Particle Technology and Engineering. An Engineer's Guide to Particles and Powders: Fundamentals and Computational Approaches PDF
Preview Particle Technology and Engineering. An Engineer's Guide to Particles and Powders: Fundamentals and Computational Approaches
Particle Technology and Engineering An Engineer’s Guide to Particles and Powders: Fundamentals and Computational Approaches Jonathan Seville Chuan-Yu Wu AMSTERDAM(cid:129)BOSTON(cid:129)HEIDELBERG(cid:129)LONDON NEWYORK(cid:129)OXFORD(cid:129)PARIS(cid:129)SANDIEGO SANFRANCISCO(cid:129)SINGAPORE(cid:129)SYDNEY(cid:129)TOKYO Butterworth-HeinemannisanimprintofElsevier Butterworth-HeinemannisanimprintofElsevier TheBoulevard,LangfordLane,Kidlington,OxfordOX51GB,UK 50HampshireStreet,5thFloor,Cambridge,MA02139,USA Copyright©2016ElsevierLtd.Allrightsreserved. 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Practitionersandresearchersmustalwaysrelyontheirownexperienceandknowledgein evaluatingandusinganyinformation,methods,compounds,orexperimentsdescribed herein.Inusingsuchinformationormethodstheyshouldbemindfuloftheirownsafety andthesafetyofothers,includingpartiesforwhomtheyhaveaprofessionalresponsibility. Tothefullestextentofthelaw,neitherthepublishernortheauthors,contributors,or editors,assumeanyliabilityforanyinjuryand/ordamagetopersonsorpropertyasa matterofproductsliability,negligenceorotherwise,orfromanyuseoroperationofany methods,products,instructions,orideascontainedinthematerialherein. BritishLibraryCataloguing-in-PublicationData AcataloguerecordforthisbookisavailablefromtheBritishLibrary LibraryofCongressCataloging-in-PublicationData AcatalogrecordforthisbookisavailablefromtheLibraryofCongress ISBN:978-0-08-098337-0 ForinformationonallButterworth-Heinemannpublications visitourwebsiteathttps://www.elsevier.com/ Publisher:JoeHayton AcquisitionEditor:FionaGeraghty EditorialProjectManager:LindsayLawrence ProductionProjectManager:NickyCarter Designer:MariaIneˆsCruz TypesetbyTNQBooksandJournals Preface Particlesdsmalldiscreteelementsofmatterdareallaroundusandareimportantas natural phenomena (mist, rain, snow, sand, etc.) and as products (food, building materials, pharmaceuticals, etc.). The study of Particle Technology has become an essentialpartoftheeducationofmanykindsofengineersandscientists.Itoverlaps significantlywithdanddrawsmaterialfromdrelatedsubjectssuchasthesciences of aerosols, colloids,and surfacesin general. ParticleTechnologyasadisciplinemustincludebothastudyofthefundamen- talsofhowparticulatematerialsbehaveandsomeindicationsofhowthesciencecan beappliedinpractice.Wehavebothbeeninvolvedinnumerousdiscussionsofthe scope and organization of Particle Technology in association with our teaching in UKuniversitiesandourworkontheEditorialBoardoftheElsevierjournalPowder Technology.Inourview,anorganizationofthesubjectofParticleTechnologywhich reflectstheinterestsofthosewhocarryoutresearchinitisasshowninthediagram below. Fundamentals Applications Mul(cid:2)phase flow, Par(cid:2)cle & Processing New including assembly materials fluidisation mechanics Organizationofresearcheffortinparticletechnology. In our view, the interaction of particles with fluids in multiphase flow needs always to be accompanied by an appreciation of the mechanics of particle-to- particlecontactandthepeculiarbehaviorofassembliesofparticles.Ontheapplica- tionside,theprocessingofproductscontainingparticlesishugelyimportant,butso arethepropertiesofmaterialscontainingparticles.Manynewmaterialscontaining particlesemergeeveryyear,andthewaysinwhichtheirmicrostructuresareformed andaresubsequentlymodifiedinuseareimportantpartsoftheemergingdiscipline of FormulationEngineering. Aswesummarize inChapter1ofthisbook,thefundamentalscienceonwhich Particle Technology draws was established in the nineteenth century by physicists such as Stokes, Smoluchowski, and Hertz, and elaborated in the twentieth century bynumerousothers,spurredonbytherapidincreaseinapplicationsofthesubject, from particulate catalysts inreactors tocomposite materials. Thenumber ofappli- cationscontinuestoincrease,butthescientificfundamentalsremainthesame.What has changed the subject very significantly in the last two decades is the rise of ix x Preface computationalmethods,enabledbythedevelopmentofbettercomputationalcodes andthe affordability ofmassivecomputerpower. BuiltuponanearlierbookauthoredbySeville,Tu¨zu¨n,andClift(1997),thisbook attemptstodotwothings:tosummarizetheessentialscientificfundamentalsandto introduce the basics required to perform computations in Particle Technology. For the former, we start by introducing the fundamental characteristics of powders in bulk form in Chapter 2, explaining the important bulk properties of powders and how they can be determined. As the bulk properties are ultimately determined by the properties ofindividual particles, we then introduce individual particle proper- ties in Chapter 3din particular, particle shape and size, how they can be defined, and how they can be appropriately measured. We then introduce the complexity of a surrounding fluid phase, first in interaction with a single particle (Chapter 4), then through considering multiple particles in gases, in applications such as gas fluidizationandpneumaticconveying(Chapter5),andmultipleparticlesinliquids, inapplicationsincludinggranulationandextrusion(Chapter6).Wealsoexplainthe fundamentalmechanicsofparticlesystems,bothatthebulklevel,suchasthedevel- opmentofstressesinstorageanddynamicsduringpowderflow(Chapter7),andat the particle level, including particleeparticle interaction (Chapter 8). Finally, we introducetwocomputationalmethods,namelythediscreteelementmethod(Chapter 9) and the finite element method (Chapter 10), both of which have been applied extensively in modeling the behavior of particle systems at low consolidation stresses, such as in powder flow and shearing, and at high consolidation stresses asencounteredindiecompactionandrollcompaction.Thelastfourchaptersfocus on mechanistic modeling of particle systems and are aimed primarily at Chemical EngineeringstudentsintheirlateryearsandatbothChemicalEngineersandother disciplines, in industry or in academia, who need to carry out mechanical analysis andcomputational work inthisfield. ParticleTechnologyisabroadandinclusivesubject.Anyworkofthiskindmust necessarily be very selective. This book focuses on fundamentals, particle mechanics, and computational aspects in Particle Technology; we refer readers to the volumes in the Elsevier series Handbook of Powder Technology for more detailed treatmentofparticular aspects. We thank all those who have contributed to our understanding of the subject, particularly Mike Adams, Roland Clift, Peter Knight and Colin Thornton, and to our long-suffering families. Thanks are also due to the universities of Surrey and Birmingham,wheremuchofthismaterialhasbeenusedinteachingundergraduate andgraduate courses. JonathanSeville andCharleyWu CHAPTER 1 Introduction Particle technology is the study of discrete elementsdusually solid particlesdthe wayinwhichtheybehaveinisolationandthewayinwhichtheyinteracttoproduce a collective effect (Seville, 2001). The ultimate goal of particle technology is to obtain predictive relationships between individual particle properties and their collectivebehavior(i.e.,behaviorinbulk),whichcanthenbeusedtodesignformu- lations,processes,andparticulateproducts.Althoughmuchprogresshasbeenmade toward achieving thatgoal, there is much stilltodo! 1.1 WHAT ARE PARTICLES? Particles are endlessly fascinating, both to scientists and engineers and to children who build sand castles on the beach. Particles surround usdthey form the foundations beneath our buildings, the soil in which we grow our food, very often thatfooditself.Theyaresuspendedintheairthatwebreatheandcancauseusgreat damage when we do so. They are extracted from the ground to obtain metals and other mineral products. They make up many of the products we consume and the wastes derived from them and are liberated in the incineration of that waste. It is estimated that over two-thirds of all chemical products are sold as, or have passed through, a particulate form. Much energy is expended in their processing. For example, crushing and grinding of minerals is estimated to use 3e4% of the world’s electricity. The first point to make about particles is the extraordinary range of scales that theyoccupy(Fig.1.1)deverythingfromlargemoleculestosmallbricksisgenerally lumped into thiscategory.Theratio betweenthe sizeofthe earth andthe sizeofa footballisroughlythesameasthatbetweenafootballandatypicalnanoparticle.As inmostbranchesofhumanendeavor,itisusefultohavesomehumanlengthscales from which to take reference: the diameter of a human hair is of order 0.1mm (or 100mm), while the diameter of a human blood cell is 6e8mm. An important physical reference from the point of view of experimental observation is the wavelength ofvisible lightdcentered at about 0.5mm. 1 ParticleTechnologyandEngineering.http://dx.doi.org/10.1016/B978-0-08-098337-0.00001-1 Copyright©2016ElsevierLtd.Allrightsreserved. 2 C H A P T E R 1 (1mµ) Particle diameter, microns (µ) (1 mm) (1 cm) I n 0.0001 0.001 0.01 0.1 1 10 100 1000 10000 t 2 34 68 2 34 68 2 34 68 2 34 68 2 34 68 2 34 68 2 34 68 2 34 68 2 ro d Electromagnetic X - rays Ultraviolet Visible Near infrared Far infrared Microwaves (radar, etc) uc waves Solar radiation tio Gas Solid: Fume Dust n Technical dispersoidsLiquid: Mist Spray definitions Atterberg or lnternational Std. Classification System Soil: adopted by lnternal. Soc. Soil Sci. Since 1934 Clay Silt Fine sand Coarse sand Gravel Common atmospheric Drizzle Smog Clouds and fog Mist Rain dispersoids Atmospheric Small dust molecules: H–CH 2 4 10 Range of lung- damaging dust FIGURE1.1 Particlescoverawiderangeofscales.(AdaptedfromSevilleetal.,1997.) 1.1 What Are Particles? 3 Sizehasaprofoundeffectonparticleproperties.Consider,forexample,thefam- ilyofatmospheric dispersionsofwaterinair: rain(diameter 1mme1cm);drizzle (100mme1mm);andfog(1e100mm).Oneobviousdifferencebetweenthemisthe effectofgravity:themotionofraindropsisdrivenbygravity;theyfallquicklytothe ground,whereasfogdropletsremainindispersionuntilariseintemperatureorwind movementremovesthem.Anothernotabledifferenceconcernstheinteractionwith light:rainisrelativelyeasytoseethrough;fogisnot.Suspendedparticlesofasize closetothatofthewavelengthoflightarestronglight-scatterersandreducevisibil- ity.Thissuggests,ofcourse,amethodformeasuringtheconcentrationofparticles, which is consideredin Chapter 3. Particles are not normally found singly, but in very large numbers, so that the scale of one particle is usually many orders of magnitude smaller than the scale of the container or process in which they sit. A “particulate solid” or “bulk solid” is an assembly of particles that may be surrounded by a continuous fluid phaseda gas (such as air) or a liquid. They may or may not be in contact with each other. It would make it much easier to predict the behavior of particles if we could considerthemaslargemolecules,forwhichmanyanalyticalpredictiveapproaches havebeendeveloped,butthisisboundtobeanoversimplification.Particle-fluidsys- tems present inherent experimental and theoretical difficulties (Grace, 1986) for which there are noanalogies inmolecular systems: (cid:129) particle shape (cid:129) particle sizedistribution (cid:129) surface effects, such as contamination with oxide layers (cid:129) regimesofmotion,causingprofoundchangesindragwithchangesinReynolds number1 inadditionto secondary effects such as: (cid:129) Brownian2 motion (cid:129) electrostatic chargedistributions Particlesystemsalsodisplay a“memory” ofprevious processing,resulting, for example, in their sticking together (agglomeration) or breaking apart (attrition), accompanied by whatmay be aprofound change inproperties. The divided state is a constant theme in philosophical discussiondhow can we exert our individuality within the collective behavior of society? More to 1The Reynolds number (Re) is a nondimensional group which describes the flow regime in fluid mechanics.ForaparticleitisdefinedasrUd/mwhererandmarethefluiddensityandviscosityrespec- tively,Uisthefluidvelocity(relativetotheparticle)anddistheparticlediameter.Foragivenparticle andfluid,therefore,“lowReynoldsnumber”implieslowfluidvelocityandviceversa.LowReynolds numberflowsaredominatedbyviscosity,whichisthecaseformicrometer-sizeddustparticlesinair, forexample.TheReynoldsnumbercanbeconsideredasaratioofmomentumforcestoviscousforces; seeChapter4;afterIrish/EnglishEngineerOsborneReynolds(1842e1912). 2Brownianmotionistherandommotionofsmallparticlescausedbycollisionswithatomsormole- culesinthesurroundingfluid;afterScottishbotanistRobertBrown(1773e1858). 4 CHAPTER 1 Introduction the point, given a large collection of individual people (or particles) how can we predict their collective behavior? The collective behavior of people is the province of economics and politics; particles are a little (but not much) easier to deal with. Taking this theme further, we are surrounded by a different sort of particled the information dot: the mark of the ink jet or the laser, the paint particle, the phosphordot,thelight-emittingdiode,thetinyunitsofdisplaywhichcollectively register an effect, provided of course that we are far enough away to make sense of the resulting pattern. There is a long line of artists who have painted in this way, including, most famously, Georges Seurat,3 the impressionist originator of “pointillism,” who based his technique on a scientific study of color analysis and visual perception. The scientist and natural philosopher Pierre-Simon Laplace4 believed that the universeisapredictablemachine(likeagreatclock)andthatifonlywecouldcalcu- late the behavior of all the parts, we could predict the future. Isaac Newton5 had made this seem possible, by showing that the motion of a set of particularly large particles obeys simple laws. At the same time, Newton knew much about matters atthescaleofthewavelengthoflightandpostulatedaworldofsubvisibleparticles behavingaccordingtoanalogoussetsoflaws.Wenowknow,ofcourse,thatmolec- ularsystemsdonotbehaveinthiswayandtheircollectivebehaviorcanonlybepre- dicted in a statistical sense. Famously, Werner Heisenberg6 pointed out that at the molecular scale not only can you not predict the future, but also you cannot even knowthe present exactly. Whathasthistodowithparticles,whichare,forthemostpart,largeenoughthat at least we know where they are, even if we must take a scanning electron micro- scope to look at them? The answer is that computational particle technology has now reached a critical point: it appears to be able to predict anything, if only the sponsors will buy us large enough computers. Maybe there is no further need for ourapproximate and semi-empirical methods. Can this betrue? The purpose of this book is to introduce the newcomer to the fundamentals of particletechnologyandtoexplainthebasicprinciplesbehindtheemergingcompu- tational approaches tothe subject. 1.2 WHAT IS KNOWN? A short answer to this question might be: “Quite a lot about isolated spheres and spheresincontact;muchlessaboutthebehaviorofnonsphericalparticlesandpar- ticles inassemblies.” 3Frenchpainter(1859e1891). 4Frenchmathematician,physicistandastronomer(1749e1827). 5Englishmathematician,physicistandastronomer(1643e1727). 6Germanphysicist(1901e1976). 1.2 What Is Known? 5 The problem of the drag on an isolated sphere at low Reynolds numbers was solved exactly in the late nineteenth century by George Stokes7dperhaps the mostimportantcontributiontothesubject.Thetwentiethcenturysawavastamount ofexperimentalworkonsphericalandnonsphericalparticlesoverthefullrangeof Reynoldsnumbers,broughttogetherbyCliftetal.(1978).Althoughthebehaviorof nonsphericalparticlesiscomplex,simpleshape-descriptorsareoftengoodenough. Forexample,spheroids aresurprisinglygoodmodelsfor morecomplicated shapes ofairborneparticles,becausetheprocessesinwhichsuchsmallparticlesparticipate nearlyalwaystakeplaceatlowReynoldsnumbers.Suchprocessescanusuallyalso bemodeledusingcomputationalfluiddynamics(CFD)codes,becausethepresence of the particleshas little effect on the motion ofthe fluid. The difficulty arises when the solids content in the fluid becomes large enough that the fluid motion is affected by the particles. In this case, the state of the art is semi-empirical, relying on the well-known equations of Ergun and Richardson-Zaki and others (see Chapter 5). This area is by no means resolved and allegedly better approaches are continually suggested. The situation is more complex still when the particles make extensive contact, as in fluidized beds. In thiscase,stress can betransmittedthroughthe particles,notjustthroughthe fluid. Attheoppositeendofthespectrumfromthesmallisolatedparticleinsuspension, then,isthecaseofadenseassemblyofparticlesincontact,flowingundergravityout of a container. In this case, unlessthe particles are small the fluidphase is of little importance;particlemotionisdominatedbyfrictionalforcesbetweentheparticles andthewallsofthecontainerandinternalfrictionbetweenthem.Particlesmoveac- cordingtowherethevacanciesarisedaprincipleknownasthe“kinematictheory” (Nedderman, 1992). ItisinterestingtocontrastthenineteenthcenturyworkofStokesonparticledrag influidflowwiththecontactmechanicsworkofHeinrichHertz8ofaboutthesame period. The former is well known in chemical engineering and is widely applied. Thelatterismuchbetterknowninmechanicalengineeringinconnectionwithbear- ings and is little applied in particle technology. The whole field of tribologydthe study of friction, lubrication, and wear at contacts between surfacesdis of great importanceinparticletechnologyandhasmovedonalongwayfromHertz’searly work on elastic contacts. The reason that the contact mechanics developed by Hertz (1896) has not been appliedinparticletechnologyasmuchasinotherfieldsisprobablythatHertz’smath- ematicalsolutionwasobtainedforthecontactoftwofrictionlesselasticspheres:that is to say for spheres which are finely polished so that the friction between them is negligible. However, real particles are rarely spherical or smooth. It was not until the1960s(i.e.,halfacenturylater)thatMindlinandDeresiewicz(1953)considered thefrictionbetweentwospheresanddevelopedmathematicalsolutionsforthecontact 7Irishmathematician,physicist,politician,andtheologian(1819e1903). 8Germanphysicist(1857e1894). 6 CHAPTER 1 Introduction betweenelasticsphereswithvariousobliqueforces.TheworkofHertz,Mindlin,and DeresiewiczwasfurtherextendedbyMawetal.(1976,1981)toanalyzetheimpact betweentwoelasticspheresatanyimpactangle,forwhichthereboundbehaviorcan bepredicted;numericalsolutionsaregiveninChapter8.Theapplicationofcontact mechanicsinparticletechnologyonlygaineditsimpetusinthe1990s,withthedevel- opmentofthediscreteelementmethod(DEM,seeChapter9).Theimplementationof thetheoriesofHertz,Mindlin,andDeresiewiczinDEMwaspioneeredbyThornton (Thornton and Barnes, 1986), who demonstrated that these theories for the contact between two spheres are useful and physically sound in predicting the collective behavior of particle systems in DEM; since then they have been applied more and more frequently in particle technology. However, even these sophisticated ap- proachesarestilllimitedtosystemsofspheres;fornonsphericalparticles,theanal- ysis is very complicated and mechanistic analysis is still an area for further study. Nevertheless,analysisusingspheresasanapproximationhasbuiltastrongtheoret- ical base inparticle technology and demonstrated the power ofDEM. Anotherimportantcomplicationinparticlemechanicsarisesfromthemanysur- faceforceswhichactbetweenparticles,duetointermolecularattractions(including Van der Waals forces), electrostatics, mechanical interlocking, and the presence of surface contaminants such as free liquids. The adhesive forces which arise in the absence of any other medium, such as a liquid layer, are collectively known as “autoadhesive.” The question here is not “How can autoadhesive forces be suffi- cientlystrongastoinfluencethebehaviorofthesystem?”but“Whydoautoadhesive forcesnothaveamorenoticeableeffectonparticlebehavior?”Theoreticalcalcula- tions of autoadhesive forces predict a force which is comparable with the particle weightforparticlesoforder1mminsize(Sevilleetal.,2000),butthefamiliarcon- sequencesofinterparticleforceeffectsdincreasedvoidfractionsandagglomeration intoclustersdarenotusuallyobservedinpracticeforparticlesaboveabout100mm in size. The reason is that adhesion between real particles is invariably limited by surface roughness (asperity contact) and contamination (Kendall, 2001). Influidization,forexample,itisgenerallyagreedthatitistheratioofinterpar- ticleforcetoparticleweightwhichgovernsthetransitionbetweenthecharacteristic fluidization behavior demonstrated by Geldart’s groups A (“aeratable”) and B (“bubbling”)(Geldart,1986)(Fig.1.2).Bystabilizingastructureofhighervoidage thanwouldotherwisebethecase,theymakeitpossibleforfluidizedbedstoshowa region of “uniform” expansion before bubbling, which is the most characteristic (and useful) feature ofgroup Apowders. If there is liquid present, at high humidity for example, “liquid bridges” may form between the particles, as shown in Fig. 1.3. This introduces a range of new problems of great complexity, even if the liquid is Newtonian9 in character and the particles are spherical. The presence of a liquid bridge between two particles introducesforcesbetweenthem.Iftheyarenotinrelativemotion,theresultingforce 9SeeChapter8
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