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The present paper introduces the mathematical model of urea plant and suggests two methods for designing special purpose controllers. The first proposed method is PID controller and the second is sliding mode controller (SMC). These controllers are applied for a multivariable nonlinear system as a Urea Reactor system. The main target of the designed controllers is to reduce the disturbance of NH3 pump and CO2 compressor in order to reduce the pollution effect in such chemical plant. Simulation results of the suggested PID controller are compared with that of the SMC controller. Comparative analysis proves the effectiveness of the suggested SMC controller than the PID controller according to disturbance minimization as well as dynamic response. Also, the paper presents the results of applying SMC, while maximizing the production of the urea by maximizing the NH3 flow rate. This controller kept the reactor temperature, the reactor pressure, and NH3/CO2 ratio in the suitable operating range. Moreover, the suggested SMC when compared with other controllers in the literature shows great success in maximizing the production of urea.