(145f) Optimal Operation of Middle-Vessel Batch Distillation

Batch distillation, due to its flexibility, has received renewed interest in the context of smart manufacturing. However, distillation operation has low thermal efficiency and thus energy integration plays a key role in improving its sustainability index. This work focuses on the operational aspects of one such integrated configuration used for the separation of a ternary mixture. Middle vessel batch distillation (MVBD), as shown in the figure below, consists of two column sections separated by a (middle) vessel [1]. It works on the principle of multi-effect operation wherein vapor from one section (effect) is used to the drive the subsequent effect, thus reducing the overall energy consumption. The entire system is operated under total reflux and at the end of the batch, the three products are accumulated in the three vessels (reflux drum, middle vessel and reboiler).

Optimization of batch distillation can be performed to maximize productivity (or separation efficiency) or minimize energy consumption (or maximize energy efficiency). Previously, it has been shown that these objectives conflict each other and can lead to practically limiting designs [2]. In this work, a new performance indicator, overall performance index (OPI), is defined for a MVBD column to capture the trade-off between separation and energy efficiency. The optimal operation policy for MVBD is then formulated to maximize this index. Specifically, the trajectories of the two reflux flows (R₁ and R₂) and reboiler duty (Q_reboiler) are obtained to maximize OPI of a batch. Subsequently, a feedback control system is designed to track this optimal operational policy.

The proposed work helps address key operational issues, such as initial feed distribution and constant versus variable holdup policy. Specifically, physical insights are drawn from the obtained optimal solutions to relate the observed performance with the properties (e.g. relative volatility, relative cost of products, etc.) of the feed. The performance of the MVBD column is compared with the conventional multi-cut operation via simulations to demonstrate the benefits of the proposed work.

References:

[1] Davidyan, A. G., Kiva, V. N., Meski, G. A., & Morari, M. (1994). Batch distillation in a column with a middle vessel. Chemical Engineering Science, 49(18), 3033-3051.

[2] Vibhute, M. M., & Jogwar, S. S. (2020). Optimal operation and tracking control of vapor‐recompressed batch distillation. AIChE Journal, 66(12), e17049.