JPS6057918B2 - Seawater desalination method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention provides a method for desalinating seawater, more specifically, a method for efficiently desalinating seawater by preventing scale from adhering to a heating evaporation device when heating and evaporating seawater to obtain fresh water. Regarding.

The conventional method of obtaining fresh water is to heat and evaporate seawater and then condense it.
For example, this is carried out using various heating evaporation devices such as a multi-stage flash method, a tube-type multiple effect method, or an immersion tube method.

However, when seawater is desalinated using the various devices mentioned above, the seawater is heated and evaporated to a high concentration, resulting in the Mg ions, Ca ions, etc. contained therein being precipitated in the form of carbonates, sulfates, hydroxides, etc. , etc. adhere as scale inside the above-mentioned apparatus, especially on the heat transfer surface. The adhesion of this scale significantly reduces the heat transfer coefficient of the heat transfer surface, extremely reducing the desalination efficiency, and eventually making operation impossible, which is a serious problem. In particular, seawater usually contains about 100 times as much salt as ordinary fresh water, and also has a large content of Mg ions, Ca ions, etc., so the tendency of scale adhesion is particularly remarkable. Therefore, in this type of seawater desalination method, a method has been developed in which an acid such as sulfuric acid is added to the seawater to be treated to adjust the pH to prevent scale formation and precipitation in the equipment. However, by adding acid, the pH
In the method of preventing scale formation by controlling the amount of water, a new problem arises: acid corrosion of equipment materials, etc. Moreover, the amount of acid required to decompose the scale components is extremely large, and since strong acids such as sulfuric acid are usually used, handling is difficult. However, it is not a practically satisfactory method as it is also dangerous. Various methods have also been proposed for adding various scale inhibitors, including polymerized phosphoric acid, which are used to prevent scale in industrial water (hard water). However, unlike general industrial water, seawater has significant scale adhesion, and ordinary scale inhibitors, when used at commonly used concentrations, are not only unable to produce sufficient scale prevention effects, but also have relatively low scale adhesion. Even when used in large quantities, it still cannot produce a practical scale-inhibiting effect. Moreover, in the desalination of seawater mentioned above, the water to be treated is highly concentrated at high temperatures, so polymerized phosphates, etc., which easily hydrolyze at high temperatures and lose their scale prevention ability, cannot withstand practical use, rather they are When used, orthophosphoric acid produced by hydrolysis forms insoluble salts with Ca ions and Mg ions, and conversely becomes a factor of scale. As described above, a satisfactory scale prevention method has not yet been developed for seawater desalination.
The current situation is that it is impossible to improve the efficiency of desalination. In view of the above-mentioned current situation, it is an object of the present invention to provide a new seawater desalination method that suppresses scale adhesion, which is a serious problem in seawater desalination, and that can be operated safely, efficiently, and at low running costs. There is something to do.

This object is achieved by adding and coexisting an isobutene-maleic anhydride copolymer or a salt thereof and a phosphonic acid or a salt thereof in an amount of 213 to 312 times the weight of the copolymer to the seawater to be treated. That is, the present invention provides that the above-mentioned isobutene-maleic anhydride copolymer has excellent scale prevention ability, and when it is used in combination with the above-mentioned predetermined amount of phosphonic acid, they act synergistically and achieve excellent scale prevention even with a small amount of addition. This product was developed based on the discovery that it exhibits a scale-preventing effect that does not deteriorate at all even under high-temperature conditions, and that it can eliminate the negative effects of scale adhesion in seawater desalination at once. In the present invention, the isobutene-maleic anhydride copolymer preferably has a molecular weight of about 1,000 to 150. The copolymerization ratio of isobutene and maleic anhydride in the copolymer is not particularly limited, but the isobutene content is usually about 20 to 70%, preferably about 50%. Further, the above copolymer can be used in the form of an alkali metal salt such as sodium or potassium, or an ammonium salt in order to further improve its solubility in water. Further, as the phosphonic acid and its salt used together with the isobutene-maleic anhydride copolymer or its salt, various phosphonic acids, phosphonocarboxylic acids, and water-soluble salts thereof can be used. Specifically, hydroxyethane-1,1-diphosphonic acid, aminoethane-1,1
Phosphonic acids such as -diphosphonic acid, hydroxypropane-1,1-diphosphonic acid, aminotrimethylphosphonic acid, ethylenediaminetetramethylphosphonic acid, hexamethylenediaminetetramethylphosphonic acid, 1-phosphonopropane-2,3-dicarboxylic acid, 1,1- Phosphonoalkanedicarboxylic acids such as diphosphonopropane-2,3-dicarboxylic acid, 2-phosphonobutane-1,2-dicarboxylic acid, 2,2-diphosphonopropane-3,4-dicarboxylic acid; 1-phosphonopropane-3,4-dicarboxylic acid; Nopropane-1,2,3-tricarboxylic acid, 2-phosphonobutane-1,2,4-tricarboxylic acid, 2-phosphono-3-(2-methylcarboxymethyl)
Examples include phosphonoalkanetricarboxylic acids such as -hexane-1J2,4-tricarboxylic acid; and alkali metal salts and ammonium salts of each compound. The combination ratio of the above copolymer or its salt and phosphonic acid or its salt is approximately 10 to 9% by weight for the former and 10 to 9% by volume for the latter, but a small amount of addition can produce a combined effect. In order to always have a stable and sufficient scale prevention effect, the former 40~(1)
The latter is preferably 40 to 6% by weight.

The amount added to seawater is about 0.5 ppm or more, preferably 0.5 to 200 ppm, more preferably 2 to 20 ppm.
It is sufficient that the amount is present at a concentration of ppm, and therefore, the amount that has the above concentration may be continuously or intermittently added to the seawater to be treated. Thus, according to the present invention, by simply adding and coexisting the above-mentioned isobutene-maleic anhydride copolymer or its salt and phosphonic acid or its salt in seawater at a predetermined ratio, they act as an unprecedented scale inhibitor. It exhibits excellent scale prevention ability without being substantially affected by thermal evaporation of seawater under high-temperature conditions and high concentration of seawater, making desalination of seawater extremely efficient.

The seawater desalination apparatus, operation, operating conditions, etc. in the present invention may be carried out in accordance with conventional methods. In addition, in the method of the present invention, it is also possible to add and use known antifoaming agents such as silicone oil and nonionic activators, if necessary. Examples will be given below to explain the present invention in more detail.

Example 1f Seawater 11 was placed in a beaker, a predetermined amount of each scale inhibitor was added thereto, a vibrator heater (100cm, 500W) was immersed as a heat source in this seawater, and heated by electricity. , Evaporate and concentrate while stirring.

Claims (1)

[Claims] 1. When heating and evaporating seawater to obtain fresh water, isobutene-maleic anhydride copolymer or its salt is added to the seawater,
A seawater desalination method characterized in that 2/3 to 3/2 times the weight of phosphonic acid or a salt thereof is added to coexist with the seawater to prevent scale from adhering to the heating evaporator.