The variables used as optimization variables describe the two streams at the bottom, at the top and two streams at the split or feed stage n s (shown in Figure as thick lines). The type of the streams at stage n s vary between the different column sections. In Figure the stage n s corresponds to a stage beneath a dividing wall where the outgoing vapor stream is split (compare Figure ). This is why an outgoing vapor stream and an ingoing liquid stream connect this column section to another column section. If stage n s describes a stage above a dividing wall, it would be an additional ingoing vapor and outgoing liquid stream. Finally, if n s corresponds to the column feed stage there would be no additional vapor stream but only one ingoing liquid stream (the column feed). These streams described by the optimization variables can be considered to be known in this section. To receive all process variables, we need to calculate all inner streams and the temperatures within the stages.