High Fe loaded supported catalysts for biosyngas Fischer-Tropsch conversion: experimental results and detailed simulation

Fischer-Tropsch synthesis (FT) is at the heart of the Biomass-to-Liquids process, one of the attractive options for the production of clean and carbon-neutral diesel from biomass. The H2/CO molar ratio of synthesis gas produced from biomass is between 1.0 and 1.5 and using this feeding iron based catalysts are more suitable in FT respect the cobalt-based ones. Supported Fe- based catalysts have several advantages (greater surface area, better dispersion of the heat developed by the reaction and better mechanical resistance) compared to massive iron catalysts adopted in the current FT industrial plants. In particular, the optimized components loading was found, in our previous researches, to correspond to 30 wt% Fe supported on silica and promoted with K (2.0 wt%) and Cu (3.75 wt%). In order to optimize the activity, selectivity and the lifetime of this kind of FT catalyst in work conditions of biosyngas feeding, a series of experimental runs were performed with different reactor temperatures and different inlet H2/CO ratios. FT reaction activity tests were carried out in a fixed bed tubular reactor, using 1 g of fresh catalyst mixed with 1 g of diluting material (α-Al2O3, Fluka). All the catalysts were reduced in situ by a flow of H2/CO (2/1) at 350°C, 500 kPa for 4 h and tested with the standard conditions of 46.8 Nml min-1 flow of syngas (H2/CO ratio of 1/1; 1.5/1 and 2/1) plus 5.0 Nml min-1 of N2 as internal standard, at 2MPa and T= 230, 250 and 260°C for 90 h. FT results confirm the good activity of these samples (CO conversion and hydrocarbons selectivity) with low H2/CO feedings. On the basis of the collected data, a rigorous simulation of the FT synthesis reactor has been developed for different purposes: (i) to support the experimentations when implemented for the experimental design of experiments; (ii) to predict the reactor yield and conversion; (iii) to optimize the performance of the reactor system with different operating conditions; and (iv) to calculate novel reliable kinetic parameters basing on the fitting of experimental data by means of model-based nonlinear regression techniques. To do so, the FT reactor is modeled as a catalytic plug-flow reactor using mass and energy balances and reaction kinetics for Fe-based catalyst defined by Zimmerman and Bukur leading to an ordinary differential equation system with structured Jacobian. Lumping techniques have been used to model heavy hydrocarbons. The system is solved by means of dedicated solvers to handle stiffness and nonlinearities of heterogeneous reactive systems.