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Ultrasonics Sonochemistry 7 (2000) 145–149
www.elsevier.nl/locate/ultsonch
Large scale sonochemical processing: aspiration and actuality
Timothy J. Mason
*
Sonochemistry Centre, School of Natural and Environmental Sciences, Coventry University, Coventry CV1 5FB, UK
Abstract
It has been recognised for many years that power ultrasound has great potential in a wide variety of processes in the chemical
and allied industries. Some of these processes have been known for many years and continue to flourish as major commercial
applications, e.g. plastic welding and cleaning. Others, like ultrasonic drilling, while showing great potential have not been widely
exploited to date. The potential for the industrial use of power ultrasound is enormous, and yet industry seems somewhat reluctant
to adopt it. In this article the existing uses of power ultrasound in processing are reviewed and the potentials are explored. © 2000
Elsevier Science B.V. All rights reserved.
Keywords: Cavitation; Industry; Power ultrasound; Reactor design; Sonochemistry; Transducer
1. Introduction key to sonochemical processing is acoustic cavitation,
but the secret of its success is the ability to control and
limit its effects to the reaction and not the reactor. ThisLarge scale processing using power ultrasound is not
a new concept. In the 1960s the industrial uses of power can often be achieved by focusing (concentrating) the
acoustic field within the liquid reaction medium.ultrasound were recognised [1,2]. There are many exam-
ples in the literature which also testify to this, for A further complication is the tendency of practi-
tioners to believe that increased input of acoustic powerexample a series of papers appeared in the journal
Ultrasonics back in the 1970s, entitled ‘Macrosonics in is related directly to increased yield or effect. This very
seldom holds true. There are many examples of an effectIndustry’ [3]. One of the more recent additions has been
a text translated from the Russian [4]. Progress over known as ‘decoupling’, where the efficiency of transfer
of power from an ultrasonically vibrating source into athe last few years has been more rapid for two reasons:
firstly the greater general awareness of the possibilities liquid suddenly decreases above a certain level. Consider
for example the production of iodine from aqueous KIfor ultrasonic processing, and secondly the ever-widen-
ing span of applications which has attracted the atten- (Fig. 1) [5]. In this classic example the initial iodine
yield first increases in a relatively linear fashion abovetion and investment of more companies.
Whatever claims are made for sonochemical process- the cavitation threshold but then reaches a plateau for
a while before decoupling sets in and the yield dropsing, there remains one term used in its description which
is a turn-off for engineers — that word is cavitation.
Certainly it is true that cavitation is normally considered
to be something to be avoided in the construction of
reactors. This is because erosion damage from hydrody-
namic cavitation (the type which affects propeller blades)
can cause problems to surfaces in contact with flowing
systems. In sonochemical processing the driving force is
acoustic (rather than hydrodynamic) cavitation, and
certainly there are many examples of surface damage
through acoustic cavitation. A typical example is the
pitting of aluminium foil when placed in the cavitating
liquid of an ultrasonic cleaning bath. Nevertheless, the
Fig. 1. The effect of ultrasonic power on iodine yield in the sonochemi-
* Tel.: +44-1203-838173; fax: +44-1203-838173. cal oxidation of I − to iodine in aqueous solution.
1350-4177/00/$ - see front matter © 2000 Elsevier Science B.V. All rights reserved.
PII: S1350-4177(99)00041-3
146 T.J. Mason / Ultrasonics Sonochemistry 7 (2000) 145–149
dramatically, despite the increased power supplied by an alloy of the rare earth elements terbium and dyspro-
sium with iron which is zone refined to produce athe transducer. Generally, for any sonochemical process,
there will be an optimum power for maximum effect. material almost in the form of a single crystal. It can be
produced in various forms, rods, laminates, tubes, etc.This will depend on a range of conditions, but will mean
that process optimisation can lead to a considerable and several major advantages have been claimed for this
material compared with the more conventional alloyssaving in overall economics of the process.
used, e.g. it can generate more power, it is more compact
(about 50% smaller) and also lighter than other magne-
tostrictives. Like other magnetostrictive materials,2. Ultrasonic transducers
Terfinol does has an upper limit of frequency response
of about 70 kHz.Whatever application of sonochemical processing is
to be studied and developed, one essential component Piezoelectric transducers are the most widely used
devices for the generation of ultrasound and can be usedof the system will be the transducer, of which there are
three main types. One is liquid driven and the other two over the whole range of ultrasonic frequencies. Current
developments using such transducers exploit their versa-are electromechanical; all of them have been in use for
many years. A fourth type of transducer has been tility in terms of both size and available frequency range.
Piezoelectric systems have been used to drive large andintroduced, the magnetically driven vibrating bar which
generates very high power vibrations in the audible powerful ultrasonic welders and cleaning baths, but are
also finding more refined uses in a new field of medicinefrequency range. This has some potential for heavy duty
processing and has been described elsewhere [6 ]. called therapeutic ultrasound [8]. One example of this
is the use of an extracorporeal array of transducers toLiquid whistles are particularly useful in applications
where homogenisation and efficient mixing are impor- produce a high intensity focus capable of destroying
tumours (HIFU ). Another is the use of a miniaturisedtant. Process material is forced under pressure generated
by a powerful pump through an orifice from which it ultrasonic device (1 MHz) attached to the end of a
catheter which can then be inserted into a blood vesselemerges, as a jet, into a mixing chamber. The jet impacts
upon a thin steel blade which is caused to vibrate and and used for the destruction of blood clots.
New possibilities exist for the development of piezo-thereby produce mixing of the process material flowing
over it. Additional mixing is produced through the electric devices using so-called 1–3 composites. These
consist of an array of piezoelectric pillars embedded inVenturi effect as the liquid rapidly expands into a larger
volume on exiting the orifice. There have been no a pliable material providing a transducer in the form of
a flexible sheet (Fig. 2) which can be moulded to fit thesignificant advances in the basic design of this type of
device in recent years, but the range of applications is shape of a reactor. A particular advantage of such a
system is that the emitting face is a combination ofbroad and new uses continue to emerge. Examples of
existing commercial processes include the preparation ceramic and plastic and can provide much better acoustic
transmission into aqueous systems.of soups, sauces, gravies and ketchup; in cosmetics
manufacture to make skin cream; and in textiles to There is also a great deal of interest in the production
of single crystal piezo material from a mixture of lead,increase the quality of dyeing in fabrics by improving
the general dispersion of the pigments. In the plastics zirconium and niobium compounds (from which they
get the name PZN ). Such single crystal transducersindustry, ultrasound has been used to disperse clay or
special thickening agents in polyester resin to give it the would provide low loss, high strain, low modulus and
have high coupling coefficients. At the time of writingrequired characteristics. It can also be used to disperse
tinting pigments, which are mixed with the basic poly- these materials are not yet commercially available.
mer, a thickening agent and a solvent to produce the
coloured finish seen on most fibre glass products.
Magnetostrictive transducers use a property of certain
materials, e.g. nickel, which reduces in size when placed
in a magnetic field (magnetostriction) and then returns
to normal dimensions when the field is removed.
Improvements in the operating efficiency of this type of
transducer have been based on finding a more efficient
magnetostrictive core. The original nickel alloys have
been replaced by more electrically efficient cobalt/iron
combinations and, more recently, aluminium/iron with
a small amount of chromium. One of the latest develop-
ments in magnetostrictive technology has been the intro-
Fig. 2. Schematic representation of a 1–3 piezoceramic array
transducer.
duction of a new material called Terfinol-D [7]. This is
147T.J. Mason / Ultrasonics Sonochemistry 7 (2000) 145–149
3. Industrial uses of power ultrasound biocide used in sterilisation, however, some bacteria are
capable of building up resistant strains which may
require a higher concentration. Power ultrasound affordsA selection of current uses of power ultrasound is
presented (Table 1). The first two of these are not driven the opportunity of increasing the efficiency of existing
biocides through two major effects. Firstly the mechan-by acoustic cavitation, but the remaining fields generally
utilise cavitation to some extent. ical effects of cavitation can break down either bacterial
clumps or agglomerates of material to which the bacteria
adhere. This will remove the protection afforded to live3.1. Cleaning and decontamination
bacteria in the centre of the clumps or agglomerates.
Secondly ultrasound can increase the permeability ofUltrasonic cleaning is now such a well established
general technology that laboratories without access to biological cell walls so that the rate of uptake of the
biocide by the bacteria is increased.an ultrasonic cleaning bath are in a minority. Although
the laboratory ultrasonic cleaning bath is familiar, the A treatment system for the water in cooling towers
has been developed which involves a combination ofindustrial applications of such cleaning are perhaps less
well known. Ultrasonic cleaning can be either delicately ultrasound and electromagnetic radiation. The system,
registered under the name Sonoxide [10], tackles twoapplied for the cleaning of microcomponents under
clean room conditions or used for very large items such major drawbacks of cooling circuits, namely the build
up of algae and limescale. It is able to kill algae onas engine blocks in factories. It is particularly effective
in the removal of biological contamination because the exposure to ultrasound at a flow rate of 2 m3 h−1 using
an acoustic power of only 450 W. The same low energycleaning action is through jets induced by cavitational
collapse on and near surfaces. These jets are easily requirement is sufficient to take care of the problem of
the accumulation of limescale because it creates a formcapable of dislodging bacteria which may be adhering
to the surfaces. The particular advantage of ultrasonic of calcium carbonate crystal which does not settle as
scale. The crystals are carried away by the water in thecleaning in this context is that it can reach crevices that
are not easily reached by conventional cleaning methods. cooling circuit and separated using a hydrocyclone. The
major advantages of such treatment are its low energyFor this reason such cleaning is used for a range of
items from large crates used for food packaging to consumption, the elimination of the need for water
softening or biocidal chemicals, and also a large reduc-delicate surgical implements such as endoscopes.
Power ultrasound is currently under investigation for tion in the requirement for make-up water in the
cooling circuit.use in the biological decontamination of water [9].
Conventional methods of disinfection involve the use of A further possibility for environmental use of sono-
chemistry is in the treatment of sewage sludge.a biocide which, for the large scale water industry, is
commonly chlorine, chlorine dioxide or ozone. Current Anaerobic fermentation is the most commonly applied
process for stabilisation of sewage sludge and providestrends are towards the reduction in quantity of the
mass reduction, methane production, and improved
dewatering properties. A disadvantage of the fermenta-
Table 1
tion technique is that it is slow, with conventional
Some industrial uses for ultrasound
residence times in anaerobic digesters of about 20 days.
Field Application
Ultrasound has been used to accelerate the anaerobic
digestion of sewage sludge [11]. Ultrasonic treatment at
Welding Fabrication of thermoplastic articles and
a frequency of 31 kHz and high acoustic intensities at
welding of metals via specific heating at the
half technical scale shortened the residence time to eight
junction between the pieces
of material.
days with a biogas production 2.2 times that of a control
Cutting Accurate cutting of all forms of material
fermenter. Once optimised it is likely that this type of
from brittle ceramics to food products.
process will also be adopted on an industrial scale.
Atomisation Water sprays for dust suppression and
Ultrasonic irradiation has also been employed for
humidifiers, low velocity spray coating, spray
chemical remediation of water. The mode of sonochemi-
drying nozzles.
Cleaning and Cleaning of engineering items,
cal degradation of organic compounds in aqueous solu-
decontamination small electronic items and jewellery using
tion depends upon their physical and chemical
aqueous based solvents.
properties. This is because there are two ways in which
Cleaning and disinfection of medical
the cavitation bubble can function. In the case of volatile
instruments and food processing
chemicals which enter the bubble, destruction occurs
equipment.
Processing Dispersion of pigments and powders
through the extreme conditions generated on collapse.
in liquid media and emulsification. Extraction,
In the case of chemicals remaining in the aqueous phase,
impregnation, crystallisation
the bubble acts as a source of radicals (H · ,HO· and
and filtration.
HOO · ) which enter the bulk solution and react with
148 T.J. Mason / Ultrasonics Sonochemistry 7 (2000) 145–149
Table 2
pollutants. This is neatly illustrated in a comparative
Species employed for extractive value experiments
study of the decomposition of phenol (to carboxylic
acids) and carbon tetrachloride (to CO
2
and Cl−)in
Plant species Main components Part of plant used
water saturated with oxygen at different frequencies
Foeniculum vulgare essential oils seeds
[12]. The results clearly show a difference in the behavi-
(fennel )
our of the chemical contaminants, with the phenol
Humulus lupulus resin, essential oils female strobiles
degradation mirroring the peroxide formation, indicat-
(hops)
ing that this reaction proceeds at the bubble interface
Calendula officinale flavonoids, resin flowers
(marigold)
or outside the bubble. The volatile CCl
4
, however, is
Mentha piperita essential oils, pigments leaves
decomposed within the bubble and increased frequency
(mint)
slightly accelerates the process.
On offshore oil drilling rigs an environmental problem
exists related to the disposal of drilling mud. This
material is a complex mixture of components which is of time with agitation, often in the form of tumbling.
Ultrasound has been shown to increase the rate ofinjected into the drill hole to serve as a lubricant and
flotation agent. After it has been used the mud, together penetration of the dye into the leather, and this offers
the commercially attractive possibility of attaining awith earth from the drill hole, is contaminated with oil
which must be thoroughly removed before the solids are more rapid turnover by reducing immersion times [15].
Since most of the processes in leather technology involvedumped at sea. Cleaning technologies exist for this, but
the cleaned material is then in the form of small agglom- liquid treatments and surface penetration, there are a
number of tanning processes which are under investiga-erates which, when dumped in the sea, sink to the sea
bed and accumulate around the platform legs. This tion with a view to increasing their efficiency by
sonication.presents a problem to sea life and a hindrance to rig
maintenance. An ultrasonic device has been built for
the de-agglomeration of the cleaned mud on site, i.e. on 3.3. Crystallisation and precipitation
the rig. The equipment comprises four modules, each
one rated at 2 kW, and consisting of a stainless steel In conventional crystallisation techniques (e.g. in
penicillin production), a solution containing materialstube 1 m in length and 12 cm in diameter with an array
of piezoelectric transducers bonded to the outside, the to be crystallised is supersaturated either by cooling or
by evaporation and is then seeded. The problem withwhole enclosed in a casing [13]. The equipment provides
a flow treatment for the drilling mud which substantially seeding is that it may be initiated non-uniformly and
this can result in crystal growth proceeding at differentreduces particle size. As a result, when the mud is
jettisoned from the oil rig it disperses across the sea bed rates at different nuclei sites. The resulting crystals may
then show a very broad and uneven crystal size distribu-and does not accumulate around the platform legs.
tion. It is also of considerable practical importance to
be able to control the onset of crystallisation in a large3.2. Extraction and impregnation
scale production process. Often it occurs in an uncon-
trolled manner simply due to a slight change in externalWhen cavitation bubble collapse occurs near a porous
material the jet which is generated is able to force liquid factors, such as a temperature or pressure fluctuation.
Ultrasound has proved to be extremely useful ininto the bulk material. This effect can be utilised to
enhance any process which involves either extraction or crystallisation processes since it can initiate seeding and
control subsequent crystal growth in a saturated orimpregnation of a liquid in a solid.
The classical techniques for the solvent extraction of supercooled medium. This is thought to be due to
cavitation bubbles themselves acting as nuclei for crystalchemical compounds from vegetable material are based
upon the correct choice of solvent and mixing condi- growth and to the disruption of seeds/nuclei already in
the medium, thus increasing the number of nuclei pre-tions, e.g. heating and agitation. A range of commer-
cially important pharmaceuticals, flavours and colorants sent. Through the correct choice of sonication conditions
it is possible to produce crystals of a uniform andare now derived from vegetable sources by solvent
extraction. One method of improving the extraction of designated size, which is of great importance in pharma-
ceutical preparations [16 ].organic compounds from within the body of plants and
seeds has been shown to be through the use of power Power ultrasound also has an additional property
which is particularly beneficial in crystallisation opera-ultrasound [14] (Table 2).
The same process of sonochemically forced impregna- tions, namely that the cleaning action of the cavitation
effectively stops the encrustation of crystals on coolingtion can be used to improve dying. The conventional
method for the dying of leather involves steeping the elements in the crystallisation vat and thereby ensures
continuous efficient heat transfer.material in an aqueous solution of the dye for a period
149T.J. Mason / Ultrasonics Sonochemistry 7 (2000) 145–149
3.4. Electrochemistry 4. Conclusions
Aspirations for sonochemical processing are those
There are many examples of the large scale use of
which all production managers would welcome: faster
electrochemistry in industry, particularly in electroplat-
reactions, better conversions, improved and perhaps
ing, but most of them are not efficient in terms of
even new products. In truth there are many large
electrical power consumed. For some years there has
ultrasonic systems available and used in processing, but
been interest in the potential for improvement of such
few are optimised. There are now several multi-disciplin-
processes by the application of ultrasound —
ary groups involved in sonochemical projects, and this
sonoelectrochemistry [17]. Electrochemical reactions are
is undoubtedly the way forward. The future for sono-
often complex but can be thought of as essentially the
chemical processing is therefore rosy, both from the
transfer of ions to and/or from the electrode surface.
point of view of greater interest in the fundamental
When an electrode is immersed in an aqueous solution
principles of its action, and in the development of
a thin diffusion layer forms at the interface and, since
international programmes in applied research and
electrochemistry must involve ions crossing this layer,
technology.
any disturbance of it will influence that process. The jet
effect produced by cavitational collapse will reduce the
thickness of this diffusion layer and assist diffusion to
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and indeed of sonochemistry itself.