Nucleation probability and memory effect of methane-propane mixed gas hydrate

Abstract

The hydrate memory effect is one of the top puzzles in the field of gas hydrate. We developed a sapphire cell to test the memory effect of hydrate under linear cooling ramp conditions. The methane-propane mixed gas with a mass ratio of 9:1 was used to form hydrate at a constant cooling rate of 0.1 K/min. Influences of superheating, heating duration, and dissociation pressure on the strength and certainty of memory effect were studied. Our experiments show little evidence of the strengthening of the memory effect with the increase of cooling/heating circles. The nucleation probability of C1/C3 mixed gas hydrate would be weakened by excessive superheating temperatures and vanishes once hydrate was forced to dissociate at a superheating temperature of 14 K for 60 min. The memory effect increases linearly with the decrease of hydrate dissociation temperature and dissociation duration. However, the dissociation temperature and duration contribute unequally to the strength of the memory effect. The hydrate nucleation subcooling temperature is found to be more predictable (i.e. less stochastic) at a relatively lower dissociation temperature, whereas the dissociation pressure has little influence on the certainties of the hydrate nucleation subcooling. However, the certainty of the memory effect becomes higher with the increase of dissociation duration, although the strength of the memory effect becomes weaker. The strength and certainty of the memory effect are supposed to be affected by the behaviors of dipoles surrounding MNBs.