Thermodynamic approach and comparison of two-step and single step DME (dimethyl ether) syntheses with carbon dioxide utilization

Abstract

DME (Dimethyl ether) synthesis from syngas with CO2 utilization through two-step and single step processes is analyzed thermodynamically. The influences of reaction temperature, H2/CO molar ratio, and CO2/CO molar ratio on CO and CO2 conversions, DME selectivity and yield, and thermal behavior are evaluated. Particular attention is paid to the comparison of the performance of DME synthesis between the two different methods. In the two-step method, the addition of CO2 suppresses the CO conversion during methanol synthesis. An increase in CO2/CO ratio decreases the CO2 conversion (negative effect), but increases the total consumption amount of CO2 (positive effect). At a given reaction temperature with H2/CO = 4, the maximum DME yield develops at CO2/CO = 1. In the single step method, over 98% of CO can be converted and the DME yield can be as high as 0.52 mol (mol CO)−1 at CO2/CO = 2. The comparison of the single step and two-step processes indicates that the maximum CO conversion, DME selectivity, and DME yield in the former are higher than those in the latter, whereas an opposite result in the maximum CO2 conversion is observed. These results reveal that the single step process has lower thermodynamic limitation and is a better option for DME synthesis. From CO2 utilization point of view, the operation with low temperature, high H2/CO ratio, and low CO2/CO ratio results in higher CO2 conversion, irrespective of two-step or single step DME synthesis.