Simon Bechtel successfully defended his PhD thesis

Mr. Bechtel was awarded with summa cum laude for his research on "Development of a novel, energy efficient process for the gas-phase electrolysis of hydrogen chloride to chlorine".

February 09, 2021

The year starts with good news. Another IMPRS student received his PhD last month. Simon Bechtel worked in the Process Systems Engineering group with Professor Sundmacher. Bechtel did research on "Development of a novel, energy efficient process for the gas-phase electrolysis of hydrogen chloride to chlorine".

On January 18, he defended his work. He passed and was awarded for his PhD thesis with distinction summa cum laude.

We congratulate him on his successful graduation.

The abstract of his work can be found below.

The overarching goal of this work is to further develop the gas-phase electrolysis of HCl to Cl2 employing an oxygen depolarized cathode on its path from a pure reactor concept towards an industrially feasible process. This is motivated by the fact that the production of Cl2 is responsible for the highest GHG emissions and energy consumption of all base chemicals. Recent studies indicated an energetic advantage of the novel gas-phase electrolyzer over the currently industrially employed liquid-phase reactor, which employs hydrochloric acid as a feed stock. Hence, the gas-phase electrolysis of HCl to chlorine, if proven to be feasible, has the potential to significantly contribute to a more energy efficient chemical industry. To reach this goal, the work focuses on the following two aspects.

The first goal is to determine, whether this efficiency advantage of the gas-phase reactor can be maintained on the overall process level, taking account all necessary separation and product purification steps. Secondly, three main phenomena observed in earlier experimental investigations of the gas-phase reactor with a significant impact on its performance are not yet understood. These three phenomena are a limiting behavior observed in previous half-cell investigations of the HCl oxidation as well as in investigations of the full-cell combining the HCl oxidation with an oxygen depolarized cathode, and, lastly, severe performance losses in the oxygen reduction reaction itself. Hence, the second goal is the detailed understanding of the origin of these phenomena and the proposal of first systematic strategies for further increasing the reactor efficiency and its suitability for an industrial application of the process. 

The first part of this work hence focuses on the development of novel separation strategies enabled by the different physical state of the now gaseous reactant HCl by means of flow sheet simulations and a rigorous exergy analysis. Three novel process variants, each based on the gas-phase reactor and a different separation strategy are proposed and benchmarked versus the industrial state-of the-art process, showing that the exergy advantage of the gas-phase reactor can not only be maintained, but even be extended on the overall process level. Furthermore, it is shown that already intermediate single pass and overall conversions of HCl are exergetically feasible, reducing the complexity of the reactor design and operation. Since all three process variants exhibit a comparable exergy efficiency, various individual advantages and disadvantages are discussed, facilitating a site-specific selection process of one of the three variants for an industrial application. Lastly, it is shown that the electrochemical reactor makes up for 88-95% of the overall exergy demand in the three process variants, motivating the dedication of the second part of this work to a detailed investigation of the electrochemical reactor.

Hence, a mathematical reactor model of the HCl gas-phase electrolyzer with a focus on the HCl oxidation kinetics, mass transfer, HCl and Cl2 crossover and the water household is developed and complemented by the design of an experimental setup. Employing a combined experimental and theoretical approach, the HCl oxidation reaction and the oxygen reduction reaction are first investigated separately and finally, in combination. 

Based on this approach, the first phenomenon, the limiting behavior in the HCl oxidation, is shown to be a pure reaction limitation. Consequently, reaction kinetics of the HCl oxidation reaction are proposed, which are so far the only kinetics published in the scientific literature that are able to emulate this limiting behavior. Secondly, the HCl and Cl2 crossover investigations suggest that under relevant process conditions especially HCl crosses over in significant amounts. This leads to poisoning of the platinum cathode catalyst employed in previous studies and hence impedes the oxygen reduction reaction, explaining the before mentioned performance losses. Thirdly, the subsequent analysis of the full-cell combining the HCl oxidation and oxygen reduction indicate that the observed imitating behavior in this case is caused by either flooding of the cathode catalyst layer or membrane dehydration. Which one of these mechanisms dominates is shown to strongly depend on the thermal and water management of the cell, due to the significant overpotentials and exothermic dissociation of HCl in water, so that already slight changes in the operating conditions are able to provoke a change from flooding to membrane dehydration. Adjusting operating and geometrical parameters in the reactor model based on these insights leads to a simulated increase in the limiting current density by more than 90%. 

These insights on all three phenomena are furthermore validated by own experiments and motivate the first application of a rhodium sulfide based cathode catalyst in the HCl gas-phase reactor, leading to significantly lower activation overpotentials. Based on the learnings from the mathematical reactor model, industrially feasible current densities of more than 5000 A/m2 are experimentally achieved for the first time, while also maintaining a feasible cell potential of 1.09 V. This is a further significant step towards an industrial realization of the process as an energy efficient alternative to the state-of-the-art liquid-phase electrolysis.

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