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The synthesis of the functionally distinct thermally coupled distillation configurations for quaternary mixtures is presented. The concept of the intended individual splits has been presented to formulate a distinct separation sequence, with which a functionally distinct thermally coupled configuration can be generated for a multicomponent distillation. Twentytwo functionally distinct thermally coupled distillation configurations for quaternary mixtures have been identified. They include the possible thermally coupled configurations in the available superstructures for quaternary distillations. Moreover, some new feasible thermally coupled configurations have been found which are not included in the known superstructures. These functionally distinct thermally coupled configurations constitute a unique space with which the other possible thermally coupled schemes can be generated. It provides a more complete search space of the possible alternatives to synthesize the global optimum distillation systems for quaternary separations.

Distillation is the largest energy consumer among process units. A large amount of research work has been done toward improving the energy efficiency of distillation systems either in terms of the design of optimal distillation schemes or improving internal column efficiency. Still, the optimal design and synthesis of multicomponent distillation systems is one of the most challenging problems in process engineering.For multicomponent distillations, the thermal-coupling technique can be used to design distillation systems that have the potential to reduce significantly both energy consumption and capital cost in comparison to the conventional simple column configurations. Since the invention of the thermally coupled distillation column by Wright (1949), numerous studies have been conducted. However, due to the complexity, research on these new configurations is mostly restricted to ternary mixtures. The most commonly studied thermally coupled schemes are the side-stripper (SS), the side-rectifier (SR), and the fully coupled scheme (FC) (the so-called Petlyuk column) (Petlyuk et al., 1965).A few interesting observations have been obtained for ternary thermally coupled schemes. For example, the FC configuration has been proved to have the minimum total vapor flow compared with other thermally coupled schemes (Fidkowski and Krolikowski, 1987). Among the three thermally coupled schemes, over a wide range of relative volatilities and feed compositions, the SR and the SS configurations can have a similar heat demand than the FC configuration (Agrawal and Fidkowski, 1999). It is also recognized that the thermodynamic efficiency of a fully thermally coupled configuration can often be inferior because all of its heat is received at the highest temperature and rejected at the lowest temperature (Carlberg and Westerberg, 1989). Thanks to the intensive research work on the thermally coupled schemes for ternary mixtures, the FC system has been successfully used in many industrial cases in its dividing-wall arrangement (Becker et al., 2001).As for thermally coupled schemes for mixtures with four or more components, there are only a few reports in the literaCorrespondence concerning this article should be addressed to B.-G. Rong ture. These are mostly concerned with finding the possible structures of the thermally coupled schemes. Sargent and Gaminibandara (1976) presented a superstructure for a four-component, fully coupled scheme. Kaibel (1987) and Christiansen et al. (1997) illustrated some distillation columns with vertical partitions for multicomponent separations. From a simple column configuration (SC), Agrawal (1996) presented some observations to generate the special fully coupled schemes with satellite column arrangements for four or more component mixtures. From a known FC configuration of a multicomponent distillation, Agrawal (2000) has illustrated how to draw its thermodynamic equivalent structures. It is interesting to note that the available works on thermally coupled schemes for mixtures with four or more compo...

The article presents an analysis between different possible sequences for the separation of bioethanol from a typical fermentor's stream. The preconcentrator column, necessary to approach the azeotropic composition was included as a fundamental part of the sequences. Starting from a recently proposed four-column configuration in the literature, a full set of alternatives was predicted and explored in detail. Different combinations of partial and total condensers were considered so as to possibly reduce the amount of equipment. It was proved that among all the simple configurations generated, two sequences with three columns are able to consistently reduce the energy demand and the capital costs. The results obtained represent the first step for the further generation of complex configuration sequences.

The thermally coupled distillation flowsheets for multicomponent separations are discussed on the basis of the intended individual splits. Then, the generation of the thermally coupled distillation flowsheets with a lower number of thermal couplings than the partially coupled scheme is presented. A formula is presented to calculate the number of feasible schemes with a lower number of thermal couplings. An integrated framework is formulated for parametric design and synthesis of the optimal thermally coupled distillation flowsheets for multicomponent separations. Based on the results of the calculations, significant insights have been made which are very helpful to the design and synthesis of the optimal thermally coupled distillation schemes for multicomponent separations. It has been found that a thermally coupled scheme with a lower number of thermal couplings could be more advantageous than one with thermal couplings introduced for all of its internal submixtures for a specific simple column configuration.