Analysis of a novel chlorine recycling process based on anhydrous HCl oxidation

Highlights

• Gas phase HCl electrolysis combined with oxygen reduction reaction (oxygen depolarized cathode) for chlorine production.

• Determination of potential losses in half-cell measurements under technically relevant conditions.

• Optimization of gas diffusion electrodes with respect to Nafion and catalyst loadings.

• The optimal Nafion content in the catalyst layer for HCl oxidation is much higher than for other gas phase reactions, such as the oxygen reduction in the cathode.

• Nafion improves catalyst dispersion, but catalyst utilization depends on the diffusion and absorption properties of the educts in the ionomer phase.

Abstract

A novel process for the recycling of chlorine based on the gaseous HCl oxidation on gas diffusion electrodes has been investigated. The cathodic and anodic reactions have been analyzed separately as half-cell reactions in a cyclone flow cell and optimized regarding catalyst as well as ionomer loadings. For the anodic hydrogen chloride oxidation, an optimal value of ca. 60 wt% Nafion has been found, whereas for the cathodic oxygen reduction a value below 37 wt% has been obtained. This discrepancy was mainly attributed to the different absorption and transport properties of HCl in Nafion compared to oxygen. The main potential losses in the reactor were identified at the cathode.

Introduction

Chlorine is an important bulk chemical, which in several processes in the plastics industry (e.g. production of PVC, PVDF, Isocyanates, etc.) is not converted completely into desired products, generating a significant amount of byproducts, where hydrogen chloride takes a lead. Its commercialization as hydrochloric acid is not profitable due to the high water volumes required and low market prices. HCl can be recycled to chlorine through chemical or electrochemical oxidation[1], [2]. Because of its milder reaction conditions, lower capital investment costs and the possibility to operate modularly, the latter one has been widely implemented.

There are two main routes to obtain chlorine back from HCl. In the first one HCl is split into chlorine and hydrogen (eq. 1), while in the second one an oxygen-consuming cathode is employed generating chlorine and water (eq. 2):

HCl →Cl₂ + H₂

4HCl + O₂ →2Cl₂ + 2H₂O

If one accounts further for the different aggregate states of HCl (aqueous or gaseous) four different process variants can be obtained (Table 1, data based on[1], [2], [3], [4]). The first process in Table 1, called Bayer-Hoechst-Uhde, is currently considered as state-of-the-art. It follows the stoichiometry of eq.1 employing aqueous HCl. The second process in Table 1, is based on the same stoichiometry and is a new development of DuPont and Denora. It employs gaseous HCl, which results in improved thermodynamics of the process, less mass transfer limitations, higher current efficiency for Cl₂ production (99%) and higher degree of process integration (there is no necessity of prior HCl absorption step) compared to the Bayer-Hoechst-Uhde process. Despite all these advantages the DuPont-Denora process has not been realized on a technical scale so far.

The last two processes in Table 1 are based on the so-called Deacon stoichiometry (eq. 2). Process 3 (Bayer-Uhdenora process) employs aqueous HCl and it is already realized on a technical scale [2], [5]. Due to the use of an oxygen consuming cathode, the reversible cell potential is significantly lower compared to the first two processes. In this process a gas diffusion electrode is utilized for oxygen reduction, while a dimensionally stable anode (DSA) is used for chlorine evolution. Anode and cathode compartments are separated by a polymer electrolyte membrane (PEM), for which Nafion is still the best choice.

The last process variant in the Table 1 combines the advantages of reactions in the gas phase and the utilization of an oxygen consuming cathode. Unlike all other processes, the last process is thermodynamically spontaneous, thus it operates theoretically in the fuel cell regime. Although this process variant has been mentioned in a patent by Dupont [6] and an article by Motupally et al. [3], the proof-of-concept for this process has been presented only recently in our publication [4]. As can be seen in the Table 1, the figures of merit of process 4 are very promising. At the same current density (4 kA m⁻²) the energy consumption of process 4 is only half of the energy consumption of the state-of-the-art process. This shows clearly the potential of the gaseous HCl oxidation for the development of a more energy efficient process for chlorine recycling [5].

For purposeful process design and optimization, it is essential to understand and allocate the potential loss distribution in the reactor. In conventional reactor setups only the total cell potential can be measured and no further information of the potential losses associated with the different parts of the reactor can be obtained. In general, the potential losses originate from kinetic and mass transfer resistances of electrode reactions and Ohmic losses induced by ionic and electronic conductors. In addition substantial losses appear even under open circuit conditions, such that the open circuit cell potential (OCV) deviates significantly from the calculated reversible cell potential.

In the present contribution we focus on the determination of potential losses originating from electrode kinetics. These losses can be assessed in half-cell measurements which in order to be meaningful, have to be performed under technical conditions, using technical electrodes. For this purpose we employed for the half-cell measurements a specially designed cell, the cyclone flow cell which allows characterization of technical electrodes under process relevant conditions as already reported in other publications, see e.g. [7]. In this work we studied the kinetics of the oxygen reduction reaction (ORR) and the hydrogen chloride oxidation (HClOR). While the former reaction due to its significance for fuel cell development has been studied extensively in the past [8], [9], [10], [11], the latter one has been studied only marginally [12]. For these reasons more emphasis has been put on the HClOR. To optimize the electrode for this reaction, Nafion and catalyst loadings have been systematically varied in a broad range from 10 to 70 wt% Nafion with Pt loadings ranging from 0.2 to 2.0 mg cm⁻² and Nafion loadings from 0.5 to 2.0 mg cm⁻². The electrode structures have been characterized by different physical and electrochemical methods.