Elsevier

Fluid Phase Equilibria

Volume 398, 25 July 2015, Pages 5-9
Fluid Phase Equilibria

High-temperature vapour–liquid equilibrium for ethanol–1-propanol mixtures and modeling with SAFT-VR

https://doi.org/10.1016/j.fluid.2015.04.009Get rights and content

Highlights

  • Measurements for water–ethanol system for temperatures between 363 K and 443 K, and pressures up to 1 MPa were obtained.

  • A flow apparatus previously reported was used.

  • Vapour pressure curves for pure fluids were studied.

  • Wagner equation was used to correlate pure fluid vapour pressure data.

  • Statistical associating fluid theory for potentials of variable range (SAFT-VR) was applied with success.

Abstract

Alcohols have a wide use in industry, and some of them are solvents for fats, oils, resins, paints, and nitrocellulose; on the other hand, others find use in the manufacture of perfumes and brake fluids. Mixtures of ethanol with 1-propanol, 1-butanol, or 1-pentanol can be used as fuel oxygenates. In addition these mixtures can be used as cryogenic fluids and as heat reservoir in cryogenic power generation systems. This justifies the importance of the knowledge of thermodynamic properties for these mixtures at various temperatures. However, properties like experimental densities, and vapour liquid equilibrium at high temperatures, for some alcohols mixtures are scarce, and when they exist do not have the claimed accuracy.

This paper reports VLE measurements for the system ethanol + 1-propanol at temperatures and pressures up to 423 K and 1 MPa, respectively, performed at our laboratories. The statistical associating fluid theory for potentials of variable range (SAFT-VR) was used to model the systems and found to accurately reproduce the experimental data.

Introduction

Many engineering problems require the knowledge of several thermodynamic and transport properties, such as heat and mass transfer coefficients, density, heat capacity, viscosity and thermal conductivity. However, the laboratory work behind such measurements is laborious and time consuming. Despite this limitation, these measurements are necessary not only for the design of chemical engineering processes [1], [2], [3], but also to check new theories about the liquid state.

In the last three decades or more, experimental programmes to measure vapour-liquid equilibria (VLE) were developed all over the world. There were numerous apparatus built to fill this need. This has provided the scientific community with a lot of experimental studies at low and room temperatures [4], [5]. However for higher temperatures, the situation is rather different, because the data are scarce, and often of lower accuracy. Nowadays it is possible to acquire more accurate experimental data, by using more convenient experimental techniques and more accurate measurements of pressure, temperature and composition.

A VLE experimental program at high temperatures was developed in collaboration between the Centre for Molecular Sciences and Materials (CCMM-FCUL) and the Experimental Thermodynamics Laboratory (CQE-IST). We have studied systems like water + ethanol [6] and water + 1-propanol [7]. The production of biodegradable fuels needs accurate measurements on water + alcohol systems, such as methanol, ethanol, propanol, butanol (linear and branched) at high temperatures. On the other hand, biodegradable fuels start to play an important role in the world global economy, as oil prices are increasing steadily and the search for alternative energies and new fuels is vital for the sustainability of the world economy.

In this work we completed the cycle of binary mixtures with water, ethanol and 1-propanol, by studying the binary mixture ethanol + 1-propanol. To the best of our knowledge, there is no reliable data for this system at high temperatures. The vapour pressures of ethanol and 1-propanol were analyzed using the Wagner equation [8].

As in previous work, we have used the statistical associating fluid theory for potentials of variable range (SAFT-VR) equation to model the experimental results. SAFT is a molecular-based equation of state (EOS), which explicitly takes into account the contribution of molecular structure, i.e. shape, association and polarity [9], [10], [11]. Within the SAFT framework, the free energy of the system is written as the sum of separate contributions:A=Aideal+Amono+Achain+Aassocwhere Aideal is the ideal free energy, Amono the contribution to the free energy due to the monomer segments, Achain the contribution due to the formation of bonds between monomer segments and Aassoc the contribution due to association interactions. The different versions of SAFT essentially correspond to different choices for the monomer fluid and different theoretical approaches to calculate the monomer free energy and structure. In this work, we have used, SAFT-VR, in which molecules are described as homonuclear chains of spherical monomers interacting through a potential of variable range, in this case a square well [12], [13].

The SAFT-VR equation has been successfully used to describe the phase equilibria of a wide range of industrially important systems, from alkanes of low molecular weight to simple polymers, perfluoroalkanes, refrigerants, carbon dioxide, water, electrolyte solutions, etc. (see Ref. [14] and references therein).

Systems involving alcohols have been successfully modeled with SAFT-VR, especially focusing on the phase behaviour of (water + alcohol) mixtures. Patel [15] studied the phase equilibria of (water + alcohol) binary mixtures and of ternary (water + alcohol + salt mixtures), including in the theory the description of electrolytes. Mac Dowell et al. [16] modeled the (water + ethanol) mixture as a starting point for developing an accurate theoretical model of ethanolamine. Very recently, Schreckenberg et al. [17] published a study on the thermodynamic and solvation properties of electrolyte solutions with SAFT-VR, including (water + alcohol) mixed solvents. In these studies, hydrogen bonding of alcohols was accounted by three association sites in each alcohol molecule, one representing the hydroxyl hydrogen atom and the other two, the oxygen unbound electron pairs; this scheme, in which association is only permitted between unlike sites, is usually referred as 3B.

Regarding mixtures of alcohols, de Villiers et al. [18] used a different version of SAFT (sPC-SAFT) to study the behaviour of mixtures of alcohols with alkanes, water and other alcohols. They proposed a new association scheme for the alcohols, which improves the description of water + alcohol mixtures without compromising the performance of the EOS for the other systems. The same group has very recently extended this line of work, explicitly including the effect of polarity in the theory [19]. In this work the (ethanol + 1-propanol) mixture was treated using the model presented by Mac Dowell et al. [16] for ethanol and for 1-propanol that developed by Patel [15].

Section snippets

Experimental

VLE measurements were carried out in the temperature range 363 K to 443 K and pressures up to 1 MPa, using a flow apparatus [6] which is an improved version of that described in a previous paper [20]. The main modifications were related with the pressure measurements and their uncertainty. The equilibrium pressure is now read using two pressure transducers (GE Druck, Model UNIK5000) with ranges of 0–0.4 and 0–1.7 MPa with uncertainties of 0.0002 MPa and 0.0009 MPa respectively, with two digital

Theory

A quantitative interpretation of the results was performed using the molecular-based statistical associating fluid theory for potentials of variable attractive range, SAFT-VR. Only a very brief description of the model will be given here and the full expressions for the SAFT-VR equation of state can be found in the original references [12], [13].

In the SAFT-VR approach, molecules are modelled as chains of m tangentially bonded hard spherical segments, with the attractive dispersive interactions

Results and discussion

In Table 3 our experimental measurements for mixtures of ethanol and 1-propanol of different composition at the temperatures 403.2, 413.2 and 423.2 K are presented. The theoretical predictions from the SAFT-VR EOS are presented in Fig. 1 along with the experimental data from this work. As can be seen, the overall agreement between the theoretical predictions and the experimental results, is very good, although the vapour pressure of pure ethanol-rich mixtures is overestimated, deviating 2.3 to

Conclusion

Experimental VLE data were obtained for the ethanol+1-propanol system for temperatures between 403.2 and 423.2 K, and pressures up to 1 MPa using a flow apparatus.

A coherence test was performed to the pure alcohols vapour pressures using the Wagner equation. The results show a good agreement between our data for the pure alcohols, previous experimental determinations and the recent equation of state of Schroeder et al. for ethanol. The results also show that Wagner equation is as said before one

Acknowledgments

One of us (AFC) is grateful to Fundação para a Ciência e a Tecnologia (Portugal) for financial support under PhD grant SFRH/BD/48596/2008. PM acknowledges financial support from Fundação para a Ciência e Tecnologia, under Post-doc grant SFRH/BPD/81748/2011.

This work was partially supported by project PEst-OE/QUI/UI0536/2011-14 and PEst-OE/QUI/UI0100/2011-14.

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