Optimization of Reverse Water-Gas Shift Chemical Looping for Continuous Production of Syngas from CO2

Syngas is widely used in industry as a building block for the production of many bulk chemicals and chemical intermediates. The production method of syngas strongly influences its composition, most importantly the H₂/CO-ratio. Consequently, the syngas composition determines for which application it is suitable. Some of the most important uses of low H₂/CO syngas include the Monsanto process for acetic acid production and hydroformylation processes. High H₂/CO syngas may be used for alcohol synthesis and methanation. Today, syngas is produced almost exclusively from fossil fuels by steam reforming of natural gas or gasification of coal, depending on the desired H₂/CO-ratio. In light of the current and future global problems associated with the rising CO₂ levels in the atmosphere, traditional processes based on fossil fuels must be substituted to minimize the anthropogenic CO₂ production.

One possible way to shift towards a more sustainable future is to use CO₂ as the main carbon source instead of fossil fuels. In the context of sustainable syngas production, the reverse water-gas shift (RWGS) reaction is of high importance, as it converts CO₂ to CO with the help of process heat and hydrogen. The traditional RWGS reaction can be improved by introducing a metal oxide as an oxygen carrier, splitting the original reaction in a reduction and oxidation reaction on the metal oxide. This cyclic two-step process is referred to as reverse water-gas shift chemical looping (RWGS-CL). It has been shown that by using this approach, efficiency improvements can be obtained because the two-step operation yields partially separated gas streams and thus simplifies downstream gas separation [1]. This is especially true for syngas production with low H₂/CO-ratio. Furthermore, unwanted side reactions can be eliminated by separating the reactants spatially or temporally and favorable thermodynamics can be obtained by choosing suitable oxygen carrier materials. Due to the cyclic switching between two process steps, RWGS-CL is more complex than the traditional RWGS process. A steady state operation in the traditional sense is not possible. However, a cyclic steady state (CSS) is achievable after prolonged cycling. Therefore, simulation and optimization techniques are used to understand the dynamic behavior and to improve the process.

Different reactor designs and configurations for the RWGS-CL process are examined and evaluated. For a fixed bed configuration, a 1D model for the two-step RWGS-CL process is derived based on [2] with kinetic information from [3]. Process simulation is used to compute the cyclic steady state of the process for different flow regimes. On top of the simulation, a multi-objective optimization problem is formulated and solved to maximize the gas conversion and the utilization of the oxygen carrier material. Since both objectives are conflicting they lead to Pareto optimal solutions. The trade-off between gas conversion and the utilization of oxygen carrier material is investigated. The influence of the decision variables and the flow regime on the optimal solution is analyzed and discussed. Constraints on the decision variables are formulated such that continuous production of CO can be achieved with the RWGS-CL process. The simulation and optimization results are compared to those of the traditional RWGS process.

References:

[1] Wenzel, M., Rihko-Struckmann, L. and Sundmacher, K. (2016), Thermodynamic Analysis and Optimization of RWGS Processes for Solar Syngas Production from CO₂. Submitted to AIChE Journal.

[2] Heidebrecht, P., Hertel, C. and Sundmacher, K. (2008), Conceptual Analysis of a Cyclic Water Gas Shift Reactor. International Journal of Chemical Reactor Engineering, 6

[3] Wenzel, M., Dharanipragada, N. V. R. A., Galvita, V. V., Poelman, H., Marin, G. B., Rihko-Struckmann, L. and Sundmacher, K. (2016), CO Synthesis From CO₂ via Reverse Water-Gas Shift Reaction Performed in a Chemical Looping Mode: Kinetics on Modified Iron Oxide. Submitted to Journal of CO₂ Utilization.