Measurements of the thermal conductivity of ethene in the supercritical region
Introduction
The goal of this work is to report new set of data and provide wide-scale correlations for the thermal conductivity of ethene that are valid over gas and liquid states. Ethene (ethylene) is the most important organic compound produced in the world, but there are very few accurate measurements of the thermal conductivity of ethene compared to other industrial fluids. One reason is probably that the temperature of the critical point is close to the ambient temperature and that the thermal conductivity in this range is very sensitive to small changes in density which require accurate measurements of temperature and pressure. In Table 1, are considered the most important experimental measurements of the thermal conductivity of ethene reported in the literature, over a wide range of temperature and pressure in the supercritical state, including the critical pressure (pc = 5.042 MPa) [1], [2], [3], [4], [5], [6], [7], [8]. A more extensive set of publications on the thermal conductivity of ethene is reported in Ref. [8]. Table 1 gives the range of measurements, the method used and the uncertainty. The uncertainty claimed by most of these authors is remarkable, less than 1.5%, in the whole range of temperature and pressure excepted in the critical region where it can rise to 3%. All these measurements were performed using absolute hot-wire instrument [1], [2], [3], [4] or absolute transient hotwire instrument [5], [6]. Golubev et al., in 1971, were among the first to report measurements of the thermal conductivity of ethene. The authors published an extensive set of data in a wide range of temperatures (293–521) K and pressure (0.1–67) MPa. Tarzimanov and Arslanov [2] and Tarzimanov and Lozovoi [3] extend the measurements at higher temperatures up to 618 K and pressures (0.1–196) MPa. The other measurements were carried out in lower pressure range (<50 MPa). Measurements were reported by Kolomiets [4] in the temperature range (180–480) K and the pressure range (0.1–50) MPa. They estimated the absence of convection with the product Gr.Pr < 1000. Prasad and Venart published measurements of the thermal conductivity of ethene from 297 to 352 K in the pressure range from 1.4 to 56 MPa [5]. More recently Millat et al. [6] reported new, absolute measurements of the thermal conductivity of ethene with an uncertainty of 0.3%. The measurements covered the temperature range 308–425 K and the pressure range 0.1–10 MPa. However their data are rather limited in density and there is only two points above the critical density.
Vargaftik et al. [7] published a handbook of thermal conductivity of liquids and gases, where tables of the thermal conductivity of ethene cover a wide range of temperature and pressure. The correlations developed by these authors are based mostly on the experimental measurements of Golubev et al. [1]. Recently general equations for the thermal conductivity of ethene as a function of temperature and density were reported [8]. The authors estimate the uncertainty on the correlation of the thermal conductivity (at the 95% confidence level) from 110 to 520 K at pressures up to 200 MPa to be 1–5% for the compressed liquid and the supercritical phases. The transient hot-wire method has been used worldwide as a main instrument for reliable measurements of the thermal conductivity of fluids within wide temperature and pressure ranges. However, if the contribution of radiation to the measurement of the thermal conductivity, can be estimated to be negligible, since ethene is generally considered a transparent fluid, natural convection is by nature inevitable, especially near the critical point where the divergence of the specific heat at constant pressure can generate large value of the Prandtl number [9].
Section snippets
Experimental apparatus and procedure
Current measurements of the thermal conductivity of ethene were performed as a function of temperature between Tc and T = 600 K and pressures up to 100 MPa, in the homogeneous critical region, using vertical coaxial cylinders, operating in the steady-state mode. This method of measurement and the related corrections have already been described and used in several experiments [10], [11], [12], [13], [14], [15], [16], [17], [18]. The uncertainties (0.95 level of confidence) of the thermal
Experimental results
Some experimental results are reported in Table 2 for the dilute gas state (in the temperature range (283–425) K, near atmospheric pressure), and in Table 3, Table 4, Table 5, Table 6, Table 7, Table 8, Table 9, Table 10 for the fluid dense state, along 8 supercritical isotherms (then covering the density range from 4 kg⋅m−3 to 400 kg m−3). The equation of state of Smukala et al. [19] was used to calculate the density ρ, with uncertainty of the order of ±0.2%.
The critical parameters of ethene
Correlations
The analyzed data are based on the residual concept, for which the thermal conductivity is expressed as a sum of three independent contributions as follows:where ρ is the density, T is the absolute temperature. λ00(T) is the thermal conductivity of the dilute-gas up to a zero density limit, which depends only on the temperature, δλ(ρ) is the residual thermal conductivity, which is independent of the temperature. δλ(ρ) represents the contribution of all non-critical
Critical enhancement
The thermal conductivities of pure fluids become infinite at the critical point and show a thermal conductivity critical enhancement over a wide range of densities and temperatures around the critical point. This thermal conductivity critical enhancement, can be calculated from several well-known theoretical crossover models [8], [21], [22], [23], [24].
In this work, the analysis of the thermal conductivity and diffusivity of ethene in the critical region was performed as a function of the
Comparison with other work
The critical enhancement Δλc(T,ρ) was calculated for the data of Prasad et al., using λb(T,ρ) given by (Eq. (3)). As can be seen in Fig. 6, there is only one isotherm at T = 298.6 K, going through the critical region which shows an increase of the thermal conductivity at the critical density, of the order of 25 mW m−1.K−1, and the value calculated by ((Eq. (19)) is 5.06 mW m−1.K−1, five time lower. In order to obtain a critical enhancement of 25 mW m−1.K−1, you must assume a critical exponent
Conclusion
New measurements of the thermal conductivity of ethene obtained with the coaxial cylinder method are presented in the supercritical region, at temperatures from 283.46 K to 425.0 K, along eight isotherms and at pressures up to 100 MPa with an estimated uncertainty (0.95 level of confidence) of ±3%. The comparison with previous works show a complete disagreement with most of them, even those which claimed uncertainty less than 1.5%. Along the critical isochore simple empirical correlations were
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