Shell-progressive model

The shell progressive model in Example 11.15, part (b) envisions a mass transfer limitation. Is the limitation more likely to be based on oxygen diffusing in or on the combustion products diffusing out ... 

Catalyst deactivation in large-pore slab catalysts, where intrapaiticle convection, diffusion and first order reaction are the competing processes, is analyzed by uniform and shell-progressive models. Analytical solutions arc provid as well as plots of effectiveness factors as a function of model parameters as a basis for steady-state reactor design. 

Fig. 8. Effectiveness factor rt as a function of a and without convection (X=0) and with convection (X IO). Shell-progressive model.

Fig. 10. Effectiveness factor t as a function of a with

Devolatilization kinetics experiments (43,44) and pellet behavior in which the unreacted-reacted interface is sharply marked indicate that the primary reactions within the solid can be described by a shell-progressive model of the type discussed by Carberry (45) and others. However, the expected secondary reactions described briefly below are quite different for volatiles escaping to the gas phase plasma or volatiles remaining in the pellet or char-residual. The differences between plasma and thermal pyrolysis regarding product distributions shown in Figure V and Tables II and III arise from the nature of the secondary reactions. 

This is just the deactivation function for the shell-progressive model. 

When the potential rate of transport of gaseous reactants is much lower than the rate of chemical reaction, a shell-progressive model (SPM) results since... 

Figure 5.2 Shell-progressive model for non-catalytic gas-solid reactions.

Suppose that a pellet is deactivated according to the shell-progressive model. Show that the activity factor can be determined from the measurements of external temperature differences ... 

---