Process Optimization of FT-Diesel Production from Lignocellulosic Switchgrass

In this paper, we present the conceptual design for the optimization of FT-diesel production process from switchgrass via gasification of biomass. We propose a limited superstructure where the process starts with the gasification of the biomass. Two different alternatives are evaluated, direct and indirect gasification. The gas obtained is cleaned up and its composition adjusted in terms of the ratio CO/H-2 considering a bypass, a PSA system and water gas shift reaction. Next, the removal of CO2 and H2S is performed using absorption in monoethanolamines and PSA. Once the syngas is prepared, the Fischer-Tropsch reaction is conducted and the products separated. Hydrocracking of the heavy products is also considered to increase the yield toward green diesel. The optimization of the system is formulated as an equation-based Mixed-Integer Nonlinear Programming (MINLP) problem that is solved for the optimal production of the biodiesel fraction while minimizing the energy consumption and the hydrogen consumption. The optimal solution requires the use of indirect gasification followed by steam reforming. The ratio of CO/H-2 is left unchanged as the one obtained after gas reforming, while adjusting the temperature at the FT reactor and the operating conditions of the hydrocracking to increase the FT-diesel production. A promising production cost of 0.71 $/gallon is obtained, and only 0.15 gal(water)/gal(bioifiuel) of water consumption is required if air cooling is implemented in the plant.