Kinetics Of Fruit Crops Drying With Instant Pressure Release

The article covers results of research of drying process of apricot fruits by the method of three-stage energy supply. The main stages of drying have been defined, including: pulsed infrared heating, instantaneous pressure impact, convective drying. Optimal technological parameters of energy supply in the process of drying apricot fruits at each stage have been developed. The choice of the method for the experimental determination of the drying speed by the parameters of the drying agent for the capillary-porous material is substantiated. The sequence of changes in moisture content in a given material under the influence of a three-stage energy supply is described. The conditions for drying fruits are established and the values of the drying speed in the first period for various methods of energy supply have been selected.

The American Journal of Engineering and Technology (ISSN -2689-0984)

INTRODUCTION
In process of drying materials, the pressure value determines the depth of its heat treatment. Due to the gradual increase in pressure, it is possible to change the drying temperature, which ensures the transition of moisture to a free state. Because of this, the boundary between bound and free moisture is shifted towards a lower moisture content when the pressure is released. At moment of pressure release, a rapid advance of moisture occurs throughout the volume of the product, a pressure drop between the center and the surface of the material being processed is aroused, which contributes to the formation of a stream of moisture directed to the surface of the particle in the form of steam. On its way, the vapor-air mixture entrains liquid droplets and is pushed to the surface [1,2].
Depending on the purpose of the process, adiabatic conditions of moisture evaporation lead to a decrease in the temperature of the material, a change in the concentration of liquid by the volume of the material being dried, and a modification or destruction of its structure. When drying with a sharp decrease in pressure under adiabatic conditions, the efficiency of moisture removal from capillary-porous materials significantly increases due to a positive temperature gradient. When the pressure is released in the drying chamber, an overpressure inside the material is formed, which corresponds to the temperature of liquid in the layers of material, in result of which a positive gradient of overpressure across the section of the dried particle is created. Accordingly, with a change in external conditions, pressure release can be achieved [1,2].
Purpose of this research is to remove moisture from a capillary-porous material, i.e. apricot fruit, using a three-stage drying method.
In the proposed drying method, the process of removing moisture from the fruit is carried out in stages [3,4]: 1) the material is pulsed infrared processing in order to soften the porous structure and open micropores; 2) the material is exposed to high pressure with instant release in order to intensify the movement of moisture from the inner layers to the surface; 3) the material is subjected to convective drying.

EXPERIMENTS
In accordance with the aim of the study, the task to study the kinetics of material drying with instant pressure release was set.
The experiments were carried out in installation, the basis of which is a sealed special chamber, where the material samples under study are placed. The pallet for the material is a container with a mesh bottom (mesh size 6x6 mm, wire diameter 0.6 mm), a sample of apricot halves with a layer thickness of 8-10 mm was placed in the pallet. The pallet is placed on angled supports. Infrared generators are placed above the pallet to heat the material. The chamber has a quartz glass window.
The chamber is equipped with a compressed air supply system with automatic support of the set pressure. To remove moisture by pressure relief, the unit has a special valve and receiver. The installation allows heating the sample under pressure up to 10 MPa and relieving pressure. The sample is heated using IR sources with a power of 1 kW each.
The unit is equipped with a control system and automatic data collection of indicators of the heating process and pressure relief. The main indicators that allows to control and manage the process are temperature, material moisture and pressure in the chamber.
The product is placed in a sealed system, in which a pressure of 0.2 to 10.0 MPa is created using a compressor. The material is processed for [2][3][4][5][6] The American Journal of Engineering and Technology (ISSN -2689-0984) Published: October 31, 2020 | Pages: 45-54 Doi: https://doi.org/10.37547/tajet/Volume02Issue10-09 IMPACT FACTOR 2020: 5. 32 OCLC -1121105677 minutes, then the pressure is released with an instant opening of the valve, and the vapor-liquid mixture is removed from the system.

RESULTS OF THE EXPERIMENTS
To study the kinetics of drying apricot fruits (halves of fruits), we carried out experiments with bringing the air pressure to various values (0.4 MPa; 0.6 MPa and 0.8 MPa), created by a compressor with instant pressure release. In all cases, the raw material was previously exposed to pulsed infrared irradiation with a radiation density q=1.25 kW/m2. The appropriateness of choosing this value of the radiation density is shown in [5]. Then the material was subjected to convective drying under the same conditions. Drying curves were constructed and subjected to comparative analysis (Fig. 1).
The curve of fruit drying consists of three stages: heating, constant and decreasing drying speed.
During the heating period, the moisture content of apricot fruits decreases slightly. At the second stage, the moisture content of the fruits decreases intensively with a straight-line dependence. This decrease in moisture is observed until the first critical The American Journal of Engineering and Technology (ISSN -2689-0984) From the curves (Fig. 1) it can be seen that the drying of fruits is more influenced by the pressure created in the working chamber of the drying installation. For example, at a pressure of P = 0.8 MPa and a thickness  = 4 mm of the product (curve 1), the drying process is reduced to 30-40 minutes compared to the process at a pressure of P = 0.4 MPa (curve 3).
To obtain data on the drying speed of apricot fruits, a tangent at any point of the curve is drawn, the slope of which is equal to the drying speed [6].
According to the results shown in Fig. 2 (curves 1,2  In order to intensify the drying process of apricot fruits, we carried out experiments on drying under infrared convection. The study of the process was carried out in the following modes: 1) Drying by convection without pretreatment under pressure in the working chamber at P=0.8 MPa; 2) Drying by infrared convection with pretreatment under pressure in the working chamber at P=0.8 MPa.
IR convective drying was carried out at a flux density q=1.25 kW/m2. In this case, the specific load was =8-10 kg/m2. It can be seen from the graph (Fig. 3a) that the drying time for apricot fruits is 4.5 hours, and its final moisture content is 19-20%. Drying time without pre-treatment was 5.5 hours. а) The 1 -method of combined thermomechanical drying at P=0.8 MPa;

-convective method
Studies have shown that a product exposed to pressure has a peculiar cellular structure. This means that due to the creation of a certain pressure and its instant release during the drying of the fruit, an intensive movement of moisture occurs between the center and the surface of the material being processed. This, in turn, predetermines the possibility of accelerated movement of moisture from the inner layers of the material to the surface. As a result, the process is accelerated 2.5-3 times than with other existing drying methods.
To find the optimal values of pressure and the time of its instantaneous release within the range P=2.0÷10.0 MPa, a number of experiments were carried out. By graphically differentiating the drying curve, the dependence of the moisture content of the fruits on the drying speed in various ranges of the generated pressure and the time of its instantaneous release was obtained.
Experimental data obtained for the drying speed in the first period, depending on the generated pressure and the time of instant pressure release in the working chamber of the dryer are presented in Table 2.

CONCLUSIONS
Analysis results showed that the drying agent is using its full potential with this drying method.
In the proposed drying method, the process of applying pressure and releasing it is quick and takes only 4-12 minutes, while 8-14% of moisture is removed from its total amount, depending on the pressure generated.
Therefore, the studies carried out using the example of drying apricot fruits show that the proposed drying technology has a number of advantages:  Reduction of energy consumption for moisture evaporation by 1.7-2.0 times;  Full utilization of heat in a closed circuit;  Preservation of biological active substances due to the lowered temperature in the circuit;  The material consumption of equipment decreases.