STUDYING THE TECHNOLOGY OF DRYING PRODUCTS
STUDYING THE TECHNOLOGY OF DRYING PRODUCTS
Andrey Ponasenko
Advisor to the Rector Belarusian National Technical University and researcher Tashkent State Technical University,
Uzbekistan, Tashkent
Jasur Safarov
DSc, professor Tashkent State Technical University,
Uzbekistan, Tashkent
Shakhnoza Sultanova
DSc, professor Tashkent State Technical University,
Uzbekistan, Tashkent
Vegetable raw materials, which include root crops, have a colloidal, capillary-porous structure, with fragments of high-molecular carbohydrates, proteins, lipids, vitamins, macro- and microelements. Preparation of vegetables for drying and the process of traditional dehydration of the product itself can lead to a significant loss of biologically active substances. In this regard, the task of improving the method of drying vegetables with the maximum preservation of physiologically valuable substances of the feedstock is very relevant. Convection drying is the most common method of dehydrating vegetable raw materials in order to extend their shelf life. The method of convective drying in the traditional version provides for the transfer of heat to the raw material to be dried using hot air. During the transfer of thermal energy, moisture is released from the raw material, which is carried away from the installation by the drying agent.
The technical literature describes various methods for dehydrating raw materials of plant and animal origin. In pilot industrial conditions, with our participation, a method for drying vegetable raw materials was developed with a theoretical justification and experimental confirmation of the expediency of preliminary blanching and processing of raw materials with a low-frequency electromagnetic field (EMF LF) to move moisture from the center to the surface. The essence of the technique lies in the use of resonant frequencies in order to maximize the redistribution of moisture from the center of the product to the surface, followed by drying the product in the microwave installation.
Drying processes occupy a very important place in industries such as food, woodworking and textiles. The features mentioned above should be taken into account to save energy when choosing a dryer. More than 200 dryers are widely used in the industry, of which only 20 are main dryers [1-4].
Drying directs a stream of hot air towards the material in the dryer, allowing heat to be transferred to the material by convection while removing evaporating water from the environment. This process continues until an equilibrium moisture content is formed in the product, depending on the relative humidity and air temperature.
Drying is a complex process consisting of simultaneous mechanisms of heat and mass transfer. The air drying process usually consists of a constant speed followed by a period of decreasing speed. During the period of constant speed, the surface is covered with water. As water evaporates, mass transfer from the surface occurs. Air velocity, temperature and relative humidity are factors that affect the drying rate in this process. During the period of velocity drop, moisture transfer is controlled by internal mass transfer mechanisms such as capillary flow, liquid and vapor diffusion. One or more of these mechanisms may operate simultaneously during the slowdown period. Air temperature, chemical composition, physical structure and thickness of the product affect the drying rate. In hygroscopic materials, 2 periods of decrease in speed are observed. During the 1st deceleration period, the wet surface area decreases as product moisture is released into the air. After the surface has dried, the second period of decline begins, and evaporation occurs inside the product [5-6].
The mass of water transferred by evaporation per unit area per unit time determines the rate of drying. When a wet product is started to dry, the surface of which is covered with a film of water, the drying rate is equal to the rate of evaporation from the surface of the water. As long as the speed, temperature and air humidity remain constant, the drying speed does not change. Humidity at the moment when the water film on the surface begins to disappear is called the first critical humidity. The constant rate period is a drying period that lasts until the critical moisture content drops. The change in absolute humidity and the rate of drying of the material over time is shown in fig. 1.
а)
b)
c)
d)
Figure 1. a) change in drying rate over time b) change in drying rate depending on humidity c) change in humidity over time d) change in temperature over time
The moisture content of the product is constantly decreasing in different periods of time. On the other hand, the surface temperature is constant in the BC range, since the external conditions (air speed, humidity, temperature) are constant depending on the dried air. This is due to the smooth evaporation of the liquid. In the BC region, heat transfer is balanced by mass transfer.
AB: The process of heating or cooling a wet product until equilibrium is reached.
BC: Stable evaporation of the liquid during the period of constant speed
A: The first critical point at which dry spots begin to form on the surface of a wet product.
CD: First fall rate period
D: The second critical point where dry patches are visible where the surface has completely evaporated.
DE: Second Fall Rate Period.
A short time after the solid begins to dry from point A, it enters a period of constant speed called "SHP". In this cycle, the drying speed is constant, while variable speed, humidity and air temperature affect the drying speed. The surface of a solid in this equilibrium state is completely covered with a layer of moisture, and its surface temperature is equal to the air temperature according to a wet bulb. However, when the amount of moisture in the material begins to decrease, the transfer of liquid from the inner region to the surface becomes more difficult due to the increase in frictional resistance in the capillary spaces and the surface does not remain wet all the time. As the solid product continues to dry, the drying rate does not remain constant and the water film begins to disappear at point C of the first critical moisture. After some time, the water film on the surface completely disappears. After this point, a continuous decrease in mass transfer is observed. This period is called the period of decreasing speed. Point D is called the second critical humidity. As drying continues, the drying rate continues to decrease depending on the rate of movement of water from the substance to the surface, and its rate becomes zero when the moisture content of the product is in equilibrium with the relative humidity of the drying air. During the period of speed reduction, as a result of dryness on the surface of the product and an increase in surface temperature, defects in structure and quality are observed in the product. Exceeding the critical value of humidity leads to hygroscopic loss of moisture. For this reason, it is preferable to dry in the BC area, where the critical moisture content of textiles is not exceeded [1-6].
The reason why it is called the critical point is that during the drying process, the internal resistance of the liquid in the solid surface limits the degree of drying. After this point, evaporation occurs on the surface and the degree of drying decreases. At the second critical point, the surface completely evaporates. During the period of decreasing velocity, the heat transferred to the surface exceeds the energy required to evaporate the liquid. The surface temperature approaches the dry bulb temperature. As a result, mass transfer is reduced and equilibrium moisture content is reached.
Products are divided into hygroscopic and non-hygroscopic substances depending on the properties of the moisture they contain. Hygroscopic materials are substances that can absorb water. The hygroscopic balance of the product with the environment changes depending on the temperature and humidity of the air. If the moisture content of the product is not balanced with that of the drying air, the product absorbs or releases water. The reason for this situation can also be an indication that the partial pressure of water in the material differs from the pressure of water vapor in the surrounding air. These materials can only be dried until they reach an equilibrium moisture content.
Non-hygroscopic materials are sand, clay, glass, and other substances that do not contain water, for example, the partial pressure of water in the material and the vapor pressure of water in air are equal to each other. During the drying process, the dry product has the least amount of moisture, depending on the air temperature and relative humidity at equilibrium humidity. Hygroscopic substances have only equilibrium moisture content.
Due to the voids in the materials, their structures are called porous. These spaces can be filled with water or air. If the water vapor pressure in the product is greater than the partial pressure of water vapor in the environment, moisture is transferred from the product to the environment. If the water vapor pressure in the product is less than the partial pressure of water vapor in the outdoor air, moisture is transferred from the external environment to the product. The balance between the humidity of the ambient air and the moisture content of the product is called the hygroscopic balance.
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