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Abstract.
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.