Tetrabutyl ammonium bromide (TBAB) forms a semi-clathrate hydrate, which can incorporate small gas molecules such as methane and nitrogen. It has recently been used for separation of gases. However, there are very limited experimental data on the phase boundaries of the gas hydrate form in the presence of TBAB. In this work, we present new experimental data at high-pressure TBAB, w ) 0.10, TBAB (w ) 0.10 and 0.43) + hydrogen, TBAB (w ) 0.05, 0.10, 0.20, and 0.30) + methane, TBAB (w ) 0.10) + nitrogen, TBAB (w ) 0.1 and 0.427) + carbon dioxide, and TBAB (w ) 0.05, 0.10, and 0.43) + natural gas semi-clathrate hydrate phase boundaries. In another part of this work, the results of visual observations of the methane + TBAB semi-clathrate hydrate morphology and the methane gas bubbles released from methane + TBAB semi-clathrate hydrates on dissociation are presented. Finally, the effect of TBAB mass fraction on hydrate promotion and the stability of the new semiclathrate hydrate are presented.

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Natural gas hydrates occur widely on the ocean-bed and in permafrost regions, and have potential as an untapped energy resource. Their formation and growth, however, poses major problems for the energy sector due to their tendency to block oil and gas pipelines, whereas their melting is viewed as a potential contributor to climate change. Although recent advances have been made in understanding bulk methane hydrate formation, the effect of impurity particles, which are always present under conditions relevant to industry and the environment, remains an open question. Here we present results from neutron scattering experiments and molecular dynamics simulations that show that the formation of methane hydrate is insensitive to the addition of a wide range of impurity particles. Our analysis shows that this is due to the different chemical natures of methane and water, with methane generally excluded from the volume surrounding the nanoparticles. This has important consequences for our understanding of the mechanism of hydrate nucleation and the design of new inhibitor molecules.

In the production of natural gas, (mono-)ethylene glycol (MEG) is commonly added to the well stream to prevent the formation of clathrate natural gas hydrates. A reduction in the amount of MEG required for hydrate prevention in industrial subsea flowlines would decrease the costs associated with natural gas production. Methyldiethanolamine (MDEA) is sometimes used for corrosion control in wet gas flowlines by increasing the solution pH, and will be typically injected with the MEG. In systems where both hydrate and corrosion control is required, hydrate inhibition via MDEA could represent an opportunity to reduce the required MEG injection rate. However, no experimental data are available to quantify the degree to which MDEA may act as a hydrate inhibitor, either in isolation or in the presence of MEG. In this work, we report 20 measurements of the hydrate phase boundary in the presence of MDEA (3 to 7 vol%) and MEG (0 or 20 vol%), performed at high pressure (6 to 9 MPa) in a sapphire autoclave cell with both ultra-high purity methane and a natural gas mixture. The results illustrate that MDEA acts as a hydrate inhibitor and, when combined with MEG, provides additional inhibition. For the systems studied, the effectiveness of MDEA as a hydrate inhibitor is approximately half that of MEG. When 20 vol% MEG was added to the aqueous phase, the MDEA became less effective as a hydrate inhibitor. However, 7 vol% MDEA still caused an average temperature shift in the hydrate phase boundary of 0.3 K, which is equivalent to the effect that would be achieved by increasing the amount of MEG in the system by 3 % (i.e. from 20 vol% to 20.6 vol% MEG).

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