Sprout Bauer Fibre Classifier

Bauer-McNettFinerClassifier Model203 INTRODUCTION Atreeismadeoffibresthatdiffergreatly inlength,widthandcoarseness.Thedifferences betweenfibreswithinatreecanbeas greatasthedifferencesbetweenspecies.However thetraditionalapproachofpreparingfibres forpapermakingistopulpthewoodandprocess thefibresasacollective–refiningthe wholepulptoshortentheaveragefibrelength, forexample.Thisapproachsimplifiestheprocess design,butneglectstheopportunitytoexploit theinherentbenefitsoftheindividualfibre fractions. Fibrefractionationisaprocessthatsegregates ablendofpulpfibresintodifferent streamsbasedonsomephysicalpropertyofthe fibres,suchastheirlengthorflexibility.The fibresofaparticulartypecanthenbedirectedto themostappropriateprocessandproduct.A pulpproducerwithtwopulpmachines,forexample, couldfractionatethefibresandincrease thecontentoflongfibresononemachineto provideahigh-valuereinforcingpulp.Alternatively apaperproducerwithamulti-layer headboxcoulddirecttheshorterfibrestothe surfacelayerstoimprovesheetsmoothness, whileplacingthelongerfibresinthecoreto providestrength.Withinthemillsystem,one couldconcentratelong,stifffibresinthefeedto therejectrefinertosaveenergyandincrease capacity,whileavoidingthedegradationof fibresthatarealreadyacceptable. Theopportunitiesforfibrefractionation areclear,buttheuseoffractionationtechnology inindustryremainslimited.Theboardindustry usesfractionationtooptimizethe characterofthedifferentlayers[1–3].Inrecycled fibreproduction,fibrefractionationand subsequentprocessinghavebeenshowntoimprove strengthandbrightness,andtoreduceenergy consumption[4].Howevertheabsenceof equipmentwhichcanprecisely,efficientlyand economicallyfractionatefibreshaslimitedthe widespreadpracticeoffractionationinmilloperations. Thepromiseoffractionationhasbeen established,inpart,throughtheuseoflaboratory fractionationequipmenttoseparateindividual pulpfractionsandformpaperwith optimalfibreblends.TheBauer-McNett classifier(BMC)isthemostwidespreadofthe laboratorydevices,anditiscommonlyusedto assesstheleveloffractionationobtainedwith pressurescreensandotherprocessequipment. TheBMCoperatesbytheselectivepassage offibresthroughascreenmesh.Itseparates fibresmainlyonthebasisoflength, althoughthefundamentalprinciplesofoperation arenotwellunderstood[5–12].Thepractice ofusingthislaboratoryscreentoevaluate pressurescreensraisesseveralimportantquestions: —DoaBMCandapressurescreenoperateusing thesameprincipleofseparation? —TowhatextentdoesaBMCprovideideal, precise,length-basedfractionation? —DoesaBMCprovideinsightsonhowpressure screenscanbeoptimizedforoptimal fractionation? Toanswerthesequestions,thisstudyreviews howfractionationisassessed,andhow indicesoffractionationareappliedtoindustrial pressurescreens.AflowmodelofaBMCis thendeveloped,andexperimentalmeasurements areusedtoassessthevalidityofthe modelandtheleveloffractionationobtained. Finally,theextentoffractionationwithaBMC iscomparedtopublishedvaluesforindustrial pressurescreens. FRACTIONATIONTHEORY Thepassageoffibresthroughscreensis describedbytheirpassageratio(P),whichis theconcentrationofaparticular classoffibres intheflowpassingthroughascreenplateaperture relativetotheconcentrationapproaching theaperture[13].Forthefractionationoffibres onthebasisoflength,onewishestohavethe shortfibrespassingfreelythroughthescreen(P equalto1)whilenolongfibrespassthrough(P equalto0).Itisappropriatetoassessfractionation bymeasuringpassageratioasafunction R.W.Gooding*andJ.A.Olson** Paprican PointeClaire,QC,Canada H9R3J9 *Nowwith: CAEForestrySystems 4635PatriciaAve. Montreal,QC,Canada H4B1Z2 (robertg@caescreenplates.com) **Nowwith: Dept.MechanicalEngin. Univ.BritishColumbia 2324MainMall Vancouver,BC,Canada V6T1Z4 AnanalyticmodeloftheBauer-McNett classifier(BMC)wasderivedandtested.ThestudyshowedthattheBMC classifiesfibresmainlybylength. Overlapinthelengthdistributionof classesreflectsthestatisticalnatureoftheBMCoperationratherthantheinfluenceofsomeotherfibrequality. Thefundamentalqualityoffibrelengthfractionationwassimilartothatfoundinanindustrialpressurescreen. Unmodèleanalytiquedu classificateurBauer-McNett(BMC)aétédérivéetmisàl’essai.L’étudeadémontréqueleBMC classelesfibressurtout selonleurlongueur.Lechevauchementdanslarépartitiondeslongueursdes classesreflètelanaturestatistiquedufonctionnementduBMCplutôtque l’influenced’uneautrequalitédelafibre.Laqualitéfondamentaledufractionnementdelalongueurdesfibresétaitsimilaireàcellequel’onretrouve dansun classeursouspressionindustriel. oflength(P(l)). Thepassageoffibresthroughscreens, andP(l)inparticular,hasbeenstudiedtheoretically andexperimentally[14–16].Acharacteristic curvehasbeenproposedbyOlsonto describethisrelationship[17]*: P(l) e 1 (1) whereandaredimensionlessconstantswith specificsignificance,asshowninFig.1.The valueofcanbeseenasa“shapeconstant”inasmuch asOlsonhasreportedavalueofequal to0.5forslottedpressurescreensandavalueof 1.0forholedscreens.Highervaluesofindicate thatthescreenapproachestheidealformof astepcurve.Thevalueofisa“sizeconstant” andexperimentaltestshaveshownthatlarger holesandslotsleadtolargervaluesof[17,18]. AkeyteachingofFig.1isthat,fora *Whilereference[17]isthefirstpublicappearance ofthisequation,itwasderivedindependently byRobertGoodingand publishedinaninternalPapricanreportin March1992. 0 0.2 0.4 0.6 0.8 1 01234567 Fibrelength(mm) Passageratio =10 1 4 A =0.5 0 0.2 0.4 0.6 0.8 1 01234567 Fibrelength(mm) =10 4 1 B =1.0 4=10 1 C =5.0 Fig.1.CharacteristicformsoftheP(l)curveforarangeofandvalues. Avalueof=0.5(A)istypicalforscreenswithslots;=1.0is typicalforscreenswithholes(B)andhighvaluesof(C)denotenearideal fractionation. Fig.2.PhotographofaBauer-McNett classifier. Fig.3.SchematicofBMCchamberinplanview.Theinletflowisshown enteringfromthetop.Thereisacirculatoryflowwhichisdriven (clockwise)bytherotorattheright.Partofthecirculatoryflowsplits offandpassesthroughthescreenmesh(dashedline)atthebottom. TABLEI BAUER-McNETTCLASSIFIERSPECIFICATIONS Samplesizeofpulp10g(o.d.) Activevolumeofachamber19.8L Standardflow-throughrate12L/min Upstreamvelocity2~2m/s Totalareaofscreenmesh0.033m2 Openareaofscreenmesh14mesh0.014m2 28mesh0.012m2 Aperturevelocity(average)0.02m/s Aperturesize14mesh1.2x1.2mm 28mesh0.6x0.6mm Wirediameter14mesh0.65mm 28mesh0.39mm 1.Thisincludestheadditionalvolumeresultingfromtheheaddriving theflowtothefollowingtank. 2.Estimatedasthetipspeedoftheimpeller. fixedvalueoftheshapeconstant,,smaller valuesofthesizeconstant,,willreducenot onlythepassageoflongfibres,butshortfibres too.Thuspurifyingthestreamofshortfibres throughthescreenplateisdoneattheexpense ofcontaminatingthestreamoflongfibreswith shortmaterial.Increasingthevalueofhowever, leadstoincreasedpassageofshortfibre andreducedpassageoflongfibre.Thusisthe essentialparameterinassessingthequalityof fractionation. ToapplyP(l)totheperformanceofindustrial pressurescreens,oneneedsamodelof howtheflowthroughindividualaperturesrelates totheoverallflowsthroughapressure screen.Apulpscreentakesasinglefeedstream anddividesitintoacceptandrejectstreams. Thequalityoffractionationismeasuredby consideringtheextentthatthelongandshort fibresgototheirtargetstreams,i.e.thatlong fibresgototherejectoutletandtheshortfibres gototheacceptoutlet.Tosimplifytheanalysis, onecanmeasurethefractionofshortfibres leavingviatherejectstreaminsteadofthefraction ofshortfibresgoingtotheacceptstream sincetheacceptandrejectstreamsarecomplementary. Aremovalfunctione(l)hasbeendefined asthemassfractionoffibresoflengthlthat passfromthefeedtorejectoutlet[19].Byassuming plugflowaxiallythroughthescreen, onefindsthat: e(l)RV() Pl (2) whereRVisthevolumetricrejectratio,equalto thevolumetricrejectflowratedividedbyfeed flowrate.Theplugflowassumptionissupported byseveralexperimentalstudies [17,18,20]. Theremovalfunction,e(l),isusedto provideadetailedunderstandingoffibrefractionation inscreening.Itcanalsoberelatedto theparameterscommonlyusedtoassessfractionation inindustrialterms.Theseparameters arethe“longfibreremovalratio”(alsoknown asthe“longfibreremovalefficiency”)andthe “shortfibreremovalratio”(whichisrelatedto themassrejectrate).Thelongfibreremovalefficiency, EL,isdefinedasthemassfractionof longfibres(i.e.fibresgreaterthanthespecified “cut-off”length)thatgofromthefeedtothereject outlet.ThevalueofELcanbeobtainedby usingalabdevicesuchastheBMCtomeasure themassfractionoflongfibresinfeedandreject samples,orbyaveraginge(l)overtherange offibrelengthsabovethe“cut-off”value.Similarly, theshortfibreremovalratio,Es,isobtained byaveraginge(l)overtheshortfibre lengths. Inidealfractionation,EL=100%andES =RV.FollowingfromEq.(2),idealfractionation impliesPL=0andPS=1,whichisconsistent withthenear-idealstep-curverelationship indicatedinFig.1C.Whileitwouldbedesirable tohaveES=0,thelimitingconditionin screeninghasverysmallparticlesfollowingthe flowsplit,whichleadstoES=RV. Fractionationparametershavebeenproposed tocombinetheimpactofELandES.One parameter,hasbeendefinedasthedifference betweenELandES[18]: ELEs(3) Thevalueofisequalto0whenthereisno fractionation,andapproaches1asfractionation increases.Thisparameterisa“valuecharacteristic” forthespecificcasewherethe economicbenefitofincreasedELisequaltothe penaltyofincreasedES.Itisofparticularusefor controlpurposes. Anotherfractionationparameter,is definedintermsofthepassageratiosofthe longandshortfibres: 1 P P L S (4) Thevalueofisa“performancecharacteristic” foraparticularscreeningconfiguration,andits usecomplementsthatof.Assumingaplug flowmodelofflowthroughapulpscreen,EL andESarerelatedasfollows: ELE1S(5) Thevalueofincreasesfromavalueof0, wherePL=PS,EL=ESandthereisnofractionation, toavalueof1foridealfractionation, wherePL=0andEL=1.Animportantattribute ofisthatitisindependentoftherejectratio, whichisthechiefoperatingvariableofascreen. Thusisafundamentalreflectionofthepulp characterandscreenplateconfiguration. Regardlessofwhetheronemonitorsor thekeypointisthatthevaluesofanddetermine PLandPS,whichhaveadirectlinkto ELandESandtheperformanceofindustrial pressurescreens.SincetheBMChasbeenused bothtoassessfibrelength(formeasurementsof EL)andasamodeloffractionation,thereisa keeninterestinassessingthevaluesofand obtainedwiththisdevice. BAUER-McNETTCLASSIFIER TheBMCisalaboratorydevicethatis usedtoassessthesizedistributionoffibres. ThedeviceisshowninFig.2andstandard methodsexistforitsuse[21,22].Itconsistsofa seriesofchambers,typicallyfourorfive,each withaprogressivelysmallerscreenmesh.At thestartofatest,aprescribedamountofpulpis pouredintothefirstchamber.Acontinuous flowofwaterpassesthroughthischamberand theseriesofchambersthatfollow.Smaller fibresarecarriedbytheflowthroughthemesh andtothefollowingchamberor,inthecaseof thefinalchamber,tothesewer.Attheendofthe testperiodtheflowisstopped,thechambersare drained,andthemassineachchamberismeasured. Thusapulpwithsmallerfibreswillhave moremassinthechamberswiththesmaller meshsizes. Forthesakeofsimplicity,theanalysis andexperimentsinthisstudyonlyconsiderthe performanceofasinglechamberintheBMC, thoughtheycouldbeextendedtotheperformance oftheoverallunit.Theelementsofasingle chamberareshownschematicallyinFig.3 andkeyparametersarelistedinTableI.Anindustrial pressurescreenandtheBMCboth causeundersizefibrestopassthroughascreen plate.Theaperturesineachdeviceareroughly thesamesize.However,therearesomeimportant differencesbetweenthesetwodevices.The BMChasanupstreamvelocityofabout2m/s andaperturevelocityof0.02m/s,whileapressure screenhasanupstreamvelocityofabout5 m/sandaperturevelocityof2to4m/s.The maximumconsistencyintheBMCis0.1%versus atypicalfeedconsistencyof1.5–3%ina pressurescreen.TheBMChasascreenmesh whileapressurescreenusesasolidmetalplate withholesorslots.Also,theBMChasanimpeller whichmovestheflowpastthemeshsurface andcausesmixinginthechamberbut, unlikeapressurescreenrotor,doesnotproduce pressurepulsationswhichbackflushthemesh apertures.ThustheBMCapertureflowisrelatively steady,whilethepressurescreenhassubstantial flowreversals. Inatypicalexperiment,theresidual oven-drymasswasmeasuredusingahot-plate technique,andthefibrelengthdistributionwas assessedusingaKajaaniFS-200FibreLength Analyzer,whichmeasuresfibrelengthwitha precisionof0.05mm.Thelength-weightedfibre lengthdistributionwasthenusedtoestimate thetotalmassoffibresinvariouslength classes.Todeterminethepassageratiooffibres ineachlength class,amixedflowmodelwas appliedtothechamber.Thustheamountof fibrespassingthroughthemesh(dM)inaperiod oftime(dt)wasassumedtobeproportional tothevolumetricflowrate(Q),the passageratio(P)andtheinstantaneousconcentration offibresinthechamber(C): dMQPCdt(6) Fibreconcentrationisthemass(M)dividedby thevolumeofthechamber(V).Substitutingthis definitionintoEq.(6)andintegratingfromthe initialmassinstalledinthechamber(M0)at timeequaltozeroyields: M M QPt V 0 e(7) Equation(7)mayberewrittenintermsofthe numberofreplacements(R)thatpassthrough thechamber: M M RP 0 e(8) where“replacements”representthetotalflow throughthechamberdividedbythevolumeof thechamber,andareassumedinthisanalysisto betheessentialfactorindepletingmassinthe chamber: R Qt V (9) Onecanalsore-arrangeEq.(8)toprovidean expressionforP: P R M M 1 0 ln(10) Bymeasuringtheinitialandfinalmassoffibres ineachsize(i.e.length)fraction,Eq.(10)canbe usedtogenerateP(l)fromexperimental measurements. EXPERIMENTALFINDINGS Sevenexperimentswereconductedin thisstudyandtheexperimentalprogramis giveninTableII.Areslushed,softwoodTMP fromtheBritishColumbiainteriorwasusedfor allexperiments.Thepulphadafreenessof200 CSFandwassubjectedtostandardprocedures fordisintegrationandlatencyremoval. DegreeofSegregation Thefirsttestwasmadetoestablishthe qualityoflengthseparationachievedwiththe BMC.AsseeninFig.4,theBMCsortsfibres onthebasisoflengthbutdoesnotprovidea preciseandcompletesegregationofthefibres. Chamberswithlargermeshsizesretainlonger fibres,asexpected.Howevertherangeoffibre lengthsinaparticular classissubstantialand thereissignificantoverlapbetweendifferent classes.Forexample,1.5–2.5mmfibrescomprise oneofthelargest classesoffibrelength withintheR28 classification,butsignificant amountsofthesefibresarealsofoundinthe R14andR48 classes.Theseobservationsare consistentwiththosebyJackson[23]. Figure5presentsthecombinedresults ofTrials1to3fortheR14 class.Oneseesthat certainfibres,suchasthoseinthe3.5–4.5mm class,arealmostcompletelyretained,irrespective ofthetimespentinthechamber.Others, suchasthe1mmfibres,havealmostcompletely passedoutofthechamberinthe20min recommendedoperatingtime.Fibresinthe veryimportant2–3mm classappeartobediminishing inagradualwaysuggestiveofarandom orstatisticalpassage. StatisticalPassage Astatisticalmodelofpassagewasassumed inthemodeloftheBMC.ThisledtoEq. (7),whichhasanexponentialform.Figures5to 7showdatapointsthatreflectthedecayinthe massoffibresfromparticular classesasafunction oftime.AlsoshowninFig.6arecurvesfit tothedatausingtheexponentialformofEq. (7).Thegoodfitprovidessupportforthestatistical modeloftheBMC. Thestatisticalmodelpresentedabove prescribesthatthediminutionofmassisdetermined bythenumberofreplacementsthatpass throughthechamber(Eq.9).Thiswasinvestigated byalsochangingtheflowratethroughthe BMCinTests4and5.Figure7includesthe datafromthechangingflowrateaswellasthe datainFig.6.Theresidualmassfractionis TABLEII EXPERIMENTALPROGRAM TestNo.MeshFlowRateDuration 1234567 All1 14 14 14 14 14 28 12L/min 12 12 15 9 12 12 20min 10 5 10 10 10 10 FibreLength(mm) Massdensity(g/mm) 0123456 0 1 2 3 4 5 Total R14 R28 R48 R100 R200 Fig.4.Fibrelengthdistribution(massdensity)ofpulpfromindividual BMC classes. FibreLength(mm) Massdensity(g/mm)0 1 2 3 4 5 6 0 1 2 3 4 5 INITIAL 5min 10min 10min(replicate) 20min Fig.5.Themassdensity–fibrelengthdistributionintheR14chamber asafunctionoftime.Notetheexcellentreproducibilityofresultsfor thereplicatetrialsof10minduration. Time(min) Massfraction 0.005101520 0.2 0.4 0.6 0.8 1.0 0-1mm 1-2mm 2-3mm 3-4mm Fig.6.ExponentialdecayofmassintheR14containerfordifferentfibre lengthranges.Massisexpressedasafractionoftheinitialmass foreachfibrelength.Replicatemeasurements(at10min)appearalmost coincident.ThefitcurvesusetheformofEq.(7). Replacements Massfraction 0510152025 0.0 0.2 0.4 0.6 0.8 1.0 0-1mm 1-2mm 2-3mm 3-4mm Fig.7.ExponentialdecayoffibremassintheR14 classasafunction ofreplacementvolumes.FitcurvesareoftheformofEq.(8).Datawere obtainedbyvaryingboththedurationofthetrialandtheflowrate. plottedasafunctionofreplacementsrather thantime.Theadditionaldatafittheexponential formofthefitcurveswell. P(l)Curves ThedatainFigs.5to7supporttheuseof astatistical/perfect-mixingmodelfortheflow offibresfromtheBMC.ItfollowsthatEq.(10) canbeusedtoprovidetheP(l)curvewhich characterizesthequalityoffractionationobtained inaBMC.ThesedataareshowninFig. 8,whichincludesacurveoftheformofEq.(1). Theexcellentmatchbetweenthetrendsinthe dataandthecurveformsupportstheapplication ofEq.(1)(proposedforindustrialpressure screens)totheBMCandsuggeststhatitmay haveageneralapplicabilitytopulpscreening devices. Thevalueofobtainedbyaleastsquares fitwas0.97,whichisveryclosetothe valueof1.0proposedbyOlsonetal.[17]for pressurescreenswithholes.Itisnotoverlysurprising thatasquarehole(intheBMCmesh) hadthesameshapeconstant(asaroundhole inthepressurescreen.Morecuriousisthatthe absenceofpressurepulsationsdidnothavean effectonHowever,qualitativeinspectionof theflowpatternswithintheBMCsuggestssubstantial mixingintheflowadjacenttothemesh, whichmightbeproducingthesameeffectas thepulses.Avalueofequalto0.82wasobtained fortheR14mesh(with1.2mmsquare openings)which,comparedtothevaluesreported byOlsonetal.[17]forpressurescreens, suggeststhattheaperturesintheBMCactlike smallerholesthaninapressurescreen. P(l)datafortheR28BMC classare showninFig.9alongwiththecurvefitusing theinitialpulpchargewasplaceddirectlyinthe R28chamberatthestartofthistestandtheR14 chamberwasnotused.Theexcellentmatchof thedatatothecurveformgivesfurthersupport totheuseofEq.(1)forcharacterizingfractionation. Moreoverthegoodfitmadeusing indicatesthefundamentalqualityoffractionation didnotchangewiththesizeoftheaperture. Thelowervalueofisconsistentwith previousfindingsforpressurescreensthatindicate lowervaluesofforsmallerscreenapertures. CONCLUSIONS Theconclusionsofthisstudycanbe statedinthecontextofquestionsposedatthe startofthisreport. DoaBMCandapressurescreenoperate usingthesameprincipleofoperation? Despitethesignificantdifferencesinthe overallconfigurationandoperationofaBMC andpressurescreen,theessentialmechanism thatgovernsfibrepassageatanapertureappears quitesimilar.Thisstudydemonstrated thatthemixed-flowmodelworkswelltodescribe theperformanceoftheBMC,andtheexponential modeloffractionationfitstheP(l) fractionationcurveverywell.Likeanindustrial pressurescreen,theBMCoperatesby“probability screening”.Thisraisesquestionsabout theuseofoneprobabilityscreen(i.e.theBMC) toevaluateanother,suchasanindustrialpressure screen.However,itestablishesthatthe BMCmaybeusedtobetterunderstandthefundamentals ofscreenoperation. TowhatextentdoesaBMCprovideideal, precise,length-basedfractionation? Thehighdegreeofdilutionandlowreject ratiointheBMCprovidesgoodsegregation ofcertainsizedfibres,butthereare substantialrangesoffibresthatareonlypartly segregatedintheBMC.Thismitigatesagainst theuseoftheBMCasanabsolutemeasurement offibrelengths.Thedegreeoffractionation,i.e. thevaluesfortheshapeconstant()andsize constant(),iscomparabletothatfoundinan industrialpressurescreen. DoesaBMCprovideinsightsonhow pressurescreenscanbeoptimizedfor optimalfractionation? Thefindingthatforthesquaremesh aperturesintheBMCisthesameasthatfora pressurescreenwithholesreinforcesthe thoughtthatrepresentsashapeconstantfor screenoperation.Sincefractionationinthe BMCandinpressurescreensoperatesbyprobability screening,theshapefactormayrepresent thedegreeofrestraintthattheaperture imposesonthesomewhatrandomwaythat fibresapproachanaperture.Aholeimposesa higherdegreeofconstraintthanaslotbecause, foraslot,longfibreshavethepotentialofbeing alignedwiththeslotlengthandpassing through.Idealfractionation(i.e.highvaluesof wouldappeartorequireamechanismforapplying furtherdegreesofconstraint.Increasing thedegreeofconstraintintheBMCorsome otherlab-scalefractionationtechnique,which canbemodifiedrelativelyeasilyandoperated undercontrolledconditions,maybeauseful steptowardsthelargergoalofincreasingfractionation onanindustrialscale. 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