Rates for Poisoned Porous Catalysts

For one catalyst = 16. It is suspected that intrapellet diffusion strongly retards both dehydrogenations. Estimate the potential improvement in selectivity if diffusion resistance could be eliminated. Make the estimate for a concentration ratio (Cg/C ) = 1.0. Neglect differences in between ethane and ethylene. 

Solution Equation (11-93) is applicable for the diffusion-limited situation. The pellet selectivity for ethylene formation with respect to ethane disappearance is 

If diffusion resistance is eliminated, Eq. (11-92) gives the point as well as the pellet selectivity 

These selectivities give the ratio of rates of formation of ethylene and total disappearance of ethane (ethylene plus acetylene). If we start with pure CjHs, a mass balance requires that the rate of formation of acetylene with respect to disappearance of ethane be 1 — Sp. Hence eliminating diffusion resistance reduces the rate of formation of undesirable C2H2 from 0.45 to 0.06. 

The discussion has been limited to selectivities existing at one position or conversion (set of concentrations) in a reactor. Wheeler has extended the development to show cumulative selectivities that would exist at any level of conversion in the reactor efSuent. 

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