Fluorinated Quaternary Ammonium Bromides: Studies on Their Tetrahydrofuran Hydrate Crystal Growth Inhibition and as Synergists with Polyvinylcaprolactam Kinetic Gas Hydrate Inhibitor

摘要

Fluorinated low-dosage hydrate inhibitors (LDHIs) have been investigated for the first time. Three novel quaternary ammonium salts with monofluorobutyl groups were synthesized and characterized, N-(4-fluorobutyl)-N, N-dipentylpentan-1-aminium bromide (2), N,N-bis(4-fluorobutyl)-N-pentylpentan-1-aminium bromide (3), and N, N, N-tris(4-fluorobutyl)pentan-l-aminium bromide (4). It was investigated whether these fluorinated ammonium salts, in which a fluorine atom replacing an end methyl group in the alkyl chains of tetra(n-pentyl)ammonium bromide (TPAB), would affect tetrahydrofuran (THF) hydrate crystal growth inhibition. We determined that the monofluoro ammonium salt N-(4-fluorobutyl)-N, N-dipentyipentan-l-aminium bromide (2) was the best THF hydrate crystal growth inhibitor, showing better performance than either TPAB or tetra(n-butyl)ammonium bromide (TBAB). However, THF hydrate crystal growth inhibition decreased as the number of fluorobutyl groups on the quaternary nitrogen atom was increased to two or three. In addition, we observed that the monofluoro ammonium salt (2) was a better kinetic hydrate inhibitor synergist than TPAB in combination with pory(N-vinyl caprolactam) (PVCap) under high-pressure rocking cell tests using a structure-Ⅱ-forming natural gas mixture. The difluoro and trifluoro salts (3 and 4) were poorer synergists in keeping with the trend from the THF hydrate crystal growth experiments. A mechanism to explain these results is proposed.