Vapor–liquid equilibrium of the ethanol + 3-methyl-1-butanol system at 50.66, 101.33 and 151.99 kPa

Abstract

In this work, the isobaric vapor–liquid equilibrium (VLE) for the ethanol (1) + 3-methyl-1-butanol (2) system at three different pressures is reported. Measurements were done in a dynamic equilibrium cell (Fischer Labodest VLE 602) at 50.66, 101.33 and 151.99 kPa. The experimental data were tested for thermodynamic consistency with total area, point, Van Ness, and Herington tests. Herington test was slightly modified considering the $\Delta H_m/\Delta G_m^E$ relation. Data were correlated using the NRTL and UNIQUAC activity models. NTRL showed the lowest deviation with respect to the experimental data at the three operating pressures. Quality of regressed parameters for this system was tested comparing results with isothermal equilibrium data reported elsewhere.

Introduction

In recent years due to the current efforts for making biofuels as an alternative for fossil fuels, ethanol industry and its by-products have been growing very fast. Among them, fusel oil obtained during bioethanol distillation can be considered a valuable raw material for several by-products of industrial interest. It is mainly composed of isoamyl alcohol, water, isobutanol, ethanol and other short-chain alcohols (C2C5). Yields of fusel oil in an industrial plant may vary between 1 and 11 L/1000 L of ethanol produced (absolute basis) depending on the substrate, nitrogen-containing additives and conditions used during fermentation, and also on distillation conditions [1]. Regarding to the composition, Patil et al. [1] presents a characterization of fusel oil in terms of two main fractions: a low boiling fraction (LBF) and a high boiling fraction (HBF). The first one corresponds to 95–98% (v/v) and is mainly composed by isoamyl alcohol (60–80%), ethanol (5–15%) and water (8–20%) [1]. The other fraction is composed by fatty acids and their esters, some pyrazines and unsaponificable material. Due to the low yield and the difficulties related to the isolation of pure alcohols from fusel oil, it is traditionally burned in steam boilers or blended with the fuel ethanol. However, fusel alcohols can be used as low cost and renewable sources for the production of biosolvents, flavoring agents, pharmaceuticals and plasticizers. These higher added-value products have the potential for improving the overall economics of the bioethanol plants [2]. According to this, it is expected that in the future, bioethanol plants will be able to tune the fermentation process in order to generate the ethanol by-products profile that maximizes profits depending on market demands.

In order to develop separation and purification strategies for isoamyl alcohol from fusel oil, a good representation of vapor–liquid equilibrium in mixtures with water and ethanol is required. Systems containing ethanol, water and other alcohols have been widely studied [3], [4], [5], [6], [7], [8]. Specifically, the system isoamyl alcohol + ethanol has been studied by Martinez et al. [9] and Sanz and Gmehling [10]. Martinez et al. [9] worked on the equilibrium with 2-methyl-1-butanol + ethanol at 33.6, 66.6 and 101.3 kPa in a modified Othmer still, while Sanz and Gmehling [10] worked on isothermal VLE of both isoamyl alcohols, 2-methyl-1-butanol and 3-methyl-1-butanol, determined experimentally by head space gas chromatography. In this work isobaric VLE for ethanol with isoamyl alcohol was measured experimentally using a VLE 602 Fischer Labodest apparatus at 50.66, 101.33 and 151.99 kPa.

In order to validate the quality of data, thermodynamic consistency tests were used. Several consistency tests have been investigated over the past years [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22]. Samuels [11] and Kojima et al. [12] have presented area tests while other authors suggested the use of point tests or tests based on activity coefficients at infinity dilution [15], [16], [17], [18], [19]. Kang et al. [13] presented a wide review of commonly used consistency tests and introduced a data–quality criterion in order to summarize different tests results. Taking into account the above, consistency of experimental data was evaluated using an area test according to the accepted criteria of Kojima, Wisniak, Redlich–Kister [12], [13], [19], [20], [21], [22], point test [15], [16] and Van Ness test [18].