in Wiley InterScience (www.interscience.wiley.com).Hybrid distillation/melt crystallization processes are widely used for the separation of isomer mixtures. The design of a hybrid separation processes is not a trivial task because of several structural and operational degrees of freedom. In this article, a new three-step design approach is proposed to address the above mentioned challenges. In the first step, process alternatives are generated by heuristic rules. In the second step, the generated process alternatives are optimized with respect to their energy consumption by using shortcut methods. In the third step, the most promising alternatives are rigorously optimized with respect to their total annualized costs, whereas the resulting mixed-integer nonlinear programming (MINLP) problem is solved using a modified generalized benders decomposition (GBD) algorithm to take the nonconvexities into account. The feasibility of this design approach is demonstrated by design of a hybrid distillation/melt crystallization process for separation of a ternary isomer mixture. 2008 American Institute of Chemical Engineers AIChE J, 54: [2925][2926][2927][2928][2929][2930][2931][2932][2933][2934][2935][2936][2937][2938][2939][2940][2941][2942] 2008 Keywords: crystallization, distillation, optimization, process synthesis, simulation, process IntroductionThe combination of distillation and melt crystallization for separation of close-boiling isomer mixtures is a well-known example for a hybrid separation process. The hybrid distillaCorrespondence concerning this article should be addressed to M. B. Franke at meikbernhard.franke@bayertechnology.com. American Institute of Chemical Engineers AIChE JournalNovember 2008 Vol. 54, No. 11 2925 tion/melt crystallization process combines advantages of the distillation, which is the working horse of the chemical process industry and the melt crystallization in which very high separation factors per stage can be reached. Simultaneously, the combination of distillation and crystallization overcomes the shortcomings of the individual unit operations, i.e. high energy-consumption at small separation factors and limitation of yield by eutectics, respectively. Several hybrid processes for the separation of terphenyl, 1 xylene, 2,3 dichlorobenzene, 4,5 and diphenylmethane diisocyanate isomers 6 are reported in literature.The detailed design of a hybrid separation process is a challenging task because of many degrees of freedom (DOF) involved (see also Figure 1), which can be divided into three classes:(1) Process structure: type of unit operations included in the flowsheet, number and sequence of unit operations, and the location of recycle streams.(2) Operating point: compositions of the process streams, so-called hand-over purities, which are discussed later in this article. As a general rule, the composition of a component which is separated in the specific unit will be chosen. For example, in Figure 1, the composition of the component P at the top of the distillation column D-1 is chosen ...

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Unter hybriden Trennverfahren versteht man die Verschaltung von mindestens zwei verschiedenen, apparativ getrennten Grundoperationen, die zur Trennaufgabe beitragen. Hybride Trennverfahren werden bei schwierigen Trennungen, wie bei engsiedenden oder azeotropen Gemischen, eingesetzt, wenn eine einzelne Grundoperation, z. B. Rektifikation, Extraktion, Kristallisation, Membranverfahren oder Chromatographie, gar nicht oder nur mit sehr großem Aufwand zum Ziel führt. Aufgrund der Struktur des Hybridprozesses, die das Vorhandensein mindestens zwei verschiedener Grundoperationen und eine entsprechende Anzahl von Rückführungen impliziert, ist die Auslegung komplex, sie wird deshalb z. Z. systematisch in der Forschung untersucht. In dieser Arbeit werden die Anforderungen an eine Auslegungsmethodik für hybride Trennverfahren dargestellt. Es wird eine Methodik vorgestellt, die auf einer detaillierten Modellierung der Grundoperationen und einer simultanen Mehrvariablen‐Optimierung beruht. Die Anwendbarkeit dieser Methodik wird anhand einer Isomerentrennung gezeigt.

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Dividing‐wall columns (DWCs) offer an interesting alternative to conventional distillation towers. Savings of up to 30 % in both energy and capital cost are reported. While these high savings are realistic in some applications, DWCs offer less or no advantage at all in other applications. In any case, it is important to make a fair comparison between the conventional system and the DWC based on an optimized process design. Mixed‐integer nonlinear programming (MINLP), especially the presented modified Generalized Benders Decomposition approach, could be the tool to make this comparison, as shown here.