Urea is an important petrochemical product which is mainly used as fertilizer. In this study, a model is developed for the synthesis section of an industrial urea plant. In the proposed model the urea reactor is divided into several continuously stirred tank reactors (CSTRs). It has been considered in this study that formation of ammonium carbamate occurs through the heterogeneous reaction of carbon monoxide and ammonia. This reaction was considered to occur only in the liquid phase in the previous works presented in the literature. The formation of biuret in the reactor is also considered in the model which has not been considered in the previous works. The validity of the proposed model was demonstrated using industrial data. It has been shown that there is a good agreement between the results of the model and the industrial data. Dynamic response of the process to some important parameters in the synthesis section was also studied. (c) 2004 Elsevier B.V. All rights reserved.