The appropriate level of modelling detail in steady state flowsheet si
mulations is an important question for the calculation of thermal sepa
ration operations such as absorption, distillation, and liquid/liquid
extraction. The generation of consistent mass and energy balances may
often be performed employing standard implementations for the various
unit operations. Surprisingly, operability and sensitivity aspects of
the process may be estimated with the help of black box mass balance c
osiderations. As soon as flowsheet simulation is to serve as a basis f
or the rating of existing or the design of new distillation units, the
level of modelling detail has to be increased significantly and a Com
plex Topology Simulation (CTS) is required. Thus, the level of the mod
elling detail depends strongly on the current project state as well as
the information desired. These aspects will be discussed through four
examples of practical relevance concerning distillation feed sections
and liquid distributors, reboiler section, condensation stage and ope
rability aspects.