Investigating purge gas recycling and optimal injection point selection in a methanol plant using operability energy exergy and environmental metrics

This research focuses on the possibility of purge gas recycling and reutilization in the methanol plant and investigates the effect of purge gas injection point on the process performance. In this regard, three scenarios, including the injection of purge gas into the methanol reactor, primary reformer and autothermal reformer are defined and simulated by Aspen Plus simulator. The accuracy of the simulation is established by comparing the results with the industrial plant data. Among all scenarios, purge gas recycling to the methanol reactor yields the most favorable stoichiometric number of 3.82, which is closest to the ideal value of approximately 2.1 for methanol synthesis. Although purge gas recycling increases the concentration of inert gases in the inlet feed stream to the methanol reactor, the superior stoichiometric ratio and higher partial pressures of CO and CO₂ achieved in the first scenario compensate for the inert dilution penalty. Based on the simulation results, purge gas recycling to the autothermal reformer decreases the CO₂ emission rate to 0.62 kg kgMeOH − 1 . Finally, the best scenario is selected from list of alternatives by TOPSIS decision making method considering production capacity, energy efficiency, rate of exergy destruction, and CO 2 emission rate as objectives. Feeding the purge gas stream to the methanol reactor enhances the methanol production rate up to 6.6% and reduces the CO₂ emission rate to 0.65 kg kgMeOH − 1 , making it the best scenario.