CC BY
1УДК 54 Babayev R.K., Meherremova A.E.
Babayev R.K.
Ph.D., Associate Professor Azerbaijan State University of Petroleum and Industry (Baku, Azerbaijan)
Meherremova A.E.
Master's degrstudent Azerbaijan State University of Petroleum and Industry (Baku, Azerbaijan)
STUDY OF THE PROCESS OF CONVECTIVE DRYING OF WET MATERIALS
Аннотация: convective drying is one of the energy-intensive processes used in the chemical, woodworking, food and other industries. Therefore, in modern conditions, when there is an increase in the deficit and growth of tariffs for energy resources, it is relevant to develop and apply new effective methods of drying wet materials in industrial production, create high-performance drying equipment, improve the operation of existing dryers, which will contribute to the rational use of natural resources, reduce the cost of finished products and increase the competitiveness of production.
Ключевые слова: drying, moisture content, clay, chamber dryer, mathematical model.
The kinetics of thermal drying of a wet material considers the issues of the mechanism and rate of moisture transfer in the form of liquid and vapor, as well as heat transfer between the material and the drying agent. Establishing the mechanism of the drying process is a fairly complex task, the solution of which requires establishing the sequence of elementary acts and the dynamics of each act. The drying rate can be affected by the conditions of the process (temperature, pressure, etc.), the peculiarity of the porous structure of the material (the number and configuration of pores, the
distribution of pores by size, the nature of their connection with each other), the degrof filling of the pores with liquid, the ability of a porous body to intermolecular interaction with moisture (lyophilicity, lyophobicity), the mobility of the body framework, etc. [1]. The movement of liquid in the pores and capillaries of porous bodies can be carried out by capillary transfer under the action of capillary forces, film flow caused by the gradient of the wedging pressure of the film, surface diffusion, thermocapillary transfer in the presence of a temperature gradient in the volume of the body and along the height of the capillary, filtration transfer under the action of the gradient of the total pressure in the material, etc. The transfer of vapor in porous bodies can occur by molecular diffusion due to the difference in vapor concentration, Knudsen diffusion in narrow pores due to the collision of molecules with the surface of the pores, Stefan diffusion in dead-end pores, thermal diffusion by thermal sliding in micro- and macropores, barodiffusion due to the molecular transfer of a component with a large mass to an area of increased pressure, etc. The rate of drying is significantly affected by the form of moisture bonding with the material. The following classification of bonding forms is conventionally accepted: chemical, physicochemical and physicomechanical. During the drying process, only moisture bonded with the material physicochemically and mechanically is removed. It is obvious that the stronger the bond, the more difficult the drying process. Therefore, moisture mechanically bonded with the material is most easily removed [2-3].
A wet body can have a movable or rigid skeleton. During the drying process, a material with a moving skeleton, such as gelatin, changes its shape and volume. Porous bodies with a rigid skeleton, when moisture is removed, are deformed within the limits of elastic deformations.
During the drying process, the temperature of the material also changes. During the warming-up period, the temperature of the material increases from the initial temperature to the wet-bulb temperature of the environment. In the first period, the temperature of the surface and the entire volume of the wet material is the same, constant, and equal to the wet-bulb temperature. In the second period, the temperature of the material gradually increases, approaching the ambient temperature [4]. The ideal
model of liquid motion in pores is Stokes' law for liquid flow in a cylindrical capillary [5]. The application of this law for practical calculations is difficult, since real porous bodies have pores and capillaries of different diameters, different tortuosity, shape and roughness. A particularly strong deviation from Stokes' law is observed during liquid flow in micropores, the radii of which are commensurate with the radius of surface molecular forces.
Other models of a capillary-porous body that can be used to study the drying process are also discussed in the literature. Among them, one can note the two-capillary model, the model with communicating capillaries etc. In these models, the intensity of moisture evaporation is usually related to the geometric surface of the capillary. For calculations, it is more correct to use the true evaporation surface. However, its experimental determination is associated with significant difficulties, since moisture evaporation usually occurs not from the entire geometric surface of the capillary, but only from the surface of the liquid meniscus, and the shape of the capillary, the configuration and surface of the meniscus also change [6-7].
To conduct experimental studies of the drying process of wet materials, a laboratory setup was created, the diagram of which is shown in Fig. 1. Outside air is fed by fan 8 into electric heater 6, where it is heated to a specified temperature. Air flow is regulated by rotameter 7 using valve 9. Then the heated air enters chamber dryer 1, which contains wet material 2. Exhaust air is discharged from the opposite end of the drying chamber. The rate of moisture removal from the material is determined by measuring the mass of a material sample, which is suspended from analytical scales 3. Temperature control of air entering the drying chamber, exhaust air, air inside the drying chamber and material is carried out using thermocouples 5, connected to multichannel potentiometer 4.
The chamber dryer consisted of a metal body 1 covered with heat-insulating material. Inside the dryer, a fan 2 was installed for mixing the air, an additional (internal) electric heater 3 for heating the air, and a grate 4 on which samples of wet material were placed.
1 1
Figure 1. Diagram of a chamber dryer: 1 - body, 2 - fan, 3 - additional electric heater (internal), 4 - grate, 5 - wet material, 6 - hot air inlet, 7 - exhaust air outlet.
The clay drying process was studied in a hot air flow dryer. In all experiments, the air flow rate was taken as constant and equal to 2.7x10-3 m3/s. Its temperature was 67, 79, 90 and 103 °C. Twelve clay samples with a diameter of 0.02 m or 4 samples with a diameter of 0.04 m with an initial moisture content of 0.3 kg/kg and an initial temperature of 20°C were placed in the dryer. Wood drying was carried out according to a scheme with air heating in the main (external) electric heater at an air flow rate of 1.39x10-3 and 1.11x10-3 m3/s. Its temperature was 71, 85, 99, 113 and 119°C. 20 wood samples with an initial moisture content of 0.35 kg/kg and an initial temperature of 20 °C were placed in the dryer. The wood samples were dried to a stable moisture content, which, depending on the process conditions, varied from 0.05 to 0.03 kg/kg.
The wood drying process was also studied under conditions when, simultaneously with the inclusion of an additional heater located inside the dryer, the volumetric flow rate of hot air entering the dryer was reduced. During the experiments, hot air was supplied to the dryer with a volumetric flow rate of L = 1.39 10-3 m3/s and a temperature of tg.in = 71 oC. At the first stage, which occurred 165 minutes after the start of the process, L decreased from 1.39x10-3 to 1.11x10-3 m3/s, td.k = 178 oC and tg increased from 60 to 70 oC. At the second stage (after 330 min) L decreased from 1.11 10-3 to 0.89 10-3 m3/s and tr increased from 70 to 80 oC. At the third stage (after 330 min) L decreased from 0.89 10-3 to 0.71 10-3 m3/s and tr increased from 80 to 90 oC.
Table 1 Summarizes the main technical characteristics of the laboratory dryer.
Name of the indicator Values
Dryer air capacity L 103, m3/s 0,71 - 2,7
Air temperature at the inlet to the device tr init, 0C 67 - 119
Initial air temperature in the dryer ,0C 20
Initial moisture content of the material uaver , kg/kg: clay wood 0,3 0,35
The mathematical description of moisture transfer in a spherical particle during a period of constant drying rate includes the following equations:
the equation of material balance of a flow-type ideal mixing apparatus:
= Gci [xcLin. — xcl ], (1)
t T u duaver.
Pel vcl ^T Pdr.m. vdr.mat.
equation of mass conductivity for a spherical body:
ди(г,т) дт
= к
д2и(г,т) 2 и(г,т)
дг2
г дг
, (2)
equation for determining the average moisture content of a solid
Uaver CO = -^¡*Г2и(Г, r)dr (3)
the results of numerical integration of the differential equation (1) under the initial condition using the Euler method with a time step An are shown in the figure 2.
Figure 2. Change in the moisture content of the air inside the dryer over time during the drying process of clay particles with a diameter of 0.02 m (a) and 0.04 m (b): L = 2.2x10-3 m3 /s, tr. in oC: 1 - 67, 2 - 103.
As can be seen from Fig.2, during the period of heating the material, the moisture content of the air quickly increases to a certain value. During the period of constant drying speed under steady-state conditions, one should expect the moisture content of the air to remain constant over time, since during this period there is intensive surface evaporation of frmoisture at a constant speed. However, for the studied conditions, in the first period of drying, the moisture content of the air inside the dryer continues to change, reaching a maximum value, since the air temperature inside the dryer increases,
As a result of the study, based on the proposed model of low-temperature drying of clay particles, the dynamics of the moisture content fields was established, which in the period of constant speed is characterized by a difference in moisture content between the surface and the center of the body within 0.03 kg/kg, and in the period of decreasing speed, the differences in moisture content can increase in comparison with the first period of drying by 4 - 5 times.
СПИСОК ЛИТЕРАТУРЫ:
1. Mushtaev V.I. Drying of dispersed materials. - M.: Chemistry, 1988. - 352 p;
2. Modern approaches to the study and description of drying processes of porous bodies. Novosibirsk: Publishing house of the Siberian Branch of the Russian Academy of Sciences, 2001. - 300 p;
3. Ilyasov U.R. Mathematical modeling of drying of wet porous material in the diffusion approximation // Thermal physics and aeromechanics. - 2008. - V. 15, No. 4. - pp. 689 - 697;
4. Mathematical modeling and numerical techniques in drawing technologies. New York: Marcel Dekker, 1997. - 696 p;
5. Kudra T. Advanced drying technologies. New York, Basel: Marcel Dekker, Inc, 2002. - 472 p;
6. Haleen Len W. Aggressive convective drying in a conical screw type mixer/dryer. Patent US 5709036 А
