Tetra(iso-hexyl)ammonium Bromide-The Most Powerful Quaternary Ammonium-Based Tetrahydrofuran Crystal Growth Inhibitor and Synergist with Polyvinylcaprolactam Kinetic Gas Hydrate Inhibitor

Tetraalkylammonium salts, particularly with several n-butyl or n-pentyl or iso-pentyl groups, have previously been shown to be excellent structure II (SII) gas and tetrahydrofuran (THF) hydrate crystal growth inhibitors. We have investigated the ability of quaternary ammonium salts with isoalkyl groups with 5 to 7 carbons atoms to inhibit the growth of tetrahydrofuran (THF) hydrate crystal. Two new quaternary salts were synthesized for the first time: tetra(iso-hexyl)ammonium bromide (TiHexAB) and tetra(iso-heptyl)ammonium bromide (TiHepAB). It was found that tetra(iso-pentyl)ammonium bromide (TiPeNB) gave poorer crystal growth inhibition than isomeric tetra(n-pentyl)ammonium bromide. (TPAB) but similar performance to tetra(n-butyl)ammonium bromide (TBAB). However, TiHexAB gave better inhibition than any quaternary ammonium previously reported, including TPAB. TiHepAB was a poorer THF hydrate crystal growth inhibitor, similar in, performance to isomeric tetra(n-hexyl)ammonium bromide. We believe the,reason for these results is related to the optimal length of the n-alkyl/isoalkyl groups and the improved van der Waals interaction with open SIT hydrate cages with the isoalkyl branching at the end of the chains, compared to a straight alkyl chain. The superior inhibition performance of TiHexAB was illustrated by testing its ability as a synergist for the well-known kinetic hydrate inhibitor (ICHI) polyvinylcaprolactam (PVCap). In high pressure rocking. cell tests using a SU-forming natural gas mixture, TiHexAB clearly outperformed TBAB, TPAB, and all the other quaternary ammonium salts tested. In addition, tetra(n-hexyl)ammonium bromide (THexAB) gave ICHI synergism with PVCap superior to that of TBAB, even though TBAB was a better THF hydrate crystal growth inhibitor. We speculate that adsorption onto hydrate crystal surfaces may not be the only synergistic mechanism operating and that the more hydrophobic THexAB is perturbing the nucleation of hydrate more than the less hydrophobic TBAB. In addition, it was investigated whether replacing a carbon atom with an oxygen atom (ether linkage) in the alkyl chains of TPAB would affect the THF hydrate crystal growth inhibition. Thus, tetra(alkoxyethyl)ammonium bromides were prepared for the first time where the alkoxy group is ethoxy or methoxy. Both of these quaternary ammonium salts gave negligible THF hydrate crystal growth inhibition.