GB2152919A - Method of controlling scale in pressurised boilers - Google Patents

Abstract

The formation of scale in aqueous pressurised boiler systems is inhibited by the addition of a copolymer of maleic acid or anhydride or fumaric acid and allyl or vinyl sulphonic acid or a water soluble salt thereof. These copolymers have the great advantage that they actually cause the removal of existing scale in such pressurised boiler systems and may be added in combination with a water soluble, hardness precipitating agent such as a carbonate or phosphate.

Description

SPECIFICATION Method of controlling scale in pressurised boilers This invention relates to the treatment of aqueous systems used in pressurised boilers.

The water used in steam generating boilers, cooling towers, desalination units and other industrial aqueous systems contains impurities. The impurities typically include alkaline earth cations, principally calcium and magnesium, and several anions including bicarbonate, carbonate, sulphate, oxalate, phosphate, silicate and fluoride. The most common impurities in industrial water supplies are the water hardening calcium, magnesium and carbonate ions although sulphate is usually also present. The salts of these metal ions, especially the carbonates, tend to precipitate forming solid accumulations on the surface of the system and these accumulations can give rise to the formation of scale on hot surfaces. The water may also contain various solids such as mud, clay, iron oxides, silt, sand and other mineral matter as well as microbiological debris which accumulate as sludge in the system.Naturally sludge and scale deposits greatly reduce heat transfer efficiency by settling at low flow points in the system and thus limiting the circulation of the water and insulating it from the heat transfer surfaces. In addition, corrosion of metal surfaces under the deposits is facilitated since corrosion control agents are unable to contact the surfaces effectively. Again, the deposits harbour bacteria. Removal of the deposits can cause expensive delays and shutdown of the system.

Since prior treatments such as softening, coagulation and filtration do not adequately remove solids and solid forming substances various chemicals have been used to counteract the adverse effects of scale and sludge in aqueous systems. In cooling water systems and desalination plants the chemicals are commonly added in order to increase the threshold at which precipitation occurs; it has also been thought that the chemical forms a film of the hot surfaces where scale formation is likely to occur thereby preventing scale forming material from adhering to the hot surfaces in question. A variety 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 quantities of these polymers can have a marked effect on the system.

Although these various chemicals are quite effective in industrial cooling towers and desalination plants and the like quite different problems arise in water which is used in pressurised boilers due to the much higher temperatures involved; the boiler point of water at the lowest pressure normally used, 80 psig, is already 324 F (about 162 C).

Because of the greater problems involved, the water used for such pressurised boilers is first subjected to a deioniser or to base exchange, the former being the more effective. Even so these treatments are not always fully effective with the result that some calcium and magnesium ions, in particular, remain in the boilerfeedwater along with their associated anions.

In view of the higher temperatures involved, it is not practical to try and keep the calcium in solution as is the case with cooling water; an exception to this is the use of chelants but these present some other problems of application in particular correct dosing. Likewise, it is much more difficult to prevent the solid deposits from contacting the very hot surfaces thereby giving rise to scaling.

As a result, different techniques have to be adopted for the prevention of scale in pressurised boiler systems.

For boilers which are subjected only to low (up to, say, 150 psig) pressures it is not unusual to add a water soluble carbonate to the feed water in order to cause all the calcium present to precipitate as carbonate and to add also a dispersant to prevent the precipitated material from settling on the hot surfaces. This is, of course, in complete contrast with the situation with cooling water where the aim is to keep the carbonate concentration to a minimum and to keep that concentration in solution. Calcium sulphate is very much more soluble than calcium carbonate so that it does not present a significant problem in cooling water and desalination systems. In pressurised boilers, however, scaling due to calcium sulphate can occur so that it is desirable to ensure that all the calcium is precipitated.

Inevitably, however, dosing with carbonate is not fully effective with the result that some scale does still form. Over a period, scale can accumulate to such an extent that the boiler has to be shut down and the scale removed. This is essential since scale deposits can cause localised overheating and even rupture in the boiler.

With moderate (e.g. 150 to 600 psig) or high (above 600 psig) pressure boilers, dosing the feed water with carbonate is not satisfactory since at the water boiler point under these pressures the carbonate will tend to decompose giving rise to carbon dioxide; at these temperatures carbon dioxide has a very corrosive effect on the metal surfaces. Accordingly, for such boilers and, indeed, for low pressure boilers as well (instead of carbonate addition) it is usual to add a soluble phosphate, typically sodium phosphate e.g. disodium phosphate, or trisodium phosphate to the feed water, although potassium phosphate and other phosphates including polyphosphates e.g. sodium hexametaphosphate and fluorophosphates can also be used. This ensures that all the calcium present precipitates as calcium phosphate which is then dispersed with a dispersant as before.This material can be removed periodically, as in the other systems, with the water drained from the boiler by blowdown where the sludge containing boiler water is removed through a valve by rapidly reducing the pressure within the boiler. Nevertheless, not all the calcium phosphate is removed in this way with the result that a scale of calcium phosphate forms which, in due course, has to be removed after shutting down the boiler.

In practice, one adds more than the stoichiometric amount of phosphate needed to reach with the calcium in the water. The aim is to add sufficient phosphate to give an excess at all times, e.g. 10 to 20 ppm in the boiler; the excess required can be established by consulting authoritative guidelines such as British Standard 2486. Such standards also specify the degree of alkalinity needed, generally a pH of 9.5 to 12. This alkalinity is needed for several reasons. First, by keeping the pH sufficiently high one ensures that all the calcium phosphate precipitates as hydroxy apatite, a basic calcium phosphate which is easy to condition and also ensures low solubility of the calcium phosphate. Second, alkaline conditions prevent corrosion. Third, such a pH ensures that any magnesium ions present are precipitated as magnesium hydroxide.

This illustrates a further difference between the way in which one tackles scale in cooling water and desalination systems, on the one hand, and pressurised boiler systems, on the other. In cooling water systems, scaling due to magnesium is seldon significant because the magnesium stays in solution in the water which rarely exceeds a temperature of about 50 C. In desalination systems magnesium scaling does become a significant problem because temperatures rise to 100 C and because salt water contains a much high magnesium content than does ordinary industrial water and, as a result, special steps have to be taken.

In effect the magnesium bicarbonate is first converted to carbon dioxide and magnesium carbonate which is hydrolysed by the hot water to magnesium hydroxide; due to the high magnesium content the solubility product is exceeded with a result that it starts to come out of solution. In contrast to the situation with pressurised boiler feed water, salt water is not significantly alkaline with the resu It that the magnesium hydroxide has a greater tendency to stay in solution. At a pH of 9.5 to 12, however magnesium hydroxide will precipitate even in the cold so that at the high temperatures involved in pressurised boilers there is usually no chance of scaling due to magnesium hydroxide.

It is also of interest to note that in cooling water systems it is not uncommon to add acid (ratherthan alkali) in order to try and keep more calcium carbonate in solution.

It has now surprisingly been found that certain specific sulphonate copolymers are effective for controlling the formation of scale in pressurised boilers. It will be appreciated that by "pressurised boilers" we mean boilers operating at a pressure of at least 50 psig, generally at least 80 psig, typically 80 to 150 psig (low pressure), generally 150 to 600 psig (moderate pressure) and above 600 and up to, say 2000 psig (high pressure). In such boilers the water will be at its boiling point which will vary from about 298 F at 80 PSIG, to about 324 F at 80 PSIG, to about 3660F at 150 PSIG, to about 4890F at 600 PSIG to about 637 F at 2000 PSIG.

Furthermore, it has been found that these particular copolymers provide the very real practical advantage that they will, in fact, when present in the boiler water, actually remove scale which is already present. In other words, these specific copolymers have an "in service" cleaning effect. This cleaning effect is not specific to the particular scale which has been deposited from the feed water currently in use; in other words the copolymers will also remove scale which may have formed from previous operation of the boiler using a different feed water.Thus if a boiler has been operated incorrectly by allowing the composition of the feed water to vary without adequate controls and, as a result calcium phosphate scale allowed to form, that is there is old scale present, regardless of the additive which may have been used, then by dosing the feed water with the specific copolymers used in the present invention the scale can be removed while the boiler is operating under load.

It should be added that this surprising in service cleaning effect is only observed when using the high temperatures involved in pressurised boilers. Thus, these same copolymers are not effective for removing scale from cooling systems even though they may be effective in inhibiting the formation of fresh scale.

According to the present invention there is provided a method of controlling scale in a pressurised boiler which comprises adding to the boiler at least a scale controlling amount of copolymer which possesses recurring units of the formula

and the formula

where Z represents -SO3H or -CH2SO3H, or a water soluble salt thereof, typically an alkali metal salt such as the sodium salt, although potassium, ammonium, zinc and lower amine salts and other water soluble salts may be used. The free acids may also be used and all of the acid hydrogens need not be replaced nor need the cation be the same for those replaced. Thus, the cation may be any one of or a mixture of NH4, H, Na, K.

etc. In another aspect the present invention provides a method of removing scale from a scaled pressurised boiler which comprises adding to the aqueous system of said boiler a scale removing amount of the specified copolymer.

The copolymers are conveniently prepared by polymerising maleic acid or anhydride orfumaric acid with the vinyl or allyl sulphonic acid or an alkali metal salt thereof using conventional procedures. Thus conventional addition polymerisation methods using light or free radical initiators may be employed.

Generally, the copolymerisation may be effected at from about 30 to about 1 200C using a peroxide catalyst such as hydrogen peroxide or benzoyl peroxide in an inert medium. The copolymer may be derived, for example, by solution polymerisation of maleic acid and sodium allyl sulphonate in the presence of hydrogen peroxide. The salts can of course be obtained using conventional methods.

The relative proportions of sulphonate and carboxylate components depend on some extent upon the degree of scaling to be treated. The copolymer generally contains from about 10 to about 80 mol per cent of sulphonate moieties and correspondingly from 90 to about 20 mol percent of the carboxylate moieties.

Preferably, the sulphonate moieties comprise about 25 to about 75 mol per cent of the copolymer and the carboxylate moieties comprise from about 75 to about 25 mole per cent. For the vinyl sulphonate copolymer, the sulphonate moieties especially comprise about 50 to about 75 mol per cent of the copolymer and the carboxylate moieties from about 50 to about 25 mol per cent.

The average molecular weight of the copolymer is not critical so long as the polymer is water soluble.

Generally, the weight average molecular weight ranges from about 500 to about 100,000. The molecular weight is preferably from about 800 to about 25,000 and especially is from about 1,000 to about 15,000. A copolymer having a mol ratio of maleic acid or anhydride to allyl sulphonic acid of about 1:1 and a molecular weight of about 2,000 is especially preferred. Other preferred copolymers include those having a mol ratio of maleic acid or anhydride to vinyl sulphonic acid of about 1:1,5 or about 1 :3, and a molecular weight of about 7,000 to 9,000. Although the best results are generally obtained with the 1:3 mole ratio, in practice because of the relatively high cost of the vinyl sulphonic acid, a mole ratio 1:1.5 is generally preferred even though the results are not quite so good.

It will be appreciated that it will sometimes be simpler to dose the feed water simultaneously with the specific copolymer and the water soluble carbonate, typically sodium carbonate, or phosphate such as those specified above or other hardness precipitating agent as appropriate to the temperatures and pressures to be used in the boiler; the pH will normally be adjusted if necessary to 9.5 to 12, preferably 10 to 11. This pH can be achieved by maintaining the recommended alkalinity value for the particular boiler employed by adding appropriate quantities of caustic soda. This alkalinity value can be determined using well known methods, such as by titration against standard acid.Accordingly, the present invention also provides a composition suitable for the addition to a pressurised boiler water system which comprises a copolymer having recurring units of the formulae set out above and a water soluble hardness precipitating agent.

Typically the copolymer is added as an aqueous solution generally containing 0.1 to 50%, preferably 2.5 to 10%, especially 3 to 5% by weight (active) of the copolymer. The amount of hardness precipitating agent in the solution is suitable from 5 to 50% (or solubility limit) preferably from 15 to 35%, especially 25 to 35% by weight. Thus the relative weight proportions ofthe copolymer and hardness precipitating agent are suitably from0.1:50to10:1,preferablyfrom 1:15to2:3,especiallyfrom 1:11 to 1:3.

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

The copolymer may be introduced at any location where it will be quickly and efficiently mixed with the water of the system although it will generally be most convenient to add it to the make-up or feed water lines through which the water enters the boilers. Typically, an injector calibrated to deliver a predetermined amount periodically or continuously to the make-up water is employed.

The amount of the copolymer added to the water is a substoichiometric amount that is effective to control, i.e., inhibit and remove the scale and naturally this depends on the nature of the aqueous system to be treated, especialy its calcium content. The amount depends to some extent on the concentration of suspended solids and existing levels of solids build up in the sytem. Typically, amounts from about 0.1 to about 400 ppm, preferably from about 1 to about 80 ppm and especially from about 5 to about 50 ppm active in the boiler water are used. In general, as the amount of precipitating agent needed for the calcium increases so does the amount of copolymer. Typically for an especially preferred composition the amount of composition added will be about 20 to about 2500 ppm.

The following Examples further illustrate the present invention. In these Examples the properties of the additives were evaluated in a small laboratory boiler which had three removable tubes as described in the proceedings of the 15th Annual Water Conference, Engineers Society of the Western Pennsylvania, pages 87-102 (1954). The feed water for the laboratory boiler was prepared by diluting Lake Zurich, Ullinois, tap water with distilled water to 40 ppm total hardness as CaCO3 and adding calcium chloride to provide a 6:1 elemental calcium to magnesium ratio. The feed water and chemical treatment solutions were fed to the boiler in a ratio of 3 volumes of feed water to 1 volume of solution giving a feed water total hardness of 30 ppm CaCO3.The scaling tests for all the treatment solutions were conducted by adjusting boiler blow-down to 10% of the boiler feed water giving an approximately 10 fold concentration of the boiler water salines and adjusting the composition of the treatment solution to give a boiler water after the 10 fold concentration having the composition shown in Table I.

TABLE I Sodium Hydroxide as NaOH 258 ppm Sodium Carbonate as Na2CO3 120 ppm Sodium Chloride as NaCI 681 ppm Sodium Sulfite as Na2SO3 50 ppm Sodium Sulfate as Na2SO4 819 ppm Silica as SiO2 less than 1 ppm Iron as Fe less than 1 ppm Phosphate as P04 10-20 ppm In the first series of tests, the boiler was run for 45 hours at a boiler water pressure of 400 psig. At the completion of each test, the boiler tubes were individually removed from the boiler and the scale or deposit present on 6" of the central length of each tube was removed by scraping, collected in a tared vial, and weighed. The results obtained are shown in Table II.

TABLE II Run Additives Active Scaling Scale No. Dosage Rate Reduction, in the gift2/hr Boiler Water ppm 1 None - 0.213 2 Ally sulphonic 5 0.063 70.4 acid and maleic acid copolymer (1:1; wt.av.M.W. = about 2,000) 3 Sodium vinyl 5 0.046 78.4 sulfonate and maleic acid copolymer (1.5:1; wt.av.M.W. = about 7,000 to 9,000) 4 Sodium 10 0.094 55.9 vinyl sulfonate and fumaric acid copolymer (1.5: :1; wt.av.M.W. = about 7,000 - 9,000) 5 Allylsulphonic 10 0.006 97.2 acid and maleic acid copolymer (as in Example 2) 6 Sodium vinyl 10 0.014 93.4 sulfonate and maleic acid copolymer (as in Example 3) These results show that the copolymers used in the present invention are effective for reducing the rate of scaling in a pressurised boiler.

In a second series of tests, the system was first run for 45 hours without any addition of polymer and then one of the three tubes was taken out and replaced by a clean tube. The system was then run for a further 45 hours but this time with polymer added. After this further period of 45 hours the scale in the three tubes is weighed as before. Thus comparison of the results of this test with those of an untreated blank (no polymer added during the second 45 hour period) enables one to assess whether the polymer is capable of removing scale and also preventing the formation of scale on a clean tube. The results obtained are shown in following Table Ill.

TABLE Ill Run Additives Active Dosage Scale No. in the Boiler Reduction Water, ppm % 7 Allylsuiphonic 30 112.0 acid and maleic acid copolymer (as in Example 2) 8 Sodium vinyl sulfonate 30 108.8 and maleic acid copolymer (as in Example 3) 9 Sodium vinyl sulfonate and 30 123.9 maleic acid copolymer (3:1; wt.av.M.W. = about 6,000) It is clear therefore, that the use of these polymers is effective not only in inhibition of scale formation but also, since the scale reduction is greater than 100%, in removing existing scale.

In order to assess the inservice cleaning ability of the copolymers in a cooling water system a clean metal heater was fixed in a glass tube assembly through which water heated to 600C. was circulated by means of a pump. The assembly formed part of a closed system provided with an expansion tank open to the atmosphere. The heater was removed and placed in dilute acid to remove the scale thereon. The acid solution was then titrated withh a standard EDTA solution to determine the amount of calcium carbonate scale (as Ca++) from which the weight of calcium carbonate scale was determined.

In the first test, a synthetic water adjusted to 400 ppm of calcium and 400 ppm alkalinity (as bicarbonate) was circulated for 6 hours. On weighing the heater it was found that 780 milligrams of calcium carbonate scale had formed.

The test was then repeated, after 6 hours 10 ppm of a copolymer of maleic acid and allyl sulphonic acid (as in Example 2) was added and circulation continued for 45 hours. The heater was then removed and tested as above; it was again found that 780 milligrams of calcium carbonate scale had formed. It is apparent, therefore, that the copolymer did not remove any scale under these conditions.

Claims (24)

1. A method for the treatment of scale in a pressurised boiler water system which comprises adding to the system at least at scale controlling amount of a copolymer which possesses recurring units of the formula -CH - CH COOH COOH and of the formula -CH2 - CH I Z where Z represents -S03H or -CH2SO3H, or a water soluble salt thereof.

2. A method according to claim 1 in which the copolymer is a copolymer of maleic acid and allyl sulfonic acid.

3. A method according to claim 1 in which the copolymer contains from about 25 to about 75 mole per cent sulphonate moieties and correspondingly from about 75 to about 25 per cent carboxylate moieties.

4. A method according to claim 3 in which the copolymer is derived from vinyl sulfonic acid and contains from about 50 to about 75 per cent sulphonate moieties and correspondingly from about 50 to about 25 per cent carboxylate moieties.

5. A method according to claim 2 in which the said copolymer has a mol ratio of allyl sulphonic acid to maleic acid or anhydride of about 1:1 and has a molecular weight of about 800 to anout 25,000.

6. A method according to claim 4 in which the said copolymer has a mol ratio of vinyl sulphonic acid to maleic acid or anhydride of about 1.5:1 and has a molecular weight of about 800 to about 25,000.

7. A method according to claim 1 in which the copolymer is added in an amount from 0.1 to about 400 ppm.

8. A method according to claim 7 in which the copolymer is added in an amount of about 1 to about 80 ppm.

9. A method according to claim 1 in which a water soluble precipitating agent is also added to the system.

10. A method according to claim 9 in which a water soluble carbonate or phosphate is added to the system.

11. A method according to claim 10 in which disodium or trisodium phosphate or sodium metaphosphate is added to the system.

12. A method according to claim 1 in which the feed water for the system is adjusted to a pH of 9.5 to 12.

13. A method for the inhibition and removal of scale in a pressurised boiler water system which comprises adding to the feed water for the system a water soluble phosphate in an amount in excess of the stoichiometric amount needed to neutralise the calcium in the water and a copolymer of maleic acid or anhydride and allyl sulphonic acid or a water-soluble salt thereof in an amount sufficient to provide about 0.1 to about 400 ppm of the copolymer in the boiler water and, if necessary, adjusting the pH of the water to 10 to 11 by addition of caustic soda.

14. A composition suitable for addition to a pressurised boiler water system which comprises a copolymer which possesses recurring units of the formula

and oftheformula

where Z represents -SO3H or -CH2SO3H, or a water soluble salt thereof, and a water soluble calcium precipitating agent.

15. A composition according to claim 14 in which the copolymer is a copolymer of maleic acid and allyl sulfonic acid.

16. A composition according to claim 14 in which the copolymer contains from about 25 to about 75 mol per cent sulphonate moieties and correspondingly from about 75 to about 25 per cent carboxylate moieties.

17. A composition according to claim 16 in which the copolymer is derived from vinyl sulphonic acid and contains from about 50 to about 75 mol per cent sulphonate moieties and correspondingly from about 50 to about 25 mol per cent carboxylate moieties.

18. A composition according to claim 15 in which the copolymer has a mol ratio of allyl sulphonic acid to maleic acid or anhydride of about 1:1 and has a molecular weight of about 8,000 to 25,000.

19. A composition according to claim 17 in which the copolymer has a mol ratio of vinyl sulphonic acid to maleic acid or anhydride of about 1.5:1 and has a molecular weight of about 800 to about 25,000.

20. A composition according to claim 14 in which the water soluble hardness precipitating agent is a water soluble carbonate or phosphate.

21. A composition according to claim 20 in which the water soluble hardness precipitating agent is di-sodium or tri-sodium phosphate or sodium metaphosphate.

22. A composition according to claim 14 which is an aqueous solution containing from about 0.1 to 50% by weight of the copolymer.

23. A composition according to claim 22 which is an aqueous solution containing from about 2.5 to 10% by weight of the said copolymer and from about 5 to 50% by weight of the said hardness precipitating agent.

24. A composition according to claim 14 which also contains at least one water treatment additive selected from the group consisting of alkalis, lignin derivatives, other polymers, tannins, biocides and corrosion inhibitors.