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possible only if the oxidation of HCl is coupled with a thermodynamically favorable reduction reaction. The Deacon catalytical oxidation process involves coupling the oxidation of HCl and the reduction of oxygen resulting in the formation of chlorine and water.1 The Deacon reaction can be represented as: 4HCl + O2 Cu catalyst >2Cl2 + 2H2O [2] Due to the sluggish reaction kinetics of the oxygen reduction reaction, the Deacon process is operated at high temperatures (400-450 °C). The process is limited to 60- 80% single pass conversions and also results in the production of a mole of water for every mole of chlorine. There have been three commercial chlorine recycle processes based on reaction 2 but using different catalyst sys- tems. The three commercial processes are Shell-Chlor (Shell com- pany, Netherlands)3, Kel-Chlor (M. W. Kellogg Company, Houston, USA)4, and MT-Chlor (Mitsui Toatsu Company, Tokyo, Japan).5 The Shell-Chlor and Kel-Chlor processes were developed in the 1960s and the MT-Chlor process was developed during this decade. Presently, only Mitsui Toatsu is operating a 50,000 tons/yr plant based on the MT-Chlor process at Omuta, Japan. Thermochemical processing of HCl is energy-intensive; it is a high temperature oper- ation and also the relatively low single pass conversions combined with the production of water vapor necessitates an extensive down- stream separation scheme. Electrochemically, HCl can be directly converted to chlorine and hydrogen. Compared to thermochemical processing, electrolysis is a low temperature operation (70-90 °C) and the amount of water asso- ciated with the electrochemically-produced chlorine is lower. Also, the co-produced hydrogen has commercial value. Until recently, the only available electrolytic recycling process was the Uhde process.6 In this process, anhydrous HCl is first absorbed in water and the resulting aqueous hydrochloric acid solution is electrolyzed in a sep- arated cell to yield chlorine at the anode and hydrogen at the cathode. Nine plants based on the Uhde technology are being oper- ated worldwide. The feed to the Uhde process is 22-wt % hydrochloric acid and typically an Uhde electrolyzer operates at a current density of 4 kA/m2 at a cell voltage of 2.0 V. This translates to an energy requirement of approximately 1500 DC kWh per ton of chlorine produced. Due to the closeness of the reversible potentials for the evolution of oxygen and chlorine, coupled with the mass transport limitations resulting from liquid feed and finite gap elec- trolysis, some oxygen is also produced in an Uhde electrolyzer. This results in a decrease in the current efficiency and also corrosion of cell components. Investment disadvantages also accrue from the up- stream HCl absorption step and the drying stages required in removing the 1-2% water typically associated with the chlorine. Researchers at the DuPont Company in Delaware and at the University of Cali- fornia at Berkeley realized that these problems with aqueous HCl electrolysis could be solved by the direct gas phase electrolysis of anhydrous HCl to chlorine in a polymer electrolyte membrane fuel cell (PEMFC) type electrolyzer.7-10 One obvious benefit of this process is the elimination of the HCl gas absorption step to form aqueous acid. Also, if the feed to the anode is anhydrous HCl gas, the water vapor associated with the chlorine produced is minimal, resulting in a simplified down- stream separation scheme. Also, gas phase diffusion coefficients are approximately three orders of magnitude higher than the corresponding values for aqueous solu- tions (10-2 vs 10-5 cm2/s). Owing to this and also due to the low concentration of water associated with the reactant, the electrolyzer can be operated at high current densities and efficiencies. High current operations reduce the size of the electrolyzer required for attaining a fixed production target of chlorine. Overall, this results in lower capital and operating costs relative to Uhde aqueous electrolysis. A U.S. patent for the electrolysis of essentially dry anhydrous hydrogen halide gases to their respective halogen gases in a PEMFC type electrolyzer was issued to DuPont in 1995.7 Though the patent issued was of a general nature dealing with all hydrogen halides (e.g., HCl, HBr, and HF), the specific examples presented dealt with the electrolysis of HCl to chlorine. Due to the existence of a vast market for the electrolysis of anhydrous HCl, DuPont engaged in an aggressive research, develop- FIG. 2: A second generation three cell prototype electrolyzer with an active area of 2.0 m2. The Electrochemical Society Interface • Fall 1998 (continued on next page) 33PDF Image | RECYCLING CHLORINE FROM HYDROGEN CHLORIDE
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