The use of diesel engines is prevalent in transportation and other industrial applications. These engines however release nitrogen oxides (NOX) into the atmosphere which poses great environmental and health risks. Among the various means studied and implemented to control NOX from diesel engine exhausts is the selective catalytic reduction (NH3-SCR) of nitrogen oxides using ammonia as a reductant over a vanadia-based catalyst. Vanadia-based catalysts have been extensively employed in the NH3-SCR process because of their high activity, relatively low cost, durability, and stability. However, the effect of various parameters such as temperature, pressure, NH3/NOX ratio, and reactor size on the performance of vanadia catalysts for SCR of NOX has not been thoroughly investigated. This work presents a comprehensive simulation study using ASPEN Plus to investigate the effects of operational parameters on the performance of an Ammonia SCR process. The main objective of this thesis is to evaluate the effects of temperature, pressure, NH3/NOX ratio, and reactor size on the performance of the SCR process and to arrive at some optimal operating conditions of the SCR process to improve the established industrial conversion while keeping the ammonia slip minimum. We begin the study by simulating some baseline models reflecting typical industrial conditions for NOX reduction and carrying out sensitivity analysis to examine the impact of four key operational parameters on the conversion of the NH3-SCR process. Subsequent simulations systematically utilize optimal values of each parameter within a realistic range to observe the corresponding effects on NOX conversion efficiency and minimize ammonia slip. Temperature is found to have a significant impact on the reaction kinetics, with higher temperatures favoring NOX conversion but also increasing the risk of ammonia oxidation to nitrogen dioxide (NO₂). Pressure variations show a positive correlation between pressure and NO conversion due to increased gas density and improved contact with the catalyst surface but reveal that higher pressures above 2atm have a less pronounced effect. The NH₃/NOX ratio is critical in avoiding ammonia slip—a condition where unreacted ammonia passes through the system. The optimal ratio that maximizes NOX conversion while minimizing ammonia slip is identified. Lastly, the reactor size is varied to determine its influence on the overall conversion efficiency. Under optimal conditions, reactor volume shows little or no observable effect on the conversion of NO to N2. The simulation results are validated against experimental data from the literature, demonstrating good agreement and thus confirming the reliability of the ASPEN Plus model. The findings of this study provide valuable insights into the operational conditions of the Ammonia SCR process using vanadia-based catalyst, contributing to the development of more efficient and environmentally friendly NOX reduction strategies for diesel engines.