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A.C. Christiansen and S. Skogestad, ``Energy savings in integrated Petlyuk distillation arrangements: Importance of using the preferred separation'' AIChE Annual Meeting, 16-21 Nov. 1997, Los Angeles, Paper 199d.

Prefractionator arrangements are often preferred from an energy point of view when separating ternary mixtures. The prefractionator performs a separation between the heaviest and lightest component, whereas the intermediate component distributes to both products. The energy usage in the prefractionator itself has a very sharp minimum for a particular distribution, which is the "preferred separation" of Stichlmair (1988). On the other hand, the energy usage in the downstream main column has a minimum when the two parts of the column, above and below the side stream, are "balanced". In the paper we derive simple analytic expression for the total energy usage of the two-column sequence as a function of the separation in the prefractionator. We find that although the preferred separation is optimal, at least for sharp splits in infinite columns, the energy usage is almost the same for any separation between the "preferred" and the balanced". The same results are shown numerically to hold for columns with infinite number of stages and non-sharp separation, as well as when the prefractionator and main column are directly coupled, as in the Petlyuk arrangement. Finally, some implications for the operation and control of such columns are discussed.