(728e) A Thermodynamic Model to Understand the Gas Exchange Process in CH4+CO2 Mixed Gas Hydrates for Methane Gas Production and Carbon Dioxide Sequestration

Dnyaneshwar R. Bhawangirkar^1,2, Jitendra S. Sangwai^2*

¹Department of Chemical Engineering, Indian Institute of Technology Bombay, Powai, Mumbai 400076, India

²Gas Hydrate and Flow Assurance Laboratory, Petroleum Engineering Program, Department of Ocean Engineering, Indian Institute of Technology Madras, Chennai 600036, India

Keywords: Gas hydrates; Thermodynamic modelling; High pressure phase equilibria

ABSTRACT

The increasing demand for energy and climate change mitigation are the two major issues which received much more attention in present days. However, it is possible to catch these two birds in one stone by recovering the huge amount of methane gas that is trapped in the form hydrates deep in the ocean reservoirs, and sequester the carbon dioxide gas in the form of hydrates as a replacement to methane gas. However, the replacement mechanism is still not very clear as it involves a complex mechanism. Here, we have made an attempt in understanding the phase equilibrium conditions under three phase coexistence i.e., liquid, hydrate and vapour phase (L-H-V) and replacement mechanism or the gas exchange process in CH₄-CO_2­ binary system by calculating the cage occupancies in small, large and in hydrate cages utilizing Klauda and Sandler’s thermodynamic model. This model is based on the concept of equal fugacities between the hydrate phase and the liquid phase. The modified UNIFAC method is used for calculating the liquid phase activity coefficients. For the vapour phase fugacity calculations the Peng-Robinson-Stryjek-Vera equation of state is used. The solubility of each guest (CH₄ and CO₂) in the liquid phase is estimated by using Henry’s law. It has been observed that the percent absolute average deviations (%AAD) in the predicted equilibrium pressures for CH₄-CO₂ mixture for a total number of 147 experimental data points has found to be 2.9% showing the robustness of this model. Methane gas molecule is small in size than carbon dioxide. However, both gas molecules either in pure form or in the mixture (at any composition) forms sI structured hydrate. sI structure has a total of eight cavities with two small and six large cavities in a unit cell. To understand the gas exchange mechanism we have calculated the cage occupancies of methane and carbon dioxide in small, large, and in hydrate cages (small and large cages together) under three-phase coexistence (L-H-V) at 280 K.

However, from cage occupancy results for the binary system (CH₄+CO₂ mixture), it has been observed that methane gas preferentially occupies in small cavities and carbon dioxide in large cavities due to their size differences. We have also calculated the averaged distribution coefficient of methane gas, i.e., [= vapour phase and = hydrate phase, and 1, 2 represents CH₄ and CO₂, respectively] and is found to be 2.06. The distribution coefficient of methane gas helped us in understanding the gas exchange process happening in vapour and hydrate phase in a much better way. We observed that the mole fraction of methane gas is found to be more significant in vapour phase than in hydrate phase, whereas the carbon dioxide has found to be more in hydrate phase than in vapour phase, which means methane gas molecules from the hydrate cages are selectively replaced by carbon dioxide molecules. This information will certainly add value during recovery of methane gas from the hydrate reservoirs, and also simultaneously sequester carbon dioxide in the form of solid hydrates deep in the ocean.