Fluidized catalyst beds axial distribution

The concept of the successive contact mechanism has been given its simplest form by dividing the fluidized catalyst bed into two parts—dense phase and dilute phase. The concept has been found to apply to bed performance, as shown in the preceding section. The reactor model has been developed on the basis of several simplifying assumptions, partly to retain mathematical simplicity as a workable design equation accounting for the relative effects of the variables, and partly due to a relative lack of information about bed performance. Further properties of the mechanism are examined here, particularly as to axial distribution of reactivity inside the bed. 

A well-defined bed of particles does not exist in the fast-fluidization regime. Instead, the particles are distributed more or less uniformly throughout the reactor. The two-phase model does not apply. Typically, the cracking reactor is described with a pseudohomogeneous, axial dispersion model. The maximum contact time in such a reactor is quite limited because of the low catalyst densities and high gas velocities that prevail in a fast-fluidized or transport-line reactor. Thus, the reaction must be fast, or low conversions must be acceptable. Also, the catalyst must be quite robust to minimize particle attrition. 

Later, Furusaki et al. (F17) studied the hydrogenation of ethylene by fluidized Ni catalyst to obtain the axial reactivity distribution. Here the samples of bed gas were removed by a traveling sampler placed at the center of the bed during steady reaction, so that the sample taken in the dense phase shows an average of the concentration in the bubble and emulsion phase. Figure 74 shows an example of the axial concentration profile. 

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