Simulation of a Natural Gas-to-Liquid Process with a Multitubular Fischer-Tropsch Reactor and Variable Chain Growth Factor for Product Distribution

Natural gas conversion to liquid transportation fuels through Fischer-Tropsch synthesis (FTS) is an alternative approach compared to the conventional crude oil refining. In a gas-to-liquid (GTL) plant, natural gas is first converted to synthesis gas (syngas) and then to liquid fuels in an FT reactor. The distribution of liquid fuels being produced in the FT reactor depends on several factors including the catalyst, reactor type, operating pressure and temperature, chain growth probability (alpha), and so forth. The change in alpha along the FT fixed-bed reactor is affected by the temperature and H-2/CO ratio. In this paper, a typical multitubular FT reactor was simulated with a variable alpha along the tube length. To do so, the reactor was discretized to 1000 small segments and the alpha model was modified to fit the wax concentration in the reference FT reactor data with a wax production capacity of 6900 kg/h. The developed FT reactor model was then used for the plantwide simulation of a GTL flowsheet with a larger wax production capacity of ca. 16,719 kg/h. The distribution of chemical species based on variable and constant a was compared. With a similar feed syngas composition, the wax mass fraction is 45.34% in the variable alpha scenario and is 41.02, 55.7, and 73.57% when alpha is fixed at 0.9, 0.92, and 0.95, respectively. In addition, sectioning of the FT reactor into two stages was considered to investigate the potential of volume reduction to produce ca. 16,719 kg/h of wax. The results suggest that the staged reactor has nearly 12% lower volume compared to the base case.