Design and operation of dividing-wall distillation columns. 1. Diminishing the black-hole problem through over-design

For a dividing-wall distillation column (DWDC) with three specifications on its top, intermediate, and bottom products, respectively, owing to the complex interactions between the prefractionator and the main distillation column involved, it is usually infeasible to enhance substantially the composition of the intermediate product from its nominal operating condition and this confines terribly the flexibility and operability of the DWDC. The issue reflects an inherent drawback of the DWDC and is termed the black-hole problem in the current work. In this paper, an attempt is made to diminish the black-hole problem through over-design and the number of stages in each section of the DWDC is employed as decision variable to balance the interactions between the prefractionator and the main distillation column involved during process synthesis and design. Three illustrative examples are used to evaluate the feasibility and effectiveness of the proposed procedure and it is demonstrated that the black-hole problem can be effectively diminished in terms of careful adjustments of the number of stages in each section of the DWDC. The proposed philosophy represents a novel way to balance process design and process flexibility and is considered to be of general significance to the design and operation of the DWDC.