Densities and derived thermodynamic properties for the (n-heptane + n-octane), (n-heptane + ethanol) and (n-octane + ethanol) systems at high pressures

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Highlights

  • Densities of n-heptane, n-octane, ethanol and their mixtures at high pressures.

  • The high pressure density modeling using the modified Tait equation.

  • Calculation of the isobaric thermal expansion coefficient and the isothermal compressibility.

  • Positive VE of n-heptane + ethanol and n-octane + ethanol mixtures.

Abstract

This work reports densities for the pure n-heptane, n-octane and ethanol at temperatures from (288.15 to 413.15) K and at pressures up to 60 MPa. The same properties were determined for the binary mixtures n-heptane + n-octane, n-heptane + ethanol and n-octane + ethanol over the temperature range (293.15–373.15) K and at pressures ranging from (0.1 to 40) MPa. Highly precise densimeter DMA HP was used for density measurements. Experimental values of densities were fitted by the modified Tammann–Tait equation. Additionally, derived volumetric properties, e.g. excess molar volumes, isothermal compressibilities and isobaric thermal expansion coefficients were calculated. The effect of pressure and temperature on the measured and derived properties was discussed.

Introduction

In order to reduce the greenhouse gases, especially CO2 emissions, the incorporation of non-fossil compounds in automotive fuels is of high interest nowadays. Worldwide, mixtures of ethanol and conventional gasoline are used due to many positive aspects of ethanol application: it has a good “well to tank” CO2 emission balance since it is extracted from the biomass, high octane number (108), high heat of vaporization, broader flammability limits and higher flame speeds [1], [2]. On the other hand, linear alkanes are typical gasoline constituents so thermodynamic and mechanical properties of liquid alkane + alcohol or alkane + alkane mixtures are of great interest, especially at elevated pressure and temperature conditions. From p, ρ, T, x data of liquid mixtures valuable derived properties can be calculated, e.g. isothermal compressibilities and isobaric thermal expansion coefficients. All these properties are essential for the design, operation, control and optimization of industrial processes.

This work presents a part of our ongoing research related to the determination of densities and derived thermodynamic properties at high pressures for pure compounds, binary and multicomponent mixtures of gasoline constituents and of biodiesel [3], [4], [5]. Here, results of the density measurements of pure n-heptane, n-octane and ethanol at temperatures from (288.15 to 413.15) K and at pressures up to 60 MPa are reported. Also, densities of binary mixtures (n-heptane + n-octane), (n-heptane + ethanol) and (n-octane + ethanol) for the mole compositions about 0.25, 0.50 and 0.75 were determined over the temperature range (293.15–373.15) K at pressures ranging from (0.1 to 40) MPa. Modified Tammann–Tait equation was applied to correlate density data. From the measured densities derived thermodynamic properties were calculated and analyzed, excess molar volumes, VE, isothermal compressibilities, κT, and isobaric thermal expansion coefficients, αp.

Densities of the n-heptane + ethanol and n-heptane + n-octane mixtures have been investigated before [6], [7], [8], [9], [10], [11] using different measuring techniques, while to the best of our knowledge density data of n-octane + ethanol at high pressures and temperatures have not been published before. Ozawa et al. [6] determined densities of n-heptane + ethanol at pressures up to 196.2 MPa over the temperature range (298.2–348.2) K, while Papaioannou et al. [7] performed measurements at 298.15 K and for pressures up to 33.81 MPa. Ulbig et al. [8] reported excess volumes of the n-heptane + ethanol system at 298.15 K and at pressures up to 60 MPa. Later on, Dzida and Marczak [9] obtained densities for the same system from speeds of sound measurements at pressures up to 90 MPa within the temperature limits (293 and 318) K. Watson et al. [10] determined densities of seven ethanol + n-heptane mixtures in the range (293.15–333.15) K and up to 65 MPa using vibrating-tube densimeter. Abdulagatov and Azizov [11] reported densities of three n-heptane + n-octane binary mixtures over the temperature range (293–557) K and at pressures up to 10 MPa.

This work provides high pressure density data for the n-octane + ethanol system that has not been analyzed in the literature. Furthermore, densities and derived property data for the n-heptane + ethanol and n-heptane + n-octane mixtures reported here cover some temperature or pressure conditions that have not been studied in the literature.

Section snippets

Materials

The chemicals in this work were used as received without further purification. n-Heptane was supplied from Sigma–Aldrich, while n-octane and ethanol were the products of Merck, all having the mass fraction purities ⩾0.99. Chemicals were degassed prior to measurements. Details of chemicals including their certified purities are listed in Table 1. Table 2 gives comparison of densities of the pure compounds, measured by using DMA 5000 densimeter [12], with literature values at atmospheric pressure.

Measured densities

Densities of the pure n-heptane, n-octane and ethanol were determined for temperatures between (288.15 and 413.15) K and at pressures ranged from (0.1 to 60) MPa. The experimental results are presented in Table 3 and some selected results are shown graphically in Figs. S1 and S2. Densities of the pure compounds measured here are compared with literature values. An absolute average percentage deviation (AAD(%)=100/Ni=1N|(ρilit/ρiexp-1)|) and maximum percentage deviation (MxD(%)=max(100|(ρilit/ρi

Conclusion

Densities, excess molar volumes, isothermal compressibilities and isobaric thermal expansion coefficients for the pure n-heptane, n-octane and ethanol and for binary mixtures of n-heptane + n-octane, n-heptane + ethanol and n-octane + ethanol were determined. Experiments were conducted at pressures up to 60 MPa for the pure compounds and up to 40 MPa for the binary mixtures. Experimental densities were fitted by the modified Tammann–Tait equation. Very good agreement between experimental and

Acknowledgments

The authors gratefully acknowledge the financial support received from the Research Fund of Ministry of Education and Science (projects No 172063), Serbia and the Faculty of Technology and Metallurgy, University of Belgrade.

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