Antiagglomerants
(AAs) are surfactants used in the upstream oil
industry to prevent gas hydrate plugging in flow lines. Most, if not
all, AAs used in the field today are cationic quaternary ammonium
surfactants and function using the “hydrate-philic”
mechanism. In this study, we have synthesized a series of butylated
mono- and bis-amine oxide surfactants with aliphatic tails with chains
of 9–17 carbon atoms and either amide or ester spacer groups.
Their performance as hydrate-philic AAs has been investigated in a
sapphire autoclave and sapphire rocking cells with a Structure II-forming
natural gas mixture. There was generally good agreement regarding
the performance and relative ranking of the surfactants between the
two sets of equipment. The AA performance of the amine oxide surfactants
depended on many factors, including the polar head and spacer groups,
tail length, subcooling at hydrate onset, salinity, and the composition
of the hydrocarbon fluid. Amido amine oxide surfactants performed
better than the equivalent ester amine oxide, probably due to the
stronger hydrogen-bonding ability of the amide group. The bis-amine
oxide surfactants were designed with optimum interamine distance for
best Structure II crystal growth inhibition and performed better as
AAs than did the mono-amine oxide surfactants. The amido bis-amine
oxide surfactants showed reasonable seawater biodegradation rates
over 28 days, giving biological oxygen demand values of 25–40%.
The bis-amine oxide surfactants in particular also showed a strong
kinetic hydrate inhibition effect, which could be very useful for
field applications. Thus, these surfactants could be used first as
kinetic hydrate inhibitors (KHIs), and if for any reason hydrate formation
does occur they could also function as AAs to prevent hydrate blockages
under certain conditions.