Design and optimization of a high-efficiency distillation process for cellulosic fuel ethanol integrated with thermal coupling and molecular sieve adsorption

Abstract

To address the challenges of high energy consumption and prominent costs in the traditional three-columns distillation process for cellulosic fuel ethanol, a distillation—molecular sieve coupling separation process is proposed. This process integrates a three-column (crude distillation column, first distillation column, second distillation column) system with a 3A molecular sieve adsorption deep dehydration unit. A thermal coupling network is constructed via differential pressure design (steam from medium/high-pressure columns as mutual heat sources, reboiler liquid waste heat for feed preheating), and molecular sieve adsorption conditions are optimized. The study first performs a thermodynamic consistency test on the ethanol—water system, determines optimal non-random two-liquid (NRTL) model binary interaction parameters via experimental data regression for Aspen Plus simulation. Aiming at minimum total annual cost (TAC), Aspen Plus is used to optimize process parameters (theoretical tray number, feed location, reflux ratio, side-draw position, etc.). Economic analysis shows this process reduces CO2 emission costs by 27.56%, TAC by 15.58% (to 5.123 × 106 USD·a-1), and increases ethanol purity to ＞99.6%, providing an effective solution for green, efficient separation.

1. Introduction

Against the backdrop of the global energy structure's accelerated transition towards low-carbon forms, cellulosic fuel ethanol, as a typical second-generation biofuel, has emerged as a vital strategic option for alleviating fossil energy crises and reducing carbon emissions in the transportation sector. It benefits from extensive raw material sources (such as lignocellulosic biomass including straw and wood chips), non-competition with arable land, and high carbon cycle efficiency [1,2]. After undergoing processes like pretreatment, enzymatic saccharification, and fermentation of lignocellulosic materials, the resulting cellulosic fuel ethanol fermentation broth contains complex components, such as water, methanol, higher alcohols, aldehydes, organic acids, and undegraded sugars. High-efficiency separation technologies are indispensable for enhancing ethanol purity [3—5]. Nevertheless, this separation process confronts significant technical challenges. Firstly, ethanol and water form an azeotropic system (azeotropic point at 78.15 °C, with an ethanol volume fraction of 95.57%), making it arduous to produce anhydrous ethanol directly via conventional distillation. Secondly, impurities in the fermentation broth interfere with the mass transfer efficiency during separation and the performance of adsorbents, rendering traditional single separation techniques inadequate to meet industrial requirements in terms of energy consumption, cost, and product stability [6—9].

Traditional separation technologies exhibit pronounced limitations in the purification of cellulosic fuel ethanol. Conventional distillation, constrained by azeotropic equilibrium, can only elevate ethanol concentration to the azeotropic composition, failing to achieve deep dehydration. Although azeotropic distillation was once applied, it has been phased out due to environmental pollution caused by toxic entrainers like benzene [10—12]. The currently widely adopted extractive distillation technology, using low-toxic solvents such as ethylene glycol, still suffers from high reboiler energy consumption (1.8—2.2 MJ·kg—1 ethanol) because of the solvent's high boiling point (197.3 °C) and large circulation volume. Moreover, the high-viscosity solvent tends to cause tray blockages, significantly increasing equipment maintenance costs. The molecular sieve adsorption method, leveraging the preferential water adsorption properties of 3A zeolite molecular sieves (water adsorption capacity 15% to 20% (mass), ethanol adsorption ＜1% (mass)), achieves dehydration through pressure-swing adsorption (PSA) or temperature-swing adsorption (TSA), cutting energy use by 30% to 40% relative to extractive distillation.