Selectivities for Porous Catalysts

The selectivity at a position in a fluid-solid catalytic reactor is equal to the ratio of the global rates at that point. The combined effect of both external and internal diffusion resistance can be displayed easily for a set of parallel reactions. We shall do this first and then consider how internal resistance influences the selectivity for other reaction sequences. 

This might be the dehydrogenation of a mixed feed of propane and n-butane, where the desired catalyst is selective for the K-butane dehydrogenation. Suppose that the temperature is constant and that both external and internal diffusion resistances affect the rate. At steady state, the rate (for the pellet, expressed per unit mass of catalyst) may be written in terms of either Eq. (10-1) or Eq. (11-44), 

Equation (11-86) expresses the combined effect of external and internal mass-transport resistance. Note that the reduction in rate due to internal diffusion (through rj) is combined with the rate constant k for the chemical step, while the external effect is separate. If the external resistance is negligible, then k a r]k. Ifinternal transport is insignificant, then rj 1. If both conditions are satisfied, the rate is determined solely by the chemical step that is, Eq. (11-85) reduces to Fp = k Cf,. 

For parallel reactions the selectivity of product B with respect to product S for a pellet in the reactor is obtained by applying Eq. (11-86) to the two reactions thus the pellet selectivity Sp is 

CHAPTER 11 REACTION AND DIFFUSION WITHIN POROUS CATALYSTS 

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