Optimal Process Design for Coproduction of Methanol and Fischer–Tropsch Liquid Fuels

The integration of a gas-to-liquid (GTL) unit with a methanol synthesis unit has been investigated. Two process structures of the integrated process have been simulated in Aspen HYSYS: 1) process without electrolyzer and 2) process with electrolyzer. In fact, the role of the electrolyzer is to increase the carbon efficiency of the overall process. An autothermal reformer is used to produce the synthesis gas for the cobalt-based Fischer–Tropsch (FT) and methanol synthesis reactors. In both of the proposed integrated process structures, the surplus energy of the processes is used to run the compressors of the methanol synthesis unit and provide the required power for the electrolyzer. Based on the simulation results of each process structure, it is possible to utilize about 46 tonnes/h of the carbon dioxide released from the GTL process. Considering the high investment cost of the electrolyzer and its trivial effect in improving the carbon efficiency (about 1.5%), the first structure was chosen as the base-case for optimization using the space filling designs approach. Two objective functions were considered: 1) carbon efficiency and 2) wax production rate. According to the optimization results, the maximum carbon efficiency of the base-case with single and multiobjective optimizations is 79.36% and 79.18%, respectively. The amount of wax production rate in these two cases was estimated 46 and 49.6 tonnes/h, respectively. The profitability index was calculated for all of the optimal points on the Pareto front. The maximum profit index of the proposed integrated process in the mode of single and multiobjective optimizations is equal to 111% and 118%, respectively.