CA2280586A1 - Method for removing scale of boiler - Google Patents
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- CA2280586A1 CA2280586A1 CA 2280586 CA2280586A CA2280586A1 CA 2280586 A1 CA2280586 A1 CA 2280586A1 CA 2280586 CA2280586 CA 2280586 CA 2280586 A CA2280586 A CA 2280586A CA 2280586 A1 CA2280586 A1 CA 2280586A1
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Abstract
The invention provides a method for removing scale during operation of a boiler by adding a chemical into boiler water. In the method, a total concentration of a chelating agent and a dispersant in the boiler water is regulated to a specified concentration under a condition that the boiler water is neutral or alkaline.
Description
MCCARTHY TETRAULT FILE: 139201-257627 APPLICANT: MIURA CO. , LTD.
INVENTORS: JUNICHI NAKAJIMA
MASAZUMI YAMASHITA
KENICHI KIMURA
TITLE: METHOD FOR REMOVING
SCALE OF BOILER
Method for Removing Scale of Boiler BACKGROUND OF THE INVENTION
The present invention relates to a method for removing scale of a boiler and, more particularly, to a method for removing scale while the boiler is running.
During the operation of a boiler, it can occur that hard, water-insoluble crystalloid solids, which are so called scale, gradually deposit to the inside of water tubes or heat transfer walls. For example, hardness such as calcium carbonate, CaC03, and calcium sulfate, CaS09, will not increase in solubility to water and, in some cases, becomes insoluble, as the water temperature is increased, so that those components precipitate by the heating and concentration of water at heat transfer walls, thus depositing on the heat transfer walls as scale. It can also occur that a relatively water-soluble salt like calcium bicarbonate, Ca(HC03)2, is converted by heating into a water-insoluble salt, that is, calcium carbonate, CaC03, thus resulting in scale.
Causative substances for the scale are exemplified by silica, SiOz, in addition to hardness such as calcium salts and magnesium salts. Silica, Si02, which is contained in most of surface water, will combine with hardness and other metal ions such as zinc, aluminum and iron, resulting in scale. For example, silica, Si02, and a hardness, when combined together, produce a substance having a low solubility in high temperature water, which precipitate by heating and concentration at heat transfer walls, thus resulting in scale. Further, iron is brought into the system in the form of hydroxides or oxides, producing floating particles or colloid, and these particles or colloid deposits on the heat transfer walls and is baked and solidified by repetitive drying and wetting or other processes, thus resulting in scale.
Scale, once having deposited on the heat transfer walls, causes various problems. First, because scale is as low in thermal conductivity as one several hundredth that of the material of water tubes (mild steel), the scale deposited on the heat transfer walls reduces heat transfer from combustion flame to the water within the water tubes, i.e. boiler water, so that the thermal efficiency lowers to a large extent, making a cause of an increase in fuel consumption. Besides, once heat transfer from flame to boiler water reduces, the temperature of the heat transfer walls elevates and the metal strength of the heat transfer wall lowers. Then, because the inside of the water tubes are normally of high pressure, the metal strength of the heat transfer walls cannot withstand the internal pressure of the water tubes, which causes tube swelling, cleavage, ruptures or other accidents.
INVENTORS: JUNICHI NAKAJIMA
MASAZUMI YAMASHITA
KENICHI KIMURA
TITLE: METHOD FOR REMOVING
SCALE OF BOILER
Method for Removing Scale of Boiler BACKGROUND OF THE INVENTION
The present invention relates to a method for removing scale of a boiler and, more particularly, to a method for removing scale while the boiler is running.
During the operation of a boiler, it can occur that hard, water-insoluble crystalloid solids, which are so called scale, gradually deposit to the inside of water tubes or heat transfer walls. For example, hardness such as calcium carbonate, CaC03, and calcium sulfate, CaS09, will not increase in solubility to water and, in some cases, becomes insoluble, as the water temperature is increased, so that those components precipitate by the heating and concentration of water at heat transfer walls, thus depositing on the heat transfer walls as scale. It can also occur that a relatively water-soluble salt like calcium bicarbonate, Ca(HC03)2, is converted by heating into a water-insoluble salt, that is, calcium carbonate, CaC03, thus resulting in scale.
Causative substances for the scale are exemplified by silica, SiOz, in addition to hardness such as calcium salts and magnesium salts. Silica, Si02, which is contained in most of surface water, will combine with hardness and other metal ions such as zinc, aluminum and iron, resulting in scale. For example, silica, Si02, and a hardness, when combined together, produce a substance having a low solubility in high temperature water, which precipitate by heating and concentration at heat transfer walls, thus resulting in scale. Further, iron is brought into the system in the form of hydroxides or oxides, producing floating particles or colloid, and these particles or colloid deposits on the heat transfer walls and is baked and solidified by repetitive drying and wetting or other processes, thus resulting in scale.
Scale, once having deposited on the heat transfer walls, causes various problems. First, because scale is as low in thermal conductivity as one several hundredth that of the material of water tubes (mild steel), the scale deposited on the heat transfer walls reduces heat transfer from combustion flame to the water within the water tubes, i.e. boiler water, so that the thermal efficiency lowers to a large extent, making a cause of an increase in fuel consumption. Besides, once heat transfer from flame to boiler water reduces, the temperature of the heat transfer walls elevates and the metal strength of the heat transfer wall lowers. Then, because the inside of the water tubes are normally of high pressure, the metal strength of the heat transfer walls cannot withstand the internal pressure of the water tubes, which causes tube swelling, cleavage, ruptures or other accidents.
This being the case, in order to prevent the deposition of scale, there have been adopted such methods as softening feedwater by a water softener or other apparatus, maintaining the concentration of scale-forming components within their solubilities by blowdown control, or making hardness into sludge with precipitating agents. These methods are explained in more detail. First, the method of softening the boiler feedwater by a water softener or other apparatus is a method in which hardness in the boiler feedwater that forms scale, such as calcium ions and magnesium ions, are exchanged for sodium ions and thereby removed by cation exchange resins in the water softener, by which the deposition of scale is prevented.
Next, the method of maintaining the concentrations of scale-forming components within their solubilities by blowdown control is a method in which the control of blowing down (discharging) the boiler water out of the system periodically is exercised more appropriately so that the concentrations of scale-forming components in the boiler water are maintained within their solubilities, by which the deposition of scale is prevented. However, excessive blowdown would cause some problems that the boiler body becomes more corrosion-prone, and that heat loss is excessive.
Next, the method of making hardness components into sludge with precipitating agents is a method in which, before the formation of scale from hardness, a chemical is used to change the hardness into sludge lower in solubility to water than scale at places far from the inner surfaces of the water tubes where scale is likely to deposit, and then the produced sludge is discharged and removed out of the system at each blowdown, thus suppressing the occurrence of scale. As the chemical .for this method, phosphate based chemicals are commonly used, but phosphorus has been put into growingly stricter effluent regulation in recent years, thus the method having become harder to apply.
As shown above, various methods for preventing the deposition of scale have been adopted. However, it is difficult to fully prevent the deposition of scale even by these methods, and there is a need for the work of removing the deposited scale.
Scale is too hard to mechanically remove, and therefore is normally removed by acid cleaning. For one type of scale that is composed singly of hardness, it is practiced to take a method of dissolving and removing deposited scale by adding hydrochloric acid and heating. For another type of scale that is produced in combination of hardness and silica, Si02, because the scale cannot be removed only by hydrochloric acid, the scale is dissolved and removed by using hydrochloric acid and a fluoride in combination. Also, for iron based scale produced from iron compounds, it is practiced to apply a method of dissolving and removing the scale by using citric acid.
Next, the method of maintaining the concentrations of scale-forming components within their solubilities by blowdown control is a method in which the control of blowing down (discharging) the boiler water out of the system periodically is exercised more appropriately so that the concentrations of scale-forming components in the boiler water are maintained within their solubilities, by which the deposition of scale is prevented. However, excessive blowdown would cause some problems that the boiler body becomes more corrosion-prone, and that heat loss is excessive.
Next, the method of making hardness components into sludge with precipitating agents is a method in which, before the formation of scale from hardness, a chemical is used to change the hardness into sludge lower in solubility to water than scale at places far from the inner surfaces of the water tubes where scale is likely to deposit, and then the produced sludge is discharged and removed out of the system at each blowdown, thus suppressing the occurrence of scale. As the chemical .for this method, phosphate based chemicals are commonly used, but phosphorus has been put into growingly stricter effluent regulation in recent years, thus the method having become harder to apply.
As shown above, various methods for preventing the deposition of scale have been adopted. However, it is difficult to fully prevent the deposition of scale even by these methods, and there is a need for the work of removing the deposited scale.
Scale is too hard to mechanically remove, and therefore is normally removed by acid cleaning. For one type of scale that is composed singly of hardness, it is practiced to take a method of dissolving and removing deposited scale by adding hydrochloric acid and heating. For another type of scale that is produced in combination of hardness and silica, Si02, because the scale cannot be removed only by hydrochloric acid, the scale is dissolved and removed by using hydrochloric acid and a fluoride in combination. Also, for iron based scale produced from iron compounds, it is practiced to apply a method of dissolving and removing the scale by using citric acid.
However, when the acid cleaning is performed, the boiler must be halted during the work and moreover some problems occur such as the corrosion of iron, which is the boiler material, by the action of acid. Also, when hydrochloric acid or, in particular, a fluoride based chemical is used, there is a need of paying severe care to the handling of these chemicals because of their great hazardousness.
Whereas chemicals composed mainly of phosphonic acid have been used to remove the scale under the operation of the boiler, these chemicals are acidic and, if excessively added into the boiler water, would cause corrosion of the boiler body, as a problem. Accordingly, there has been a desire for a method of effectively removing scale with a chemical of neutral or alkaline, which is the water quality of normal boiler water, without causing the aforementioned corrosion problem.
In view of these and other problems, an object of the present invention is therefore to provide a method for removing scale under the operation of the boiler by adding a chemical into boiler water.
In order to achieve this object, according to the present invention, there is provided a method for removing scale of a boiler characterized in that a total concentration of chelating agent and dispersant in boiler water is regulated to a specified concentration under a condition that the boiler water is neutral or alkaline.
Whereas chemicals composed mainly of phosphonic acid have been used to remove the scale under the operation of the boiler, these chemicals are acidic and, if excessively added into the boiler water, would cause corrosion of the boiler body, as a problem. Accordingly, there has been a desire for a method of effectively removing scale with a chemical of neutral or alkaline, which is the water quality of normal boiler water, without causing the aforementioned corrosion problem.
In view of these and other problems, an object of the present invention is therefore to provide a method for removing scale under the operation of the boiler by adding a chemical into boiler water.
In order to achieve this object, according to the present invention, there is provided a method for removing scale of a boiler characterized in that a total concentration of chelating agent and dispersant in boiler water is regulated to a specified concentration under a condition that the boiler water is neutral or alkaline.
In an embodiment of the present invention, the chelating agent is any one compound or a combination of two ( 2 ) or more compounds selected from a group consisting of:
a) ethylenediaminetetraacetic acid (EDTA) and/or its salts;
b) polyalkylene polyamine and/or its salts;
c) bipyridine and/or its salts;
d) glycine and/or its salts;
e) acetylacetone;
f) nitrilotriacetic acid (NTA) and/or its salts;
g) aminotri(methylphosphonic acid) and/or its salts; and h) 1-hydroxyethylidene-1,1-diphosphonic acid and/or its salts.
In an embodiment of the invention, the dispersant is any one compound or a combination of two ( 2 ) or more compounds selected from a group consisting of:
a) polymaleic acid and/or its salts;
b) 2-phosphonobutane-1,2,4-tricarboxylic acid (PBTC) and/or its salts;
c) bis(poly-2-carboxyethyl) phosphinic acid and/or its salts;
d) phosphinocarboxylic acid copolymer and/or its salts;
e) acrylamide acrylate copolymer and/or its salts; and f) polyacrylic acid and/or its salts.
_ 7 In an embodiment of the invention, the total concentration of the chelating agent and the dispersant in boiler water is within a range of 500 - 5000 ppm.
Further, in an embodiment of the invention, a weight ratio of the chelating agent to the dispersant is within a range of 3 . 97 - 97 . 3.
BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 is a graph showing scale removal effects in a first embodiment of the present invention; and Fig. 2 is a graph showing scale removal effects in a second embodiment of the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Embodiments of the present invention are described below. In the invention, while the boiler is running, a scale remover containing a chelating agent and a dispersant is added into the boiler water at a time. More specifically, with a chemical feeder provided in the boiler feedwater line, the scale remover is stored in a chemical tank, and the pump of this chemical feeder is operated to inject the scale remover to the boiler feedwater. At the time of this addition, other chemicals such as oxygen scavengers may be added concurrently.
The present invention has been achieved by obtaining the following findings as a result of hard studies. The findings are as follows. First, in the method of removing scale under the operation of the boiler, too high a rate of scale _ g _ removal would cause the removed scale of large piece accumulate at narrowed portions of water tubes and the like, which may cause blockage of the water tubes. Conversely, too low a rate of scale removal may cause deterioration of the thermal efficiency as well as tube swelling, cleavage, ruptures or the like of the water tubes. Therefore, the rate of scale removal should be such an appropriate level as can avoid the above-described problems . Also, the chelating agent indeed has a function of dissolving scale, but is liable to cause re-deposition of the dissolved scale, which in turn may cause blockage of the water tubes. Further, the dispersant also has a function of dispersing and removing scale but is low in scale removal rate.
From these findings, the inventors have invented a method for removing scale by adding a combination of a particular chelating agent and a particular dispersant to the boiler water.
For the present invention, the chelating agent refers to a compound which has two or more bonding sites capable of forming coordinate bonds with metal ions and which is water-soluble. This chelating agent is exemplified by ethylenediaminetetraacetic acid (EDTA), such polyalkylene polyamines as ethylenediamine, propylenediamine, diethylenetriamine, and triethylenetetramine, bipyridine, glycine, acetylacetone, nitrilotriacetic acid (NTA), aminotri(methylphosphonic acid), 1-hydroxyethylidene-1,1-diphosphonic acid, and the like as well as salts of these compounds (except acetylacetone). Among others, ethylenediaminetetraacetic acid (EDTA) and nitrilotriacetic acid (NTA) as well as their salts are preferable.
Then, the dispersant is exemplified by polymaleic acid, 2-phosphonobutane-1,2,4-tricarboxylic acid (PBTC), bis(poly-2-carboxyethyl) phosphinic acid, phosphinocarboxylic acid copolymers, acrylamide acrylate copolymers, sodium polyacrylate, and the like as well as their salts.
Among these, for the removal of scale that is composed of hardness with carbonate or hydroxide and scale that is composed of hardness and silica, Si02, in combination (hereinafter, referred to as hardness/silica based scale), polymaleic acid, 2-phosphonobutane-1,2,4-tricarboxylic acid (PBTC), bis(poly-2-carboxyethyl) phosphinic acid, 1-hydroxyethylidene-1,1-diphosphonic acid, acrylamide acrylate copolymers, sodium polyacrylate, and the like as well as their salts are preferable, and polymaleic acid or its salts are particularly preferable. Also, for the removal of iron based scale that is composed of iron compounds, bis(poly-2-carboxyethyl) phosphinic acid and phosphinocarboxylic acid copolymers as well as their salts are preferable, and bis(poly-2-carboxyethyl) phosphinic acid or its salts are particularly preferable.
The ratio of chelating agent to dispersant in the scale remover is 3 : 97 - 97 : 3, preferably 15 : 85 - 85 : 15, and more preferably 25 . 75 - 75 . 25.
The scale remover is generally dissolved in water, manufactured as an aqueous solution and preserved for use.
Concentrations of the chelating agent and the dispersant in the aqueous solution are generally about 1 - 40 wt o, respectively, but not particularly limited. The scale remover, which can be obtained by mixing the chelating agent and the dispersant, is generally obtained by adding and dissolving specified quantities of the chelating agent and dispersant into water, another aqueous solvent or the like. The chelating agent and the dispersant may be dissolved into water after mixed together.
It is also possible to add, to the scale remover, a corrosion inhibitor such as dimethyl-p-substituted benzylsulfoniumchloride within such a scope as will not impair the object of the present invention.
It is still also possible to add, to the scale remover, other components within such a scope as will not impair the object of the present invention. These other components are exemplified by pH regulators such as sodium hydroxide, NaOH, potassium hydroxide, KOH, sodium carbonate, Na2C03, and potassium carbonate, KZC03. Because some of chelating agents and dispersants mentioned above show acidity, these pH
regulators are added to neutralize the acidity and thereby make the scale remover neutral.
The scale remover is added into the boiler water so that its concentration in the boiler water is maintained at 500 - 5000 ppm, preferably 2000 - 3000 ppm. An example of the addition process is that, with a blowdown rate of 10 0, the scale remover is added continuously so that the quantity of the scale remover becomes 200 - 300 ppm relative to the total quantity of the boiler feedwater. Another example is that the scale remover is added collectively so that the quantity of the scale remover becomes 2000 - 3000 ppm relative to the total quantity of the boiler water, where the boiler water is fully blown down periodically.
Examples:
Concrete examples of the present invention are described in detail below. However, the present invention is not limited to these examples. In the description of the examples, the term "parts" represents parts by weight.
First, four types of scale removers as described below were prepared. As a first chemical, 15 parts of disodium ethylenediaminetetraacetate (EDTA), 7.5 parts of polymaleic acid, 6 parts of sodium hydroxide and 71.5 parts of water were mixed and dissolved together, by which a scale remover (hereinafter, referred to as chemical A) was prepared. Also, 15 parts of disodium ethylenediaminetetraacetate (EDTA), 6 parts of bis(poly-2-carboxyethyl) phosphinic acid, 4.1 parts of sodium hydroxide and 74.9 parts of water were mixed and dissolved together, by which a scale remover (hereinafter, referred to as chemical B) was prepared. Also, 15 parts of disodium ethylenediaminetetraacetate (EDTA), 9 parts of 1-hydroxyethylidene-1,1-diphosphonate, 3 parts of potassium hydroxide and 73 parts of water were mixed and dissolved together, by which a scale remover (hereinafter, referred to as chemical C) was prepared. Further, 15 parts of disodium ethylenediaminetetraacetate (EDTA), 9 parts of acrylamide acrylate copolymer, 1 part of polyacrylate and 75 parts of water were mixed and dissolved together, by which a scale remover (hereinafter, referred to as chemical D) was prepared.
Next, a first example of the present invention is described. A boiler (experimental boiler) having a holding water quantity of about 0.15 L', to which scale composed of hardness and carbonate and scale composed of hardness and silica, SiOz, in combination had been deposited, was fed with soft water with a hardness of 0.4 ppm and operated under atmospheric pressure. During this operation, the chemical A
was added and the boiler was fully blown down every two hours while the concentration in the boiler water was maintained at 2000 - 3000 ppm. The pH value of the boiler water in this case was about 11.5. Results of measuring the removal Pffect after an eight-hour operation are shown in Fig. 1. Also, the same operation was performed with the chemical B or the chemical C
instead of the chemical A, and besides, for comparison, the same operation was performed without adding any chemical. Results of those cases are shown also in Fig. 1 together. As apparent from Fig. l, excellent scale removal effects can be obtained by the chemicals according to the present invention. In particular, the chemical A, which uses polymaleic acid as the dispersant, is superior in performance. In this connection, the case in which no chemicals were added resulted in zero removal amounts.
Next, a second example of the present invention is described. A boiler (experimental boiler) having a holding water quantity of about 0.15 ~, to which iron based scale composed of iron compounds had been deposited, was fed with soft water with a hardness of 0.4 ppm and operated under atmospheric pressure. During this operation, the chemical B was added and the boiler was fully blown down every two hours while the concentration in the boiler water is maintained at 2000 - 3000 ppm. The pH value of the boiler water in this case was about 11.5. Results of measuring the removal effect after an eight-hour operation are shown in Fig. 2. Also, the same operation was performed with the chemical A or the chemical D
instead of the chemical B, and besides, for comparison, the same operation was performed without adding any chemical. Results of those cases are shown also in Fig. 2 together. As apparent from Fig. 2, an excellent scale removal effect can be obtained by the chemical B, which is a chemical according to the present invention and which uses bis(poly-2-carboxyethyl) phosphinic acid as the dispersant . In this example also, the case in which no chemicals were added resulted in zero removal amounts.
As described above, by maintaining certain concentration of the scale remover composed of chelating agent and dispersant during the operation of the boiler, the scale remover of the present invention can attain excellent performance in scale removal.
Chelating agents we mentioned here have some bonding sites where they coordinate to metal ions. These bonding ~i tP~
are oxygen atoms in carboxylate, nitrogen atoms, oxygen atoms in phosphate, and the like. Bonding energies of coordinations are basically electric, common to all the coordinations.
Therefore, the same function is achieved in both cases where a single chelating agent is used and where a combination of two (2) or more chelating agents are used.
Also, it is generally known that when we use dispersing agent to prevent scale, the dispersant acts to inhibit the scale nucleus from combining with another. The reason why the combination of chelating agent and dispersant achieves an excellent function of scale removal is thought that the scale removed by the chelating agent is maintained as fine particles in the boiler water by the dispersant. Therefore, the same function is achieved in both cases where a single dispersant is used and where a combination of two (2) or more dispersants are used.
As shown above, according to the present invention, there can be provided a method which allows scale of a boiler to be removed under normal operation and without halting the boiler, by adding a chemical into the boiler water. Also, the scale removal method according to the present invention is free from hazards as would be involved in acid cleaning, hence high safety, and moreover is carried out under a condition that the boiler water is neutral or alkaline, thus never causing the boiler to corrode. Further, the scale removal method according to the present invention has an appropriate scale removal rate, thus well suppressing a problem that the removed scale deposits on narrowed portions of the water tubes so as to make the water tubes blocked, as well as such problems of reduction in thermal efficiency and swelling, cleavage, ruptures or the like of the water tubes.
a) ethylenediaminetetraacetic acid (EDTA) and/or its salts;
b) polyalkylene polyamine and/or its salts;
c) bipyridine and/or its salts;
d) glycine and/or its salts;
e) acetylacetone;
f) nitrilotriacetic acid (NTA) and/or its salts;
g) aminotri(methylphosphonic acid) and/or its salts; and h) 1-hydroxyethylidene-1,1-diphosphonic acid and/or its salts.
In an embodiment of the invention, the dispersant is any one compound or a combination of two ( 2 ) or more compounds selected from a group consisting of:
a) polymaleic acid and/or its salts;
b) 2-phosphonobutane-1,2,4-tricarboxylic acid (PBTC) and/or its salts;
c) bis(poly-2-carboxyethyl) phosphinic acid and/or its salts;
d) phosphinocarboxylic acid copolymer and/or its salts;
e) acrylamide acrylate copolymer and/or its salts; and f) polyacrylic acid and/or its salts.
_ 7 In an embodiment of the invention, the total concentration of the chelating agent and the dispersant in boiler water is within a range of 500 - 5000 ppm.
Further, in an embodiment of the invention, a weight ratio of the chelating agent to the dispersant is within a range of 3 . 97 - 97 . 3.
BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 is a graph showing scale removal effects in a first embodiment of the present invention; and Fig. 2 is a graph showing scale removal effects in a second embodiment of the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Embodiments of the present invention are described below. In the invention, while the boiler is running, a scale remover containing a chelating agent and a dispersant is added into the boiler water at a time. More specifically, with a chemical feeder provided in the boiler feedwater line, the scale remover is stored in a chemical tank, and the pump of this chemical feeder is operated to inject the scale remover to the boiler feedwater. At the time of this addition, other chemicals such as oxygen scavengers may be added concurrently.
The present invention has been achieved by obtaining the following findings as a result of hard studies. The findings are as follows. First, in the method of removing scale under the operation of the boiler, too high a rate of scale _ g _ removal would cause the removed scale of large piece accumulate at narrowed portions of water tubes and the like, which may cause blockage of the water tubes. Conversely, too low a rate of scale removal may cause deterioration of the thermal efficiency as well as tube swelling, cleavage, ruptures or the like of the water tubes. Therefore, the rate of scale removal should be such an appropriate level as can avoid the above-described problems . Also, the chelating agent indeed has a function of dissolving scale, but is liable to cause re-deposition of the dissolved scale, which in turn may cause blockage of the water tubes. Further, the dispersant also has a function of dispersing and removing scale but is low in scale removal rate.
From these findings, the inventors have invented a method for removing scale by adding a combination of a particular chelating agent and a particular dispersant to the boiler water.
For the present invention, the chelating agent refers to a compound which has two or more bonding sites capable of forming coordinate bonds with metal ions and which is water-soluble. This chelating agent is exemplified by ethylenediaminetetraacetic acid (EDTA), such polyalkylene polyamines as ethylenediamine, propylenediamine, diethylenetriamine, and triethylenetetramine, bipyridine, glycine, acetylacetone, nitrilotriacetic acid (NTA), aminotri(methylphosphonic acid), 1-hydroxyethylidene-1,1-diphosphonic acid, and the like as well as salts of these compounds (except acetylacetone). Among others, ethylenediaminetetraacetic acid (EDTA) and nitrilotriacetic acid (NTA) as well as their salts are preferable.
Then, the dispersant is exemplified by polymaleic acid, 2-phosphonobutane-1,2,4-tricarboxylic acid (PBTC), bis(poly-2-carboxyethyl) phosphinic acid, phosphinocarboxylic acid copolymers, acrylamide acrylate copolymers, sodium polyacrylate, and the like as well as their salts.
Among these, for the removal of scale that is composed of hardness with carbonate or hydroxide and scale that is composed of hardness and silica, Si02, in combination (hereinafter, referred to as hardness/silica based scale), polymaleic acid, 2-phosphonobutane-1,2,4-tricarboxylic acid (PBTC), bis(poly-2-carboxyethyl) phosphinic acid, 1-hydroxyethylidene-1,1-diphosphonic acid, acrylamide acrylate copolymers, sodium polyacrylate, and the like as well as their salts are preferable, and polymaleic acid or its salts are particularly preferable. Also, for the removal of iron based scale that is composed of iron compounds, bis(poly-2-carboxyethyl) phosphinic acid and phosphinocarboxylic acid copolymers as well as their salts are preferable, and bis(poly-2-carboxyethyl) phosphinic acid or its salts are particularly preferable.
The ratio of chelating agent to dispersant in the scale remover is 3 : 97 - 97 : 3, preferably 15 : 85 - 85 : 15, and more preferably 25 . 75 - 75 . 25.
The scale remover is generally dissolved in water, manufactured as an aqueous solution and preserved for use.
Concentrations of the chelating agent and the dispersant in the aqueous solution are generally about 1 - 40 wt o, respectively, but not particularly limited. The scale remover, which can be obtained by mixing the chelating agent and the dispersant, is generally obtained by adding and dissolving specified quantities of the chelating agent and dispersant into water, another aqueous solvent or the like. The chelating agent and the dispersant may be dissolved into water after mixed together.
It is also possible to add, to the scale remover, a corrosion inhibitor such as dimethyl-p-substituted benzylsulfoniumchloride within such a scope as will not impair the object of the present invention.
It is still also possible to add, to the scale remover, other components within such a scope as will not impair the object of the present invention. These other components are exemplified by pH regulators such as sodium hydroxide, NaOH, potassium hydroxide, KOH, sodium carbonate, Na2C03, and potassium carbonate, KZC03. Because some of chelating agents and dispersants mentioned above show acidity, these pH
regulators are added to neutralize the acidity and thereby make the scale remover neutral.
The scale remover is added into the boiler water so that its concentration in the boiler water is maintained at 500 - 5000 ppm, preferably 2000 - 3000 ppm. An example of the addition process is that, with a blowdown rate of 10 0, the scale remover is added continuously so that the quantity of the scale remover becomes 200 - 300 ppm relative to the total quantity of the boiler feedwater. Another example is that the scale remover is added collectively so that the quantity of the scale remover becomes 2000 - 3000 ppm relative to the total quantity of the boiler water, where the boiler water is fully blown down periodically.
Examples:
Concrete examples of the present invention are described in detail below. However, the present invention is not limited to these examples. In the description of the examples, the term "parts" represents parts by weight.
First, four types of scale removers as described below were prepared. As a first chemical, 15 parts of disodium ethylenediaminetetraacetate (EDTA), 7.5 parts of polymaleic acid, 6 parts of sodium hydroxide and 71.5 parts of water were mixed and dissolved together, by which a scale remover (hereinafter, referred to as chemical A) was prepared. Also, 15 parts of disodium ethylenediaminetetraacetate (EDTA), 6 parts of bis(poly-2-carboxyethyl) phosphinic acid, 4.1 parts of sodium hydroxide and 74.9 parts of water were mixed and dissolved together, by which a scale remover (hereinafter, referred to as chemical B) was prepared. Also, 15 parts of disodium ethylenediaminetetraacetate (EDTA), 9 parts of 1-hydroxyethylidene-1,1-diphosphonate, 3 parts of potassium hydroxide and 73 parts of water were mixed and dissolved together, by which a scale remover (hereinafter, referred to as chemical C) was prepared. Further, 15 parts of disodium ethylenediaminetetraacetate (EDTA), 9 parts of acrylamide acrylate copolymer, 1 part of polyacrylate and 75 parts of water were mixed and dissolved together, by which a scale remover (hereinafter, referred to as chemical D) was prepared.
Next, a first example of the present invention is described. A boiler (experimental boiler) having a holding water quantity of about 0.15 L', to which scale composed of hardness and carbonate and scale composed of hardness and silica, SiOz, in combination had been deposited, was fed with soft water with a hardness of 0.4 ppm and operated under atmospheric pressure. During this operation, the chemical A
was added and the boiler was fully blown down every two hours while the concentration in the boiler water was maintained at 2000 - 3000 ppm. The pH value of the boiler water in this case was about 11.5. Results of measuring the removal Pffect after an eight-hour operation are shown in Fig. 1. Also, the same operation was performed with the chemical B or the chemical C
instead of the chemical A, and besides, for comparison, the same operation was performed without adding any chemical. Results of those cases are shown also in Fig. 1 together. As apparent from Fig. l, excellent scale removal effects can be obtained by the chemicals according to the present invention. In particular, the chemical A, which uses polymaleic acid as the dispersant, is superior in performance. In this connection, the case in which no chemicals were added resulted in zero removal amounts.
Next, a second example of the present invention is described. A boiler (experimental boiler) having a holding water quantity of about 0.15 ~, to which iron based scale composed of iron compounds had been deposited, was fed with soft water with a hardness of 0.4 ppm and operated under atmospheric pressure. During this operation, the chemical B was added and the boiler was fully blown down every two hours while the concentration in the boiler water is maintained at 2000 - 3000 ppm. The pH value of the boiler water in this case was about 11.5. Results of measuring the removal effect after an eight-hour operation are shown in Fig. 2. Also, the same operation was performed with the chemical A or the chemical D
instead of the chemical B, and besides, for comparison, the same operation was performed without adding any chemical. Results of those cases are shown also in Fig. 2 together. As apparent from Fig. 2, an excellent scale removal effect can be obtained by the chemical B, which is a chemical according to the present invention and which uses bis(poly-2-carboxyethyl) phosphinic acid as the dispersant . In this example also, the case in which no chemicals were added resulted in zero removal amounts.
As described above, by maintaining certain concentration of the scale remover composed of chelating agent and dispersant during the operation of the boiler, the scale remover of the present invention can attain excellent performance in scale removal.
Chelating agents we mentioned here have some bonding sites where they coordinate to metal ions. These bonding ~i tP~
are oxygen atoms in carboxylate, nitrogen atoms, oxygen atoms in phosphate, and the like. Bonding energies of coordinations are basically electric, common to all the coordinations.
Therefore, the same function is achieved in both cases where a single chelating agent is used and where a combination of two (2) or more chelating agents are used.
Also, it is generally known that when we use dispersing agent to prevent scale, the dispersant acts to inhibit the scale nucleus from combining with another. The reason why the combination of chelating agent and dispersant achieves an excellent function of scale removal is thought that the scale removed by the chelating agent is maintained as fine particles in the boiler water by the dispersant. Therefore, the same function is achieved in both cases where a single dispersant is used and where a combination of two (2) or more dispersants are used.
As shown above, according to the present invention, there can be provided a method which allows scale of a boiler to be removed under normal operation and without halting the boiler, by adding a chemical into the boiler water. Also, the scale removal method according to the present invention is free from hazards as would be involved in acid cleaning, hence high safety, and moreover is carried out under a condition that the boiler water is neutral or alkaline, thus never causing the boiler to corrode. Further, the scale removal method according to the present invention has an appropriate scale removal rate, thus well suppressing a problem that the removed scale deposits on narrowed portions of the water tubes so as to make the water tubes blocked, as well as such problems of reduction in thermal efficiency and swelling, cleavage, ruptures or the like of the water tubes.
Claims (5)
1. A method for removing scale of a boiler characterized in that a total concentration of a chelating agent and a dispersant in boiler water is regulated to a specified concentration under a condition that the boiler water is neutral or alkaline.
2. The method for removing scale of a boiler according to Claim 1, wherein the chelating agent is any one compound or a combination of two (2) or more compounds selected from a group consisting of:
a) ethylenediaminetetraacetic acid (EDTA) and/or its salts;
b) polyalkylene polyamine and/or its salts;
c) bipyridine and/or its salts;
d) glycine and/or its salts;
e) acetylacetone;
f) nitrilotriacetic acid (NTA) and/or its salts;
g) aminotri(methylphosphonic acid) and/or its salts; and h) 1-hydroxyethylidene-1,1-diphosphonic acid and/or its salts.
a) ethylenediaminetetraacetic acid (EDTA) and/or its salts;
b) polyalkylene polyamine and/or its salts;
c) bipyridine and/or its salts;
d) glycine and/or its salts;
e) acetylacetone;
f) nitrilotriacetic acid (NTA) and/or its salts;
g) aminotri(methylphosphonic acid) and/or its salts; and h) 1-hydroxyethylidene-1,1-diphosphonic acid and/or its salts.
3. The method for removing scale of a boiler according to Claim 1, wherein the dispersant is any one compound or a combination of two (2) or more compounds selected from a group consisting of:
a) polymaleic acid and/or its salts;
b) 2-phosphonobutane-1,2,4-tricarboxylic acid (PBTC) and/or its salts;
c) bis(poly-2-carboxyethyl) phosphinic acid and/or its salts;
d) phosphinocarboxylic acid copolymer and/or its salts;
e) acrylamide acrylate copolymer and/or its salts; and f) polyacrylic acid and/or its salts.
a) polymaleic acid and/or its salts;
b) 2-phosphonobutane-1,2,4-tricarboxylic acid (PBTC) and/or its salts;
c) bis(poly-2-carboxyethyl) phosphinic acid and/or its salts;
d) phosphinocarboxylic acid copolymer and/or its salts;
e) acrylamide acrylate copolymer and/or its salts; and f) polyacrylic acid and/or its salts.
4. The method for removing scale of a boiler according to any one of Claims 1 to 3, wherein the total concentration of the chelating agent and the dispersant in boiler water is within a range of 500 - 5000 ppm.
5. The method for removing scale of a boiler according to any one of Claims 1 to 3, wherein a weight ratio of the chelating agent to the dispersant is within a range of 3 : 97 - 97 : 3.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| JP250406/1998 | 1998-08-19 | ||
| JP25040698 | 1998-08-19 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA2280586A1 true CA2280586A1 (en) | 2000-02-19 |
Family
ID=17207432
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA 2280586 Abandoned CA2280586A1 (en) | 1998-08-19 | 1999-08-19 | Method for removing scale of boiler |
Country Status (2)
| Country | Link |
|---|---|
| CN (1) | CN1262487C (en) |
| CA (1) | CA2280586A1 (en) |
Cited By (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| WO2006028917A2 (en) * | 2004-09-02 | 2006-03-16 | Ashland Licensing And Intellectual Property Llc | Process for inhibiting scale on metal surfaces |
| EP2982655A4 (en) * | 2013-04-02 | 2016-10-26 | Kurita Water Ind Ltd | Method for removing scales in steam generation facility |
Families Citing this family (3)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN103965098A (en) * | 2014-05-22 | 2014-08-06 | 山东昆达生物科技有限公司 | Energy saving technology in pyridine and 3-methylpyridine preparation processes |
| CN109354221A (en) * | 2018-10-29 | 2019-02-19 | 爱环吴世（苏州）环保股份有限公司 | A kind of ooze regulator for removing the old old dirt of boiler system |
| CN110241430A (en) * | 2019-06-27 | 2019-09-17 | 湖南省湘电试验研究院有限公司 | A kind of composite organic acid contaminant removing cleaning agent and its application |
-
1999
- 1999-08-19 CA CA 2280586 patent/CA2280586A1/en not_active Abandoned
- 1999-08-19 CN CN 99119632 patent/CN1262487C/en not_active Expired - Lifetime
Cited By (3)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| WO2006028917A2 (en) * | 2004-09-02 | 2006-03-16 | Ashland Licensing And Intellectual Property Llc | Process for inhibiting scale on metal surfaces |
| WO2006028917A3 (en) * | 2004-09-02 | 2007-03-15 | Ashland Licensing & Intellectu | Process for inhibiting scale on metal surfaces |
| EP2982655A4 (en) * | 2013-04-02 | 2016-10-26 | Kurita Water Ind Ltd | Method for removing scales in steam generation facility |
Also Published As
| Publication number | Publication date |
|---|---|
| CN1262487C (en) | 2006-07-05 |
| CN1252388A (en) | 2000-05-10 |
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Legal Events
| Date | Code | Title | Description |
|---|---|---|---|
| FZDE | Discontinued | ||
| FZDE | Discontinued |
Effective date: 20040819 |