The Driving Forces of Guest Substitution in Gas Hydrates?A Laser Raman Study on CH4-CO2 Exchange in the Presence of Impurities

Resumen

The recovery of CH₄ gas from natural hydrate formations by injection of industrially emitted CO₂ is considered to be a promising solution to simultaneously access an unconventional fossil fuel reserve and counteract atmospheric CO₂ increase. CO₂ obtained from industrial processes may contain traces of impurities such as SO₂ or NO_x and natural gas hydrates may contain higher hydrocarbons such as C₂H₆ and C₃H₈. These additions have an influence on the properties of the resulting hydrate phase and the conversion process of CH₄-rich hydrates to CO₂-rich hydrates. Here we show results of a microscopic and laser Raman in situ study investigating the effects of SO₂-polluted CO₂ and mixed CH₄-C₂H₆ hydrate on the exchange process. Our study shows that the key driving force of the exchange processes is the establishment of the chemical equilibrium between hydrate phase and the surrounding phases. The exchange rate is also influenced by the guest-to-cavity ratio as well as the thermodynamic stability in terms of p-T conditions of the original and resulting hydrate phase. The most effective molecule exchange is related to structural changes (sI-sII) which indicates that hydrate decomposition and reformation processes are the occurring processes.