NL8403714A - METHOD FOR COMBATING BOILERY IN PRESSURE BOILERS. - Google Patents

Description

ί * S 'Γ ί - 1 -

Method for combating scale in pressure boilers.

The invention relates to the treatment of aqueous systems used in pressure boilers.

The water used in steam boilers, cooling towers, 5 desalination units and other technical and industrial water systems contains various contaminants. In particular, the impurities are alkaline earth cations, mainly calcium and magnesium, and various anions including hydrogen carbonate, carbonate, sulfate, oxalate, phosphate, silicate and fluoride. The most common impurities in industrial water supplies are the calcium, magnesium and carbonate ions, which cause the hardness of water, although sulfate is usually also present. The salts of these metal ions, especially the carbonates, tend to precipitate to form solid build-ups on the surfaces of the system, and these build-ups can lead to the formation of scale on hot surfaces. The water may also contain various solids such as mud, clay, iron oxides, sludge, sand and other inorganic substances as well as microbial residues that build up as sludge in the system. Naturally, sludge and scale deposits greatly reduce heat transfer efficiency by settling in areas of the system with low flow, thus limiting water circulation and isolating the water from the surfaces where heat transfer occurs. In addition, corrosion of metal surfaces under the deposits is facilitated since corrosion inhibitors cannot efficiently come into contact with the surfaces. Furthermore, the deposits contain bacteria. Removal of deposits can cause costly delays and system shutdown.

8403714

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Since known treatments such as softening, coagulation and filtration do not sufficiently remove the solids and solids, various chemicals have been used to overcome the adverse effects of scale and sludge in aqueous systems.

In cooling water systems and desalination plants, the chemicals are usually added to raise the threshold at which precipitation forms; it has also been thought that the chemical forms a film on the hot surfaces where scale formation is likely to occur, thereby preventing scale formation from adhering to the hot surfaces. A number of different chemicals have been used for this purpose, including polycarboxylates and other soluble polar polymers such as acrylate and methacrylate polymers. The presence of small amounts of these polymers can have a noticeable effect on the system.

While these various chemicals are very effective in industrial cooling towers and desalinators and the like, "in" water used in pressure boilers presents quite different problems, due to the much higher temperatures involved; the boiling point of water at the lowest pressure normally used, 560 kPa gauge, is already 162 ° C.

Because of the greater problems involved, the water used for such pressure vessels is first subjected to a deionization or base exchange, the first of which is the most efficient. However, these treatments are not always fully effective with the result that in particular some calcium and magnesium ions with their associated anions remain in the boiler feed water.

In view of the higher temperatures occurring, it is not practical to try to keep the calcium in solution as is done with cooling water; an exception to this is the use of chelating agents, but these pose some other problems when used, in particular §403714-3 - correct dosage. Likewise, it is much more difficult to prevent the solid deposits from contacting the very hot surfaces, thereby forming scale.

As a result, it is necessary to switch to other 5 techniques for preventing scale in pressure boiler systems.

For boilers and boilers in which only low overpressure (up to 1050 kPaty pressures occur), it is not uncommon to add a water-soluble carbonate to the feed water to ensure that all the calcium present precipitates as carbonate and also a add dispersant to prevent the precipitated substance from settling on the hot surfaces This is of course completely in contrast to the situation with cooling water where the carbonate concentration is to be kept as small as possible and in solution 15 Calcium sulfate is much more soluble than calcium carbonate, so that it does not present a major problem in cooling water and desalination systems, however, in boilers, scale formation by calcium sulfate can occur, so that one would like to ensure that all calcium precipitates.

However, the treatment with carbonate is inevitably not entirely effective, with the result that some scale is nevertheless formed. Over time, scale can build up to such an extent that the kettle must be stopped and the scale removed. This is necessary since scale deposits can cause local overheating and even breakage in the kettle.

Treating the feed water with carbonate is not sufficient for boilers operating at moderate (for example 1050 to 4200 kPa overpressure) or high (above 4200 kPa 30 overpressure) pressure, since at the boiling point of water the carbonate will decompose to carbon, among other things. -dust dioxide; at these temperatures, carbon dioxide has a very corrosive effect on the metal surfaces. Consequently, for such boilers, and indeed also for boilers operating at low pressure, it is customary to use (instead of adding carbonate) a soluble phosphate, in particular an n 4 0 3 7 1 4

V

Add its sodium salt, for example, sodium hydrogen phosphate, or sodium phosphate to the feed water, although potassium phosphate and other phosphates, including polyphosphates, for example, sodium hexametaphosphate and fluorophosphates, may also be used. This causes all of the calcium present to precipitate as calcium phosphate, which is then dispersed with a dispersant as described above. This substance, as in the other systems, | removed from time to time by draining the water from the kettle, removing the sludge-containing kettle water through a valve by rapidly reducing the pressure in the kettle. Nevertheless, not all the calcium phosphate is removed in this way, with the result that a calcium phosphate deposit forms, which must be removed in due course after stopping the kettle.

In practice, more is added than the stoichiometric amount of phosphate required to react with the calcium in the water. Efforts are made to add sufficient phosphate so that an excess is present at any time, for example 10 to 20 ppm in the kettle; one can determine the required excess by consulting authoritative guidelines such as British Standard 2486. Such standards also state the alkalinity required, generally a pH of 9.5 to 12. This alkalinity is necessary for several reasons. First, keeping the pH sufficiently high ensures that all of the calcium phosphate precipitates as hydroxyapatite, a basic calcium phosphate that is easy to handle, and also ensures low solubility of the calcium phosphate. Second, alkaline conditions prevent corrosion. Thirdly, such a pH ensures that any magnesium ions present, such as magnesium hydroxide, will imitate the pree.

This illustrates another difference between the way in which scale is tackled in cooling water and desalination systems on the one hand, and pressurized boiler systems on the other side. In cooling water systems, scale formation by magnesium is rarely important, because the magnesium in the water, which rarely has a temperature higher than 50 ° C, remains in solution. In desalination systems, scale formation by magnesium does become a major problem, because the temperatures rise to 100 ° C, and because salt water contains a much higher magnesium content than ordinary water for industrial use, and special measures must be taken as a result. Actually, the magnesium hydrogen carbonate is first converted to carbon dioxide and magnesium carbonate, which is hydrolyzed by the hot water to magnesium hydroxide, and because of the high magnesium content, the solubility product is exceeded, so that it starts to come out of the solution. Unlike feed water for a pressure-15 boiler, salt water is not significantly alkaline, so magnesium hydroxide tends to remain in solution. However, at a pH of 9.5 to 12, magnesium hydroxide precipitates even in the cold, so there is usually no chance of scale forming by magnesium-20 hydroxide at the high temperatures that occur in pressure vessels.

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

It has now surprisingly been found that certain specific sulfonate copolymers are effective in combating scale build-up in pressure vessels. It is to be understood that we mean by "pressure boilers" boilers operating at a pressure of at least 350 kPa gauge pressure, generally at least 560 kPa gauge pressure, in particular 560-1050 kPa gauge pressure (low pressure), in particular general 1050-4200 kPa gauge (moderate pressure) and 4200-14,000 kPa gauge (high pressure) Such boilers contain the water at the boiling point, which ranges from 148 ° C at 350 kPa gauge, to 162 ° C at 560 kPa gauge , to 186 ° C at 1050 kPa gauge pressure, to 254 ° C at 0 * ^ 3714

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- 6 - 4200 kPa gauge pressure to 336 ° C at 14,000 kPa gauge pressure. Moreover, it has been found that these special copolymers provide the very substantial useful advantage of actually removing scale already present when present in the boiler water. In other words, these specific copolymers have an "in-operation" cleaning effect. This cleaning effect is not specific to the particular scale, which is deposited from the feed water then in use; in other words, the copolymers also remove scale which may have formed in an earlier boiler operation using a different feed water.

If one has worked improperly with a boiler, by varying the composition of the feed water without sufficient control, and consequently deposition of calcium phosphate 15, if therefore old boiler stone is present, the boiler stone can be removed, regardless of the additive one may have used by dosing the specific copolymers used in the present invention into the feed water while the kettle is operating under load.

It should be noted here that this surprising cleaning effect is observed during operation only when using the high temperatures that occur in pressure vessels. Thus, these same copolymers are not effective in removing scale from cooling systems, although they may be effective in preventing new scale formation.

In accordance with the invention there is provided a method of controlling scale in a pressure boiler, wherein at least one scale-fighting copolymer containing repeating units of the formula -CH-CH- is added to the kettle

I I

COOH CÖOH

35 and the formula - CHn - CH -

2 I

8403714 has 2 # ♦ - 7 - where 2 represents -SO ^ H or -C ^ SO ^ H, or a water-soluble salt thereof, especially an alkali salt such as the sodium salt, although potassium, ammonium, zinc and lower amine salts and other water-soluble salts can be used. The free acids can also be used and all the acidic hydrogen atoms need not be replaced, and the cations replacing the hydrogen atoms need not be the same. Thus, the cation can be a mixture of NH 2, H, Na, K, etc. or just one of them. In addition, the invention provides a method for removing scale from a pressure scale in which scale is deposited, wherein a kettle-removing amount of the copolymer described is added to the aqueous system of the kettle.

The copolymers are conveniently prepared by polymerizing maleic acid or maleic anhydride or fumaric acid with the vinyl or allyl sulfonic acid or an alkali salt thereof by conventional methods. Conventional addition polymerization using light or free radical initiators can thus be used. Generally, copolymerization can be performed at 30-120 ° C with a peroxide catalyst such as hydrogen peroxide or benzoyl peroxide in an inert medium. The copolymer can be obtained, for example, by solution polymerization of maleic acid and sodium allylsulfonate in the presence of hydrogen peroxide.

The salts can, of course, be obtained by conventional methods.

The interrelationships of the sulfonate and carboxylate components depend to some extent on the degree of scale formation to be treated. The copolymer generally contains 10-80 mol% sulfonate residues and accordingly 90-20 mol% carboxylate residues. Preferably the sulfonate residues form 25-75 mol% of the copolymer and the carboxylate residues form 75-25 mol%. In the case of the vinyl sulfonate copolymer, the sulfonate residues, in particular, form 50-75 mol% of the copolymer and the carboxylate residues 50-25 mol%.

The average molecular weight of the copolymer is not critical, provided the polymer is water soluble. Generally, the weight average molecular weight ranges from 500 to 100,000. The molecular weight is preferably 800-25,000 and especially 1000-15,000. A copolymer with a molar ratio of maleic acid or maleic anhydride to allylsulfonic acid of 1: 1 and a molecular weight of 2000 is especially preferred. Other 10 copolymers. preferred ones are, for example, those with a molar ratio of maleic acid or maleic anhydride to vinyl sulfonic acid of 1: 1.5 or 1: 3, and a molecular weight of 7000-9000. Although the best results are generally obtained with the molar ratio 1: 3, in practice a molar ratio of 1: 1.5 is generally preferred, because of the relatively high price of the vinyl sulfonic acid, even though the results are not equally be good.

Men. understands that it is sometimes easier to simultaneously meter into the feed water the appropriate copolymer and the water-soluble carbonate, especially sodium carbonate or phosphate, as described above, or other precipitating softener suitable for the temperatures and pressures used in the boiler; the pH is adjusted to 9.5-12 norma, preferably 10-11, if necessary. This pH can be achieved by maintaining the recommended alkalinity for the particular kettle used by adding appropriate amounts of sodium hydroxide. This alkalinity can be determined by known methods, such as by titration with standard acid. Consequently, the invention also provides a suitable preparation for adding to a water system of a pressure vessel, which comprises a copolymer with repeating units of the above formulas and a water-soluble precipitating softener. The copolymer 35 is typically added as an aqueous solution with generally 0.1-50, preferably 2.5-10, most preferably 3-5-8403714% - 9% by weight (active) copolymer. The appropriate amount of precipitating softener in the solution is 5-50 (or solubility limit), preferably 15-35, and most preferably 25-35% by weight. The mutual weight ratios of the copolymer 5 and precipitating softener are thus expediently 0.1: 50 to 10: 1, preferably 1:15 to 2: 3, most preferably 1:11 to 1: 3.

Of course, other conventional additives can also be included in the water, including alkaline constituents, lignin derivatives, other polymers, tannins, biocides and corrosion inhibitors.

The copolymer can be introduced at any point where it is mixed quickly and efficiently with the system water, although it is generally most effective to add the copolymer to the make-up or feed water lines, thereby water enters the boiler. Typically, an injection device is calibrated to deliver a predetermined amount from time to time or continuously to the make-up water.

The copolymer is added to the water in a sub-stoichiometric amount effective to combat, ie prevent and remove the limestone, and of course this depends on the nature of the aqueous system to be treated, especially the calcium-25 content of it. The amount depends to some extent on the concentration of suspended solids and existing levels of solids formed in the system. In particular, amounts of 0.1-400 ppm, preferably 1-80 ppm and most preferably 5-50 ppm active in the boiler water are used. As the amount of precipitating agent required for the calcium increases in h1-1-3-general, the amount of copolymer increases. For a particularly preferred preparation, the amount of preparation added is typically 20-2500 ppm.

The following examples further illustrate the invention. In these examples, the properties of the additives were evaluated in a small laboratory kettle with 8403714-10-three removable tubes, as described in the reports of the 15th Annual Water Conference, Engineers Society of the Western Pennsylvania, biz. 87-102 (1954). The laboratory boiler feed water was prepared by diluting tap water from Lake Zurich, Illinois, with distilled water to a total hardness such as CaCO2 of 40 ppm, and adding calcium chloride so that the atomic ratio of calcium to magnesium was 6: 1 amounts. The feed water and the chemical treatment solutions were fed to the kettle in a ratio of 3 volumes of feed water to 1 volume of solution, giving a total feed water hardness of 30 ppm CaCO 2. The scale formation tests were performed for all treatment solutions by adjusting the boiler blowdown to 10% of the boiler feed water to obtain an approximately 10-fold concentration of the boiler water salts, and by measuring the composition of the treatment solution. so that a kettle water was obtained after the 10-fold concentration with the composition shown in Table A.

20 Table A

sodium hydroxide as NaOH 258 ppm sodium carbonate as 120 ppm sodium chloride as NaCl 681 ppm sodium sulfite as ^ 280 ^ 50 ppm 25 sodium sulfate as Na2SO ^ 819 ppm silica as SiC ^ less than 1 ppm iron as Fe less than 1 ppm phosphate as PO ^ 10-20 ppm

In the first series of tests, the kettle was run for 45 hours at a kettle water pressure of 2800 kPa gauge. At the end of each run, the boiler houses were removed separately from the kettle, and the scale or deposit 15 cm from the center of the length of each tube was removed by scraping, collected in a tared vial, and weighed. The results obtained are shown in Table B.

8403714 Λ * ♦ - 11 -Table Β

Corridor Additives Effective Boilerstone- Reduction of dosage formation, reduction in the g / m2 / hour of boiler water, counting ppm stone __.__% 1 none - 2,293 ----- 2 allylsulfonic acid and 5 0.678 70 .4 maleic acid copolymer (1: 1, wt.

M = 2000) 15 3 sodium vinyl sulfo 5 0.495 78.4 nate and maleic acid copolymer (1.5: 1, wt. M = 7000-9000) 20 4 sodium vinyl sulfo 10 1,012 55,9 nate and fumaric acid copolymer (1, 5: 1, wt. M = 7000-9000) 5 allylsulfonic acid and. 0.065 97.2 maleic acid copolymer (as in step no. 2) 30 6 sodium vinyl sulfonate 10 0.151 93.4 and maleic acid copolymer (as in step no. 3) 35 These results demonstrate that the copolymers used in the invention are effective in reducing the rate of scale formation in a pressure vessel.

In a second series of tests, the system was first run for 45 hours without any polymer addition, then one of the three tubes was removed and replaced with a clean tube. The system was run 8403714 * <12 - for an additional 45 hours, but now with added polymer. After this second period of. The scale in the three tubes is weighed for 45 hours as before. By comparing the results of this test with those of an untreated blank (no polymer added during the second 45 hour period), it is possible to determine whether the polymer is capable of removing scale, and also the formation of scale on a clean tube. The results obtained are shown in the following table C.

10 Table C

Corridor Additives Effective Reduction no. Dosing ring in the boiler water. stone, 15% by weight 7 allylsulphonic acid and 30-112.0 maleic acid copolymer (as in step no. 2) 20 8 sodium vinyl sulfonate 30 108.8 and maleic acid copolymer (as in step no. 3) 9 sodium vinyl sulfonate and 123.9 25 maleic acid copolymer (3: 1, wt. M = 6000)

It is therefore clear that the use of these polymers is effective not only in preventing scale formation but also in removing existing scale since the scale reduction is greater than 100%.

To determine the cleaning performance "during operation" of the copolymers in a cooling water system, a clean metal heater in a glass tubing through which heated water was circulated to 60 ° C by means of a pump was mounted. The system was part of a closed system, equipped with an expansion vessel, which had an open connection to the atmosphere.

8403714 - 13 -

The heater was removed and placed in dilute acid to remove the scale. The acidic solution was then titrated with a standard EDTA solution to determine the amount of calcium carbonate scale (as Ca ++) from which the weight of calcium carbonate scale was determined.

The first run artificially circulated water set at 400 ppm calcium and 400 ppm alkalinity (as hydrogen carbonate) for 6 hours. When weighing the heater, it was found that 780 mg of calcium carbonate kettle stone had been formed.

The test was then repeated, after 6 hours 10 ppm of copolymer of malexic acid and allylsulfonic acid (as in step no. 2) / was added and the water was allowed to circulate for another 45 hours. The heater was then removed and examined as described above; again it was found that 780 mg of calcium carbonate scale had been formed. It is therefore evident that the copolymer did not remove scale under these conditions.

20 8403714

Claims (13)

1 I has COOH COOH 10 and the formula -CH 1 -CH 2 -iz 15 wherein Z 1 represents SO 2 H or -Q 2 SO 2 H, or a water-soluble salt thereof, preferably 25-75 mol% sulfonate residues and correspondingly 75-25 mol% carboxylate residues. A method for treating boiler stone in a water system of a pressure boiler, characterized in that a copolymer is added to the system, which repeating units of the formula 5 - CH - CH - 2 I z 30, wherein Z represents -SO ^ H ° f "CH ^ SO ^ H, or water-soluble salt thereof, preferably a copolymer having 25-75 mol% sulfonate residues and correspondingly 75-25% carboxylate- remains. 2. Process according to claim 1, characterized in that the copolymer is a copolymer of maleic acid and allylsulfonic acid, preferably a copolymer with a mol ratio of allylsulfonic acid to maleic acid or maleic anhydride of 1: 1, and a molecular weight of 800-25,000 . 3. Process according to claim 1, characterized in that the copolymer is derived from vinyl sulfonic acid and contains 50-75% sulfonate residues and accordingly contains 50-25% carboxylate residues, preferably a copolymer with a molar ratio of vinyl sulfonic acid to maleic acid or maleic anhydride of 1, 5: 1 and a molecular weight of 800-25,000. 4. Process according to any one of claims 1-3, 30, characterized in that the copolymer is added in an amount of 0.1-400 ppm, preferably 1-80 ppm. 5. Process according to any one of claims 1-4, characterized in that a water-soluble 84 0 3 7 1 /; -φ -c -15- precipitating agent, preferably a water-soluble carbonate or phosphate, especially sodium hydrogen phosphate or sodium phosphate or sodium metaphosphate. 6. Process according to any one of claims 1-5, 5, characterized in that the feed water for the system is adjusted to a pH of 9.5-12. 7. A method according to claim 1 for preventing and removing scale in a water system in a pressure boiler, characterized in that a water-soluble phosphate is added to the feed water for the system in an amount greater than the stoichiometric amount required. to neutralize the calcium in the water, and add a copolymer of maleic acid or maleic anhydride and allylsulfonic acid or a water-soluble salt thereof in an amount sufficient to provide 0.1-400 ppm copolymer in the boiler water, and if necessary, adjust the pH of the water to 10-11 by adding sodium hydroxide. 8. A suitable preparation for addition to a pressurized boiler water system, characterized in that it comprises a copolymer comprising repeating units of the formula -CH-CH-! I C00H C00H 25 and the formula - CEL - GH - 9. A composition according to claim 8, characterized in that the copolymer is a copolymer of maleic acid and allylsulfone-8403714-16. η is acid, preferably a copolymer with a molar ratio of allylsulfonic acid to maleic acid or maleic anhydride of 1: 1 and a molecular weight of 800-25,000. 10. A composition according to claim 8 or 9, characterized in that the copolymer is derived from vinyl sulfonic acid and contains 50-75 mol% sulphonate residues and accordingly contains 50-25 mol% carboxylate residues, preferably a copolymer with a molar ratio of vinyl sulfonic acid to maleic acid or maleic anhydride of 1.5: 1 and a molecular weight of 800-25,000. 11. A preparation according to any one of claims 8-10, characterized in that the water-soluble precipitating softener is a water-soluble carbonate or phosphate, preferably sodium hydrogen phosphate or sodium phosphate or sodium metaphosphate. 12. A composition according to any one of claims 8-11, characterized in that it is an aqueous solution containing 0.1-50 wt% copolymer, preferably 2.5-10 wt% copolymer and 5-50 wt% of the precipitating softener. 13. A composition according to any one of claims 8-12, 20, characterized in that it also contains at least one water treatment additive, namely an alkali, lignin derivative, other polymer, tannin, biocide or corrosion inhibitor. 25 8403714