The drying process is to remove moisture from the material. This is usually carried out by heating. Drying of solid, bulk and other materials are distributed in a wide variety of technological processes. In most cases, heated air is used as a coolant. But when dried by many chemical products, biological objects, fertilizers, it is impossible to carry drying at elevated temperatures, since substances either decompose or lose their bactericidal properties.

When it is impossible to allow increasing temperature during drying, drying is carried out under the influence of ultrasound oscillations.

The impact of ultrasonic oscillations contributes to the removal of moisture and allows you to speed up the drying process many times. The laboratory team has extensive experience in the development and creation of practical designs for acoustic (ultrasonic) drying, more detailed information can be found in the monograph "Application of ultrasound in industry" Chapter 6.4 "Acoustic drying processes".

6.4 Acoustic drying processes

(Khmelev V.N., Popova O.V. Multifunctional ultrasound devices and their use in conditions of small industries, rural and household: scientific monograph / Alt. State Tehn Un-t. them. I.I. Polzunova. - Barnaul: ed. AltGTU, 1997. - 160 p.)

The drying process is to remove moisture from the material. This is usually carried out by heating. Drying of solid, bulk and other materials are distributed in a wide variety of technological processes.

In most cases, heated air is used as a coolant. But when dried by many chemical products, biological objects, fertilizers, it is impossible to carry drying at elevated temperatures, since substances either decompose or lose their bactericidal properties.
When it is impossible to make a temperature increase when drying, drying is carried out under the influence of ultrasound oscillations. The first results were obtained back in 1955 P. Gregohem (Hungary). At a frequency of 25 kHz, it received a 10-fold acceleration of the drying of the wet cotton fiber. Later, it was also obtained to accelerate wood drying.

The drying process consists of two main periods. The first is to evaporate moisture from the surface of the material and diffusion of steam into the surrounding space. The evaporation of moisture leads to the fact that a gradient of humidity is created in the material, as a result of which moisture from the inner layers begins to move to the surface. This is the second period.

Ultrasound exposure to high intensity allows you to intensify both periods of drying process.
During the first period of drying, the oscillations allow you to reduce the thickness of the hydrodynamic boundary layer. In the ultrasound field, the hydrodynamic border layer can be significantly less than diffusion. This means that oscillations penetrate inside the diffusion layer, turbulizes it, thereby speeding up the process of evaporation. Along with a decrease in the thickness of the boundary layer, the drying method has another important advantage - the oscillations penetrate the material and create quickly changing zones of increased and reduced pressure in it, which intensifies moisture transfer processes from deep layers to the surface in the second drying period. Other acting factors of ultrasound impacts are (Figure 6.5): Reducing the viscosity of the fluid under the action of oscillations, which contributes to the transfer of moisture from the depth layers to the surface; extrusion of moisture from the material by cavitation bubbles arising in fluid under the action of oscillations; Radiation pressure, extruding liquid from material, etc.

Figure 6.5 - Existing Factors of Ultrasonic Drying

Smaller energy costs of bonds drying, compared with convective, are due to the fact that the liquid from the surface is removed not only by evaporation (which requires significant energy costs for the implementation of the phase transition), but also at the expense of ultrasound spraying in the form of an aerosol (without phase transition) which occurs as a result of highly intense elastic oscillations of ultrasonic frequency.

Therefore, bond drying with a comparable power of the energy exposure proceeds several times faster than the convective dryer. Patented methods of drying capillary and porous materials in ultrasound fields of high intensity, confirmed the following advantages of acoustic drying:

- high intensity of the process at less energy costs;
- Ability to provide high-quality and efficient drying at low temperatures or, in principle, without increasing temperature.

Thus, the main existing factors are:

1. Reducing the viscosity of the fluid under the action of ultrasound contributing to the accelerated movement of moisture on capillaries from the depth of the body to the surface.
2. The oscillations of gas bubbles located in the liquid, which, extrude moisture from the capillaries.
3. Radiation pressure directed in capillaries from a liquid in a gas that moves the capillary fluid column by moving it to the surface.

Drying in bond field occurs without heating the material. That is why this is the only way to dry thermal and easily oxidizing materials. This speed method differs from ordinary methods. For example, when drying silica gel with an initial humidity of 25%, the radiation intensity of 152 dB, at a frequency of 8 kHz, T \u003d 15 min - complete drying. Figure 6.6 presents dependencies on the performance of the drying process of ethyl cellulose.

Figure 6.6 - dependence on the performance of the drying process of ethylcellulose

Comparison with vacuum drying and drying with heated air (92 ° C): During the same time, it was possible to remove only 10-15% of the moisture contained.
When drying enzymes (not withstanding 40 ° C), the process in the acoustic field took 14 minutes and the speed in comparison with the vacuum method increased by 3-4 times.

Features of acoustic drying (equipment requirements).

1. There is a lower intensity limit at which a noticeable acceleration of the process begins (about 130 ... 145 dB).
2. There is no dependence of the speed of drying from frequency in the range from 2 to 25 kHz.
3. The drying is most effective for thin layers (about 2-3 cm).

Consider practical diagrams of dryers (Figure 6.7)

Figure 6.7 - Practical Dryer Schemes

Basic - these are sounded dryers with a boiling layer for drying powdered substances, drum dryers and vibration.

Consider the design of the dryer with a boiling layer. In the vertical cylinder in the side and upper part emitters are installed.

Through the feeder, the material is loaded into the dryer on the grille. Air for drying is fed from the bottom through the distribution grid. When air passes through the grid and the material, it begins to mix intensively, "boiled", forming a pseudo-liquefied layer. Constantly stirred material is exposed to narrow and dried.

As the particles are drying, it becomes easier, climb over the upper partition and refer to the bunker. Removal of wet air is performed through the upper nozzle.

Acoustic drying - This is a way of dehydrating the product by means of intensive ultrasound. This is a cyclic way of removing moisture. In the primary processing of the moisture product is removed from the surface, then when using the second ultrasonic wave, the moisture is distributed over the capillaries. This is happening until the product contains the required fraction of moisture.

Acoustic drying has found a successful use in agriculture, in the pharmaceutical, chemical and food industry. In agriculture through acoustic drying, grain crops, vegetables and fruits are treated. In the food industry, the acoustic drying is used in the production of dry milk. The most widespread ultrasonic drying was obtained in pharmaceuticals. Expensive drugs - powders, antibiotics, tablets - are produced in ultrasound chambers. The high cost of drugs is due to the high performance of the equipment and, as a result, a high degree of energy consumption. In the chemical industry, the ultrasound drying is used for the production of coal powder. With the help of such cameras, drying paper, cotton, wood.

Features of acoustic drying

Acoustic drying has a number of benefits: the product is not exposed to heat treatment, it is processed in a cold form; Due to the lack of temperature effects, the product retains almost all nutrients and vitamins, does not lose its initial properties, is not oxidation.

Acoustic drying is the only way to work with thermally sensitive materials. Thanks to this method, their structure is fully maintained without losing its original form.

Acoustic drying - high-speed processing method. Compared to vacuum drying, acoustic reduces processing time four times. Due to this, the quality of the finished product increases.

Acoustic drying technology

Drying material must have a capillary-porous structure. Various materials have a different moisture content, so the intensity and amount of ultrasonic waves are calculated in accordance with the percentage of humidity of the product.

If the product contains a large amount of moisture, use a wave of high impact force, as a result, the moisture literally "shake out" from the product. This is because the wave appears not only at the surface of the material, but also inside the capillaries, leading to the intensive loss of moisture.

If the capillary-porous material has moderate humidity, acoustic oscillations occur more intensely in the first stage and less intensively on the second. During the first stage, the drying speed does not change, so the moisture is constantly being replenished. The upper layers of the product are losing it, and the lower layers "throw out" moisture to the surface. Thus, moisture exchange does not stop until the optimal moisture content is reached.

During the second stage, the drying speed decreases, so the fluid from the inside enters the weakly and its loss is no longer replenished or replenished, but weakly.

Acoustic drying is most effective during the first stage of product processing. Thanks to her, the physico-chemical and consumer properties of the product are improved. For example, during acoustic processing of seeds, their germination increases.

Thus, the acoustic method of drying is successful for some types of industries and is most effective at the first stage of product processing to increase the speed of moisture exchange and improve the quality of the finished product.

Equipment for drying products you can purchase from us. Delivery in Russia and Belarus. .

Removal of moisture from the material under the influence of intense acoustic oscillations

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Description

Ultrasonic drying - removal of moisture from the material under the influence of intense acoustic oscillations. To a large extent, the efficiency of ultrasonic drying is associated with the acceleration of heat exchange processes in the ultrasound field. In this case, the dried material is subjected to an ultrasonic field with an intensity level of І 145 dB, usually generated by gas-gas emitters.

The mechanism of the effect of elastic waves on moisture depends on the aggregate state of the material, its humidity, the size of the particles of the dried material, the type of moisture with it and the characteristics of the acoustic field.

With very high humidity (moisture content) of capillary and porous materials (200-500%), there is a purely mechanical removal of moisture, which comes down to a peculiar "shaking" liquid from capillaries. This is due to the crushing of the drops when the surface of the material of strong acoustic flows and the appearance of capillary waves occurs. To a certain extent, these processes are similar to the processes flowing with ultrasound spraying, with the difference that in the latter case the energy of ultrasonic oscillations is supplied by the liquid. The mechanical effect depends on the intensity of the acoustic wave, strongly, with an increase in its level above 165 dB and weakens with the advent of the frequency; It is most strongly manifested in the velocities of a standing wave, where the acoustic flows are maximal.

With moderate humidity of capillary and porous material (10-70%), the effects of acoustic oscillations on the drying process manifests itself with a high and low degree of intensification at the first and second stage, respectively.

The first stage, characterized by a constant speed of drying, is distinguished by the fact that the moisture removed from the surface of the dried material is continuously replenished by incoming from its internal layers. The drying rate is determined in this stage with a gradient of the fluid concentration in the diffusion boundary layer. Under the influence of ultrasound, the process of evaporation of fluid from the surface is sharply accelerated, since in a wet surface, acoustic flows arise, causing the deformation of the diffusion boundary layer at the same time, the layer becomes thinner, the concentration gradient grows, which leads to the acceleration of moisture removal from the surface. The essential influence of acoustic flows in the first period of drying is associated with a relatively small thickness of their boundary layer. Comparison of ultrasonic drying with convective with a constant blowing of the material surface shows that even when the speed of acoustic flows is comparable at a constant air flow rate when blowing, the ultrasonic drying proceeds much faster in the fact that the thickness of the boundary layer for acoustic flows is less than the thickness of the hydrodynamic boundary layer ( The latter is approximately equal to the thickness of the diffusion boundary layer).

The acoustic effect in the first stage of drying begins with a certain threshold value of the sound pressure of R CR, depending on the configuration of the body, the type of acoustic flows and the difference of fluid concentrations (that is, the differences in humidity) on the surface of the material with M and in the environment. For example, for powdered materials with conditionally spherical particles with a diameter D smaller than the length of the sound wave:

,

where R C is the wave resistance of the gas environment;

g - acceleration of free fall;

r - gas density.

Typically, the critical level of sound pressure lies within (130-140 dB). The range of the frequencies used depends on many factors, but is mainly determined by the attenuation of the sound in the medium and the permissible noise standards of the operating equipment of 8-18 kHz.

The second stage of drying, designated usually as a period of incident speed, is characterized by a low moisture content of the material and the weak flow of fluid from the inside, in connection with which its decrease on the surface and the effects of acoustic oscillations are reduced to an increase in the diffusion coefficient of fluid as a result of its heating when the ultrasound is absorbed in McCapillary and pores. However, the heating of the material in the sound field is small and an increase in the diffusion coefficient does not exceed 100-200% and the substantial acceleration of drying at this stage is not observed.

Ultrasound use is most efficient during the period of constant drying speed, i.e. In the first stage.

The dignity of ultrasonic drying includes the possibility of accelerating the process in 2-6 times without a significant increase in the temperature of the material, which is especially important when drying easily oxidizing heat sensitive products.

The most appropriate is the ultrasonic drying for fine materials in the process of visualing in suspension or in a state of continuous mixing, because At the same time, there is little value of R CR and ensures uniform processing of the product. Drying speed decreases with an increase in the thickness of the layer being processed.

Temporary characteristics

Initiation time (log to 0 to 1);

Existence time (Log Tc from 1 to 6);

Degradation time (Log TD from 0 to 1);

The time of optimal manifestation (Log TK from 1 to 5).

Diagram:

Technical implementation effect

Ultrasonic drying powder material

Fig. one

Designations:

1 - fine medium;

2 - emitter;

3 - ultrasonic bath.

To implement ultrasonic drying, a standard laboratory ultra-sounding bath fill in a thin (order of centimeter) with a layer of wet powder (quartz sand will go well). Above part of the layer surface at a distance of about 1 cm, lay a flat ultrasonic emitter (magnetostriction or piezoceramics). Turn on the emitter. Make sure that the population in the zone of its irradiation will dry faster than outside it.

Application effect

Ultrasonic drying is a specific type of drying process used in the implementation of many technological processes in industry, agriculture and construction.

The feature of ultrasonic drying is due to a rather high cost of energy used and low efficiency (20-25%) emitters operating in gas environments. That is why it is used mainly in the production of expensive biological and pharmaceutical preparations, in particular, heat-sensitive powders from antibiotics, hormonal drugs, etc.

In recent years, work in the direction of using ultrasonic drying during dehydrating coal dust, drying grain, in the production of dry milk, etc. For drying, ultrasonic dryers are used, which, as a rule, differ from traditional convective dryers only in that in them, a powerful acoustic field is created in the location of the product using a gas resource emitter. When drying, dryers with a "boiling" layer, tunnel, spray and drums are most effective.

Literature

1. Ultrasound / Ed. I.P. Holling. - M.: Soviet Encyclopedia, 1979.

Ecology consumption. Comm: Washing and drying of linen is not such an interesting topic as scientific advances in energy storage or renewable energy sources, but these processes occupy a sufficiently large amount of time, and with it a significant share of the family budget.

Washing and drying of linen is not such an interesting topic as scientific advances in energy storage or renewable energy sources, but these processes occupy a sufficiently large amount of time, and with it a significant share of the family budget.

Modern clothing drying technology that uses heat generated with electricity to evaporate water from clothing, it has long been modifications. There are ways to reduce the amount of water in clothing before drying, for example, high-speed centrifuges.

The new decision was offered scientists from the National Laboratory of Oak Ridge, with the support of the US Department of Energy and GE Appliances, which developed a prototype of innovative clothing drying technology. It can reduce the drying time of linen until 20 minutes, and reduce the amount of energy used for each load by 70%.

Instead of using a high temperature to remove water from clothes, this prototype uses high-frequency oscillations - ultrasonic waves - produced using piezoelectric transducers with a custom amplifier.

"This drying method gives stunning results," said the scientist Laboratory Ayob Momen (Ayyoub Momen), who developed the prototype.

"We were able to dry a piece of fabric in just 14 seconds. If you want to do this in a thermal drying chamber at different temperatures, you will need a few minutes. "

The ultrasonic laundry dryer is effective with a quick removal of water from clothes and has low power consumption, but produces a "cool fog" instead of warm wet air, as happens with ordinary dryers (though, it causes the problems of humidity and mold). For this reason, scientists advise the "emissions" of the dryers outside the building, but who knows, perhaps the next stage for scientists will be the development of a method for re-using a condensed fog for washing the next load load.

According to the US Department of Energy, the laboratory and GE work together on the development of the commercial version of the product, and plan to have a prototype in the fall of this year. If we assume that the ultrasonic dryer will be a cost-effective method for the modernization of commercial washing machines, it will significantly reduce the cost of electricity to drying the linen. Published

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Modern drying machines are not deprived of deficiencies: many hostesses have come across a significant shrinkage after washing. As a rule, fibers of the fabric are reduced as a result thermal impact. A completely different principle of operation has an ultrasound drying, developed at the Oak Ridge National Lab with General Motors.

As it is clear from the title, the drying device works with ultrasonic wavesUnlike traditional hot air devices. After loading the wet linen, the piezoelectric transducer begins to emit high-frequency ultrasound-waves, which completely dry the fabric in less than half an hour. Depending on the density and type of material, the volume of loading, the product will dry from 1 to 20 minutes. Water extracted from the linen turns into steam and flows into a special container where it is condensed. The hostess will only drain the resulting liquid and get absolutely dry products. Thanks to this technology, shrinkage of tissue fibers, as well as the formation of rollers on its surface is excluded.

Due to the high speed, the drying machine has excellent energy efficiency indicators: compared with traditional devices, it spends 70% less electricity.

When to wait for sale

At the moment it is only an invention, but its creators work on the launch of the commercial version of the device, which is planned in 2017. It is assumed that mini-laundry will be the main buyers of this product - they will attract low electricity consumption and available cost Devices. By the way, about the cost: the estimated retail price will be set about 500 US dollars.