Fischer-Tropsch-synthesis with nitrogen-rich syngas - Fundamentals and reactor design aspects

An option in bringing remote natural gas reserves to the market is its conversion by Fischer-Tropsch (FT)-synthesis into diesel oil and wax. The use of nitrogen-rich syngas (50 vol.%) could be an alternative to classical processes with nitrogen-free syngas because the investment costs are probably lower: syngas is produced by partial oxidation with air, which eliminates the need for an air separation plant, and a process with nitrogen-rich syngas does not utilize a recycle loop and a recycle compressor. For the development of such a process, the kinetics of F-T-synthesis was studied on an Fe-catalyst, indicating that nitrogen only dilutes syngas, and therefore, has no influence on the kinetics if the partial pressures of carbon monoxide and hydrogen are kept constant. Subsequently, the FT-synthesis with nitrogen-rich syngas was investigated in wall-cooled single tube reactors. Based on the experimental data, a mathematical model for industrial multitubular F-T-reactors was developed. Model calculations indicate that nitrogen plays an important role in the operation of multitubular reactors by helping to remove the heat generated by the F-T-reaction. This leads to an optimum diameter of the tubes of 70 mm for nitrogen-rich syngas with respect to a stable and safe operation of the reactor, whereas for nitrogen-free syngas, the diameter is limited to about 45 mm. The production rate of diesel oil and wax per tube is, in case of nitrogen-rich syngas, about three times higher, which will decrease the number of tubes and the investment costs of industrial multitubular reactors. Detailed economic studies are still necessary to validate or disprove whether and under which circumstances the proposed process with nitrogen-rich syngas is an attractive alternative to classical processes with nitrogen-free syngas, especially in areas with remote natural gas resources. (C) 1999 Elsevier Science B.V. All rights reserved.