The Successive Contact Mechanism for Catalytic Reaction

Early attempts to approximate gas-solid contacting in fluid catalyst beds were based on the assumption either of isothermal plug flow of the fluidizing gas through the bed with the catalyst uniformly distributed or of isothermal complete mixing of the gas within the bed. The simple dispersion model, falling between the above two cases, was also used (G8, R4). Evidence from both large-scale and laboratory observations (G9a, L12), 

When a fixed bed of catalyst is operated isothermally as an id al piston-flow reactor without radial concentration gradients and without change in the gas density, the differential mass balance for reactant along the bed is given by 

When the bed is not fixed, but is fluidized freely at the same gas velocity as above, with Uq much greater than the minimum fluidization velocity Umt, the bed becomes aggregative and deviates from the piston-flow reactor. As a consequence, the apparent overall rate constant of reaction for the fluidized bed is smaller than k, and the extent of reaction is expressed by 

Usually, the fluidized catalyst bed constitutes an aggregative dense phase forz between 0 andLf, and a dilute phase forz L(. Hence, another apparent overall rate constant A or s defined on the basis of Lf. Taking L( instead ofL, Eq. (7-3) is rewritten  

The object of the following treatment is to establish a physically sound reactor model to obtain A or based on the flow and transport properties of fluidized catalyst beds. Bed performance for chemical kinetics other than the first-order reaction may be computed after a sound bed performance has been established. 

---