Heat transfer compositions

The invention provides a heat transfer composition comprising (i) from about 45 to about 75% by weight 2,3,3,3-tetrafluoropropene (R-1234yf); and (ii) from about 25 to about 55% by weight 1,1,1,2-tetrafluoroethane (R-134a). A heat transfer composition comprising, optionally consisting essentially of, (i) from about 20 to about 90% by weight R-1234yf; (ii) from about 10 to about 60% by weight R-134a; and (iii) from about 1 to about 20% by weight R-32 is also provided.

SUMMARY OF THE INVENTION

The invention relates to heat transfer compositions, and in particular to heat transfer compositions which may be suitable as replacements for, or alternatives to, existing refrigerants such as R-134a, R-152a, R-1234yf, R-22, R-410A, R-407A, R-407B, R-407C, R-507 and R-404A. Certain of the compositions are particularly suitable as alternatives for replacing R-134a.

Mechanical refrigeration systems and related heat transfer devices such as heat pumps and air-conditioning systems are well known. In such systems, a refrigerant liquid evaporates at low pressure taking heat from the surrounding zone. The resulting vapour is then compressed and passed to a condenser where it condenses and gives off heat to a second zone, the condensate being returned through an expansion valve to the evaporator, so completing the cycle. Mechanical energy required for compressing the vapour and pumping the liquid is provided by, for example, an electric motor or an internal combustion engine.

In addition to having a suitable boiling point and a high latent heat of vaporisation, the properties preferred in a refrigerant include low toxicity, non-flammability, non-corrosivity, high stability and freedom from objectionable odour. Other desirable properties are ready compressibility at pressures below 25 bars, low discharge temperature on compression, high refrigeration capacity, high efficiency (high coefficient of performance) and an evaporator pressure in excess of 1 bar at the desired evaporation temperature.

Dichlorodifluoromethane (refrigerant R-12) possesses a suitable combination of properties and was for many years the most widely used refrigerant. Due to international concern that fully and partially halogenated chlorofluorocarbons, such as dichlorodifluoromethane and chlorodifluoromethane, were damaging the earth's protective ozone layer, there was general agreement that their manufacture and use should be severely restricted and eventually phased out completely. The use of dichlorodifluoromethane was phased out in the 1990's.

Chlorodifluoromethane (R-22) was introduced as a replacement for R-12 because of its lower ozone depletion potential. Following concerns that R-22 is a potent greenhouse gas, its use is also being phased out. R-410A and R-407 (including R-407A, R-407B and R-407C) have been introduced as a replacement refrigerant for R-22. However, R-22, R-410A and the R-407 refrigerants all have a high global warming potential (GWP, also known as greenhouse warming potential).

1,1,1,2-tetrafluoroethane (refrigerant R-134a) was introduced as a replacement refrigerant for R-12. However, despite having a low ozone depletion potential, R-134a has a GWP of 1430. It would be desirable to find replacements for R-134a that have a lower GWP.

R-152a (1,1-difluoroethane) has been identified as an alternative to R-134a. It is somewhat more efficient than R-134a and has a greenhouse warming potential of 120. However the flammability of R-152a is judged too high, for example to permit its safe use in mobile air conditioning systems. In particular its lower flammable limit in air is too low, its flame speeds are too high, and its ignition energy is too low.

R-1234yf (2,3,3,3-tetrafluoropropene) has been identified as a candidate alternative refrigerant to replace R-134a in certain applications, notably the mobile air conditioning or heat pumping applications. Its GWP is about 4. R-1234yf is flammable but its flammability characteristics are generally regarded as acceptable for some applications including mobile air conditioning or heat pumping. In particular its lower flammable limit, ignition energy and flame speed are all significantly lower than that of R-152a. However the energy efficiency and refrigeration capacity of R-1234yf have been found to be significantly lower than those of R-134a and in addition the fluid has been found to exhibit increased pressure drop in system piping and heat exchangers. A consequence of this is that to use R-1234yf and achieve energy efficiency and cooling performance equivalent to R-134a, increased complexity of equipment and increased size of pipe is required, leading to an increase in indirect emissions associated with equipment. Furthermore, the production of R-1234yf is thought to be more complex and less efficient in its use of energy and fluorinated and chlorinated raw materials than R-134a. So the adoption of R-1234yf to replace R-134a will consume more raw materials and result in more indirect emissions of greenhouse gases than does R-134a. Moreover, R-1234yf is known to be only poorly miscible with several standard polyalkylene glycol (PAG) lubricants used with R-134a, such as Nippon Denson ND8.

Whilst heat transfer devices of the type to which the present invention relates are essentially closed systems, loss of refrigerant to the atmosphere can occur due to leakage during operation of the equipment or during maintenance procedures. It is important, therefore, to replace fully and partially halogenated chlorofluorocarbon refrigerants by materials having zero ozone depletion potentials.

In addition to the possibility of ozone depletion, it has been suggested that significant concentrations of halocarbon refrigerants in the atmosphere might contribute to global warming (the so-called greenhouse effect). It is desirable, therefore, to use refrigerants having relatively short atmospheric lifetimes as a result of their ability to react with other atmospheric constituents such as hydroxyl radicals or as a result of ready degradation through photolytic processes.

The environmental impact of operating an air conditioning or refrigeration system, in terms of the emissions of greenhouse gases, should be considered with reference not only to the direct GWP of the refrigerant, but also with reference to the indirect emissions, meaning those emissions of carbon dioxide resulting from consumption of electricity or fuel to operate the system. Several metrics of this total GWP impact have been developed, including those known as Total Equivalent Warming Impact (TEWI) analysis, or Life-Cycle Carbon Production (LCCP) analysis. Both of these measures include estimation of the effect of refrigerant GWP and energy efficiency on overall warming impact.

There is a need to provide alternative refrigerants having improved properties, such as low flammability. Fluorocarbon combustion chemistry is complex and unpredictable. It is not always the case that mixing a non-flammable fluorocarbon with a flammable fluorocarbon reduces the flammability of the fluid. For example, the inventors have found that if non-flammable R-134a is mixed with flammable R-152a, the composition can be flammable even if the amount of R152a is less than the lower flammable limit of pure R-152a (SeeFIG. 1). By contrast, the effect of mixing R-152a with another non-flammable fluorocarbon (R-1225ye(Z)) in a similar test is shown inFIG. 2. It is clear from this study that R-134a can contribute to flame chemistry of other fluorocarbons and cannot therefore be considered simply as an inert flame suppression agent. The situation is rendered even more complex and less predictable if ternary or quaternary compositions are considered.

There is also a need to provide alternative refrigerants that may be used in existing devices such as refrigeration devices with little or no modification.

A principal object of the present invention, therefore, is to provide a heat transfer composition which is usable in its own right or suitable as a replacement for existing refrigeration usages which should have a reduced GWP, yet have a capacity and energy efficiency (which may be conveniently expressed as the “Coefficient of Performance”) ideally within 20% of the values, for example of those attained using existing refrigerants (e.g. R-134a, R-1234yf, R-152a, R-22, R-410A, R-407A, R-407B, R-407C, R-507 and R-404a, particularly R-134a), and preferably within 10% or less (e.g. about 5%) of these values. It is known in the art that differences of this order between fluids are usually resolvable by redesign of equipment and system operational features without entailing significant cost differences. The composition should also ideally have reduced toxicity, acceptable flammability and/or improved miscibility with lubricants, compared to existing refrigerants.

The invention addresses the foregoing and other deficiencies by the provision of a heat transfer composition comprising:(i) from about 45 to about 75% by weight 2,3,3,3-tetrafluoropropene (R-1234yf); and(ii) from about 25 to about 55% by weight 1,1,1,2-tetrafluoroethane (R-134a).

The (fluoro)chemicals described herein are commercially available, for example from Apollo Scientific (UK).

Unless otherwise stated, these compositions will be referred to hereinafter as (binary) compositions of the invention.

An advantageous composition of the invention comprises from about 45 to about 65% R-1234yf by weight; and/or from about 35 to about 55% by weight of R-134a.

A preferred composition of the invention comprises from about 45 to about 60% R-1234yf by weight; and/or from about 40 to about 55% by weight of R-134a.

One aspect of the invention is a composition comprising from about 50 to about 55% R-1234yf by weight; and/or from about 45 to about 50% by weight of R-134a. A related aspect of the invention is a composition comprising from about 50 to about 56% R-1234yf by weight; and/or from about 44 to about 50% by weight of R-134a.

An advantageous composition of the invention comprises from about 54 to about 58% R-1234yf by weight; and/or from about 42 to about 46% by weight of R-134a. A related aspect of the invention is a composition comprising from about 54 to about 56% R-1234yf by weight; and/or from about 44 to about 46% by weight of R-134a.

EXAMPLES

Flammability Testing

The ASHRAE Standard 34 methodology for flammability testing was employed in this test work. The method used is based on ASTM E681-04 “standard test method for concentration Limits of Flammability of Chemicals (vapours and gases)” Annex 1 “Test Method for Materials with Large Quenching Distances, which may be difficult to Ignite” (incorporated herein by reference). A video camera was used to record the tests and review of the record was carried out to establish a final determination of flammability. Spark ignition using 1 mm L shaped tungsten electrodes with a ¼-inch spark gap, powered by 30 mA at 15 KV was used. The spark duration was set via an electronic timer at between 0.2-0.4 seconds.

All tests were carried out in a 12 Liter short-necked round-bottomed flask. The stirring device was as described in E681. The atmospheric pressure was taken before each run and that pressure used to calculate the partial pressure of each component required to give the desired composition in the flask: since volume fraction and partial pressure are related by the gas law. Pressure measurement was by a calibrated 2 bar Druck pressure transducer capable of measuring accurately to 0.01 psi. The air humidity in the flask was regulated to be equivalent to 50% of saturation humidity at 23° C. The test temperature used was 60° C. for all tests.

Results from the flammability testing are shown graphically inFIGS. 1 to 7. The figures represent ternary compositions of fuel, diluent and air on a triangular composition plot, where the axes are scaled on volume fraction of each component. The curved (except forFIG. 2) lines plotted on the diagram represent the flammable region of compositions.

FIG. 1shows the flammability behaviour of R-152a (fuel), R-134a (diluent) and air at 23° C.FIG. 2shows the flammability behaviour of R-152a (fuel), R-1225ye(Z) (diluent) and air at 100° C. It can be deduced from the shape of the flammable region of R-152a with R-134a, in particular the downward curvature of the bottom, that R-134a is playing an active role in the flame chemistry and is not acting to suppress flammability. By contrast the shape of the flammable region inFIG. 2shows that R-1225ye(Z) is acting to reduce the flammability of R-152a.

We have studied the flammability of R-1234yf in air at 23° C. and 60° C. using the ASHRAE Standard 34 flammability test protocol and found it to be quite sensitive to temperature.

The flammability of R-1234yf may be suppressed using inert carbon dioxide CO2, as shown inFIGS. 3 and 4. At 23° C., mixtures of R-1234yf and CO2having at least 52% v/v CO2were found to be non-flammable. At 60° C. by contrast (as shown inFIG. 4), the minimum quantity of CO2required to render the mixture of R-1234yf/CO2non-flammable is 66% v/v. It may also be seen that the area of flammable region in the triangular map of fuel/air/diluent compositions has increased substantially from that at 23° C.

We also studied the effect of adding R-134a to R-1234yf. The results are shown inFIGS. 5 and 6. At 23° C. the R-1234yf can be rendered non-flammable if mixed with at least 30% v/v of R-134a. At 60° C. the R-1234yf can be rendered non flammable if mixed with at least 48% v/v R-134a. Furthermore, the size of flammable region is significantly reduced as compared to that observed with CO2.

FIG. 6illustrates the following data of interest for mixtures of R-1234yf/R-134a/air at 60° C.:

Lower flammable limit of R-1234yf in air: 6% v/v

Upper flammable limit of R-1234yf in air: 15% v/v

Minimum R-134a content for a non flammable R-1234yf/R-134a mixture: 45% v/v (42% w/w). This mixture has a GWP of 600.

In order for a mixed refrigerant to pass the ASHRAE flammability assessment as non flammable, a fractionation analysis must be undertaken and flammability assessed of both the worst case formulation that can be made in the manufacture of refrigerant and the worst case fractionated composition that can result from handling of this mixture. The test temperature for assessment of the worst case fractionated composition is 60° C. and that for assessment of the worst case formulation is 100° C. Further details are given in Appendix B of ASHRAE standard 34-2007, which is incorporated herein by reference.

The vapour-liquid equilibrium behaviour of R-1234yf with R-134a was studied by measurement of vapour pressure of a series of binary compositions in a static equilibrium cell apparatus. This consisted of a stirred sample cell of accurately known volume, held in a thermostatic bath, and charged with known amounts of R-1234yf and R-134a. The vapour pressure of mixtures of the fluids was determined over a range of temperatures and these data were then regressed to a suitable thermodynamic model using Barker's method as outlined inThe Properties of Gases and Liquids4thedition (Reid, R C; Prausnitz, J M; Poling, B E pub. McGraw Hill 1986), which is incorporated by reference herein.

The system was found to form a minimum boiling azeotrope whose composition is approximately 15% v/v (13.7% w/w) R-134a at 1 atmosphere pressure, with an azeotropic normal boiling point of approximately −29.4° C. The data obtained in this experiment were fitted to a vapour liquid equilibrium model, and the ability of this model to reproduce the observed data was demonstrated by regression. The model was based on the Wilson equation to represent liquid phase fugacities and the Redlich Kwong equation of state to represent vapour phase fugacities. This thermodynamic model was then used to test the behaviour of R-1234yf/R-134a mixtures.

Mixtures containing higher proportions of R-134a than the azeotrope content at 1 atmosphere pressure were found to be nonazeotropic and to exhibit composition difference between vapour and liquid. If the liquid composition of R-134a is in the region of 40% v/v, the composition of the vapour phase is enriched in R-1234yf compared to the liquid phase. This means that the worst case fractionated composition for any assessment is the vapour in equilibrium with a specified liquid composition. This vapour composition must be at least 45% v/v R-134a in order to pass the nonflammability test at 60° C. ASHRAE Standard 34 Appendix B specifies that the composition should be determined at a temperature of 10 degrees Kelvin above the atmospheric bubble point of the mixture. This corresponds to a temperature of −19° C. for the 45% v/v R-134a mixture.

The liquid composition in equilibrium with a 45% v/v R-134a composition in the vapour phase is approximately 47% v/v, or 44% w/w at −19° C. It is anticipated therefore that compositions of at least 44% w/w R-134a will be required to ensure that the R-1234yf/R-134a binary mixture passes ASHRAE flammability assessment. The GWP of such a 44% R-134a mixture is 631, based on fourth assessment report (AR4) GWP values for R-1234yf and R-134a of 4 and 1430, respectively.

The experiment above illustrated inFIG. 6with R1234yf as fuel was repeated at 60° C. for a binary fuel mixture of R-32 with R-1234yf in the volumetric proportions 12:88. The results are shown inFIG. 7. R-32 is also flammable: its lower and upper flammable limits in air are 14% and 30% respectively; its flame speed in air is approximately 7 cm/s; and its minimum ignition energy is between 30 and 100 milliJoules. It can be considered to be more flammable in some respects than R-1234yf, whose flame speed in air is less than 2 cm/s and whose minimum energy of ignition is more than 500 milliJoules

In this case the key properties for the binary fuel mixture consisting of R-32/R-1234yf in the volumetric proportions 12:88 were found to be:

Lower flammable limit of fuel in air 7.5% v/v

Upper flammable limit of fuel 15% v/v

Minimum content of R-134a to be added to the fuel to give a non-flammable mixture: 34% v/v (33% w/w

It is evident by comparingFIG. 7withFIG. 6that for the R-1234yf/R-32/R-134a ternary mixture: (i) the flammable region for this fuel when mixed with R-134a is smaller than the flammable region for the R-1234yf/R-134a binary mixture, (ii) the lower flammable limit in air is significantly higher than that of the R-1234yf/R-134a binary mixture, and (iii) the amount of R-134a required to create a non flammable mixture is lower than for the R-1234yf/R-134a binary mixture.

For a binary mixture of R-32 and R-1234yf in the proportions 12:88 on a volumetric (molar) basis, the minimum R-134a content to yield a non-flammable composition was found to be 34% v/v at 60° C. This corresponds to a ternary composition of 4% R32, 33% R-134a and 63% R-1234yf on a weight basis. Surprisingly, the amount of R-134a needed to be added to the mixture of R-32 and R-1234yf to render it non flammable (33% w/w) is considerably less than that needed for pure R-1234yf (42-44% w/w), even though on the basis of flame speed and ignition energy the R-32 component can be considered more flammable than R-1234yf.

The GWP of the ternary composition of 4% R32, 33% R-134a and 63% R-1234yf (w/w) using the AR4 data of 675 for R-32 and 1430 for R-134a is 501. A similar ternary composition of 4% R32, 34% R-134a and 62% R-1234yf (w/w) has a GWP of 516. Thus it is possible by adding R-32 to a R-1234yf/R-134a system to generate a nonflammable formulation having improved environmental impact (such as reduced GWP).

Air Conditioning Performance

The performance of selected compositions of the invention was evaluated in a theoretical model of a vapour compression cycle. The model used experimentally measured data for vapour pressure and vapour liquid equilibrium behaviour of mixtures, regressed to the Peng Robinson equation of state, together with correlations for ideal gas enthalpy of each component to calculate the relevant thermodynamic properties of the fluids. The model was implemented in the Matlab software package sold in the United Kingdom by The Mathworks Ltd. The ideal gas enthalpies of R-32 and R-134a were taken from public domain measured information, namely the NIST Fluid Properties Database as embodied in the software package REFPROP v8.0. The ideal gas heat capacity of R-1234yf was experimentally determined over a range of temperatures.

These calculations were performed following the standard approach as used in (for example) the INEOS Fluor “KleaCalc” software (other available models for predicting the performance of refrigeration and air conditioning systems known to the skilled person in the art may also be used), using the following conditions:

Suction line pipe diameter: 16.2 mm

In the calculation it was assumed that the pressure drops in evaporator and condenser were negligible.

The results are shown in the following table, where the compositions are quoted on a weight basis.

The non-flammable binary R-1234yf/R-134a offers improved performance relative to R-1234yf. If admixed into an existing system to replace refrigerant lost by leakage, performance will stay close to that of R-134a.

The non-flammable ternary R-32/R-1234yf/R-134a composition is the closest match to R-134a of these compositions and offers improved energy efficiency, reduced pressure drop and reduced GWP relative to the non flammable binary R-1234yf/R-134a mixture. This means that the overall environmental impact as assessed by LCCP analysis will be reduced by the addition of R-32 to the system.

Non-flammable compositions of R-1234yf/R-134a or R-32/R-1234yf/R-134a defined herein exhibit improved miscibility with standard PAG lubricants as compared to R-1234yf.

Furthermore their thermodynamic performance is improved relative to R-1234yf, and is sufficiently close to R-134a that they may be used in systems designed for R-134a with only slight loss in air conditioning performance. They may therefore be used in technology designed for R-134a with only slight system modifications in contrast to flammable R-1234yf.

The invention is defined by the following claims.