The common way to obtain low-order models of distillation columns by identifying the transfer function elements independently may result-in models that are inconsistent with the physical reality. The description of the process directionality may be poor, and the model is likely to contain an excessive number of slow poles. In this paper, the directionality of nonideal (real) distillation columns is studied with the help of a number of published experimental and simulated models. Based on the results, a multivariable model structure for capturing process directionality in continuous nonideal distillation is proposed. Further, a method is given for how to avoid multiple representation of single states in the model, when flow lags are explicitly included.