Crystal growth inhibition of tetrahydrofuran hydrate with bis- and polyquaternary ammonium salts

Small quaternary ammonium salts, particularly with butyl or pentyl groups, have been shown previously to be good Structure II tetrahydrofuran hydrate crystal growth inhibitors. This ability has been exploited in their use as synergists for certain kinetic hydrate inhibitor polymers, and in the design of commercial gas hydrate anti-agglomerants. We have now investigated the effect of bis- and polyquaternary ammonium salts, in which all the nitrogen atoms are in the backbone of the molecule, on the crystal growth of tetrahydrofuran hydrates. The results are compared with those of monomeric quaternary compounds previously reported. The new compounds tested contain 2 to over 100 quaternary centres with various structural geometries including linear and branched polymers. The size of the alkyl groups on the nitrogen atoms was varied and contained from 2 to 6 carbon atoms. Improved performance over monoquaternary ammonium salts was obtained with both bis-quaternary and polyquaternary ammonium products. The optimal distance between the nitrogen atoms in bis-quaternary ammonium salts for best THF hydrate inhibition appears to be with a chain of 6–8 aliphatic carbon atoms.

► Bis- and polyquaternary ammonium salts are effective hydrate crystal growth inhibitors.
► Some of them are more effective than mono-quaternary ammonium salts.
► They work best with butyl and pentyl groups.
► The optimum inter-nitrogen distance is a chain of about 6–8 aliphatic carbon atoms.