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 / H2 considering a bypass, a PSA system and water gas shift reaction. Next, the removal of CO2 and H2S is performed using absorption in ethanolamines 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 towards green diesel. The optimization of the system is formulated as an equation-based Mixed-Integer Non-linear 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 CO /H2 is left 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 0.72 $/gallon is obtained and only 0.15 gal Water / gal bioifuel of water consumption is requried if air cooling is implemented in the plant.