The synergetic effect of a range of different solvents on the kinetic hydrate inhibitor (KHI) performance of poly­(N-vinylcaprolactam) (PVCap) has been investigated. The equipment used was a high-pressure (76 bar) rocking cell apparatus using slow constant cooling (approximately 1 °C/h from 20.5 °C) and a synthetic natural gas mixture forming structure II hydrate. The synergetic effect was investigated by adding 5000 ppm of a range of alcohols, glycol ethers, and ketones to a solution of 2500 ppm of PVCap (M w = 10 000 g/mol). For many of the additives, the ranking of the synergetic effect can be explained with reference to the size, shape, and hydrophobicity of the main alkyl group (“tail”) in the molecule as well as the presence of a glycol ether group. Among all of the solvents investigated, the best synergetic effect was achieved by 4-methyl-1-pentanol. When 5000 ppm of 4-methyl-1-pentanol was added to 2500 ppm of PVCap, no hydrate formation occurred down to the minimum test temperature of 3 °C (subcooling at ca. 16.3 °C) in 15 parallel experiments compared to 10.4 °C for pure PVCap. Predictions for improved glycol ether synergists are given.

The synergistic effect of a range of solvents on the kinetic hydrate inhibitor (KHI) performance of poly­(N-isopropyl methacrylamide) (PNIPMAm) has been investigated in slow (ca. 1 °C/h) constant cooling high-pressure (76 bar) rocking cell experiments using a structure II forming synthetic natural gas mixture. We have tested the synergistic effects of several monoglycol ethers and compared them to those of corresponding diglycol ethers and alcohols. Compounds containing lactate, carboxylate, ammonium, and pyrrolidone headgroups have also been investigated as synergists. In general the monoglycol ethers were found to function best as synergists with PNIPMAm, and the performance improved going from n-butyl to isobutyl, cyclopentyl, and cyclohexyl alkyl substituents. The best performing synergist in this study was found to be 2-(cyclohexyloxy)­ethanol (CHexGE). Addition of 5000 ppm CHexGE to 2500 ppm PNIPMAm-II (made in isopropyl alcohol and precipitated out, M n = 8100 g/mol) dropped T o from 6.8 °C for pure polymer to the situation where no hydrate formation occurred in 9 out of 10 experiments down to 2 °C, the minimum test temperature and a subcooling of 17.3 °C. PNIPMAm identically made with similar molecular weight and kept in n-butyl glycol ether (nBGE), isobutyl glycol ether (iBGE), and tert-butyl glycol ether (tBGE) reaction solvents gave similar performances as KHIs. The performance of PNIPMAm sometimes varied depending on whether the polymer was made and kept in synergist solvent, or synergist was added to the isolated polymer. For example, nBGE added to pure PNIPMAm performed worse than PNIPMAm made in nBGE, even when the polymer molecular weights were similar. In contrast, PNIPMAm made in iBGE gave a performance similar to that of iBGE added to pure PNIPMAm. The seawater biodegradability according to the 28 day OECD306 test protocol is reported for some of the solvents tested for synergism. In particular, the high flash point solvent n-butyl lactate was found to be readily biodegradable. Combined with a reasonable effect as synergist with PNIPMAm, this makes n-butyl lactate a component to consider when developing more environmentally friendly KHI formulations.

A series of homopolymers of N,N-dimethylhydrazidomethacrylamide (DMHMAM) and copolymers with (N-isopropylmethacrylamide) (IPMAM) have been synthesized and investigated as kinetic hydrate inhibitors (KHIs) with a structure-II-forming synthetic hydrocarbon gas mixture in high-pressure steel rocking cells. The same polymerization method to give similar low molecular weights was used to aid KHI performance comparison. The homopolymer polyDMHMAM was found to give good KHI performance, significantly better than the related homopolymer, poly(N,N-dimethylhydrazidoacrylamide) (polyDMHAM), which does not have methyl groups in the backbone. This is in agreement with previous studies on N-alkylmethacrylamides. PolyDMHMAM also has no cloud point in deionized water up to 95+ °C, making it suitable for injection in high-temperature wells. A 1:1 N,N-dimethylhydrazidomethacrylamide/N-isopropylmethacrylamide copolymer (1:1 DMHMAM/IPMAM copolymer; M n = 2100) also exhibited no cloud point in deionized water and showed an improved KHI performance over polyDMHMAM homopolymer (M n = 2300) and poly(N-isopropylmethacrylamide) (M n = 1300) of similar low molecular weights. Other ratios of monomers in DMHMAM/IPMAM copolymers did not show an improvement on the performance compared to the other copolymers. Protonation of polyDMHMAM to give only quaternary dimethylhydrazinium groups strongly lowers the KHI performance. High pH (8−9) also gave a worse performance than at pH 6.5, suggesting that partial protonation of polyDMHMAM is optimal for the best KHI performance.

Homo- and copolymers of N-alkyl methacrylamides with varying alkyl pendant groups have been synthesized and investigated for their performance as kinetic hydrate inhibitors (KHIs) in slow constant cooling experiments in rocking cells using a structure II-forming synthetic gas mixture at an initial pressure of 76 bar. We have tried to polymerize disubstituted N,N-dialkyl methacrylamides; however, these were found to not homopolymerize using radical initiators. For monosubstituted N-alkyl methacrylamides, the KHI performance was found to improve as the size of the alkyl pendant group increased from methyl to ethyl and then propyl (n- or iso-). The best performing homopolymer was poly­(N-iso-propyl methacrylamide) (PIPMAm-I) which gave an onset temperature (average of 9 experiments) of 10.0 °C at 2500 ppm. The four structurally different N-butyl methacrylamides (n-, iso-, sec-, and tert-) were copolymerized with hydrophilic comonomers in order to investigate their potential as KHIs. The best performing copolymer containing butyl groups was a 1:1 copolymer of N-tert-butyl methacrylamide (tBuMAm) and N-methyl methacrylamide (MMAm) (P­(tBuMAm-co-MMAm)), which gave an average onset temperature of 8.3 °C at 2500 ppm. The KHI performance of the N-propyl methacrylamide polymers (n- and iso-) was further investigated by adjusting the synthetic route. For example, poly­(N-iso-propyl methacrylamide) (PIPMAm-IV) made and kept in n-butyl glycol ether (nBGE) as a 16.6 wt % solution gave an excellent performance, with an average onset temperature of 4.2 °C at a polymer concentration of 2500 ppm. The nBGE concentration was 12 500 ppm.

A series of polyglyoxylamides (PGAms) have been synthesized and investigated for performance as kinetic hydrate inhibitors (KHIs) in slow constant cooling high-pressure rocking cell experiments using a structure II-forming synthetic natural gas mixture at 76 bar initial pressure. We found that the KHI performance improved as the size of the alkyl pendant group was increased. The best performing PGAm, poly(pyrrolidinyl glyoxylamide) (PPyGAm-I), gave an onset temperature of 8.2 °C at 2500 ppm. The KHI performance was improved by increasing the polymer concentration or by adding the high-flashpoint solvent n-butyl glycol ether (nBGE) as a synergist. For example, using 7500 ppm of PPyGAm-I, the onset temperature was lowered to 3.8 °C (giving a subcooling of 15.7 °C, compensating for the drop in pressure at T o ). A mixture of 2500 ppm PPyGAm-I and 7500 ppm nBGE gave an onset temperature of 5.1 °C. Combined with a high cloud point (T Cl = 79 °C), this makes PPyGAm-I a strong candidate for potential industrial use. The seawater biodegradability of the PGAms in this study was found to be low, 4−17%, according to the 28-day marine OECD306 test protocol. However, this can be improved by changing the polymer's end groups, grafting, or by utilizing the polymer's susceptibility to acid hydrolysis.