JPH03127699A - Method for preventing scale of multieffect sea water desalting device - Google Patents

Abstract

PURPOSE:To economically obtain fresh water from the sea water by heating the deaerated sea water, which is added with an acid to lower its pH, continuously adding the acid thereto to maintain the water a <=pH2 for a short period of time and adding an alkali thereto where the temp. of the concd. sea water attains <=110 deg.C. CONSTITUTION:The acid is added to the sea water to lower its pH and to obtain the deaerated sea water of pH 7 to 8. The acid is continuously added to the deaerated sea water which is heated by continuous preheaters until the temp. rises to >=90 deg.C, by which the pH thereof is maintained at <=5.5. The amt. of the acid to be added is intermittently increased to maintain the pH of the deaerated sea water of >=90 deg.C at <=2 for a short period of time. Further, the alkali is added to the concn. sea water to increase the pH thereof to <=6.5 where the temp. of the sea water concd. by evaporation of a part of the water in the deaerated sea water attains <=110 deg.C. The number of the effect stages is increased as compared with the conventional method and the economy is improved.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Field of Application] The present invention relates to a method for preventing scale in a multi-effect seawater desalination device.

[Conventional technology]

Examples of seawater desalination methods include multiple effect method, multistage flash method, reverse osmosis method, and self-vapor compression method. The multiple effect method has the potential to reduce the cost of water generation the most compared to the ground type method, but there is a problem that the amount of water produced decreases over time due to scale precipitation on the heat transfer surface.

FIG. 2 shows a configuration diagram of a seawater desalination apparatus using the multiple effect method, and the conventional method will be explained based on this diagram.

Seawater 10 was adjusted to pH 5 by adding sulfuric acid 17 from sulfuric acid tank 2.
6, oxygen and carbon dioxide gas in the seawater 10 is degassed in a degassing tower 1. The degassed seawater 11 is sequentially transferred to the preheater 4.
a, 4 b, 4 c, 4 d.

4e and 4f, and is heated and heated, and finally heated and heated in a preheater 5 by steam 15 generated from an external steam generator (not shown), and introduced into a heat exchanger 6.

In the heat exchanger 6, the degassed seawater 11 undergoes flash evaporation to reach the pressure of the chamber constituting the heat exchanger 6, and then is heated by the steam 15 and partially evaporates. heat exchanger 6
12g of steam generated in the chamber constituting the chamber is sent to a preheater 4f and a heat exchanger 7f.

The steam 15 is condensed by heating the degassed seawater 11,
The condensate 16 is returned to the external steam generator.

Concentrated seawater 13 after some of the water in degassed seawater 11 has evaporated
g is sent to the heat exchanger 7f. The pressure in the chamber constituting the heat exchanger 7f is set lower than the pressure in the chamber constituting the heat exchanger 6. Therefore, 13g of concentrated seawater is transferred to heat exchanger 7.
After flash evaporation in the chamber constituting f, further vapor 1
After a portion of the water in the concentrated seawater 13g is evaporated in a chamber constituting the heat exchanger 7f, the concentrated seawater is sent to the heat exchanger 7e.

13g of concentrated seawater is sequentially transferred to heat exchangers 7e, 7d, 7c, and 7b.
, 7a, and finally becomes concentrated seawater 13 and is discharged outside the system. Heat exchanger 7f.

7e, 7d,? The steam generated in steps c and 7b is condensed by heating the concentrated seawater in the next stage and preheating the degassed seawater 11 in a preheater, and is finally collected to become manufactured water 14.

[Problem to be solved by the invention]

Conventional multi-effect seawater desalination equipment adds acid to the seawater to prevent the precipitation of alkali scales such as calcium carbonate and magum cilum hydroxide from the seawater when the seawater temperature reaches above 90°C. Dissolved carbon dioxide was expelled, and polyphosphoric acid-based or polycarboxylic acid-based scale inhibitors were added. Although the above prevention method can suppress the precipitation of alkali scale, it is not a method for preventing the precipitation of calcium sulfate, and the upper limit of seawater temperature is about 100°C.

In view of the above-mentioned state of the art, the present invention is aimed at seawater desalination, which makes it possible to increase the temperature of seawater to 100°C or more, thereby increasing the number of stages of multiple effects, and making it possible to economically obtain freshwater from seawater. The present invention aims to provide a method for preventing scaling of equipment.

[Means to solve the problem]

In the present invention, when obtaining fresh water using a multi-effect seawater desalination device,
8. Obtain deaerated seawater, ■ Heat the deaerated seawater with a continuous preheater until the temperature reaches 9.
Continuously add the left to the degassed seawater at a temperature of 0°C or higher to make its pH 5.5 or lower, and intermittently increase the amount of acid added to the degassed seawater at a temperature of 90°C or higher p) I is maintained at 2 or less for a short time, and when the temperature of the concentrated seawater obtained by evaporating part of the water in the deaerated seawater becomes 110°C or less, an alkali is added to the concentrated seawater to reduce its p)l. This is a method for preventing scale in a multi-effect seawater desalination device, characterized in that the desalination temperature is 6.5 or more.

In carrying out the present invention, titanium-based materials are used for heat transfer surfaces with a pH of 5.5 or less, and titanium-based materials can of course be used for heat transfer surfaces with a pH of 6.5 or higher; cheaper materials can be used.

[Effect]

In chemical equilibrium, calcium sulfate has a total salt concentration (TDS) of seawater.
) becomes around 38,000 ppm, the seawater temperature decreases by 10
When the temperature exceeds 0 to 105°C, scale precipitation begins, but p
By lowering H to about 5.5, the precipitation rate of calcium sulfate can be significantly slowed down. This is because the so-called supersaturated state becomes more stable as the pH is lowered.
This is Ca, which becomes the crystal nucleus when calcium sulfate precipitates.
It is presumed that this is because scale precipitation of CO5 or Mg (OHL) becomes less likely to occur.

Therefore, as in the present invention, the pH of seawater at 90°C or higher is
By setting the value to 5.5 or less, the precipitation rate of calcium sulfate, which precipitates at 90° C. or higher, is reduced. In this case, even if scale is precipitated, the scale is porous, so by intermittently reducing the pl+ of seawater to 2 or less, the precipitated scale can be easily redissolved. Therefore, the maximum temperature of degassed seawater is approximately 13
It can be raised to 0°C.

When the seawater temperature is over 100℃, the only device material available is titanium, regardless of the pH; however, when the seawater temperature is below 100℃,
．． If it is 5 or more, it becomes possible to use a material that is cheaper than titanium-based materials, and there is no fear of scale at 110° C. or lower even if the pH is 6.5 or more. Therefore, in the present invention, an alkali is added to the concentrated seawater at a temperature of 110° C. or lower to adjust its pH to 6.5 or higher.

〔Example〕

An embodiment of the present invention will be described with reference to FIG.

In addition to the conventional multi-effect seawater desalination apparatus, the apparatus of this embodiment has the following features:
8 and a line for adding base 19 from base tank 8 to concentrated seawater 13e in heat exchangers 7e and 7d.

In the case of the embodiment shown in FIG. 1, the temperature of the degassed seawater 11 is 90°C or less at the outlet of the preheater 4d, and
The temperature at the outlet is 90°C or higher.

Moreover, the concentrated seawater 13e at the outlet of the heat exchanger 7e is 110°C.
Above, the concentrated seawater 13d at the outlet of the hot exchanger 7d is 1
The temperature will be below 10℃. In normal operation according to this embodiment,
The degassed seawater between the deaerator 1 and the preheater 4d is pt17.
．． 0, the pH of the fluid between the degassed seawater 11 and the concentrated seawater 13e after the preheater 7e is 5.0 to 5.5, and the fluid downstream from the concentrated seawater 13d is operated at a pH of 6.5 to 8. The reason for setting the pH to 6.5 or higher is to reduce the corrosivity caused by seawater, and this allows the use of low-cost materials such as cupronickel. Heat exchanger in high temperature section 6.7
If the scale of f and 7e becomes longer and the overall amount of fresh water decreases, increase the amount of acid 18 added and add preheater 4.
p of the fluid between degassed seawater 11 and concentrated seawater 13e after e
) by setting I to 1 to 2 and increasing the amount of base 19 added,
The pH downstream from the concentrated seawater 13d is 6.5 to 7.0. Degassed seawater 11 to concentrated seawater 1 after preheater 4e
It is sufficient to take about 5 minutes to adjust the I)H of the fluid between 3e and 3e to 1 to 2.

By using titanium or titanium with palladium as the material for the heat transfer part from the preheater 4e onwards to the heat exchanger 7e, material corrosion could be prevented even if the pH was lowered.

The maximum temperature of degassed seawater 11 is now 130°C.

Even if the material used for seawater temperatures below 90°C was made of cupronickel, which is cheaper than titanium, material corrosion could be prevented because the seawater pH was 6.5 or higher.

By appropriately selecting the pH of the seawater and the materials that make up the device depending on the temperature of the seawater, it has become possible to manufacture a water generating device that is both scale-preventable and inexpensive.

〔Effect of the invention〕

Even if the maximum temperature of degassed seawater is increased to 130℃, there is no decrease in the amount of fresh water produced due to scale precipitation, and the maximum temperature is 130℃.
By setting the temperature to 0°C, it becomes possible to increase the number of effect stages compared to conventional methods, and it is possible to provide an economical multi-effect seawater desalination apparatus.

[Brief explanation of the drawing]

FIG. 1 is an explanatory diagram of a seawater desalination apparatus for explaining an embodiment of the present invention, and FIG. 2 is an explanatory diagram of one aspect of a conventional seawater desalination apparatus.

Claims (1)

[Claims] When fresh water is obtained by a multi-effect seawater desalination device, (1) an acid is added to seawater to lower the pH, and then degassed to raise the pH;
(2) Heat the degassed seawater with a continuous preheater, and then continuously add acid to the degassed seawater whose temperature reaches 90°C or higher to adjust its pH to 5. 5 or less, and intermittently increased the amount of acid added to maintain the pH of the degassed seawater at 90°C or higher at 2 or less for a short period of time, (3) evaporated part of the water in the degassed seawater. A method for preventing scale in a multi-effect seawater desalination apparatus, which comprises adding an alkali to the concentrated seawater to adjust its pH to 6.5 or higher when the temperature of the concentrated seawater becomes 110° C. or lower.