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32 FIG. 1: The first generation three cell electrolyzer stack with an active area of 2.0 m2. The electrolyzer was designed by DuPont and DeNora. RECYCLING CHLORINE FROM HYDROGEN CHLORIDE A New and Economical Electrolytic Process by Sathya Motupally, Dennie T. Mah, Francisco J. Freire, and John W. Weidner In the chemical industry, chlorine is one of the most widely used chemicals. The chlorine molecule is often used as a modifier or as an aid to manufacture other chemicals. Oftentimes, some or all of the chlorine used is reduced to hydrogen chloride, a waste byproduct. Processes used to manufacture polyurethanes, flu- orocarbons, hydro-fluorocarbons and white pigments constitute some examples where all of the chlorine used is reduced to hydrogen chloride.1 Polyurethane resins and rubbers are produced by the polymerization reaction of isocyanates with polyethers or polyesters.2 Toluene diisocyanate and methylene-diphenyl diiso- cyanate are produced by reacting the corresponding diamines with phosgene. The corresponding reactions can be represented as: R(NH2)2 + 2 COCl2 ➞ R(NCO)2 + 4 HCl where R is either toluene [C6H5CH3] or methylene-diphenyl [CH2(C6H5)2]. It can be seen from the above reaction that the diiso- cyanates produced do not contain any chlorine. Chlorine in the phosgene molecule functions only as an oxidizing agent and is reduced to HCl. For every pound of toluene and methylene- diphenyl diisocyanate, 0.83 and 0.58 pounds of HCl are also pro- duced, respectively. The current world production of chlorine is approximately 42 million metric tonnes per year. Approximately 50% of the chlorine produced is reduced to HCl or chloride salts. The formation of HCl has severe economic and environmental implications on the overall process. There are three options to deal with the HCl produced: (1) recover the material value of chlorine in HCl, (2) sell the HCl in the form of concentrated hydrochloric acid, or (3) discharge the acid or the chloride ions (scrubbed acid) into waste water streams. The third option is environmentally unsound and is, therefore, never followed or encouraged. Option 2 is feasible as long as there is a market for hydrochloric acid. The demand for the aqueous acid has to be located close to the manufacturing facility as shipping the acid over long distances may not be eco- nomically attractive. The economics for selling the acid are also affected by the excessive costs associated with the necessity of de- mineralized water for the hydrochloric acid and also a scheme to separate trace organics from the HCl gas. For large-scale manufac- turers who produce vast quantities of HCl, the supply of hydrochloric acid exceeds demand and therefore, all of the acid pro- duced cannot be sold. For large-scale manufacturers, option 1 is oftentimes the best. There are two possibilities for recovering the material value of the chlorine from HCl. The waste HCl produced can be used as the chlo- rine source for making more of the desired product or the HCl can be reoxidized to chlorine. Most chlorination reactions are thermodynamically unfa- vorable when HCl is substituted as the chlorine source. Recycle of HCl to chlorine can be carried out thermochemically or electrochemically. The direct oxidation of HCl to hydrogen and chlorine is thermodynamically unfavor- able (∆G>0) at all practical temperatures. Therefore, thermochemical processing is The Electrochemical Society Interface • Fall 1998 [1]PDF Image | RECYCLING CHLORINE FROM HYDROGEN CHLORIDE
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