Electrochemical gas phase oxidation of hydrogen chloride to chlorine: Model-based analysis of transport and reaction mechanisms

The electrochemical conversion of hydrogen chloride to chlorine plays a significant role within major industrial processes like the polyurethane or polycarbonate production. Our recent studies demonstrated that the direct electrolysis of gaseous HCl in combination with novel strategies for product purification leads to significant exergetic savings (36–38%) compared to the Bayer UHDENORA state-of-the art process, which employs aqueous hydrochloric acid as a feedstock. Furthermore, we showed that despite of the improvements in the efficiency of the electrochemical reactor, it still has by far the greatest exergy demand of all process units. While the oxygen depolarized cathode (ODC) utilized in both process variants has been investigated in great detail in the scientific literature, the oxidation reaction of gaseous HCl (HClOR) has not yet received much attention and is hence the major subject of this work. In earlier experimental investigations of the HClOR, a limiting current was observed that has the potential to critically reduce the performance of the overall reactor. In the past, membrane dehydration and mass transfer resistances on the anode side were suggested as possible reasons. In order to shed light on this phenomenon, a dynamic, one dimensional agglomerate model of the gas phase HCl oxidation half-cell, considering detailed micro kinetics, as well as the mass transfer of HCl in the different cell parts, is developed. The modeling results suggest that neither mass transfer resistance nor membrane dehydration but a kinetic limitation is the underlying reason for the observed limiting behavior. This has a significant impact on the strategy for further reactor optimizations and harbors the potential of major energy savings in an industrial application of this highly relevant process.