BACKGROUND OF THE INVENTION

This invention relates to azeotrope-like or constant-boiling mixtures of 1 ,1 , 1 ,2-tetrafluoroethane (HFC-134a) or 1 , 1 ,2,2-tetrafluoroethane (HFC- 134) and ammonia. These mixtures are useful as refrigerants for heating and cooling. HFC-134a and HFC-134 shall collectively be referred to herein as "tetrafluoroethane".

Fluorocarbon based fluids have found widespread use in industry for refrigeration applications such as air conditioning and heat pump applications. Vapor compression is one form of refrigeration. In its simplest form, vapor compression 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.

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 chlorofluoromethane and chlorofluoroethane derivatives have gained widespread use as refrigerants in applications including air conditioning and heat pump applications owing to their unique combination of chemical and physical properties. Ammonia is often used in industrial

refrigeration. The majority of refrigerants utilized in vapor compression systems are either single component fluids or azeotropic mixtures.

Azeotropic or azeotrope-like compositions are desired as refrigerants because they do not fractionate upon boiling. This behavior is desirable because in the previously described vapor compression equipment with which these refrigerants are employed, condensed material is generated in preparation for cooling or for heating purposes. Unless the refrigerant composition exhibits a constant boiling point, i.e. is azeotrope-like, fractionation and segregation will occur upon evaporation and condensation and undesirable refrigerant distribution may act to upset the cooling or heating.

The art is continually seeking new azeotrope-like mixtures which offer alternatives for refrigeration and heat pump applications. Fluorocarbon and hydrofluorocarbon based azeotrope-like mixtures are of particular interest because they are considered to be environmentally safe substitutes for the presently used fully halogenated chlorofluorocarbons (CFC's) which are suspected of causing environmental problems in connection with the earth's protective ozone layer. Ammonia is a well known refrigerant however its flammability properties and high discharge temperatures, when used in vapor compression machines, are of some disadvantage.

Refrigerants ideally must possess properties like chemical stability, low toxicity, non-flammability, and efficiency in-use. The latter characteristic is important in refrigeration and air-conditioning especially 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. Furthermore, the ideal CFC refrigerant substitute should not require major engineering

changes to conventional vapor compression technology currently used with ammonia or CFC refrigerants. Mathematical models have substantiated that hydrofluorocarbons, such as 1 , 1 , 1 ,2-tetrafluoroethane (HFC-134a) or 1 ,1 ,2,2-tetrafluoroethane (HFC-134), will not adversely affect atmospheric chemistry, being a negligible contributor to ozone depletion and to green¬ house global warming in comparison to the fully halogenated species.

U.S. patent 3,732, 150 published on May 8, 1973 teaches a non- azeotropic composition of HFC-134 and ammonia.

DETAILED DESCRIPTION OF THE INVENTION

Our solution to the need in the art for substitutes for existing refrigerants is azeotrope-like mixtures consisting essentially of tetrafluoroethane and ammonia.

The term "azeotrope-like" as used herein is intended to mean that the composition behaves like an azeotrope, i.e. has constant-boiling characteristics or a tendency not to fractionate upon boiling or evaporation. Thus, in such compositions, the composition of the vapor formed during boiling or evaporation is identical or substantially identical to the original liquid composition. Hence, during boiling or evaporation, the liquid composition, if it changes at all, changes only to a minimal or negligible extent. This is to be contrasted with non-azeotrope-like compositions in which during boiling or evaporation, the liquid composition changes to a substantial degree.

The azeotrope-like compositions of the invention are advantageous for the following reasons. Each component is a negligible contributor to ozone

depletion. Also, because the present compositions exhibit essentially constant-vapor pressure characteristics as the liquid mixture is evaporated and show relatively minor shifts in composition during evaporation, the compositions are shown advantageous in a vapor compression cycle as they mimic the performance of a constant-boiling single component or azeotropic mixture refrigerant. This is to be contrasted with the performance of the non-azeotropic composition of HFC-134 and ammonia disclosed in U.S. patent 3,732, 150.

The preferred HFC-134a azeotrope-like compositions are shown in the following Table I. In the Table, the numerical ranges are understood to be prefaced by "about":

Table I

COMPONENTS PREFERRED MORE MOST BOILING RANGE PREFERRED PREFERRED POINT (WT.%) RANGE RANGE (oC/14.7

(WT.%) (WT.%) PSIA)

HFC- 134a 25 - 85 35 - 80 45 - 75 -37 ± 1

Ammonia 75 - 15 65 - 20 55 - 25

The preferred HFC-134 azeotrope-like compositions are shown in the following Table II. In the Table, the numerical ranges are understood to be prefaced by "about":

Table II

COMPONENTS PREFERRED MORE MOST BOILING RANGE PREFERRED PREFERRED POINT (WT.%) RANGE RANGE (°C./14.7

(WT.%) (WT.%) PSIA)

HFC-134 5 - 80 10 - 70 15 - 65 -33.6 ± Λ

Ammonia 95-20 90 - 30 85 - 35

The tetrafluoroethane azeotrope-like compositions have boiling points which are lower than that of ammonia (-33.3°C) or that of the tetrafluoro¬ ethane component. The cooling capacity of the azeotrope-like compositions is therefore higher than that of either component. Unlike ammonia, certain compositions of ammonia and tetrafluoroethane mixtures are also nonflammable. Addition of tetrafluoroethane to ammonia also causes the discharge temperature of the compressor in a vapor compression machine to decrease, an advantage from the refrigeration engineer's point of view.

Additional components may be added to the mixture to tailor the properties of the mixture according to the need. For example, in the art propane has been added to refrigerant compositions to aid oil solubility, but is not considered to substantially affect the refrigeration properties of the mixture.

In one process embodiment of the invention, the azeotrope-like compositions of the invention may be used in a method for producing refrigeration which comprises condensing a refrigerant comprising the

azeotrope-like compositions and thereafter evaporating the refrigerant in the vicinity of a body to be cooled.

In another process embodiment of the invention, the azeotrope compositions of the invention may be used in a method for producing heating which comprises condensing a refrigerant comprising the azeotrope- like compositions in the vicinity of a body to be heated and thereafter evaporating the refrigerant.

The tetrafluoroethane and ammonia components of the novel azeotrope-like compositions of the invention are known materials and are commercially available or may be manufactured by procedures well known in the art. Preferably, the materials should be used in sufficiently high purity so as to avoid the introduction of adverse influences upon the cooling or heating properties or constant-boiling properties of the system.

EXAMPLE 1

The following example establishes that HFC-134a and ammonia form an azeotrope since a minimum occurs in the boiling point curve for this system. The temperature of boiling liquid mixture was measured using an ebulliometric technique similar to that described by W. Swietoslawski in Ebulliometric Measurements, p.4, Reinhold Publishing Corp. (1945).

The ebulliometer was first charged with a weighed amount of ammonia. The condenser was cooled with dry-ice and ethanol mixture. The temperature was measured with a platinum resistance thermometer. The boiling temperature and pressure were recorded after steady state was achieved. An aliquot of HFC-134a was introduced into the ebulliometer and the temperature recorded after the attainment of equilibrium. The process

was repeated again with addition of additional aliquots. The boiling point data showed a minimum at a -37°C. _±. 1 °C at 14.7 psia., about 4°C below the boiling point of ammonia, in the boiling temperature versus composition curve, i.e., ammonia and HFC-134a form a positive azeotrope.

EXAMPLE 2

This Example shows that HFC-134 and ammonia also form an azeotrope. The experiment was done in a manner identical to the one in Example 1 , except that HFC-134 was used instead of HFC-134a.

The boiling point data showed a minimum at a -33.6°C. ±_ .1 at 14.7 psia, about 0.3° C below the boiling point of ammonia, in the boiling temperature versus compositions curve, i.e., ammonia and HFC-134 also form a positive azeotrope.