Abstract:
A polymeric residue with one or more metals, salts, and oxides mixed therein, deposited on refrigeration system surfaces by a circulating halogenated hydrocarbon refrigerant, is removed from a surface by contacting the residue with a saturated aqueous solution or slurry with a pH from 2 to 4 of ethylenediaminetetraacetic acid at temperatures from 30° C to 100° C.

Description:
BACKGROUND OF THE INVENTION 
     The invention pertains generally to cleaning methods and especially to methods for removing residues produced in refrigeration systems. 
     Refrigeration systems operate by alternately compressing and vaporizing a refrigerant, such as trichlorofluoromethane (CFCl 3 ). Usually, a small amount of compressor oil is admixed with the refrigerant. Also to be found with the circulating refrigerant are small amounts of water, air, metal (e.g., iron, aluminum, and copper), and oxides and salts of those metals. During the operation of the refrigeration systems, these materials slowly react and accumulate on the surfaces, such as, walls and impeller blades of the refrigeration system. This residue resembles varnish or coke. It is these polymers and halogenated polymers with metals, salts, and oxides admixed therein that render cleaning refrigeration systems nearly impossible. 
     Presently, several chemical techniques are used, e.g., solutions of citric acid, salts of ethylenediaminetetraacetic acid, modified solutions of hydrochloric acid or phosphoric acid. The major disadvantages associated with the use of these cleaning solutions are poor cleaning and surface corrosion. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of this invention to effectively clean refrigeration systems. 
     Another object of this method is to remove polymeric and halogenated polymeric residues from metallic surfaces quickly, effectively, and with little corrosion of the surface. 
     These and other objects are achieved by contacting a polymeric residue having one or more salts, metals, and oxides mixed therein with a saturated aqueous solution with a pH from about 2 to about 4 of ethylenediaminetetraacetic acid (EDTA) at a temperature from about 30° to about 100° C. 
     DETAILED DESCRIPTION OF THE INVENTION 
     The surfaces inside a refrigeration system consist of a wide range of materials. In view of this situation, the cleaning composition of this invention was tested and found to be completely non-corrosive or only slightly corrosive to surfaces made of stainless steel, carbon steel, copper, brass, a silver alloy, aluminum, polyethylene, polyvinyl chloride. Obviously, many other surfaces could have been tested and found corrosion-resistant to ethylenediaminetetraacetic acid. 
     To clean a refrigeration system, the refrigerant is drained from the system. A saturated aqueous solution of ethylenediaminetetraaectic acid (EDTA) having a pH from about 2 to about 4, preferably from 2 to 3, and most preferably from 2 to 2.5 is circulated through the system. The water is preferably demineralized or nearly demineralized. A surfactant may also be added to the solution in an amount of at most about 1 weight percent of the total solution weight. The surfactant should be nonionic with an HLB number from about 13 to about 13.5. Exemplary of surfactants are octylphenoxypolyethoxyethanol, ethoxylated octylphenol, and ethoxylated castor oil. In the description of the present invention, a saturated solution is intended to include a slurry, i.e,, a saturated solution with additional solute. The pH and the saturation of the solution are maintained by occasionally adding ethylenediaminetetraacetic acid to the solution. Monitoring the pH provides a convenient method of determining the progress of the cleaning. As the solution removes the residue, the pH increases and when pH remains constant at a low value the refrigeration system is cleaned. 
     The temperature of the solution should be in excess of room temperature (30° C.) in order to have a desirable cleaning rate, but not too much in excess of 100° C. because of the increased corrosion of the surfaces. Preferably, the temperature is from 55° to 85° C. and most preferably, the temperature is from 65° to 80° C. The temperature may be maintained by any means. One possible technique is to circulate the cleaning solution through a thermostated container filled with the solution and an excess of solid ethylenediaminetetraacetic acid (EDTA). 
     The aqueous saturated ethylenediaminetetraacetic acid (EDTA) solution is circulated through the refrigeration system until the system is clean. For other applications, e.g., cleaning bilges, the method of this invention can be easily modified to meet any new requirements. What is important is the temperature and concentration of the aqueous EDTA solution. The solution does not have to move across or flush a surface in order for the present invention to work, but faster results are obtained if there is this movement or flushing. Hence if an object is submerged in a saturated aqueous solution of EDTA, it should be agitated in some manner for the fastest results. 
     After the system is clean, the EDTA solution is drained out and the system is flushed with water or with slightly alkaline buffered water and water in sequence. Thereafter, the system is dried by passing warm air through it. 
    
    
     In order to demonstrate the improvement provided by the method of this invention and the effect of varying the condition of the present method, the following experiments were conducted. It is understood that the results to follow are presented by way of illustration and are not meant to the disclosure or the claims to follow. 
     Experiment I: Residue Removal Comparison 
     Weighted portions of residue obtained from a refrigeration system were added to stirred aqueous solutions at various times and temperatures. At the completion of each reaction, the solution was filtered. The residue was then dried and reweighed to determine the percentage of residue removed. The results are shown in Table I. 
     
                                           TABLE I__________________________________________________________________________            Initial                 Final                     Amount inSample    Cleaning Agent            pH   pH  Solution, wt%                             Conditions__________________________________________________________________________1   Ammonium citrate            2.4  2.5 78      Heated and stirred at    (controlled pH)               70° C for 2 hours. pH                             readjusted after 1 hour.2   Citric Acid  2.2  3.6 41      Heated and stirred at                             60° C for 1 hour.3   Citric Acid  2.2  2.4 77      Heated and stirred at    (controlled pH)               60° C for 2 hours. pH                             readjusted after 1 hour.4   EDTA, pH adjusted            5.3  7.8 28      Heated and stirred at    with ammonium hydroxide       60° C for 1 hour.5   EDTA, pH adjusted            3.7  2.8 81      Heated and stirred at    with ammonium hydroxide       70° C for 3 hours, pH    (controlled pH)               readjusted after 1 and                             2 hours.6   EDTA, Tetrasodium            10.8 10.8                     16      Heated and stirred at    salt                          60° C for 1 hour.7   EDTA, Disodium salt            4.6  8.4 28      Same as above.8   EDTA, saturated with            2.7  2.4 91      Heated and stirred at    an excess of acid             70° C for 2 hours. pH                             readjusted after 1 hour.__________________________________________________________________________ 
    
     Experiment II: Corrosion Comparison 
     Samples of 50% carbon steel type 515, red brass, a copper-nickel alloy, copper, and a silver alloy were placed in cleaning solutions. Residue obtained from a refrigeration system was added to each solution in an amount of approximately three weight percent of the total weight of solution. Under these conditions only the steel showed any appreciable weight loss. All of the non-steel samples exhibited less than a 0.5 weight percent loss. The loss in weight percent of the steel sample along with the loss in weight percent of the residue in parenthesis is listed in Table II. 
     
                       TABLE II______________________________________    Time = 24 hours  Time = 48 hours    Temperature      TemperatureCleaning Agent      60° C                70° C                        80° C                                80° C______________________________________Citric Acid      10(75)    8(82)   10(84)  22(84)EDTA        5(84)    7(90)   12(91)  12(91)______________________________________ 
    
     The 90 weight percent loss of residue and approximately 7 weight percent loss of the most susceptible metal demonstrate that the method of this invention provides a highly effective technique for cleaning residues from refrigeration systems and other apparatus and constructions. The one- or two-hour cleaning times of TABLE I show the practicality of this method. 
     Obviously many modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described.