WO2003101609A1

WO2003101609A1 - Ultrasonic reactor and process for ultrasonic treatment of materials

Info

Publication number: WO2003101609A1
Application number: PCT/IL2003/000450
Authority: WO
Inventors: Evgeny Marhasin
Original assignee: Nano-Size Ltd.
Priority date: 2002-05-30
Filing date: 2003-05-29
Publication date: 2003-12-11
Also published as: AU2003231351A1; US7504075B2; IL149932A0; US20050260106A1

Abstract

An ultrasound reactor (30) having a reactor body (1), a reactor tube (9) disposed within the reactor body and a magnetostrictor transducer (11) comprising at least one annular element (28a-d) concentric with, and mounted to an external wall of said tube and a process for ultrasonic treatment of a reaction material are disclosed. The transducer is mounted on the reactor tube in such a way so as to transmit ultrasound radiation to the interior of the tube and to induce an upwardly flowing cavitation stream whose longitudinal axis substantially coincides with a longitudinal axis of said tube, causing physical changes to the reaction material.

Description

ULTRASONIC REACTORAND PROCESS

FOR ULTRASONIC TREATMENT OFA REACTION

MATERIALS

ULTRASONIC REACTOR AND PROCESS FOR ULTRASONIC TREATMENT OF MATERIALS

Field of the Invention

The present invention relates to the field of ultrasonic treatment of

materials. More particularly, the invention relates to a reactor which

comprises a reactor tube and at least one annular magnetostrictor

transducer element mounted on the external wall of the tube, such that

the transducer is capable of transmitting ultrasound radiation to the

interior of the tube and inducing a vertically flowing cavitation stream

whose vertical axis substantially coincides with the longitudinal axis of the tube.

Background of the Invention

Ultrasounds have many applications in present-day technology in physical

and chemical processes. Some general references are :

1) K.S. Suslick, Sonochemistry, Science, 247, pp. 1439-1445 (23 March 1990);

2) W.E. Buhro et al., Material Science Eng., A204, pp. 193-196 (1995);

3) K.S. Suslick et al., J. Am. Che . Soc, 105, pp. 5781-5785 (1983);

4) Telesonic Co., Products Bulletin. There are several types of ultrasonic reactors. One of them is the loop

reactor, described e.g. in D. Martin and A.D. Ward, Reactor Design for

Sonochemical Engineering, Trans IChe E, Vol. 17, Part A, May 1992, 29,

3. Inside this reactor, a liquid which is to be subjected to ultrasound

treatment, is caused to flow in a closed loop formed by a vessel provided with a stirrer and by a conduit in which the ultrasound generator is

housed.

Also, several transducers may be placed around an elongated enclosure, as

in USP 5,658,534 and USP 6,079,508.

This invention relates to a type of reactor in which the reaction occurs in a

localized space filled with a material that is generally a liquid phase, which

may contain solid particles. By the term "reaction" is meant herein whatever phenomenon is caused or facilitated by the ultrasound radiation, viz. not necessarily a chemical phenomenon, but a physical one or a

combination of the two, as well. A reactor of this type is coupled to a

transducer, wherein an oscillating, generally alternating, magnetic field is

generated by an oscillating, generally alternating, current — hereinafter called "the exciting current." The reactor contains a material to be treated by ultrasound, which will be called hereinafter "reaction material". The

reaction material generally comprises a liquid phase and fills the process

chamber. The transducers of ultrasound devices can be of various types. Most

common transducers are piezo-electric ones. Therein, the generator of the

ultrasound typically consists of a piezo-electric element, often of the

sandwich type, coupled with a horn having a generally circular emitting

face. Piezo-electric transducers, however, have a maximum power of about

2 kW and a low maximum oscillation amplitude dictated by the fragihty of

piezo-electric elements, which tend to break under prolonged working load.

They are also not reliable compared to magnetostrictive transducers, to be

described hereinafter, because their amplitude drifts with operation, causes

breakdowns and lower energy output and has to be manually corrected.

Similar properties are also possessed by electrostrictive materials polarized

by high electrostatic fields.

Another type of transducer is that based on the use of a magnetostrictive

material, viz. a material that changes dimensions when placed in a

magnetic field, and conversely, changes the magnetic field within and

around it when stressed. When a magnetostrictive material is subjected to

a variable magnetic field, the material will change dimensions with the same frequency with which the magnetic field changes.

Magnetostrictive materials, to be quite suitable, must present a sufficiently large magnetic stricture at the temperature at which the ultrasonic reactor is intended to be used. To achieve this, proposals have been made to use

special magnetostrictive materials, for example, alloys containing rare

earth materials: see, e.g., US Patent Nos. 4,308,474, 4,378,258 and

4,763,030. Such alloys are expensive, and, in spite of their better elastic

properties, they suffer major drawbacks, one of them being that they will

break if subjected to relatively high power, e.g. 5 Kw, and at lower levels of

power they do not transduce enough electromagnetic energy to acoustic

energy.

A magnetostrictive transducer must comprise a magnetostrictive element, e.g. a rod or another elongated element, located in a space in which an

oscillating magnetic field is produced. In its simplest form, such a

transducer would comprise a nucleus of magnetostrictive elements and a coil disposed around said element and connected to a generator of

oscillating electric current. However, different forms of transducers can be

devised to satisfy particular requirements: for instance, USP 4,158,368

discloses a toroidal-shaped core of magnetic metal, about which a coil is

wound, which toroid defines with its ends an air gap in which a

magnetostrictive rod is located.

The magnetostrictive transducer transduces the electromagnetic power it receives into ultrasonic power, which it transmits to an irradiating device -

the wave guide or horn. It will be said hereinafter that the horn irradiates the ultrasound into a process chamber, but no limitation is intended by

said expression, which is used only for the sake of brevity. Generally, the

horns of the prior art have a slim frusto-conical shape or a stepped or

exponential shape. In every case, he3^ concentrate the ultrasonic

vibrations and irradiate them from their tip, which is generally circular

and anyway of reduced dimensions. The ultrasonic waves have therefore a

high intensity only at the tip of the horn and spread out from it in a conical

configuration, so that they reach only a part of the process chamber and at

any point of said chamber their intensity is reduced, generally

proportionally to the square of the distance from the horn tip. At their region of maximum intensity various phenomena occur, including heating,

cavitation, evaporation, and so on, which absorb and waste a large portion of the ultrasound energy, resulting in a limited efficiency, which is

generally in the order of 20-30%. Additionally, some desired phenomena

that are produced by the high energy density at the tip of the horn may become reversed at a distance from said tip: for instance, if it is desired to

fragment solid particles, contained in a liquid phase, into smaller ones,

such smaller particles may be produced at the tip of the horn, but then

migrate through the liquid phase and coalesce to some extent at a distance

from said tip, so that the particles finally obtained are not as small as desired. Material treatment of various materials with high intensity ultrasound has

recently achieved a significant role in different industries. Ultrasound

treatment has been applied to particle dispersing, emulsification, mixing

and dissolving of various materials. Other types of processes suitable with

ultrasonic radiation are disclosed in US Patent Nos. 4,131,238, 4,556,467,

5,520,717, 6,035,897,6,066,328 and 6,168,762.

USP 4,071,225 discloses an apparatus for material treatment by the application of ultrasonic longitudinal pressure oscillations, consisting of an

enclosure for material to be treated therein having an interior with two

closely-spaced walls at least one of which is made to oscillate at ultrasonic

frequencies. The spacing between the walls is such that the pressure

oscillations produced at the oscillating wall are reflected by the other wall

before they are attenuated to a negligible value, thus producing waves with periodically changing frequencies and resulting in thorough dispersion of

fine particles in the liquid vehicle. Intense cavitation appears near the

reactor walls of this apparatus and therefore interferes with the transfer of ultrasonic energy to the working volume.

USP 6,079,508 discloses an ultrasonic processor, which comprises a hollow

elongated member with a plurality of transducers fixed to the exterior of the elongated member and a control means for regulating the frequency of the ultrasonic waves produced by the transducers. Although the processor focuses ultrasonic energy onto the center of the reactor, so that cavitation

disperses from the center to the periphery of the reactor, thereby protecting

the reactor from cavitation damage, the processor suffers from several

disadvantages.

Firstly, the transducers are preferably piezoelectric elements which are

produced from a ceramic material, and these types of transducers tend to exhibit flexural failure upon exposure to high oscillating amplitudes.

Piezoelectric transducers do not facilitate automatic control of the

resonance frequency. In piezoelectric transducers a direct relationship

between current and magnetic field is non-existent, and therefore the resonance frequency cannot be automatically adjusted in response to

changes in the system during operation. Therefore if the piezoelectric

material cannot withstand the oscillation amplitude at which it is

vibrating, resulting in cracking or fracture due to the stress acting thereon,

the transducers are liable to fail without warning.

Secondly, the affixing of the transducers by bonding or by soldering induces

a high intensity of ultrasonic energy at those surfaces on the reactor to

which the transducers are affixed. This high intensity results in cavitation to those surfaces on the reactor surface, causing pitting to the irradiator and shortening its life. Thirdly, piezoelectric transducers require a high level of operating voltage due to their relatively high resistivity so that a

predetermined amount of current aj^ pass therethrough.

Co-pending International Publication No. WO 03/012800 by the Applicant

discloses an ultrasound device for the production of material having dimensions in the order of nanometers, comprising a magnetostrictive

transducer and a hollow horn which transmits ultrasonic radiation in a

substantially uniform manner throughout a reaction chamber. In response

to ultrasonic vibrations, the walls of the horn oscillate elastically and

produce alternate compression and decompression. It is desired to provide a

reactor in which the irradiator does not oscillate elastically. It is also

desired to provide a reactor for effecting other processes, in addition to the

production of nano-products.

It is a purpose of this invention, therefore, to provide an ultrasound device

that is free from the drawbacks of the prior art ultrasound devices.

It is another purpose of the invention to provide such an ultrasound, device

which has a higher power than the prior art devices.

It is another purpose of the invention to provide an ultrasound device wherein the transducer is of a new design, namely, a magnetostrictive annular ring. It is an additional purpose of this invention to provide an ultrasonic device

in which intense cavitation does not appear near the reactor walls.

It is a further purpose of the invention to provide such an ultrasound device

comprising a transducer that is durable and has a high oscillation amplitude, up to 45 microns.

It is a further purpose of the invention to provide such an ultrasound device

comprising a transducer that is not of the piezoelectric type.

It is a still further purpose of this invention to provide a reactor for

emulsification, particle dispersion and deagglomeration.

It is a still further purpose of this invention to provide a reactor that is

capable of effecting speedy ultra-fine grinding.

It is a still further purpose of this invention to provide means for the

acceleration of reactions.

Other objects and advantages of the invention will become apparent as the description proceeds. Summary of the Invention

An ultrasound reactor of the present invention comprises a reactor body, a

reactor tube disposed within said reactor body and a magnetostrictor

transducer comprising at least one annular element concentric with, and

mounted to an external wall of said tube, said transducer mounted on said

tube in such a way so as to transmit ultrasound radiation to the interior of

said tube and to induce an upwardly flowing cavitation stream whose

longitudinal axis substantially coincides with a longitudinal axis of said tube.

The diameter of the cavitation stream ranges from 5-30 mm, and preferably from 5-10 mm. The diameter of the cavitation stream is

preferably essentially uniform throughout the length of the transducer.

Each magnetostrictor element preferably comprises a plurality of plates

made from a ferromagnetic material. In one aspect, the ferromagnetic

material is permendur. In one aspect, each magnetostrictor element ■

preferably comprises 150-300 plates, each of which has a thickness of

approximately 0.2 mm. Each plate has an inner diameter ranging between

25-200 mm.

In another aspect, the total length of the magnetostrictor transducer is approximately 40 cm. An outer wall of the reactor tube and an inner wall of each magnetostrictor

element are preferably burnished, such that the outer diameter of the

reactor tube has a dimension of 40 microns greater than the inner

diameter of each magnetostrictor element.

Each magnetostrictor element is mounted on the reactor tube following a

hot adjustment.

The ultrasound reactor preferably further comprises a cylindrical absorber mounted on the outer periphery of each transducer element. As referred to

herein, an "absorber" is an element that partially absorbs ultrasonic

radiation, while the remaining energy that is not absorbed is reflected.

The absorber is preferably made of a suitable material such that an ultrasonic wave outwardly radiating from the transducer will be partially

absorbed in said absorber and partially inwardly reflected into the reactor tube. As a result, a reflected inwardly directed wave has the same

frequency and amplitude as an outwardly radiating wave. In one aspect,

the material of the absorber is partially absorbent (x %) and partially

reflective (100-x %) and may be selected from the group of rubber and polyurethane. The efficiency of the reactor ranges from 30 to 40%. With the addition of

the absorber, the efficiency of the reactor ranges from 60 to 80%.

The reactor is capable of irradiating a range of 1 to 8 kW, and preferably^*

at least 2.5 kW, of ultrasonic energy into the reactor tube. The frequency

range of the ultrasonic energy preferably ranges from 8 to 25 kHz.

The transducer is cooled by a cooling liquid flowing between the

transducer and reactor body.

The reactor preferably further comprises a stirrer, said stirrer capable of operating at a pressure of up to 5 kg/cm² within liquid introduced into the

reactor tube. An impeller of the stirrer is disposed within the reactor tube,

preferably at a location radially outwards of the cavitation stream. The

impeller is preferably provided with two vertical blades and two blades

inclined at 45 degrees with respect to the vertical blades.

The present invention also comprises a processor for material processing in the aforementioned ultrasonic reactor.

The processor preferably comprises a generator for generating waves of alternating current at variable ultrasonic frequencies ranging from 8 to 25

kHz, the frequency and intensity of which is controllable in response to process conditions. The generator preferably comprises means for generating and maintaining electromagnetic energy at a predetermined

resonance frequency essentially throughout the duration of a process.

In one aspect, the processor preferably further comprises a pump for

pumping liquid into the reactor tube at a pressure of up to 8 atmospheres.

In another aspect, the processor preferably further comprises a tank for

storing cooling liquid and means for controlling the temperature of the

cooling liquid.

The present invention also involves a process for ultrasonic treatment of a

reaction material, comprising the following steps:

a) Providing an ultrasound reactor comprising a reactor body, a

reactor tube having a longitudinal axis disposed within said reactor

body and a magnetostrictor transducer comprising at least one

annular element concentric with, and mounted to an external wall

of said tube;

b) Introducing a reaction material comprising at least one liquid into said reactor tube;

c) Transmitting waves of electromagnetic energy at ultrasonic

frequencies to said transducer; d) Allowing said transducer to transduce the electromagnetic

energy into ultrasonic energy and to irradiate waves of said

ultrasonic energy;

e) Allowing inwardly directed ultrasonic waves to converge at

approximately the longitudinal axis of said reactor tube, thereby

inducing an upwardly flowing cavitation stream within said reaction

material, the longitudinal axis of said cavitation stream

substantially coinciding with the longitudinal axis of said reactor

tube;

f) Allowing said cavitation stream to cause physical changes to

said reaction material; and g) Removing ultrasonically treated reaction material from said

reactor tube.

Inwardly directed ultrasonic waves radiating from each transducer

element propagate through a wall of the reactor tube and preferably induce a zone of low intensity cavitation outwardly disposed relative to the

cavitation stream, said zone being defined by a boundary, located radially

inwards from said wall.

The diameter of the cavitation stream is essentially uniform and continuous throughout the length of the transducer. The length of the

cavitation stream is approximately equal to the length of the transducer. The cavitation stream is preferably fountain-like, due to high-intensity

implosions that are induced thereat.

The process preferably further comprises the step of allowing an ultrasonic

wave radiating outwardly .from the transducer to be partially inwardly

reflected by a cylindrical absorber mounted on each element. In one aspect,

an outwardly radiating ultrasonic wave is absorbed and reflected by the

absorber. In another aspect, a reflected inwardly directed wave has the

same frequency and amplitude as an outwardly radiating wave.

The process is selected from the group of ultra-fine grinding, agitation, dispersion, emulsification and de agglomeration.

Ultra-fine grinding is preferably effected with particles of a hardness of up

to 9 in the Mohs Scale. The particles are for example selected fro the group of alumina, silver, silica, and ceria.

In one process, alumina particles having a size ranging from 1-6 microns

are reduced in size to a range of 0.1-1.5 microns in 1 hour.

Brief Description of the Drawings

In the drawings: Fig. 1 is a cross sectional view of an ultrasonic reactor, in

accordance with the present invention;

Fig. 2 is a perspective view of a magnetostrictor element which is

mounted on a reactor tube, in accordance with the present

invention;

Fig. 3 is a simulation of a focused cavitation stream, on which

the present invention is based;

Fig. 4 is a cross sectional view of a stirrer; and

Fig. 5 is a schematic diagram of a processor used in accordance

with the present invention.

Detailed Description of Preferred Embodiments

Fig. 1 illustrates a reactor according to the present invention, which is

generally designated as 30. Reactor 30 comprises body 1, e.g. stainless steel of 1.5 mm thickness, into which reactor tube 9, e.g. made of ASTM

306L stainless steel, is insertable. The interior of reactor tube 9 defines process chamber 19, and the volume external to reactor tube 9 and internal

to body 1 constitutes cooling chamber 26. An annular focused transducer

which is designated by numeral 11 is mounted onto the external wall of

reactor tube 9 so as to be symmetric with respect to the axis of reactor tube 9. Reactor bottom 18 is provided with openings for cooling liquid inlet pipe 3

and cooling liquid outlet pipe 4, wherein the diameter of outlet pipe 4 is

greater than inlet pipe 3 to thereby produce a pressure differential which

facilitates the circulation of the cooling liquid. A centrally located opening is formed in bottom 18 for the insertion therethrough of fitting 14, which is

welded thereto, on which cylinder 15 is mounted. Cylinder 15 functions as

an electrical insulator for tube 9, and is made of e.g. rigid plastic reinforced

by glass fibers. Cylinder cover 16, which is made of a massive material, e.g.

stainless steel, in order to withstand the high pressures that are induced during material processing, is welded to the lower inner wall of reactor

tube 9. Disc 17, which is mounted on cylinder cover 16, is made of acoustic

rubber and has an outer diameter essentially equal to the inner diameter

of reactor tube 9, to prevent the flow of processed material within the

clearance between tube 9 and cover 16. Body 1 is formed with hermetically

sealed cable conduit 8 for coil wires 29.

Annular bottom flange 2, the inner diameter of which is slightly larger

than the outer diameter of transducer 11, is welded to the top of body 1.

Top flange 10 is connected to bottom flange 2, e.g. by a plurality of cotter

pins 5 and corresponding nuts 6, and is sealed by means of rubber packing 7. Top flange 10 is also annular, and the diameter of its opening is equal to

the outer diameter of reactor tube 9. Cover 21 is connected to top flange 10, e.g. by a plurality of cotter pins 24 and corresponding nuts 25. Reactor cover 21 is formed with several openings to allow for the communication

with process chamber 19 of feed tube 20, secondary tube 22, thermocouple

23, a manometer (not shown) and stirrer 39.

A suspension or any other liquid under pressure is injected into process

chamber 19, via feed tube 20, which is welded to cover 21. The

ultrasonically treated liquid may be automatically discharged from process

chamber 19 by means of tube 22, as a result of the pressure differential that results from the difference in diameters between feed tube 20 and the

narrower secondary tube 22. Alternatively, the ultrasonically treated

liquid may be discharged semi-automatically, viz. by the depressing of control buttons. Accordingly, secondary tube 22 is closed as liquid is

introduced into process chamber 19 by feed tube 20. Following the

ultrasonic treatment and the ensuing reaction, a pressurized fluid, e.g. air

at 5 atmospheres, is injected into tube 22 and entrains the ultrasonically

treated liquid through tube 20.

Transducer 11 is cylindrical and is advantageously divided into four elements 28a-d to allow for, on one hand efficient ultrasonic transmission

into process chamber 19 and on the other hand increased heat transfer to the surrounding cooling liquid to prevent any transducer overheating or

failure. It will be appreciated that the transducer may be comprised of one or more elements. The reactor efficiency is defined as the percentage of ultrasonic energy

that is utilized in the process chamber, i.e. the ratio between the

ultrasonic energy generated by transducer 11 and the ultrasonic energy

influx into process chamber. 19. In order to maximize reactor efficiency, the energy influx into the process chamber needs to be maximized and the

energy loss from the transducers needs to be minimized. Energy influx is

maximized by focusing inwardly radiating ultrasonic energy onto

longitudinal axis 32 of reactor tube 9. The reactor efficiency is further

increased by the addition of cylindrical absorber 12, which is mounted on the outer periphery of each transducer element 28. The efficiency of the

reactor ranges from 60-80%, and ranges from 30-40% when absorbers are

not emplo3'ed.

Absorber 12 is made of a partially absorbent material, e.g. rubber or

polyurethane, which results in absorbance and reflectance of incident

ultrasonic radiation, to reduce energy loss to cooling chamber 26.

Consequently, an outwardly radiating ultrasonic wave, that is, in the direction of cooling chamber 26, will be partially absorbed by absorber 12

and then reflected into process chamber 19. A reflected inwardly directed

wave has the same frequency and amplitude as an outwardly radiating wave. Since the absorbent material causes heat dissipation of the absorbed

ultrasonic energy, the transducer is advantageously divided into a plurahty of elements so that the top and bottom of each absorber 12 may

be cooled by the surrounding cooling water. Absorber 12 also adds to the

longevity of the reactor 30 by preventing pitting at the cooling water

jacket, which would have resulted from cavitation induced by an

outwardly directed wave.

Fig. 2 illustrates an element 28 of transducer 11, which is mounted on the outer wall of reactor tube 9. Each magnetostrictor element 28 is preferably

a magnetostrictor and comprises a plurality of annular permendur plates

31, although any ferromagnetic alloy is suitable as well for the production

of the transducer. Each element 28 comprises, by example, of 150-300 plates, with each plate having a thickness of approximately 0.2 mm, such

that the total length of four transducer elements is by example approximately 40 cm. Each plate 31 has an inner diameter ranging from

25-200 mm. Each plate 31 is formed with a plurality of apertures for the

insertion of electrical wire 29 therethrough so that a coil 13 may be formed

by the wire and wound about magnetostrictor element 28.

Each plate 31 is cut with a general tolerance, including its thickness and

outer diameter, of 0.05 mm, so that all plates of element 28 will be aligned. In order to generate a homogeneous magnetic field in the reactor, the magnetic properties of each plate, including hysteresis, induction, magnetic flux density and magnetic field strength, need to be substantially identical. Plates with similar magnetic properties are placed adjacent one

to another, and then the plates undergo thermal treatment in a vacuum

furnace so that all of the plates will have substantially identical magnetic

properties. The plates are held under pressure in order to retain their

dimensions during the thermal treatment. Following the thermal

treatment, the plates are bonded together, e.g. with ceramic paste or any

other suitable bonding material, and baked in a vacuum furnace at a

vacuum of approximately 10"⁵ torr in order to prevent oxide contamination thereof. The plates are baked at a temperature no greater than 280°C so as

not to weaken the bonding. This process is then repeated for the other

magnetostrictor elements.

The outer wall of reactor tube 9 and the inner wall of all of the magnetostrictor elements are burnished, such that the outer diameter of

reactor tube 9 has a dimension of 40 microns greater than the inner

diameter of each magnetostrictor element 28. A so-called hot adjustment is

then performed, whereby the magnetostrictor elements are heated to a .

temperature of approximately 280°C and reactor tube 9 is cooled to a temperature of approximately -75°C, such as. by application of liquid

nitrogen, so that the magnetostrictor elements may be mounted by pressure on the reactor tube with the necessary adjustment tension. The

tension imposed on the reactor tube enables the latter to undergo elastic deformation, and thereby transfers substantially all of ultrasonic energy transmitted by the transducer. It will be appreciated that with the

aforementioned manufacture and assembly process, ultrasonic energy may

be transmitted to reactor tube 9 and focused along longitudinal axis 32

thereof with a minimal amount of losses due to the tight fit between the

magnetostrictor elements -and the reactor tube. As a result, uniform

cavitation may be achieved along longitudinal axis 32.

Following the mounting of the magnetostrictor elements, coil 13 is wound

about transducer 11, such that conducting wire 29 passes through each of

the vertically stacked plates 31 before being wound for an additional loop,

and then is directed through cable conduit 8 (Fig. 1). Top flange 10 is then connected to bottom flange 2, after which cover 21 is connected to top

flange 10.

Operationally, current, which is produced by generator 45 and passes through coil 13, generates a magnetic field parallel to the plane of each

permendur plate 31. The magnetic field in turn induces each transducer

element 28 to vibrate, and as a result transducer 11 transduces the

electromagnetic power it receives into ultrasonic power. Ultrasonic waves

are transmitted radially inwards, some of which after being reflected by absorber 12 (Fig. 1), through the wall of reactor tube 9. Referring now to Fig. 3, which is a simulation of a phenomenon on which

the present invention is based, inwardly directed ultrasonic waves

radiating from each permendur plate 31 (Fig. 2) propagate through wall 33

of reactor tube 9 and induce zone 34 of low intensity cavitation within the

liquid medium that has been introduced into the reactor tube. Zone 34 of

low intensity cavitation is defined by boundary 36, located radially inwards

from wall 33. Cavitation is inhibited between wall 33 and boundary 36 due

to the hydrostatic pressure that results from inwardly radiating

compression waves which compress the liquid medium from wall 33 to boundary 36. Since cavitation is not induced at wall 33, erosion of the

reactor tube wall is substantially non-existent.

As the inwardly directed ultrasonic waves continue to propagate, they converge at axis 32 and induce a localized compression wave, due to the

high intensity energy influx thereat. The localized compression wave

results in a focal vaporous transient cavitation region concentric to axis 32,

in which bubbles smaller than the resonant size are created. These bubbles

are filled with vapor and grow as the pressure falls through the rarefaction

half-cycle of the ultrasonic wave and then collapse rapidly. Consequently,

fine and uniform upwardly flowing cavitation stream 37, having a similarity to a fountain due to the rapidly rising high-intensity implosions

is induced along axis 32 and is capable of causing physical changes to the liquid medium. Cavitation stream 37 has a diameter of approximately 5-30 mm, and preferably 5-10 mm, depending upon the frequency of the

ultrasonic waves and the diameter of reactor tube 9. '

The length of cavitation stream 37 is substantially equal to the length of

transducer 11 (Fig. 1). Since ultrasonic waves inwardly radiate from each

plate 31 (Fig. 2) and cause high-intensity cavitation at the center of the

corresponding plate, coincident with axis 32, the rapidly collapsing bubbles

mix along the length of the transducer and result in a continuous

cavitation stream. When transducer 11 is comprised of separate elements 28 as depicted in Fig. 1, the spacing between adjacent elements is

sufficient to provide cooling of the elements while retaining continuity of

cavitation stream 37. It has been surprisingly found that even though the

diameter of low intensity cavitation zone 34 is reduced along the spacing

between adjacent elements 28 due to the lack of plates 31, cavitation

stream 37 continues to be continuous with a uniform diameter along its

entire length because of the high momentum of the rapidly collapsing bubbles.

In a preferred design, the diameter of reactor tube 9 is inversely

proportional to the ultrasonic frequency. The wavelength λ of the ultrasonic

radiation is given by λ = v/γ, wherein γ is the frequency and v is the velocity

of the ultrasound propagation in the material of which reactor tube is made. For example, a process chamber having a volume of 5 L is suitable for an ultrasonic frequency range of 8-12 kHz, and a process chamber

having a volume of 1 L is suitable for an ultrasonic frequency range of 18-

22 kHz.

The intensity I of the ultrasound radiation corresponding to an energy W,

assumed to be uniformly distributed, is I = W/S, wherein S is the area from

which the ultrasound is irradiated. In ideal cases, the intensity I can be

calculated from the formula I = vpγ²A², wherein v is the ultrasound velocity

in the environment, p is the density of the environment, γ is the ultrasound

frequency and A the ultrasound amplitude. In the reactor of the present invention, the intensity of the energy influx into the process chamber

ranges from 2.6-6.6 W/cm² and the intensity per volume of the energy

influx into the process chamber ranges from 0.7-1.8 W/cm³.

In order to maximize the energy influx into the process chamber, the

intensity and amplitude of the ultrasonic radiation should preferably be as large as possible. The present invention is capable of operating at a power

level ranging from 1-8 kW, and preferably greater than 2.5 kW, which is

irradiated into the process chamber, although the maximum power level

should be limited to 5 kW to ensure structural integrity of the transducer.

Prior art devices are not capable of providing an influx of 5 kW into a corresponding reactor, and therefore are not capable of achieving speedy

ultra-fine grinding of individual particles. The reactor of the present invention is capable of achieving the ultra-fine grinding of a single particle

having a hardness of up to 9 in the Mohs Scale, as well as agitation,

dispersion, emulsification and deagglomeration, i.e. separation of

agglomerated particles.

Fig. 4 illustrates stirrer 39. At times the ultrasonic energy generated by

transducer 11 is not sufficient for satisfactorily suspending the powder

particles in the process chamber. Stirrer 39 is used to prevent

sedimentation, and is capable of operating at a pressure of up to 5 kg/cm²

within process chamber 19. The stirrer is disposed radially outward from axis 32, within low intensity cavitation zone 34 (Fig. 3), and ameliorates

dispersion of relatively heavily suspended particles.

Stirrer 39 comprises motor 43, drive means for imparting torque to shaft

46, and impeller 48. Shaft 46 is made, by example, of Ti-4V-6A alloy. Impeller 48 is made from a non-corrosive material such as polypropylene,

and is provided with two vertical blades and two blades inclined at 45

degrees with respect to shaft 46. This configuration of blades produces a

turbulent stream of suspension particles and directs the flow from the top

to bottom of process chamber 19. Motor 43, by example, is a 24V direct current motor that is capable of generating 200 W at a rotational speed of

2000 rpm. Fig. 5 schematically illustrates a continuousry feeding processor, generally

indicated by 50, for effecting several processes with the use of reactor 30,

according to the present invention. Ultrasonic generator 45 transmits

waves of alternating current at ultrasonic frequencies, e.g. ranging from 8

to 25 kHz, the frequency and intensity of which can be set and controlled in

response to process conditions, to the magnetostrictor elements by means of a cable passing through conduit 8. After an allowable range of frequency

is programmed, ultrasonic generator 45 is capable of generating an initial

frequency of ultrasonic energy and of regulating the generated energy in

order to achieve a predetermined resonance frequency. The generator is

capable of maintaining the resonance frequency throughout the remaining

processing time.

Labyrinth pump 48 pumps liquid suspension under pressure, of up to 8

atmospheres, the flow rate and pressure of which can be regulated by

pump 48 or by a capillary tube (not shown), depending on the characteristics of the suspension or the required process duration, through

feed tube 20 into reactor 30 to undergo ultrasonic treatment within its

process chamber. Ultrasonically treated liquid, which is withdrawn from

reactor 30 by discharge tube 51, may be released from storage vessel 52 for sampling and further processing by opening valve 53. Valve 53 also allows for the injection of liquid into storage vessel 52 during the next processing

run. Cooling liquid, the temperature of which is set and controlled by means of thermostat 51, is retained in tank 49, and is introduced through

inlet pipe 3 into the cooling chamber. The cooling liquid is discharged from

the cooling chamber via outlet pipe 4.

Processor 50 is suitable for producing ultra-fine powder. For example,

alumina particles ranging from 1-6 microns may be reduced in size to a

range of 0.1-1.5 microns in 1 hour with implementation of the present

invention. Similarly the present invention may be utilized for particle

dispersing, emulsification, agitating and dissolving various materials.

Example 1

Ultra-fine Grinding 85 g of silver particles are mixed with 780 ml of deionized water. The

suspension is fed into a 1 L chamber, and is treated for a period of 3 hours

at a pressure of 3.5 atmospheres with an influx of ultrasonic energy having a power of 3 kW, wherein the temperature within the process chamber is

44°C.

The following is the particle size distribution, in microns, wherein the

subscript refers to the percentage of total particles having a size equal to or less than the listed value:

. Example 2

Ultra-fine Grinding

1000 g of silica particles for use in glazed coatings are added to 4600 ml of

deionized water. The suspension is fed into a 5 L chamber, and is treated

for a period of 90 minutes at a pressure of 3.2 atmospheres with an influx

of ultrasonic energy having a power of 4.6 kW, wherein the temperature

within the process chamber is 42°C.

The following is the particle size distribution in microns, wherein the

subscript refers to the percentage of total particles having a size equal to or

less than the listed value:

Example 3

Ultra-fine Grinding

600 g of ceria particles (CeOz) needed for Chemical Mechanical Planarization (CMP) are added to 4600 ml of deionized water. The suspension is fed into a 4.5 L chamber, and is treated for a period of 60

minutes at a pressure of 3.0 atmospheres with an influx of ultrasonic

energy having a power of 4.6 kW, wherein the temperature within the

process chamber is 32°C.

The following is the particle size distribution in microns, wherein the

subscript refers to the percentage of total particles having a size equal to or

less than the listed value:

Example 4

Ultra-fine Grinding

1000 g of silica particles, a cristalolite mineral, are added to 4600 ml of

deionized water. The suspension is fed into a 4.5 L chamber, and is treated

for a period of 90 minutes at a pressure of 4.0 atmospheres with an influx

of ultrasonic energy having a power of 4.6 kW, wherein the temperature within the process chamber is 45°C.

The following is the particle size distribution in microns, wherein the subscript refers to the percentage of total particles having a size equal to or less than the Hsted value:

Example 5

Homogeneous Dispersion

42 g of polytetrafluoroethane (PTFE) are vigorously mixed together with

600 ml of distilled water within an intermediate container to thereby

prepare a first suspension, which is then fed into a 5 L process chamber. A

second suspension is prepared from 273 g of active carbon, 29 g of metal

oxide and 1400 ml of distilled water. The second suspension is then fed to the process chamber, in addition to the first suspension, to thereby prepare

a slurry. The slurry is treated for a period of 30 minutes with an influx of

ultrasonic energy having a power of 3 kW, wherein the temperature within

the process chamber is ambient temperature. The resulting dispersion has

a degree of homogeneity of 98%.

Example 6

Emulsion

2.5 L of deionized water and 2.5 g of polyoxyethylene sorbitan (a surface

active material of an organic acid type) are fed to a process chamber of 5 L.

2.5 L of vegetable oil are then gradually added for a period of 10 minutes to the process chamber and vigorously mixed with a stirrer until achieving a

pressure of 2 atmospheres, to thereby produce a suspension. The suspension is treated for a period of 10 minutes with an influx of ultrasonic

energy having a power of 5 kW, wherein the temperature within the

process chamber is 40°C. A stable emulsion of 50% vegetable oil resulted.

While some embodiments of the invention have been described by way of

illustration, it will be apparent that the invention can be carried into

practice with many modifications, variations and adaptations, and with

the use of numerous equivalents or alternative solutions that are within

the scope of persons skilled in the art, without departing from the spirit of the invention or exceeding the scope of the claims.

Claims

1. Ultrasound reactor comprising a reactor body, a reactor tube disposed

within said reactor body and a magnetostrictor transducer comprising

at least one annular element concentric with, and mounted to an

external wall of said tube, said transducer mounted on said tube in

such a way so as to transmit ultrasound radiation to the interior of

said tube and to induce an upwardly flowing cavitation stream whose

longitudinal axis substantially coincides with, a longitudinal axis of

said tube.

2. Ultrasound reactor of claim 1, wherein the diameter of the cavitation

stream ranges from 5 to 30 mm.

3. Ultrasound reactor of claim 2, wherein the diameter of the cavitation

stream is essentially uniform throughout the length of the transducer.

4. Ultrasound reactor of claim 2, wherein the diameter of the cavitation

stream ranges from 5 to 10 mm.

5. Ultrasound reactor of claim 1, wherein each magnetostrictor element comprises a plurality of plates made from a ferromagnetic material.

6. Ultrasound reactor of claim 5, wherein the ferromagnetic material is

permendur.

7. Ultrasound reactor of claim 5, wherein each magnetostrictor element

comprises 150 to 300 plates.

8. Ultrasound reactor of claim 6, wherein with each plate has a thickness

of approximately 0.2 mm.

9. Ultrasound reactor of claim 8, wherein the total length of the

magnetostrictor transducer is approximately 40 cm.

10. Ultrasound reactor of claim 5, wherein with each plate has an inner

diameter ranging from 25 to 200 mm.

11. Ultrasound reactor of claim 1, wherein an outer wall of the reactor

tube and an inner wall of each magnetostrictor element are burnished,

such that the outer diameter of the reactor tube has a dimension of 40

microns greater than the inner diameter of each magnetostrictor

element.

12. Ultrasound reactor of claim 11, wherein each magnetostrictor element

is mounted on the reactor tube following a hot adjustment.

13. Ultrasound reactor of claim 1, further comprising a cylindrical

absorber mounted on the outer periphery of each transducer element.

14. Ultrasound reactor of claim 13, wherein the absorber is made of a suitable material such that an ultrasonic wave outwardly radiating

from the transducer will be partially absorbed in said absorber and

partially inwardly reflected into the reactor tube.

15. Ultrasound reactor of claim 14, wherein the absorber is selected from

the group of rubber and polyurethane.

16. Ultrasound reactor of claim 14, wherein a reflected inwardly directed wave has the same frequency and amplitude as an outwardly radiating wave.

17. Ultrasound reactor of claim 1, wherein the efficiency of the reactor

ranges from 30 to 40%.

18. Ultrasound reactor of claim 13, wherein the efficiency of the reactor ranges from 60 to 80%.

19. Ultrasound reactor of claim 1, wherein the power range of ultrasonic

energy which is irradiated into the reactor tube is 1 to 8 kW.

20. Ultrasound reactor of claim 19, wherein the power range of ultrasonic

energy which is irradiated into the reactor tube is 2.5 to 5 kW.

21. Ultrasound reactor of claim 1, wherein the frequency range of

ultrasonic energy which is irradiated into the reactor tube is 8 to 25

kHz.

22. Ultrasound reactor of claim 21, wherein the diameter of the reactor

tube is inversely proportional to the frequency of the ultrasonic energy.

23. Ultrasonic reactor of claim 1, wherein the transducer is cooled by a

cooling liquid flowing between the transducer and reactor body.

24. Ultrasound reactor of claim 1, further comprising a stirrer.

25. Ultrasound reactor of claim 24, wherein an impeller of the stirrer is

disposed within the reactor tube, at a location radially outwards of the cavitation stream.

26. Ultrasound reactor of claim 25, wherein the impeller is provided with

two vertical blades and two blades inclined at 45 degrees with respect

to the vertical blades.

27. Processor for material processing in an ultrasonic reactor according to

claim 1 to 26.

28. Processor of claim 27, further comprising a pump for pumping liquid

into the reactor tube at a pressure of up to 8 atmospheres.

29. Processor of claim 27, further comprising a tank for storing cooling

liquid and means for controlling the temperature of the cooling liquid.

30. Processor of claim 27, further comprising a generator for generating

waves of alternating current at variable ultrasonic frequencies ranging from 8 to 25 kHz, the frequency and intensity of which is

controllable in response to process conditions.

31. Processor of claim 30, wherein the generator comprises means for

generating and maintaining electromagnetic energy at a predetermined resonance frequency essentially throughout the duration of a process.

2. Process for ultrasonic treatment of a reaction material, comprising the

following steps:

a) Providing an ultrasound reactor comprising a reactor body, a

reactor tube having a longitudinal axis disposed within said

reactor body and a magnetostrictor transducer comprising at

least one annular element concentric with, and mounted to an

external wall of said tube; b) Introducing a reaction material comprising at least one liquid

into said reactor tube;

c) Transmitting waves of electromagnetic energy at ultrasonic

frequencies to said transducer;

d) Allowing said transducer to transduce the electromagnetic energy into ultrasonic energy and to irradiate waves of said

ultrasonic energy; e) Allowing inwardly directed ultrasonic waves to converge at

approximately the longitudinal axis of said reactor tube, thereby inducing an upwardly flowing cavitation stream within said

reaction material, the longitudinal axis of said cavitation stream substantially coinciding with the longitudinal axis of said

reactor tube; f) Allowing said cavitation stream to cause physical changes to said reaction material; and g) Removing ultrasonically treated reaction material from said

reactor tube.

33. Process of claim 32, wherein inwardly directed ultrasonic waves

radiating from each transducer element propagate through a wall of

the reactor tube and induce a zone of low intensity cavitation

outwardly disposed relative to the cavitation stream, said zone being

defined by a boundary, located radially inwards from said wall.

34. Process of claim 32, wherein the diameter of the cavitation stream is

essentially uniform and continuous throughout the length of the

transducer.

35. Process of claim 32, wherein the length of the cavitation stream is

approximately equal to the length of the transducer.

36. Process of claim 32, wherein the cavitation stream is fountain-like,

due to high-intensity implosions that are induced thereat.

37. Process of claim 32, further comprising the step of introducing a cooling liquid within the reactor body, external to the reactor tube,

wherein said cooling liquid flows within spacing between each

adjacent element.

38. Process of claim 32, further comprising the step of allowing an

ultrasonic wave radiating outwardly from the transducer to be

partially inwardly reflected by a cylindrical absorber mounted on each

element.

39. Process of claim 38, wherein a reflected inwardly directed wave has the same frequency and amplitude as an outwardly radiating wave.

40. Process of claim 32, further comprising the step of stirring the reaction

material.

41. Process of claim 32, wherein the process is selected from the group of

ultra-fine grinding, agitation, dispersion, emulsification and

deagglomeration.

42. Process of claim 41, wherein ultra-fine grinding is effected with

particles of a hardness of up to 9 in the Mohs Scale.

43. Process of claim 42, wherein the particles are selected from the group

of alumina, silver, silica, and ceria.

4. Process of claim 43, wherein alumina particles having a size ranging

from 1 to 6 microns are reduced in size to a range of 0.1 to 1.5 microns

in 1 hour.