Calcium looping (CaL) is a competitive thermochemical energy storage (TCES) alternative allowing uninterrupted power production from solar heat. In this work, a model based approach of the very fast carbonation reaction (CaO (s)+CO2(g)?CaCO3(s)) is employed to simulate a drop tube as the carbonator reactor. A one dimensional (1D) heterogeneous plug flow reactor (PFR) model for gas and solid phase is developed at steady state conditions. The model handles gas and solids as two distinct phases characterized by individual physical properties each. Model simulations export useful information about crucial process variables variation along the reactor. A case study of carbonator operation is examined through process simulation subjected to different CO2/CaO molar ratios at the inlet of the reactor. In this way, the capabilities of the model are demonstrated related to different operating conditions providing insights for preliminary designs of similar reactors.