Nonequilibrium models combine material and energy balances for each phase with properly formulated models of simultaneous mass and energy transfer in multicomponent systems. An improved nonequilibrium stage model has been implemented for simulating multicomponent separation processes and is described in this paper. The improved model adds to an existing model the phenomena of entrainment and weeping in tray columns and the calculation of the pressure profile. A design mode has been implemented to eliminate the need for equipment design data to be specified in advance. Numerical results show that design mode does not impose an unnacceptable computational penalty. Design mode allows nonequilibrium models to be used for simulating single columns still under consideration or as part of a flowsheet simulation of an entire plant. Nonequilibrium simulations sometimes converge more easily in design mode.