Second Law Analysis of Conventional and Nonconventional Distillation Sequences

C. G. Hernández-Gaona, J. C. Cárdenas, J. G. Segovia-Hernández, S. Hernández and V. Rico-Ramírez

Abstract
In this work thermodynamic efficiency calculations for the separation of ternary and quaternary mixtures of hydrocarbons in conventional, heat-integrated and thermally coupled distillation sequences, are presented. When a ternary mixture was considered, energy savings achieved in the thermally coupled and heat-integrated distillation sequences ranged from 10 to 40 % in comparison to conventional direct and indirect distillation sequences. Regarding thermodynamic efficiency, thermally coupled and heat-integrated distillation sequences presented the highest values in almost all of the cases considered. As the analysis was extended to the separation of quaternary mixtures, the thermally coupled distillation sequences show acceptable energy savings and the thermodynamic efficiency of a thermally coupled distillation sequence linked to a side stripper and a side rectifier was better than that obtained in the conventional distillation sequence; however, the second law efficiency for the thermally coupled distillation with prefractionator (Petlyuk-type column) was low, when compared to that of the conventional distillation sequence for a mixture with low content of intermediate components. When both thermal links and heat integration were introduced in the thermally coupled distillation sequence with side columns, the energy saving and second law efficiency increased too.

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Keywords
Thermally coupled distillation, heat-integrated distillation, second law efficiencies