Hydrate-Phase Equilibria and ¹³C NMR Studies of Binary (CH₄ + C₂H₄) and (C₂H₆ + C₂H₄) Hydrates

Three-phase equilibria of hydrate + liquid water + vapor phases were investigated at various gas compositions for binary gas mixtures of (CH₄ + C₂H₄) and (C₂H₆ + C₂H₄). Hydrate-phase equilibria of the binary (CH₄ + C₂H₄) hydrate show significant changes with changing composition, whereas those of binary (C₂H₆ + C₂H₄) hydrates show little difference because of the similar physical properties of the two guest species. In addition to macroscopic equilibrium measurements, solid-state ¹³C NMR spectra of hydrate samples were collected to identify both cavity occupancies of guest components and hydrate structures. For the binary (CH₄ + C₂H₄) hydrate, the large-cavity occupancy of C₂H₄ molecules increased nonlinearly with increasing C₂H₄ concentration, which supports nonlinear shifts of the equilibrium curves. Meanwhile, the large-cavity occupancy of C₂H₄ molecules from the (C₂H₆ + C₂H₄) gas mixtures increased linearly with increasing C₂H₄ concentration, which is attributed to linear changes in the distribution or density

Competing Occupation of Guest Molecules in Hydroquinone Clathrates Formed from Binary C₂H₄ and CH₄ Gas Mixtures

When reacted with pure ethylene (C₂H₄) and pure methane (CH₄) at 2.0 and 4.0 MPa, respectively, pure hydroquinone (HQ) was converted into β-form clathrate compounds. Experimental solid-state ¹³C NMR spectra and powder X-ray diffraction patterns provided direct evidence of C₂H₄ and CH₄ enclathration in the β-form HQ clathrates. On the basis of cage occupancy from the solid-state ¹³C NMR spectra, C₂H₄ (cage occupancies of 0.81–0.88) molecules are more likely to occupy the clathrate cages than CH₄ molecules (cage occupancies of 0.38–0.39). The selective occupation by C₂H₄ was also observed for HQ clathrates formed from C₂H₄ and CH₄ gas mixtures of 10, 30, 50, 70, and 90 mol % concentrations of C₂H₄. The experimental results from this study could be applied to a clathrate-based process for separating and concentrating C₂H₄ from gas mixtures

Synergetic Effect of Ionic Liquids on the Kinetic Inhibition Performance of Poly(<i>N</i>‑vinylcaprolactam) for Natural Gas Hydrate Formation

To identify the synergetic inhibition effects of ionic liquids (ILs) containing tetrafluoroborate anion (BF₄^–), various ILs, poly­(<i>N</i>-vinylcaprolactam) (PVCap), commercially available polymeric hydrate inhibitor, and their mixtures, were tested as kinetic hydrate inhibitors (KHIs) for natural gas hydrate formation. The experimental results revealed that PVCap–IL mixtures exhibited significantly higher KHI performance. In particular, the mixture of PVCap and 1-hexyl-1-methylpyrrolidinium tetrafluoroborate (HMP-BF₄) showed the best hydrate inhibition effectiveness, even under higher pressures. As HMP-BF₄ also exhibited the highest hydrate-nucleation-inhibiting performance when it was used alone, further experiments were performed using the mixtures of PVCap and HMP-BF₄ at various combinational concentrations. As a result of the experiments, the combination of 1.0 wt % PVCap and 0.5 wt % HMP-BF₄ was found to provide the longest induction time. The excellent synergetic effect of the ILs on natural gas hydrate inhibition may arise from the prevention of methane-containing 5¹² cage formation by the ILs, inducing inhibition of metastable structure I hydrate formation

Synergetic Effect of Ionic Liquids on the Kinetic Inhibition Performance of Poly(<i>N</i>‑vinylcaprolactam) for Natural Gas Hydrate Formation

To identify the synergetic inhibition effects of ionic liquids (ILs) containing tetrafluoroborate anion (BF₄^–), various ILs, poly­(<i>N</i>-vinylcaprolactam) (PVCap), commercially available polymeric hydrate inhibitor, and their mixtures, were tested as kinetic hydrate inhibitors (KHIs) for natural gas hydrate formation. The experimental results revealed that PVCap–IL mixtures exhibited significantly higher KHI performance. In particular, the mixture of PVCap and 1-hexyl-1-methylpyrrolidinium tetrafluoroborate (HMP-BF₄) showed the best hydrate inhibition effectiveness, even under higher pressures. As HMP-BF₄ also exhibited the highest hydrate-nucleation-inhibiting performance when it was used alone, further experiments were performed using the mixtures of PVCap and HMP-BF₄ at various combinational concentrations. As a result of the experiments, the combination of 1.0 wt % PVCap and 0.5 wt % HMP-BF₄ was found to provide the longest induction time. The excellent synergetic effect of the ILs on natural gas hydrate inhibition may arise from the prevention of methane-containing 5¹² cage formation by the ILs, inducing inhibition of metastable structure I hydrate formation

Experimental Measurement of Phase Equilibrium of Hydrate in Water + Ionic Liquid + CH₄ System

With the goal of discovering a more effective type of thermodynamic hydrate inhibitors (THIs), the phase equilibrium conditions of CH₄ hydrates were examined in the presence of morpholinium and piperidinium ionic liquids (ILs) at a mass fraction of 0.1. It was observed that the addition of ILs shifted the hydrate equilibrium conditions toward higher pressure and lower temperature compared with those of hydrates formed from pure water. Both cationic and anionic species influenced the equilibrium conditions of the CH₄ hydrate. The piperidinium ILs showed better inhibition effect than did the morpholinium ILs at the same species of counteranions. The result may be due to the more hydrophobic nature of piperidinium ILs, which have a higher affinity for CH₄ molecules. It was also seen that the inhibition effect of BF₄^– ions was stronger than that of Br^– ions for both piperidinium and morpholinium ILs. Thus, the inhibition effect became stronger in the order: <i>N</i>-ethyl-<i>N</i>-methylpiperidinium tetrafluoroborate ([EMPip]­[BF₄]) > <i>N</i>-ethyl-<i>N</i>-methylpiperidinium bromide ([EMPip]­[Br]) > <i>N</i>-ethyl-<i>N</i>-methylmorpholinium tetrafluoroborate ([EMMor]­[BF₄]) > <i>N</i>-ethyl-<i>N</i>-methylmorpholinium bromide ([EMMor]­[Br]). The best among these ILs had inhibition effectiveness comparable with ethylene glycol and triethylene glycol, which are used commercially as THIs