This is a continuation-in-part of copending Serial No. 334,007, filed April 5, 1989.

The present invention is directed to a novel heat transfer fluid based on potassium and/or sodium chloride, containing sodium molybdate as a corrosion inhibitor.

BACKGROUND OF THE INVENTION

Water may be used as a heat-transfer medium in refrigeration and other heat transfer systems because it affords high rates of heat transfer at relatively small expense, is thermally stable and nontoxic. However, water is limited in that it is useful only over a limited temperature change (undergoing a phase change to ice) and has a tendency to corrode the metals with which it is in contact. Various brines are used in refrigeration systems since they gain or lose heat energy without changing into another phase. Historically, many brine systems use calcium chloride as the salt because of its ability to be used at much lower temperatures than other brines, i.e., below about 10°F, and because of its thermal and chemical compatibility with sodium bichromate, a widely used

corrosion inhibitor. Both calcium chloride and sodium chloride brines usually include hexavalent chromium as a corrosion inhibitor in the refrigeration system at relatively high levels, i.e., about 1500 pp or greater, in order to control corrosion of equipment and transport piping. Other common corrosion inhibitors such as phosphates, silicates, or carbonates react with these brines or form undesirable precipitates with naturally occurring contaminants in brine salts.

Ethylene glycol is commonly used in many industrial and commercial systems, particularly which operate at 15"F or higher, however it suffers at a disadvantage in that it requires high concentrations of ethylene glycol to attain lower operating temperatures and ethylene glycol containing certain corrosion inhibitors is a toxic material.

Since most refrigeration systems utilizing brine or ethylene glycol as a heat transfer medium utilize underground storage tanks, these systems are subject to recent regulations requiring that any container bearing a liquid stored underground must be contained in a manner to prevent groundwater contamination in the case of leakage if the tank contains a hazardous material as defined by the appropriate regulating authority. Since most calcium chloride and sodium chloride brine systems are designed to have a substantial portion of their volume stored in tanks below grade, and since hexavalent chromium is a toxic substance, the use of these brines in connection with a hexavalent chromium inhibitor has been prohibited in some areas.

Moreover, with the increasing need to conserve energy or to shift peak power demand, many industries have

been moving into use of ice storage systems, which require even lower operating temperatures and, on an operational basis, create a demand for an effective, inexpensive, nontoxic antifreeze material. Initially ethylene glycol has been used because of its availability and relatively low cost. However, for various reasons, the cost and availability of ethylene glycol has driven its price to a point at which it is now becoming impractical to use.

For the foregoing reasons there is a need to develop a safe, nontoxic, inexpensive brine and corrosion inhibitor combination which would be readily available for use in all regions.

It is therefore an object of the present invention to provide such a safe, nontoxic, inexpensive brine and corrosion inhibitor combination which both is useful because of its operable temperature range and because it is environmentally acceptable.

This and other objects of the invention will be apparent from the following description and from operation of the invention.

SUMMARY OF THE INVENTION

The present invention also relates to a single phase, thermally stable, corrosion inhibiting heat transfer brine based on sodium and/or potassium chloride and containing a corrosion inhibiting amount of sodium molybdate.

The present invention provides a method for charging a heat transfer storage and circulation system with a sodium molybdate/potassium (and/or sodium) chloride heat transfer fluid.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

For the purpose of this invention, corrosion inhibiting means that metals immersed in the composition for 7 days at 350'F suffer a corrosion of less than 10 mil per year (MPY) , preferably less than 2 MPY. The corrosion rate is determined by weighing a metal specimen and then immersing the specimen in a vessel containing a solution of 160 ml of heat transfer fluid and 140 ml of corrosive water which contains 100 ppm each of chloride, sulfate and bicarbonate ions derived from their sodium salts. The vessel is pressurized to 150 psig with nitrogen and heated to a solution temperature of 350 ^β F for 7 days. At the end of 7 days, the metal specimen is removed, cleaned, dried and reweighed. The weight loss of the specimen is determined and the corrosion rate in mils per year which is determined by (weight loss in grams) (3.46x10 ⁶ )/(surface area of the specimen in cm ² ) (duration of test in hours) (specimen density in grams/cm ² ) .

It has been surprisingly found that a heat transfer storage and circulation system utilizing the heat transfer fluid according to the present invention must be charged in a particular way to benefit from the corrosion inhibiting characteristics of the sodium molybdate. Initially a system (comprising a storage tank and circulation system therefor) should be washed and cleaned thoroughly to be free of debris or any residual salts of the previously used refrigeration fluid. The system is then filled to approximately its fluid capacity, but less than full capacity, with fresh water, reserving sufficient unused volume for adding the corrosion inhibitor concentrate as described below.

The corrosion inhibiting concentrate will be a solution containing sodium molybdate. Alternatively, the molybdate may be made in situ by neutralization of molybdic acid with a base, such as sodium hydroxide. A sufficient volume of this concentrate will be added to the refrigeration system containing fresh water to attain a concentration in the range of about 300-1200 ppm sodium molybdate. A preferred concentration is approximately 600 ppm sodium molybdate. The concentrate also optionally and preferably contains small amounts of sodium carbonate and an azole (such as potassium benzotriazole) , sufficient to attain a concentration within the system of less than about 50 ppm, preferably less than about 2 ppm sodium carbonate, and less than about 200 ppm, preferably less than about 60 ppm of azole. Exemplary azoles include benzotriazole or other triazoles such as Reomet 42 (Ciba-Geigy) . Trace amounts of other stabilizing agents, chelators and buffering agents may be also added to the system such as phosphonic acids, polyacrylic acids and phosphates.

A particularly preferred concentrate will comprise (all percentages in weight by volume) 5% sodium molybdate, 8% potassium polyacrylic acid, 1.7% potassium l-hydroxyethylidene-l,l-diphosphonic acid, 0.55% potassium benzotriazole and 0.02% sodium carbonate. This concentrate may be conveniently stored in conventional 55 gallon drums and is characterized by a pH of approximately 9.0. The concentrate containing the sodium molybdate will be utilized in the amount of about 1 gallon to 100 gallons water in the system. If diluted 1 to 100, then the final concentration will be about equivalent to 240 ppm as molybdate or 600 ppm as sodium molybdate. The corrosion characteristics of the

above concentrate are approximately 0.30 MPY on copper and 1.05 MPY on steel, which are conventional metals utilized in storage and circulation systems for brine.

A typical preoperational cleaner should first be circulated within the system for a period of time sufficient to ensure contact with the interior metal surfaces in all areas of the system. This period of time will usually be at least 8 hours, more preferably about 48 hours. During this initial pretreatment of the metal surfaces, it is also preferred that the temperature of the cleaner be heated to a range of about 110 ^β F ^" to 130°F, preferably at 120 ^β F.

If heating of the fluid is not practical or heating means is not available, then the cleaner will be circulated at ambient temperature, which is usually between about 40 ^β F to 90°F, but then the concentration of the preoperational cleaner should be doubled.

After this preoperational cleaning, the Na ₂ Mo0 ₄ solution is circulated at ambient temperature (after dumping and flushing to remove the cleaner) for a minimum of 48 hours, but preferably 168 hours (7 days) . After this passivation, solid potassium and\or sodium chloride salt is then added to provide the desired salt concentration to attain the desired freezing point depression of the solution. Typically this will be about 5% to 25% weight by volume, preferably about 18% weight by volume. To ensure thorough dissolution of the salt and mixing, the fluid should then be circulated throughout the system for approximately 24 hours at ambient temperature.

The pH of the heat transfer fluid will normally be between about 8 and 9, depending upon the pH of the water used to fill the system.

In addition to the ingredients described above, the heat transfer fluid concentrate compositions of this invention may contain other additives such as antifoam agents, acid base indicators, dyes, and the like, provided that these additives are soluble in water and are thermally stable at high temperatures.

Heat transfer fluids, according to the present invention, are especially useful in underground storage containers. The heat transfer fluids are also useful in applications where there is no mechanical circulation of heat transfer fluid. Among the metals to be protected by the heat transfer fluid are brass, copper, solder, steel, iron and aluminum.

Acute Aquatic Toxicitv Bioassav

A heat exchange liquid sample containing 25% weight by volume of potassium chloride, 600 ppm sodium molybdate, 60 ppm azole, 2 ppm sodium carbonate, 2000 ppm potassium polyacrylic acid and 50 ppm potassium l-hydroxyethylidene-l,l-diphosphonic acid was tested according to the California Department of Health Services 96-Hour Acute Aquatic Toxicity Bioassay in accordance with the California Administrative Code, Title 22, Section 66696, Article 11. To pass the test there must be a finding that there is greater than 50% survival of test fish at the 500 mg/1 concentration of the potential contaminant. It was found that the test sample resulted in 100% survivorship at each of the concentration levels of 250, 500 and 750 mg/1.

Heat Transfer Properties

An 18% by weight solution of potassium chloride has the following characteristics: specific gravity at 60"F = 1.125; viscosity at 20'F - 2.49 centipoise; viscosity at 60"F = 1.27 centipoise; specific heat at 20'F = 0.77 Btu/lb-f; specific heat at 60'F = 0.86 Btu/lb-f; thermal conductivity at 20"F = 0.22 Btu/hr- ft-f; thermal conductivity at 60'F = 0.24 Btu/hr-ft- f; crystallization commences at 6 ^β F; weight per gallon of solution = 9.43 lbs.

While the invention has been described with reference to certain specific embodiments, it will be recognized by those skilled in the art that many variations are possible without departing from the scope and spirit of the invention. It will be understood that it is intended to cover all changes and modifications to the invention disclosed herein for the purposes of illustration which do not constitute departures from the spirit and scope of the invention.