Urea is one of the most important fertilizers  and chemical materials. In this age of energy    crisis, research on the urea process becomes more and more important in the world. One of the research focuses is to introduce simulation and optimization techniques into the urea industrial process so as to discover the bottleneck, and optimize the operational parameters and equipment parameters.
Because of the complex thermodynamic system and  reaction conditions of high temperature and high pressure, it is much more difficult to model the urea synthesis process exactly. In this paper, an extended UNIQUAC equation is introduced to predict the vapor-liquid equilibrium model and the results have agreed well with experimental data.
An equilibrium-stage model is proposed to simulate the unit in urea high-pressure loop (H.P. loop). The sequential-modular approach may become extremely slow and it is difficult to tear the streams and evaluate the initial value of streams because of the complex nature of the process with a number of recycle streams. In this paper, we attempt to simulate the high-pressure loop of the urea process with a simultaneous-modular approach. The simulation result is compared with the design data.