Elsevier

Fluid Phase Equilibria

Volume 481, 15 February 2019, Pages 66-102
Fluid Phase Equilibria

Measurements of the thermal conductivity of n-hexane in the supercritical region

https://doi.org/10.1016/j.fluid.2018.10.006Get rights and content

Highlights

  • We presented the first measurements of the thermal conductivity of n-hexane in the supercritical region, along ten isotherms ranging from 508.17 to 553.00 K and at pressures up to 10 MPa with an estimated experimental uncertainty of 3 %.

  • These measurements represent 1321 new data of the thermal conductivity, which are analyzed to address the development of wide-ranging correlating equations.

  • The complete correlating equations are able to provide calculated thermal conductivity of n-hexane from 293 to 612 K and densities up to 670 kg.m-3.

  • The resulting percentage standard deviation is 1.0388% from 1898 experimental thermal conductivity data, including thus 577 data previously measured in the gaseous and liquid n-hexane.

Abstract

Measurements of the thermal conductivity of supercritical n-hexane performed in a coaxial cylinder cell operating in steady state conditions are reported. The present 1321 data of the thermal conductivity of n-hexane were carried out along ten quasi-isotherms above the critical temperature. These data cover the temperature range from 508.17 K to 553.00 K and the pressure range 0.l to 10 MPa. An analysis of the various sources of error leads to an estimated uncertainty that do not exceed 4% (95 level of confidence). The parameters of a background equation previously determined from 577 data of the thermal conductivity of gas and liquid n-hexane are reused here in order to analyze the critical enhancement of the thermal conductivity as a function of temperature and density. A set of theoretical Ising-like equations are re-formulated to describe the thermal conductivity critical enhancement along the near-critical isochoric lines in terms of the effective power laws fitting the singular behavior of the needed fluid properties as functions of the finite temperature distance to the critical point. Assuming the knowledge of the regular temperature behavior of the viscosity, the critical enhancement of the thermal conductivity appears then only characterized by a single effective amplitude. Such a separated analysis of the well-defined temperature effects provides the empirical functional form of the density effects, as a 6th-order polynomial density function, which can be normed whatever the temperature. Finally, our complete formulation of the total thermal conductivity describes, within three standard deviations (with SD = 1.0388%), the 1898 thermal conductivity measurements of n-hexane from 293 to 612 K and densities up to 670 kg m−3.

Introduction

The goal of this work is to report new set of data and to provide wide-scale correlations for the thermal conductivity of n-hexane that are valid over an extended pressure-temperature-density domain covering the supercritical conditions. This work complements our previous measurements and correlations of the thermal conductivity of gaseous and liquid n-hexane reported in Ref. [1]. To our knowledge, no self-consistent measurements of the thermal conductivity of n-hexane were reported in such a large compressed fluid state. In particular, we have noted that the reference correlation for the thermal conductivity of n-hexane over a large range of temperature and pressure reported by Assael et al. [2] was constructed without any extended validation from experimental data obtained in the surrounding domain of the n-hexane critical point.

Section snippets

Experimental set UP

The present thermal conductivity measurements of n-hexane were carried on as a function of temperature between Tc+0.35K and Tc+45.18K and pressures up to 10 MPa, in the homogeneous supercritical region, using vertical coaxial cylinders, operating in the steady-state mode. Tc is the critical temperature of n-hexane. This method of measurement and the applied corrections were described in several papers [[3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15]].

We briefly recall that

Experimental results

Our new experimental results (1321 data) are reported in column 3 of Table 1, Table 2, Table 3, Table 4, Table 5, Table 6, Table 7, Table 8, Table 9, Table 10 for the dense fluid state characterized by the pressure (column 1) - density (column 2) values along 10 isotherms at 508.17 K (93), 508.92K (193), 509.85 K (133), 510.36 K (127), 511.55 K (145), 511.79 K (147), 513.09 K (167), 515.56 K (141), 525.42 K (106), and 553.00 K (69), respectively. The numbers of data per isotherm are indicated

Background correlations

The two first contributions of Eq. (1) represent the background thermal conductivity:λb(T,ρ)=λ00(T)+δλ(ρ)The formulations of Eq. (4) were obtained from our previous measurements of the thermal conductivity of gaseous and liquid n-hexane, as detailed in Ref. [1] and briefly recalled below.

The thermal conductivity for the zero-density limit was expressed by the following polynomial form:λ00=c11T+c0+c1T+c2T2+c3T3where λ00(T) and T were expressed in mW⋅m−1⋅K−1and K, respectively. The values of the

Critical enhancement correlations

The values of Δλc(T,ρ) were estimated by subtracting the background thermal conductivity λb,cal(T,ρ) calculated using Eqs. (4)–(6), from the measured data λexp(T,ρ) reported in column 4 of Table 1, Table 2, Table 3, Table 4, Table 5, Table 6, Table 7, Table 8, Table 9, Table 10, i.e.,Δλc(T,ρ)=λexp(T,ρ)λb,cal(T,ρ)The resulting Δλc(T,ρ) data are reported in column 5 of Table 1, Table 2, Table 3, Table 4, Table 5, Table 6, Table 7, Table 8, Table 9, Table 10. Even though Δλc(T,ρ) was a calculated

Thermal conductivity calculated from Eq. (1)

The total thermal conductivity λ(T,ρ) estimated from Eq. (1), with Δλc(T,ρ)Δλc,e(t,ρ), are reported in column 6 of Table 1, Table 2, Table 3, Table 4, Table 5, Table 6, Table 7, Table 8, Table 9, Table 10 The residuals Rλi,total%=100×λi(exp)λi(cal)1 are shown (open red circles) in Fig. 15. The evaluations of the correlation of Eq. (1) are reported in Table 15 for each isotherm and for all the 1321 data measurements in supercritical n-hexane. The standard deviation is 1.152%. Due to the large

Conclusion

New measurements of the thermal conductivity of n-hexane are presented in the supercritical region, at temperatures from 508.17 to 553.00 K along ten isotherms and at pressures up to 10 MPa with an estimated uncertainty lower than 4% (95 level of confidence). Their careful analysis is based on the (background + critical enhancement) additive form of the thermal conductivity data. The temperature and density dependences of the background term have been already determined from previous

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