Analysis of a novel chlorine recycling process based on anhydrous HCl oxidation
Graphical abstract
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 + H24HCl + 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.
Section snippets
Electrochemical setup
All electrochemical measurements have been performed in the experimental set-up shown in Fig. 1a. Gas inlets were regulated by mass flow controllers (Bronkhorst). Hydrogen chloride or oxygen were mixed with nitrogen in T-connectors to achieve the desired concentrations, after which they were directed to the cyclone flow cell, as described in [7]. The cell was kept in a convection oven (UNP500) with integrated temperature control (Mammert) in order to assure the desired reaction temperature. The
Catalyst ink characterization
In order to understand the effect of the catalyst/Nafion ratio on catalyst utilization, some insight into the formation of the Nafion network in the CL is needed. The organization of Nafion and catalyst particles in the CL is based on self-assembly and it is currently not well understood [5]. Still, it can be assumed that this process starts already in the catalyst ink. Consequently, investigation of the agglomerate formation in the catalyst ink might be helpful to better understand this
Conclusions
DLS analysis of catalyst inks indicates an improvement of the catalyst dispersion with an increase of the Nafion loading. This technique shows a decrease of the agglomerate size with an increase of Nafion loading, implying a substantial enhancement of the active surface area. The increase of the EASA was also observed in cyclic voltammetry experiments under nitrogen atmosphere. These experiments revealed additionally a strong extension of the Nafion/carbon interface with an increase of the
Acknowledgment
The authors are grateful to the German Research Foundation (Deutsche Forschungsgemeinschaft, DFG) for financial support of this research work under the project grants SU 189/4-1 and KU 853/5-1.
References (34)
- et al.
Electrochemistry Communications
(2013) - et al.
J. Power Sources
(2004) - et al.
Journal Power Sources
(2004) - et al.
Journal Power Sources
(2007) - et al.
Journal Power Sources
(2008) - et al.
Electrochimica Acta
(1998) - et al.
Journal of Power Sources
(2004) - et al.
Int. J. Hydrog. Energy
(2008) - et al.
Electrochimica Acta
(2010) - et al.
International Journal of Hydrogen Energy
(2010)
Journal of Power Sources
Journal of Membrane Science
Journal of Electroanalytical Chemistry
Electrowinning of Other Inorganic Compounds
Journal of The Electrochemical Society
Membranes
Cited by (10)
Synthesis of sustainable production chains for phosgene-related byproducts
2022, Journal of Cleaner ProductionCitation Excerpt :Two main techniques to produce chlorine from hydrogen chloride are electrolysis and oxidation (Ding et al., 2013). Electrolysis is divided into dry and wet methods, including diaphragm electrolysis and oxygen cathodic electrolysis (Motupally et al., 1998), Oxidation methods are divided into direct oxidation and catalytic oxidation (Martinez et al., 2014). Both diaphragm (Zhao et al., 2015) and oxygen cathode (Mohammadi et al., 2009) electrolyses must absorb of HCl into hydrochloric acid before electrolysis and have cumbersome processes, high energy consumption, and large investments (Paidar et al., 2016; Singh et al., 2017).
Fully anhydrous HCl electrolysis using polybenzimidazole membranes
2022, International Journal of Hydrogen EnergyCitation Excerpt :Ultimately, however, this process suffered from corrosion issues that prevented its adoption. More recently, Bechtel et al. [9–11] demonstrated an electrolyzer that utilized anhydrous HCl as the anode feed and oxygen as the cathode feed. With this ODE, they demonstrated cell operation at approximately 1 V, although the estimated per pass conversion efficiency was ∼3%.
Electrochemical gas phase oxidation of hydrogen chloride to chlorine: Model-based analysis of transport and reaction mechanisms
2019, Electrochimica ActaCitation Excerpt :As discussed in the introduction of this article, the major contributions are expected to come from the mass transfer within the GDL and within the agglomerates. The concentration gradient within the porous catalyst layer can be assumed to be markedly smaller [36], since the CL in the experiments of Martínez et al. [16,19,20] is about 40 times thinner than their GDL. The experimentally observed limiting behavior at a temperature of 60 °C is starting at current densities between 700 and 800 mA/cm2.
Recent advances in energy-efficient chlorine production via HCl electrolysis
2024, Journal of Materials Chemistry AModel-Based Analysis of the Limiting Mechanisms in the Gas-Phase Oxidation of HCl Employing an Oxygen Depolarized Cathode
2020, Journal of the Electrochemical SocietyEnergy-Efficient Gas-Phase Electrolysis of Hydrogen Chloride
2019, Chemie-Ingenieur-Technik