The main goal of this work was to develop a computational module capable to evaluate Themkin-Phyzev modified rate expression in two ammonia synthesis reactors models: adiabatic and autothermal. Main hypothesis are that the system is at steady-state, with negligible heat and mass transfer in the catalyst to bulk gas. So, the system of differential equations that was solved consisted of: limiting reactant conversion, pressure and temperature. Numerical strategy to solve the system was based on Euler method, implemented at Wolfram Mathematica®. Initially, a comparison between industrial data and models was done. After that, a sensibility analysis was performed, varying inlet pressure and heat transfer coefficient. In adiabatic case, the product temperature was above 800 K, the maximum supported by catalysts. Results showed that autothermal case was safer and inlet pressure had more effect in output.