Heat-exchange technological processes are widely used in various industries to create the necessary conditions for processing or changing the temperature state of equipment, as well as production blanks. At enterprises where the tasks are set to change the characteristics of liquid media, heat can be used as a means of maintaining boiling. Technically, similar problems are solved with the help of evaporators provided with a special set of functional components for organizing the heat exchange process.
What is the evaporation process?
In the industrial sector, evaporation is considered as a method of concentrating liquid solutions, which are based on low-volatile or non-volatile substances dissolved in volatile active mixtures. This process is carried out as a resultevaporation of the solvent during boiling. This procedure is most often subjected to alkalis, s alts, as well as high-boiling liquids. But in each case, the main task of the process is to obtain a pure solvent or individual substances in a high degree of concentration. If we are talking about targeted purification of a specific component of the solution, then the evaporation process can be supplemented by a crystallization operation, in which the formation of the target substance in solid form is possible.
From a technological point of view, evaporation is a combination of a number of heat exchange operations. The complexity of the technical organization of this process necessitates the use of special equipment. In this capacity, a vacuum evaporator plant with an optimized design is used, designed to perform the main evaporation processes, as well as auxiliary operations. It is important to keep in mind that evaporation involves the use of aggressive media - hot liquids, gases, water vapor, etc. Added to this is the unfavorable background from target chemically active substances. These and other factors of adverse technological impact require the use of special materials for the assembly of evaporators, which increases the protective properties of the structures.
Basic device of the evaporator
Most modern industrial evaporators use a multi-component system based on a heat exchanger with a condenser and an evaporation chamber. To optimize the process and more efficient concentration of solutions, the presence ofseparator is a unit that is connected in a separate order through the gas duct and organizes the removal of secondary steam. Separators of external type are more commonly used, which operate under conditions of centrifugal force. What is fundamentally different vacuum evaporator? Creating a vacuum allows you to achieve the effect of soft evaporation. This provides two positive points - the acceleration of the evaporation process (the serviced solution spends less time in the chamber) and an increase in the quality of the concentrated substance. The output products can be used in other technological operations at the same target processing enterprise. To do this, they organize the connection of individual modules with outlet flows, thanks to which not only the removal of excess gas mixtures is carried out, but the regulation of the flow is ensured with the necessary delivery parameters in terms of pressure force and movement speed. What's more, many evaporators can optionally be paired with pre-treatment or waste dilution units to meet the requirements of processes where the same gas can be reused.
Apparatus with forced circulation
The design is based on a vertical or horizontal shell-and-tube heat exchanger with a heating chamber and a concentric separator. The working process is supported by a circulation pump station and a flash vessel. Usually, the forced process of movement of working mixtures is implemented in double-shell evaporators withcountercurrent circulation scheme. As part of such devices, there is also a device for distillation and steam purification from organic and s alt compounds. The average capacity of the forced circulation evaporator is about 9000 kg/h, and the concentration ratio reaches 65%.
During the operation of such a unit, the liquid circulates along the contours of the heating chamber due to the force provided by the pump. In the chamber, the temperature of the liquid is brought to the boiling point, after which the pressure in the separator block is sharply reduced. From this moment, the process of active evaporation of part of the liquid begins. What are the benefits of using this type of unit? This is the most effective solution when handling viscous and problematic contaminated mixtures. For example, for the evaporation of saline solutions, this option is more appropriate than single-effect evaporators, which can show a higher circulation rate, but their power will not be enough to provide even an average level of productivity. By the way, modern evaporators with forced circulation perform boiling and evaporation operations not on the heating walls in the main chamber, but in the separator, so the contamination of the main working unit is minimized.
Evaporators with plate heat exchanger
The design feature of such installations is the presence of special plates, due to which the working medium is directed through the heating chamber along alternating channels. To seal the plates, special gaskets are used - they are alsoperform the function of thermal insulation, which increases the efficiency of heat transfer.
As a rule, these are multi-effect evaporators with a capacity of about 15 t/h. The heating flows of water and the target product move in countercurrent along their channels, giving off part of the energy. The force for the movement of media is generated by the same circulation pump, however, the design of the plates is designed to support the effect of turbulence in the circuit, which reduces the required power potential to support the transfer of the product and coolant. As a result of active heat exchange, the working medium boils, after which steam is formed. Residual liquid products are cut off in the separator block due to centrifugal force.
This is one of the few cases when it comes to a universal evaporator in terms of the ability to work with different technological media. In particular, the principle of operation of an evaporator plant with a plate heat exchanger allows the use of steam-gas and aqueous media. At the same time, a high concentration quality is ensured, since evaporation is carried out evenly in a gentle mode in one pass. The design itself is maximally optimized in size, which facilitates installation and technical measures. So, the height of the installation space with all communications and connecting piping for such a device is 3-4 m.
Three-effect natural circulation evaporators
Structurally, such devices are distinguished by the presence of a shortvertically located heat exchanger and the upper placement of the separator. The working fluid is supplied from below, after which it rises through the heating pipes through the chamber. The principle of an ascending film or gas lift is implemented. If in oil and gas fields the associated gas carries out the product, then in the case of a three-vessel evaporator, hot vapors lift the liquid along the shell-and-tube circuits. The whole process takes place against the backdrop of boiling. The liquid separated from the vapor is carried out through the return pipe to the heat exchanger, after which it is again sent to the separator for the next separation session. This process is repeated several times until the desired concentration level is reached.
The evaporation rate in this case is determined by the temperature difference in the heating chamber and the boiling unit. Both indicators can be adjusted by automatic control. The natural circulation in the vacuum evaporator allows a high specific capacity with a fast start-up. But one should not rely on the maintenance of solutions containing complex or thermally unstable compounds. This is a highly specialized equipment, the calculation of which is made for the chemical and food industries, where it is required to perform point separation operations with a small capacity load. For example, glycerin evaporators provide a processing speed of 3600 kg/h.
How a barometric condenser works
Varietymixing heat exchangers, which do not perform surface separation of working media in the process of overflow, but allow their mixing. In other words, at the moment of heating, the conditional concentrated solution can come into contact with the process hot medium represented by steam or water. The barometric condenser itself is part of a complex evaporator plant, which performs the processes of mixing cooling water and secondary steam. Since the volumes of the newly formed condensate are less than the volume of steam, a natural vacuum occurs. To maintain it, it is necessary to remove atmospheric air from the condenser, which is sent there along with the coolant flows. In some designs, air can also enter through defects in the capacitor case. The output of mixed mixtures from the condenser is carried out through a barometric tube. It is pre-immersed in the liquid and forms a hydraulic seal that prevents the passage of air into the condenser.
The principle of operation of the capacitive apparatus
A special kind of equipment for technological evaporation processes. The main difference between capacitive units in terms of the principle of operation is the support of free circulation mode, which is achieved due to the internal configuration of the location of the circuits in the heat exchange system. The infrastructure of the heat exchange network is formed by tube bundles, coils and other elements that create conditions for a multi-stage and in many respectsdifficult process of transfer of thermal energy. By the way, capacitive evaporators are practically not used in work with viscous, heat-sensitive and crystallizing solutions precisely because of the free, but slow circulation of flows. Moreover, the heat transfer coefficients in this system are small, which negatively affects the overall evaporation performance. How do capacitive devices justify themselves? They are successfully used in small-tonnage industries, where the heat transfer coefficient is not so significant with output volumes. The internal structure of capacitive evaporators, with all its shortcomings, opens up a lot of opportunities for organizing directed circulation, which is very important in enterprises with low structural mobility when connecting communication channels.
Evaporator calculation
In an integrated evaporator design, individual calculations are made for each component, since the characteristics of the production process can change at each stage. As a rule, the following are used as initial data:
- approximate steam pressure;
- heat of concentration;
- properties of the initial solution;
- heat loss level;
- heat transfer coefficient;
- design parameters that are already set and cannot be changed.
For three-effect evaporator plants, the calculation with the above-mentioned initial data can be carried out using several parameters at once, including the power of the circulation pump, the volume of the heating chamber,the maximum amount of serviced liquid, etc. The most important design tasks include design calculation of the same barometric condenser, separator and determination of the characteristics of the piping elements. In particular, the intensity of evaporation in systems with continuous evaporation will depend on the diameters of the nozzles and the length of the transition pipes.
Workflow Requirements
Calculated indicators for the organization of the evaporation process may not give the expected effect if the requirements for the external environment are not met. A lot will depend on the conditions inside the room where the equipment is used. According to the requirements, once-through evaporators can only be used in rooms with an area of at least 4.5 m2 and a height of 3.2 m. like a chimney. It will not be superfluous to provide an adjustable hood with a gate and a thrust setting.
The ventilation system is designed according to special rules. It should include inflow channels and exhaust systems with direct connection to the areas where the evaporation process is directly performed. It is obvious that a complex ventilation system operating in a regular mode in two directions will require serious power support. But at the same time, the emitted noise from channels and operating equipment should not exceed 75 dB. And this is not to mention compliance with the requirements of fire andelectrical safety. If the evaporator plant regularly works with gas mixtures, then a special air degassing system must be organized. It can be part of a single complex of heat exchange communications, which will allow, in some operational aspects, to complement the functions of both systems.
Conclusion
Evaporation and concentration operations have long been used in industries both as main and as secondary technological processes. In most cases, materials are prepared in this way for further stages of manufacturing products or preparing technical means. Vacuum evaporators and installations can be put among the most productive tools for solving such problems. High performance indicators are explained by the presence of the function of a circulation evaporator operating from an external power source in the form of a pumping station. There are different combinations of the interaction of the circulation group with the heating chamber and separator, but in principle, multicomponent systems of this type provide the highest performance of the technological operation, both in terms of the quality of the product concentration and the dynamics of the evaporation process.
