Elsevier

Fluid Phase Equilibria

Volume 409, 15 February 2016, Pages 207-241
Fluid Phase Equilibria

Vapor–liquid equilibrium data for the carbon dioxide and nitrogen (CO2 + N2) system at the temperatures 223, 270, 298 and 303 K and pressures up to 18 MPa

https://doi.org/10.1016/j.fluid.2015.09.034Get rights and content

Abstract

A new setup for the measurement of vapor–liquid phase equilibria of CO2-rich mixtures relevant for carbon capture and storage (CCS) transport conditions is presented. An isothermal analytical method with a variable volume cell is used. The apparatus is capable of highly accurate measurements in terms of pressure, temperature and composition, also in the critical region. Vapor-liquid equilibrium (VLE) measurements for the binary system CO2 + N2 are reported at 223, 270, 298 and 303 K, with estimated standard uncertainties of maximum 0.006 K in the temperature, maximum 0.003 MPa in the pressure, and maximum 0.0004 in the mole fractions of the phases. These measurements are verified against existing data. Although some data exists, there is little trustworthy data around critical conditions, and our data indicate a need to revise the parameters of existing models. A fit made against our data of the vapor–liquid equilibrium prediction of GERG-2008/EOS-CG for CO2 + N2 is presented. At 223 and 298 K, the critical region of the isotherm are fitted using a scaling law, and high accuracy estimates for the critical composition and pressure are found.

Introduction

Knowledge about how CO2-rich mixtures behave under different conditions is important for the development of carbon capture, transport and storage (CCS) processes. For instance, an accurate equation of state (EOS) describing the thermodynamic properties of these mixtures is needed to model and dimension the various processes along the CCS value chain. Moreover, an EOS can be used to set requirements on the amount of impurities present in the CO2 to be transported. Even with the recent progress of molecular modeling, empirical EOSs still provide the most accurate description of thermodynamic properties of such systems. Unfortunately, even for relatively simple binary mixtures, the data situation is not satisfactory for all relevant mixtures and conditions [1], [2], [3]. Hence, new and accurate experimental data are needed in order to improve the thermodynamic property predictions, by developing new EOS models or modifying the parameters and structure of existing ones.

Even small amounts of impurities in CO2-rich mixtures can significantly affect the behavior of the fluid [3], [4]. As an example, the maximum pressure at which a mixture of CO2 and only 5% N2 can be in the two-phase region, the cricondenbar, will increase to approximately 8.4 MPa compared to the critical pressure of CO2, 7.3773 MPa [4], [5], [6].

Until recently, the most accurate EOS model describing CO2-rich mixtures has been the GERG-2008 [7], [8]. This EOS [7] covers most of the relevant mixtures expected in CO2 conditioning and transport found in CCS [8], [3], [9]. The structure and parameters in this EOS were developed and fitted with focus on natural gas mixtures.

In the works by Gernert and Span [1] and Gernert [2], an equation of state called EOS-CG (Equation Of State for Combustion Gases and combustion gas like mixtures) has been developed specifically for CO2-rich mixtures. The EOS was based on the structure of GERG-2008, with modifications for the binary CO2-rich systems found within CCS. The EOS was fitted against a significantly extended literature data base for CO2-rich mixtures compared to the GERG-2008 data base [1], [2].

However, as Gernert and Span [1] and Gernert [2] pointed out in the review of available literature data, large gaps occur in the experimental data for thermophysical properties of CO2-rich mixtures [3], [10]. Moreover, some of the existing data from different authors are systematically inconsistent with those of other authors within the stated uncertainty estimates. As a consequence, the accuracy of the equations of state fitted to the data could be increased significantly by reconciling the inconsistencies and filling in the gaps in the available data.

The work to be presented here is part of a project called CO2Mix. As described by Løvseth et al. [5], the CO2Mix project aims at performing accurate vapor–liquid equilibrium (VLE), speed of sound and density measurements of CO2-rich mixtures at conditions relevant for transport and conditioning in CCS [3], [9]. As part of this project, a setup has been specifically designed and constructed in order to perform highly accurate phase equilibria measurements on CO2-rich mixtures under relevant conditions for CCS.

The present paper reports the results of VLE measurements on the CO2 + N2 binary system, with measurements over the whole VLE pressure region at the temperatures 223, 298 and 303 K, and one VLE data point at 270 K. For some conditions, high quality literature data exist for this system, making it suitable to validate the operation of the experimental setup. Furthermore, several measurements were taken at conditions where no previous data or only data of dubious quality could be found, for instance at pressures close to the critical point of the binary mixture at the measured temperatures. Additionally, measurements were performed at temperatures close to the critical temperature of CO2. The results are compared to existing EOS models, and new fits are presented.

Special care has been taken by the authors to present the results and analysis in accordance with the IUPAC Guidelines for reporting of phase equilibrium measurements given in the work by Chirico et al. [11]. One of the most important aspects of this is the thorough estimation of the standard uncertainties, as specified in the ISO Guide for the Estimation of Uncertainty in Measurement, commonly referred to as “GUM” [12]. Error-free dissemination of the resulting experimental data with the uncertainty estimates is ensured by supplying the data in a file written in the NIST ThermoML format [13], [14], [15], [16].

In the current work, the experimental setup and the operational procedures applied will be described in detail in Section 2. In Section 3, an analysis of the pressure, temperature and composition measurement uncertainty will be presented, with references to further details in the appendix. The measurement results will be provided in Section 4, before an analysis of the data with regards to existing data and models in Section 5. Section 5 will also present fitting of existing models to the new data.

Section snippets

Description of setup

The experimental setup has been described briefly in Ref. [17]. A more detailed description will be given here. Additional details necessary for the uncertainty analysis for the measurement of pressure, temperature, and composition will be given in Sections 3.2 Pressure, 3.3 Temperature, 3.4 Composition, respectively.

The vapor–liquid equilibrium measurements were carried out using an isothermal analytical method with a variable-volume cell, as described by Ref. [18]. This method involves

Definitions

The “GUM” [12] terms and definitions will be used in the following analysis. For ease of reading, and, since several of the estimation methods will be used repeatedly, some of the symbols used will be defined here.

The uncertainty components will be evaluated as standard uncertainties, with symbol u(y), where y is the estimate of the measurand Y, that is, the measurement result. Standard uncertainty is the uncertainty of the result of a measurement expressed as an estimated experimental sample

Results

VLE measurements at 223.14, 270.00, 298.17 and 303.16 K were conducted.

The existence of liquid and vapor phases was confirmed visually before the sampling of the phase compositions. Furthermore, the volumes occupied by the liquid and vapor phases inside the cell were measured visually. This visual inspection also assisted in determining the proximity to the critical point, that is, when the liquid and vapor phases for the CO2 + N2 system become clouded due to the small density difference of the

Comparison with literature data

Identified literature data around the temperatures 223, 270, 298 and 303 K are plotted together with the measurement data and uncertainties of this work in Fig. 7, Fig. 8, Fig. 9, Fig. 10.

The only literature data found in the vicinity of 223.14 K were the bubble and dew point measurements at 5 and 10 MPa by Weber et al. [27]. Their measurements at 5 MPa were in very good agreement with our measurements. Their measurements at 10 MPa seemed to be slightly off in composition, compared with our

Conclusions

This work describes a new facility for the measurement of vapor–liquid equilibria (VLE) of CO2-rich mixtures, and reports the measurements of this setup on mixtures of CO2 and N2. More accurate VLE data will be required for a number of relevant mixtures in order to build better predictive models to be used in order to optimize the design and operation of various processes needed within CCS.

Our data covers a large range of VLE liquid and vapor phase CO2 compositions, spanning from approximately

Acknowledgments

This publication has been produced with support from the research program CLIMIT and the BIGCCS Centre, performed under the Norwegian research program Centres for Environment-friendly Energy Research (FME). The authors acknowledge the following partners for their contributions: Gassco, Shell, Statoil, TOTAL, ENGIE and the Research Council of Norway (193816/S60 and 200005/S60).

The research leading to these results has also received funding from the European Community's Seventh Framework

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