食品工程原理 liquid absorbtion and ion exchange

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1、Chapter12Adsorption&.IronExchangeAdsorptionIntroductionAdsorption,inwhichmoleculesdiffusefromthebulkofthefluidtothesurfaceofthesolidadsorbentformingadistinctadsorbedphase.Typically,gasadsorbersareusedforremovingtracecomponentsfromgasmixtures.Adsorptionmaybee

2、quallyeffectiveinremovingtracecomponentsfromaliquidphaseandmaybeusedeithertorecoverthecomponentorsimplytoremoveanoxioussubstancefromanindustrialeffluent.Thekeycharacteristicoftheadsorptionisthetransferenceofthesolutebetweendifferentphases.Themainapplicationof

3、adsorptionisfoodprocessisseparationorpurificationofsolutions.FunctionsandCharacteristicsofAdsorptionFunctionsremovingtracecontaminantsdissolvedinwaterofinfoods。organicsColloidalparticlesHeavymetalRadioactiveelementothers（microorganisms,chlorine,odor,color）Bas

4、icprinciplesofadsorptionSolidsurfaceadsorption不同相表（界）面剩余表面能溶质浓集吸附剂吸附质解吸或脱附PrinciplesAdsorptionoccurswhenmoleculesdiffusinginthefluidphaseareheldforaperiodoftimebyforcesemanatingfromanadjacentsurface.Thesurfacerepresentsagrossdiscontinuityinthestructureoftheso

5、lid,andatomsatthesurfacehavearesidueofmolecularforceswhicharenotsatisfiedbysurroundingatomssuchasthoseinthebodyofthestructure.TheseresidualorVanderWaalsforcesarecommontoallsurfacesandtheonlyreasonwhycertainsolidsaredesignatedadsorbentsisthattheycanbemanufactu

6、redinahighlyporousform,givingrisetoalargeinternalsurface.Theprinciplesofadsorptionistoocomplex,andthereisstillnoaccuratefinaltheoriestoexplainit.AdsorptionmechanismandclassificationReasonscausingadsorptionVanderWaalsForcesBondstrengthElectrostaticattractionCl

7、assificationofadsorptionPhysicaladsorptionChemisorptionsExchangeadsorptionPhysicaladsorptionVanderWaalsforcescausedNoselectivityTheamountofheatliberatedaccompaniedwithphysicaladsorptionissmall,andissimilarinmagnitudetotheheatofcondensation.About42kJ／molMulti-

8、molecularlayeradsorptionAdsorbentspecificsurfaceareaandporedistributionaffecttheadsorptiongreatly.BecausetheVanderWaalsforcesarenotstrong,theadsorptionmaybeeasilyreversed.Chemisorptionsch