CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation-in-part of U.S. Application Serial No. 07/628,000 filed December 17, 1990.

FIELD OF THE INVENTION This invention relates to constant boiling mixtures for use as refrigerants, aerosol propellants, heat transfer media, gaseous dielectrics, fire extinguishing agents, blowing or expansion agents for polymers such as polyolefins and polyurethanes and as power cycle working fluids. More particularly, it relates to constant boiling mixtures of fluorinated hydrocarbons. Specifically this invention relates to the use of mixtures of pentafluoroethane (HFC-125) and difluoromethane (HFC-32) as replacements for Refrigerant 502 (R-502), a commercial binary azeotrope of chlorodifluoromethane (HCFC-22) and chloropenta- fluoroethane (CFC-115) that has been used as the refrigerant in numerous commercial applications..

BACKGROUND OF THE INVENTION Recently the long-term environmental effects of chlorofluorocarbons have come under substantial scientific scrutiny. It has been postulated that these chlorine-containing materials decompose in the stratosphere, under the influence of ultraviolet radiation, to release chlorine atoms. Chlorine atoms are theorized to undergo chemical reaction with the ozone layer in the stratosphere. This reaction could

deplete or at least reduce the stratospheric ozone layer, thus permitting harmful ultraviolet radiation to penetrate the earth's protective ozone layer. A substantial reduction of the stratospheric ozone layer could have a serious deleterious impact on the quality of life on earth.

Refrigerant 502, the azeotropic mixture of about 47-50 weight percent HCFC-22 and 53-50 weight percent CFC-115 (the azeotrope is composed of 48.8 weight percent HCFC-22 and 51.2 weight percent

CFC-115) has long been used as the refrigerant in most of the country's supermarket refrigeration cases. However, since CFC-115 is a chlorofluorocarbon compound which is being phased out by the year 2000, the industry is required to replace Refrigerant 502 with environmentally safer fluorinated hydrocarbons.

The tetrafluoroethanes (HFC-134 and its isomer (HCFC-134a) have been mentioned as possible substitutes. However, the low vapor pressures (rela- tively high boiling points) limit the refrigeration capacity of these compounds, making them undesirable in R-502 applications. Also, pentafluoroethane (HFC-125) has been suggested as a replacement for R-502, but its' energy efficiency (heat removed by the evaporator divided by the power to compress the vapor) is 10% lower than R-502. Consequently, newly designed equipment would be required to achieve the refrigera¬ tion currently needed for these supermarket applica¬ tions. Mixtures of environmentally safe materials might also be used if the desired combination of properties could be attained in a simple (not constant boiling) mixture. However, simple mixtures create problems in the design and operation of the equipment used in refrigeration systems. These problems result

primarily from component separation or segregation in the vapor and liquid phases.

Azeotropic or constant boiling mixtures of two or more components, where the composition of the vapor and liquid phases are substantially the same at the temperatures and pressures encountered in the refrigeration cycle, would appear to be the answer. Included in the definition of constant boiling mixtures are near-azeotropic mixtures. U.S. Patent No. 4,810,403 teaches that near-azeotropic mixtures maintain a substantially constant vapor pressure even after evaporative losses, thereby exhibiting constant boiling behavior.

It is an object of the present invention to provide a constant boiling composition of at least two hydrofluorocarbons that is low boiling, is non-flammable, and suitable for use as a refrigerant, aerosol propellant, a heat transfer medium, a gaseous dielectric, a fire extinguishing agent, an expansion or blowing agent for polymers and as a power cycle working fluid.

SUMMARY OF THE INVENTION According to the present invention, a substantially constant boiling composition has been discovered that comprises about 10-90 weight percent pentafluoroethane, CF,CHF- also known as HFC-125, and about 90-10 weight percent difluoromethane, CH-F- also known as HFC-32, that is suitable for the aforemen- tioned uses, particularly for use in the refrigeration cases found in supermarkets. The preferred composi¬ tions comprise about 13-61 weight percent HFC-125 and about 39-87 weight percent HFC-32. More preferred compositions comprise about 13-23 weight percent HFC-125 and about 77-87 weight percent HFC-32; but the

ost preferred is the azeotropic composition itself of about 18.5 weight percent HFC-125 and about 81.5 weight percent HFC-32 determined at -15.3 ^β C at 70.2 psia. The preferred, more preferred and most preferred compositions described above are based on their close proximity to the azeotropic composition. However, the commercial applications of this invention will be as a replacement for R-502 in current commercial equipment. Unexpectedly, it has been found that compositions rather distant from the azeotropic composition remain substantially constant boiling; are less flammable (since they contain far less than 60% of HFC-32) ; operate in refrigeration equipment at lower compression temperatures; match the surface tension of R-502; in short, operate without any substantial changes in the commercial equipment that currently employs R-502.

Such appealing compositions for commercial operations comprise about 10-45 weight percent HFC-32 and about 55-90 weight percent HFC-125. The more preferred commercial compositions comprise about 15-40 weight percent HFC-32 and about 60-85 weight percent HFC-125; and the most preferred comprise about 20-30 weight percent HFC-32 and about 70-80 weight percent HFC-125.

The compositions of this invention are particularly useful in refrigeration applications since they maintain their stability and their azeotrope-like properties at temperatures of -30 ^β F to 115°F and pressures of 24 psia to 415 psia as shown in Examples 3-9 hereinafter. As a matter of information, the compositions of this invention may be used successfully at temperatures as low as -50 ^β F to temperatures as high as 350 ^β F.

The novel azeotrope and substantially constant boiling compositions of the invention exhibit dew and bubble points with virtually no pressure differentials. As is well known in the art, the difference between dew point and bubble point pressures is an indication of the constant boiling behavior of mixtures. The pressure differentials demonstrated by the substantially constant boiling mixtures of the invention are very small when compared with those of several know, non-azeotropic, binary compositions, namely, (50+50) weight percent mixtures of pentafluoroethane (HFC-125) and 1,1,1,2-tetra- fluoroethane (HFC-134a) and chlorodifluoromethane (HCFC-22) and l-chloro-l,l-difluoroethane (HCFC-142b) , respectively. The pressure differentials demonstrated by the substantially constant boiling mixtures of the invention are also smaller than values for near azeotropic mixtures of HCFC-22, HFC-152a and HCFC-124 described in U.S. Patent No. 4,810,403. These data, which are shown in Table 1, confirm the azeotrope-like behavior of the composi¬ tions claimed in this invention.

For the purpose of clarifying this dis¬ closure, "azeotropic* or "constant boiling" is intended to mean also essentially azeotropic or essentially constant boiling. In other words, included within the meaning of these terms are not only the true azeotrope described above, at -15.3 ^β C at 70.2 psia, but also other compositions containing the same components in different proportions, which are true azeotropes at other temperatures and pressures, as well as those equivalent compositions which are part of the same azeotropic system and are also

substantially constant boiling. As is well recognized in the art, there is a range of compositions which contain the same components as the azeotrope, which not only will exhibit essentially equivalent properties for refrigeration and other applications, but which will also exhibit essentially equivalent properties to the azeotrope composition at -15.3'C and 70.2 psia in terms of constant boiling characteristics or tendency not to segregate or fractionate on boiling.

The novel azeotropic mixtures may be used to produce refrigeration by condensing the mixtures and thereafter evaporating the condensate in the vicinity of a body to be cooled. The novel azeotropic mixtures may also be used to produce heat by condensing the refrigerant in the vicinity of the body to be heated and thereafter evaporating the refrigerant.

The use of azeotropic mixtures eliminates the problem of component fractionation and handling in system operations, because azeotropic mixtures behave essentially as a single substance. Several of the substantially constant boiling mixtures also offer the advantage of being essentially nonflammable.

It should be understood that one or more of the compounds shown in Table 2 can be combined with the substantially constant boiling binary mixtures of HFC-125/HFC-32 to provide ternary or higher substantially constant boiling mixtures for similar uses while adding advantageous properties unique to the added component(s) .

The invention will be more clearly under¬ stood by referring to the examples which follow.

EXAMPLE I A phase study was made on pentafluoroethane and difluoromethane wherein the composition was varied and the vapor pressures measured at a constant temperature of -15.3*C. An azeotropic composition was obtained, as evidenced by the maximum vapor pressure observed, and was identified as follows:

Pentafluoroethane = 18.5 + 2 weight percent Difluoromethane = 81.5 + 2 weight percent Vapor Pressure = 70.2 psia at -15.3 ^β c.

EXAMPLE 2 A phase study was made on pentafluoroethane and difluoromethane to verify minimal fractionation and change in vapor pressure during a vapor loss.

A blend was prepared in a 75 cc stainless steel cylinder consisting of pentafluoroethane and difluoromethane. The cylinder was agitated with a magnetic stirrer and submerged in a constant temperature bath at 23.8 ^D C. The vapor space was

allowed to leak at a slow rate. The vapor pressure was constantly measured using a pressure transducer and the vapor was sampled at various times during the experiment and analyzed using a standard gas chromatography method. Initial and final liquid concentrations were also analyzed by gas chromatography. Initial liquid (IQ) , final liquid (FQ) , vapor compositions, vapor pressure data, and change in vapor pressure from the initial vapor pressure are presented in Table 3.

These data demonstrate that with more than 80% of the original charge depleted, the vapor pressure has remained substantially constant (2.87% change) . It is important to note that the difluoro- methane concentration has gone down in both the liquid and vapor phases during leakage. Therefore, since the initial concentration is nonflammable, recognizing that difluoromethane is flammable, the blend will not become flammable in the event of vapor loss.

EXAMPLES 3-9 Evaluation of the refrigeration properties of the azeotropic mixtures of the invention versus HCFC-22, Refrigerant 502 and pentafluoroethane (HFC-125) alone, are shown in Table 4.

TABLE 4 COMPARISON OF REFRIGERATION PERFORMANCE

Control Control Control Ex. 3 Ex. * Ex. 5 Ex. 6 Ex. 7 Ex. 8 Ex. 9 Weight Percentage

10/90 18.5/81.5 50/50 60/40 70/30 80/20 90/10 BCFC-22 R-502 HFC-125 125/32 125/32 125/32 125/32 125/32 125/32 125/32 Evaporator Tcεαp. (deg F) -30 -30 -30 -30 -30 -30 -30 -30 -30 -30 Evaporator Pressure

(psia) 19.6 2..0 26.7 33.9 33.8 32.9 32.2 31.3 30.1 28.5

Condenser Temp.

(deg F) 115 115 115 115 115 115 115 115 115 115 Condenser Pressure

(psia) 258 282 327 415 .12 396 388 379 365 349 Coαpressor Discharge Temp.

(deg f) 303 239 223 357 345 300 284 269 254 239 Coefficient of Per¬ formance 1.97 1.89 1.69 1.81 1.81 1.80 1.79 1.77 1.75 1.71 Refrigeration Capacity

(Btu/rαln) 76.6 80.0 79.4 124 122 112 107 102 95 87 Sur ace Tension (dyne/cm) — 14.7 — 18.2 ~ 16.2 15.7 — 14.4

"Coefficient of Performance" (COP, is the ratio of net refrigeration effect to the compressor

work. It is a measure of refrigerant energy efficiency.

Net refrigeration effect is the change in enthalpy of the refrigerant in the evaporator, i.e., the heat removed by the refrigerant in the evaporator. Refrigeration Capacity is based on a fixed compressor displacement.

For a refrigeration cycle typified by the conditions shown in Table 4 for the evaporator and the condenser, the COP's shown in the examples of the invention are higher than the COP of pentafluoroethane (HFC-125) alone.

Important considerations in evaluating the performance data are compressor discharge temperature, surface tension, capacity and condenser pressure.

R-502 was originally developed to replace HCFC-22 in applications with long refrigerant return lines to the compressor. Use of HCFC-22 resulted in high compressor discharge temperatures and early compressor failures. Lower compressor discharge temperatures are produced with R-502 due to the higher heat capacity of the CFC-115 component. Since one of the objectives of this invention was to develop a refrigerant to replace R-502 in existing commercial equipment with minimal changes, the replacement refrigerant must produce lower compressor discharge temperatures than with HCFC-22.

The data in Table 4 indicate that the compressor discharge temperature of HCFC-22 is matched by the mixture of 50 weight percent HFC-32. Higher concentrations of HFC-32 would result in even higher discharge temperatures. It is obvious that concentrations of HFC-32 lower than 50 weight percent should be used to approach the discharge temperature of R-502. A match of compressor discharge

temperatures for R-502 and the HFC-32/HFC-125 mixture occurs at about 10 weight percent HFC-32, and compressor design allowances would likely permit operation at somewhat higher temperatures, possibly up to 275*F, resulting in an HFC-32 concentration of about 35 weight percent.

Surface tension is another factor to consider. It is important in heat transfer performance of the refrigerant in condensers and evaporators where bubbles and droplets occur, in turn being related to system energy efficiency. In fact, it has been said that "Surface tension is one of the most important physical properties, especially when two-phase heat transfer occurs with bubble or droplet generation on surfaces." D. Jung and R. Radermacher, Transport Properties and Surface Tension of Pure and Mixed Refrigerants, ASHRAE TRANSACTIONS 1991., Vol. 97, Pt. l. Since the object of this invention was to identify a refrigerant to replace R-502, preferably for use in existing commercial equipment with minimal changes, it would be advantageous to have similar values of surface tension for R-502 and the replacement mixture. Surface tension values were calculated by the method of Brock and Bird, AICHE Journal, Vol. 1, p. 174 (1955), and are shown in Table 4. A match of surface tension values with that of R-502 occurs at about 25% HFC-32. Higher values of surface tension are less desirable, as more energy is required to remove the bubbles or droplets from heat exchanger surfaces.

The data in Table 4 also indicate that the lower concentrations of HFC-32 provide capacity and condenser pressures closer to that of R-502. The mixtures of HFC-32 and HFC-125 may also be considered as replacements for HCFC-22. The data in Table 4

again indicate that the lower concentrations of HFC-32 (10-30%) provide capacity and condenser pressures closer to that of HCFC-22.

Additives such as lubricants, corrosion inhibitors, stabilizers, dyes and other appropriate materials may be added to the compositions of the invention for a variety of purposes, provided they do not have an adverse influence on the substantially constant boiling nature of the composition.