(55g) 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, d_cat = 50 μm)
to 10 (large catalyst particle, d_cat = 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.