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SuperheatedsteamfluidisedbedpaddydryingChaiyongTaechapairoja,,IsaresDhuchakallayaa,SomchartSoponronnarita,SomboonWetchacamaa,SomkiatPrachayawarakornbaThermalTechnologyDivision,SchoolofEnergyandMaterials,KingMongkutsUniversityofTechnologyThonburi,Suksawat48Road,Thungkaru,Bangkok10140,ThailandbFacultyofEngineering,KingMongkutsUniversityofTechnologyThonburi,Suksawat48Road,Thungkaru,Bangkok10140,ThailandReceived18February2002accepted24July2002AbstractFluidisedbedpaddydryingusingsuperheatedsteamisanewlyalternativeapproachinsteadofusingtheconventionalhotair.Themechanismofmasstransferforpaddydryinginarangeofinitialmoisturecontentbetween25and44.5d.b.isstronglycontrolledbyinternalmoisturemovementinsidethekernelandatwoseriesexponentialequationissuitablyusedtoexplainitsmovement.Dryingparametersintheequationareafunctionoftemperatureandbeddepth.Forthepaddyquality,headriceyieldfromthesuperheatedsteamdryingismoresustainableandhashighervaluesthanthoseobtainedfromthehotairdrying,whereasthecolourofwhitericebecomesdarker,makingitpoorerquality.Thepercentageofwhitebellyissignificantlyaffectedbytheinitialmoisturecontent.C2112002ElsevierScienceLtd.Allrightsreserved.KeywordsDehydrationGrainHeadriceQualitySuperheatedsteamWhitebelly1.IntroductionPaddyisanimportantfoodcropintheworld.Thiscerealisprocessedintoseveralproducts,inadditiontoastaplefood.Beforebeingprocessed,freshlyharvestedpaddyneedstobedriedinordertoavoidthequalitydeteriorationbymicroorganismsandrespiration.Adryingmethodwhichisutilisedeffectivelyforessentiallyhighmoisturegrains,forexample,paddy,parboiledrice,soybeanandmaize,isafluidisationtechnique,inwhichairandsolidparticlesarerigorouslymixed.Ithasbeensuggestedthathighmoisturepaddyshouldbedriedquicklytoapproximate23d.b.topreventtheyellowingofricekernelswhichiseasilyoccurredathighmoisturelevelandthensubjectedtoambientairdryinginastoragebinuntilitsmoisturecontentisdownto16d.b.SoponronnaritPrachayawarakorn,1994.SutherlandandGhaly1990undertookanintensiveinvestigationintothefeasibilityofusingahotairfluidisedbeddryingtechniqueforpaddy.Theirexperimentsshowedthattheheadriceyieldrelatedtothefinalmoisturecontenttheheadriceyieldwas58–61whenpaddywasdriedfrom28.2to20.5d.b.butwas15–24forafinalmoisturecontentlowerthan20d.b.Thedryingmediumwhichisusedforcarryingtheevaporatedwaterandfluidisingsolidparticlescouldbehotairorsuperheatedsteam.SuperheatedsteamdryingoffersimportantadvantagesoverhotairdryingItheenergysuppliedtothedryercanbereducedeconomicallybyrecyclingtheexhauststeaminaclosedloop,IItheenergyfromtheexhauststeamresultingfromtheevaporationofmoistureinsidethesolidscanberecoveredandusedintheothersections,IIIenvironmentalpollutionorodouremissiontotheatmospherecanbeeliminatedsincedryingoccursinaclosedchamberwithoutair.Thegeneralcharacteristicsofthedryingprocesschangesignificantlywhensuperheatedsteamisusedinsteadofhotair.ThemajordifferencesincludeIsteamcondensationatthematerialsurfaceduringtheinitialdryingperiod,IIhigherdryingrateinsuperheatedsteamthaninhotairataboveinversiontemperature,thetemperatureatwhichtheevaporationrateofwaterintosteamandhotairbecomesequalYoshidaHyodo,1963,IIInogasfilmresistancearoundthesolidsurfacetherebyincreasingthemasstransferrate.JournalofFoodEngineering58200367–73www.elsevier.com/locate/jfoodengCorrespondingauthor.Tel./fax6624708663.Emailaddresschaiyong_tyahoo.comC.Taechapairoj.02608774/02/seefrontmatterC2112002ElsevierScienceLtd.Allrightsreserved.PIIS0260877402003357Theuseofahighsteamtemperaturemaycauseaseriousproblemforthermallysensitivefoodmaterials,suchasbrowning,discolourationandproteindenaturation.Insomecases,however,itchangesthetexturalpropertybeneficiallyforexample,shrimpdryingwithsuperheatedsteampresentedalowerdegreeofshrinkagethanthatusinghotairPrachayawarakorn,Soponronnarit,Jaisut,2002.Thepurposeofthepresentworkwastomaketheparboiledricebyasuperheatedsteamfluidisedbeddryer.Thedryingcharacteristicsofpaddywereinvestigated,alongwiththequalitiesintermsofheadriceyield,whitenessandwhitebellytobedetermined.2.MaterialandmethodsAbatchsuperheatedsteamfluidisedbeddryerisschematicallyillustratedinFig.1.Itconsistedoffivemaincomponentsacylindricalchamberwithaninnerdiameterof15cmandaheightof100cm,a13.5kWelectricalheaterforconvertingsaturatedsteamtosuperheatedsteam,abackwardcurvedbladecentrifugalfandrivenbya2.2kWmotor,areverseflowcycloneandasmallboilerwitha31kg/hcapacityofgeneratingsteam.Aperforatedsheet,with10holespercm2,wasusedfordistributingthedryingmedium.Theratioofthediameterofdryertograindiameterdesignedwaslargeenoughtominimisethewalleffect.However,thewalleffectbecomesimportantwhentheratioislowerthan10Geankoplis,1995.SuperheatedsteamtemperaturewascontrolledbyaPIDcontrollerwithanaccuracyofC61C176C.Beforeusingsteamfordrying,hotairwasusedforwarmingupthesystemuntilthetemperatureineverypartreachedthedesiredlevel.Then,theairwasreplacedbysteam.Thesteamgeneratorgeneratedthesaturatedsteamat106kPaabsolutepressurewiththecorrespondingtemperatureof100C176C.Whenthesaturatedsteamwasflowedthroughtheelectricalheater,additionalheatwassuppliedtoraisethesteamtemperatureuptothedesiredlevel.Itwassubsequentlypassedthroughthefluidisedbeddryer.Afterthat,thesmalldustparticlesandtheimmaturegrainssuspendedintheexhauststeamwerecollectedinthecyclone.Finally,allthecleanedexhauststeamwasreusedagain.Thepaddywasrewettedtoadesiredmoisturecontentandthenkeptinacoolroomatthetemperatureof3–5C176Cfor5–7daystoensureuniformmoisturecontentthroughoutthekernels.Theexperimentalconditionsweresetupasfollowsinitialmoisturecontentsof25–45d.b.,beddepthsof10–15cm,superheatedsteamtemperaturesof150–170C176Catafixedsuperficialvelocityof3.1m/s.TemperaturesatdifferentpositionsweremeasuredbyChromel–AlumelthermocouplesTypeKconnectedtoadataloggerwithanaccuracyofC61C176C.Afterdrying,paddykernelswereslowlycooleddowntoambienttemperatureandkeptinapolypropylenebag.Then,theyweregentlyventilatedwithambientairuntiltheirmoisturecontentreached16d.b.Finally,a300gsamplewaskeptinasealplasticbagfor2weeksbeforetestingforheadriceyield,whitenessandwhitebelly.Themoisturecontentofpaddywasdeterminedbyanelectricalairovenatatemperature103C176Cfor72h,accordingtoAACCmethod,1995.Paddyqualitiesintermsofheadriceyieldandwhitenessweredeterminedquantitativelyandcomparedtothereferencesamplepaddydriedbytheambientair.ThemethodsfollowedtheguidelineoftheMinistryofAgriculturalandCooperatives,Thailand.Headriceisdefinedasmilledricehavingakernellengthatleast75ofitsoriginallength.Theheadriceyieldwasdefinedasthemassofwhitericethatremainsasheadriceaftercompletemillingdividedbythemassofpaddysample.Fromtheexperiments,theheadriceyieldoffreshpaddysamplesobtainedeachtimewasdifferentanditwasthusdifficulttocomparethepaddyqualityobtainedfromdifferentdryingconditions.Tomakeaclearcomparison,theheadricequalitywasthereforerepresentedasarelativeheadriceyield.TherelativeheadriceFig.1.Aschematicdiagramofthesuperheatedsteamfluidisedbeddryer1fluidisedbeddryer,2heater,3fan,4cyclone,5boilerand6bypassline.68C.Taechapairojetal./JournalofFoodEngineering58200367–73yieldisdefinedastheratioofheadriceobtainedfromartificialdryingtothatfromthenaturalairdrying.Inadditiontotheheadriceyield,theothertwoqualitiesofpaddy,translucence,whichwasdirectlymeasuredbywhiteness,andwhitebelly,werealsoexamined.Theopaquewhitespotatthepaddykernelcanbelooselydividedintothewhitebellyandthechalkygrain.Thewhitebellyarisesfromincompleteorpartiallygelatinisedpaddy,whilethechalkygrainhasitsowncharacteristicdependinguponthegenotypeofpaddyandcanstillexistinthegrainevenwithcompletegelatinisation.ThewhitericesamplesweremeasuredforthecolourbyusingaKettdigitalwhitenessmeterModelC300.Beforemeasuringthecolourofasample,thewhitenessmetermustbecalibratedwithawhitecolouredreference,havingastandardvalueof86.3.Throughoutthiswork,thecolourofthesamplewasrepresentedintermsoftherelativewhiteness,whichwasdefinedasthewhitenessofthesamplefromtheartificialdryingdividedbythatfromnaturaldrying.3.Resultsanddiscussion3.1.FluidisationconditionsAtfirst,theminimumvelocityrequiredforfluidisinginthesuperheatedsteamwasdeterminedbymeasuringthepressuredrop.TheresultsareshowninFig.2forbeddepthsof10,12.5and15cm,indicatingthattheminimumsuperficialvelocityforfluidisationðUmfÞ,determinedfromthepointwherethepressuredropbeginstobeconstant,isindependentonthebeddepthwithavalueof2.6m/sforasteamtemperatureof150C176C.Thisminimumvelocityishigherthanthatusinghotairforwhichthevelocitywas1.65m/saspreviouslyreportedbySoponronnaritandPrachayawarakorn1994.Therequiredhigherfluidisingvelocityisprobablyduetothelowerdensityandviscosityofsuperheatedsteam.Thelowerphysicalpropertiesofsteamresultintheratiooftheverticalforceonthepaddybecominglowerforthesamesuperficialvelocity.However,theforceactingonthepaddykernelintheoppositedirectioncanbeoffsetbyincreasingthesuperficialsteamvelocity,thusrequiringahigherminimumfluidisingvelocityforsteam.AscanbeseeninFig.2,belowtheminimumfluidisingvelocity,thebedisastaticbed,correspondingtotheregionwherepressuredrophasalinearincreasewithsuperficialvelocity.However,attheonsetoffluidisation,thepaddywaspartiallyfluidisedanditrequiredavelocityatleast1.3timeshigherthantheminimumsuperficialfluidisingvelocitytoachieveallparticlemovement.WhenapplyingtheErgunequation1952,thecalculatedminimumfluidisationvelocitywas2.30m/satatemperature150C176C,slightlylowerthantheexperimentallydeterminedvalue.Thismightbebecauseoftheagglomerationofpaddykernelscausedbythesteamcondensation.Fig.3showstheminimumfluidisingvelocityforairandsuperheatedsteam,andindicatestheslightdependenceoffluidisingvelocityontemperature,butthestrongdependenceonthecharacteristicfluidmedium,withthehigherfluidisingvelocityforsteamoverawiderangeoftemperatures.Thesmalleffectoftemperatureonthefluidisingvelocitymightbebecauseofasmallchangeinthedensityandviscosityofthemediumsfortherangeoftemperatureconsidered.ThecharacteristicpropertiesofpaddyusedinthecalculationofthefluidisingvelocityareshowninTable1.Fig.2.Relationshipbetweenpressuredropandsuperficialsuperheatedvelocityatdifferentbedheightsuperheatedsteamtemperature¼150C176C.Fig.3.Effectofdryingmediaonthefluidisingvelocity.Table1PropertiesoflonggrainpaddyPhysicalpropertiesEquivalentdiametermm3.5Truedensitykg/m31414VoidemfSoponronnaritPrachayawarakorn,19940.557Sphericity0.68C.Taechapairojetal./JournalofFoodEngineering58200367–73693.2.DryingbehaviourFig.4showsthebehaviourofpaddykernelsdriedinsuperheatedsteamatdifferenttemperaturesandbedheights.Inthefirsthalfaminute,thepaddykernelsadsorbasmallamountofthecondensedwaterandthus,theirtemperaturerapidlyincreasestothesaturationpointcorrespondingto100C176C.Ascanbeseeninthisfigure,thesamplescangainwaterevenlyalthoughtheoperatingconditionssuchastemperatureandbeddeptharechanged.SuchresultsaredissimilartotheworkreportedbyTangandCenkowski2000,whostudiedpotatodryinginabatchtraydryerusingsuperheatedsteam.Intheirwork,theinitialgaininmoisturecontentvariedwiththetemperaturethesamplescouldgainmoremoistureatlowersteamtemperatures.Apossiblereasonforthiscontradictioncanbethatusingahighsteamvelocityinthepresentwork3m/sforthefluidisedbedleadstothecombinationofhighheatandmasstransferrates.Thus,thelargeamountofcondensedwaterexistingbetweenthekernelsevaporatesrapidly,insteadofdiffusingintothegrains.FromFig.4,theinitialgainsinmoisturecontentforsamplesatdifferentconditionsareapproximately0.02d.b.Fig.5representsthecomparativedryingcharacteristicsofpaddyinsuperheatedsteamandhotair.Afterpassingthroughthecondensationstage,themoisturecontentreducesexponentiallywithdryingtime,indicatingthatthemainresistancetomoisturemovementisintheinterior.Inthismoisturereductionstep,thedryingcurveisverysimilartothatfoundinthehotairdrying,butthetotaldryingtimeforsteamisshorterinspiteoftheexistenceofcondensationperiod.Thefastermoisturereductionisduetothehigherheattransferratewithsuperheatedsteamatthistemperature.Thetotaldryingtimeforobtainingagivenmoisturecontentdecreaseswithincreaseinsteamtemperature,asexpected.Thisisconsistentwiththefundamentaldryingtheory,whichexpectsahigherheattransferfluxtothebedofpaddywithhighertemperature.Besidestheeffectoftemperature,thebeddepthor,intheotherwords,thetotalsurfaceareaofgrainsinthebedisanotherfactorthatreflectstheamountofwaterevaporationfromthegrainsthickergrainbeddepthsleadtolowermoisturereductionratesforasinglegain,therebygivingalongerdryingtime.Thisisbecausetheheatsupplyisrathersmall,alongwiththepoorerheatconvection.AspreviouslyshowninFig.4,itclearlydemonstratedthattemperaturehadagreatercontributionthanbeddepth.3.3.ModifiedthinlayerdryingequationDuringthecourseofdryinginthefluidisedbeddryer,paddywassuspendedinthehighsteamvelocityexceptforinitialdryingperiodwithconstantsteamtemperature,whichfullystratifiesthethinlayerdrying.Insuchcircumstances,thebedshouldbeverythin.However,accordingtoourexperiments,thebeddepthhadaneffectonmoisturecontent.Therefore,thethinlayerequationismodified,sothatthedryingconstantsaccountforthebeddepth,inadditiontotemperature.Thetwoseriesexponentialmodelforthinlayerdryingofgrainshasbeenchosentocorrelatetheexperimentaldata,MR¼MðtÞC0MeqMinC0Meq¼AexpðC0k1tÞþBexpðC0k2tÞð1ÞwhereA,k1,Bandk2arethedryingconstantsvaryingwiththeoperatingconditions.Meq,MinandMðtÞaretheequilibriummoisturecontentandthemoisturecontentatt¼0andt,respectively.Inthepresentwork,thesteamtemperatureabovethenormalboilingpointwasused,sothatitmightbereasonabletoassumethemoisturecontentatequilibriumtobezero.Amultipleregressiontechniquewasusedtodeterminethedryingconstants.Togetamoreaccurateresult,thepossiblecorrelationequationsbetweenthedryingconstantsandtheoperatingconditionsweresubstitutedintoEq.1andthenstatisticallyfittedwithFig.4.Variationinmoisturecontentofpaddyatdifferentbedheightsandinletsuperheatedsteamtemperatures.Fig.5.Comparisonofdryingratesofpaddydriedundersuperheatedsteamandhotairdryingtemperature¼150C176C,bedheight¼10cmandvelocity¼13Umf.70C.Taechapairojetal./JournalofFoodEngineering58200367–73
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