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from the top of the distillation column is recycled back to the electrolyzer. Hydrogen in the dilute acid stream is used as a chemical stock or as fuel for a fuel cell. Due to the transfer of a small amount of HCl from the anode to the cathode, through the Nafion membrane, purge of some amount of the catholyte along with the addition of some fresh water is required. An experimental polarization curve illustrating the applied current densities achievable in the DuPont electrolyzer versus the corresponding energy requirements per ton of chlorine is shown in FIG. 4. As seen in the figure, the electrolyzer can be operated at current densities as high as 15.0 kA/m2. Even at the highest current den- sity shown in the figure, the DC voltage is less than 2.0 V. Due to such high rates at which chlorine can be recycled from HCI gas, the foot-print of the DuPont elec- trolytic process is rather small. Also, by using non-metallic materials for wetted parts, the process can withstand corrosive environments. Therefore, when inte- grated into an existing process (for example, a diisocyanate producing plant), on shut down, it gives the operators ample time to work on the main process before attending to the electrolyzer. Electrode and Membrane Processes The electrolyzer consists of current collectors with flow channels and gas diffu- sion backings on the anode and the cathode. The flow channels and the gas diffu- sion backings facilitate the distribution of the process fluids and also carry the electronic current out of the cell. The anode and the cathode are separated by a Nafion membrane. The primary reac- tions take place in the thin catalyst layers on the membrane or in the gas diffusion backings. The Nafion mem- brane in the electrolyzer acts as the elec- trolyte facilitating the transport of protons from the anode to the cathode and also as a separator to keep the anolyte and the catholyte from mixing. A schematic of a cell with a catalyst- coated membrane (CCM) along with the accompanying electrode processes is shown in FIG. 5. The HCl gas from the flow channels diffuses through the gas diffusion backing and reacts at the anode catalyst of the CCM, producing chlorine gas. The reaction can be represented as: 2HCl(g) ➞ Cl2(g) + 2 H+(aq) + 2e- [3] The protons produced at the anode reaction site are transported to the cathode catalyst of the CCM, where two protons combine to form H2 gas. The reaction at the cathode and the overall reaction can be represented as: 2H+(aq) + 2 e- ➞ H2(g) [4] FIG. 5: Schematic of the DuPont Anhydrous HCl electrolyzer and the corresponding electrode and membrane processes. 2HCl(g) ➞ H2(g) + Cl2(g) [5] A good catalyst system that minimizes the kinetic overpotential losses is used for the anode and the cathode. Due to the corrosive nature of the reactants in the anhy- drous HCl electrolyzer, chemical and mechanical stability of the CCM is an impor- tant factor in the catalyst selection. Nafion polymers contain conductive perfluorosulfonic acid groups with a Teflon backbone and their ion conducting properties are dependent on the amount of water associated with the sulfonic acid groups.11 Therefore, during anhydrous HCl electrolysis, in an attempt to keep the CCM hydrated, water or dilute hydrochloric acid is fed to the cathode. The presence of desiccating HCl on the anode side and water on the cathode side results in a water activity gradient across the membrane. This results in the diffusion of water from the cathode to the anode. At zero current, the flux of water from the cathode to the anode is solely governed by diffusion. However, with the application of a cur- rent, water in contact with the anode side of the membrane is dragged along with the The Electrochemical Society Interface • Fall 1998 (continued on next page) 35PDF Image | RECYCLING CHLORINE FROM HYDROGEN CHLORIDE
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