The modified expanding grain model, presented in the part-2 of this series of papers, is extended to describe the sorbent behavior in a long-term cyclic process using the experimental data obtained in a 25-cycle sulfation/regeneration test. The model extension uses one adjustable parameter to describe the changes in the diffiisional resistances occurring within the sorbent particle to predict the behavior of the sorbent undergoing an extended cyclic process. The model suggests that during the cyclic process, because of the cracks and fissures developing within the product layer of the grain, the product layer porosity and the product layer diffusivity of the grain as well as the tortuosity parameter gradually increase, resulting in lower intergrain diffusivity and lower overall sorbent reactivity. The model was used to predict the long-term performance of the sorbent to estimate the fresh sorbent makeup rate in the regenerative copper-oxide process. The results indicate that the sorbent makeup rate generally correspond to a useful sorbent life of higher than two years, but is highly sensitive to the slope of the extrapolation line for the sorbent deactivation.