Thermodynamic simulation represents an important tool in the process engineering treatment of distillative separation processes, The degree of detail in the representation of the real process is orientated upon the question of the stage of process development at which thermodynamic simulation is utilized, For three examples of practical relevance it is shown that an improper representation of process reality in the simulation may lead to ambiguous and even false interpretations. In most cases problems arise when process-relevant functions of single units are not incorporated adequately It is shown that a Langrange-type consideration of the process avoids most of the problems discussed