(54f) Control of a Batch Distillation Column

The primary challenge associated with the control of a batch distillation process is that process gain for the overhead product decreases continuously as the overhead product is removed. An analysis of the process gain of a rectifying section of a column using the Smoker equation assuming a binary separation shows that the overhead gain varies somewhat linearly with the composition of the light key in the base of the column. Based on this generalized analysis, an approach for scheduling the gain of the overhead composition controller for a batch distillation column was developed. The process gain at the beginning of the batch and at the end of the batch are measured experimentally. These results are used to estimate the current process gain using the online estimate of the composition of the light key in the column bottom and assuming that the process gain varies linearly with the composition of the light key in the column bottom. The product composition and flow rate are used online to continuously estimate the composition of the light key in the column base. The control configuration used in this study is applied by cascading the overhead composition controller to a tray temperature controller for a selected tray near the upper portion of the column. The gain of the composition controller is adjusted by linearly adjusting the process gain during the production phase while a fixed gain temperature controller is used.

This approach for batch distillation control was tested using a detailed dynamic simulation of a multi-component batch distillation column. The batch column contained ethane, propane, iso-butane, n-butane and n-pentane. The initially an ethane and propane product was produced. Startup of the column under total reflux conditions was consider followed by transition to the production phase for the propane and ethane product. At the end of the ethane and propane production phase, a transition to the production phase for iso-butane was simulated. Finally, the complete production phase for the iso-butane product was simulated. In spite of the fact that the process gain for this system changes by over an order of magnitude, the proposed gain scheduled controller demonstrated very tight composition control for both batch production phases.