Nonlinear low-order models are developed for two coupled distillation columns separating a ternary mixture of methanol, ethanol and 1-propano!. The approach is based on well-known spatio temporal pattern formation phenomena in distillation processes also termed as nonlinear wave propagation. In a first step, the energy balance is neglected assuming constant molar flow rates in each column section. Further, constant molar holdups are assumed. In a second step, the wave approach is extended for columns with variable flow rates and constant volumetric holdups. It is shown that these assumptions are more realistic for the process considered here. In both cases the low-order models are in good agreement with the corresponding reference models with constant and variable flow rates, respectively. Further, the reduction of the system order is considerable in both cases.