Compressed liquid densities for the (n-heptane + n-decane) and (n-octane + n-decane) systems from T = (313 to 363) K

doi:10.1016/j.jct.2011.08.016

The Journal of Chemical Thermodynamics

Volume 44, Issue 1, January 2012, Pages 133-147

https://doi.org/10.1016/j.jct.2011.08.016 Get rights and content

Abstract

Densities (p, ρ, T, x₁) of two binary n-alkane systems are reported from T = (313 to 363) K in the compressed liquid phase up to 25 MPa over the whole range of composition. The binary mixtures {x₁n-heptane + (1 − x₁)n-decane} and {x₁n-octane + (1 − x₁)n-decane} were prepared at compositions of (x₁ = 0.0531, 0.2594, 0.5219, 0.777, 0.952), and (x₁ = 0.0616, 0.2801, 0.5314, 0.7736, 0.9623), respectively. A measuring system based on a vibrating tube densimeter, DMA HPM from Anton Paar with data acquisition system was developed in order to obtain experimental densities. Water and nitrogen were used as reference fluids to calibrate the densimeter. Experimental methodology was checked by comparing the n-heptane and n-decane densities against multi-parameter equations proposed in the literature. Differences between both sets of data show a maximum deviation of 0.07%. Excess molar volumes, isothermal compressibility and isobaric thermal expansivity were computed from experimental densities.

Highlights

► We built an equipment which consists of a variable volume cell and a VTD Anton Paar DMA–HPM. ► Compressed liquid densities are reported for n-heptane and n-decane. ► Binary (n-heptane or n-octane + n-decane) systems were studied in the whole range of composition. ► Derived properties were calculated from experimental data.

Introduction

Thermodynamic properties obtained through experimentation are the fundamental basis for the development of empirical, semi-empirical or theoretical models used to represent and predict the behaviour of fluids. Volumetric properties are important in chemical engineering for the design, simulation and optimisation of various separation processes, such as distillation, absorption, desorption, drying, liquid–liquid extraction, extraction with supercritical fluids. Knowledge of the density at different temperatures and pressures is also essential for the calculation of properties such as solubility or viscosity, both crucial in different fields of technology.

A survey of studies during the last century of liquid densities for n-alkane mixtures has been collected by Aalto et al. [1]. Most of the density data for binary n-alkane mixtures reported in the literature were studied at atmospheric pressure and low temperatures between (283 and 298) K [2], [3], [4], [5], [6], [7], [8], [9], [10], [11]; therefore the corresponding ternary systems are scarce even for temperatures higher than 373 K [11], [12], [13].

Volumetric properties at high pressure were published elsewhere for a homogeneous liquid phase [14], [15], [16], [17], [18], [19]. Abdulagatov and Azizov [14] investigated the (p, ρ, T, x₁) behaviour for the (n-heptane + n-octane) system at a maximum pressure of 10 MPa over a wide temperature range of (293 to 557) K. The volumetric behaviour of (n-hexane + n-decane, n-octane + n-dodecane, n-decane + n-tetradecane) systems is reported by Takagi and Teranishi [15] for no more than three pressures (0.1, 50, and 100) MPa at T = 298 K. Experimental densities for the binary (n-decane + n-tetradecane, n-dodecane + n-hexadecane) and ternary (n-decane + n-tetradecane + n-hexadecane) mixtures were studied at a maximum pressure of 430 MPa in reference [16]. Densities for n-pentane, n-hexane and n-heptane and their binary and ternary mixtures were measured elsewhere [17] from (0.1 to 40) MPa at T = (298, 323, and 348) K. A bellows volumometer was used to obtain density data for the (n-hexane + n-dodecane, n-octane + n-dodecane and n-hexane + n-hexadecane) systems at temperatures T = (298, 323, 348, and 373) K over the pressure range of (0.1 to 500) MPa [18], [19].

Densities for n-alkane mixtures from the above mentioned references were measured in piezometer devices [14], [15], [16] and a commercial vibrating tube densimeter [17]. The excess molar volumes exhibit the highest negative values approximately in the equimolar composition. However, some systems present both negative and positive values [16]. For binary mixtures at atmospheric pressure, the values of excess molar volume are closer to zero when the carbon number of both compounds is nearly identical.

The only density data for the (n-heptane + n-decane and n-octane + n-decane) system are reported at atmospheric pressure and T = 293 K by Cooper and Asfour [3] using a vibrating tube densimeter. Compressed liquid density data for the (n-heptane + n-decane) system are reported by Assael et al. [20] at pressures from (0.1 to 71) MPa and temperatures of (313 and 323) K; nevertheless those values are calculated from densities of pure compounds assuming ideal behaviour.

The purpose of this work is to present the experimental density behaviour, computed excess molar volume and derivative properties of n-alkane binary mixtures in the compressed liquid region. Densities of the binary (n-heptane + n-decane) and (n-octane + n-decane) systems were obtained over the whole range of composition from T = (313.15 to 363.15) K and up to 25 MPa.

Section snippets

Materials

The n-alkanes and water were supplied by Sigma–Aldrich. Infra Air Products supplied nitrogen. A Karl-Fischer coulometer (831, Metrohm) was used to determine the water content of n-alkanes. Chemicals were not purified further and were used as received. Certified purities and water content of the compounds are given in table 1.

Apparatus

A schematic diagram of the experimental apparatus developed in this work is shown in figure 1. The measuring principle is based on the static – synthetic method. A

Results and discussion

Experimental densities of high purity compounds and binary mixtures were measured at constant temperature and pressures from (1 to 25) MPa. In order to check for the performance of the vibrating tube densimeter, densities of n-heptane and n-decane were obtained at T = (313.18, 333.04, and 353.06) K; and from (313 to 363) K in temperature intervals of 10 K, respectively. These values are reported in TABLE 2, TABLE 3.

Experimental densities for n-heptane were compared with those calculated by the

Conclusions

A static–synthetic cell coupled to a DMA–HPM densimeter from Anton Paar was used with a data acquisition system for density measurements. Values of density for n-heptane and n-decane were found to be in agreement and within the deviation reported for the proposed equations and experimental data from the literature over the range studied from which it is considered that the apparatus is able to measure densities with good precision. Ideal behaviour can be assumed for the two binary (n-heptane + n-

Acknowledgments

The authors thank the financial support of this research provided by Mexican Institutions: CONACyT and Instituto Politécnico Nacional.

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