Can cyclopentane hydrate formation be used to rank the performance of kinetic hydrate inhibitors?

Abstract

The equilibrium temperature for cyclopentane hydrate formation is approximately 7.7°C. Since these hydrates are Structure II and are formed at atmospheric pressure it was of interest to determine if cyclopentane hydrate-forming fluids could be used in the performance ranking of kinetic hydrate inhibitors (KHIs), avoiding the use of high pressure equipment and flammable gases. We have carried out a series of tests with well-known classes of KHIs in four parallel cells using cyclopentane hydrate-forming fluids. The superheated hydrate method was used since it was found that cyclopentane hydrate formation was inconveniently slow using fluids with no hydrate history. The amount of KHI needed to obtain significant kinetic inhibition was found to be in the region of 100–200ppm, a fraction of that needed for high pressure Structure II gas hydrate inhibition with the same KHI. This may be due to the fairly low subcooling used (7.8°C) as well as the lower solubility of cyclopentane in water compared to small hydrocarbon hydrate-formers at elevated pressures. The results of testing a range of KHIs using this superheated CP hydrate test method correlated fairly well with tests carried out on Structure II-forming gas hydrates in high pressure stirred autoclaves, but there were some notable exceptions, which meant that this method is unlikely to be reliable for ranking all KHIs under atmospheric conditions. Furthermore, raising the KHI concentration from 100ppm to 400ppm did not lead to the expected increase in KHI performance for all the KHIs tested.