Fischer-Tropsch reaction-diffusion in a cobalt catalyst particle: aspects of activity and selectivity for a variable chain growth probability

The reaction-diffusion performance for the Fischer-Tropsch reaction in a single cobalt catalyst particle is analysed, comprising the Langmuir-Hinshelwood rate expression proposed by Yates and Satterfield and a variable chain growth parameter alpha, dependent on temperature and syngas composition (H-2/CO ratio). The goal is to explore regions of favourable operating conditions for maximized C5+ productivity from the perspective of intra-particle diffusion limitations, which strongly affect the selectivity and activity. The results demonstrate the deteriorating effect of an increasing H-2/CO ratio profile towards the centre of the catalyst particle on the local chain growth probability, arising from intrinsically unbalanced diffusivities and consumption ratios of H-2 and CO. The C5+ space time yield, a combination of catalyst activity and selectivity, can be increased with a factor 3 (small catalyst particle, d(cat) = 50 mu m) to 10 (large catalyst particle, d(cat) = 2.0 mm) by lowering the bulk H-2/CO ratio from 2 to 1, and increasing temperature from 500 K to 530 K. For further maximization of the C5+ space time yield under these conditions (H-2/CO = 1, T = 530 K) it seems more effective to focus catalyst development on improving the activity rather than selectivity. Furthermore, directions for optimal reactor operation conditions are indicated.