A literature review of three-phase distillation calculation procedures highlights the large number of efforts that have been made to account for the appearance of a second liquid phase on a distillation tray. The numerical problems due to the introduction of a second liquid phase during an iteration scheme are summarized in a table containing the known 15 solution sets. Many of these solutions contain a liquid-phase-splitting algorithm. A new algorithm based on Michelsen's phase-stability test and phase-splitting algorithm has been developed. New procedures in the iteration method and to identify liquid-phase instability have been produced and verified. These new procedures use analytical partial derivatives which can be determined from modern thermodynamic models to describe nonideal liquid-phase behavior. The predicted separation performance of a three-phase azeotropic distillation column using this new algorithm is in good agreement with the experimental data collected in part 1 of this series.

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A new simulation method using an extended phase-stability and phase-splitting algorithm has been programmed, and new three-phase regions are identified in published azeotropic distillation examples.If phase splitting is ignored, then an erroneous composition profile can be produced. The modern thermodynamic models UNIFAC-VLE, UNIFAC-LLE, modified UNIFAC, UNIQUAC, NRTL, ASOG, and modified ASOG have been investigated in the simulation model. The sensitivity of the selection of thermodynamic model on temperature and composition profiles has been studied for a wide range of systems. The changes in the number of theoretical stages required for the separation is dependent on the choice of thermodynamic model. A worked example of ethanol dehydration with 2,2,4-trimethylpentane shows the importance of reflux ratio, feed plate location, and number of stages on the separation. Multiple solutions for azeotropic distillation columns reported in the literature can be rationalized by the inclusion of a phase-splitting algorithm in the simulation model.

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