Porosity Alumina

Minimum and Full Fluidization Velocity for Alumina Used in the Aluminum Smelter Regular Paper

Paulo Douglas S. de Vasconcelos1,* and Andr L. Amarante Mesquita2

1 Albras - Alumnio Brasileiro S/A 2 Federal University of Par * Corresponding author E-mail: [email protected] Received 27 Jul 2011; Accepted 02 Nov 2011

AbstractFluidizationisanengineeringunitoperationthatoccurswhenafluid(liquidorgas)ascendsthroughabedofparticles,andtheseparticlesgetavelocityofminimumfluidization mfV enoughtostayinsuspension,butwithoutcarryingthemintheascendingflow.Asfromthismomentthepowderbehavesas liquidatboilingpoint,hence thetermfluidization.Thisoperation iswidelyused in thealuminumsmelterprocesses,forgasdryscrubbing(masstransfer)and inamodernplant for continuousaluminapot feeding (particles momentum transfer). Theunderstandingofthealuminafluoriderheologyisofvitalimportance in the design of fluidized beds for gastreatmentandfluidizedpipelinesforpotfeeding.This paper shows the results of the experimental andtheoretical values of theminimum and full fluidizationvelocities for the alumina fluoride used to project thestateoftheartroundnonmetallicairfluidizedconveyorofmultiplesoutlets.KeywordsFluidizedbed,Minimumfluidizationvelocity,Permeameter,Maximumpressuredrop1.IntroductionGassolidflowoccursinmanyindustrialoperations.Themajority of chemical engineering units operations, such

asdrying, separation,adsorption,pneumatic conveying,fluidizationandfiltrationinvolvegassolidflow.Poorpowderhandlinginanindustrialprocessoperationmayresultinabadperformance,leadingtoerrorsinthemassbalance,erosioncausedbyparticles impacts in thepipelines, attrition and elutriation of fines overloadingthe bag houses. Lack of a good gassolid flow ratemeasurement can cause economic and environmentalproblemduetoairborneparticles.This paper focuses on the applications of powdertechnology related to the aluminum smelters processessuch as dry scrubbing of gases and pot feeding toproduceprimaryaluminum.To optimize the residence time in the gasadsorptionprocessandminimizetheenergyconsumptioninthepotfeeding, theprecisedeterminationsof theminimumandfullfluidizationvelocitiesaswellastheaeratedandnonaerated angle of repose of the fluoride alumina aremandatory.2.FundamentalsofpowderfluidizationInthepowderprocessesdiscussedabove, it is importantto know the hydrodynamic behavior of the particles influidizedbeds.Itisknownthatthefluidizationbehavior

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ofagassolidbeddependson theparticlediameteranddensity. (Geldart, (19721973))1 classified powders intofour types: C, A, B and D, based on their fluidizationbehavior.Geldartsdiagramisillustratedinfigure1.

Figure1.Powderclassificationdiagramforfluidizationbyairsource:(Geldart,(19721973))1.

Thefluidizedbedregimemapisillustratedinfigure2.Group C particles have small diameters and are verydifficulty to fluidize. Channeling the bed of particlesbecoming difficult to determine the minimumfluidization velocity ultrafine alumina ( 20pd m ).GroupAparticlesareaeratableandreadilyfluidizefinealumina( mdp 5.40~ ).GroupBaluminaformsabubblebed after the fixed bed with increasing gas velocity

md p 70 .GroupDparticlesare relatively largebeanshape particles ~1000pd m . This bed of particles isspoutable,recirculatingtheparticlesinthefluidizedbed

Figure2.Flowregimemapforvariouspowders2.1MinimumfluidizationvelocitycalculationThisisgenerallydefinedasminimumsuperficialvelocityatwhich the drag force and the upward buoyant forceduetothefluidisbalancedbytheweightoftheparticlesillustratedinfigure3.

Figure 3.Fixed and a fluidizedbed ofparticles at aminimumfluidizationvelocitysource:(VasconcelosandMesquita,2011)2

2.1.1Theoreticalmodelequationsforpredict: mfV (Ergun,1952)3developed,forfixedbeds,apressuredropcorrelation at incipient fluidization based ondrag forceconsiderationsofasingleparticle inabedofparticlesofthesamesizeandneglectingthebuoyantforceasfollowsinequation1.

3 22

3 2 3(1 ) (1 )(1 )( ) 150 1.75( )

mf g mf mf g mfmf s g mf mf

mf s p mf s p

V Vg AV BVd d

(1)Calculating C by equation 2 one gets an equation ofsecond power,which the positive solution is calculatedbyequation3.

(1 )( )mf s gC g (2)

2 42mf

A A BCV B (3)

Where A and B aretheviscousandtheinertialfactoroftheErgun equation,C is theweightperunitvolumeofthebedofparticles.Fluidization is relatedwith smallvelocities, the factorB isnegligibleandtheErgunequationcanbesimplifiedwithanerrorlessthan5%relatedtoequation3bytheequation4.

3 2( ) ( )150(1 )

s g mf s pmf mf

mf g

g dCV VA

(4)

Theprediction of mfV byErguns equation forpressuredroprequirestheknowledgeofsphericityoftheparticle( s ) and bed voidage ( mf ).Many researchers has beentryingtooverreachthedifficulttodeterminethevoidageandsphericityoftheparticle,combiningequations1and2,plus theArquimedes,Froude andReynoldsnumberswith theirexperimental resultsadjusting theirequationstopredict mfV ascanbeseenintable1.


Table 1. Semiempirical equations for predicting minimumfluidizationvelocity mfV Foranincipientfluidization,whentheweightofparticlesequalsthedragforce,itisagoodattempttoconsidertheporosityattheminimumfluidizationvelocity mf equalsorclose theporosity of the fixedbed.Theporosityofthefixedbediscalculatedbytheequation10.

1 bnvs

(10)

sbnv

total

MV (11)

Where bnv is the nonvibrated bulk density, s is thesolid real density derived in a laboratory by apycnometer, sM isthetotalmassofparticlesweightedonan electronic scale, totalV is the total volume of particlesand voids in the sample previously weighted on anelectronicscale, pd istheparticlemeandiameterobtainedby sieve analysis in a laboratory, s is the particlesphericity, that can be estimated from figure 4consideringanormalpackingbedofparticles.Based on thework of (Biswal et al, 2007)9 anddata foraluminafluorideobtaineduponathesisby(Vasconcelos,2011)10fordoctoratedegreeatFederalUniversityofPar,equation 12 was proposed to predict the minimumfluidizationvelocity.

5.07.025.0 05.121.0 gdAV psrmf

(12)Where:

ArquimedesNumber:

g

ggspr

gdA

3(13)

Figure 4. Voidage in uniformly sized and randomly packedbedsadaptedfrom(KuniiandLevenspiel,1991)11And ggg ;; are respectively the real density andviscosityofthegas(air);accelerationduetogravity.Theexperimentalvalueof mfV andminimumvelocityoffullfluidization mffV areobtainedusingapermeameterasshowedinfigure5measuringthegassuperficialvelocityandthepressuredropthroughthebedofparticles.

Figure 5. Pressure drop through a bed of uniformly particlesversussuperficialairvelocitysource:(Mills,1990)12.2.1.2Experimentalsdetails

Aschematicdiagramoftheexperimentalsetupisshowninfigure6.Thearrangementofthepermeametermadeoffiberglasswithacrylicsheets toallowvisualobservationof the fluidized particles. Two rectangular and onecircular shapepermeameterswerebuiltas illustrated infigure 7.Air flow rate ismeasured by rotameterswithprecisionof+3% full scaleand thepressuredropwiththe appropriate dust filter in the pressure taps is

Paulo Douglas S. de Vasconcelos and Andr L. Amarante Mesquita: Minimum and Full Fluidization Velocity for Alumina Used in the Aluminum Smelter

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measuredbydigitalpressure transmittersataprecisionof + 1% of full scale. Air at a temperature of 303 K,( 3189.1 kgmg and sPaxg .10602.18 6 ) under oneatmosphericpressure,usedasthefluidizingmediumwaspassedthroughadustfilterandhumidityeliminator.Thepressure was regulated at 2 bars in the inlet of therotameters.

Figure6.PermeametersusedatAlbraslaboratorytosurveytheminimum fluidization velocity of the powders used in theprimaryaluminumindustrysource:(Vasconcelos,2011)10.

Figure 7. Experimental setup: 1) airmover; 2) air receiver; 3)manometer;4)airpressureregulator(03)bar;5)ballvalve;6)air flowregulator;7)rotameters (0 10;0 25;0 100;0 700)LPM; 8) ball valves; 9) thermometer; 10) polyester porousmembrane;11)pressuredroptransmitters(0125;01250;05000)Pa;12)permeameter.

Table 2. Fluoride alumina sieve analysis 100 samples(minimum/maximumsizeandstandarddeviation):(68.68/78.23and2.7) m )source:(Vasconcelos,2011)10.

2.1.3ProceduresAlumina isamixtureofmultisizeparticles,sobeforetostartthefluidizationsurvey,itwasnecessarytoperformasieveanalysistocharacterizetheparticlessizeascanbeseenintable2.Firstly,adetermined initialmass (1553kg)ofaluminafluoride was introduced in the big rectangular 14permeameter and (3 12 kg) in the circular 8permeameterinordertoensureabedheightofabout0.10.4m.Prior to actual fluidization tests, thebedwaspreconditionedtoreducetheinfluenceofinterparticlesforcesin thedensebedpacking.To this end, the superficialairvelocitywas rapidly increased around thepredicted mfV and thendecreased inmore or less 30 s.Thisprocedureguarantees better homogeneity of the bed. After thispreparation the air flow ratewas increased in steps of 2LPM (liters per minute) for about one minute andsubsequently the pressure drop in bed of particles wasmeasured until the pressure drop stays constant at avariationlessthan5%.Then,theairstreamwasdecreasedin steps of 2 LPM until 0 LPM see figure 8. Eachfluidization cycle test (batches of alumina fluoridesummarizedintable3)wasrepeatedtwice.

Figure 8. Experimental pressure drop in the bed of aluminafluoride (72.6 + 2.7) m /3387.3 3.kg m versus superficial airvelocitysource:(Vasconcelos,2011)10.From pointA to point B in figure 8 the pressure dropincreases with velocity (fixed bed). The transition fromthestagnantbedtofluidizedbedoccursatpointB,i.e.atthe minimum fluidization velocity mfV (52 cm/min)intersection with the maximum pressure drop. FrompointB thepressuredrop increasesuntilpointC, i.e.atthe minimum velocity of full fluidization mffV (153cm/min). The pressure drop remains constant withvariation around 5% of the maximum pressure drop.AfterpointCthebedisfullyfluidized.The same procedures explained above were used toobtain the experimental values of mfV and mffV


summarizedintable3forothersdiametersofthealuminafluorideusedatAlbrasaluminumsmelter.3.ResultsandDiscussionsThe minimum fluidization velocity was predict usingequation 12 proposed by (Vasconcelos, 2011) 10with anaverage absolute error less than 0,5% comparewith theexperimentalshowedinfigure8.Forthegranulometriessub37 m (400mesh),itwasuseda small rectangular permeameter illustrated in figure 9,becauseaftermore than150sieveanalysisof the fluoridealuminawegotonlyonekilogramofthisfractiontostudy.As can be observe in figure 9 it was impossible todetermine the minimum fluidization velocity in this testduetochannelinginthebedofparticles.The experimental data obtained for the minimumfluidizationvelocity mfV werecomparewith theequationsproposedby(Vasconcelos,2011)10,(Ergun,1952)3andothersresearchers listed in table 1.The comparison of calculatedandexperimental mfV forastagnantbedofparticleswiththedatarelatedintable2isshownintable4withtherespectiveabsoluteerror(%)basedonexperimentalvalue.Figures10and13showthegraphicanalysis.

Figure9.Permeameterof50x200mmused to fluidizealuminafluoridesub37 m (GeldartgroupC).

Table 3.Parameters of the fluidized bed of alumina fluoridesparticlessource:(Vasconcelos,2011)10.

Figure 10. Comparison of experimental values of mfV andpredictedbyVasconcelosandColttersandRivas.

Figure 11.Apparatus tomeasure the nonaerated and aeratedangleof reposealumina fluoridenonaeratedangleof repose(32.73+0.437).

Figure 12.Albras nonmetallic air fluidized conveyor of threeoutlets



Table 4.Comparison of experimental andpredictedminimum fluidization velocity for stagnant bed of alumina fluoride source:(Vasconcelos,2011)10.

Figure13.Comparisonofexperimentalvaluesof mfV withpredictedvalues 4.ConclusionsExperimentals were carried out with alumina fluorideand four fractions of this alumina in the fluidizationlaboratory erected at Albras smelter to survey thealumina rheology in order to optimize energyconsumption of the state of art air fluidized conveyordevelopedandshowedinfigure12.An equation was developed to predict the minimumfluidization velocity upon a thesis by (Vasconcelos,2011)10 compared with other researchers and theexperimental values.Thepredict values byVasconcelosand Coltters and Rivas adjusted by the experimental

result for md p 6.70 are quite satisfactory with theexperimentalsresultswithanerrorlessthan0,5%ascanseeintable4andinfigure10and13.ThecorrelationsofAbrahamsenandGeldartfitverywellwith finepowders of groupC andA.The equations ofWen and Yu and Miller and Logwinuk show betterresultsforlargeparticlesofgroupBandD.TheequationofLevafitbetterwithgroupBpowders.The correlations of Ergun and Vasconcelos are verysensitivetoporosityandthesphericityoftheparticlesandhence care should be taken tomeasure the porosity veryaccuratelyandalsoinestimatethesphericityusingfigure4.


The proposed correlation by this paper could also findpracticalutility indesigningandoperationsof fluidizedbed forgasdryscrubbing toestimate theresidence timeof theparticles incontactwithgasesandalsooptimizedthesizeandenergyconsumptionofairslides.Otherimportantparameteristhenonaeratedandaeratedangle of repose illustrated in figure 11 to estimateflowabilityofthepowdertobetransportinconventionalor new development of air fluidized conveyors ofmultiplesoutlets.5.AcknowledgmentTheauthorswould like to thanks theLordGod for thisopportunity, Albras Alumnio Brasileiro SA for theauthorizationtopublicthispaper,theFederalUniversityofPar formydoctorate in fluidizationengineeringandtomymanagerMarcelinoVasconcelos and the generalmanagers Lus Carlos Carvalho Costa and Braz MileoFerraioli.6.References[1] Geldart, D. Types of Gas Fluidization Powder

Technology,7,285292(19721973).[2]Vasconcelos,P.D.andMeaquita,L.A.Gassolidflow

applications for powder handling in industrialfurnacesoperations.HeatTransfer/Book5,ISBN9789533075853. Intech open access publisher, Rijeka,(2011).

[3]Ergun,S.FluidFlowthroughPackedColumns,Chem.Engrg.Progress,Vol.48,No.2,pp.8994(1952).

[4]Leva,M.,Fluidization,McGrawHill,NewYork(1959).[5]Abrahamsen,A.R. andGeldart,D. Behavior of gas

fluidized beds of fine powders. I. Homogenousexpansion,Powdertechnology,Vol.26,35,(1980).

[6] Coltters, R. and Rivas, A. L. Minimum fluidizationvelocity correlations inparticulate systems,PowderTechnologyVol.147pp.3448(2004).

[7]Wen,C.Y.andYu,Y.H.,MechanicsofFluidization,Chem.Engrg.ProgressSymp.Series,Vol.62,No.62,pp.100111(1966).

[8]Miller,C.O.andLogwinuk,A.K.Ind.Eng.Chem.,43(1951)1220.

[9] Biswal et al. Minimum fluidization velocities andmaximumbedpressuredrops forgassolid taperedfluidizedbeds.

[10] Vasconcelos, P. D. Doctorate Thesis, FederalUniversityofParatBelm,2011.

[11] Kunii, D. & Levenspiel O. Fluidization Engineering,secondedition,ButterworthHeinemann,Boston(1991).

[12] Mills, D. Pneumatic Conveying Design Guide,Butterworths,London,(1990).



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