GB2110659A

GB2110659A - Boiler scale control

Info

Publication number: GB2110659A
Application number: GB08231071A
Authority: GB
Inventors: Walter F Lorenc; John A Kelly; Frederick S Mandel
Original assignee: Nalco Chemical Co
Current assignee: ChampionX LLC
Priority date: 1981-11-05
Filing date: 1982-10-29
Publication date: 1983-06-22
Also published as: ES517116A0; DE3240780C2; MX160592A; IT1149113B; AT386594B; AU555892B2; FR2515631B1; FR2515631A1; SE8206274L; ES8500192A1; ATA405082A; IT8249421A0; AU8963682A; SE8206274D0; SE450954B; DE3240780A1; GB2110659B; CA1198027A

Description

1 GB 2 110 659 A 1

SPECIFICATION Boiler scale control

One of the biggest challenges in boiler water treatment lies in the development of simple, easily monitored, and easily controlled programs. Ideal' would be one product which can prevent scale, provide heat transfer surface protection, and protect condensate systems. However, the state-of-theart practices have not been able to meet this challenge. Chelant programs, for example, are capable of eliminating hardness deposits; they are also known, however, to cause corrosion under certain conditions. While the chelants are capable of solubilizing hardness metal ions, their strong affinity toward iron ions may actually be the corrosion mechanism. Excessive residual chelants may not only prevent the formation of magnetite but also strip the boiler of its protective magnetite films.

The present invention provides a method of treating boiler water which contacts heat transfer surfaces to prevent or remove scale due to water hardness, the method comprising treating the boiler water with watersoluble anionic vinyl polymer which contains at least 30% by weight of carboxylate functionality and has an average molecular weight of 500 to 50,000. By the term "hardness" herein we include soluble and insoluble compounds of calcium, magnesium, iron, copper, aluminum, and the like.

In addition to the above characteristics, the water-soluble anionic vinyl polymer should interact with hardness ions to sequester them. The sequestration is preferably such as to yield a chelation value of at least 200 as measured by specific ion electrodes.

In a preferred embodiment of the invention, the molecular weight range of the carboxylate 20 polymers is 1,000 to 30,000. Polymer molecular weights herein are average molecular weights.

In another preferred embodiment, there is utilized, in combination with the anionic water-soluble vinyl polymers, other water-soluble polymer having. dispersant properties such as a sulphonate containing polymer which is capable of acting as a dispersant for any excess hardness not acted upon by the sequestrant anionic water-soluble vinyl polymer.

The water-soluble sequestrant anionic vinyl polymers The polymers may be homopolymers or copolymers of vinyl carboxylate- containing monomers.

"Carboxylate-containing monomers" means that the carboxylic acid groups are either in the form of the free acid or of a water-soluble salt thereof such as alkali metal, ammonia or amine. In the case of acrylic acid polymers, it would include the amide.

Thus, the homopolymers of acrylic acid, methacrylic acid, maleic acid, fumaric acid, itaconic acid, and the like may be used. Polyacrylamide, when added to the boile, r water, undergoes hydrolysis to convert some or all of the amide groups to carboxylate groups, and as such, is also included.

In addition to using these homopolymers, water-soluble copolymeric forms may also be employed. When the copolymers are used, the amount of carboxylate should be at least 30% by 35 monomer weight ratio of the copolymers.

A preferred group of carboxylate polymers are those derived by the hydrolysis of the corresponding polyacrylamides. These materials, after either caustic or acid hydrolysis, will contain between about 10-30% by weight of amide groups. A most preferred group of carboxylate polymers are those obtained by polymerizing acrylic acid with acrylamide at a 3:1 monomer weight ratio.

The amount of polymers used to treat the hardness contained in the boiler waters is preferably 1-30 ppm per ppm of hardness.

It has been found that the preferred water-soluble anionic vinyl polymers exhibit a chelation value in excess of 200, most preferably in excess of 300. When applied to the invention, chelation value means the average chelation value from both calcium and magnesium determinations.

Chelation value Chelation value is defined as the milligrams of calcium or magnesium expressed in terms of calcium carbonate complexed by one gram of active sequestrant. in this work, it is measured by specific ion electrode techniques. A known increment of sequestrant is added to a system containing a known amount of free (uncomplexed) calcium or magnesium. The decrease in calciurn/magnesium 50 activity (concentration) is then a direct measure of complexed species. This amount is then converted (ratioed) to yield the chelation value The effective mole ratio can also be computed using this information. By dividing the chelation value into 100,000 an equivalent weight for the sequestrant is determined. If the molecular weight is known, then the mole ratio is found by dividing the molecular weight by the equivalent weight. For 55 EDTA and NTA, the value should approximate unit. For polymers, this number varies with the molecular weight and is generally greater than unity.

The polymeric dispersants In a preferred mode of the invention, the carboxylate polymers described above are used in conjunction with a watersoluble polymer which is capable of dispersing hardness.

The polymeric dispersants used in this preferred mode of the invention are anionic water-soluble 2 GB 2 110 659 A 2 vinyl polymers. To be operative, they must be capable of dispersing suspended matter that normally occurs in boiler waters. They may be further characterized as containing carboxylate functionality and/or sulphonate functionality. Preferably they have an average molecular weight of at least 500 to about 50,000.

The water-soluble dispersing polymers useful in this invention may be chosen from the carboxylate containing water-soluble vinyl polymers such as, vinyl sulphonate-acrylic acid copolymers, vinyl sulfonate-methacrylic acid copolymers, sulfonated styrene-maleic anhydride copolymers, and acrylamide/acrylate copolymers.

The preferred water soluble dispersing polymer is a vinyl sulphonate copolymer synthesized from vinyl sulphonate and acrylic acid. This dispersant molecule generally contains from 5-25 mole percent of the vinyl sulphonate or its alkali metal (preferably Na) salts and from 95-75 mole percent of acrylic acid and its watersoluble alkali metal or ammonium salts. Preferably, the acrylic acid-vinyl sulphonate copolymers contain 10-20 mole percent of the vinyl sulphonate and from 90-80 mole percent of acrylic acid. The molecular weights of these preferred dispersant polymers range from as low as 500 to as high as 50,000. Molecular weight ranges of from 750-50, 000 are preferred with a 15 molecular weight range of approximately 900-15,000 being especially preferred. Ideally, the molecular weight will range from 1,000-6,000. It is surprising to find that these dispersant molecules may or may not be chelant or sequestrant molecules if treatment levels are drastically increased.

Another class of polymeric dispersants are the low molecular weight polyacrylic acids and their water-soluble salts. These materials have a molecular weight range of 1, 000-5,000. The ratios in 20 which these materials are used with the carboxylate polymers are the same as described above for the acrylic acid-vinyl sulphonate copolymers.

Yet another class of polymeric dispersants are the low molecular weight sulphonated copolymers of styrene and maleic anhydride. These materials are preferably present as the sodium salt of sulphonated copolymers of styrene and maleic anhydride and are typically known and commercially sold as Versa TL-3 products (National Starch and Chemical Corporation). Other sulphonated copolymers of styrene and malelc anhydride are also found useful in this application when combined with the above-described carboxylate polymers having sequestrant properties.

The preferred method of treating hardness present in boiler waters which are in contact with heat transfer surfaces to prevent and remove scale caused by such hardness comprises treating such waters 30 with:

(a) a water-soluble anionic vinyl polymer containing at least 30% by weight of a carboxylic functionality, said polymer having a molecular weight within the range of 500-50,000 and (b) a second anionic watersoluble vinyl polymer dispersant.

The boiler waters are preferably simultaneously treated with both the sequestrant (chelant) water-soluble anionic vinyl polymer mentioned above and a second anionic water-soluble vinyl polymer dispersant chosen from carboxylate-containing water-soluble vinyl polymers, vinyl sulphonate-acrylic acid copolymers, vinyl sulphonate-methacrylic acid copolymers, sulphonated styrene-maleic anhydride copolymers, and acrylamide-acrylic acid copolymers.

It is particularly interesting to note that most of the chelant or sequestrant anionic vinyl polymers show an ability to disperse solids in boiler water systems if treatment levels are below those required to chelate hardness ions. As a result of this observation, the chelant or sequestrant polymer may be used in quantities above those quantities necessary to chelate all of the hardness initially found in the boiler system. When this occurs, it has been observed that any hardness ions or scale existing in the boiler system may be removed from the system and the additional chelant polymer may act as a dispersant, as well as a sequestrant for hardness contamination.

Thus, reference to the addition of a second anionic water-soluble vinyl polymer dispersant in combination with the sequestrant polymer is meant to include the phenomenon described above, i.e., use of excess sequestrant polymer to provide both sequestration and dispersion.

Ratio of sequestrant to dispersant polymers The ratio of carboxylate polymer to acrylic acid-vinyl sulphonate copolymer, when they are used, is for example 30:1 to 1:30, with 20:1 to 30:1 being a preferred range and 20:1 being most preferred.

In general, the ratio of sequestrant carboxylate polymer to dispersant polymer is also suitably 30:1 to 1:30. A preferred sequestrant polymer to dispersant polymer ratio is 30:1 and 10: 1 with a most preferred ratio of sequestrant polymer to dispersant polymer being 20A. In all cases, this 55 ratio is on a weight:weight basis.

The invention is illustrated by the following experiments and Examples:

Experimental Various polymers and known boiler water treatment chemicals were evaluated in two series of testing programs. The first testing program involved the measurement of the chelation values of 60 2 3 GB 2 110 659 A 3 various polymers so as to determine by initial screening the potential of each polymer to function adequately, as a boiler transport material.

The experimental design used to test the chelation value of a series of polymers was as follows:

Solutions of calcium or magnesium ions were titrated with solutions of various polymeric and other sequestering agents. The residual unsequestered metal!on concentration (or, more correctly, 5 activity) was measured by means of a Specific Ion Electrode (henceforth S. I.E.). This data was ultimately converted to graphical representations of sequestrant performance. Sequestrant performance for Ca ion was measured by a calcium specific electrode, manufactured by Orion Research, Model 93-20. Sequestrant performance for Mg!on was measured by a Divalent cation electrode, Model 93-32, again manufactured by Orion Research. The electrode response is measured10 as the sequestrant solution is added incrementally to the hardness solution. The desired solution pH is automatically maintained by feeding potassium hydroxide solution from a Mettler DV1 0 which is controlled by a Mettler DK1 0/11 system.

A short period of time is allowed after each sequestrant addition before taking a reading so that the electrode can come to equilibriu m with the solution. Noise levels are typically 0.2 mV using 15 mechanical stirring (higher using a magnetic stirrer).

Prior to each titration, the S.I.E. was calibrated with standard solutions containing 1,000, 100, 10, and 1 ppm calcium or magnesium.

The S.I.E. responds to activity rather than concentration. For calcium measurements, a high, constant, ionic strength is maintained by addition of 6 g/I potassium chloride to all solutions (i.e., standards, sequestrant, and calcium sample). This maintains a constant activity coefficient for the calcium ion. The divalent sensing electrode used for magnesium measurements is subject to interference from both sodium and potassium ions at fairly low concentrations so no ionic strength buffer can be used in thii case.

Typical operating conditions were 2 or 3 g/I active polymeric sequestrant titrated against (in all 25 cases) 100 ml; of 100 ppm metal ion. Under these conditions, most titrations were essentially complete after the addition of 40-50 ml of sequestrant. Sequestrant solutions were usually added in 2 to 3 ml increments. Sequestrant was added slowly so as to avoid the formation of bubbles which could be trapped at the base of the electrodes and result in incorrect readings.

All measurements were made at room temperature (measurements above about 4011C will result 30 in rapid electrode deterioration).

The data from these experiments was graphically displayed or preferably converted to a usable form by a computer program which was written specifically for these experiments. The computer program obtains the best straight line fit through the origin using a reiterative, least-squares approach, allowing calculation of chelation value for each polymeric species.

For calcium measurements, a pH of 10 was used in most cases. Initial studies gave results indicating chelation greater than theoretical. At a pH of 9, the results were in good agreement with theory. The discrepancy at pH 10 may be due to a competing reaction (e.g., magnesium hydroxide formation). All magnesium measurements were made at a pH of 9 after this.

An attempt was made to correct magnesium results for the effects of any sodium present. 40 However, when corrections obtained from sodium chloride solutions were applied to an NTA. Na3. H,0 titration, the "corrected" results were very unreasonable (much greater than theoretical chelation). No further attempts to correct for sodium were made.

Chelation values were determined from the initial slope of the titration curve which plots percent metal ion sequestered versus grams of active polymer added. This calculation gives practical chelation 45 values in that no consideration has been given to which complexes are formed, the effects of competing equilibria, or various stability constants.

In the case of the polymers studied, a comparison of chelation values may be more valid thar, they might be in the case of strong complexing agents, such as EDTA or NTA, where a simple 50 comparison of chelation values is not necessarily a good guide to chelation performance.

No evidence of "threshold effects" was observed for any of the polymers tested. As later results will show, to be effective as a transport agent in boilers, the chelation value for the polymeric sequestrant preferably exceeds 200 and gives clear solutions for both calcium and magnesium ion test solutions. The most preferred average chelation value is above 300. Most of the polymers tested 55 appeared to sequester magnesium ion better than they were able to sequester calcium!on.

The data in Table I compares different sequestrants and the chelation values obtained using the above described test. As can be seen from this Table, the sequestrants tested include not only wellknown complexing agents such as EDTA and NTA but also polymeric sequestrants, as well as other sequestrants.

Some sequestrants on the list are not satisfactory as transport agents because of known thermal 60 degradation in a boiler system. Such agents include the phosphate containing compounds listed in Table 1.

Table 11 identifies each of the polymeric species tested.

Table Ill lists results for the polymeric sequestrants of this invention, as well as other more common sequestrants versus magnesium ion. T-fibse polymeric sequestrant agents which have 65 4 GB 2 110 659 A 4 chelation values above 200 and are thermally stable give excellent results in boiler transport. Of particular note in Table Ill is the result for citric acid. Although a very large chelation value is obtained, this material does not effectively transport magnesium or calcium hardness when tested in the boiler. It is believed that this is due to citric acid thermally decomposing when exposed to boiler operating conditions. The advantage of the low molecular weight polymeric carboxylate polymers used in this invention may well be that the thermal stability is obtained while maintaining sequestrant activity for hardness ions at proper dosages.

Of particular note in Table Ill is the fact that Polymer C, though giving a chelation value in excess of 200, yields a somewhat cloudy solution with magnesium at pH 9 and would not be expected to perform as well as the other carboxylate containing polymeric sequestrants of this invention. This problem might be solved by increasing the concentration of this polymer or by combining this polymer with other materials giving improved results. The polymeric materials that did not perform well with calcium were not tested for magnesium since both ions must be complexed before adequate boiler transport systems can be achieved.

Table 1.

Sequestrants vs. Ca',pH=10 Mole Product %Active M.W C.v. Ratio EDTA 100 292 347 1.01 NTA 100 191 541 1.03 20 Citric Acid 100 192 392 0.75 1,2,4-tricarboxy-2 phosphono-butane 50 256 610 1.56 amino-tri (methylene phosphonic) acid 50 299 559 1.67 25 diethylene triamine penta (methylene phosphonic) acid 50 573 701 4.01 hexa-potassium salt of hexamethylenediamine 30 tetra (methylene phosphonic) acid 23 492 282 1.39 1 -hydroxy ethylidene-1, 1 -di phosphonic acid 60 206 961 1.98 Sodium tri-poly phosphate 100 368 553 1.4 35 Polymer A 25.5 1000-5000 479 13.9 (ave 2300) Polymer B 50 2500-7500 374 19.1 (ave 5100) Polymer C 100 1000-2000 294 4.7 40 2 (ave 1600) Polymer D 25 2500-7500 386 18.9 (ave 4900) Polymer E 65 1000-3000 300 6.3 (ave 2100) 45 Polymer F 50 Not available 300 - Polymer G 22.75 Not available 291 - Polymer H 31.6 Not available 252 - Polymer 1 - 145 - Polymer J 105 - 50 Polymer K 19 Chelation values calculated on the basis of 100% active material for all cases.

Produced cloudy solutions.

GB 2 110 659 A 5 Table 11

Polymer Identification Polymer Chemical Designation mole wt.

Polymer A Polyacrylic Acid 1000-5000 (ave 2300) 5 Polymer B Polyacrylic Acid 2500-7500 (ave 5100) Polymer C Styrene-maleic anhydride 1: 1000-200 copolymer (ave 1600) 10. Polymer D Vinyl sulfonate-acrylic acid 2500-7500 10 1:3 copolymer (ave 4900) Polymer E Polyacrylic acid 1 OOP-3000 (ave 2100) Polymer F Polymaleic anhydride (est.) 700-3500 Polymer G Acrylic acid-acrylamide 4:1 not avail. 15 copolymer -below 50,000 Polymer H Acrylic acid-acrylamide 3:1 not avail.

copolymer -below 50,000 Polymer I Hydrolyzed polyacrylonitrile not avail.

-below 50,000 20 Polymer J Sodium salt of sulfonated co polymer of styrene and maleic not avail.

anhydride -below 50,000 Polymer K Acrylic acid-acrylatnide 1:3 copolymer (esft.) 5000-15,000 25 Ratios are monomer weight ratios Product Table Ill Sequestrants vs. Mgl-, pH=9 %Active M. W. C.V. Mole Ratio EDTA Acid 100 292 345 1.01 30 NTA Acid 100 191 461 0.88 Citric Acid 100 192 762 1.46 Polymer A 25.5 - 910 - Polymer B 50 2500-7500 691 35 (eve 5100) 35 Polymer C 100 1000-2000 281 4.5 (ave 1600) Polymer D 25 2500-7500 603 29.5 (eve 4900) Polymer E 65 1000-3000 527 9.5 40 (eve 2100) Polymer F 50 - 607 Polymer G 22.75 291 Polymer H 31.6 493 4All results expressed in terms of 100% active materials. C.V. and Mole Ratio expressed as CaC03 Produced cloudy solutions Examples

Some of the more promising carboxylate-containing polymers having chelation values above 200 were tried in experimental boiler water systems. The experimental boiler is described in the paper,---The50 investigation of Scaling and Corrosion Mechanisms Using Process Sirnulation," by J. A. Kelly, P. T.

Colombo, and G. W. Flasch, paper No. IWC-80-1 0, given at the 41 st Annual Meeting, International Water Conference, Pittsburgh, Pennsylvania, October 20-22, 1980.

Table IV indicates the formulations and sequestrant polymers chosen to be tested in the experimental boiler program. Included in these tests were a phosphorus- containing sequestrant, as 55 well as a water-soluble polymer which does not contain measurable amounts of carboxylate functionality.

6 GB 2 110 659 A 6 Table IV Test Ingredients for Experimental Boiler Work Example ComposItion Ingredients 1 1 20/1 active ratio Composition IVIV 2 11 Polymer H 3 Ill Polymer A 4 IV Polymer D v Polyacrylamide (M.W.=4000) 6 V1 Polymer E VII Diethlenetriaminepenta (methylene 10 phosphonic acid) Vill Ethylene dichloride-Ammonia copolymer (M.K-25000-60000) EDTA Ethylenediaminetetraacetic acid The experimental scale boiler Most of the experiments were conducted at 1,000 psig, 110,000 BtU/ft2 -hr heat flux, and 10 concentration cycles. The Composition I polymer was tested more extensively at 250, 600, and 1,500 psig. This laboratory boiler is of the type described in U.S. 3,296,027, to which reference is directed for more detail.

Feedwater was typically delonized water containing 1 ppm Ca, 0.5 ppm Mg, and 0.5 PPM S'02- 20 Sulfite residual was maintained at 25 5 ppm at 600 psig and 10 5 ppm at 1, 000 psig. Boiler water 0' alkalinity was maintained at 160-180 ppm. The pH of the polymers was adjusted to 9.

The experimental scale boiler results A. Dosage profiles Dosage Profiles of a number of polymers were obtained under three conditions. It is apparent in 25 Fig. 1 that:

1. the recommended dosage of Comp. I combination polymer for hardness control is about 5.3 ppm active polymers/ppm total hardness at 1,000 psig, and 2. at dosages below the recommended, the combination polymer preferentially transports Ca rather than Mg ions.

Figs. 1, 2, and 3 indicate that Comp. I as well as Comp. III and Comp. IV have threshold inhibition capability at low dosages for the Ca ions, and chelate hardness ions at high treatment levelssequestration may be a more appropriate terminology, but chelation appears to be the mechanism.

In general, all the tested acrylate-, acrylamide-, and vinyl sulphonatebased polymers give excellent hardness control at high dosages, as long as they contain sufficient carboxylate functionality. 35 Among those, Comp. 11 and IV are the most effective. Results are listed below.

Condition 1:

Boiler Pressure=1,000 psi, Heat Flux=1 10,000 BtU/ft2-hr, Ca=11, Mg=0.5, SiO,.=0.5 ppm in the feedwater.

1 c Treatment Trea tm en t ra tio % Ca recovery % Mg recovery 40 Comp. 1 0.53 16 trace 1.05 41 6 2.1 75 47 3.15 40 33 6.3 129 141 45 Comp. Ill 4.8 66 44 6.72 84 73 8.64 106 104 Treatment ratio is defined as ppm active polymer per ppm total hardness.

7 GB 2 110 659 A 7 Condition 2:

Boiler Pressure=l;000 psi, Heat flux=250,000 BtU/ft2 -hr Feedwater contained 1 ppm Ca' 0.5 ppm Mg, and 0.5 PPM Si02' Treatment Treatment ratio % Ca recovery % Mg recovery None 47 trace 5 Comp. 11 6.72 118 101 8.64 122 114 Comp. 1 4.2 90 78 5.25 96 89 6.3 107 107 10 7.25 109 117 8.09 107 112 9.03 104 109 Comp. Ill 8.64 102 105 9.6 99 106 15 Comp Ill 8.64 101 102 +1 ppm 9.6 102 108 Comp. VII Comp. V 8.64-9.6 127 118 Comp. V 8.64 82 120 20 Comp. IV 8.64 112 105 Condition 3:

Boiler Pressure=600 psi, Heat Flux=l 10,000 Btu/h r_ft2, Ca=l, Mg=0.5, S'020.5 ppm in the feedwater Treatment Treatment rptio % Ca recovery % Mg recovery - 25 Comp. IV 0.15 104 trace 0.3 87 trace 0.6 41 trace 1.2 94 8 2.4 87 41 30 4.8 120 lk Comp. 111 0.15 67 trace 0.3 55 trace 0.6 42 trace 1.2 73 -4 35 2.4 53 29 4.8 93 90 Comp. VI 0.15 64 trace 0.3 57 trace 0.6 42 trace 40 1.2 72 trace 2.4 67 32 4.8 104 86 Comp. Vill 0.3 53 trace 4.8 54 trace 45 25.0 114 16 B. Comp. 1 performance at 1,500 psig Higher recommended dosage is required for hardness control at higher boiler pressure. The increase in treatment level is probably due to the decomposition of polymer. 50 As the data below indicate, the dosage required for a complete hardness recovery increased at 50 1,500 psig. Corrosion rate, as measured by the iron content in the blowdown, did not increase. Ca=l, Mg=0.5, S'02==0.5 ppm in the feedwater. Pressure=1,500 psig, heat Flux=1 10,000 BtU/ft2 -hr Polymer Treatment ratio % Ca recovery % Mg recovery Comp. 1 15.75 - 89 105 55 21.0 99 108 26.25 99 107 8 GB 2 110 659 A 8 C. Comp. 1 performance at 250 psig The combination polymer had no problem controlling hardness at low pressure (250 psi) boiler applications.

Heat flux was 110,000 BtU/ffi -hr.

Condition 1:

Feedwater contained C=3, Mg=1.5, Na2S04=42.6, NaCl=1 0, S'02=5, Fe=l ppm, and enough NaHC03 to give an M alkalinity 40 in the feedwater or 400 in boiler water, S03=-30 ppm in the blowdown.

Recovery Treatment ratio % Ca % M9 % S102 % Fe % Na2S04 10 5.04 116 87 101 94 113 10.08 113 107 95 95 ill 20.16 115 122 92 96 109 Condition 2:

Feedwater contained 3 ppm Mg and all the other components listed in 1.

Recovery Trea tm en t ra tio % Ca % mg % Si02 % Fe % Na2S04 3.78 113 57 83 96 107 7.56 112 79 88 91 116 15.12 113 136 99 85 116 20 D. Effect and hardness and silica upsets on comp. 1 Comp. 1 treatment can recover from moderate hardness, silica, and treatment upsets.

Condition 1:

Pressure of the boiler--1,000 psig, Heat flux--l 10,000 Btu/hr-ft'. Comp. I treatment ratio 7.88 polymer/ppm hardness when initial hardness was 1 ppm Ca and 0.5 ppm Mg. Total polymer held 25 constant throughout the test as hardness was varied.

Feedwater Recovery Totalhardness Ca mg Si02 Fe by PpM PPM PPM % Ca % M9 % S102 PPM byAA titration 1 1.0 0.5 0.5 116 112 102 0.5 17.2 16.6 30, 2.0 1.0 1.0 70 66 62 0.4 20.7 19.5 1.0 0.5 1.0 111 102 97 0.3 16.2 - 5.0 2.5 2.5 39 32 45 0.3 27.2 26.5 1.0 0.5 0.5 116 128 134 0.1 18.1 - Condition 2:

Ca/M9/S'02 Swing Effect 1,000 psi and 250,000 BTu/h r_ft2 CaIMgISIO, ppm Recovery Polymer Treatment ratio in F. W. %Ca%Mg Comp. 11 8.64 1/0.5/0.5 122 114 40 6.72 1/0.5/0.5 118 101 6.72 0.5/l/0.5 146.87 6.72 0.5/l/2.0 147 82 Comp. IV 8.64 1.0/0.5/0.5 132 107 8.46 0.5/l/0.5 109 95 45 8.64 0.5/l/2.0 94 81 17.28 2/l/l 91 84 9 GB 2 110 659 A 9 Condition 3-for comp. 1: ft2. Feedwater contained 1 ppm Ca, 0.5 ppm M9, and 2.5 ppm S'02, 1,000 psi and 110,000 Btu/hr- Treatment ratio % Ca recovery % Mg recovery - % S102 recovery 7.88 100 97 103 5 15.75 106 149 102 E. Scale removal using comp. 1 Scale removal using Comp. 1 appears feasible if hardness and silica can be discharged by the blowdown. Adequate Comp. 1 treatment can transport boiler deposits in addition to the hardness in the feedwater. It enhanced passivation of the boiler heat transfer surface and formed a black, magnetite 10 film.

Condition 1:

Feedwater contained Ca=l, Mg=0.5, and S'02=0.5 PPM Pressure=1,000 psig, Heat flux=l 10,000 Btu/hr_ft2 Recovery To&tal hardness -15 Fa by Treatment ratio % Ca % Mg % SIO,. PPM byAA titration 3.93 88 71 112 trace 12.7 12.8 7.88 140 183. 104 trace 21.2 17.8 15.75 168 218 156 0.6 27.7 24.6 20.

Condition 2:

Feedwater contained no hardness and no silica, a badly fouled boiler.

Pressure=1,000 psig, Heat f lux-1 10,000 Btu/hr_ft2.

Recovery Treatment ratio (assume hard- Ca S102 Fe P04 ness= 1 PPO PPM PPM PPM PPM PPM byAA Total hardness by titration 23.63 10.2 16.4 6.5 0.7 10.7 23.4 23.4 47.25 18.1 15.9 6.8 0.8 12.2 35.9 24.1 70.88 24.1 21.9 8.1 1.6 16.1 46.7 44.3 30 Condition 3:

Feedwater contained no hardness and no silica and the boiler was relatively clean.

Pressure=1,000 psig, Heat flux--1 10,000 Btu/hr-ft2 Treatment=23.63 ppm Comp. 1 per ppm total hardness, assuming total hardness=1 ppm.

Recovery -35 No. of days Ca ppm Mg ppm S102 PPM Fa ppm 0 3.6 5.0 11.5 5.8 1.8 1.7 2.9 1.3 6 1.7 1.4 1.5 1.4 7 2.1 1.4 1.1 0.9 40 8 2.2 1.3 0.5 1.2 F. The effect of heat flux on comp. I performance Heat flux in the range of 110,000 to 250,000 Btu/hr-ft' had little influence on hardness recovery. At heat fluxes greater than 300,000 Btu/h r_ft2 there was a thin film of deposition on the heat transfer surface. Test Condition: 1,000 psi Feedwater contained 1 ppm Ca, 0.5 ppm Mg, and 0.

5 PPM S'02 Recovery Heat flux Treatment Brulhr-ft2 ratio % Ca % Mg % S102 50 100,000 9.03 112 140 117 300,000 9.03 121 171 122 GB 2 110 659 A 10 G. Performance of other combination polymers Con.p. 111/Comp. IV combination polymer, although less effective than Comp. 1, gave reasonable hardness control at 1,000 psig. This combination polymer could be used in NH3 sensitive applications.

Conditions: 5 Ccll, Mg05, Si020,5 ppm in the feedwater. Pressure=1,000 psig, Heat Flux=l 10,000 BtU/ffi Treatment ratio % Ca recovery % Mg recovery 7.88 86 91 10.5 99 ill 13.12 102 108 10 Experimental boiler results The combination polymer Comp. I initially added to the boiler at a dosage of 7.9 pprn active per ppm total hardness. The dosage was maintained for eight days. The average calcium and magnesium recoveries were 118% and 101 %, respectively. Initial hydrogen level was 11 ppb but dropped to 1.5 ppb the same day. It leveled off to 0.4 ppb. Hydrogen values of 1.0-1.2 ppb are equivalent to 15 background levels. The high initial rise of hydrogen frequently occurs in a boiler just brought on line. In addition, sulfite residuals for the first few days were lower than desired and contributed to hydrogen generation. Iron in the blowdown started off high at 2-3 ppm and declined to 1.1 ppm after eight days. The condensate frequently had a pH greater than 9, and contained small amounts of ammonia.

At this point the polymer dosage was decreased to 3.9 and the test was continued at this condition for six days. This treatment level is less than 2/3 the recommended. The average calcium and magnesium recoveries for this period were 96% and 81 %, respectively. It was anticipated from the experimental scale boiler results that hardness recoveries would decline and that magnesium would be more affected than calcium. Hydrogen dropped to 0.3 ppb and iron decreased to 0.3 ppm. The temperature of the high heat flux area remained constant, indicating no scaling.

At this point, the polymer dosage was further reduced to 2.6 and held at this condition for three days. Calcium and magnesium recoveries declined to 89% and 78%, respectively, while hydrogen and iron levels were fractionally lower. During low level treatment, the temperature in the horizontal test section increased 30OF which indicated that scale was being deposited.

The polymer dosage was then restored to the original level of 7.9 for fifteen days. Within the first 30 day, the temperature of the horizontal test section dropped 300F. As anticipated, calcium and magnesium recoveries increased dramatically and averaged 122% and 111 %, respectively. These high recovery values suggest that the polymer treatment program is removing deposits previously laid down when under-treating. Similarly, it is postulated that at the start of this test, the 118% calcium recovery was due to removal of boiler deposits that remained in the system from the previous test. Hydrogen 35 and iron remained at relatively low levels.

Figs. 4, 5, and 6 depict percent hardness recovery, H2 level, and iron level in the blowdown as a function of days of test. Polymer treatment was then stopped for 7 days. Only trace amounts of calcium and magnesium were recovered during this period, while hydrogen and iron remained virtually unchanged.

During the remaining period of the test, polymer dosage at 7.9 was alternated with no treatment.

In the presence of polymer, recoveries of calcium and magnesium averaged 115% and 106%, respectively.

A

Claims

Claims 45 1. A method of treating boiler water which contacts heat

transfer surfaces, to prevent or remove 45 scale due to water hardness, the method comprising treating the boiler water with water-soluble anionic vinyl polymer which contains at least 30% by weight of carboxylate functionality and has an average molecular weight of 500 to 50,000.
2. A method according to claim 1 wherein the amount of such polymer used is 1 to 30 ppm per pprn hardness present in the boiler water.
3. A method according to claim 2 where the said amount is 3 to 10 ppm per ppm hardness present in the boiler water.
4. A method according to claim 1 or 2 or 3 which uses such polymer having a chelation value of at least 200.
5. A method according to claim 4 wherein the said chelation is of at least 300.
6. A method according to any preceding claim which uses such polymer having a molecular weight of 1,000 to 30,000.
7. A method according to any preceding claim wherein the water-soluble anionic vinyl polymer is selected from hydrolyzed polyacrylamides, copolymers of acrylic acid and vinyl sulphonate, and homopolymers of acrylic acid and methacrylic acid.

i 9 1 I.

11 GB 2 110 659 A 11
8. A method according to any preceding claim wherein the boiler water is also treated with dispersant selected from other anionic water-soluble vinyl polymers.
9. A method according to claim 8 wherein the average molecular weight of the dispersant anionic water-soluble vinyl polymer is 500 to 50,000.
10. A method according to claim 8 or 9 wherein the dispersant polymer is chosen from carboxylate containing water-soluble vinyl polymers, vinyl sulphonate- acrylic acid copolymers, vinyl sulphonate-methacrylic acid copolymers, sulphonated styrene-maleic anhydride copolymers, and acrylamide-acrylic acid copolymers.
11. A method according to claims 9 and 10 wherein the dispersant polymer is chosen from water-soluble acrylic acid-vinyl sulphonate copolymers of average molecular weight of 900 to 15,000 10 and acrylic acid polymers of average molecular weight of 2000 to 4000.
12. A method according to claim 11 wherein a water-soluble acrylic acidvinyl sulphonate copolymer dispersant is used having a a 5-25 mole percent vinyl sulphonate content, a 95-75 mole percent acrylic acid content, and an average molecular weight of 1000 to 6000.
13. A method according to claim 11 wherein a water-soluble acrylic acid polymer dispersant is 15 used having an average molecular weight of 2000 to 4000.
14. A method of treating hardness in boiler water, the method being substantially as hereinbefore described in any one of Examples 1 to 6.

Printed for Her Majesty's Stationery Office by the Courier Press, Leamington Spa, 1983. Published by the Patent Office, 25 Southampton Buildings, London, WC2A lAY, from which copies may be obtained