Accurate (p, ρ, T) data for two new (carbon dioxide + nitrogen) mixtures from (250 to 400) K at pressures up to 20 MPa

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Abstract

Recently our group published a set of (p, ρ, T) data for two (carbon dioxide + nitrogen) mixtures with a low carbon dioxide content (xCO2=0.10,0.15). These data showed larger relative deviations from the GERG-2008 equation of state than expected, specially at low temperatures, high pressures, and for the mixture with higher carbon dioxide content (xCO2=0.15). In order to analyze whether the mentioned deviations from the equation of state increase with the carbon dioxide content, it was decided to measure the (p, ρ, T) behavior of two additional mixtures with higher carbon dioxide molar fractions (xCO2=0.20,0.50). The new experimental data show again an appreciable disagreement with the GERG-2008 equation of state at low temperatures and high pressures. Relative deviations, which depend on temperature, arise to 0.4% at 250 K and 20 MPa and to 0.24% at 275 K and 20 MPa for the xCO2=0.20 and xCO2=0.50 mixture, respectively. Second virial coefficients are calculated for the two new mixtures presented in this work and also for those presented in our previous paper.

Highlights

► New (p, ρ, T) data of two mixtures of nitrogen and carbon dioxide are reported. ► Experimental data show a disagreement with the equation of state at low temperatures and high pressures. ► Relative deviations in density increase with the carbon dioxide molar fraction of the mixture. ► Only relative deviations at pressures below 10 MPa are within a 0.1% band.

Introduction

Thermodynamic (p, ρ, T) behavior of mixtures of carbon dioxide with nitrogen has been widely studied and reported by several authors since 1940 due to the industrial and scientific interest of these binary mixtures, first in the natural gas and oil production industry and currently in the exploitation of alternative fuels. Nevertheless the amount of available density data is still low comparing with the density data reported for the pure gases, nitrogen and carbon dioxide, and for other binary mixtures such as methane with carbon dioxide, ethane or propane [1]. Main contributions to the data base of density data for mixtures of carbon dioxide with nitrogen come from Brugge et al. [2], [3], Duarte-Garza et al. [4], Esper et al. [5] and Jaeschke et al. [6]. Therefore any contribution to the (p, ρ, T) database is of great interest since it can help to the future testing or improvement of the present reference equation of state for these mixtures.

Recently our group reported in a previous paper [7] a set of (p, ρ, T) data of two binary mixtures of carbon dioxide with nitrogen with a low concentration of the former component, xCO2=(0.10,0.15). Results of the measurements showed unexpected high relative deviations in density between experimental data and the GERG-2004 equation of state [1] at high pressures below 300 K. These deviations appeared to increase with the molar fraction of carbon dioxide, since they were higher for the xCO2=0.15 mixture. It was also observed that the relative deviations in density depended on the temperature, decreasing at higher temperatures.

In this paper accurate density experimental data of two new mixtures of carbon dioxide with nitrogen are reported to contribute to the current data base of thermodynamic properties of binary mixtures and also provide enough data for the testing and improvement of the GERG-2008 equation of state. In order to determine whether divergences observed in the previous paper between experimental data and calculated densities depended on the carbon dioxide molar composition of the mixtures, additional density measurements of two new binary mixtures with higher molar fractions of carbon dioxide were carried out. This paper reports accurate (p, ρ, T) data for these two new binary mixtures of carbon dioxide with nitrogen xCO2=(0.20,0.50) from (250 to 400) K and pressures up to 20 MPa. Measurements of the xCO2=0.50 mixture were not performed at 250 K to avoid a potential partial condensation of the carbon dioxide contained in the mixture in the colder points of the equipment. These measurements were carried out using an improved accurate single sinker densimeter with magnetic suspension coupling, which operates in the gaseous phase.

Experimental data are compared with the density values calculated from the GERG-2008 equation of state [8], which is the current international reference equation of state for natural gases and related mixtures. Though the experimental data presented in our previous work [7] were compared with the GERG-2004 equation of state, the relative deviations reported there are still valid for comparison since the new version GERG-2008 only differs from the previous one in the addition of three components which are not present in our mixtures (n-nonane, n-decane and hydrogen sulfide).

Deviations from the GERG-2008 equation of state of the experimental data reported in this work for the xCO2=0.50 mixture and by other authors for the same composition are compared and statistically analyzed. Mixture second virial coefficients Bmix are reported for the two mixtures presented in this paper and also for the two mixtures presented in the previous one [7].

Section snippets

Experimental

Density measurements were carried out by using a high accuracy single sinker densimeter with magnetic suspension coupling. This single sinker densimeter, originally developed by Kleinrahm and Wagner [9] and successively improved, applies the Archimede’s principle by weighting the apparent mass of a sinker which is affected by the buoyancy force produced by the pressurized measuring fluid. The innovation of this densimeter is that the sinker mass is determined by means of a magnetic suspension

Results

Experimental (p, ρ, T) data of the (0.20 CO2 + 0.80 N2) and (0.50 CO2 + 0.50 N2) mixtures, together with their relative deviations in density from the GERG-2008 equation of state [8], and with their uncertainties are presented in TABLE 2, TABLE 3. Each pressure step consisted of 30 measurement points. The value of the three state point magnitudes (p, ρ, T) was calculated as the average of the last ten values of the magnitude for each pressure step. The repeatability of the measurements was checked by

Discussion

(p, ρ, T) data of two binary mixtures of carbon dioxide with nitrogen with molar compositions xCO2=(0.20,050) in the working range (250 to 400) K and pressures up to 20 MPa were reported. The mixtures were measured by using an improved single sinker densimeter with magnetic suspension coupling. These measurements were carried out to complete a previous study of mixtures of carbon dioxide with nitrogen.

Experimental data were compared with the GERG-2008 equation of state. Relative deviations from the

Acknowledgments

Support for this work came from the Programa Nacional de Formación de Profesorado Universitario (FPU), Project ENE2009-14644-C02-01 of the Spanish Ministry of Science and Innovation and from the Junta de Castilla y León reference GR 152. We thank the Spanish national metrology institute (Centro Español de Metrología, CEM) for the preparation of the two binary mixtures studied in this work.

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