Kinetic Studies on Methane Hydrate Formation in the Presence of Kinetic Inhibitor via in Situ Raman Spectroscopy

To fully understand the influence of kinetic inhibitor, poly-N-vinylcaprolactam (PVCap), on methane hydrate formation, gas uptake measurement (macroscopic point of view) and in situ Raman spectroscopic analysis (microscopic point of view) were carried out simultaneously in a semibatch stirred tank reactor at constant temperature (T) and pressure (P). The capturing the behavior of guest molecules (large to small cavity ratio or cage variation) is one of the most important properties of gas hydrate studies. However the real-time properties of the cage variation under constant T and P conditions in an agitation system have not been reported previously. In this study, we measured this property (i.e., the large to small cavity ratio) from the in situ Raman spectra during hydrate formation in an agitation system instead of a static system, which provided valuable information on the time-dependent hydrate kinetic behavior. The study reveals that the presence of PVCap prevents the rate of large cavity encapsulation at an early stage of hydrate formation. The influence of PVCap from microscopic and macroscopic points of view is also presented.