ASPEN simulation of the SNG production process in an indirect coal-liquefaction plant

Abstract

The synthetic natural gas (SNG) production process (methanation, CO-shift, and hydrogen removal) in an indirect coal-liquefaction plant was simulated using the Advanced System for Process Engineering (ASPEN). The simulation of the methanation unit agreed to within 12% of Fluor's design for converting carbon monoxide and carbon dioxide. A parametric study examined the effect of four important operating parameters on product composition, process thermal efficiency, and outlet temperature from the second methanation reactor. The molar split of gas feed to the CO-shift unit before methanation was varied from 0.2 to 0.6; variations of molar recycle ratio (0.01 - 0.67), molar steam-to-feed ratio (0.04 - 0.19), and feed temperature (478 - 533 K, 400-500/sup 0/F) to the first methanation reactor were also studied. A 50%-lower split improved thermal efficiency by 6%, but the mole % hydrogen and carbon monoxide in the product SNG required to meet pipeline-quality standards and temperature constraints were not met. Increasing the steam-to-feed ratio from 0.04 to 0.19 improved product quality but decreased thermal efficiency by 8%. By decreasing the feed temperature from 533 to 477 K (500 to 400/sup 0/F), product specifications and temperature constraints were met with no effect on thermal efficiency. However, it may bemore » impractical to operate the reactor at 477 K (400/sup 0/F) because the kinetics are too slow. Increasing the recycle ratio from 0.4 to 0.67 had no effect on thermal efficiency, and temperature constraints and product specifications were met. The SNG production process should be optimized at recycle ratios above 0.67.« less