NET Equations for Distillation Columns

Distillation columns are made to separate at least two different components. There are several different types of columns. The assumption of equilibrium between the liquid and vapour that leave each tray is still in common use to model tray distillation. The work of Krishna and Wesselingh shows, however, that non-equilibrium models give results that are very different from those obtained with the equilibrium assumption. 

A description consistent with NET is the Maxwell-Stefan equations. In the application of Maxwell-Stefans equations, one frequently neglects the heat of transfer, q This assumption may be good for gases. It is not so good for liquid mixtures. According to Section 3, it is likely that the heat of transfer plays an important role in flux equations of interface transport. Olivier showed that failure to include the heat of transfer leads to an error of up to 20% in the heat flux calculated for some typical phase transition conditions. 

With constant fluxes of energy and mass through the interface and adjacent films, the overall resistance coefficients were defined through the parentheses in the following equations  

With these relations, the combined films and interface is regarded as an effective interface . There is no need to assume phase equilibrium between liquid and vapour. The entropy production rate can alternatively be expressed by the measurable heat flux in the vapour and fluxes of mass. - This set of flux equations was used to explain the entropy production in tray distillation columns. However, it has not yet been used for predictive purposes. Much work remains to be done to include these equations in a software that is useful for industrial purposes. 

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