CFCs - The Search. For Alternatives. Fluorocarbons

Transcription

1 CFCs - The Search For Alternatives f Fluorocarbons

2 by J Hugo Steven, New Fluorochemicals Research Manager and Archie McCulloch FRSC, Assistant Environmental Adviser, C1 Chemicals and Polymers Ltd., P 0 Box 8, The Heath, Runcorn, Cheshire WA7 4QD United Kingdom How is it that a compressor seal leaking CFC 12, one of the most innocuous chemicals known, is supposed to contribute to a hole in the middle of the ozone layer 10,000 miles away and years later? How can we keep the benefits of safe refrigeration without using a potential ozone depletor as the working fluid? CFCs (chlorofluorocarbons) are a family of different products, each with a unique set of properties designed to meet important market needs, many of which are essential to modern industrialized society. At the same time they are non-flammable, very stable, of low toxicity, show excellent thermodynamic performance in use and are cheap. No other class of compound can combine these desirable characteristics, but their very stability is at the root of their environmental problems. The simple parts of the problem are that, to be implicated in ozone depletion, a compound must be volatile and be stable in the lower atmosphere; it must contain chlorine or bromine and must release this by photolysis in sunlight in the ozone layer, at an altitude of 12 to 25 miles. The fully halogenated CFCs and carbon tetrachloride satisfy all of these criteria. n contrast, some other chlorocarbons such as methylene chloride, trichloroethylene and perchloroethylene are not as stable in the lower atmosphere and so may not be transported to the stratosphere. Differentiation between existing compounds in such a qualitative sense is trivial. Evaluation of compounds which could replace CFCs requires a more robust analysis, but attempts to measure the effects and to quantify the role of individual compounds show the complexities. 2

3 Until 1987, analysis of measurements from ground level of the ozone concentration in the layer had found no trend. Natural summer to winter variability is in the range 25 to 35%. t took a new statistical approach to show that, over 17 years, ozone levels may have dropped by an average of 2% in total, over the Northern Hemisphere. Direct measurement of the effect of individual compounds is impossible, so their contributions are estimated by mathematical models which incorporate up to 250 chemical reaction equations together with approximations for the mixing of the atmosphere. They take account of the many reactions, other than the ozone depletion cycle, into which stratospheric chlorine enters. Most of these yield compounds such as hydrogen chloride or chlorine nitrate which are normally inactive. When possible, the models are calibrated against measurements of the real life atmosphere; it is estimated that the atmospheric lifetime of a new compound can be predicted to within t- 50%. Many CFCs are used as the working fluid in refrigerators, freezers and air conditioning units. Without such temperature control, modern food processing, delivery and storage would be impossible, many hospital operations would be unsafe and large computer applications unreliable. Some CFCs are used in producing a wide variety of foams for use in the insulation of homes, offices and in the refrigeration industry. Other uses include propellants in medical aerosols and solvents in the electronics industry. The problem facing the researcher is to replace the CFCs in these application areas, where their unique properties are required, with a new family of chemicals which offer the same benefits and none of the potential environmental disadvantages. Alternatives to CFCs need to meet several criteria, e.g., 0 Performance - similar performance to products being replaced - user industry requires minimal cost of change - minimal energy losses due to fluid in refrigeration, heat pumps and air conditioning - insulation energy benefits need to be maximized 3

4 Safety - low toxicity - non-flammable - environmentally benign Production Technology - safe - rejiable - economical to produce, competitive, affordable products - operable when market requires product The old-fashioned refrigerants such as SO, CH,C, CO, or NH, which the CFCs replaced in many systems, fail the criteria on the grounds of toxicity, flammability or technical considerations such as high system pressure or corrosiveness. Solutions The two CFCs most widely used are CFC 12 (CF,C,) and CFC 11 (CFC,); both of these compounds are produced by the fluorination of carbon tetrachloride (CC,) using the technique of halogen exchange. They differ widely in boiling point (CFC 12's is -29.8"C and CFC 11 's, +23.8"C) and in critical temperature, (112 C and 198 C respectively). The compounds being suggested as replacements are: For CFC 12 HFC 134a CF,CH,F (boiling point "C) For CFC 11 HCFC 123 CF,CHC, (bpt "C) and HCFC 141 b CH3CFC, (bpt + 32 C) Other non-direct replacements for specialist areas could be: HCFC 124 CFSCHCF (bpt -1 2 C) HFC 125 CF,CHF, (bpt -48.5"C) HCFC 22, which is currently available, will also be used in some of the market sectors. 4

5 Compounds which contain hydrogen have the possibility of reacting with the hydroxyl radicals naturally present in the lower atmosphere. Thus it is possible to build atmospheric instability into a compound in a more or less controlled manner, thereby resulting in short atmospheric lifetimes. (Atmospheric lifetime is equivalent to the reduced lifetime if one treats the lower atmosphere as a single continuous stirred tank reactor.) Current estimates of atmospheric lifetimes are shown in Table 1 [ 1,2] which also shows ozone depletion potentials, defined as the potential ozone depletion arising from one ton of the compound compared to that of one ton of CFC 11, over the whole of their atmospheric lifetimes. Any effect depends on how much of each of the compounds is emitted. ndividual contributions to stratospheric ozone depletion can be approximately represented by the amount of chlorine which the compound could introduce into the stratosphere. n turn this is related to the atmospheric concentration of the compound, which may be calculated simply. Figure 1 shows both historic chlorine contributions and the outcome of a scenario in which the effects provided by CFCs are provided in the future by alternatives. This example has been chosen to illustrate the effect of eliminating CFCs. Reductions and phase-out of releases of 1, 1,1 -trichloroethane and carbon tetrachloride will lead to atmospheric levels of chlorine below those shown. Replacement in this way would enable production of CFCs to be phased out. 5

6 Figure 1. Potentially Active Chlorine in the Stratosphere / Original Montreal Protocol (a) All Alternatives (b) All CFCs (c) 1,1,1 -trichloroethane (d) Carbon tetrachloride (e) Year Methyl Chloride (f) 6

7 KEY a) Original provisions of Montreal Protocol; ozone depletion potential of CFCs to be reduced by 50% by reducing production and consumption. No controls on carbon tetrachloride or 1, 1,l -trichloroethane. b) Alternative fluorocarbons (HCFCs and HFCs) supply 40% of former CFC market. Remainder of demand is met by more efficient use of fluids (recycling rather than emissions) and non-fluorocarbon fluids. This scenario was developed by C1 but is similar to that of the Alliance for Responsible CFC policy r31. c) Historic emissions of CFCs are based on CMA data [4] and industry estimates. Atmospheric concentrations post-phaseout are calculated from these according to the formula: Quantity remaining in year T = original emission x exp (-T/P), where P is the atmospheric lifetime standardized to the 1986 concentrations given in [5]. d) Historic emissions of 1,l,l -trichloroethane are taken from [6] and the formula contained in this used to calculate future emissions. e) Current atmospheric concentrations of carbon tetrachloride given in [5] equate to a constant annual emission of 100,000 tons/year. After phaseout the residual.concentrations are calculated as in (c) above. f) Methyl chloride arises from natural sources. The concentrations were obtained from [5]. 7

8 For the purposes of this illustration, CFCs are assumed to be produced in accordance with the current provisions of the original version of the Montreal Protocol only up to the year After that year, worldwide, no more CFCs are manufactured or used. Carbon tetrachloride production and use follows the same pattern with dispersive uses ceasing in The production of 1,l,ltrichloroethane, which has a short atmospheric lifetime, is assumed to remain cons tan t. The difference in potentially active chlorine between that from provisions of the original Protocol and that which the alternative fluorocarbons might provide is striking. With a truly global phaseout of CFCs and carbon tetrachloride, the decay rate of the chlorine which is already in the stratosphere is limited only by atmospheric chemistry. f phaseout is incomplete the downslope would not be as steep. Fifteen percent of current production would remove the downslope altogether and stratospheric chlorine concentrations would remain constant. Having identified some potential replacements from basic research work, a number of activities must begin, e.g., toxicity and environmental chemistry studies, synthesis of product for customer evaluation and toxicity trials, process chemistry studies, application study work on physical data, material compatibility, lubrication, etc., leading to more increased scale equipment work. This area requires careful design of equipment to enable the critical semi-technical information to be obtained. A summary of the steps is shown in Figure 2 on the next page. 8

9 1 PRELMNARY PROCESS ENGNEERNG PRODUCE TEST MATERALS SEM-TECH RESEARCH PLANT DESGN STE SELECTON PLOT PLANT MATERALS COMPATBLTY PROTOTYPE TRALS (C1 + CUSTOMERS) LONG-TERM TESTS (CUSTOMERS) CONSTRUCTON COMMSSONNG PRODUCT COMMERCALZATON 9

10 To mount a viable program on this scale for the replacement compounds costs C1 in the region of $160 million.this figure is overshadowed by the many billions of dollars user industries throughout the world will need to invest in order to switch from CFCs. The schedule for the work is unique. Taking the project for the beneficial production of HFC 134a in 1991 as an example, it is clear that with a starting date in 1987 it was not possible to use the normal sequential scheme of process development, as this could not be completed in this short time scale. The approach which C1 used was to introduce process engineers into the chemistry route research programs so that close integration of chemical and process knowledge was achieved early, leading to early flowsheeting of processes. Fast decisions could then be taken on design of semi-technical plant, etc., and development studies implemented quickly. Clearly, under such tight timescales it was necessary to run many activities in parallel, with definitive high quality process data becoming available at the lust moment to meet plant construction targets. Technically, therefore, we were set a tremendous challenge. This will be successfully completed with the arrival of material in the market in 1991 from the plant at Rocksavage Works, Runcorn, England. HFC 134a, being a more complex molecule with a higher fluorine content than CFC 12, is technically more difficult to manufacture and involves more process steps. The physical characteristics, although similar, differ in key areas of materials compatibility and lubricity. n our applications area, the fundamental work in refrigeration and air conditioning focused on acquiring the physical property data necessary for users to design their equipment. This has included developments in lubricant technology to overcome problems with insolubility of mineral oils in HFC 134a. The mineral oils used with the CFC 12 proved to be insoluble in HFC 134a, so a research program was undertaken to develop a lubricant. Cl s lubricants business has developed new lubricants, Emkaroxa and EmkarateB, which show themselves to be suitable and which are now in lifetime trials with customers. n the foam blowing area two products are being evaluated by customers; HCFC 123 and HCFC 141 b. Neither is a direct replacement, with hurdles of 10

11 stronger solvent properties or flammability (in the case of 14 1 b) to be considered and rationalized before they become accepted replacements. n summary, much has been done in the short time since these compounds have been seriously considered as products; much remains to be done to introduce a complete family to fully replace the chlorofluorocarbon family of products. REFERENCES 1. Alternative Fluorocarbon Acceptability Study (AFEAS). Scientific Assessment of Stratoshperic Ozone: World Meteorological Organization Global Ozone Research and Monitoring Project Report No UNEP. Synthesis Report. Open-ended Working Group of the Parties to the Montreal Protocol. UNEP/OzL.Pro.WG. (1)/4 14 Nov HCFCs and HFCs Provide the Balance, available from the Alliance for Responsible CFC Policy, 1901 North Fort Meyer Drive, Suite 1 204, Rosslyn, - Virginia 22209, USA Production and Sales of Chlorofluorocarbons CFC-11 and CFC-12, Chemical Manufacturers Association, Fluorocarbon Program Panel, M Street NW, Washington, DC 20037, USA. 5. Watson, R.T., et al., Present State of Knowledge of the Upper Atmosphere 1988, NASA Reference Publication No 1208, August 1988, p Midgley, P.M. The Production and Release to the Atmosphere of 1,1,1- Trichloroethane (Methyl Chloroform), Atmospheric Environment 23, , This article, in its original form, was published in ssue 003 (November 1989) of the Environmental Protection Bulletin, published by the nstitution of Chemical Engineers. For a sample copy and subscription details contact: John O Hara, Editor, Environmental Protection Bulletin, The nstitution of Chemical Engineers, Railway Terrace, RUGBY CV21 3HQ, United Kingdom. 11

12 For further information please contact: C1 Americas nc. Chemicals and Polymers Group Fluorocarbons Business Wilmington, DE (800) The contents of this note are given in good faith but without warranty and users must satisfy themselves that the product is entirely suitable for their purpose. Freedom from patent rights must not be assumed. FBG/