Shortcut methods for the design of heteroazeotropic distillation of multicomponent mixtures

Abstract:
When designing a distillation process for an azeotropic multicomponent mixture, numerous alternative flowsheets and entrainer candidates have to be evaluated in order to determine the most energy-efficient flowsheet. It is obvious that not all alternatives can be covered with simulation studies and a MINLP flowsheet optimization comprising all alternatives would be far too complex to solve. Consequently, shortcut methods are required for a rapid flowsheet evaluation in the conceptual design phase. These shortcut methods need to be based on rigorous thermodynamics, applicable to multicomponent mixtures and automatable for an optimization based process synthesis. The rectification body method (RBM)1 is a powerful shortcut method that meets these requirements for the distillation of homogeneous azeotropic mixtures. However, the RBM cannot be applied for most heterogeneous mixtures since the approximation of column profiles by linear rectification bodies does not hold in this case. The boundary value method (BVM)2 as an alternative shortcut method is in practice restricted to three-component mixtures due to the graphical inspection of intersecting profiles and sensitivity towards trace components in the products. More recently, Lucia et al.3 introduced the shortest stripping line method for the determination of the minimum energy demand for multicomponent separations. In this work, we will present the novel feed angle method (FAM), which combines elements from the RBM, the minimum angle4 and the zero volume criterions5. The FAM is a further development of the feed pinch method (FPM)6 presented earlier. It is applicable to heterogeneous mixtures of any number of components and to sharp as well as non-sharp splits. The FAM is also based on rigorous thermodynamics, insensitive towards trace components and automatable. First the relevant separation pinch points are determined by an initialization step with the RBM. Then the liquid composition on the feed tray is optimized such that the tray above and the tray below the feed tray point towards the respective saddle pinches. This is achieved by a minimization of the angles between the trays adjacent to the feed tray and the respective saddle pinches. The minimum angle then corresponds to a feasible separation with the minimum energy demand. The method is illustrated by several examples of sharp and non-sharp splits for heterogeneous ternary and quaternary mixtures as well as heterogeneous mixtures with more then four components and multicolumn separation processes. Separation feasibility and the minimum energy demand are determined with paramount robustness and efficiency. The performance of the novel method is benchmarked against the alternative methods.