(499g) Phase Behaviour Studies of the System (CH4 + CO2) and (CH4 + H2S + CO2) at Temperatures Between (220.15 and 303.15) K

Introduction

Over the last decade, different types of decarbonisation technologies, including post-combustion, pre-combustion and oxy-combustion, have been developed for use with thermal power plant. Pre-combustion capture is usually coupled with Integrated Gasification Combined Cycle (IGCC) power plants and the CO₂ is then to be separated from the syngas that is produced by the gasification of coal. However, in common with the other technologies mentioned, separation and compression of the CO₂ involves a large energy penalty. To moderate the parasitic load of pre-combustion carbon capture, a new process, known as Next Generation Carbon Capture Technology (NGCT) was recently proposed [1] wherein CO₂ separation from the effluent stream is based on low-temperature high-pressure physical separation. This will offer significant cost savings compared with alternative processes. To gain a better understanding in terms of both energy efficiency and cost, and to help improve the design of this process, accurate experimental phase-equilibrium data and predictive modelling is essential. Unfortunately, the vapour-liquid equilibrium (VLE) and solid-vapour-liquid equilibrium (SVLE) data available in the literature for systems comprising CO₂ with both major components (such as H₂, N₂) and minor components (such as CO, CH₄ and H₂S) present significant gaps and discrepancies, especially near the three-phase SVLE line, which is important for the design of this process. Therefore, new VLE and SVLE data are required to determine the optimal operating conditions and to avoid solid formation in the separation process.

In this work, we have studied the phase behaviour of CO₂ and some relevant minor components at temperature and pressure conditions relevant to those existing in both the NGCT process itself and typical pipeline scenarios.

Experimental Work and Measurements

The experimental measurement was carried out using a state-of-the-art computer-controlled low-temperature VLE apparatus designed for a maximum working pressure of 20 MPa at temperatures down to 183 K. The experimental technique was based on a static-analytic method with sampling of coexisting equilibrium liquid and vapour phases through a pair of ROLSI valves and on-line composition measurement by gas chromatography (GC). The apparatus design is described in detailed by Fandiño, et al. [2].  

The temperature was measured using a platinum resistance thermometer calibrated at the triple-point of water and by comparison in a constant-temperature bath with a standard platinum resistance thermometer over the full working temperature range. The overall standard uncertainty of the temperature was 0.006 K. The pressures were measure using a pressure transmitter calibrated by the manufacture and verified by us by means of comparison with a primary pressure standard having a relative uncertainty of 0.016 % of reading. The overall standard uncertainty of the pressure was 0.003 MPa. An absolute area method was used for the GC calibration in this work. Samples of pure gases at different pressures, varying from (0.1 to 0.5) MPa, were injected in the GC via two different gas sampling loops (nominal volume 0.05 and 0.25 cm³) and the area response of the thermal conductivity detector (TCD) was recorded. For the H₂S measurement, a flame photometric detector (FPD) was used instead of the TCD. The estimated uncertainties of the mole fraction x associated with the calibration method was less than 10^-4 or 0.01x as determined by the method discussed in detail by Al Ghafri et al. [3].

The VLE measurements for the binary system (CH₄ + CO₂) proceeded as follows. First, CO₂ was introduced into the equilibrium cell kept at the desired temperature. After the pure CO₂ system achieve thermal equilibrium, the second gas (CH₄) was add to increase the pressure of the system. The gases were allowed to mix under stirring until equilibrium was achieved. Several samples of liquid and vapour were than withdrawn and analysed by GC. After equilibrium data were obtained at one temperature and pressure, the pressure were increased by adding more CH₄ to the system until the last measurement, as close to the critical point as possible. The measurements of VLE for the ternary system (CH₄ + H₂S + CO₂) was similar to the binary but instead of starting the measurement with pure CO₂, a mixture of CO₂ and H₂S, with fixed molar ratio, was introduced into the equilibrium cell.

VLE has been measured for the binary system (CH₄ + CO₂) at seven temperatures between (220.15 and 303.15) K and at pressure in the range of the vapour pressure of CO₂ to approximately 10 MPa. These new data will help to resolve discrepancies, especially in the vapour-liquid critical region. Further measurements were performed on the ternary system (CH₄ + H₂S + CO₂) with low concentration of H₂S (< 100 ppm) over similar ranges of temperature and pressure to assess the effect of this impurity on the phase behaviour.

Modelling Work

The experimental data obtained for these mixtures have been compared with calculations based on different models. We examined first the performance of the Peng-Robinson equation of state (PR EoS) [4] combined with the classical one-fluid mixing rules incorporating either one or two binary parameters. The results showed that the model can predict the vapour-liquid equilibria of (CH₄ + CO₂) and (CH₄ + H₂S + CO₂) mixtures with good accuracy, except in the critical region. We also examined the predictive performance of SAFT-&gamma-Mie [5], which is based on the Statistical Associating Fluid Theory implemented with a group-contribution approach and the generalized Mie potential to represent segment-segment interactions. The results showed that the model can predict the VLE of (CH₄ + CO₂) and (CH₄ + H₂S + CO₂) mixtures with similar accuracy to those of the PR EoS. We also compare our SVLE results with appropriate models.

References

1. Jackson, S. R., C. Corden and P. W. Howells; (2011). "Process and apparatus for the separtion of carbon dioxide and hydrogen." UK Patent Application - GB2490476.
2. Al Ghafri, S. Z., E. Forte, G. C. Maitland, J. J. Rodriguez-Henriquez and J. P. Trusler (2014). "Experimental and modeling study of the phase behavior of (methane + CO2 + water) mixtures." J Phys Chem B 118(49): 14461-14478.
3. Fandiño, O., J. P. M. Trusler and D. Vega-Maza (2015). "Phase behavior of (CO2+H2) and (CO2+N2) at temperatures between (218.15 and 303.15)K at pressures up to 15MPa." International Journal of Greenhouse Gas Control 36: 78-92.
4. Peng, D.-Y. and D. B. Robinson (1976). "A new two-constant equation of state." Ind. Eng. Chem. Fundamentals 15: 59−64.
5. V. Papaioannou, T. Lafitte, C. Avendano, C.S. Adjiman, G. Jackson, E.A. Muller, A. Galindo (2014). "Group contribution methodology based on the statistical associating fluid theory for heteronuclear molecules formed from Mie segments." J. Chem. Phys, 140.

Acknowledgment

We gratefully acknowledge the funding provided by of Science Without Border (Ciência Sem Fronteiras) and their permission to publish this research.