Boronic and Organic Acids as Synergists for a Poly(N-vinylcaprolactam) Kinetic Hydrate Inhibitor

Abstract

A range of boronic acids have been investigated as synergists for the kinetic hydrate inhibitor (KHI) polymer, poly(N-vinylcaprolactam) (PVCap, Mw ≈ 10,000 g/mol) using high pressure rocking cells, a natural gas mixture, and a slow constant cooling (1 °C/h) test method from 76 bar. Surprisingly, unlike other classes of synergists such as alcohols and quaternary ammonium salts, the boronic acids that gave the best synergy had an alkyl or cycloalkyl tail with a maximum of a 3 carbon atom distance from the boron atom. The tail-branched iso-butylboronic acid was the best of these, yet it showed a negligible KHI effect when tested alone. However, consistent with the other classes of synergists, tail branching or use of a cyclic alkyl group was beneficial. Interestingly, boronic acids with chains of 5 to 6 carbon atoms, i.e., n-pentyl- and n-hexylboronic acids, were antagonistic to the PVCap KHI performance. For comparison, several organic acids were also investigated as synergists with PVCap. The same trend as for the boronic acids regarding the size and branching of the acid was seen. 3-Methylbutanoic acid gave the best synergy although worse than that of iso-butylboronic acid. The synergistic performance of sodium salts of some organic acids differed markedly to that of the free organic acids. Sodium 3,3-dimethylbutanoate gave the best synergy with PVCap.