(572d) Simulative Investigation of Heat Management in Methanol Synthesis from Carbon Dioxide with Intermediate Condensation

In the transition from fossil fuels to renewables, energy production will become more decentralized due to the widespread availability of wind and solar power. At the same time, energy storage will become increasingly important. Green hydrogen is a promising option for medium- and long-term energy storage. However, in a decentralized energy system, hydrogen may face significant challenges due to the need for compression and cooling during storage. Further hydrogen valorization in the synthesis of simple molecules, such as methanol, can be advantageous. Methanol not only serves as an energy storage solution but also enables the production of various chemicals. Thus, methanol can become a key component in a fossil-free economy.^[1]
Traditionally, large-scale methanol synthesis is performed from syngas with a high CO and only a low CO₂ content. Higher levels of CO₂ in the syngas reduce the single-pass CO_x conversion in the reactor and lead to an increased formation of the catalyst-damaging by-product water.^[2] However, concentrated and unavoidable CO₂ point sources such as cement production or fermentation, combined with decentralized energy production, have potential for small and medium scale methanol production in a future carbon neutral economy.^[1] To achieve higher conversions and remove water from the system, a multi-reactor setup with product condensation in between could be used.^[3] Allowing the product to condense and then reaching the required reaction temperatures of 200-300 °C again requires intelligent heat management.

Methods

Process simulations and 2D-CFD simulations were combined to model methanol synthesis followed by product condensation. Conversions close to the thermodynamic equilibrium were simulated for CO₂-rich feeds employing different kinetic models. Physical properties were modeled using the Soave-Redlich-Kwong equation of state method. The resulting outlet compositions were then used as the inlet for a coupled modeling of fluid flow, mass transport, and heat transfer in a condensing section. Convective and diffusive transport were simulated using Fuller-Scheddler-Giddings and Wilke mixing rules for the description of the gas properties.^[4]

Results and Discussion

A conceptual design for a condensation system manufacturable by metal 3D-printing was developed. The gas mixture enters the heat exchange unit under process conditions at the top and is split up into multiple channels that are passed in parallel. Rich and lean gas streams are guided through the unit, resulting in a countercurrent heat exchange. This pathway enables the lean gas to be heated back up to the process temperature, allowing for conversion of the mixture in a second reactor section. A stream of water at 288.15 K is placed at the bottom to provide continuous cooling of the gas mixture. Once the dew point of the mixture is reached, liquid formation by condensation begins. Liquid and gas are modeled pseudo-homogeneously as two separate coexisting species connected by mass transfer at the saturation temperature. The liquid is collected at the bottom in a porous layer and is continuously removed from the system. The liquid holdup serves as barrier for the gas stream, forcing it to exit the system at the top. The simulations indicate that the performance of the heat exchanger is strongly affected by the length of the contact area between the hot and cold gas streams. Parameter variation showed that with proper design, up to 80 % of the methanol entering the unit can be removed while ensuring that the lean gas is reheated to process conditions. The simulation results will be used to manufacture an experimental set-up enabling more information for further optimization of the system.

Acknowledgments

The research was supported by the Federal Ministry for Economic Affairs and Climate Action from Germany (BMWK) through the 3D-Process project. We would like to thank our partners at Siemens and KIT-IKFT for providing valuable input on how to design and implement the ideas.

[1] G. A. Olah, A. Goeppert, G. K. S. Prakash, Beyond Oil and Gas: The Methanol Economy, Wiley-VCH, Weinheim, 2018.
[2] M. Bowker, ChemCatChem 2019, 11, 4238–4246.
[3] B. Lacerda De Oliveira Campos, Multiscale Modeling of the Methanol Synthesis: From Surface Reaction Kinetics to Techno-Economic Analysis, Karlsruher Institut für Technologie, 2023.
[4] E. N. Fuller, P. D. Schettler, J. C. Giddings, Industrial & Engineering Chemistry, 1966, 58, 18.