Inhibition of tetrahydrofuran hydrate crystal growth by tetraalkylphosphonium salts—superior performance to equivalent tetraalkylammonium salts

Abstract

A series of tetraalkylphosphonium bromide salts have been tested as tetrahydrofuran (THF) hydrate crystal growth inhibitors at varying concentrations and temperatures. In general, the inhibition performance is better than that of tetraalkylammonium bromide salts. For example, tetra(n-butyl)phosphonium bromide is a much superior inhibitor than tetra(n-butyl)ammonium bromide. Tetra(n-pentyl)phosphonium bromide was the best phosphonium salt investigated and had a performance close to that of the best reported ammonium salt, tetra(iso-hexyl)ammonium bromide. Although we have not yet succeeded in synthesising tetra(iso-hexyl)phosphonium bromide, the improved performance of phosphonium salts with iso-hexyl groups was demonstrated by synthesising and testing a series of tri(n-butyl)alkylammonium bromides. The alkyl group was varied from 2 to 6 carbon atoms including normal- and iso-alkyl groups. The best THF hydrate crystal growth inhibition for this series was obtained when the alkyl group was iso-hexyl. These results suggest that the best tetraalkylphosphonium salts could be used as improved low dosage hydrate inhibitors, both in kinetic hydrate inhibitor formulations or as anti-agglomerants, compared to tetraalkylammonium salts.