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Noise emission of suspended particles in an agitated environment
Noiseemissionof suspendedparticles in an agitated environment W.GUINTO, T.HIRAJIMA*, M.TSUNEKAWA, K.TADANO and I. NAKAJIMA Hokkaido Resources Kita-ku, Department ofMineral Development Engineering, University, 060, Sapporo Japan 1992 Received forAPT 12June 25August 1992; accepted Abstract-The mechanism ofnoise theagitation ofasuspension was generation during investigated. inaparaffin-type Glass beads with known diameters andofknown number were suspended organic low and noises emitted theagitation ofthesuspension The thehigh liquid. frequency frequency during were characterized and correlated with thenumber and size ofthebeads. Low noise measured, frequency contained mechanical andhydraulic noises thatinterfered with themonitoring ofthenoise signals of In noise exhibited characteristic contrast, signature suspended particles. high frequencysignals Anincrease inthesize relative tothesize andnumber ofsuspended orinthenumber particles. changes ofbeads oscillation relative andfrequency increased theevent rate, rate, rate, energy energy power Thefundamental ofvibration were estimated. Theorder ofmagnitude ofthe spectra. frequencies calculated ofvibration from bead-wall and bead-bead was ingood frequencies impacts agreement arising with thatofthemeasured Itisconcluded thatthehigh noises emitted the frequencies. frequency during ofasuspension model aremainly andbead-bead particles generated bybead-wall agitation containing Anempirical andthediameter andnumber ofthe collisions. relation between thenoise relative energy beads isalso derived. 1.INTRODUCTION Several haveusednoise oracoustical forthedetermination of investigators signals inpowder sizeormonitoring some Leach etal.[1] particle processes technology. utilized thesound fromtheelastic collision ofrigidparticles fordeteroriginating Thefrequencies inthe theparticle sizeandsizedistribution. ofinterest were mining theapplication of50-200 kHz.Wakimoto etal.[2]reported ofnoise measurerange mentformonitoring thecoating ofpharmaceutical usedthe They process products. todescribe thedegree ofcoating. results oflowfrequency The amplitude analysis wasusedbyHidaka al. et [3]to monitor the acoustic emission (AE)technique theAEcountrateandhigh of a powder bed.Theyrelated compaction process to powder andcompression of theAEwaves properties frequency components thatdistinct noises areemitted conditions. Most theauthors demonstrated recently, inliquid thatthechanges theprocess ofagglomeration [4].Itwassuggested during inthenoises ofagglomerate ortheproperties of emitted aremanifestations growth In particular, thevariations in thenoiseemission exhibited theagglomerates. andnumber. themechanism totheagglomerate diameter of However, sensitivity of agglomeration is notwellunderstood. noisegeneration duringtheprocess * To whom should beaddressed. correspondence
144 been havegenerally ofnoisegeneration Fundamental studies onthemechanism ofballona plateor thenoiseemitted bytheimpact byanalyzing performed aboutthenoise no literature is available ball-ball collisions [5-9].However, inavessel. inliquid andagitated number ofparticles suspended generated byalarge ofgeneration ofthesenoises themechanism toclarify itisnecessary Consequently, in theprocess ofagglomeration thistechnique formonitoring inordertoutilize andhighfrequency noises emitted thelowfrequency Inthisrespect, by liquid. inveswereexperimentally environment model inanagitated suspended particles thephenomenon ofnoise emission andfullyunderstand inordertoverify tigated inliquid. theprocess ofagglomeration during 2.EXPERIMENTAL 2. IMaterials . Some wasanaliphatic Thesuspending containing isoparaffin. hydrocarbon liquid 750kg/m3, interfacial of thisorganic liquidare;density: important properties with 7-12.Highly beads andcarbon number: tension: 43.01 spherical glass mN/m, wereused.The a density of2500 kg/m3, andmeansizesof255,412and755 um off-size andensure sieved toremove beads were sharpsizedistribution. particles 2.2.Equipment theglass thesuspension usedforagitating 2.2. I.Agitator. Thedevice containing ofceramic which wasdesigned fortheproduction wasa horizontal beads agitator inliquid [4].Ashaftfittedwiththreefour-bladed microspheres byagglomeration Theinternal volume ofthe atthehorizontal axisofthevessel. waslocated impellers was3000 cm3. oftheshaft-impeller vessel minus thevolume assembly cylindrical steel. The fromstainless Thevessel consisted oflower andupper partsconstructed toprevent andtocontrol witha cooling lower wasfitted overheating jacket portion filled withtheorganic Thevessel wascompletely thetemperature during agitation. a specific number ofbeads wascarried ofsuspensions Agitation containing liquid. to form wassimilar to therotational outat 1840 speedneeded r.p.m.which withgoodproperties [4]. agglomerates noiseemission Thelowfrequency 2.2.2.Lowfrequency system. monitoring were detected inFig.1.Noise isillustrated bya transducer signals system acquisition holder. Thetransducer was totheexternal wallofthevessel attached bya magnetic witha sensitivity of3.14 pC/go Endevco anaccelerometer (7701-100, Corp.) (gois of7-22kHz± 5dB.The acceleration dueto gravity) anda frequency response of100 mV/go ina charge wasamplified fromtheaccelerometer bya factor signal filter androuted toa programmable Endevco bandpass Corp.) (2721B, amplifier
emission 1.Schematic ofthelow noise frequency acquisition system. diagram Figure
145 NFCorp.). andthelowpassfilter Thehighpassfilterwassetat 100 Hz (FV-665, ina cassette wasfixed at30kHz.Thefiltered wasstored signal taperecorder (TCwasalsotransmitted Thefiltered toa trigger D5M, SonyCorp.). signal generator Ifthesignal NFCorp.). exceeded a setthreshold thetrigger level, (9622, generator released aneventpulsewhich wasincremented ina universal counter (TR5822, Advantest Off-line oftherecorded wascarried out Corp.). frequency analysis signal ina fastFourier transform Iwatsu (FFT) analyzer (SM-2701, Corp.). 2.2.3.Highfrequency Thediagram ofthehighfrequency monitoring system. acoustic emission is shown in Fig.2. Sound emissions were acquisition system NFCorp.) detected fixed atthesideofthevessel. The byasensor (AE-900S-WB, sensor wasa widebandpiezoelectric transducer witha ±10 dB overthe response silicon-rubber frequency range100kHzto2MHz.A hightemperature gluewas usedto attachthesensor to a flange outside thevessel. TheAEsignal fromthe sensor wasprocessed inanAEtester(9501, NFCorp.)anda disseparately criminator NFCorp.). TheAEtester isa compact version ofthe (AE-922, portable andwasusedas a backup discriminator Thedifferent noisesignal system. characterization formats ofthediscriminator areshown inFig.3.Thesignal from thesensor wasamplified of40dBinapreamplifier NFCorp.), byafactor (AE-912, before tothediscriminator. Inthediscriminator, thepreamplified inputting signal isfiltered a variable andlowpassfilter(LPF). The through highpassfilter(HPF) HPFwassetat 100kHzandtheLPFwasnotused.Thefiltered signalwas attenuated andamplified tothelevel tothegainsetting. Thegainsetting according ofthediscriminator could bevaried from-20to60dB.Intheseexperiments the were Theamplified andfiltered i.e.RF,was 20,10and0 dB. gainsettings signal, routed toanenvelope Theriserateoftheenvelope was10V///s processor. processor andthetimeconstant whenfalling was100,us. Theenvelope, a fluctuating d.c. wasrouted toanaverager circuit andtohighlevel,V ,andlowlevel,VL, output, Theoutput fromtheaverager circuit iscalled therelative or comparators. energy,
2.Schematic ofthehigh noise emission Figure diagram frequency acquisition system.
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formats ofthediscriminator. characterization 3.Noise Figure signal whenever theenvelope exceeds thehigh Inthecomparator circuits, Enofthesignal. unituntilthe control discrimination levelVH, anevent isreleased bya digital signal levelYLThe were downtothelowdiscrimination decays . eventsignals envelope asevent rateand NFCorp.) andformatted incremented indigital counters (AE-932, eventtotal.If the periodof oneeventT, failedto satisfythe condition aftertheevent isreleased immediately Tj < T.< Tx , a nogood(NG)signal In todisregard/cancel theevent TheNGsignal isa cuetothecounter signal. signal. 3.5V,3.4V,10 ps and100ms,respecandTMAX were ourexperiments VH, VL, TMIN where itwasconwasalsorouted toa zero-cross TheRFsignal comparator tively. the wereoutputted Theoscillation verted tooscillation pulses onlyduring pulses. cominanevent, ofcycles itindicates thenumber duration ofanevent Thus, signal. wereincremented ina digital Oscillation asringdown counts. known pulses monly rate(pulse/s). andformatted astheoscillation counter anenergy wasmeasured content oftheRFsignal Theenergy byusing processor andgenerates thesignal NFCorp.). Theenergy pulse squares processor (AE-972, to theenergy of thesignal. Eachpulseis trainswitha frequency proportional were forvoltpeak.These to6.25x 10-6V2s,pwhere pulses Vpstands equivalent The andformatted astheenergy rate(pulse/s). counter transmitted to a digital ofthefour andtheoutputs fromthediscriminator andrelative energy envelope routed rateandenergy i.e.theevent rate,were total,oscillation rate,event counters, inthedatalogger converter toananalog/digital computer. (A/D)andstored were oftheemitted waveforms Foranoff-line noises, digitized frequency analysis NFCorp.). Thedatalength was ina wave andtemporarily stored memory (9620, timewas1ms timewas500ns.Thus,thesweep andthesampling twokilowords of1MHz.Datafromthewave measurable witha maximum memory frequency in floppy disks.A FFT to a personal andrecorded weretransferred computer forthepower wasprepared spectrum analysis. program 3.RESULTS AND DISCUSSION 3.1.Low frequency thevessel is inside thenumber ofbeads rateversus Theplotoftheevent placed tothenumber ofbeads rateappears tobeinsensitive inFig.4.Theevent shown
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thenumber of255 and755 beads noises 4.Event rateoflow pm in against frequency plotted Figure suspension. inFig.5alsoreveal that FFTanalysis results inthevessel. depicted Typical present inthe ofbeads ofthenoise isnotaffected thepower bythenumber signal spectrum inFig.5isthatofthesignal obtained Thefirstspectrum inthez-axis). vessel (shown This without media wasagitated whenonlythesuspending glassbeadspresent. andhydraulic noises. Thefactthatthe tomechanical canbeattributed spectrum didnotexhibit obtained forsuspensions ofthesignals any subsequent power spectra of thenoises thatthelowfrequency increase norchange components suggests suchas areoverwhelmed emitted byothersources particles bythesuspended when waveforms ofthenoise noises. andmechanical only signals Typical hydraulic were of255,um beads with4000 thesuspending media anda suspension glass pieces areshown inFig.6. agitated
of755 beads noises fordifferent numbers ofthelow 5.Frequency pm frequency Figure power spectrum insuspension.
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6.Typical waveforms ofthelow noises obtained of(a)suspending Figure frequency during agitation media and(b)suspension with 4000 of255 beads. only pieces pm 3.2.High frequency ThedatafromtheAEtesterareofgoodresemblance tothoseobtained fromthe discriminator. Forsimplicity, fromthediscriminator willbeusedin onlytheresults thefollowing discussion. Thelog-log ofbeads event rateforgains of20,10and plotsofthenumber against 0dBareshown inFig.7.Forallbeadsizes, thenumber ofbeads inthe increasing increased eventrate.Bigger beadsgenerated noisesevenin lesser suspension number. Theincrease inthenumber ofbeads increased thenumber ofimpingements andcollisions, theevent rateortheamount ofnoise emitted thereby increasing per unittime.Withtheincrease innoise theenergy rateandoscillation ratealso activity,
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412 rateagainst thenumber of255, and noises event ofhigh 7.Logarithmic plots frequency Figure fordifferent 755 (a)20,(b)10and(c)0dB. settings: gain umbeads
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8.Logarithmic ofhigh noises rateagainst thenumber of255, 412 and Figure plots frequency energy beads for20 dB 755,um gain setting. asshown inFigs8 and9.These showthatbigger beadshave increased, graphs Theimpingements of bigbeadsgenerate noiseswith higherimpactenergy. thatexceed thethreshold level. Thisexplains beads amplitudes Fig.7,where 755 um exhibited eventrateseveninlesser number than255 um beads. Agreater higher number ofsmall beads isneeded togenerate noises withamplitudes to highenough exceed thethreshold level.Thetapering of thecurves indicates thelimitofthe effective ofthenoise device. Theamplifier range monitoring gainmustbeadjusted toavoid andoverloading thecounting device. the over-amplification Byreducing thenumber ofbeads thatcanbemonitored inside thevessel increased. gainsetting, noise waveforms obtained when of412,um Typical 20,160and3120 signal pieces beads were usedareshown inFig.10.Thelinesdrawn inFig.10indicate the glass discrimination levels. It canbeseenthatasthenumber of beadsincreases the
9.Logarithmic ofhigh noises oscillation rate thenumber of255, and Figure plots 412 frequency against 755 beads for20dBgain 11m setting.
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and(c)3120 of412um 10.Typical noise waveforms obtained with (a)20,(b)160 pieces Figure beads. glass number ofwaves thediscrimination levels alsoincreases. theevent Thus, exceeding in thenumber of beads.In rateandoscillation rateincrease withan increase multitudinous of a largenumber of beadsgenerate addition, impingements ofa harmonic noises withamplitudes thanthosegenerated bytheimpact greater bead.Thisisevident inthehighnumber concentration where waves withhigh single arenoticeable. increases theinitial Asthenumber ofbeads amplitudes burst-type emission a pseudo-continuous-type emission (Fig.l0a)becomes (Fig.lOc)dueto
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ofhigh noises fordifferent numbers of255, 412 and 11.Frequency power spectra frequency Figure 20dB. 755 beads when thegain was 11m setting intheenergy theincrease rateandtherelative ofevents. Thisexplains overlapping withincreasing number ofsuspended beads. energy forthethreebeadsizes when the Thepower ofthewaveforms obtained spectra inFig.11.Thepower increases with was20dBareillustrated spectrum gainsetting Thiscanbeverified noise thenumber ofbeads, shown asthez-axis. bythetypical
153 inFig.10.Itisevident waveforms shown fromthewaveforms thatasthe signal number ofbeadsincreases, theinitialburst-type emission to a pseudochanges emission duetooverlapping ofevents. Thisoverlapping ofnoise continuous-type Inthecase events causes theincrease ofthepower ofthenoisesignal. spectrum inthespectrum, of255,um threedistinct i.e.264,534and989 kHz, beads, peaks canbeobserved. With412,um werenotedat beads,thepeaksinthespectrum of166, 272and532kHz.With755 um at242and533kHz beads, frequencies peaks wereobserved. Hertz's lawofcontact wasutilized to approximate thefrequency ofvibration fromtheimpingement ofglass beads tothevessel wall.Theequation forthe arising duration of contact fortheHertzian of a (Lbw) reported byGoldsmith impact ofdiameter without spherical object (D)andmass(m)ona thickplatewillbegiven proof[10] : Fortwospherical oneachother,theduration ofcontact is objects impinging (zbb) as: given where v is therelative and and6. arethematerial velocity during collision, constants ofthesphere andtheplane Itisdefined as: surface, property respectively. ratioandE theYoung's modulus ofelasticity. Forglass, whereispthePoisson's = 0.19;while forstainless steel, Es= 2.1x 10"Paand Eg= 6.9x 10"Paand I1g [10]. ,us = 0.28 oftheparticle inanagitated environment wascarried out Approximation velocity theempirical obtained andProbstein foreddies byusing equation byDelichatsios thantheKolmogorov microscale ina homogeneous turbulent flow larger isotropic Themagnitude ofrelative isgiven [11]. particle velocity by: therateofturbulent Thevalue for was where is energy dissipation perunitmass. obtained thepower inthevessel, i.e. bydividing input(P)bythemassoffluid(mf) of0.1634 microscale = Plm[12],which gavea value J/kg s.TheKolmogorov wascalculated tobe62 um. Inconjunction withtheabove thefundamental ofvibration equations, frequency wascalculated tobethereciprocal oftwice theduration ofcontact [8]: Thecalculated of vibration frequencies arisingfrombead-wall and inTable bead-bead forthethreebeads usedaresummarized 1.Inthe impacts (Fbb) caseoftwobeads eachother, thevelocity wasapproximately colliding against equal toEq.(4)multiplied ofthecalculated for by2.Theorderofmagnitude frequencies
154 Table 1. Calculated frequencies
obtained from isinagreement withthatofthepeakfrequencies allthreebeadsizes inFig.11. theFFTanalysis shown in iswellpronounced ofthebeads Thedependence offrequency onthediameter did themeasured fromtheFFTanalysis values. thecalculated However, frequencies onthe oftheparticle diameter indicates thattheeffect notshow which bigshifts, caused Thiscanbeunderstood asanattenuation effect could belesser. by frequency unaccounted forpeakfrequencies There arealsosome theliquid inside thevessel. to either the which canprobably beattributed intheresults oftheFFTanalysis Itisalso ofbeadcollisions. wallortoothermodes flexural vibration ofthevessel of the fundamental the harmonics thatthe otherpeaksrepresent possible thevessel tobea massive Inourcalculations weassumed ofvibration. frequencies Dimensional thickplatewithnegligible orminimal flexation. analysis byGoodier oftheplate(h)overtwice the thattheratioofthethickness andRipperger showed as bodies radius 16,i.e.hl2a> 16,fora platetobehave (a)mustexceed impinging ina thick Flexural behavior isnotobserved solid[13]. a thickplateorsemi-infinite ofimpact islessthanthetime ortheduration thetimeofcontact platebecause the toreflect fromtheothersideoftheplate.Forthinplates, needed forthewave backtothepointof fora wave to reflect timeexceeds thetimeneeded contact andflexural vibrations tendto dominate soundradiation contact [9].When ofthevessel wallcanbe thethickness tothesizeofimpinging beads, compared thatthe eitherasa thickplateor a thinplate.Thusit is probable categorized aredueto flexural behavior ofthewalls. unaccounted formeasured frequencies itisdifficult toobtain a ofthevessel, thecomplicated However, shape considering ofthisflexural vibration. goodtheoretical approximation andare ratesaredependent onthethreshold Theevent rateandoscillation setting emitted of suspended idealforburst-type bylargenumber signals [14].Noises which make oftheamplifier arecontinuous-type adjustments gain signals particles isindependent level ofthesignal andthethreshold verycritical. Envelope processing ofsuspended it suitable formonitoring noiseemission ofthethreshold, making d.c.output whose is Theenvelope ofa signal isa fluctuating amplitude particles. inthesignal. Since noises emitted inherent totheenergy bysuspended proportional ofparticles, the ofenergy duetotheimpact areattributed totherelease particles related totheenergy content ofthesignal, suchas oftheparameters measurement oftheparticles. oftheimpact isconsidered tobeanindication energy envelope, itthrough anaveraging circuit. intheenvelope iseliminated Fluctuation bypassing circuit iscalled therelative theoutput oftheaveraging Inthisstudy, henceforth, energy, EI. ofbeads in thenumber oftheemitted noises Therelative plotted against energy exceeds 200.This becomes a straight linewhen thenumber ofbeads a log-log scale
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12.Logarithmic ofrelative ofhigh noises thenumber of255, 412 Figure plot energy frequency against and755 beads. pm isdepicted inFig.12.Theslopes ofthelinesforallthethreebeadsizes relationship aresimilar. Theempirical relation between therelative andthediameter and energy number ofbeads isgiven asfollows: where istherelative andNthenumber of energy [V],Dthebeaddiameter (J1mJ
13.Comparison ofexperimental andcalculated 412 and755,um beads. Figure E,for255,
156 between thecalculated andexperimental relative beads. Thegoodagreement energy isevident inFig.13. arespherical andclosely Ncanbeexpressed interms Since theglass beads sized, ofmass W andtheabove relation becomes: [g](N= W/(nD3p/6» insuspension, it Thisexpression shows thatwitha known massofparticles clearly diameter ofthenoise ispossible todetermine theparticle fromtherelative energy withanother instrumentation method Itcanalsobesaidthatwhen combined signal. in suspension, which themassof particles thetechnique of noise canmeasure emission canbeutilized foron-line oftheprocess ofagglomeration in monitoring thispossibility andtheresultsarereported liquid.Theauthorsinvestigated elsewhere [15]. 4.CONCLUSION ina liquid ina Thenoiseemission ofparticles medium andagitated suspended stirredvessel wasstudied Lowfrequency noisesignals contain experimentally. noises withthemonitoring ofthenoise mechanical andhydraulic thatinterfere Incontrast, noise exhibited ofsuspended signature particles. highfrequency signals relative tothesizeandnumber ofsuspended An corresponding changes particles. increase inthesizeorinthenumber ofbeads increases theevent rate,energy rate, oscillation andfrequency rate,relative energy power spectra. Thefundamental ofvibration were estimated based onHertz's lawof frequencies contact inconjunction withwellestablished Theorderof hydrodynamic equations. thecalculated of vibration frombead-wall andbead-bead frequencies arising withthatof themeasured isingoodagreement It canbe impacts frequencies. deduced thatthehighfrequency noises emitted theagitation ofsuspension during aremainly andbead-bead collisions. generated bythebead-wall Relative fromthethreshold a broader level, provides energy, being independent thediameter ofthesuspended andsimple means ofapproximating range particles. Therelative of noises emitted energy bysuspended glassbeadsin an agitated environment isproportional totheirdiameter andnumber raised topower 2.80and These results demonstrate thepossibility ofusing themethod of 0.53,respectively. noise emission foron-line oftheprocess ofagglomeration inliquid. monitoring REFERENCES 1.M.Leach, G.Robin andJ.Williams, from acoustic Powder Particle size determination emissions. Technol. 1977. 16,153-158, 2.T.Wakimoto, A.Takeda and A.Otsuka, The utilization ofnoise inmonitoring thecoating emission J.Soc. Powder 1980. Technol., 17, 693-697, process. Japan 3.J.Hidaka, T.Kinboshi andS.Miwa, Parameters ofacoustic emission thecompact ofa during 1989. J.Soc. Powder Technol. 26,238-244, powder. 4.W.Guinto, T.Hirajima, M.Tsunekawa, Y.Nishisu andM.Nakamura, Production ofzirconia anewly semi-continuous ofagglomeration inliquid. J.Mining microspheres using developed system Mater. Proc. Inst. No. in 109, 1,1993,press. Japan, Areview 5.A.Akay, ofimpact noise. J.Acoust. Soc. Am. 1978. 64, 977-987, 6.J.Hidaka, A.Shimosaka andS.Miwa, The effects oftheparticle ontheparameters of properties 1987. sound between two J.Soc. Powder Technol., 24,655-663, impact particles. Japan
157 7.J.Hidaka, H.Ito,A.Shimosaka andS.Miwa, Parameters ofthesound ofa duetotheimpact onacircular J.Soc. Powder 1991. Technol., particle Japan 284, 78-84, plate. 8.J.Tillet, Astudy Proc. oftheimpact onspheres ofplates. Soc. 1954. 67,677-688, Phys. (London) 9.T.Takahagi, M.Yokoi and M.Nakai, The sound ofaball onacircular byatransverse impact plate. J.Acoust. Soc. 1980. 2,121-132, (E)1 Japan 10.W.Goldsmith, Edward London: Arnold, 1960, 82-144. Impact. 11.M.Delichatsios andR.Probstein, inturbulent flow: and J.Colloid Coagulation theory experiment. Sci. 1975. 51,394-405, Interface 12.H.Schubert inunit and K.Muhle, The role ofturbulence ofparticle In:Proc. operations technology. 55-67. Vol. III,1990, of2ndWorld Congr. ofParticle Technology, 13.J.Goodier and E.Ripperger, of a to Transition slab from surface wave to flexural Response impact. behavior. J.Appl. Mech. 26, 146-147, 1959. 14.R.Kline, Acoustic emission characterization. In:Acoustic Emission. Vol. 2,J.Matthews signal New York: Gordon andBreach, 105-138. (Ed.). 1983, 15.T.Hirajima, W.Guinto, M.Tsunekawa, K.Tadano, 1.Nakajima andM.Nakamura, On-line foragglomeration inorganic J.Soc. Powder 1992. monitoring Technol., Japan 29,428-433, liquid.
144 been havegenerally ofnoisegeneration Fundamental studies onthemechanism ofballona plateor thenoiseemitted bytheimpact byanalyzing performed aboutthenoise no literature is available ball-ball collisions [5-9].However, inavessel. inliquid andagitated number ofparticles suspended generated byalarge ofgeneration ofthesenoises themechanism toclarify itisnecessary Consequently, in theprocess ofagglomeration thistechnique formonitoring inordertoutilize andhighfrequency noises emitted thelowfrequency Inthisrespect, by liquid. inveswereexperimentally environment model inanagitated suspended particles thephenomenon ofnoise emission andfullyunderstand inordertoverify tigated inliquid. theprocess ofagglomeration during 2.EXPERIMENTAL 2. IMaterials . Some wasanaliphatic Thesuspending containing isoparaffin. hydrocarbon liquid 750kg/m3, interfacial of thisorganic liquidare;density: important properties with 7-12.Highly beads andcarbon number: tension: 43.01 spherical glass mN/m, wereused.The a density of2500 kg/m3, andmeansizesof255,412and755 um off-size andensure sieved toremove beads were sharpsizedistribution. particles 2.2.Equipment theglass thesuspension usedforagitating 2.2. I.Agitator. Thedevice containing ofceramic which wasdesigned fortheproduction wasa horizontal beads agitator inliquid [4].Ashaftfittedwiththreefour-bladed microspheres byagglomeration Theinternal volume ofthe atthehorizontal axisofthevessel. waslocated impellers was3000 cm3. oftheshaft-impeller vessel minus thevolume assembly cylindrical steel. The fromstainless Thevessel consisted oflower andupper partsconstructed toprevent andtocontrol witha cooling lower wasfitted overheating jacket portion filled withtheorganic Thevessel wascompletely thetemperature during agitation. a specific number ofbeads wascarried ofsuspensions Agitation containing liquid. to form wassimilar to therotational outat 1840 speedneeded r.p.m.which withgoodproperties [4]. agglomerates noiseemission Thelowfrequency 2.2.2.Lowfrequency system. monitoring were detected inFig.1.Noise isillustrated bya transducer signals system acquisition holder. Thetransducer was totheexternal wallofthevessel attached bya magnetic witha sensitivity of3.14 pC/go Endevco anaccelerometer (7701-100, Corp.) (gois of7-22kHz± 5dB.The acceleration dueto gravity) anda frequency response of100 mV/go ina charge wasamplified fromtheaccelerometer bya factor signal filter androuted toa programmable Endevco bandpass Corp.) (2721B, amplifier
emission 1.Schematic ofthelow noise frequency acquisition system. diagram Figure
145 NFCorp.). andthelowpassfilter Thehighpassfilterwassetat 100 Hz (FV-665, ina cassette wasfixed at30kHz.Thefiltered wasstored signal taperecorder (TCwasalsotransmitted Thefiltered toa trigger D5M, SonyCorp.). signal generator Ifthesignal NFCorp.). exceeded a setthreshold thetrigger level, (9622, generator released aneventpulsewhich wasincremented ina universal counter (TR5822, Advantest Off-line oftherecorded wascarried out Corp.). frequency analysis signal ina fastFourier transform Iwatsu (FFT) analyzer (SM-2701, Corp.). 2.2.3.Highfrequency Thediagram ofthehighfrequency monitoring system. acoustic emission is shown in Fig.2. Sound emissions were acquisition system NFCorp.) detected fixed atthesideofthevessel. The byasensor (AE-900S-WB, sensor wasa widebandpiezoelectric transducer witha ±10 dB overthe response silicon-rubber frequency range100kHzto2MHz.A hightemperature gluewas usedto attachthesensor to a flange outside thevessel. TheAEsignal fromthe sensor wasprocessed inanAEtester(9501, NFCorp.)anda disseparately criminator NFCorp.). TheAEtester isa compact version ofthe (AE-922, portable andwasusedas a backup discriminator Thedifferent noisesignal system. characterization formats ofthediscriminator areshown inFig.3.Thesignal from thesensor wasamplified of40dBinapreamplifier NFCorp.), byafactor (AE-912, before tothediscriminator. Inthediscriminator, thepreamplified inputting signal isfiltered a variable andlowpassfilter(LPF). The through highpassfilter(HPF) HPFwassetat 100kHzandtheLPFwasnotused.Thefiltered signalwas attenuated andamplified tothelevel tothegainsetting. Thegainsetting according ofthediscriminator could bevaried from-20to60dB.Intheseexperiments the were Theamplified andfiltered i.e.RF,was 20,10and0 dB. gainsettings signal, routed toanenvelope Theriserateoftheenvelope was10V///s processor. processor andthetimeconstant whenfalling was100,us. Theenvelope, a fluctuating d.c. wasrouted toanaverager circuit andtohighlevel,V ,andlowlevel,VL, output, Theoutput fromtheaverager circuit iscalled therelative or comparators. energy,
2.Schematic ofthehigh noise emission Figure diagram frequency acquisition system.
146
formats ofthediscriminator. characterization 3.Noise Figure signal whenever theenvelope exceeds thehigh Inthecomparator circuits, Enofthesignal. unituntilthe control discrimination levelVH, anevent isreleased bya digital signal levelYLThe were downtothelowdiscrimination decays . eventsignals envelope asevent rateand NFCorp.) andformatted incremented indigital counters (AE-932, eventtotal.If the periodof oneeventT, failedto satisfythe condition aftertheevent isreleased immediately Tj < T.< Tx , a nogood(NG)signal In todisregard/cancel theevent TheNGsignal isa cuetothecounter signal. signal. 3.5V,3.4V,10 ps and100ms,respecandTMAX were ourexperiments VH, VL, TMIN where itwasconwasalsorouted toa zero-cross TheRFsignal comparator tively. the wereoutputted Theoscillation verted tooscillation pulses onlyduring pulses. cominanevent, ofcycles itindicates thenumber duration ofanevent Thus, signal. wereincremented ina digital Oscillation asringdown counts. known pulses monly rate(pulse/s). andformatted astheoscillation counter anenergy wasmeasured content oftheRFsignal Theenergy byusing processor andgenerates thesignal NFCorp.). Theenergy pulse squares processor (AE-972, to theenergy of thesignal. Eachpulseis trainswitha frequency proportional were forvoltpeak.These to6.25x 10-6V2s,pwhere pulses Vpstands equivalent The andformatted astheenergy rate(pulse/s). counter transmitted to a digital ofthefour andtheoutputs fromthediscriminator andrelative energy envelope routed rateandenergy i.e.theevent rate,were total,oscillation rate,event counters, inthedatalogger converter toananalog/digital computer. (A/D)andstored were oftheemitted waveforms Foranoff-line noises, digitized frequency analysis NFCorp.). Thedatalength was ina wave andtemporarily stored memory (9620, timewas1ms timewas500ns.Thus,thesweep andthesampling twokilowords of1MHz.Datafromthewave measurable witha maximum memory frequency in floppy disks.A FFT to a personal andrecorded weretransferred computer forthepower wasprepared spectrum analysis. program 3.RESULTS AND DISCUSSION 3.1.Low frequency thevessel is inside thenumber ofbeads rateversus Theplotoftheevent placed tothenumber ofbeads rateappears tobeinsensitive inFig.4.Theevent shown
147
thenumber of255 and755 beads noises 4.Event rateoflow pm in against frequency plotted Figure suspension. inFig.5alsoreveal that FFTanalysis results inthevessel. depicted Typical present inthe ofbeads ofthenoise isnotaffected thepower bythenumber signal spectrum inFig.5isthatofthesignal obtained Thefirstspectrum inthez-axis). vessel (shown This without media wasagitated whenonlythesuspending glassbeadspresent. andhydraulic noises. Thefactthatthe tomechanical canbeattributed spectrum didnotexhibit obtained forsuspensions ofthesignals any subsequent power spectra of thenoises thatthelowfrequency increase norchange components suggests suchas areoverwhelmed emitted byothersources particles bythesuspended when waveforms ofthenoise noises. andmechanical only signals Typical hydraulic were of255,um beads with4000 thesuspending media anda suspension glass pieces areshown inFig.6. agitated
of755 beads noises fordifferent numbers ofthelow 5.Frequency pm frequency Figure power spectrum insuspension.
148
6.Typical waveforms ofthelow noises obtained of(a)suspending Figure frequency during agitation media and(b)suspension with 4000 of255 beads. only pieces pm 3.2.High frequency ThedatafromtheAEtesterareofgoodresemblance tothoseobtained fromthe discriminator. Forsimplicity, fromthediscriminator willbeusedin onlytheresults thefollowing discussion. Thelog-log ofbeads event rateforgains of20,10and plotsofthenumber against 0dBareshown inFig.7.Forallbeadsizes, thenumber ofbeads inthe increasing increased eventrate.Bigger beadsgenerated noisesevenin lesser suspension number. Theincrease inthenumber ofbeads increased thenumber ofimpingements andcollisions, theevent rateortheamount ofnoise emitted thereby increasing per unittime.Withtheincrease innoise theenergy rateandoscillation ratealso activity,
149
412 rateagainst thenumber of255, and noises event ofhigh 7.Logarithmic plots frequency Figure fordifferent 755 (a)20,(b)10and(c)0dB. settings: gain umbeads
150
8.Logarithmic ofhigh noises rateagainst thenumber of255, 412 and Figure plots frequency energy beads for20 dB 755,um gain setting. asshown inFigs8 and9.These showthatbigger beadshave increased, graphs Theimpingements of bigbeadsgenerate noiseswith higherimpactenergy. thatexceed thethreshold level. Thisexplains beads amplitudes Fig.7,where 755 um exhibited eventrateseveninlesser number than255 um beads. Agreater higher number ofsmall beads isneeded togenerate noises withamplitudes to highenough exceed thethreshold level.Thetapering of thecurves indicates thelimitofthe effective ofthenoise device. Theamplifier range monitoring gainmustbeadjusted toavoid andoverloading thecounting device. the over-amplification Byreducing thenumber ofbeads thatcanbemonitored inside thevessel increased. gainsetting, noise waveforms obtained when of412,um Typical 20,160and3120 signal pieces beads were usedareshown inFig.10.Thelinesdrawn inFig.10indicate the glass discrimination levels. It canbeseenthatasthenumber of beadsincreases the
9.Logarithmic ofhigh noises oscillation rate thenumber of255, and Figure plots 412 frequency against 755 beads for20dBgain 11m setting.
151
and(c)3120 of412um 10.Typical noise waveforms obtained with (a)20,(b)160 pieces Figure beads. glass number ofwaves thediscrimination levels alsoincreases. theevent Thus, exceeding in thenumber of beads.In rateandoscillation rateincrease withan increase multitudinous of a largenumber of beadsgenerate addition, impingements ofa harmonic noises withamplitudes thanthosegenerated bytheimpact greater bead.Thisisevident inthehighnumber concentration where waves withhigh single arenoticeable. increases theinitial Asthenumber ofbeads amplitudes burst-type emission a pseudo-continuous-type emission (Fig.l0a)becomes (Fig.lOc)dueto
152
ofhigh noises fordifferent numbers of255, 412 and 11.Frequency power spectra frequency Figure 20dB. 755 beads when thegain was 11m setting intheenergy theincrease rateandtherelative ofevents. Thisexplains overlapping withincreasing number ofsuspended beads. energy forthethreebeadsizes when the Thepower ofthewaveforms obtained spectra inFig.11.Thepower increases with was20dBareillustrated spectrum gainsetting Thiscanbeverified noise thenumber ofbeads, shown asthez-axis. bythetypical
153 inFig.10.Itisevident waveforms shown fromthewaveforms thatasthe signal number ofbeadsincreases, theinitialburst-type emission to a pseudochanges emission duetooverlapping ofevents. Thisoverlapping ofnoise continuous-type Inthecase events causes theincrease ofthepower ofthenoisesignal. spectrum inthespectrum, of255,um threedistinct i.e.264,534and989 kHz, beads, peaks canbeobserved. With412,um werenotedat beads,thepeaksinthespectrum of166, 272and532kHz.With755 um at242and533kHz beads, frequencies peaks wereobserved. Hertz's lawofcontact wasutilized to approximate thefrequency ofvibration fromtheimpingement ofglass beads tothevessel wall.Theequation forthe arising duration of contact fortheHertzian of a (Lbw) reported byGoldsmith impact ofdiameter without spherical object (D)andmass(m)ona thickplatewillbegiven proof[10] : Fortwospherical oneachother,theduration ofcontact is objects impinging (zbb) as: given where v is therelative and and6. arethematerial velocity during collision, constants ofthesphere andtheplane Itisdefined as: surface, property respectively. ratioandE theYoung's modulus ofelasticity. Forglass, whereispthePoisson's = 0.19;while forstainless steel, Es= 2.1x 10"Paand Eg= 6.9x 10"Paand I1g [10]. ,us = 0.28 oftheparticle inanagitated environment wascarried out Approximation velocity theempirical obtained andProbstein foreddies byusing equation byDelichatsios thantheKolmogorov microscale ina homogeneous turbulent flow larger isotropic Themagnitude ofrelative isgiven [11]. particle velocity by: therateofturbulent Thevalue for was where is energy dissipation perunitmass. obtained thepower inthevessel, i.e. bydividing input(P)bythemassoffluid(mf) of0.1634 microscale = Plm[12],which gavea value J/kg s.TheKolmogorov wascalculated tobe62 um. Inconjunction withtheabove thefundamental ofvibration equations, frequency wascalculated tobethereciprocal oftwice theduration ofcontact [8]: Thecalculated of vibration frequencies arisingfrombead-wall and inTable bead-bead forthethreebeads usedaresummarized 1.Inthe impacts (Fbb) caseoftwobeads eachother, thevelocity wasapproximately colliding against equal toEq.(4)multiplied ofthecalculated for by2.Theorderofmagnitude frequencies
154 Table 1. Calculated frequencies
obtained from isinagreement withthatofthepeakfrequencies allthreebeadsizes inFig.11. theFFTanalysis shown in iswellpronounced ofthebeads Thedependence offrequency onthediameter did themeasured fromtheFFTanalysis values. thecalculated However, frequencies onthe oftheparticle diameter indicates thattheeffect notshow which bigshifts, caused Thiscanbeunderstood asanattenuation effect could belesser. by frequency unaccounted forpeakfrequencies There arealsosome theliquid inside thevessel. to either the which canprobably beattributed intheresults oftheFFTanalysis Itisalso ofbeadcollisions. wallortoothermodes flexural vibration ofthevessel of the fundamental the harmonics thatthe otherpeaksrepresent possible thevessel tobea massive Inourcalculations weassumed ofvibration. frequencies Dimensional thickplatewithnegligible orminimal flexation. analysis byGoodier oftheplate(h)overtwice the thattheratioofthethickness andRipperger showed as bodies radius 16,i.e.hl2a> 16,fora platetobehave (a)mustexceed impinging ina thick Flexural behavior isnotobserved solid[13]. a thickplateorsemi-infinite ofimpact islessthanthetime ortheduration thetimeofcontact platebecause the toreflect fromtheothersideoftheplate.Forthinplates, needed forthewave backtothepointof fora wave to reflect timeexceeds thetimeneeded contact andflexural vibrations tendto dominate soundradiation contact [9].When ofthevessel wallcanbe thethickness tothesizeofimpinging beads, compared thatthe eitherasa thickplateor a thinplate.Thusit is probable categorized aredueto flexural behavior ofthewalls. unaccounted formeasured frequencies itisdifficult toobtain a ofthevessel, thecomplicated However, shape considering ofthisflexural vibration. goodtheoretical approximation andare ratesaredependent onthethreshold Theevent rateandoscillation setting emitted of suspended idealforburst-type bylargenumber signals [14].Noises which make oftheamplifier arecontinuous-type adjustments gain signals particles isindependent level ofthesignal andthethreshold verycritical. Envelope processing ofsuspended it suitable formonitoring noiseemission ofthethreshold, making d.c.output whose is Theenvelope ofa signal isa fluctuating amplitude particles. inthesignal. Since noises emitted inherent totheenergy bysuspended proportional ofparticles, the ofenergy duetotheimpact areattributed totherelease particles related totheenergy content ofthesignal, suchas oftheparameters measurement oftheparticles. oftheimpact isconsidered tobeanindication energy envelope, itthrough anaveraging circuit. intheenvelope iseliminated Fluctuation bypassing circuit iscalled therelative theoutput oftheaveraging Inthisstudy, henceforth, energy, EI. ofbeads in thenumber oftheemitted noises Therelative plotted against energy exceeds 200.This becomes a straight linewhen thenumber ofbeads a log-log scale
155
12.Logarithmic ofrelative ofhigh noises thenumber of255, 412 Figure plot energy frequency against and755 beads. pm isdepicted inFig.12.Theslopes ofthelinesforallthethreebeadsizes relationship aresimilar. Theempirical relation between therelative andthediameter and energy number ofbeads isgiven asfollows: where istherelative andNthenumber of energy [V],Dthebeaddiameter (J1mJ
13.Comparison ofexperimental andcalculated 412 and755,um beads. Figure E,for255,
156 between thecalculated andexperimental relative beads. Thegoodagreement energy isevident inFig.13. arespherical andclosely Ncanbeexpressed interms Since theglass beads sized, ofmass W andtheabove relation becomes: [g](N= W/(nD3p/6» insuspension, it Thisexpression shows thatwitha known massofparticles clearly diameter ofthenoise ispossible todetermine theparticle fromtherelative energy withanother instrumentation method Itcanalsobesaidthatwhen combined signal. in suspension, which themassof particles thetechnique of noise canmeasure emission canbeutilized foron-line oftheprocess ofagglomeration in monitoring thispossibility andtheresultsarereported liquid.Theauthorsinvestigated elsewhere [15]. 4.CONCLUSION ina liquid ina Thenoiseemission ofparticles medium andagitated suspended stirredvessel wasstudied Lowfrequency noisesignals contain experimentally. noises withthemonitoring ofthenoise mechanical andhydraulic thatinterfere Incontrast, noise exhibited ofsuspended signature particles. highfrequency signals relative tothesizeandnumber ofsuspended An corresponding changes particles. increase inthesizeorinthenumber ofbeads increases theevent rate,energy rate, oscillation andfrequency rate,relative energy power spectra. Thefundamental ofvibration were estimated based onHertz's lawof frequencies contact inconjunction withwellestablished Theorderof hydrodynamic equations. thecalculated of vibration frombead-wall andbead-bead frequencies arising withthatof themeasured isingoodagreement It canbe impacts frequencies. deduced thatthehighfrequency noises emitted theagitation ofsuspension during aremainly andbead-bead collisions. generated bythebead-wall Relative fromthethreshold a broader level, provides energy, being independent thediameter ofthesuspended andsimple means ofapproximating range particles. Therelative of noises emitted energy bysuspended glassbeadsin an agitated environment isproportional totheirdiameter andnumber raised topower 2.80and These results demonstrate thepossibility ofusing themethod of 0.53,respectively. noise emission foron-line oftheprocess ofagglomeration inliquid. monitoring REFERENCES 1.M.Leach, G.Robin andJ.Williams, from acoustic Powder Particle size determination emissions. Technol. 1977. 16,153-158, 2.T.Wakimoto, A.Takeda and A.Otsuka, The utilization ofnoise inmonitoring thecoating emission J.Soc. Powder 1980. Technol., 17, 693-697, process. Japan 3.J.Hidaka, T.Kinboshi andS.Miwa, Parameters ofacoustic emission thecompact ofa during 1989. J.Soc. Powder Technol. 26,238-244, powder. 4.W.Guinto, T.Hirajima, M.Tsunekawa, Y.Nishisu andM.Nakamura, Production ofzirconia anewly semi-continuous ofagglomeration inliquid. J.Mining microspheres using developed system Mater. Proc. Inst. No. in 109, 1,1993,press. Japan, Areview 5.A.Akay, ofimpact noise. J.Acoust. Soc. Am. 1978. 64, 977-987, 6.J.Hidaka, A.Shimosaka andS.Miwa, The effects oftheparticle ontheparameters of properties 1987. sound between two J.Soc. Powder Technol., 24,655-663, impact particles. Japan
157 7.J.Hidaka, H.Ito,A.Shimosaka andS.Miwa, Parameters ofthesound ofa duetotheimpact onacircular J.Soc. Powder 1991. Technol., particle Japan 284, 78-84, plate. 8.J.Tillet, Astudy Proc. oftheimpact onspheres ofplates. Soc. 1954. 67,677-688, Phys. (London) 9.T.Takahagi, M.Yokoi and M.Nakai, The sound ofaball onacircular byatransverse impact plate. J.Acoust. Soc. 1980. 2,121-132, (E)1 Japan 10.W.Goldsmith, Edward London: Arnold, 1960, 82-144. Impact. 11.M.Delichatsios andR.Probstein, inturbulent flow: and J.Colloid Coagulation theory experiment. Sci. 1975. 51,394-405, Interface 12.H.Schubert inunit and K.Muhle, The role ofturbulence ofparticle In:Proc. operations technology. 55-67. Vol. III,1990, of2ndWorld Congr. ofParticle Technology, 13.J.Goodier and E.Ripperger, of a to Transition slab from surface wave to flexural Response impact. behavior. J.Appl. Mech. 26, 146-147, 1959. 14.R.Kline, Acoustic emission characterization. In:Acoustic Emission. Vol. 2,J.Matthews signal New York: Gordon andBreach, 105-138. (Ed.). 1983, 15.T.Hirajima, W.Guinto, M.Tsunekawa, K.Tadano, 1.Nakajima andM.Nakamura, On-line foragglomeration inorganic J.Soc. Powder 1992. monitoring Technol., Japan 29,428-433, liquid.