Ultrasonics Sonochemistry 18 (2011) 197–208

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Ultrasonics Sonochemistry

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

A calorimetric study of energy conversion efficiency of a sonochemical reactorat 500 kHz for organic solvents

Maricela Toma a,*, Satoshi Fukutomi a, Yoshiyuki Asakura b, Shinobu Koda a

a Department of Molecular Design and Engineering, Graduate School of Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya, Aichi 464-8603, Japanb Honda Electronics Co. Ltd., 20 Oyamazuka, Oiwa-cho, Toyohashi, Aichi 441-3193, Japan

a r t i c l e i n f o

Article history:Received 18 December 2009Received in revised form 14 May 2010Accepted 17 May 2010Available online 24 May 2010

Keywords:Sonochemical reactorOrganic solventsCalorimetryEnergy conversionAtomizationEnergy balance

1350-4177/$ - see front matter � 2010 Elsevier B.V. Adoi:10.1016/j.ultsonch.2010.05.005

* Corresponding author. Tel.: +81 52 789 3275; faxE-mail address: maricela_toma@yahoo.com (M. To

a b s t r a c t

It would seem that the economic viability is yet to be established for a great number of sonochemical pro-cesses, owning to their perfectible ultrasonic equipments. Industrial scale sonoreactors may becomemore important as a result of mastering the parameters with influence on their energy balance. This workrelated the solvent type to the energy efficiency as the first step of a complex study aiming to assess theenergy balance of sonochemical reactors at 500 kHz. Quantitative measurements of ultrasonic power forwater and 10 pure organic solvents were performed by calorimetry for a cylindrically shaped sonochem-ical reactor with a bottom mounted vibrating plate. It was found that the ultrasonic power is stronglyrelated to the solvent, the energy conversion for organic liquids is half from that of water and there isa drop in energy efficiency for filling levels up to 250 mm organic solvents. Surface tension, viscosityand vapor pressure influence the energy conversion for organic solvents, but it is difficult explain thesefindings based on physical properties of solvents alone. The apparent intensity of the atomization processshows a good agreement with the experimentally determined values for energy conversion for water andthe solvent group studied here. This study revealed that to attain the same ultrasonic power level, moreelectrical energy is need for organic solvents as compared to water. The energy balance equation has beendefined based on these findings by considering an energy term for atomization.

� 2010 Elsevier B.V. All rights reserved.

1. Introduction

It is important to recognize that a key issue to be addressed forevery sonochemical process with potential industrial application isthe efficiency as compared with the classical pathway. Sonochem-istry and its hybrid technologies are advanced techniques that relyupon the acoustic cavitation effects and yields. Cavitational fieldintensity is known to be under the strong influences of physicaland chemical properties of the host solvent, treatment conditionand ultrasonic irradiation characteristics [1–6]. It is the under-standing of these parameters mode of interaction together withthe optimization of the acoustic cavitation equipment that will fi-nally authorize successful application of sonochemistry [7,8].

Given the concept of true and false sonochemical processes[9,10], there are two types of ultrasound applications: those basedon the chemical effect (sonochemistry) and those based on thephysical effects generated by bubble collapse (sonoprocessing).The entrenchment of power ultrasonic devices in industrialprocessing is an important gain but investigations that seek toscale-up for processes that take advantage on chemical effects ofcavitation are always worthy of consideration in the endeavor to

ll rights reserved.

: +81 52 789 3273.ma).

achieve a full recognition of sonochemistry in industry [11]. Thefield of sonochemical equipment modeling has been active formany years but earlier efforts focused mostly on facilitating thechemistry rather than getting better acoustics parameters [8].

It is almost two decades ago that Bernard and Mason, assessinga large range of ultrasonic devices concluded that the equipmentfor industrial scale sonochemistry can easily be derived from thetype of those existing at laboratory level [12]. Ultrasonic systemsmanufacturing was seen to follow the expansion of ultrasoundapplications by tailoring reactors to fit new strategies [13] butthe main goal: sonoreactors featuring high energy efficiency, isnot materialized yet.

The energy conversion is known to be a critical factor in indus-trial applications. In order to lower the electric energy demands forthe scaled-up version of sonochemical reactors, their energy bal-ance is important to be throughly evaluated. To resolve the energybalance of the sonochemical reactors it is not a simple matter asthere is a simultaneous input of mechanical and chemical energyinto the reaction medium during sonication [12] and more rigorousand realistic characterization of the energy consumed to producecavitations together with the thermal, viscous and radiation lossesis needed. Instead, the sonochemical efficiency (SE) concept [14]was introduced to determine the energy conversion toward therequired effect in lieu of the calorimetrically measured acoustic

198 M. Toma et al. / Ultrasonics Sonochemistry 18 (2011) 197–208

energy available in the sonicated medium. Beside being an elegant,reliable method to express the sonoreactors performance, SE(mol dm�3)/(J dm�3) is a basic parameter to compare differentultrasonic systems (the lack of a standard methodology in the pastallowed many confusions when trying to correlate chemical withacoustical information).

Whenever is the case of calibration or scale-up, the acousticpower (W) dissipated by the ultrasonic wave into the bulk mediumis a key parameter required to express the efficiency of a sono-chemical reactor, also considered to have the main contributionon the sonochemical yield [15,16]. Therefore, a variety of investiga-tive techniques has been proposed during the years to evaluate theoutput ultrasonic power. According to the type of cavitational out-come investigated, these methods can be categorized as following:(a) methods dealing merely with the physical effect of cavitationsuch as the thermal effect (calorimetry [17–20]) and acoustic pres-sure (aluminium foils erosion [21–23,40] or hydrophone probe[24]); (b) methods that are sensitive to radical species producedduring solutions sonolysis (chemical dosimetry [25–27] and elec-trochemical dosimetry [28–30]), and (c) methods that combinethe above mentioned effects [31]. However, comparative tests ofcalorimetry and chemical dosimetry carried out on the samesonoreactors demonstrated that these two methods provide simi-lar prediction of the ultrasonic power [32,33].

The energy conversion in sonochemistry is strongly dependentupon the type and operation mode of the sonication equipmentand sonication medium involved [10,31,34,35]. Nevertheless, an-other critical parameter to be assessed for optimizing or fordesigning reactors with adequate ultrasonic fields is the energydistribution on the sonication volumes [36,37]. There are indica-tions that by controlling the geometric and operating conditionsof the sonoreactor, the required intensity of cavitation can beachieved with maximum energy efficiency [38,39]. The height ofthe sonoreactor is regarded to play an important role since it wasreported that there is an axial and radial distribution of the cavita-tional field intensity [40–44].

Having achieved an increased understanding over many aspectsof sonoreactors efficiency [14,45–49] our research group is aimingat developing industrial scale sonochemical reactors with highultrasonic efficiency. In pursuing this aim, a special attention waspaid at easing the acoustics admittance in the benefit of chemistry[48,49]. The efficiency of energy conversion from electricity to ultra-sound was 70% for the tower type sonoreactor that we developed ina previous work [47]. Therefore, we were interested to determinewhich changes in the reactor sonication medium might affect theenergy conversion. Since the relative significance of the parametersthat influence energy conversion depends upon the physical andchemical characteristics of the liquid being irradiated [34], we wereinterested to obtain informations about the energy conversion fororganic solvents. Back in 1965, it was revealed by Weissler et al.[50] in their pioneering study that cavitation could occur in organicsolvents at 800 kHz and 3 W/cm2. The non-aqueous sonochemistryis an active research field related to polymer chemistry and solventdecomposition [1,51] but information concerning ultrasonic energyconversion in organic solvents is sparse. This study focused on theinvestigation regarding the ultrasonic energy dissipation into vari-ous organic solvents sonicated at 500 kHz in a cylindrically shapedsonoreactor with a bottom mounted transducer.

2. Method and materials

2.1. The sonochemical reactor system

The ultrasonic equipment used for this study was a custom-build sonoreactor whose design incorporates a cylindrical glass

pipe (4 mm thick and 58 mm inner diameter) vertically mountedon a 0.1 mm stainless steel (SUS304) vibrating plate attached toa PZT disk type transducer with a resonance frequency of490 kHz and 50 mm diameter, manufactured by Honda ElectronicsCo., Ltd. A fan was incorporated into the transducer housing toavoid the over-heating during the sonication. The reactor was con-figured to accept a 500 mm liquid column, and equipped with a lidin order to minimize the solvent evaporation during sonication.The measurements at 20 kHz frequency were performed byemploying a Langevin type transducer in connection with a2 mm vibration plate. The transducer was driven by a continuoussinusoidal wave produced by a signal generator (W1942, NF Corp.)and delivered through a high-frequency power amplifier (L-400BM-H, Honda Electronics Co., Ltd.). The effective electric powerinput to the transducer was accurately calculated from the voltageat both ends of the transducer and the current as measured with anoscilloscope (TDS3012B, Tektronix Inc.) connected to a currentprobe (TCP202, Tektronix Inc.). The temperature measurementswere performed using two kinds of thermocouples (Sheath T-typewith a diameter of 2.3 mm, Takahashi Thermo Sensor Ltd. or cop-per–constantan with a diameter of 0.2 mm, Omega EngineeringInc.) and a thermometer (NR-500, NR-TH08 Keyence Corp.) con-nected to a personal computer equipped with an in-house devel-oped software for post processing. Precise movements betweentwo successive measurement positions were provided by a SGSP-33-200 Sigma Koki motorized 3D stage. The scheme shown inFig. 1a illustrates the experimental set-up details.

2.2. Reagents

The chemicals were purchased as follows: ethanol (99.5%), tol-uene (99.5%), cyclohexane (99.5%), propanol (98.5%) methanol(99.5%), benzene (99.5%), xylene (99.5%) and 1,2,3,4-tetrahydro-naphthalene (tetralin) (99.5%) from Chameleon Reagent; hexane;n-butanol (99.5%) from Nacalai Tesque and decane from WakoPure Chemical Industries Ltd. All solvents used here were analyti-cal grade and were used without extra purification. Distillate water(conductivity less that 0.1 mS/m) was prepared on the laboratorysite employing a Yamato Autostill WG 25 system. Prior to sonica-tion, samples were air-saturated by bubbling the air for a periodof 30 min at 25 ± 1 �C to provide uniform distribution for the gasin the bulk liquid as a precaution raised from the known influenceof bubble dynamics on the liquid media [52]. Moreover, in order tofacilitate the gas transfer into the relatively large sample volumeshandled here (ranging from 100 to 1500 ml), solutions were alsoslowly stirred during the purging for reproducibility concerns. Pre-cautions were taken to avoid solvents temperature variation priorthe measurement starting point.

2.3. Quantification of dissipated ultrasonic power

The procedure we adopted here for acoustic power measure-ments was the calorimetric method. Based on the assumption thatthe mechanical energy generated by the ultrasonic waves is re-duced to heat, the dissipated ultrasonic power Up was calculatedfrom the rate of temperature increase as:

Up ¼ CpMdTdt

ð1Þ

where Cp is the heat capacity of the solvent at constant pressure(J kg�1 K�1), M is the mass of solvent (kg) and dT/dt is temperaturerise per second [53].

The choice of the quantification method was based upon theneed to make a correct comparison on acoustic power for the sol-vents involved in this work. We found it desirable to avoid theexperimental errors arising from using solvent specific chemical

Current probeOscilloscop marewoPe plifier

Cylindrical glass

vessel

Signal generator

Cooling

fan

58

Vibration plate

Transducer

Thermometer

PC

Liq

uid

heig

ht:

30-

500

mm

x

Motorized 3D stage

Thermocouples

Lid

A

B

Fig. 1. Experimental set-up for calorimetric measurements of ultrasonic powerdissipated: general assemble (A) with details on the reactor unit (B).

M. Toma et al. / Ultrasonics Sonochemistry 18 (2011) 197–208 199

dosimetry methods. At the moment, the literature offers few dataabout the dosimetry in non-aqueous solvents [43,54,55] and a gen-eral method for capturing and quantifying the cavitational effectson organic solvents has not yet been validated. Despite the re-ported drawbacks (the convective cooling measurements, theheating of transducer and the sensor that may disturb the mea-surements), calorimetry is an universal, yet precise method forquantifying the acoustic power [18]. This method, however, doesprovide a way of determining the power dissipation based on thedependence of the temperature rise (dT/dt) upon the solventacoustic and thermal properties.

This sonoreactor had previously been characterized by alterna-tive measurement techniques, such as KI dosimetry or hydrophoneprobe and the details can be seen in Ref. [48].

2.3.1. Experimental procedures2.3.1.1. Electrical power input measurements. By using the aboveexperimental set-up, the electric power change could be held with-in ±0.5 W by continually tuning the voltage at both ends of thetransducer as observed on the oscilloscope display reading.

2.3.1.2. Temperature measurements. A set of experiments was car-ried out by recording the temperature evolution under sonicationfor every organic solvent investigated and for water. Temperature

readings were taken within the bulk liquid for the first 100 s son-ication with a thermocouple reproducibly positioned at the middleof the liquid sample and 15 mm apart from the central axis as toavoid the larger intensity reported from the reactor centerline[39]. The measurements were repeated three times and theirmeans were used in calculations. For an accurate picture of the li-quid height influence on the ultrasonic power dissipation, the tem-perature measurements have been carried out stepwise, while thefilling levels were gradually varied with a constant increment of10 mm in a range from 10 mm up to 500 mm. It is worthwhile not-ing that the reactor cell was refilled with a fresh portion of air-sat-urated solvent prior to each measurement to assurereproducibility. To simplify the ultrasonic waves environment,the measurements for this study where carried out without areflector. It is known that this can affect the topology of soundand bubble field in the reactor [8,56]. Previous studies have shownthat the calorimetric method is rather independent of the bulk li-quid temperature (below 50 �C), reactor shape and insulation,and concluded that the barely discernible heat gains or losses tothe experimental environment are negligible for the short periodof measurement [18,32,35,55]. Accordingly, thermal insulationwas omitted for the sonoreactor glass cell. All measurements weremade at ambient temperature and pressure. The details of temper-ature measurement set-up are depicted in Fig. 1b.

3. Results and discussion

High-frequency ultrasound (200–600 kHz) generates chemi-cally active bubbles but their applications came much later as com-pared to the low frequency ultrasound, owning to their inefficiencyin processing applications. However, this ultrasonic region becameintensively approached in scale-up studies soon after their effec-tiveness for pollutants decomposition was revealed [57]. The en-ergy conversion is known to be a critical factor in industrialapplications. It should be emphasized, however, that the energyterms for ultrasonic systems are not always evident and the mod-els reported in literature are likely to be applicable to a specific setof operating parameters. This work focused on the effect of liquid,electrical power and liquid height on energy efficiency at 500 kHz.

3.1. The influence of solvent type on ultrasonic power

Previous studies on the effect of changes in reactor shape andsolvent characteristics on the energy conversion and power densitysuggest that carefully investigation are required to understand thesystem in order to evaluate its energy balance [12,42]. Thus, thedependence of energy conversion on the type of solvent will be ad-dressed herein. Although the liquid properties influencing cavita-tional events are well studied, the sonochemical literature haslittle to tell about the liquid parameters that influence the powerdissipation, especially for the non-aqueous systems. The liquidselection for this study included distilled water and pure organicsolvents, namely: ethanol, butanol, propanol, hexane, cyclohexane,toluene, benzene, tetralin, xylene and pentadecane.

Seeking for accurate informations on the energy conversion fororganic solvents, comparative tests were conducted at 500 and20 kHz. The ultrasonic power has been estimated calorimetricallyfrom the initial temperature rise (dT/dt) that gives a reasonableindication of the quantity of energy effectively dissipated intothe sonicated liquid. At this point, the measurements were carriedout for 100 ml liquid as the electrical power supplied to the trans-ducer was set constant at 30 W. The temperature rise dependenceon the type of liquid under sonication at 500 and 20 kHz is de-picted in Fig. 2. The plots clearly show that for both frequenciesthe slope of temperature rise depends strongly on the liquid

24

26

28

30

32

34

36

38

0 50 100 150 200Time [s]

Tem

pera

ture

[o C

]

WaterTetralinEthanolButanolHexaneBenzenePropanolCyclohexanePentadecaneTolueneXylene

24

28

32

36

0 50 100 150Time [s]

Tem

pera

ture

[o C

]

Water

Ethanol

Propanol

Butanol

Cyclohexane

ΔTo

ΔTo

Fig. 2. Experimental curves of temperature evolution vs. time for different solvents at 500 kHz. Depicted in the graph inlet are the experimental curves at 20 kHz.

200 M. Toma et al. / Ultrasonics Sonochemistry 18 (2011) 197–208

investigated and there is a significant difference between the waterand the non-aqueous medium. Immediately after the ultrasonicsource was turned on, a dynamic linear rise in temperature (DTo)was observed for the first ms sonication at high-frequency. Thisbehavior is attributed to the solvent shear viscosity stress [59].During the experimental work carried out for this study, wenoticed that DNo depends on the viscosity of the liquid beingsonicated, ultrasonic frequency and liquid height.

In examining the influence of liquid on the ultrasonic powerdissipated into de sonication medium, we found that water accom-modates more ultrasonic power than the organic solvents asshown in Fig. 3 and this difference is more evident at 500 kHz ascompared to 20 kHz.

3.1.1. Influence of physical propertiesContrary to what one might intuitively expect, our experimen-

tal data show little dependence on the acoustic impedance. Viscos-ity, surface tension and vapor pressure are among the most

500kHz

0

5

10

15

20

25

30

Wate

r

Ethano

l

Hexan

e

Benze

ne

Xylene

Toluen

e

Propa

nol

Pentad

ecan

e

Butano

l

Tet

C

Ultr

ason

ic p

ower

[W

]

Fig. 3. Dependence of ultrasonic pow

important variables of the sonicated medium to influence theultrasound effect [35,58,60].

As shown in Fig. 4, the plots of ultrasonic power at 500 kHzagainst the liquids physical properties (calculated from data inRef. [61]) are in keeping with the previous ones. The data was distrib-uted into two distinct groups: organic liquids and distillate waterindicating again a significant difference between water and theorganic solvents investigated here. The ultrasonic power fororganic solvents decreases as following: hexane > ethanol > benzene> xylene > toluene > propanol > butanol > tetralin > cyclohexane.

The difference in viscosity cannot explain the difference seenbetween water and organic solvents ultrasonic power values sincewater and cyclohexane have similar viscosity. On the other hand, itwould seem that the ultrasonic power in the organic solventsgroup decreases for solvents with elevated viscosity such as propa-nol, butanol and tetralin.

Vapor pressure is an important property that controls the cavi-tational activity in solvents [63]. Hexane and cyclohexane show

ralin

ycloh

exan

e

20kHz

0

5

10

15

20

25

30

Wate

r

Ethano

l

Propa

nol

Butano

l

Cycloh

exan

e

er on solvent at 20 and 500 kHz.

0 5 10 15 20 250

5

10

15

20

25

30

0.5 1.0 1.5 2.0 2.5 20 30 40 50 60 70 80

Water

Ethanol

Propanol

Butanol

Hexane

Cyclohexane

Toluene

Tetralin

Benzene

Xylene

SurfaceTension [mN·m-1]

Viscosity [mPa·s]Vapor Pressure [kPa at 298K]

Ultr

ason

ic P

ower

[W

]

Fig. 4. The influence of vapor pressure, viscosity and surface tension on the ultrasonic power.

M. Toma et al. / Ultrasonics Sonochemistry 18 (2011) 197–208 201

close values for vapor pressure and surface tension, but the viscos-ity of hexane is three time lower than those of cyclohexane andthis can be a reason for their difference in the ultrasonic power.The ultrasonic power dissipated decreased for solvents with highvolatility such as tetralin and butanol but again, it is hard to ex-plain the difference between water and the organic solvents.

A variation in the dissipated ultrasonic power with their surfacetension values can be seen for water and organic solvents.

These results suggested that it is difficult to explain the energydissipation in organic solvents by looking at a particular physicalproperty. Instead, it would be logical to consider a complex inter-ference of liquid viscosity, surface pressure and vapor pressure to-gether with their variation during the sonication responsible forthe energy conversion. Since the surface tension (another colliga-tive property) shows a good fit with the ultrasonic power variationamong the investigated liquids, we looked at the atomization effectfor a better explanation of our experimental results.

3.1.2. Influence of ultrasonic atomizationThe fluctuation of the atomization intensity with the liquid and

sonication parameters detected by visual observations made dur-ing the calorimetric measurements also suggested to us a certaininfluence of atomization on the dissipated ultrasonic power. There-fore, a detailed description of the solvent atomization that takesplace during the calorimetric measurements for the ultrasonicpower will be presented here.

Ultrasonic excitation of the solvent results in a break of capil-lary surface waves followed by a liquid protuberance and a subse-quent expulsion of drops (atomization fountains). It is known thatthe instability of capillary surface waves increases at high-fre-quency owing to the difference in ultrasonic beam directivity[52] and that the drop size is small at high frequencies wherethe capillary wavelength is short and vice versa [62–64]. This canexplain the fact that the discrepancy between the ultrasonic powerfor water and organic solvents is more reduced at 20 kHz as com-pared to 500 kHz (Fig. 4).

An illustration of the capillary wave disintegration at 500 kHzduring a typical experimental run for calorimetric measurementsis shown in Fig. 5. The sonication resulted in large volumes of ultra-sonic mist from cyclohexane, ethanol and toluene where liquidsplashes were also observed for the last two (Fig. 5A). Decaneand tetralin displayed vigorous splash fountains while a gentlysplashing fountain was observed for butanol but the capillarywaves for water never teared up to form drops under the experi-mental conditions employed here. It is worth mentioning thatthe difference in the drop size can be clearly attributed to the dif-

ference in vapor pressure and surface tension as a heavy mist layerwas formed for solvents with high vapor pressure and low surfacetension. However, it is unclear why decane, a solvent with low val-ues of surface tension, viscosity and density developed the higherliquid crest with large sized drops but no mist. We assumed thatnot only the mist formation but also the liquid splashes have a surecontribution to the temperature decrease during the measurementperiod.

These experimental results made obvious the dependence ofthe ultrasonic power dissipated into the solvents at high-frequencyon the colligative properties (viscosity and surface pressure) andvapor pressure rather than on acoustic impedance. This finding isin good agreement with the dependence shown by other authors[35,58]. The fact that the power transferred into the sonicatedmedium decreased at high volatilities and high viscosities of the li-quid, cleared the link between the energy efficiency and theatomization.

3.2. The influence of electrical power delivered to the transducer on theultrasonic power

The electrical power supplied at transducer was varied in arange from 10 to 50 W. As it was expected, the ultrasonic power in-creases linearly with the electric power but there is a significantdifference in the curve slope for water and organic solvents asshown in Fig. 6. The slope is 0.82 for water and about 0.3–0.4 fororganic solvents indicating that the energy base efficiency for sol-vents is half compared to the efficiency of water. This is in agree-ment with a low energetic yield for toluene sonicated at 40 Welectrical power reported previously [55]. The energy based effi-ciency has been calculated from ultrasonic power (Pu) and electri-cal power (Pe) delivered at transducer as following:

ge ¼Pu

Peð2Þ

The relative energy efficiency has been also calculated from:

gr ¼gs

gwð3Þ

where gw is the energy efficiency for water and gs is energy effi-ciency for organic solvents. The experimental results of the influ-ence of liquid on energy conversion at 500 kHz and forcomparison, at 20 kHz are shown in Table 1. The calculated energyefficiency is half of that for water under the same sonication param-eters or less than half as for the rest of solvents measured here.

Fig. 5. An illustration of liquid atomization of the solvents during the calorimetric measurements: (A) the dependence on solvent; (B) the influence of electrical powersupplied at transducer on atomization of tetralin and distillate water and (C) the influence of liquid height on vaporization.

202 M. Toma et al. / Ultrasonics Sonochemistry 18 (2011) 197–208

It is important to emphasize at this stage that an increase inelectrical power is followed by an increase in atomization inten-sity, as well (Fig. 5B). The apparent intensity of the atomizationprocess is in good agreement with the experimentally determinedvalues for ultrasonic power dissipated in water and the solventgroup. Thus, the energy conversion is not automatically a pureelectrical power input effect since the ultrasonic atomization isalso involved and consequently, a part of the input energy is lostdue to atomization. Low cavitation based efficiencies were previ-ously reported for organic solvents as shown in Table 1 but it is dif-ficult to compare the results for solvents individually since theultrasonic frequency is different. These low energy efficiencies cor-respond well with those found in the sonolysis of organic com-pounds [51,63].

It is known that more energy is needed to achieve a certaincavitational intensity in organic solvents than in water, owning

to the difference in cavitation threshold [65]. Here it was foundthat to attain the same ultrasonic power level, more electricalenergy is needed at transducer in the case of organic solvents thanfor water.

3.3. The influence of liquid height on the ultrasonic power

Webber and Chon [39] were first to observe variation of theacoustic power density along the sonication volumes with a pat-tern of the standing waves that were later confirmed in many pa-pers. For more information on the atomization influence on energyconversion we find it vital to investigate on the influence of liquidheight on dissipated acoustic power.

Quantitative measurements of absorbed acoustic energy werecarried out with different levels of liquid in reactor cell for waterand organic solvents. The electrical power input to the transducer

0 10 20 30 40 500

5

10

15

20

25

30

35

40

45

50

Electrical Power [W]

Ultr

ason

ic P

ower

[W

]

Water Ethanol Propanol Butanol Hexane Cyclohexane Pentadecane Benzene Toluene Xylene Tetralin

Fig. 6. The influence of electrical power on the ultrasonic power dissipated into thesonicated medium for water and organic solvents.

M. Toma et al. / Ultrasonics Sonochemistry 18 (2011) 197–208 203

was set constant at 30 W over the measurements period and theamount of acoustic power that a particular solvent can accommo-date was measured for water, tetralin, butanol, hexane, decane,ethanol and cyclohexane.

0

0.005

0.01

0.015

0.02

0.025

0.03

0.035

0.04

0.045

0 100 200Liquid

dT/d

t

Fig. 7. Experimental data of calorimetric measurements at three differe

Table 1Calculated values for the energy based efficiency.

Solvent ge gr

500 kHz 20 kHz 500 kHz 20 kHz

Water 0.82 0.27Tetralin 0.32 0.39Butanol 0.34 0.21 0.41 0.77Decane 0.41 0.5Ethanol 0.38 0.23 0.46 0.85Toluene 0.34 0.41 0.75 [55Cyclohexane 0.34 0.20 0.41 0.74Propanol 0.33 0.19 0.40 0.70Xylene 0.30 0.36Benzene 0.34 0.41Hexane 0.42 0.51

* Cavitational based efficiency at 46 kHz (water = 100) when controlled at k/2 from Ref

3.3.1. Local vs. overall temperature investigationCalorimetry is a method based on bulk liquid temperature and

assumes uniformity in the liquid temperature. However, it hasbeen shown that the calorimetric data are compromised by highviscous media and insufficient convection [58].

A characteristic of sonoreactors operating with bottommounted transducers is the location of the active zone in a cylin-drically shaped hollow defined by the directivity of the ultrasonicbeam [44,49]. The sonochemical reactor employed here has theadvantage of hosting the bulk liquid in this region since the differ-ence between the diameter of transducer and the diameter of glasscell is only 8 mm. Moreover, our previous measurements on thevelocity profile of the ultrasonic streaming [49] are indicating highliquid velocities: 1–1.8 cm s�1 for this zone. Boldo et al. [44] havealso reported a well mixing effect for the bulk solvent during thesonication in a similar sonoreactor as a result of the convective cur-rents in association with the cavitational hydrodynamicphenomena.

In order to get a valid temperature value of the bulk liquid, themeasurement location was set intuitively at the sample heightmedian point (h/2) and 10 mm apart from the reactor centerlinewere the liquid velocity is optimal. However, a concern about tem-perature uniformity for the relatively large volumes and heights ofliquid employed led us to investigate the local temperature varia-tions for this point. We also suspected that the significant acousticpressure variations at positions hosted in the near-field and closeto the transducer can affect the temperature data along the liquid

300 400 500height [mm]

bottommiddletopaverage value

nt points of the sample and their variation along the liquid height.

ge Cavitation efficiency* %from water500 kHz

10–300 mm 300–500 mm

0.74 0.680.56 0.59 700.57 0.62 430.59 0.810.52 0.70 46

] 0.59 0.77 710.49 0.61

426443

. [60].

24

25

26

27

28

29

30

31

32

33

0 20 40 60 80 100 120Time [s]

Tem

pera

ture

[o C

]

50mm

100mm

150mm

200mm

250mm

300mm

350mm

400mm

450mm

Fig. 8. The temperature radiated by the transducer body during measurements fordifferent liquid height.

204 M. Toma et al. / Ultrasonics Sonochemistry 18 (2011) 197–208

column. Thus, the temperature variation along the cylinder centralaxis has been measured.

ANEAR-FI

-0.075

-0.025

0.025

0.075

0.125

0.175

0.225

0.275

0.325

0.375

0 20 40

T

Δτ

BFAR-FI

-0.075

-0.025

0.025

0.075

0.125

0.175

0.225

0.275

0.325

0.375

0 20 40

T

Δτ

Fig. 9. Experimental data of local variation of temperature at the calorimetric m

Temperature readings were made simultaneously at differentlocations within the bulk liquid with three sensors located as fol-lowing: (a) at 15 mm from vibrating plate, (b) in the middle of li-quid column and (c) at 15 mm beneath the liquid surface. Thetemperature raise (dT/dt) for these locations as a function of liquidheight can be seen in Fig. 7 together with the average value. As weexpected, the albeit small difference in the temperature distribu-tion along the liquid height increases with increasing the liquidvolume. The temperature at the sonochemical reactor bottom staysrelatively distant from the averaged value, compared with the topand middle positions, thus being less influenced by the liquidheight.

A first explanation for this behavior is an additional input ofheat from the transducer body and to confirm this hypothesis,the temperature radiated from the transducer body was been mea-sured. The results indicate that the temperature change upon theliquid height are in the range of 0 to 1 �C and can be considerednegligible (Fig. 8). As this cannot explain the bottom temperaturevariation with the liquid height especially after 250 mm, we as-sumed that the temperature at this location is controlled mainlyby the frictional heat between the thermocouple or the liquids fric-tional heat of vibrating particles in the thermocouple.

In an effort to better understand the mixing effect, the localtemperature variations considered as Ds = s(n+1) � sn where sn is

ELD

60 80 100

ime [s]

50mm

100mm

150mm

200mm

250mm

ELD

60 80 100

ime [s]

300mm

350mm

400mm

450mm

500mm

easurements points for: A – near-field locations and B – far-field locations.

M. Toma et al. / Ultrasonics Sonochemistry 18 (2011) 197–208 205

the liquid temperature and s(n+1) is the consecutive temperaturerecorded after one second at h/2 location were plotted againstthe time. The near-field for this reactor was calculated to belocated in first 300 mm near the transducer [48] but there is nosignificant difference in the temperature variation recorded inthe near-field locations (Fig. 9A) as compared to the far-field(Fig. 9B), except for the first 10 s, when they are significantly larger.The results show that the temperature variation around thermo-couple position is hold within 0.03 �C under the experimentalconditions employed here and support our presumption that anefficient mixing process takes place in the reactor cell as a resultof strong convection currents made possible by the reactor design.The signal recorded over the first 10 s exhibited turbulentvariations also attributed to viscosity stress associated phenomena.Therefore, the temperature rise for the first 10 s of sonication wasignored in the calculations of the dissipated acoustical power.

Considering these experimental evidences in support of theabove assumption, all the correlations developed in the present

Water

0 100 200 300 400 500Liquid height [mm]

0 100 200 300 400 500Liquid height [mm]

0 100 200 300 400 500Liquid height [mm]

0 100 200 300 400 500Liquid height [mm]

average value

Tetralin

0

5

10

15

20

25

30

Up

[W]

05

1015

2025

30

Up

[W]

0

510

15

2025

30

Up

[W]

0

5

10

15

2025

30

Up

[W]

Up

[W]

Up

[W]

average value

Cyclohexane

average value Up

[W]

Butanol

average value Up

[W]

Fig. 10. The influence of liquid height on the ultr

work are based on the fairly uniform distribution of the tempera-ture in the available sample volume.

The sample height in the sonochemical reactor cell was variedfrom 30 to 500 mm and an average value of dissipated ultrasonicpower for different levels along the sonoreactor cell was calcu-lated. The influence of liquid height on the dissipated acousticpower, together with their average value is illustrated individuallyin Fig. 10.

The results indicate that the average values do not vary muchbetween the organic solvents and water and this is a departurefrom what was observed for 100 ml samples (shown in Section 3.1).It has been reported that the cavitation intensity in organic sol-vents at 46 kHz is strongly influenced by the liquid height and itis possible to induce in some organic liquids cavitation of intensityalmost equal to that achieved in water by modeling the thicknessof the irradiated liquid layer in terms of k/2 [60]. This pattern ofacoustic energy density involving the standing waves has beenreported by many authors in investigations concerning with the

0 100 200 300 400 500Liquid height [mm]

0 100 200 300 400 500Liquid height [mm]

0 100 200 300 400 500Liquid height [mm]

0 100 200 300 400 500Liquid height [mm]

Ethanol

average value

05

101520

25

30

05

1015

2025

30 Toluene

average value

Decane

0

5

1015

20

25

30

average value

0

10

20

30

40

50

water toluenebutanol cyclohexenetetralin decaneethanol

atomization

asonic power dissipated in organic solvents.

206 M. Toma et al. / Ultrasonics Sonochemistry 18 (2011) 197–208

effect of liquid height on the ultrasonic systems efficiency [28,41].Also, there are evident fluctuations around their average valuesalong the reactor cell filling levels, but our experimental data arein disagreement with the standing wave pattern of this system.An explanation for this can be found in the different measurementmethods or in the difference of the sonication parameters. Most ofthe reported data focused on cavitational energy or were recordedby screening the energy distribution in the system after the equi-librium was established but our investigation focused on withthe energy conversion based on the rate of initial temperature rise.It is know that heating is a bulk effect, whereas cavitation isregarded as a local effect [58]. Furthermore, the variations we

1.15

1.2

1.25

1.3

1.35

1.4

0

Liquid height 30-250mm

1.15

1.2

1.25

1.3

1.35

1.4

0 20 40 60 80

Surface tension [mNom-1]

Log

ultr

ason

ic p

ower

[W

1.15

1.2

1.25

1.3

1.35

1.4

0

Liquid height: 30-250mm

1.15

1.2

1.25

1.3

1.35

1.4

0 1 2 3Viscosity [mPaos]

Log

ultr

ason

ic p

ower

[W

Liquid height 30-250mm

1.15

1.2

1.25

1.3

1.35

1.4

0 2 4 6 8 10 12 14 16Vapor pressure[kPa at 298K]

Log

ultr

ason

ic p

ower

[W

1.15

1.2

1.25

1.3

1.35

1.4

0

Fig. 11. The averaged ultrasonic power from two regions of liquid height: 30–250 mm

recorded cannot be clearly associated with a power density distri-bution in a standing wave pattern under the experimental condi-tion employed here since the k/2 is only 1.52 mm and theultrasonic power is relatively high. Thus, the experimental datavariation along the liquid column can be only attributed to the per-fectible experimental condition.

It is expected that for a certain input of electric power, the ultra-sonic power dissipated into the liquid medium would diminish asthe sample height increases. However, this behavior can be seenonly for water as there is an opposite trend for the organic solvents.It is also observed that there is a clear difference in the ultrasonicpower dissipated in water and organic liquids for filling levels

Liquid height: 250-500mm

1 2 3Viscosity [mPaos]

Water

Toluene

Decane

Butanol

Tetralin

Ethanol

Cyclohexane

Liquid height 250-500mm

20 40 60 80

Surface tension [mNom-1]

Water

Toluene

Decane

Ethanol

Butanol

Tetralin

Cyclohexane

Liquid height 250-500mm

2 4 6 8 10 12 14 16Vapor pressure[kPa at 298K]

Water

Toluene

Decane

Ethanol

Butanol

Tetralin

Cyclohexane

and 250–500 mm as a function of viscosity, vapor pressure and surface tension.

M. Toma et al. / Ultrasonics Sonochemistry 18 (2011) 197–208 207

located in the first 250 mm of the reactor cell. Fig. 10 contains alsoa comparative plot emphasizing on the difference between the sol-vents and water. A plausible explanation for this difference is theatomization phenomena for water and organic solvents as theatomization intensity is strongly influenced by solvents height[61]. Fig. 5C shows the influence of liquid height on atomizationobserved in the case of ethanol. It can be seen that as the liquid le-vel in column increases, the liquid protuberance gradually dimin-ishes and it is no longer visible for levels that exceed 300 mm.However, it is worthwhile to notice that a disturbance of the uppersurface is visible at every filling level of the reactor for all the par-ticular liquids investigated here.

We observe that there is a different variation in the energy con-version for organic liquid and water along the reactor cell. Asshown in Table 1, the conversion values for a particular liquidare different in the 30–250 mm segment, where the atomizationis an active process as compared to the 250–500 mm segment ofreactor cell. Furthermore, the energy data plotted against the vis-cosity, vapor pressure and surface tension (Fig. 11) individuallyfor these two column segments also suggest an influence of atom-ization on power conversion. Good correlations are found betweenenergy conversion for toluene and cyclohexane and their viscosity.Their low viscosity can be responsible for the decrease of ultrasonicenergy due to atomization at low filling levels as compared withtetralin, a viscous solvent that show little variation with the sam-ple height. The high volatile cyclohexane and ethanol show a big-ger difference in the energy absorbed for the upper and lower partof the cell as compared to tetralin. It is the difference in surfacetension of water and organic solvents that can be seen as a reason-able explanation for the energy conversion variation from the low-er to the upper part of the reactor cell due to atomization. Based onthese evidences, we consider that the assumable decline in theultrasonic power with increasing the sample height has been re-versed by a drop in the dissipated ultrasonic power due to solventsatomization at low filling levels of the reactor cell. As we expected,this study also confirmed the influence of atomization on the en-ergy conversion of organic liquids.

Therefore, it would appear reasonable to consider a certaininfluence of ultrasonic atomization on the energy balance for or-ganic solvent and high-frequency systems. In defining the energybalance for a sonochemical reactor running on organic solventsat high-frequency, an energy term for atomization should be con-sidered. Based on the observations of this work, the energy balanceis considered as:

Ielectrical ¼ Idissipated þ Iatomisation þ Itransducer þ Ilost ð4Þ

where Ielectrical is the electrical power applied at transducer, Idissipated

is the ultrasonic power accommodated into the sonicated liquid,Iatomisation is the power lost through the liquid atomization, Itransducer

is the heat stored at transducer and Ilost stand for the missing partthat is more difficult to quantify such as the heat lost to the envi-ronment, liquid vaporization (apart from atomization) or ultrasonicwave attenuation. Our research group is now developing a separateexperimental work to quantify atomization term from energybalance.

4. Conclusions

The intensity of the cavitation is obviously linked to the acous-tic power and any increase in ultrasonic power entering the reac-tion will be associated with an increase in cavitational effectwithin the system [34]. The purpose of this work was to obtaininformation about the energy conversion for organic solvents at500 kHz as a step of a complex study aiming to ascertain the en-ergy balance for the sonochemical reactors. By using the calorimet-

ric method, it was possible to study the effects of the type ofsonicated liquid, electrical power and liquid height on the ultra-sonic power. This experimental work revealed a difference in theenergy efficiency between water and the organic solvents. Theultrasonic power conversion for organic liquids depends on the vis-cosity, surface pressure and vapor pressure rather than on acousticimpedance but the energy conversion for organic solvents cannotbe attributed to a particular physical property as there is acombined influence from these parameters. Under the experimen-tal conditions employed here, the energy conversion for organicsolvents at 500 kHz is only half or less from the energy conversioncalculated for water. Contrary to what may be expected, there is adrop in the energy conversion efficiency in organic liquids for lowfilling levels as compared to the 250–500 mm segment of the reac-tor cell. All these findings appear to indicate a strong impact ofultrasonic atomization on the energy conversion for organic sol-vents. Therefore, an energy term for atomization was consideredfor the energy balance of a sonochemical reactor running on organ-ic liquids at high-frequency.

The interest in this topic arises from our previous work on sono-chemical reactors efficiency and the findings of these experimentshave inspired the development of a research project to quantify theinfluence of atomization on the energy balance at high-frequency.

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