Kinetic mechanism of dimethyl ether production process using syngas from integrated gasification combined cycle power plant

摘要

In a 1-step synthesis gas-to-dimethyl ether process, synthesis gas is converted into dimethyl ether (DME) in a single reactor. Three reactions are involved in this process: methanol synthesis, methanol dehydration and water gas shift, which form an interesting reaction network. The interplay among these three reactions results in excellent syngas conversion or reactor productivity. The higher syngas conversion or reactor productivity in the syngas-to-DME reaction system, compared to that in the syngas-to-methanol reaction system, is referred to as chemical synergy. This synergy exhibits a strong dependence on the composition of the reactor feed. To demonstrate the extent of this dependence, simulations with adjusted activity for each reaction were performed to reveal the relative rate of each reaction. The results show that the water gas shift reaction is the most rapid, being practically controlled by the equilibrium. Both methanol synthesis and methanol dehydration reactions are kinetically controlled. The role of the dehydration reaction is to remove the equilibrium barrier for the methanol synthesis reaction. However, the role of the water gas shift reaction is more complex; it helps the kinetics of methanol dehydration by keeping the water concentration low, which in turn enhances methanol synthesis. It also readjusts the H_2: CO in the reactor as the reactions proceed. In the CO-rich regime, the water gas shift reaction supplements the limiting reactant and H_2 by reacting water with CO. This enhances both the kinetics and thermodynamic driving force of the methanol synthesis reaction. In the H_2-rich regime, water gas shift consumes the limiting reactant, CO, which harms both the kinetics and thermodynamics of methanol synthesis. An understanding of these complex roles of the methanol dehydration and water gas shift reactions and of their dependence on the syngas composition explains why the synergy is high in the CO-rich regime, but decreases with the increasing H_2 or CO_2 content in the reactor feed. The analysis shows that the optimal H_2: CO for the LPDME reactor is around 1-to-l, in good agreement with the results from the simulation. While the 1-to-l feed provides a good foundation for some process configurations, it does not match the composition of syngas, which typically has a H_2: CO of 3 : 1 or greater.