Ultrasonics Sonochemistry 34 (2017) 504–518

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Ultrasonics Sonochemistry

journal homepage: www.elsevier .com/ locate/ul tson

Investigation of mass transfer intensification under power ultrasoundirradiation using 3D computational simulation: A comparative analysis

http://dx.doi.org/10.1016/j.ultsonch.2016.06.0261350-4177/� 2016 Published by Elsevier B.V.

⇑ Corresponding author.E-mail addresses: azizraman@um.edu.my (A.A.A. Raman), rchrp@rmit.edu.au

(R. Parthasarathy).

Baharak Sajjadi a, Seyedali Asgharzadehahmadi a, Perumal Asaithambi a, Abdul Aziz Abdul Raman a,⇑,Rajarathinam Parthasarathy b

aDepartment of Chemical Engineering, Faculty of Engineering, University of Malaya, 50603 Kuala Lumpur, Malaysiab School of Civil, Environmental and Chemical Engineering, RMIT University, Victoria 3000, Australia

a r t i c l e i n f o

Article history:Received 17 February 2016Received in revised form 24 May 2016Accepted 20 June 2016Available online 23 June 2016

Keywords:UltrasoundStirred vesselMass transferGas-liquidAcoustic streamingComputational fluid dynamics (CFD)

a b s t r a c t

This paper aims at investigating the influence of acoustic streaming induced by low-frequency (24 kHz)ultrasound irradiation on mass transfer in a two-phase system. The main objective is to discuss the pos-sible mass transfer improvements under ultrasound irradiation. Three analyses were conducted: i) exper-imental analysis of mass transfer under ultrasound irradiation; ii) comparative analysis between theresults of the ultrasound assisted mass transfer with that obtained from mechanically stirring; and iii)computational analysis of the systems using 3D CFD simulation. In the experimental part, the interactiveeffects of liquid rheological properties, ultrasound power and superficial gas velocity on mass transferwere investigated in two different sonicators. The results were then compared with that of mechanicalstirring. In the computational part, the results were illustrated as a function of acoustic streaming beha-viour, fluid flow pattern, gas/liquid volume fraction and turbulence in the two-phase system and finallythe mass transfer coefficient was specified. It was found that additional turbulence created by ultrasoundplayed the most important role on intensifying the mass transfer phenomena compared to that in stirredvessel. Furthermore, long residence time which depends on geometrical parameters is another key formass transfer. The results obtained in the present study would help researchers understand the role ofultrasound as an energy source and acoustic streaming as one of the most important of ultrasound waveson intensifying gas-liquid mass transfer in a two-phase system and can be a breakthrough in the designprocedure as no similar studies were found in the existing literature.

� 2016 Published by Elsevier B.V.

1. Introduction

Ultrasound irradiation has been proven as a reliable techniquefor intensifying a wide range of chemical and biological processes[1–3]. Ultrasound effects are generally attributed to its differentnoninvasive nature and strong acoustic effects including cavita-tion, micro streaming, shock waves or oscillating fluid motionand acoustic streaming when travelling across a medium [4].Acoustic cavitation refers to the growth of nuclei in a liquid duringlow-pressure cycles and subsequent collapse of bubbles duringhigh-pressure cycles. A large amount of energy that is accumulateddue to gas compression in the bubbles is freed during the collapsecycle. At the point of the maximum compression, the liquid ele-ments converged towards the bubble are reflected back from theinterface creating high-pressure shock waves. Micro-streaming

phenomenon occurs when a micro-bubble, surrounded by a liquidenvironment, undergoes direct oscillatory action when exposed toultrasound. Oscillatory action forms rapid, toroidal eddy currentsdue to the displacement of liquid around the micro-bubble. Whenultrasound energy is applied in the liquid media, the fluid flows inthe same direction as the propagation of ultrasound waves. Thesteady flow induced by the absorption of acoustic energy duringthe passage of acoustic waves is usually referred as acousticstreaming [5,6]. This phenomenon is identified by its transportproperties: appearance of liquid motion without any mechanicalimpact. Employment of acoustic streaming in an appropriate waymay cause high efficient mixing especially in gas-liquid systems[7]. Among different ultrasound effects, cavitation phenomenahave gained much attention and have been subjected to differentresearch both in experimental and theoretical contexts, whileacoustic streaming and its effects have been ignored in most stud-ies. For example, Vanhille, et al. [8] developed a three-dimensionalnumerical model to simulate nonlinear interaction between ultra-sonic waves and cavitation bubbles. The analysis of nonlinearity,

Nomenclature

c Sound speed [m/s]db Bubble diameter [m]DL Diffusion coefficient [m2/s]E Cumulative dissipated energy [m2/s3]F Force [kg.m/s2]g gravitational acceleration, [m/s2]K Kinetic rate constant [dm3/mol.min]P(t) Sinusoidal acoustic pressure [kg/m.s2]Pg Gas pressure inside the bubble [kg/m.s2]P0 Static pressure [kg/m.s2]P Pressure [kg/m.s2]R Mass transfer Source [kg/(m2 s)]t Time [min]T Temperature [�C]u Liquid velocity [m/s]ui Instantaneous Velocity [m/s]�u Mean velocity [m/s]Re Reynolds number, [Re ¼ ND2=m] [-]

Greek lettersa Volume fraction [-]q Liquid bulk density [kg/m3]

l Molecular viscosity [Ns/m2]lðtÞ Turbulent viscosity [kg/m s]S/ Source-sink term [-]e Turbulent dissipation rate, [m2/s3]m Kinematic viscosity [m2/s]ss Surface stress [N/m2]sv Vicious stress [N/m2]/SI Source term for interfacial area density [1/m.s]

Subscripts and Superscriptsc Continuous phased Dispersed phasem Mixture

AbbreviationsCFD Computational Fluid DynamicUS UltraSound (UltraSonicator)SV Stirred VesselHP Horizontal Ultrasound ProbeVP Vertical Ultrasound Probe

B. Sajjadi et al. / Ultrasonics Sonochemistry 34 (2017) 504–518 505

attenuation, and dispersion of ultrasound waves due to the pres-ence of bubbles in the liquid was the main objective of that work,which was accomplished through a three dimensional numericalsimulation. The impact of cavitation, its characteristics and totalvolume fraction have also been studied in the previous work ofauthors using CFD Simulation [9]. It was found that the cavitationbubbles reached the maximum volume fraction of 0.016% in thesonoreactor. Accumulation of cavitation bubbles was also observedin the regions in the vicinity of the transducer. In the author’s sec-ond work, acoustic streaming, its propagation and turbulent inten-sity in a liquid media under ultrasound energy were investigatedand compared with the fluid flow pattern and turbulence undermechanical mixing. It was observed that the turbulence underultrasound irradiation almost doubled that of the mechanical stir-ring. Besides, the pressure distribution profile under ultrasoundirradiation, which was affected by the sinusoidal behavior of theultrasound mechanical pressure waves, contributed to the turbu-lence in the system. These phenomena could be associated withthe mass transfer increment under ultrasound energy [10], whichis the objective of the current study. Similar work has been pre-sented by other researchers using numerical analysis. For example,the liquid flow distribution in a sonochemical reactor at high fre-quency (490 kHz) and low acoustic power (10–50 W) was investi-gated by Xu et al. [11]. The authors used the time harmonic waveequation (inhomogeneous Helmholtz equation) to numericallysimulate the acoustic pressure. However, the focus of that workwas only on analyzing the effect of input power and liquid heighton liquid velocity and pressure distribution, in the 2D structure.

Although different effects of ultrasound on solid surface aregenerally well-understood, the effects of ultrasound on gas/liquidsystems have not been clarified and again although mass transferis one of the most important phenomena which can be observedin almost every engineering field, very few papers have discussedgas-liquid mass transfer under ultrasound irradiation. The mostimportant works with on mass transfer under ultrasound wavesare summarized in Table 1. Based on existing researches, ultra-sound energy has appeared as a promising way to overcome trans-fer limitations and enhance mass transfer compared to the same

process in the absence of ultrasonication. Some researchers havesuggested possible alteration of mass transfer steps under ultra-sound waves [12,13]. Most researchers have also reported thatultrasonically induced effects (i.e. acoustic cavitation, acousticstreaming and fluid particles oscillations) play a key role for inten-sifying the transfer phenomenon. Focusing on mass transferenhancement, Moholkar et al. [14] described a semi-empiricalmethodology for a quantitative estimation of mass transferenhancement in ultrasonic textile treatments. The physical mech-anism of ultrasonic adsorption of organic pollutants on activatedcharcoal and ultrasonic desorption of aromatic pollutant from acti-vated charcoal and Amberlite XAD-4 were also identified byChakma et al. [15] and Midathana et al. [16], respectively. Theauthors have attempted to discriminate between the contributionsmade by the various physical effects of ultrasound and cavitationthat generate high convection in the medium towards enhance-ment in desorption of pollutants. Adewuyi [17], in his review onthe utilization of sonic and ultrasonic waves in chemical synthesesand processes concluded the critical role of ultrasound energy onozone mass transfer, especially in large-scale reactors. Althoughvery promising results have been reported in the literature, thereis limited industrial application of ultrasound energy. The studyof ultrasound irradiation is a challenging research field since itinvolves the multiphysics nature of ultrasound which includesthe oscillating pressure field, formation of acoustic cavitation,scattering and attenuation. The mentioned phenomena change atdifferent power intensities and liquid media. Furthermore,high-power ultrasonicated systems are commonly used under highReynolds number. To-date, different theoretical and numericalapproaches have been used to predict the mentioned phenomenain order to improve, optimize and standardize the operation anddesign of ultrasound processes. However, most of the research onlyconsider the cavitation phenomenon as the main effect of ultra-sound energy and neglect the effect of other phenomena such asacoustic streaming. Besides, most theoretical works consider cavi-tation bubbles as the only dispersed phase in the system. However,in real industrial applications of ultrasound, the systems include asecond dispersed phase (either gas bubbles or liquid droplets) with

Table 1The main computational and CFD works on mass transfer under ultrasound irradiation.

Geometry Ultrasoundproperties

Analyzing Parameters Comment/objective Ref

US-Bubble ColumnHT: 0.15 mDT: 0.088–0.15 mV: 0.5–20LH/D:0.3–3

fR: 20 kHzPower Density:16–460kw.m�3

� D:0.088-0.15 m� HL:0.04-1.02 m� T: 20–45 �C� Usg:0.001-0.01 m.s�1

� USD:16–460 kw.m�3

� Analsying the effect of reactor diameter and liquid height on Kla under US.� No significant variation of KLa was observed between 25 and 45 �C.� KLa linearly increased with the gas flow rate.� The improvement in mass transfer only observed at low H/D.� The effect of US in Kla was about 21.47 ± 11.71% for HL:0.08 m.� No significant improvement in Kla was observed for HL:0.16 & 0.34 m.

[30]

US autoclave withmechanicalagitation

fR: 20 kHzPowerAmplitude:62.6 W

� N:0–3000RPM� T: 298–368 K� P: 2.5–10.5 bar

� Ultrasound effect on the apparent solubility is very low, <12%.� Sonication effect is five times higher for degassing than regassing.� KLa is multiplied by 11 with ultrasound.� Temperature & pressure did not show any significant influence on KLa.

[31]

Three-Phase SpargedReactorsH: 180 mmD: 65 mm

fR: 18 kHzPower Density:140 W

� Usg: 1–20 m.s�1

� Gas hold up� Ultrasound pressure did not show any significant effect on gas hold up and masstransfer.

� Any enhancement in mass transfer caused by ultrasound was attributed to thephenomena of cavitation or acoustic streaming.

[32]

High-power USmicroreactorh:74 mm

fR: 20 kHzPower Density:100W

� Bubble oscillation � Under ultrasound slug bubble’s surface vibrated fiercely without collapsing, indi-cating that it undergoes non-inertial cavitation.

� Cavitation with multiple surface wave oscillation enhanced the overall masstransfer coefficient by 3.3–5.7 times.

[33]

Horn-US reactorH � 4.8 cm,D:12 cmBath-US reactorH:4.4–11.1,D:15 cm

fR: 20 kHz,500 kHzPower Amplitude:0–65WPower Density:0.01–0.13 W/ml

� Usg:5e�4–3e�3 m.s�1

� Power density� Deisnace betweensparger & horn Nacladding

� Ultrasound horn was more effective than ultrasound bath.� Use of ultrasound horn results in about 50–110% enhancement in KLa.� Sparger position should be as such all introduced gas come under the influence ofthe cavitational activity or in the path of acoustic streaming.

� Low-frequency reactor appeared more favorable than high-frequency.� The enhancement of mass transfer allotted to reduction of gas bubble size.

[34]

Pulsed US in highpressure systemH: 16 cmD: 9.5 cm

fR: 20 kHzPower Amplitude:300–1500 W

� Reaction time:0–60 min

� Power density:0.1–0.3 w.cm�3

� Standoff distance

� Analyzing the effect of surfactant on mass transfer enhancement under US� Hydrolysis time of benzoyl chloride reduced from 27 h to 30 min using US.� Processes that rely on the physical effects of sonication and lead to dispersion ofheterogeneous phases can be safely scaled-up.

� Low quantities of a bio-compatible surfactant complemented the US effect.

[35]

Horn-US reactorHL:60 mmDR:50 mm

fR: 25 kHzPower Amplitude:25W

� Surface of the fabric� Trend of washing

� Analyzing the mechanism of mass transfer enhancement under US in textile� The beneficial action of US on textile washing is due to cavitation activity.� Ultrasonication can be most efficient if the fabric is placed in the vicinity of theacoustic pressure antinodes.

� Intense microconvection due to the transient bubble motion enhances fluid flowand, thus, mass transfer through the textile.

[36]

Horn-US reactorD: 0.08-0.1 mV:340–680 ml

fR: 22 kHzPower Amplitude:0–600 W

� Power density: 3500–13000 W.m�3

� Power dissipation:� 7–35 W

� Analyzing the mass transnfer in 3 different system of air-water (A-W), air-aqueousNaCl (A-NaCl), air-aqueous surfactant (A-S) solutions.

� Liquid phase physicochemical properties play a dominant role in deciding theextent of air entrainment. A-S > A-NaCl > A-W.

� Mass transfer increase with power density.� Optimum distance of the horn tip from the liquid surface exists at which theextent of induction of air is the maximum.

[18]

Horn-US reactorV:500 ml

fR: 20 kHzPower Density: 9–18W.cm�2

� Diffusion layerthickness

� Power density� Cathode potential

� Analyzing mass transfer by ultrasound agitation during electrodeposition (copperdeposition) on electrodes separated by a narrow inter-electrode gap.

� The distortion in polarisation data was caused by the close placement of themetallic US probe to the two parallel electrodes.

[37]

US-Heat exchangerDouble-tube heatexchanger, 35 kHzIndustrial plateheat exchanger,24 KHz

fR: 20 kHzPower Density:2 W.cm�2

� Hot/Cold water flowrate

� Thermal resistance� cumulative exposuretime

� Analyzing heat transfer enhancement under US in heat exchanger based on energybalances in steady state.

� Direct US irradiation on the heat transfer surface prevented undesired fouling� Analyzing mass transfer under US to intensify membrane separation process.� Four types of colloidal suspensions (Laponite XLG, Skim milk, Nanocrystal, Naturalclay) were investigated.

� Overall permeate flux improvement by US are all severe, above 95%.

[38]

Sono-ozonation open,closed, andsparged,

fR: 20Power Density:71–431W.cm�2

fR: 500 kHzPower Density:14.5–96 W.cm�2

� Degradation RateConstants

� Gas flow rate� Power density

� Analaysing the possibility of changing mass transfer steps (absorption of ozone tothe solution, degradation of ozone to radicals, and degassing of ozone) under US.

� The main effect of US was to increase the degradation of ozone, and mass transferthrough the concentration gradient, not through the mass transfer coefficient.

[12]

Horn-US reactor+ magnetic stirrerV: 100 ml

fR: 20 kHzPower Density:250 W

� Power density: 0–200 W

� Transient absorption and desorption experiments, with or without injection ofgas.

� An increase in the kinetics of degassing of oxygen (by 6–20) linked to the emittedpower.

� Major effect of ultrasound was on the kinetics of degradation of ozone.� No acceleration of absorption could occur through the chemical degradation ofozone.

[13]

Horn-US reactorV:1000 ml

fR: 20 kHzPower Density:600W

� Gas breakage� Gas Hold-up

� Ultrasound and stripping have a synergistic effect on treatment of VOC.� Ultrasound through increasing of gas holdup played an important role.� Ultrasound did not present any effect on the size of bubbles.� Volatility was much more altered by ultrasound than diffusivity in the liquid,mostly for low volatility compounds.

[39]

506 B. Sajjadi et al. / Ultrasonics Sonochemistry 34 (2017) 504–518

Table 1 (continued)

Geometry Ultrasoundproperties

Analyzing Parameters Comment/objective Ref

Horn-US reactorV:1000 ml

fR: 20 kHzPower Density:600 W

� Henry Constant� Diffusion Coefficient� Power density

� Henry’s constant exerts a greater influence on sonolytic degradation compared tothe effect of the diffusion coefficient.

� Henry’s constant with the value of greater than 1 (volume/volume ratio)does notmuch influence on the degradation process.

[40]

Ultrasound bathV:2000 ml

fR: 35 kHzPower Density:35 W

� Adsorption ofcontaminant

� Bubble radius� Bubble internalTemperature

� Acoustic waveamplitude

� Micro-turbulencevelocity

� Analyzing the individual contribution of micro-streaming, micro-turbulence,shock waves, and micro-jets to the enhancement of the adsorption process

� The micro-turbulence generated by cavitation bubbles makes a useful contribu-tion to the enhancement of adsorption

� acoustic waves emitted by the cavitation bubbles render an adverse effect on theprocess

[16]

Horn-US reactor fR: 25 kHzPower Density:15–20W

� Textile treatments(soil removal)

� Bubble radius� Oscillation velocity

� Analyzing the mass transfer enhancement in the ultrasonic textile treatments.� The mass transfer in the textile during was characterized by two different convec-tive diffusion coefficients.

� The mass transfer enhancement factor, defined as ratio of convective diffusioncoe8cient to molecular di7usion coefficient of soil particles

[14]

Ultrasound bathV:2000 mlConical flaskinside the bath:250 ml

fR: 35 kHzPower Density:35 W

� Desorption of aro-matic pollutants

� Bubble radius� Bubble internalTemperature

� Acoustic waveamplitude

� Micro-turbulencevelocity

� The highest desorption rate for 3 aromatic pollutants and the adsorbents is attrib-uted to highest level of turbulence in the medium.

� Marginal reduction in the desorption rate with medium being saturated is attrib-uted to reduction in the magnitude of shock waves.

� The rate of desorption does not increase with increasing micro-convection indi-cates that this phenomena does not play a primary role in the process.

[15]

Usg: Superficial gas velocity, USA: ultrasound power Amplitude, USD: ultrasound power density. T: temperature, P:Pressure, D: Reactor Diameter

B. Sajjadi et al. / Ultrasonics Sonochemistry 34 (2017) 504–518 507

a diameter and volume fraction much greater than cavitation bub-bles. The current study aims to fill those gaps by developing athree-dimensional simulation model to analyse the effect of ultra-sound waves on the mass transfer phenomenon in a system con-taining high volume fraction of gas bubbles. There are two maindifferences between the current work with the other works in thisarea. Firstly, in this study, a two-phase system was considered asthe media to which gas bubbles were injected from an externalsource. Accordingly, a special range of bubble that cannot beemployed as cavitation nuclei was used. Secondly, in the currentstudy, the pure effect of acoustic streaming on mass transfer wasinvestigated and the effect of cavitation phenomena was ignored.Therefore, bubbles in the macro-meter range were used so thatthey could not assist the cavitation phenomenon by acting asnuclei for bubble formation. Accordingly, a computational andcomparative study was conducted in the present study to investi-gate the complex structure of wave propagations of low-frequencyultrasound waves in a two-phase system: liquid media in the pres-ence of gas phase. The attempt of the current work was to measurethe overall volumetric mass-transfer coefficient (KLa) in two speci-fic geometries of ultrasonic horn. The effects of rheological proper-ties, superficial gas velocity, ultrasound power and ultrasoundconfiguration on the overall gas-liquid volumetric oxygen transfercoefficient, pressure balance, liquid velocity and gas hold-up werealso investigated. The results were further compared with that of astirred vessel. In the next work, the authors would analyse the con-tribution of cavitation on mass transfer in the same system.

2. Methodology

2.1. Experimental setup

In this study, three glass batch reactors with a diameter of 6 cmwere used for the experiments The first reactor was an ultrasonica-tor equipped with a central-down ward horn (V-US); the secondwas equipped with a side ward horn placed within 15 mm from

the vessel bottom (H-US) to investigate and clarify the possibleinfluence that ultrasound might have on the gas hold up and masstransfer. The ultrasound in both systems was emitted by a 24 kHzcylindrical-horn (Diamter: 22 mm) equipped with a generator withthe maximum power input of 400 W (Ultrasonic Processor modelUP400S; Dr. Hielscher GmbH, Stuttgart, Germany). The third reac-tor was a stirred vessel (SV) equipped with 45 pitched-bladeimpellers with 6 blades (Diameter: 25 mm) mounted on a shaftwith a diameter of 4 mm placed at the centerline of the contactorand located 15 mm from the bottom of the contactor. The liquidtemperature in the systems were controlled via a water bath jacketand maintained at 25 �C. Experiments were conducted using threedifferent solutions of glycerol (0, 25 and 50 wt%) in water in orderto investigate the effect of viscosity on gas-liquid mass transfercoefficient. Air was used as the gas stream in all systems and itwas supplied through a porous rubber cylindrical-shaped spargerlocated at the bottom of the systems, centred horizontally. Theinlet gas flow-rate was measured and controlled with a mass flowcontroller (SMC flow switch, PF2A7, Japan).

2.2. Experimental design

In this study, Central Composite Design (CCD) provided byDesign-Expert software Version 9.0.4.1 (Stat-Ease Inc., USA) wasused to design the experimental conditions for mass transfer anal-ysis in V-US, H-US and SV. The selected independent parameterswere liquid viscosity (0.001–0.006 e�3 kg.m�1s�1), superficial gasvelocity (0.117–0.235 m/s), mixing intensity (power in sonicators:200–400W) and rotating speed in the stirred vessel (180–300 RPM). The independent variables were coded at three levels:1(minimum), 0 (center), +1(maximum). Eventually, a three-level-three-factor CCD, which required 60 experiments was obtainedfor each category. The center point with viscosity: 0.0035 e�3 kg.m�1s�1; superficial gas velocity: 0.235 m/s; ultraosound power:300 W and mechanically stirring speed: 240RPM was repeated 6times to determine the experimental error. Considering the levels

508 B. Sajjadi et al. / Ultrasonics Sonochemistry 34 (2017) 504–518

of the operating conditions, 12 factorial tests of 60 tests wereselected for CFD simulation. In this study the dispersed phaseincludes the air bubbles injecting from an external source. Bubblessize distribution was analyzed through photographic techniques.Accordingly, the bubbles were considered in the ranges of0.0007–0.003 m under ultrasound irradiation and stirred vessel.The continuous phase was already depleted from oxygen and sothe mass transfer was analyzed from the rate of oxygenating of liq-uid by the air bubbles. It should also be noted that, superficialvelocity is the velocity of fluid (herein gas) moving through a pipe,defined as the volumetric flow rate of that fluid divided by thecross sectional area and defined as us = Q/A, where us is the super-ficial velocity of a given phase, m/s, Q is the volume flow rate of thephase, m3/s and A is the cross sectional area, m2.

Table 2 lists the experimental tests selected for CFD simulationalong with the responses of mass transfer for V-US, H-US and MS. Itshould also be noted that the mass transfer data were collectedwithin 80 s in experimental tests. However, it was not possible tosimulate this duration. Therefore, the results related to the dura-tion of 5 s of experimental data were considered for the CFDsimulation.

2.3. Determination of gas-liquid mass transfer coefficient (kLa)

In this study, the volumetric gas-liquid mass transfer coefficientwas determined through the dynamic method of gassing-in. Theconcentration of dissolved oxygen was measured with apolarographic-membrane probe (Portable Dissolved OxygenMeters, Thermo Scientific, United States) and monitored using StarCom 1.0 (Thermo Scientific, United States). In this method, the con-centration of dissolved oxygen in the liquid was initially reduced tozero (de-aerated) by sparging nitrogen gas. After letting the nitro-gen bubbles exit the system and stabilizing the system, which tooka few minutes, the liquid was re-aerated by sparging gas and ultra-sonication was started simultaneously. The concentration of dis-solved oxygen over time was recorded and aeration wascontinued until the liquid was saturated with dissolved oxygen.A similar operating procedure was repeated by using mechanicalmixing. Volumetric gas-liquid mass transfer coefficient in the tran-sient state in the system was obtained using the below equation[18]:

KLaVðC � �CtÞ ¼ V � dCt

dtð1Þ

After integration, Eq. (1) transforms into the following form:

lnC � �C0

C � �Ct

� �¼ KLa � t ð2Þ

Table 2Experimental design matrix and the final Mass transfer results in V-US, H-US and MS.

Run Type Viscosity kg m�1.s�1 Vgas (m/s) Mixing Intensity kLa

US-Powera1 V-US 0.001 0.117 200 0.042a2 V-US 0.001 0.117 400 0.0624a3 V-US 0.001 0.353 200 0.072a4 V-US 0.006 0.117 200 0.0196

b1 H-US 0.001 0.117 200 0.033b2 H-US 0.001 0.117 400 0.052b3 H-US 0.001 0.353 200 0.0482b4 H-US 0.006 0.117 200 0.0221

MS-Rotating ratec1 MS 0.001 0.117 180 0.0146c2 MS 0.001 0.117 300 0.031c3 MS 0.001 0.353 180 0.0188c4 MS 0.006 0.117 180 0.0312

Herein, KLa, V, C⁄, Ct and C0 are the liquid volume in the reactor,saturated dissolved oxygen concentration, dissolved oxygen con-centration at any time t and initial dissolved oxygen concentrationin the contactors, respectively. A plot of the left-hand side of Eq. (2)versus time provides a straight line with a slope of KLa.

2.4. Computational fluid dynamic study

2.4.1. Theoretical backgroundContinuity. Euler-Euler multiphase model was used in this

study. In the Euler-Euler multiphase model, the continuous phaseand the dispersed phase are both treated as a continuous interpen-etrating media and described in terms of their volume fractions.The interactions between the phases are explained through thesource terms and the flow fields are solved for both phases. Theinterfacial momentum transfer between dispersed and continuousphases in this approach includes investigations on the turbulentfluctuations [19,20]. In our previous studies, ‘‘Mixture model”,which could be combined with the Keller-Miksis or Rayleigh mod-els in order to investigate the cavitation generation and measurethe total cavity volume fraction was employed [9,10]. However,the focus of this study is on analyzing the contribution and theeffect of acoustic streaming in a gas-liquid system where the gasbubbles were injected from an external source. Therefore, the Eule-rian scheme was more reliable.

Accordingly, the mass conservation equation for each phase isdefined as follows:

@

@tðqiaiÞ þ r � ðaiqi

~UiÞ ¼ 0:0 ð3Þ

Gas-Liquid Mass transfer: The volumetric mass transfercoefficient was calculated based on the liquid mass transfer coeffi-cient kL and the interfacial area a, using Higbie’s penetrationtheory:

kL ¼ 2ffiffiffiffip

pffiffiffiffiffiffiffiffiDO2

q kqL

lL

� �1=4

ð4Þ

where DO2 is oxygen diffusion coefficient (2.01 � 10�9 m2/s) and k isthe turbulence kinetic energy. ql and ll denote the liquid densityand viscosity, respectively. The interfacial area a, is also defined asa function of the local Sauter mean diameter (d32) and local gas vol-ume fraction (aG):

a ¼ 6aG

d32ð5Þ

Momentum: The momentum conservation equation for phase iis defined as:

@

@tðqiai

~UiÞ þ r � ðaiqi~Ui

~~UiÞ ¼ �airpþr � s~*effi þ~Ri þ~Fi þ aiqi~g

ð6Þwhere, the terms p, Ri and g denote the pressure shared by the twophases, interphase momentum exchange and gravity acceleration,respectively.

The term ~~seffi on the right hand-side of Eq. (6) represents Rey-nolds stress tensor related to the mean velocity gradients usingBoussinesq hypothesis which is defined as

s~*

effi ¼aiðllam;iþlt;iÞðr~Uiþr~UTi Þ�

23aiðqikiþðllam;iþlt; iÞr�~UiÞ~~I

ð7ÞTurbulence model equations: In Eq. (7), lt,L represents the

turbulent liquid viscosity which is formulated through

lt;L ¼ qLClðk2L=eLÞ. In this study, standard k-e turbulence modelwas employed for prediction of kL and eL through:

3

5

2 (b)

1 3

4

5

1. Pressure Outlet 2. Velocity Inlet 3. Pressure Inlet 4. Symmetry 5. Wall 6. Rotating Region 7. Stationary Region

1

2 (c)

6

7

5

2 (a)

4

1

Fig. 1. Computational domains of (a) Stirred vessel (b) HP-ultrasonicator, (c) VP-ultrasonicator geometries.

B. Sajjadi et al. / Ultrasonics Sonochemistry 34 (2017) 504–518 509

@

@tðqLaLkLÞ þ r � ðqLaL

~ULkLÞ ¼ r � aLlt;L

akrkL

� �þ aLGkL

� aLqLeL þ aLqLPkL ð8Þ

@

@tðqLaLeLÞ þ r � ðqLaL

~ULeLÞ ¼ r � aLlt;L

akreL

� �þ aLðC1eGkL � C2eqLeLÞ þ aLqLPeL ð9Þ

C1e ¼ 1:44; C2e ¼ 1:92; C3e ¼ 0:09; rk ¼ 1:0 and rk ¼ 1:3 arethe turbulent constants according to the recommendations of[19–21].

Interfacial momentum exchange: Among different interphaseforces, drag force plays an important role in the hydrodynamicstudies. In this study, drag coefficient was formulated throughthe Schiller-Neumann model as follows:

CD ¼24 1þ0:15Re0:687ð Þ

Re Re 6 10000:44 Re > 1000

(ð10Þ

where Re is the relative Reynolds number for the dispersed phase(G) and the continuous phase (L). According to the acoustic theory[22], the sound pressure was obtained through the plane wave aspresented in Cai’s model [23], Eq. (11). This model has also beenused by other researchers [24,25]. Therefore, in this study, the ultra-sound effect was introduced to the media as a boundary conditionon which the pressure profile changes using Eq. (11) and so noattenuation coefficient was taken into account. However, attenua-tion of ultrasound was not ignored since the interaction of pressureprofile with other factors was considered through continuity equa-tion, momentum balance and parameters such as turbulent inten-sity. It should also be noted that the effect of liquid propertieswas also considered through Eq. (11) in which density is directlyeffective while the effect of viscosity is considered though theparameter of c (sound speed in the media).

PðtÞ ¼ �Pa sinxðt þ y=cÞ ð11Þwhere x ¼ ð2pf Þ, c and y denote angular frequency, sound speedand space coordinate. In Eq. (11), Pa refers to the intensity of ultra-sound source which is defined as:

Pa ¼ffiffiffiffiffiffiffiffiffiffiffiffi2qIUS

pð12Þ

Herein, IUS refers to the ultrasound intensity (W/m2) which isdefined as (Pus/A). Pus (W) is the ultrasonic power and A (m2) isthe transducer area. In this study also ultrasound power denotesthe Pus. In doing so, acoustic pressure was written in a User DefinedFunction UDF and compiled in FLUENT to simulate the ultrasoundwaves and their interactive effect with the surrounding gas and liq-uid in this study. The programming codes were generated in VisualC++ (Version 6.0, Mathworks, Natick, MA).

2.5. Geometry and boundary conditions

As mentioned earlier, acoustic pressure was introduced throughan inlet pressure with two different power amplitudes on piezo-electric transducer surface which was settled vertically in onegeometry and horizontally in the other. Since the flow was axisym-metric in both sonicators, only half of the contactors were simu-lated and so a symmetrical boundary condition was considered(Boundary No#4). In the stirred tank, rotation of the pitched bladeimpeller was modeled with the Sliding Mesh (SM) method inwhich almost 20% of the vessel was considered stationary andthe remaining (about 80%) was considered as the rotating region.

For all three geometries, a gas inlet with the boundary condi-tions of velocity inlet was defined for the sparger section (Bound-ary No#2) and atmospheric pressure outlet was considered for thevessels outlet (Boundary No#1). The transducer surface in sonica-

tors was introduced through pressure inlet boundary conditions onwhich the Eq. (11) was introduced through a User Defined FunctionUDF (Boundary No#3). In case of stirred vessel, rotating and sta-tionary regions were defined instead. The other boundary condi-tions for these geometries consisted no-slip wall conditions forthe lateral boundaries. The details of the boundary conditions arealso provided in Fig. 1(a, b, c) and summarized in Table 3. Besides,the initial gas holdups and the velocities of the fluids in all thethree systems were set to zero.

Among different grid types and sizes tested in all the three sys-tems, tetrahedral cells provided the best results. It was also foundthat pressure distribution was dramatically influenced by the gridsize in all the three systems. Accordingly, 144,488, 154,223 and330,700 tetrahedral cells were generated for VP-Ultrasonicator,HP-Ultrasonicator and stirred vessel, respectively; consisting ofthe minimum and maximum of 0.3 and 1.5 mm for the elementsquality. Finer grids were settled in the vicinity of the probe inthe ultrasonicator and the rotating region in the stirred vessel toobtain stable and reliable results. The quality of the meshes waspresented in Fig. 1(a, b, c).

2.5.1. Simplification assumptions and justificationIn the present study, the fluid flow in all systems was assumed

adiabatic, turbulent and transient with the initial velocity of zero.Besides, in this study, the secondary Bjerknes force was negligiblein the system. The secondary Bjerknes force can induce repulsionor attraction between two bubbles, depending upon the averagevolume oscillation changes and the distance between them.According to [26], the secondary Bjerknes force between twoapproaching bubbles become significant at close distance (lessthan 2 mm). However, in this study, the bubbles were injected tothe system from a location with the maximum distance from thehorn tip and the designs of the sonoreactors were in a way to makethe bubbles far from each other and spread them within the sys-tem uniformly. The liquid properties were set as pure water forglycerol 0%, qL = 1083.7 kg m�3and qL = 1123.6 kg m�3, lL = 0.006 -kg m�1.s�1 for glycerol 50%. The properties of air was also set asqG = 1.225 kg m�3, lG = 1.789�10�5 kg m�1.s�1. Constant density

Table 3Defined boundary conditions in low frequency sonicator and mechanical stirrer vessel.

Ultra-Sono Reactor

Boundary zone Number Type Phase Applied condition

Vessel – Fluid Fluid StationaryVessel top 1 Pressure Outlet – Pgauge = 0 (pa)Vessel bottom 2 Wall – Stationary, No slipVessel bottom center Velocity Inlet Gas Flow = 0.117-0.353 m/sVessel wall 3, 4 Wall – Stationary, No slip

5 Symmetry SymmetryTransducer 6 Pressure inlet – PðtÞ ¼ �Pa sinxðt þ y=cÞ

Power = 200–400 W

Mechanical Stirrer VesselVessel – Stationary Fluid StationaryVessel top 1 Pressure Outlet – Pgauge = 0 (pa)Vessel bottom 2 Wall – Stationary, No slipVessel bottom center 2 Velocity Inlet Gas Flow = 0.117–0.353 m/sVessel wall 3 Wall – Stationary, No slipPitched blade impeller Wall – Stationary, No slipImpeller motion model 4 SM Fluid Rotational velocity

Rate = 180–300 RPM

510 B. Sajjadi et al. / Ultrasonics Sonochemistry 34 (2017) 504–518

was assumed since the pressure was almost the same throughoutthe vessel. It should be noted that, the focus of this study was toanalyse the effect of acoustic streaming generated by high powerultrasound energy on fluid flow pattern and the motion of gas bub-bles. It also focused on the interaction of gas and liquid phasesthrough mass transfer analysis. The main parameter analyzed inthis study was the turbulent intensity generated by ultrasoundenergy and its release in the system through and in the directionof acoustic streaming. Accordingly, the cavitation phenomenonand generation of micro cavitation bubbles under high powerultrasound were ignored and the pure effects of acoustic streamingand turbulent intensity on mass transfer in a two-phase systemsincluding liquid and injecting gas (air) bubbles were considered.

2.5.2. Numerical solution of equationsThe solution domains investigated in this study are presented in

Fig. 1. ANSYS FLUENT (version 13.5) was used for geometry andmesh generating as well as combining and solving Eqs. (1) to(14) numerically. The tanks’ domains were discretized by anunstructured finite volumemethod, in order to convert the govern-ing equations to algebraic equations that could be solved numeri-cally [19,20]. The SIMPLE pressure-velocity coupling algorithmalong with the second-order upwind discretization scheme wasused for all discretized terms. The solutions were considered tobe converged when the normalized residuals of all the variableswere less than 1 � 10�4.

In unsteady-state simulations of the wave motion in the liquid,the time period of the sound wave should be considered biggerthan the time step (ss ¼ 1=f ). Accordingly, the value was set toquarter period of the ultrasound frequency (time step = 1 e�6).Although higher time step could be used in the stirred vessel, thestability of the solution mainly depended on the time step at thestarting point. Therefore, the calculation started with a very smalltime step size (1 e�7) followed by increment to 1 e�5 to speed upthe computation.

3. Results and discussion

3.1. The effect of operational conditions on mass transfer

Table 2 presents the experimental results of mass transfer in theVP-sonicator, HP-Sonicator and stirred vessel. As observed, themass transfer value in both the sonicators almost doubled that ofmechanically stirring, which was attributed to the high ability of

ultrasound in increasing the interfacial surface and turbulentintensity. Besides, the results suggests that the vertical positionof ultrasound probe played a positive role in increasing the masstransfer. On the other hand, increasing mixing intensity and super-ficial gas velocity posed synergistic effects on mass transfer whileviscosity presented an antagonistic effect. Higher liquid viscositydid not only reduce the turbulent intensity in the system but alsolimit the breakage of gas bubbles, reducing the mass transfer phe-nomenon. Analysis of the effect of operating parameters on masstransfer depicted that they did not follow a similar pattern in thethree systems and the intensity of their influence on each systemwas quite different. As shown in Table 2 (and also in the last Fig-ure), superficial gas velocity critically affected the mass transferin VP-S, whereas, ultrasound power played the most importantrole in HP-S. Similar pattern as in VP-S was followed in stirred ves-sel. In other words, in SV and HP-S in which circular fellow patterndominated the system, the power or rotating rate are the mostimportant parameters clarifying the impact of gas bubbles resi-dence time. While in VP-Sonoreactor in which gas bubbles directlycame toward acoustic jet like streaming, superficial gas velocity isthe key which was attributed to the ability of ultrasound inbreaking gas bubbles and increasing the mass driving force usinghigher turbulence intensity. Accordingly, the maximum masstransfer coefficient of 0.072, 0.052 and 0.0312 s�1 were obtainedunder the operating conditions of viscosity:0.001 kg m�1.s�1,ultrasound power: 200 W and Vgas:0.353 m/s (Run a3) in H-USand viscosity:0.001 kg m�1.s�1, ultrasound power: 400 W andVgas:0.117 m/s (Run b2) in V-US and viscosity:0.001 kg m�1.s�1,mixing intensity: 300 RPM and Vgas:0.117 m/s (Run c2) in stirredvessel respectively.

3.2. Fluid flow pattern and liquid velocity

Dynamic pressure is the kinetic energy per unit volume of afluid that represents fluid kinetic energy, while static pressure rep-resents hydrostatic effects (Ptotal = Pdynamic + Pstatic), which werepresented in Fig. 2. As observed in this figure, pressure pulses gen-erated by the transducer started dispersing from surface of theultrasound probe. It should be considered that ultrasound energydistributes in a fluid through mechanical pressure waves. In otherwords, ultrasound waves are sinusoidal mechanistic waves con-sisting of both expansion (negative) and compression (positive)pressure waves. Hence, irradiation of ultrasound waves affectsthe static pressure value in the liquid. Accordingly, the static

(a) Static Pressure, Power:200W Dynamic Pressure Power:200W

(b) Static Pressure, Power:200W Dynamic Pressure Power:200W

(c) Static Pressure, Speed:180RPM Dynamic Pressure Speed:180RPM

Fig. 2. Distribution of pressure pulses inside the a) VP-Sonicator. b) HP-Sonicator, c) Stirred vessel under superficial gas velocity of 0.117 m/s.

B. Sajjadi et al. / Ultrasonics Sonochemistry 34 (2017) 504–518 511

pressure under ultrasound energy provided in Fig. 2 a and b are notequal to ambient pressure. However, in the system containingmechanical stirring, the surface static pressure represents thevalue, which is equal to the ambient static pressure. In regions

below and above the impeller, the static pressure was affected bythe impeller rotating motion, which leads to pulling and pushingthe liquid, creating suction and discharge section and influencingthe balance of pressure in those regions. It should be noted that,

(a) Power: 200 W (a) Power: 400 W

(b) Power: 200 W (b) Power: 400 W

(c) Impeller rotating rate: 180 RPM (c) Impeller rotating rate: 300 RPM

Fig. 3. Fluid flow pattern and velocity profiles induced by a) VP- Sonicator, b) HP- Sonicator, c) mechanically stirring.

512 B. Sajjadi et al. / Ultrasonics Sonochemistry 34 (2017) 504–518

all values are gage pressure. The quality of these pressure wavesdetermined the turbulence energy dissipation within the system,which subsequently intensified the mass transfer phenomenon.These positive and negative pressure pulses were converted to

each other in turn (for more detail please refer to [10]), multiplyingthe turbulent energy within the system. Fig. 2 clearly shows themagnitude and propagation of static pressure within the systemand its interaction with the dynamic pressure in ultra-sonicators

B. Sajjadi et al. / Ultrasonics Sonochemistry 34 (2017) 504–518 513

and stirred vessels. Considering the pressure distribution withinthe systems, different fluid flow patterns were imposed to the sur-rounding. Fig. 3 shows the fluid flow pattern dominating the twoultra-sonicators compared to that in the mechanically stirred ves-sel. The corresponding velocity magnitude versus contactor radiusis presented in Fig. 4. Generally, the rotating impeller generated astrong convective flow in the mechanically agitated contactor,whereas ultrasound energy mostly produced oscillatory (vibratory)flow. This type of flow pattern was also observed in the CFD simu-lation by following the fluid flow in sequential time-steps withvery short durations (in the level of 10�7 s). However, an overallfluid movement dominated the systems (as presented in Fig. 3).In other words, acoustic streaming caused by high-power gener-ated strong convective flow in both ultra-sonicators. Althoughthe streamlines were deformed by gas injection, the flow in theVP-sonicator was mainly axial. Besides, acoustic streaming con-ducted a strong recirculation flow from the probe surface towardthe bottom of the contactor and then to the surface of the contac-tor. This type of flow pattern dragged the gas bubbles toward thewall of the contactors, which increased gas bubbles spreadingwhile speeding up the release of bubbles from the system. The

(a)

(b)

(c)

00.050.10.150.20.250.30.350.40.450.5

-0.04-0.03-0.02-0.0100.010.020.030.04

Vel

ocit

y M

agni

tude

(m/s

ec)

R (m)

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

-0.04-0.03-0.02-0.0100.010.020.03

Vel

ocit

y M

agni

tude

(m

/sec)

R(m)

00.050.10.150.20.250.30.350.40.450.5

-0.04-0.03-0.02-0.0100.010.020.030.04

Vel

ocit

y M

agni

tude

(m/s

ec)

R(m)

Fig. 4. Velocity magnitude at 3 cm elevation from the tank bottom induced by: a)VP-Sonicator, b) HP- Sonicator, c) mechanically Stirrer. Grey filled: Case 1, Blackfilled: Case 2, White filled: Case 3 Pattern filled: Case 4.

same flow pattern, although in horizontal direction, dominated inthe system of the HP-sonicator, generating a circulating flowwhichkept gas bubbles in the HP-sonicator for a longer residence timeand increased the gas-liquid contact time and mass transfer even-tually. However, the increment of mass transfer in this flow patterndepends on the number of bubbles imprisoned in the circulatingflow. The numerical simulation showed that the liquid velocityvaried within the range of 4–47 and 9–52 cm/s in the VP-sonicator and HP-sonicator, respectively. Furthermore, the maxi-mum velocity magnitudes were observed in the acoustic streamingdirection. The liquid velocity magnitude and fluid flow profileswere measured and validated in the non-aerated VP-Sonicatorusing PIV analyses [10]. There was only a marginal difference inthe simulation results with those of PIV analyses.

On the other hand, a strong spiral and circulating flow wascaused by agitation in the mechanically stirring contactor. Thecharacteristics of the path-lines illustrated in Fig. 4 clarify two dif-ferent flow patterns induced by the impeller rotating at 180 and300 RPM. In the former, the flow was upward among the impellerblades and it was downward along the vessel walls, assisting thegas bubbles to leave the system easier. In the latter, the directionof the central flow was downward (below the impeller), pullingthe bubbles into the vicinity of the impeller to experience the high-est turbulence value. However, the fluid flow pattern was notaffected by the variation in ultrasound amplitude in both sonica-tors. It can be concluded that, the flow was discharged both axiallyand radially in the stirred vessel, with the highest value of 45.1 and61.9 cm/s (Fig. 3) at impeller rotating speed of 180 and 300 RPM(below impeller). Note that the values presented in Fig. 4c relateto the velocity magnitude above the impeller. A point should beconsidered in this figure. Velocity magnitude related to case 3(with highest gas injection) and case 4 (with highest viscosity)demonstrate highest velocity magnitude in center and left side ofthe figure which is caused by the accumulation of gas bubbles. Inother words the gas bubbles in some regions accumulate and gotoward liquid surface, which affect the symmetrical distributionof velocity. This challenge is more intensive in liquid with higherviscosity as it is observed in Figs. 4c and 6c and affected on themass transfer profile as observed in Fig. 8c, which will be discussedin next parts.

3.3. Turbulent intensity dissipation

The results obtained for turbulent intensity dissipated into thesystem are shown in Fig. 5. As observed, the turbulent intensitydissipation values in the HP-sonicator and VP-sonicator werealmost three times greater than that in the stirred vessel, where

0

0.2

0.4

0.6

0.8

1

1.2

1.4

-0.04-0.03-0.02-0.0100.010.020.030.04

Tur

bule

nt I

nten

sity

(%)

R (m)

Fig. 5. Distribution of turbulence intensity at 3 cm elevation from the tank bottominduced by:s) VP- Sonicator,D) HP-Sonicator, }: mechanically stirring. Grey filled:Case 1, Back filled: Case 2, White filled: Case 3 Pattern filled: Case 4.

(a1) Vg:0.117,P:200, :1e-3 (a2) Vg:0.117,P:400, :1e-3 (a3) Vg:0.353,P:200, :1e-3 (a4) Vg: 0.117,P:200, :6e-3

(b1)Vg:0.117,P:200, :1e-3 (b2) Vg:0.117,P:400, :1e-3 (b3) Vg:0.353,P:200, :1e-3 (b4) Vg: 0.117,P:200, :6e-3

(c1)Vg:0.117,P:200, :1e-3 (c2) Vg:0.117,P:400, :1e-3 (c3) Vg:0.353,P:200, :1e-3 (c4) Vg: 0.117,P:200, :6e-3

Fig. 6. Flow pattern of gas bubbles distribution within the a) VP-Sonicator, b) HP-Sonicator, c) mechanically stirring.

0.0510.041

0.097

0.058

0.024 0.0290.0397

0.027

0.07

0.12

0.08965

0.06186

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

a1 a2 a3 a4 b1 b2 b3 b4 c1 c2 c3 c4

Fig. 7. Gas hold up volume fraction within the a) VP- Sonicator, b) HP- Sonicator, c)mechanically stirring.

514 B. Sajjadi et al. / Ultrasonics Sonochemistry 34 (2017) 504–518

the systems contained liquid with a viscosity of 0.001 kg m�1.s�1.The maximum turbulent intensity dissipation rate was observedin the direction of acoustic jet streaming starting from the trans-ducer’s surface in sonicators and in vicinity of the impellers inthe stirred vessel. Increasing the liquid viscosity in sonicators lim-ited the jet-like motion of acoustic streaming, resulting in lowerturbulent intensity dissipation. The same happened in stirred ves-sel. In other words, higher turbulence observed in regions near theimpeller. However, turbulence dissipation into regions away fromthe impeller was limited and reduced. Although, no significantincrease in the turbulent intensity dissipation in HP-S and SVwas observed with increased superficial gas velocity, the presenceof entrapped air slightly altered the turbulent intensity dissipation

in some regions. In term of input power, higher turbulent intensitydissipation was observed in the HP-sonicator, VP-sonicator andstirred vessel, respectively. It should be noted that, the values pre-sented in Fig. 3 are the exact turbulence intensity at 3 cm elevationfrom the tank bottom which is in the direction of horizontal probesonoreactor. However, greater increment in turbulent intensity (asthe average value in whole contactor) with ultrasound power wasobtained by placing the horn tip vertically instead of horizontally.

3.3.1. Gas-liquid interfacial areaThe CFD results of the flow patterns in the investigated system

are demonstrated in Fig. 6. As observed, in the system of VP-sonicator, acoustic jet improved the gas volume fraction by creat-ing a flow pattern in which gas bubbles were significantly spreadalong the wall. In parallel, this flow pattern had an antagonisticeffect through which the gas bubbles were directly conducted tothe free surface of the system. In contrast, HP-sonicator exploitedof two advantage: i. acoustic streaming imprisoned the gas bubblesin the HP-sonicator (Fig. 6b2 and b3), which increased the residencetime and subsequent mass transfer, ii) a section of the bubbles fol-lowed the flow pattern circulating within the system. However,this flow pattern suffered from the weak spreading of the gas bub-bles in the system and most of which got rid of the acoustic jetstreaming and moved to the free surface of the system.

Gas hold up bubbles in a gassed liquid is the volume fraction ofgas bubbles and their residence time. Besides, gas hold up in con-junction with the knowledge of mean bubble diameter allows thedetermination of interfacial area and thus, the mass transfer ratebetween gas and liquid phase. The results related to the volumefraction of air (gas hold-up) in the systems are provided in Fig. 7.

(a) VP-Sonicator, Case#a1

(b) HP-Sonicator, Case#b1

(c) Mechanically Stirrer, Case#c1

Fig. 8. Liquid mass transfer coefficient within the a) VP-Sonicator, b) HP- Sonicator,c) mechanically stirrer. Case 1.

B. Sajjadi et al. / Ultrasonics Sonochemistry 34 (2017) 504–518 515

As observed, increasing the ultrasound power in the VP-sonicator(a2) had a negative effect on the presence of gas bubbles in thatsystem. In this case, higher power helped the bubbles leave thesystem sooner. However, increment of gas hold up with ultrasoundpower in the HP-sonicator (b2) indicated that more bubbles wereimprisoned under the acoustic jet or in the circulating flowinduced by higher-powered jet streaming. The same situation hap-pened by increasing the impeller rotating rate (c2). The figure alsoindicates that an increase in the superficial gas velocity led to anobvious increment in the value of the gas volume fraction with alinear trend. These changes were up to 90.2% (a3), 65.4% (b3) and28.1% (c3) in the system of VP-sonicator, HP-sonicator and stirred

vessel, respectively. Higher increments in sonoreactors corre-sponded to the homogeneous regime induced in these systems.But as observed in Fig. 6(c3), the impeller rotating rate of180RPM is not sufficient to imprison the bubbles in the flow pat-tern dominated in the system and most of the bubbles easily leavethe stirred vessel. The simulation results suggest that the incre-ment of liquid viscosity from 0.001 to 0.006 cP slightly increasedthe gas hold up in the sonicators (a4 and b4) due to the incrementof residence time. However, reduction of gas hold-up with viscos-ity in the stirred vessel related to the accumulated bubbles whichleave the system faster as shown in case c4 (in Fig. 7). Generally,Fig. 7 shows that there was an average increment of 97.7% in thegas hold-up in the sonicators by changing the geometrical condi-tions (horizontal position of the probe to vertical position) underthe same operating conditions. The same difference was observedin the stirred vessel due to the circular liquid flow pattern thatdominated in this system. Accordingly, the highest gas bubblesvolume fraction with the values of 9.7% and 12% were observedin two cases: VP-sonoreactor with the gas injection of 0.353 m/s(a3) and stirred vessel with the impeller rotating rate of 300 RPM(c2). The differences in terms of velocity magnitude and turbulentintensity dissipation appeared to be the main reasons for theobserved variation in the mass transfer rate in these reactors.

3.4. Volumetric gas-liquid mass transfer

The momentary gas-liquid mass transfer coefficient (KL) after5 s is presented in Fig. 8. As observed, the maximum potentialpoints of the liquid mass transfer coefficient in the VP-sonicatorand HP-sonicator were observed in two regions i) in vicinity ofthe sparger due to the direct interaction of the acoustic streamingwith the gas bubbles; ii) in vicinity of the transducer and in thedirection of the acoustic jet like streaming due to large accumula-tion of kinetic energy in these regions. Comparing the VP-sonicatorand HP-sonicator contours also demonstrated that more regions inthe HP-sonicator were potential for higher gas-liquid mass transfercoefficient. The opposite was observed in the stirred vessel.

The volumetric mass transfer coefficient (KLa) simulated under12 different operating conditions, at different positions above thesparger, middle of the reactor and close to the free surface, isdemonstrated in Fig. 9. The exact and averaged values of boththe CFD and experimental results are reported and validated inFig. 10. It should be noted that only regions in which gas bubblesexist can contribute to the transfer phenomenon. Therefore, thearea with zero value of KLa demonstrated no presence of gas bub-bles. Generally, the volumetric mass transfer coefficient in the son-icators almost doubled that in the mechanically stirred system.Similar was reported by Coleman and Roy in their study on masstransfer under ultrasound irradiation who reported that agitationprovided by ultrasound significantly increased the mass transferrate by about 10 times [27]. Although gas hold-up in the stirredvessel was greater than in the sonicators, the volumetric and over-all mass transfer rate were prominently less than that in both thesonicators due to lower turbulent kinetic energy. Between the son-icators, a slight increment was observed in mass transfer value inthe HP-sonicator. Generally the results predicted by CFD simula-tion confirm the results obtained by experimental tests, except inone case (b2). This difference was caused by the low value of gashold-up predicted by CFD simulation due to weak and incompletecirculation in the system in this case. Experimental analysis of gasvolume fraction due to especial geometrical properties of contac-tors in this study was not possible. However, the authors clearlyobserved that the flow pattern predicted by CFD simulation agreedwell with those in experimental tests which confirm the resultspredicted by CFD simulation. The other possible reason may berelated to the bubbles size distribution. In mass transfer analysis,

(a1) Vg:0.117,P:200, µ:1e-3 (a2) Vg:0.117,P:400, µ:1e-3 (a3) Vg:0.353,P:200, µ:1e-3 (a4) Vg: 0.117,P:200, µ:6e-3

(a) VP-Sonocator

b1)Vg:0.117,P:200, µ:1e-3 (b2) Vg:0.117,P:400, µ:1e-3 (b3) Vg:0.353,P:200, µ:1e-3 (b4) Vg: 0.117,P:200, µ:6e-3

b) HP-Sonocator

(c1)Vg:0.117,P:200, µ:1e-3 (c2) Vg:0.117,P:400, µ:1e-3 (c3) Vg:0.353,P:200, µ:1e-3 (c4) Vg: 0.117,P:200, µ:6e-3

c) Mechanically Stirrer

Fig. 9. Volumetric mass transfer coefficient within the a) VP-Sonicator, b) HP-Sonicator, c) mechanically stirring. d: Above sparger, : Middle of the reactor, s: Close to thefree surface.

Case No.

00.010.020.030.040.050.060.070.080.09

a1 a2 a3 a4 b1 b2 b3 b4 c1 c2 c3 c4

kLa(

1/s)

Fig. 10. Volumetric mass transfer coefficient within the a) VP-Sonicator, b) HP-Sonicator, c) mechanically stirring.

516 B. Sajjadi et al. / Ultrasonics Sonochemistry 34 (2017) 504–518

computationally or experimentally, bubble size distribution andturbulent intensity are the keys which affect the mass transferresults. The other obvious underestimation was observed in casea2. Since, cases of a2 and b2 are accompanied by higher ultrasound

power compared to other cases, the reason of under estimationsmore likely caused by deviation in bubble size analysis.

The influence of superficial gas velocity, liquid viscosity andultrasound power amplitude upon the overall mass transfer arealso shown Fig. 10. The simulation and experimental resultsdepicted an increase in the overall mass transfer with the superfi-cial gas velocity. This effect was mostly due to an increase in thegas volume fraction. Higher viscosity inhibited the mass transferbecause of its significant reducing effect on deploying turbulentkinetic energy, which led to a decrease in the mass transfer coeffi-cient and overall mass transfer. Besides, higher viscosity producedanother negative effect upon the mass transfer rate since it slightlyincreased the bubble size distribution. It also reduced the value ofgas diffusivity in the liquid phase [28]. Generally, the effect of liq-uid viscosity and ultrasound power amplitude upon the masstransfer value was mostly related to the turbulent kinetic energy,while the influence of superficial gas velocity was more relatedto the gas volume fraction. In the stirred vessel, the rotating rateof impellers affected both the turbulent kinetic energy and gas vol-ume fraction within the liquid. However, in the stirred vessel with

B. Sajjadi et al. / Ultrasonics Sonochemistry 34 (2017) 504–518 517

an impeller rotating speed of 180 RPM, it was observed that mostof the bubbles rose with low dispersion or were almost undis-turbed throughout the central area. In the sonicators, turbulentfluid motion that assisted the general gas-liquid mixing causedby acoustic jet streaming increased with ultrasound power ampli-tude and enhanced the distribution of the entrapped gas bubbles.Experimentally, increased ultrasound power amplitude reducedthe average size of bubbles. In this study, the overall mass transfercoefficient (KLa) increased by about 18.3% and 28.3% with a corre-sponding increase in the power amplitude by about 200 W in theVP- and HP-sonicators, respectively. Similar observation was foundin the literature [18], indicating the linear increment of kLa withthe power dissipated per unit volume.

Generally, any improvement in the mass transfer caused byultrasound could be attributed to acoustic streaming or cavitation.Both the simulation and experimental results indicated that addi-tional turbulence created by ultrasound was significant enough tomake a non-ignorable enhancement in the mass transfer phe-nomenon compared to that in the stirred vessel. The simulationresults in the presence of ultrasound irradiation showed a satisfy-ing agreement with that of the experiments. However, a non-ignorable difference in the kLa values was likely attributed to thecavitation phenomenon which was not considered in this study.The other reason was only one bubble size was used in simulatingeach system. According to Kerodous et al. [29], employing popula-tion balance equation gives the best prediction results as the equa-tion considers different bubble sizes, the bubble coalescence/breakup phenomena and other interactions (i.e. bubble–turbulenceInteraction). The other important point that should be consideredis that the mass transfer could be significantly increased whenmore bubbles were entrapped in the circulation flow or imprisonedunder acoustic streaming. Accordingly, a modified geometry inwhich a horizontal ultrasound probe makes an optimum anglewith the contactor wall can maximize the mass transfer. As aresult, such system can benefit from all the associated advantagessuch as increment of gas hold up, turbulent intensity and bubblesresidence time.

4. Conclusion

This study discussed the needs to know the effects of acousticstreaming on the mass transfer enhancement in macro-scalesono-reactors. Two types of sonicators (horizontally- andvertically-probe ultrasonicator) were considered and the resultswere then compared with that of a stirred vessel. 3-D CFD simula-tion was used to present the overall gas-liquid volumetric oxygentransfer coefficient as a function of ultrasound power amplitude,fluid rheological properties and superficial gas velocity. The mainobservations and results are summarized as follows:

1. A significant enhancement in the mass transfer was observed inultrasonicators, which was attributed to the kinetic energyimposed on the system by acoustic streaming.

2. The mass transfer improvement in case the H-sonoractor wasnot as significant as in the V-sonicator, implying the importanceof propagation quality of kinetic energy within the system.

3. The volumetric mass transfer coefficient presented a linear rela-tionship (with the slope of +1) with the ultrasound amplitude.However, this increment was more significant with the rotatingrate of impeller in the stirred vessel.

4. The rheological properties in liquid phase played a dominantrole in deciding the residence time of gas bubbles and dissipa-tion of turbulent kinetic energy. This issue was more criticalin sonoreactors, which showed the effect of viscosity on reduc-ing the propagation depth of acoustic jet-like streaming.

5. A combination of the modified approach for the gas induction(by placing the horn tip just above the sparger) and theconventional approach (by making an optimum angle betweenthe horizontal horn tip and the HP-Sonicator’s wall) in whichthe horizontal circulation of liquid along with the imprisonedgas bubbles became the maximum appeared to be the opti-mum strategy for the operation of gas/liquid sonochemicalcontactors.

Acknowledgement

The authors are grateful to the University of Malaya HighImpact Research Grant (HIR-MOHE- D000038-16001) from theMinistry of Higher Education Malaysia and University of MalayaBright Spark Unit which financially supported this work.

References

[1] S. Pilli, P. Bhunia, S. Yan, R.J. LeBlanc, R.D. Tyagi, R.Y. Surampalli, Ultrasonicpretreatment of sludge: a review, Ultrason. Sonochem. 18 (2011) 1–18.

[2] B. Thokchom, A.B. Pandit, P. Qiu, B. Park, J. Choi, J. Khim, A review onsonoelectrochemical technology as an upcoming alternative for pollutantdegradation, Ultrason. Sonochem. 27 (2015) 210–234.

[3] F.J. Trujillo, P. Juliano, G. Barbosa-Cánovas, K. Knoerzer, Separation ofsuspensions and emulsions via ultrasonic standing waves – a review,Ultrason. Sonochem. 21 (2014) 2151–2164.

[4] B. Sajjadi, A.R. Abdul Aziz, S. Ibrahim, Mechanistic analysis of cavitationassisted transesterification on biodiesel characteristics, Ultrason. Sonochem.22 (2015) 463–473.

[5] W.L. Nyborg, Acoustic Streaming due to attenuated plane waves, J Acoust SocAm 25 (1953) 68–75.

[6] S. Tjøtta, Theoretical investigation of heat and streaming generated by highintensity ultrasound, Acta Acustica United with Acustica 85 (1999) 780–787.

[7] F.J. Trujillo, K. Knoerzer, A computational modeling approach of the jet-likeacoustic streaming and heat generation induced by low frequency high powerultrasonic horn reactors, Ultrason. Sonochem. 18 (2011) 1263–1273.

[8] C. Vanhille, C. Campos-Pozuelo, Numerical simulations of three-dimensionalnonlinear acoustic waves in bubbly liquids, Ultrason. Sonochem. 20 (2013)963–969.

[9] B. Sajjadi, A.A.A. Raman, S. Ibrahim, Influence of ultrasound power on acousticstreaming and micro-bubbles formations in a low frequency sono-reactor:mathematical and 3D computational simulation, Ultrason. Sonochem. 24(2015) 193–203.

[10] B. Sajjadi, A.A.A. Raman, S. Ibrahim, A comparative fluid flow characterisationin a low frequency/high power sonoreactor and mechanical stirred vessel,Ultrason. Sonochem. 27 (2015) 359–373.

[11] Z. Xu, K. Yasuda, S. Koda, Numerical simulation of liquid velocity distributionin a sonochemical reactor, Ultrason. Sonochem. 20 (2013) 452–459.

[12] L.K. Weavers, M.R. Hoffmann, Sonolytic decomposition of ozone in aqueoussolution: mass transfer effects, Environ. Sci. Technol. 32 (1998) 3941–3947.

[13] H. Zhang, L. Duan, D. Zhang, Absorption kinetics of ozone in water withultrasonic radiation, Ultrason. Sonochem. 14 (2007) 552–556.

[14] V.S. Moholkar, M.M.C.G. Warmoeskerken, Investigations in mass transferenhancement in textiles with ultrasound, Chem. Eng. Sci. 59 (2004) 299–311.

[15] S. Chakma, V.S. Moholkar, Mechanistic features of ultrasonic desorption ofaromatic pollutants, Chem. Eng. J. 175 (2011) 356–367.

[16] V.R. Midathana, V.S. Moholkar, Mechanistic studies in ultrasound-assistedadsorption for removal of aromatic pollutants, Ind. Eng. Chem. Res. 48 (2009)7368–7377.

[17] Y.G. Adewuyi, Sonochemistry in environmental remediation. 1. Combinativeand hybrid sonophotochemical oxidation processes for the treatment ofpollutants in water, Environ. Sci. Technol. 39 (2005) 3409–3420.

[18] A. Kumar, P.R. Gogate, A.B. Pandit, A.M. Wilhelm, H. Delmas, Investigation ofinduction of air due to ultrasound source in the sonochemical reactors,Ultrason. Sonochem. 12 (2005) 453–460.

[19] B. Sajjadi, A.A.A. Raman, R.S.S.R.E. Shah, S. Ibrahim, Review on applicablebreakup/coalescence models in turbulent liquid–liquid flows, Rev. Chem. Eng.29 (2013).

[20] B. Sajjadi, A.A.A. Raman, S. Ibrahim, R.S.S.R.E. Shah, Review on gas–liquidmixing analysis in multiscale stirred vessel using CFD, Rev. Chem. Eng. 28(2012) 171–189.

[21] B.E. Launder, D.B. Spalding, The numerical computation of turbulent flows,Comput. Methods Appl. Mech. Eng. 3 (1974) 269–289.

[22] N. Feng, Ultrasonics Handbook, Nanjing University Press, Nanjing, 1999.[23] J. Cai, X. Huai, R. Yan, Y. Cheng, Numerical simulation on enhancement of

natural convection heat transfer by acoustic cavitation in a square enclosure,Appl. Therm. Eng. 29 (2009) 1973–1982.

518 B. Sajjadi et al. / Ultrasonics Sonochemistry 34 (2017) 504–518

[24] M. Abolhasani, M. Rahimi, M. Dehbani, A.A. Alsairafi, CFD modeling of heattransfer by 1.7 MHz ultrasound waves, Numer. Heat Transfer Part A Appl. 62(2012) 822–841.

[25] M. Rahimi, N. Azimi, F. Parvizian, A.A. Alsairafi, Computational fluid dynamicsmodeling of micromixing performance in presence of microparticles in atubular sonoreactor, Comput. Chem. Eng. 60 (2014) 403–412.

[26] J. Jiao, Y. He, T. Leong, S.E. Kentish, M. Ashokkumar, R. Manasseh, J. Lee,Experimental and theoretical studies on the movements of two bubbles in anacoustic standing wave field, J. Phys. Chem. B 117 (2013) 12549–12555.

[27] S. Coleman, S. Roy, Effect of ultrasound on mass transfer duringelectrodeposition for electrodes separated by a narrow gap, Chem. Eng. Sci.113 (2014) 35–44.

[28] D. Gómez-Díaz, N. Gomes, J.A. Teixeira, I. Belo, Oxygen mass transfer toemulsions in a bubble column contactor, Chem. Eng. J. 152 (2009) 354–360.

[29] F. Kerdouss, A. Bannari, P. Proulx, R. Bannari, M. Skrga, Y. Labrecque, Two-phase mass transfer coefficient prediction in stirred vessel with a CFD model,Comput. Chem. Eng. 32 (2008) 1943–1955.

[30] N. Sainz Herrán, J.L. Casas López, J.A. Sánchez Pérez, Gas–liquid mass transferin sonicated bubble columns. Effect of reactor diameter and liquid height, Ind.Eng. Chem. Res. 51 (2012) 2769–2774.

[31] F. Laugier, C. Andriantsiferana, A.M. Wilhelm, H. Delmas, Ultrasound ingas–liquid systems: effects on solubility and mass transfer, Ultrason.Sonochem. 15 (2008) 965–972.

[32] S.V. Jadhav, V.G. Pangarkar, Gas–liquid and solid–liquid mass transfer in three-phase sparged reactors with and without ultrasound, J. Oil Fat Ind. 66 (1989)362–364.

[33] Z. Dong, C. Yao, X. Zhang, J. Xu, G. Chen, Y. Zhao, Q. Yuan, A high-powerultrasonic microreactor and its application in gas–liquid mass transferintensification, Lab Chip 15 (2015) 1145–1152.

[34] A. Kumar, P.R. Gogate, A.B. Pandit, H. Delmas, A.M. Wilhelm, Gas�liquid masstransfer studies in sonochemical reactors, Ind. Eng. Chem. Res. 43 (2004)1812–1819.

[35] S.M. Cenci, L.R. Cox, G.A. Leeke, Ultrasound-induced emulsification ofsubcritical carbon dioxide/water with and without surfactant as a strategyfor enhanced mass transport, Ultrason. Sonochem. 21 (2014) 401–408.

[36] V.S. Moholkar, M.M.C.G. Warmoeskerken, Mechanism of mass-transferenhancement in textiles by ultrasound, AIChE J. 50 (2004).

[37] S. Coleman, S. Roy, Effect of ultrasound on mass transfer during electrodeposition for electrodes separated by an arrow gap, Chem. Eng. Sci. 113 (2014)35–44.

[38] N. Gondrexon, L. Cheze, Y. Jin, M. Legay, Q. Tissot, N. Hengl, S. Baup, P. Boldo, F.Pignon, E. Talansier, Intensification of heat and mass transfer by ultrasound:application to heat exchangers and membrane separation processes, Ultrason.Sonochem. 25 (2015) 40–50.

[39] O. Ayyildiz, P.R. Anderson, R.W. Peters, Laboratory batch experiments of thecombined effects of ultrasound and air stripping in removing CCl4 and 1,1,1-TCA from water, J. Hazard. Mater. 120 (2005) 149–156.

[40] O. Ayyildiz, R.W. Peters, P.R. Anderson, Sonolytic degradation of halogenatedorganic compounds in groundwater: mass transfer effects, Ultrason.Sonochem. 14 (2007) 163–172.