Surfactant effects on methane solubility and mole fraction during hydrate growth

Abstract

Investigations are still ongoing to discover the mechanism by which surfactants promote hydrate growth. This paper investigates the effects of sodium dodecyl sulfate (SDS), a common surfactant for promoting hydrate growth, on methane solubility and mole fraction in the bulk liquid phase. Hydrates were formed in a stirred 600 cm3 isobaric/isothermal reactor containing 343 cm3 of liquid. Bulk solubility experiments under hydrate–liquid, liquid–gas, and hydrate–liquid–gas equilibria were performed at temperatures ranging from 275.1 K to 283.3 K and pressures ranging from 3049 kPa to 6500 kPa with pure water as well as SDS solutions. Kinetic experiments were also performed with water and 360-ppm solutions of SDS at temperatures of 275.1 K, 277.1 K and 279.1 K and pressures of 4545 kPa, 5180 kPa and 6080 kPa respectively. Measurements of the mole fraction of methane in the bulk liquid were taken at 0 s, 225 s and 450 s after hydrate nucleation. Experiments showed that SDS has no effect on bulk methane solubility at concentrations that significantly promote hydrate growth. SDS was found to increase methane mole fraction in the bulk liquid during hydrate growth following nucleation. Results were analyzed using the solubility model previously developed by Bergeron and Servio. The increase in methane mole fraction was found to be the major contributor to the increase in hydrate growth rate. It is estimated that other factors, such as changes in hydrate particle surface area, may also affect the growth rate and should be investigated further.