Effect of the impeller shape on the performance of the mixer-type disperser

Effect of the impeller shape on the performance of the mixer-typedisperser KUNIAKI HITOSHI ASAOKA andHIROAKI MASUDA GOTOH, 606-01, Yoshida-honmachi, University, Sakyo-ku, Kyoto Department ofChemical Engineering, Kyoto Japan 13May 1994 Received forAPT hasbeen studied with ofthemixer-type Abstract-The disperser experimentally particular performance showed andtheblade The results attention totheeffect oftheimpeller position angle. experimental andnarrow were better forattaining The therelatively impeller good performance. higher position a noncorrelated introduced waswell variation ofdispersion bya newly parameter, efficiency theamount ofrecirculating flow dimensional residence time. The non-dimensional parameter represents toanon-dimensional airflow which isdefined inthevessel, because itwas rate, inversely proportional from theimpeller. Itwas ratetotheflow rateexhausted ofthedischarged airflow bytheratio hasnorecirculating flow. Asfortheeffect oftheangle, theblade concluded thatthebest disperser and as a result of no short cut to the bottom attained the best particles perpendicular performance norecirculating particles. NOMENCLATURE B width [mm] impeller and thebottom ofvessel clearance between C [mm] impeller vessel diameter D [mm] tube diameter [mm] Din inlet tube diameter outlet [mm] Do diameter d [mm] impeller distribution ofresidence time obtained 0 #0Is-'] '] frequency f time obtained 0=0Is-'] distribution ofresidence '] frequency f vessel H height [mm] effective (=H [ mm] height H,) H. inlet height [mm] Hi oftheoutlet tube [mm] position Ht inEq.3(=2)[-] coefficient k tube andtheimpeller between thelower L tipoftheinlet [mm] space rotational n speed [rad/s] impeller f low rate air discharged [m3/s] Qs exhausted airflow rate (= mr2d2B 0/2) [m3/s] cos bytheimpeller Qim non-dimensional rate(= Q.IQim) airflow I-] Ri short cutratio defined byEq.6[-] Rs defined ratio byEq.7[-] recirculating R, ofresidence time distribution time oftheintersecting [s] point 11,12,13,14 V volume ofvessel [m3] mass flow rate[g/min] W powder time ofaparticle mean residence f [s] residence IS] apparent time (=VlQa) sa time non-dimensional residence [-] ( = T/Ta) Tt blade 0 angle [deg]

354 1.INTRODUCTION oftheperformances ofthreedifferent ofdisperser, i.e.themixerComparison types andanarrow-tube theadvantages ofthemixertype,afluidized-type type,revealed wasthemostconvenient fortheconcentration control typedisperser-the disperser ofdispersed particles [1]. Astable withparticular attention tothe operation rangewasalsoinvestigated stateofthedischarged aerosol andthepowder inthevessel layerdeposited [2].The showed thatthefullydispersed aerosol wasobtained intheoperating study region, andnodeposition orspeckled of satisfying homogeneous discharging deposition These conditions wereachieved when theimpeller rotational was particles. speed maintained andpowder massflowratewaslower thanthespeckled higher depositionlimit. Ontheotherhand, thedispersion mechanism ofthemixer-type hadbeen disperser studied andtheoretically, where itwasconfirmed that experimentally experimentally thedispersibility ofagglomerated bothonthesizeoftheprimary particles depended andthemorphological structure of agglomerated particles particles [3].These results were wellexplained model thedispersion force bya dispersion [4]inwhich caused offluidmotion isassumed. itisexpected that byanacceleration Therefore, themixer-type which canaccelerate disperser, agglomerated particles strongly, havea highperformance. might Theairflowandparticle movement inthedisperser areexpected tobestrongly affected ofthedisperser. Westudied theeffect ofthevessel bytheconfiguration andtheinlettubelength onthedisperser height performance [5].Theexperimental results revealed thatboththedouble-walled section formed andthe bythevessel inlettube,andthespace between theinlettubeandtheimpeller caused a deteriorationofthedispersion weconcluded thattheparticle should Therefore, performance. fromtheinlettotheimpeller without a shortcutandgooutassoonasposgodown sibleafterpassing theimpeller. through From these theconfiguration ofthedisperser wasimproved. Thenewdisresults, hasa small vessel andtheinletposition issetaslowaspossible. Thisstudy perser aimed to improve theperformance theshapeandposition of the bychanging impeller. 2.EXPERIMENTAL APPARATUS AND PROCEDURE Anewly isshown inFig.1andthedimensions are designed mixer-type disperser listed inTable 1.Apaddle-type ofavessel impeller (4)wassetatthebottom (1)and rotated motor. Itbrings about ahighspeed airflow, resultbyahighspeed rotating atthecentral oftheimpeller. airandapowder inginnegative pressure region Then, aresucked intothevessel an inlettube(2)bythenegative through pressure. aredispersed atthevicinity oftheimpeller anddischarged Agglomerated particles fromanoutlet tube(3). Inourstudy thatthespace Lbetween theinlettubeandthe [5],itwasconfirmed should besetasshortaspossible. Inthisdisperser, thespace is impeller however, restricted atthetopoftheimpeller shaft.Therefore, theinletposibyanutlocated tionH;wassetat theheight of50 mm. Ontheotherhand,theoutlettubewas

355

1.Improved mixer-type disperser. Figure Table 1 List ofdisperser dimensions (inmm)

tubewasset asthelower sideoftheinlettubeandtheoutlet setatthesamelevel H is 65 mm andthe 15 mm fromthetopofthevessel. Thus,thevessel height effective height He( =H;)is50 mm. Allimpellers 2were tested inthisstudy. listed inTable Several ofimpellers types inFig.2. asshown areofthepaddle-type andsomeofthemhavetiltedblades, 1-5havea bladeangle 0 =0 andeachofthemhasa different impeller Impellers Cis oneofthetwo inFig.1.Clearance widthB and/ora clearance C,shown L of theimpeller. Theotheris thespace theposition parameters representing which is between thelower tipoftheinlettubeandtheuppertipoftheimpeller,

356 Table 2 dimensions Impeller

2.Definition ofblade Figure angle. Band/ortheclearance theimpeller width C.Thespace Lwas changed byadjusting inimpellers setconstant 1-3.Ontheotherhand,theimpeller width Binimpellers 3-5 was setconstant. Asforimpellers C andspace L werekeptconstant. 6-12,widthB,clearance theseimpellers havedifferent bladeangles 0.Theimpeller with0 >0 However, theairupwards, while theimpeller with0 <0discharges downwards. discharges Performance testsof dispersers werecarried outbythesameapparatus and asthepreceding conditions arelisted inTable 3. procedures study [5].Experimental Thetested wasFly-ash rotational waskept powder (JISno.10).Theimpeller speed constant atthemaximum value ofthisdisperser andthepowder massflowrate was intherangeof24-60 oftheparticle residence changed g/min.Themeasurement timewasalsocarried outbythesame asinthepreceding procedure study[5]. Table 3. conditions Experimental

357 AND DISCUSSIONS 3.RESULTS 3.1.Effect position ofimpeller obtained 3shows theexperimentally efficiency p.Thedispersion dispersion Figure = 100% thefullydispersed stateofthedischarged aerosol efficiency fl represents oneofthetested coincides withthefullydispersed whose sizedistribution particle sedimentation method obtained [6]. bya centrifugal powder C oftheclearance asa function thedispersion efficiency Figure 3(a)shows width B.Thedispersion flowrateWandconstant under constant impeller powder

with different ofthedisperser. 3.Variation ofdispersion configurations efficiency Figure

358 withtheclearance C.These results showthehigher efficiency p increases setting oftheimpeller isbetterforattaining betterdispersion asfarasthewidth position Bisconstant. Ontheotherhand,Fig.3(b)shows thedispersion asa funcefficiency tionoftheimpeller width under constant flowrateandconstant L.Thedisspace withincreasing thewidth shows thatthe persion efficiency /3decreases B,which intheexperimental narrower isbetter impeller range. oftheclearance Candthewidth affects Thevariation B notonlythedispersion butalsothemean timeoftheparticles residence f andthedischarged air efficiency, flow rateQameasured Seiki asshown inFig.4. Co.,DC-5) byagasmeter (Shinagawa

4.Mean residence time and airflow rate asafunction ofdisperser Figure configurations.

359 constant width airflow Withanincrease intheclearance Cunder B,thedischarged timeofparticles decreases. rateincreases andthemeanresidence with thedischarged airflowrateincreases Asfortheeffect oftheimpeller width, width. Theincrement ofQais,however, smaller thanthat theimpeller increasing airflowrateexhausted fromanimpeller, i.e.theairflow expected bythetheoretical isonlyabout1.25times ofthatof B= 10 mm. Themean rateQaofB = 20 mm Thisfact theairflowrateincreases. residence timeisalmost constant, although inthevessel waschanged withtheimpeller width thattheairflowpattern suggests wasalsoaffected. andtheparticle movement airflowrateRiwasdefined asaratioofthedischarged thenon-dimensional Here, theoretical airflowrateQ;m*; airflowrateQatotheexhausted isdischarged from thatalloftheairexhausted bytheimpeller where =R;1represents inthedisintheflowrecirculating thedisperser. Adecrease inR;means anincrease residence timeT,wasdefined Ontheotherhand,thenon-dimensional bya perser. timeaa(= V / Qa) timet totheapparent residence ratioofthemeasured mean residence asfollows; thatthemeasured timelargerthanunitymeans A non-dimensional residence airflowrate. timeislonger thanthatexpected residence bythedischarged airflowrateR;and between thenon-dimensional 5shows thecorrelation Figure timerr.Thedataforthevarious bladeangles are residence thenon-dimensional lineona log-log included inFig.5.Alldataarerepresented bya straight paper defined theproduct ofthevolume ofrotating theoretical airflow rate was * The exhausted impeller by cos e/2. xBcos 0=nn2d2 B andtherotational speed: Qi. 2nn = x7rd2/4

residence time. rateandnon-dimensional 5.Correlation between non-dimensional airflow Figure

360 width andtheangle. Theslope isabout - 1. oftheimpeller position, independently residence timeisinversely to thatthenon-dimensional Thisfactshows proportional flowrate.Then,thefollowing was thenon-dimensional experimental equation obtained; where, = 1,thefollowing relation isalsoobtained from Ontheotherhand,when R; ofT,andR; : onthedefinition Eq.(3)based timeofparticles istwice thatofthe thatthemeanresidence (5)shows Equation residence timeofairinthedisperser when therecirculating flowdoesnot average thattheparticle inthevessel istwice aslongas exist. Thisfactsuggests trajectory theaverage airpath. 6 shows thecorrelation between non-dimensional residence timeandthe Figure Thedataunder constant flowratearerepresented efficiency. powder by dispersion alineindependently oftheclearance andthewidth. Thisfactsuggests thatthevariaisalmost dueto thedifference in thenontionof thedispersion efficiency dimensional residence time.Thenon-dimensional residence timecorresponds tothe flowrateasmentioned above. itcanbeconcluded thatthe Therefore, recirculating inFigs.3and6 iscaused variation ofthedispersion shown efficiency bythereflow. Itisexpected based ontheprevious circulating report [5]thattherecirculating flowcauses ofthedispersed theshorter the Therefore, re-agglomeration particles. non-dimensional residence thedispersion time,thehigher efficiency expected. thattheextended line(dotted lineinFig.6)intersected Itwasalsofound theline atthepointofT,= 2.Thus,it isexpected thatthedisperser which of,8= 100% satisfies a fullydispersed aerosol aslongasthepowder flow -r,= 2candischarge

6.Dispersion asafunction ofnon-dimensional residence time. efficiency Figure

361 theadheThissupposition isrealized when theexperimental rateiskeptbelow range. JISno.10(adhesion ofpowder islessthanthatofFly-ash sioncharacteristic value characteristic value= 1.6[Pa],[6]). 3.2.Effect ofimpeller angle asa function ofthebladeangle 9.These 7shows the dispersion efficiency Figure forW= 24g/minisattained show thatthemaximum results dispersion efficiency atboth0 = -15°to -30°and efficiencies taketheminimum at0 = 0°.Dispersion ofthedisthevariation flowrateW= 40g/min, 0 = 15°.Asforthehigher powder attained therelatively thesmaller issmall. However, higher angle persion efficiency 0 = 0°isthebestforthedisperser theblade Therefore, angle efficiency. dispersion performance. value atthe thedispersion hasapeakat0 =0°andminimum efficiency Although ofthenon-dimensional resito - 30'and0= 15°,thevariation of0 = - 15' vicinity thevariation ofthenoninFig.8.Therefore, dence timeisquite asshown different, ofthe 0cannot thetendency timewiththeblade dimensional residence explain angle timedistributions inFig.7.Then,theresidence variation ofdispersion efficiency

ofblade 7.Dispersion asafunction angle. efficiency Figure

ofblade residence time asafunction 8.Non-dimensional angle. Figure

362 for0 = -15°and -45° arecompared withthatfor0 = 0°asshown inFig.9.Each Alloftheresidence sothattheintegrated value takes ofthedataisnormalized unity. timedistributions havea sharppeak.It canbefoundthattheresidence time of 0 = - 15°hasa peakjustbefore thepeakfor0=0° andthe distribution afterabout 0.2sislonger thanthecorresponding value for0 = 0°.Asfor frequency 0 = -45°,themaximum islarger thanthatfor0=0°,although thepeak frequency timeisalmost thesame. Thedistribution of0 = -45°hasa second peakaround inthelargefrequency 0.2s,resulting afterthattime. thesedifferences ofdistributions Soastoevaluate theshortcut quantitatively, ratioR,andtherecirculating ratioRrwere introduced. tothe R,andRrareequal hatched areainFig.9,andexpressed bythefollowing equation:

- Figure 9.Comparison ofresidence time distributions.

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10.Short cutratio andrecirculating ratio asafunction ofblade Figure angle. where timedistribution forB= 0°which attained thehighest foistheresidence several bladeangles. thetime dispersion efficiency among represent t1,tz t3 , and t4 oftheintersecting inthisorder. pointofthetwodistributions 10shows theshortcutratioR,andtherecirculating ratioRrasa function Figure oftheblade 0.Asfortherecirculating ratioRr,itincreases withincreasing the angle absolute value oftheangle 0.Ontheotherhand,theshortcutratio7?sshows a correnegative peakat0 = -15°andasmall negative peakat0 =15°.These angles tothose theminimum inFig.7.Therefore, itis spond giving dispersion efficiency considered thattheminimum efficiencies around 0 = - 150to -30°and dispersion 15°were caused where mainly bytheshortcutofparticles, agglomerated particles were without thedispersion action ofthese discharged experiencing [5].Asa result themaximum wasobtained at0 =0°,where boththe effects, dispersion efficiency shortcutparticles andtherecirculating weremaintained attheminimum particles amounts. 4.CONCLUSIONS Theperformance withparticular ofthemixer-type hasbeenstudied attendisperser tionto theeffectoftheimpeller andthebladeangle. Themainresults position obtained aresummarized asfollows: forattaining oftheimpeller isbetter better (1)Thehigher setting position disperser aslongastheimpeller width isconstant. Awideimpeller isnot performance thespace theimpeller andthebottom ofthevessel because between preferable, becomes toonarrow. residence soastorepresent theeffect of time,introduced (2)Thenon-dimensional tothenon-dimensional airflow flow,wasinversely recirculating proportional means thatthelonger timeresults residence fromthelargeamount rate,which oftherecirculating flow.

364 theaverage residence timeofair residence timeofparticles istwice (3) Themean flowdoesnotexist. aslongasrecirculating ofthedispersion mentioned in(1)iscaused efficiency bythe (4) Thevariation residence variation ofthenon-dimensional time,i.e.themaincauseofthe deterioration is attributable to therecirculating flow.Thebest efficiency flowinthevessel. hasnorecirculating disperser themaximum is (5) Asfortheeffectofthebladeangle, dispersion efficiency at0= 0°.However, thedispersion efficiencies tooktheminimum at attained lowpowder flowrate. both0 = - 15° to -30°and0 = 15°,under relatively Itwasfound theshortcutratioandtherecirculating ratiothat byintroducing ofthedispersion wasmainly caused theminimum efficiency bytheshortcutof inthelarge theparticles, andthedeterioration ofthedispersion efficiency flow. ofthese wasmainly caused Asa result angle region bytherecirculating themaximum wasobtained at 0=0°, where effects, dispersion efficiency oftheshortcutparticles andtherecirculating werekeptto amounts particles a minimum. REFERENCES 1.K.Iinoya and H.Masuda, K.Willeke Ann Arbor: Ann Arbor Generation Sci. Pub. of Aerosols, (ed.). 189-202. Inc., 1980, 2.H.Masuda, S.Kawaguchi and K.Gotoh, Effect ofpowder feed rate and rotational ofimpeller speed Powder ontheperformance ofamixer-type J.Soc. 1990. Technol., 27,515-519, disperser. Japan 3.K.Gotoh, M.Takahashi and H.Masuda, The mechanism ofamixer-type J.Soc. dispersion disperser. Powder 1992. Technol., 29,11-17, Japan 4.Y.Kousaka, K.Okuyama, A.Shimizu and T.Yoshida, mechanism ofaggregate Dispersion particles inair.J.Chem. 1979. Eng. Japan 12,152-159, 5.K.Gotoh, T.Yoshida andH.Masuda, oftheMixer-type Power Adv. Improvement Disperser. Powder inpress. Technol., 6.H.Masuda and K.Gotoh, Performance evaluation ofdry J.Soc. Powder dispersers. Technol., Japan 1993. 30, 703-708,