Fluidization of Fine Powders: Cohesive versus Dynamical Aggregation PDF
Preview Fluidization of Fine Powders: Cohesive versus Dynamical Aggregation
Fluidization of Fine Powders Particle Technology Series Volume 18 Manymaterialsexistintheformofadispersesystem,forexamplepowders,pastes, slurries,emulsionsandaerosols,withsizerangingfromgranularallthewaydown to the nanoscale. The study of such systems necessarily underlies many technolo- gies/productsanditcanberegardedasaseparatesubjectconcernedwiththeman- ufacture, characterization and manipulation of such systems. The series does not aspiretodefineandconfinethesubjectwithoutduplication,butrathertoprovidea goodhomeforanybookwhichhasacontributiontomaketotherecordofboththe theory and applications of the subject. We hope that engineers and scientists who concernthemselveswithdispersesystemswillusethesebooksandthatthosewho becomeexpertwillcontributefurthertotheseries. The Springer Particle Technology Series is a continuation of the Kluwer Particle TechnologySeries,and thesuccessorto theChapman&Hall PowderTechnology Series. Forfurthervolumes: www.springer.com/series/6433 José Manuel Valverde Millán Fluidization of Fine Powders Cohesive versus Dynamical Aggregation Prof.Dr.JoséManuelValverdeMillán FacultyofPhysics UniversityofSevilla Sevilla,Spain ISSN1567-827X ParticleTechnologySeries ISBN978-94-007-5586-4 ISBN978-94-007-5587-1(eBook) DOI10.1007/978-94-007-5587-1 SpringerDordrechtHeidelbergNewYorkLondon LibraryofCongressControlNumber:2012950314 ©SpringerScience+BusinessMediaDordrecht2013 Thisworkissubjecttocopyright.AllrightsarereservedbythePublisher,whetherthewholeorpartof thematerialisconcerned,specificallytherightsoftranslation,reprinting,reuseofillustrations,recitation, broadcasting,reproductiononmicrofilmsorinanyotherphysicalway,andtransmissionorinformation storageandretrieval,electronicadaptation,computersoftware,orbysimilarordissimilarmethodology nowknownorhereafterdeveloped.Exemptedfromthislegalreservationarebriefexcerptsinconnection with reviews or scholarly analysis or material supplied specifically for the purpose of being entered and executed on a computer system, for exclusive use by the purchaser of the work. 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Printedonacid-freepaper SpringerispartofSpringerScience+BusinessMedia(www.springer.com) To mybeloveddaughterSofia Preface Granularmaterialsareperfectexamplesofcomplexsystems.Eventhoughtheme- chanicalbehaviorofasinglegrainiswellunderstood,thebehaviorofalargecol- lectionofgrainsexhibitsarichvarietyofyetunexplainedphenomena.Complexity isboostedwhenparticlesizeisdecreasedbelowafewtensofmicrons.Finepowder cohesiveness leads to poor flowability, clumping, difficulty in fluidizing, irregular avalanching behavior, etc. Despite all the inconveniences, fine powder processes pervade the chemical, pharmaceutical, agricultural and mining industries among others. This book is mainly devoted to demonstrate the rich phenomenology ex- hibited by fine powders when they are fluidized by a gas flow. Due to its marked interdisciplinarycharacter,thebehavioroffluidizedbedsoffinepowderscannotbe understood without the sharing of knowledge between specialists on a variety of disciplines. An overall aim of the book is to contribute to develop the capacity of interdisciplinaryresearchfieldsinordertounderstandthesecomplexsystems.This book is thus intended for academic and industrial researchers in applied physics, mechanical, chemical, and environmental engineering, who are interested in the sharing and integration of knowledge bases from this disparate set of disciplines intoasingleinterdisciplinarysubjectareaorientedtothespecialcharacteristicsof finepowders. Mostempiricalobservationsandnumericalanalysishavedemonstratedthatgas- fluidizedbedsofgranularmaterialscanonlybestabilizedifinterparticleattractive forcesreachanorderofmagnitudesimilartoparticleweight,whichhappenswhen particle size is typically of the order of a few tens of microns. In the absence of sufficientlystrongnaturalattractiveforces,gas-fluidizedbedsofgranularmaterials exhibitanunstablebubblingbehavior.However,interparticleforcesmaybeinduced byanexternalfield,whichleadstothesuppressionofbubblesandsubsequentstabi- lizationasseenwhenamagneticfieldisappliedtoabubblingbedofmagnetizable particles. On the other side, powders with particle size smaller than about 20 mi- cronscannotbefluidizedbyagasbecauseinterparticleforcesareexceedinglylarge ascomparedtoparticleweight,whichleadstocohesiveaggregation.Cohesiveag- gregates may reach a size comparable to system size and cannot be fully broken by the gas flow. Instead, the gas flow becomes heterogeneously distributed in the vii viii Preface bedandusuallybypassesitthroughchannelsthathinderthegas-solidscontacteffi- ciency. In the last years, a numberof reports haveappearedon the novelbehaviorof a newclassoffinepowders,whichexhibitauniformgas-fluidizationbehaviorresem- blingthenonbubblingfluidlikebehaviorofgranularmaterialsfluidizedbyliquids. Asopposedtocohesiveaggregation,thesefineparticlesundergoadynamicalpro- cessofaggregationinthefluidizedbedwhichyieldstheformationofporouslight aggregates.Theseaggregates,whichreachanequilibriumsizeruledbythebalance betweenthelocalshearforceexertedbythegasflowontheirsurfaceandinterpar- ticleattraction,canbefluidizedbythegasinanonbubblingfluidlikeregime. Anefficientmethodtoassistfluidizationoffinecohesivepowdersistheaddition of surface additives, which serves to decrease interparticle adhesion thus helping thegas flow tobreak cohesiveaggregatesandallowingfor dynamicalaggregation oftheparticlesinafluidlikenonbubblingregime.Afundamentalquestionhowever is whether the nonbubbling fluidlike state displayed by these special fine powders canbereallyconsideredasastablestate. Different techniques have been developed in the last years to assist fluidization by helping the gas flow to mobilize and break cohesive aggregates, which help to homogenizefluidization.Themechanismbywhichinterparticleadhesiveforcescan be reduced to allow for dynamic aggregation instead of cohesive aggregation will beanalyzedinthisbook.Turningcohesiveaggregationintodynamicalaggregation may have a remarkable impact on novel processes based on fluidized beds of fine powders with relevant applications on leading edge technologies such as Atomic LayerDepositiononnanoparticlesandCO capturebygas-fluidizedbedsofadsor- 2 bentpowders. Seville,Spain JoséManuelValverdeMillán Acknowledgements Mostoftheresultsreviewedinthisbookwereobtainedbyourresearchgroupatthe UniversityofSeville.Ourresearchactivityonfinecohesivepowderswasinitiated more than 20 years ago from a collaboration with Dr. P. Keith Watson at the Wil- sonResearchCenterof XeroxCo.(Webster,NY).Membersofour groupinitially involvedinthisnovelresearchatthattimewereDr.A.Castellanos,Dr.A.T.Perez, and Dr. A. Ramos. I would like to acknowledge their pioneer contribution to this workandmakeaspecialmentiontoDr.WatsonandDr.Castellanos,whosparked thisfruitfulcollaboration.Itisalsoacknowledgeablethehighlyqualifiedassistance of Xerox engineers Frank Genovesse and Mike Morgan, who helped us with the experiments carried out at the Wilson Research Center. The later incorporation of Dr.M.A.S.Quintanillarepresentedaremarkableimpulsetothisresearchthatwas consolidated at the University of Seville with the contribution of his diligent and brilliant dedication. Further members of our group that have contributed to some of the specific topics covered in this book are Dr. M.J. Espin, Dr. C.S. Hoyo, Dr. F. Gilabert and Dr. F. Pontiga and team’s members. A fundamental part of our research has been upheld by the contribution of external collaborators, among whichImustacknowledge(inchronologicalorder):Dr.Mills(UniversityMarne-la- Valle,France),Dr.Lemaire(UniversityofNice,France),Dr.PfefferandDr.Dave (New Jersey Institute of Technology), Dr. Hristov (University of Sofia, Bulgary), Dr.Maqueda(CSIC,Spain),andDr.vanOmmen(TUDelft,Holland).Thanksalso to Michael Stephen Pohen, student from the Texas A&M University, who helped me with formatting the references of the book during his stay at the University of Seville. Atthemomentofwritingthisbook,theresearchofourgrouponfinecohesive powders is being supported by the “Consejeria de Innovacion, Ciencia y Empresa (Junta de Andalucia)” within the European Regional Development Fund contracts FQM-5735 and by the Spanish Government Agency “Ministerio de Ciencia e In- novacion” (contract FIS2011-25161). Previous research activities were supported by these institutions through diverse public research projects as well as by Xerox FoundationandXeroxCorporation. ix Contents 1 Introduction.TheClassicalGeldart’sDiagramandtheNewType ofGas-FluidizationBehavior . . . . . . . . . . . . . . . . . . . . . . 1 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 2 TheStructureofGeldartAGas-FluidizedBeds . . . . . . . . . . . . 7 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 3 MagneticStabilizationofFluidizedBedsofMagnetizableParticles . 13 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 4 TheFluidlikeBehaviorofGranularMaterialsFluidizedbyLiquids . 23 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27 5 TheFluidlikeBehaviorofFineandUltrafinePowdersFluidized byGas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38 6 OntheQuestionofFluid-LikeFluidizationStability . . . . . . . . . 41 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45 7 DynamicAggregationofFineParticlesinGas-FluidizedBeds . . . . 47 7.1 InterparticleForcesontheGasPhase . . . . . . . . . . . . . . . . 47 7.2 TheSizeofDynamicAggregatesinGas-FluidizedBeds . . . . . . 49 7.3 TheModifiedRichardson-ZakiEquation . . . . . . . . . . . . . . 51 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53 8 TheModifiedGeldart’sDiagram . . . . . . . . . . . . . . . . . . . . 55 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63 9 FluidizationofNanopowders . . . . . . . . . . . . . . . . . . . . . . 65 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71 10 EffectofGasViscosityontheFluidizationBehaviorofFinePowders 75 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82 xi xii Contents 11 FluidlikeFluidizationasAffectedbyExternalFields . . . . . . . . . 85 11.1 EffectofExternalElectricFieldsonFluidization . . . . . . . . . . 85 11.2 EffectofExternalFieldsonDynamicalAggregation . . . . . . . . 91 11.2.1 EffectofanOscillatingElectricField . . . . . . . . . . . . 91 11.2.2 EffectofVibration . . . . . . . . . . . . . . . . . . . . . . 94 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 96 12 The Use of Additives to Control Powder Flow. Mechanical PropertiesofFinePowderBeds . . . . . . . . . . . . . . . . . . . . . 99 12.1 EffectofSurfaceAdditivesonPowderFlowability . . . . . . . . . 99 12.2 InterparticleContactMechanics . . . . . . . . . . . . . . . . . . . 102 12.2.1 MeasurementofBulkStresses. . . . . . . . . . . . . . . . 104 12.2.2 EstimationofContactForces . . . . . . . . . . . . . . . . 108 12.3 TheFluid-to-SolidTransitionofDynamicAggregates . . . . . . . 112 12.3.1 DistributionofContactForcesatVeryLowConsolidations 115 12.4 EffectofLoadingTimeonPowderCohesion . . . . . . . . . . . . 115 12.5 EffectofPreviousConsolidationonFluidization . . . . . . . . . . 116 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 118 13 FluidizationAssistanceTechniques . . . . . . . . . . . . . . . . . . . 121 13.1 AtomicLayerDepositioninAssistedFluidizedBeds . . . . . . . . 125 13.2 Improving the CO Capture Performance of Fine Powders 2 byAssistedFluidization . . . . . . . . . . . . . . . . . . . . . . . 126 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 132
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