A multi-phase dynamic model has been developed for Fischer-Tropsch synthesis in a slurry bubble column reactor operating in a heterogeneous flow regime. The catalyst particles concentration distribution, deactivation, and on-line replacement were considered in this model. The population balance equation model has been utilized to calculate the catalyst age distribution. Based on the dynamic model, the process of catalyst replacement has been investigated, and the effect of catalyst replacement on the design and operation of the reactor has been studied. A correlation between the catalyst replacement rate and the catalyst activity has been developed in this research [A(C) = (1-alpha(infinity))e(-kdt) + alpha(infinity)].