Rigorous and accurate simulation of Liquid Phase Catalytic Exchange (LPCE) is required for water detritiation process design and analysis. The Two-Fluid model simulates exchange between gas and liquid using an overall mass transfer coefficient model. The Three-Fluid model simulates liquid/vapor and vapor/gas mass transfer explicitly with separate mass transfer coefficients. Both Two-Fluid and Three-Fluid models are presented. The Two-Fluid model combines liquid and vapor flow, resulting in accuracy close to the more rigorous Three-Fluid model. Mass transfer coefficients are estimated from Maxwell-Stefan theory of multicomponent diffusion across films at the liquid/vapor and catalyst interfaces.