EXPERIMENTS ON IMPROVING THE EVAPORATOR DESIGN FOR FRUIT JUICE CONCENTRATION

ABSTRACT

It has been demonstrated that, in the concentration of food product solutions, the efficiency of the complete evaporation process can be improved by installing an “aerator” (a device that divides the liquid flow into small channels) in the inlet pipe of the evaporator. A schematic diagram of the proposed evaporation unit is presented, which makes it possible to obtain a final product in the solid phase with a concentration of up to 90 °Brix. Using apple juice as an example, it was determined that, during the evaporation process under reduced pressure and with the use of an aerator, the evaporation rate remains stable, and the concentration can be increased to 80÷85 °Brix. The fact that the product temperature did not exceed 35÷40°C indicates that its high nutritional value was preserved.

АННОТАЦИЯ

Показано, что при концентрировании растворов пищевых продуктов эффективность процесса выпаривания может быть повышена за счет установки «аэратора» (устройства, разделяющего поток жидкости на мелкие каналы) во входном патрубке выпарного аппарата. Представлена конструкция предложенной выпарной установки, позволяющей получать конечный продукт в твердой фазе с концентрацией до 90 °Brix. На примере яблочного сока установлено, что в процессе выпаривания при пониженном давлении и с использованием аэратора скорость испарения остается стабильной, а концентрация может быть увеличена до 80÷85 °Brix. То, что температура продукта не превышала 35÷40°C, свидетельствует о сохранении его высокой пищевой ценности.

Keywords: food industry, energy efficiency, evaporation, concentration, moisture removal, aerator, food concentrates.

Ключевые слова: пищевая промышленность, энергоэффективность, выпаривание, концентрирование, удаление влаги, аэратор, пищевые концентраты.

Introduction. Expanding the range of dried products, introducing innovative and energy-efficient drying equipment and technologies, developing the scientific foundations of product concentration, and creating new types of equipment are among the pressing global challenges [1].

Concentrates from solutions are usually obtained through evaporation and subsequent drying. The amount of energy required to convert 1 kg of water into steam is approximately 2.5 MJ. Compared to evaporation, drying is a less intense process, with energy consumption being 1.6–3 times higher. In evaporation, at least 85% of the energy supplied to the raw material is used effectively, whereas in drying, this figure is at most 40%. The two processes differ in their physical nature: in evaporation, the driving force is the temperature difference, while in drying it is a mass transfer process driven by the difference in concentrations.

The production of dried products is usually carried out in two stages: first, the product concentration is increased through evaporation, and then the remaining moisture is removed in specialized drying equipment. In this case, the final concentration of the finished product does not exceed 35–60%. The concentrated product from the evaporator is then sent to drying units.

In solution concentration technology, there are three main methods for removing moisture: membrane methods, evaporation, and cryoconcentration. Each has its own advantages and disadvantages. Membrane technologies are widely used for producing fresh drinking water and purifying wastewater [2]. However, in the concentration of food product solutions, evaporation technologies are preferred in industry because they have been tested over time, are reliable, and can operate at large capacities. The main problem with evaporators is that, as the solution concentration increases, its viscosity also rises, the circulation of the solution in the apparatus slows down, and the thermal resistance and temperature in the boundary layer increase. Effective solutions to this problem have not yet been found. Therefore, in practice, the final concentration of the finished product is limited to 25÷60%. High-quality products can be obtained using cryoconcentration technologies, particularly block freezing [3,4]. However, these technologies are also limited to achieving a final concentration of up to 50 °Brix.

Research Objective. The aim of the study is to develop a new concentration technology and equipment that ensures higher final concentrations than traditional technologies, while maintaining the nutritional value of the product and achieving efficient energy utilization; as well as to investigate the kinetics and hydrodynamics of the evaporation process.

Materials and Methods.. The main differences between the proposed method and conventional evaporation equipment are presented in Figure 1.

Figure 1. Schematic diagram of the evaporation process

The proposed evaporator differs from conventional equipment in the following way: the process is significantly intensified due to the increased contact surface area between the liquid phase and the vapor phase. This can be represented in the form of a physical diagram (Figure 2). In a conventional evaporator, the feed solution is supplied to the inlet pipe of the evaporation chamber, and the energy of the heating steam is spent on producing secondary vapor from the solution. In this process, complex heat transfer occurs from the condensing steam to the solution. The efficiency of heat transfer is determined by the total sum of thermal resistances, with the primary resistance being the thermal resistance of the boundary layer.

Figure 2. Elements of conventional (a) and proposed (b) evaporator designs

During the evaporation process, the concentration of the solution increases, and its viscosity rises. This leads to an increase in the thickness of the boundary layer and a corresponding rise in product temperature within this layer. As a result, a “cooked” flavor characteristic of overheating during high-temperature processing may develop, or the product composition may deteriorate. Even organizing circulation of the solution to enhance flow does not solve this problem. Therefore, in practice, the final concentration of the finished product is limited to a specified threshold [5-8].

In the proposed design, this process occurs differently. In a conventional evaporator, the liquid flows as a single stream, whereas in the proposed unit, the liquid flow is divided into several small channels (Figure 2b). Due to the increased contact surface area between the liquid and vapor phases, components that pass into the vapor phase are generated more intensively. This makes it possible to reduce the moisture content in the solution to as low as 5÷10% [6]. Experimental studies were carried out based on the parametric model presented in Figure 3.

 Figure 3. Parametric model of the evaporator: a_s - thermal diffusivity coefficient of the solution, m²/s; c_s - specific heat capacity of the solution, kJ/(kg·°C); ρ_s - density of the solution, kg/m³; λ_s - thermal conductivity coefficient of the solution, W/(m·°C); ν_s - kinematic viscosity of the solution, m²/s; r_b - latent heat of evaporation (for acetone r = 525 J/kg);  W_s - solution flow rate; x_b, x_о - initial and final concentrations of the solution; H - apparatus height; D - diameter; V - product volume; d - aerator diameter, m; n - number of slots; α - slot angle; W - steam flow rate; j - specific energy consumption for concentrating 1 kg of product, J/kg.

Experiments on the process of fruit juice concentration were carried out using a special test installation (Figure 4). The working chamber volume of the experimental unit was 0.042 m³, and the volume of the solution in the chamber was 0.014 m³. The evaporation chamber and the condenser were connected by a steam pipeline.

Figure 4. Experimental setup diagram for the evaporation process:

1 - heating chamber, 2 - separator, 3 - condenser, 4 - cold-water tank, 5 - condensate collector

The experiment was carried out as follows: The initial solution was fed into the heating chamber (1). In the heating chamber, the solution was heated to its boiling point and then directed tangentially into the separator (2). An aerator (mesh) installed at the inlet nozzle of the separator served to split the liquid into several small channels. This increased the contact surface between the liquid and vapor phases, causing water molecules in the solution to pass into the vapor phase.

In the separator, the secondary steam separated from the liquid phase was directed into the condenser (3) and condensed. The resulting condensate was collected in the condensate collector (5). The condensation of the secondary steam was achieved by circulating the liquid from the cold-water tank (4).

The cooling system consisted of a compressor refrigeration unit, a cold-water storage tank, a temperature control regulator, and a circulation pump, which supplied cold water to the condenser. The system was equipped with flow meters to measure the consumption of the initial solution, concentrated solution, water vapor, and condensate, as well as standard control and measuring instruments to monitor and measure the operating temperature in the heating chamber, the temperature and operating pressure in the separator, and the vacuum level in the condenser.

Experiments were conducted at a pressure of 10÷20 kPa, a temperature of 40÷60°C, and a solution flow rate of 0.6÷0.8 kg/s. Apple and apricot juices were chosen as the study objects (Table 1).

Table 1.

Initial and final concentrations of fruit juices

The following objectives were set for conducting the experiments: To determine the effect of operating pressure, temperature, solution concentration, aerator dimensions, and chamber loading level on the evaporation kinetics. To establish the dependence of evaporation rate (steam generation) on the input parameters. To analyze the experimental results and present them in the form of a generalized variables model.

Research results. Let us consider the results of the apple juice concentration experiment. In the first stage of the experiment, the influence of the input parameters on the amount of moisture removed from the apple juice, the kinetics of juice concentration increase, and the evaporation rate during the process was determined (Figures 5–7). At the initial stage, the experiments were carried out under low pressure (Fig. 5), which is of practical interest for most juices and extracts. According to Fig. 5, the systems for delivering energy to the solution and transferring energy during secondary steam condensation operate in a coordinated manner, ensuring that the evaporation temperature remains stable. The design of the equipment meets the requirements for tightness — no air leakage from the external environment into the unit was detected. The evaporation process temperature complies with the technological requirements for producing high-quality concentrate.

Figure 5. Dependence of the operating temperature of the apple juice evaporation process on pressure

The effect of pressure on the kinetics of apple juice concentration growth is shown in Fig. 6, from which it can be seen that at 10 kPa the rate of concentration growth is higher. The difference in juice concentration between 10 kPa and 20 kPa reaches 15–20%.The main indicator of the evaporation process — steam generation capacity — was determined by the weight of the collected condensate (Fig. 7). This dependence was plotted over the entire range of obtained concentrations.

Reducing the pressure in the chamber increases the evaporation rate. In the studied range, this difference reaches up to 20%.

Conclusions on the effect of operating pressure on fruit juice evaporation:

• Reducing the pressure in the evaporation chamber has a positive effect on all process characteristics: the temperature decreases, steam generation intensity increases, processing time shortens, and the final concentration becomes higher.
• The studied pressure range corresponds to practically feasible operating conditions for the developed unit.

Conclusions on the effect of the aerator on fruit juice evaporation:

• The second factor that determines evaporation efficiency and allows process control is the aerator size.
• In the experiment, the pressure was maintained at 10 kPa, while the initial juice concentration and the loading volume remained constant (Fig. 8).

Figure 8. Aerator shape (a – square, b – rhombic) and installation scheme in the apparatus (c)

In the investigated range, the results corresponded to the regime for obtaining high-quality concentrate. The process kinetics can be controlled by adjusting the processing time and the final concentration (Figure 9).

With different aerator designs, the use of a rhombic shape compared to a square one allows influencing the steam output capacity more than threefold (Figure 10).

Conclusion
The main problem of evaporation equipment is that the formation of a boundary layer causes a sharp increase in product viscosity and temperature, making it impossible to achieve high concentrations. To solve this problem, it was proven that installing an “aerator” (a device that splits the liquid flow into small channels) at the inlet pipe of the evaporator enables complete evaporation of the product. It was confirmed that installing an aerator at the inlet pipe of a multi-stage evaporator is technically feasible. The developed design prevents the formation of a traditional boundary layer. Moreover, the design makes it possible to obtain apple juice concentrate of up to 85 °Brix at low temperatures (up to 40 °C). The typical “cooked” taste, which usually appears when processing products at high temperatures, is not observed.

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