NOx gases are by products of numerous technologies, such as nitric acid production, but are also present in the flue gas. NOx is known to have a strong negative environmental impact; therefore, its reduction in process technologies is a top priority. Various technologies have been developed for the reduction of the NOx content, including absorption, adsorption and catalytic removal. The kinetics and hydrodynamics pay a key role in the efficiency of all processes above. For instance, the geometry and the position of the inlet gas distributor have a significant effect on the Fenton reagent absorption process. In this case, the optimal solution must provide homogeneous feed gas distribution in the vessel, which leads to maximal phase contact, therefore improved removal efficiency. The objective of this work is the CFD based optimization of a laboratory scale NOx removal absorption vessel. Different inlet geometries were designed and evaluated using CFD modeling. 12 geometries with 4 nozzle number (1, 4, 37 and 64), and 3 inlet areas (5.8e 5 m2 (50 %), 1.2e 4 m2 (100 %) and 1.7e 4 m2 (150 %)) were simulated, in COMSOL CFD environment. The geometries were evaluated based on the analysis of the outlet variables (gas volume fractions, well-mixed areas). The 64-nozzle construction with medium inlet area gave the best performance, which has the maximal averaged well-mixed area from all constructions.