Abstract: This paper presents a polynomial model for the characteristic curves of the synthesis gas compressor of Kellogg-type ammonia plants and a regression model for vapor-liquid equilibria in ammonia separation as well as for loop pressure drop based on plant data. The refrigeration required for separation of ammonia is also estimated. Steady state simulation of the ammonia synthesis loop by combining the sequential modular approach and the equation-solving approach, is carried out. Case studies on the basis of design conditions indicate that the simulation results match well with plant operation. Effects of the main operation variables on loop performance are studied by simulation analysis under 53 operation conditions. Multi-objective and multilevel optimization, with minimal compressor power and maximal production rate as object, and optimal temperature profiles of the ammonia synthesis converter as constraint, is explored.