Method for controlling scale formation and deposition in aqueous systems

This application is a continuation-in-part of application Ser. No. 09/808,679 filed on Mar. 15, 2001 now U.S. Pat. No. 6,444,747.

The present invention relates to methods of inhibiting corrosion and controlling the formation and deposition of scale imparting compounds in aqueous systems such as cooling, boiler and gas scrubbing systems. The novel polymeric compositions which are useful in accordance with the present invention comprise water-soluble or water-dispersible copolymers of ethylenically unsaturated monomers with sulfate, phosphate, phosphite or carboxylic terminated polyalkylene oxide allyl ethers.

The problems of corrosion and scale formation and the attendant effects have troubled water systems for years. For instance, scale tends to accumulate on internal walls of various water systems, such as boiler and cooling systems, and thereby materially lessen the operational efficiency of the system.

Deposits in lines, heat exchange equipment, etc., may originate from several causes. For example, precipitation of calcium carbonate, calcium sulfate and calcium phosphate in the water system leads to an accumulation of these scale-imparting compounds along or around the metals' surfaces which water circulating through the system. In this manner, heat transfer functions of the particular system are severely impeded.

Corrosion, on the other hand, is a degradative electrochemical reaction of a metal with its environment. Simply stated, it is the reversion of refined metals to their natural state. For example, iron ore is iron oxide. Iron ore is refined into steel. When steel corrodes, it forms iron oxide which, if unattended, may result in failure or destruction of the metal, causing the particular water system to shut down until the necessary repairs can be made.

Typically, in cooling water systems, the formation of calcium sulfate, calcium phosphate and calcium carbonate, among others, has proven deleterious to the overall efficiency of the cooling water system. Recently, due to the popularity of cooling treatments using high levels of orthophosphate to promote passivation of the metal surfaces in contact with the system water, it has become critically important to control calcium phosphate crystallization so that relatively high levels of orthophosphate may be maintained in the system to achieve the desired passivation without resulting in fouling or impeded heat transfer functions which would normally be caused by calcium phosphate deposition.

Silica (SiO 2 ) is present in most natural waters. When these waters are cycled in a cooling tower, the silica level increases and often a level is reached where precipitation of a silica species occurs. Sometimes the precipitation proceeds by the polymerization of silica itself, resulting in a silica gel. For this to occur, a relatively high SiO 2 concentration is required, usually greater than approximately 200 ppm. However, when certain cations are present, silica species can precipitate at much lower concentrations. Cations that promote silica precipitation include, but are not limited to, Al 3+ , Mg 2+ , Zn 2+ and Fe 3+ . Aluminum is very insoluble in water and readily precipitates under cooling water conditions. When aluminum gets into a cooling system (such as by carryover) it can cause serious precipitation problems. One such problem is the precipitation of phosphate species which may be present as a corrosion inhibitor. Such precipitates can be problematic due to both deposition and corrosion effects.

Although steam generating systems are somewhat different from cooling systems, they share a common problem in regard to deposit formation.

As detailed in the Betz Handbook of Industrial Water Conditioning, 9 th Edition, 1991, Betz Laboratories Inc., Trevose, Pa., Pages 96-104, the formation of scale and sludge deposits on boiler heating surfaces is a serious problem encountered in steam generation. Although current industrial steam producing systems make use of sophisticated external treatments of the boiler feedwater, e.g., coagulation, filtration, softening of water prior to its feed into the boiler system, these operations are only moderately effective. In all cases, external treatment does not in itself provide adequate treatment since muds, sludge and hardness-imparting ions escape the treatment, and eventually are introduced into the steam generating system.

In addition to the problems caused by mud, sludge or silt, the industry has also had to contend with boiler scale. Although external treatment is utilized specifically in an attempt to remove calcium and magnesium from the feedwater, scale formation due to residual hardness, i.e., calcium and magnesium salts, is always experienced. Accordingly, internal treatment, i.e., treatment of the water fed to the system, is necessary to prevent, reduce and/or retard formation of scale imparting compounds and their resultant deposition. In addition to carbonates of magnesium and calcium being a problem as regards scale, having high concentrations of phosphate, sulfate and silicate ions either occurring naturally or added for other purposes cause problems since calcium and magnesium, and any iron or copper present, react and deposit as boiler scale. As is obvious, the deposition of scale on the structural parts of a steam generating system causes poorer circulation and lower heat transfer capacity, resulting in an overall loss in efficiency.

U.S. Pat. No. 4,471,100 to Tsubakimoto et al. discloses a copolymer consisting of maleic acid and polyalkyleneglycol monoallyl ether repeat units useful as a dispersant for cement and paint and as a scale inhibitor for calcium carbonate.

U.S. Pat. Nos. 5,180,498; 5,292,379; and 5,391,238 to Chen et al., disclose copolymers of acrylic acid and polyethyleneglycol allyl ether for boiler water treatment and metal pretreating applications.

U.S. Pat. No. 5,362,324 describes terpolymers of (meth)acrylic acid and polyethyleneglycol-monomethylether-(meth)acrylate and polypropyleneglycol di(meth)acrylate for superplasticizer applications. U.S. Pat. No. 5,661,206 and EP448717 disclose similar technology but using diepoxy based compounds as crosslinking agents. Japanese Patents 93660, 226757 and 212152 disclose acrylic acid terpolymers with sodium methallylsulfonate and methoxy polyethylene glycol-monomethacrylate for superplasticizer applications.

U.S. Pat. No. 5,575,920 to Freese et al. discloses terpolymers of acrylic acid, allyloxy-2-hydroxypropylsulfonic ester (AHPS) and polyethyleneglycol allyl ether for cooling water treatment as calcium phosphate inhibitors.

U.S. Pat. No. 3,875,202 to Steckler discloses polymerizable ammonium and alkali metal salts of sulfated monoethylenically unsaturated alcohols of from 3 to 12 carbon atoms and of the alkenoxylated adducts of such alcohols. The polymerizable monomers are useful as co-polymerizable surfactants for self-stabilizing latexes and as comonomers in the copolymerization with other monomers in the preparation of co- or ter-polymeric films and fibers, especially as receptors for basic dyes and to build in anti-static properties. Monomers such as vinyl chloride, ethyl acrylate, 2-ethylhexyl acrylate, vinyl acetate and N-methyl acrylamide are disclosed in the patent to be copolymerizable with the ammonium salt of sulfated monoethylenically unsaturated alcohols. The copolymers disclosed are not water-soluble.

U.S. Pat. No. 5,705,665 to Ichinohe et al. relates to organic silicon compounds having as one of the components ethoxylated allyl alcohol with alkali metal salt of sulfonate group in the molecule. The resulting compound is useful as a surface treating agent and modifier for inorganic material. The copolymers disclosed are not water-soluble or dispersible.

In accordance with the present invention, it has been surprisingly discovered that water-soluble or water dispersible polymers, which contain pendant functional groups are effective in controlling the formation and deposition of mineral deposits and in inhibiting corrosion in various aqueous systems. The novel polymers useful in the present invention are copolymers or terpolymers having the structure of Formula I.

Wherein E is the repeat unit remaining after polymerization of an ethylenically unsaturated compound; preferably, a carboxylic acid, sulfonic acid, phosphonic acid, or amide form thereof or mixtures thereof. R 1 is H or lower (C 1 -C 4 )alkyl. G is —CH 2 — or —CHCH 3 —; R 2 is CH 2 —CH 2 —O n or CH 2 —CHCH 3 —O n where n ranges from about 1 to 100, preferably about 1 to 20. X is an anionic radical selected from the group consisting of SO 3 , PO 3 , or COO; Z is H or hydrogens or any water soluble cationic moiety which counterbalances the valence of the anionic radical X, including but not limited to Na, K, Ca, or NH 4 . F, when present, is a repeat unit having the structure of Formula II.

In Formula II, X and Z are the same as in Formula I. R 4 is H or lower (C 1 -C 4 )alkyl. R 5 is hydroxy substituted alkyl or alkylene having from about 1 to 6 carbon atoms.

With respect to E of Formula I, it may comprise the repeat unit obtained after polymerization of a carboxylic acid, sulfonic acid, phosphonic acid, or amide form thereof or mixtures thereof. Exemplary compounds include but are not limited to the repeat unit remaining after polymerization of acrylic acid, methacrylic acid, acrylamide, methacrylamide, N-methyl acrylamide, N, N-dimethyl acrylamide, N-isopropylacrylamide, maleic acid or anhydride, fumaric acid, itaconic acid, styrene sulfonic acid, vinyl sulfonic acid, isopropenyl phosphonic acid, vinyl phosphonic acid, vinylidene di-phosphonic acid, 2-acrylamido-2-methylpropane sulfonic acid and the like and mixtures thereof. Water-soluble salt forms of these acids are also within the purview of the present invention. More than one type of monomer unit E may be present in the polymer of the present invention.

Subscripts c, d, and e in Formula I are the molar ratio of the monomer repeating unit. The ratio is not critical to the present invention providing that the resulting copolymer is water-soluble or water-dispersible. Subscripts c and d are positive integers while subscript e is a non-negative integer. That is, c and d are integers of 1 or more while e can be 0, 1, 2 . . . etc.

A preferred copolymer of the present invention, that is where e=0, is acrylic acid/polyethyleneglycol monoallyl ether sulfate of the structure:

Wherein n ranges from about 1 to 100, preferably about 1 to 20. Z is hydrogen or a water-soluble cation such as Na, K, Ca or NH 4 .

Molar ratio c:d ranges from 30:1 to 1:20. Preferably, the molar ratio of c:d ranges from about 15:1 to 1:10. The ratio of c to d is not critical to the present invention providing that the resulting polymer is water-soluble or water-dispersible.

A preferred terpolymer of the present invention, that is where e is a positive integer, is acrylic acid/polyethyleneglycol monoallyl ether sulfate/1-allyloxy-2-hydroxypropylsulfonic acid of the structure.

Wherein n ranges from about 1-100, preferably about 1-20. Z is hydrogen or a water-soluble cation such as, Na, K, Ca or NH 4 . Z may be the same or different in c, d and e. The mole ratio of c:d:e is not critical so long as the terpolymer is water-soluble or water-dispersible. Preferably the mole ratio c:d:e ranges from about 20:10:1 to 1:1:20.

The polymerization of the copolymer and/or terpolymer of the present invention may proceed in accordance with solution, emulsion, micelle or dispersion polymerization techniques. Conventional polymerization initiators such as persulfates, peroxides, and azo type initiators may be used. Polymerization may also be initiated by radiation or ultraviolet mechanisms. Chain transfer agents such as alcohols, preferably isopropanol or allyl alcohol, amines or mercapto compounds may be used to regulate the molecular weight of the polymer. Branching agents such as methylene bisacrylamide, or polyethylene glycol diacrylate and other multifunctional crosslinking agents may be added. The resulting polymer may be isolated by precipitation or other well-known techniques. If polymerization is in an aqueous solution, the polymer may simply be used in the aqueous solution form.

The molecular weight of the water-soluble copolymer of Formula I is not critical but preferably falls within the range Mw of about 1,000 to 1,000,000. More preferably from about 1,000 to 50,000 and most preferably from about 1,500 to 25,000. The essential criteria is that the polymer be water-soluble or water-dispersible.

Use of the Polymers

The polymers of the invention are effective for water treatment in cooling water, boiler and steam generating systems as deposit control and/or corrosion inhibition agents. The appropriate treatment concentration will vary depending upon the particular system for which treatment is desired and will be influenced by factors such as the area subjected to corrosion, pH, temperature, water quantity and the respective concentrations in the water of the potential scale and deposit forming species. For the most part, the polymers of the present invention will be effective when used at levels of from about 0.1-500 parts per million parts of water, and preferably from 1 about to 100 parts per million of water contained in the aqueous system to be treated. The polymers may be added directly into the desired water system in an aqueous solution, continuously or intermittently.

The polymers of the present invention are not limited to use in any specific category of aqueous system. They would be expected to inhibit the formation and deposition of scale forming salts in any aqueous system prone to that problem. For instance, in addition to boiler and cooling water systems, the polymers may also be effectively utilized in scrubber systems and the like wherein corrosion and/or the formation and deposition of scale forming salts is a problem. Other possible environments in which the polymers of the present invention may be used include heat distribution type seawater desalting apparatus, dust collection systems in iron and steel manufacturing industries, mining operations and geothermal systems.

The water-soluble or dispersible polymers of the present invention may be used in combination with topping agents in order to enhance the corrosion inhibition and scale controlling properties thereof. For instance, the polymers of the present invention may be used in combination with one or more compounds selected from the group consisting of inorganic phosphoric acids or salts thereof, phosphonic acid salts, organic phosphoric acid esters, and polyvalent metal salts or mixtures thereof. Such topping agents may be added to the system being treated in an amount of from about 1 to 500 ppm.

Examples of inorganic phosphoric acids include condensed phosphoric acids and water-soluble salts thereof. Examples of phosphoric acids include orthophosphoric acids, primary phosphoric acids and secondary phosphoric acids and salts thereof. Examples of inorganic condensed phosphoric acids include polyphosphoric acids such as pyrophosphoric acid, tripolyphosphoric acid and the like, metaphosphoric acids such as trimetaphosphoric acid and tetrametaphosphoric acid and salts thereof.

Examples of other phosphoric acid derivatives, which can be combined with the polymers of the present invention include aminopolyphosphonic acids such as aminotrimethylene phosphonic acid, ethylene diaminotetramethylene phosphonic acid and the like, methylene diphosphonic acid, hydroxyethylidene diphosphonic acid, 2-phosphonobutane 1,2,4, tricarboxylic acid, etc. and salts thereof.

Exemplary organic phosphoric acid esters which may be combined with the polymers of the present invention include phosphoric acid esters of alkyl alcohols such as methyl phosphoric acid ester, ethyl phosphoric acid ester, etc., phosphoric acid esters of methyl cellosolve and ethyl cellosolve, and phosphoric acid esters of polyoxyalkylated polyhydroxy compounds obtained by adding ethylene oxide to polyhydroxy compounds such as glycerol, mannitol, sorbitol, etc. Other suitable organic phosphoric esters are the phosphoric acid esters of amino alcohols such as mono, di, and tri-ethanol amines. The water-soluble polymers may also be used in conjunction with molybdates such as, sodium molybdate, potassium molybdate, lithium molybdate, ammonium molybdate, etc.

The polymers of the present invention may be used in combination with yet other topping agents including corrosion inhibitors for iron, steel, copper, and copper alloys or other metals, conventional scale and contamination inhibitors, metal ion sequestering agents, and other conventional water treating agents. Examples of other corrosion inhibitors include tungstate, nitrites, borates, silicates, oxycarboxylic acids, amino acids, catechols, aliphatic amino surface active agents, benzotriazole, halogenated triazoles and mercaptobenzothiazole. Other scale and contamination inhibitors include lignin derivatives, tannic acids, starches, polyacrylic acids and their copolymers including but not limited to acrylic acid/2-acrylamido-2-methylpropanesulfonic acid copolymers and acrylic acid/allyloxy-2-hydroxypropane-3-sulfonic acid copolymers, maleic acids and their copolymers, polyepoxysuccinic acids and polyacrylamides, etc. Examples of metal ion sequestering agents include polyamines, such as ethylene diamine, diethylene triamine and the like and polyamino carboxylic acids, such as nitrilo triacetic acid, ethylene diamine tetraacetic acid, and diethylenetriamine pentaacetic acid.

U.S. Pat. Nos. 4,659,481; 4,717,499; 4,759,851; 4,913,822; and 4,872,995 disclose the use of specific copolymers in treating cooling, boiler, steam generating and other aqueous heat transfer systems to inhibit deposition of scales such as calcium phosphate, calcium phosphonate, calcium oxalate, iron oxide, zinc oxide and silica. Based upon the deposit control efficacy exhibited by the polymers of the present invention, it is believed that they could be substituted for the polymers disclosed in the above and other similar patents to provide improved performance in a wide variety of water based treatment applications.

The copolymers of the present invention can be used alone or in combination with conventional cleaning agents such as surfactants, chelating agents, citric acid, phosphoric acid and other common reagents to remove deposit and prevent fouling on membranes used in the micro filtration, ultra filtration and reverse osmosis applications.

The present invention will now be further described with reference to a number of specific examples which are to be regarded solely as illustrative and not as restricting the scope of the present invention.