CatSys: Development of a process for the gas phase electrolysis of hydrogen chloride to chlorine

Chlorine is a base chemical with a production capacity of 66 million tons worldwide in 2014, which is expected increase to up to 76 million tons per year until 2019 [1,2]. Interestingly, one third of all substances produced with the aid of chlorine do not contain Cl2themselves and 50 % of the employed chlorine is being discharged in form of side products like hydrogen chloride or chloride salts [1,2]. Great example processes with a further growing industrial relevance are the isocyanate and polycarbonate production [1,2]. Due to the steadily increasing production capacities of polycarbonates and polyurethanes, the volume of HCl, emerging as a by-product of these processes, is growing rapidly. Owing to the oversaturation of the market for hydrochloric acid, answering the question on how to utilize this by-product in a sustainable and energy-efficient way is becoming more urgent [1,2]. 

Fig. 1: Scheme for the utilization of renewable energy sources for the recycle of hydrogen chloride to chlorine which then again enters into main industrial processes like the polyurethane and polycarbonate production [1]. 

The oxidation of HCl to chlorine, which can be recycled to the above mentioned processes, offers a feasible solution to this problem. It can be carried out either through heterogeneous catalysis at high temperatures or through electrolysis. Especially in the transition scenario from fossil to renewable energy sources, the electrolysis of HCl allows for the electricity stemming from these renewable sources to be employed for the sustainable production of chlorine as shown in Fig. 1. Up to now, the most energy-efficient industrially employed electrochemical variant is the Bayer UHDENORA process, employing aqueous hydrochloric acid as feedstock [1,2].

Fig. 2: Contributions of the different unit operations to the exergy demand of the state of the art process (shaded) and the novel IL process (filled) [4].

As an alternative, our group recently showed in cooperation with TU Clausthal that the electrolysis of gaseous hydrogen chloride is much more efficient and leads to exergy savings of 36% on the reactor level [3]. In order to investigate whether this energetic advantage can be maintained on the process level, different separation strategies for the purification of chlorine were analyzed via systematic flowsheet simulations. Subsequently, a detailed exergy assessment was carried out. The analysis revealed significant exergetic savings of up to 38% in total, compared to the Bayer UHDENORA process [1,2,4]. Fig. 2 shows the exergy demand of the different unit operations in one of the proposed novel process configurations where HCl is separated from Cl2by absorption in a specific ionic liquid (see Fig. 3), identified by a systematic, quantum chemical screening procedure [4], compared to the exergy demand of the state-of-the-art Bayer UHDENORA process. Based on these findings our group has recently filed a patent [5]. As Fig. 2 shows, more than 90 % of the exergy demand of the overall process can be traced back to the electrochemical reactor. Hence, the investigation and simulation of different separation methods is very important in order to evaluate the feasibility of the overall process, since an industrial application of the process is only possible under the condition of a viable separation of reactants and products. However, for reducing the overall exergy demand of the process, the optimization of the reactor is of much greater significance. For this reason, we are currently working on a detailed numerical reactor model, focusing on the interaction between reaction kinetics and mass transfer of the involved substances [6], in order to gain a better understanding of the underlying processes on the reactor and phase level to subsequently utilize this knowledge for further increasing the exergetic efficiency of the process.

Fig. 3: 3D Structure of the ionic liquid (diethyl-methylsulfonium methanesulfonate) used for the absorption of HCl during the separation of the HCl and Cl2containing reactor outlet stream.



Bechtel, S., Vidakovic-Koch, T., Sundmacher, K. (2018). Novel process for the exergetically   efficient recycling of chlorine by gas phase electrolysis of hydrogen chloride, Chem. Eng. J., 346535-548.


Bechtel, S., Vidakovic-Koch, T., Sundmacher, K. (2018). Energy efficient gas phase electrolysis of hydrogen chloride, Chem. Ing. Tech., under review.


Kuwertz, R., Martinez, I. G.,Vidaković-Koch, T.,  Sundmacher, K., Turek,  T., Kunz, U. (2013). Energy-efficient chlorine production by gas-phase HCl electrolysis with oxygen depolarized cathode, Electrochem. Commun., 34, 320-322.


Bechtel, S., Song, Z., Zhou, T., Vidaković-Koch, T., & Sundmacher, K. (2018). Integrated process and ionic liquid design by combining flowsheet simulation with quantum-chemical solvent screening, Comput. Aided Chem. Eng., 44, pp.2167-2172.


Bechtel, S., Vidakovic-Koch, T., Sundmacher, K. (2017). A method and an apparatus for  separating chlorine gas from a gaseous anode outlet stream of an electrochemical reactor. PatentEP 17203967.9.


Bechtel, S., Bayer, B., Wiser, A.,Vidakovic-Koch, T., Vogel, H., Sundmacher, K. (2018). Precise determination of Lennard-Jones parameters and Eucken correction factors for a more accurate modeling of transport properties in gases, J. Chem. Eng. Data., under review.

Go to Editor View