JPH0762266B2 - Ion blocking agent for boiler water system and method for preventing scale formation in boiler water system using the same - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial field of application] The present invention relates to an ion sequestering agent for a boiler water system for preventing scale formation of a boiler having a pH of boiler water of 9.0 or more, and to prevent the production of scale using the same. On how to do.

[Conventional technology]

In the boiler water system, dissolved ions contained in the feed water are concentrated and exposed to high temperature of 150 ° C or higher, so that the relatively insoluble compound becomes supersaturated and precipitates as scale on the heat transfer surface. If scale deposits on the heat transfer surface, the thermal conductivity will decrease, which will increase the amount of combustion used and the amount of steam generated, and also the temperature on the surface of the heat transfer surface will rise due to scale accumulation, increasing the strength of the metal material. Boiler may be ruptured due to a decrease in the. In recent years, as the performance of boilers has increased, the amount of evaporation per unit heat transfer area has tended to increase, and the tendency of scale formation has increased.

In order to prevent the corrosion of carbon steel which is a constituent material of the boiler, the pH of the boiler water is usually operated in the range of 9-12.
In addition, especially when raw water or softened water is used as makeup water, it is necessary to keep the alkalinity of the boiler water high in order to prevent silica scale.
It will be in the range of 10-12.

Under such conditions of high temperature and high pH, the precipitation tendency of scales such as calcium carbonate, calcium silicate, magnesium silicate, magnesium hydroxide, iron hydroxide, iron oxide, zinc hydroxide, zinc oxide becomes strong. Therefore, the conventional technique for preventing scale formation has not been found to have sufficient effect of preventing scale formation. For example, conventionally, calcium and magnesium are insolubilized as suspended solids in the form of calcium phosphate and magnesium hydroxide, respectively, by using a phosphate and an alkali to prevent scale troubles due to precipitation of a hardly soluble compound on the heat transfer surface. Tannins to prevent occasional deposition of suspended solids,
A method of using a dispersant such as a natural polymer such as lignin and starch, an anionic synthetic polymer such as poly (meth) acrylic acid, and a styrenesulfonic acid-maleic acid copolymer has been adopted. The effect of such a treatment is to prevent the generation of scale by producing a solid content in the can, so that the scale generation preventing effect was insufficient.

The so-called threshold treatment, which treats organic phosphonic acids and anionic synthetic polymers in an amount much smaller than the amount required to block calcium ions in water, is successful in high-temperature water systems with a pH of 9.0 or higher like boilers. I didn't. On the other hand, the method using a chelating agent such as EDTA and NTA has an excellent scale-preventing effect because the chelating agent forms a stable chelate compound with metal ions such as calcium, magnesium and iron in water and solubilizes it. On the other hand, if a chelating agent is added in excess of the amount required to chelate metal ions in water, there is a drawback that corrosion of the iron surface of the boiler is promoted.

In order to overcome the corrosion problem caused by the chelating agent, an attempt has been made to use an anionic polymer containing a carboxylate functional group as a sequestering agent for metal ions in water in place of the existing chelating agent. Japanese Unexamined Patent Publication No. 56-2897 discloses a scale control agent containing an itaconic acid polymer. Further, JP-A-56-20169 discloses a scale control agent containing an itaconic acid copolymer composed of 75 mol% of itaconic acid and 25 mol% or less of another unsaturated monomer. Such an itaconic acid-based polymer is excellent in the effect of controlling scale on the calcium-based scale, but is inferior in the effect of controlling the magnesium-based scale. JP 58-84099
JP-A No. 1989-242 discloses a method of using polyacrylic acid or an acrylic acid-acrylamide copolymer as an ion sequestering agent for a boiler. Not only do these polymers have an insufficient blocking effect, but when the addition amount is insufficient, the polymer itself reacts with calcium to form a sparingly soluble calcium salt of the polymer.

[Problems to be Solved by the Invention]

The object of the present invention is to solve the above-mentioned problems, and to simultaneously produce not only both calcium-based scale and magnesium-based scale in a boiler water system having a pH of 9.0 or more, but also the production of a scale containing a metal excluding an alkali metal. It is an object of the present invention to provide an improved ion-sequestering agent for a water system for boilers and a method for preventing scale formation using the same. The present inventors have synthesized a large number of copolymers with various combinations of monoethylenically unsaturated monomers and different copolymerization composition ratios by utilizing synthetic techniques. The present invention has been achieved as a result of systematically investigating the relationship between the concentration ratio of the metal component and the scale prevention effect and conducting intensive experimental research.

[Means for Solving the Problems]

The present invention is an ion sequestering agent containing a copolymer of (meth) acrylic acid and itaconic acid for preventing generation of boiler water-based scale having a pH of boiler water of 9.0 or more, and (meth) acrylic. The composition ratio of acid and itaconic acid is 9
An ion sequestering agent containing a copolymer in the range of 9: 1 to 60:40 mol% as an active ingredient, and 2 to 1 ppm of a metal component (excluding alkali metal) contained in water to be treated using the same It is constructed by a method to prevent the generation of scale used in the range of 30 ppm.

Here, (meth) acrylic acid may be either or both of acrylic acid and methacrylic acid, but acrylic acid is preferred.

The composition ratio of (meth) acrylic acid and itaconic acid of the copolymer used in the present invention is in the range of 99: 1 to 60:40 mol%,
More preferably, it is in the range of 90:10 to 70:30 mol%. The ion sequestrant of the present invention can be produced by a known radical polymerization method. For example, a polymerization initiator and, if necessary, a chain transfer agent are added to a comonomer solution prepared by dissolving a predetermined amount of monomers in a suitable solvent so that the total concentration of the monomers is 10 to 60% by weight, and the mixture is mixed under an inert gas stream. It is obtained by stirring and mixing for about 2 to 8 hours while maintaining the temperature at 60 to 120 ° C. As the solvent, water, lower alcohols such as isopropyl alcohol, ethyl alcohol and methyl alcohol, and mixed solvents thereof are used. Examples of the polymerization initiator include persulfates (including sodium salts, potassium salts, ammonium salts), hydrogen peroxide, t-butyl hydroperoxide, peroxides such as benzoyl peroxide, and various azo compounds (for example, 2, 2 ′). -Azobis (2-amidinopropane)
Hydrochloride) or the like can be used, and a redox catalyst system may be used. The optimum amount of the polymerization initiator depends on the kind of the initiator, but usually 0.1 to 10% is used with respect to the total amount of the monomers.

The weight average molecular weight of the copolymer is preferably in the range of 1,000 to 100,000, more preferably 2,000 to 50,000. Here, the weight average molecular weight is measured by a method of gel permeation chromatography (GPC) using polyethylene glycol having a known molecular weight as a standard.

The molecular weight of the copolymer can be controlled by using a chain transfer agent and adjusting the amount used. Known compounds as chain transfer agents, for example, thioglycolic acid and its esters, β-mercaptopropionic acid and its esters, mercapto compounds such as alkyl mercaptans,
Allyl compounds such as (meth) allyl sulfonic acid and (meth) allyl alcohol, hypophosphite, bisulfite and the like are used.

Another method for controlling the molecular weight of the copolymer is a method in which the comonomer solution and the polymerization initiator solution are separately and continuously fed into a reaction vessel whose temperature is kept constant at a constant flow rate. In this method, the molecular weight can be adjusted by changing the conditions such as the addition rate of the initiator and the monomer, the reaction temperature, the concentration of the initiator and the concentration of the comonomer solution.

The copolymer of (meth) acrylic acid and itaconic acid produced by the known radical polymerization method as described above is a linear random polymer.

The addition amount of the ion sequestering agent of the present invention is such that the copolymer as an active ingredient is added in the range of 2 to 30 ppm, preferably 3 to 2 with respect to 1 ppm of the metal component contained in the water to be treated.
It is in the range of 5 ppm. Here, the metal component is an alkali metal such as calcium, magnesium, iron, zinc, copper, manganese, or aluminum which is present in ionic or fine suspension particles (oxides, hydroxides, various salts, etc.) in boiler water. The metal components other than are shown.

The ion sequestrant of the present invention exhibits not only the function of sequestering metal ions present in water, but also the function of removing scale already attached to the boiler without corroding the iron surface of the boiler. The ion sequestering agent of the present invention does not promote corrosion of the iron surface of the boiler even when over-implanted. The ion sequestrant of the present invention is a polyelectrolyte and exhibits not only an ion sequestering action but also a good dispersing action, and therefore, it is suitable for holding the generated precipitated particles in a dispersed state even when the ion sequestering effect is not sufficient. .

The ion sequestering agent of the present invention is usually used in a form neutralized with alkali metal hydroxides such as caustic soda and caustic potassium, ammonia and alkali compounds such as various amines. The amount of alkali used for neutralization is preferably adjusted so that the pH of boiler feed water and boiler water is in the range of 7 to 12, more preferably 9 to 12.

The ion sequestering agent of the present invention can be used in combination with a known boiler treating agent such as an oxygen scavenger, a condensate neutralizer, a condensate film-type anticorrosive agent, a dispersant and an antifoaming agent.

〔Example〕

Example 1 100 pp of the ion sequestering agent shown in Table 1 was added to 100 ml of deionized water.
m, and the following reagents were dissolved. Then, the pH was adjusted to 11.8 with 0.5N sodium hydroxide.

_{CaCl 2 .2H 2 O 44.0mg / 1} MgCl 2 .6H 2 O 30.6mg / 1 NaHCO 3 840 mg / 1 Na 2 SiO 3 .5H 2 O 1060 mg / 1 Na 2 SO 4 444 mg / 1 FeCl 3 29.1mg / 1 This solution has a calcium hardness of 22 ppm (calcium 8.8
ppm), magnesium hardness 14ppm (as magnesium 3.
4ppm), total iron 9.5ppm, silica 300ppm, sulfate ion 300pp
m, M Contains 500ppm alkalinity. The concentration ratio of the sequestering agent to the metal component was 4.61. M above
The alkalinity acid consumption (pH 4.8) was determined by adding methyl red / brom cresol green mixed solution as an indicator to the sample and then using N / 50 sulfuric acid.

This solution was set in an autoclave and allowed to stand in a constant temperature oil bath maintained at a temperature of 180 ° C. After 18 hours, the autoclave was cooled, the content liquid was taken out, and the concentration of calcium and magnesium was measured by an atomic absorption spectrometry with respect to the filtrate filtered through a 0.45 μ membrane filter, and the inhibition rate was calculated by the following formula.

Where C _A : measured value of calcium or magnesium with the addition of the test agent C _B : measured value of calcium or magnesium without the addition of the test agent C _O : initial concentration of calcium or magnesium Test results and co-weight used for the test 18% aqueous solution (Na
The Brookfield viscosity at 25 ° C. of pH adjusted to 10 with OH) is also shown in Table 1.

Example 2 An ion sequestering agent having the concentration shown in Table 2 was added to 100 ml of deionized water to dissolve the following reagents. Then, the pH was adjusted to 11.8 with 0.5N sodium hydroxide.

_{CaCl 2 .2H 2 O 88.0mg / 1} MgCl 2 .6H 2 O 61.2mg / 1 NaHCO 3 840 mg / 1 Na 2 SiO 3 .5H 2 O 1060 mg / 1 Na 2 SO 4 444 mg / 1 FeCl 3 29.1mg / 1 This solution has a calcium hardness of 60ppm (24p as calcium
pm), magnesium hardness 30ppm (as magnesium 7.3
ppm), total iron 9.5 ppm, silica 300 ppm, sulfate ion 300 pp
m, M Contains 500ppm alkalinity.

This solution was set in an autoclave and allowed to stand in a constant temperature oil bath maintained at a temperature of 180 ° C. After 18 hours, the autoclave was cooled, the content liquid was taken out, and the concentration of calcium and magnesium was measured by an atomic absorption spectrometry with respect to the filtrate filtered through a 0.45 μ membrane filter, and the inhibition rate was calculated in the same manner as in Example 1. The test results are also shown in Table 2.

Example 3 To 100 ml of deionized water, 30 ppm of the ion sequestering agent shown in Table 3 was added, and the following reagents were dissolved. Then, the pH was adjusted to 11.8 with 0.5N sodium hydroxide.

NaHCO ₃ 840 mg / 1 FeCl ₃ 29.1 mg / 1 This solution contains 9.5 ppm total iron and 500 ppm M alkalinity.

The concentration ratio of the sequestering agent to the metal component was 3.16. This solution is set in an autoclave and the temperature is 180 ° C.
It was allowed to stand in a constant temperature oil bath maintained at. After 18 hours, the autoclave was cooled, the content liquid was taken out, and the concentration of iron was measured by atomic absorption spectrometry with respect to the filtrate filtered with a 1 μ membrane filter. The inhibition rate was calculated in the same manner as in Example 1. The test results are shown in Table-3.

Example 4 The experiment was carried out using the experimental boiler shown in FIG.
The boiler heat transfer tube (9) is cylindrical with a diameter of 28 mm and a length of 345 mm, and the material is carbon steel (S25C). 3.5KW inside the heat transfer tube
The electric heater of No. 1 was inserted to continuously heat. The pressure of the boiler is controlled at 20 kgf / cm ² , and the steam generation amount is about 3.7 l /
The temperature of the can water was 210 ° C. As the make-up water, the water quality shown in Table 4 was used.

Make-up water was mixed with the ion sequestrant shown in Table 5 in the water supply tank (3) and 3.5 ppm of hydrazine as an oxygen scavenger, and the feed water pump (4) kept the liquid level constant so that the experimental boiler ( It was automatically supplied to 8), and continuous blowdown was carried out so that the concentration of canned water became 5 times. The boiler can water was sampled at regular intervals to measure the concentration (ratio of feed water to chloride ion concentration in can water) and the calcium and magnesium concentrations, and the inhibition rate was calculated from the following formula. The test time was 8 hours.

Here, N: Concentration Z _B : Concentration of calcium or magnesium in can water Z _F : Concentration of calcium or magnesium in feed water Table 5 shows the results of the scale weight attached to the heat transfer tube (9) and the inhibition rate.

[Effects of the Invention] As shown in the examples, the ion sequestrant of the present invention and the method for preventing the production of scale using the same have an effect of preventing the production of scale in a high-temperature water system as compared with the conventionally known production inhibitors for scale. Is excellent, contributes to the safe operation of the boiler, improves the heat transfer efficiency, and can increase the concentration of boiler can water, so that the fuel consumption can be greatly reduced. Further, not only the generation of scale is prevented but also the action of removing existing scale is exhibited, so that the cost and construction period required for chemical cleaning can be reduced.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 shows a flow chart of an experimental boiler used in Examples of the present invention and Comparative Examples. (1) ... Make-up water tank, (2) ... Make-up water pump (2) ... Water supply tank, (4) ... Water supply pump (5) ... Water supply heater, (6) ... Chemical solution tank (7) ) …… Drug injection pump, (8) …… Boiler for experiment (9) …… Heat transfer tube, (10) …… Electric heater (11) …… Level gauge, (12) …… Automatic level controller ( 13) Blow water cooler (14) Mist separator (15) Pressure gauge, (16) Automatic pressure regulator (17) Pressure regulator, (18) Condenser

Claims (3)

[Claims] 1. A method for treating boiler water having a pH of 9.0 or more to prevent the formation of scales due to metal components in the boiler water, 60 to 99 mol% of (meth) acrylic acid and 1 to 1 of itaconic acid. 40 mol%, wherein the copolymer has a weight average molecular weight in the range of 1,000 to 100,000, and is composed of a linear random polymer, containing the copolymer as an active ingredient An ion sequestrant characterized by the following. 2. An ion sequestering agent containing a copolymer consisting of 60 to 99 mol% of (meth) acrylic acid and 1 to 40 mol% of itaconic acid as an active ingredient, which has a pH of 9.0 or more. Generation of a scale of a boiler water system characterized by being applied to the boiler water within a range of 2 to 30 ppm with respect to 1 ppm of metal components excluding alkali metals present in ionic or fine suspended particles contained in the boiler water How to prevent. 3. A boiler water system according to claim 2 wherein the metal component is present in the boiler water as one or more ionic or finely suspended particulate compounds from calcium, magnesium, iron or a combination of two or more thereof. How to prevent scale formation.