Performance Enhancement of N‑Vinylcaprolactam-Based Kinetic Hydrate Inhibitors by Synergism with Alkylated Guanidinium Salts

The synergistic effect of hexa-n-butylguanidinium chloride (n-Bu6GuanCl) and related compounds in different ratio mixtures with a series of three commercially available N-vinylcaprolactam-based polymer kinetic hydrate inhibitors (KHIs) against structure-II-forming synthetic natural gas hydrates has been evaluated by carrying out constant cooling and isothermal KHI screening experiments in high-pressure rocking cells. Formulations containing n-Bu6GuanCl/poly­(N-vinylcaprolactam) with weight ratios of 4:1, 9:1, and 14:1 at 3000 ppm total concentrations were found to give the best gas hydrate inhibitory performances, with average onset temperatures (T o) of 3.4, 3.1, and 3.3 °C, respectively. In comparison, 3000 ppm of poly­(N-vinylcaprolactam) alone gave an average T o of 7.7 °C. Using 9% white spirit lowers the T o values of the best synergistic mixtures by 2.1 °C. However, this effect is at least partly due to shifting of the hydrate phase equilibria to a lower equilibrium temperature and higher equilibrium pressure. n-Bu6GuanCl was found to be a better synergist than n-Bu6GuanCH3COO, n-Bu6GuanHCOO, and diethyl-di-n-pentylguanidinium chloride [Et2(n-Pe)4GuanCl] when used with poly­(N-vinylcaprolactam) for gas hydrate kinetic inhibition. The synergistic capacity of n-Bu6GuanCl is dependent upon the chemical structure of the N-vinylcaprolactam-based polymer, although the molecular weight differences may also play a part. A mixture of 300 ppm of high-cloud-point N-vinylcaprolactam-based terpolymer with 2700 ppm of n-Bu6GuanCl lowered the average T o by 8.7 °C compared to using 3000 ppm of this terpolymer alone. In long-term isothermal experiments at 8 °C subcooling and 30 bar, 5000 ppm of the synergistic mixture Bu6GuanCl/poly­(N-vinylcaprolactam) in a 9:1 weight ratio showed a hold time of over 13.6 days. Long hold times were also achieved with various ratios of n-Bu6GuanCl/N-vinylcaprolactam/N-vinylpyrrolidone copolymer or n-Bu6GuanCl/N-vinylcaprolactam terpolymer mixtures.