Method for controlling deposits and corrosion in water treatment applications

Abstract: The invention concerns a method for treating water to prevent corrosion and or scaling, characterized in that use is made of carbondioxide as pH regulation, together with one or more of the following components : a) one or more water soluble polymers b) one or more water soluble phosphonates c) one or more chemicals capable of preventing corrosion and/or deposits.

Description: The most important scale forming reaction in water treatment applications is the calciumcarbonate formation by the well known reaction : Ca²⁺ + CO₃²⁻ → CaCO₃ ↓ (1) Carbonate can be formed in the water, such as cooling water, under the influence of heat by : 2 HCO₃ CO₃²⁻ + CO₂ ↑ + H₂O (2) If carbondioxide can escape from the water the equilibrium reaction (2) is shifted to the right, with the result that carbonate ions are generated. The most common practice to avoid excessive carbonate formation is the dosage of strong mineral acids such as hydrogen sulfate or hydrogen chloride. Still another method in use today is to not control pH at all : calciumcarbonate formation is then controlled by the addition of calciumcarbonate scale inhibition formulations with high performance. In cooling water applictions this method is becoming very popular today, through the development of all organic formulations. Both methods have their inconveniences : dosage of strong mineral acids makes the water very corrosive because bicarbonate ions are eliminated from the water and replaced by sulfate and/or chloride ions. To control the corrosion, water treatment companies have developed a lot of corrosion inhibitors: formulations which contain one or more of the following chemicals: chromate, nitrite, silicate, phosphorus containing chemicals, zinc, molybdate, polymers. These formulations may contain besides these chemicals other products as topping agents such as surfactants, iron complexants etc... Water treatment without pH regulation has the advantage that the water is less corrosive than in the former case. As a consequence quence lower amounts of corrosion inhibitors or less toxic chemicals can be used. The disadvantage of not using a pH regulation however is that cycles of concentration are most of the time rather limited: today a Ryznar Stability Index of not lower than about 3 is achievable. Another, but not widely used method, is the injection of carbon dioxide in the water, whereby equilibrium (2) is shifted to the left. Dosage of carbondioxide is a less common practice: an extensive description of this practice for cooling water applications is given by E. Wahnschaffe (VGB Speisewassertagung, p. 21-27 (1970)). To suppress calciumcarbonate formation in an effective way, pH must be regulated well below 8.3. In that case carbondioxide consumption is excessive and the corrosion tendency of the water is becoming a serious problem. FR-A-2509714 describes a method of treating an aqueous system for reducing scale formation by adding to the system an antiscaling agent and carbon dioxide at pH between 8.7 and 9.3 and maintaining the P-alkalinity between 20 and 90 ppm. We now surprisingly found, according to the present invention, that regulating the pH of the system to a value between 7 and 9 using carbon dioxide and maintaining the TAC value (m-alkalinity) at 500 to 1000 ppm expressed as calcium carbonate, combined with one or more other water treatment chemicals gives better overall performance of water treatment programs than in the case where the same water treatment chemicals are used together with strong mineral acid pH regulation or no pH regulation : corrosion and scaling can be eliminated almost completely. Lower dosages of toxic water treatment chemicals are needed than in the classical treatment modes; moreover most of the time waters with higher salt contents can be handled without problems or in the case of cooling water treatment higher cycles of concentration are possible. So blowdown can be lowered and less chemicals are needed, which saves money but also a smaller amount of water treatment chemicals is blown down into the environment. The water treatment chemicals which can be combined with carbondioxide include (a) water soluble polymers, (b) phosphonates and (c) other water treatment chemicals which can be used as scale and or corrosion inhibitors. Our invention can be used in a variety of different water treatment applications : cooling water treatment, process water treatment and waste water treatment for example. Besides corrosion and or scale inhibitors, biocides, antifoaming agents, biodispersants and, tensioactive molecules, for example, may also be used in this technique. The carbondioxide can be supplied to the water in any form, whether it is a gas, solid or liquid. The carbondioxide can also be supplied from a wide range of sources : combustion gases or, carbondioxide in containers for example. The gas can also be generated through decomposition of carbondioxide releasing chemicals or by the action of mineral or organic acids on carbondioxide releasing chemicals such as calciumcarbonate. If the carbondioxide is not clean enough, it can be purified by filters, or in the case of combustion gases from heavy fuel oil, freed from SO₃ through condensation. The SO₂ present in combustion gases of sulfur containing fuels can be used as biocide. The carbondioxide pH regulation may also be combined with classical pH regulation with mineral acids such as sulfuric or hydrochloric acid, or organic acids. The strong mineral acid can be formed in situ for example if SO₃ resulting from combustion of sulfuur containing fuels is led into the water together with the carbondioxide. The carbondioxide may be mixed with the water by several means : it can be blown into the water by a ventilator : the carbondioxide can be dosed directly in the water or mixed with the air entering the cooling tower. Another very effective method consists in mixing the carbondioxide with water by means of a venturi system, if desired mixed in two successive steps. Any other mixing technique however may be used. The water soluble polymers may be of any kind. The only prerequisite is that they have calciumcarbonate stabilizing and or anticorrosive properties, alone or in combination with other chemicals. They may be prepared from anionic, non ionic, cationic or betaine containing monomers or mixtures thereof. Examples of anionic monomers are : acrylic acid, methacrylic acid, maleic acid, itaconic acid, 3-acrylamido-3-methyl-butanoic acid, sulfonate containing monomers such as vinylsulfonic acid, methallysulfonic acid, 2-acrylamido-2-methylpropanesulfonic acid, allylhydroxypropylsulfonate ether, styrenesulfonic acid, sulphate containing monomers such as the sulphate ester of hydroxypropyl (meth)acrylate, phosphonate containing monomers such as 2-acrylamido-2-methylpropanephosphonic acid and isoproprenylphosphonic acid, phosphate containing monomers such as allyl-2-hydroxypropylphosphate ether. Examples of non ionic monomers are acrylamide, t-butylacrylamide, acrylonitrile, maleic anhydride, vinylacetate,vinylalcohol, allylalcohol, esters of acrylic and methacrylic acid such as ethylacrylate and methylmethacrylate, hydroxyethylacrylate, 2-hydroxypropylacrylate, hydroxyethylmethacrylate, allylamine, vinylamine, polyalkylene glycol (meth)acrylate. Betaine containing monomers such as N-methacroyloxyethy-N-(3-sulfopropyl)-N,N-dimethylammonium betaine, N-methacryloxyethyl-N-(3-phosphonopropyl)-N, N-dimethylammonium betaine, N-methacroyloxyethy-N- (3-sulfatopropyl)-N,N-dimethylammonium betaine, N-methacroyloxyethy-N-(3-phosphatopropyl)-N, N-dimethylammonium betaine. Examples of cationic monomers are dimethyl diallyl ammoniumchloride, diethyldiallyl ammonium chloride, methacryloyloxyethyl trimethyl ammonium chloride. European patent application 0 181 151 gives an extensive list of which can be used cationic polymers. Besides the above mentioned monomers the polymers may also contain other groups such as phosphonate, sulfonate or sulfate end groups or phosphino groups. Mixtures of different polymers are also included in the invention. The phosphonates may also be of any kind as long as they have antiprecipitant and or anticorrosive properties. The most well known phosphonates are nitrilotrimethylenephosphonic acid, hydroxyethylidenediphosphonic acid, phosphonobutanetricarboxylic acid and 2,2-hydroxyphosphono-acetic acid. Mixtures of different phosphonates can also be employed in the invention. European patent application 0 093 508 further gives an extensive list of suitable phosphonates. The corrosion and scale inhibitors mentioned under (c) can be of different chemical nature. The most well known corrosion and or scale inhibitors included in (c) are : zinc salts cerous and or ceric salts phosphorus containing inorganic chemicals such as phosphates, phosphates, polyphosphates, orthophosphorous acid or its salts. silicates chromates molybdates lignosulfonates tannins nitrites organic carboxylic acids such as cycloaliphatic carboxylic acids, aliphatic carboxylic acids, aromatic or heterocyclic carboxylic acids polymeric diols such as polyalkyleneglycol, copolymers of ethyleneoxide and propyleneoxide, ethoxylated and or propoxylated organic molecules. iron complexants such as citric acid, gluconic acid, glucoheptonic acid, aminocarboxylic acids such as disodiumethanol diglycine, EDTA. copper corrosion inhibitors such as benzotriazole, tolyltriazole, butylbenzotriazole, 2-mercaptobenzothiazole. polyhydroxy compounds such as mannitol, sorbitol, phosphate esters such as polyol phosphate esters, alkyl acid phosphates, ethoxylated alkyl acid phosphates. amines such as fatty amines and or neutralizing amines The list of the above mentioned products serves just as an illustration and should not in any way be interpreted as a limitation of the invention. The method of this invention can also be combined with other supplementary techniques to save water, such as reverse osmosis, side stream softening, make-up softening. The invention will be further illustrated by a few Examples. The Examples can be found in table 1 and 2. The abreviations used in these tables are given below : Legend for table 1 and 2 NTMPA : Nitrilotrimethylenephosphonic acid PBTC : Phosphonobutanetricarboxylic acid HEMA-S-AA : copolymer sulphate ester of hydroxyethylmethacrylate (40 %)-acrylic acid (60 %) o-PO₄ : orthophosphate AA-AMPS : copolymer 2- acrylamido-2-methylpropanesulphonic acid (40 %)-acrylic acid (60 %) Belclene 283 : acrylate copolymer marketed by Ciba-Geigy HPA : 2,2-hydroxy-phosphono-acetic acid TTZ : Tolyltriazole Versa-TL3 : copolymer styrenesulphonate-maleic acid marketed by National Starch PE-6200 : ethylene oxide-propyleneoxide copolymer marketed by BASF AA-HPA : copolymer acrylic acid (75 %)-hydroxypropylacrylate (25 %) AA-AHPSE : copolymer acrylic acid (62 %)-allylhydroxypropylsulfonate ether (38 %) A simulated process water containing 1000 ppm calcium as calciumcarbonate and 1000 ppm TAC expressed as calcium-carbonate was followed as a function of time at different pH. If the pH had to be adjusted it was always done with carbondioxide : this is the case, where the pH of the solution is lower than 9. These tests were also performed in the presence of water treatment antiscalants. Tests were run at a water temperature of 60° C. The results are brought together in table 1. This table contains also data obtained without pH correction in the absence and presence of antiprecipitants. TAC and TCa were created in the water by mixing a calciumchloride containing solution together with a sodiumbicarbonate-carbonate mixture. From these results it can be seen that in the absence of calciumcarbonate stabilizing chemicals a pH regulation with carbondioxide is not very effective. On the other hand combined with polymers and or phosphonates as antiprecipitants very good calciumcarbonate stabilization results. This is in contrast with no pH regulation where lower TCa and TAC values already give calciumcarbonate precipitation at higher RSI values, even in the presence of a scale inhibitor. The improvement of corrosion and scale inhibition of water treatment formulations containing water soluble polymers and or phosphonates in combination with pH regulation by means of CO₂ over other methods is demonstrated next. These experiments are performed with a heat transfer unit (HTU): water under study is pumped to a reservoir of 10 1. From here the water flows to a heat exchanger (heat flux : 10 W cm⁻²; water velocity : 90 cm sec⁻¹) and pumped back to the reservoir, where it is cooled to 30° C. In the tank air is bubbled through the water. The reservoir is equipped with a pH device which allows pH regulation by means of hydrogensulfate or carbondioxide. Deposit formation and corrosion are visually followed. The HTU test unit is also equipped with corrosion coupons and with an electrochemical measuring device. Most of the time an inhibitor is dosed to the water. The test duration always was 10 days. In the next table the results are brought together with different corrosion inhibitors under different methods of pH regulation. From the examples it can be seen that every formulation containing water soluble polymers and or phosphonates, we have tested, performs better under CO₂ control than by other pH control methods. Under CO₂ pH regulation it is possible to run the system under higher cycles of concentration or with formulations containing a lower amount of polluting chemicals. As can be seen from the examples zinc and phosphorus and nitogen can eventually completely suppressed from water treatment formulations.

Claims: A method of treating an aqueous system thereby to reduce corrosion and/or scaling which comprises incorporating in the system an agent which controls corrosion and/or scaling characterised by regulating the pH of the system to a value which is at least 7 but less than 9 using carbon dioxide so as to enhance the effectiveness of the agent and maintaining the TAC value (m-alkalinity) at 500 to 1000 ppm expressed as calcium carbonate. A method according to claim 1 in which the pH is regulated to 7 to 8.5. A method according to claim 1 or 2 in which the agent is one or more of the following: (a) a water soluble polymer (b) a water soluble phosphonate (c) a chemical which is a zinc salt, an inorganic phosphorus containing compound, a cerous or ceric salt, a chromate, a molybdate, a lignosulfonate, a tannin, a nitrite, an organic carboxylic acid, a polymeric diol, an iron complexant, a copper corrosion inhibitor, a polyhydroxy compound, a phosphate ester or an amine (d) a water soluble silicate. A method according to any one of the preceding claims in which a part of the carbon dioxide is replaced by an inorganic and/or an organic acid. A method according to any one of the preceding claims in which the carbon dioxide is generated by the decomposition of carbon dioxide releasing chemicals. A method according to any one of claims 3 to 5 in which the polymer contains one of the following groups: phosphonate, sulfonate, sulfate or phosphate. A method according to any one of claims 3 to 5 in the polymer contains phosphinic acid groups. A method according to any one of claims 3 to 5 in which the polymer has calcium carbonate stabilising and/or anticorrosive properties, alone or in combination with one or more of the other chemicals mentioned in paragraph (c). A method according to any one of claims 3 to 8 in which the phosphonate has calciumcarbonate stabilizing and/or anticorrosive properties, alone or in combination with one or more of the chemicals mentioned in paragraph (c). A method according to any one of the preceding claims in which the system also has incorporated therein one or more of the following components: biocides, surface active agents, antifoaming agents, or biodispersants. A method according to any one of the preceding claims, used in combination with reverse osmosis, side stream softening, make-up softening, blowdown softening, sidestream filtration or ozone dosage. A method according to any one of the preceding claims in which the aqueous system is for cooling water treatment, waste water treatment, or process water treatment.

Inventors: THEVISSEN PETER, THEVISSEN, PETER
Applicants: DENAC NV, DENAC N.V.
Application year: 1990
Main classification: C02F1
IPC codes: C02F5, C02F1, C23F11