Process exploration and assessment for the production of methanol and dimethyl ether from carbon dioxide and water

Abstract

A thermodynamic, model-based, study was carried out to assess the relative performance of methanol and dimethyl ether (DME) synthesis systems using CO- and CO₂-based syngas feeds. The upstream production of a range of syngas feed compositions was simulated using CO₂ and H₂O as the sole chemical building blocks, a requirement motivated by the increasing constraints on permissible CO₂ emissions and the successful adaptation by some industrial methanol plants to the direct utilisation of CO₂. The objective was to establish whether the energy requirements and CO₂ emissions associated with upstream conversion of CO₂ to CO were justified by increased productivity in the methanol/DME systems. In the first part of the study, the performance of four systems was evaluated and compared in terms of energy efficiency and CO₂ conversion: (1) methanol synthesis system, (2) direct DME synthesis system, (3) two-step DME synthesis system with an interposed syngas separation step between the methanol production reactor and methanol dehydration reactor and (4) two-step DME synthesis system with no separation step between the two reactors. Based on equilibrium yields at 250 °C and 50 bar, the direct DME synthesis system was found to exhibit the highest energy conversion efficiencies with both CO₂- and CO-based syngas. Although this system demonstrated the lowest CO₂ emissions per methanol equivalent product with a CO-based feed, the benefits were offset by emissions associated with the upstream conversion of H₂O and CO₂ to H₂ and CO, evaluated in the second part of the study. It was determined that CO₂ could be utilised directly in the direct DME synthesis route, whereas upstream conversion of CO₂ to CO was necessary to achieve effective yields in the methanol/two-step DME systems. CO-based syngas production via high temperature co-electrolysis of H₂O and CO₂, or alternatively high temperature CO₂ electrolysis coupled with the water–gas shift process, was identified as the best technology based on energy consumption and CO₂ utilisation.