Isothermal (liquid + liquid) equilibrium data at T = 313.15 K and isobaric (vapor + liquid + liquid) equilibrium data at 101.3 kPa for the ternary system (water + 1-butanol + p-xylene)

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Highlights

  • The phase equilibria of the system (water + 1-butanol + p-xylene) have been studied.

  • Isothermal (liquid + liquid) equilibrium data at T = 313.15 K was determined.

  • Isobaric (vapor + liquid + liquid) equilibrium data at 101.3 kPa was determined, too.

  • A heterogeneous ternary azeotrope, not previously referenced, has been found.

  • All experimental data have been correlated with thermodynamic models.

Abstract

The (vapor + liquid), (liquid + liquid) and (vapor + liquid + liquid) equilibria of the ternary system (water + 1-butanol + p-xylene) have been determined. (Water + 1-butanol + p-xylene) is a type 2 heterogeneous ternary system with partially miscible (water + 1-butanol) and (water + p-xylene) pairs. By contrast, (1-butanol + p-xylene) is totally miscible under atmospheric conditions. This paper examines the (vapor + liquid) equilibrium in both heterogeneous and homogeneous regions at 101.3 kPa of pressure. (Liquid + liquid) equilibrium data at T = 313.15 K have also been determined, and for comparison, the obtained experimental data have been calculated by means of several thermodynamic models: UNIQUAC, UNIFAC and NRTL. Some discrepancies were found between the (vapor + liquid + liquid) correlations; however, the models reproduced the (liquid + liquid) equilibrium data well. The obtained data reveal a ternary heterogeneous azeotrope with mole fraction composition: 0.686 water, 0.146 1-butanol and 0.168 p-xylene.

Introduction

Ever since the economic crisis of 1973, due to a sharp rise in the price of oil, and especially during the late 80s, concerns regarding the lack of energy supplies and the effects of man’s use thereof on the environment have taken on greater importance. Various factors, including a reliance on oil producing regions, the inexorable drop in world oil reserves, the rise in carbon dioxide (CO2) air concentration – resulting in an increased global surface temperature (greenhouse effect) – have pushed developed countries toward the goal of reducing their fossil fuel consumption.

To achieve this goal, and because economic development is energy intensive, the only viable alternative appears to be to substitute part of the fossil fuel consumed by other energy sources. In several sectors, such as electricity production, fossil fuels have been substituted by nuclear energy on the one hand, but also by renewable energies such as solar farms or wind generators on the other. However, the transport sector is one of the most dependent on fossil fuels and it is important to note that approximately 33% of total CO2 emitted to the environment by human activities in 2012 [1] came from this sector.

Thus, initiatives to reduce polluting emissions in the transport sector are of key importance if governments are to fulfill their promises [2] regarding the control of CO2 emissions.

Over the short term, substitution of oil by a renewable fuel seems the only viable alternative to achieving the emissions rate goal.

One of the most widely used renewable fuels over the last few years has been ethanol, produced from crop fermentation. Employing ethanol as a fuel is associated with several problems: it has corrosive properties, making it difficult to transport through pipelines; it can damage engines; it is easily hydrated by moisture in the air, which means it has to be handled with care in order to avoid hydration; and it has a lower energy content than gasoline. Finally, the cost of producing it (including agricultural, fermentation and subsequent purification processes) makes it difficult to compete with petrol as a fuel.

An alternative to bioethanol in recent years has been biobutanol. This is because butanol has more desirable properties as a fuel than ethanol (its energy content is 86% of gasoline’s, versus only 67% in the case of the latter) and, in addition, it is not associated with the same problems (as highlighted in the previous paragraph).

To use biobutanol as a fuel, it must be separated from the other substances present during its production, especially water. Several processes, which are more or less energy consuming, can be used for the purpose of purifying biobutanol. If it is intended to be used as a gasoline component, a possible technique to accomplish this is azeotropic distillation, using gasoline components as entrainers – to obtain a blend of butanol and gasoline that is very low in water content.

To follow previous studies [3], [4] on the viability of using hydrocarbons as entrainers in the dehydratation of butanol, it would be useful to obtain experimental data on the (vapor + liquid), (liquid + liquid) and (vapor + liquid + liquid) equilibria for another hydrocarbon, such as p-xylene. The purpose of this would be to determine the ternary system (water + 1-butanol + p-xylene), if p-xylene were the hydrocarbon to be used as entrainer in a separation step.

(Water + 1-butanol + p-xylene) is a type 2 heterogeneous ternary system with partially miscible (water + 1-butanol) and (water + p-xylene) pairs. However, the pair (1-butanol + p-xylene) is totally miscible under atmospheric conditions. The present paper is concerned with the determination of the (vapor + liquid) equilibria in both heterogeneous and homogeneous regions at 101.3 kPa, and the (liquid + liquid) equilibrium data at T = 313.15 K.

Section snippets

Chemicals

Ultrapure water, prepared using a MiliQPlus system, was employed in experiments The rest of the chemicals were used as supplied. 1-Butanol was provided by Merck at a chemical purity higher than 99.5%. p-Xylene was provided by Merck at a chemical purity higher than 99%. The internal standard 2-propanol was provided by Merck at a purity of more than 99.8%. The moisture content of all compounds was measured by the Karl Fisher titration technique and was found to be 640 ppm, 290 ppm and 530 ppm for

(Liquid + liquid) equilibrium determination

The experimental procedure that was followed to obtain the (liquid + liquid) equilibrium data can be found in a previous paper [3]. However, for the sake of convenience, the most important aspects of it are reproduced here:

Mixtures of water, 1-butanol and p-xylene of known mass were put inside glass tubes and sealed with septum caps. These tubes were then introduced in a thermostatic bath to maintain their temperature constant at T = 313.15 K. After shaking them, their contents were allowed to

(Liquid + Liquid) equilibrium results

The results obtained from the (liquid + liquid) equilibrium experiments at T = 313.15 K are presented in table 1, and plotted as a ternary phase diagram in figure 1. For comparison, figure 1 also includes the (liquid + liquid) equilibrium data obtained by Letcher et al. [5] at T = 298.15 K. As can be seen, the size of the heterogeneous region of this system does not vary with rising temperature in the range of temperatures studied.

The obtained data has been correlated by means of two thermodynamic

Conclusions

The (liquid + liquid) equilibrium of the (water + 1-butanol + p-xylene) system has been determined at T = 313.15 K. Contrary to the data and findings of other authors, the present authors have found this equilibrium to be little affected by temperature. In addition, the (vapor + liquid) equilibria of both the homogeneous and heterogeneous regions have been determined experimentally. The observed behavior of these equilibria implies that the vapor in equilibrium with the liquid phases in this system tends

Acknowledgment

The authors thank the DGICYT of Spain for the financial support of project CTQ2009-13770.

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