CFC phase-out: have we met the challenge?

Transcription

1 Journal of Fluorine Chemistry 114 (2002) CFC phase-out: have we met the challenge? Richard L. Powell * UMIST, P.O. Box 88, Manchester M60 1QD, UK Received 18 September 2001; received in revised form 14 December 2001; accepted 15 December 2001 Abstract In 1974 Nobel prize winners Rowland and Molina proposed that chlorofluorocarbons (CFCs) were stable enough to reach the stratosphere, where, under intense solar radiation they released Cl atoms that could destroy stratospheric ozone protecting the earth s surface from UV rays. The CFC industry funded both scientific studies to test the Rowland and Molina hypothesis and programmes to identify potential replacements, from which the HFCs emerged as likely candidates. After 5 years it was concluded, on the best scientific evidence available, that stratospheric ozone was being depleted at 3% per decade, but sufficient time was available for an orderly phase-out. Although the USA and a few other countries stopped the use of CFCs in aerosols little further work was done until 1985 when the CFC debate was renewed following the discovery of stratospheric ozone depletion over the Antarctic during its spring. Manufacturers restarted their RD programmes; governments negotiated the Montreal Protocol in 1987 agreeing the partial phase-out of the CFCs. As a result of subsequent amendments CFCs have now been phased-out in the developed world and HCFCs will follow over the next two decades. This paper reviews what has been achieved and what remains to be done. Has the world-wide effort been successful in protecting the ozone layer? Have acceptable alternatives been found for the CFCs/HCFCs in their various applications? # 2002 Published by Elsevier Science B.V. Keywords: Chlorofluorocarbon; Hydrochlorofluorocarbon; Hydrofluorocarbon; Refrigerant; Montreal Protocol; Ozone depletion; Global warming; Trifluoroacetate 1. Introduction Concern about the impact of chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs) on the atmosphere goes back almost 30 years, and phase-out of HCFCs in the industrially developed nations will be completed by 2030 at the latest. Clearly this half-way point is an opportune time to review progress and to ask whether the programme will be successful. In the context of this paper it is not possible to explore all the major applications of CFCs/HCFCs and to discuss their replacements comprehensively. My own experience mainly lies in the area of refrigerants so this paper is biased towards these products. Furthermore, I shall illustrate the issues involved using the major refrigerants CCl 2 F 2 (CFC-12) and CHClF 2 (HCFC-22) as examples. Similar considerations apply to lower tonnage refrigerants although the details vary from case to case. 2. History In the 1970s Lovelock and co-workers, using his newly developed electron capture detector, demonstrated that * Tel.: þ ; fax: þ address: dick@powell10055.freeserve.co.uk (R.L. Powell). CFCs 11 and 12 were trace constituents in the atmosphere [1 3]. In hindsight, perhaps this observation should not have been surprising given that a significant fraction of CFC production was either purposely vented from aerosol packages or inadvertently lost from leaking refrigeration and air-conditioning systems. In 1972, at a meeting initiated by DuPont, representatives of the leading CFC manufacturers discussed the environmental fate of their products. Ray McCarthy succinctly summarised their remit in the following way: Fluorocarbons are intentionally or accidentally vented to the atmosphere world-wide at a rate approaching one billion pounds per year. These compounds may be either accumulating in the atmosphere or returning to the surface, land or sea, in pure form or as decomposition products. Under any of these alternatives it is prudent that we investigate any effects which the compounds may produce on plants or animals now or in the future. As a result the CFC manufacturers formed the Fluorocarbon Panel to investigate the environmental impact of CFCs, a programme which was pointed in a very specific direction when Molina and Rowland published their 1974 paper proposing that CFCs could destroy stratospheric ozone [4]. Although the chemical industry is often depicted as riding rough-shod over environmental concerns, in case of the CFCs the fluorocarbon producers adopted a demonstrably /02/$ see front matter # 2002 Published by Elsevier Science B.V. PII: S (02)

2 238 R.L. Powell / Journal of Fluorine Chemistry 114 (2002) responsible attitude [5]. Through the Fluorocarbon Panel they jointly funded an extensive and high quality atmospheric research programme to test the validity of the Rowland and Molina hypothesis using independent experts who were free to publish their results as peer reviewed papers in reputable journals. Simultaneously, each company initiated its own research programme to identify CFC replacements which retained the advantages of CFCs, notably low toxicity and non-flammability, while avoiding their implied threat to the ozone layer. The atmospheric research programme confirmed that CFCs were likely to deplete stratospheric ozone, as predicted by Rowland and Molina, with the best computer models available at the time indicating a rate of about 3% per decade. The observational techniques in the late 1970s were thought to be incapable of detecting any depletion that had already occurred. It was concluded that CFCs should be phasedout, but that this could occur over a sufficiently long period to minimise the economic impact of the change to CFC users. Nevertheless, several countries, most notably the USA, unilaterally banned the use of CFCs in most aerosols and by 1980 this had significantly reduced world CFC production. The company programmes identified that hydrofluorocarbons (HFCs) offered the desired combination of properties, especially CF 3 CH 2 F (HFC-134a) which was reasonably similar in thermodynamic properties to CFC-12 and might therefore replace it as a refrigerant. CH 2 F 2 (HFC-32), CF 3 CH 3 (HFC-143a) and CF 3 CF 2 H (HFC-125) also emerged as candidate low boiling refrigerants. However the atmospheric science studies suggested that CHClF 2 (HCFC-22), the major low boiling refrigerant currently in production, could continue, because hydroxyl radicals largely destroyed it in the troposphere, so little reached the stratosphere. Since the CFCs producers were generating the same candidate replacements they shared safety data, notably toxicology results, under the auspices of the Fluorocarbon Panel. By 1980 companies had designed pilot plants for HFC-134a production; but a combination of the early 1980s recession and the change in the American political climate after the 1980 presidential election resulted in the shelving of these projects. Despite the reduction in interest, in 1981 the Vienna Convention to protect the ozone layer was signed by interested states although CFCs were only mentioned in an annexe as compounds that needed to be watched. In 1984 a remarkable and totally unpredicted phenomenon was discovered by the British Antarctic Survey, the so-called ozone hole [6]. During the Antarctic spring in October, when the sun first rises above the horizon, a major, although temporary, loss of stratospheric ozone was observed. Chlorine injected into the stratosphere by the CFCs was the prime suspect. In 1985 this unexpected observation was discussed at a meeting of the Vienna Convention when it was decided that world-wide regulations were required to control the production and emissions of chlorine containing gases. In 1987 governments negotiated the Montreal Protocol, the first international treaty to protect the global environment [7]. This agreement originally mandated a 50% reduction in CFC production and consumption by 1 July 1999, but, importantly, allowed for future revision in light of new scientific evidence. Subsequent atmospheric studies and computer models superior to those available 10 years previously suggested that ozone was likely to be depleted at faster rate than had been previously thought. Following revisions of the Protocol the complete phase-out of CFCs and HCFCs were also mandated; Table 1, based on information from [7], summarises the present position. Although the European Union did not convince other signatories to the Protocol to accept proposals for earlier CFC and HCFC phase-out, it decided to include them in its own regulations. The phase-out of CFCs in the developed countries proved to be slower than was anticipated by national governments. Quite legally, users and distributors stockpiled considerable quantities of CFCs to delay the ultimately inevitable switch to alternative fluids. Totally illegally, significant amounts of CFCs were smuggled into developed countries from developing countries, who were allowed to continue production. Ironically, the high tax imposed by the USA on CFCs to reduce their use made smuggling even more profitable. Smuggled virgin CFC was often labelled as re-cycled product since the trading of this material was not illegal. To overcome the problem the European Union unilaterally banned the use of all recycled CFCs from January CFC recycling is still allowed in the USA and it has even become attractive to recover CFC-12 from its azeotrope with CHF 2 CH 3 (HFC-152a) known as R Selecting fluorocarbon replacements for CFC refrigerants In response to renewed concern over CFCs following the discovery of Antarctic ozone depletion the manufacturing companies restarted their development programmes for the environmentally acceptable replacements. Since their introduction in the early 1930s equipment designs had been specifically optimised around the properties of CFCs and HCFCs. Through careful attention to detail and the adoption of conservative design practices, the refrigeration and air-conditioning industry had developed reliable, safe products. Not surprisingly, what the equipment manufacturers required were new fluids having properties as close as possible to the current fluids to minimise design changes. The generally accepted, key requirements for the replacement fluids were the following. Thermodynamic properties as close to original refrigerants: in particular maximum operating pressures should not be significantly above those of the chlorinated refrigerants. Non-flammable. Non-toxic. Miscible with acceptable lubricants.

3 R.L. Powell / Journal of Fluorine Chemistry 114 (2002) Table 1 Montreal Protocol control measures Date Control measure 1 July 1989 Freeze of annex A a CFCs 1 January 1992 Freeze of halons 1 January 1993 Annex B CFCs b reduced by 20% from 1989 levels Freeze of methyl chloroform 1 January 1994 Annex B CFCs reduced by 75% from 1989 levels Annex A CFCs reduced by 75% from 1986 levels Halons c phased-out d Methyl chloroform reduced by 50% 1 January 1995 Methyl bromide frozen at 1991 levels Carbon tetrachloride reduced by 85% from 1989 levels 1 January 1996 HBFCs d phased-out e Carbon tetrachloride phased-out d Annex A and B CFCs phased-out d Methyl chloroform phased-out d HCFCs f frozen at 1989 levels of HCFC þ 2.8% of 1989 consumption of CFCs (base level) 1 January 1999 Methyl bromide reduced by 25% from 1991 levels 1 July 1999 * Freeze of annex A CFCs at average levels g 1 January 2001 Methyl bromide reduced by 50% from 1991 levels 1 January 2002 * Freeze of halons at average levels g * Freeze of methyl bromide at average levels 1 January 2003 Methyl bromide reduced by 70% from 1991 levels * Annex B CFCs reduced by 20% from average consumption h * Freeze in methyl chloroform at average levels 1 January 2004 HCFCs reduced by 35% below base levels 1 January 2005 Methyl bromide phased-out * Annex A CFCs reduced by 50% from average levels g * Halons reduced by 50% from average levels g * Carbon tetrachloride reduced by 85% from average levels * Methyl chloroform reduced by 30% from average levels 1 January 2007 * Annex A CFCs reduced by 85% from average levels g * Annex B CFCs reduced by 85% from average levels h 1 January 2010 HCFCs reduced by 65% * CFCs, halons and carbon tetrachloride phased-out * Methyl chloroform reduced by 70% from average levels 1 January 2015 HCFCs reduced by 90% * Methyl chloroform phased-out * Methyl bromide phased-out 1 January 2016 * Freeze of HCFCs at base line figure of year 2015 average levels 1 January 2020 HCFCs phased-out allowing for a service tail of up to 0.5% until 2030 for existing refrigeration and air-conditioning equipment 1 January 2040 * HCFCs phased-out Developed countries (); developing countries (*). a Five CFCs in annex A: CFCs 11, 12, 113, 114 and 115. b Ten CFCs in annex B: CFCs 13, 111, 112, 211, 212, 213, 214, 215, 216 and 217. c Halons 1211, 1301 and d With exemptions for essential uses. Consult the Handbook on Essential Use Nominations prepared by the Technology and Economic Assessment Panel, 1994, UNEP, for more information. e A 34 hydrobromofluorocarbons. f A 34 hydrochlorofluorocarbons. g Calculated level of production of 0.3 kg per capita can also be used for calculation, if lower. h Calculated level of production of 0.2 kg per capita can be also be used for calculation, if lower.

4 240 R.L. Powell / Journal of Fluorine Chemistry 114 (2002) Each CFC refrigerant to be substituted by a single environmentally acceptable product. The rapid expansion of the refrigeration and air-conditioning industries since 1945 had largely been based on only four major refrigerants, CFC-12, HCFC-22, CFC-11 and R502 (an azeotrope of CFC-115 (CF 3 CF 2 Cl) and HCFC-22). The industry considered that the proliferation of different refrigerants for the same application would cause confusion by increasing the number of products that a service engineer would need to carry. Initially, attention was focused on HFC-134a as a replacement for the major refrigerant CFC-12 used in automobile air-conditioning systems, domestic fridge freezers and commercial refrigeration. The single, most important item of thermodynamic data, and fortunately the one which chemists traditionally record, is boiling point. HFC-134a boils at C compared to C for CFC-12. Simple theoretical cycle calculations initially suggested that 134a would be slightly less energy efficient, but this was disproved when experimental results in optimised systems became available. The real problem with 134a was its very low miscibility (<1%, w/w) in the mineral oil lubricants used for CFC-12. In typical compressor designs refrigerant returning from the evaporator passes through the oil sump before being sucked into the compressor. This excellent design ensures efficient lubrication of the compressor, but 2% of the total oil charge circulates through the system. The complete miscibility of the oil with the refrigerant ensured that none accumulated in the refrigeration circuit. Unfortunately with HFC-134a, reliable return of the lubricant to the sump could not be guaranteed, with the attendant risk that oil depletion would cause compressor failure. New oils miscible with 134a were required and two classes emerged, the polyalkylene glycols and the polyol esters (POEs). Many compressor systems contain aluminium alloy components which proved susceptible to attack by polyalkylene oxides, because the protective oxide layer was removed. Fortunately, the POEs proved to be satisfactory providing both good lubrication and oil return. Their major technical disadvantage is their ability to absorb typically up to 2000 ppm of water, so special care has to be taken to prevent moisture ingress which would cause corrosion and consequent compressor failure. By the late 1980s HFC-134a had emerged as the preferred the non-ozone depleting replacement CFC-12 essentially based upon the consensus of the major refrigerant users, the refrigerant manufacturers and not least national governments who wanted an effective programme to replace CFCs. There were, and still are, dissenting opinions that will be discussed later in this paper. 4. Selecting fluorocarbon replacements for HCFC refrigerants By 1990 continuing observations of the Antarctic ozone hole and increasingly sophisticated atmospheric models Table 2 Input parameters for cycle calculations Parameter Value Mid-point evaporator temperature (8C) 7 Mid-point condenser temperature (8C) 45 Cooling output (kw) 10 Compressor efficiency 0.7 Electric motor efficiency 0.85 Parasitic power (fans and controls) (kw) 0.8 Table 3 Calculated values of key output parameters Refrigerant HCFC-22 30% HFC-32, 70% HFC-134a 23% HFC-32, 25% HFC-125, 52% HFC-134a (407C) Evaporator pressure (bar) Discharge pressure (bar) Discharge temperature (8C) COP (system) Capacity (kw/m 3 ) Glide in evaporator (8C) Glide in condenser (8C) made it clear that the CFCs needed to be phased-out completely, earlier than had been anticipated. Furthermore, the HCFCs, which had been considered as part of the long-term solution, also needed to be regulated. The major HCFC, CHClF 2 (HCFC-22) is used extensively in building air-conditioning systems and has a boiling point of 42 8C. Although three simple HFCs have low boiling points, namely CF 3 CF 2 H (HFC-125, 48 8C), CF 3 CH 3 (HFC- 143a, 48 8C) and CH 2 F 2 (HFC-32, 51 8C), there is no single compound which can provide a direct substitute for HCFC-22. To a chemist the obvious solution is to mix a high and a low boiling point HFC, so that the blend has a boiling point similar to that of HCFC-22. Independent work by several industrial groups, including the author s, identified that a blend containing 70 wt.% HFC-134a and 30 wt.% HFC-32 had thermodynamic properties similar to HCFC-22. Furthermore, simple cycle calculations suggested that its energy efficiency (coefficient of performance, COP 1 ) and cooling capacity were close to those of HCFC-22. Table 2 summarises the input parameters for a typical air-conditioning refrigerant cycle. 2 Table 3 gives the values of key output parameters. Although such calculations cannot fully represent a real cycle, they provide a useful first screen for comparing candidate compositions. Clearly this HFC-134a/32 blend has 1 The COP of a unit is defined as the ratio of the cooling duty delivered to the power input. 2 Cycle calculations reported here were performed using the commercially available CYCLE D modelling programme developed by the National Institute of Science and Technology (NIST), USA.

5 R.L. Powell / Journal of Fluorine Chemistry 114 (2002) properties close to that of R22. As formulated, the blend was also non-flammable. Since HFC-134a was already being developed as the replacement for CFC-12, it was clearly attractive commercially for it to also be the main component of the substitute for the other major refrigerant, HCFC-22. However, the proposed blend differed from HCFC-22 in one important aspect: when it evaporated its boiling point changed, i.e. it was a zeotrope. Equipment manufacturers had a long-standing aversion towards zeotropic refrigerants because of several anticipated problems. Differential leakage Liquid refrigerant in a unit would be richer in 134a than the blend while the vapour would be richer in 32. If the unit experienced a liquid leak then the average composition of the blend would change which would both affect the performance and make topping-up more difficult. If the original blend were used, then the composition of the mixture in the unit would drift. In practice this fear proved to be unfounded. Real leaks do not change the composition of the refrigerant. Temperature glide A one-component refrigerant evaporates or condenses at a constant temperature. A zeotrope, however, evaporates or condenses over a temperature range, which is known in refrigeration technology as the temperature glide. The greater the difference between the boiling points of the components, the greater the glide. For many years enthusiastic academics had advocated the use of zeotropic blends as a way of improving energy efficiency, for which there was good theoretical thermodynamic justification. In practice non-equilibrium effects and poorer heat transfer often wiped-out any potential gains in efficiency, or required more expensive heat exchangers to realise the improved performance. Equipment manufacturers were concerned that the same problems would be experienced in replacing HCFC-22 by a zeotrope. A further potential consequence of the glide was that the temperature in the portion of the evaporator immediately after the expansion device might be below 0 8C, although the average temperature over the evaporator was higher. If this happened, ice would form, blocking part of the heat exchanger surface and thus reducing its efficiency. Fortunately in practice, the glides for the HFC134a/32 blend were sufficiently small not to cause these problems. Air-conditioning circuit An air-conditioning circuit sometimes contains a receiver immediate after the evaporator to prevent slugs of unevaporated liquid refrigerant from entering the compressor where they could cause a serious damage. However, when using a zeotropic refrigerant the higher boiling component might accumulate preferentially in the reservoir thus altering the composition of the circulating refrigerant, which might adversely affect the performance of the unit. None of these anticipated problems proved to be significant in practice, but the blend was rejected, because it was capable of generating a flammable vapour under exceptional circumstances. If an air-conditioning unit is exposed to winter temperatures of 40 to 20 8C, typically experienced in the northern Great Plains of the USA, then the lower boiling HCFC-32 can distil preferentially from the evaporator within the building into the external condenser. In effect the air-conditioner fractionates the refrigerant. The vapour in the condenser can become sufficiently rich in HFC-32 to be flammable. The flammability hazard was deemed to be unacceptable, so a new formulation was developed containing HFC-32, HFC-125 and HFC-134a in the mass ratio 23/25/52. The nonflammable HFC-125 suppressed the flammability of R32, while still giving thermodynamic properties that were reasonably similar to those of HCFC-22. This HFC-32/125/134a blend has been assigned the code number 407C 3 by American Society for Heating Refrigeration and Air-conditioning Engineers (ASHRAE 4 ). Its energy efficiency was considered to be slightly worse than that of the original HFC-32/134a blend, but the COP values in Table 3 indicate that the effect is likely to be marginal. The calculated cooling capacity was slightly higher than of HCFC-22, which was good. Although under conditions used for the example, the discharge temperature of the compressed vapour is relatively low, even for HCFC-22, the even lower temperature calculated for R407C suggests that under more extreme operating conditions there is little risk of excessive discharge temperatures. The major disadvantage with R407C as a direct replacement for HCFC-22 was its higher discharge pressure. Refrigeration engineers normally consider that equipment designed for HCFC-22 can accommodate, at most, an extra 2 bar of pressure, a value which R407C attains at a condensing temperature of 55 8C. Although design modifications allow R407C to be used in new equipment it is not generally recommended for retrofitting into R22 units already in service. 5. Long-term blends Once the principle of zeotropic refrigerants had been accepted then an infinite number of products was in theory 3 The 400 series numbers are reserved for zeotropes. When a new combination of fluids is first submitted to ASHRAE it is given the next available number; thus the first combination containing 32/125/134a was assigned the number 407. To distinguish blends containing the same components but in different ratios, each specific composition is given an upper case letter which is assigned in the order that the blends are registered by ASHRAE; thus R407C was the third HFC-32/125/134a blend to be registered. The composition of a zeotrope can only be determined from its refrigerant code number by reference to a list. As with the azeotropes of the 500 series no direct link exists between chemical structures and the number. 4 Although ASHRAE is an American organisation whose membership is strictly limited to individuals, not industrial corporations, its standards on refrigerant nomenclature and refrigeration practice are recognised worldwide.

6 242 R.L. Powell / Journal of Fluorine Chemistry 114 (2002) Table 4 Long-term refrigerant blends based on HFCs ASHRAE code Component Composition (wt.%) Refrigerant replaced Application 404A 125/143a/134a 44/52/4 R502 Supermarket R502 (CHClF 2 /CF 3 CF 2 Cl) 407A 32/125/134a 20/40/40 R502 Supermarket 407B 32/125/134a 10/70/20 R502 Supermarket 407C 32/125/134a 23/25/52 R502 Supermarket 407D 32/125/134a 15/15/70 R500 Industrial R500 (CCl 2 F 2 /CH 3 CF 2 H) 410A 32/125 50/50 HCFC-22 Air-conditioning 410B 32/125 45/55 HCFC-22 Air-conditioning /143a 50/50 R502 Supermarket 508A 23/116 a 39/61 R503 Biomedical freezer R503 (CF 3 H/CF 3 Cl) 508B 23/116 46/54 R503 Biomedical freezer a PFC-116 is hexafluoroethane. possible. Not surprisingly therefore, several different products appeared in the market place intended to replace the same CFC or HCFC refrigerant, a selection of which is listed in Table 4. Previously the fluorocarbon refrigerant market was satisfied by two major tonnage products (CFC-12, HCFC-22), two moderate tonnage products (R502, 5 CFC- 11) and three low tonnage products (R503, 6 CClF 2 CClF 2 (CFC-114), CF 3 Br (BFC-13B1)). Service engineers could buy CFC-12 as Freon TM 12 from DuPont or as Arcton TM 12 from ICI, knowing they were the same material. Of course this is also true of the blends in Table 4, but not all blends are available from all suppliers, because in some cases they are still restricted by patents and licensing agreements. Several refrigerants are designed to replace R502, a key supermarket freezer refrigerant, but it is not acceptable to top-up a unit containing one product with another intended for the same application. Although they contain the same components in different ratios, R508A and R508B are both azeotropes. This is possible because the azeotropic composition is markedly temperature dependent. The R508A composition was selected to be azeotropic at 80 8C close to its typical evaporating temperature in low temperature medical freezers, while the composition of R508B corresponds to an azeotrope at 40 8C. Although both components of these azeotropes have high global warming potentials (GWPs) (Table 5) they have been accepted as long-term replacements for R503 in biomedical freezers offering a high degree of containment and from which they can recovered efficiently for recycling. By refrigeration standards, global yearly usage is very small, of the order of 100 te. Furthermore such freezers are considered an essential use. CFC-11 is used in low pressure (<2 bar) chiller airconditioning systems based on centrifugal compressors and 5 An azeotrope of CF 3 CF 2 Cl (CFC-115) and HCFC An azeotrope of CFC-12 and CHF 2 CH 3 (HFC-152a). Table 5 Global warming potentials of fluorinated compounds [13] ASHRAE code Compound Estimated atmospheric lifetime (year) GWP a (100 year integrated time horizon) CFCs CFC-11 CCl 3 F CFC-12 CCl 2 F CFC-113 CClF 2 CCl 2 F CFC-114 CClF 2 CClF CFC-115 CF 3 CF 2 Cl HCFCs HCFC-22 CHClF HCFC-123 CF 3 CHCl HCFC-124 CF 3 CHClF HCFC-141b CH 3 CCl 2 F HCFC-142b CH 3 CClF HFCs HFC-23 CHF HFC-32 CH 2 F HFC-43-10mee CF 3 CHFCHFCF 2 CF HFC-125 CF 3 CF 2 H HFC-134a CF 3 CH 2 F HFC-143a CF 3 CH HFC-152a CHF 2 CH HFC-227ea CF 3 CHFCF HFC-236fa CF 3 CH 2 CF HFC-245ca CHF 2 CF 2 CH 2 F a GWP values are referenced to the absolute GWP for CO 2 at 100 year integrated time horizon; typical uncertainty is 35%. using a cold water circuit to distribute coolth 7 around large buildings. A successful replacement must satisfy the low pressure requirement and have a comparable molecular weight to be compatible with centrifugal compressors. 7 Coolth is a term used in refrigeration and air conditioning to mean the opposite of heat. The temperature of a room is raised by supplying heat and cooled by supplying coolth. It can be defined as negative heat.

7 R.L. Powell / Journal of Fluorine Chemistry 114 (2002) FC-1,1,2,2,3-pentafluoropropane (CHF 2 CF 2 CH 2 F, bp 27 8C) appears to be the single fluid with the closest match in properties [8,9], but is flammable over a modest range of concentrations in moist air. 6. Transitional products Although new refrigeration and air-conditioning equipment now being sold in the developed world contains HFC refrigerants, or, in the case of European domestic fridge freezers, hydrocarbons, a huge number of installed commercial and industrial units still operate using CFCs. In the USA, and also in Europe up to the end of 2000, this equipment could be serviced with recycled refrigerants. However, refrigerant suppliers recognised that as the pool of recyclable CFCs and HCFCs diminished or recycling was banned, products would be required that could be retrofitted into existing systems, which still had many years of useful service life. The result has been a proliferation of zeotropic blends aimed at the retrofit market. Table 6 lists some blends that are being, or have been, offered commercially, but is by no means comprehensive. Some blends were originally developed in response to equipment manufacturers and users customer demands to eliminate CFCs as soon as possible, even ahead of the Montreal Protocol requirements. Thus the author, when working for ICI Klea (now Ineos Fluor), developed R509 in collaboration with Sanyo as a replacement for R502 in biomedical freezers. A key objective in the development of these transitional products was that they should be at least partly miscible with the hydrocarbon lubricants used with the CFCs being replaced to provide adequate oil return from the refrigeration system. The inclusion of chlorinecontaining HCFCs in most of the listed formulations ensured this would happen. Table 6 Transitional products ASHRAE code Component Composition (wt.%) Refrigerant replaced 401A 22/152a/124 53/13/34 CFC A 125/propane/22 60/2/38 R B 125/propane/22 38/2/60 R A Propane/22/218 5/56/39 R A 125/143a/134a 44/52/4 R A 125/143a/22 7/46/47 R A 22/124/142b 60/25/15 CFC B 22/124/142b 65/25/10 CFC A Propene/22/152a 1.5/87.5/11 HCFC A 22/218/142b 70/5/25 R A 218/iso-butane/134a 9/3/88 CFC-12 No code 125/134a/n-butane 47.5/49/3.5 HCFC /218 a 44/56 R CF 3 CHCl 2 CFC-11 a PFC-218 is perfluoropropane. In anticipation of the phase-out of HCFCs, formulations are being developed which contain no chlorinated fluids. In this case, to ensure good oil return, a small quantity of hydrocarbon is added which is sufficient to promote oil return but does not generate a flammable vapour or liquid even when the zeotrope fractionates during evaporation Foaming blowing CFC phase-out has had an equally important impact on the poly-urethane foam used to insulate refrigerators. In the past, CFC-11 was the universal foam blowing for refrigerant applications. In the USA it been has replaced by HCFC-141b (CCl 2 FCH 3 ), a transitional product which provides an insulating effect approaching that of CFC-11. In Europe, however, the strong environmental lobby has influenced manufacturers to adopt c-pentane as the preferred blowing agent. In the USA the possible flammability hazard posed by this hydrocarbon has militated against its use. Clearly American and European views differ when dealing with environmental problems. Along with other HCFCs, 141b will be largely phased-out over the next decade. A possible replacement is CF 3 CH 2 CHF 2 (HFC-245fa); but vacuum panels and aerogels are potential not-in-kind alternatives. 7. Other HFCs and hydrofluorocarbon ethers Although HFC-134a and R407C emerged as the most important replacements for CFC-12 and HCFC-22, respectively, various alternative fluids were mooted, especially in the late 1980s and early 1990s. HFC-134 (CHF 2 CHF 2,bp 20 8C) was considered for automobile air-conditioning, where its critical temperature of 112 8C, 12 8C higher than that of HFC-134a (101 8C), was seen as advantage since vehicle air-conditioner condensers operate at temperatures up to 80 8C. HFC-227ea (CF 3 CHFCF 3,bp 20 8C) was originally advocated by Hoechst as an alternative to HFC- 134a. Although it was not accepted as a major replacement refrigerant, it has emerged as a fire-fighting agent to replace CF 2 BrCl (halon 1211) and as a propellant for medical aerosols. The American EPA favoured low boiling, partially fluorinated dimethyl ethers, funding academic programmes to synthesise candidates and measure thermodynamic properties. Of particular interest were CF 3 OCH 3 (E143a, bp 24 8C) and CF 3 OCHF 2 (E125, bp 37 8C). These compounds appeared to offer lower GWPs than HFC-134a and HFC-125 and appeared to be promising refrigerants, either alone, or perhaps formulated with other fluids [10]. So why was none of these compounds developed further as replacements for CFC-12 and HCFC-22? Reasons include the following. HFC-134, HFC-227ea and E143a have lower refrigeration capacities than CFC-12 so would need larger compressors.

8 244 R.L. Powell / Journal of Fluorine Chemistry 114 (2002) This requirement would have a cost penalty, and in the case of vehicles a size and weight penalty thus increasing fuel consumption. For ease of servicing, especially in mobile and packaged air-conditioning applications, single products, which were direct replacements for R12 and HCFC-22, were considered important. In the early 1990s when these alternative compounds were being mooted, the refrigerant producers had already committed considerable effort to introducing HFC-134a as the major replacement refrigerant in order to meet the deadlines for CFC phase-out set by the Montreal Protocol. Any delay to consider alternative products would have meant that CFC-12 would not have been phased-out as planned. The synthetic routes used for producing lab samples of E134a and E125 were not economically viable for large-scale production processes. Although manufacturing routes for 134a had proved to be difficult, they had been able to exploit existing technology. The atmospheric lifetimes of E125 and E134a were lower than those of HFC-125 and HFC-134a and so had lower GWPs. In reality this was only a marginal advantage since most of the global warming contributions of refrigeration and air-conditioning units were a result of burning fossil fuel to power them, not from the refrigerant, even assuming that this was released and not recycled (v.i.). Environmental lobbyists, especially in Europe, were advocating the banning of fluorine-containing fluids as a matter of principle, so choosing fluorine containing refrigerants with modestly lower GWPs would not have lessened their opposition. 8. The present state of the ozone layer The concentrations of stratospheric ozone over much of the globe are being monitored continuously by the satellitemounted total ozone mapping spectrometer (TOMS). Both the latest daily readings and historical data from 1980 onwards are readily accessed on the CMDL web-site [11]. Fig. 1a shows the minimum ozone readings over Antarctica for the period Stratospheric ozone concentrations are expressed in Dobson units (DUs). 8 The minimum ozone concentration over the Antarctic occurs in its spring and is generally observed in October, although it can occur also in late September or 8 The DU is named after Dobson, a pioneer of atmospheric ozone research in the period A 1 DU is defined as a 0.01 mm thickness of ozone at STP. To understand the definition of the DU, imagine a column covering the land area of France and stretching up into the higher atmosphere. If all the ozone in the stratospheric region of this column was compressed to STP and the gas spread evenly over the whole area, then the thickness of the resulting gas layer expressed in millimetres would be the average concentration of stratospheric ozone in DUs over France. early November. Fig. 1a demonstrates that the Antarctic ozone minimum dropped from about 210 DUs in 1980 to roughly 90 DUs in late But the ozone hole was only discovered in 1984 so how is it possible to provide results for earlier years? The evidence for the Antarctic ozone depletion was already present in the pre-1984 TOMS data, but this was not analysed until after Farman s team made their discovery using ground based equipment. Fig. 1b shows the expansion the area of ozone depletion over the same period. Clearly Antarctic spring ozone depletion remains severe. So are the controls on the emissions of chlorine-containing gases imposed by the Montreal Protocol having any effect? Fig. 2 shows that the measured concentrations of chlorine-containing gases in the atmosphere have reached a maximum and are starting to decline. There can be no doubt that the controls introduced by the Montreal Protocol are having the desired effect, but the atmospheric chlorine concentration is still close to its maximum level so ozone depletion over the Antarctica is unlikely to improve significantly in the immediate future. Fig. 3 shows both the historical increase of chlorine and bromine containing gases and their predicted decline over the next century. The consensus of scientific opinion is that the ozone problem has been solved by the world-wide regulation of CFC and HCFC production and emission under the terms set by the Montreal Protocol. By 2050 the Antarctic ozone hole is expected to be much less extensive. The concentrations of the chlorine-containing gases HCFCs 141b and 142b, adopted especially in the USA as transitional products to facilitate the phase-out of CFCs, are presently increasing (Fig. 4) [11]. However, their concentrations are only 2% of that of CFC-12 and will start decreasing following HCFC phase-out over the next decade. Measurements are made at eight sites around the world, including the South Pole, which are distant from major centres of fluorocarbon production and use. The upper and solid lines in the diagram show the range of values obtained. The dotted lines show the average trends in the atmospheric concentrations of the compounds. Not surprisingly the concentration of HFC-134a is also growing rapidly following the start of large-scale manufacture and use in the mid-1990s. Of the HFC-134a emitted 7 20% is converted by atmospheric hydroxyl radicals to trifluoroacetyl fluoride, which is subsequently hydrolysed to trifluoroacetic acid. Although the detection of trifluoroacetate in land surface water is therefore not unexpected, it is remarkable that quantities present already exceed those estimated for 2010 having taken into account all man-made sources [12], including compounds such as halothane (CF 3 CHBrCl), CFC-114a (CF 3 CIF 2 ), CFC-115 (CF 3 CF 2 Cl) and HCFC-123 (CF 3 CHCl 2 ). This suggests that a natural source of trifluoroacetate exists [13]. Oceanic concentrations strongly support this view with values of 200 ng/l

9 R.L. Powell / Journal of Fluorine Chemistry 114 (2002) Fig. 1. (a) The deepening and (b) the growth of the ozone hole. in both mid-atlantic and the southern ocean of Antarctica at depths down to 4150 m [14], corresponding to more than 100 mte of trifluoroacetate which far exceeds any historical industrial production of fluorochemical precursors. Fig. 2. Atmospheric concentrations of two major CFCs [11]. The source of this trifluoroacetate is unknown, although Harnisch et al. has speculated that it could have a geological origin [15], but has not presented a chemical mechanism. Clearly this discovery represents an exciting challenge to fluorine chemists. Given that trifluoroacetate is a natural product it is perhaps not surprising that from extensive testing of the effect of trifluoroacetate on living organisms, it has been concluded the amounts of this ion generated from the breakdown of HFCs, such as 134a will not adversely affect the environment. But this is not a justification for the uncontrolled release of these valuable compounds to the environment. The well-established policy of minimising HFC emissions to the atmosphere, by not using them in nonessential, dispersive applications, preventing leaks and recycling material recovered from defunct equipment, is both environmentally responsible and economically sound practice [11].

10 246 R.L. Powell / Journal of Fluorine Chemistry 114 (2002) Fig. 3. Halogen loading as equivalent stratospheric chlorine [11]. Fig. 4. Atmospheric concentrations of HFC-134a and three HCFCs [11]. 9. The impact of the HFC replacements on global warming Around 1990 global warming resulting from the release of man-made gases became a major environmental issue. Although the largest contributor is carbon dioxide from the burning of fossil fuels, it was estimated that CFCs accounted for 15% of global warming in the early 1980s. Although their effects would obviously disappear, the question is whether their commercially preferred replacements, the HFCs, would contribute significantly to global warming. From Table 4 the GWP of HFC-134a relative to CO 2 is 1300 which is only 6.5 times less than that of CFC-12 which it has replaced. Seizing upon the perceived high GWP

11 R.L. Powell / Journal of Fluorine Chemistry 114 (2002) of HFC-134a, environmental campaigners and some governments have urged that this product and other HFCs should be regarded as transitional products and that, like the CFCs, a time-table should also be adopted for their eventual phase-out. During the negotiations leading to the Kyoto Treaty, however, a basket of green-house gases was defined that included HFCs as well as CO 2, methane, perfluorocarbons, SF 6 and nitrous oxide. Each country committed to achieving a specific level of total global warming gas emissions in 2010 based on its emissions in Flexibility was allowed in the combination of gases which were emitted to achieve this target; for example, a country could choose to control all the six types of gases prorata; alternatively it might reduce its CO 2 emissions by a greater extent, but increase its HFC emissions. Arguing for the phase-out of HFC-134a refrigerant purely on the basis of its GWP is scientifically simplistic. Fig. 5a shows the rates of elimination of HFC-134a from the atmosphere relative to carbon dioxide assuming that equal masses of each are emitted at time zero. Whereas essentially all the 134a has been eliminated after 100 years, over 30% of the carbon dioxide remains after 500 years. Obviously the fossil fuel derived energy consumed by the refrigerator over its 20 year lifetime has greater consequences for global warming than the HFC-134a in its refrigeration circuit. Fig. 5b demonstrates this very clearly for a domestic refrigerator similar to those currently sold in the USA, which are insulated by foam blown with CCl 2 FCH 3 (141b). The vertical axis is a measure of the actual contribution of the refrigerator to global warming over 500 years which takes into account not only their GWPs, but also the quantities of each associated with the appliance. Furthermore, it assumes that the HFC-134a is released at the end of the 20 year life; in practice this will not happen since all developed countries have strict regulations requiring the recover and recycle of HFC refrigerants. Despite biasing the case against HFC-134a the area under the CO 2 curve is much greater than that under the combined fluorinated Fig. 5. (a) Elimination of HFC-134a and CO 2 from the atmosphere [16]. (b) Total impact of a refrigerator on global warming [16].

12 248 R.L. Powell / Journal of Fluorine Chemistry 114 (2002) fluids curve, i.e. the two smaller areas above the CO 2 curve. Replacing HFC-134a by lower GWP refrigerant will have only a small effect on the over-all contribution of the refrigerator to global warming. Much greater improvements could be achieved by improving the insulation or increasing the heat exchanger area to reduce temperature differentials. Even better, environmentally, would be the use of energy from non-fossil sources, such as wind generators or solar cells. The idea of considering both the direct and the indirect contributions from the refrigerant agent and the power generation, respectively, plus the contribution from manufacturing a unit, was developed by the Alternative Fluorocarbons Environmental Acceptability Study (AFEAS) and the US Department of Energy (DOE). The sum of these contributions is called the total equivalent warming impact (TEWI) of the technology being considered [16]. The value of the concept lies in its ability to compare, on a common basis, systems operating on different physical principles. For example, standard mechanical refrigerators can be assessed against absorption units in which a heat source, such as a gas flame, is used to distil ammonia from water. TEWI ¼½refrigerant contribution converted to CO 2 te equivalentš þ½te CO 2 from fossil fuel over lifetime of cooling unitš þ½te CO 2 equivalent to energy to build unit and produce the refrigerant fluidš The calculated TEWI is sensitive to assumptions of the system lifetime, emission losses, and the integration time horizon chosen to calculate GWP values, as well as the source and consumption of energy. For example, an electric refrigerator in France, where 60% of electricity obtained from nuclear power, will have a lower TEWI than the same unit in the UK, where only 20% of the electricity is derived from this source. The TEWI concept includes the energy required to build a unit although this is typically small compared with the energy consumed by the unit during its working life. 10. Natural refrigerants Environmental campaigners strongly support so-called natural refrigerants which are listed in Table 7. The adjective natural describes their presence in the environment from biological and geological sources; but the commercially available products sold for refrigeration are typically derived from non-renewable sources: hydrocarbons from oil cracking; ammonia and CO 2 from natural gas. All these refrigerants were in extensive use until the rapid growth of the CFCs and HCFCs post In 1930, 80% of the British merchant refrigerated shipping fleet relied upon carbon dioxide because of its relatively low hazards. Ammonia has remained the refrigerant of choice in large-scale food freezing plants to the present day. In the 19th century most small refrigeration units used hydrocarbons. Obviously there is nothing new about the natural refrigerants. They are readily available commercially and are certainly no more expensive than HFCs. In equipment design specifically around their particular thermodynamic properties they can also achieve acceptable energy efficiencies. So why did equipment manufacturers not revert to these compounds when the need to phase-out CFCs and HCFCs become apparent? In Europe domestic refrigerator manufacturers did adopt hydrocarbons under consumer pressure following adverse publicity about the high GWP of HFC- 134a from environmental lobbyists; many freezers contain i-butane in the refrigeration circuit and have insulating foam blown with c-pentane. Although the refrigerator/freezer is the most obvious manifestation of refrigeration to the general public, this application only accounts for 4% of total refrigerant use. Table 7 Natural refrigerants Compound Boiling point (8C) Hazard Typical application Hydrocarbons C 2 H Flammability Biomedical freezers C 3 H Flammability Building air-conditioning i-c 4 H Flammability Domestic freezers n-c 4 H Flammability Refrigeration, p-styrene foam blowing c-c 5 H Flammability Refrigerator foam blowing Inorganics CO (sublimation) Mobile air-conditioning NH 3 33 Toxicity Food processing Flammability Supermarket refrigeration Absorption refrigerators Air Air-conditioning H 2 O 100 Absorption chillers Adsorption air-conditioning

13 R.L. Powell / Journal of Fluorine Chemistry 114 (2002) The largest single cooling application is automobile air-conditioning, which in the USA takes 30,000 te of HFC-134a a year in new vehicles alone. Although various hydrocarbon compositions have been offered both as retrofit replacements for CFC-12 and for new vehicles, tests have shown that if these fluids escaped into the passenger compartment during an accident and ignited the resulting explosion would cause serious injury. Not surprisingly, hydrocarbon refrigerants are banned from mobile air-conditioning units in USA and in some Australian states. Carbon dioxide is being considered by some auto-manufacturers in Germany, although the very high pressures require a radical engineering redesign. Tests suggest that it is not energy efficient as 134a at high ambient temperatures where airconditioning is especially appreciated. Furthermore, if the gas entered the cabin its physiological effects, notably shallow breathing, could be worse than those for HFC-134a. Regulations typically require that building air-conditioning or refrigeration systems in direct contact with the general public (e.g. in supermarkets) should not contain hazardous refrigerants. The problem can be overcome by installing a secondary circuit containing glycol or calcium chloride brine to transport the coolth from the refrigeration plant to the building or display units. Such systems have been used for many years, but involve an extra temperature drop in the heat exchanger between the primary and secondary circuits. This means that the evaporator is operating at a lower temperature than in a simple direct cooling system and thus the energy efficiency of the system is lower. The net result is more carbon dioxide generation and a higher TEWI. Absorption refrigeration or air-conditioning units, which are driven by the direct application of a heat source, are sometimes pushed as a more environmentally acceptable system. Two technologies exist: (1) the evaporation of ammonia from water; and (2) water from lithium bromide solution. The former is often used in the minibar refrigerators in hotel rooms because of its quiet operation. The latter is used for providing chilled water to air-condition buildings. Absorption units are based on intrinsically inefficient thermodynamic cycles although it is sometimes assumed, wrongly, that using a direct source of heat avoids the fundamental thermodynamic inefficiency in a power station. However, absorption units can be environmentally attractive in niche applications where there is an otherwise useless source of low temperature heat, or a natural source such as solar or geothermal. Although rapid and complete replacement of HFCs with natural refrigerants or other refrigeration technologies is strongly advocated by environmental activists, the scientific case for doing so is debatable. In the context of reducing global warming the real target must be the reduction of the fossil fuel derived-energy consumed by refrigerators and air-conditioners. Merely replacing HFCs with natural refrigerants will have little impact or in some cases could increase global warming. It could also be counterproductive, because it would divert key technical expertise from making refrigeration and air-conditioning units more energy efficient. 11. Conclusions In this paper I have described the factors which have influenced the selection of the HFCs as the major replacements for the CFCs and HCFCs especially in cooling applications. Atmospheric concentrations of major CFCs appear to have stopped increasing and are expected to decline within the next decade. Models suggest that the annual Antarctic ozone hole will also start to diminish and by 2050 will have largely disappeared. In answering the question posed in the title of this paper, I conclude that we have met the challenge. We depend crucially upon refrigeration for the safe storage and distribution of our foodstuffs, for the preservation of vaccines and drugs, as well as for the comfort afforded by air-conditioning. The CFCs have been replaced by safe, efficient HFC refrigerants in a transition that has caused no disruption to our way of life. To move from the position in 1986 when CFC-12 was the major refrigerant to 1996 when the developed world had essentially stopped production and replaced it by HFC-134a has been a remarkable achievement. The dialogue between the refrigerant producers and refrigeration equipment industry identified appropriate CFC and HCFC replacements; collaboration between refrigerant producers themselves ensured the efficient generation of key toxicity and environmental data without unnecessary duplication of effort. Producers competed to introduce HFCs into the market place, which ensured that market forces drove down prices, especially of HFC-134a. Confidence to invest in expensive new plants based on innovative technologies was provided by a strong global regulatory framework provided by the Montreal Protocol. Of course hitches and misunderstandings occurred; this was to be expected in a complex world-wide programme. A significant problem was the stock-piling of CFCs prior to phase-out in the USA and Europe and the smuggling of CFCs from plants in the developing world. Consequently the market for HFC-134a grew more slowly than anticipated; from 1994 to 1998 the installed capacity was greater than demand leading to prices which were too low to provide an adequate return on the newly invested capital. The next substantial landmark is the reduction of HCFC production in HCFC-22 will be replaced by various refrigerant blends based on HFC-134a, 125, and 32, especially R407C described above. Since this change has been anticipated for over 5 years, refrigerant companies have been able to plan their investments which are now being publicly announced; for example, Ineos Fluor (originally the ICI Klea Business) has indicated that it had commenced HFC-125 production in early The phase-out of remaining HCFC production in accordance with the Montreal Protocol can be assured.