Rate-Based Model: The rate based model eliminate the necessity of using efficiencies and is capable of predicting the actual performance of the process. In this model the mass and energy conservation equations are split into two parts, one for each phase. The equations for each phase are connected by mass and energy balances around the interface and by the assumption that the interface be at the thermodynamic equilibrium. The process of simultaneous mass and energy transfer through the interface is modeled by means of rate equations and transfer coefficients.The mass transfer coefficients for the liquid phase and vapour phase are combined to calculate the overall mass transfer coefficient. Resistances to mass and energy transfer offered by fluid phases can be accounted for by using separate equations for each phase.