AZEOTROPE-LIKE COMPOSITIONS OF 1,1,1,2-TETRAFLUOROETHANE AND 1,1-DIFLUOROETHANE

Abstract: Novel azeotrope-like compositions of 1,1,1,2-tetrafluoroethane and 1,1-difluoroethane which are useful in heating and cooling applications.

Description: AZEOTROPE-LIKE COMPOSITIONS OF 1.1.1.2-TETRAFLUOROETHANE AND 1.1-PIFLUOROETHANE FIELD OF THE INVENTION This invention relates to azeotrope-like compositions of 1,1,1,2-tetrafluoroethane and 1.1-difluoroethane. These mixtures are useful as refrigerants for heating and cooling applications. BACKGROUND OF THE INVENTION Fluorocarbon based fluids have found widespread use in industry for refrigeration, 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. 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 vapor formed 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 addition of mechanical energy by the compressor. The high pressure vapor then passes to the condenser whereupon heat exchanges with a cooler medium. 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. Single component fluids and azeotropic mixtures are characterized as constant-boiling because they exhibit isothermal and isobaric evaporation and condensation. The use of azeotropic mixtures as refrigerants is known in the art. See. for example. R.C. Downing, "Fluorocarbon Refrigerants Handbook", pp. 139-158. Prentice-Hall. 1988. and U.S. Patents 2.101.993 and 2.641.579. Azeotropic or azeotrope-like compositions are desired because they do not fractionate upon boiling or evaporation. 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, and unless the refrigerant composition is constant boiling, 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. Non-azeotropic mixtures have been disclosed as refrigerants, see. e.g.. U.S. Patent 4,303.536. but have not found widespread use in commercial applications even though the ability of non-azeotropic refrigerant blends to exhibit improved thermodynamic performance has often been discussed in the literature. See, e.g., T. Atwood. "NARBS - The Promise and the Problem", American Society of Mechanical Engineers. Winter Annual Meeting, paper 86-WA/HT-61. 1986 and M.O. McLinden et al., "Methods for Comparing the Performance of Pure and Mixed Refrigerants in the Vapor Compression Cycle", Int. J. Refriq. 10, 318 (1987). Because non-azeotropic mixtures may fractionate during the refrigeration cycle, they require certain hardware changes. The added difficulty in changing and servicing refrigeration equipment is the primary reason that non-azeotropic mixtures have been avoided. The situation is further complicated if an inadvertent leak in the system occurs during such use or service. The composition of the mixture could change, affecting system pressures and system performance. Thus, if one component of the non-azeotropic mixture is flammable, fractionation could shift the composition into the flammable region with potentially adverse consequences. The art is continually seeking new fluorocarbon based azeotrope-like mixtures which offer alternatives for refrigeration and heat pump applications. Currently, environmentally acceptable fluorocarbon-baβed refrigerants are of particular interest because the fully halogenated chlorofluorocarbons have been implicated in causing environmental problems associated with the depletion of the earth's protective ozone layer. Mathematical models have substantiated that hydrofluorocarbons like 1.1.1.2-tetrafluoroethane (HFC-134a) and 1.1-difluoroethane (HFC-152a). will not adversely affect atmospheric chemistry because their contribution to stratospheric ozone depletion and global warming in comparison to the fully halogenated species is negligible. The 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 refrigeration applications like air conditioning where a loss in refrigerant thermodynamic performance or energy efficiency may produce secondary environmental effects due to increased fossil fuel usage arising from an increased demand for electrical energy. Furthermore, the ideal CFC refrigerant substitute would not require major engineering changes to conventional vapor compression technology currently used with CFC refrigerants. It is accordingly an object of this invention to provide novel azeotrope-like compositions based on 1,1.1.2-tetrafluoroethane and 1.1-difluoroethane which are useful in cooling and heating applications. Another object of the invention is to provide novel environmentally acceptable refrigerants for use in the aforementioned applications. Other objects and advantages of the invention will become apparent from the following description. SUMMARY OF THE INVENTION The invention relates to novel environmentally acceptable azeotrope-like compositions of 1,1.1,2-tetrafluoroethane and 1.1-difluoroethane which are useful in heating and refrigeration applications. DESCRIPTION OF THE INVENTION In accordance with the invention, novel azeotrope-like compositions have been discovered comprising 1.1.1.2-tetrafluoroethane and 1,1-difluoroethane. The azeotrope-like compositions comprise from about 5 to about 90 weight percent 1,1.1,2-tetrafluoroethane and from about 10 to about 95 weight percent 1,1-difluoroethane and have a vapor pressure of about 76 psia ± 5 psia at 20°C. These compositions are azeotrope-like because they exhibit essentially constant vapor pressure versus composition and essentially identical liquid and vapor compositions over the aforementioned ranges. In a preferred embodiment of the invention, such azeotrope-like compositions comprise from about 40 to about 85 weight percent 1.1,1.2-tetrafluoroethane and from about 15 to about 60 weight percent 1.1- difluoroethane and have a vapor pressure of 76 psia + 3 psia at 20°C. Vapor phase compositions containing in excess of about 80 weight percent 1.1.1.2-tetrafluoroethane were determined to be nonflammable in air at ambient conditions using the Bureau of Mines - style eudiometer apparatus. The azeotrope-like compositions of this invention, comprised of HFC-152a and HFC 134a, do not segregate. In addition, they exhibit a number of advantages over dichlorodifluoromethane (CFC-12), HFC-134. and HFC-152a. For example, the azeotrope-like mixtures are non-flammable above 80 weight percent HFC-134a thereby reducing the hazard of explosion which might occur if flammable HFC-152a vapors were used, stored, or handled in pure form. These azeotrope-like mixtures also exhibit zero ozone depletion potential and low atmospheric lifetime hence they contribute negligibly to the greenhouse warming effect. This is contrasted with the high ozone depletion potential and correspondingly high greenhouse warming potential of CFC-12. . . . The energy efficiency and cooling capacity of the azeotrope-like compositions of the invention are superior to those of pure HFC-134a and. in addition, the 134a/152a compositions provide significantly reduced direct and indirect greenhouse warming potential over pure HFC-134a. The azeotropic compositions of this invention uniquely possess all of the desireable features of an ideal refrigerant i.e.. safe to use, non-flammable, zero ozone depletion potential, negligible greenhouse warming effect, and attractive energy/cooling performance compared to the most relevant pure fluoromethane or fluoroethane fluids; i.e.. fluorocarbon refrigerants boiling between -19 C and o . . . -30 C (this range represents the boiling point range for the majority of currently used air conditioning and refrigerant working fluids). In summary, when HFC-152a/HFC-134a are combined in effective amounts, a non-flammable, non-segregating, environmentally acceptable azeotrope-like refrigerant having improved thermodynamic performance results. The term "azeotrope-like" is used herein for mixtures of the invention because in the claimed proportions, the compositions of 1.1.1.2-tetrafluoethane and 1.1-difluoroethane are constant boiling or essentially constant boiling. All compositions within the indicated ranges, as well as certain compositions outside the indicated ranges, are azeotrope-like, as defined more particularly below. From fundamental principles, the thermodynamic state of a fluid is defined by four variables: pressure, temperature, liquid composition, and vapor composition, or P-T-X-Y, respectively. An azeotrope is a unique characteristic of a system of two or more components where X and Y are equal at a stated P and T. In practice this means that the components cannot be separated during a phase change, and therefore are useful in the cooling and heating applications described above. . . . For the purposes of this discussion, by azeotrope-like composition is intended to mean that the composition behaves like a true azeotrope in terms of this constant boiling characteristics or tendency not to fractionate upon boiling or evaporation. Thus, in such systems, the composition of the vapor formed during the 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 slightly. This is contrasted with non-azeotrope-like compositions in which the liquid and vapor compositions change substantially during evaporation or condensation. If the vapor and liquid phases have identical compositions, then it can be shown, on a rigorous thermodynamic basis, that the boiling point versus composition curve passes through an absolute maximum or an absolute minimum at this composition. If one of the two conditions, identical liquid and vapor compositions or a minimum or maximum boiling point, are shown to exist, then the system is an azeotrope, and the other condition must follow. One way to determine whether a candidate mixture is azeotrope-like within the meaning of this invention, is to distill a sample thereof under conditions (i.e.. resolution - number of plates) which would be expected to separate the mixture into its separate components. If the mixture is non-azeotrope or non-azeotrope-like. the mixture will fractionate, i.e.. separate into its various components with the lowest boiling component distilling off first, and so on. If the mixture is azeotrope-like, some finite amount of the first distillation cut will be obtained which contains all of the mixture components and which is constant boiling or behaves as a single substance. This phenomenon cannot occur if the mixture is not azeotrope-like. i.e., it is not part of an azeotrope system. It follows from the above that another characteristic of azeotrope-like compositions is that there is a range of compositions containing the same components in varying proportions which are azeotrope- like. All such compositions are intended to be covered by the term azeotrope-like as used herein. As an example, it is well known that at different pressures the composition of a given azeotrope will vary at least slightly as does the boiling point of the composition. Thus, an azeotrope of A and B represents a unique type of relationship but with a variable composition depending on the temperature and/or pressure. As is readily understood by persons skilled in the art. the boiling point of an azeotrope will vary with the pressure. In one process embodiment of the invention, the azeotrope-like compositions of the invention may be used in a method for producing cooling which comprises condensing a refrigerant comprising the azeotrope-like compositions and thereafter evaporating the refrigerant in the vicinity of the body to be cooled. In another process embodiment of the invention, the azeotrope-like compositions of the invention may be used in a method for producing heating which utilizes condensing a refrigerant in the vicinity of the body to be heated and thereafter evaporating the refrigerant. For purposes of this application, the process embodiments for producing cooling or heating, discussed above, will generally be referred to as heat exchange applications. The 1.1.1.2-tetrafluoroethane and 1.1-difluoroethane components of the novel azeotrope-like 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 constant boiling properties of the system. It should be understood that the present compositions may include additional components so as to form new azeotrope-like compositions. Any such compositions are considered to be within the scope of the present invention as long as the compositions are essentially constant boiling and contain all the essential components described herein. In addition, the azeotrope-like compositions of the invention may include components which may not form new azeotrope-like compositions. In particular, lubricants like those discussed in U.S. Patent 4,755.316 may be added without departing from the scope of the invention.. The present invention is more fully illustrated by the following non-limiting Examples. -10- EXAMPLE 1 This example shows that certain compositions of 1.1.1.2-tetrafluoroethane and 1.1-difluoroethane are azeotrope-like, i.e., exhibit essentially identical liquid and vapor compositions, and are constant boiling, i.e., exhibit essentially constant vapor pressure versus composition within this range. 0 Vapor liquid equilibrium experiments were performed by preparing mixtures of HFC-134a and HFC-152a in an approximately 150 cubic centimeter vessel. The vessel, equipped with a magnetically driven stirrer and a 0-300 psia pressure transducer _ accurate to ± 0.2*. was submerged in a constant temperature bath controlled to within + 0.02°C. Once thermal equilibrium was attained, as determined by constant vapor pressure readings, vapor and liquid samples were withdrawn from the vessel and analyzed by standard gas chromatographic techniques. This 0 procedure was repeated at three nominal compositions of approximately 25. 50 and 70 mole percent HFC-134a in HFC-152a. and at three temperatures, -20. 20 and 60°C. Table I summarizes the results of these experiments. 5 The data shown in Table I indicate that the vapor and liquid compositions are essentially identical within the experimental uncertainty of +.2.0 weight percent unit associated with the chromatographic Q analysis. The vapor pressure data measured at -20°C show a minimum versus composition which is evidence of azeotropic behavior. The vapor pressures of the blends are essentially constant to within +.5 psia over the composition range from about 5 to about 90 weight 5 percent HFC-134a and from about 10 to about 95 weight percent HFC-152a. that is, these blends are constant boiling or azeotrope-like. EXAMPLE 2 This example shows that azeotrope-like HFC-134a/HFC-152a blends have certain performance advantages when compared to 'FC-134a alone. The performance of a refrigerant at specific operating conditions can be measured by the coefficient of performance and the capacity of the refrigerant. 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 TAB I I HFC-134a/HFC-152a Vapor Liquid Equilibria Data Temperature Liquid Composition* Vapor Composition* Vapor Pressure CO (vt. % HFC-134a) (wt. * HFC-134a) (psia) 0.0 17 .8 35.9 17 .1 63.4 17 .2 79.5 17 . 7 100.0 19.3 0.0 74. 7 36 .4 75.0 63.4 76 . 7 80.1 79.0 100.0 82.9 0.0 218.9 36. 7 219.5 63.2 225 . 7 79.4 233.5 100.0 243.9 •Weight percent HFC-134-a in HFC-152a or condensation of the refrigerant. In refrigeration engineering this term expresses the ratio of useful refrigeration to the energy applied by the compressor in compressing the vapor. The capacity of a refrigerant represe- s 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. The performance of a 78/22 HFC-134a/HFC-152a by weight azeotrope-like blend was evaluated in a typical automotive air conditioning unit operated under controlled laboratory calorimeter conditions. The compressor and condenser sections of the air conditioning cycle were maintained in a controlled environment of 100°F. Thermocouples were used to measure the temperature of the air flowing to and from the condenser and the temperature of the refrigerant at the discharge and suction ports of the compressor as well as at the condenser outlet. The compressor was operated at a constant speed of 1188 revolutions per minute with an electric motor. A Watt-hour meter was used to determine the mechanical work input to the compressor. Heat removal from the condenser environment was achieved using chilled water flowing at a measured flow rate to a cooling coil in the condenser room. The capacity of the air conditioning system is determined by performing an energy balance over the condenser room. The evaporator and expansion valve section of the air conditioning cycle were maintained at 100°F and 40* relative humidity. Thermocouples were used to measure the temperatures of the refrigerant leaving the evaporator and leaving the accumulator. Four pressure transducers, located in the compressor suction and discharge lines and just after the condenser and accumulator, were used to measure the refrigerant pressure throughout the cycle. Two sets of electrical heaters in the evaporator room were continuously adjusted to balance the heat removed by the evaporator. Tests were performed at the above specified . . conditions for three refrigerants, CFC-12 (dichlorodifluoromethane) , HFC-134a and the 78/22 HFC-134a/HFC-152a azeotrope-like blend. CFC-12 is a fully halogenated chlorofluorocarbon which has been used widely in air conditioning and refrigeration applications. CFC-12 has been determined to be a contributor to the depletion of the Earth's stratospheric ozone layer. Each test consisted of at least two consecutive experiments where the measured COP's agreed to within 1*. The data for these tests is reported in Table II. The data listed in this table for Tests 1-3 show that the 78/22 HFC-134a/HFC-152a azeotrope-like blend provides a 6.6* increase in COP over HFC-134a and a capacity similar to that of HFC-134a. The blend also provides a COP and capacity comparable to that attained with CFC-12. The compressor discharge pressure for the blend is lower than that exhibited by HFC-134a, which aliminates the need to design air conditioning equipment to withstand higher operating pressures. Tests 4-6 represents a more extreme condition where the air flow over the condenser has been reduced which results in greater discharge pressures and temperatures. The HFC-134a/HFC-152a blend provides a 4* improvement in COP and a slight (2*) drop -14- Tabla II Performance Of λ 78/22 HFC-124a/HFC-152a Bland Pressure Temperature Cooling Compressor Discharge Accumulator Discharge Acccumulator Capacity Work COP (psia) (psia) CF) CF) (Watt) (Watt) Test 1: CFC-12 255.2 60.8 174.0 53.2 5194.7 2153.4 2.41 Test 2: HFC-134a 284.0 59.5 163.6 51.3 4948.8 2224.2 2.23 Test 3: HFC-134a/HFC-152a(78/22) 270.9 57.1 171.2 52.3 5069.7 2128.6 2.38 Test 4: CFC-12 313.7 60.7 187.8 53.1 3957.4 2332.8 1.70 Test 5: HFC-134a 352.7 59.6 180.4 51.3 3538.9 2881.0 1.49 Test 6: HFC-134a/HFC-152a(78/22) 339.1 57.2 194.4 51.6 3603.1 2321.7 1.55 in capacity compared to HFC-134a as well as a reduction in discharge pressure. 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.

Claims: Claims: 1. Azeotrope-like compositions comprising 1,1,1,2-tetrafluoroethane and 1,1-difluoroethane which have a vapor pressure of about 76 psia ± 5 psia at 20°C. 2. Azeotrope-like compositions comprising 1,1,1,2-tetrafluoroethane and 1,1-dichloroethane, which have a vapor pressure of about 76 psia ± 5 psia at 20°C, in effective amounts for heat exchange applications. 3. Azeotrope-like compositions consisting essentially of 1,1,1,2-tetrafluoroethane and 1, 1-difluoroethane which have a vapor pressure of about 76 psia ± 5 psia at 20°C. 4. Azeotrope-like compositions comprising from about 5 to about 90 weight percent 1, 1,1,2-tetra- fluoroethane and from about 10 to about 95 weight percent 1,1-difluoroethane which have a vapor pressure of about 76 psia at 20°C. 5. The azeotrope-like compositions of claim 4 wherein said compositions contain from about 40 to about 85 weight percent 1,1,1,2-tetrafluoroethane and from about 15 to about 60 weight percent 1, 1-difluoroethane and have a vapor pressure of about 76 psia at 20°C. 6. Azeotrope-like compositions consisting essentially of from about 5 to about 90 weight percent 1, 1,1,2-tetrafluoroethane and from about 10 to about 95 weight percent 1, 1-diflLiorσethane which have a vapor pressure of about 76 psia at 20 C. 7. The azeotrope-like compositions of claim 6 wherein said compositions contain from about 40 to about 85 weight percent 1, 1,1,2-tetrafluoroethane and from about 15 to about 60 weight percent 1,1-difluoroethane which have a vapor pressure of about 76 psia at 20°C. 8. A method of producing cooling comprising condensing a composition of claim 1 and thereafter evaporating said composition in the vicinity of a body to be cooled. 9. A method of producing heating comprising condensing a composition of claim 1 in the vicinity of a body to be heated and thereafter evaporating said composition.

Inventors: KNOPECK GARY M, LUND EARL A E, NG FUN Y, ORFEO S ROBERT, WILSON DAVID P, KNOPECK, GARY, M., LUND, EARL, A., E., NG, FUN, Y., ORFEO, S., ROBERT, WILSON, DAVID, P.
Applicants: ALLIED SIGNAL INC, ALLIED-SIGNAL INC.
Application year: 1990
Main classification: C09K5
IPC codes: C09K5