1. A medium having instructions for performing steps for creating a regular model for a multi-stage separation process having feed trays, reboiler return trays and condenser return trays and one or more additional trays that allows said regular model to converge to a solution comprising: (a) lumping each of said one or more additional trays into an associated compartment that comprises one or more of said one or more additional trays that are not in another associated compartment; (b) treating said feed trays, reboiler return trays and said condenser return trays as one tray compartments; (c) designating one of said one or more additional trays in each of said associated compartments comprising one or more additional trays as a sensitive tray; (d) equating liquid holdup in each of said compartments to the total liquid holdup of the trays in each of said compartments; (e) ignoring vapor holdup in each of said compartments; (f) calculating phase equilibria in an associated compartment only for said sensitive tray; (g) presuming that all trays other than said sensitive trays in each of said associated compartments respond instantaneously to liquid and vapor flow; and (h) basing temperatures of all trays other than said sensitive trays in each of said associated compartments on linear interpolation between the temperature at each of two adjacent sensitive trays.