(449m) 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: ilungchien@ntu.edu.tw

Keywords: n-Propanol; Dehydration; Hybrid Separation Process; Extraction;
Azeotropic Separation; Design and Control

Abstract

        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.

References:

Y. An, W. Li, S. Huang, J.
Ma, C. Shen, C. Xu; Design/optimization of energy-saving extractive
distillation process by combining preconcentration column and extractive
distillation column; Chem. Eng. Sci., 135, 166−178, 2015.

J.
Pla-Franco, E. Lladosa, S. Loras, J. B. Monton; Approach to the 1-propanol
dehydration using an extractive distillation process with ethylene glycol; Chem. Eng. and Process., 91, 121−129,
2015.

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.