Process Analysis for the Carbon Dioxide Hydrogenation into Dimethyl Ether

In the last years, dimethyl ether (DME) has been revealed as a promising clean alternative fuel for several energy applications as well as an interesting chemical feedstock. DME is traditionally produced from syngas (CO+H2), but an interesting alternative would be its synthesis by using CO2 with this option possibly representing an effective contribution to the mitigation of carbon dioxide emissions. DME can be produced from CO2 by two routes: via a two-step process, involving the methanol synthesis and its subsequent dehydration, or by a single-step process where the two reactions occur simultaneously. Moreover, being both the reactions equilibrium-limited, reactive distillation seems to be suitable because it would allow overcoming the equilibrium, by bringing the conversion to completion. In this work, the four possible processes, one and two-step, with and without reactive distillation, were developed through the simulation software Aspen Plus; they were then compared in term of conversion, selectivity, energy efficiency and emissions, as well as the safety risks associated to the design and operation of the processes. From these analyses, the most attractive process resulted to be the one in which both steps are carried out in two different reactive distillation columns, with this solution giving the best performances under all the aspects considered, with a CO2 conversion of 89.62 % and the lowest specific energy consumption, global warming potential and safety concerns. On the other hand, even though the one-step process is thermodynamically attractive, it is not possible to perform it in a single reactive distillation column, being the relative volatility of DME in the middle of all the other compounds. At the same time, if the one-step is performed in a traditional reactor the difficulty in the post-reaction separation makes the process not economically feasible and the least convenient solution under almost every point of view.