Model-Based Analysis of the Limiting Mechanisms in the Gas-Phase Oxidation of HCl Employing an Oxygen Depolarized Cathode

Abstract

The electrochemical oxidation of HCl to Cl2 plays an important role in the production of polycarbonates and polyurethanes. Recently, the gas-phase oxidation of HCl proved to be significantly more efficient than the current state-of-the-art process based on the oxidation of hydrochloric acid. In experimental investigations of this gas-phase reactor, a limiting current can be observed that is so far not understood but impedes the overall reactor performance. In the present work, a nonisothermal multiphase agglomerate model is developed to investigate the underlying reasons for this limiting behavior in more detail. It is shown that the thermal management of the cell plays a significant role and that minor changes to its thermal resistance lead to the limiting behavior being caused by either flooding of the cathode or dehydration of the membrane and anode. An optimization of operational and structural parameters of the cell based on these insights leads to an increase in the limiting current by more than 90%. Interestingly, under these conditions a third phenomenon, the rate determining Tafel step in the microkinetic reaction mechanism of the HCl oxidation, limits the overall reactor performance. These insights harbor the potential for enormous energetic savings in this industrially highly relevant process.

On the evolution and application of the concept of electrochemical polarization

“…Similarly to the term of polarization, depolarization has also undergone some meaning change, though it is relatively minor. The recent terminology often speaks about oxygen depolarized cathodes in aqueous electrolysis cells [41][42][43]. This terminology simply means that the cathode reaction is oxygen reduction instead of hydrogen evolution, which results in a diminished cell voltage due to the difference in the potential regime of O 2 and H + reduction.…”