(716a) Experimental and Modelling Study of the Phase Equilibria of (CO2 + methylcylohexane + N2) at High Pressures and Temperatures

In order to design and operate both CO₂ storage and CO₂-EOR processes, an in-depth understanding of the phase behaviour of multicomponent mixtures containing CO₂ and hydrocarbon is essential [1]. Numerous experimental and modelling studies exists in the literature for the phase behaviour of binary mixtures of CO₂ with various hydrocarbons, resulting in generally good understanding of these systems. However there still remains a scarcity of data on the phase behaviour of multicomponent mixtures of CO₂ and hydrocarbons, including mixtures containing diluents such as N₂. Such systems are more representative of real CCS and EOR processes in which the CO₂ stream usually contains impurities that can be of significant impact. Accordingly, the current work is aimed at providing new data on multicomponent mixtures that can be used to assess the impact of diluents on the phase behaviour of CO₂-hydrcarbon systems for a better of CO₂ storage and CO₂-EOR processes.

2. Equipment Design

In this work, the phase behaviour of the multicomponent mixture (CO₂ + Methylcyclohexane + N₂) was studied in an apparatus combining the synthetic and analytical measurement capabilities at temperature up to 423 K and pressure up to 50 MPa.

During synthetic measurements, mixtures of precisely known composition are prepared and their phase behaviour is observed visually in the variable-volume cell. This allow for the determination of various types of phase boundary including vapour-liquid, liquid-liquid and vapour-liquid-liquid loci, critical curves of mixtures as well as cloud curves. In the analytical approach, the compositions of the coexisting bulk phases at equilibrium are determined by means of fluid sampling devices connected to on-line gas chromatography for identification and quantification.

The apparatus consisted of a cylindrical equilibrium variable volume cell made of titanium and driven by a computer controlled servo motor. The cell was designed with five high-pressure ports, two of which were used to allow for the inlet and outlet of the mixture. Two other ports were used for sampling, to which Rolsi sampling valves were fitted, while a fifth port is used for a temperature sensor in direct contact with the fluid. Composition analysis was by gas chromatography with both thermal-conductivity and flame-ionisation detectors arranged in parallel. Visual observation was enabled by a sapphire window and a CCD camera.

Temperature control of the equilibrium cell was achieved by encasing the cell in an aluminium heating jacket with axial holes for cartridge heaters and PT100 temperature sensors which were in turn connected to a PID process controller. Equilibration of the contents of the cell was assisted by a magnetic stirrer. Four calibrated syringe pumps with a maximum service pressure of 70 MPa were used for quantitative fluid injection. One pair of pumps was configured to accept high-pressure gases or liquefied gases, while a second pair was configured to accept components that are liquid under ambient conditions.

3. Experiment and Modelling

The apparatus was validated by means of comparison with published isothermal saturated vapour pressure measurements on methanol, as well as isothermal vapor-liquid equilibrium measurements on (CO₂ + methylcyclohexane).

The vapour-liquid phase behaviour for the ternary mixture of (CO₂ + methylcyclohexane + N₂) were studied over the temperature range (323 and 423) K and pressures up to the critical pressure. Three different molar ratios between CO₂ and N₂ were fixed (0.25, 0.5 and 0.75) in the ternary system and the bubble-curve and dew-curve were determined along the target isotherms at different amounts of methylcyclohexane.

The predictive capability of SAFT-g-Mie [2] was also tested in order to model the phase equilibria of this mixture. The Statistical Associating Fluid Theory, stemming from the first-order perturbation theory of Wertheim [3], was implemented in this work with a group contribution approach and the generalized Mie potential to represent segment-segment interactions. In the resulting SAFT-g-Mie, complex molecules are represented by fused segments representing the functional groups from which the molecule may be assembled. All interactions, both like and unlike, as implemented in this work were determined from experimental data of systems comprising the constituent groups. In addition to the above, the capability of the Predictive Peng-Robinson equation of state [4] was also tested.

References

1. S. W. Løvseth, H. G. J. Stang, A. Austegard, S. F. Westman, R. Span, R. Wegge, Measurements of CO2-rich mixture properties: Status and CCS needs, Energy Procedia, 86 ( 2016 ).

2. V. Papaioannou, T. Lafitte, C. Avendano, C.S. Adjiman, G. Jackson, E.A. Muller, A. Galindo, Group contribution methodology based on the statistical associating fluid theory for heteronuclear molecules formed from Mie segments, J. Chem.Phys, 140 (2014).

3. M. S. Wertheim, Fluids with highly directional attractive forces. II. Thermodynamic perturbation theory and integral equations, J. Stat. Phys, 35 (1984).

4. D. Y. Peng, D. B. Robinson, A new two-constant equation of state, Ind. Eng. Chem. Fundam, 15 (1976).