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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.

We present pure CH4 and CH4/C3H8 mixed hydrate phase equilibria formed from a mixture of NaCl (10 wt %) and monoethylene glycol (MEG, 10 and 30 wt %) solutions. As expected for thermodynamic inhibitors, the mixture of salt and glycol causes the hydrate phase equilibrium boundary to shift to lower temperatures and higher pressures, and on increasing the MEG concentration, the hydrate stable region shifted more. The measured experimental data are also compared with a thermodynamic model recently developed, named the Hu–Lee–Sum correlation, showing that the data match well with the predictions. The experimental data were used to calculate the enthalpy of hydrate dissociation. The enthalpies of CH4 hydrates in the mixture of 10 wt % NaCl brine and 10 or 30 wt % MEG were found to be ∼58.7 and 54.63 kJ/mol, respectively, corresponding to structure I hydrates, whereas for the CH4/C3H8 (91.98:8.02 mol %) mixed gas system, the enthalpies of dissociation were found to be ∼101.10 kJ/mol (10 wt % NaCl + 10 wt % MEG) and 95.34 kJ/mol (10 wt % NaCl + 30 wt % MEG), confirming the mixed hydrates formed structure II. We also performed Raman analysis for CH4 hydrates and CH4/C3H8 mixed hydrates in the NaCl and MEG system and investigated their spectroscopic behavior and hydrate structure.

The current knowledge of guest−guest interactions and co-occupation in clathrate hydrates is exclusive for the same guests (H 2 or N 2 ) at moderate pressure. Here, we introduce the unusual co-occupation of distinct guests in the metastable state of hydrates. With controlled hydrate fraction, particle size, and intensification of the sintering of SF 6 hydrate particles formed from water and SF 6 gas as a help gas, we observed an abnormal but unique synchronous behavior in Raman intensities of two guest molecules (SF 6 and N 2 /H 2 ) in hydrates consistently and repeatedly; over time, the scattering intensity for the guests (i) increases, (ii) decreases, and (iii) finally reaches the stable level. Without a concentration change of SF 6 , this abnormal behavior must arise from the possible changes in the scattering cross section of the molecules, suggesting that N 2 /H 2 strongly interacts with SF 6 in the large cages, resulting in a possible co-occupation during the metastable transition. These observations on the metastability of gas hydrate attest the importance of the sintering effect as a barrier to prevent fast gas diffusion for reaching equilibrium, which could have significant implication in increasing overall gas storage in clathrate hydrates.