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42254-AC2
Investigating the Mechanisms of Gas Hydrate Inhibition
Background
The ability to control gas hydrate formation or dissociation are key issues for both pipeline flow assurance and the efficient and safe release of gas from natural hydrate deposits [Sloan, Nature 426, 353, 2003; Koh, AIChE Journal 53, 1636, 2007; Sloan and Koh, Clathrate Hydrates of Natural Gases, CRC Press, 2007]. It is known that hydrate nucleation and growth are significantly delayed by adding low concentrations of water-soluble polymers (about 0.5 to 1 wt.%), called kinetic hydrate inhibitors (KHI) [Kelland, Energy & Fuels 20, 825, 2006]. Thus, the development of high performance KHI is of major importance for the industry. However, there are only a limited number of studies on this phenomenon at the molecular level [Koh, Chem. Soc. Rev. 31, 157, 2002], and little is known about the effect of KHI on the time dependent structural transitions, or the role of structure and guest composition on the KHI performance.
The overall aim of this work is to advance a molecular-scale understanding of gas hydrate inhibition by studying a range of single and binary hydrate systems with molecular-level techniques, which include NMR and Raman spectroscopy and neutron diffraction. The major goals of this study are: (1) to examine the effect of model inhibitors on the kinetics and growth pathways of gas hydrates on a molecular scale and (2) to determine the effect of the structure and composition of gas hydrates on the inhibition process.
Summary of Achievements
1. Structure and Cage Occupancy Ratio of Reforming sI Methane-Ethane Hydrates with PVCap
13C MAS NMR spectroscopy has been employed to determine the structures and cage occupancies of structure I methane-ethane hydrates, which has not been studied previously in terms of kinetic hydrate inhibition, formed from gas mixtures and liquid water with polyvinylcaprolactam (PVCap). In order to investigate kinetics of hydrate inhibition hydrate samples under reformation have been measured.
The cage occupancy ratios were found to be almost the same as those expected for the system without PVCap (i.e. LC/SC ratio = 1.35±0.18), suggesting that PVCap does not affect the cage filling. This finding is different from observations of inhibited methane hydrate [Subramanian and Sloan, Fluid Phase Equilibria 158-160, 813, 1999]: PVCap decreases the large to small cavity occupancy ratios in this case. This discrepancy may be due to differences in guest-gas compositions because in both the studies hydrates have the same structure (sI) and were measured on a similar time scale.
It was also found that the reformation rate of the sample with PVCap is several times faster compared with the pure system, indicating that PVCap promotes the hydrate reformation even though this chemical is well-known as a good hydrate-nucleation inhibitor. This finding suggests that the function of KHI strongly depends on hydrate structure.
2. Effect of PVCap on Methane-Ethane Hydrate Meta-Stability
Methane-ethane hydrate structure was measured as a function of time using Raman spectroscopy to investigate how PVCap affects the meta-stability of the hydrate: in this binary system structure I (sI) and II (sII) often coexist as a metastable states. Hydrates were formed from gas mixtures (90% or 60% CH4 in a CH4/C2H6 mixture) and liquid water.
In all experiments the first hydrates observed were sI and sII mixtures. At 90% methane (sII is thermodynamically stable; Subramanian et al., Chem. Eng. Sci. 55, 5763, 2000; Ballard and Sloan, Chem. Eng. Sci. 55, 5773, 2000), for the hydrates formed from pure water complete conversion from sI into sII occurs within a few hours, while for the inhibited samples the conversion rate was two orders of magnitude slower compared with the pure samples. These results indicate that PVCap greatly retards the inter-conversion of methane-ethane hydrate. On the other hand, at 60% methane (sI formation predicted), it was found that PVCap does not affect the conversion from sII to sI considerably, indicating that the KHI's performance on the inter-conversion depends on crystal structure.
The effectiveness of KHI on the methane-ethane hydrate meta-stability has not been previously investigated. This has implications to the industry since the formation of a sI and sII mixture and subsequent inter-conversion are likely to occur in pipelines. Information which is essential for the pipeline flow assurance industry such as dissociation pressure (or temperature) and heat capacity of the hydrate are strongly dependent on crystal structure.
Future Plans
To further investigate the mechanism of hydrate inhibition, more systematic and comprehensive studies of the hydrate formation kinetics, structure and occupancy ratios will be performed for a range of temperature, pressure and gas composition conditions. The NMR studies will be extended to measurements of methane-ethane sII hydrates and to other binary hydrate systems. Other chemicals with different hydrate-inhibition performance, e.g. polyvinylpyrrolidone, will be tested for the meta-stability of methane-ethane hydrates using the Raman spectroscopy. Also, selected systems will be examined with high pressure DSC to complement the spectroscopic studies.
