In addition to simple distillation columns, complex distillation columns can be considered to help save energy in sequences of distillation columns. These can have more than 2 product streams, or have linking between columns by two-directional flows with one column having no condenser, and/or no reboiler - often referred to as thermally coupled distillation columns.
In sequencing distillation with complex columns, heuristics for simple columns are still available and useful as they provide qualitative guidance with insights; however, extra heuristics for the complex columns are also available, though it is often useful to think about thermally coupled columns outside of these heuristics. The key types of complex columns are:
Side-Stream Columns -
Heuristic 1 - For a pure sidestream product, for a three component mixture, A-B-C, if the feed has B > 50% and and C < 5%, use a vapour sidestream, as B is more volatile than the other dominant component, C, at the draw stage.
Heuristic 2 - For a pure sidestream product, for a three component mixture, A-B-C, if the feed has B > 50% and and A < 5%, use a liquid sidestream, as B is less volatile than the other dominant component, A, at the draw stage
Side-Stripper Srrangement -
Heuristic 3 - For a three component mixture, A-B-C, if the feed has B < 30% and there is more C than A, use a side-stripper.
Side-Rectifier Arrangement -
Heuristic 4 - For a three component mixture, A-B-C, if the feed has B < 30% and there is more A than C, use a side-rectifier.
Pre-Fractionation Arrangement -
Heuristic 5 - For a three component mixture, A-B-C, when the middle-boiling component comprises a large fraction of the feed and αAB is similar to αBC, it is then useful to consider a pre-fractionator arrangement.
Energy savings for using these systems can be examined for a three component system (A,B,C) based on the assumption of ideal separations. The energy use of the complex configuration can be compared to the energy use for the direct or indirect equivalent sequence, the section marked D is where the energy use of the direct sequence is less, and the section marked I is where the energy use of the indirect sequence is less. The graphs below show the percent saving (based on vapour flow rate) for using the complex configuration, and line to represent the relevant heuristics for comparison.