464407 Potentials of Significant Energy-Saving Via Hybrid Extraction−Distillation Separation System: N-Propanol Dehydration
Potentials of Significant Energy-Saving via Hybrid Extraction−Distillation Separation System: n-Propanol Dehydration
Wei-Lun Chang, Bor-Yih Yu and I-Lung Chien*
Department of Chemical Engineering, National Taiwan University, Taipei 10617, Taiwan
*Corresponding Author¡¦s E-mail: email@example.com
Keywords: n-Propanol; Dehydration; Hybrid Separation Process; Extraction; Azeotropic Separation; Design and Control
n-propanol is one of the common solvents widely used in chemical processes. However, it forms a minimum-boiling azeotrope with water that makes the dehydration impossible with only one distillation column. There are two recent literatures to tackle this separation task via extractive distillation, one using NMP [An et al., 2015] and the other one using ethylene glycol [Pla-Franco et al., 2015] as entrainer. Unfortunately, the enhancement on relative volatility of n-propanol over water is not that large compared to other extractive distillation systems, resulting in a large entrainer-to-feed ratio and also high energy consumption.
Hybrid extraction−distillation process was reported to be an energy-saving design for pyridine dehydration [Chen et al., 2015]. In this work, this improved separation method is studied for the purpose of n-propanol dehydration. This hybrid process can be viewed as a derivative from heterogeneous azeotropic distillation method, because they both require adding an appropriate third component to form a new minimum-boiling heterogeneous azeotrope and large liquid-liquid envelope. The difference is that the needing of a pre-concentration column in heterogeneous azeotropic distillation is replaced in the hybrid process with an extraction column. Note that the solvent-rich stream into the extraction column comes from the organic reflux of the heterogeneous overhead from the azeotropic column. Most importantly, the solvent flow rate is an additional degree-of-freedom (DOF) that can significantly affects the energy consumption in the hybrid process, while this DOF is not available for heterogeneous azeotropic distillation. In this proposed hybrid process, DIPE is selected as the extraction solvent considering its separation performance on extraction, heat of vaporization, density, and less toxicity. Compared to the extractive distillation with NMP, steady-state results show that this improved design saves at least 64.1% reboiler duty using also much less column stages.
As for the dynamic control of this kind of hybrid separation process, Chen et al. (2015) made a trade-off between optimal steady-state design and dynamic controllability to reject the feed flow rate or composition disturbances with a fixed solvent-to-feed ratio. In this work, a novel control strategy is proposed based on closed-loop and open-loop sensitivity tests. Here, an adjustable solvent-to-feed ratio during dynamic control allows the operation of the optimal steady-state design instead of a trade-off design. Dynamic simulation results show that both n-propanol and water products can still be maintained at high-purities despite large variations in feed flow rate and feed composition changes.
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Y. C. Chen, K. L. Li, C. L. Chen, I. L. Chien; Design and Control of a Hybrid Extraction−Distillation System for the Separation of Pyridine and Water; Ind. Eng. Chem. Res., 54, 7715−7727, 2015.