Vapor and liquid split flexibility in dividing wall columns in relation to the theoretical stage allocation

[Display omitted] •Dividing wall columns have flexibility of vapor and liquid split in operation.•Flexibility equals multiplicity of energetically optimal specifications.•Extent of flexibility is strongly dependent on the stage allocation.•Non-optimal stage allocation can restrict flexibility significantly.•Optimal stages in dividing wall columns can be estimated with Fenske equation. Dividing wall columns operated close to the energetic optimum can achieve their product specification within finite ranges of the liquid and vapor split. This split flexibility is important because it enables a stable operation in practice. The impact of non-optimal stage allocations on the split flexibility, which has not been treated satisfactorily in the literature before, is investigated in this article. It is found that if the stages are non-optimally distributed over the column sections and if operation is nearly energetically optimal, the originally high split flexibility at infinite overall number of stages becomes strongly limited with decreasing stage number. The flexibility can be regained with a much higher energy input or a more suitable stage allocation. In this context, the applicability of the Fenske equation to determine the minimum stage number of a dividing wall column is proven for the employed mixtures.