Ammonia separation from water by a membrane contactor was simulated on transient state and compared with experimental data. Aqueous low concentrated solution of ammonium with high pH has been pumped inside a hydrophobic hollow fiber (lumen), sulfuric acid solution in the outside part and the feeding solutions are in closed loop configuration. In order to simulate the separation process, the equations were developed considering radial and axial diffusion and convection in the lumen with a well-developed parabolic velocity profile. The model proposed shows minimal deviations when is compared against experimental data. This study shows that the most important parameters to control during the operation are the flow rate and the pH, mainly the last one, due to high dependence in chemical equilibrium of ammonium reaction to ammonia.