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Eprints ID : 9910
To link to this article : DOI:10.1016/j.cep.2012.12.004
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http://dx.doi.org/10.1016/j.cep.2012.12.004
To cite this version :
Ammarcha, Chawki and Gatumel, Cendrine and Dirion,
Jean-Louis and Cabassud, Michel and Mizonov, Vadim and
Berthiaux,
Henri
Transitory powder flow dynamics during emptying of a
continuous mixer.
(2013) Chemical Engineering and Processing :
Process Intensification, vol. 65 . pp. 68-75. ISSN 0255-2701
Any correspondence concerning this service should be sent to the repository
Transitory
powder
flow
dynamics
during
emptying
of
a
continuous
mixer
C.
Ammarcha
a, C.
Gatumel
a, J.L.
Dirion
a,
M.
Cabassud
b,
V.
Mizonov
c,
H.
Berthiaux
a,∗aCentreRAPSODEE,FRECNRS,EcoledesMinesd’Albi-Carmaux,CampusJarlard,routedeTeillet,81000Albi,France bLaboratoiredeGénieChimique,UMR5503CNRS-INPT-UPS,BP1301,5ruePaulinTalabot,31106Toulouse,France cIvanovoStatePowerEngineeringUniversity,DepartmentofAppliedMathematics,Rabfakoskaya34,153003Ivanovo,Russia
Keywords: Semi-batchmixing Powder Markovchain Transitoryregime
a
b
s
t
r
a
c
t
Thisarticleinvestigatestheemptyingprocessofacontinuouspowdermixer,frombothexperimental andmodellingpointsofview.TheapparatususedinthisworkisapilotscalecommercialmixerGericke GCM500,forwhichaspecificexperimentalprotocolhasbeendevelopedtodeterminetheholdupin themixerandtherealoutflow.Wedemonstratethatthedynamicsoftheprocessisgovernedbythe rotationalspeedofthestirrer,asitfixescharacteristicvaluesofthehold-upweight,suchasathreshold hold-upweight.ThisisintegratedintoaMarkovchainmatrixrepresentationthatcanpredictthe evo-lutionofthehold-upweight,aswellasthatoftheoutflowrateduringemptyingthemixer.Depending ontheadvancementoftheprocess,theMarkovchainmustbeconsideredasnon-homogeneous.The comparisonofmodelresultswithexperimentaldatanotusedintheestimationprocedureofthe param-eterscontributestovalidatingtheviabilityofthismodel.Inparticular,wereportresultsobtainedwhen emptyingthemixeratvariablerotationalspeed,throughstepchanges.
1. Aquickstate-of-the-art
Themixingofsolidsisacommonoperationinseveral indus-trialapplications.Itfrequentlyrepresentsacrucialunitoperationin manyindustries(foodstuffs,cosmetics,ceramics,detergents, pow-deredmetals,plastics,drugs,etc.).Whilemixersareoperatedeither inbatchorincontinuousmodeforindustrialapplications,itis
cur-renttoseesuchmachinesoperatinginsemi-batchmodeduring
limitedperiodsoftime,eitherbecausethefeedhasbeenstopped
while keeping a continuous withdrawal, or conversely keeping
acontinuousfeedwitha blockedoutlet.Thishasbeenrecently pointed out by Berthiauxet al. [4], who studiedthe impactof feedingperturbationsonmixturequalityforanOTCdrug.Because loss-in-weightfeedersloosetheirregularitywhenthemassof pow-derinthehopperbecomeslessthanapproximately15–20%ofits apparentvolume,the fillingperiod of thefeedersresultsin an importantsourceofmixtureheterogeneity. Itwassuggestedto stopthefeederswhenfillingandoperatethecontinuousmixerin semi-batchmode,atleastnottodisturbthefollowingoperations liketabletting.Anotherexampleoftheimportanceofsemi-batch modeintheindustryisthedischargeofbatchblendersattheend ofoperation.Duringemptying,thebatchmixeristherefore trans-formedintoasemi-batchsystem.Inthecaseofbatchconvective mixers,itisworthnotingthatoperatorswillruntheengineathigh
∗ Correspondingauthor.Tel.:+33563493144.
E-mailaddresses:henri.berthiaux@mines-albi.fr,henri.berthiaux@mines-albi.fr
(H.Berthiaux).
speedtoacceleratetheemptyingprocesswithoutpaying atten-tiontotheeffectthisprocedurecanhaveonmixturequality.Inthe pharmaceuticalindustry,thisisallthemoreimportantbecauseof the“principleofreconciliation”statingthateachportionofeach materialshouldbetrackedfromitsenteringintothesystemtoits leaving.
IfweexcludethepaperbySudahetal.[13],veryfewstudieshave beenpublishedconcerningtheemptyingprocessofamixerandits effectonblendhomogeneityorpowderflow.Moregenerally, con-tinuousprocessingofpowdershasneverbeenextensivelystudied fromtheviewpointoftransitoryregimephasesthatarecurrently takingplaceduringemptying,startingordosagemodechanging. Thisisallthemoreastonishingthatthediscretenatureofgranular medialogicallyinducestransitorymotionandtransitoryflow.
Behaviour of granular materials in mixers or in other
pro-cess equipment,is still poorly understood from a fundamental
standpoint, as it is hardly possible to portray perfectly all the detailsoftheprocessinamathematicalformandinareasonable time.However,somedynamicmodelsarepresentinthechemical engineeringliterature,tryingtoexplainanddescribegloballythe processratherthanintofulldetails.Markovchainspertaintothis system’sapproachtoolbox.
Twocasesareunderlinedbehindthetermmixingandmustbe distinguishedwhendealingwithmodelsofanytype:bulkparticle flowandtransportinanytypeofvessel,includinginmixers; mix-ing/blendingofvariousflowsrelatedtodifferenttypesofparticle
[3].SeveralresearchersusedtheMarkovchaintheorytodescribe “pure”powderflowpatternsinmixers.Some40yearsago,Inoue andYamaguchi[8]describedthegeneralflowpatternofglassbeads
insideabatchtumblermixer.Asinglecolouredbeadwasadded anditspositionwasrecordedateverytransition(revolutionofthe mixer).Theexperimentaldatawereusedtodeterminethe transi-tionmatrix,andservedtoderiveatwodimensionalhomogeneous Markovchainmodel.Morerecently,Aounetal.[1]appliedthistype ofmodelforalaboratoryhoopmixer,whichwasdividedinto33 elementarycells(11axialcompartmentsthatwereinturn sepa-ratedinto3radialcells).Theexperimentaldataallowedtoderive thetransitionmatrixandservedagaintodiagnosetheflow pat-ternofcouscousparticlesinthevessel.OtherexamplesofMarkov chainsforthedescriptionofpowderflowinmixerscanbefoundin Chenetal.[5]andLaiandFan[9].
Attheverysametime,otherresearchersweremodellingthe mixingofdifferentparticulateflows,principallybinarymixtures.At theendoftheseventies,FanandShin[7]studiedthebinarymixing processofsphericalluciteparticleswithtwodifferentparticlesize inatumblermixerdividedinto10sectionsofequalvolume.They linkedthetransitionprobabilitiesofaMarkovchaintothediffusion coefficientandthedriftvelocity.Thetransitionmatrixwas deter-minedexperimentallyandallowedtodescribetheconcentration profilesforeachsystem.Morerecently,atwo-dimensionalmodel oftheflowandmixingofparticulatesolidshasbeendevelopedon thebasisoftheMarkovchaintheoryforanalternatelyrevolving staticmixer[12].Thismodelrepresentedthedistributionof com-ponentduringmixingoperation,in bothverticalandhorizontal directions.Severalsimulationswereperformedtoinvestigatethe effectoftheinitialloadingofthecomponents,andtheeffectofthe valuesofthetransitionprobabilities.Otherexamplesarepublished inOyamaandAgaki[11],WangandFan[14]orWangandFan[15]. Inthepresentarticle,wewillconcentrateonstudyingthe dis-chargeprocessofacontinuouspilot-scaleconvectivemixer.We willfocusontheeffectoftheinitialpowdermass,aswellasonthe rotationalspeedofthestirrerwhileemptyingthemixeronboththe hold-upweightofa“pure”bulkmaterialanditsoutletflowrate. Thisisapreliminarytoabetterunderstandingofthedynamicsof blenddischarginganditsimpactonmixturehomogeneity.Wewill alsoinvestigatetheeffectofstepvariationsintherotational stir-rer’sspeedduringemptyingofthemixer.Allthiswillbeintegrated intoaMarkovchainasatoolforpredictionofhold-upweightand outflowratefromoperatingvariablesvaluesandvariability.
2. Experimentalset-upsandmethods
TheexperimentswereperformedinaGerickeGCM500 continu-ouspilotscalepowdermixer,whichcanberuneitherincontinuous, batchorsemi-batchmode.Thismixerisequippedwiththree loss-in-weightfeederstoensureaverypreciseandregulardosage.In
thiswork,allexperimentswereperformedinsemi-batchmode,
whichmeansthatparticleshavebeeninitiallyplacedintheblender andfeederswerenotused.Attheoutletofthemixeran analyti-calbalanceisplacedtomeasuretheoutflowmass inrealtime. Thisbalancecommunicateswithacomputerthroughaserialport RS232wehaveconfiguredintheMatlabControlToolbox. There-forealldataaremeasuredandsavedinrealtime.Experimentshave beenperformedwithfoodproducts,namelycouscousparticles.Full descriptionofequipmentandpowderusedcanbeseeninMarikh etal.[10]andinFig.1a.Themixeritselfisahemi-cylindricaltankof 50cmlong,16.5cmheightand20cmdiameter.Particlemotionis duetothestirringactionofthemobile,sothismixercanbe classi-fiedintothecategoryofconvectivemixers.Thestirrerisconstituted of14rectangularpaddlesinclinedata45◦angleandinstalledona
framesupportinganinternalscrew.Thedimensionsofthisframe are45cmlengthand18cmwidth.Thescrewisplacedatthecentre oftheframeandallowsaconvectiveaxialmotionoftheparticles insidethevessel.
Theoutletofthemixerconsistsinagatevalvewhoseposition playsanimportantroleasitdeterminesthesectionthroughwhich particleshavetoflowoutoftheapparatus.Ifthissectionisreduced, theholdupinthemixerwillbehigherforfixedconditions,asless particlesareallowedtoexitfromthemixer.Inthepresentwork,all experimentshavebeenperformedwiththeoutletgatevalveatthe maximumofopening.Insemi-batchmode,theholdupweightM1
isafunctioninfluencedaprioribythestirrer’sspeedofrotationNm
andtheinitialloadingmass(seeFig.1b).Inthisstudytheeffectof thesevariablesontheholdupM1willbeexaminedforthefollowing
ranges:
–fivevaluesoftherotationalspeedofthemobileNm,whichcan
beexpressedbythefrequencyoftheengine,N:10,20,30,40and 50Hz;
– threeinitialmassesintroduced4,6and8kg.
TherelationshipbetweentherotationalspeedofthemobileNm
andthefrequencyoftheengineisgivenby:Nm(rpm)=2.6N(Hz).
3. Markovchainmodellingofthesemi-batchprocess
3.1. Generalarchitectureofthemodel
The essenceof a Markov chain model is that ifthe present
statesofa systemareknown—forexamplethepowdermassin
eachstate—andiftheprobabilities—forexampleflowrateratios—to transittoanyotherstatesarealsogiven,thenthefuturestateof thesystemcanbepredicted.Whenprobabilitiesareunchanged,the chainissaidtobehomogeneousandasimplematrixrelationcan bederivedtolinkthefuturestatestotheinitialstate.Conversely, time-dependentorstate-dependentprobabilitieswillresultina step-by-stepcalculationofthestatesofthesystem.
Whenappliedtomixing,thegeneralprinciplelaysindividinga vesselintoncells(Fig.2),orstatesofthesystem,andtoexaminethe possibletransitionsofaparticleproperty(massorconcentrationin akeycomponent)betweeneachcellsduringafixedtimeinterval 1t.
LetthepropertyunderobservationbethemassMn(t)of
parti-clesincellnattimet,andletM(t)bethevectorofelementsMn(t).
Aftereachtransition,aparticlecantransittoanothercellandthe observedpropertycanchangeineachcell.Thisissummarisedbya (n×n)matrixP(t)oftransitionprobabilities,thepj,k(t)
correspond-ingtothetransitionfromstatektostatejattimet.Ifweassume thattransitionscanonlyoccurfromonecelltoaneighbouringcell, thefuturestateM(t+1t)canbepredictedbythefollowingmatrix equation: M(t+1t)=P(t)M(t) (1) P(t)=
P1,1(t) P1,2(t) .... .... 0 0 0 P2,1(t) P2,2(t) ··· ··· 0 0 0 0 P3,2(t) ··· ··· 0 0 0 ··· ··· ··· 0 ··· ··· ··· ··· ··· Pi−1,i(t) Pi,i(t) Pi+1,i(t) ··· ··· ··· ··· ··· ··· ··· ··· ··· 0 0 ··· ··· ··· Pn−2,n−1(t) 0 0 0 ··· ··· ··· Pn−1,n−1(t) 0 0 0 ··· ··· 0 Pn,n−1(t) 1
Therepresentationofthisprocedureundermatrixform,givesa practicaldescriptionandaneasyhandlingoftheprocessdynamics
Fig.1. GerickeGCM500equipmentshowingthestirringmobile(a).Generalprocessvariables(b).
Fig.2.GeneralMarkovchainrepresentationofthesemi-batchoperationofthemixer,includingaflowmodelinsidethemixer.
whatevertheregimeconsidered:batch,continuous,semi-batch, multiplestagedfeed,transitoryfeed,etc.Fulldescriptionof
blend-ing modelling using Markov chains is detailed in thework by
Berthiauxetal.[2].
Asaforementioned,themixeris usuallydividedinto several cells.Inthepresentrepresentation,wewillonlyconcentrateona generalframeformodellingabletogiveinformationonthe“black box”behaviourofthemixerunderunsteadyconditions,andusea simplifiedMarkovchainmodelwithonlytwocells.Thisfirstmodel willbeimplementedinthefuturetoenterintomoredetailsofthe flowstructure,uptotheleveloftheparticlesthroughdistinct ele-mentmodelling(DEM),givingthenrisetoahybridmodelsuchas theonerecentlydevelopedbyDoucetetal.[6].
Thefirstcellrepresentsthewholemixer,thelevelof discreti-sationbeingthewholeapparatus.The secondcellis theoutlet of the mixer, acting as an absorbingstate. This cell is shown, for example,in Fig.3 as thelast cellon theright.If a particle reachestheabsorbingstate,itstaysinitforever,i.e.,pii=1exists
forthisstate.Inourcase,theabsorbingstateisjoinedtothechain artificially.
LetM1bethemassofparticlesinmixer(orincell1),andM2the
massofparticlesintheabsorbingstate(orincell2).Thetransition ofaparticlefromstate1tostate2willobeytothegeneralMarkov chainrelation: M1(t+1t) M2(t+1t)
!
= p11(t) p12(t) p21(t) p22(t)!
× M1(t) M2(t)!
= 1−p21(t) 0 p21(t) 1!
× M1(t) M2(t)!
(2)InordertorepresenttheemptyingprocessthroughEq.(2),itis necessarytoknowtheinitialloadingmassM1(0).Astheabsorbing
statedoesnotcontainparticlesinitially,thenM2(0)isequaltozero.
Fig.3.SimplifiedMakovchainrepresentationoftheemptyingprocessofthemixer.
canbedonebythefollowingstep-by-stepprocedure: P21(t+1t)= M2(t+M1t)−M2(t)
1(t)
= [M1(0)−M1(t+1t)]M −[M1(0)−M1(t)]
1(t)
(3)
3.2. Implementingthemodelfromexperimentalresults
AsstatedbyEq.(3),p21(t)canbedeterminedfromthe
experi-mentalresultsofM1(t).Fig.4givesanexampleoftheexperimental
resultsobtainedatN=20Hzfortheevolutionofthehold-upweight incell1(mixer)fordifferentinitialweight.P21isalsopresentedin
thesamegraphforatransitiontime1t=1.6s.Thistransitiontime hasbeenarbitrarilyfixedafterseveraltries,toensureasufficient
resolutionofthemodel.Twoothersimilarexamplesareshown
inFig.5forhigherrotationalspeeds.Itcanbeseenthatthe ini-tialmassM1(0)donotaffecttheresultofprobabilityp21,which
meansthatthedynamicsofemptyingofthemixeronlydependson therotationalspeed.Whenexaminingtheseresults,wecandefine threeregimesofprobabilitycorrespondingtothecurrentholdup weightinthemixer.Thereexistsaminimumholdupbelowwhich
nopowdercanflowoutofthemixer,andathresholdvalueabove whichp21isconstant(andequalsp21max).LetMminandMthreshold
bethesecharacteristicholdups.Intermsoftransitionprobability,
wehave
for M1>Mthreshold, p21=p21max (4)
and for M1<Mmin, p21=0 (5)
WhenM1 ishigherthanMthreshold,thevalueofp21isstablewith
time,sothattheMarkovchainisa timehomogeneouschainor
aMarkovchainswithtime-homogeneoustransitionprobabilities. ThetransitionmatrixPalwaysremainsthesameateachstep,so thek-steptransitionprobabilitycanbecomputedasthek’thpower ofthetransitionmatrix,Pk.
WhenthemassM1islowerthanMthresholdandhigherthanMmin,
theprobabilityP21increasesalmostlinearlywithhold-upweight
M1:
for Mmin<M1<Mthreshold, p21(t)=aM1(t)+b0 (6) Inthiscase,theprobabilityoftransitioninasinglestepfromstate 1tostate2dependsontheparticlemassinstate1,whichmakes thetransitionstime-dependentthroughitsstate-dependency.The
Fig.4.ExperimentalevolutionofM1andp21duringemptyingofthemixer(N=20Hz).
Fig.6.EvidenceofalinearrelationbetweenparameteraandN(a);relationbetweenMminandN(b).
Markovchainisthereforenothomogeneous,andstatesmustbe
calculatedstep-by-step.However,this isfarfrombeinga prob-lemfromacomputationalpointofview.AsfarasM1isinferiorto
Mthreshold,thenewprobabilityp21(t)dependsonlyontheprevious
calculationofthestatesthroughM1(t)andbothparameters(aand
b)oflinearEq.(6).
ParameteraonlydependsontherotationalspeedNasitcan beseeninFig.6a,andthesecondparameterbcanbecalculated forM1=Mminasb=−a×Mmin.Inturn,MminhasbeenrelatedtoN
throughapowerlaw,asindicatedinFig.6b.Thepowerhasbeen identifiedtobeclosedto0.5,sothatweconsideredthisvalueas beenfixedandrecalculatetheotherparametervaluesk1andk2:
a=k1×N (7)
b=−a×Mmin=−k1×k2N0.5 (8)
Inthisnon-linearregime,theemptyingprocessofthemixer there-forevarieswiththehold-upweightandtherotational speedof themobilethroughthefollowinggeneralrelationshipthatincludes twoconstantsdeterminedformthewholesetofexperimentaldata: p21(t)=k1N
M1(t)−√k2 N (9) Itmustbeemphasizedthatthislatterequationmakesthe over-allMarkovchainnon-homogeneous,asthetransitionprobability dependsonthestateofthesystem.Ifthereisnoapriori obvi-ousphysicalexplanationforthis, itis clearthat fromaprocess standpoint,thisbehaviourmustbeknowntomonitortheprocess. IfM1ishighertoMthreshold,theprobabilityp21becomessteadyandcanbelinearlyrelatedtoN,asshowninFig.7a:
p21max=k3×N (10)
Finally,thethresholdmassMthresholdcanbedeterminedfromthe
aboveprocedureandlinkedtotherotationalspeed:
a×Mthreshold+b=p21max (11) k1×N×Mthreshold+(−k1×N×k2×N0.5)=k3×N (12) Mthreshold= (k3+k1×k2×N 0.5) k1 (13)
4. Resultsanddiscussion
Despitethefactthattheabovemodelhasbeendevelopedusing
ageneralframework,itcontainsunknownparametersthathave
beenfoundfromtheexperimentsdescribedintheprevious sec-tion.Inthissection,wewilltestthevalidityofthisprocedureby eitherconsideringtheabilityofthemodeltodescribesecondary variables,suchasoutflowrate,oroperatethemixerunder condi-tionsfarfromthoseunderwhichparametershavebeenobtained, likeduringstep-likeperturbations.
4.1. Predictingoutletflowrateduringemptying
Fortheproductselected,e.g.,afree-flowingmaterial,the rota-tionalspeedofthestirrercompletelydefinesthedynamicsofthe emptyingprocessofthemixerbyfixingthevalueofthe
thresh-oldhold-upweight.Belowthisvalue,anon-homogeneouschain
canbeemployed.Aboveit,ahomogeneouschaingivestheruleof semi-batchprocessing.Thisprocedurealsogivesfullaccesstothe evolutionoftheoutletflowrateQout,throughthefollowing:
Qout(t)= M2(t+1t)1t−M2(t) (14)
As M2(t+1t)=M2(t)+p21(t)×M1(t) (15)
Qoutcanthenbelinkedtop21:
Qout(t)= p211t(t)×M1(t) (16)
Anexampleofcomparisonbetweenexperimentalresultsofthe
flow rate,obtainedby Eq.(14),and modelsimulation formthe MarkovchainrepresentationisshowninFig.8fordifferent rota-tionalspeedsofthestirrer.Thedifferentflowregimes,depending
onthe value ofthe actualhold-up weightas compared to the
thresholdvalue,areagainpointedout: –ForM1>Mthreshold
Qout(t)= p211tmaxM1(t) (17)
Qout(t)= k31t×NM1(t) (18)
0 20 40 60 80 100 120 140 0 50 100 150 200 250 300 350 400 M1(g) Qout kg/h) c 0 50 100 150 200 250 300 350 400 0 200 400 600 800 1000 1200 1400 M1(g) Qout model: 10Hz Qout exp: 10Hz Qout model: 20Hz Qout exp: 20Hz Qout model: 30Hz Qout exp:30Hz Qout model:35Hz Qout exp: 35Hz Qout moedel: 40Hz Qout exp: 40Hz Qout (kg/h) c 0 20 40 60 80 100 120 140 0 50 100 150 200 250 300 350 400 M1(g) Qout kg/h) c 0 50 100 150 200 250 300 350 400 0 200 400 600 800 1000 1200 1400 M1(g) Qout model: 10Hz Qout exp: 10Hz Qout model: 20Hz Qout exp: 20Hz Qout model: 30Hz Qout exp:30Hz Qout model:35Hz Qout exp: 35Hz Qout moedel: 40Hz Qout exp: 40Hz Qout (kg/h) c
Fig.8.ComparisonofsimulatedandmeasuredoutflowrateaccordingtoretainedmassM1.
–ForMmin<M1<Mthreshold
Qout(t)=p211t(t)M1(t) (19)
Qout(t)=k11t×N
#
M21(t)−Mmin×M1(t)(20) Inthislattercase,asthetransitionprobabilitychangeslinearlywith M1,theoutflowrateincreaseswiththehold-upweightaccording
toasecondorderpolynomiallaw.
4.2. Predictingtheeffectofstep-likeperturbations
Thepredictabilityofthemodelcanbebetterdemonstratedif itsresultsareplottedagainstdatathathavenotbeenusedinthe determinationofitsparameters.Inthislastpart,wewillcompare modelresultsandexperimentaldataobtainedwhenemptyingthe mixerthroughnegativeorpositivestepsontherotationalspeed, asitmaybethecaseintheindustry.
Fig.9 reports theresults obtainedwhile changing instanta-neouslytherotationalspeedfrom20to40Hzduringanemptying process.Asitcanbeobserved,thepredictionofbothoutflowrate
Qout and holdupweightM1 isexcellentwhencomparedtothe
experimentaldata.Thestepinrotationalspeedinducesajumpin flowrate,butalightdecreaseinhold-up.Thisisvalidated experi-mentallyforawiderangeofrotationalspeedofthestirrerdevice (10–50Hz),notreportedhereforclarity.Anotherexampleof com-parisonisshowninFig.10,forwhichanegativestepchangeinthe rotationalspeedfrom35to15Hzhasbeenperformed.Thesame conclusionscanbedrawnonthemodel’sperformance.
The evolution of the outflow rate with the hold up weigh
M1 during thesetwotypes ofperturbationis shownin Fig.11.
The lines predicted by the model with or without
perturba-tion are also figured to better illustrate the effect of the step changes.
Thetransitionprobabilities,usedinthesesimulations,are deter-minedfromtheempiricalequations(4)–(6),andarerepresentedin
Fig.12forthepositivestep.Inthisgraph,wecandifferentiatethree phasesdependingonNandM1(seeFig.13forarepresentationin
termsofoutflowrate):
• Initially,Nisfixedat20Hz.Inthiscase,asthehold-upweightin themixerM1ishigherthanMthreshold,thevalueofp21isstable
(a)
(b)
0 100 200 300 400 500 600 700 800 900 1000 0 20 40 60 80 100 Temps(s) 0 10 20 30 40 50 Qout model: 20_40Hz Qout exp: 20_40Hz N(Hz) Qout (kg/h) N 20Hz 40Hz 0 1000 2000 3000 4000 5000 6000 0 20 40 60 80 100 Temps (s) 0 10 20 30 M1 model:20_40Hz M1exp: 20_40Hz N (Hz) M1 ( N (H 20Hz 40Hz 0 100 200 300 400 500 600 700 800 900 1000 0 20 40 60 80 100 Temps(s) 0 10 20 30 40 50 Qout model: 20_40Hz Qout exp: 20_40Hz N(Hz) Qout (kg/h) N 20Hz 40Hz 0 1000 2000 3000 4000 5000 6000 0 20 40 60 80 100 Temps (s) 0 10 20 30 40 50 M1 model:20_40Hz M1exp: 20_40Hz N (Hz) M1 (g) N (Hz) 20Hz 40HzFig.9.Comparisonofsimulatedandmeasuredresultsduringemptyingwithapositivestepchangeinrotationalspeed(N=20–40Hz).Effectontheoutflowrate(a)andon theholdupweight(b).
0 100 200 300 400 500 600 700 800 900 1000 0 20 40 60 80 100 120 140 0 10 20 30 40 50 Qout model: 35_15Hz Qout exp: 35_15Hz N(Hz) Qout (kg/h) (kg/h) N (Hz) 35Hz 15Hz 1000 2000 3000 4000 5000 6000 0 20 40 60 80 100 0 10 20 30 40 50 M1 model:35_15Hz M1exp: 35_15Hz N (Hz) M1 (g) N (Hz) 35Hz 15Hz 0 100 200 300 400 500 600 700 800 900 1000
(a)
(b)
0 20 40 60 80 100 120 140 Temps(s) 0 10 20 30 40 50 Qout model: 35_15Hz Qout exp: 35_15Hz N(Hz) Qout (kg/h) (kg/h) N (Hz) 35Hz 15Hz 0 1000 2000 3000 4000 5000 6000 0 20 40 60 80 100 Temps (s) 0 10 20 30 40 50 M1 model:35_15Hz M1exp: 35_15Hz N (Hz) M1 (g) N (Hz) 35Hz 15HzFig.10. Comparisonofsimulatedandmeasuredresultsduringemptyingwithanegativestepchangeinrotationalspeed(N=35–15Hz).Effectontheoutflowrate(a)and ontheholdupweight(b).
(a)
(b)
0 200 400 600 800 1000 1200 1400 0 500 1000 1500 2000 2500 3000 3500 4000 M1 (g)0 10 20 30 40 Qout model: 35Hz Qout model: 15Hz Qout model: 35_15Hz Qout exp: 35_15Hz N(Hz) Qout (kg/h) N(Hz) 35Hz 15Hz 0 200 400 600 800 1000 1200 1400 0 1000 2000 3000 4000 5000 M1 (g) 0 10 20 30 40 Qout model: 40Hz Qout model: 20Hz Qout model: 20_40Hz Qout exp: 20_40Hz N(Hz) Qout (kg/h) N(Hz) 40Hz 20Hz 0 200 400 600 800 1000 1200 1400 0 500 1000 1500 2000 2500 3000 3500 4000 M1 (g)0 10 20 30 40 Qout model: 35Hz Qout model: 15Hz Qout model: 35_15Hz Qout exp: 35_15Hz N(Hz) Qout (kg/h) N(Hz) 35Hz 15Hz 0 200 400 600 800 1000 1200 1400 0 1000 2000 3000 4000 5000 M1 (g) 0 10 20 30 40 Qout model: 40Hz Qout model: 20Hz Qout model: 20_40Hz Qout exp: 20_40Hz N(Hz) Qout (kg/h) N(Hz) 40Hz 20HzFig.11.Evolutionoftheoutflowrateaccordingtohold-upweightafterapositive(a)andoranegative(b)stepchangeinrotationalspeedN.
(a)
(b)
0 200 400 600 800 1000 1200 1400 0 500 1000 1500 2000 2500 3000 3500 4000 M1 (g) 0 10 20 30 40 Qout model: 35Hz Qout model: 15Hz Qout model: 35_15Hz Qout exp: 35_15Hz N(Hz) Qout (kg/h) N(Hz) 35Hz 15Hz 0 200 400 600 800 1000 1200 1400 0 1000 2000 3000 4000 5000 M1 (g) 0 10 20 30 40 Qout model: 40Hz Qout model: 20Hz Qout model: 20_40Hz Qout exp: 20_40Hz N(Hz) Qout (kg/h) N(Hz) 40Hz 20Hz 0 200 400 600 800 1000 1200 1400 0 500 1000 1500 2000 2500 3000 3500 4000 M1 (g) 0 10 20 30 40 Qout model: 35Hz Qout model: 15Hz Qout model: 35_15Hz Qout exp: 35_15Hz N(Hz) Qout (kg/h) N(Hz) 35Hz 15Hz 0 200 400 600 800 1000 1200 1400 0 1000 2000 3000 4000 5000 M1 (g) 0 10 20 30 40 Qout model: 40Hz Qout model: 20Hz Qout model: 20_40Hz Qout exp: 20_40Hz N(Hz) Qout (kg/h) N(Hz) 40Hz 20HzFig.12.Comparisonofsimulatedandmeasuredvaluesofp21duringtheemptyingprocessafterapositivestepchangeinrotationalspeedN,onthebasisoftimechange(a)
orhold-upchange(b).
(p21max=0.07),andthecorrespondingoutflowratechanges
lin-earlywithM1accordingtoEq.(18).
• ThenextphasebeginswhenNchangesfrom20to40Hz,att=40s. Immediatelyafterthisstepchange,thevalueofp21increasesto
remainstable(p21=0.14)aslongasM1ishigherthanMthreshold
forthisnewvalueofN.
• Finally,whenM1becomeslowerthanthethresholdmass,the
probabilityincreasesalmostlinearlywiththehold-upweight, andtheoutflowratechangeswithM1accordingtoasecondorder
polynomialfunction(Eq.(20)).
The resultsobtainedfor thenegative step changein rotational speed,whilenotpresentedherforclarityreasons,alsosupports theparameteridentificationresults,andconfirmsthatthe proba-bilitytoleavethemixerdependsonlyontherotationalspeedand onthecurrentparticlemassinthemixer.Theempiricalcorrelations thathavebeendevelopedinthisworkseemtoberobustenough todescribethedynamicsoftheemptyingprocess,evenwhenit includesastrongvariationontheprocessvariables.Inotherwords, the“history”oftheprocess,doesnotinfluenceitsdynamics,which correspondstotheMarkovproperty.
0 50 100 150 200 250 300 0 200 400 600 800 1000 1200 1400 1600 M1 (g) 10 20 30 40 50 60 70 Qout model: 20_40Hz Qout exp: 20_40Hz N Qout (kg/h) N(Hz)
Fig.13.Simulatedandmeasuredevolutionoftheoutflowratewiththehold-upweightM1duringapositivestepchangeinrotationalspeedN,focussingonthesmallmasses.
5. Concludingremarks
In this study, we have developed a Markovchain approach
torepresentthedynamicsoftheemptyingprocessofa contin-uouspowdermixer.Thechainisonlyheretoprovideageneral frameworkoftheproblem,andincludescorrelationsobtainedfrom experimentaldatafitting.Thetransitionmatrixonlydependson therotationalspeedofthestirrerandthehold-upweight,asthe
chaincaneitherbehomogeneousornon-homogeneousduringthe
emptyingprocess.Allobtainedfunctionalrelationshipsarebased
oninformationgatheredexperimentallyfromcontinuoussystem
operatinginsemi-batchmode,andmayservetothebetter under-standingandmodellingofcontinuousregime.
Animportantindustrialissueisconcernedwiththeeventuality ofstoppingthefeedingsystemduringitsfillingtoavoid pertur-bationduetolow-qualitydosage,lettingthemixeroperateinthe meantime.Fromthepresentstudy,itcanbearguedthat,assoonas theholdupweightisabovethethresholdvalue,thechainis homo-geneousandthemixermaystilloperateadequately,butwitha changedoutflowrate.Butiftheholdupweightpassesthislimit,the chainbecomesnon-homogeneous,andthenon-linearcharacterof theemptyingprocessmaygiverisetosomefluctuationsand per-hapshomogeneitylossofthefinalproduct,astransitionmatrices arechangingateverystep.
Thisworkand themethodologypresented alsosuggest that
simple emptying experimentscan berun to determinetheset
ofgoverningparameters, atleastfor similarmixinggeometries andfree-flowingpowders.Thiswouldsufficetoelaboratea sim-plemodelofglobalbehaviourofacontinuousmixerthatmaybe usefulforaprocessengineer,asafirststep.
Finally, it may be thought that a number of principles lay-ingbehindpowderblendinginsemi-batchmode,orintransitory regimesingeneral,wouldplayanimportantroleinunderstanding andmodellingthesteady-stateoperationofacontinuoussystem anditsprocesscontrol.Futureworkfromourteamwillbe concen-tratedonthispoint.
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