ChemicalhgitzeeringScimce.Vol.42,No.7,pp.1815-1822,1987.
000%2509/87S3.00+0.00 PrintedinGreatBritain.
01987PergamonJournalsLtd. GAS/LIQUIDMASSTRANSFERINSPARGEDAGITATED
SLURRIES H.OGUZ,A.BREHMandW.-D.DECKWER*
FachgebietTechnischeChemiederUniversitatOldenburg,Postfach2503,D-2900Oldenburg,F.R.G.
(Recei ved17 J anuary1986;accepted/orpubl i cati on12 December1986)
Abstract-Volumereferredmasstransfercoefficientsk,aweredeterminedforsixslurrysystemsinan
agitatedvessel ofstandard
configuration.Themeasurementswerecarriedoutunder variation
ofpowerinput, gasflowrate andsolidsconcentration.Thek,adata canbe
fittedwell by correlationsproposedforagitated
gas/liquidtanks.Preferenceis given tothe conceptintroduced by
Zlokarnik(Ado.Bi ochem.Engng8,133, 1979)asbymeansofthis the
experimentalStantonnumberscanbe
describedexcellentlyasfunctionofonly
onedimensionlessgroupwhichinvolvestotalpowerinputandslurryphaseproperties.
INTRODUCTION Multiphasemechanicallyagitatedvesselsarevery
versatileand,hence,widespreadinchemicalindustry
andfermentationtechnologyaswell.Inmultiphase processescarriedoutin
stirredvesselsinterphasemass
transferisoftenanimportantstepandmayberate
determiningfortheoverallprocess.Particularly,in
processeswhereareactivegaseousspeciesisinvolved
knowledgeofliquidmasstransferratescharacterized bythe
volumereferredmasstransfercoefficientsk,ais
neededforreliabledesignThek,avaluesinmechan-
icallyagitatedreactorsdependonvariousparameters
likesystemproperties,vesselandstirrerdimensions
andarrangement,andoperatingconditions. Hydrodynamicsandmass
transfercharacteristicsof
mechanicallyagitatedgas/liquidreactorshavebeen
criticallyreviewedbyJoshietal.(1982)andSteiffand
Weinspach(1982).Gas/liquidmasstransferinstirred
vesselsfortheair-watersystemhasbeenanalyzedby
Judat(1982)whocouldderivereasonableconclusions
forscale-uppurposes.Often,highlyviscousmediaas,
forinstance,dilutedpolymersolutionsareusedto
simulatefluidmediawhichareprocessedinbio-
technology,fermentations,foodandpharmaceutical
industry.Theworkongas/liquidmasstransferin
stirredvesselsapplyingthosemediahas beenreviewed bySchiigerl(1981).
Inslurryreactors,asolidphase(usuallycatalyst)is
presentinadditiontothegaseousandliquidphase. Forslurryreactorsitis
importanttofindouthowthe
presenceofsolidsaffectsgas/liquidmasstransfer.A
particularquestionis whetherthe k,a-correlationsfor
mechanicallyagitatedgas/liquidcontactorscanbe
appliedforslurrysystemsaswell,orunderwhat
conditionsthesecorrelationsmustbemodified.
OnleavefromChemicalEngineeringDepartment,
AnkaraUniversitv.Ankara.Turkev.
*Presentadd&s:GBF-Ge&ehaftfurBioteehno-
log&heForschungmbH.,MaaeheroderWeg1,3300 Braunsehweig,F.R.G.
Severalinvestigationsontheinfluenceofsuspended particlesonk,ain
agitatedvesselswerereportedin the literature(Joostenetal
.,1977;KiirtenandZehner, 1979; Alperetal .,1980; Miyachiet
al.,1981; Steiffand Weinspach,1982; Wiedmann,1982; Albaletal.,1983;
Chapmanetal .,1983;Ledakowiczetal .,1983; GollakatoandGuin,1984;
Iwanakaetal .,1985; Brehm etal.,1984,1985).
AselectionisgiveninTable1. The
solidseffectonk,adependsonparticletype,size,
concentrationandoperatingconditions.Particularly,
atlowparticleconcentrationsdifferenteffectscanbe
observedfordifferingparticles(Alperetal.,1980, Ledakowiczetal
.,1983;Brehmetal .,1984,1985). However,severalstudies(Alperet
al.,1980; Miyachiet al.,1981;Albaletal.,1983,Chapmanetal.,1983)
confirmtheresultsofJoostenetal .(1977)withinert
particles.Theseauthorsreportedthatinthe
He-kerosine-solidsystemk,aincreasesslightlyat low
particleconcentrationsunderotherwiseconstantcon-
ditionsbutdropssharplyiftheviscosityoftheslurry
systemrelativetotheliquid(ps,/pL)exceedsa valueof roughly4.
Correlationswhichaccountproperlyforthe
effectofsolidsonk,ainagitatedslurrysystemshave notbeenreported.
Inthis study,newdataongas/liquidmass transferin
variousaqueousslurrieswerereported.Powerinput
(stirrerspeed),gasflowrateandparticleconcentration
wasvaried.Onlysmallsizedparticlestypicallyof
catalyticslurryreactorswereemployed.Themain
objectivewastoprovidemorecomprehensiveinfor-
mationontheeffectoffinesolidsongas/liquidmass
transferandtochecktheapplicabilityofk,acorre-
lationsdevelopedforagitatedgas/liquidvessels. EXPERIMENTALSETUP
Thestirredtankusedinthisstudywasavesselof standardconfiguration.Its
diameterTwas0.145 m, the totalvolumebeingabout4dm3.Theotherlength
relationsare givenin Fig.1 andwerechosenas givenby
Chapmanetal.(1983).Theimpellerwithfourflat
blades(D=7/2)wasmountedwithanoffbottom 1815 Table 1. Selected
studies on k,a in three-phase mechanically agitated vessels Vessel
Reference G/L/S-system Stirrer CTn) DIT Remarks Joosten el III.
(1977) He/kerosine/glass beads, standard Rushton 0.125 l/3 small
increase of kLa up to 4 of 20 to 30% vol. sugar, polypropylene then
decrease Kiirten and Zehner (1979) air/Na2SOJ soln/&ss 6.blade
turbine 0.33 0.4 reduction of a and gas holdup with increasing
solid beads concentration Alper et al. (1980) Miyachi et al. (1981)
air/water/activated standard Rushton 0.1 114 threefold increase of
kLa with activated carbon, carbon, sand, acrylic beads other solids
little effect OJMVM& two 8-blade 0.133 0.526 little effect,
agreement with Joosten et al. (1977) impellers Steiff and Weinspach
air/Na2S03 soln/glass Cblade turbine 0.2 (1982) beads Albal et al.
(1983) O,/water/glass beads, oil pitched blade 0.1 shale particles
turbine Chapman et al. (1983) O,/water/glass beads standard Rushton
0.56 Gollakota and Guin (1984) 02/Na2S03 soln/silica gas-inducing
powder and sand turbine lwanaka et al. (1985) air/water/glass
heads, vibrating disc 0.1-0.5 PVC, sugar 113 0.45-0.57 l/2 213 0.8
decrease of a at low N, increase of a at high N increase of kLa at
small 4 then decrease agreement with Joosten et al. (1977) a is not
affected significantly, addition of solids increases k,a
significant reduction of k,a Gas/liquid mass transfer in sparged
agitated slurries1817 H= > "G E1928ml Vs,=2285ml Gas out UAL
Gasi n I - T = 0.115m -I Fig.1. Experimentalset up. clearanceof0.3
T. Formeasuringthevolumetricmass PARTICLESYSTEMSSTUDIED
transfercoefficientsk,aunderspargingconditionsaTheslurrysystemsemployedinthisstudyaregiven
ringwith12 holesof0.5 mmdiameterwaslocatedatin Table2.
Theviscositiesofthe slurriesweremeasured thebottomofthevessel.witha
rotationalviscosimeter(Contraves115) at shear
Oxygenmasstransferratesintotheaqueousslurry
systemsweredeterminedbythedynamicmethod.The
slurryphasewasdeoxygenatedbyspargingwithN1.
Thenairflowandagitationwasstartedandthe
transientO2concentrationintheslurryphasewas measuredwitha
polarographicprobe(WTWEO90). Duetothefastresponseoftheprobek,acanbe
determinedfrom in (cz-cL)=-k,at+ In (cr-cLo)_ (1)
Duringtheoxygenabsorptionofni-
trogentakesplaceastheslurryphasewasinitially saturatedwithNJ
.Itwasreportedearlier(Lineket al ., 1981;Brehmetal
.,1985)thatthesimultaneousNa
desorptionleadstolowerk,avalues.Dependingon the
residencetimeofthebubblesthe differencescanbe as largeas 28 %.
Therefore,whenapplyingthe dynamic
methodwithsimultaneousN2desorptionthe k,adata
determinedfromeq.(1)werecorrectedas describedby Brehmetal
.(1985).Allmeasurementsweredoneat
25Cundervariationofgasflowrate(50-2501/b),
stirringspeed(SOO-SOOrpm)andparticleconcen- tration(upto10%byvol.).
Themasstransfercoefficients(k.a)evaluatedfrom
eq.(1)arereferredtotheliquidvolume.Masstransfer
coefficients(k,a)referredtotheslurryvohtmeare obtainedfrom k,a=
k,a(l -4) (2) wherer$ is thesolidsvolumefraction.Inwhatfollows
thek,adataarereportedandcorrelatedthroughout.
Table2.Particlesystems studied at25C Slurry system Sea sand/H,0
Kieselguhr/HZO &G3/HzG %03/&O TiOZ/H20 ZnO/HzO
VolumeViscosityparameters DiameterDensityfractionK (pm) ( 103
kg/m3) 4 ?I(mPa 5)
