Optimization of forced periodic operations in milli-scale fixed bed reactor for Fischer-Tropsch synthesis

One-dimensional pseudo-homogenous dynamic reactor model, incorporating detailed Fischer-Tropsch kinetics, was applied in a theoretical analysis of forced periodic operations. A milli-scale fixed-bed reactor was analyzed, using design and operation parameters, obtained previously in a steady-state optimization. Dynamic optimization and NLP methods were utilized to obtain optimal values of amplitude(s), frequency and phase shift(s) of sine-wave variation of inputs, around the corresponding optimal steady-state values, which maximize the productivity of C5+ hydrocarbons. Inlet variables that were modulated are: coolant temperature, reactants molar ratio, mass flow rate and pressure. In addition to the single input forcing, simultaneous modulations of multiple inputs were also considered, with combinations of the listed inlet variables. Among the single input cases, periodic variation of the coolant temperature resulted in the highest relative improvement of C5+, productivity by 30%. Multiple inputs forcing showed additional potential for improvement, resulting in relative c(5+) productivity increase of 52% for synchronized modulation of the coolant temperature, reactants molar ratio and mass flow rate. However, the increase in C5+ productivity is accompanied with relative increase in methane selectivity of 22-33% (relative to the steady-state value). The results suggest that, in the case of multiple input variations with high amplitudes, modulation of the inlet reactants molar ratio mainly contributes to the increase of CO conversion (e.g. reaction rate), the coolant temperature forcing slightly increases selectivity towards the desirable higher hydrocarbons (C5+), while the variation of the inlet mass flow rate enables better reaction temperature control and prevents a thermal runway.