Maximizing the profitability of integrated Fischer-Tropsch GTL process with ammonia and urea synthesis using response surface methodology

Abstract

The integration of a natural gas to liquids (GTL) process with ammonia and urea synthesis units was conducted to utilize the emitted CO2 of the GTL process for the urea synthesis. The feedstocks of the ammonia synthesis unit including hydrogen and nitrogen were provided by a polymer electrolyte membrane (PEM) electrolyzer and air separation unit (ASU) of the GTL process, respectively. The required power for the PEM modules was assumed to be supplied by the surplus generated power of the GTL process. To enhance the overall carbon efficiency and profitability of the three processes, the emitted CO2 from the GTL process was utilized in the urea synthesis unit. Multi-objective optimization approach was conducted to determine the optimal values of carbon efficiency and wax production rate of the GTL process. Objective functions were calculated by response surface methodology with second-order polynomial regression. The degrees of freedom were defined as follows: unpurged ratio of recycled tail gas from Fischer-Tropsch (FT) reactor, recycle ratio of the GTL tail gas to the FT reactor, CO2 removal percentage from the GTL process synthesis gas (syngas) section, steam to carbon ratio to pre-reformer, molar flow of feed to the ammonia synthesis unit, and CO2 intake to the urea unit. The presented integration results in the production of about 434,000 tonnes/year urea in addition to the FT-derived products. 13.71% (37 tonnes/h) of the produced CO2 in the GTL process is utilized in the urea production unit and the profitability of the integrated process is enhanced by 8%.