Experimental Study on the Dissociation Equilibrium of (CH₄ + CO₂ + N₂) Hydrates in the Mixed Sediments

Abstract

The recovery of CH 4 from gas hydrate by injecting flue gas would inevitably lead to coexistence of CH 4 , CO 2 , and N 2 in the hydrate phase. To simulate the actual situation and investigate the stability condition of (CH 4 + CO 2 + N 2 ) hydrates, their dissociation equilibrium in the mixed sediments of quartz sand and kaolin clay was measured with the isochoric pressure search method. Six gas samples were tested and analyzed with respect to the CH 4 content and the mole ratio of CO 2 /N 2 in the gas phase. The results indicated that the hydrate dissociation equilibrium pressure at a given temperature shows a decreasing trend as the CH 4 content increases at a lower CO 2 /N 2 ratio, which was contrary to that at a higher CO 2 /N 2 ratio. The dissociation equilibrium temperature of (CH 4 + CO 2 + N 2 ) hydrates for the mixed sediments is shifted lower than those for bulk phase at the same pressure. The effect of the mixed sediment composition and water saturation on the hydrate dissociation equilibrium was investigated. The results showed that the temperature shift increases with the increase of kaolin content in the mixed sediments, while it decreases as the water saturation increases. The maximum temperature shift observed in this work was 1.56 K in the mixed sediments (the mass ratio quartz sand and kaolin clay was 1:0.20) with a water saturation of 20%. The hydrate dissociation enthalpies are calculated with the Clausius−Clapeyron equation. It was observed that the hydrate dissociation enthalpies exhibited a decreasing trend with the increase of CH 4 content in the gas phase.

Phase Equilibria of Natural Gas Hydrates in Bulk Brine and Marine Sediments from the South China Sea

“…The authors attributed these differences mainly to the forces between water molecules and different sediments media. On the other hand, Mu and Cui compared the differences of CH 4 hydrate phase equilibrium conditions in bulk water and in quartz sand systems with different salinity, which is similar to our comparison. However, there was no such coupling effect between the sediments and salt in their experiment, and the temperature shift in sediments and bulk phase at different salinities was almost equal.…”

“…It reveals that the presence of marine sediments does not change the inhibiting effect of saline ions on hydrate. The influences of porous sediments on hydrate phase equilibrium conditions are related to many factors, including pore size, medium features, the content of mixed porous media, and water saturation in the sediments. ,, The similarities and differences of hydrate phase equilibrium being affected in marine sediments compared with the generally used artificial media will be analyzed in detail in what follows.…”

“…In addition to porous sediments, gas compositions and brine components can also significantly affect the hydrate phase equilibrium conditions. The experiment of Mu and Cui 10 showed that the increase of CO 2 gas content in CO 2 /CH 4 systems made the hydrates more stable. Dickens and Quinby-Hunt 14 reported that the decomposition temperatures of CH 4 hydrates in seawater were reduced by about 1.1 K compared to that in the pure water system.…”

“…Only in small-grained quartz sand can the sediments themselves affect the phase equilibrium conditions. Mu and Cui studied the hydrate decomposition conditions in mixed sediments containing quartz sand and kaolinite. They found that the equilibrium temperature increased with the higher kaolinite content.…”

Experimental Study on CO2 Capture from Simulated Flue Gas with an Adsorption–Hydration Method

“…Subsequently, the prepared wet activated carbons were placed in the hydrate reactor. The hydrate equilibrium conditions were determined by using an isochoric pressure searching method, which has been specifically described in our previous work. , After evacuation, the simulated flue gas was injected into the hydrate reactor. The temperature was decreased at a certain cooling rate to form hydrates.…”