NOVEL COMPOSITIONS COMPRISING

PENTAFLUOROETHANE AND MONOCHLORODIFLUOROMETHANE

This is a continuation-in-part application of co- pending, commonly assigned application Serial No. 656,376 filed February 19, 1991.

Field of the Invention

This invention relates to novel compositions of pentafluoroethane and chlorodifluoromethane. These mixtures have unusual efficiency and capacity as fluids for heating and cooling.

Background of the Invention

Fluorocarbon based fluids have found widespread use in industry for refrigeration, air conditioning and heat pump applications.

Vapor compression cycles are one form of refrigeration. In its simplest form, the vapor compression cycle involves changing the refrigerant from the liquid to the vapor phase through heat absorption at a low pressure, and then from the vapor to the liquid phase through heat removal at an elevated pressure. First, the refrigerant is vaporized in the evaporator which is in contact with the body to be cooled. The pressure in the evaporator is such that the boiling point of the refrigerant is below the temperature of the body to be cooled. Thus, heat flows from the body to the refrigerant and causes the refrigerant to vaporize. The formed vapor is then

removed by means of a compressor in order to maintain the low pressure in the evaporator.

The temperature and pressure of the vapor are then raised through the additional of mechanical energy by the compressor. The high pressure vapor then passes to the condenser whereupon heat exchange takes place with a cooler medium and the sensible and latent heats are removed with subsequent condensation. The hot liquid refrigerant then passes to the expansion valve and is ready to cycle again.

While the primary purpose of refrigeration is to remove energy at low temperature, the primary purpose of a heat pump is to add energy at higher temperature. Heat pumps are considered reverse cycle systems because, for heating, the operation of the condenser is interchanged with that of the refrigeration evaporator.

Certain chlorofluorocarbons have gained widespread use in refrigeration applications including air conditioning and heat pump applications owing to their unique combination of chemical and physical properties. The majority of refrigerants utilized in vapor compression systems are either single component fluids or azeotropic mixtures.

The art is continually seeking new fluorocarbon based mixtures which offer alternatives for refrigeration and heat pump applications. Currently, of particular interest, are fluorocarbon based mixtures which are considered to be environmentally acceptable substitutes for the presently used fully halogenated chlorofluorocarbons (CFC's) . The latter are suspected of causing environmental problems in connection with

the earth's protective ozone layer. Mathematical models have substantiated that hydrofluorocarbons, such as pentafluoroethane (HFC-125) will not adversely affect atmospheric chemistry, being negligible contributors to stratospheric ozone depletion and global warming in comparison to the fully halogenated species. The ozone depletion potential of monochloro- difluoromethane (HCFC-22) is low.

The alternative or substitute materials must also possess those properties unique to the CFC's including chemical stability, low toxicity, non-flammability, and efficiency in use. The latter characteristic is important, for example, in air conditioning and refrigeration where a loss in refrigerant thermodynamic performance or energy efficiency may have secondary environmental impacts through increased fossil fuel usage arising from an increased demand for electrical energy.

The majority of refrigerants utilized in vapor compression systems are either single component fluids or azeotropic mixtures. The latter are binary mixtures, but for all refrigeration purposes behave as single component fluids. Nonazeotropic mixtures have been disclosed as refrigerants for example in U.S. Patent 4,303,536 but have not found widespread use in commercial applications.

The condensation and evaporation temperatures of single component fluids are defined clearly. If we ignore the small pressure drops in the refrigerant lines, the condensation or evaporation occurs at a single temperature corresponding to the condenser or evaporation pressure. For mixtures ^' being employed as

refrigerants, there is no single phase change temperature but a range of temperatures. This range is governed by the vapor-liquid equilibrium behavior of the mixture. This property of mixtures is responsible for the fact that when nonazeotropic mixtures are used in the refrigeration cycle, the temperature in the condenser or the evaporator has no longer a single uniform value, even if the pressure drop effect is ignored. Instead, the temperature varies across the equipment, regardless of the pressure drop. In the art this variation in the temperature across an equipment is known as temperature glide.

It has been pointed out in the past that for nonisothermal heat sources and heat sinks, this temperature glide in mixtures can be utilized to provide better efficiencies. However in order to benefit from this effect, the conventional refrigeration cycles has to be redesigned, see for example T. Atwood "NARBS - The Promise and the Problem", paper 86-WA/HT-61 American Society of Mechanical Engineers. In most existing designs of refrigeration equipment, a temperature glide is a cause of concern. Therefore nonazeotropic refrigerant mixtures have not found wide use. An environmentally acceptable nonazeotropic refrigerant mixture with a very small temperature glide and with an advantage in refrigeration capacity over other known pure fluids would be of substantial value. This will be more important if the mixture retains these advantages over extreme operating conditions.

The nonazeotropic mixture of HFC-125 (pentafluoro- ethane) and HCFC-22 (monochlorodifluoromethane) has been mentioned as a mixture suitable for use as a

nonazeotropic refrigerant in Research Disclosures, pp. 13-14 (June, 1976) . However this reference fails to disclose a specific composition or compositions range which may be useful as a refrigerant. Moreover, this reference teaches away from using equimolar amounts of the components in mixed refrigerants for refrigeration.

It is an object of this invention to provide novel compositions comprising pentafluoroethane and mono- chlorodifluoromethane which are useful in cooling and heating applications which have high refrigeration capacities.

Another object of the invention is to provide such compositions for use in the aforementioned applications which are environmentally acceptable.

Still another object of the invention is to provide such compositions which exhibit a small temperature glide.

Yet another object of the invention is to provide such compositions which may be used as a replacement for Refrigerant 502. Refrigerant 502 is a blend of 48.8 weight percent HCFC-22 and 51.2 weight percent CFC-115 (monochlorodifluoromethane) .

Other objects and advantages of the invention will become apparent from the following description.

Description of the Invention

In accordance with the invention, novel compositions have been discovered comprising from about 25 to about 75 weight percent pentafluoroethane (HFC-

125) and from about 75 to about 25 weight percent ono- chlorodifluoromethane (HCFC-22) which are useful in cooling and heating applications. These compositions are nonazeotropic because they do not exhibit a maximum or a minimum in the vapor pressure versus composition curve.

In a preferred embodiment of the invention, the compositions of the invention comprise from about 30 to about 70 weight percent HFC-125 and from about 70 to about 30 weight percent HCFC-22.

In a still preferred embodiment of the invention, the compositions of the invention comprise from about 40 to about 60 weight percent HFC-125 and from about 60 to about 40 weight percent HCFC-22.

In the most preferred embodiment of the invention, the compositions of the invention comprise about 50 weight percent of HFC-125 and about 50 weight percent of HCFC-22.

In one process embodiment of the invention, the compositions of the invention may be used in a method for producing refrigeration which involves condensing a fluid comprising the compositions and thereafter evaporating the refrigerant in the vicinity of a body to be cooled. Other than the choice of fluid, this process is conventional.

In another process embodiment of the invention, the compositions of the invention may be used in a method for producing heating which involves condensing a fluid comprising the compositions in the vicinity of a body to be heated and thereafter evaporating the

refrigerant. Other than the choice of fluid, this process is conventional.

The HFC-125 and HCFC-22 components of the novel compositions of the invention are known materials, preferably they should be used in sufficiently high purity so as to avoid the introduction of adverse influences upon the properties of the system.

Surprisingly, the novel compositions of the invention exhibit, at certain operating conditions, a higher refrigeration capacity than that of either of its HFC-125 or HCFC-22 components. This result is particularly surprising in view of the fact that these mixtures exhibit negative deviations from Raoult's Law which suggest lower vapor pressures and correspondingly of the invention also exhibit an unexpectedly low temperature glide.

It should be understood that the present compositions may include one or more additional components, such as auxiliary refrigerants or heating media, lubricants or other additives. Such additional components may or may not form azeotropic compositions with the HFC-125 and HCFC-22 components.

One example of such an additive is propane which may be added to enhance solubility of lubricants (particularly mineral oils) in the HFC-125/HCFC-22 compositions of this invention. If propane is used as an additive, it should be present in an amount effective to improve the solubility of the lubricant in the claimed composition. Generally, about 1.1 to about 7.6 and, preferably about 2.2 to about 6.6 weight percent of the propane based on the weight of the HFC-

125 and HCFC-22 is effective for this purpose (or from about l to about 7 and, preferably about 2 to about 6 weight percent based on the entire composition) . The propane which is present forms an azeotrope with the HFC-125 component of the compositions of the invention.

The present invention is more fully illustrated by the following non-limiting Examples.

EXAMPLE 1

This example of the vapor-liquid equilibrium (VLE) shows that a negative deviation from Raoult's Law occurs in the HFC-125 and HCFC-22 system and that the system is nonazeotropic.

The VLE of the system was measured by charging a stainless steel cell of approximately 150 cubic centimeter volume with a known amount of HFC-125. The vessel was equipped with a magnetically driven stirrer and a 0-3000 kPa pressure transducer accurate to + 0.2%. The VLE cell was frozen to reduce the vapor pressure of the first component and then a known amount of HCFC-22 was added. The vessel was submerged in a constant temperature bath controlled to within + 0.03K. The vapor pressure measurement was recorded once thermal equilibrium was attained. The vapor phase and the liquid phase samples were taken and analyzed on a gas chromatograph after shutting down the stirrer. This procedure was repeated at different HFC-125 and HCFC-22 compositions. Table I summarizes the results of these experiments.

Interpolation among the data listed in Table I indicates that at low temperatures the dew and bubble

points are very close, implying small temperature glides in the evaporator. The data also indicate a negative deviation from Raoult's Law which normally would imply a lower refrigeration capacity for the mixture. This VLE data is important because it can be fitted to a mixture equation of state. From this mixture equation of state, we can obtain necessary thermodynamic information for estimating the efficiency of a refrigeration cycle using the compositions of the invention as refrigerants. Such estimates are provided in Example 2 herein.

TABLE I

Composition

EXAMPLE 2

This example shows that HFC-125/HCFC-22 blends have certain performance advantages when compared to HFC-125 or HCFC-22 alone.

The theoretical performance of a refrigerant at specific operating conditions can be estimated from the thermodynamic properties of the refrigerant using standard refrigeration cycle analysis techniques, see for example, "Fluorocarbons Refrigerants Handbook", ch. 3, Prentice-Hall. (1988), by R.C. Downing. The coefficient of performance, COP, is a universally accepted measure, especially useful in representing the relative thermodynamic efficiency of a refrigerant in a specific heating or cooling cycle involving evaporation or condensation of the refrigerant. In refrigeration engineering this term expresses the ratio of the useful refrigeration to the energy applied by the compressor in compressing the vapor. The capacity of a refrigerant represents the volumetric efficiency of the refrigerant. To a compressor engineer this value expresses the capability of a compressor to pump quantities of heat for a given volumetric flow rate of refrigerant. In other words, given a specific compressor, a refrigerant with a higher capacity will deliver more cooling or heating power. A similar calculation can also be performed for nonazeotropic refrigerant blends.

We have performed this type of calculation for a low temperature refrigeration cycle where the condenser temperature is typically 100"F to 160"F and the evaporator temperature is typically -40"F. We have further assumed isentropic compression and a compressor inlet temperature of 65"F. Such calculations were performed for a 75/25 by weight bleϊid and a 25/75 by

weight blend, as well for 100 percent HCFC-22 and for 100 percent HFC-125. The highest temperature glide was, as expected, in the condenser but in no case exceeded 1.4°F. The temperature glide due to the nonazeotropic nature of the mixture is smaller than the temperature profile already present due to the pressure drops in existing machinery.

Therefore the temperature glide of the mixture compositions claimed is small enough to be negligible and does not pose a problem for conventional refrigeration units. This was an unexpected discovery since it is known that for ideal mixtures the temperature glide is approximately one-third of the boiling point difference. (Ref: P.S. Burr and G. G. Haselden, "Proceedings of the Institute of Refrigeration, pp. 18-26, Vol. 71, 1974-5) . Based on this rule, the temperature glide for the mixture compositions claimed would be expected to be about 4°F. Surprisingly, the actual temperature glide for the claimed mixtures is about 1°F which is much smaller than what would have been expected.

Reference is hereby made to accompanying Tables II and III which form a part of this specification. Table II depicts the relationships of COP, capacity, compressor discharge pressure and the compressor discharge temperature, respectively, as a function of the mixture composition at an average condensing temperature of 100"F. Similarly, Table III depicts the relationship of the same quantities as a function of compositions at an average condensing temperature of 140°F. We also show in these Tables the values of the same quantity when Refrigerants 502,125 and 22 are used under the same conditions.

As can be further seen from Tables II and III, in the composition ranges studied, the blends provide a modest improvement in COP compared to that attainable with Refrigerant 502. The blends also produce discharge temperatures and pressures similar to that produced by Refrigerant 502 used in the art. It is noted from these Tables that HCFC-22 alone gives a higher discharge temperature in a compressor. A high discharge temperature usually results in a loss of compressor reliability. We have already noted the loss in capacity with HFC-125 at high condensing temperatures. The compositions of the invention give low discharge temperatures similar to those obtained with Refrigerant 502 without the loss in capacity associated with HFC-125 alone. This shows that one could replace the ozone layer destroying Refrigerant 502 with the claimed compositions of HFC-125 and HCFC- 22 in existing refrigeration machines without substantial modifications.

Thus Tables II and III document the unexpected advantages that result from combining HCFC-22 with HFC- 125 in certain proportions. The condensing temperatures in air cooled low temperature refrigeration systems located in hot climates are in the order of 140 ^β F. In such cases the refrigeration capacity of the nonazeotropic mixtures containing 30 to 70 percent by weight HFC-125 is higher than that of HCFC-22 or HFC-125 alone, as the data in Table III indicate. As previously indicated, this result is surprising in view of the fact that these mixtures exhibit negative deviations from Raoult's Law which suggest lower vapor pressures and correspondingly lower refrigeration capacities.

TABLE II

Blend Performance Relative to Refrigerant 502 at Condenser Temperature 100"F

TABLE III

Blend Performance Relative to Refrigerant 502 at Condenser Temperature 140°F

Having described the invention in detail and by reference to preferred embodiments thereof, it will be apparent that modifications and variations are possible without departing from the scope of the invention defined in the appended claims.