Many classic operating units such as distillation columns or reactors can be modeled this way. They can be combined in a flow sheet and simulated safely. Introducing an objective function, the resulting flow sheets can be optimized. However there are operating units which still cause difficulties and can not be simulated safely. In order to nevertheless find a design for such units, a lot of effort is spent. For example one can try to find a thermodynamic consistent flow sheet. Good starting values are needed to find a convergent simulation. If an optimization has to be performed, the convergent area has to be explored carefully and bounds have to be set in such a way that the optimization doesn't end up with non convergent simulations. An example for these more demanding unit operations in terms of simulation are dividing wall columns Generally dividing wall columns are intensified distillation processes. Conventionally, multi-component mixtures are separated in sequences of simple distillation columns where the number of columns is one below the number of components in the original mixture. Figure shows an example for the separation of a quaternary mixture using a conventional split sequence. In the figure the placeholders A, B, C and D denote the components in the feed mixture sorted according to their boiling temperature where A is the light boiler. Applying internal partition walls enables the separation of the same mixtures in only one column shell . This way pure products can also be obtained from side flows. In Figure an example with three dividing walls to separate quaternary mixtures is shown. A distillation column with one dividing wall (to split ternary mixtures) is called simple dividing wall column. If more than one dividing wall is present, the column is called multiple dividing wall column (mDWC). In both cases the internal vapor and liquid flows have to be split at the partition walls. Like this, remixing of intermediate boiling components, which usually occurs in column sequences, is avoided resulting in energy savings. For the simple dividing wall the savings are around 30 % M. A. and for the mDWC up to 50 % .