HEAT TRANSFER COMPOSITIONS

The invention relates to heat transfer compositions, and in particular to heat transfer compositions comprising at least one flammable component.

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 dichlorofiuoromethane was phased out in the 1990's.

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 global warming potential (GWP) of 1300.

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 which have 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.

There is a need to provide alternative refrigerants having improved properties, such as low fiammability. 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.

In accordance with one aspect of the invention, there is provided a heat transfer composition comprising

(i) at least one flammable component selected from R-1234yf and R- 152a,

(ii) CF ₃ I ₅ and

(iii) at least one non-flammable (hydro)fluorocarbon, wherein the composition has a GWP of less than that of the at least one nonflammable (hydro)fluorocarbon.

^" PreferabTy7tEe ^" composltion ^' comprises substantially no R--12-34yfí

Conveniently, the composition comprises substantially no R-152a.

Advantageously, the at least one non-flammable, (hydro)fluorocarbon is selected from R-134a, R-134, R-125, R-245fa, R-245ca, R-245cb, R-236fa and R-227ea.

Preferably, the at least one (hydro)fluorocarbon is selected from R- 134a and R-245fa.

Conveniently, the composition comprises the at least one (hydro)fluorocarbon in an amount of from about 1 to about 30% by weight of the composition.

Advantageously, the composition comprises the at least one (hydro)fluorocarbon in an amount of from about 1 to about 10% by weight of the composition.

Preferably, the composition comprises the at least one (hydro)fluorocarbon in an amount of from about 1 to about 6% by weight of the composition.

Advantageously, the composition comprises the at least one (hydro)fluorocarbon in an amount of from about 1 to about 5% by weight of the composition.

Preferably, the composition is azeotrope-like.

Conveniently, the composition has a GWP of about 750 or less.

Advantageously, the composition has a GWP of about 500 or less.

Preferably, the composition has a GWP of about 250 or less.

Conveniently, the composition has a GWP of about 150 or less.

Advantageously, the composition has a GWP of about 100 or less.

Preferably, the composition further comprises a lubricant.

Conveniently, the lubricant is selected from the group consisting of mineral oil, silicone oil, polyalkyl benzenes (PABs) ₅ polyol esters (POEs), polyalkylene glycols (PAGs), polyalkylene glycol esters (PAG esters), polyvinyl ethers (PVEs), poly (alpha-olefms) and combinations thereof.

Advantageously, the composition further comprises a stabiliser.

Preferably, the stabiliser is selected from the group consisting of diene- based compounds, phosphates, phenol compounds and epoxides, and mixtures thereof.

Conveniently, the composition further comprises an additional flame retardant.

Advantageously, the additional flame retardant is selected from the group consisting of tri-(2-chloroethyl)-phosphate, (chloropropyl)phosphate, tri-

(2,3 -dibromopropyl)-phosphate, tri-( 1 ,3 -dichloropropyl)-phosphate, diammonium phosphate, various halogenated aromatic compounds, antimony oxide, aluminium trihydrate, polyvinyl chloride, a fluorinated iodocarbon, a fluorinated bromocarbon, trifluoroiodomethane, perfiuoroalkyl amines, bromo-fluoroalkyl amines and mixtures thereof.

Preferably, the composition is a refrigerant composition.

According to another aspect of the invention, there is provided a heat transfer device containing a composition of the invention.

Preferably, the heat transfer device is a refrigeration device.

Conveniently, the heat transfer device is selected from group consisting of automotive air conditioning systems, residential air conditioning S3fstems, commercial air conditioning systems, residential refrigerator systems, residential freezer systems, commercial refrigerator systems, commercial freezer systems, chiller air conditioning systems, chiller refrigeration systems, heat pump systems.

Advantageously, the heat transfer device contains a centrifugal-type compressor.

According to a further aspect of the invention, there is provided a blowing agent comprising a composition of the invention.

According to another aspect of the invention, there is provided a foamable composition comprising one or more components capable of forming foam and a composition of the invention.

Preferably, the one or more components capable of forming foam are selected from polyurethanes, thermoplastic polymers and resins, such as polystyrene, and epoxy resins.

According to a further aspect of the invention, there is provided a foam obtainable from the foamable composition of the invention.

Preferably the foam comprises a composition of the invention.

According to another aspect of the invention, there is provided a sprayable composition comprising a material to be sprayed and a propellant comprising a composition of the invention.

According to a further aspect of the invention, there is provided a method for cooling an article which comprises condensing a composition of the invention and thereafter evaporating said composition in the vicinity of the article to be cooled.

According to another aspect of the invention, there is provided a method for heating an article which comprises condensing a composition of the invention in the vicinity of the article to be heated and thereafter evaporating said composition.

According to a further aspect of the invention, there is provided a method for extracting a substance from biomass comprising contacting the biomass

with a solvent comprising a composition of the invention, and separating the substance from the solvent.

According to another aspect of the invention, there is provided a method of cleaning an article comprising contacting the article with a solvent comprising a composition of the invention.

According to a further aspect of the invention, there is provided a method for extracting a material from an aqueous solution comprising contacting the aqueous solution with a solvent comprising a composition of the invention, and separating the substance from the solvent.

According to another aspect of the invention, there is provided a method for extracting a material from a particulate solid matrix comprising contacting the particulate solid matrix with a solvent comprising a composition of the invention, and separating the substance from the solvent.

According to a further aspect of the invention, there is provided a mechanical power generation device containing a composition of the invention.

Preferably, the mechanical power generation device is adapted to use a Rankine Cycle or modification thereof to generate work from heat.

According to another aspect of the invention, there is provided a method of retrofitting a refrigeration device comprising the step of removing an existing heat transfer fluid, and introducing a composition of the invention.

s mentioned above, due to concerns about potentially harmful environmental effects of existing heat transfer compositions, there is an on-

going trend towards the use of fluids which have an improved (i.e. lower) global warming potential (GWP). However, although fluids which have been proposed to replace traditional heat transfer fluids may have a reduced GWP ₅ many of them have increased flammability.

It is known to include in heat transfer compositions substances which reduce the flammability of the composition. For example, trifhioroiodomethane (CF ₃ I) has been proposed for use as a flame retardant in heat transfer compositions. However, CF ₃ I has a number of disadvantages. Firstly, CF ₃ I is relatively expensive, partly due to the relative scarcity of the element iodine. Also, CF ₃ I has a high molecular weight of 195.91. In practice, this means that a relatively large and expensive amount of CF ₃ I must be combined with a flammable heat transfer fluid in order to result in significantly reduced flammability. Also, the use of such relatively high amounts of CF ₃ I can lead to increased weight of fluid to be used in a heat transfer device, such as an automotive air- conditioning system, as compared to a traditional heat transfer composition.

We have unexpectedly found that blending a non-flammable (hydro)fluorocarbon fluid with a flammable (hydro)fluorocarbon fluid and CF ₃ I can result in a composition having an acceptable GWP and an acceptable flammability. In particular, the use of at least one nonflammable (hydro)fluorocarbon fluid with a relatively high GWP can result in a heat transfer composition of reduced flammability and acceptable GWP. Also, the level of CF ₃ I needed to give the required level of non- flammability can be reduced.

In some circumstances, it is preferred for the compositions to have a GWP of about 150 or less. However, for other applications, it may be acceptable

for composition to have a higher GWP, for example a GWP of up to 250, 500 or 750.

Preferred flammable refrigerants are 2,3,3,3-tetrafluoropropene (R-1234yf) and 1,1-difmoroethane (R-152a).

Preferred non-flammable (hydro)fluorocarbon fluids are R- 134a, R- 134, R- 125, R-245fa, R-245ca, R-245cb, R-236fa andR-227ea.

The non-flammable (hydro)fluorocarbon components may be present in the composition preferably in a range from about 1 to about 30% by weight. The maximum proportion preferred for any given application will be determined by the fluid's Greenhouse Warming Potential (GWP) and the desired GWP of the formulated blend of flammable fluid, CF ₃ I and non- flammable fluid.

Typical but non-limiting preferred ranges for specific non-flammable fluids (by % weight) are set out below:

The flammability of R-1234yf when mixed with air and CF ₃ I has been determined according to the methodology of Addendum p to ASHRAE Standard 34, using the ASTM E681 protocol in a 12 litre flask with electronic ignition. The test temperature used was 60 ⁰ C and the humidity was equivalent to 50% relative humidity at 23 ⁰ C.

The flammability limits of R-1234yf in air were found to be 5.5% to 13.4% v/v. The minimum quantity of CF ₃ I required to render the fuel nonflammable in air at all concentrations was found to be 19% v/v when mixed with R-1234yf.

The test was repeated, using as fuel a mixture of R-1234yf with R-134a in the molar ratio 86:14. The flammability limits of this fuel mixture in air were found to be 6.5% to 15% v/v. The minimum quantity of CF ₃ I required to render the fuel non-flammable in air at all concentrations was found to be 18% v/v.

Thermodynamic Property Estimation

The key thermodynamic properties of a fluid for estimation of its performance as a refrigerant or air conditioning system working fluid are:

• The critical temperature - the highest temperature at which liquid and vapour phases can co-exist. • The vapour pressure of the fluid over the temperature range of operation of the refrigerant/air conditioning equipment. ® The latent heat of vaporisation of the fluid, β The variation of the gas density with pressure and temperature

• The heat capacity of the gas and liquid states. Two mixtures have been studied. The first mixture is a binary mixture of R- 1234yf with CF ₃ I, such that the quantity Of CF ₃ I in the mixture is sufficient to render the mixture non-flammable in air at all concentrations according to the flammability test data determined above. This yields a mixture composition of R-1234yf/CF ₃ I as 71%/29% on a weight basis.

The above binary mixture was compared with a ternary mixture of the invention, comprising R-1234yf/R-134a/CF ₃ I, selected such that the molar ratio of R-1234yf:R-134a is 86:14 and the mixture is non-flammable in air at all concentrations. This }άelds a mixture composition of R-1234yf/R- 134a/CF ₃ I as 63 %/9%/28% on a weight basis.

The thermodynamic properties of the mixtures have been calculated as follows. Literature values of the properties of R- 134a and CF ₃ I were used. The properties for R-134a were taken from the REFPROP7.0 program supplied by the National Institute for Standards and Technology (NIST). The properties of CF ₃ I were obtained from the MST Web book website (http ://webbook.nist. gov) and from the academic literature, in particular from Duan et al Fluid Phase Equilibria (1996) vl21 pp227-234 and J Chem EngData (1999) v44 pρ501-504.

For R-1234yf the critical point, saturated liquid vapour pressure and liquid density have been measured. The vapour pressure has been used to determine the acentric factor for R-1234yf. The boiling point for R-1234)'f and its chemical structure, have been used to estimate the ideal gas enthalpy of R-1234yf by using Joback's estimation method, which is described in more detail in the textbook "The Properties of Gases and Liquids" 4th edition, editors RC Reid, JM Prausnitz, BE Poling, published by McGraw- Hill 1987.

The critical temperature, critical pressure and the calculated acentric factor data have been used in conjunction with the Peng Robinson equation of state to generate estimates of vapour pressure data for the fluid over the range of temperatures -50 ⁰ C to the critical point. These predicted vapour pressures agreed closely with the observed data, which were measured over the range -20 to +-6O ⁰ C ₃ giving confidence that this equation of state can

adequately predict vapour pressure for the temperature range of interest. The Peng Robinson equation of state has been used in conjunction with the estimated ideal gas heat capacity to calculate the density, enthalpy and entropy of the real gas, and to estimate the latent heat of vaporisation from use of the Clapeyron relationship between vapour pressure and the difference in volume between saturated liquid and saturated vapour. The Peng Robinson equation has also been used to determine the phase equilibrium behaviour of the mixtures. The binary interaction constants that are used by this equation to correlate vapour-liquid equilibrium between pairs of fluids were determined as follows. For R-1234yf with CF ₃ I ₅ boiling point data for mixtures of the two fluids contained in US Patent Application US2005/0233934A1 were used to estimate the interaction constant. For the other interactions (those concerning R-134a) the constants were set to zero.

The refrigeration performance of the two mixtures was compared using a standard idealised air conditioning cycle calculation as discussed below. An idealised vapour compression cycle, consisting of four steps as outlined below, was used to perform the calculations necessary to provide these evaluation parameters:

(a) Isothermal evaporation of a refrigerant liquid at constant temperature followed by warming of the vapour to ambient temperature.

(b) Compression of the refrigerant vapour in a positive displacement compressor of specified isentropic efficiency and clearance volume.

(c) Cooling and condensation of the vapour to liquid with the average condensation temperature and the final liquid temperature specified.

(d) Throttling of the liquid refrigerant from condenser to evaporator pressure to complete the cycle.

The conditions used are typical of a mobile air conditioning (MAC) application and are as follows for all fluids:

• Evaporation temperature: 5 ^D C. • Temperature of the vapour at the inlet to the compressor: 25 ⁰ C.

• Condensation temperature of the vapour: 45 ⁰ C.

• Compressor isentropic efficiency: 65%.

• Compressor clearance volume: 4%.

The volumetric capacity of the ternary mixture comprising R-1234yf/R- 134a/CF ₃ I (composition 63%/9%/28% weight basis) is equivalent to that of the binary composition comprising R-1234yf/CF ₃ I (71%/29% weight basis). In other words, the compressor displacement required to deliver a given rate of cooling is equivalent for both blends.

The energy efficiency of the ternary mixture is enhanced (102%) compared to the binary mixture of R-1234yf and CF ₃ I, representing an additional benefit to the invention. The temperature glide (difference between dewpoint temperature and bubblepoint temperature of the mixture at 1 atmosphere pressure) is negligible for both mixtures, indicating that the ternary mixture is azeotrope-like.

The GWP of the ternary mixture, calculated assigning a GWP of 10 to the tetrafluoropropene, is 123, which is less than 150 and therefore suitable for certain applications such as automotive air conditioning systems.