Measurement of the speed of sound in supercritical n–hexane at temperatures from (509.17–637.99) K and pressures from (3.5–7.5) MPa

Highlights

• New experimental data of the speed of sound in supercritical n-hexane along five isobaric lines were presented.

• An advanced optical method for measuring the speed of sound in supercritical fluid was reported.

• The EOS proposed by Span and Wagner was assessed by the experimental speed of sound in supercritical n-hexane.

Abstract

n-Hexane is a significant component of industrial fluids. The speed of sound in supercritical n–hexane was measured by Brillouin light scattering (BLS) technique in the temperature range from (509.17–637.99) K along five isobars at p = (3.5, 4.5, 5.5, 6.5 and 7.5) MPa. The relative expanded measurement uncertainty in the speed of sound at the 95% confidence level is estimated to be 0.018. For the studied supercritical n–hexane, the change regularities of the speed of sound with increasing temperature and pressure are illustrated. Moreover, the well-known equation of state (EOS) proposed by Span and Wagner is assessed, and the absolute average deviation (AAD) between the experimental results and the calculated values is 0.89% for the whole examined p – T region.

Introduction

Normal alkanes are significant constituents of industrial fluids and transportation fuels. As a key member of n-alkanes, n-hexane is well-known as an important component of petroleum and gasoline and becomes a promising alternative to aviation kerosene [[1], [2], [3]]. Moreover, n-hexane is widely used as solvent, adhesive, diluent, extractant, denaturant and cleaning agent in many fields [[4], [5], [6], [7]]. As a solvent, n-hexane is capable of improving the mass transfer rate in the transesterification reaction of waste oil recovery [8]. In food industry, n-hexane can efficiently extract fats from oilseeds such as soybeans, peanuts and corn germs [9]. Meanwhile, n-hexane in the critical region can significantly improve the rate of lipid extraction [10].

The speed of sound in fluid is extremely significant to obtain several important thermodynamic properties such as the heat capacity at constant pressure, the virial coefficient and the isentropic compressibility. The speed of sound also plays an important role in the derivation and verification of fundamental equations of state (EOS) [11,12]. Moreover, the speed of sound is particularly essential to estimate the fuel injection timing that is critical for the accurate design and improvement of the injection system [13].

In the past decades, speeds of sound in n-hexane have been studied to some extent because of its widespread applications. According to the investigated p-T region in this work, only studies related to elevated temperature or pressures are summarized, which are listed in Table 1. Zotov et al. employed the pulse-type ultrasonic method to measure the speed of sound in saturated n-hexane at T = (193.15–473.15) K [14]. Khasanshin et al. measured the speed of sound in n-hexane at pressures up to 100.1 MPa within the temperature range from (298.15–433.15) K by the acoustic pulse method [15]. Daridon et al. supplemented 275 data points of speeds of sound in liquid n-hexane in the temperature interval (293.15–373.15) K and at pressures up to 150 MPa by a pulse transmission-reflection apparatus [16]. Ball et al. measured the speed of sound in liquid n-hexane over the temperature range from (298.15–373.15) K and at pressures from (0.1–101) MPa by using the acoustic pulse method [17]. Boelhouwer et al. obtained the speed of sound in n-hexane at pressures up to 140 MPa within the temperature limits of (253.15–333.15) K and by using a pulse method with constant path length [18]. Wang et al. reported the speed of sound in n-hexane at temperatures from (265.15–368.15) K under atmospheric pressure by using the ultrasonic pulse transmission method [19]. Hawley et al. developed a pulse method to measure the speed of sound in n-hexane at T = 303.15 K and pressures up to 392 MPa [20]. To sum up, the temperature and pressure distribution of the literature data of the speed of sound in pure n–hexane is shown in Fig. 1, which indicates that there is a lack of experimental speeds of sound in supercritical n–hexane.

As mentioned above, previous studies mainly focus on the speed of sound in saturated liquid/vapor and liquid n–hexane. However, the speed of sound in fluids varies sharply with the temperature and pressure in the near critical region, where the speed of sound is hardly to be accurately measured by the acoustic resonator method and the ultrasonic pulse-echo method. The Brillouin light scattering (BLS) technique is capable to measure the speed of sound over a wide thermodynamic region, especially in the vicinity of the critical point [[21], [22], [23], [24], [25]]. Our group has investigated the speed of sound in saturated liquid/vapor, liquid and near-critical n–hexane by the BLS technique over the temperature range from (300.15–506.15) K and at pressures up to 8.5 MPa [26]. On the basis of such background, this work aims to continue and extend the previous study by filling in the gaps of experimental speeds of sound in supercritical n–hexane within the temperature limits of (509.17–637.99) K along 5 isobaric lines at p = (3.5, 4.5, 5.5, 6.5 and 7.5) MPa. Finally, the new data can subsequently be applied to verify and improve the formulation of the Helmholtz energy for n–hexane. In this work, the applicability of a famous multi-parameter equation of state (EOS) proposed by Span and Wagner in predicting the speed of sound in supercritical n–hexane was verified [27,28].