Electrostatic phase separation: A review

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ContentslistsavailableatScienceDirect

Chemical

Engineering

Research

and

Design

jo u r n al ho m e p a g e :w w w . e l s e v i e r . c o m / l o c a t e / c h e r d

Electrostatic

phase

separation:

A

review

S. Mhatre

a

_,

_V.

_Vivacqua

a

_,

_M.

_Ghadiri

b,∗

_,

_A.M.

_Abdullah

a

_,

_M.J.

_Al-Marri

c

_,

A. Hassanpour

b

_,

_B.

_Hewakandamby

d

_,

_B.

_Azzopardi

d

_,

_B.

_Kermani

e

a_Center_for_Advanced_Materials,_Qatar_University,_Doha_2713,_Qatar

b_Institute_for_Particle_Science_and_Engineering,_University_of_Leeds,_Leeds_LS2_9JT,_UK

c_Gas_Processing_Center,_Qatar_University,_Doha_2713,_Qatar

d_Department_of_Chemical_and_{Environmental}_Engineering,_University_of_Nottingham,_Nottingham_NG7_2RD,_UK

e_Keytech,_Camberley_GU15_2BN,_UK

a

r

t

i

c

l

e

i

n

f

o

Articlehistory:

Received17October2014 Receivedinrevisedform12 February2015

Accepted19February2015 Availableonline25February2015

Keywords:

Electrocoalescence Phaseseparation Emulsionbreak-up Crudeoiltreatment

a

b

s

t

r

a

c

t

The currentunderstanding anddevelopmentsinthe electrostaticphaseseparation are reviewed.Theliteraturecoverspredominantlytwoimmiscibleandinter-dispersedliquids followingthelastreviewonthetopicsome15years.Electrocoalescencekineticsand gov-erningparameters,suchastheappliedfield,liquidproperties,dropshapeandflow,are considered.Theunfavorableeffects,suchaschainformationandpartialcoalescence,are discussedindetail.Moreover,theprospectsofmicrofluidicsplatforms,non-uniformfields, coalescenceonthedielectricsurfacestoenhancetheelectrocoalescenceratearealso con-sidered. Inadditionto theelectrocoalescenceinwater-in-oilemulsionstheresearchin oil-in-oilcoalescenceisalsodiscussed.Finallythestudiesinelectrocoalescerdevelopment andcommercialdevicesarealsosurveyed.

TheanalysisoftheliteraturerevealsthattheuseofpulsedDCandACelectricfieldsis preferredoverconstantDCfieldsforefficientcoalescence;buttheselectionoftheoptimum fieldfrequencyaprioriisstillnotpossibleandrequiresfurtherresearch.Somerecentstudies havehelpedtoclarifyimportantaspectsoftheprocesssuchaspartialcoalescenceand drop–dropnon-coalescence.Ontheotherhand,somekeyphenomenasuchasthinfilm breakupandchainformationarestillunclear.Somedesignsofinlineelectrocoalescershave recentlybeenproposed;howeverwithlimitedsuccess:theinadequateknowledgeofthe underlyingphysicsstillpreventsthistechnologyfromleavingtherealmofempiricismand fullydevelopinginonebasedonrigorousscientificmethodology.

1. Introduction

Dispersionsofoneﬂuidinanother immiscibleﬂuidcanbe foundinmanynaturalaswellassyntheticproducts,suchas milk,petroleum,foodproducts,drugs,paints,etc.The sep-aration ofthe twophasesbecomesnecessarytorecoveror purifytheproduct,e.g.waterseparationfromcrudeoil,phase separationinsolventextraction,glycerolseparationfromthe bio-diesel(Abeynaikeetal.,2012),etc.

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moreover,theseparationcannotbetothedesiredextent.In anemulsion,phaseseparationspeedcanbegovernedbythe probabilityofdrop–dropcontact.Therefore,thephase sepa-rationprocess canbeenhancedbystimulatingthe relative motionbetweenthedroplets,usingexternalforcessuchas mechanical,thermal,electrostaticandchemicalor combina-tionofsomeofthesemeans(Eowetal.,2002;Klassonetal., 2005;Sunetal.,1999;Wuetal.,2003).

Inspiredbytheinventionofanelectrostaticprecipitatorby FrederickCottrellinearly 20thcentury(Cottrelland Speed, 1911),theuseofelectrostaticforcesforthephaseseparation ledtoaflurryofpublicationsinthe firstthirdofthat cen-tury(Speed,1919;Muth,1927).Thetechniqueinvolvingthe useoftheelectricfieldinphaseseparationofliquidphases, commonlyknownaselectrocoalescence,hasbeendeveloped andextensivelyinvestigatedduetoitsfast,cleanandefficient coalescencecapabilities(Eowetal.,2001).Apartfromwater separationfromoil,electrcoalescencecanalsobeemployed in phase separation operations such as solvent extraction and dispersion as well as to fractionate mixed oils (Scott and Wham,1989). Inindustrial applicationssuch ascrude oildemulsification,electricfield iscommonlyusedtohelp thesmall waterdroplets tocomeclosertoeach other and eventuallymergeinto eachother orwithaninterface. The largerdropscanthenbeeasilysettledbygravity,resultingina finalproductthatcontainswaterbelowaprescribedlevel.The applicationofelectricfieldincreasesthecoalescencerateas wellasenhancingthemigrationspeedofthedropletstowards theelectrodes,facilitatingphaseseparation.

Researchintoelectrcoalescencehasresultedinmany find-ings,whichhavehelpedtomakethephaseseparationfaster. Severalstudies,identifyingthechallengesinthe electrocoa-lescenceprocessandthemethodstoresolvethem,havebeen carriedout(Pearce,1953;Zhangetal.,2011;RayatandFeyzi, 2012;Noik et al., 2002;Fjeldly et al.,2008; Midtgard, 2009). Thefactors directlyaffectingtherate ofelectrocoalescence aremany;namelythechainformation,fluidmotion,type(DC orAC)andfrequencyoftheappliedfield,partialcoalescence, electricalandphysicalpropertiesoffluids,etc.Thephysical andelectricalpropertiesofthecrudeoilfromdifferent reser-voirscanbedifferent;alsotheycanvarywiththeageofan oilwell(Bergetal.,2010).Sincethesepropertiesgovernthe stabilityoftheemulsion,identifyingtheoptimumoperating parametersisdifficult.Itfollowsthatdesigningauniversal electrocoalescerwhichcanhandlecrudeoilfrom varietyof oilreservoirsisstillverychallenging.

2. Electrocoalescence

dynamics

2.1. Electrostaticforceofattractionbetweendrops

Themechanismofcoalescenceoftwodropsinthepresence ofanelectricfieldinvolvesthreedistinctsteps.Theapplied electricfieldpolarizesanindividualdropandeachdropacts as adipole withinduced positive and negative charges at twopolarends.Thedipolealignsinthedirectionofapplied electricfield.Thefirststepofelectrocoalescenceinvolvesthe interactionoftwodropsduetoattractionbetweenopposite polaritypoles. In the large separation limits, the different forcesactingonadropcanbeelectrostaticforce,dragforce andgravitationalforce(ifdropsarecoarse).Whentwodrops move towards each other, atsmall separation thereexists aninterstitialfilmofthemediumfluidbetweentheleading

faces of the drops. The second step of electrocoalescence involves thesqueezingoffluidattheplateauborderofthe thin film.As drops come closer, the thickness ofthe film reducesfurther.Inthethirdstep,whenthefilmbecomesvery thin,thecoupledactionofelectrostaticandmolecularforces breakthefilmallowingthetwodropstomergetogether.

If drops carry inherentcharges, ‘migratory coalescence’ resultsduetotheelectrophoresis(WilliamsandBailey,1986). In addition to driving drops closer, an electric field also enhances the thin filmbreakup. Inoneofthe first studies of electrocoalescence, Berget al. (1963)found that fortwo anchoreddrops,thecoalescenceratewasproportionaltothe strengthoftheappliedelectricfield(E0)whenE0wassmall; whereasifE0washigh,theratewasfoundtobeproportional

toE02.Apartfromthemagnitudeoftheappliedelectricﬁeld,

there are many other parameterswhich can inﬂuence the forceofattractionbetweenthecoalescingdrops.Theyarethe inter-dropseparation,sizeofdrops,shapedistortionandﬂuid propertiessuchasconductivity,permittivity,viscosity, interfa-cialtension,etc.Thesubsequentstudiesinelectrocoalescence were focusedon these parameters, their optimization and consequentlymakingtheprocessfaster.

Theradial(Fr)andangular(F)componentsofthe

electro-static forceofattractionbetweentwoconductingspherical dropsofradiiaandbinadielectricmedium(asinFig.1)are givenas(Waterman,1965;Atten,1993),

Fr=−12∈mb3E20 a3

(d+a+b)4(3cos

2 ₋₁₎_, ₍₁₎

F=−12∈mb3E20 a3

(d+a+b)4sin2. (2)

Eqs.(1)and(2)canberewrittenfortheuniformsizeddrops (a=b)alignedinthedirectionoftheappliedelectricﬁeld(=0) as(Waterman,1965;Atten,1993),

Fe=−24∈ma2E20 a4

(d+a+b)4. (3)

The force Fe (which is proportional to E02) is

dielec-trophoretic in nature. Itis a short range force and Eq. (3) becomesinvalidwhentheseparationbecomessuchthatd/a

1.InthatcaseDipole-Induced-Dipole(DID)modelgivesmore accurateestimateoftheelectrostaticforceofattraction(Yu andJones,2000;Siuetal.,2001).DIDmodelisbasedonthe assumptionthatwhentwodipolesareincloseproximity,a point-dipole inducesits multiplereﬂections.Theradial(Fr)

andangular(F)componentsofelectrostaticforce,usingDID

model,canbeexpressedinamorecompactformasfollows (Lundgaardetal.,2006),

Fr=−12∈mb3E20 a3

(d+a+b)4(3K1cos

2 ₋₁₎_, ₍₄₎

F=−12∈mb3E20 a3

(d+a+b)4K2sin2. (5)

ThecoefﬁcientsK1andK2areexpressedas,

K1=1+ a3s5

(s2₋_b2₎4+ b3_s5

(s2₋_a2₎4+

3a3_b3₍₃_s2₋_a2₋_b2₎

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a b

d

s

r ρ

d σ ε d μ _d _d

μ m

ρ

m

m σm

E0

0

Drop Phase

ε Medium Phase

Fig.1–Interactionoftwodropsinanelectricﬁeld.

and

K2=1+ a3_s3

(s2₋_b2₎3 + b3_s3

2(s2₋_a2₎3+

3a3_b3

(s2₋_a2₋_b2₎3. (7)

When the drop–drop separation is large (d>a, b), the coefﬁcientsK1andK2areequalto1andEqs.(4)and(5)reduce toEqs.(1)and(2),respectively.

Theeffectofseparationondropshapeandinturnonthe rateofelectrocoalescencehasbeenthetopicofmany stud-ies (Latham and Roxburgh, 1966; Atten, 2005; Atten et al., 2006;Raisinetal.,2008).Sincetheelectrostaticforcebetween neighboring drops is short range, for two drops to attract andcoalesce,theymustbewithinanambitwhichis deter-minedbyacombination ofthe appliedfield,dropsizeand fluidproperties.Inanemulsion,theinter-dropseparationis determinedbythecontentandthesizeofthedispersedphase (PanchenkovandVinogradov, 1970). Theelectrostatic inter-actionoftwofalling dropsin aquisant oilwas studiedby Pedersenetal.(2004)usingnumericalsimulationsand experi-ments.Theirresultssuggestedthatthepointdipolemodelcan beusedforaccurateestimationofelectricforceswhend>a. Attheseparationsd<a,theactualelectricfieldbetweeninner polesandtheeffectoffilmdrainageshouldbeconsidered.

Theestimationoftheactualelectricpotentialdifference (V)betweenleadingpolesofthetwoapproachingdropsin anexternalelectricfield(E0)hasbeenacrucialfactorin elec-trcoalescencecalculations. However,thepresent models to estimate electrostatic interaction force between two drops are basedon externallyappliedelectric field.The relation-shipbetweenVandE0suggestedbyDavis(1964)forclosely spaceddrops(0.001<d/a<0.1)ismathematicallyhardtouse. Themagnitudeoftheinduced charge atthe interface ofa coalescingdrop,duetotheappliedfield,canbeestimatedas,

±Q=a2∈mE0. (8)

Assuming the value ofcoefﬁcient =5 for twouniform sized drops aligned in the directionof ﬁeld forseparation 0.001<d/a<0.1,AttenandAitken(2010)derivedasimpler rela-tionshipbetweenVandE0as,

V∼=₂ 2aE0

log

1.78a_s

. (9)

Eq.(9)isingoodagreementwithDavis’s(1964)expression intheseparationrangeof0.001<d/a<0.1.

Atheoreticalstudy oftwocolliding sphereswascarried out byFriesenand Levine(1992), who developeda method

to calculate the interaction energyand the force between twocharged,conductingspheresinauniformelectricfield. Thedeformationandaggregationofdropletsinanemulsion alteritsviscoelasticpropertiessuchasyieldproperties(Mason etal.,1996).Detailedanalysisofthemotionandinteractionof twodrops,fallinginaquiescentmedium,paralleland perpen-diculartotheappliedexternalfieldwasdoneexperimentally aswellasnumericallybyChiesaetal.(2005).Thestudywas morefocusedonflowcirculations,dipolesandsurfacetension gradients.Theanalysisindicatedthattheexpressionsfordrag, buoyancyandfilm-drainage,applicablefortherigidparticles donotgivetheaccurateresultsforthetwo-dropinteraction duetotheinternalflowsaswellasvariationinthesurface tensioninthepresenceofaelectricfield(Chiesaetal.,2005). Thesimultaneouseffectsofthehydrodynamicsandelectric stressesonapairofdropswerestudiedbyRaisinetal.(2011a). Theysuggestedanexperimentalsetupforgenerationofapair ofunchargeddrops,puttingtheminthePoiseuilleflowand applyinganelectricfieldtostudytheelectrohydrodyanamic interactions.

2.2. Shapesofcoalescingdrops

Animmediateeffectadropshowsuponapplicationofan elec-tricﬁeld isshapedeformation. Twoclosely placeddropsin anelectric ﬁeldexhibitdeformationwhen the electrocapil-lary number (orelectricalWeber number), CaE=εmaE02/ is

large.AtsmallCaE,thedeformationcanbeobservedonlyat

theleadingpolesofthecoalescingdrops.Suchdeformation isthe resultofhigh electricﬁeldbetweenleading edgesof thetwodropsandthushighchargedensity.TheTaylor’s fac-tor(E0

(2aεm/))forthestabilityinanelectricﬁeld(Taylor,

1964)whichis0.648forasingledropislowerforadropin apair(LathamandRoxburgh,1966;Brazier-Smith,1971)and dependsontheinter-dropseparation.Adropcanalsoshow foreandaftasymmetricdeformationwhenitisneartothe electrodesurface(ImanoandBeroual,2006).Thepresenceof dropsintheproximityofelectrodesnotonlyinduces defor-mation,butalsoshieldstheinnerdropsintheemulsion.

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numericalmethodsindicatethatforcloselyhelduniformsize drops,coalescence occurs whenleadingfaces deformsuch thatd/d0≈0.5. Bjorklund(2009) usedthe coupledLevel-Set

methodandGhost-Fluidmethodforthenumerical investiga-tionoftwodropsinanelectricfield.Inauniformfield,when equalsizedropsaligninverticaldirectionwiththeelectrodes, dropscaninteractonlywhentheyarewithinacritical sep-aration.Ifdropsarefarapart,theyattracttotheelectrodes due tothe mirrorcharges across the electrode (Bjorklund, 2009).ImanoandBeroual(2006)reportedsimilarobservations fora single as well as multipledrops restingon a dielec-tricsurfaceinACelectricfield.Theycarriedouttheoretical andexperimentalinvestigationofinfluenceofanelectrode on shapes and coalescence of drops. A drop nearer to an electrodestretchasymmetricallyduetounbalanceofelectric forces.Thecontacttimeoftwofreelysuspendedaswellas anchoreddropsinanelectricfieldwasstudiedbyRaisinetal. (2011b).Theyfoundthatthecontacttimewasinversely pro-portionaltotheinitialmaximumelectrostaticpressure.They alsostudiedthedeformationofwater–airandwater–oil inter-facesinelectricfieldusingFiniteElementArbitraryLagrangian Eulerian(FE-ALE)methodwithmoving meshesinCOMSOL Multiphysics(Raisinetal.,2011c).Inaflowingemulsionunder a uniform electric field the coalescence of drops depends on deformation of facing surfaces, their motion and the drainageoftheoilfilmbetweendrops.Thetimeofcontact forverysmallandcloselyplaceddropletswithhigh viscos-ityratio(= d/ m)islargercomparedtodeformablelarger

drops.

2.3. Dampingforces

Whentwoapproachingdropsareatafairlylargeseparation distancethedragforce(FD)opposesthemotion.For arigid

sphereinStokesregimeFDcanbegivenas(Davisetal.,1989),

FD=6 ma . (10)

However,inthecaseofadropmotioninviscousmedium theactualdragforceexperiencedbydropislowerthanthat givenbyEq.(10)onaccountofthecirculationsonbothsidesof theinterface.Hadamard–Rybczynskiequationgivesan accu-rateestimateofthedragforceondrop,whichiswrittenas,

FD=4 mac , (11)

wherec=(3+2)/(2(+1))

Theresistancetothemotionofthecoalescingdropsatthe largeseparationdistances(d>a)ispredominantlyduetodrag forcewhereas atda, it isgovernedbythe ﬁlm thinning force.Filmthinningforce(Ff)istheforceduetodrainageof

theinterstitialliquid filmbetweenabouttocoalescedrops. Ifthedistancebetweentheleadingedgesisverysmall rela-tivetodropradiiandtheflowiswithintheStokesregime,the expressionfortheresistivefilmthinningforcecanbewritten as(Davisetal.,1989;Chiesaetal.,2006),

Ff =−6 m

d

_ab

a+b

2

f, (12)

InEq.(12),f=1ifdropsaretreatedasrigidspheres.The force Ff resists the drainageof ﬂuidfrom the ﬁlm trapped

betweenthetwodrops.DifferentexpressionsofFfhavebeen

proposed for the drops (Vinogradova, 1995; Barnocky and Davis,1989).

Eq.(12)indicatesthatapartfromdropradiusand separa-tion(d),themediumphaseviscosity( m)playsamajorrolein

thefilmdrainagestageofdrop–dropcoalescence.Chiesaetal. (2006)reportedthatintheabsenceofanelectricfield,thefilm thinningforce(Ff)increaseswithdecreasingviscosityofthe

mediumﬂuid.IncreaseinFfcanbeattributedtotheincrease

inrelativevelocityofdropsonthedecreasingmedium vis-cosity.Onapplyinganelectricﬁeld,thedipolarforcebetween leadingedgesofdropsopposestheﬁlmthinningforce.The viscosityeffectdiminishesasdropsapproacheachotherand completelyvanishesatonsetofthecoalescence(Chiesaetal., 2006).

Thepresenceofsurfactantsattheinterface,accompanied by drop elongation,causes the interfacial tension gradient leading toMarangonistresses. Thisinhibitsthe generation ofinternalcirculations.Levan (1981)tookintoaccountthe effectsofinducedcirculationsandinterfacialtensiongradient togetarevisedexpressionfordragcoefﬁcient.Thepresence ofsurfactantsstronglyaltersthedragforcebytheformation of stagnant caps (Hamlin and Ristenpart, 2012). Depend-ingontheadsorptionanddesorption rateofthesurfactant molecules,atverylowandveryhighconcentrationsadrop obeys Hadamard–Rybczynskimodel.Howeverat intermedi-ateconcentrationsStokesexpressiongivesbetterestimates fordragforce.

Thedeformabilityofthecoalescingdropsplaysacrucial role inthe film drainage stage of the coalescence. With a numericalstudy ofthe coalescenceoftwodropsina flow-ing emulsion in uniformelectric field, Raisin et al. (2011c) concludedthat therateofelectrocoalescence dependedon thedeformationoftheleadingsurfaces,dropmotionandthe drainageoftheoilfilmbetweenthedrops.Thetimeofcontact forcloselyspacedverysmalldropletswithhighviscosityratio islongercomparedtodeformablelargerdrops.Theexternally inducedfluidflowalsodeterminestherateoffilmthinning. GiljarhusandMunkejord(2011)usedFEMtosolvethehead-on collisionsoftwodropsintheflowingmediumandpredicted thatastheflowcapillarynumber(Ca= ma/)increases,drop

deformsmoreandthecontactareabecomeslarger.Ittakesa longertimetodraintheﬁlm,andthusthecoalescencetime islonger.However,thecoalescencetimedecreaseswithan increaseintheelectrocapillarynumber(CaE).Similar

observa-tionswerereportedbyDongetal.(2002).Theirexperimental observations suggest that decreasing interfacial tension in the absence of electric field can resist the coalescence as large deformations (due to high Ca) inhibit the film thin-ning.However,inthepresenceofelectricfieldtheincreased deformation on decreasing interfacial tension assists the coalescence.

Depending onthe Ohnesorge number, Oh= d/

(da),

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torigidlytranslatetwoinitiallystationarydropstowardseach other.

Therateofdrop–drop approachcanbeestimatedusing expressionsforelectrostaticforceofattractionanddragforce. Atten(1993)proposedtheuseofthepointdipole approxima-tion(Eq.(3))andStokesexpression(Eq.(10))toestimatethe timeofdrop–dropapproach(t)whentheinitialinter-drop sep-arationislarge(d0≥a).Hisexpressionfortforuniformsized rigiddropsisgivenas

t= 8

15

m

εmE20

_s0 2a

5 −1

. (13)

Consideringtheﬂowcirculationsinandaroundthedrop andvalidityofpointdipoleapproximationonlyatseparations

d0≥a,Eq.(11)canbeusedalongwithEq.(3)tocalculatethe timeofdropmotionbetweenseparationss=s0ands=3a.The

resultingexpressionfortime(t1)foruniformsizedropsis,

t1= 81₂₀ mc

εmE20

_s

0

3a

5 −1

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Atthelower drop–dropseparations(s<a),theresistance tosqueezingofmediumfluidinthinfilmplaysavitalrole. When the viscosity ratio 1, the thinning of the film induces convection rings inside the drops. Film thinning betweenundeformeddropsandelectrostaticpressure distri-bution(=εmE2/2)nearthedropinterfaceresultintheviscous

forceproportionaltotheproduct ma(Atten,2012).Thus,the

timeofﬁlmdrainageuntilcontactoftwodropsisgivenas (Raisinetal.,2010;Raisin,2011;Atten,2012),

t2≈ 1 B

m

εmE2₀

_s0

a

1.7

, (15)

whereBisanondimensionalconstant.Timefordrop–drop contact(t)inanelectricﬁeldcanbemoreaccuratelyestimated as(t1+t2)thanbyusingEq.(13).

2.4. Thinﬁlmbreakup

Theapproachingdrops containafilmofthe mediumfluid betweentheirleadingfaces.Asdropsmovecloser,the thick-nessofthe film continuouslyreducesby squeezing atthe plateau border. On further reductionin the film thickness below1000 ˚A,molecularforcesstartplayingrole.The attrac-tivevanderWaalsforcehelpstoreducethethicknesswhile the double-layer repulsion tries to push drops apart. The filmattainsametastablestatewhentheplateauborder suc-tion,vanderWaalsattractionanddouble-layerrepulsiondo balance each other. Instability can set in due to the ther-mal/mechanicalshocksorthepresenceofimpuritiesatthe interfacewhichresultsintothebreakupofthefilmseparating thetwodrops.

Whentwodropsarewithinacriticaldistancefromeach other,themicroscopicallythinﬁlmseparatingthemcan rup-turerapidly,followedbythedrop–dropcoalescence.Acritical thicknessofﬁlmruptureisgivenbytheexpression(Chesters, 1991), tc=(Aa/8)1/3, where Ais the Hamaker constant. A

numberofmechanismsforthefilmruptureandsubsequent mergingoftwodropshavebeen proposedintheliterature. Oneofthehypotheses suggests thatthe filmbreakswhen theelectricfieldacrossthefilmattainsthedielectric break-downstrengthofthemediumphase(Pearce,1953).Similarly,

Sartor(1954)andAllanandMason(1962)suggestedthespark dischargeasacauseofthefilmbreakup.Thesehypotheses offilmbreakupwereprovedwrongbyPriestetal.(2006)by their study ofselective coalescenceofdropsina microflu-idicschannel.Fromtheanalysisoftheexperimentaldatait wasprovedthat,notthedielectricbreakdownbuttheelectric field-induceddynamicalinstabilityofthewater–oilinterface drivesthecoalescence. Anotherhypothesis(Fordedalet al., 1996)statesthatwhentheappliedelectricfieldishigh,the ions in the dispersed phase are pulled through the inter-face.Therupturedinterfaceleadsthecoalescenceofdrops. AccordingtoBergetal.’s (1963)hypothesis, thecoalescence involvescontinuousmaking,breakingandrearrangementof theintermolecularbondsoverthetwointerfacesincontact. Theoppositelychargeddropsattracteachother;butwhether theycoalesceorretreatbackisdependentontheconeangle theymakeon contact.Theexpressionforthecritical cone angleatdropcontact,whichisafunctionofthe electrocap-illarynumber,wasderivedbyBirdetal.(2009).Theirsurface energymodelpredictedthecriticalconeangleatdropcontact as30.8◦whichwasclosetotheexperimentallyobservedvalue. AccordingtoJungandKang(2009)thestrengthoftheelectrical forceandthesurfacetensionforcedeterminethecoalescence. Whentheelectricalforceisweakerthanthesurfacetension force, drops cancoalesce;whereas stronger electricalforce mayresultintheretreatofdropsaftertheircontact.However, presenttheoriesfailtoconvincinglyexplainthemechanism ofthinfilmbreakup.

3. Critical

conditions

for

electrocoalescence

Thethresholdconditionsforadrop-pairinelectricﬁeld,above whichtheshapedistortionandcoalescencecanoccur,have been suggested in literature. Latham and Roxburgh (1966) obtainedanexpressionforthecriticalappliedﬁeldfortwo closelyspaceddropsasEcrit˛d01.3andthecriticalseparation

asdcrit≈0.63d0.Duetothemotionofthecoalescingdropswith

time,thereisnostaticsolutionforthedeformeddropsatvery small separations.Latham–Roxburgh’s(1966)assumptionof fullelongationofadropwasrefutedbyTaylor(1968)withan argumentthattheneighboringdropsinapairdonotdeformas awholebutonlythenearestpolarsurfacesshowdeformation. Taylor,thus,didtheinstabilitystudywithtwoanchoreddrops inanelectricﬁeld.Withtheassumptionsofverysmall sepa-ration(d0a)andtheﬁeldbeingthepotentialdifferenceV

dividedbythecenter-to-centerdistance,Taylorcouldgetthe staticsolutionforthesmallseparations(Taylor,1968).Taylor’s analysisresultedintothecriticalparametersas,dcrit≈0.5d0

and criticalpotentialdifferenceas,Vcrit≈0.38d0

(/εma).

ThesamerelationfordcritwasobtainedbyAttenandAitken (2007)usingmathematicalanalysisandbyReboudetal.(2008) using numerical method and experiments. Brazier-Smith (1971)suggestedthatforevery valueoftheseparationratio (d0/a),thereexistsacritical valueofE

(a/), abovewhich dropscannotremainstable.Brazier-Smithetal.(1971) numer-ically investigatedthe possibilities of the coalescence and possibleshapesduringinteractionoftwodropsinanelectric ﬁeld.Theyconcludedthatfortheseparationratiod0/a<1.2, dropsdeform,readilyattractandeventuallycoalesce,whereas for d0/a>1.2, the facing surfaces deform, assume conical

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ofelectrocoalescence.Attenetal.(2006)obtainedexpressions forthecriticalseparationandthecriticalelectricfield simi-lartodcritandVcritbyTaylor.Theyalsosuggestedthatfora pairofdropsinanelectricfieldthereexistacriticalfieldfor deformationforeveryseparationvalue,givenas,

_d0

a

crit=

8 E0

∈ma

1.22

. (16)

BasedonTaylor’sassumptionofdeformationonlyatthe innerpoles,Atten(2005)extendedhiswork(Attenetal.,2006) tounequalsizedropswithd0/a1.Hetheoretically

inves-tigatedthecriticalconditionsforthedropdeformationand thecoalescenceoftwodrops(equalaswellasunequalsize) inelectric ﬁeld.The electrocoalescenceoccurs if,afterthe deformation,thefacingpolescomecloserthan45%oftheir initialseparation(Atten,2005).Attenproposeda dimension-lessnumber,

Be∼₌∈m

2

_V

d0

2_a

, (17)

whichisaratiooftheelectrostatic andcapillaryforces. Thecriticalconditions,Becrit anddcrit,arelogarithmic

func-tionsoftheinitialseparationtodrop-radiusratio,d0/a.With theassumptionofthequasistaticdeformationofan approa-chingdropinafreelysuspendedaswellasanchoreddrop-pair, AttenandAitken(2010)obtainedananalyticalexpressionfor theinterfacialdeformation.Thisanalysissuggestedthatthe shapeoftheapproachingfacesisslightlydependentonthe separationratiod0/a;however,theinterfacialtensionplays a major role in drop deformation. Thenumerical solution fortwoanchoreddropsbyRaisinetal.(2008)suggestedthat asmallpressuregradientexistsinsidethedispersedphase (neartheinnerface)duetohighelectrostaticstresses.This pressuregradientisresponsiblefordeformationatinnerpoles ofthecoalescingdrops.

Inacoalescingemulsion,theorientationoftwo neighbor-ingdropswithrespecttodirectionofappliedfieldalsoaffects thepossibilityofcoalescence(FriesenandLevine,1992).Inan electricfield,fortwodropstoattracteachother,thepotential energyofdipole moment(M=P·E)must benegative,which canonlybesatisfiedwhen(3cos2₋₁₎_>_0._Two_drops_cannot

interactif arccos(1/√3)<<arccos(−1/√3).For adrop-pair inelectricﬁeld,coalescencecanbepossiblewhentheangle madebythelinejoiningthecentersofthetwodropswiththe directionoftheappliedﬁeldissuchthat,54.71◦>>125.19◦ (Atten,1993;EowandGhadiri,2003).

4. Effect

of

turbulence

and

shear

ﬂow

Aneffectivedesignofawater–oilseparatorstronglydepends on the waterfraction inthe emulsion and the size ofthe droplets.Theelectrostaticforceofattractionbetween oppo-site polarity poles of two dipoles is a short range force. In a dilute emulsion, inter-drop separation is large and therefore drops need to be moved closer to expedite the electrocoalescence.Inducingtheshearﬂoworturbulencein the electrocoalescing emulsionincreases the probability of drop–dropcontact.

Otherwaysofincreasingtheprobabilityofinter-drop con-tact is by applying shear(Atten, 1993; Urdahl et al., 1996, 2001)orbygeneratingﬂowbypurginggas(Thoroddsenand Takehara,2000).Inanemulsion,theapplicationofshearforces

or turbulencealsoresiststhechainformation(Chenet al., 1994).However, veryintense shearcaninduce drop break-up(Galinatetal.,2005;MhatreandThaokar,2014).Fernandez (2009)reportedacoupledeffectofshearandelectrostaticsfor emulsionswithlessconductingdispersedphase.Theeffects of viscosity as well as electrical properties on the coales-cencerateoftwodropsaswellasinemulsionswerestudied numerically.Their directnumerical simulation results sug-gestthatshearforcecausestiltingandbreakupofchains.The commercialACelectrocoalescersbefore1960sweredesigned tomaintaintheturbulentflowconditionsinordertoresist thechainformation.However,themodernelectrocoalescers are designedtoprevaillaminarflowssoastofacilitatethe simultaneous coalescence and gravitysettling ofdispersed phase(Noiketal.,2006).Thesimultaneouseffectoffluidflow andelectricfieldcanspeeduptheelectrocoalescence.Bailes and Kuipa (2001) used sparged airbubbles to enhance the drop–dropinteractioninapulsedDCelectricfield.Therateof electrocoalescencewasfoundtobehighattheoptimum fre-quencyandincreasedwithincreasingtheairflowrateupto acriticalflowrate.However,excessiveairflowinanemulsion canresultinunfavorableeffectsasitdoesnotallowdropsto beincontactforenoughtime.Urdahletal.(2001)reviewed cor-relationstoestimatethemaximumstabledropletdiameterin laminarandturbulentflows.Itwasalsoshowedthatdifferent mechanismscancontributetotheelectrostaticcoalescence, suchasBrownianmotion,sedimentation,laminarshear, tur-bulentshearorturbulentinertia.

MelheimandChiesa(2006)usednumericalsimulationsto showthatturbulenceenhancestheelectro-coalescencerate overawiderangeofwatercutsintheoil.Thesimultaneous effectsoftheelectricandshearforcesonthecoalescenceof twodrops aswellasemulsionsconsistingofless conduct-ing dropsinthemoreconductingmediumwere studiedby Fernandez(2009)usingdirectnumericalsimulations.

5. Chain

formation

Chain formationhas been considered as oneofthe major retarding factors in electrocoalescence (Fig. 2). The stable chainsnotonlyreducetherateofcoalescencebut alsocan extend and bridge the electrodes, leading to short circuit. Chain formation in electrocoalescing emulsions has been attributed to the presence of impurities which alters the propertiesofaninterface.Inacrudeoilthepresenceof com-ponents suchas asphaltenes and resins stabilize the drop interface(Hannisdaletal.,2006)andhindertheﬁlmthinning, leadingtochainformation(Taylor,1988;Mohammedet al., 1993).Thepresenceofasphaltenesisalsoconsidered respon-sibleforincreasingconductivityofthecrudeoil(Lesaintetal., 2010).

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Fig.2–Chainformationin20%water-in-Buchancrudeoil emulsionat50Hzand1.6kV.Photomicrographsshowing emulsionat;(a)0sand(b)22s(afterChenetal.,1994).

contactattainsdielectricbreakdownlimit.Inanotherwork, BezemerandCroes(1955)usedasimilarexperimentalsystem toPearce’sexperiment(Pearce,1953)butwithlargerelectrodes separation and very dilute emulsions. Contrary toPearce’s observation,theyfoundthatdropletsmoveinthedirection ofthemaximumfieldstrength.Theyalsoreportedothernew observations: large droplets move faster than the smaller ones,velocityofthedropletsincreaseonincreasingthe elec-tricfield,dropletscollectnearthehighfieldelectrodeandform radialprojections,etc.Theseobservationsconfirmedtherole ofdielectrophoresisaswellaselectrophoresisinthe electro-coalescenceandchainformation.LaterGalvin(1986)claimed thattheelectrophoresisdoesnotplayanyroleinthe coales-cenceastwodropsmoveinoppositedirectionsinorderto contacteachother.However,thepresenceofsurfactantsor otherimpuritiescanimpartchargeatthewater–oilinterface. HoweandPearce(1955)usedsimilarelectrodesystemsasin referencesPearce(1953)andBezemerandCroes(1955)to elec-trocoalescewaterinveryviscoustarand observedthatthe carbonparticlesinthetarinhibitedthewaterdropletsfrom coalescing.

Chain formation is an undesirable effect in electrocoa-lescence and hasbeen addressed in a number of studies. Nevertheless,aconvincing mechanismforchainformation isnotavailableintheliterature.However,thereexist stud-iessuggestingmethodstoavoidthechainformation.Bailes

and Larkai(1981)firstusedthepulsedDCelectricfieldsfor coalescenceofawater-in-oilemulsion.Theyfoundthatfor thepulsedDCelectricfieldinfrequencyrangebetween0.5and 60Hz,8Hzgaveoptimumcoalescencerate.BasedonBailes andLarkai’s(1981)experimentalsetup,Midtgard(2009,2012) subsequentlyproposedtheelectrostaticfieldtheoryaswellas thecircuitanalysistheoryandsuggestedtheoptimumpulsed field parameters which did notresult in the chain forma-tion.ApplicationofthepulsedDCfieldshasbeenproposed asasolutiontoavoidthechainformation(Mohammedetal., 1993;Eowetal.,2002).InthepresenceofapulsedDCfield, coalescence can beobservedonlyduringrising and falling edgeswhilenocoalescenceoccursinthemiddleofthepulse width (Taylor,1996).Like analternatecurrent,apulsed DC fieldexhibitsanoptimumfrequencyanditisdependenton thepropertiesofthecrudeoilandelectrodesaspects.

TheACelectricfield hasanadvantageovertheDC field becauseofitsabilitiestosuppresshydrodynamicflowsinan emulsion. Moreover,theACfield aboveacriticalfrequency doesnotresultinthechainformationandsuppressesthe sub-sequentshort-circuitingoftheelectrodes(Chenetal.,1994). HighfrequencyACfieldhasbeensuggestedasasolutionto avoidthechainformation.Chenetal.(1994)studiedthechain formation andcoalescence inACelectric fieldboth experi-mentally aswellasusingmoleculardynamicssimulations. They also investigated the effect ofstabilizing reagents in crudeoil,suchasasphaltenesandresins,ontherateof coales-cence. Theyattributed the chainformationto the induced dipolesintheelectricfieldandnotedthatthehighfrequency ACfieldresistedthechainformationintheelectrocoalescing emulsions.

IntheACelectricfield,findinganoptimumfrequency(f0) ofcoalescenceisanimportantaspectofinvestigation(Zhang et al.,2011). Inthe electrocoalescenceliteraturethereisno unanimityonthevalueofsuchanoptimumfrequency. Appar-entlyf0 isaffectedbymanyfactors,includingphysicaland electricalproperties,percentageofdispersedphase,droplets size, polydispersity, etc. Ingebrigtsen et al. (2005) observed thatforawater-in-oilemulsion,atalowfrequencyf<100Hz the electrophoretic force induced fluidmotion. The result-ing advectionsreduced thecoalescenceefficiency. Contrary topreviousobservations,Holtoetal.(2009)assertedthatthe coalescenceatlowfrequencydoesnotallowchainformation. Athigherfrequencies,electrophoreticmovementofacharged dropcoversashortdistanceduringhalfcycle,thereby preven-tinglarge-scalemotionandloweringthefrequencyofcontact withneighboringdrops(Holtoetal.,2009).

6. Partial

coalescence

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2006). Ohnesorgenumber,Oh= d√ad, istheratioofthe

viscous to the inertial and surface tension forces. In the absenceofexternalforces,asystemwithlargeOh, exhibits completecoalescence(AryafarandKavehpour,2009;Rayetal., 2010).Highervalueoftheviscousforce(Stokesﬂowregime,

Oh1)retards the secondarydrop formation(Aryafarand Kavehpour,2006).Theelectrocoalescenceofanaqueousdrop ororganicdropwithitshomophasewasstudiedbyDongetal. (2002). They reported that the coalescence time decreased withincreasingtheappliedelectricﬁeldforaqueousdrops; however,inthecaseoforganic drops,thecoalescencetime is dependent on polarity of the ﬁeld. They attributed the polaritydependenceoforganicdrop-interfacecoalescenceto theelectricdoublelayeratinterfaceinaqueousphase.

Partialcoalescenceinanelectrocoalescencingemulsionis consideredasoneofthe mostundesiredeffects. The pres-ence ofthe high electric field expedites the drop-interface coalescencebutitcanalsoinducepartialcoalescence.Minute dropletsaregeneratedasaresultofelongationandbreakup oftheneckbetweentwocoalescingdropsorbetweenadrop andaninterfacewhichleadstopartialcoalescence.The sec-ondarydropletsaremuchsmallerthantheprimarydroplets ofanemulsion.Theseparationofsuchminutedropletsfrom anemulsionmakestheelectrocoalescenceprocessmore dif-ficult.Althoughpartialcoalescenceintheabsenceofelectric fieldhasbeenstudiedextensivelyusingexperimentaland the-oreticaltechniques; there are very fewstudies available in literatureonthephenomenoninthepresenceoftheelectric field(Allanand Mason,1961; Aryafarand Kavehpour,2007; Mousavichoubehetal.,2011a,2011b).

Thepartial coalescence ofa drop at a flat interface in thepresenceofelectricfieldwasfirstreportedbyAllanand Mason(1961).AryafarandKavehpour(2007)reportedthatthe filmbetweenadropandaninterfacecanbecomemore per-turbedinthe presenceof aDC electricfield. For asystem withconstant Oh, the volumeofsecondary drop inpartial coalescencecanbemoreinthepresenceofelectricfield com-paredto that in the absence ofelectric field. Interestingly atbulkphasehighviscosities,dropsshowcomplete coales-cenceintheabsence ofelectric fieldbut applyingastrong field resultsinemergenceofsecondary drops(Aryafarand Kavehpour,2007).Likeindrop–dropelectroalescence,initial timeofapproachbetweendropandtheinterfaceisa func-tionofelectrostaticforceofattraction.Afterthefilmbetween thedropandinterfaceattainsacriticalthickness,the film-drainage becomesrate limiting(Lukyanets and Kavehpour, 2008). Lukyanets and Kavehpour (2008) concludedthat the drop-interfacecoalescencecanbecollectivelyaffectedbythe magnitudeaswellasfrequencyoftheappliedelectricfield. Thehighelectric field strengthatthe pointofcoalescence caninducelocalinstabilityattheinterface.Thecompetition betweentheelectrostaticstressesandgravitationalforceat theinterface resultsintoacolumnextendingtowardsthe drop. The critical field of interfacial instability is given as (TaylorandMcEwan,1965),

Ecrit, int≈(g)1/4

2

∈m

. (18)

In drop-interface coalescence studies, the initial drop-velocityand release heightabovean interface signiﬁcantly affectthepartialcoalescence(AryafarandKavehpour,2009). Theseeffectsneedtobenulliﬁedinordertounderstandthe electrohydrodynamicsofthedrop-interfacecoalescence.That

canbeachievedbyapplyingtheelectricfieldattheonsetof coalescence.AryafarandKavehpour(2009)withtheir exper-imentalwork showedthatinthe Stokesflow regimeif the appliedelectricfieldishigh,asecondarydropletundergoes breakupwithajetatitstopsurface.Thejeteventuallybreaks in the surrounding dielectric medium as a stream of fine droplets. Strikingly, the jet exhibitswhippinginstability as theternarydropletsarehighlycharged.Thetimeof coales-cenceremainsunaffectedbyvaryingtheappliedelectricfield; however, a coalescing drop can stretch more if E0 is high (Aryafarand Kavehpour, 2009).Samegroup experimentally demonstratedthatthehighstrengthDCelectricfieldcangive asemistablejetemergedfromthe trailingendofthedrop, coalescingintointerface(AryafarandKavehpour,2010).Such a jetcan givethewhippingelectrospray dependingon the strength ofthe field.Thedrop-interface interaction canbe analogizedwithametalsphereheldnearaflatliquid–liquid interface(Attenetal.,2005;Reboudetal.,2008;Attenetal., 2008).Insuchasystemtheinterfacerisesattheaxisof sym-metryandtheheightoftheliquidcolumnisgovernedbythe appliedpotentialandtheseparationbetweensphereandthe interface(Reboudetal.,2008).

Mousavichoubehetal.(2011a)studiedthepartial coales-cenceandexpressedthephenomenonintermsofelectrical clampingmechanism(Ghadirietal.,2006).Theelectriccurrent withinanarrowpathbetweennearestfacesofadropandan interfacecausesacompressiveforcesqueezingthemedium fluidoutandmakingmicroscopicfilmunstable.Aholeformed inthefilmactsasaconduitthroughwhichthedropfluidis pumpedintothebulkfluidduetothesurfacetensionforce (Mousavichoubehetal.,2011a;HoneyandKavehpour,2006).At thesametimethedropaftercontactingtheinterfaceacquires the same polarity charge and start experiencing Columbic repulsion.ThestrengthoftheColumbicrepulsionand rate ofthefluidpumpingdecidestheoccurrenceofpartial coales-cence.Inpartialcoalescence,whentheappliedelectricfield ishigh,theneckingoccursfasterandthevolumeofthe resul-tantsecondarydropcanbelarger.Also,thelengthofatail emergingfromasecondarydropcanbelongeranditcan fur-therbreakintofinerprogenydroplets(Fig.3qandr).Itwasalso observedthatthevolumeofthesecondarydropletwas pro-portionaltosizeoftheprimarydropaswellasthedistance betweentheinterfaceandthepointofdrop-injection.

In another study Mousavichoubehet al. (2011b) investi-gatedtheeffectoftheinterfacialtensionandappliedelectric ﬁeldonthevolumeofthesecondarydroplet.Sincethe pres-ence ofsurfaceactiveimpuritiesinthe crudeoilstabilizes thewater–oilinterface,investigatingofeffectofinpartial coalescenceisimportant.Thepresenceofthesurfactants low-erstheinterfacialtension,resultinginthetip-streamingon bothpolarends.Thisgivesrisetoverytinysecondarydroplets oforder1␮m.Furthermore,thelowervalueofreducesthe criticalelectricﬁeld(Ecrit),abovewhichthesecondarydrops becomeunstable.Mousavichoubehetal.(2011b)obtaineda newdimensionlessnumber,WO,bycouplingtheWeber num-ber(We=2aεmE02/)andOhnesorgenumber(Oh)toexpressthe

tendencyofpartialcoalescence.ThenumberWOisdeﬁnedas,

WO=We×Oh=2∈mE20 d

√ a 1.5√

d

. (19)

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Fig.3–Partialcoalescenceofadropletofsize1196±4␮minthepresenceofelectricﬁeld181V/mm(afterMousavichoubeh etal.(2011a).

Whenadropwithinherentchargescomesintothecontact withaninterfaceofthesamefluidbutwithopposite polar-ity, the twoshould readily coalesce.However,two charged interfaceswithoppositepolaritiesattracttoeach otherbut donotalwayscoalesce.Ristenpartetal.(2009)reportedthat underaveryhighelectricfieldtwooppositelychargedbodies repelaftercontact.Hamlinetal.(2012)withtheir experimen-talresultsconcludedthatinadditiontotheappliedelectric field,thedrop-interfacecoalescenceisalsogovernedbyionic conductivity.Thereexistsacriticalionicconductivity,below which partialcoalescence canbe observed,whereas above it,thedropbouncesagainsttheinterface.Surprisingly,ionic conductivitydoesnotdeterminethechargeandsizeofthe secondarydroplets.Hamlinetal.(2012)alsogavean explana-tionofthegenerationandchargingofthesecondarydrops.In thepartialcoalescenceaninterfacedoesnotimpartchargeto thesecondarydropbutthechargecomesfromtheinduced charge on the primary drop prior to coalescence. On con-tactingtheinterface,theleadingedge(whichhaschargeof polarityoppositetothatofinterface)losesitschargeby con-vection, whereas,the top edgeofdrop which hasresidual dipolarchargesnowactsasachargedentityandexperiences electrophoreticpullintheoppositedirectionleadingto for-mationofsecondarydroplet.Atten(2012)proposedasimilar reasonforthenon-coalescencebehavioroftwodropsinahigh electricfield.

7. Effects

of

operating

parameters

and

ﬂuid

properties

Themajorityoftheearlystudiesinelectrocoalescencewere motivatedbytheuseincrudeoildemulsification.Sincethe crudeoilsfromdifferentoilfieldsareblackincolor, visual-izationandmicroscopyoftheprocessisdifficult.Asaresult, mostoftheexperimentsusedemulsionsinmineraloilssuch asparaffinoil,NynasNytro10×,Modeloil,etc.Nearinfra-red (NIR)microscopes havealsobeenemployedinexperiments with crude oils to overcome low refractive index problem (Lesaintetal.,2009).

Thedifferent operatingparameters thatgovernthe rate ofelectrocoalescencearethemagnitude,waveformand fre-quency of the applied electric field, coalescer geometry, electrodeconfiguration,floworturbulence,etc.(Hanoetal., 1988; Lee et al., 2001). Also, the emulsion properties such asinterfacialtension,densitydifference,viscosities, conduc-tivitiesandpermittivitiesoffluids,dropsize,polydispersity, percentageofdispersedphaseplaysignificantrolesin electro-coalescence(Leeetal.,2001;Al-Sabaghetal.,2011;Hosseini andShahavi,2012;Zhangetal.,2012).

The effect of various parameters on coalescence effi-ciency in continuous process in an AC electric field was studied by Kim et al. (2002). Apart from the electric field strength, the effect of parameters such as frequency of applied field, demulsifier concentration, temperature, and contact time, on the separation rate was studied. They attributed the increase in the coalescence rate with tem-peraturetothereductionintheviscosityofthecontinuous phase.

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Electrorheology of water-in-model oil emulsion under theactionofACelectricfield wasstudiedbyLesaintetal. (2009).Alongwithviscosityeffect,theyalsoinvestigatedthe influenceofother parameters including frequency,time of fieldapplication,temperature,etc.Theviscosityofthe emul-siondecreaseswithtimeoffieldapplicationaswellaswith temperature (within the range of 20–60◦C). The efficiency of coalescence is found increasing with frequency in the range of 50–5000Hz. Type of the waveform ofthe applied ACfieldcanalsobeasignificantfactoraffectingtherateof coalescence. Among three waveforms used, Lesaint et al. (2009) observedthat the coalescence efficiencywas higher withsquarewavesthan withthe sinusoidaland triangular waves.Thewaveformwithmoreareaoftheformresultsin morecoalescence. However,contrary resultswere reported byMousavietal.(2014)whofoundthatthetriangularwaves weremoreeffectiveindrop-interface coalescencethan the sinusoidal and triangular waves. They observed that the volumeratioofsecondaryandprimarydropswaslowerwith triangular waves than that with sinusoidal and triangular waves.Inthepresenceofdissolvedsaltsinwater,therateof coalescenceinACfieldischaracterizedbyasharpthreshold frequency(fth)(Szymborskietal.,2011).Atf>fth thereisno

coalescence,whereasatf<fthcoalescencedependson

magni-tudeaswellasfrequencyoftheappliedfield.Leeetal.(2001) investigatedtheperformanceofanelectrocoalescerunderAC andpulsedDCfields.TheyconcludedthattheACfieldresults inthebettercoalescenceratethanthepulsedDCwhichwas contrary to the earlier findings (Waterman, 1965; Hsu and Li,1985; Bailesand Larkai, 1981;Wakeman, 1986;Figueroa andWagner,1997).Thedifferentparameterstheyexamined wereconductivityandviscosityofemulsions,electricalfield (strength,wave formsand frequencies) and emulsionfeed rate keeping the particle size distribution constant. They also reported the actual power consumption to demulsify crude oilunder AC and pulsed DC fields inthe frequency range60–600Hz.Nooptimumfrequencywasobservedinthis frequency range; however the coalescence rate was found increasedonincreasingthefrequency(Leeetal.,2001).

In electrostatic demulsiﬁcation the coalescence rate dependsonstabilityofthe water–oilinterface. Thisis pre-dominantlygovernedbyinterfacialtensionandinturnbythe presenceofsurfactants,impurities,etc.Thecharacterization ofcrudeoilsamplesforsuchimpuritiesandtheireffectson emulsion stability have been studied using different tech-niques(Sjoblometal.,1990;Sjoblomet al.,1992;Mingyuan etal.,1992;Fordedaletal.,1996).Thepresenceofimpuritiesin thecrudeoilalterstheelectricalaswellasphysicalproperties likeviscosity, interfacialtension, elasticity, etc.(Berget al., 2010). In the batch electrocoalescence, the concentration ofimpurities in crude oil can increase as the coalescence progressesandemulsioncaneventuallybemorestable(Noik etal.,2002).

Asphaltenesareamajorcontributortotheconductivityof thecrudeoil.However,nottheamountofasphaltenesbutits aggregationstatedecidestheconductivityofcrudeoil(Lesaint etal.,2010).Fromtheconductivity(m)versusviscosity( m)

behaviorofdilutedcrudeoilsamples,Lesaintetal.(2010) con-cludedthatm˛(1/ m),ashigherviscosityresiststhemobility

ofthe charge carriers. Fjeldlyet al. (2008) investigatedthe electrocoalescenceinheavyandmediumcrudeoilaswellas multistageseparationofthreephaseemulsioninanindustrial unit.Highpercentageofwaterincrudeoilleadstoshort cir-cuitingtheelectrodes.Useofcoatedelectrodesnotonlyhelps

toovercomethisproblembutalsogreatlyenhancesthewater separationandimprovestheproducedwaterquality(Fjeldly et al.,2008). Fromtheanalysisofindustrialandpilotplant dataforthedifferentcrudeoilsamples,Suemaretal.(2012) concludedthattheseparationefﬁciencyisdependentupon appliedﬁeldaswellasonthetimeofresidenceinthe coa-lescer.

Theeffectofphysicalandelectricalpropertieson electro-coalescencewasnumericallyinvestigatedbyLinetal.(2012) usingacoupledphasefieldandleakydielectricmodel.Their analysisshowedthat,whentheouter fluidismoreviscous thanthedropphase,thetimescaleforcoalescenceislonger asitishardertodrainthemicroscopicfilmbetweenthedrops. Onreducingtheinterfacialtension,thebiggerdropsformed afterthecoalescencecanbesusceptibletofurtherbreakup. Thedeformedfacesofthecoalescingdropsintheabsenceof electricfield resistthe filmdrainage.Sincethe lesser inter-facial tension()causes moredeformation, inthe absence of electricfield, decreasing slows downthe coalescence. However,inelectrocoalescence,increasingenhances coales-cencerate(Dongetal.,2002).Recently,RayatandFeyzi(2012) usedBarker–Hendersonperturbationtheorytoproposea ther-modynamicmodelforthepredictionofcriticalconditionsfor breaking anemulsion. Theyalso studiedthe role of defor-mationonthedemulsificationandthedifferentparameters, suchastemperature,size,interfacialtension,viscosity,Gibbs elasticity,etc.,affectingthedeformationofdrops.

8. Electrocoalescence

on

dielectric

surfaces

Distortion and coalescence of water drops on insulation surfacesinhighelectricpowerapplicationsisconsidered trou-blesome. Theenhanced field atthe poles ofthe deformed drops leadstotheignitionofcoronadischarges whichcan adverselyaffecttheinsulationquality(Ndoumbeetal.,2012). Moreover,dropsinvicinitycancoalesceandtheresulting big-gerdropcanbridgetheelectrodes.Suchkindofbehaviorof waterdropsrestingonthedielectricsurfacewasinvestigated byNdoumbeetal.(2012).Thecoalescenceofsessiledropson insulationsurfaceundertheactionofDCelectricfield was foundtobedependentontheappliedelectricfield,volumeof dropsandpositionofdropintheelectricfield.Thecoalescence rateonhydrophobicinsulatorsurfaceincreaseswiththefield strengthaswellasvolumeofdrops.Orientationofdropsin theelectricfielddeterminestheprobabilityofcoalescence.If theline joiningcenters oftwodropsmakeanapproximate angle45◦ withthedirectionoftheappliedfield,thechances ofdrop–dropattractionarehighest.However,iftheangleis around90◦,dropshardlyrespondtotheelectricfield.

9. Effects

of

drop

size

and

polydispersity

Anemulsionwithcoarserdropsbreaksfaster(Pearce,1953; BezemerandCroes,1955;Eowetal.,2003;Ingebrigtsenetal., 2005).Moreover,thepresencesoffewbigdropsalsoenhance thedemulsiﬁcationasminutedropsareeasilysuckedintothe interfaceofalargedrop.Thecriticalﬁeld(Ecrit)forasingle con-ductordropabovewhichitbecomesunstable(Taylor,1964)is,

Ecrit=0.648

2a∈m. (20)

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(Eowetal.,2003).Theinstabilitycanbeasymmetricdueto theinhomogeneityofﬁeldintheemulsion(Holtoetal.,2009). Suchinstabilities makethe waterseparationmore compli-catedastheyintroduceveryﬁnerdropletsthantheoriginal dropsintheemulsion.Thedeformationofdropsaffectsthe coalescencerate.Thiseffectisobservedmoreinemulsions withsmallerdropletsthanemulsionsofcoarserdrops(Rayat andFeyzi,2012)althoughthedeformationcanbemoreinthe lattercase.

Twodissimilarsizedropsinanelectricﬁelddonotcoalesce at the same speed as the equal size drops. The electro-static forces(Eqs. (1) and (2)) aswell as mechanicalforces (Eqs.(10)–(12))arefunctionsofthedropsize.Emulsionsare essentiallypolydispersed in nature.Therefore, inthe elec-trocoalescerdesign,the observationsforthecoalescenceof uniformsizedropscannotnecessarilybetrueforactual emul-sions.

10. Non-uniformity

of

the

applied

ﬁeld

Electricfieldsemployedintheearlierelectrocoalescence stud-ies werepredominantly ofuniform kind.Theeffectiveness ofthenon-uniformfieldsinelectrcoalescencehasnotbeen exploredverymuch.Aconcentricnon-uniformelectricfield was first used for emulsion breaking by Pearce (1953). In anotherstudy,EowandGhadiri(2003)suggestedfiveelectrode designsforcoalescenceofflowingdropsandinvestigatedthe effectofdirectionofthe appliedfield ontwodrops coales-cence.Theyalsostudiedtheeffectofthefieldandfrequency onoscillationsofamovingdropintheirelectrodesystems (EowandGhadiri,2003).Noik etal.(2002),workingatpilot plant-scale,designedacompactcoalescerwithtwoannular cylindersaselectrodesandhavingacentrifugalflowof emul-sion.Theystudiedeffectsofelectricfieldstrengthaswellas flowontherateofelectrocoalescenceandfoundthattheshear forces or turbulence due to the hydrodynamic conditions caninducedropletbreak-uporcandisturbthedipole–dipole interaction.HosseiniandShahavi(2012)usedconcentric cylin-dricalelectrodestoseparatetinysunfloweroildropletsfrom water.

11. Electrocoalescence

in

leaky

dielectric

emulsions

Theuseofoil-in-oilemulsionsisgettingprominenceinmany industrial applications particularly in emerging areas like polymerblendsandelectrorheological(ER)fluids(Blockand Kelly,1988;Zukoski,1993;Aidaetal.,2010).Inpolymerblends, twoormoreimmisciblepolymersaremixedtogethertogeta resultingproductwithimprovedproperties.Thesizecontrol ofthedispersedphaseplaysavitalroleindefiningthe quali-tiesofthefinalproduct.Electrocoalescenceinpolymerblends can beusedto controlthe drop size and dispersityofthe dispersedpolymer.ERfluidisanelectro-magnetoresponsive fluidwhichexhibitsshearthinningorthickeningof suspen-sionundertheinfluenceofelectricfield(BlockandKelly,1988; Zukoski,1993).Thesefluidshavethepotentialtobedeveloped foractivecontroldevicessuchasdampers,shockabsorbers, clutches,brakes,etc.Thepolydispersityandspatial distribu-tionoftheparticlesgoverntherheologicalpropertiesofthe suspensioninsignificantmanner.Applicationoftheelectric fieldhelpstoeffectivelycontrolthedispersity,segregationand mergingofdispersedphase.

Theelectrocoalescenceofleakydielectric(LD)fluids,i.e. those withvery low but finite electricalconductivity, such asvegetableandmineraloils,organicsolventsandperfectly dielectric(PD)systemswasfirstaddressedbyBaygentetal. (1998)usingtheBoundaryElementMethod(BEM).Theshapes, motion and flows in and around the interface were stud-iedfortheequalsizedropsinauniformelectricfield.Ina PD system,the dropvelocityapproaches(∝1/s4₎_when_the

deformationissmall;whereasforaLDdroppair,the veloc-ityscalesas (s/a)−2forlargerinter-dropseparation.Attraction or repulsion ofLD dropsisdecidedbyelectricalproperties suchasconductivityandpermittivity.When (d/m)<(εd/εm),

dropsattracttoeachother;whiletheyrepelwhen (d/m)>

(∈d/∈m).Adamiak(2001)conductedasimilarstudywiththe

Finite DifferenceMethod and theBEM, withprimary focus oninterdependenceofdeformationanddroppositioninPD andperfectlyconducting(PC)systems.Heobservedthatthe point-pointmodeldidnotgivetheaccurateestimateof elec-trostaticforcewheninter-dropseparationwassmall;whereas, sphere-spheremodelwasaccurateatlowE0andlarges.Lin etal.(2012)usedphaseﬁeldmodeltoinvestigatetheeffectsof physicalandelectricalpropertiesontheshapesofthe coalesc-ingdropsinLDsystems.Theshapeoftheelectrocoalescing dropsisgovernedbypermittivitiesofﬂuids.ALDdropina pairassumesoblateshapewhen (∈d/∈m)>1anddeformation

increasesonincreasingtheratio,∈d/∈m.Whenthemedium

phaseviscosity( m)islow,inertiaassiststheﬁlmdrainage

whilehighermagnitudeof mresultsinthenon-trivialshapes

oftheinnerfaces.

Klingenbergetal.(1991)studiedtherheologicalproperties of the electrorheological (ER) suspensions using experi-mentsandmoleculardynamicssimulations.Therheological responsedependsontheconcentrationaswellasdielectric constantsoftheﬂuids.“HierarchicalModel”proposedbyAida etal.(2010)toderivethecoalescencerateinimmiscible poly-merblendssuggestedthattherateincreasedwiththeelectric ﬁeldandthevolumefractionofthedispersedphase.

12. Electrocoalescence

in

microﬂuidics

devices

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Theydetaileddifferenttypesofmicrochanneldesignssuitable forthepassivemerging(mergingofunstabilizeddropsinthe absenceofexternalforces)aswellasfortheactivemerging (mergingofstabilizeddropsbyapplyingexternalforces). Elec-tricﬁeldsarepredominantlyusedintheactivemerginginside themicroﬂuidicchannels(Guetal.,2011).

Electrodescanbeembeddedinsidethemicrofluidics chan-nelsinsuchawaythatneitherdispersedphasenorcarrier phase come in direct contactwith electrodes. Thisaspect lowersthechancesofcross-contaminationinbiological appli-cations.Electricfieldhelpsdropstocoalesceinamicrofluidics channelwheredropsizeiscomparabletothechannelsize. The microfluidic channels are mostly fabricated in poly-dimethylsiloxane(PDMS)whichishighlyhydrophobic.Inthe microreactorapplicationsthecarrieroilphasewetsthe chan-nelsurfacesandpreventsthecontactofthedispersedphase withthechannelwalls(Linketal.,2006).

Chabert et al. (2005a) studied drop–drop coalescence in stationary phase as well as in flowing phase in microflu-idicschannel.TheyobservedthatinDCelectricfield,drops didnotcoalescence;insteadtheymigratedtotheelectrode duetotheacquiredcharge.UnderanACelectricfield,drops showedcoalescence;however,velocityoftheirapproachwas constant due to the proximityof the electrodes. In an AC electricfielddropsinachannelexhibitedshapeoscillations. Aboveacriticalfrequency,adropdidnotoscillatebutit exhib-itedDCfield-likeelongationinthe directionofthe applied field.Chabertetal.(2005a)plottedcriticalmagnitudeversus frequencyof the applied electric field to get the region of coalescence.Lowerlimitoftheregionisindependentof fre-quencybuttheupperlimitvarieswithfrequency.Thesame groupproposed amicrofluidicssystemforcontinuousflow high-throughput polymerase chain reaction (PCR) (Chabert etal.,2005b).Inthemicroreactorapplicationtheelectricfield canbeusedtochargethedropsofreactantstoopposite polar-ity. Theappliedelectricfield notonlyfacilitatethe precise controlofmotionofchargeddropsbutalsoeasetheir coales-cenceinthe presence ofstabilizing impurities(Link et al., 2006).

Similartotheelectrocoalescenceinfreespace(Atten,2012), inmicrofluidicschannel,whentheappliedfieldisveryhigh, thelongandnarrowbridgebetweendropscanbreakanddrops repelviolently.Thebreakupofthebridgeresultsinminute satellitedroplet.Thecharge-exchangebetweendropsinduces thecyclesofattractionandrepulsion.Wangetal.(2010)used DC,non-uniformelectricfieldtoselectivelytrapandfusethe dropson microfluidicsplatform.Themagnitudeofapplied voltage needed for trapping and coalescence increases on increasingtheflowrateofcarrierfluid.However,acarefully designedmicrofluidicschannelcanhelptoenhance coales-cenceevenatlowelectricfield(Niuetal.,2009).

13. Available

technologies

Thestateoftheartofelectrostaticdemulsificationwasearlier reviewedbyEowandGhadiri(2002a,b).Theyalsodeveloped anovelandcompactelectrocoalescertohandletheflowing emulsions(Eowetal.,2002)(showninFig.4).Theseparation ofaqueousphasewasbasedondrop–dropanddrop-interface coalescence.Thewaterlayerthataccumulatedatthebottom ofthe device notonly acted as aninterface forthe drop-interfacecoalescencebutalsoactedasthegroundelectrode. TheirresultswiththepulsedDCfieldindicatedthatforevery

Fig.4–SchematicofelectrocoalescerbyEowetal.(2002).

magnitude of applied potential, there exists an optimum frequencywhichgivesmaximumseparationefficiency.They alsofoundthatseparationefficiencyincreasedwith increas-ing the applied field and size ofdroplets. However, above certainappliedfield,theefficiencystarteddiminishingdueto short-circuitingandfurtherbreakupofdrops.Usingaforce balanceonadropinanelectricfield,Eowetal.(2002)useda parameterIptodescribetheaccelerationduetotheapplied

electricﬁeld.Ipisdeﬁnedas,

Ip=1+

g(d−m)

d +

9 4

∈md3

dE 2 0

_dL4 −

18 m

_dd2

d 18 m

dd2_d

, (21)

whereLisdistancebetweendropandthegroundelectrode,

ddisdiameterofdrop,isvelocityofthedroprelativetothe

mediumﬂuid,anddandmarethedensitiesofdropphase

and mediumphase, respectively. Ip increases withapplied

electricfieldaswellasthesizeofthedispersedphase. Although not commercialized, additional novel electro-coalescers were designed by Eow and Ghadiri (2002a) and Eow(2002).Thefirstdesignwasbasedonastandard gravi-tationalseparator,commerciallyavailabletoseparatewater dropsfrom theoil,whichwasmodifiedandequippedwith electrodes (Fig. 4). This design has combined effects of thegravitational andelectrically-inducedforcestoenhance the separation of disperseddrops bydrop–drop as well as drop–interfacecoalescence.Theaccumulatedaqueousphase atthebottomoftheseparatoractsasagroundelectrodeand italsofacilitatesthedrop–interfacecoalescence.

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Fig.5–ElectrocoalescersbyEowandGhadiri(2002a)andEow(2002).(a)Schematicdiagramandmovementofanaqueous dropinthegravitationalelectrocoalescer-separator.(b)Movementofaqueousdropsinthecentrifugal

electrocoalescer–separator.

wheretheycoalesceundertheinfluenceoftheelectricfield. Coalescencealsotakesplaceatthewater–oilinterfaceatthe bottomofthe separator.In bothdesigns,theheightofthe accumulatedwaterlayeratthebottomoftheseparatorcan becontrolledbyavalvetooptimizetheseparationefficiency. Inbothdevices,anoptimumfrequencywasobservedtoexist withpulsedDCelectricfield.

TrapyandNoik(2007),NoikandTrapy(2004)proposedan electrostaticseparatorforefﬂuentscontainingphasesof dif-ferentdensities andelectricalconductivities.Theproposed devicecontainscylindricalelectrodesplacedalongacommon axisandahelicalchannelsituateddownstreamofthe coales-cence section which separates the dispersedand medium phases.

Intheearlydays,electrocoalescencedevicesinthecrude oilindustry were bulky; divided into a ‘treating space’ for the droplet growth and a ‘settling zone’ forphase separa-tion(Urdahletal.,2001).Duetothecompellingneedoffast andefﬁcientwaterseparation,newconceptsweredeveloped and various technologies are now availablein the market. ThesurveyofLessandVilagines(2012)wasfocusedon com-mercialtechnologiesdevelopedinrecentyears.Alltherecent technologiesuse insulatedelectrodes topreventthe short-circuitingduringelectrocoalescence.

VesselInternalElectrostaticCoalescer(VIECTM₎_technology

developed byHamworthy (aWartsila company) is claimed tobe theﬁrst electrocoalescence technologywhich can be used in the inlet separators (WVIEChttp://www.wartsila. com/en/oil-separation/oil/viec; Viechttp://www.hamworthy. com/products-systems/oil-gas/oil-separation-systems/viec-separation-technology/viec).ThebeneﬁtsofVIECTM _include,

it can process high viscosity oils, reduces chemical con-sumption, it can handle light aswell as heavycrude oils. TheelectrodesusedinVIECarefully insulatedthereforeit can withstand 100%water or gas without short-circuiting. Aibel Vessel Internal Electrostatic Coalescer was tested by Lessetal.(2008)andfoundtoreducetheseparationtimeto aquarter. Combinationofchemical andelectrostatic treat-ments showed signiﬁcant improvement in the separation.

The electrodes usedare isolated bymoldingin epoxy and thereforecanbeusedtodemulsifycrudeoilsofdifferentkind withoutshort-circuiting.

Three electrocoalescence products are commercialized by Hamworthy: the Vessel Internal Electrostatic Coalescer (VIECTM_),_the_High_Temperature_VIEC_(HT_VIECTM₎_and_the_Low

WaterContentCoalescer(VIECLVTM_)._The_VIECTM_technology

Fig.6–VIECTM_Technology_(image_source:

http://www.hamworthy.com).(a)SchematicofVIECTM

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Fig.7–CECTM_{electrocoalescers}_by_Aker_Solutions_(Image_source:_{http://www.akersolutions.com).}

(Fig.6)integratesaconventionalupstreamseparationvessel andisbuiltasawallofthecoalescermodulessittingvertically acrossthecross-sectionofthevessel.TheHTVIECTM_has_a

similarstructurebutitissuitableforhightemperature(150◦C) andhighpressure(150bar)operations.TheVIECLVTM

tech-nologymakesuseofdielectrophoreticforceandisdesigned tooperatedownstreamtheVIECTM_or_HT_VIECTM_to_meet_the

crudeoilspeciﬁcationquality.

AcompactelectrocoalescerdevelopedbyFMC Technolo-gies can beﬁtted into pipelineupstream of separator and claimedto givehigh separation efﬁciencywith less power consumption.VetcoAibeldevelopedLowWaterContent Coa-lescer(LOWACC)tobe useddownstreamofVIEC.LOWACC technologyenhancestheheavyoilseparation,improves pro-ducedwaterqualityanditissuitableforsubseaoperations (Fjeldlyetal.,2006).

Other commercial technologies involve Statoil patented CompactElectrostaticCoalescer (CECTM₎_technology₍_Fig.₇_).

Natco’sDualPolarity®_{electrocoalescer}_uses_simultaneous_AC

andDCelectricﬁeld.Recently,Natcopatentedanupgraded DualFrequency®_technology_which_uses_a_high_base_frequency

optimizedtolimitthetimeofvoltagedecay.

Startingwiththechoiceoftheelectriccurrent,DCfieldis consideredfavorablewhendropsaresparselydistributedinan emulsion;whereas,ACfieldsaremoresuitableforemulsions withhighpercentageofdispersedphase.However,the opti-mumfrequencyoftheappliedACfieldneedstobeidentified, which isgoverned bythe emulsion properties. If the elec-trodesareinsulated,theoptimalfrequencyalsodependson theinsulationproperties.Adropinanelectricfieldundergoes disintegrationwhentheelectrocapillarynumberCaEexceeds

acriticalvalue(HaandYang,1999;MhatreandThaokar,2014; Karyappaetal., 2014), solargeCaE shouldbeavoided. The

magnitudeoftheappliedelectricfieldinelectrocoalescence should not exceed a critical value which may lead to the dropbreakupandadverselyaffectthecoalescencerate.Partial coalescencecanbeovercomebyavoidingexcessiveelectric fieldstrengthandusingpulsedDCfieldsatsufficientlyhigh frequency(Mousavi et al., 2014). Also the chainformation couldbeavertedbytheuseofpulsedDCelectricfields.

Theprobabilityofdrop–dropcontactcanbeincreasedby gaspurging,shearing, agitation,etc., or internally induced electrohydrodynamicﬂows.Theuseofnon-uniformelectric

fields could generate electrohydrodyanamicflows and also resultsin thedielectrophoretic segregation ofthedroplets. However,careneedstobetakenwhenapplyingthe strong flowsastheshearstressesstimulatethedropbreakup(Mhatre andThaokar,2014).Somerelativelysimplecorrelationsto esti-mate themaximum stabledropletdiameterunder laminar andturbulentconditionsarediscussedbyUrdahletal.,2001. Consideringthevarious designelements foranefficient electro-coalescence,requirementsforanewcompactdesign are:

• reducingmigrationofdroplets;

• promotingmultipleelectrocoalescence;

• immediatepushingawayofthelargedroplets,mitigating short-circuiting.

14. Conclusions

Differentaspectsofelectrostaticphaseseparationhavebeen criticallyreviewed.Electrocoalescenceoftwodropsinvolves three majorstages;drop–drop approach,film drainage and thin film breakup. Accelerating every stage can make the coalescencefaster.Thecharacteristicsoftheappliedfieldsuch asstrength,frequency,and kindoffield directlydetermine theelectrostaticforceofattraction.Otherfactorsinvolvedare electricalandphysicalpropertiesoffluids,turbulence,water content,polydispersity,etc.Therealsoexistsomephenomena whichadverselyaffectthecoalescenceprocess;examplesare chainformation,partialcoalescence,electrohydrodyanamic drop breakup. Current developments in electrocoalescence research addressing all the above mentioned aspects are reviewed.