Key Structural Features for an Effective Kinetic Hydrate Inhibitor─You Need Them All!

Kinetic gas hydrate inhibitor (KHI) liquid formulations have been used since the mid-1990s as a method to prevent gas hydrate formation in oil and gas production flow lines. All commercial KHIs contain one or more polymers as the key active ingredient. Among the commercial homopolymers, poly(N-vinylpyrrolidone) (PVP) is considered a mild KHI (low performance), whereas poly(N-vinylcaprolactam) (PVCap) or poly(N-isopropylmethacrylamide) (PNIPMAM) are powerful KHIs (high performance). Our contention is that KHI formulations only give high performance if they contain one or more water-soluble molecules with a high density of multiple amphiphilic groups with the correct size hydrophobic groups adjacent to strong hydrogen-bonding groups. This often means oligomers or polymers (as well as copolymers), but other molecules with a high density of the correct multiple amphiphilic groups can also be used. Blends of such molecules, or addition of certain molecules that do not conform to the theory, can be added to boost the performance. Here we review past work related to this claim and present experimental results on some molecules recently reported to be active KHIs, but which fall outside our structural definition of what makes a good KHI. These molecules all gave very poor KHI performance in our steel rocking cell equipment using either methane or a natural gas mixture in slow (1.0 °C/h) constant cooling tests when compared to classic KHIs such as PVCap. They include safranine O, several amino acids, l-ascorbic acid, alanine-glycine dipeptide, 18-crown-6 ether, hydroxyethylcellulose, pectin, carboxymethylguar, iota- and lamba-carrageenans, and poly(vinyl alcohol) (PVA) plus several more hydrophobically modified PVA derivatives. Finally, based on 34 years of KHI experience, we give some advice to new KHI researchers.