Composition Useful as Corrosion Inhibitor, Anti-Scalant and Continuous Biocide for Water Cooling Towers and Method of Use

Water cooling systems, particularly water cooling towers, require continuous maintenance in order to avoid fouling caused by sludge, scale and microorganisims, and corrosion. These problems are treated by adding various chemicals to the water cooling towers.

One class of compounds useful as dispersants, antiscaling agents and corrosion inhibitors are organophosphorous compounds, particularly organophosphorous carboxylic acids and the phosphonates. These organic phosphorus compounds have the ability to complex metal ions by sequestration or threshold treatment. They also form inhibitive films along metal surfaces. This prevents formation of the crystalline structure in scale. Phosphonates are a well defined class of threshold inhibitors used to prevent scale formation. Typically used phosphonates include aminomethylenephosphonic acid and 1-hydroxyl ethylidene-1,1-diphosphonic acid. A commonly employed organophosphorous carboxylic acid (phosphenite) is 2-phosphonobutane- 1,2,4-tricarboxylic acid which can be purchased from Bayer AG.

Alkali and alkaline earth metal molybdate salts are superior corrosion inhibitors to chromate and nitrite salts used at higher concentrations. Sodium molybdate is an anodic inhibitor and is the most commonly used molybdate inhibitor.

Also used for corrosion inhibition are the aromatic azoles. These are specific corrosion inhibitors for copper and its alloys. They are chemisorbed on the metal surfaces and are capable of excellent protection at a minimal dosage of 1 to 2 parts per million. Generally included within the aromatic azoles are benzotriazole, tolyltriazole, and mercaptobenzothiazole.

Organophosphorous carboxylic acids, phosphonates and azoles are used in combination to provide a corrosion and scale inhibition program. Functionally these are all anionic in solution and therefore generally quite compatible.

In addition to corrosion and scale inhibitors and dispersants, water cooling towers are generally treated with certain biocides or biostats to prevent or inhibit algae, fungi and bacteria. Two general types of these are oxidizing and nonoxidizing. Typical oxidizing biocides are ClO.sub.2, chlorine and chlorine release compounds such as chlorinated isocyanurates, hypochlorites and chlorinated hydantoins. Quaternary ammonium compounds are the primary nonoxidizing biocides and biostats. The quaternary ammonium salts are cationic surface active chemicals which are most effective against algae and bacteria in alkaline pH ranges.

One particular class of quaternary ammonium complexes are polymeric quaternary ammonium salts. These complexes are generally nontoxic and accordingly safer to use. They are formed by reacting a bis tertiary amine with a dihalo compound or a hydroxy halocompound. Several of these compounds are currently used to treat water cooling towers. One such product is WSCP from Buckman Laboratories which is poly[oxyethylene(dimethylimminio)ethylene(dimethylimminio)ethylenedichlori de]. This product is used as a swimming pool algicide at two parts per million(ppm) as a maintenance dose and 5 to 8ppm to rid pools of heavy objectionable algae growth. Swimming pools are more easily treated than water cooling towers. The chlorination of swimming pools and other biocidal treatment prevents acclimatization. Further, swimming pools are not actively aerated. Water cooling towers inherently aerate the water which promotes growth of aerobic microorganisms. The manufacturers of WCSP recommend use of WCSP at 20 to 40 parts per million of active product as a biocide for water cooling towers.

Another polymeric quaternary ammonium compound is Mirapol A-15. This product is poly[N-[3(dimethylammonio)propyl]-N'-[3-(ethyleneoxyethylenedimethylammoni o)propyl] urea dichloride]. This is inhibitory against certain bacteria at 100 parts per million. A third polymeric quaternary ammonium compound is ETC-PQ from Onyx Chemical Company which is .alpha.-4-[1 tris(2-hydroxy ethyl)ammonium chloride-2-butenyl]poly[1 dimethylammonium chloride-2-butenyl]-.omega.-tris(2 hydroxyethyl)ammonium chloride. This also acts as a bacteriostat.

Quaternary ammonium salts are excellent biocides for use in water cooling towers. However because quaternary ammonium compounds are cationic in nature and when combined with corrosion inhibitors, dispersants and antiscalants which are anionic in nature, the compounds tend to salt out via chemical reaction thereby destroying functionality. The corrosion inhibitors and antiscalants are used continuously and remain effective even though used continuously. Biocides and biostats on the other hand are not used continuously. Bacteria and other microorganisms can acclimatize to a particular biocide rendering the biocide totally ineffective. This is particularly a problem when low levels of a biocide are used as is required for continuous application. Of course at extremely high concentration, certain biocides would be effective on a continuous basis. But this would be environmentally objectionable and economically unfeasible. Other biocides and biostats are totally ineffective at low levels used in continuous treatment. For these reasons biocides and biostats are generally applied periodically in high concentration, slug doses which kill a high proportion of bacteria or algae. A typical treatment for a water cooling tower will use multiple slug doses of different biocides to prevent acclimatization to any particular biocide.

The present invention is premised upon the discovery that a particular quaternary ammonium complex, a poly[oxyalkylene(dialkylimminio)alkylene(dialkylimminio)] salt, is effective as a continuous biocide/biostat at low levels.

The invention is further premised on the discovery that this quaternary ammonium compound is compatible at high concentrations with organophosphorous carboxylic acid dispersants and scale inhibitors, as well as with molybdate corrosion inhibitors and azole type corrosion inhibitors.

Thus the present invention provides a means to continuously treat a water cooling system, particularly a water cooling tower, with one composition which prevents corrosion, scaling and acts as a bacteriostat and an algicide. These and other advantages of the present invention will be appreciated in light of the detailed description.

According to the present invention an all-in-one treatment agent for a water cooling system includes a polymeric quaternary ammonium compound as a biocide/biostat, an organophosphorous carboxylic acid compound which acts as a scale control agent, dispersant and corrosion inhibitor and an azole type corrosion inhibitor. These are provided in high concentrations for storage and shipment and are applied at low level concentration on a continuous basis in a water cooling tower.

Further the unique polymeric quaternary ammonium compound used in the invention can be used alone as a low level continuous bacteriostat and algicide.

For use in the present invention the polymeric quaternary ammonium compound is a poly[oxyalkylene(dialkylimminio)alkylene(dialkylimminio)alkylene] compound having the following general formula: ##STR2## wherein: R.sub.1, R.sub.4 and R.sub.7 are C.sub.1 -C.sub.10 alkylene and preferably C.sub.1 -C.sub.3 alkylene;

R.sub.2, R.sub.3, R.sub.5 and R.sub.6 represent C.sub.1 -C.sub.5 alkyl and preferably methyl; and

n is 5-12 and preferably 7-10.

The +2 charge is preferably offset by anionic halogen atoms preferably C1.sup.-.

The preferred polyquaternary ammonium compound for use in the present invention is poly[oxyethylene(dimethylimminio)ethylene(dimethylimminio)ethylenedichlori de]. The terminal groups of the polymer may vary but generally should be hydrogen or a halogen such as chlorine. This compound is produced by reacting dichloroethyl ether with the following compound in an aqueous medium via a rate/temperature controlled reaction: ##STR3##

The amount of polymeric quaternary ammonium compound(active) used in the present invention will generally be about 5%-60% and preferably 10% based on the total amount of organophosphorous compound, azole corrosion inhibitor and polymeric quaternary ammonium compound.

The antiscalant dispersant of the present invention is one or more organic phosphorous compounds. Organophosphorous antiscalants are a known class of compounds. The two preferred compounds are phosphonates and organophosphorous carboxylic acids and their salts. Suitable phosphonates are water soluble phosphonates which have the following general formula: ##STR4## wherein R.sub.8 represents C.sub.1 -C.sub.5 alkyl or substituted alkyl such as amine substituted alkyl and phosphonic acid substituted alkyl. Preferred phosphonates are polyphosphonates where R.sub.8 is substituted with one or more phosphonate groups. The most commonly used is 1-hydroxyethylidene-1, 1-diphosphonic acid(HEDP).

The organophosphcrous carboxylic acid compounds suitable for use in the present invention are the water soluble organic phosphorous carboxylic acids typically used as antiscalants in the water treatment industry. These have the following general formula: ##STR5## wherein R.sub.9 and R.sub.10 represent hydrogen, C.sub.1 -C.sub.5 alkyl or C.sub.1 -C.sub.5 alkyl substituted by an amine, carboxylic acid group or ester group. In use, since the final composition is alkaline, the compound will be present as alkali or alkaline earth metal salts or C.sub.1 -C.sub.5 alkyl esters.

The relative percentage of organophosphorous antiscalant based on the total amount of poly quaternary ammonium compound, azole corrosion inhibitor and organophosphorous compound in the present formulation should generally range from 20% to 85%(actives) preferably 60%. At higher concentrations it tends to salt out.

The organophosphorous antiscalants can be added as a combination of phosphonates and organophosphorous carboxylic acid for applications encountering hard water. In such applications the two organophosphorous antiscalants preferred are 1-hydroxyethylidene-1, 1-diphosphonic acid(HEDP) and 2-phosphonobutane-1, 2,4 tricarboxylic acid(PBS-AM).

Further the all in one treatment composition of the present invention includes a water soluble aromatic azole type corrosion inhibitor. These include benzotriazole, C.sub.1 -C.sub.10 alkylbenzotriazoles such as tolyltriazole and thiozoles such as mercaptobenzothiozole. Preferred azoles are benzotriazole and tolyltriazole.

The aromatic azole corrosion inhibitors should be present in a percentage of 2 to 20 based on combined amounts of polyquaternary ammonium compound, organophosphorous compound, and azole.

In the composition of the present invention the relative percentages of organophosphorous compound, aromatic azole and polyquaternary ammonium compound are provided so that in use in a water cooling tower when the polyquaternary ammonium compound is present at a concentration of less than or equal to about 15 ppm, the concentration of organophosphorous compound and aromatic azole will be high enough to provide effective corrosion and scale inhibition. The use levels are usually at least about 0.5ppm for the aromatic azole and at least about 5ppm for the organophosphorous compound.

A preferred composition for use in the present invention would include 32% poly[oxyethylene(diethylimminio) ethylene(dimethylimminio)ethylenedichloride] as the quaternary, 24% HEDP and 38% PBS-AM as the organophosphorous and 6% benzotriazole, based on the total actives content.

Further the above compositions may also include a base in an amount effective to establish the pH at about 7 to 8.5. Suitable bases include the alkali metal hydroxides particularly sodium and potassium hydroxide.

In use in a water cooling tower, it is desirable to have the following approximate concentrations of components:

For water cooling towers which employ low hardness or low alkalinity water, a modified all-in-one treatment composition can be used. Specifically, a molybdate corrosion inhibitor can be added. The molybdates are compatible with the polyquaternary compound used in the present invention. Because this formulation is adapted for use in soft water, it can be used with a single organophosphorous component. Accordingly PBS-AM may be employed alone as the organophosphorous antiscalant.

In the composition of the present invention the relative percentages of organophosphorous compound, molybdate aromatic azole and polyquaternary ammonium compound are provided so that in use in a water cooling tower when the polyquaternary ammonium compound is present at a concentration of less than or equal to about 15ppm the concentration or organophosphorous compound, molybdate and aromatic azole will be high enough to provide effective corrosion and scale inhibition. The use levels are usually at least about 0.5ppm for the aromatic azole and at least about 5ppm for the organophosphorous compound.

In this formulation the aromatic azole is present at 1 to 15% of total active and preferably 7%, the organophosphorous compound is present at 5% to 60% or higher of total actives and preferably 11%; the polyquaternary compound is present at 5% to 40% of total active preferably 16% and the Na molybdate is present in 30% to 80% of total active and preferably 70%.

In use in a water cooling tower it is desirable to have the following concentration of individual components:

The poly[oxyalkylene(dialkylimminio)alkylene(dialkylimminio)] compound previously described is useful alone as a continuous biocide/biostat at low levels of use. For use in the present invention this polyquaternary ammonium compound should be added to a water cooling tower at 2-15ppm and preferably 3-5ppm.

The water cooling system should be maintained with this concentration of the polyquaternary ammonium compound on a continuous basis. Under these conditions acclimatization by algae and bacteria does not occur. The preferred polyquaternary ammonium compound is poly[oxyethylene(dimethylimminio)ethylene(dimethylimminio)ethylene dichloride].

According to the present invention a poly[oxyalkylene(dialkylimminio)alkylene(dialkylimminio)] can be used at low levels to provide a continuous biostat/biocide treatment of cooling system water or it can be incorporated into an all in one water cooling system treatment system to control corrosion, sludge buildup, scale formation and microorganisms.