Raul Rink¹, Dr. Sergey Shestakov²

¹Oil Tech Production OY, Estonia, 11913, Tallinn, Kressi Tee, 34 A,

E-mail: info@oiltech-nordic.eu

²Moscow state university of Technologies and Management, Russian

Acoustical Society, Russia, 109004, Moscow, Earth wal, 73,

E-mail: sdsh@mail.ru

The Cavitation Reactor with Symmetric Solid-State Oscillatory System of the Acoustic Cell for Processes of the Food Sonochemistry

In comparison with well-known, describes in the technical and patent literature counterparts acoustic cavitation reactors suitable for use in food sonochemistry, consider examples of the executed on the basis set forth in the [1] of theoretical bases of projects reactors, mastering the production of which started in Estonia and prepared in the republic of Belarus. They are intended for physic-chemical treatment by ultrasonic cavitation true and colloidal solutions, as well as disperse systems (emulsions and suspensions) by initiating in them the sonochemical reactions and cavitation erosion of their phases. In addition to these solutions, emulsions and suspensions in these reactors can be processed chemically pure water and other chemically pure solvents. In them is the impact on their physical-chemical state of liquids, the intensification of in them chemical reactions and initiation of new by changing the dipole-dipole and ion-dipole interactions in their environments and phases, as well as increasing the dispersion of multiphase systems, the destruction of bacteria and stabilization of their number.

A typical example of a well-known classical design of the reactor is, for example, a device, patented in the united states for nearly half a century ago, and is called the "vibrations device" [2]. It is equipped the reflecting wall, mounted so that his surfase is aimed in the liquid, is in one plane with the surface of ultrasound radiator, surrounding it. That is, surface of the front of ultrasonic wave, containing surface of the radiator, coincides with surface of the wall, which has contact with liquid. The second of reflective wall, located opposite first and radiator in the direction of propagation wave is part of the corps of reactor. But the energy of cavitation is the cause of erosion fracture of solids [3]. Erosion may be subjected the structural elements design of cavitation reactor. The products of erosion, falling in the treated liquids, can irreversibly change its physical and chemical properties, which are strictly not allowed in the processing of medicinal and food raw materials, as well as the medical products and food products.

Housing this device is exposed to cavitation erosion, caused by his contact with the perimeter of the cavitation areas, which related to it work [4]. In addition, because the reflective the wall which is lying opposite the radiator is part of the hard corps, it not may fully reflect the elastic wave and its fluctuations partially transmitted the on the corps, which leads to energy losses. The surface of the radiator and the walls are exposed to erosion under the action of cavitation, which occurs near them.

Known the analog of reactor, consisting from a working camera (the corps), the source of ultrasound for creating cavitation and reflective wall, between which is placed elastic layer, absorbing penetrating part of the ultrasonic wave and turns its into heat [5]. In such a device rigid mechanical connection between reflective wall and the corps is absent, which allows you to dissipate some of the energy of ultrasonic waves in the film between them, and prevent the dissipation of the energy of elastic waves on the cavitation near her. However, here, as in the previous case, the walls of the corps are exposed to cavitation, formed in the volume of fluid. Cavitation near the sides of the corps causes erosion of the metal from which it is made and the ingress of its ions in the work environment, which prevents the use of a similar reactor in the field of food sonochemistry.

There is the design of the cavitation reactor, representing a camera, the volume of which is limited surface wall of corps, at least one of the reflective wall, at least one emitter of acoustic waves and is filled with in the work the treated liquids [6]. The size of the corps in the plane of any front of the elastic wave is equal to the minimal positive root of a transcendental equation, obtained by equating to zero singular generalized function, which approximates the function of the integral of a dimensionless stiffness [4]. The root of this equation determines the size of the housing, in which the potential energy density of the cavitation at its surface is equal to zero. That is the erosion of the walls of the corps in such a reactor is completely excluded. However, the surface of the transducer ultrasonic waves still exposed to erosion, since cavitation on its surface occurs because of the difference of the amplitude-phase characteristics of radiated and fall on him the wave reflected от opposite wall, about the means and ways of equalization of which in the description of this reactor nothing no.

Known similar in design cavitation reactor [7], where made the alignment by the largest vibration amplitudes of fluids near reflective walls and these walls by selection the acoustic resistance of the absorbing material layer between them and the housing. This allowed to avoid cavitation also have the radiator surface of ultrasonic waves and thus exclude it erosive destruction. But this reactor also has the disadvantage of preventing its use for the treatment of liquids in the food processing industry. The alignment of the amplitudes of the oscillations of a radiator and reflecting walls and liquid near them here achieved through the organization the regime, such as energy regime of traveling wave, when the whole falling on the wall of the wave passes through it, are not reflected back inside the reactor. The wall is here called reflective only conditionally. A portion of the energy of wave which transmitted through the wall into layer between her and the corps, scattered on the internal friction of the material from which made this layer. And only part of the energy transferred by a wave from the emitter to the treated liquid will be dissipated in it on cavitation. Thus, the exclusion erosion of the material reactor is achieved here due to the loss of part energy of the wave radiated in the fluid of its source. For this additional costs are required energy, replenishes these losses. And diffused in the layer between wall and the corps the energy is converted into heat, which is gradually heats up liquid, lowers threshold of cavitation and intensity of the latter [2]. This is partly remedied in the acoustic cell cavitation reactor, where reflective wall is regarded as an integral part of oscillatory system from the source of the waves and located in this cell the fluid [8]. It is a solid-state mechanical element with zero reactive components of acoustic impedance and high mechanical q-factor. This allows to avoid thermal scattering, as the power, the equivalent to dissipated on the internal friction, with her help of is transformed into a reactive power, which can be compensated on the electrical side included in the source of ultrasound electro acoustic transducer, which is acoustically connected with the liquid and reactive sonoabsorber in the one oscillatory system. But still, a part of this system draws part of the oscillation power into the reactive power, that is, brings it out the cavitation process in the other part of it - in the liquid, for whom, and there is a cavitation reactor.

Such disadvantage is absent in the other reactor, where all the parts of the vibrating system of acoustic cell is solid-state [9]. In its design the surface to facing the transducer reflective wall and itself emitting surface belongs to the general solid-state resonator with resonance frequency equal to the frequency of fluctuations of a radiating surface and posted near antinodes of vibrational displacement, located between them fluid at a frequency of resonance of its fluctuations. With such a cell in principle are possible to implement a resonant mode of operation together with a liquid. It is clear that the wave energy dissipated on the cavitation is the maximum. However, in [9] there are no requirements to the structures providing such a regime. There installed the distance between the planes of the resonator with which the oscillations propagate in fluid equal to the length of their waves in it. But the distance between antinodes of oscillatory displacement of solid-state resonator which are near them is specified equal to half of length of a wave in the metal. That is, these surfaces are commit fluctuations in the fluid in opposite directions. Using the method of summing up the oncoming waves, in which acts a cavitation, as described in [10], we can calculate that in this case the average on height of the reactor, the amplitude of the sound pressure of the fluid in it will reach 87% from the average amplitude of the sound pressure in each of the waves from these surfaces, if they escape into an infinite half-space (see fig. 2). This is a disadvantage, the main reason of which is that it created the conditions for the resonance of the solid part of the oscillatory system of the acoustic cell of reactor but conditions of resonance in the processed liquid is not provided. Therefore, the power of a cavitation is not maximal possible, which are may be get from these sources of fluctuations.

Considering the volume of the processed in the reactor of the liquid, placed between the planes of the solid-state resonator, with which in her emanate the fluctuations, as part of oscillatory system the acoustic cell of reactor, you can create the optimum conditions for the superposition of these oscillations. For such conditions the formed near the antinodes of pressure of the total wave areas of cavitation [4] will occupy a larger total volume of [1,10]. This can be done, given the well-known fact [10,11], that is in the water flat-elastic wave gives able to generate cavitation energy at a distance of not more than three half-waves, and on the length of the half-wave fluctuations in metal (when the ends of segment is fluctuate in opposite directions) fit about three half-wave of oscillations of the same frequency in the liquid. That is, there is no sense to do the volume of working chamber of the cell reactor length more than 1.5 length of wave elastic oscillations in liquid, when the radiating surface are from each other by distance half wavelength fluctuations of metal. Then the height of the acoustic cell (the distance between the radiating surfaces) must be exactly equal to half the length of wave oscillations of metal on the same frequency. Such conditions is easier to implement when the solid part of oscillatory system of the acoustic cell is symmetrical about the center of mass, that is, the sources of fluctuations (converters) are located on both sides of the volume of treated water in the reactor. It is known [3] that the geometrically symmetric relative to its center of mass of a system of elastic fluctuations holds better resonance. Education cavitation erosion of radiating surfaces for it with virtually no will.

Using the mathematical model of cavitation reactor and the principle of similarity of the cavitation processes, described in the first chapter, can compare such a reactor with a benchmark by arming the computer experiment. The Estonian company "Oil Tech Production OY" (Tallinn) made a reactor with two converters connected to one power supply (ultrasonic generator) and doing of coherent oscillations [12], which form the standing elastic wave by in the flow water through workspace. He is shown in fig. 1 and consists of the camera body 1, which is made from the standard seamless stainless steel pipes of the outside diameter 90 mm, a wall thickness of 5 mm. Acoustic booster 2 with emitting the ends of a diameter of 79 mm, such, as described in [13], is fixed in the casing with flanges 2 through the seals with the help of studs 4. As the sources of fluctuations used converters MPI 5050F-20L (5).

Fig. 1. Cavitation reactor of the company Oil Tech Production OY, described in [13]:

on the left is the design of the reactor;

on the right is a photo of the sample of the reactor, fixed on a tripod.

As a etalon was selected reactor, with a design similar to shown in the figure. 1. This reactor is a reactor of a plane wave and is calculated by the formulas for unsteady cavitation mode, since the time of its effect on the volume of the processed water is very little for accounting of acoustic flows and under hydrostatic pressure in it approximately equal to the atmospheric. Height of the working chamber H standard (the distance between surfaces radiating the coherent flat-elastic waves ) is selected to 1.5l in a liquid that has a static pressure p_h = 1 atm, the speed of propagation of flat-elastic oscillations c and density r close to the parameters of chemically clean water (Fig. 2, a). Total power consumption of radiators 2.4 kW. Dimensions of the etalon chosen so that when consumed his converters of electric power is the maximum amplitude of the sound pressure in the fluid meets the concept of food sonochemistry. Through the working volume of the reactor the fluid passes through the special nozzles.

The reactor, with which made comparison (Fig. 2, b) contains a monolithic oscillatory system of the acoustic cell, similar to the systems of the company Hielscher Systems GmbH. The height of its single cell (the distance between the radiating oscillations of the circular surfaces) is equal to two полуволнам in the liquid. It can contain a several of such cells, components of total oscillatory system. The liquid in it passes through the volume of the camera along the oscillatory system between it and the wall of the chassis.

Reactors UIP Hielscher Systems GmbH contain six cells, depicted in fig. 2, b, and source - wave piezoelectric transducer is attached to the oscillatory system with one side of her, as shown in fig. 3.

Fig. 2. a – design of the etalon. The tonal pattern shows the distribution of the volume density power cavitation erosion in the axial cross section; b – a fragment of a reactor design firm Hielscher Systems GmbH. Dimensions of working volume of a single acoustic cells are surrounded by a dotted line; c –design of the reactor with a symmetric solid-state oscillatory system of the acoustic cell. The tonal pattern shows the distribution of the volume density power cavitation erosion. At the bottom of figure on the charts shows the graphs of pressure:

▬ in emitted each transformer waves, if they were distributed in an open half-space liquid;

▬ in the resulting wave of the superposition of these waves. On the x-axis shows the phase in radians at the resonant frequency of oscillatory system.

Fig. 3. Asymmetric oscillatory system reactors UIP 4000 with a piezoelectric converter power consumption 4000 kW in the housing system of cooling (on the left).

Developed in accordance with the model [1] the reactor with monolithic oscillatory system of the acoustic cell, the symmetrical about the center of mass (Fig. 2, c), also consists of the camera body, maded from welded seamless stainless steel pipes with a diameter 102 mm wall thickness of 6 mm and 146 mm whis wall thickness of 7 mm, with a flanges with hole for fixing other flanges with sealing by means of bolts and nuts (six on each side) waveguide transformers with diameters of radiating surfaces 88 mm, which transmit the vibrations sources (not shown) in the liquid with the a decrease of amplitude and also interconnected barbell (Ø 32 mm) into a solid-state oscillatory system. Compared reactors have a capacity of oscillation sources, the maximum amplitude of the pressure of wave and the variance of the spatial distribution of the power density of cavitation in the same as the etalon.

As a result of comparison were modeled performances for all these reactors for the treatment of water used for the needs of food and pharmaceutical industry, and pollutions her to the products of erosion in relation to the indicators of the etalon reactor. Relative performance, so as in all reactors used one and the same, the oscillation frequency f, is calculated as the ratio of works average volume density of erosion capacity of cavitation in the working volume of the chamber V

(1)

where q_er – volume density erosion power of cavitation in point of space кавитирующей fluid with coordinates x, a, y in the volume V of the product the indicators of etalon reactor:

(2)

Instead of absolute values of qer in the calculation of volumetric power density cavitation erosion, because the liquid in the variants of one and the same, is used contingent values:

, (3)

where: a – the average of n cavitation areas of the attenuation the total perturbations of pressure from all of the bubbles in the point for which is calculated;

Sv_i = S×Sh_i – total volume of the i = 0...n cavitation areas (S - the area of the front of the resulting wave in a reactor, equal to the square of the surfaces radiating);

t – average of the dimensionless time of arrival perturbations of pressure from all the bubbles n cavitation areas in this point;

– amendment to the phase of the cavitation field. Square brackets denote a whole, and curly - fractional part of the number. Lengths of the free run of fluctuations of the pressure from cavitation for period of the harmonic wave are equal to the length of the wave.

Dimensions h_i cavitation areas on the beam of the waves were calculated, as recommended [1,10] in angular units phase as the diference between the values of the even and odd positive roots of the transcendental equation:

(4)

where: А_max, А₀ – the maximal amplitude of the sound pressure the wave when the radiation of fluctuations doing in half-space of the liquid and the threshold for the education of cavitation, respectively; h – scattering coefficient of the energy of the waves on the cavitation [11]. In the linear units, they were translated by dividing the on wave number 2p/l. Water pollution the products of erosion calculated proportional to the integral value volume density erosion power E in contact with water metal surfaces of the reactors. For reference it is equal to:

(5)

and for compared reactors:

(6)

where: R₂, R₁ – outer radius of radiating surfaces and the radius of the connecting rod, respectively;

Н – the distance between the radiating surfaces;

D – her diameter;

q_er,1, q_er,2, q_er,3 – density of erosion power of the cavitation in the points of the radius of a radiating surface and in the points of rays are separated from the axis of symmetry at a distance of diameter of the walls of the camera and the diameter connecting rod, respectively.

The results of comparison in the values in relation to the etalon shown in the table.

OPTION REACTOR THE RELATIVE INDICATOR	UIP (Fig. 2 b)	Fig. 2 c
The performance of water treatment	1,8	3,5
The variance of the spatial distribution of the density of the power erosion*	1,02	0,87
The erosion of the elements of design	2,5	3,9

*the variance was adopted as much as possible close to 1.00.

From the table it can be seen that the reactor UIP 4000 which has 1,7 times more, than at etalon power consumption of the piezoelectric converter and the same (slightly larger) the variance of the spatial distribution of the density of power of the erosion in the working chamber it has 1.8 times greater productivity of processing of water, and 2.5 times larger erosion of the details of the construction. The reactor with a symmetric oscillatory system acoustic cells with similar (slightly smaller) the distribution of the density of power of the erosion in the chamber has the performance in 3.5 times, and the erosion of the details of the construction in 3.9 times higher than that of the etalon. In other words, the change of these parameters is almost to proportion to each other. Thus, when performing the above requirements to the size of the solid parts of oscillatory system acoustic cell cavitation reactor his power of cavitation, performance and cavitation erosion surfaces of the parts are increased proportionally, which testifies to the fact that the absolute contamination of treated water will not change, as it flows through the reactor proportionally faster, but useless loss of energy elastic wave is reduced.

R E F E R E N C E S

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2. Patent US 3519251, 1970

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4. Shestakov S. The basic technology of cavitation disintegration.-M: EVA-Press, 2001 (in Russian)

5. Patent US 4618263, 1986

6. Patent RU 2209112, 2002

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8. Patent RU 2392047, 2010

9. Patent ЕР 1810747, 2007

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13. Shestakov S. // Х session of the Ross. Acoust. Society, Vol.2 .-M.: GEOS, 2000, p.p. 115-119 (in Russian)