Calculation of evaporator for evaporation of calcium chloride - exchange rate

Contents

CONTENTS

INTRODUCTION

1 LITERARY OVERVIEW ON THE THEORY AND TECHNOLOGY OF THE EVAPORATION PROCESS

1.1 Theoretical basis of evaporation process

1.2 Main Process Flow Charts

1.3 Modern hardware and technological design of evaporation process

2 RATIONALE FOR SELECTION AND DESCRIPTION OF PRODUCTION FLOW CHART

2.1 Justification for selection of double-hull evaporator

2.2 Description of production flow chart

3 SELECTION OF APPARATUS STRUCTURAL MATERIALS

4 MATERIAL BALANCE OF THE PLANT

5 DETERMINATION OF HEATING STEAM FLOW RATE

6 DEFINITION OF HEAT TRANSFER SURFACE, SELECTION OF EVAPORATOR DESIGN TYPE

7 CALCULATION AND SELECTION OF HEAT EXCHANGERS OF INITIAL MIXTURE AND BAROMETRIC CONDENSER

7.1 Calculation of heater by extrapar

7.2 Calculation of hot steam heater

7.3 Calculation of barometric capacitor

8 SELECTION OF EVAPORATOR AUXILIARY EQUIPMENT

8.1 Condensate sink selection

8.2 Selection of feed pump

8.3 Vacuum Pump Selection

9 CONCLUSION

10 LIST OF LITERATURE USED

Literary overview on the theory and technology of the evaporation process

Theoretical basis of evaporation process

Evaporation is the process of concentrating liquid solutions of substantially non-volatile substances by partially removing the solvent by boiling evaporation.

The essence of the evaporation consists in converting the solvent to a vapor state and removing the obtained steam from the remaining concentrated solution. Evaporation is usually carried out at reflux, i.e. under conditions where the vapor pressure above the solution is equal to the pressure in the working volume of the apparatus.

Evaporation is carried out in such a way that, at a given capacity, a thickened solution of the required concentration of the proper quality is obtained without loss of dry matter and at the lowest possible fuel consumption.

Concentration of solutions by evaporation is one of the most common technological processes in chemical, food, metallurgical and other industries. This is due to the fact that many substances, for example sodium hydroxide, potassium hydroxide, ammonium nitrate, ammonium sulfate, etc., are obtained in the form of diluted aqueous solutions, and for further processing and transport (in order to reduce container volumes and transportation costs) they must be supplied in the form of concentrated products. A huge amount of heat is used to evaporate the solutions, and a large amount of carbonaceous and alloyed steels, nickel and other metals are used to create evaporation plants. Therefore, a rational evaporation process is required on a case-by-case basis to ensure maximum evaporator capacity with minimal heat and metal consumption.

A feature of the evaporation process is that only pure solvent vapors are normally contained in vapors of boiling solutions, and the solute is non-volatile. This provision, which is the basis of the theory and methods of calculating evaporators for most solids solutions, is quite justified.

The solvent removed in the vapor state is most often water vapor, called secondary steam.

Heat for evaporation can be supplied by any heat transfer agents used in heating. However, in the vast majority of cases, saturated or slightly superheated water vapor, which is called heating or primary, is used as the heating agent during evaporation. The primary steam is either steam obtained from the steam generator or spent steam, or steam from the intermediate extraction of steam turbines. The heat required to evaporate the solution is usually supplied through the wall separating the heat carrier from the solution. In some industries, solutions are concentrated by directly touching the evaporated solution with flue gases or other gaseous heat carriers. Electric heating can also be used.

Evaporation processes are carried out under vacuum, at elevated and atmospheric pressures. By vacuum evaporation, it becomes possible to carry out the process at lower temperatures, which is important in the case of concentrating solutions of substances prone to decomposition at elevated temperatures. In addition, under vacuum, the useful temperature difference between the heating agent and the solution increases, which allows reducing the heating surface of the apparatus. The use of vacuum makes it possible to use the secondary steam of the evaporator itself as a heating agent, except for primary steam, which reduces the consumption of the primary heating steam. However, the vacuum application increases the cost of the evaporator because additional costs are required for vacuum devices (condensers, vacuum pumps) as well as increased operating costs.

By evaporating at a pressure above atmospheric pressure, secondary steam can also be used, both for evaporation and for other needs not related to the evaporation process. However, evaporation under excessive pressure involves an increase in the boiling point of the solution. Therefore, this method is used only for evaporation of thermally stable substances.

In atmospheric evaporation, secondary steam is not used and is usually removed to the atmosphere. This evaporation process is the simplest but least economical.

Evaporation under atmospheric pressure, and sometimes evaporation, under vacuum is carried out in single evaporators (single-body evaporators). However, the most common are multi-body evaporators, consisting of several evaporators, or housings in which the secondary steam of each previous housing is sent as heating to the subsequent housing. In these installations, only the first housing is heated with primary steam. Consequently, significant primary steam savings are achieved compared to single-hull units of the same capacity.

Primary steam savings can also be achieved in single-hull heat pump evaporators. In such installations, the secondary steam at the outlet of the apparatus is compressed by a heat pump (for example, a thermocompressor) to a pressure corresponding to the temperature of the primary steam, after which it is returned to the apparatus.

Main process diagrams

The principle of operation of multi-hull devices is the repeated use of heat of heating steam supplied to the first housing of the plant by sequentially connecting several single-hull devices, which allows using the secondary steam of each previous housing to heat the next. In order to practice such reuse of the same amount of heat, the secondary vapor temperature of each subsequent housing is required to be higher than the boiling point of the solution in the subsequent housing. This requirement is easily met by lowering the operating pressure in the housings from the first to the last. To this end, a relatively high boiling point is established in the first housing and a temperature of 5060 ° C in the last housing of the discharge evaporator, which is connected to a condenser equipped with a vacuum pump.

If heating steam and liquid solution enter the first, "head," body of the evaporator, then the latter is called straight-through.

Most evaporators operate according to this principle. If heating steam enters the first body in order, and liquid solution enters the last and passes from the last body to the first, then the installation is called countercurrent.

This counter movement of steam and solution is used in the case of evaporation of solutions with high viscosity and high temperature depression in order to increase heat transfer coefficients. At the same time, however, the maintenance of the apparatus is complicated because such a scheme requires the installation of pumps between each two housings for pumping fluid moving in the direction of increasing pressures, not to mention the additional cost of energy consumption for the pumps.

During evaporation of crystallizing solutions, their transfer from the housing to the housing can be accompanied by plugging of connecting pipelines and disruption of normal operation of the plant. In this case, devices with parallel chassis power are often used. Here, the solution is evaporated to the final concentration in each housing, and the steam, as in the previous two schemes, moves sequentially from the first housing to the last. In the same direction, the operating pressures and boiling points of the solution in the housings are reduced.

According to the principle of operation, evaporators are divided into continuously and periodically operating ones. In continuous installations, non-concentrated solution is continuously supplied to the apparatus, and the evaporated solution is continuously withdrawn from it. Such devices are more heat efficient because they do not have heat loss associated with periodic heating of the device. During periodic evaporation, a certain amount of initial concentration solution is loaded into the apparatus, heated to boiling point and evaporated to a given concentration. The evaporated solution is then removed from the apparatus, refilled with fresh solution, and the process is repeated .

In the chemical industry, mainly continuous evaporators are used. Only in small scale industries, as well as when evaporating solutions to high final concentrations, periodical devices are sometimes used.

Modern hardware design of evaporation process

Steam ejectors or mechanical compressors are used in modern evaporators to save heating steam. In this case, the secondary steam is compressed to the required pressure. The ejector or compressor can be installed behind any unit housing. The lower the intake steam pressure, the more heat recovery, but more compression energy is required. The location of the compression stage installation shall be determined based on feasibility calculations. Installation of a steam ejector in a three-hull evaporator allows to achieve the same steam savings as the installation of another additional housing.

In 1991, a evaporator for salt-containing solutions was invented at the research and design institute of metallurgical heat engineering of non-ferrous metallurgy and refractories. It serves for evaporation of salt-containing solutions and can be used in metallurgical, chemical and food industries. The apparatus is characterized in that an injection nozzle is installed on the end of the ascending circulation pipeline facing the second heating chamber, and the second heating chamber is located in the lower part of the separator. Use of this invention provides higher efficiency of evaporation due to conversion of rectilinear flow into vortex flow, reduced splash and dimensions of apparatus.

In 2002, V.P. Chernykh invented an evaporator, which differs from its analogues in that a solution inlet pipe is installed on the cover. The evaporator vessel does not have a "condenser," so the entire working space in the vessel is used rationally to carry out the evaporation process of the solution. This improves the specific performance of the machine. The evaporator of the new design allows to obtain polymer organometallic compounds of rare metals of high chemical frequency.

Currently, the role of automation in the evaporation process has increased dramatically. This is due to the high speed of the process, as well as the need to constantly maintain a number of parameters within a given range. Therefore, recently, automatic evaporation control systems have been actively developing.

JUSTIFICATION OF SELECTION AND DESCRIPTION OF PRODUCTION FLOW CHART

Justification for selection of double-hull evaporator

In industry, multi-shell evaporators are used to save primary heating steam. With an increase in the number of housings, the specific consumption of steam and, therefore, fuel consumption decreases. However, with an increase in the number of enclosures, metal consumption, initial installation costs and depreciation charges, ongoing repairs costs, and, in addition, operational complexity increase. The selection of the number of stages of the evaporation station is made on the basis of technical and economic calculations. In most cases, evaporators with heating surfaces with 2-4 housings are used in practice.

Evaporators with steam heating can be divided into three groups: with natural circulation of solution, with forced circulation of solution and film apparatuses. Natural circulation evaporators are highly efficient and are widely used to evaporate solutions with relatively low viscosity. In such devices, circulation is carried out due to the difference in densities at individual points of the device.

Based on the above, a two-hull continuous evaporator with natural circulation is selected to concentrate the aqueous calcium chloride solution.

Description of Production Flow Chart

The initial solution from the tank is supplied by centrifugal pump (1) to the preheater by extrapar (3), and then to the preheater by heating steam (4). In heat exchangers, the initial solution is heated to a temperature close to the boiling point, and then supplied to the first housing of the evaporator (5). Preheating the solution increases the boiling rate in the evaporator.

The first case is heated by sharp steam (dry saturated). Secondary steam generated by concentration of solution in the first housing is directed as heating to the second housing (6). A partially concentrated solution from the first housing also comes here.

Spontaneous flow of solution and secondary steam into subsequent housings is possible due to common pressure drop resulting from creation of vacuum by condensation of secondary steam of the last housing in barometric condenser (9) of mixing, where specified pressure is maintained by supply of cooling water and suction of non-condensing gases by vacuum pump (12). The mixture of cooling water and condensate is removed from the condenser using a barometric pipe with a hydraulic lock. Concentrated solution formed in the second case is fed into the tank for evaporated solution (8).

Condensate of heating vapors from evaporators is removed by means of condensate dischargers (7).

The technological scheme is provided on sheet A1 of a graphic part.

Selection of structural materials of the apparatus

The choice of structural materials for the designed apparatus is determined by the peculiarities of the technological process flowing in it, the properties of working substances, their parameters and the nature of the mechanical load. In turn, the process properties of the structural material predefine the method of making parts of the apparatus therefrom.

Structural material is selected, which is stable in medium of boiling solution of calcium chloride within the range of concentration variation from 10 to 30%.

When selecting a steel grade, first of all, its corrosion resistance in the working environment is taken into account. It is recommended to use carbon or alloyed steels with a corrosion rate of not more than 0.1 mm/year, i.e. quite stable. Steel of grades 35, 45 is applicable as a material.

Conclusion

During the course project, the installation for concentrating the aqueous glycerin solution was calculated, as a result of which, according to GOST 1198781, an evaporator with natural circulation and with a removed heating chamber was selected.

The design of the evaporator meets a number of general requirements. These include:

High efficiency and heat transfer intensity with minimum volume of the apparatus and metal consumption for its manufacture;

Simplicity of the device;

Reliability in operation;

Easy cleaning of the heat exchange surface;

Easy inspection, repair and replacement of individual parts.

Initial mixture heat exchangers and barometric condenser, as well as auxiliary equipment: condensate traps, feed mixture pump and vacuum pump, were calculated and selected for the apparatus.

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