Selectivity residence time distribution, limits

The single-bed reactor is the simplest catalysis reactor. It is completely filled with catalyst and is mainly used for thermally neutral and autothermal gas reactions. Owing to its design, the pressure drop is high, and the residence-time distribution has a major influence on the selectivity and conversion of the reaction. Of particular importance is the maintenance of temperature limits, both axially and radially, as heat removal is naturally poor. An advantage is the ease of catalyst regeneration. 

Hydrocarbon distributions in the Fischer-Tropsch (FT) synthesis on Ru, Co, and Fe catalysts often do not obey simple Flory kinetics. Flory plots are curved and the chain growth parameter a increases with increasing carbon number until it reaches an asymptotic value. a-Olefin/n-paraffin ratios on all three types of catalysts decrease asymptotically to zero as carbon number increases. These data are consistent with diffusion-enhanced readsorption of a-olefins within catalyst particles. Diffusion limitations within liquid-filled catalyst particles slow down the removal of a-olefins. This increases the residence time and the fugacity of a-olefins within catalyst pores, enhances their probability of readsorption and chain initiation, and leads to the formation of heavier and more paraffinic products. Structural catalyst properties, such as pellet size, porosity, and site density, and the kinetics of readsorption, chain termination and growth, determine the extent of a-olefin readsorption within catalyst particles and control FT selectivity. 

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