Ice crystallization in a fluidized bed heat exchan

MAXIMUMTEMPERATUREDIFFERENCEWITHOUTICE-SCALINGINSCRAPEDSURFACECRYSTALLIZERSDURINGEUTECTICFREEZECRYSTALLIZATIONP.Pronk1,C.A.InfanteFerreira1,M.RodriguezPascual2,G.J.Witkamp21EngineeringThermodynamicsSection,DelftUniversityofTechnology,Mekelweg2,2628CD,Delft,TheNetherlands.2LaboratoryofProcessEquipment,DelftUniversityofTechnology,Leeghwaterstraat44,2628CA,Delft,TheNetherlands.E-mail:[e-mail]ystallization(EFC)inscrapedsurfacecrystallizers,iceandsaltarecrystallizedsimultaneouslyaroundtheeutecticpoint.Duetothestrongtendencyoficetoadhereontocoldheattransfersurfaces,scalelayerstendtoformincrystallizerscausingaseveredecreaseinheattransferrates.Preventionoficescalingthereforeplaysamajorroleindesignandoperationoffreezecrystallizationprocesses.Aparallelpaperrevealsthatboththesolutetypeandconcentrationplayanimportantroleinicescaleformationonheatexchangersurfaces.Inthiswork,themaximumtemperaturedifferencebetweenthecooledsurfaceandtheslurrywithouticescalingisdeterminedforKNO3solutionsinacontinuouslyfedcrystallizerwithfourPTFEscraperbladesrotatingonacooledplate.Atlowconcentrations,theresultsareincorrespondencewiththeparallelpapershowingthatthemaximumtemperaturedifferencewithouticescalingincreaseslinearlywiththesoluteconcentration.However,atsoluteconcentrationsneartheeutecticpoint,themaximumtemperaturedifferencewithouticescalingdecreases.Thelatterisascribedtosaltcrystallizationwhichacceleratestheicegrowthrateresultinginicescaling.1.IntroductionEutecticfreezecrystallization(EFC)isaninterestingtechniquetoseparateaqueouselectrolytesolutionsintoiceandsalt.Duringeutecticfreezecrystallization,acrystallizeroperatesateutecticconditionsasaresultofwhichiceandsaltcrystallizesimultaneously.Previousstudieshaveshownthatthisseparationtechniqueissuitablefornumerouselectrolytesolutionsandhasthepotentialtoreduceprimaryenergyconsumptioncomparedtootherseparationtechniques[HAM98].Furthermore,experimentshaveshownthatthepurityoftheproducedsaltandicecrystalsisveryhigh[HAM04,VAE03].Animportantaspectofeutecticfreezecrystallizationisicescalingprevention,sinceicecrystalshaveastrongtendencytoadheretocoldheatexchangersurfaces.Ificescalingisnotprevented,ascalelayerwillformontheheatexchangingwallsresultinginastrongdecreaseofheattransferrates.Icescalingisnotonlyofimportanceforeutecticfreezecrystallizationbutalsoforothericecrystallizationprocesses,suchasfreezeconcentrationoffoods[VER02]andiceslurryproductionforcoldthermalstorageapplications[BEL05].Toavoidicescaling,mosticecrystallizersareequippedwithmovingpartswhichcontinuouslyremoveicecrystalsfromheattransfersurfaces[STA05].Acommonlyusedicecrystallizerisascrapedsurfacecrystallizerinwhichrotatingscraperbladespreventice1scaling.Experimentsonicecrystallizationfromelectrolytesolutionswithascrapedsurfacecrystallizerrevealedthatthescrapingrateandtheionconcentrationareimportantparametersthatdeterminewhethericescalingoccursornot[VAE02].Analternativeicecrystallizerisafluidizedbedheatexchangerinwhichfluidizedsteelparticlesremoveicecrystalsfromthewalls[PRO05].Experimentshaveshownthatthemaximumtemperaturedifferencetopreventicescalinginthistypeofcrystallizerslinearlyincreaseswiththesoluteconcentration.TheobjectiveofthispaperistostudyicescalinginscrapedsurfacecrystallizersduringicecrystallizationfromdifferentaqueousKNO3solutionsatdifferentrotationalspeedsofthescraperblades.Theresultsareusedtoobtainmoreunderstandingonicescalingclosetoeutecticconditionsandtocomparetheicescalingpreventionabilityofscrapedsurfacecrystallizersandfluidizedbedheatexchangers.2.ExperimentalSet-upandProcedureTheexperimentalset-upmainlyconsistsofa10-literscrapedsurfacecrystallizerasshowninFigure1.Thecrystallizerhasa1mmstainlesssteelbottomplatewithaheattransferareaof0.031m2whichisscrapedbyfourrotatingPTFEscraperbladesthataredrivenbyaverticalshaft.Halfwaythisshaft,aturbinemixerisinstalledtokeeptheslurryhomogenous.Thebottomplateiscooledbya50wt%potassiumformatesolutionwhichfollowsaspiralchannelbelowthebottomplateofthecrystallizer.Theheightandwidthofthecoolantchannelmeasure5and17mmrespectively.Thecoolantflowrateis1000l/handitsinlettemperatureiscontrolledwithin0.1Kbyacoolingmachine.Thecrystallizeroverflowstoanicemeltingvesselweretheproducedicecrystalsaremeltedandfromwhichaqueoussolutionispumpedbacktothecrystallizer.Figure1:Experimentalset-upwithscrapedsurfacecrystallizer[VAE03]Duringtheexperiments,theshafttorque,thecoolantflowrateandthetemperaturesinthecrystallizerandattheinletandoutletofthecoolantweremeasured.Thetotalamountoftransferredheatthroughthebottomplatewascalculatedfromthemeasuredflowrateandcoolanttemperatures.Subsequently,theoverallheattransfercoefficientwascalculatedfromthetransferredheatandthetemperaturedifferencebetweenthecoolantandtheslurry.23.ResultsandDiscussionIcecrystallizationexperimentswereperformedatdifferentKNO3concentrationsanddifferentrotationalspeedsofthescraperblades.Firstofall,oneexperimentisdescribedindetail,afterwhichtheinfluencesofKNO3concentrationandrotationalspeedonicescalingarediscusse