Novel process for the exergetically efficient recycling of chlorine by gas phase electrolysis of hydrogen chloride

Abstract

Due to the steadily increasing production capacities of polycarbonates and polyurethanes, the volume of hydrogen chloride (HCl), which emerges as byproduct from these processes, is experiencing a significant growth. Owing to the oversaturation of the market for HCl and hydrochloric acid, the question on how to utilize this byproduct in a sustainable and energyefficient way is becoming more and more important. The oxidation of HCl to chlorine, which can be redirected as an educt to these above mentioned processes, offers a feasible solution to this problem and can be carried out either heterogeneously catalyzed at high temperatures or through electrolysis. Up to now, the most energy-efficient industrially employed electrochemical variant is the Bayer UHDENORA process, based on aqueous hydrochloric acid as a feed stock. The major objective of this work is to propose a novel electrochemical process utilizing HCl as a gaseous reactor feed in order to significantly reduce the electrical energy demand of the reactor combined with new and more energy-efficient subsequent separation sequences. For this purpose, flowsheet simulations of the Bayer UHDENORA process and two novel process variants based on the gas phase reactor and two different separation sequences were carried out and followed by a detailed exergy analysis. The analysis shows significant exergetic savings of up to 38 % in this novel process variants compared to the Bayer UHDENORA process, including not only the electrochemical reactor but also the subsequent separation steps. The novel process therefore allows for a more sustainable and energy-efficient production of the base chemical chlorine and, contrary to the heterogeneously catalyzed high temperature process, for the utilization of electrical energy from advancing renewable sources like wind and sun.

Energy‐Efficient Gas‐Phase Electrolysis of Hydrogen Chloride

“…However, the effect of the temperature on the reaction kinetics and also the water management in the cell has to be taken into account. Furthermore, Bechtel et al [7] have shown that operating the reactor at 80°C instead of 40°C while assuming a comparable performance would decrease its exergy demand by 18 % due to the higher temperature level of the discharged heat. In addition to the temperature, also the humidification of the cathode stream proofed to be critical for the water management of the cell, leading to overall optimal process conditions that clearly differ from the optimal settings from a purely thermodynamic point of view, as will be further discussed in Sect.…”

“…The three process variants are namely the ELEC-TRA-DIST, the ELECTRA-ABS and the ELECTRA-IL process as briefly discussed in the following. More detailed process flowsheets and further information on the proposed process variants can be found in [7,36].…”

“…The obtained chlorine can then be refed into these production processes. Reprinted from [7] with permission from Elsevier B.V. lizes aqueous hydrochloric acid as a reactor feed. In the cathode chamber, hydrogen is produced, following the overall stoichiometry depicted in Eq.…”

“…3) based on a gas-phase reactor and according separation strategies, a flowsheet of the Bayer UHDENORA process is depicted in (Fig. 2) [7].…”

“…The latter problem is even amplified due to the high heat capacity of the acid solution, which makes the separation step very energy inefficient. For a detailed exergy analysis and a more exhaustive description of the Bayer UHDENORA process, the reader may refer to [7]. Owing to these inefficiencies of the current state of the art, there have been efforts in recent years to develop a novel and more energy-efficient reactor and overall process for the industrially highly relevant electrolysis of hydrogen chloride.…”