RU2548696C1 - Method of highly intensive acoustic drying of capillary and porous materials and device for method implementation - Google Patents

Abstract

FIELD: heating, drying.

SUBSTANCE: invention refers to acoustic drying method for capillary and porous materials by high-intensity sound vibration and can be applied in any industries and agricultures requiring drying of materials in amounts of dozens of cubic metres. Drying chamber is made of heavy materials with high acoustic resistance (such as concrete), with wall of sufficient thickness minimising sound vibration penetration, so that the sound reflects from walls, structures and dried material inside the chamber, and acoustic energy affecting material drying increases. Powerful sound source generating sound field with 160-170 dB intensity in 70-15000 Hz range is mounted on a chamber wall. Reflector is mounted on the wall opposite to the sound source.

EFFECT: maintenance of required air exchange parameters in the chamber, allowing to reduce minimum drying time and prevent diffusion-blocking area formation in the drying material and moisture evaporation, adjustable air flow speed at the material surface in drying chamber and air temperature and humidity allowing for levelling of material moisture evaporation and diffusion rates.

2 cl, 2 dwg, 1 tbl

Description

FIELD OF THE INVENTION

The invention relates to acoustic methods for drying capillary-porous materials by high-frequency sound frequency vibrations and can be applied at enterprises of various industries and agriculture.

BACKGROUND

It is known that the drying process consists of two main periods. In the first period, moisture on the surface of the material evaporates into the surrounding space. The second period begins from the moment when the surface moisture has mainly evaporated, a moisture gradient has formed in the material and moisture diffuses from the underlying layers to the surface.

, т.е. как функцию квадратаThe acoustic effect during drying significantly accelerates the processes of evaporation and diffusion, which is explained by a number of effects that occur both on the surface and inside the material. The main factors accelerating the removal of moisture from the surface of the material to be dried at the first stage of drying are the arising turbulent flows and acoustic flows near the surface of the material, and the instability of the surface liquid layer [1]. Under the influence of alternating pressures brought by an acoustic wave to the surface of the material being drained, elastic vibrations arise in the volume of the latter, as a result of which moisture is distributed throughout the volume faster and more evenly. M. Smolukhovsky and A. Einstein obtained the following expression for the acoustic diffusion coefficient:

, i.e. as a function of the square

amplitude A and frequency - v ₀ particle oscillations [2].

It was experimentally established that when the intensity of acoustic vibrations is more than 136 dB, the drying speed increases exponentially with the increase in the intensity of the sound field [1]. According to other data obtained, including during the operation of a pilot plant for drying wood, this growth begins with a sound pressure level greater than 150 dB.

, где: D_t - коэффициент молекулярной диффузии, D₀ - предэкспоненциальный множитель, Q - энергия активации диффузии, k_b -постоянная Больцмана, Т - температура.On the other hand, the drying rate is exponentially dependent on temperature, as evidenced by a qualitative analysis of the Arrhenius equation

Where: D _t - molecular diffusion coefficient, D ₀ - the pre-exponential factor, Q - diffusion activation energy, k _b is the Boltzmann constant, T - temperature.

Thus, the speed of acoustic drying for most materials is most affected by the intensity of the sound field and the temperature in the chamber.

Humidity does not affect the drying intensity as brightly as the sound intensity and temperature, but has its own characteristics that are significant for drying some materials.

Existing methods of acoustic drying can conditionally be classified according to the frequency of the used signal as sound from 70 to 15000 Hz and ultrasonic from 15000 Hz and above.

The use of multiple reflections of ultrasonic vibrations, their focus on the material being dried and its periodic evacuation characterizes the method of drying capillary-porous materials, which consists in the fact that the material for drying is placed in a sealed technological volume made in the form of a toroid, which is placed in an even larger volume. Depending on the physicochemical properties of the materials and the specified degree of drying, the power of the mechanical vibrations of the ultrasonic frequency is selected in the range from 120 to 170 dB at an oscillation frequency of 15 to 25 kHz, a larger volume is evacuated to a residual pressure in the range of 1 to 10000 Pa four]. The disadvantages of this method are:

- a small volume of the chamber for drained material, measured in tens of liters, associated with strong absorption of ultrasonic vibrations by air;

- in connection with the toroidal form of the drying chamber, the list of drained materials is limited mainly to bulk materials;

- the need to create a sealed chamber of complex shape reduces reliability, increases material consumption, increases the complexity of manufacturing the device;

- an additional compressor and energy costs for creating a vacuum in the chamber, a large consumption of compressed air from gas-jet emitters and their low efficiency increase the cost of the device and the cost of its operation;

- the presence in the design of foci determines the uneven effect on the drained material with a deterioration in the quality of drying.

Acoustic dryers operating in the sound range of 70-15000 Hz are characterized by the ability to dry all types of capillary-porous materials in incomparably larger volumes than ultrasonic dryers, measured in tens of cubic meters.

A known method of acoustic drying, in which the drying of materials is carried out by intervals of exposure in time and pauses between them. Interrupting the process of sound irradiation of the drained material for a while, part of the moisture from its inner layers, without any energy expenditure, enters the surface [5]. However, the practical implementation of this patent is difficult due to the complexity of the technical definition of the "moisture difference along the pore length of the dried material in the interval", which is included in the formula of this invention. If we take as a basis the data from the description of the invention about a pause during drying of 1 day, then the practical value of such a device is doubtful. In addition, the method of achieving a sound intensity in the chamber of 160 dB, which is indicated in the description of the patent, is not known, which complicates its industrial application.

In order to intensify the process of acoustic drying, the use of a combination of acoustic and thermal effects in the acoustothermal method of drying materials [6], which is carried out cyclically by applying an acoustic field to a preheated material to be dried, is first proposed. The sequence of cycles can be continuous, which, in essence, means the alternation of heating and acoustic processing. The disadvantages of this drying method were "inherited" from the method of acoustic drying [5]. In addition, when heating the material in the traditional way, energy is consumed, the process itself takes a long time, and then the material is cooled again during sounding. All this reduces the effectiveness of this method.

A device for acoustothermal drying of capillary-porous materials [7], adopted as a prototype, in which the camera consists of 4 sections separated by soundproof partitions, and is equipped with a heater. The device first heats the material, and then it is voiced in an acoustic field, after which the procedure is repeated. At one end of each section are sound sources that operate alternately, and sound absorbers at the other. An invention that is very similar in nature to the prototype [7] is a method of multi-chamber drying of materials using acoustic, thermal and convective effects on the material to be dried, in which heated air is supplied to the sound source chamber and simultaneously creates an acoustic field and convective flow in it, directed into the common sound duct [8].

The air flow is supplied to each drying chamber separately or to all at the same time until the desired moisture content of the material is obtained. Also, as in the prototype, its speed in the chamber is determined by the pressure of the air supplied to the gas-jet emitter, the cross-sectional areas of its nozzle and drying chamber. The difference between this device and the prototype is that the gas-jet emitter is located in the center of the device, and the chambers are crosswise adjacent to it. In one sentence, the material is heated by warm air pumped into the chamber before acoustic exposure, in the other during exposure. Both of these devices involve periodic acoustic exposure. In pauses, the moisture levels out naturally in the volume of the material. To ensure uniformity of sounding of the drained material in the device [7] and method [8], the concept of the formation of a plane traveling wave in the chamber is adopted. Concerning:

- at the end of the chamber, opposite the sound emitter, a sound absorber is installed in which a significant part of the energy of sound waves is irretrievably lost, which is the main disadvantage of these devices of acoustic (sound) drying;

- the maximum dimensions of the cross-section of the camera are limited to half the wavelength of the operating frequency of the emitter;

- in accordance with the data [12] on the maximum acoustic power of gas-jet emitters of 2 kW, it can be argued that 155 dB is the ultimate intensity in such installations and the achievement of an optimal intensity of 160-170 dB is not possible with reasonable energy consumption.

Other disadvantages of the prototype and the above devices for acoustic drying of the sound range include:

- energy loss of sound waves in the walls of the chamber and partitions made of sheets of metal and sound-absorbing material located between them;

- a significant excess of sanitary standards for noise, requiring additional costs for sound insulation;

- deterioration in the uniformity of the sound field due to the reflection of acoustic vibrations from the structures in the chamber to accommodate the drained material and the material itself;

- all of the aforementioned devices of acoustic drying use gas-jet emitters, which have a high consumption of compressed air and low efficiency, which reduces the technical and economic performance of the devices;

- large power consumption reduces the availability of new technology for small enterprises;

- drying of some materials in the described devices is difficult or impossible due to the fact that the compressed air for the emitters comes from the compressor with a high temperature, raising the temperature in the chamber to values that are undesirable for thermolabile materials, and its cooling requires a lot of energy. For example, when drying medical biological products, heating above 40 ° C leads to their chemical decomposition;

SUMMARY OF THE INVENTION

The present invention is the creation of such a method of acoustic drying of capillary-porous materials, which:

the needs of industrial enterprises for high-performance drying of volumes from units to tens of cubic meters are provided by creating in the cameras a sound field with an intensity of 160-170 dB in the range of 70-15000 Hz;

- achieved high quality drying of any capillary-porous materials with the possibility of optimizing the minimum energy consumption and drying speed;

- simple measures are implemented permissible sanitary standards for noise.

The technical solution consists in the fact that the walls of the drying chamber of sufficient thickness are made of heavy materials with high acoustic impedance (for example concrete), ensuring minimal penetration of sound vibrations, with a reflector located on the chamber wall opposite to the sound source made of the same material as and walls, together providing both the necessary noise protection, and the reflection of sound vibrations from walls, structures, and drained material inside the chamber, increasing the share of acoustic oh energy affecting the drained material. As a sound source, a sound generator is used in the range of 70-15000 Hz, which allows to obtain acoustic power to achieve the task. The lower limit of the range is determined by the capabilities of the source and the approach to the resonant frequencies of building structures that can cause them damage. The upper limit is limited by the necessary chamber volume for production needs, since with increasing frequency the sound absorption by air increases, the power consumption increases, and the technical and economic performance of the device deteriorates accordingly. An exponential horn is used to match the sound source to the camera. The technical result of the proposed proposal is to achieve an intensity of sound energy of 160-170 dB, at which all the advantages of acoustic drying are realized to the maximum.

The more the acoustic resistance of the air and the wall material are different, the smaller the fraction of sound energy penetrates the interface

, где β - коэффициент проникновения звуковых колебаний, c₁ - скорость звука в воздухе, с₂ - скорость звука в материале стены, ρ₁ - плотность воздуха, ρ₂ - плотность материала стены [9].these two environments. This conclusion can be drawn from the Euler formula, which for the case of a large difference in the acoustic impedances of the two media is transformed into an approximate formula:

where β is the penetration coefficient of sound vibrations, c ₁ is the speed of sound in air, c ₂ is the speed of sound in the wall material, ρ ₁ is the air density, ρ ₂ is the density of the wall material [9].

, где f - рабочая частота излучателя, Гц. (Получено из закона массы R=20 log mf - 47,5, R - звукоизоляция, дБ; m - масса 1 м² ограждения, кг; f - частота колебаний, Гц. [10]). Для частоты в 2 кГц толщина стенки камеры будет около 500 мм, что вполне допустимо. Применяя комбинированную шумозащиту, можно получить необходимый уровень интенсивности звука в камере заданного размера и приемлемую толщину стены камеры.So that the wall does not play the role of a large membrane and does not transmit sound energy to the surrounding space, its thickness should be sufficiently large. The wall thickness of the chamber depends on the frequency used and the mass per unit area of the wall. For example, to comply with sanitary standards for noise in industrial premises (SN 2.2.4 / 2.1.8.562-96 - 80 dB) when manufacturing the chamber only from concrete (specific gravity 2400 kg / m), the formula for calculating the wall thickness h will have the form :

where f is the operating frequency of the emitter, Hz. (Obtained from the law of mass R = 20 log mf - 47.5, R - sound insulation, dB; m - mass of 1 m ² fencing, kg; f - vibration frequency, Hz. [10]). For a frequency of 2 kHz, the thickness of the chamber wall will be about 500 mm, which is quite acceptable. Using combined noise protection, it is possible to obtain the necessary level of sound intensity in a chamber of a given size and an acceptable wall thickness of the chamber.

Another part of the proposed technical solution relates to measures for a more uniform distribution of the acoustic field inside the chamber, by installing a reflector on the opposite wall of the sound source, made of the same material as the wall. The shape of the reflector is in the form of convex inhomogeneities, the geometrical dimensions of which approach the wavelength of the emitter, preferably made in the form of three, tetrahedral prisms or cones, and their number is a multiple of the transverse dimensions of the chamber.

The longest process that determines the drying time is the diffusion of moisture to the surface of the material. As noted above, elastic oscillations occur in the volume of the material to be drained, as a result of which moisture is distributed throughout the volume faster and more evenly. In this regard, the last part of the proposed technical solution is aimed at optimizing the drying speed and energy efficiency, for which the acoustic effect on the material to be drained should be, as a rule, constant. In addition to the strong influence of sound intensity and temperature, air humidity has a smaller effect, but has a pronounced maximum, which changes during drying depending on the difference in the moisture content of the medium and material [3]. This is important to consider when drying certain materials to prevent the formation of zones that impede diffusion and evaporation of moisture. For example, due to a large moisture gradient on the surface of the drained fish and dry air entering the chamber, a crust formed on its surface, which drastically slowed down the drying process and led to the rejection of products. Drying in a wetter atmosphere was normal. A subsequent gradual increase in temperature and a decrease in the humidity of the air injected into the chamber achieve the best organoleptic qualities of the product.

Based on the restrictions imposed by the properties of the processed material, in devices implementing this drying method, the air flow rate at the surface of the material, its temperature and humidity are subject to regulation in order to equalize the rates of evaporation and diffusion of moisture of the material to the surface. This is achieved in addition to using the air exhausted by the sound source by using an additional ventilation system that provides the necessary air exchange parameters in the chamber, thereby achieving high quality drying of any capillary-porous materials and a minimum drying time.

Drained material is loaded into the chamber in any convenient way, for example, on a trolley like wood (see Figure 1). For drying materials of viscous consistency, granular, such as peat pressed into granules, mineral-organic fertilizers, sewage sludge, the material is poured into a cylindrical rotating chamber through a loading device (see Figure 2). In this case, the device is made of steel of sufficient thickness, with the emitter frequency selected to fulfill the requirements of the present method in terms of sound intensity in the chamber and sanitary standards for noise outside. The drying process of each specific material occurs in accordance with a previously developed technology.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 Sketch design of acoustic drying of wood by the proposed method. The figure 1: 1 - camera, 2 - a dynamic siren and a horn, 3 - a cart with lumber, 4 - a reflector, 5 - a protective screen, 6 - a mechanism for moving the cart, 7 - additional insulation. Camera ventilation system not shown.

Figure 2 is a diagram of a device for drying dry and viscous materials in a rotating chamber according to the proposed method. In the figure 2: 1 - camera, 2 - dynamic siren and horn, 4 - reflector, 8 - drive rotation of the cylindrical chamber. A loading / unloading device is not shown.

DETAILED DESCRIPTION OF THE INVENTION

The following examples and explanations are given only to illustrate the principles of design and operation of the device and nothing in this section should be construed as limiting the scope of claims.

With the active participation of one of the authors, in 2001-2008, a pilot industrial plant for drying wood was built in Novosibirsk based on the “Method of Acoustic Drying of Capillary-Porous Materials” [5], which subsequently underwent a series of reconstructions. In the chamber with internal dimensions of 1.7 × 1.7 × 6.5 meters, a gas-jet emitter managed to achieve a sound intensity of just over 148 dB, at which changes in wood moisture during periodic sounding were insufficient for drying. In order to intensify the drying process, the pilot plant was equipped with a heating system and additional ventilation in accordance with the acoustothermal drying method [6], which practically did not change the drying process, the installation did not work. In order to increase the sound field intensity in the chamber, a device was proposed for acoustothermally drying capillary-porous materials [7], according to which the chamber of the pilot plant was divided into 4 parts by soundproof partitions, and equipped with an additional system for supplying warm air to the chamber. As a result of all these changes, with a power consumption of 200 kW, the sound intensity in each section reached 155 dB, the drying process, for example, of a pine board with a thickness of 50 mm lasted 10-12 hours to a moisture content of 14-16 percent. In this form, the installation worked quite successfully as part of a woodworking enterprise for about 2 years.

The difference between the pilot fish drying plant, built in 2011 in the Ryazan region, from the prototype installation was the use of a dynamic siren as a sound source, a reflector in the form of a black-faced prism in the place of the absorber and the absence of soundproof partitions. In the chamber of this installation, with a power consumption of just over 37 kW, the sound field intensity reached the same 155 dB, but in the entire 19 cc chamber volume. Doubling the air pressure supplied to the dynamic siren increased the intensity to 160 dB with a satisfactory uniformity of the sound field in the chamber volume.

Sounding time for drying a pine board at 165 dB at the experimental installation of the Institute of Theoretical and Applied Mechanics of the SB RAS was a little over 1 hour [13]. At the pilot industrial installation for drying wood, the sound time of the pine board is 4-6 hours, the total drying time of the material was 10-12 hours. It should be expected that the drying time according to the claimed method with a sound intensity of 160-170 dB will be reduced by at least 2-3 times and reach 4-5 hours.

Table 1 below shows the data of wood dryers of various types with a volume of about 10 m ^{3 of} drained material [14, 15] *.

Compared to steam drying with equal volumes of drained material, only energy and heat savings exceed 2 million rubles per year. Given the 11-fold higher plant productivity, it pays off in less than one year.

A similar comparison with the prototype shows the possibility of saving electricity in the amount of more than 4 million rubles. per year with a 2 times greater productivity.

Energy consumption of 40-80 kW is available for most small industrial enterprises.

In the literature, the authors did not find descriptions of devices and methods that allow acoustic drying of any capillary-porous materials at a sound field intensity of 160-170 dB in the range of 70-15000 Hz, which provide significant volumes of dried material for production, with energy consumption per unit of production and drying time less than shown in Table 1. The inventive method involves the possibility of regulating the parameters of the air at any stage of drying, which when exposed to an acoustic field, a significant intensity vnosti, unattainable reasonable energy consumption in cameras and other prior art devices known acoustic (sound) drying, enables effective drying of any capillary-porous materials. The above allows us to conclude: the claimed technical solution meets the first condition for patentability of the invention - novelty.

The sound field characteristics in the range of 70-15000 Hz with an intensity of 160-170 dB, which provide all the advantages of acoustic drying in volumes significant for production, are achieved by manufacturing walls and a reflector of sufficient thickness from heavy materials with high acoustic impedance. This technical solution, in addition to creating a powerful sound source of the required intensity of the sound field in the drying chamber, allows you to simultaneously ensure sanitary standards for the noise of the device. If the mechanism for achieving a high-intensity sound field providing energy efficiency, drying speed, significant volumes of material to be dried, and the breadth of application of the proposed method of acoustic (sound) drying, confirmed by the presented calculations and experimental data on the operation of acoustic drying devices, was previously unknown, and the performance of the new method unattainable by other drying methods and devices, while the proposed technical solutions that achieve this result do not imply explicitly Zoom from known to date prior art that solves a similar problem of acoustic (sound) drying at least an equivalent way, the proposed technical solution corresponds to the second condition of patentability of the invention - an inventive step.

The experiments conducted by the authors on the installation with the partial use of the elements of the claimed invention when drying fish, vegetables, fruits and some other materials proved the effectiveness of the proposed solutions. The results obtained in this way correspond to the calculations and data obtained earlier during the pilot operation of the prototype installation for wood drying and at the experimental installation at the Institute of Theoretical and Applied Mechanics SB RAS. All this indicates compliance with the third condition of patentability of the invention - industrial applicability.

The application of the proposed method allows to intensify the drying of capillary-porous materials at lower energy and labor costs in volumes that satisfy the needs of industrial production and agriculture.

INFORMATION SOURCES

1. “Physical fundamentals of ultrasound technology” Under. ed. L.D. Rosenberg. M .: Nauka, 1970.

2. Kardashev G.A., Mikhailov P.E. "Heat and mass transfer acoustic processes and devices." M .: Engineering, 1973.

3. Lykov A.V. "Theory of drying." M .: Energy, 1968.

4. RF patent for the invention No. 2239137 "METHOD FOR DRYING CAPILLARY-POROUS MATERIALS".

5. RF patent for the invention No. 2062416 "METHOD FOR ACOUSTIC DRYING OF CAPILLARY-POROUS MATERIALS".

6. RF patent for the invention No. 2215953 "ACOUSTHERMIC METHOD OF DRYING MATERIALS".

7. RF patent for invention No. 2283995 "DEVICE FOR DRYING CAPILLARY-POROUS MATERIALS IN ACOUSTIC-THERMAL METHOD".

8. RF patent for the invention No. 2270966 "METHOD OF DRYING MATERIALS AND DEVICE FOR ITS IMPLEMENTATION".

9. Putilov K.A. "Course of Physics" Vol. 1, State Publishing House of Physics and Mathematics, 1963.

10. "The fight against noise in the workplace", ed. E.A. Yudina, "Engineering", 1985.

11. "Ultrasonic equipment for industrial use", D. Gershgal Energy, 1967.

12. “Ultrasound. Little Encyclopedia. " Ed. Golyamina IP, “Soviet Encyclopedia”, 1979.

13. Website of the Institute of Theoretical and Applied Mechanics SB RAS http://www.itam.nsc.ru/applications/acousticdrying.html.

14. L .: Forest expert. 2004 No. 16.

15. Website of NPO Viteks LLC (Scientific-Industrial-Commercial Association for the Development and Implementation of Vacuum-Pulse Technologies) http://www.witeks.ru/technology/kvis 15 / tehno.html.

Claims (2)

1. A method of drying capillary-porous materials, characterized in that the material to be dried is placed in a chamber with a powerful sound source installed in it, characterized in that the acoustic power of the said source allows you to create a sound field in the range of 70-15000 Hz with an intensity of 160-170 dB a chamber having walls of sufficient thickness made of heavy materials with high acoustic impedance, ensuring minimal penetration of sound vibrations, with a reflector located on the chamber wall, counter aschey sound source, made from the same material as the walls, together providing both the necessary schumozaschitu and reflected sound vibrations from walls, structures and material to be dried within the chamber, increasing the proportion of the acoustic energy acting on the material to be dried. 2. The method according to claim 1, characterized in that the chamber, in addition to its ventilation with air exhausted by the sound source, can be further equipped with ventilation means with the ability to control both air humidity and (or) temperature and its speed in the chamber.

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