Modeling and simulation of multi-phase flows in Fischer-Tropsch synthesis
Maleic anhydride (MA) is industrially produced by selective oxidation of n-butane over a vanadium-phosphorus-oxide (VPO) catalyst. The reaction is carried out in salt bath cooled shell-and-tube reactors with up to 30,000 individual tubes. Due to the highly exothermic reaction, the complex reaction kinetics and the small tube-to-particle diameter ratio, the classical reactor models reach their limits in this process. Therefore, a particle resolved CFD simulation is used for a detailed description of the phenomena occurring in the catalytic fixed bed. A reaction kinetics measured in a transport limitation free microreactor are implemented. A specific focus is put on the non-uniformity of the flow in the catalyst bed as well as on the mass and heat transport within the fixed bed and the catalyst particles. The results of the CFD simulation are validated with experimental measurements.
With the data generated from the CFD simulation, drawbacks of the classical reactor models can be identified and improved by modifying the models. The current main problem of industrial MA synthesis from n-butane is the overheating of catalyst particles and the formation of a distinct hot spot at the beginning of the fixed bed due to insufficient heat removal. This leads to increasing over-oxidation and losses in MA yield. CFD simulation can be used to investigate the impact of particle geometry on heat transport and MA yield to find a more suitable particle shape for the process.