Chlorine Production by HCl Oxidation in a Molten Chloride Salt Catalyst

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Kinetics, Catalysis, and Reaction Engineering

Chlorine Production by HCl Oxidation in a Molten Chloride Salt Catalyst Shizhao Su, Davide Mannini, Horia Metiu, Michael J Gordon, and Eric W. McFarland Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b01141 • Publication Date (Web): 22 May 2018 Downloaded from http://pubs.acs.org on May 22, 2018

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Industrial & Engineering Chemistry Research

Chlorine Production by HCl Oxidation in a Molten Chloride Salt Catalyst Shizhao Su1, Davide Mannini2, Horia Metiu2, Michael J. Gordon1, Eric W. McFarland1,* 1

Department of Chemical Engineering, University of California, Santa Barbara, CA 93106-5080, USA.

2

Department of Chemistry and Biochemistry, University of California, Santa Barbara, CA 93106-9510, USA

*E-mail: [email protected]

Abstract A molten salt mixture containing 45 mol% KCl and 55 mol% CuCl2 was investigated as a catalyst for the reaction of HCl with O2 to produce Cl2. The HCl conversion for an HCl:O2 molar feed ratio of 1:2, at 450°C and a total pressure of 1 atmosphere, was 80% at a residence time of less than 1 second in a lab scale bubble column reactor. The equilibrium conversion at this temperature and pressure is 84%. The catalyst system was found to remain stable throughout a continuous 24-hour experiment. The use of a mixed transition metal/alkali metal molten salt catalyst for HCl oxidation reduces the volatility of supported chlorides and may avoid the mechanical stability limitations of solid catalysts caused by volume changes between the halide and the oxide.

1

Introduction

Chlorine is a vital commodity chemical for the global economy. The worldwide demand for chlorine in 2015 was 71 million tons and is expected to exceed 100 million tons by 20241. Chlorine (Cl2) is produced predominantly by the electrochemical chlor-alkali process2–5. Most chlorine is used for the production of vinyl chloride monomer (VCM), polyvinyl chloride (PVC),

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isocyanates, and oxygenates (e.g. propylene oxide and propane-1,3-diol). Hydrogen chloride (HCl) is a common byproduct of chlorine use and there is significant interest in cost-effective methods to recover Cl2 from HCl. Although electrochemical processes are used today, direct catalytic oxidation of HCl with oxygen to produce Cl2 and water was first developed in the late 1800’s using solid CuO/CuCl2 catalyst6,7 and is known as the Deacon process. The reaction is exothermic and equilibrium limited in practice. Separation of the final products and reactor heat transfer are challenging and add to the process cost. In attempts to reduce reaction temperatures, several catalysts have been developed and tested in semi-commercial processes, namely, SiO2 supported copper-didymiumpotassium chloride (Shell Chlorine Process)8–11, Cr2O3/SiO2 catalyst (Mitsui Process)12,13, and rutile oxide (RuO2) (Sumitomo Process)14–20. Catalyst volatilization is the main cause of deactivation for chloride-based catalysts21, while volume change associated with the solid oxide to chloride conversion are thought to contribute to deactivation22,23. Figure 1 shows the process flowsheet of the state-of-the-art Sumitomo HCl oxidation process using a fixed bed tube reactor and RuO2/rutile TiO2 as catalyst. The product has to go through multiple separation steps to obtain pure Cl2 and recycle unreacted O2 and HCl24.


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Figure 1: Flowsheet of the Sumitomo HCl oxidation process in a fixed bed tube reactor using RuO2/rutile TiO2 as catalyst24. The product mixture goes through multiple separation steps: (1) HCl absorption, (2) Drying, and (3) O2/Cl2 separation, to obtain pure Cl2 product. If limitations in catalyst lifetime and reactor heat transfer were reduced, catalytic HCl oxidation might be cost competitive with electrochemical chlorine production. We have investigated molten chloride salts as catalysts for HCl oxidation. Whereas the solid catalyst undergoes continuous cyclic conversion between the solid halide and solid oxide, each with different unit cell volumes thought to promote mechanical degradation, the melt eliminates the structural fatigue and provides a continuously renewed gas-liquid interface with effectively unlimited lifetime. Further, molten salts have excellent heat transfer properties, avoiding “hot spots”, and when relatively volatile molten transition metal halide salts are mixed with alkali metal halides, the volatility of the transition metal halide is decreased. Molten halide salts have been used as catalysts in a number of chemical processes25,26, and the unit operations and system management are well understood. Molten salt catalysts have been used in extraction of ores27, metal



production28–32, catalytic coal gasification33, Wacker oxidation of ethylene34, diesel soot catalytic oxidation35 and oxidative dehydrogenation of alkanes 25,26,36. For HCl oxidation, the Deacon reaction mechanism over CuCl2 has been previously described 10,37–39

as  → 2CuCl + Cl2 2CuCl2 ←  

(1)

2CuCl + ½O2 → CuO + CuCl2

(2)

CuO + 2HCl → CuCl2 + H2O

(3)

Combining reactions (1) through (3) gives the overall reaction:  → Cl2 + H2O 2HCl + ½O2 ←  

(4)

The free energy change, ∆G0, as a function of temperature for the equilibrium-limited reaction is shown in Figure 2 for reactions (1) – (4). ∆G0 for reaction (1) is positive and large. However, reaction (2) consumes CuCl and shifts the equilibrium of reaction (1) towards the products. In addition, reaction (3) favorably shifts the equilibrium of reaction (2). It is believed that the reaction rates are such that CuCl2 is in thermodynamic equilibrium with CuCl and Cl2, even when the overall reaction is run at steady state9. It is noteworthy that the detailed reaction mechanism is more complicated than reaction (1) – (3). The oxychloride (Cu2OCl2), and hydroxyoxichlorides (Cu(OH)Cl, Cu2(OH)3Cl) may be important reaction intermediates.40.


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Figure 2: The Gibbs free energy changes, ∆G0, of reactions (1) 2CuCl2 ↔ 2CuCl + Cl2, (2) 2CuCl + ½O2 → CuO + CuCl2, (3) CuO + 2HCl → CuCl2 + H2O, and the overall reaction (4) 2HCl + ½O2 ↔ Cl2 + H2O as a function of reaction temperature. Values calculated using HSC chemistry software (Outotec Research Oy, Finland)41. The original work with supported copper chloride catalysts was limited by the volatility of the chloride, heat management, and degradation of the supported catalyst due to cyclic volumetric stresses.

In this article, we address the following questions: (1) Can a mixture of copper

chloride and potassium chloride provide stable activity in a molten state for HCl oxidation? (2) How does the reaction activity and stability of the molten salt change as a function of the HCl/O2 feed ratio? (3) In a lab-scale bubble column, how does conversion vary with the height of the bubble column? (4) How to characterize the catalytic reaction by measuring the HCl conversion as a function of time and/or residence time at different temperatures and feed concentration?



2

Experimental Methods

2.1 Reactor System. Molten salts were contacted with gas phase reactants in a small bubble column reactor consisting of a quartz tube sealed at the bottom, with a diameter of 0.88cm and length of 20cm. Reagent grade (≥99%) anhydrous potassium chloride (KCl) used in the experiments was from Honeywell Fluka® Chemicals. Analytical reagent grade (≥99.8%) cupric chloride (CuCl2⸱2H2O) used in the experiments was from Mallinckrodt®. The cupric chloride was dried in a box furnace at 120°C42–45 overnight (>12 hours) to produce anhydrous CuCl2. 15g of powdered anhydrous salt (45 mol% KCl – 55 mol% CuCl2) was loaded in the reactor and heated above 400°C in a tubular ceramic fiber heater (Watlow) to form a liquid. The experiments were operated in a fume hood to avoid any potential safety hazards caused by HCl and Cl2 gas. According to the binary phase diagram of KCl–CuCl246 the liquidus temperature of the salt mixture is lower than 365°C . The liquid height in the reactor was 10.5cm. To ensure that the salt was dehydrated, argon was sparged through the melt for 30 minutes. The reactant feed stream, containing HCl and O2, was prepared by bubbling a mixture of Ar and O2 through concentrated hydrochloric acid via a Pyrex® gas sparger. The mixed gas flow rate of Ar and O2 was set to 20 SCCM. The total pressure of the reactant feed stream was one atmosphere at all times. Trace amounts of H2O vapor (

Chlorine Production by HCl Oxidation in a Molten Chloride Salt Catalyst

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