A Mathematical model has been developed for predicting the outlet gas composition of a fluidized bed reactor in which the oxychlorination of ethylene process taking place. In the present work, a mathematical model is developed for predicting the outlet concentration and conversion of ethylene in the first reactor, which is fluidized bed reactor. A three phase fluidization has been assumed in which bubble phase, cloud-wake phase and emulsion phase exist.Further the model predicts the variation of concentration of gaseous species along with bed height. The model results have been compared with bed height. The model results have been compared with the results available in the literature and the agreement between the model and the literature found to be good.