Surfactant effects on methane solubility and mole fraction during hydrate growth Jonathan Verrett, Dany Posteraro, Phillip Servio n Department of Chemical Engineering, McGill University, 3610 University Street, Montreal, Canada H3A 2B2 HIGHLIGHTS c Measure the effects of SDS on hydrate growth in a stirred tank reactor. c SDS has no effect on methane solubility but increases methane mole fraction. c Increases in growth rate are attributed to the increase in methane mole fraction. c Other factors, such as increased particle surface area, may also increase growth. article info Article history: Received 27 June 2012 Received in revised form 30 July 2012 Accepted 7 August 2012 Available online 14 August 2012 Keywords: Clathrate hydrate Gas hydrate Kinetics Surfactant Crystallization Energy 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 cm 3 isobaric/isothermal reactor containing 343 cm 3 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. & 2012 Elsevier Ltd. All rights reserved. 1. Introduction Methane can form structure I crystalline gas hydrates when combined with water under suitable thermodynamic conditions (Sloan and Koh, 2008). These structures form naturally in ocean floors and permafrost regions and are estimated to represent an amount of organic carbon larger than all other sources on earth combined (fossil fuel, soil, peat and living organisms) (Suess et al., 1999). Methane hydrates could prove to be very useful in a multitude of industrial applications; one of the most notable is replacing liquefied natural gas transport in ocean tankers (Gudmundsson and Borrehaug, 1996). The main challenge to using hydrates in industrial processes is their slow formation rates (Rogers and Zhong, 2000). Studies have shown surfactants, in particular sodium dodecyl sulfate (SDS), have a pronounced effect on hydrate formation (Kalogerakis et al., 1993; Zhong and Rogers, 2000). There is now much interest in under- standing and characterizing the mechanism by which surfactants promote hydrate growth (Okutani et al., 2008). This information could help to guide the synthesis or selection of surfactants with properties better suited to promote hydrate growth. It has been observed that the presence of surfactants in quiescent systems allows porous hydrate layers to form at the liquid–gas interface instead of a nonporous film (Mel’nikov et al., 1998; Kutergin et al., 1992). This increases hydrate growth by Contents lists available at SciVerse ScienceDirect journal homepage: www.elsevier.com/locate/ces Chemical Engineering Science 0009-2509/$ - see front matter & 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ces.2012.08.009 n Corresponding author. Tel.: þ1 514 3981026; fax: þ1 514 398 6678. E-mail address: phillip.servio@mcgill.ca (P. Servio). Chemical Engineering Science 84 (2012) 80–84