Tramontana, Manuel and Gachagan, Anthony and Nordon, Alison and Littlejohn, David and O ...

12 Pages·2017·2.17 MB·English

SensorsandActuatorsA228(2015)159–169

ContentslistsavailableatScienceDirect

Sensors and Actuators A: Physical

journal homepage: www.elsevier.com/locate/sna

System modelling and device development for passive acoustic

monitoring of a particulate-liquid process

ManuelTramontanaa,AnthonyGachagana,∗,AlisonNordonb,DavidLittlejohnb,

RichardO’Learya,AnthonyJ.Mulhollandc

aDepartmentofElectronicandElectricalEngineering,UniversityofStrathclyde,204GeorgeStreet,Glasgow,G11XW,UK
bWestCHEM, De partment ofPu reandAp pliedChemis tryandCen tr eforProcess Ana lyticsan dCont rolTechn olo gy(CP ACT),UniversityofStrathclyde,295

CathedralStreet,Glasgow,G11XL,UK
cDepartm entofM athemat ics and Statistics,UniversityofStrathclyde,26RichmondStreet,Glasgow,G11XH,UK

a r t i c l e i n f o a b s t r a c t

Articlehistory: Thispaperpresentsthedevelopmentofapassiveultrasonicmonitoringsystemforthedetectionof
Receiv ed1December2014 acou sticem ission(A E)cr eatedbychem ica l particles strikingth einnerwall ofareac tor vess el.Thefin ite
RAeccceepivteedd i1n 3 rMevaisrechd 2fo0 r1m5 13 March 2015 element (FE) code PZFl ex was u se d to analy ze the co mplex in ter action s bet we e n chem ical par ticle s and
thevesselwall.A4-layer2Dmodelwasdevelopedcomprisingaliquidloadmediumandaglass-oil-glass
Availableonline22March2015
combinationcorrespondingtothejacketedvesselreactor.Themodelhasbeenexperimentallyvalidated

withexcellentcorrelationachieved.TheexcitationfunctionwasderivedfromHertz’stheoryandusedas
Keywords:
themodelstimuluscorrespondingtoparticlesstrikingtheinnerglasswall.AnalysisoftheFEsimulations
Passiveacoustics
providedthetransducerspecificationsforapassiveultrasonicmonitoringsystem.Thesystemcomprises
Heterogeneousreactionmonitoring
PFianrittiecleel esimzee nantda n caolnycsiesnt ration stwenos titriavnitsyd.uIcmerpso wrtiathn tcloym,tphleemseennstiatirvyi tcyhaorfatchteerrisetsiocsn:a nnatrrtroawn sbdauncdewripdrtohv/hidigehs sdeinscsriitmiviintya;t wioindeobfapnadr/tliocwle
Stacke dtransd ucer concentrati on.Moreover ,the broaderb and wid thofthe off-resonan tdeviced emonstratespo te ntialfor

Off-resonanttransducer insituestimationofparticlesize.Theperformanceaffordedbythisapproachhasconsiderablepotential

forreal-timeprocessmonitoringinthechemicalsandpharmaceuticalindustries.

©2015TheAuthors.PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBYlicense

(http://creativecommons.org/licenses/by/4.0/).
1. Introduction of vessel or pipe walls [2], although collisions with any internal

structuresorbetweenparticlesmayalsocontribute,todetermine

Acoustic monitoring techniques offer a significant advantage informationrelatedtothestatusoftheprocess.Measurementof

over optical techniques such as near infrared and Raman spec- AEhasbeenusedsuccessfullytomonitor,forexample,high-shear

trometriesforprocessmonitoringinthattheycanbeappliedto granulation[3–6],powderblending[7],drying[8]andvariousflu-

samplesthatareopticallyopaquewithouttheneedforanysample idizedbedprocesses[9–15],heterogeneousreactions[16–20],and

preparation. In particular, the ability of acoustic waves to pene- thetransportofpowders[21,22],tablets[23]andslurries[24].

trateopticallyopaquemediasuchasstainlesssteelenablesacoustic A number of experimental and theoretical studies have been

techniquestobeconfiguredtooperateinanon-invasivemodeof publishedtounderstandhowdifferentfactorsaffecttheAEsignal

operation,e.g.bysensorattachmentorlocationofamicrophone generated. In a series of papers, Leach et al. [25–30] investi-

closetotheoutervesselwall,withouttheneedforincorporationof gatedAEgeneratedbycollisionsbetweenspherical,cylindricaland

awindowinthevessel.Whilethereareonlyafewreportsoftheuse irregular-shapedparticlesinarotatingvessel.Acondensermicro-

ofnon-invasiveactiveacoustictechniquesforinsituprocessmoni- phonewasusedtodetecttheAEgeneratedbythecollisions,with

toring[1],passiveacousticshasbeenusedmorewidelyparticularly the frequency of emission inversely related to particle size. The

forthenon-invasivemonitoringofparticulateprocesses.Passive samerelationshipwasalsoobservedforthecollisionoftwosteel

acousticmonitoringtechniquesusetheacousticemission(AE)gen- balls[31],andforcollisionsbetweenglassspheres[32,33]andsed-

erated by collisions of particles primarily with the inner surface imentgravelinwater[34].Inaddition,itwasdeterminedthatthe

amplitudeofAEincreasedwiththenumberofcollidingspheres,

butthatthefrequencyofAEwasunaffected[32].AEgeneratedby

∗ theimpactofobjectswithsurfaceshasbeeninvestigatedusinga
CEo-mrraeislpaodnddreinssg: aau.gtahcohra. gTaenl.:@ +s4tr4a 0th1.4a1c. u5k48(A 2.5G3a5c.hagan). mic rophon eo ratrans ducer attached tot hesu rfaceoftheim pacte d

http://dx.doi.org/10.1016/j.sna.2015.03.022
0924-4247/©2015TheAuthors.PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBYlicense(http://creativecommons.org/licenses/by/4.0/).
160 M.Tramontanaetal./SensorsandActuatorsA228(2015)159–169

Table1

Particleinformationusedinboththeexperimentalandsimulationinvestigations.

Mnemonic Experimental Simulated((cid:2)m)
distribution((cid:2)m)

Size(cid:3) 0<x<251 200
Size (cid:4) 2 51 < x <5 00 400
Size (cid:5) 500 < x < 853 600

structure.TheamplitudeofAEarisingfromcollisionsbetweena

steelplateandaballbearingincreasedasthenumberofimpacting

objectswasincreased,butasforcollisionsbetweenspheres,the

frequencyofAEdidnotchange[35].Duringthepneumaticcon-
veyanceof co al, them ax imumfr equen cyofth evi brationalm odes Fig.1. MeasuredAEpowerspectraof40gdm−3ofitaconicacidintoluenewith
ofthest ruc ture exc itedonimp actwasfo un dto decreasew ithan part icl e sizes (cid:3), (cid:4) and (cid:5).

increaseinparticlesize[36,37].Instudiesofthemixingofdrypow-

ders[7,38]andparticlesinaliquid[17],theAEsignalincreasedwith

anincreaseinthemassandsizeofparticles,whiletheportionof
onthebroadbandAEsignals,eachparticlesizerangeofitaconicacid
AEatlowerfrequenciesincreasedwithparticlesize.
wasaddedstepwise,upto200g,totoluene.
In previous experimental work [16,17], AE generated by the E xampl esofAEs pec tra ofd if fer entparticlesizes,(cid:3),(cid:4)and(cid:5),
impact of particles in a fluid with the internal wall of a 1 L jacketed of 40gdm−3 of ita conic ac id in toluen e are ill ustrate d in Fig. 1.
glass reactor was studied. More recently, mathematical models
The principal frequencies of interest lay in the 0–350kHz range,
werederivedtodescribetheAEgeneratedbyparticleimpactswith
withcomponentsabove200kHzdemonstratinglesssensitivityto
the vessel wall [39–41]. The vessel wall was modelled as a sin-
changesinparticlesizestothoseatlowerfrequencies.
glelayercircularplateandanalyticalexpressionswerederivedto
Avarietyofdataanalysismethodswereemployedtodetermine
describetheimpactofparticleswiththeplate.Whileitwaspos-
both particle size and concentration information. Analyzing the
sibletoobtainparticlesize[39,40]andparticleconcentration[41]
signalareaundereachAEspectrum,asillustratedinFig.1,demon-
information,itbecomesincreasinglydifficulttoderivemathemat-
stratedthebestopportunityforparticleconcentrationandsizing
icalexpressions,whichcanbesolvedanalytically,describingthe
information.Particlesizeinformationwasextractedbyconsider-
propagationofacousticwavesformorecomplexgeometriesand
ingtheratioofsignalareabetween55and200kHz,wherethereis
materialproperties.Insuchcases,ithasbeenshownthatanumer-
cleardiscriminationbetweenthedifferentparticlesizeranges,to
icalmethodlikefiniteelement(FE)modellingcanbeused[42,43].
theoverallsignalareabetween55and500kHz.Fulldetailsofthe
Therefore,inthiswork,thegenerationofAEfromtheimpactof
resultsobtainedaregiveninreference[17].
particles with a reactor vessel wall for the experimental set-up
Tocharacterizethefrequencyresponseofdifferentcomponents
described in references [16,17] was investigated using FE mod-
of the experimental equipment (the jacketed reactor vessel, the
elling.AFEmodeloftheexperimentalset-upcomprisingaliquid
Nano30(PhysicalAcoustics)transducerandthepreamplifier),the
loadmediumandtheglass-oil-glasscombinationcorrespondingto
inner face of the vessel was excited using a ‘pencil lead break’
thereactorvesselwallstructurewasdevelopedusingPZFlex(Wei-
(Hsu-Nielson source) [44] to simulate impulse excitation of the
dlingerAssociatesInc.,NewYork,USA).TheFEmodelwasusedto
system. Fig. 2 shows the measured frequency spectrum of the
investigateparticlesizeandconcentrationcharacteristicsthrough
systemimpulseresponse.OncomparingtheexperimentalAEspec-
analysisofthefrequencyspectraassociatedwiththegeneratedAE
tra(Fig.1)withthismeasuredimpulseresponse,thesimilarityis
fromtheparticle–wallcollisions.Theresultsoftheinvestigation
clearlyevident.Therefore,itcanbeconcludedthattheacousticAE
havebeenusedtodefinetheultrasonictransducersystemspeci-
signalsobservedaremodifiedbythefrequencytransferfunction
ficationforanewpassiveacousticmonitoringapproach,through
ofthetransducer,andbythefilteringeffectsofthereactorvessel
whichapairofultrasonictransducershavebeendesignedandfab-
materialandelectronicdevices.
ricated.Interestingly,thecomplementarycharacteristicsofthese

transducers can provide additional particulate information from

theheterogeneoussystemunderinvestigation.

2. Experimentalpilotstudy

Theexperimentalset-upthatwasmodelledinthepresentstudy

isdescribedindetailinreferences[16,17],withthemainfindings

summarizedhere.Theexperimentalapparatusemployedconsisted

of:a1Lglassreactor(VWRInternational,Dorset,UK)withanoil

jacket,whichwasconnectedtoaheater–chillerunitfortemper-

ature regulation; a glass stirrer rod and paddle connected to an

overhead stirrer motor; a Nano30 AE sensor (Physical Acoustics

Limited,Cambridge,UK)attachedtotheouterwallofthereactor

vessel;andaPCfordataacquisitionandprocessing.

BroadbandAEsignalswerecollectedofitaconicacidparticles

(SigmaAldrich,Dorset,UK)mixingin500mLoftoluene(Bamford
Laborat ories,Ro chdale, UK) at20◦C .T oge ner ate differen tparticle

sizeranges,theitaconicacidwassievedintothreefractions(see
Tabl e1).Toi nve stigateth eeffe ctso fpartic lesiz eand concentra tion rFeiga.c t2o.r ,Etxrapnesridmuceenrtaalnlyd mpreeaasmurpelidfi esrp.ectral profile of the impulse response of the
M.Tramontanaetal./SensorsandActuatorsA228(2015)159–169 161

totheparticlesizedistributionsutilizedintheearlierexperimen-

tal work, as described in Table 1. Fig. 4 illustrates the temporal

andspectralprofilescorrespondingtoparticleimpactonthewall

ofthereactorforthethreesimulatedparticlessizesdescribedin

Table1.Itisimportanttonotethatasparticlesizeincreases,the

impactforceamplitudeincreasesandthedurationofthecontact

islonger.Thus,theAEassociatedwithsmallparticleshavewider

bandwidthwhencomparedtolargerparticleimpacts.Itisacknowl-

edgedthatparticle–wallcollisionsdooccuratanglesotherthan

thenormaldirectionandmoreover,therewillbeatangentialcom-

ponentinvolvingtheparticlesliding,orrolling,againstthewall.

Interestingly,iftheparticleandwallaresufficientlyhard,asinthe

Fig.3. Finiteelementmodelofthevesselreactor. caseconsideredhere,thenthesetangentialcomponentswillbesec-

ondorder.Hence,tosimplifytheexcitationschemeforthiswork

Table2 onlyelasticnormalimpactsareconsidered.

Materialproperties.

Material Density (kg m−3) (Lmonsg−i1tu)dinal velocity (Smhesa−r1 v)elocity 3.3. FE model validation

Model validation and verification are critical in the develop-
OToilluene 982690 1926225 00 mentofa simulatedm ode l.Themathe mat icalmo del ofth eimpact

Glass 2484 6192 3143 of the particle striking a surface has been validated in the liter-

ature[46],henceonlythevalidationofthereactorvesselmodel

was performed for this work. It was necessary to reproduce an
3. Simulationmethodology
identicalinputfortheexperimentaldataandpredictedmodel,and

thencomparethefunctionaloutputofthetwosystems.Forthis
3.1. Modellingofthevesselreactor
purpose,acustomizedtankwasbuilttakingintoaccountthe2D

nature of the computerized model. The tank was manufactured
Theapproachundertakentoanalyzethecomplexinteractions
usingPlexiglass.Onewallwascomposedoftwoparallelsheetsof
betweenthechemicalparticlesandthevesselwallusedFEmod-
glass of 3mm thickness and the 4.5mm cavity in between con-
elling techniques based on PZFlex software. Fig. 3(a) shows the
tained 500mL of the same oil that was employed in the reactor
threedimensional(3D)4-layermodeldeveloped,comprisingaliq-
heater–chiller.Duringthevalidationexperiments,a250kHzwide-
uidloadmediumandaglass-oil-glasscombinationcorresponding
band immersion transducer (Alba Ultrasound Ltd., Glasgow, UK)
to the jacketed vessel reactor. Due to computational demand of
waspositionedinthemiddleofthevesselandemployedasatrans-
such a 3D FE model, a 2D longitudinal section of the cylindrical
mitter.APVDFhydrophoneimmersedinsidethetankandaPVDF
model was utilized throughout this work as illustrated in Fig. 3(b). strip(s ize 30m m×80mm) attachedo ntheo uter wall oft he con-
Samplesofglassandoilfromthereactorwerecollectedandtheir
taineractedasbroadbandreceivers.Analuminiumcasewasalso
acousticandphysicalpropertieswerecharacterized.Thesevalues
usedtoshieldthePVDFstripfromtheenvironmentalnoiseinthe
wereobtainedbymeasuringthetimeofflightinaglasssampleof
lab.Moreover,thePVDFhydrophoneimmersedinthetankcanbe
thickness10mmandasampleof50mLofoil.Table2reportsthe
consideredacousticallytransparenttothepropagatingultrasonic
measuredacousticmaterialproperties(density,longitudinalveloc-
energyattheoperatingfrequencyof250kHz.Thereceiverswere
ity,shearvelocity)utilizedthroughouttheFEmodellingdiscussed
directlyconnectedviaacoaxialcabletotheAgilentInfiniiumoscil-
inthispaper.
loscopewhichwasinterfacedtoaPCthroughaGPIBconnectionto

collectreceivedwaveformsfromthePVDFsensors.
3.2. Simulationoftheexcitationfunction
AsrepresentedinFig.5,thevalidationofthevesselreactorwas

conductedintwodifferentstages.Inthefirststage,a10Vpp20-
ExcitationoftheFEmodelisillustratedinFig.3(b)andcorre-
cycletoneburst,withacentrefrequencyof250kHzwasusedto
spondstoapressureloadingattheinnerglasswallusingsignals
drive the ultrasonic transducer and the propagation waveforms
derivedfromthemathematicalmodelofimpactofparticlesofdif-
werecollectedbythehydrophoneandthePVDFstripreceivers.
ferentsizes.TheexcitationfunctionusedasthesourceofAEwas
Thefirstarrivalwaveformonthehydrophonewasselectedman-
derivedfromHertz’stheory[40,45]andthemodeloutputisthe
uallyandstoredintoafile.Inthesecondstageofthevalidation,the
out-of-plane displacement on the outer glass wall in the centre
signal output acquired by the hydrophone was employed as the
of the model. Assuming an elastic normal impact, each particle
signalinputintheFEmodelasillustratedinFig.5(b).Inthissim-
colliding against the wall of the vessel generates a force normal
pletechnique,theultrasonictransducerresponsecanbesimulated
to the surface. The determination of the impact source function
withoutknowingitsinternalcharacteristicasthesignalreproduced
as a function of time yields the impact time and the magnitude
isdependentuponthefrequencyresponseoftheacoustictransmit-
oftheforce,bothofwhichdependuponparticlesizeandveloc-
ter.Thus,thepredictedoutputpressurevariationwascollectedat
ity of collision. Table 3 shows the model predictions for contact
thecentreofthesimulatedmodel,wherethePVDFsensorwastheo-
timeandtheimpactforceforparticlesizeschosentocorrespond
reticallysituatedandcomparedwiththeexperimentallymeasured

waveform.
Table3 TheexperimentaldataandFEmodelpredictionarecompared

Parametersofthepredictedparticleimpact.
inFig.6todetermineiftheFEmodelhassufficientaccuracyforuse

Sizeofparticle Experimental Contacttime((cid:2)s) Force(N) inthissimulationprogramme.Theplotsshowastandingwavepat-
((cid:2)m ) distribution ter nes tablishedb ythelayerst ruct ure.T heses ta ndingwa vemo des

200 Size(cid:3) 7.08×10−6 4.25×10−7 arisefrominterferencebetweenthereflectedwavesandtheinci-
400 Size (cid:4) 1.20 × 10−5 9.60 × 10−7 dent wave s.Themaxim umofth ew aveisloc atedat 265 kHz and
600 Size (cid:5) 1.65 × 10−5 1.50 × 10−6 thisc orrespo nds tothethick ne ssm odeof th eultraso ni ctra nsm itter
162 M.Tramontanaetal./SensorsandActuatorsA228(2015)159–169

Fig.4. Predictedimpactprofiles(temporalandspectral)for200,400and600(cid:2)mparticlesemployedasinputexcitationfunction.

used during the validation process. In general, the model shows studyandthepublishedtheoryintheliteratureregardingparticle

excellentcorrelationwiththeexperimentalresponse;theerrorin concentrationandparticlesizeeffects.Theparticlesizesusedinthe
thepredi ctedmaxim apos itio nsislessthan 9%forth em ainfo ur simulationstu dyw ere200 ,40 0and6 00(cid:2) m,whic hcor respo nd to

peaks. This good agreement is in part due to the careful charac- theexperimentalparticledistributionrangesusedinthefeasibility

terizationofalloftherelevantmaterialpropertiesutilizedinthe study(seeTable1).

modelforeachlayer.

4.1. Concentrationeffects

4. FEsimulationresults

The effects of different numbers of particles hitting the inner

OncetheFEmodelwasvalidated,anumberofparameterswere wallofthereactorvesselweresimulated.Anexampleofthepre-

selectedtoprovideaninsightintotheAEgeneratedbytheparticle dictedAEspectracausedby1,3and7particlesofafixedparticle
impacts on theinne rw allofav esse l.O neo bjectiveof th emo delling diamet er of200(cid:2) m,imp act ing i nth e middleof th e inner wallof

workwastoascertainiftheconceptualmodelpredictionofthesys- thevesselisshowninFig.7.Here,themultipleexcitationscheme

temoutputwouldbeconsistentwiththeexperimentalfeasibility ispurelytheoretical,withthepressureloadingfunctionassociated

Fig.5. Illustrationofexperimentalandsimulationconfigurationsusedinthevalidationprocedure.
M.Tramontanaetal./SensorsandActuatorsA228(2015)159–169 163

1
F E M p red ic tio n
0 .9 E x p e rim e n ta l res pon s e
0 .8
0 .7
a.u. 0 .6
e
d
plitu 0 .5
Am 0 .4
0 .3
0.2
0 .1
0
0 1 2 3 4 5
F r equ e ncy [H z] x 1 05 Fig. 8. Histogram of predicted AE spectra arising from the impact of one particle of
differentsizes.

Fig.6. ComparisonofmodelvalidationresultsforexperimentalresponseandFEM

prediction.
4.2. Particlesizeeffects

Toevaluatetheeffectsofparticlesize,theexcitationfunction
with each particle size applied to the same node in the model
correspondingtooneparticleofeachsizedetailedinTable3was
tosimulatedifferentconcentrationeffects.Thepressurevariation
appliedastheinputtotheFEmodel.Theresultantnormalizedsig-
of the external wall was considered as the output of the sys-
nalprofilesareplottedinFig.8andextendbetween1and170kHz.
tem; hence, the shape of the predicted frequency spectra were
Moreover,onethousandparticlesoffixedsizeimpingingagainst
notinfluencedbythetransferfunctionofthetransducer,aswas
thefirstglasslayerofthevesselwerealsosimulatedtoevaluate
thecasefortheexperimentalresultsobtainedduringthefeasibil-
the effect of multiple particle impacts. In this work, the number
ity study. Here, only the filtering effects of the reactor structure
of events was limited to 1000 for computational efficiency. To
itselfmoderatethesimulateddata.Inparticular,thereactorves-
simulateascenarioclosertoapracticalsituation,thesimulation
selbehavesasamechanicallowpassfilter,withthegeneratedAE
softwarerandomizedboththespatialimpactpositionandrelative
restrictedtothefrequenciesbelow180kHz.Interestingly,theorig-
timesforeachAEeventassociatedwiththe1000particlesample.
inalexperimentalworkdemonstratesfrequencycomponentsupto
Fig.9illustratesthepredictedoutputwhenthesystemissubjected
380kHz,seeFig.1andthesehigherfrequencycomponentscorre-
tomultipleparticleimpacts.Theplotisnormalizedtohighlightthe
late directly with resonant modes in the transducer itself. From
spectra content. A frequency shifting effect is evident, where an
Fig.7,theFEmodelhaspredictedtworesonantmodesat74.4and
increaseinthediameteroftheobjectimpactingthevesselresults
89kHzcorrespondingtotheresonancesassociatedwiththeglass-
inadecreaseinAEfrequency.Again,thiseffectisconsistentwith
oil-glassconfiguration,whichwerecalculatedtobe75and90kHz,
theexperimentalworkundertakenbytheauthorsanddiscussedin
respectively.Anincreaseinthenumberofimpactingparticlescor-
Section2[17].
respondstoanincreaseinparticleconcentrationandresultedinan
Thechangesinsignalareabetween0and150kHzforthethree
overallincreaseofAEspectralenergy.Importantly,thefeasibility
differentparticlesizerangesasafunctionofnumberofparticles
studyofthisheterogeneousreactionproducedasimilarrelation-
(N)isillustratedinFig.10.Anincreaseinthenumberofimpact-
ship[17].
ingparticlescorrespondstoanincreaseinparticleconcentration

andresultedinanoverallincreaseofAEspectralenergy.Impor-

tantly,theseeffectsareconsistentwithpublishedtheoreticaland

experimentalstudiesofcollisionsbetweenglassspheres[32],aball

Fig.7. PredictedAEspectraarisingfromtheimpactof1,3and7particleswiththe Fig.9. PredictedAEspectraarisingfromtherandomizedimpactof1000particles

vesselwall. ofdifferentsizes.
164 M.Tramontanaetal./SensorsandActuatorsA228(2015)159–169

Fig.10. RelationshipbetweentheAEspectralenergyandthenumberofimpactingparticlesandtheparticlesize.

bearingandaplate[35],andotherinvestigationsofthemixingof monitoringsystemthatprovidesenhancedsensitivityandoperates

particulatesystems[7,16,17,38]. overthebroadfrequencyrangeof0–180kHz.

Itwasdecidedthatthemostappropriatewaytomatchtheultra-

4.3. DiscussionofFEsimulationresults sonicsystemperformancewiththesignificantfindingsoftheFE
simul ationpr ogrammewas tode velo padual-tra nsducera pp roa ch.

The FE simulation programme has been used to enhance To enhance the sensitivity of the monitoring system, a resonant
the un ders tanding of the underlyin g dy namic s of t his complex 40 kHzultra son ictransduc er was proposed.W hereas, th ebroad-
ultr asonic wave pr op agat ion system. Important ly, the following ban dsy stemrequ irementsco uldb ematched usingano ff-re sonant
statement shave tobeconside redinth econtextoft hisw ork:only mode ofope ration.Itwas consid er edthatth eset wo transducers

a 2D FE model has been utilized; a well-mixed system has been

assumed;andtheentiresystemoperatesatacommontempera-

ture. The validated model has provided the following properties

associatedwithparticleimpactsonaglassreactorvessel:

• Thereactorvesselisahighlyattenuatingsystem,withonly1.1%

oftheenergyinthehorizontalplanetransferredthroughtothe

receptionsensorposition.
• Thepredic tedsys temsensitivitytoparticleimpactatthefurthest

excitationpointinthemodelisonly3%weakerthananimpact

in the centre of the model, thus the AE sensor position is not

themostcriticalparameterassociatedwithsystemSNR.Inter-

estingly,thissupportstheexperimentalresultsacquiredduring

thepilotstudyandpresentedinreference[17].
• Part icles izeha sad irectcorre lat iontothe energyinthesystem.
• TheAEs pect ral pr ofiles arestrongly de pen denton p arti clesize.
• The AE spectralr esponse sar ewithina 0–180kHz fre quency band,

withthemaincontributionsbetween10and60kHzandapeak

around40kHz.

4.4. Ultrasonictransducerspecification

Analysis of the simulation results provided the basis of
Fig. 11. Magnitude of electrical impedance for (a) stacked device and (b) off-
ultrasonic transducer design specifications for use in a passive reso nan cedevice.
M.Tramontanaetal./SensorsandActuatorsA228(2015)159–169 165

Fig.12. AEspectraofitaconicacidintoluene,andtheimpulseresponsesofthetransducerandsystemacquiredusing(a)thestackedtransducerand(b)theoff-resonance

transducer.
offeredcomplementarityintheirmonitoringcapabilitiesandcould propertiestomatchthedesiredspecification.Ineachdevice,the

be used to provide both particle concentration and particle size loadmediumwastheglasswallofthereactor.

informationonheterogeneoussystemsfoundinthechemicalsand The resonant 40kHz transducer design (stacked device) uti-

pharmaceuticalindustries. lizedastacked3–1connectivitycompositetransducercomprising

fivelayersofPZT4Dceramicconnectedmechanicallyinseriesand

electricallyinparallel,withasingleslotineachverticalsideincor-

5. Experimentalresults poratingasoftpolymerCIBA/GEIGYCY208/HY956[48].Thedevice
had an a c tive lateral d imension o f 9mm×9mm , en caps ulated

5.1. Transducerfabrication withinapolymercylinderofdiameter45mmandthetransducer
hadan o verallthic knessof 37 mm.

The two ultrasonic transducer designs utilized piezoelectric To accommodate the broadband ultrasonic receiver design
cerami c com posite con figurations [47] to t ailor the piezoelectric specifi cation,a1–3c onne ctivitypiez oelectricce ramiccom posite
166 M.Tramontanaetal./SensorsandActuatorsA228(2015)159–169

Fig.13. Powerspectraofdifferentmassesofitaconicacidin500mLoftoluenemeasuredusingthestackedtransducer.

transducer [47,49–51] was designed to operate below its fun- response,i.e.forthetransducer,vesselandpre-amplifier,isdomi-

damental resonant mode (off-resonance device). Again, PZT4D natedbythevesselresonancesasobservedintheoriginalfeasibility

ceramicwastheactivephaseandthesamesoftpolymerwasused study[17].

asthepassivephaseina30%ceramicvolumefractiondevicewith
dim en sionsof 18mm × 1 8m m×7.8m m.The fundam entalt hick- 5.2. Concentrationeffects

ness mechanical resonance frequency was designed to occur at

240kHz,withnoothersignificantresonancesbelowthethickness Fig.13showsthespectracorrespondingtodifferentconcentra-
mod edu etoth eh ighla teralattenu ationofthe softp olym erincor- tionso fita conic acid ofafixe dparticlesize (cid:3) in500mL oftoluene

poratedintothepiezocompositedesign. andemployingthestackedtransducerastheAEsensor.Asexpected

Fig. 11 illustrates the magnitude of the electrical impedance, the peak sensitivity is at the resonance frequency of the device,

both experimental and simulated, for each transducer config- with discrimination between each particle concentration clearly

uration. There is excellent correlation between the theory and observed.

experimental data for the stacked configuration, as shown in The difference in signal energy for the particle size ranges

Fig.11(a).Theagreementisnotasgoodfortheoff-resonancedevice, described in Table 1, as a function of concentration, employing

Fig. 11(b), although it is clear that there is only one significant thestackedtransducerisillustratedinFig.14.AssumingAEaris-

resonancemodebelow200kHz. ingfromNparticlesimpactingwithconstantvelocityontheinner

Moreover,apre-amplifierwasintegratedintoeachtransducer layeroftheglassreactor,theintensityorenergywillbepropor-

housingtoimprovesystemSNR.Thepre-amplifierhadavoltage tionaltothesquarerootofN[35].However,powerspectrawere

gainof40dB,withlownoisecomponentschosenandpass-band calculatedinthiswork,sotheintensityofpowerspectrashould

filters to complement the transducer operating characteristics, varylinearlywiththemassconcentrationofparticlesandhenceN.

includedinthedesign. Thevariationofenergyisapproximatelylinearwithconcentration

To evaluate the transducer performance, three experimental

measurementswereacquiredforeachconfiguration:theimpulse
1100
responseoftheunloadedtransducer;theimpulseresponseofthe <251um
transduce r atta ched to th e vessel an d th e AE de tected by e ach 1000 >251um <500um
>500um <853um
devicefromparticulateinteractioninthevessel.TheHsu-Neilson
900
pencilleadtestwasusedasawidebandexcitationsourceforboth

oftheimpulsemeasurements.Thetwomeasurementswerethen 800

(csoimzep(cid:4)ar)esdu swpietnhd tehde AinE5 s0p0ecmtrLao afctqouluireende fraonmd a1g0i0ta gt eodf iatatcaosntiicr aractide a.u. 700
y
of250rpm.Fig.12(a)and(b)illustratesthespectralcharacteristics g 600
er
forthestackedandoff-resonanttransducerconfigurations,respec- n
E 500
tively. It is clearly evident that the stacked transducer provides

superiorsensitivityandtheoff-resonantcompositeoperatesovera 400

widerfrequencyrange.Itshouldbenotedthatthefrequencyspec-
300
trumoftheoff-resonantdeviceisnotflatbetween0and180kHz.

First, the resonant mode of the manufactured device is slightly 200

lowerat170kHzandtherearesomelateralmodeactivitypresent
100
at 20 and 35kHz. Nevertheless, this device is considered appro- 0 20 40 60 80 100
pr iate for this app licationasind icate dbyth ew idebandre sponse Mass [g]

associatedwiththeAEfromtheparticulatesuspensionshownin
Fig.14. EffectsofbothconcentrationandparticlesizeontheAEsignalenergy
Fig.12(b).Itisalsointerestingtonotethatthe‘system’impulse mea sure dusing th estac kedtransducer .
M.Tramontanaetal./SensorsandActuatorsA228(2015)159–169 167

shiftingeffectsarenotimmediatelyobviousfromFig.15andhence,

thespectraweredissectedintoanumberoffrequencyrangesto

examinetheirpotentialtodiscriminatebetweendifferentparticle

sizes.
The energy of each spectrum in the range between 60 and

200kHz was computed as a percentage of energy over the total

bandwidth (0–200kHz). Fig. 16 illustrates this approach for dif-

ferent particle sizes; the percentage of area was invariant with

concentration,butincreasedwiththeparticlesizerangeandcould

beusefulforestimatingthemeansizeofparticlesinliquid.Most

commercialAEmonitoringsystemswouldmissthistypeofinfor-

mationastheyoftenconvertthesignaltoaDClevel.Importantly,

theshifttowardslowerfrequencies,correspondingtoanincrease

inparticlesize,couldbeanimportantindexforinsituparticlesize

characterization.
6. Conclusions

A model-based approach has been described through which
tFhige. o1f5f-. reAsEo pnoawntetrr aspnesdcturcae orf. different sizes of itaconic acid particle measured using thea cousticemissi oncharact erist icsass ociatedwi thparticl e–wall

impacts in a reactor vessel have been analyzed to provide a

upto20gdm−3 (i.e.10gin500mL)Abovethisconcentrationthe transducerdesignspecificationforanon-invasivepassiveacous-
sen sit ivit y changes due to the satu ration of pa rticles in tolu ene ticmonitor ingsys tem.Thetran sdu ce rspecification lendsi tselfto
or the pre sence of othe r s ourc es of AE ( par ticle–imp ell er colli- im plementatio n throug h tw o separate devices: a l ow-fre quen cy
sio ns, particle–p art icles c ollisions ) [1 7]. In fact, at values above (40kHz)sensitiv econfigu ratio nandabr oadband (1 0–180kHz)off-
20gd m−3thedegreeofn on-lineari tydep en dson th epartic lesize, reso nant structure .Thecomplem en ta rityofthes etwode vices has
wi th thelarges tpartic les givingamore linearre spo nse .FromFi g.14 been sho wn to pro vide discrimination o f p articl e co ncentrat ion
alone the reisno improve ment o nthe inform ationthat isach iev ed comb inedwi th particles izequalification .
using RMSs ig na lsbecauseitw ou ldn otbepossibl eto di stinguish Theabi lityt omonit orch angesinparticlesizeandconcentra-
betwe en,fo rexam ple,thes ig nalene rgy of ahighco nce ntrationof tioninf ormati on isofgre atinteres tt omany phar mac euticaland
smallpar ticle sfromal ow concen tration of l argep articles. food industrypro ce sse s.The reistyp ica llyac ompromisebetw een
the s ensing c apabilities of su ch a monit or ing system, with the

following key priorities desired: non-invasive; low power; high
5.3. Particlesizeeffects
sensitivity; and particle size discrimination. The dual transducer

approach described here offers a route to in situ process moni-
Interestingspectralfeatureswereapparentwhenemployingthe
toringbycombiningallthesedesirableattributesusingultrasonic
off-resonanttransducer.Fig.15illustratesthespectrafor100gof
transducers. Importantly, it would be possible to integrate both
differentparticlesizesofitaconicacidin500mLoftoluenecollected
transducer configurations described in this paper into a single
using the off-resonant transducer. Note that the peak at 90kHz
package,withadequatelateraldampingbetweenthetwophases
correspondstotheresonanceoftheoillayer.Theincreaseinthe
ofthetransducervitaltominimizecrosstalk.
relativeintensityofthesignalsinthelowerfrequencyregionsas
Thispaperhasinvestigatedparticleconcentrationandsizeand
particle size increased is in agreement with previously reported
notaddressedthepracticalissuesofmixtureswithdifferentparti-
studies of the impact of objects with surfaces [7,17,35–38]. An
clematerialproperties.Forexample,theAEspectralcharacteristics
increaseinparticlesizecausesanincreaseinAEmagnitudeand
fortwoparticlesofthesamesize,butdifferentdensity,illustrate
adecreaseinAEfrequencyowingtoadependenceontheparti-
that the frequency content is similar, with some of the higher
clesurfaceareaandimpacttime,respectively[33].Thefrequency
frequencycomponentsreducedinamplitudeforthedenserpar-

ticlecase.Thiswouldbeanadditionallevelofcomplexityofusing
1 the acous tic t echniq ue de scribed in this pa per. Howev er , it is
<251um
0.9 >251um <500um the authors’ proposition that if the mixtures contained different
>500um <853um
materials,thisiswhereadditionaltechniquessuchasRamanspec-
0.8
trometry would be needed to provide chemical information and
ge 0.7 hence,co mplem ent theinfo rm ationpr ovidedby AE.Ifapro cess
a
cent 0.6 ccuonltttaoindeedd muciexttuhreeps aorft idcilfefesrizeento fmeaatcehrimalast, ewrihaillefr oitm wtohueldA Ebes idginfafil-,
er 0.5 AEc ou ldbeus edt oprovid ea sig natur eofthe proce ssa nd hence,
a p 0.4 cou ldbe use dtod et ectanyp r ocesschan ge s.
e
r
A 0.3
0.2 Acknowledgements
0.1
The authors would like to acknowledge EPSRC (GR/S8599/01,
00 20 40 60 80 100 GR/N27 644/01) and a CPA CT industrial cas e stude ntship for the
Mass [g] fundingthrough wh ich thisco llaborative proj ectwasesta blish ed.

Inaddition,theRoyalSocietyisthankedfortheawardofaUniver-
Fig.16. PercentareaofAEspectra,measuredusingtheoff-resonanttransducer,for
sityResearchFellowshiptoAlisonNordon.
differentconcentrationsandparticlesizesofitaconicacidintoluene.
168 M.Tramontanaetal./SensorsandActuatorsA228(2015)159–169

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