Minimum Reflux Calculation for Non-Sharp Splits In Multicomponent Distillation Systems Using Column Profile Maps

Minimum Reflux Calculation for Non-sharp Splits in Multicomponent Distillation Systems using Column Profile Maps.

Ma, Chan Yee; Abbas, Ronald*; Hildebrandt, Diane

Centre of Material and Process Synthesis

School of Chemical and Metallurgical Engineering

University of the Witwatersrand Johannesburg

Private Bag 3

WITS 2050

Although distillation is a highly energy intensive process, it is the most widely used in industrial separation techniques. Thus, minimizing the energy consumption in a distillation column is of interest, both environmentally and economically. In a distillation process, the minimum energy corresponds to the minimum reflux, or boil-up ratio. Previous minimum reflux calculations, and comparisons to techniques, have relied considerably on the methods proposed by the classical Underwood equations for constant relative volatility systems. Numerous methods exist to calculate minimum reflux for non-ideal or azeotropic mixtures; however, these methods require multiple iteration steps. Column Profile Maps (CPM) offer a range of graphical tools to synthesize and analyse the behaviour of simple, as well as complex, distillation configurations. The eigenvectors associated with CPMs have proven to be highly useful for the analysis and, therefore, synthesis of distillation configurations. The eigenvectors for a CPM are a function of thermodynamics only and are thus fixed for a defined system, and do not vary with reflux. Previous research pertaining to the CPM techniques have shown that the eigenvectors of CPM's can be used to non-iteratively determine the minimum reflux requirement for any zeotropic distillation process relating to sloppy split product specifications. Furthermore this method, called the CPM-E technique, can be used to determine the feasible composition region for product specifications given a particular feed. In this work we investigate and extend the use of this the CPM-E technique to more general systems involving non-sharp splits as well as non-ideal thermodynamics. Though the CPM-E technique can calculate the minimum reflux accurately for sloppy split product specifications, without modification to the technique, it becomes a close approximation, not an exact solution, of the minimum reflux for non-sharp split product specification. This is caused by the disappearance of the co-linearity criteria that exists for sloppy splits. This work proposes the use of other geometrical conditions in combination with CPMs to calculate the minimum reflux.

Keywords: Minimum reflux, non-sharp splits, column profile maps, eigenvectors

*Corresponding author contact details:

Email:  rabbas@dual-h.com