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

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