Design of vacuum evaporator

Contents

Introduction

1 Analytical Overview

2 Project Objective and Objectives

3 Process diagram

4 Engineering calculations

4.1 Evaporator calculation

4.1.1 Material balance of evaporation process

4.1.2 Evaporator temperatures and pressures

4.1.3 Calculation of heat balance

4.1.4 Heating steam flow rate

4.1.5 Calculation of heat exchange surface area of the heating chamber of the apparatus

4.2 Calculation of barometric capacitor

4.2.1 Cooling water flow

4.2.2 Barometric capacitor diameter

4.2.3 Speed of water in barometric pipe

4.2.4 Barometric pipe height

4.3 Vacuum Pump Calculation

4.4 Calculation of heat exchangers

4.4.1 Approximate calculation of evaporated solution cooler

4.4.2 Approximate calculation of initial solution heater

4.4.3 Detailed calculation of initial solution heater

5 Conclusions to the project

List of sources used

Introduction

Evaporation is the process of concentrating solutions of substantially non-volatile or minor substances in liquid volatile solvents. The evaporation process is widely used in the chemical, food and related industries. Solutions of solid substances, as well as high-boiling liquids having low vapour pressure at the evaporation temperature (mineral and organic acids, alcohols, etc.) are subjected to evaporation. Sometimes evaporation is used to isolate the solvent in pure form (desalination of seawater). Production of highly concentrated solutions, practically dry and crystalline products makes their transportation and storage easier and cheaper.

Heat for evaporation is supplied by some kind of heat carrier, which ensures boiling of the evaporated solution. The most widespread was water (heating) steam.

The evaporation process is carried out in evaporators at atmospheric or elevated pressure, or under vacuum. The simplest method is evaporation at atmospheric pressure, but the least economical. Evaporation under vacuum is most effective, since solutions of substances prone to decomposition at high temperatures can be evaporated; increasing the useful temperature difference between the heating steam and the solution, which makes it possible to reduce the heating surface.

The work aimed at calculation of evaporator, auxiliary equipment and preparation of technical documentation for evaporator is the current task of this project.

Analytical Overview

Modern evaporators (VP) are complex, high-tech, automated systems made of high-quality materials, providing safe and effective concentration of solutions of various substances.

You can classify ARs by different characteristics:

by the number of cases (single and multi-case);

by the type of heating surface (steam jackets, coils, etc.) and its location in space (with a vertical, horizontal, inclined heating chamber);

by heat carrier type (water steam, electric current, etc.);

by type of heating (VP with steam heating, with thermal or mechanical compression of secondary steam [1]), etc.

A more significant feature of the classification of VP, characterizing the intensity of their action, is the type and multiplicity of the circulation of the solution. According to this classification, VP with 2 types of circulation are distinguished:

directed natural (free)

compulsory

Evaporation processes are carried out under vacuum, at elevated and atmospheric pressures. The choice of pressure is related to the properties of the evaporated solution and the possibility of using heat of secondary steam .

Evaporation under vacuum has certain advantages over evaporation at atmospheric pressure, although the heat of evaporation of the solution increases slightly with a decrease in pressure and, accordingly, the steam consumption for evaporation of 1 kg of solvent (water) increases. The use of vacuum makes it 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. It also makes it possible to use the secondary steam of the evaporator itself as a heating agent, 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, traps, 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. This evaporation method allows better use of heat than in vacuum evaporation. This method is used only for evaporation of thermally stable substances. In addition, higher temperature heating agents are required.

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

The simplest evaporators with free circulation of solution are periodically operating open evaporator bowls with steam jackets (for working under atmospheric pressure) and closed boilers with jackets operating under vacuum. The heating surfaces of the jackets and, accordingly, the loads of these devices are very small. Coil evaporators have a much larger heating surface per unit volume. Evaporators with free circulation of solution are currently supplanted in most industries by evaporators of more advanced designs, in particular vertical tubular apparatuses.

In vertical apparatuses with directed natural circulation of the solution, evaporation is carried out with multiple natural circulation of the solution. They have several advantages compared to devices of other designs, due to which they are widely used in industry. The main advantage of such devices is the improvement of heat transfer to the solution with its repeated organized circulation in a closed circuit, which reduces the rate of scale deposition on the surface of the pipes. In addition, most of these devices are compact, occupy a small production area, are convenient for inspection and repair.

In devices with an internal heating chamber and a central circulation pipe, the circulation pipe, like the boiling pipes, is heated with steam, which reduces the difference in density of the solution and the vapor-liquid mixture and can lead to undesirable steam formation in the circulation pipe itself. Their disadvantage is also the rigid attachment of the boiling pipes, which does not allow a significant difference in the thermal elongations of the pipes and the body of the apparatus.

In the apparatus with the suspended heating chamber, the annular channel has a large cross section and is located outside the heating chamber, which has a favorable effect on the circulation of the solution.

The intensity of circulation in the apparatus with the suspended heating chamber (as in the apparatus with the central circulation pipe) is not sufficient for the effective evaporation of highly viscous and especially crystallizing solutions, treatment, which leads to frequent and long-term stops of these apparatus for cleaning working surfaces.

The structures of the remote circulation tubes are somewhat more complex, but they achieve more intensive heat transfer and reduce metal consumption per 1 m2 of the heating surface compared to the apparatus with the suspended heating chamber or the central circulation tube.

The apparatus in the remote heating chamber operates at a more intense natural circulation due to the fact that the circulation pipe is not heated, and the lifting and lowering sections of the circulation circuit have a significant height.

In the outfitted boiling zone apparatus, the boiling solution does not contact the heat exchange surface, which reduces scale deposition. In these devices, splash is significantly reduced, a high rate of circulation of the solution is achieved, which leads to an increase in productivity and intensification of heat exchange. The boil-out apparatus can be effectively used to evaporate crystallizable solutions of moderate viscosity.

The fundamental difference of straight-flow devices with natural circulation is that evaporation in them occurs when the evaporated solution passes through the pipes of the heating chamber once, evaporation is carried out without circulation of the solution. Such apparatuses achieve a reduction in temperature losses due to hydrostatic dispersion.

In rotary direct-flow apparatuses, intensive heat exchange is achieved with a small entrainment of liquid by secondary steam. At the same time, rotary apparatuses are difficult to manufacture and differ in the relatively high cost of operation due to the rotating parts (rotor).

In forced circulation devices, its speed is determined by the capacity of the circulation pump and does not depend on the height of the liquid level in the pipes, as well as on the intensity of steam generation.

Therefore, in forced circulation apparatuses, evaporation proceeds at small useful temperature differences not exceeding 3-5 K and at significant solution viscosities.

In evaporators with a heat pump, with the help of a heat pump, which is a heat transformer, the efficiency of the single-body apparatus is increased by compressing secondary steam at the outlet of the apparatus to the pressure of fresh (primary) steam and directing it to the heating chamber of the same apparatus. In some cases, evaporators with a heat pump can compete with multi-body evaporators.

Project Objective and Objectives

Objectives of the course project:

- design a continuous single-hull vacuum evaporator to evaporate the magnesium sulfate solution with an initial mass concentration of 0.06;

- provide heating of the initial solution before feeding into the evaporator and cooling of the concentrated solution after the evaporator. Use secondary steam to preheat the initial solution. Select the heating steam pressure.

Course Project Tasks:

- calculate the material and heat balances of the evaporator;

- perform approximate calculations of heat exchanger-cooler and heat exchanger-heater;

- perform detailed calculation of heater, perform drawings of general view of heat exchanger-heater and process diagram of evaporator.

Process Diagram

The initial dilute solution from the previous shop is fed to preheater T1 and then to evaporator VA. Primary heating steam is used as heat carrier in evaporator, and secondary steam is used in heater. The resulting condensate is returned to the boiler room through the mechanical condensate pump CH. The concentrated solution is withdrawn from the evaporator separator through the cooler T2 and fed alternately to the tanks E1 and E2. Throttling to atmospheric pressure occurs in the tanks, and then the pumps H1 and H2 are supplied further through the process line. Cooling of the solution in the refrigerator is carried out by water. Since the evaporator is operated under vacuum, the vessels E1 and E2 are connected to the vacuum line to allow the solution to be discharged from the apparatus by gravity. To create a vacuum, a barometric BC mixing condenser is used, into which secondary steam is supplied, formed when the solution is concentrated in the evaporator and cooling water. The mixture of condensate and cooling water is discharged from the condenser to the tank E3 by gravity, using a barometric pipe with a hydraulic lock. To maintain the specified pressure in the barometric condenser, a HV vacuum pump is used. For operation of vacuum pump water supply is provided to it

. Project Conclusions

Following the results of the course work, the following devices were selected:

Evaporator with natural circulation and extended heating chamber (type 1, version 2) as per GOST 1198781. Pipes shall be made of stainless steel. The area of ​ ​ the apparatus is 560 m2, the length of the pipes is 5 m, the diameter of the pipes is 38x2 mm.

Barometric condenser with segment shelves with a diameter of 1.2 m with a barometric pipe with a diameter of 0.25 m and a height of 6.96 m.

Vacuum pump of VNN12 type. Residual pressure - 23 mmHg, capacity - 12 m3/min, power - 20 kW.

Heated solution cooler as per GOST 1512079. Six-way, with a heat transfer area of ​ ​ 60 m2, pipe length of 3 m, flow section of pipe space of 0.009 m2, inner casing diameter of 600 mm.

Initial solution heater as per GOST 1512279, version M8. Six-way with heat exchange area - 40 m2, length of pipes - 2 m, outer diameter of casing - 630 mm, area of flow section - 0.009 m2.

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