Desiccant - Wikipedia

A desiccant is a hygroscopic substance that is used to induce or sustain a state of dryness (desiccation) in its vicinity; it is the opposite of a humectant ... Desiccant FromWikipedia,thefreeencyclopedia Jumptonavigation Jumptosearch Substanceusedtoinduceorsustaindryness Fortheuseofchemicalsinagriculturetoaidharvesting,seecropdesiccation. Canistersarecommonlyfilledwithsilicagelandothermolecularsievesusedasdesiccantindrugcontainerstokeepcontentsdry. Silicagelinasachetorporouspacket Adesiccantisahygroscopicsubstancethatisusedtoinduceorsustainastateofdryness(desiccation)initsvicinity;itistheoppositeofahumectant.Commonlyencounteredpre-packageddesiccantsaresolidsthatabsorbwater.Desiccantsforspecializedpurposesmaybeinformsotherthansolid,andmayworkthroughotherprinciples,suchaschemicalbondingofwatermolecules.Theyarecommonlyencounteredinfoodstoretaincrispness.Industrially,desiccantsarewidelyusedtocontrolthelevelofwateringasstreams. Contents 1Typesofdesiccants 1.1Performanceefficiency 1.2Coloredsaturationindicators 2Applications 3Dryingofsolvents 4Seealso 5References 6Furtherreading Typesofdesiccants[edit] Mainarticle:Listofdesiccants Althoughsomedesiccantsarechemicallyinert,othersareextremelyreactiveandrequirespecializedhandlingtechniques.Themostcommondesiccantissilicagel,anotherwiseinert,nontoxic,water-insolublewhitesolid.Tensofthousandsoftonsareproducedannuallyforthispurpose.[1]Othercommondesiccantsincludeactivatedcharcoal,calciumsulfate,calciumchloride,andmolecularsieves(typically,zeolites).Desiccantsmayalsobecategorizedbytheirtype,eitherI,II,III,IV,orV.Thesetypesareafunctionoftheshapeofthedesiccant'smoisturesorptionisotherm. Alcoholsandacetonesarealsodehydratingagents. Performanceefficiency[edit] Onemeasureofdesiccantefficiencyistheratio(orpercentage)ofwaterstorableinthedesiccantrelativetothemassofdesiccant. Anothermeasureistheresidualrelativehumidityoftheairorotherfluidbeingdried. Theperformanceofanydesiccantvarieswithtemperatureandbothrelativehumidityandabsolutehumidity.Tosomeextent,desiccantperformancecanbepreciselydescribed,butmostcommonly,thefinalchoiceofwhichdesiccantbestsuitsagivensituation,howmuchofittouse,andinwhatform,ismadebasedontestingandpracticalexperience. Coloredsaturationindicators[edit] Indicatingsilicagel Sometimesahumidityindicatorisincludedinthedesiccanttoshow,bycolorchanges,thedegreeofwater-saturationofthedesiccant.Onecommonlyusedindicatoriscobaltchloride(CoCl2).Anhydrouscobaltchlorideisblue.Whenitbondswithtwowatermolecules,(CoCl2•2H2O),itturnspurple.Furtherhydrationresultsinthepinkhexaaquacobalt(II)chloridecomplex[Co(H2O)6]Cl2. Applications[edit] Oneexampleofdesiccantusageisinthemanufactureofinsulatedwindowswherezeolitespheroidsfillarectangularspacertubeattheperimeterofthepanesofglass.Thedesiccanthelpstopreventthecondensationofmoisturebetweenthepanes.Anotheruseofzeolitesisinthedryercomponentofairconditioningsystemstohelpmaintaintheefficacyoftherefrigerant. Baggeddesiccantsarealsocommonlyusedtoprotectgoodsinbarrier-sealedshippingcontainersagainstmoisturedamage:rust,corrosion,etc.[2][3]Hygroscopiccargo,suchascocoa,coffee,variousnutsandgrains,andotherfoods[4]canbeparticularlysusceptibletomoldandrotwhenexposedtocondensationandhumidity.Becauseofthis,shippersoftentakemeasuresbydeployingdesiccantstoprotectagainstloss.Pharmaceuticalpackagingoftenincludessmallpacketsofdesiccanttokeeptheatmosphereinsidethepackagebelowcriticallevelsofwatervapor. Desiccantsinducedrynessinanyenvironmentandreducetheamountofmoisturepresentinair.Desiccantscomeinvariousformsandhavefoundwidespreaduseinthefood,pharmaceuticals,packing,electronicsandmanymanufacturingindustries. Airconditioningsystemscanbemadebasedondesiccants.[5] Desiccantsareusedinlivestockfarming,where,forexample,new-bornpigletsarehighlysusceptibletohypothermiaowingtotheirwetness.[6] Dryingofsolvents[edit] Tolueneisheatedunderrefluxwithsodiumandbenzophenonetoproducedry,oxygen-freetoluene.Thetolueneisdryandoxygenfreewhentheintensebluecolorationfromthebenzophenoneketylradicalisobserved. Mainarticle:Air-freetechnique Desiccantsarealsousedtoremovewaterfromsolvents,typicallyrequiredbychemicalreactionsthatdonottoleratewater,e.g.,theGrignardreaction.Themethodgenerally,thoughnotalways,involvesmixingthesolventwiththesoliddesiccant.Studiesshowthatmolecularsievesaresuperiorasdesiccantsrelativetochemicaldryingreagentssuchassodium-benzophenone.Sievesoffertheadvantagesofbeingsafeinairandrecyclable.[7][8] Seealso[edit] Desiccator Humiditybuffering Humidityindicatorcard Moisturesorptionisotherm Solarairconditioning Oxygenscavenger(oxygenabsorber) Sorbent Volatilecorrosioninhibitor Cromercycle References[edit] ^OttoW.Flörke,etal."Silica"inUllmann'sEncyclopediaofIndustrialChemistry,2008,Weinheim:Wiley-VCH,.doi:10.1002/14356007.a23_583.pub3. ^Rollo,P(1996).AProtectivepackagingevaluationinvolvingahighbarrierfilmlamiation,desiccantsandoxygenabsorbers(MSc).RochesterInstituteofTechnology.RetrievedAugust8,2021. ^MIL-D-3464E,MILITARYSPECIFICATION:DESICCANTS,ACTIVATED,BAGGED,PACKAGINGUSEANDSTATICDEHUMIDIFICATION,1987,retrievedAugust8,2021 ^Hirata,T(1985)."SimulationofMoistureandChlorophyllChangesinDriedLaver,PorphyraYezoensis,inaDesiccant-EnclosingPackagingSystem".NipponShokuhinKogyoGakkaishi.32(4):266–273.doi:10.3136/nskkk1962.32.4_266.S2CID 101082998.Retrieved11August2021. ^Daou,K;Wang,Xia(2005)."Desiccantcoolingairconditioning:areview".RenewableandSustainableEnergyReviews.10(2):55–77.doi:10.1016/j.rser.2004.09.010. ^VandePol,KatherineD.;Tolosa,AndresF.;Shull,CalebM.;Brown,CatherineB.;Alencar,StephanAS.;Ellis,Michael(2020)."Effectofmethodofdryingpigletsatbirthonrectaltemperatureoverthefirst24hafterbirth1".TranslationalAnimalScience.4(4):txaa183.doi:10.1093/tas/txaa183.PMC 7672461.PMID 33241187. ^Chai,ChristinaLiLin;Armarego,W.L.F.(2003).Purificationoflaboratorychemicals.Oxford:Butterworth-Heinemann.ISBN 978-0-7506-7571-0. ^Williams,D.BradleyG.;Lawton,Michelle(2010)."DryingofOrganicSolvents:QuantitativeEvaluationoftheEfficiencyofSeveralDesiccants".TheJournalofOrganicChemistry.75(24):8351–8354.doi:10.1021/jo101589h.PMID 20945830.S2CID 17801540. Furtherreading[edit] Lavan,Z.;Jean-BaptisteMonnier&Worek,W.M.(1982)."SecondLawAnalysisofDesiccantCoolingSystems".JournalofSolarEnergyEngineering.104(3):229–236.doi:10.1115/1.3266307. S.Sadik;J.W.White(1982)."Truepotatoseeddryingoverrice".PotatoResearch.25(3):269.doi:10.1007/BF02357312.S2CID 41039047. 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