Methanol is a highly demanded chemical essential for chemical production, energy storage, and fuel applications. As a clean fuel alternative, it has gained traction in the transportation and marine industries due to its high-octane number and low emissions. The demand for methanol has increased exponentially due to advancements in methanol-to-olefin, methanol-to-aromatic, and methanol-to-gasoline processes[1]. Conventionally, methanol is synthesized from syngas (CO2 + CO +H2) using an industrial Cu/ZnO/Al2O3 catalyst. Despite being well-established, there is growing interest in utilizing CO2 for methanol production due to environmental benefits. Methanol production is an exothermic process requiring high pressure and low temperatures. However, due to the chemical inertness of CO2, it requires high energy (>240℃), leading to a drop in overall CO2 conversion and methanol yield. Additionally, CO2 hydrogenation produces large amounts of water, leading to catalyst deactivation and posing equilibrium limitations for high methanol formation[2].
There are two ways to tackle the following issue: developing a water-tolerant, highly selective catalyst or developing an intensified process. This work explores the latter using a sorption-enhanced (SE) process to improve CO2 conversion and methanol yield. According to Le Chatelier's principle, selectively removing water from the reaction environment can shift the equilibrium toward higher methanol production. This is achieved by incorporating a solid adsorbent within the reactor, which captures water in situ, thus mitigating catalyst deactivation and thermodynamic constraints. This work employed a mixture of industrial Cu/ZnO/Al2O3 catalysts and zeolite 3A adsorbent to facilitate the sorption-enhanced reaction. Reactant gases were introduced at elevated pressures, where methanol synthesis and water adsorption occurred simultaneously. Once the adsorbent reached saturation, pressure-swing technology was utilized for regeneration while maintaining catalyst stability. The adsorbed water was desorbed by lowering the pressure to 1 bar and introducing an inert gas, allowing the system to be reset for the next cycle.
Additionally, we developed a mathematical model to precisely simulate the numerous phenomena occurring within the reactor during SE methanol synthesis operations. This model is based on a set of equations that capture the complex interactions involved in the synthesis process. The model is structured as a one-dimensional framework, simplifying the analysis while capturing essential dynamics. It is categorized as pseudo-homogeneous, indicating that the properties of the mixture can be treated as uniform throughout despite the presence of different phases. Additionally, the model operates under non-isothermal and non-isobaric conditions, reflecting the realities of heat and pressure exchanges in the reactor. The simulations were performed to understand the effect of temperature, pressure, catalyst:adsorbent ratio, CO2:CO ratio and gas-hourly space velocity (GHSV).
Our findings from simulating the SE process showed many promising results. The maximum yield from the SE methanol synthesis process was 65.32% at 220, 30 bar and 1:1 wt.% catalyst-adsorbent ratio, which was 330.46% higher than conventional at the similar process. Adsorption of water propels both CO2 hydrogenation reaction and reverse-water gas shift reaction (RWGS). Therefore, we observed extremely high CO2 conversion of 92.2% at 220, 30 bar and 1:1 wt.% catalyst-adsorbent ratio, which was 472.67% higher than the conventional process. In addition, we observed that the SE process enhanced methanol concentration and CO concentration, since the CO2 hydrogenation and the adsorption processes are exothermic, the rise in the system's temperature favours the endothermic RWGS reaction. The adsorption of water, therefore, also pushes the equilibrium of the RWGS reaction, producing higher CO. Furthermore, our analysis of the effect of pressure revealed that the increase in pressure enhanced the performance metric of the system as CO2 hydrogenation has a drop in the total number of moles, and an increase in pressure also leads to high water adsorption. On the other hand, since high pressure leads to high water formation, the SE phase is shortened as the adsorbent will get saturated faster. The RWGS reaction is equimolar; therefore, pressure has no effect on CO formation.
Additionally, an increase in temperature led to higher methanol formation; however, temperature over 235 led to an overall drop in methanol yield due to the faster kinetic of RWGS due to its endothermicity. Higher temperature is favourable for CO2 conversion. Due to the exothermic nature of the adsorption process, higher water adsorption was observed at lower temperatures than at higher temperatures. This also led to a shortened SE phase at higher temperatures compared to lower temperatures. The higher temperature leads to high CO formation, leading to a drop in the overall selectivity of methanol. Varying catalyst-adsorbent ratios revealed that a higher catalyst-adsorbent ratio favoured methanol synthesis over CO formation but led to a shorter SE phase. The high amount of adsorbent can lead to high heat release, favouring RWGS reaction and leading to a drop in methanol concentration. Adding CO to the feed changes the overall dynamic of the reaction; since methanol is mainly formed from CO2 and not CO, the presence of CO will lead to a drop in methanol concentration. On the other hand, as CO content increases, the water-gas shift reaction becomes favourable, leading to a drop in water concentration and negating the use of adsorbents in the system.
Lastly, shifting the equilibrium using the SE process substantially improves CO2 hydrogenation to methanol. By employing an adsorbent in the system, we achieved extremely high methanol yield and CO2 conversion at a relatively low temperature and pressure compared to the conventional process. The process, however, has a limitation with high CO formation, and the process condition needs to be optimized to minimize CO formation.
References
[1] G. Bozzano, F. Manenti, Efficient methanol synthesis: Perspectives, technologies and optimization strategies, Progress in Energy and Combustion Science 56 (2016) 71–105. https://doi.org/10.1016/j.pecs.2016.06.001.
[2] A. Zachopoulos, E. Heracleous, Overcoming the equilibrium barriers of CO2 hydrogenation to methanol via water sorption: A thermodynamic analysis, Journal of CO2 Utilization 21 (2017) 360–367. https://doi.org/10.1016/j.jcou.2017.06.007.