JPS60139397A - Method of controlling scale in pressure boiler - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to the treatment of water systems used in pressurized boilers.

Water is used in steam generation boilers, cooling towers, desalination equipment and other industrial hydrogen containing various impurities. Typical impurities include alkaline earth metal cations, primarily calcium and magnesium, and several anions such as bicarbonate, carbonate, sulfate, oxalate, phosphate, silicate, and fluoride. It will be done. The most common impurities in industrial water sources are calcium, magnesium and carbonate ions, which harden the water, and sulfate ions are usually also present. Salts of these metal ions, especially carbonates, tend to precipitate and form solid deposits on the surface of the system, which can give rise to scale on hot surfaces.

This water contains, in addition to the above, various solids such as mud, clay, iron oxides, sils, sand and other minerals, and microbial debris in the system.
It may also contain substances accumulated as dge).

Naturally, sludge and scale build-up creates points of low flow in the system, restricting water circulation, and increasing heat transfer efficiency by isolating water from heat exchange surfaces.
]. In addition, at such deposits, the corrosion control agent does not come into contact with the metal surface efficiently, making the metal surface more likely to corrode. In addition, this deposit provides a habitat for bacteria (bαcte-rjα).

Removal of deposits is expensive, time consuming, and may require system outage.

Since pre-treatments such as water softening, flocculation, and filtration do not adequately remove solids and solid-forming materials, various chemicals are used to combat the detrimental effects of scale and sludge in water systems. It has been used. Chemicals are widely used in cooling water systems and desalination plants to raise the threshold for precipitation. It is also believed that the chemicals form a thin film on scale-prone hot surfaces, preventing scale-forming substances from adhering to the hot surfaces in question. A variety of chemicals have been used for this purpose, including polycarxylates as well as soluble, polar polymers such as acrylate and methacrylate polymers. The presence of small amounts of these polymers can have a significant effect on the system.

Although these various chemicals are highly effective in industrial cooling towers and desalination plants, completely different challenges arise when water is used in pressurized boilers, which require much higher temperatures (e.g., Even at the lowest pressure commonly used, 80 psig, the boiler
point) is already 324'F (approx. 162°C)
).

The water used in the irrigator must first be treated in a deionizer or base exchanger (bα
se exch, αngtt), but the former is more effective. Even so, these processes
In particular, some of the calcium and magnesium ions remain in the boiler feed water together with the coexisting anions, so it cannot always be said to be sufficiently effective.

From the point of view of high temperature applications, it is impractical to try to keep calcium in solution as in cooling towers. An exception is the use of chelating agents, which present some additional practical challenges, especially accurate dosing. Similarly, it is much more difficult to prevent solids from depositing and forming scale in contact with extremely hot surfaces.

As a result, several techniques have been applied to prevent scaling of pressurized gaylor systems.

For boilers used only at low pressures (e.g., up to 150 psig), soluble carbonates may be added to the feed water to precipitate any calcium present as carbonate, and to prevent the precipitated material from settling on hot surfaces. Therefore, it is common practice to add a dispersant at the same time.

This is, of course, in complete contrast to the situation in cooling towers. In a cooling tower, the objective is to keep the carbonate concentration to a minimum and maintain that concentration in solution. Calcium sulfate is much more soluble than calcium carbonate, so there are no significant problems in cooling water or desalination systems. However, in a pressurized boiler, scale due to calcium sulfate may occur, so it is desirable to precipitate all the calcium.

However, adding carbonate is not sufficiently effective as some scale inevitably still forms. Over a period of time, scale can accumulate to such an extent that the error must be stopped and the scale removed. Removal of scale deposits is essential, as they can cause local overheating and even destroy the boiler.

Moderate pressure (e.g. 150 to 6 o psig
) or high pressure (above 600 psig) boilers, the boiling point of water at this pressure
Carbonates tend to decompose and produce carbon dioxide at temperatures above 100 to 200 m, and carbon dioxide has the effect of severely corroding metal surfaces at these temperatures, so adding carbonates to the water supply will not produce satisfactory results. I can't do it. therefore,
In such boilers, and indeed in low-pressure boilers as well (in addition to adding carbonate), soluble phosphates (potassium phosphate as well as other phosphates, including polyphosphates, such as hexametaphosphate Although sodium and sodium fluorophosphate can also be used),
Typically, a sodium phosphate salt, such as disodium phosphate or trisodium phosphate, is commonly added to the feed water. This ensures that any calcium present is precipitated as calcium phosphate and then dispersed with a dispersant as described above. As in other systems, this material is used to remove sludge-containing boiler water from the giller by blowdown, which rapidly reduces the pressure in the boiler and removes it through the pulp. It can also be removed periodically. However, this method does not remove all the calcium phosphate, and as a result, calcium phosphate scale is formed, which must be removed by stopping the boiler at appropriate times.

In practice, it is necessary to add more than a stoichiometric amount of phosphate corresponding to the amount of calcium in the water. The purpose is to add enough phosphate to the
always in excess, for example 10 to 20 ppm.
The amount of excess required is based on the British standard.
It can be set against an authoritative index such as 2486. This standard value also determines the required alkalinity (α1
kalinity), generally a pH of 9,s to 12.

This degree of alkalinity is necessary for several reasons.

First, by keeping the pH high enough, all of the calcium phosphate can be converted into hydroxyapatite, a basic calcium phosphate that is easy to process.
and ensure that the solubility of calcium phosphate is low. Second, alkaline conditions prevent corrosion. Third, such a pH ensures that any magnesium ions present will precipitate as magnesium hydroxide.

The following is cooling water and desalination system on the one hand, and pressurization on the other? Explain other differences between the scale and approach of the Iler system. In cooling water systems, magnesium remains in solution in water where the temperature rarely exceeds about 50°C.
Magnesium-based scale formation is of little significance. In desalination systems, magnesium scale formation becomes a significant problem, both because the temperature rises to 10°C and because the brine has a higher magnesium content than normal industrial water. Therefore, as a result, a special process has to be set up.In fact, magnesium bicarbonate is first converted into carbon dioxide and magnesium carbonate, and then magnesium carbonate is hydrolyzed with hot water to form magnesium hydroxide. .Due to the high magnesium content, the solubility product is exceeded and as a result it begins to precipitate out of solution.In contrast to the situation of pressurized boiler feed water, brine is not significantly alkaline, so as a result magnesium hydroxide does not remain in solution. However, at pHta, 5 to 12, magnesium hydroxide precipitates even when cold, so at the high temperatures set in pressure boilers there is usually no chance of scale formation with magnesium hydroxide.

It is also worth noting that in cooling water systems it is not uncommon to add acid (rather than alkali) to keep more calcium carbonate in solution.

Surprisingly, it has been discovered that certain specific sulfonate copolymers are effective in controlling scale formation in the pressurized beer. 6. What the inventors mean by the term "pressurized boiler" is at least so psig.
pressure, i.e. generally at least f30 pg
tgz typically 80 to 150 psig (low pressure), typically 150 to 600 psig (medium pressure),
It should even be considered a boiler that operates at pressures above 600, for example up to 2000 psig (high pressure). In such a tank, the water will rise to its boiling point, which will range from about 298'F at SO psig to about 324'F at 80 psig, 150 pszg.
At about 366″F, at 600 psig it’s about 48
9〒, it changes to about 637'F at 2000 pajg. Furthermore, it has been found that these particular copolymers, if actually present in boiler water, provide a very real practical benefit of actually removing scale that is already present. In other words, these specific copolymers have a 1nserτice cleaning effect. This cleaning effect has limited effectiveness against specific scales deposited from currently used water supplies. There are so many things.

In other words, the present copolymer also removes scale produced by previous boiler operations using different water supplies. Therefore, if 13? If the error was caused to operate inaccurately by varying the composition of the feed water without proper control, resulting in the formation of calcium phosphate scale, i.e. if old scale was present;
Whether or not additives are used, the particular copolymers used in the present invention can be added to the water supply to remove scale while operating the reiler under load. It is.

It should be added that this surprising in-service cleaning effect is only observed when used at the high temperatures required in pressure boilers.

Therefore, while the same copolymer may be effective in preventing the formation of new scale in a cooling system, it is not effective in removing old scale.

According to the present invention, the formula % formula % and the formula -CH2-CH- where Z is 1503H or 10H, -503H, or a water-soluble salt thereof, typically, such as the IJium salt. A method for controlling scale in a pressure boiler is provided in which at least a scale-controllable amount of a copolymer having repeating units of , representing an alkali salt, is added to the boiler. In Z in the above formula, the water-soluble salt is typically the sodium salt, although potassium, ammonium, zinc and lower amine salts and other water-soluble salts may also be used.

The free acid may be used, all hydrogens of the acid need not be replaced, and the replaced cations need not be the same. Therefore, the cation may be any one of NH4, H, Na, K, etc. or a mixture thereof. From another point of view, the present invention is characterized in that the above-mentioned specific copolymer is added to the water system of the scale-attached water system in an amount necessary for removing scale. To provide a method for removing scale from.

The above copolymers are conveniently produced by polymerizing maleic acid or its anhydride or fumaric acid with vinylsulfonic acid or allylsulfonic acid or their alkali metal salts using conventional methods. Conventional addition polymerization methods using photo or free radical initiators may also be applied.

Generally, this copolymerization can be carried out at about 30 to 120°C using a peroxide catalyst such as hydrogen peroxide or benzoyl peroxide in an inert medium. This copolymer can be derived, for example, by solution polymerization of maleic acid and sodium allylsulfonate in the presence of hydrogen peroxide. Salts can of course also be obtained using conventional methods.

The relative proportions of the sulfonic acid component and the carboxylic acid component are
To some extent it depends on the degree of scale generation to be processed. Generally, the subject copolymers contain from about 10 to about 80 moles/-9-cents of sulfonic acid moieties and correspondingly from about 90 to about 20 mole percent of carboxylic acid moieties. Preferably, the sulfonic acid moieties account for about 25 to about 75 mole percent of the copolymer and the carboxylic acid moieties account for about 75 to about 25 mole percent. In copolymers of vinyl sulfonic acid, the sulfonic acid moiety specifically accounts for about 50 to about 75 mole percent of the copolymer, and the carboxylic acid moiety accounts for about 50 to about 25 mole percent.

The average molecular weight of the copolymer is not critical as long as the polymer is water-soluble. Generally, the weight average molecular weight ranges from about 500 to about i o o, o o o. The molecular weight is preferably about 800 to about 25,000,
Especially about 1. OOO to 15,000. Particularly preferred are copolymers of maleic acid or its anhydride and allylsulfonic acid in a molar ratio of about 1:1 and a molecular weight of about 2,000. Other preferred copolymers include a molar ratio of maleic acid or anhydride to vinyl sulfonic acid of about 1:1.
5 or about 1=3 and the molecular weight is about 7.000 to 9.00
0 is included. Best results are generally 1:
However, since vinyl sulfonic acid is relatively expensive, it is possible to obtain a molar ratio of 1:3 even if the results are not as good.
A molar ratio of 1.5 is generally more preferred.

The above specified copolymers and water soluble carbonates, typically sodium carbonate, or phosphates as specified above or 7+? It is sometimes considered easier to simultaneously charge the water supply with other hard water component precipitants suitable for the temperatures and pressures used in the boiler. The pH is adjusted to usually 9.5 to 12, preferably 10 to 11, if necessary. This pH can be achieved by adding an appropriate amount of caustic soda to the particular boiler used to maintain the recommended alkalinity value. This alkalinity value can be determined using well known methods such as titration with standard acids. Accordingly, the present invention also provides a composition suitable for addition to pressure boiler hydrogen containing a copolymer having repeating units of the formula shown above and a water-soluble hard water component precipitant. Typically, the subject copolymers generally contain 0.1 to 50,
Preferably? - added as an aqueous solution containing 5 to 10, especially 3 to 5% by weight (active) of the copolymer. The amount of the hard water component precipitant in the solution is suitably 5 to 50% by weight (or solubility limit), preferably 15 to 35%, especially 25 to 35% by weight. Therefore, the relative weight ratio of the copolymer to the hard water component precipitant is preferably 0.1:50 to 10:1, preferably 1:15 to 2:3, particularly 1:11 to 1:3. . □Of course, other conventional additives can also be incorporated into the water, including alkalis, lidanine derivatives, other polymers, tannins, biocides, corrosion inhibitors, and the like.

Although it will generally be most convenient to add the subject copolymers to the make-wp or water line through which the water passes before entering the boiler,
Moreover, they can be introduced anywhere as long as they can be mixed effectively. Typically, a syringe is used that is calibrated to deliver a predetermined amount of make-up water periodically or continuously.

The amount of copolymer added to the water is less than stoichiometric, but this amount is effective in controlling scale, i.e., preventing scale formation and removing scale formed; Naturally, the amount added depends on the nature of the aqueous system to be treated, in particular its calcium content. This amount depends to some extent on the concentration of suspended solids and the amount of solid build up (7) present in the system. Typically, amounts of from about 0.1 to about 400 boiler water widths, preferably from about 1 to about 80, and especially from about 5 to about 50 ppm active, are used. Generally, as the amount of precipitant required for calcium increases, the amount of copolymer also increases.

In typical particularly preferred compositions, the amount of composition added will be from about 20 to about 2500 ppm.

The following examples further illustrate the invention. In these examples, the properties of the additives are described in the Western Pennsylvania Society of Engineers 15th Annual Hydraulic Engineering Conference Proceedings.
dings of the 15th AnnrbaL
TI'ater Conference, Engine
eers 5ociety of ten Wester
The experiments were carried out in a small laboratory boiler with three removable tubes as described in J. Pennsylvania, pp. 87-102 (1954). The water supply for the laboratory boiler is Illinois (llinois).
Dilute tap water from Lake Zxrich with distilled water to obtain a total hardness of 40p in terms of CαCO8.
pm and added calcium chloride to give an elemental ratio of calcium to magnesium of 6:1. Feed water and chemical treatment solution are fed to the boiler at a ratio of 3 volumes of feed water to 1 volume of solution, with a total hardness in the feed water of 30 ppm as CaCO3. For all treatment solutions, the boiler effluent was adjusted to 10% of the boiler feed water, the concentration of boiler water salts was increased by 10 times, and the composition of the treatment solution was adjusted so that the concentration of boiler water was 10 times as high as before. A scale formation test was conducted using the composition shown in the figure below.

Table ■ Sodium hydroxide as NaOH 2587)7)”
I Sodium carbonate 120ppm as Nα2CO3
Sodium chloride 681pp as NαC1? ? 'l
.

Sodium sulfite Na, So, 50pptn
Sodium sulfate Ha250. as 819ppm silica Sin, as 17)'I) less than m iron as Fe less than ippm phosphate ion as po4 10-20ppm test 1st series, e (the 1st 5eries
) The boiler water pressure is 400 psi.
It was operated for 45 hours at g. At the completion of each test, the Deiler tubes are individually removed from the boiler and the center length (cent
The scale or deposits present in the 6" container are scraped off and the weight is collected in a container and weighed. The results obtained are shown in Table 3.

These results demonstrate that the copolymers used in the present invention are effective in reducing scale formation rates in pressure boilers.

In the second series of tests, the system is first run for 45 hours without any polymer addition, then one of the three tubes is removed and replaced with a clean tube. The system is then run for an additional 45 hours, this time with the addition of polymer. This second 4
After 5 hours, weigh the amount of scale in the three tubes as before.

In this way, a comparison of the results of this test with an untreated blank (in which no polymer was added during the second 45 hours) shows that the polymer has removed the scale and transferred to the clean tube. It becomes possible to evaluate whether the generation of scales can be prevented.

The results obtained are shown in Table 3 below.

Therefore, it is clear that the use of these polymers is effective not only for preventing scale formation, but also for removing existing scale since the scale reduction rate is greater than ioo%. .

In order to evaluate the in-action cleaning ability of the copolymer in a cooling water system, a glass tube assembly (gtα88tqbeα
Fix a clean metal heater inside the
Water heated to 60°C is circulated through it using a pump. This assembly forms part of a closed system with an expansion tank open to the atmosphere. Remove the heater and place it in dilute acid to remove the scale on it. The amount of calcium carbonate scale is then determined by titrating the acid solution with a standard EDTA solution (as Cα), and the amount of calcium carbonate scale is calculated from this.

In the first test, 400 ppm calcium and 40
0 ppm alkalinity
The water formulation (as bicarbonate) is circulated for 6 hours. The heater was weighed and found to have formed 780 milligrams of calcium carbonate scale.

Then, after 6 hours, 10 ppm of a copolymer of maleic acid and allylsulfonic acid (same as in Example 2) is added, and circulation is continued for 45 hours. Next, remove the heater and test as above. Again, 780 milligrams of calcium carbonate scale formation was found.

It is therefore clear that under these conditions the copolymer does not remove any scale.

Claims (1)

[Claims] 1. The system has a repeating unit of the formula % formula % and the formula -CH, -CH-, where Z represents -50,H or -CHt-5o3H or a water-soluble salt thereof. A method for treating scale in a pressurized boiler water system, characterized by adding a polymer. 2. The method of claim 1, wherein the λ copolymer is a copolymer of maleic acid and allylsulfonic acid. 1 The above copolymer has a molecular weight of about 800 to about 25. The method according to claim 1 or 2, having a molecular weight of OOO. 4. The method according to any one of claims 1 to 3, wherein the copolymer is added in an amount of 0.1 to about 400 ppm. 5. Claim 1 characterized by a water-soluble precipitant in the system
The method described in any of Items 4 to 4. 6. A copolymer having a repeating unit of the formula % formula % and the formula -CH-CH- 'I where Z represents 15O,H or 10H,SO,H, still a water-soluble salt thereof. A composition suitable for addition to a pressure boiler water system containing a coalescent and a water-soluble calcium precipitant. 7. The composition according to claim 6, wherein the copolymer is a copolymer of maleic acid and allylsulfonic acid. 8. The composition of claim 6 or 7, wherein the copolymer has a molecular weight of about 800 to 25,000. 9. The composition according to claim 7 or 8, wherein the water-soluble hard water component precipitant is a water-soluble carbonate or phosphate. 10. The water-soluble hard water component precipitant is diphosphoric acid, trisodium phosphate, or metaphosphoric acid.
The composition according to claim 9, which is Jum. 11. The method according to any one of claims 6 to 10, which is an aqueous solution containing about 25 to 10% by weight of the copolymer and about 5 to 50% by weight of the hard water component precipitant. composition. 12. Claims 6 to 11 contain at least one water treatment additive that is an alkali, a lignin derivative, another polymer, a tannin, a biocide, or a corrosion inhibitor.
The composition described in any of the paragraphs.