Are thermally coupled distillation columns always thermodynamically more efficient for ternary distillations?

The thermodynamic efficiency of five ternary distillation configurations to distill ideal saturated liquids into pure product streams are calculated and compared. A striking result of this study is that for the fully coupled column (Petlyuk) configuration, which is known to have the lowest heat demand for ternary distillation, the range of values of feed composition and relative volatilities for which it is the most thermodynamically efficient configuration is quite limited. Among the three thermally coupled column configurations, the side-rectifier and side-stripper configurations tend to provide the most efficient configuration more often than the fully coupled configuration. Generally, the modified direct and indirect split configurations together provide the most thermodynamically efficient configuration for more feed compositions and relative volatilities than do the three thermally coupled column configurations. The high thermodynamic efficiency of these two configurations is primarily due to their ability to either accept or reject heat at the intermediate temperatures of binary mixtures.