acoustically aided separation of oil droplets from aqueous emulsions

Chemical Engineering Science 59 (2004) 3183–3193www.elsevier.com/locate/ces

Acoustically aided separation of oil droplets from aqueous emulsions

Gautam D. Pangu, Donald L. Feke∗

Department of Chemical Engineering, Case Western Reserve University, Cleveland, OH 44106, USA

Received 4 April 2003; received in revised form 15 January 2004; accepted 31 March 2004

Abstract

A novel method for recovering the oil phase from aqueous emulsions has been developed. The method applies a low-intensity, resonantultrasonic 5eld within a rectangular chamber, which is optionally 5lled with a highly porous medium. Oil droplets dispersed in waterhave negative acoustic contrast factor and thus are driven to the pressure antinodes of the standing wave 5eld under the in7uence ofacoustic radiation forces. Subsequent coalescence and/or wetting onto the internal surfaces of the chamber occur. Three types of porousmedia (an unconsolidated bed of 3-mm glass beads, aluminum mesh or reticulated polyester mesh) having pore sizes two to three ordersof magnitude larger than droplets being collected were used. The oil collection was found to be sensitive to the natural a9nity betweenthe oil and the porous medium as well as its porosity. Of the three media studied, the polyester mesh was found to be the best in termsof the percentage oil collection while the bed of glass beads performed the poorest. The oil collection was found to be highly sensitive tothe residence time of the emulsion in both the porous medium and acoustic 5eld. Oil collection also showed expected trends with appliedelectrical power, but it was not found to be strongly dependent on the internal surface area of the mesh for the range of feed concentrationtested. These experiments enable a preliminary understanding about the mechanisms underlying the separation process.? 2004 Elsevier Ltd. All rights reserved.

Keywords: Emulsion; Droplet; Acoustic 5eld; Porous medium; Separation; Coalescence

1. Introduction

Many chemical, material and biological process appli-cations involve multiphase systems where a 7uid phase isin contact with a particulate or immiscible liquid phase.There can be several instances where the separation of thedispersed phase from its suspending 7uid is of interest inindustrial processes. Conventional separation techniquesfor solid suspensions involve physical screening techniques(membranes or beds of 5ltration media, mechanical sieves),gravity-driven methods that utilize a density di;erence forseparation, or methods that involve external 5elds (e.g. cen-trifugal or magnetic) to increase the rate and sharpness ofseparation. The method used to recover a dispersed phasefrom a liquid emulsion depends on the type of the emulsion(e.g. oil-in-water or water-in-oil) and other factors such asthe viscosity, density di;erential and relative proportion ofthe two phases as well as the age of the emulsion (Lissant,1983; Schramm, 1992). Chemical methods involve the useof additives that enhance the phase separation by altering

∗ Corresponding author. Tel.: +1-216-368-2750;fax: +1-216-368-3016.

E-mail address: [email protected] (D.L. Feke).

the molecular-scale interactions within the system. Whilethese methods are usually fast, they have the disadvantagesof cost and the complexity of having to remove the additivein subsequent processing steps. Physical methods includetraditional gravity settlers, which may require large resi-dence times or large physical spaces. Other physical meth-ods involve the application of external 5elds (e.g. centrifugalor electric (Jang and Lee, 2000; Kim et al., 2002)), whichenhance phase-separation by assisting migration, collisionsand subsequent coalescence of dispersed phase droplets,or the use of porous membranes to break the emulsion(Cheryan and Rajagopalan, 1998). A combination of theabove-mentioned methods has also been reported in the lit-erature (Edmondson, 1998; Eow and Ghadiri, 2002). How-ever, di9culties involved in scale-up of these processes andmaintaining high e9ciencies on a large scale over longerperiods of time may pose problems in using these processesfor practical applications.During the past few decades, the ability of low intensity

ultrasonic standing wave-5elds to manipulate small parti-cles in liquid suspension has been used to as the basis ofvarious fractionation and 5ltration methods. The suspendedparticles respond to the resonant acoustic 5eld if there is

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3184 G.D. Pangu, D.L. Feke / Chemical Engineering Science 59 (2004) 3183–3193

a non-zero acoustic contrast between the dispersed phaseand the suspending 7uid. In several of these methods, aone-dimensional sound 5eld is used to organize the parti-cles into thin parallel bands separated by a one-half acousticwavelength spacing. The particles are separated from theirsuspending 7uid by either placing closely spaced physicalbarriers between the bands of particles and transporting theacoustically produced particle-free and particle-rich streamsinto separate exit streams (Mandralis and Feke, 1993a,b;Gupta et al., 1995), or transporting particles in the oppo-site direction of the 7owing 7uid by using slowly movingpseudo-standing waves (Tolt and Feke, 1992; Benes et al.,1991). Other approaches involve the use of acoustic methodsto induce agglomeration of particles which can then be re-covered by conventional physical screening or gravity drivenmethods (Frank et al., 1993; Allman and Coakley, 1994).Certain separation techniques are a blend of acoustic and

physical screening methods in which a resonant acoustic5eld is applied to a porous medium through which a par-ticulate suspension 7ows. The interaction of the acoustic5eld with the porous medium improves its performanceas a 5lter and collection of particles up to two orders ofmagnitude smaller than the pore size can be achieved. Thetrapped particles are recovered by deactivating the acoustic5eld and 7ushing the porous medium with processing 7uid(Gupta and Feke, 1997).In the work cited above, the ability of ultrasonic stand-

ing wave 5elds to manipulate solid particles in liquid sus-pensions was studied and described. Here we report on thedevelopment of the 5rst ultrasonically aided process that in-tends to recover the oil phase from aqueous emulsions (hav-ing droplet size ranging from 1 to 15 �m). This is in directcontrast to the more common use of ultrasonic 5elds andsonication processes to produce or stabilize emulsions. Inour method, the ultrasonic 5eld causes oil droplets to coa-lesce and/or to be transported to the internal surfaces of thechamber where the oil phase accumulates, and clari5ed wa-ter 7ows out from the chamber.The principal aim of the work described here was to

demonstrate proof-of-concept for our separation method andto investigate the feasibility and practicality of the processby obtaining some relevant separation performance data.Another goal of our study was to determine whether the ma-terial of construction of the porous media would a;ect theseparation performance in a manner based on the interfaciala9nity between the porous media and the oil droplets. Theseresults provide insights into the fundamental mechanism ofthe separation process.

2. Concept

The susceptibility of a suspended particle to respond toa resonant ultrasonic 5eld depends on the acoustic con-trast factor, F , relative to the suspending 7uid (Yosioka andKawasima, 1955). The acoustic contrast F for the particle,

under the conditions of particle size R � acoustic wave-length �, is given by

F =�+ 2(�− 1)=3

1 + 2�− 13�2�

; (1)

where � is the ratio of particle density to the 7uid densityand � is the ratio of longitudinal sound speed in a particleto that in the 7uid. Due to the acoustic contrast, the particleexperiences a time-averaged force known as the primaryacoustic force, F1;ac that is given for a one-dimensional 5eldas

F1;ac = 4�R3EacF sin(2x) (2)

where R is the particle radius, is the wave number of theacoustic 5eld, Eac is the energy density of the acoustic 5eld,F is the acoustic contrast factor and x is the distance froma pressure antinode of the standing wave. This force actsin the direction parallel to the direction the acoustic 5eldpropagates. A simple stability analysis of Eq. (2) indicatesthat the sound 5eld will drive the particles into a pressurenode if F ¿ 0 or to a pressure antinode if F ¡ 0. Hencethe particles subjected to a resonant standing acoustic 5eldwill be pushed towards these stable equilibrium points andbecome concentrated into regions located at pressure nodesor antinodes, depending on the sign of F .Suspended particles subjected to a one-dimensional reso-

nant 5eld may also scatter that sound 5eld. The interactionof particles with the scattered 5eld from a neighboring par-ticle gives rise to secondary acoustic forces (Weiser et al.,1984). The secondary acoustic force, F2;ac is attractive whenboth particles are either more or less compressible than the7uid and repulsive in any other case. The secondary acous-tic force between two particles (indices 1 and 2) is given as

F2;ac =2Eac2�

(1− �p1

�f

) (1− �p2

�f

)Vp2Vp1d2

(3)

where �f the compressibility of the 7uid, �p the compressi-bility of the particle, Vp is the particle volume, and d is thecenter-to-center distance between the interacting particles.Usually the secondary acoustic force is at least an order ofmagnitude smaller than the primary acoustic force imme-diately after the sound 5eld is applied. But as the particlesare gathered at nodes (or antinodes) due to primary acous-tic force, agglomeration is induced between the particlesand the particle concentration inside the separation equip-ment increases. As particle-agglomerates grow larger andinterparticle distances decrease due to particle build-up, thesecondary acoustic force can become signi5cantly large inmagnitude inducing further agglomeration between the par-ticles or agglomerates.As described in the Introduction, acoustic forces have

been previously used to manipulate micron-sized solid par-ticles suspended in a liquid. Also, resonant ultrasonic 5eldspropagated through a highly porous medium have beenused to entrap very small (relative to the pore size) solid

G.D. Pangu, D.L. Feke / Chemical Engineering Science 59 (2004) 3183–3193 3185

particles within the porous medium. High collection e9cien-cies for solid particles of 70–80% have been achieved withthe use of aluminum and polyester mesh (Gupta and Feke,1998). Various possible mechanisms of the particle trappingphenomena within the porous medium have been proposed(Gupta and Feke, 1997; Grossner et al., 2003). The use ofporous medium inside the chamber can in7uence the pro-cess in di;erent ways. For example, the acoustic 5eld insidethe chamber could undergo internal re7ections and scatter-ing due to the presence of the porous medium, producing acomplex three-dimensional 5eld.For the vegetable-oil emulsions used for this work, F =

−0:10. Thus, when an aqueous emulsion of vegetable oil issubjected to a standing ultrasonic wave 5eld, the primaryacoustic force is expected to drive the oil drops towards thepressure antinodes of the standing wave 5eld. The oil dropsare expected to coalesce due to secondary acoustic forces.Furthermore, the coalesced drops are expected to be retainedin the void spaces of the porous medium or collected on theinternal surfaces of the porous medium depending upon theirwetting properties, giving rise to signi5cant oil retentioninside the porous medium.

3. Experimental methods

The experiments were carried out in a rectangular acous-tic chamber, the schematic of which is shown in Fig. 1. The

Fig. 1. Schematic of the acoustic chamber.

chamber consisted of a rectangular PZT transducer (Navytype I, Model EC-64, 77:5×46:0×10:03 mm, EDO ElectroCeramics Corporation, Salt Lake City, Utah) and a stain-less steel re7ector (82:0 × 48:1 × 1:5 mm) supported ontwo polyethylene support structures, one each for transducerand re7ector. The transducer and re7ector were recessedinto the acrylic centerpiece. When the chamber was assem-bled, a watertight seal was created between the transducer,the re7ector and their support structures by thin latex mem-branes (0:4 mm thick) that were glued around the edges ofthe transducer and re7ector. The spacing between the trans-ducer and re7ector was adjusted to be 12:2 mm for all ex-periments. According to the mathematical model developedto predict resonant frequencies and energy densities in dif-ferent layers (transducer, re7ector and the 7uid layer) of aparallel-plate con5guration (shown in Fig. 1) of the acous-tic chamber as a function of chamber dimensions and mate-rial properties (Rusinko, 2001), the spacing of 12:2 mm wasfound to give high acoustic energy density in the 7uid layeras compared to the other layers for this experimental con-5guration. The chamber was equipped with ports at the topand bottom for the emulsion feed and eOuent removal. Theultrasonic 5eld was produced by energizing the transducerat 680 kHz frequency using a continuous sinusoidal signalgenerated by a KROHN-HITE 2100 A signal generator andampli5ed by an ENI 240 L power ampli5er. The voltage,current, power and power factor of the signal was recordedby a Clarke-Hess 2330 sampling V-A-W meter. To ensure


that the chamber stayed at a resonance condition and witha high fraction of the power delivered to the emulsion, ahome-built automatic electronic controller was used. Thequality of resonance was characterized in terms of the powerfactor, which is de5ned as the cosine of the phase di;erencebetween the voltage applied to the chamber and the resultingelectrical current. The electronic controller senses the powerfactor and accordingly corrects the operating frequency inorder to keep the power factor close to one.Video imaging of the acoustic chamber was accomplished

using a CCD video camera mounted on a micrometer ad-justable x–y–z positioning platform. The video images wereviewed on a digital monitor and recorded digitally on a PCusing Sphinx Pro Video Capture Software.The oil/water emulsions used as feed for the experiments

were prepared from vegetable oil (commercially obtainedpure Wesson Soybean oil) and deionized water. The oil andwater were mixed such that the volume percentage of oil inthe mixture was 0.50. The mixture was stirred vigorouslyfor about 1 min and then it was ultrasonicated (using a MIS-ONIX XL 2020 20 kHz Sonicator) for the period of 7–8 min. This procedure typically produced an emulsion withdroplet sizes ranging from 1 to 10 �m. Prior to performingthe experiments, these emulsions were diluted with deion-ized water in a ratio of 1:10. The dilution was required toenable the visual observation of the response of oil dropletsto ultrasound and also to accommodate the analytical tech-nique that was used to determine the droplet size distribu-tion and oil concentration in the feed and the eOuent stream.A peristaltic pump was used to deliver the emulsion to theacoustic chamber.The emulsions were characterized in terms of droplet

size distribution and oil concentration using a SPECTREXILI-1000 Laser Particle Counter. The counter can be used todetermine the number of particles/droplets in 1 cm3 of sam-ple (henceforth referred to as droplet count) as a function ofdi;erent size ranges. By maintaining su9cient accuracy inpreparing and diluting the emulsions, this information canbe used to determine the droplet size distribution and oilconcentration (volume fraction of oil) in the feed and ef-7uent emulsions. For example, if di is the droplet diameterand ni is the number concentration of droplets that have di-ameter di (measured by the droplet counter), then the totalvolume concentration of the oil would be

Coil =∑i

ni

(16�d3i

); (4)

where Coil would be expressed in (volume of oil/volume ofsample).If the feed and the eOuent are sampled in similar fash-

ion, the corresponding oil concentration in the feed andthe eOuent would be Cfeedoil and CeOuentoil . Then the instanta-neous percentage oil collection inside the chamber could becalculated as

�inst =(1− CeOuentoil

Cfeedoil

)× 100 (5)

Before starting an experiment, a small sample of feed wasanalyzed using the particle counter. A steady 7ow of emul-sion through the chamber was 5rst established and then theacoustic 5eld was activated. After 20 min, a small instan-taneous sample of eOuent was collected and immediatelyanalyzed using the counter.

4. Results and discussion

4.1. Experiments with no internal porous media

As a control, experiments in which no porous mesh waspresent in the chamber were performed. Emulsions were fedat the 7ow-rate of 35 cm3=min from the bottom to the topof the chamber. When the acoustic 5eld was activated, theoil droplets were found to respond almost instantaneously.Even with no porous medium inserted inside the chamber,the acoustic 5eld was found to entrap the small oil dropletsinside the chamber and once they were retained, they showedstrong tendency to coalesce. Photographs of the verticalcross-section of the chamber before and after the activationof the acoustic 5eld are shown in Fig. 2. Very soon after theacoustic 5eld was turned on; comparatively larger dropletsbecame visible inside the chamber. These coalesced dropletswere also found to show a strong tendency of migrating to-wards the internal surfaces as well as towards the transducerand re7ector of the chamber. Some of the coalesced droplets,being lighter than water, became entrained with the eOuent.It was also observed that even after the acoustic 5eld wasterminated, the droplets within the interior of the chamber7owed out but the larger droplets remained on the chamberwalls.A typical droplet size distribution of the feed and the

eOuent emulsion is shown in Fig. 3. The eOuent emulsionsare seen to have a higher percentage of the larger dropletsthan the feed, which indicates the coalescence of the dropletsdue to acoustic 5eld. Since the 7ow through the chamber(upwards) was in the same direction that buoyancy acts onthe oil droplets, coalesced droplets might have escaped inthe eOuent. Based on this droplet size distribution and thedroplet count shown by the counter, the volume fraction ofoil in the eOuent (calculated using Eqs. (4) and (5)) wasfound to decrease approximately by 30% as compared tothat in the feed in these trials.The emulsions used as feed in these experiments were pre-

pared without the addition of any emulsifying agent. Hencethe natural coalescence of oil drops is expected to occur overthe period of time. The observation that the eOuent containsa higher percentage of larger droplets than the feed couldbe the result of natural coalescence. To study this, an emul-sion similar in characteristics with the emulsions used in theexperiments with the acoustic chamber was prepared. Theemulsion was left undisturbed in a beaker and the dropletsize distribution was determined again after 20 min. Thetwo distributions are shown in Fig. 4. A shift in the distri-


Fig. 2. Photographs of the vertical cross-section of the chamber for the experiments with no porous medium: (a) before activation of the acoustic 5eld;and (b) one minute after the acoustic 5eld is activated. The droplet coalescence and retention inside the chamber can be seen.

Fig. 3. Droplet size distributions for feed and eOuent for the experiments with no porous medium inside the chamber. The corresponding oil retentionis 30%.

bution is indeed observed, but only to a very small extent.This result can be attributed to the small density di;erencebetween the dispersed and continuous phases (0:1 g=cm3)and that the emulsions under consideration are su9cientlydilute (volume percentage of oil is less that 0.1). Thus weconclude that natural coalescence is not very prominent forthese emulsions and is not a signi5cant contributor to the dif-ference in the droplet size distributions observed in acoustic5eld experiments.In the experiments with no porous medium inside the

chamber, migration and collection of oil drops on the inte-

rior surfaces of the chamber were observed. Some experi-ments were performed in which a thin polyethylene sheetwas used to cover the re7ector of the chamber. Oil dropsshowed a strong tendency to migrate towards the re7ectorduring these experiments and careful observations showedthe existence of thin oily layer on the plastic sheet at theend of the experiment. Hence it was thought that the oil col-lection would improve if more hydrophobic area inside theacoustic chamber is provided. Subsequently, a series of ex-periments to explore the e;ect of providing more area insidethe chamber for oil collection was performed. In addition to


Fig. 4. Droplet size distribution for the emulsion immediately after it was prepared and 20 min after it was prepared. In between the two measurements,the emulsion was left undisturbed in a beaker.

studying the role of wetting in oil recovery by using di;er-ent types of porous media inside the chamber, these experi-ments were also aimed at quantifying the e;ect of process-ing variables on separation performance.

4.2. Experiments with porous media

Three di;erent types of porous media were used in theexperiments to study the role of the properties of porousmedium on oil collection e9ciency:

(1) 3 mm diameter glass beads (Fisher Scienti5c, Pitts-burgh, PA)

(2) Aluminum mesh (ERG Materials & Aerospace Corp.,Oakland, CA)

(3) Polyester mesh (Polyester-polyurethane reticulatedmesh) (Meshex International, Inc., Linwood, PA19061).

In the experiments with glass beads, the beads were simplypoured into the open volume between the transducer andre7ector. This resulted into a randomly packed bed havingporosity ∼ 0:4. For the aluminum mesh and polyester mesh,the porosity was greater than 0.9. For all experiments withdi;erent porous media, the dimensions of the porous regionwere 75 mm long, 45 mm wide and 12 mm thick in thedirection of propagation of acoustic 5eld. The feed 7ow ratewas kept at 35 cm3=min and the feed was passed from topto bottom of the acoustic chamber. The applied power was25:8 W (Table 1).Fig. 5 shows a typical result for an experiment with

polyester mesh. The result is reported in terms of dropletsize distribution of feed (analyzed before the start of the ex-

Table 1Values of important parameters for the experiments with no porous media

Parameter Value

Spacing between transducer and re7ector 12:2 mmFrequency 680 kHzFlow rate 35 cm3=minPower to the transducer 25:8 WLinear 7ow speed 1:08 mm=s

periment) and the eOuent (a small sample obtained 20 minafter the activation of the acoustic 5eld). Qualitatively, thedroplet size distributions were found to be similar for theexperiments with all three porous media. In all three cases,the eOuent has a higher percentage of smaller dropletsand a lower percentage of larger droplets than the feed. Itshould be noted that this trend is opposite to the one that isobserved for the experiments with no internal porous mediaand with the feed 7owing upwards (shown in Fig. 3). Basedon the droplet size distribution and droplet count of the feedand eOuent, the instantaneous percentage oil collection in-side the chamber was found to be 31% for the experimentsusing glass beads, 50% when 20-ppi aluminum mesh wasused and 75% for the case of 20-ppi polyester mesh. Tocon5rm these trends, experiments were also performed atapplied power of 47.2 W keeping all other operating condi-tions the same. Qualitatively, similar trends were observedin the droplet size distributions and the percentage oil col-lection values were found to be 52%, 55% and 80% forglass beads, 20-ppi aluminum mesh and 20-ppi polyestermesh respectively. These results are summarized in Table 2.


Fig. 5. Droplet size distributions for feed and eOuent for the experiment with the 20-ppi polyester mesh inside the chamber. The corresponding oilretention is 75%.

Table 2Results for the experiments with three porous media

Porous medium No. of experiments, n Collection e9ciency Collection e9ciency(Power = 25:8 W) (Power = 47:2 W)

3 mm glass beads 3 31± 5:56% 52± 7:55%20-ppi aluminum mesh 3 50± 3:61% 55± 2%20-ppi polyester mesh 3 75± 4:00% 80± 4:00%

The contrast in the droplet size distribution of the feedand eOuent may be explained as follows. The oil dropletsthat are being retained inside the chamber due to the acousticforce coalesce. Due to the presence of the porous mediuminside the chamber, the large drops thus formed plus the largedroplets that are already present in the feed have a largeravailable surface area on which spreading can occur. Theoil retention is further assisted by gravity because the 7owof emulsion within the chamber is in the opposite directionto that of buoyancy. Thus, the larger the drop is, the higheris the probability of its being retained inside the chamber.Hence the eOuent is seen to contain a higher percentage ofsmaller drops as compared to the feed. However the actualdroplet count for the eOuent is much less than for the feed,which indicates signi5cant oil retention inside the chamber.The di;erence in the oil collection performance of three

porous media in presence of acoustic force can be explainedbased on the acoustic as well as wetting properties of threemedia. Of all the three porous media used for experimen-tation, polyester has the acoustic impedance closest to thatof water (Table 3). Glass and aluminum have the acous-

Table 3Important acoustic properties of the materials used in this work

Material Density, � Bulk sound speed, c Acoustic impedance(103 kg=m3) (103m=s) Z = �c(106 kg=m2 s)

Water 1.0 1.48 1.48Vegetable oil 0.9 1.43 1.29Aluminum 2.7 6.27 17.0Glass 3.6 4.26 15.4Polyester 1.2 2.43 3.0

tic impedances that are vastly di;erent from that of water.So it can be expected that the acoustic 5eld will have min-imal internal re7ections and scattering inside the polyestermesh. Thus the probability of the existence of a strongeracoustic 5eld throughout the whole interior of the chamberis greater in the case of the polyester mesh. This might haveresulted in better performance of polyester mesh in terms ofoil collection. Vegetable oils mainly contain triglycerides,which are tri-esters of glycerols and fatty acids. The wetting


properties of triglycerides on solid surfaces of di;erent hy-drophobicities have been studied by others (Michalski andSaramago, 2000). In general, it is found that the wetting be-havior improves with increasing hydrophobicity of the solid.Speci5cally it has been found that triglycerides exhibit lowervalues of contact angles on polymers like polyethylene (PE)and polyethylene terephthalate (PET) than on glass surfaces.Hence it can be expected that vegetable oil would show bet-ter wetting behavior on polyester mesh rather than on glassleading to the better oil collection on the polyester mesh.The better performance of the aluminum mesh compared tothe glass beads could also be the result of the higher porosity(greater than 0.9) of aluminum mesh.

4.3. E7ect of residence time

The e;ect of residence time of the suspension in the acous-tic 5eld and in the porous medium was studied in two dif-ferent ways: (a) by changing the volumetric 7ow rate of thefeed and keeping the dimensions of porous region constant;and (b) by changing the dimensions of the porous mediumin the 7ow direction and keeping the volumetric 7ow rate ofthe feed constant. For these experiments, 20-ppi polyestermesh was used and 25.8 W of electrical power was appliedto the chamber. In the experiments performed to study thee;ect of volumetric 7ow rate, the two 7ow rates used were35 and 70 cm3=min. These 7ow rates correspond to the lin-ear velocities of 1.08 and 2:16 mm=s respectively and thecorresponding values of the residence time within the porousmedium are 69:4 s and 34:7 s respectively.The separation performance was found to be very sen-

sitive to the residence time. For the case of a 7ow rateof 70 cm3=min, the instantaneous percentage oil collectionwithin the porous medium dropped to 27% from its valueof 75% for the 7ow rate of 35 cm3=min. Qualitatively, thedroplet size distributions for two cases are also di;erent. TheeOuent at the 7ow rate of 70 cm3=min (shown in Fig. 6)was found to contain a higher percentage of larger dropletsthan the feed, which is opposite to the droplet size distri-bution observed using the 7ow rate of 35 cm3=min (shownin Fig. 5). This suggests that the porous medium does notstrongly enhance the oil retention process at low residencetimes. The coalescence of smaller droplets in the acoustic5eld to form larger droplets occurs on a time scale that ismuch smaller than that required for the retention of thesedrops within the porous medium. The relatively short resi-dence time within the porous medium is not large enoughfor the larger droplets (either originally present in the feedor formed due to both the natural coalescence of smallerdroplets and induced coalescence due to acoustic 5eld) to beretained within the porous medium. This occurs even thoughthe 7ow direction is opposite to that of the buoyancy e;ect.In another type of experiment performed to study the e;ectof residence time within the porous medium, the path lengthof the emulsion was changed by altering the dimensions of

porous region in the 7ow direction. The experiments wereperformed with the polyester mesh cut to half of its length(in the 7ow direction) compared to that used in previous ex-periments. Thus the dimensions of the porous region were37 mm long, 45 mm wide and 12 mm thick in the directionof propagation of acoustic 5eld. The emulsions were fed tothe acoustic chamber from the top and the polyester meshwas present in the upper half of the chamber. Thus, for a7ow rate of 35 cm3=min, the residence time of the emulsionwithin the porous medium decreases to 34:7 s. However theresidence time within the acoustic 5eld was the same as be-fore. Qualitatively, the droplet size distributions are similarto those observed for full-length mesh (Fig. 5), but the per-centage oil collection dropped to 49%. The results of theseexperiments once again indicate that the percentage oil col-lection and thus the separation performance are highly sen-sitive to the residence time within porous medium and theacoustic 5eld.

4.4. E7ect of electrical power

The acoustic 5eld intensity in the chamber is directlyrelated to the electrical power delivered to the piezoelec-tric transducer. However the exact relationship between thetwo is complex, so the electrical power consumption wasused as a measure of acoustic intensity in the chamber.Table 4 summarizes the results for these experiments. Forthese experiments, 20-ppi polyester mesh was used and thefeed 7ow rate was 35 cm3=min. Once again, the dropletsize distributions are qualitatively similar to those shown inFig. 5, but the percentage oil collection increases from 62%to 75% as applied power is increased from 6.3 to 25:8 W.However, with an increase in power to 47:2 W, the per-centage oil collection increases only to 80%. This limitedincrease in retention could be attributed to acoustic stream-ing that can take place at high power (Gould et al., 1992),which might o;set the e;ect of larger acoustic forces.

4.5. E7ect of pore size

The pore size of polyester mesh a;ects the absolute 7owvelocity as well as the internal surface area. Smaller poresprovide a large internal surface area, but also higher ab-solute velocities. To study the e;ect of pore size on sepa-ration performance, experiments were performed with 10,20 and 30-ppi polyester meshes, The feed 7ow rate was35 cm3=min and the applied power was 25:8 W. It was foundthat the percentage oil collection improves only slightly withdecrease in pore-size with values for the 10, 20 and 30-ppimeshes being 72%, 75% and 77%, respectively.The collection of oil droplets inside the porous medium

is expected to be aided by their spreading on the internalsurface of the porous mesh. This is in contrast to the caseof solid particles, in which case the collection inside theporous medium is due to the entrapment of particle 7ocs


Fig. 6. Droplet size distributions for feed and eOuent for the experiments with 20-ppi polyester mesh for the feed 7ow rate of 70 cm3=min. Thecorresponding oil retention is 27%.

Table 4Results for the experiments with di;erent applied powers

Power (W) No. of experiments, n Oil retention (%)

6.3 3 62± 3:6025.8 3 75± 4:0047.2 3 80± 4:00

within the void spaces of porous medium or the attachmentof individual particles or particle 7ocs to the internal sur-face of the porous medium (Gupta and Feke, 1997). Sincethe retained oil can actually spread over the surface of theporous medium thereby forming a 5lm, the porous mediumis unlikely to saturate quickly. This might also mean that forthe feed concentration tested, the 5ltration e9ciency is notstrongly dependent on pore size and available internal areaand hence only small improvement in 5ltration e9ciency isobserved with a decrease in pore size.To study whether the porous medium can become sat-

urated, experiments were performed with 20-ppi polyestermesh at the feed 7ow rate of 35 cm3=min and an appliedpower of 25:8 W. The eOuent samples were collected at10, 20 and 30 min after the acoustic 5eld was activated (inthree separate experiments) and analyzed for droplet sizedistribution and oil concentration. Oil retention within thechamber corresponding to the three cases was found to be69%, 75% and 58%. These results indicate that the perfor-mance of the porous medium initially improves with timebut then it deteriorates. However the decline in the collectione9ciency is slow compared to the analogous experimentswhere the porous medium was used to trap solid particles in

a suspension (Gupta, 1997). In that case, the instantaneouscollection e9ciency eventually dropped to zero signalingthat the porous medium was saturated with particles. Suchconditions were never reached in any experiments using oildroplets. Thus the current results with the oil droplets indi-cate that the wetting of the porous media by oil plays a vitalrole in the collection of oil inside the porous medium.

4.6. Experiments with no applied 9eld

To study the tendency of the porous mesh to retain oildroplets in absence of any acoustic force, experimentswere performed with no acoustic 5eld and a feed 7ow rateof 35 cm3=min. All three types of porous media (20-ppipolyester mesh, 20-ppi aluminum mesh and glass beads)were used in these experiments. The droplet counts for thefeed and eOuent are similar and the percentage oil col-lections observed are less than 10% for all three types ofporous media. This indicates that in the absence of acoustic5eld, the porous medium cannot be e;ectively used to re-cover the oil droplets. The pore size in the media used in theexperiments is much larger compared to the droplet sizesand the droplets can be collected on the porous mediumonly after the acoustic force traps them, Also there is nosigni5cant di;erence in the performance of three media inthe absence of acoustic force while in the presence of sound5eld, the performance di;ers signi5cantly. This indicatesthat the natural a9nity between the porous medium and oilphase may not be highly e;ective for oil recovery in theabsence of acoustic force.


5. Conclusions

A novel method for recovering the oil phase from aqueousemulsions (oil droplets ranging in size from 1 to 15 �m) hasbeen developed. A resonant ultrasonic 5eld of mild intensity,propagated within a rectangular chamber that could be 5lledwith a porous medium can be used to trap the oil droplets.These trapped oil droplets tend to coalesce and collect onthe internal surfaces of the chamber as well as the porousmedium used to 5ll the chamber. Three types of porous me-dia were studied: glass beads, aluminum mesh and polyestermesh and the separation performance was found to be better(instantaneous oil retention e9ciency of up to 80%) withthe use of polyester mesh. The trends of 5ltration e9ciencywith respect to emulsion 7ow rate, path length, power andinternal area available for collection were studied. The 5ltra-tion e9ciency was found to be strongly dependent on boththe emulsion 7ow rate and the path length, which impliesits sensitivity with respect to the residence time within theporous medium. These experimental trends suggest that thecoalescence of oil droplets occurs much faster than their re-tention inside the porous medium. The separation e9ciencyalso shows expected trends with electrical power, but wasnot found to be strongly dependent on internal surface areafor the feed concentration tested which suggests that thespreading of oil on porous medium plays a signi5cant rolein oil retention. These insights into the process would helpthe detailed theoretical study of the process, which is thesubject of ongoing research. Higher separation e9cienciesmay be achieved by increasing the residence time withinthe porous medium either by using a lower 7ow rate or byincreasing the length of porous medium in the 7ow direc-tion. This could also be achieved by using a multi-stage ap-proach. The continuous nature of operation, reasonable sep-aration e9ciencies without the addition of third componentfor phase separation, reduced tendency for saturation of theporous medium with oil and adaptability to large-scale op-eration makes these methods potentially useful for practicalapplications.

Notation

c longitudinal sound speed, m/sC volume concentration, m3=m3

d center-to-center distance between interactingparticles, m

di droplet diameter, mEac acoustic energy density, J=m3

F acoustic contrast factor, dimensionlessF1;ac primary acoustic force, NF2;ac secondary acoustic force, Nni number concentration of dropletsR radius of the particle, mVp volume of the particle, m3

x distance from a pressure antinode of a standingwave, m

Z acoustic impedance, kg=m2s

Greek letters

�f compressibility of the 7uid, m2=N�p compressibility of the particle, m2=N wave number of the acoustic 5eld, m−1

� acoustic wavelength, m� ratio of particle density to 7uid density, dimen-

sionless� density, kg=m3

� ratio of longitudinal sound speed in a particle tothat in a 7uid, dimensionless

Acknowledgements

The authors are grateful to Nestle R& D for its supportof this work through a Nestle Fellowship.

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