(277a) Minimum Reflux Behavior of Multicomponent Mixture Separation Using Complex Distillation Columns

To separate a multicomponent mixture, a series of distillation columns are usually required. Each sequence or arrangement of distillation columns generates a distillation configuration. In many distillation configurations, one or more columns involve more than one feed stream and/or one or more sidedraw streams. These multi-feed, multi-product columns, which are often referred to as complex columns, are very common in many industrial applications, such as multi-effect distillation systems. On the other hand, the minimum reflux ratio of a column is related to its energy consumption and capital cost, and thus is a key parameter in distillation design. Therefore, to design and build energy efficient and cost effective distillation configurations, it is practically very important to an efficient and accurate method to determine the minimum reflux ratio for complex columns.

Despite its practical significance, this problem is intellectually difficult to solve. Most existing approaches involve either rigorous tray-by-tray calculations or iterative guessing of reflux ratio which are computationally expansive. Here, we present a simple and easy-to-use algorithmic method to determine the minimum reflux ratio for a general complex distillation column for ideal or close-to ideal multicomponent mixture separations. The classic Underwood’s shortcut method is applicable for simple columns with exactly one feed and a top and a bottom product. Our method uses Underwood’s ideas as a starting point and then formulates new concepts and incorporates them to create an algorithm that can solve for minimum heat duty of a distillation column with multiple feeds and multiple products including those from intermediate locations of the column. Compared with other existing shortcut based approaches, our method does not require guessing of pinch location or controlling split, which may lead to incorrect solutions.

Through several case studies, we demonstrate the accuracy and effectiveness of our new approach. Furthermore, we show that this method can be easily incorporated into a global optimization framework to facilitate the calculation of the minimum heat duty requirement for a multicomponent distillation configuration under minimum reflux. Finally, we present some useful heuristics to help industrial practitioners design energy efficient and cost effective distillation systems.