This study presents a dynamic model for a polyesterfication reaction in a system containing a semi-batch wise operating reaction vessel connected to a distillation unit. This model can be used for multi-product batch production. The reaction between maleic anhydride and 1, 2-propylene glycol has been taken as a case study. The model has been developed in Aspen custom modeller. The reaction scheme is complex and the proposed model takes four types of reactions into account, ring opening, polyesterfication, isomerisation and saturation reactions. The model includes kinetics, describing the change of rate order during the reaction, simple thermodynamics and mass balances. The parameter describing the changing rate order during the reaction is fitted. The acid value and water content have been used to validate the model and the model predicts these important variables reliably.