Optimization of technological processes depends on the relevant process design, properly selected column internals and sufficient understanding of the process behavior. This can only be achieved with the help of accurate and reliable process models. Along these lines, the present article suggests a new modelling concept - complementary modeling - for a large class of fluid engineering processes. Due to diversity of process conditions and criteria, it is impossible to develop a unified modelling approach. Instead, an efficient combination of different modeling approaches is advantageous. The complementary modeling is discussed in detail and illustrated with several case studies.