A nonequilibrium model for fractionation trays with lateral dispersion is analyzed by a collocation method. Mobile-interface turbulent boundary-layer theory is used to describe the local heat and mass transfer rates with allowance for vapor superheat, liquid subcooling, and net molar transfer between the phases. Multicomponent transport is incorporated by a linearized matrix method. Numerical integration of the gradients through the tray permits use of local flows and states, as well as local phase equilibrium expressions. A collocation scheme solves the model efficiently; two grid points normally suffice to represent a tray. The model contains just two adjustable coefficients, a00 and Pe; the dependence of these on tray geometry and hydrodynamics remains to be investigated.