Chapter 4: Production of Second- or Third-Generation Fluorine-based Refrigerants from (Photo)-Dechlorination of Fluorocarbon Wastes

Abstract

Abstract Chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs) are called first-generation fluorocarbons because they contain chlorine atoms. Because they deplete the ozone layer, the production of CFCs has been banned and HCFCs will soon share the same fate. From this trend one can predict that hundreds of thousands of tons of CFCs or HCFCs presently being used will become harmful wastes in the near future. On the other hand, if these chlorinated wastes could be converted into either nonhazardous or useful compounds, CFCs or HCFCs could be regarded as chemical feedstocks. In this work, our first objective was to develop a decomposition method for CFCs or HCFCs at moderate conditions. Combustion of CFCs is an obvious choice, but has the disadvantage of producing toxic compounds such as dioxins. In our work, we found that photodecomposition of chlorinated compounds in dissolved alcohol–NaOH solutions under ultraviolet (UV) irradiation is possible under mild conditions of room temperature and atmospheric pressure. Chlorine atoms are easily removed yielding NaCl. Among the 12 chlorinated compounds tested, low-pressure mercury lamps were more effective than high-pressure ones. Chlorodifluoromethane (HCFC-22) and dichlorotrifluoroethane (HCFC-123a) is decomposed by only bubbling into methanol–NaOH solution and did not require UV irradiation. The second objective of our work was to determine the reaction products and their possible application to environmental fields. Three kinds of solutions, methanol plus NaOH, 2-propanol plus NaOH, and 2-propanol, were tested using 10 types of CFCs and HCFCs. Results showed that the main products were either hydrofluorocarbons (HFCs) or fluoroethers. HFCs are known as second-generation fluorocarbons because – although they have low effects on the ozone layer – they still have a large greenhouse gas potential. Fluoroethers are known as third-generation refrigerants, because they have neither the ozone layer depleting potential nor the greenhouse gas potential. Our work showed the possibility that wastes such as CFCs and HCFCs could be converted into resources. From our results, a possible system for producing 1,1-difluoromethyl ether, CH 3 OCHF 2 , is via HCFC-22, because the reaction occurs in the absence of UV irradiation at room temperature and atmospheric pressure, and is widely used in air conditioners. As vapor pressure of the ether is similar to that of chlorodifluoroethane (HCFC-142b), the ether has a good possibility of use as an alternative refrigerant for HCFC-142b. The third objective of our work was to know the effect of process factors such as agitation in addition to reaction mechanism for designing a photochemical reactor. The process is composed of two steps: a mass transfer process of fluorocarbons dissolving in solutions from vapor bubbles and the reaction rate in a solution. To estimate the mass transfer, we measured the solubility of fluorocarbons in alcohol–NaOH solutions. Using these values, a stationary state model, in which mass transfer rates and reaction rates are balanced, was set up. Using the stationary state model with the kinetic data, we obtained a model for the reaction rate and were able to propose a reaction mechanism.