Feedback control strategy of Fischer–Tropsch process in a micro-GtL plant

Abstract

Gas-to-liquid technology monetizes flared natural gas to produce green fuel while reducing carbon emissions. In the first step, oxygen reforms methane to CO and H2 followed by Fischer–Tropsch synthesis (FTS), which is highly exothermic and requires a tight temperature control to avoid thermal runaway. Bench-scale units are insufficient to generate basic data to design dynamic control systems for pilot-plants, particularly for intensified processes, because of uncertainties related to heat losses and the spatial and temporal reaction rates. Here, we propose a simulation-based feedback controller design strategy to initially estimate the parameters of a proportional Integral (PI) controller. We applied a power law model to design the controller in Aspen V12 and tested its robustness with a fractional lumped model as an alternative. An Excel ‘solver module’ estimated heat exchange parameters considering the fluidized bed hydrodynamics at steady state. Then, through dynamic simulation, we identified the parameters of a ‘first-order plus dead time’ model via a step response of the reactor temperature to design the PI temperature controller. The controller parameters were characterized by the internal model control (IMC) approach, which is robust to upstream disturbances, kinetic model mismatch, and heat exchange uncertainties.