CFC substitutes

CFCs substitute for ozone destroyers dangerous refrigerants no hospital refrigerators something spectacular needed Midgley group discovers other CFCs and halons lion s share uses for CFCs aerosols and halons in World War II. 

Olson, M.J. and S.E.Surbrook, Jr. 1991. Defluorination of the CFC-substitute 1,1,1,2-tetrafluoroethane comparison in human, rat and rabbit hepatic microsomes. Toxicol. Lett. 59 89-99. 

Surbrook, S.E., and M.J.Olson. 1992. Dominant role of cytochrome P-450 2E1 in human hepatic microsomal oxidation of the CFC-substitute 1,1,1,2-tetrafluoroethane. Drug Metab. Dispos. 20 518-524. 

Simple hydrocarbons can also be used as CFC substitutes. Hydrocarbons such as propane, 2-methylpropane (common name isobutane), and butane are efficient aerosol propellants. These hydrocarbons are stable and inexpensive, but they are extremely flammable. 

Nonfluorine CFC substitutes have been considered, but few are fully satisfactory. For example, we could go back 50 years to the use of anhydrous ammonia as a refrigerant, but NH3 is as toxic now as it ever was. Cyclopentane could be used as a foam-blowing agent, but it is less effective than HCFC-141b and besides would contribute to the volatile organic compound load in the troposphere, which is the root cause of ozone pollution (Section 8.3.2). On the other hand, supercritical CO2 is emerging as an alternative to CFCs in various steps in the preparation of fluorocarbon polymers (Section 8.1.3). 

Fluorinated ethers have been widely tested as anesthetics,3 4 solvents and CFC substitutes and numerous toxicity data have been collected (Tabic 19). 

Staff ICI Plans U.S. Plant for CFC Substitute, Chem. Eng. Progress, 11 (February 1990). 

An ideal CFC substitute is one that offers identical performance properties of the CFC it replaces with the added stipulation that it does not destroy the ozone layer and has low global warming potential. Low toxicity, a property of CFCs, and shorter atmospheric lifetimes compared to CFCs are also preferred. Summarized, the desired characteristics include short atmospheric lifetimes, low toxicity, ozone compatibility, low global warming potential, thermal and chemical stability, equivalent physical properties, and amenability to realistic processes. 

Sulfur removal from natural gas Separation of MFCs (CFC substitutes)... 

An intense effort is under way to find CFC substitutes that are not harmful to the ozone layer. One of the promising candidates is called hydrochlorofluorocarbon-123, or HCFC-123 (CF3CHCI2). The presence of the hydrogen atom makes the compound more susceptible to oxidation in the lower atmosphere, so that it never reaches the stratosphere. Specifically, it is attacked by the hydroxyl radical in the troposphere ... 

Although the hydroxyl radical (OH) is present only in a trace amount in the troposphere, it plays a central role in its chemistry because it is a strong oxidizing agent and can react with many pollutants as well as some CFC substitutes (see p. 704). (a) The hydroxyl radical is formed by the following reactions ... 

USEPA Information on CFC substitutes, the methyl bromide, tbe UV index etc. http //www.epa.gov/docs/zozne/index.html... 

FIGURE 6.20 Atmospheric degradation of HFCs, HCFCs, and other CFC substitutes. Timescales for different processes are given in italics. 

Theoretical calculations of a CFC substitute, CHC12CF3, have been carried out by Cabral180 using DFT. The calculated polarizability agrees well with an... 

Greenhouse effect CO2, CFC, CH4, NgO, O3 Combustion of fuel Decrease of forest Eruption of volcano Use and loss of CFC Substitution and collection of CFC Saving energy Improving efficiency Changing fuel energy Forest conservation... 

The research has shown that not all HFCs and HCFCs are equally environmentally acceptable from a gas-phase atmospheric chemistry standpoint. Most are only slowly removed, allowing the possibility that a fraction of what is released at the earth s surface reaches the stratosphere. For HCFCs, subsequent photolysis releases chlorine, making these compounds less attractive as CFC substitutes. Even if the parent HCFC undergoes tropospheric degradation, some of the chlorine-containing by-products can reach the stratosphere and release Cl atoms. It is up to the atmospheric modelers to ascertain the ozone depletion potentials implied by these chemical processes. 

The gas-phase chemistry reviewed in this chapter shows how the pattern of substitution of chlorine and fluorine atoms on the parent hydrocarbon affects its atmospheric degradation. Although numerous reaction details need to be investigated and some discrepancies resolved, a core of reliable data is available from which one can deduce which HFC or HCFC may have deleterious atmospheric consequences and which potential substitute appears environmentally acceptable. However, the work is not finished. This work has dealt only with the gas-phase chemistry. Research is still needed into the heterogeneous chemistry of HFC and HCFC degradation products and into the biological effects that they may have. It is then up to the atmospheric modelers to combine this scientific information with emission scenarios and meterology to ascertain the feasibility of a particular CFC substitute. 

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