Heat transfer technological processes are widely used in various fields of industry to create the necessary processing conditions or change the temperature state of equipment, as well as production blanks. In enterprises where the task is to change the characteristics of liquid media, heat can be used as a means of supporting boiling. Technically, such tasks 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 field, evaporation is considered as a method of concentrating liquid solutions, the basis of which are volatile or non-volatile substances dissolved in volatile active mixtures. This process is performed as a result of evaporation of the solvent during boiling. This procedure is most often subjected to alkali, salts, as well as high-boiling liquids. But in each case, the main objective of the process is to obtain a pure solvent or individual substances in a high degree of concentration. If we are talking about the targeted purification of a specific component of the solution, the evaporation process can be supplemented by a crystallization operation, in which it is possible to form the target substance in solid form.
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 quality, a vacuum evaporation unit with an optimized design, designed to perform basic evaporation processes, as well as auxiliary operations, is used. It is important to keep in mind that evaporation involves the use of aggressive media - hot liquids, gases, water vapor, etc. To this is added an unfavorable background from the 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 structures.
The basic structure of the evaporator
Most modern industrial evaporators use a multicomponent system based on a heat exchanger with a condenser and an evaporation chamber. To optimize the process and a more effective concentration of solutions, a separator is also provided - this is a unit that is connected in a separate order through the gas duct and organizes the removal of secondary steam. Often used remote type separators that operate under centrifugal force. What is fundamentally different vacuum-evaporation installation? Creating a vacuum allows you to achieve the effect of soft evaporation. This provides two positive aspects - accelerating the evaporation process (the served solution spends less time in the chamber) and improving the quality of the concentrated substance. Output products can be used in other technological operations at the same enterprise of target processing. To do this, organize the connection of individual modules with exhaust flows, due to which not only the removal of excess gas mixtures is performed, but also the flow control with the necessary delivery parameters by pressure force and speed of movement is ensured. Moreover, optionally many evaporators allow the possibility of interfacing with blocks for pre-treatment or dilution of waste in accordance with the requirements of technological processes in which the same gas can be used again.
Forced circulation devices
The design is based on a vertical or horizontal shell-and-tube heat exchanger with a heating chamber and a concentric separator. The workflow is supported by a circulation pumping station and a flash vessel. Usually the forced process of the movement of working mixtures is implemented in double-shell evaporators with a counter-current circulation scheme. In the composition of such devices there is also a device for the distillation and purification of steam from organic and salt compounds. The average productivity of evaporation equipment with forced circulation is about 9000 kg / h, and the concentration coefficient reaches 65%.
During the operation of such an aggregate, 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 drops sharply. From this moment, the process of active evaporation of part of the liquid begins. What is the advantage of using units of this type? This is the most effective solution if maintenance is required for viscous and problem contaminated mixtures. For example, for the evaporation of saline solutions, this option is more appropriate to use than single-shell evaporators, which can show a higher circulation rate, but their capacity will not be enough to ensure even an average level of performance. By the way, modern forced-circulation evaporators perform boiling and evaporation operations not on the heating walls in the main chamber, but in the separator, therefore, pollution of the main working unit is minimized.
Plate Heat Exchanger Evaporators
The design feature of such plants is the presence of special plates, due to which the working medium is directed through the heating chamber through alternating channels. Special gaskets are used to seal the plates - they also perform the function of thermal insulation, which increases the efficiency of heat transfer.
As a rule, these are multi-case evaporators with a capacity of about 15 t / h. The heating water flows and the target product move countercurrently through their channels, giving up some 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 product and coolant. As a result of active heat transfer, the working medium boils, after which steam is formed. Residual liquid products are cut off in the separator unit due to centrifugal force.
This is one of the few cases when it comes to a universal apparatus for evaporation from the point of view of the possibilities of working with different technological environments. In particular, the principle of operation of the evaporator with a plate heat exchanger allows the use of steam-gas and aqueous media. At the same time, a high quality of concentration is ensured, since evaporation is carried out uniformly in soft 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 the connecting harness for such an apparatus is 3-4 m.
Naturally-circulating three-case evaporators
Structurally, such devices are characterized by the presence of a short vertically arranged 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 the oil and gas fields the associated gas carries the product, then in the case of a three-case evaporator, hot vapors lift the liquid along the shell-and-tube circuits. The whole process takes place against a background of boiling. The liquid separated from the steam is discharged through the return pipe to the heat exchanger, after which it is again sent to the separator for the next separation session. So the process is repeated several times until the desired level of concentration 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. Natural circulation in a vacuum evaporator allows for high specific productivity with quick 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 on the chemical and food industries, where it is required to perform point separation operations with a small capacity load. For example, apparatus for evaporating glycerin mixtures provides a processing speed of 3600 kg / h.
The principle of operation of the barometric capacitor
A kind of mixing heat exchangers that do not perform superficial separation of the working media during the flow, but allow their mixing. In other words, the conditioned concentrate solution may at the time of heating contact with the process hot medium represented by steam or water. The barometric condenser itself is part of an integrated evaporator that performs mixing processes of cooling water and secondary steam. Since the volumes of the newly formed condensate are less than the volume of the vapor, a natural vacuum arises. To maintain it, it is necessary to remove atmospheric air from the condenser, which is sent there together with coolant flows. In some designs, air may also penetrate through defects in the capacitor housing. The withdrawal of mixed mixtures from the condenser is carried out through a barometric pipe. It is pre-immersed in a liquid and forms a hydraulic shutter, which prevents the passage of air into the condenser.
The principle of operation of the capacitive apparatus
A special kind of equipment for ensuring technological evaporation processes. The main difference of capacitive units from the point of view of the principle of operation is the support of a free circulation mode, which is achieved due to the internal configuration of the arrangement of circuits in the heat exchange system. The infrastructure of the heat exchange network is formed by tube bundles, coils and other elements that create the conditions for a multi-stage and largely difficult process of transferring thermal energy. By the way, capacitive evaporators are practically not used in working 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-scale industries where the heat transfer coefficient with output volumes is not so significant. The internal structure of capacitive evaporators, with all its drawbacks, opens up a lot of opportunities for organizing directional circulation, which is very important in enterprises with low structural mobility when connecting communication channels.
Calculation of the evaporator
In the integrated design of evaporation equipment, it is envisaged to perform separate calculations for each component, since the characteristics of the production process can change at each stage. As a rule, the following data are used as initial data:
- approximate vapor pressure;
- the heat of concentration;
- properties of the initial solution;
- level of heat loss;
- heat transfer coefficient;
- design parameters that are already installed and cannot be changed.
For three-case evaporator plants, the calculation with the above initial data can be carried out immediately according to several parameters, among which the capacity of the circulation pump, the volume of the heating chamber, the maximum amount of fluid served, etc. Design calculation of the same barometric condenser, separator can be referred to the most responsible design tasks. and characterization of strapping elements. In particular, the evaporation rate in systems with flow evaporation will depend on the diameters of the nozzles and the length of the transition pipes.
Workflow requirements
The calculated indicators for the organization of the evaporation process may not give the expected effect if the environmental requirements are not met. A lot will depend on the indoor conditions where the equipment is used. According to the requirements, direct-flow evaporators can only be used in rooms with an area of at least 4.5 m 2 and a height of 3.2 m or more. A technological channel for the discharge of waste like a chimney is mandatory to remove evaporated harmful substances. It will not be superfluous to provide an adjustable hood with a gate and traction adjustment.
The ventilation system is designed according to special rules. It should include inflow channels and exhaust systems with a direct connection to the zones where the evaporation process is directly carried out. Obviously, an integrated ventilation system, operating regularly in two directions, will require serious power support. But at the same time, the emitted noise from the channels and operating equipment should not exceed 75 dB. And this is not to mention compliance with fire and electrical safety requirements. If the evaporation plant regularly works with gas mixtures, a special air degassing system must be organized. It can be included in a single complex of heat exchange communications, which will allow in some operational aspects to mutually complement the functions of both systems.
Conclusion
The operations of evaporation and concentration have long been used in factories as both primary and secondary processes. In most cases, in this way the preparation of materials for the further stages of manufacturing products or the preparation of technical means is carried out. Vacuum evaporators and plants can be placed among the most productive tools for solving such problems. High operational performance is 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 various combinations of the interaction of the circulation group with the heating chamber and the separator, but in principle multicomponent systems of this type provide the highest rates of technological operation, both in terms of the quality of the concentration of the product and the dynamics of the evaporation process.