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 SII 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 (KHI) polyvinylcaprolactam (PVCap). In high pressure rocking cell tests using a SII-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 KHI 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. ■ INTRODUCTIONIt has been known for about two decades that certain onium salts (quaternary ammonium and phosphonium salts) are capable of perturbing and inhibiting the growth of structure II (SII) clathrate hydrates. 1−4 Shell, the oil and energy company, was the first to discover this fact from tests with a study on SII tetrahydrofuran (THF) hydrates. The most active inhibitors of THF hydrate were found to have either n-butyl, n-pentyl, or iso-pentyl groups on the quaternary N or P atom (Figure 1). Shell's patents on this subject also cover sulphonium salts, which can have a maximum of three alkyl groups attached to the sulfur atom, althoug...

Poly(N-vinyl azacyclooctanone) (PVACO) has been synthesized for the first time. Dependent upon the method of polymerization and polymer molecular weight, the cloud point of a 1.0 wt % solution in water can be varied between approximately 14 and 22°C. Using identical polymerization conditions for the four N-vinyl lactams with 5−8-membered rings, the polymer molecular weight decreases as the ring size increases. This is probably due to the relative steric effect of the monomers in the polymerization process. In high-pressure rocking cell experiments with a structure-II-forming hydrocarbon gas mixture, PVACO was shown to be a more powerful kinetic hydrate inhibitor (KHI) than the other 5−7-ring poly(N-vinyl lactam)s of similar molecular weight made using an otherwise identical method to PVACO. The synergistic effect of mono-nbutyl glycol ether with PVACO and the effect of the polymer molecular weight on KHI performance are also discussed. ■ INTRODUCTIONKinetic hydrate inhibitors (KHIs) are now a well-known technology for preventing gas hydrate plugs in upstream oilfield operations. 1−4 KHIs are water-soluble polymers, often with added synergists that improve their performance. KHIs delay the nucleation and usually also the crystal growth of gas hydrates. The nucleation delay time (induction time), which is the most critical factor for field operations, is dependent upon the subcooling (ΔT) in the system: the higher the subcooling, the lower the induction time. The absolute pressure is also an important factor. 5−8Probably the commonest class of polymers used in commercial KHI formulations are homo-polymers and copolymers of the N-vinyl lactams N-vinyl pyrrolidone (VP) and N-vinyl caprolactam (VCap). 9−16 Recently, we showed that the homo-polymer of the 6-ring N-vinyl lactam monomer N-vinyl piperidone (VPip) had an intermediate KHI gas hydrate performance between that of poly(N-vinyl pyrrolidone) and poly(N-vinyl caprolactam). 17,18 Thus, the KHI performance increases with an increasing lactam ring size. The structures of the homo-polymers of VP, VPip, and VCap are given in Figure 1. It was therefore of great interest to investigate whether the homo-polymer of an even larger ring N-vinyl lactam would perform better than homo-polymers with the smaller lactam rings. The 8-ring N-vinyl lactam monomer N-vinyl azacyclooctanone (VACO) has not been reported previously nor have any polymers from this monomer. We were also uncertain if the homo-polymer poly(N-vinyl azacyclooctanone) (PVACO) was even water-soluble and, therefore, could be tested as a KHI, because the cloud points of the poly(N-vinyl lactam)s decrease with lactam ring size. For example, PVCap as a 1.0 wt % solution in water has a cloud point of about 30−40°C depending upon the polymerization method and molecular weight, whereas cloud points for PVPip are generally in the range of 60−80°C. 17,18 In this paper, we report the synthesis and structure II (SII) gas hydrate KHI performance of PVACO for the first time and compare the results to other poly(N-vinyl la...

Plugging of flow lines by gas hydrates is a costly and challenging problem for the oil and gas industry. One of the options available is the injection of anti-agglomerant (AA) low-dosage hydrate inhibitor (LDHI). LDHIs are injected in much lower concentration and amount in comparison to thermodynamic hydrate inhibitors (THIs). Quaternary ammonium surfactants currently dominate the AA market. In this paper, we report the first systematic study of laboratory tests on single-tail quaternary tributylammonium bromide surfactants for their ability to prevent structure II gas hydrate agglomeration in highpressure rocker cells. The effect on AA performance by varying the alkyl chain length from 8 to 18 carbon atoms and the effect of salinity have been investigated. It was shown that there is an optimum alkyl tail chain length for the anti-agglomerating efficacy of this surfactant class and that increased salinity improves the AA performance. The performance of the AAs could also be further improved by the addition of the anionic surfactant, sodium dodecyl sulfate (SDS), particularly at low salinity.

Kinetic hydrate inhibitors (KHIs) have been used successfully for about the last 16 years to prevent gas hydrate formation mostly in gas and oilfield production lines. They work by delaying the nucleation and often also the growth of gas hydrate crystals for periods of time dependent mostly on the subcooling in the system. Poly(N-alkyl-N-vinylamide)s have been briefly investigated previously but no work has previously been published detailing a systematic study of structure versus performance. In this paper we report the KHI performance of a series of poly(N-alkyl-N-vinylacetamide)s polymers with alkyl side groups up to five carbon atoms. The study includes hydrate crystal growth tests on structure II tetrahydrofuran hydrate crystals as well as high pressure nucleation and crystal growth studies on a synthetic natural gas mixture giving structure II hydrates. The SII gas hydrate inhibition results correlate well with the THF hydrate crystal growth inhibition results since the polymers with the larger isopentyl or isobutyl groups performed best in both equipments but not significantly better than a commercial low molecular weight Nvinyl caprolactam-N-vinyl pyrrolidone 1:1 copolymer.

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