Performance Characteristics of Eggshell Co/SiO2 Fischer-Tropsch Catalysts: A Modeling Study

We investigate the performance characteristics of cobalt catalyst (Co/SiO2) pellets with eggshell morphology used in Fischer-Tropsch synthesis (FTS). An analytical strategy is developed first, to estimate the optimum thickness of active shell. When the system is strongly mass transfer limited (Thiele Modulus >> 1) it can be represented by slab geometry in order to determine the shell thickness. Otherwise, the analysis is done on a spherical geometry. In the latter case, the continuity equation simplifies into the Emden Fowler equation which was solved via a numerical iterative technique based on prior investigations. A more accurate detailed numerical solution was also developed for this intrapellet model. This numerical result is compared with the analytical solution as well as with a previously published model. The sensitivity of the catalyst performance to temperature and pressure variations was also examined. Performance analysis was carried out considering both rate of reaction and a chain growth probability as yardsticks. These performance parameters were examined using contour maps with Thiele modulus and H-2/CO ratio as their coordinates. On this map there exists an ideal combination of gas ratio and Thiele modulus that fulfills the design requirements of product selectivity and high catalyst activity. This study confirms the need for, and the advantage of, using eggshell morphology for Fischer-Tropsch synthesis applications.