(500f) Process Modeling and Design for Methanol Synthesis Using Membrane Reactor and Vacuum Pressure Swing Adsorption

Various Carbon dioxide Capture and Utilization (CCU) approaches have been proposed to reduce CO₂ emissions. Conversion of CO₂ to methanol is one of CCU approaches. It is a promising CCU approach because methanol is a raw material for many chemicals, including chemical fibers, pharmaceuticals, and plastics. Methanol, which is currently produced from coal and natural gas, can be synthesized from CO₂, which would reduce dependence on fossil fuels and mitigate CO₂ emissions simultaneously. However, methanol production by CCU is more expensive than the current production method. This is due to the high cost of CO₂ separation from the exhaust gas and the severe thermodynamic constraint in the methanol synthesis from CO₂.

To reduce the cost of CO₂ separation, chemical absorption, and physical adsorption methods have been studied. Among them, Vacuum Pressure Swing Adsorption (VPSA), one of the physical adsorption methods, allows flexible operations to reduce costs when high-purity CO₂ is not required. There have been many studies in the past on purifying CO₂ from various emission sources [1,2].

Furthermore, a solution to the thermodynamic constraints in methanol synthesis is to use a membrane reactor. This concept is a multifunctional reactor that integrates a membrane and a reactor into a single unit. By continuously removing a reaction product by the membrane, the reaction equilibrium can be shifted in the direction where the conversion is higher, exceeding the thermodynamic limit.

This study performs multi-objective optimization to analyze the cost reduction of a methanol synthesis process using blast furnace gas from a steelwork with a membrane reactor and VPSA. The CCU system is illustrated in Fig.1. which includes a water gas shift (WGS) reactor, VPSA unit, and membrane reactor. These units are modeled mathematically by taking material and energy balances. In particular, the VPSA model requires time-consuming calculations because it is complex and must be calculated to the cycle steady state (CSS). To overcome this challenge, we employed a surrogate model to approximate the input-output relationship to reduce the computational cost. A surrogate model was created using training data from the detailed VPSA model simulated under various operating parameters. The entire flowsheet including these units is optimized to minimize the energy consumption and maximize productivity, where the economic trade-offs are revealed as Pareto fronts.

References

[1] Z. Liu, C. A. Grande, P. Li, J. Yu, and A. E. Rodrigues, “Multi-bed Vacuum Pressure Swing Adsorption for carbon dioxide capture from flue gas,” Separation and Purification Technology, vol. 81, no. 3, pp. 307–317, Oct. 2011.
[2] N. Shigaki, Y. Mogi, H. Kijima, T. Kakiuchi, T. Yajima, and Y. Kawajiri, “Performance evaluation of gas fraction vacuum pressure swing adsorption for CO2 capture and utilization process,” International Journal of Greenhouse Gas Control, vol. 120, p. 103763, Oct. 2022.

Acknowledgment

This study was funded by New Energy and Industrial Technology Development Organization (Grant Number JPNP 16002).

Fig. 1. Process flow of CCU-methanol system