Trade-off assessment between controllability and energy savings in internally and externally heat integrated distillation structures

摘要: Distillation is known as a separation process, which inherently entails high energy consumption. To alleviate this situation, heat integration between vapor streams at high pressure and liquid streams at low pressure have been proposed and extensively researched in the past decades. Most of these studies have been done at steady state, and little has been researched at dynamic state, particularly, when heat integration is done at locations other than condensers and reboilers in the columns. Thus, it is our aim to assess the trade-off between economic and/or energy savings and controllability at an early design stage by combining a design procedure, which minimizes the total annual cost, and an analysis at zero frequency, which calculates the theoretical control properties of distillation columns. The relative gain array (RGA) was used to determine the most appropriate pairing between controlled and manipulated variables while the singular value decomposition (SVD) technique was used to assess the theoretical control properties of conventional and heat-integrated structures. A ternary mixture with two different feed compositions was evaluated, the results showed that for an almost equimolar feed composition, the controllability can be improved by sacrificing the cost and energy savings if the number of intermediate heat exchangers is decreased while for a feed composition high in the intermediate component, the distillation column with the highest energy savings also attained the best controllability performance.