RU2775695C1 - Method for washing tubes in film evaporator from sediments formed as a result of water evaporation from saline solution - Google Patents

Abstract

FIELD: chemistry.

SUBSTANCE: invention relates to water evaporation from saline solutions in a film evaporator; it can be used for concentrating both aqueous solutions of mineral salts and organic media, including hardly soluble compounds. The method consists in that water evaporation from saline solutions is carried out with periodical hydrodynamic washing of tubes. Between regeneration of tubes by chemical washing, hydrodynamic washing of tubes is carried out with a steam-water stream by supplying overheated water together with the saline solution with a temperature of overheated water 10÷20°C higher than a boiling temperature of the saline solution. A volume of overheated water is 1.0÷1.5 of the inner volume of tubes.

EFFECT: increase in the period before chemical washings and reduction in their number, absence of a negative impact on equipment material, increase in the performance of an evaporator by distillate.

2 cl, 4 ex, 5 tbl, 2 dwg

Description

The invention relates to the evaporation of water from salt solutions in a film evaporator and can be used to concentrate both aqueous solutions of mineral salts and organic media, including sparingly soluble compounds.

Known methods of washing tubes in a film evaporator from deposits formed as a result of evaporation of water from a saline solution in a descending film mode, including hydroabrasive washing, periodic regeneration of tubes by chemical washing [1].

The closest in technical essence and achieved technical result to the proposed method is a method of washing, including periodic regeneration of tubes chemical cleaning [2 (prototype)].

This method is intended for washing tubes in a film evaporator from deposits formed as a result of evaporation of water from a salt solution in a descending film mode, and allows you to effectively restore the performance of a film evaporator, but has the disadvantage of the need for frequent chemical washes to remove salt deposits on heat transfer surfaces and restoration of the heat transfer coefficient, and, as a consequence, the high cost of water evaporation.

The aim of the invention is to reduce the cost of evaporating water from saline solutions by reducing the operating costs for chemical washing during tube regeneration.

This goal is achieved by the fact that between the regeneration of the tubes by chemical washing, a hydrodynamic washing of the tubes is carried out with a steam-water flow by supplying superheated water together with a salt solution with a temperature of superheated water 10÷20°C higher than the boiling point of the salt solution, with a volume of superheated water equal to 1, 0÷1.5 internal volume of the tubes.

It is known from the literature data [3] that during the operation of heat exchange equipment, problems often arise associated with a decrease in heat transfer due to the formation of various types of deposits and scale inside pipelines. This is especially true for pipelines of small diameter, in particular the heat exchange tubes of the evaporator module of evaporators.

Operation at elevated temperatures and the simultaneous action of aggressive media (solutions of mineral salts and organic media) leads to the fact that over time, rather strong layers of various deposits form on the inner surfaces of the tubes, which include hardness salts, corrosion products, and other types of mechanical impurities. This leads to the occurrence of thermal distortions in various heat exchange zones and has a negative impact on the operation of the apparatus as a whole.

To remove deposits from heat transfer surfaces, chemical washing is carried out using acidic detergents. These include mainly aqueous solutions of "strong" mineral acids: nitric acid, sulfuric acid, amidosulfonic (sulfamic) acid, inhibited hydrochloric acid.

However, the frequent use of acidic detergents, in addition to increasing the operating costs of the evaporation process, can lead to corrosion damage to the heat transfer surfaces of the evaporation module tubes.

An increase in the period between chemical washes and a reduction in their number can be achieved by carrying out hydrodynamic washing (cleaning) of the tubes.

Hydrodynamic cleaning is the cleaning of surfaces using a vapor-liquid flow moving in tubes at high speed. The resulting turbulent vortices provide a sufficiently effective removal of deposits from the inner surface of the heat exchange tubes. For washing, distillate is used without the addition of chemicals. Before serving, the distillate is heated to a temperature of 10÷20°C above the boiling point of the brine. The advantage of this method over mechanical and chemical ones is the absence of a negative effect on the material of the equipment.

The efficiency of hydrodynamic washing depends primarily on the degree of turbulence of the vapor-liquid flow, which is formed during the instantaneous boiling of superheated water supplied to the zone of evaporation of the salt solution, which has a lower pressure. The proportion of steam will be higher, the higher the temperature of the superheated water. However, this increases the cost of energy for heating water. In addition, the pressure in the shell-and-tube evaporator may exceed 0.7 atm, which will require obtaining a permit from Rostekhnadzor for the operation of pressure vessels. With a decrease in the proportion of steam in the vapor-liquid flow, the turbulence of the flow decreases, and, accordingly, the degree of cleaning of the tubes of the shell-and-tube heat exchanger from salt deposits.

It has been experimentally established that the temperature of superheated water should exceed the boiling point of the saline solution by 10÷20°C.

The efficiency of hydrodynamic washing, and, consequently, the completeness of washing off the formed salt deposits and scale from the surface of heat exchange tubes depends on the volume of superheated water supplied.

With an increase in the amount of superheated water, a bubbling mode of liquid movement is created in the evaporator tubes, which raises loose deposits from the surface of the tubes and transfers them to a suspended state. In this case, the evaporator performance is fully restored. However, this increases the consumption of superheated water and, as a consequence, the energy consumption for its heating.

With a decrease in the amount of superheated water, the formed deposits are not completely washed away and the heat transfer is restored.

It has been experimentally established that the optimal volume of superheated water is 1.0÷1.5 of the internal volume of the tubes.

The frequency of hydrodynamic washing depends on the degree of reduction in the performance of the film evaporator. The decrease in productivity will be the less, the more often hydrodynamic washings are carried out. However, this increases the consumption of water for washing, as well as the energy costs for its heating. The less often hydrodynamic cleaning is carried out, the greater the reduction in evaporator performance will occur and the faster the chemical cleaning will be required. An increase in the frequency of chemical washes will increase the operating costs of the evaporation process, as well as gradual corrosion damage to the heat transfer surfaces of the evaporation module tubes.

It has been experimentally established that it is optimal to carry out hydrodynamic washing with a decrease in the productivity of the film evaporator by 5-10%.

Examples

Example 1

The saline solution was fed for evaporation into a film evaporator with a downward film liquid flow regime, the performance of which was restored by hydrodynamic washing. Washing was carried out by supplying superheated water along with a saline solution. The temperature of the superheated water was 5, 10, 15, 20, 25°C higher than the boiling point of the salt solution. The distillate capacity increases as the superheated water temperature rises 20°C above the boiling point of the brine. A further increase in the temperature of superheated water is impractical. The test results are shown in table 1.

Example 2

The saline solution was fed for evaporation into a film evaporator with a downward film liquid flow regime, the performance of which was restored by hydrodynamic washing. Washing was carried out by supplying superheated water together with brine at a temperature of superheated water 20° C. above the boiling point of the brine. The volume of superheated water during hydrodynamic washing was 0.5; one; 1.25; 1.5; 2.0 internal volume of tubes. The distillate capacity increases with an increase in the volume of superheated water up to 1.5 of the internal volume of the tubes. Further increase in the volume of superheated water is impractical. The test results are shown in table 2.

Example 3

The saline solution was fed for evaporation into a film evaporator with a downward film liquid flow regime, the performance of which was restored by hydrodynamic washing. Washing was carried out by supplying superheated water together with brine at a temperature of superheated water 20° C. above the boiling point of the brine. The volume of superheated water during hydrodynamic washing was 1.5 of the internal volume of the tubes. Hydrodynamic washing was carried out with a drop in the performance of the evaporator by 3, 5, 7, 10, 15%.

The duration of the evaporator operation before the need for chemical washing increases with an increase in the frequency of hydrodynamic washing up to 1 time / 2 days. A further increase in the frequency of hydrodynamic washes is not advisable. The test results are shown in table 3.

Example 4

The saline solution was fed for evaporation into a film evaporator with a downward film liquid flow regime. When the performance of the evaporator dropped by 5%, hydrodynamic washing was carried out. Washing was carried out by supplying superheated water together with brine at a temperature of superheated water 20° C. higher than the boiling point of the brine, with the volume of superheated water being 1.5 times the internal volume of the tubes. The test results are shown in table 4.

After 14 days (2 weeks) of operation of the evaporator, its productivity decreased by 30%, after which the tubes were chemically washed using a nitric acid solution. When carrying out hydrodynamic washes 1 time / 2 days, a decrease in productivity by 30% occurred after 56 days (8 weeks). Thus, carrying out hydrodynamic washes can reduce the frequency of chemical washes by 4 times.

The test results are shown in Table 4 and in the graph of FIG. one.

Thus, hydrodynamic washes can reduce the frequency of chemical washes by at least 4 times.

In FIG. 2 is a diagram showing the proposed method for cleaning tubes in a film evaporator from deposits resulting from the evaporation of water from a salt solution.

The initial salt solution with a total salt content of 50 g/l is fed into a film evaporator with a distillate capacity of 30 l/h.

According to the known method, water is supplied to the film evaporator, distributed through the tubes, the evaporation of water from the salt solution is carried out in a descending film mode, the output of the secondary steam coincides with the movement of the salt solution, the tubes are periodically regenerated from salt deposits by chemical washing.

According to the proposed method, water is supplied to the film evaporator, distributed through the tubes, the evaporation of water from the salt solution is carried out in a descending film mode, the output of the secondary steam coincides with the movement of the salt solution, the tubes are periodically regenerated from salt deposits by chemical washing.

Between the regeneration of tubes by chemical washing, hydrodynamic washing of the tubes is carried out with a steam-water flow by supplying superheated water together with a salt solution with a temperature of superheated water 10÷20°C higher than the boiling point of the salt solution, with a volume of superheated water constituting 1.0÷1.5 internal tube volume.

The initial solution from tank 1 flows by gravity into the lower chamber of evaporator 2, from where it is pumped by pump 3 to the flow distributor in the upper part of evaporator 4 and flows down in the form of a thin film along the inner walls of the pipes. Steam from the electric steam generator 5 is supplied to the annular space for heating the solution and evaporating the liquid. The resulting vapor-liquid mixture at the outlet of the tube is divided into liquid and vapor phases.

Water vapor from the evaporator enters the heat exchanger 6, where it is condensed, cooled to a temperature of 45°C and drained into the distillate tank 7, from which it is constantly pumped out by the pump pos.8. The amount of distillate pumped out is measured by flowmeter 9.

Part of the saturated salt solution from the bottom of evaporator 2 is taken as a finished product into container 10. The sampling volume is automatically adjusted according to the level in the bottom of evaporator 2.

With a decrease in distillate productivity by 5-10%, an operation of hydrodynamic washing of the evaporator tubes is performed, for which superheated water is supplied together with brine with a temperature of 10÷20°C higher than the boiling point of the brine, from tank 11 to the flow distributor in the upper part of the evaporator 4. The volume of superheated water is 1.0÷1.5 of the internal volume of the tubes.

The operation of chemical washing of the equipment is performed with a decrease in distillate productivity by 30%, for which purpose a washing solution is poured into the container 12, which flows by gravity into the lower chamber of the evaporator 2, after which it is circulated for 5÷8 hours using a pump 3.

Thus, the use of the proposed method allows to reduce the operating costs for the evaporation of 1 m of water by 16.7%

Bibliography

1. Film evaporators. Udyma P.G. / Ed. A.M. Baklastov. - M.: Mosk. energy Institute, 1985.

2. USSR author's certificate No. 799840, C23G 5/00 01/30/81 / Method for washing a vacuum evaporator - prototype.

3. Modern technologies for water treatment and protection of equipment from corrosion and scale formation. Materials of the II conference, part I, Moscow, 2007

Claims (2)

1. A method for washing tubes in a film evaporator from deposits formed as a result of evaporation of water from a salt solution in a descending film mode, including periodic regeneration of the tubes by chemical washing, characterized in that between regeneration of the tubes by chemical washing, a hydrodynamic washing of the tubes is carried out with a steam-water flow by supplying superheated water together with saline solution with superheated water temperature 10÷20°C higher than the boiling point of saline solution, while the volume of superheated water is 1.0÷1.5 of the internal volume of the tubes. 2. The method according to p. 1, characterized in that hydrodynamic washing is carried out with a decrease in the performance of the film evaporator by 5-10%.

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