(474b) A New Minimum Reflux Calculation Method for Multiple-Feed Distillation Columns Distilling Ideal Multicomponent Mixtures

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 have more than one feed streams. These multiple feed 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 have an efficient and accurate method to determine the minimum reflux ratio for multiple feed 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 multiple feed distillation column distilling ideal multicomponent mixtures. This method uses and extends the classic Underwood’s method, which is a short-cut method originally developed to describe simple columns with exactly one feed. The geometric interpretations of the Underwood’s equations are exploited, which provide some valuable insights in calculating the minimum reflux ratio for a multiple feed column. Compared with other existing short-cut based approaches, our method does not require guessing of pinch location or controlling split, which may lead to incorrect solutions.

Through several case studies for ternary separation involving columns with two and three feed streams, 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 envision that this approach can be further extended to a general complex column with multiple feeds and multiple side-product withdrawals.