Much higher specificity and much lower energy consumption and investment costs - these requirements for modem industrial separation operations cannot be achieved without proper process modelling. Industrial separation, as a rule, occurs in multicomponent multiphase fluid systems. Multicomponent distillation, absorption, extraction, liquid membranes, etc provide examples. Multicomponent nature and multiphase character of separation processes radically complicate the pattern of the process and call for the theoretical description by strongly coupled matrix mathematical models. In this paper, a general analysis of the modelling concept is given with regard to the multicomponent multiphase separation. Some important problems arising in modelling and simulation of such processes are discussed and several examples are presented which illustrate the application of the rate-based modelling approach.