Tris(dialkylamino)cyclopropenium chlorides: Tetrahydrofuran hydrate crystal growth inhibition and synergism with polyvinylcaprolactam as gas hydrate kinetic inhibitor

A series of tris(dialkylamino)cyclopropenium chloride salts with different alkyl groups of up to 5 carbon atoms has been synthesised and their performance studied as low dosage hydrate inhibitors for the first time. We have investigated the ability of these organic salts to inhibit the growth of tetrahydrofuran (THF) hydrate crystals. The hexa-n-butylated derivative gave the best crystal growth inhibition, with a similar performance to tetrabutylammonium bromide (TBAB). The hexa-pentylated derivatives (both normal and iso) were found to be water-insoluble. Sparingly soluble cyclopropenium derivatives with a 2:1 mixture of di-n-pentylamino:diethylamino groups gave poor THF hydrate crystal growth inhibition.This series of cyclopropenium salts was also investigated for their ability to act as synergists for the kinetic hydrate inhibitor (KHI), polyvinylcaprolactam (PVCap). In a series of high pressure rocking cell tests using a structure II-forming natural gas mixture, the hexa-butylated cyclopropenium derivative performed well, but the best synergist was the cyclopropenium derivative with a 2:1 mixture of di-iso- and di-n-pentylamino:diethylamino groups. As has been shown in previous work with the synergism of tetraalkylammonium salts with PVCap (P.C. Chua, M.A. Kelland, 2012. Energy Fuels 26, 1160), we presume that adsorption onto hydrate crystal surfaces is not the only KHI mechanism operating and that perturbing the water structure and thus the nucleation of gas hydrates with the cyclopropenium salts is also a key KHI mechanism.

► A series of tris(dialkylamino)cyclopropenium chloride salts have been synthesised.
► The salts were investigated as synergists for the gas hydrate kinetic inhibitor, poly(N-vinyl caprolactam).
► They perform best with peripheral butyl or pentyl groups as long as the salt is water-soluble.
► The best tris(dialkylamino)cyclopropenium chloride salts performed better as synergists than tetrabutylammonium bromide.