Direct vs. indirect dimethyl ether synthesis from sustainable H$_2$ and CO$_2$: kinetic modeling, exergy analysis and techno-economic assessment

Dimethyl ether (DME) is a basic chemical, promising energy carrier and potential intermediate for chemical energy storage that can be produced from renewable hydrogen and captured CO$_2$, either indirectly (methanol synthesis followed by dehydration) or directly with both steps coupled in a single bifunctional reactor. In published comparisons, opinions differ as to which route is preferable, and it is well known to be sensitive to a number of assumptions: feedstock, flowsheet design, heat integration, cost model, and kinetics. This work contributes to a comparison in which several of these elements are strengthened simultaneously. High-pressure experiments over a Cu/ZnO/ZrO$_2$ + H-FER-20 catalyst bed, spanning 1200 h time on stream including 1320 operating points, were used to reparametrize a kinetic model, extending its validity to CO$_2$-rich feeds and representing the produced DME molar fractions with a mean relative error of 8 %. Both routes were simulated for an industrial large scale DME plant with detailed heat integration, including a water Rankine cycle, purge-gas combustion, and a two-stage refrigeration cycle for the cryogenic CO$_2$/DME separation. ... mehrA cost model for a production capacity of 836.8 kton DME·a$^{−1}$ for the location of Germany was developed. At this configuration, both plants are fully self-sufficient in heat, in contrast to earlier reports. The direct route converts CO$_2$ with higher overall DME yield (95.3% vs. 91.7%), reducing purge losses, but its reactor is twice as large, owing to water inhibition, and the operation requires 21% more net electrical energy. On balance the trade-off favors the direct route, which reaches a higher overall exergy efficiency (87.1% vs 84.9%) and lower production costs (2195.4 vs 2234.9 €/t$_{DME}$, or 704.42 vs 743.97 €/t$_{DME}$ on a conversion-cost basis) for hydrogen costs of 5300 €/t.