Contamination-free high capacity converging waves sonoreactors for the chemical industry

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Contamination-free high capacity converging waves sonoreactors for thechemical industry

Jean-Luc Dion *

SonerTec Inc., 3760 Rue de Montpellier, Trois-Rivières, Québec, Canada G8Y 3P2

a r t i c l e i n f o

Article history:Received 27 August 2007Received in revised form 19 May 2008Accepted 13 July 2008Available online 29 July 2008

PACS:81.0778.67.Bf43.2543.35.Hl43.20.Mv43.3543.35.Ei46.4047.4082.40.Fp43.35.Bf43.35.Zc43.38.Fx43.25.Cb43.25.Vt43.25.Yw47.2789.30.Jj64.70.fh43.35.Ei62.60.v87.50.Y

Keywords:Cylindrical SonoreactorAcoustic cavitationCylindrical wavesIndustrial pilot plantHigh capacity sonoreactorSonochemistryHigh pressure processChemical processing

a b s t r a c t

A new sonoreactor technology is presented here which should give a decisive impulse to sonochemistryin various areas of chemical processing. These exclusive systems use high power converging acousticwaves in a tube to produce a relatively large volume confined acoustic cavitation zone in flowing liquidreagents under pressure. It is well known that numerous chemical reactions are strongly acceleratedwhen they take place inside such a zone. The new cylindrical sonoreactors do not contaminate the pro-cessed liquids with erosion products as most other devices do since the cavitation zone is maintainedaway from the wall of the tube. The processing conditions can be widely varied with pressure, power,temperature, and flow rate. The processing capacity of the largest models may be up to several tonsper hour, depending on the required cavitation energy per unit volume to produce the desired processenhancement, using an electric power input of about 50 kW.

� 2008 Elsevier B.V. All rights reserved.

1. Introduction

Ever since the first reported chemical effects of acoustic orultrasonic cavitation by Richards and Loomis in 1927 [1], expecta-

tions have risen continuously in the chemical process industry ingeneral without being really fulfilled, particularly during the lasttwenty years. During this last period, a considerable number ofpublications on experimental work have shown the advantagesof ultrasonic activation for many chemical reactions and processes,mainly in the field of organic chemistry [2], crystallization [3], food[4], and pharmaceutical industry [5]. Some reaction rates have

1350-4177/$ - see front matter � 2008 Elsevier B.V. All rights reserved.doi:10.1016/j.ultsonch.2008.07.009

* Tel./fax: +1 819 378 1853.E-mail address: JL.Dion@TR.cgocable.ca

Ultrasonics Sonochemistry 16 (2009) 212–220

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been shown to be increased by factors up to 100,000 due to ultra-sonic cavitation [6]. The catalytic effect of ultrasonic cavitation inmany processes is now well demonstrated [7]. Many other applica-tions are also known, for example: extraction of valuable mole-cules from plants or animals [8], production of nanoparticles [9],sludge disruption [10], water decontamination [11], etc.

One of the main reasons for the slow development of the indus-trial applications of sonochemistry is the fact that ultrasonic cavi-tation, a violent phenomenon, takes place on the metallic vibratingsurfaces of most sonoreactors, and produces severe erosion ofthese surfaces as the driving power is increased. Even the costlytitanium alloy vibrators or sonotrodes are eroded. Consequently,the reagents are contaminated by erosion products, so barringthe application to most fine chemical and food processes. Thiswas until now the most serious barrier to the use of sonoreactorsin industrial chemical processing besides low volume treatmentcapacity. One of the unique features of the sonoreactor system thatwe present here is the absence of erosion and erosion products.

2. The known industrial sonoreactors

Fig. 1 was taken from the Hielscher company website [12] andshows the principle of their sonoreactors which are similar to mostothers, except SonerTec’s. This photograph represents a specialunit where the outside casing is transparent. This casing surroundsthe sonotrode which is the concentric cylinder seen through. Thepiezoelectric elements to the left (not showing) are driven by anelectric current at a frequency such that the sonotrode vibrates

in an upper longitudinal mode. This figure illustrates the abovementioned limitations.

As shown schematically in Fig. 2, it is observed that cavitation(clear zones) is induced at points where the acoustic displacementis high (region A) or maximum (region B). There is none where thevibration amplitude is weak or minimum (region C). We can seethat the relative volume of the cavitation zones is rather low. Sincethe acoustic waves produced are divergent, their intensity dropsquickly away from the sonotrode. It is then easily understoodwhy acoustic cavitation is produced only in the immediate neigh-borhood of the surface, causing erosion, with a relatively small cav-itation volume. It is also observed that cavitation appears mostlyalong broken lines, practically always at the same places, so accel-erating the erosion effects.

3. An exclusive sonoreactor system

We will describe here a new type of patented sonoreactor withits associated system, which is radically different from others in itsoperating principles. Two models presently exist: 5 kW and 50 kWas described below with large volume processing capacity. Thesesonoreactors are particularly suited for chemical processing in gen-eral since they cannot contaminate reagents with erosion productsextracted by acoustic cavitation from metallic surfaces as in otherdevices [13]. The reason is that the powerful and efficient large,and centered cylindrical cavitation zone produced under variablehydrostatic pressure is kept away from the walls of the tube madeof PTFE (polytetrafluoroethylene, TeflonTM) which transmits theacoustic energy into the freely flowing liquid (Fig. 3). In othersonoreactors, the acoustic cavitation taking place on the vibratingmetallic surfaces or sonotrodes, gradually destroys them, and theyhave to be replaced more or less frequently. Furthermore, in mostconfigurations, the liquid is forced to change direction severaltimes during transit which is not ideal for thick liquids or sludges[13].

However, due to the lack of appropriate sonoreactors, only afew limited applications to industrial chemical processing existat the moment on a small scale, mostly for crystallization, mixing,etc. [14,15]. The vast domain of industrial sonochemical processing

Fig. 1. Cavitation on a common sonotrode.

Fig. 2. Principle of a common sonotrode.

Fig. 3. Schematic cross-section of the cylindrical sonoreactor.

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has still to be occupied. But the situation could be transformed bythe new type of cylindrical sonoreactors presented here.

4. Using cylindrical converging waves

One basic exclusive feature of the new sonoreactors is shownschematically in Fig. 3 [16]. The special prismatic piezoelectrictransducers Tr produce high quality converging, and nearly cylin-drical, ultrasonic waves in the pressurized liquid circulating inthe central processing tube T. Consequently, the acoustic intensityis normally a maximum at the center of the tube where cavitationstarts. This achievement is essentially the result of an initial con-cept which was extensively simulated with a special softwarecalled Atila which is a finite element computation code appliedto 2- and 3-D electromechanical structures with piezoelectric ormagnetostrictive elements [17]. The 2-D computations were donefor an assembly composed of one transducer applied on a tube sec-tion followed by an infinite water medium. This approximation isvalid since that in cavitation regime the cavitation zone absorbs al-most totally the acoustic energy.

The mechanical design of the sonoreactor being one of theexclusive features of the device which is not yet commercialized,it is hardly possible at this time to disclose details of the mounting.But appropriate means are taken so that all the twelve special pris-matic transducers are evenly pressed on the polymer tube allaround. The internal hydrostatic pressure in the tube partiallyequilibrates the push of the transducers.

The vibration amplitude of the transducer end applied on thetube is impossible to measure. But it could be approximately de-duced from the measured acoustic power dissipated in the flowingliquid. It is this power which is important. From the observed cav-itation zone symmetry (Fig. 4) and the erosion produced on alumi-num plates (Figs. 9 and 10), the amplitude difference betweenadjacent transducers must be rather low. For the same reason,the phase difference between adjacent transducers must be lessthan 60�. This is due to the good acoustic impedance matching be-tween the transducers and the load. This is achieved with a pat-ented particular design. The cavitation zone is therefore confinedor centered in the tube, as shown in Fig. 4. This photograph is ex-tracted from a video taken through a process window, end-on, withclear water flowing. We can see the cavitation zone produced at aninput power of about 4 kW at 45 kHz, and medium pressure(4 bars = 400 kPa) in the 5 kW model. The photograph clearlyshows that the cavitation zone is confined, away from the wall ofthe tube: an exclusive feature of these sonoreactors.

The intensity distribution in this zone as measured by its ero-sion effect is illustrated in Section 7. The diameter of the tube is75 mm. The relative size of the cavitation zone depends mainlyon power, frequency, and pressure in the freely flowing liquid:there is no obstruction in the tube. Cavitation intensity along theradius is a highly non-linear function of acoustic input power, fre-quency, tube radius, pressure, temperature, viscosity, and othervariables. For certain conditions, the intensity may even be lowerat the center, since cavitation in a centered annular region stronglyabsorbs acoustic energy: a cavitation zone is a sort of ‘‘black hole”for acoustic energy. That is why, for a given chemical process, onehas to determine the particular optimum conditions.

The zone in Fig. 4 looks black since the light from below isblocked by the thousands of microscopic cavitation bubbles per cu-bic centimetre which appear, oscillate during a few periods, andimplode violently. Imploding bubbles generate shock waves withacoustic pressures well over 1000 atm (bars) and temperaturesover 5000 K due to adiabatic compression of the residual gases,even at atmospheric pressure in laboratory conditions [18]. The

Fig. 4. Cavitation zone in the low power SR-31 sonoreactor.

Fig. 5. Effect of pressure on the eroded mass of aluminum due to acoustic cavitationat the focus of converging spherical ultrasonic waves, as a function of thetransducer driving voltage, for various values of hydrostatic pressure in water(Sirotyuk). It is to be noted that for the same driving voltage, 20 V (thick verticalline), the cavitation efficiency is about 40 times higher at 4 bars that at 1 bar(1 bar = 100 kPa), and 80 times higher at 10 bars. If the voltage is then doubled at4 bars, this graph shows that the cavitation intensity is then about 250 times that at20 V, 1 bar.

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design of the transducers with their supporting structure and thechoice of building materials was quite an interesting challenge toachieve that type of cylindrical waves. It should be mentioned thatthe production of these waves requires the use of a rather largenumber of piezoelectric ceramics. So, even to produce very largepower, each ceramic is driven at a small fraction of its maximumpossible power and temperature. In fact, the temperature of thecooling oil flowing on the transducers is kept well below 60 �C. Thisway, the useful life of these ceramics and that of the sonoreactorsis considerably extended.

The bubbles are formed in violently agitated filaments radiatingfrom the center which look like electric corona discharges (Fig. 4),and a strong typical cavitation noise is heard through the massivesteel and aluminum alloy structure of the sonoreactor. Varying thepressure and driving power allows a wide variety of processingconditions where the relative volume of the cavitation zone ischanged without ever destroying the wall of the tube, and liberat-ing contaminants. Of course, this zone is where the interestingphysical and chemical reactions take place. It is now well knownthat the extreme conditions produced by the implosion of cavita-tion bubbles can break any organic molecules, and generate veryactive free radicals, particularly in water. It has even been reportedby Taleyarkhan and Lahey et al. [19,20] that nuclear fusion actuallytakes place in cavitation bubbles produced in a deuterated liquid atatmospheric pressure in a low power sonoreactor.

One interesting feature of the new technology is the fact thatthe processed liquids flow freely through the tube, without anyobstructions as in most other systems [21]. The flow rate cantherefore be varied through a wide range. For example, a very highflow rate may be required for multiple passes through the sonore-actor in certain batch processes. This is so since (as in any othersonoreactor) not all the liquid passes through the cavitation zonein one pass. However, the relative volume of the concentric cylin-drical zone is large as compared to that of the active volume inother sonoreactors (Fig. 1).

5. Operating under pressure

Another exclusive feature of this new sonochemical sonoreactoris its capability of operating largely above atmospheric pressure inthe circulating liquid, where the effective acoustic cavitation effi-ciency is radically increased with the same electric power input,

without auto-destruction of the device. This has been demon-strated in the former USSR in the early-1960s, particularly in theexperimental work of Sirotyuk [22]. In the hemispherical deviceused to produce converging acoustic waves, it was demonstratedthat cavitation intensity at 4 bars (400 kPa) was about 40 times lar-ger than at atmospheric pressure for the same driving voltage, theratio being still much larger for higher driving power (Fig. 5). Thecavitation efficiency was measured by the eroded mass from asmall aluminum sphere placed at the focus of the device. Erosionof an aluminum specimen is still a simple and convenient way tocompare cavitation efficiencies as we will see.

More recently, Matula and Crum observed that an ‘‘increase ofup to two orders of magnitude may be achieved by simply increas-ing the static pressure over the cavitating solution” in relation withsonoluminescence activity, but apparently didn’t know about Sir-otyuk’s experiments [23].

The sonoreactors described here were designed with this phe-nomenon in view, and to operate normally above atmosphericpressure, where the cavitation activity is really efficient. Actually,they were operated up to 6 bars (600 kPa) with the present design.In future models, the operating pressure could be raised, if needed,rather easily to high limits such as 50 bars where the active ordestructive power of cavitation may be thousands of times largerthan at atmospheric pressure (Fig. 5). This will be particularly use-ful for the decontamination of large volumes of water: for example,a considerable number of publications show that ultrasonic cavita-tion can be efficiently used to reduce the concentrations of chlori-nated and fluorinated pollutants to less than 1 ppm [24,25]. Strongcavitation is particularly useful in the decontamination of waterstrongly polluted by organic compounds or microorganisms.

However, for operation in food processing and other delicateprocesses, it should be desirable to use lower pressure and power,taking advantage of a larger cavitation zone in the free-flowingtube to process large quantities of material.

6. The actual systems

Two models of this type of sonoreactors have been designed,built and operated. The smaller one is a 5 kW, 1 l treatment volumesonoreactor system (SR-31, 75 mm tube) which is well adapted forpilot plant and process development operations. Its control systemis particularly suited for these operations, where a close control of

Fig. 6. The SonerTec SR-31 5 kW process development sonoreactor system.

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power, pressure, temperature, time and flow volume is required inthe batch or open circuit modes. The operation is entirely auto-matic after set-up. One version is shown in Fig. 6 where we cansee the cylindrical sonoreactor to the right (yellow), mounted onthe cabinet containing the medium frequency (45 kHz) 5 kWpower generator and the control system. The core of the sonoreac-tor is composed of 12 prismatic transducers, each of which can bedriven with over 400 W of high frequency power. The system canoperate in closed circuit with the tank to the left containing the li-quid to be treated under pressure. In this particular model used forsludge treatment process development, a progressive volumepump force the liquid through the central free-flowing tube in

the sonoreactor, and an automatically adjusted pinch valve main-tains its pressure. This unit does not have observation windows.The appropriate combination of power, pressure and flow rate ismaintained automatically as required. Other configurations arealso possible: for example, using a centrifugal pump and com-pressed gas to set the flow rate, and the operating pressure witha pressurized reservoir. The photograph of the cavitation zone inFig. 4 was taken with a similar system equipped with observationprocess windows.

The other model is a 50 kW system for industrial applications(SR-42, 100 mm tube). The sonoreactor core is composed of twogroups of 12 concentric prismatic transducers, each of which beingabout 900 mm long, and each transducer being able to be drivenwith 2 kW of high frequency current (shown rotated 90� inFig. 7). Its operating frequency is normally about 39 kHz. Oneexperimental unit can be seen in Fig. 8, as installed for testing ina water treatment plant. It is experimented for sludge disruptionin front of an anaerobic digester, to reduce the mass of residues,and increase the biogas output. The sludge is fed into the 2 m highsonoreactor from below, and the pressure is maintained by a largepinch valve partly seen behind the sonoreactor. To the left, we cansee the automatic control system, and the high frequency current

Fig. 7. The core of the 50 kW sonoreactor (rotated 90�).

Fig. 8. The ‘‘DigestSonic” SR-42 system installed in a sludge treatment plant in France.

Fig. 9. Erosion of a 0.6 mm aluminium alloy plate during a 3 min exposure in model SR-31.

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generator. This system can operate at any internal pressure from 1to 6 bars (100–600 kPa). In this model, the pressure is also limitedby the type of pipes used, presently CPVC.

As with the smaller model, the operating pressure, power, flowrate, etc. and the type of associated systems (pumps, generator,etc.) can be varied widely to accommodate various processes.

7. The powerful effects of confined acoustic cavitation

When a 0.6 mm thick hard aluminum alloy plate is inserted intothe cavitation zone of Fig. 4, it is actually observed that the centeris pierced in less than 1 min. The photograph in Fig. 9 shows clearlythe effect of a 3 min exposure in the smaller sonoreactor describedabove (SR-31). The total length of the eroded zone (narrow ends) isabout 240 mm; the central strong erosion zone is about 100 mmlong; while the length of the surrounding prismatic transducersis 210 mm. This figure illustrates the end effects as predicted bythe theory of diffraction of waves. In this case, the end effects ofcavitation also depends on the end effects of the transducers whichare relatively short compared to the tube radius. If the same tubewere much longer, the end effects would be more or less the same,while the relatively uniform intensity region would be much long-er and efficient.

This has been well confirmed in the 50 kW model using a1800 mm long tube which is described above. Fig. 10 shows the ef-

fect of acoustic cavitation in this long PTFE tube of the SR-42sonoreactor operating at 40 kHz, 50 kW, 6 bar pressure: the hardaluminum alloy 1.5 mm thick plate was submitted to a 5 minexposure. These last two photographs may be actually the bestdemonstrations of the unique possibilities of the new technologywhere industrial capacity and power are required.

Another impressive effect of the powerful confined cavitationzone in these sonoreactors is the disruption of sludge particles ascan be seen in Fig. 11. It shows eight closed glass cylinders filledwith secondary sludge from a paper mill (Kruger, Trois-Rivières)which has been treated with the SR-31 sonoreactor during earlyexperiments at rather low power. At far left is the untreated sludge(1). The other cylinders contain sludge sonicated with increasingspecific cavitation energy (SCE), from 10 to 200 J/cm3 from left toright (2–9). This energy is computed as the product of the electricpower input and time. The maximum power used for these exper-iments at the time was only about 1200 W, with a hydrostaticpressure of about 2.5 bars. Only later could we raise the usefulpower up to nearly 5 kW with a 4.5 bar pressure. It can be seen thatwith an SCE of only 10 J/cm3 (second from the left), the dark solidpart of the sludge is already modified, and floats on the liquid part.The microphotographs at 100� in Fig. 12 compare untreatedsludge (left) with sonicated sludge at 10 J/cm3 SCE (right). It canbe seen that the flocs present in the untreated sludge are largelydisrupted with this weak SCE. Most living organisms as the elon-

Fig. 10. Erosion of a 1.5 mm aluminum alloy plate during a 5 min exposure in model SR-42 (50 kW).

Fig. 11. Effect of acoustic cavitation on secondary sludge from a paper mill (Kruger).

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gated one close to �100� (photo to the left) are blown up with thislevel of SCE.

When the specific cavitation energy is increased, another effectof cavitation appears due to the highly oxidizing effect produced bycavitation in water: sterilization. After 15 days at room tempera-ture, in the last two hermetically closed cylinders to the right,no. 8, 9 (�100, and �200 J/cm3), no moulds appeared above thesludge, as they did increasingly in the other cylinders from rightto left, demonstrating efficient destruction of microorganisms.

We also experimented with the oxidizing effect of acoustic cav-itation in the SR-31 sonoreactor with small batches of un-maturedwhisky and brandy. We could observe that a 20 min treatment hada mellowing effect, which some tasters compared to an 8–10 yearaging.

We have also experimented briefly with the production of cop-per and silicon nanoparticles in the SR-31 sonoreactor. The basicmaterial was firstly copper powder used for brazing, and secondly,silicon powder provided by a manufacturer (Industries SKW Inc.,Bécancour). The initial size of these particles was roughly 0.1 mm(100 lm). In both cases a slurry was prepared by adding 3 kg ofpowder to about 40 l of demineralised water. At the time of theseexperiments, the maximum driving power was less than 2 kW,with a 3 bar pressure. A 40 min sonication was applied. The result-ing particles were compared with the original ones using an opticalmicroscope with a maximum enlargement of 800�. It could be ob-served in both cases that the distribution of sizes was stronglyshifted to small sizes, an estimated 25% being smaller than10 lm. We were not equipped to make measurements of smallerparticles.

8. Processing conditions and versatility

For each particular process, the operating conditions have to bedetermined with a laboratory unit before scale-up in most cases ofchemical processing. Depending on the process, mild or strong cav-itation may be required. For a given driving power, operating pres-sure above the cavitation threshold, and temperature, eachparticular process requires a specific cavitation energy (SCE). Forexample, we have experimented with sugar crystallization, and ob-served that the increased production of fine uniform crystals re-quired low pressure, and low power above cavitation threshold.It is expected that strong cavitation destroys crystals as soon asthey are formed.

Another practical feature of this type of sonoreactors is the factthat they can be used for almost any application with relativelyminor modifications in the sonoreactor and outside tubing andpumps, with the same electronic systems. For example, if corrosiveor highly pure liquids have to be processed, the outside tubing

would be made with appropriate stainless steel, or PTFE. Otherpractical models are also on the drawing board, with variousimprovements over the actual systems. New designs will alsoachieve larger relative volumes of the cavitation zone for increasedefficiency. It should be noted that these sonoreactor systems cannormally be simply inserted into an existing process with littlechanges. The floor space occupied by one SR-42 unit is typically2 � 3 m only. Any number of units can be operated in series or par-allel, or both ways, to process very large quantities.

9. Efficiency

The energy E (joules, J) used to raise the temperature of a liquidby DT �C in the sonoreactor is

E ¼ mcDT ð1Þ

where m is the mass of heated water in grams, c is the specific heat(4.18 J g�1 �C�1 for water).

In the present type of sonoreactors, this energy is very largelycontributed by the dissipated acoustic energy and cavitation inthe liquid, since the tube provides thermal insulation from thetransducers. The following relationship between dissipated acous-tic power Pa, specific mass q, specific heat c, flow rate d, and tem-perature rise DT during transit can then be easily demonstrated

DT ¼ Pa

qcdð2Þ

The efficiency e of a sonoreactor is a characteristic which may havevarious definitions. Most of the time, it is simply measured by thisenergy E divided by the electrical energy input Ee:

e ¼ EEe

ð3Þ

But in most other configurations, much of the temperature rise isdue to heat dissipated by the transducers which are cooled by theflowing processed liquid. Therefore, the result is an overestimationof the cavitation efficiency. In the present systems, the transducers,which are designed for the highest efficiency, are cooled separatelyby an oil flow. The quantity of heat energy propagating through theTeflonTM tube should be only a small unknown fraction of the totaldissipation since PTFE is a good insulator. On the other hand, thePTFE tube absorbs a certain fraction of the acoustic energy propa-gating through its wall, a fraction of which goes into the liquid: itis a fairly high dissipative medium for acoustic waves at 40 kHz.That is why its thickness is kept below 5 mm. So, the major partof the temperature rise in the present sonoreactors is due to theeffective acoustic and cavitation energy Ea. If we call the electrical

Fig. 12. Microphotographs, 100�. Left, untreated sludge; Right, sludge treated with a 10 J/cm3 cavitation specific energy.

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energy input Ee, the cavitation efficiency ea is then defined approx-imately as

ea ¼Ea

Eeð4Þ

where

Ea ¼ mcDT ð5Þ

m is the mass of the liquid, c is its specific heat, and DT the temper-ature rise.

Of course, such a definition should give lower efficiency valuesthan those published for other sonoreactors. However, it should bemore realistic. Our measurements gave values of ea up to nearly50%. Various improvements in the future models are expected toprovide higher figures.

It would certainly have been interesting to measure the acousticcavitation efficiency using the classical free iodine productionmethod [26], but we did not have the laboratory equipment todo this test. The cavitation efficiency was essentially measuredby its physical effects: erosion of aluminum plates, and particlefragmentation. We think that the observation of these effects isfairly clear evidence for persons familiar with acoustic cavitationeffects and applications.

But, finally, the real and effective efficiency of a sonoreactor sys-tem is measured essentially by the total cost of operation per pro-cessed ton of materials as an improvement over the higher cost of aregular process. This has yet to be done.

10. Processing capacity

The volume of liquid that can be processed per hour dependsessentially on the specific cavitation energy needed to achievethe required chemical or mechanical effect. Let us assume, forexample, that a given process requires a reasonable SCE (definedabove) S = 20 J/cm3 = 20 � 106 J/m3, and that the SR-42 is operatedat maximum power P = 50 kW. The total input energy E during 1 his then

E ¼ Pt ¼ 50� 103 J=s � 3600s=h ¼ 1:8� 108 J=h ð6Þ

The possible flow rate D is then easily computed

D ¼ ES¼ 1:8� 108 J=h

20� 106 J=m3 ¼ 9m3=h ¼ 9000l=h ð7Þ

This flow rate is inversely proportional to the SCE. If larger flowrates are required, any number of these units can be easily installedin series or parallel.

11. Costs of operation

At the moment, the known costs of operation of these two newsonoreactor systems are mainly those of the electric driving en-ergy. Let us suppose that the 50 kW system (SR-42) is operatedat nominal power. Allowing an upper limit of 15 kW for peripheralsystems (pumps, cooler, control system, etc.) the energy used in1 h in kW/h is then 65 kW/h. If the cost of energy is $0.15/kW/h(per kW/h), the cost of operation is $9.75/h.

With S = 20 J/cm3, the cost of sonication per litre is thereforeabout $0.0011. This cost is directly proportional to the SCE. Foran SCE of 100 J cm�3, it is simply multiplied by 5 to give $0.0055/l, meaning that the processing time is multiplied by 5 if the systemis operated at full power. Or, the other way, for the same inputpower and same time, the processed volume is 5 times less forthe same cost of operation. The smaller SR-31 (5 kW) system hastherefore a processing capacity which is about ten times lowerthan the SR-42.

At the moment, if the driving power is kept below the possiblelimit of 60 kW, the lifetime of the tube may be well over one yearof continuous operation according to various observations. Let usalso mention that the efficiency of the type of HF current generatorused is over 90%: the ratio of HF power over the input low fre-quency power. If necessary, the tube can normally be replaced inless than a day, at a reasonable cost.

12. Conclusion

It has been known for a long time that acoustic cavitation con-siderably accelerates many chemical processes, largely in organicchemistry, and produce energetic disruption effects on particlesand molecules. Acoustic cavitation can also improve crystalliza-tion, extraction, mixing, emulsifying, etc. We presented a radicallynew sonoreactor technology which should open innovating andexciting perspectives for the modern food and chemical industrylooking for significant and competitive process improvements withsonochemistry. It is based on cylindrical converging ultrasonicwaves producing a powerful concentric and confined, chemicallyactive cavitation zone in a TeflonTM tube, away from the wall, in sucha way that there is no erosion and contaminating products. Thenew cylindrical converging waves sonoreactors have industrialprocessing capabilities, measured in tons per hour for the largermodel, depending on the cavitation energy per unit volume re-quired to produce a given effect.

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