275714 Reaction-Diffusion in a Cobalt Catalyst Particle: Aspects of Activity and Selectivity for a Variable Chain Growth Probability in Fischer-Tropsch Synthesis
Selecting appropriate conditions and catalyst dimension for the heterogeneously catalyzed Fischer-Tropsch (FT) reaction is crucial for realizing optimum catalyst selectivity, indicated by the chain growth probability (α), effectiveness (η), and productivity (space time yield), a combination of selectivity, effectiveness, and activity. Here, we present the results of a reaction-diffusion analysis to investigate the effects of intrinsically unbalanced diffusion and consumption ratios on catalyst selectivity, activity and productivity with the purpose to optimize operating conditions.
We analyze the reaction–diffusion performance for the Fischer–Tropsch reaction in a single cobalt catalyst particle, applying the Langmuir–Hinshelwood rate expression proposed by Yates and Satterfield and a variable chain growth parameter α, dependent on temperature and syngas composition (H2/CO ratio). The goal is to explore regions of favorable 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 H2/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 H2 and CO. The C5+ space time yield, a combination of catalyst activity and selectivity, can be increased with a factor 3 (small catalyst particle, dcat = 50 μm) to 10 (large catalyst particle, dcat = 2.0 mm) by lowering the bulk H2/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 (H2/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.