This paper focuses on developing a mathematical model for a seasonal thermal storage system based on sorptive materials. The storage has the shape of a continuously rotating drum and is filled with Zeolite as thermochemical storage material. The mathematical model is a particular realization of a model used to describe fixed bed reactors. The aim of the model is to use it in finding optimal operating strategies for an entire heating system. The resulting model consists of four coupled linear, constant coefficient differential equations. The simulation study presented here highlights the differences in operating conditions between a rotating drum reactor and a fixed bed storage system.