(729c) Combination of Electrolysis and Catalytic Oxidation to Simultaneously Produce Hydrogen and Chlorine Using Waste Hydrogen Chloride Gas

Authors: 
Wang, X., UC Davis
Palazoglu, A., University of California, Davis
El-Farra, N. H., University of California, Davis

Hydrogen chloride is widely produced as a byproduct of processes where chlorine is used as a primary raw material. The recovery of hydrogen chloride presents a significant opportunity in terms of economic yields and environmental impact.

A number of processes are available for producing Cl2 from HCl. Two traditional approaches are [1]:

(1) Electrolysis of HCl with production of Cl2 and H2.

(2) Oxidation of HCl by air or O2 in the presence of a catalyst, e.g., Deacon-type processes.

Direct electrolysis requires extensive electricity and the energy consumed to produce hydrogen exceeds the energy released by the same hydrogen when used as a fuel. The strength of this path is that both hydrogen and chlorine can be produced at the same time. On the other hand, Deacon-type processes have a simple flowsheet and low power requirements, while only chlorine is achieved as the product. A combination of these two approaches is feasible to achieve multiple products as well as achieving some energy savings. Furthermore, using renewable energy sources to power electrolysis reactions and to realize thermochemical splitting of HCl is a promising dierction, especially when solar energy, as a renewable resource, applied to supply heat and electricity at the same time from one integrated system, called photovoltaic/thermal hybrid solar systems (or PVT systems) [2].

In the present work, we propose a combination of electrolysis and catalytic oxidation to simultaneously recover hydrogen and chlorine through waste HCl gas. Thermochemical decomposition of hydrogen chloride generally involves three distinct reaction steps: hydrogen production, oxygen production and materials regeneration. We examine various design schemes and present a thermodynamic analysis. The proposed process is tested using advanced process simulations tools (Aspen Plus®). A preliminary set of reaction steps are given below:

Block

Description

Process

S1H2

Step1 H2 Production

2 Cu (s) + 2 HCl (g) → 2 CuCl (l) + H2 (g)  450°C

S2ELE

Step 2 Electrolysis

2 CuCl (s) +  H2O (l) → CuCl2 (aq) + Cu (s) + H2O (l)  25°C

S3CL2

Step 3 Cl2 Production

CuCl2 + 1/2O2 ↔ CuO + Cl2   25°C

S4FB

Step 4 Fluidized Bed Reactor

2CuCl2(aq) + H2O(g) → 2CuCl(l) + 2HCl(g) + 1/2O2(g)  450°C

S5

Step 5 CuCl2 recovery

CuO + 2HCl ↔ CuCl2 + H2O  100°C

Further developments of the reaction cycles are being conducted to improve efficiency and facilitate eventual commercialization.

[1] Pan H Y, Minet R G, Benson S W, et al. Process for converting hydrogen chloride to chlorine. Industrial & engineering chemistry research, 1994, 33(12): 2996-3003.

[2] Chow T T. A review on photovoltaic/thermal hybrid solar technology. Applied Energy, 2010, 87(2): 365-379.