阅读说明：本技术 一种多效蒸发工艺及其系统 (Multi-effect evaporation process and system thereof ) 是由 李智杰 李想 罗樟芝 郭志军 于 2019-10-31 设计创作，主要内容包括：本发明公开了一种多效蒸发工艺及其系统,所述工艺两个交替进行的工况：工况a：前一效蒸发器产生的蒸汽引入下一效蒸发器,外来溶液依次经过第一效蒸发器、第二效蒸发器和第三效蒸发器,最后进入过滤工序；工况b：蒸汽路线和工况a相同,外来溶液依次经过第一效蒸发器、第三效蒸发器和第二效蒸发器,最后进入过滤工序。所述系统第一效蒸发器的管程下端并联连通第二效蒸发器和第三效蒸发器的溶液入口,第二效蒸发器的管程下端并联连通第三效蒸发器的溶液入口和外部过滤系统,第三效蒸发器的管程下端并联连通第二效蒸发器的溶液入口和外部过滤系统。本发明提工况a和工况b交替进行,对溶液蒸馏浓缩的同时防止和清除多效蒸发器内的结晶。(The invention discloses a multiple-effect evaporation process and a system thereof, wherein the process has two working conditions which are alternately carried out: working condition a: introducing steam generated by the previous effect evaporator into the next effect evaporator, and enabling the external solution to sequentially pass through the first effect evaporator, the second effect evaporator and the third effect evaporator and finally enter a filtering process; working condition b: the steam route is the same as the working condition a, and the external solution sequentially passes through the first effect evaporator, the third effect evaporator and the second effect evaporator and finally enters the filtering process. The lower end of a tube side of a first effect evaporator of the system is communicated with solution inlets of a second effect evaporator and a third effect evaporator in parallel, the lower end of a tube side of the second effect evaporator is communicated with a solution inlet of the third effect evaporator and an external filtering system in parallel, and the lower end of a tube side of the third effect evaporator is communicated with a solution inlet of the second effect evaporator and the external filtering system in parallel. According to the invention, the working condition a and the working condition b are alternately carried out, and the solution is distilled and concentrated while the crystallization in the multi-effect evaporator is prevented and removed.)

1. A multi-effect evaporation process is characterized by comprising the following two working conditions which are alternately carried out:

working condition a: introducing steam generated by the first effect evaporator (1) into a second effect evaporator (2), introducing steam generated by the second effect evaporator (2) into a third evaporator, introducing a solution discharged from the first effect evaporator (1) into the second effect evaporator (2), introducing a solution discharged from the second effect evaporator (2) into the third effect evaporator (3), and introducing a solution discharged from the third effect evaporator (3) into a filtering process;

working condition b: the method comprises the following steps that steam generated by a first effect evaporator (1) is introduced into a second effect evaporator (2), steam generated by the second effect evaporator (2) is introduced into a third evaporator, solution coming out of the first effect evaporator (1) is introduced into the third evaporator, solution coming out of the third effect evaporator (3) is introduced into the second evaporator, and solution coming out of the second effect evaporator (2) enters a filtering process.

2. A multiple effect evaporation system, characterized by: the device comprises a first effect evaporator (1), a second effect evaporator (2) and a third effect evaporator (3), wherein the upper ends of tube passes of the first effect evaporator (1), the second effect evaporator (2) and the third effect evaporator (3) respectively comprise a solution inlet and a steam outlet, the first effect evaporator (1), the steam outlet of the second effect evaporator (2) is respectively communicated with the second effect evaporator (2), the shell pass inlet of the third effect evaporator (3), the lower end of the tube pass of the first effect evaporator (1) is communicated with the solution inlets of the second effect evaporator (2) and the third effect evaporator (3) in parallel, the lower end of the tube pass of the second effect evaporator (2) is communicated with the solution inlet of the third effect evaporator (3) and an external filtering system in parallel, and the lower end of the tube pass of the third effect evaporator (3) is communicated with the solution inlet of the second effect evaporator (2) and the external filtering system in parallel.

3. The multi-effect evaporation system of claim 2, wherein: the lower end of the tube side of the first effect evaporator (1) is communicated with the second effect evaporator (2) through a first effect solution outlet pipe (4) and a second effect solution inlet pipe (11), and is communicated with the third effect evaporator (3) through the first effect solution outlet pipe (4) and a third effect solution branch pipe (13); the lower end of the tube side of the second-effect evaporator (2) is communicated with the third-effect evaporator (3) through a second-effect solution outlet pipe (5) and a third-effect solution inlet pipe (12); the lower end of the tube side of the third-effect evaporator (3) is communicated with the second-effect evaporator (2) through a third-effect solution outlet pipe (6) and a second-effect solution branch pipe (14).

4. The multi-effect evaporation system of claim 3, wherein: the lower end of the tube side of the first effect evaporator (1) is communicated with one end of a first effect solution outlet tube (4), the other end of the first effect solution outlet tube (4) is communicated with one end of a second effect solution inlet tube (11) and one end of a third effect solution branch tube (13) in parallel, the other end of the second effect solution inlet tube (11) is communicated with a solution inlet of the second effect evaporator (2), the other end of the third effect solution branch tube (13) is communicated with one end of a third effect solution inlet tube (12), and the other end of the third effect solution inlet tube (12) is communicated with a solution inlet of the third effect evaporator (3).

5. The multi-effect evaporation system of claim 4, wherein: the lower end of the tube side of the second effect evaporator (2) is communicated with one end of a second effect solution outlet pipe (5), and the other end of the second effect solution outlet pipe (5) is communicated with one end of a third effect solution inlet pipe (12).

6. The multi-effect evaporation system of claim 5, wherein: the lower end of the tube side of the third-effect evaporator (3) is communicated with one end of a third-effect solution outlet tube (6), the other end of the third-effect solution outlet tube (6) is communicated with one end of a second-effect solution branch tube (14), and the other end of the second-effect solution branch tube (14) is communicated with one end of a second-effect solution inlet tube (11).

7. The multi-effect evaporation system of claim 3, wherein: the first effect solution outlet pipe (4), the second effect solution outlet pipe (5) and the third effect solution outlet pipe (6) are all provided with pumps.

8. The multi-effect evaporation system of claim 6, wherein: the other end of the second effect solution outlet pipe (5) is communicated with an external filtering system through a second effect solution collecting pipe (15), and the other end of the third effect solution outlet pipe (6) is communicated with the external filtering system through a third effect solution collecting pipe (16).

9. The multi-effect evaporation system of claim 8, wherein: be equipped with first A valves (7) on second effect solution inlet pipe (11), be equipped with second A valves (8) on third effect solution inlet pipe (12), be equipped with third A valves (9) on third effect solution collecting pipe (16), be equipped with first B valves (17) on third effect solution branch pipe (13), be equipped with second B valves (18) on second effect solution collecting pipe (15), be equipped with third B valves (19) on second effect solution branch pipe (14).

10. The multi-effect evaporation system of claim 9, wherein: the other end of the second effect solution branch pipe (14) is connected to one end, close to the solution inlet of the second effect evaporator (2), of the first valve group A (7), and the other end of the third effect solution branch pipe (13) is connected to one end, close to the solution inlet of the third effect evaporator (3), of the second valve group A (8).

Technical Field

The invention belongs to the technical field of evaporators, and particularly relates to a multi-effect evaporation process and a system thereof.

Background

The multi-effect evaporator is characterized in that a plurality of evaporators are connected in series to operate, so that the heat energy of steam can be utilized for many times, and the utilization efficiency of the heat energy is improved. The working flow of the multi-effect evaporator is that saturated heating steam is introduced into a first effect to heat the solution in the first effect, the generated steam is introduced into a second effect to serve as heating steam, so that the solution in the second effect is evaporated at a lower temperature than that of the first effect, and the process is repeated until the last effect. Taking three effects in a multi-effect evaporator as an example, the multi-effect evaporator comprises a first effect evaporator, a second effect evaporator and a third effect evaporator, the solution concentration of the next effect is greater than that of the previous effect, the evaporator with high solution concentration can generate crystallization, and the efficiency of a heat exchanger is reduced.

Disclosure of Invention

Aiming at the problems in the prior art, the invention provides a multi-effect evaporation process and a system thereof, wherein a working condition a and a working condition b are alternately carried out, the solution is distilled and concentrated, and simultaneously, the crystallization in a multi-effect evaporator is prevented and removed, so that the efficiency and the progress of distillation and concentration are not influenced; the technological process is simple and easy to operate, and the multi-effect evaporation system is simple, reliable and low in cost.

In order to achieve the purpose, the invention adopts the technical scheme that:

a multi-effect evaporation process comprises the following two working conditions which are alternately carried out:

working condition a: introducing steam generated by the first effect evaporator into a second effect evaporator, introducing steam generated by the second effect evaporator into a third evaporator, introducing a solution discharged from the first effect evaporator into the second effect evaporator, introducing a solution discharged from the second effect evaporator into the third effect evaporator, and introducing a solution discharged from the third effect evaporator into a filtering process;

working condition b: and introducing steam generated by the first effect evaporator into the second effect evaporator, introducing steam generated by the second effect evaporator into the third evaporator, introducing the solution coming out of the first effect evaporator into the third evaporator, introducing the solution coming out of the third effect evaporator into the second evaporator, and introducing the solution coming out of the second effect evaporator into the filtering process.

A multi-effect evaporation system adopts the multi-effect evaporation process and comprises a first effect evaporator, a second effect evaporator and a third effect evaporator, wherein the upper ends of tube passes of the first effect evaporator, the second effect evaporator and the third effect evaporator respectively comprise a solution inlet and a steam outlet, the steam outlets of the first effect evaporator and the second effect evaporator are respectively communicated with shell pass inlets of the second effect evaporator and the third effect evaporator, the lower end of the tube pass of the first effect evaporator is communicated with solution inlets of the second effect evaporator and the third effect evaporator in parallel, the lower end of the tube pass of the second effect evaporator is communicated with a solution inlet of the third effect evaporator and an external filtering system in parallel, and the lower end of the tube pass of the third effect evaporator is communicated with a solution inlet of the second effect evaporator and an external filtering system in parallel.

As a further improvement of the above technical solution:

the lower end of the tube side of the first effect evaporator is communicated with the second effect evaporator through a first effect solution outlet pipe and a second effect solution inlet pipe, and is communicated with the third effect evaporator through a first effect solution outlet pipe and a third effect solution branch pipe; the lower end of the tube side of the second-effect evaporator is communicated with the third-effect evaporator through a second-effect solution outlet pipe and a third-effect solution inlet pipe; the lower end of the tube side of the third effect evaporator is communicated with the second effect evaporator through a third effect solution outlet pipe and a second effect solution branch pipe.

The lower end of the tube side of the first effect evaporator is communicated with one end of a first effect solution outlet tube, the other end of the first effect solution outlet tube is communicated with one end of a second effect solution inlet tube and one end of a third effect solution branch tube in parallel, the other end of the second effect solution inlet tube is communicated with a solution inlet of the second effect evaporator, the other end of the third effect solution branch tube is communicated with one end of a third effect solution inlet tube, and the other end of the third effect solution inlet tube is communicated with a solution inlet of the third effect evaporator.

The lower end of the tube side of the second effect evaporator is communicated with one end of a second effect solution outlet pipe, and the other end of the second effect solution outlet pipe is communicated with one end of a third effect solution inlet pipe.

The lower end of the tube side of the third effect evaporator is communicated with one end of a third effect solution outlet pipe, the other end of the third effect solution outlet pipe is communicated with one end of a second effect solution branch pipe, and the other end of the second effect solution branch pipe is communicated with one end of a second effect solution inlet pipe.

The first effective solution outlet pipe, the second effective solution outlet pipe and the third effective solution outlet pipe are all provided with pumps.

The other end of the second effect solution outlet pipe is communicated with an external filtering system through a second effect solution collecting pipe, and the other end of the third effect solution outlet pipe is communicated with the external filtering system through a third effect solution collecting pipe.

The second is imitated and is equipped with first A valves on the solution inlet pipe, and the third is imitated and be equipped with second A valves on the solution inlet pipe, is imitated and is equipped with third A valves on the solution collecting pipe on the third, is imitated and is equipped with first B valves on the solution branch pipe on the third, is imitated and is equipped with second B valves on the solution collecting pipe on the second, is imitated and is equipped with third B valves on the solution branch pipe on the second.

The other end of the second effect solution branch pipe is connected to one end, close to the solution inlet of the second effect evaporator, of the first valve bank A, and the other end of the third effect solution branch pipe is connected to one end, close to the solution inlet of the third effect evaporator, of the second valve bank A.

The first effect evaporator, the second effect evaporator and the third effect evaporator are internally provided with a solution to be evaporated and concentrated, and the shell pass is internally provided with steam for absorbing the heat of the solution in the tube pass.

The solution inlet of the first-effect evaporator is communicated with an external solution supply system.

The shell pass inlet of the first effect evaporator is communicated with an external steam supply system.

And a steam outlet of the third-effect evaporator is communicated with the condenser.

The first effect evaporator, the second effect evaporator and the shell pass outlet of the third effect evaporator are communicated with an external condensate collecting system.

Compared with the prior art, the invention has the beneficial effects that: the working condition a and the working condition b are alternately carried out, so that the solution is distilled and concentrated while the crystals in the multi-effect evaporator are prevented and removed, and the efficiency and the progress of distillation and concentration are not influenced; the process is simple and easy to operate, the multi-effect evaporation system is simple, reliable and low in cost, and when the multi-effect evaporation system is applied to the existing system, only the existing multi-effect evaporation system needs to be locally adjusted, and the existing multi-effect evaporation system does not need to be greatly changed.

Drawings

Fig. 1 is a schematic structural diagram of an embodiment of the present invention.

Detailed Description

The multi-effect evaporation process and the system thereof provided by the invention are further described in detail and fully with reference to the following examples. The following examples are illustrative only and are not to be construed as limiting the invention.

A multi-effect evaporation process comprises the following two working conditions which are alternately carried out:

working condition a: the method comprises the following steps that steam generated by a first effect evaporator 1 is introduced into a second effect evaporator 2, steam generated by the second effect evaporator 2 is introduced into a third evaporator, solution discharged from the first effect evaporator 1 is introduced into the second effect evaporator 2, solution discharged from the second effect evaporator 2 is introduced into a third effect evaporator 3, and solution discharged from the third effect evaporator 3 enters a filtering process;

working condition b: the method comprises the following steps that steam generated by a first effect evaporator 1 is introduced into a second effect evaporator 2, steam generated by the second effect evaporator 2 is introduced into a third evaporator, solution discharged from the first effect evaporator 1 is introduced into the third evaporator, solution discharged from the third effect evaporator 3 is introduced into the second evaporator, and solution discharged from the second effect evaporator 2 enters a filtering process.

An external solution supply system is introduced into the first-effect evaporator 1, and the first-effect evaporator 1 is communicated with an external steam supply system.

A multi-effect evaporation system adopts the multi-effect evaporation process and comprises a first effect evaporator 1, a second effect evaporator 2 and a third effect evaporator 3. The middle parts of the first effect evaporator 1, the second effect evaporator 2 and the third effect evaporator 3 are respectively provided with a plurality of pipelines which are arranged in parallel at intervals to form a shell pass and a tube pass which are arranged at intervals, and the two fluids are respectively positioned in the shell pass and the tube pass to exchange heat. The technical solutions of the shell side and the tube side of the evaporator are well known to those skilled in the art and will not be described herein. The first effect evaporator 1, the second effect evaporator 2 and the third effect evaporator 3 are filled with solution to be evaporated and concentrated in tube pass and steam for absorbing heat of the solution in the tube pass in shell pass.

The upper ends of the tube passes of the first effect evaporator 1, the second effect evaporator 2 and the third effect evaporator 3 respectively comprise a solution inlet and a steam outlet, the steam outlets of the first effect evaporator 1 and the second effect evaporator 2 are respectively communicated with the shell pass inlets of the second effect evaporator 2 and the third effect evaporator 3, the lower end of the tube pass of the first effect evaporator 1 is communicated with the solution inlets of the second effect evaporator 2 and the third effect evaporator 3 in parallel, the lower end of the tube pass of the second effect evaporator 2 is communicated with the solution inlet and the external filtering system of the third effect evaporator 3 in parallel, and the lower end of the tube pass of the third effect evaporator 3 is communicated with the solution inlet and the external filtering system of the second effect evaporator 2 in parallel.

The lower end of the tube side of the first effect evaporator 1 is communicated with the second effect evaporator 2 through a first effect solution outlet pipe 4 and a second effect solution inlet pipe 11, and the lower end of the tube side of the first effect evaporator 1 is also communicated with the third effect evaporator 3 through a first effect solution outlet pipe 4 and a third effect solution branch pipe 13. The lower end of the tube side of the second-effect evaporator 2 is communicated with a third-effect evaporator 3 through a second-effect solution outlet pipe 5 and a third-effect solution inlet pipe 12; the lower end of the tube side of the second-effect evaporator 2 is also communicated with an external filtering system through a second-effect solution outlet tube 5 and a second-effect solution collecting tube 15. The lower end of the tube side of the third-effect evaporator 3 is communicated with the second-effect evaporator 2 through a third-effect solution outlet pipe 6 and a second-effect solution branch pipe 14; the lower end of the tube side of the third effect evaporator 3 is also communicated with an external filtering system through a third effect solution outlet pipe 6 and a third effect solution collecting pipe 16.

The lower end of the tube side of the first effect evaporator 1 is communicated with one end of a first effect solution outlet tube 4, the other end of the first effect solution outlet tube 4 is communicated with one end of a second effect solution inlet tube 11 and one end of a third effect solution branch tube 13 in parallel, the other end of the second effect solution inlet tube 11 is communicated with a solution inlet of the second effect evaporator 2, the other end of the third effect solution branch tube 13 is communicated with one end of a third effect solution inlet tube 12, and the other end of the third effect solution inlet tube 12 is communicated with a solution inlet of the third effect evaporator 3.

The lower end of the tube pass of the second effect evaporator 2 is communicated with one end of a second effect solution outlet tube 5, the other end of the second effect solution outlet tube 5 is communicated with one end of a third effect solution inlet tube 12 and one end of a second effect solution collecting tube 15 in parallel, and the other end of the second effect solution collecting tube 15 is communicated with an external filtering system.

The lower end of the tube pass of the third effect evaporator 3 is communicated with one end of a third effect solution outlet tube 6, the other end of the third effect solution outlet tube 6 is communicated with one end of a second effect solution branch tube 14 and one end of a third effect solution collecting tube 16 in parallel, the other end of the second effect solution branch tube 14 is communicated with one end of a second effect solution inlet tube 11, and the other end of the third effect solution collecting tube 16 is communicated with an external filtering system.

And the first effect solution outlet pipe 4, the second effect solution outlet pipe 5 and the third effect solution outlet pipe 6 are respectively provided with a pump for providing power for the solution in the system.

The system is characterized in that a plurality of valve banks are arranged on pipelines of the system and used for controlling the opening and flow regulation of corresponding pipelines. Specifically, the second effect solution inlet pipe 11 is provided with a first valve bank 7, the third effect solution inlet pipe 12 is provided with a second valve bank 8, the third effect solution collecting pipe 16 is provided with a third valve bank 9, the third effect solution branch pipe 13 is provided with a first valve bank 17, the second effect solution collecting pipe 15 is provided with a second valve bank 18, and the second effect solution branch pipe 14 is provided with a third valve bank 19.

For the specific position of the valve group, the other end of the second effect solution branch pipe 14 is connected to one end of the first A valve group 7 close to the solution inlet of the second effect evaporator 2, and the other end of the third effect solution branch pipe 13 is connected to one end of the second A valve group 8 close to the solution inlet of the third effect evaporator 3.

For the initial solution and steam source, the solution inlet of the first effect evaporator 1 is communicated with the external solution supply system through a mother liquor pipe 20, the shell pass inlet of the first effect evaporator 1 is communicated with the external steam supply system through an external steam pipe 21, and the steam outlet of the third effect evaporator 3 is communicated with the condenser 10. The shell side outlets of the first effect evaporator 1, the second effect evaporator 2 and the third effect evaporator 3 are all communicated with an external condensate collecting system.

Based on the above system structure, when working condition a, the first a valve group 7, the second a valve group 8 and the third a valve group 9 are opened, and the first B valve group 17, the second B valve group 18 and the third B valve group 19 are closed.

During this operating mode, outside steam passes through outside steam pipe 21 and gets into in the shell side of first effect evaporimeter 1, and the concentrated solution of outside waiting to distill gets into the tube side of first effect evaporimeter 1 through mother liquor pipe 20 in first effect evaporimeter 1, and in first effect evaporimeter 1, after steam in the shell side transmitted heat for solution, steam condensation in the shell side to discharge through the condensate pipe of first effect evaporimeter 1, solution evaporation or boiling in the tube side produce one effect steam and one effect residual solution. The first effect steam enters the shell pass of the second effect evaporator 2 and is used as heating steam of the second effect evaporator 2, the first effect residual solution enters the tube pass of the second effect evaporator 2 through the second effect solution inlet pipe 11, and similarly, in the second effect evaporator 2, after the first effect steam in the shell pass transfers heat to the first effect residual solution, the steam in the shell pass is condensed and is discharged through the condensate pipe of the second effect evaporator 2, and the solution is evaporated or boiled to generate second effect steam and second effect residual solution. The second-effect steam enters the shell pass of the third-effect evaporator 3 and is used as heating steam of the third-effect evaporator 3, the second-effect residual solution enters the tube pass of the third-effect evaporator 3 through the third-effect solution inlet pipe 12, similarly, in the third-effect evaporator 3, after the second-effect steam in the shell pass transfers heat to the second-effect residual solution, the steam in the shell pass is condensed and is discharged through a condensate pipe of the third-effect evaporator 3, the solution is evaporated or boiled to generate third-effect steam and third-effect residual solution, the third-effect steam is condensed and then discharged through the condenser 10, and the third-effect residual solution is evaporated and concentrated to be qualified and is discharged to the filtering system.

Therefore, when the working condition a is adopted, the external solution sequentially passes through the first effect evaporator 1, the second effect evaporator 2 and the third effect evaporator 3, the concentration of the solution discharged from the first effect evaporator 1, the second effect evaporator 2 to the third effect evaporator 3 is gradually increased, and after the working condition a operates for a period of time, the content of the third effect evaporator 3 is easy to crystallize, the heat transfer efficiency is influenced, blockage occurs seriously, and therefore the working condition a operates for a period of time and is switched to the working condition b.

And under the working condition B, the first A valve group 7, the second A valve group 8 and the third A valve group 9 are closed, and the first B valve group 17, the second B valve group 18 and the third B valve group 19 are opened. Different from the working condition a, the first-effect residual solution generated by the first-effect evaporator 1 cannot pass through the first valve bank 7, but enters the third-effect evaporator 3 through the third-effect solution branch pipe 13 and the third-effect solution inlet pipe 12, the concentration of the first-effect residual solution is low, the first-effect residual solution is washed and redissolved after entering the third-effect evaporator 3, crystallization in the third-effect evaporator 3 is prevented and relieved, the third-effect residual solution generated by the third-effect evaporator 3 enters the second-effect evaporator 2 through the second-effect solution branch pipe 14 and the second-effect solution inlet pipe 11 for evaporation, and the generated second-effect residual solution enters an external filtering system through the second-effect solution collecting pipe 15.

Two kinds of operating mode switch go on to change mother liquor concentration in the evaporimeter, reach the purpose of washing and redissolution, prevent that last effect concentration is high and last crystallization lasts, reduce heat exchange efficiency. Finally, the continuous and efficient operation of the multi-effect evaporation system is realized.

The working principle of the invention is as follows: when the traditional multi-effect evaporator operates, under the working condition a, namely the tube passes of the first effect evaporator 1, the second effect evaporator 2 and the third effect evaporator 3 are sequentially communicated, and external solution sequentially flows through the tube passes of the first effect evaporator 1, the second effect evaporator 2 and the third effect evaporator 3 and is heated by steam at the shell pass side of the corresponding evaporators. The method comprises the steps that a solution in a tube pass of a first-effect evaporator 1 is heated to generate a first-effect residual solution and steam, the first-effect residual solution enters a tube pass of a second-effect evaporator 2 from the bottom of the first-effect evaporator 1, the steam rises and flows out of the top of the first-effect evaporator 1 and enters a shell pass of the second-effect evaporator 2 through a corresponding pipeline to heat the first-effect residual solution on the tube pass side of the second-effect evaporator 2, and similarly, the first-effect residual solution is heated to generate the steam and the second-effect residual solution, the second-effect residual solution enters a tube pass of a third-effect evaporator 3 from the bottom of the second-effect evaporator 2 and is heated to generate the steam and the third-effect residual solution in the third-effect evaporator 3, the steam source of the first-effect evaporator 1 is external steam, and the steam heat on the shell pass side of each effect evaporator generates condensate and the condensate is discharged out of the evaporator. As can be seen from the above, although the temperatures of the steam in the first effect evaporator 1, the second effect evaporator 2 and the third effect evaporator 3 are sequentially reduced, the steam in the second effect evaporator 2 and the third effect evaporator 3 is derived from the moisture in the solution, that is, the concentrations of the solution in the external solution, the first effect residual solution, the second effect residual solution and the third effect residual solution are sequentially increased, and the concentrated solution is convenient for the subsequent treatment.

However, since the solution concentration is decreased in this order, a crystallization site is liable to occur in the third effect evaporator 3, and after a long run, clogging and corrosion may occur in the tube side of the third effect evaporator 3. Therefore, after operating the working condition a for a period of time, the operating mode of the working condition b needs to be adopted.

And under the working condition b, the steam flow direction is unchanged, and the solution flow direction is changed. The first effect residual solution generated by the first effect evaporator 1 enters the tube pass of the third effect evaporator 3, because the concentration of the first effect residual solution is low and the water content is more, the crystals in the shell pass of the third effect evaporator 3 can be washed and dissolved, the first effect residual solution is heated and evaporated in the shell pass of the third effect evaporator 3 at the same time, and because the steam temperature in the third effect evaporator 3 is low and the heat is less, the first effect residual solution is evaporated less and cannot generate crystals, the third effect residual solution generated in the third effect evaporator 3 enters the tube pass of the second effect evaporator 2 and is heated by the steam with higher temperature in the second effect evaporator 2, and the concentration of the generated second effect residual solution is greatly improved.

Working conditions a and b are performed alternately, so that heat is fully utilized, and crystallization in the second effect evaporator 2 and the third effect evaporator 3 can be prevented.

Finally, it must be said here that: the above embodiments are only used for further detailed description of the technical solutions of the present invention, and should not be understood as limiting the scope of the present invention, and the insubstantial modifications and adaptations made by those skilled in the art according to the above descriptions of the present invention are within the scope of the present invention.