Two principles for describing catalyst deactivation are discussed, one base
d on the deactivation mechanism, the other based on the activity and cataly
st age distribution. When the model is based upon activity decay, it is com
mon to use a mean activity developed from the steady-state residence time d
istribution. We compare control-relevant properties of such an approach wit
h those of a model based upon the deactivation mechanism. Using a continuou
s stirred tank reactor as an example, we show that the mechanistic approach
and the population balance approach lead to identical models. However, com
mon additional assumptions used for activity-based models lead to model pro
perties that may deviate considerably from the correct one.