Speeds of sound in {(1  x)CH4 + xN2} with x = (0.10001, 0.19999, and 0.5422) at temperatures between 170 K and 400 K and pressures up to 30 MPa

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Abstract

The speed of sound in {(1  x)CH4 + xN2} has been measured with a spherical acoustic resonator. Two mixtures with x = (0.10001 and 0.19999) were studied along isotherms at temperatures between 220 K and 400 K with pressures up to 20 MPa; a few additional measurements at p = (25 and 30) MPa are also reported. A third mixture with x = 0.5422 was studied along pseudo-isochores at amount-of-substance densities between 0.2 mol · dm−3 and 5 mol · dm−3. Corrections for molecular vibrational relaxation are discussed in detail and relaxation times are reported. The overall uncertainty of the measured speeds of sound is estimated to be not worse than ±0.02%, except for those measurements in the mixture with x = 0.5422 that lie along the pseduo-isochore at the highest amount-of-substance density. The results have been compared with the predictions of several equations of state used for natural gas systems.

Introduction

Accurate knowledge of the thermodynamic properties of methane-rich gas mixtures is of major importance in the production, processing, storage and transport of natural gas. In particular, precise measurements of the speed of sound can contribute to the establishment of new and more-reliable equations of state from which a wide range of thermodynamic properties may be computed. The present work was initiated in response to requirement set out by the Groupe Europeen de Recherches Gaziers (GERG), Working Group on Development of a Reference Equation of State (EOS) for Thermal and Caloric Properties of Natural Gases. The objective of the measurements was to fill notable gaps in the available sound-speed data on methane-rich gas mixtures, thereby helping to facilitate the development of a new global equation of state for natural gas systems [1].

We studied three mixtures of methane and nitrogen in ranges of temperature, pressure and composition chosen to maximise the value of the results for equation-of-state development. For two of the mixtures studied, precise pressure–density–temperature measurements were also made and those results are presented in an accompanying paper [2].

Section snippets

Experimental

The speeds of sound were measured using the spherical-resonator and associated apparatus described earlier [3], [4], [5]. The pressure of the gas was measured by means of two quartz-crystal manometers (Paroscientific Digiquartz transducers) with full-scale readings of 21 MPa and 42 MPa, respectively. The lower-pressure transducer, which was used for all measurements except those at p > 20 MPa, was housed in an oven maintained at T = 313.15 K and calibrated in situ against Desgrange & Huot Model 26410

Acoustic model

The speeds of sound were obtained from the measured resonance frequencies by means of the well-established acoustic model [6]. The model includes the usual corrections for the thermal boundary layer and for the coupling of gas and shell motion, as well as very-small corrections for the presence of the tubular opening through which the gas was admitted.

The thermodynamic properties needed in the calculation of these corrections were computed from the Lee–Kesler equation of state [7] with the

Results

The results are given in TABLE 3, TABLE 4, TABLE 5 (preliminary results were reported previously for the mixtures with x = 0.19999 and x = 0.5422 [5]). Consideration of the uncertainties in temperature, pressure and resonance frequency, and also of the uncertainties of the calibration measurements and those associated with ‘correction’ terms in the acoustic model, leads us to an overall estimated uncertainty of not more than ±0.02%. We have also considered the possibility of sorption errors leading

Discussion

Given the relatively small experimental uncertainties, the present results constitute a very sensitive test of any proposed equation of state applicable to mixtures of methane and nitrogen. At the present time, the AGA8-DC92 equation of state [14] is probably the most widely accepted thermodynamic model for natural gases of known composition. FIGURE 5, FIGURE 6 compare the present speed of sound in {(1  x)CH4 + xN2} for x = 0.1 and x = 0.2 with the prediction of the AGA8-DC92 equation. The comparison

Acknowledgements

We are pleased to acknowledge the financial assistance of Rurhgas AG, and we are grateful to O. Kunz for providing comparisons with the GERG-2004 equation of state prior to its publication.

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