Key Takeaways & Executive Findings
- •• • The TCED-IDC process achieved a 41.2% reduction in total annualized cost (TAC) and a 50.4% reduction in CO2 emissions compared to the conventional three-column extractive distillation (TCED), demonstrating significant economic and environmental benefits for industrial wastewater treatment. • • Thermodynamic efficiency (η) was improved by 102% relative to TCED, indicating superior energy utilization and reduced irreversibility, which is critical for sustainable process design. • • Ethylene glycol was identified as the optimal extractant based on vapor-liquid equilibrium analysis, achieving product purities of ≥99.9 wt% and extractant recycle purity of ≥99.99 wt%, ensuring high separation performance and solvent recovery. • • The TCED-IDC configuration also outperformed the four-column pre-concentration extractive distillation (FCED) with a 10.5% lower TAC, 13.5% lower CO2 emissions, and 12.1% higher thermodynamic efficiency, highlighting the advantage of process intensification via thermal coupling.
Abstract
The separation of multicomponent azeotropic mixtures remains a persistent challenge in industrial wastewater treatment due to complex phase equilibrium behavior involving minimum/maximum boiling azeotropes and liquid-liquid phase separation. Pharmaceutical wastewater often contains a ternary mixture of acetonitrile, n-propanol, and water, which exhibits significant non-ideality and multiple azeotropic points, including binary azeotropes for acetonitrile-water, n-propanol-water, and acetonitrile-n-propanol pairs, as well as a ternary azeotrope. This study conducted a comprehensive investigation encompassing thermodynamic modeling, solvent screening, process design, and multi-objective optimization. A reliable thermodynamic framework was established using an activity coefficient model (NRTL), validated against experimental data. Systematic analysis of vapor-liquid equilibrium diagrams identified ethylene glycol as the optimal extractant due to its superior selectivity. Three distinct separation processes were developed: a conventional three-column distillation sequence (TCED), a four-column pre-concentration extractive distillation configuration (FCED), and an innovative three-column integrated pre-concentration extractive distillation system incorporating a thermally coupled column (TCED-IDC). Multi-objective optimization using the improved nondominated sorting genetic algorithm (NSGA-II) targeted total annualized cost (TAC), CO2 emissions (ECO2), and thermodynamic efficiency (η), subject to stringent purity constraints (≥99.9 wt% for products and ≥99.99 wt% for extractant recycle). The integrated three-column configuration achieved 41.2% lower TAC, 50.4% lower CO2 emissions, and 102% higher thermodynamic efficiency compared to the conventional TCED process. This integrated pre-concentration extractive distillation process is established as an industrially viable, energy-efficient solution for acetonitrile-n-propanol-water separation, aligning with green chemistry principles.
1. Introduction
The separation of ternary azeotropic mixtures such as acetonitrile-n-propanol-water is a critical bottleneck in pharmaceutical wastewater treatment. Conventional distillation approaches are energy-intensive and often fail to achieve required purities due to the presence of multiple azeotropes and non-ideal phase behavior. Existing commercial processes typically rely on entrainer-based extractive distillation but suffer from high energy consumption and environmental impact, necessitating the development of more efficient and sustainable separation schemes.
This study addresses these limitations by systematically screening solvents and designing an integrated pre-concentration extractive distillation process with a thermally coupled column (TCED-IDC). Through rigorous thermodynamic modeling and multi-objective optimization using NSGA-II, the proposed configuration achieves substantial reductions in total annualized cost and CO2 emissions while enhancing thermodynamic efficiency, offering a viable industrial solution that aligns with green chemistry principles.
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Shoushi BO, Meiyu WANG, Ying LI, Lanyi SUN (2026). Simulation and optimization of pre-concentration extractive distillation for the separation of acetonitrile-n-propanol-water ternary azeotropic system. The Chinese Journal of Process Engineering. https://doi.org/10.12034/j.issn.1009-606X.225195
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Frequently Asked Questions
What are the specific purity constraints for the products and the extractant recycle in the optimized process?
The process must achieve product purities of at least 99.9 wt% for acetonitrile, n-propanol, and water, and the extractant (ethylene glycol) recycle purity must be at least 99.99 wt%. These stringent constraints ensure high-quality separation and efficient solvent recovery, which are critical for industrial viability.
How does the TCED-IDC configuration achieve lower energy consumption compared to conventional extractive distillation?
The TCED-IDC configuration integrates a pre-concentration column and a thermally coupled column, which reduces the number of columns from three to three (compared to four in FCED) and improves heat integration. This leads to a 41.2% reduction in total annualized cost (TAC) and a 50.4% reduction in CO2 emissions relative to the conventional TCED process, primarily due to reduced reboiler duty and improved thermodynamic efficiency.
What is the basis for selecting ethylene glycol as the optimal extractant?
Ethylene glycol was selected based on systematic analysis of vapor-liquid equilibrium diagrams, which showed that it provides the highest selectivity for separating the acetonitrile-n-propanol-water mixture compared to other candidates such as NMP, DMF, and DMSO. Its selectivity enhances the relative volatility of the components, enabling efficient separation with lower energy requirements.
How was the thermodynamic model validated, and what is its reliability for process simulation?
The NRTL activity coefficient model was used and validated against experimental data from NIST. The model accurately predicted vapor-liquid equilibrium behavior, as confirmed by parity plots showing good agreement between experimental and calculated compositions. This validation ensures the reliability of subsequent process simulations and optimization results.
What are the scalability implications of the TCED-IDC process for industrial application?
The TCED-IDC process demonstrates significant improvements in economic and environmental performance, with a 41.2% reduction in TAC and 50.4% reduction in CO2 emissions compared to conventional TCED. These benefits, combined with the use of a common solvent (ethylene glycol) and standard distillation equipment, indicate that the process can be scaled up for industrial wastewater treatment, offering a cost-effective and sustainable solution.
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