Experimental and modeling vapor-liquid equilibrium for the binary systems {ethanol(1) + glycerol(2)} and {tert-butanol(1) + glycerol(2)} at high pressures

https://doi.org/10.1016/j.jct.2018.03.023Get rights and content

Highlights

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    High-pressure VLE data for alcohols (1) + glycerol (2) system.

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    Asymmetric binary systems modeling using PR-vdW2.

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    Determination of the operating conditions in the process of glycerol etherification.

Abstract

This work reports the experimental vapor-liquid equilibrium data of {ethanol(1) + glycerol(2)} and {tert-butanol(1) + glycerol(2)} systems. Boiling pressures were measured using a static synthetic method over the temperature range of 423–523 K. The experimental data were fitted using the Peng Robinson equation of state (PR-EoS) coupled to classical van der Waals mixing rules, quadratic regarding the composition. Results presented in this work may be relevant in process design of glycerol etherification.

Introduction

Biodiesel has been seen as the most important biofuel supplied in the transportation sector due to its improvements and sustainability issues [1]. The most used method to produce biodiesel is the transesterification of vegetables oils using short chain alcohols which generates glycerol as by-product [2]. A large increase in the amount of glycerol produced in the world has been observed, causing its output to become greater than the demand, and consequently affecting the glycerol market [3]. If biodiesel is produced on a large scale, new outlets to convert the excess glycerol into high-value-added products are needed since the current application of glycerol is mainly limited to cosmetics and pharmaceuticals [4].

The transformation of glycerol into different chemicals can occur by a series of reactions such as hydrogenolysis, etherification, oxidation, polymerization, dehydration and acetylation. In particular, the etherification is the most promising among these processes because it can convert glycerol into oxygenated additives for diesel fuel. The addition of oxygenated additives to diesel could enhance the combustion efficiency in internal combustion engines with a significant reduction of pollutant emissions [5], [6].

The most used route to get these oxygenated compounds is the glycerol etherification with alcohols or short-chain olefins which generate mono, di- and tri-alquil ethers of glycerol [4]. The alcohols that stand out for this purpose are tert-butanol and ethanol. Glycerol etherification with ethanol have been reported on literature in previous studies and presented low values of yield to ethers even using long reaction times [7], [8], [9]. Klepacova et al. [10], Kiatkittipong et al. [11], Frusteri et al. [6], Gonzalez et al. [12] and Srinivas et al. [4] studied the transformation of glycerol with tert-butanol into ethers over a series of catalyst. Frusteri et al. [6] reported the positive influence of pressure and temperature on product selectivity. It is well known that the problems related to mass transfer between catalyst and reagents are minimized in a reaction under pressure. Furthermore, there is an enhancement in contact among reagents.

The knowledge of phase behavior for the mixtures containing alcohol and glycerol under high pressure is a key step in the development and optimization of etherification process using pressurized reagents. Experimental phase equilibrium data for binary systems {ethanol(1) + glycerol(2)} and {methanol(1) + glycerol(2)} were investigated by Shimoyama et al. [13] but, the measurements were restricted to temperatures near the critical temperatures of the alcohols that do not meet the conditions of the etherification reaction. There are no experimental phase equilibrium data for {tert-butanol(1) + glycerol(2)} system at high pressures.

In this context, the aims of the present work were to measure phase equilibrium data for the binary systems {ethanol(1) + glycerol(2)} and {tert-butanol(1) + glycerol(2)} at temperatures ranging from 423 to 523 K and to correlate data using the Peng Robinson equation of state (PR-EoS) [14] coupled to van der Waals mixing rules, quadratic regarding the composition.

Section snippets

Materials

Table 1 presents some information about the chemicals used in this work. All chemicals were used without further treatment.

Apparatus and experimental procedure

The experimental data of {ethanol(1) + glycerol(2)} and {tert-butanol(1) + glycerol(2)} phase equilibria were conducted by static synthetic method. The experimental apparatus and procedure have been used in previous studies [15], [16], [17]. The experimental apparatus consists of a high-pressure syringe type pump (ISCO 260D), a volumetric cell with side and front windows of

Thermodynamic modeling

In order to adjust the experimental phase equilibrium data, the PR-EoS [14] coupled to classical van der Waals mixing rules was used. The expression for PR-EoS explicit for pressure is as follows:P=RTv-b-av(v+b)+b(v-b)Where, P is the absolute pressure, T is the absolute temperature, v is the molar volume, a and b are the attractive energy parameter and the repulsive co-volume parameter respectively, and R is the universal gas constant. The attractive parameter a and the repulsive parameter b in

Results and discussion

Table 3, Table 4 exhibit the experimental data for the binary systems {ethanol(1) + glycerol(2)} and {tert-butanol(1) + glycerol(2)}, respectively. These tables present information on phase transition at five different temperatures (423–523 K). The experimental data refer to vapor-liquid coexistence curve where one can notice the occurrence of biphasic vapor-liquid transition (in these experiments only bubble point transition type).

The interaction parameters that minimize the objective function

Conclusions

In this study, the binary systems {ethanol(1) + glycerol(2)} and {tert-butanol(1) + glycerol(2)} were studied at different temperatures (423–523 K) using a static synthetic method. Results presented in this report can be useful in the development and improvement of etherification process using pressurized reagents. The experimental data obtained in this work provides the knowledge of the homogeneous region limits (liquid region, where the chemical reactions are carried out in an esterification

Acknowledgements

The authors are grateful to CNPq, FAPEMIG (Project # APQ-02263-15) and CAPES for the financial support and scholarships.

References (18)

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