The preliminary isotope exchange unit (PIEU) of the mono-thermal ammonia hydrogen chemical exchange process has been modeled as a reactive absorption system based on a rate-based approach. The model parameters are estimated using the operating plant data. The developed model is utilized to quantify the effects of the operating variables such as temperature, pressure, catalyst concentration, and gas load on deuterium extraction. The model predicts an equal influence of mass transfer and reaction kinetics on the rate of deuterium extraction. Therefore, a higher operation temperature of the PIEU and higher catalyst concentration increases the extraction of deuterium substantially due to improved kinetics.