A nonequilibrium model for the dynamic simulation of distillation columns is described. The nonequilibrium model includes the direct calculation of the rates of mass and energy transfer and is better able to model the actual physical processes occurring on a real distillation tray than is the conventional equilibrium stage model. Example calculations show that heat-transfer limitations and the vapor holdup above the froth cannot be neglected at elevated pressuress. Back-computed Murphree tray efficiencies are not constant over time which implies that the equilibrium model should not be used for dynamic simulations.