High-Temperature Fe-Based Fischer-Tropsch Synthesis: Experimentally Validated Kinetic Models Implemented at Pellet and Reactor Scales

Pellet- and reactor-scale models for Fischer-Tropsch synthesis (FTS) with a Fe-K/silica catalyst were developed to investigate the sensitivity of the hydrocarbon products and carbon dioxide selectivity to process conditions and feed composition at high temperature (350-400 degrees C), moderate pressure (1-10 bar), and a range of H-2/CO ratios (3-1). The major objective of this paper is to develop, validate, and evaluate a high-temperature FTS model that is then used to assess the feasibility of process integration with syngas production. Since there is limited kinetic data available, in literature at these conditions, bench-scale reactor tests were conducted to obtain operational data for parameter fitting of kinetic expressions used in the model. This resulting kinetic model demonstrated agreement with the experimental data with an R-2 of 0.97 to the testing data set and, thus, was feasible to apply at pellet and reactor scales. Multiple pellet sizes were modeled to detail the role of transport limitations as the sphere's diameter approached and exceeded 1 mm. Application of the reactor model indicated that hydrocarbon selectivity depended strongly on temperature, whereas the ratio of olefin to paraffin products decreased with increasing temperature, pressure, and H-2/CO ratio. Product selectivity was not sensitive to the conversion of carbon monoxide. Furthermore, the roles of the pressure and H-2/CO ratio were closely coupled. At a H-2/CO ratio of 3, only slight variations in selectivity occurred over a pressure range of 1-20 bar, whereas at a ratio of 1, selectivity could vary by as much as 30% over the same pressure range. At pressures below 5 bar and temperatures above 350 degrees C, minimal selectivity to heavy hydrocarbons (C12+) is obtained, and selectivity to midrange products (C5-11) rapidly declined as pressure dropped below 5 bar, which indicated that an operational pressure of at least 5 bar is needed to achieve reasonable yields in this temperature range. These results, while tentative, provide guidelines for further experimentation and evaluation of integrated FTS processes.