Abstract The NOx absorption process is very complex with over forty reactions having been identified. The NOx absorption tower is a key operation in the production of nitric acid. In this work, we proposed a simple yet theoretically sound reaction model consisting of only four reactions (one kinetic and three equilibrium). We apply the Soave-Redlich- Kwong equation of state and NRTL activity-coefficient model, respectively, to account for the thermodynamic vapor and liquid non-idealities for the NOx-HNO3-O2-N2-H2O system. Based on the reaction model and thermodynamic framework in this work, the NOx absorption tower has been simulated statically by SimSci PRO/II and dynamically by SimSci DYNSIM. The simulation results agree well with the reference data. For the dynamic simulation, the simulation speed is well above the stringent speed requirement by a special dynamic application. In addition, the simulation of the tower azeotropic phenomenon has been discussed.