughalargecentralopenspaceordowncomer.Thisnaturalcirculationincreasestheheattransfercoefficient.Itisnotusedwithviscousliquid.Thistypeisoftencalledtheshort-tubeevaporator.Avariationofthisisthebaskettype.Whereverticaltubesareused,buttheheatingelementisheldsuspendedinthebodysothereisanannularopenspaceasthedowncomer.Thebaskettypediffersfromtheverticalnaturalcirculationevaporator,whichhasacentralinsteadofannularopenspaceasthedowncomer,thistypeiswidelyusedinthesugar,salt,andcausticsodaindustries.Long-tubevertical-typeevaporatorsincetheheattransfercoefficientonthesteamsideisveryhighcomparedtothatontheevaporatingliquidside,highliquidvelocitiesaredesirable.Inalong-tubevertical-typeevaporatortheliquidisinsidethetubes.Thetubesare3to10mlongandtheformationofvaporbubblesinsidethetubescausesapumpingactiongivingquitehighliquidvelocities.Generally,theliquidpassesthroughthetubesonlyonceandisnotreticulated.Contacttimescanbequitelowinthistype.Insomecase,aswhentheratioofthefeedtoevaporationrateislow.Naturalrecirculationoftheproductthroughtheevaporatorsdonebyaddingalargepipeconnectionbetweentheoutletconcentratelineandthefeedline.Thisiswidelyusedforproducingcondensedmilk.Falling-filmevaporatoravariationofthelongtubetypeisthefalling-filmevaporator,whereintheliquidisfedtothetopofthetubesandflowsdownthewallsasathinfilm.Vapor-liquidseparationusuallytakesplaceatthebottom.Thistypeiswidelyusedforconcentratingheat-sensitivematerialsuchasorangejuiceandtheotherfruitjuices,becausetheholduptimeisverysmall(5to10sormore.andtheheat-transfercoefficientsarehigh.Forced-circulationtypeevaporatorthisliquidfilmheattransfercoefficientcanbeincreasedbypumpingtocauseforcedcirculationoftheliquidinsidethetubes.Thiscouldbedoneinthelongtubeverticaltypebyaddingapipeconcentratewithapumpbetweentheoutletconcentratelineandthefeedline.However,usuallyinaforced-circulationtype,theverticaltubesareshorterthaninthelong-tubetype.Also,inothercasesaseparateandexternalhorizontalheatexchangerisused.Thistypeisveryusefulforviscousliquids.阅读材料14蒸发1、介绍蒸发的目的是浓缩不易挥发的溶质和易挥发的溶剂组成的溶液。蒸发所用的绝大多数溶剂是水。蒸发是通过蒸发部分溶液来得到溶质的浓溶液。蒸发不同于烘干的地方在于其残余物是液体—有时是粘稠度很高的液体—而不是固体。蒸发和蒸馏的不同在于—蒸汽的成分往往仅有一种,即使有多种,也不会在蒸发时刻意分离出来。蒸发和结晶的不同在于—其重点是浓缩溶质,而不是形成晶体。在特定的场合下,比如蒸发浓盐水获取食盐时,蒸发和结晶基本木有区别。在饱和母液中,蒸发有时会产生晶体的浆液。通常,蒸发所得的浓溶液是有价值的产品,蒸气则被压缩和废弃。然而,在一些特殊场合下,情况却相反。矿物质水通常通过蒸发获得无固相的产品制成锅炉供料、特殊过程的必需品、或者供人食用的消费品。这种技术通常叫做水的蒸馏,但其实还是蒸发。现在已经在发展大规模的蒸发来从海水中回收饮用水。在这里浓缩水是我们希望得到的产品。回收的只是馏分,剩余部分回归海洋。2、液体特性蒸发遇到的实际问题明显受到即将浓缩的液体的影响。由液体性质可以将操作广泛分类(要求在设计和操作蒸发器时的经验和判断,从简单的换热到分离。以下是蒸发液体的一些最重要的性质。即使供给蒸发器的稀溶液足够稀,以致其物理性质和水差不多,但随着其浓度越来越大,情况会大不一样。其密度和粘度会随着固体含量的增加而增加,直至饱和或满足换热器。持续加热饱和溶液会引起结晶;这必定会引起分离或管路堵塞。溶液的沸点会随着固体量的增加而显著升高,所以在相同温度下浓溶液的沸腾温度会比水高得多。起泡一些材料,尤其是有机物质在蒸发过程中会起泡。伴随着蒸气的流出会有稳定的泡沫,引起强烈的反应。在一些极端情况下大量的液体会被带出出气口并消失。温度灵敏度很多纯净的化学药品和食物若在相对较短的时间内加热到一定的温度会变质。因此在浓缩这样的材料时就需要特殊的技术来降低液体的温度和加热的时间。水垢一些溶液会沉淀在加热的表面形成水垢。各种参数会稳定地降低,直到蒸发器关闭、管路清洗干净。水垢变硬以及难溶解后,清洗工作会很难,费时费力。构造材料在允许的情况下,蒸发器会用一些钢材制成。然而,一些溶液会侵蚀铁合金或被它们污染。一些特殊的材料比如铜、镍、不锈钢、铝防水石墨和铝在这个时候就会派上用场。虽然这些材料费用高,但其高传热率使其在减少设备成本方面而倍受青睐。蒸发器的设计者还必须考虑液体的其他特性。其中包括比热、凝固点、沸点、气体挥发点、毒性、爆炸危险性、放射性和必要的消毒操作。由于液体的多种特性,现已发展设计出很多不同的蒸发器。针对特定问题的解决的根本依据都是液体的特性。3、单效及多效操作大多数蒸发器都是通过管道的压缩蒸气加热。需蒸发的材料在管道内流动。蒸气的压力一般较低,在3个