JP2812623B2 - Boiler water treatment method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for treating boiler water, and more particularly to a method for effectively preventing corrosion and scale from occurring in a boiler can using pure water or soft water as feed water. is there.

[0002]

2. Description of the Related Art Corrosion inhibitors such as hydrazine, sodium sulfite, tannins, lignins, saccharides, etc. and water-soluble compounds are used to suppress corrosion and scale formation in boilers using pure or soft water as feed water. A scale dispersant mainly composed of a polymer compound is used together with an alkali agent and inorganic phosphates.

On the other hand, a water treatment agent containing a copolymer derived from a monomer having a phosphonic acid and a carboxyl group and having an ethylenically unsaturated bond, or a water treating agent containing a phosphonic acid and a maleic acid-based copolymer. Water treatment methods using a lignin derivative, a tannic acid, a polysaccharide such as starch as a treating agent and a scale inhibitor and a combination of these derivatives are disclosed in JP-A-51-146341 and JP-A-53-1.
No. 46279.

A metal anticorrosive mainly comprising a mixture of a phosphonic acid and a tannin or a lignin is disclosed in JP-A-48-71741 by adding a zinc compound, a lignin compound and a maleic acid polymer in combination. An anticorrosion method for underwater metals has been proposed in JP-A-56-62974.

[0005]

2. Description of the Related Art In recent years, there has been an increasing interest in not only the effects on water treatment chemicals but also their safety and energy saving effects, and water treatment in boiler water systems is no exception. For the purpose of energy saving,
As boiler high-concentration operation and condensate recovery and utilization are progressing, scaling and local corrosion in boiler cans caused by iron oxide brought in from make-up water or condensate have become a major problem.

[0006] Hydrazine and sodium sulfite are the main corrosion inhibitors used in boiler water systems. When hydrazine is used, the generated steam or high-temperature water may come into direct contact with human bodies or products due to its toxicity. Cannot be used in hospitals or food factories.

[0007] Although sodium sulfite has no problem in safety, it does not always exert a sufficient effect since it reacts with oxygen to produce sulfate ions which cause corrosion. As a result of significantly increasing the concentration and the electric conductivity, the amount of blow is increased, and there are disadvantages such as an increase in energy cost.

[0008] On the other hand, natural product-based corrosion inhibitors such as tannic acid have no problems of safety or excessive rise of dissolved solids concentration and electric conductivity of boiler water, but stable corrosion prevention. It is difficult to obtain the effect, and particularly when the scale mainly composed of iron oxide adheres to the metal surface, a sufficient corrosion inhibiting effect is not exhibited due to local corrosion caused by formation of the oxygen concentration cell.

Acrylic acid-based polymers and maleic acid-based polymers are mainly used as scale dispersants in boiler water systems together with hardeners such as alkali agents and inorganic phosphates. It exerts an excellent dispersing effect on calcium-based scales such as calcium carbonate and calcium phosphate, which are generated when it is used, but hardly exhibits a dispersing effect on metal oxide-based scales such as iron oxide.

Further, the water treatment method disclosed in JP-A-51-146341 or JP-A-53-146279 specifically relates to a method for preventing corrosion, scale and dirt in a cooling water system, and is relatively limited. Insoluble precipitation scale composed of phosphonic acids and calcium ions that precipitate on metal surfaces in cooling water with high calcium hardness is suppressed by using a water-soluble polymer compound and other scale inhibitors to suppress corrosion and scale simultaneously. I'm trying to figure it out,
Generally, pure water or soft water is used as feed water, and the expected effect cannot be obtained in a boiler water system having no calcium hardness.

The metal anticorrosive and the water treatment method proposed in JP-A-48-71741 or JP-A-56-62974 are concretely similar to the above-mentioned water treatment methods in cooling water. Regarding corrosion inhibition, the former is to improve the anticorrosion effect by adding phosphonic acids to tannins or lignins, reduce the required addition concentration, and prevent water from coloring at the use concentration,
In the latter case, by blending the maleic acid polymer with the combination of the zinc compound and the lignin compound, the dispersibility of the zinc and the lignin compound in water is improved, the anticorrosion property is improved, and the treatment at a lower concentration is realized. is there. These inventions relate to the anticorrosion effect, but do not mention any other major problem of water treatment, that is, the effect of controlling scale.

In addition, polysaccharides such as lignin derivatives, tannic acids and starch used in the above-mentioned water treatment method are used as scale inhibitors, and reduce their functions by hydrolysis under high temperature and high pressure such as in boiler water systems. However, there is no suggestion about the idea of acting as a corrosion inhibitor by removing dissolved oxygen. An object of the present invention is to solve the above-mentioned drawbacks of the conventional water treatment method and to provide a water treatment method capable of effectively suppressing scale formation and preventing corrosion.

[0013]

Means for Solving the Problems The method for treating boiler water according to the present invention, which has been made to achieve the above object, comprises a method for boiler water containing at least one or more natural substances selected from tannins, lignins and sugars. It is characterized in that a substance-based corrosion inhibitor, a phosphonic acid or a salt thereof, and an unsaturated carboxylic acid polymer are added together.

[0014] Of the three components used in the present invention, treatment with one or a combination of two does not provide sufficient anticorrosion and scale control effects due to the following technical grounds.

That is, with respect to corrosion prevention of metals, corrosion inhibitors of natural compounds such as tannic acid suppress corrosion by forming an insoluble film on the metal surface together with the deoxidizing action. If a scale such as iron oxide adheres to the surface, local corrosion due to the formation of an oxygen concentration cell occurs, and a sufficient corrosion suppressing effect is not exhibited.

In terms of scale suppression, phosphonic acid or a salt thereof exerts a high dispersing effect on calcium carbonate, especially iron oxide scale, due to its excellent chelating effect, but almost disperses on calcium phosphate scale. No effect, on the other hand, unsaturated carboxylic acid polymer exhibits an excellent dispersing effect on calcium-based scales such as calcium carbonate and calcium phosphate,
Almost no dispersing effect on iron oxide.

The method for treating boiler water of the present invention prevents the adhesion of iron oxide and other scales to a metal surface by combining these three components having different characteristics.
It is intended to prevent the occurrence of local corrosion due to the formation of an oxygen concentration cell, thereby preventing corrosion of metals and suppressing scale.

The addition amount of the above components used in the present invention is as follows:
Tannins, lignins and sugars are each 20-500
mg / l, preferably 50 to 300 mg / l, phosphonic acid or a salt thereof is 5 to 100 mg / l, preferably 10 to 10 mg / l.
50 mg / l, 5-100 m of unsaturated carboxylic acid polymer
g / l, and preferably 10 to 50 mg / l, from the viewpoint of the effect of suppressing corrosion and scale or from the viewpoint of economy.

The above components may be added separately so as to have a predetermined concentration, or may be added by mixing them in advance. Further, the above-mentioned components may be added to the makeup water line or directly to the boiler.

The unsaturated carboxylic acid polymer used as a component of the present invention is a homopolymer of acrylic acid, maleic acid and methacrylic acid from the viewpoint of exhibiting an excellent dispersing effect on calcium carbonate and calcium phosphate scales. And / or copolymers and / or copolymers with other compounds that can be copolymerized with these monomers.

Other compounds copolymerizable with the above monomers include compounds having an ethylenic double bond, such as fumaric acid, itaconic acid, acrylamide, acrylate, vinyl acetate, styrene, ethylene oxide, ethylene, n -Butylene, isobutylene, allylsulfonic acid, vinylsulfonic acid and styrylsulfonic acid.

The molecular weight of these polymers is from 500 to
100,000, preferably 1,000 to 20,000
Is preferred from the viewpoint of exhibiting an excellent scale dispersion effect.

Among the natural compounds used as corrosion inhibitors in the present invention, tannins include hydrolyzable tannins such as gallotannin and condensable tannins such as kevracho tannin, and lignins include ligninsulfonic acid. Sodium and saccharides include monosaccharides such as glucose, polysaccharides such as starch, and derivatives thereof.

The phosphonic acid or a salt thereof used as a dispersant on a metal oxide scale such as iron oxide in addition to calcium carbonate in the present invention includes aminotrimethylphosphonic acid, aminodimethylphosphonomonoethylphosphonic acid, ethylenediamine Tetramethylphosphonic acid, diethylenetriaminepentamethylphosphonic acid, triethylenetetraaminehexamethylphosphonic acid, hydroxyethylidenediphosphonic acid, hydroxypropylidenediphosphonic acid, 1,2,4-tricarboxybutane-2-phosphonobutanetricarboxylic acid, Examples include 2-dicarboxybutane-2-phosphonic acid, 1,2,4-tricarboxyhexane-2-phosphonic acid, and salts thereof.

In addition to the above three components used in the treatment method of the present invention, alkaline agents such as sodium hydroxide and sodium carbonate, inorganic phosphates such as sodium tertiary phosphate and sodium hexametaphosphate, glycolic acid, lactic acid, It can be used in combination with a corrosion inhibitor such as citric acid, tartaric acid, malic acid, gluconic acid, cyclohexylamine, morpholine, and octadecylamine, or can be used by mixing them in advance.

[0026]

[Action] Natural corrosion inhibitors, such as tannic acid,
As an oxygen scavenger, it removes dissolved oxygen in water, which is the main cause of corrosion, and forms an insoluble film on the metal surface, thereby isolating the metal from the environment containing the corrosive factors, thereby suppressing metal corrosion.

The phosphonic acids and unsaturated carboxylic acid polymers used as scale dispersants are adsorbed on the surface of the scale particles to increase the charge repulsion between the particles,
Scaling is suppressed by altering the crystal structure to prevent crystal growth.

In the treatment method of the present invention, adhesion of scales such as iron oxide to metal surfaces is prevented by the action of phosphonic acids and unsaturated carboxylic acid polymers, and furthermore, these scale inhibitors and tannic acid are included. Due to the synergistic effect of the natural product corrosion inhibitor, an insoluble adsorption film or oxide film with excellent stability is formed on the metal surface, and scale suppression and corrosion prevention are achieved by isolating the metal from the environment containing corrosion factors. It is thought that.

An embodiment of the present invention will be described below.

[0030]

EXAMPLES [Corrosion Test] A mild steel test tube (inner diameter 50 mm, length 500 mm) and a mild steel test piece were placed in an experimental boiler having a capacity of 10 liters, and a corrosion test was performed. The soft water shown in Table 1 was used as the make-up water of the boiler, and each chemical used in the treatment method of the present invention was added to the make-up water so as to have a predetermined concentration in the boiler water, and was supplied to the experimental boiler.

[0031]

[Table 1]

In addition, adjustment was performed by adding sodium hydroxide to the makeup water so that the pH of the boiler water became 11.0 or more. The experimental boiler was operated at a pressure of 1 MPa, a temperature of 183 ° C. and a 10-fold concentration for 150 hours.

After the test, the test tube and the test piece were taken out, the state of occurrence of corrosion was observed, and the corrosion rate and the corrosion inhibition rate of the test piece were measured by the following formulas.

[0034]

(Equation 1)

[0035]

(Equation 2)

Table 2 shows the test results.

[0037]

[Table 2]

[Scale Dispersion Test] In order to evaluate the dispersing power of various scale components (calcium carbonate, calcium phosphate, iron hydroxide), a dispersion test using synthetic water shown in Table 3 was performed. 200ml
Put synthetic water each drug was given concentration added to the tall beaker, which was left in a constant temperature bath kept at 80 ° C. 20 h,
The scale component concentration in the supernatant was determined, and the scale dispersion was determined by the following equation.

[0039]

(Equation 3)

[0040]

[Table 3]

Table 4 shows the test results.

[0042]

[Table 4]

[0043]

[Effects] The boiler water treatment method of the present invention has high safety, for example, using natural compounds such as tannic acid as a corrosion inhibitor, and is effective in high concentration operation and condensate recovery. Since it can be sufficiently used, it can greatly contribute to a reduction in energy cost.

Furthermore, the treatment method of the present invention provides a stable and excellent scale inhibition by the synergistic effect of the combined use of two kinds of scale dispersants having different dispersing characteristics and a natural product-based corrosion inhibitor such as tannic acid. Anticorrosion treatment can be realized at the same time.

Continuation of the front page (51) Int.Cl. ⁶ identification code FI C23F 14/02 C23F 14/02 A F22B 37/00 F22B 37/00 A (58) Field surveyed (Int.Cl. ⁶ , DB name) C23F 11/12 C23F 11/167 C23F 11/173 C23F 14/02 C23F 15/00 C02F 5/08 C02F 5/10 F16L 58/00 F22B 37/00

Claims (3)

(57) [Claims] 1. To boiler water, at least one or more natural corrosion inhibitors selected from tannins, lignins and sugars, phosphonic acid or a salt thereof, and an unsaturated carboxylic acid polymer are added together. A method for treating boiler water. 2. In boiler water, 20 to 500 mg / l of at least one natural compound selected from tannins, lignins and sugars, 5 to 100 mg / l of phosphonic acid or a salt thereof, and unsaturated carboxylic acid 2. The method for treating boiler water according to claim 1, wherein the polymer is added together so as to be 5 to 100 mg / l. 3. The unsaturated carboxylic acid polymer is a homopolymer of acrylic acid, maleic acid and methacrylic acid and / or
3. The method for treating boiler water according to claim 1, wherein the method is a copolymer and / or a copolymer with another compound copolymerizable with these monomers.