ARTICLE pubs.acs.org/EF
Natural Gas Hydrate Formation and Decomposition in the Presence of Kinetic Inhibitors. 1. High Pressure Calorimetry Nagu Daraboina,† John Ripmeester,‡,§ Virginia K. Walker,§ and Peter Englezos*,† †
Department of Chemical and Biological Engineering, University of British Columbia, Vancouver, British Columbia, Canada Steacie Institute for Molecular Sciences, National Research Council Canada, Ottawa, Ontario, Canada § Department of Biology, Queen’s University Kingston, Ontario, Canada ‡
ABSTRACT: The effect of kinetic inhibitors, both chemical (PVP and H1W85281) and biological (type III antifreeze protein), on natural gas hydrate formation was investigated using high pressure differential scanning calorimetry (HP-DSC). The presence of inhibitors decreased the overall formation of methane/ethane/propane hydrate compared to systems without added inhibitors. As well, all of the inhibitors significantly delayed hydrate nucleation as compared to water controls. However, the two classes of inhibitors were distinguished by the formation of hydrates with different stabilities. A single hydrate melting peak was seen with the antifreeze protein (AFP), and this was consistent after recrystallization. In contrast, multiple hydrate melting events, some indicating the formation of hydrate structures with high stability, were observed in the presence of the chemical inhibitors, and these varied depending on the crystallization cycle. This heterogeneity suggests that the use of these chemical inhibitors (PVP and H1W85281) may present a special challenge to operators depending upon the gas mixture and environmental conditions and that AFPs may offer a more predictable, efficacious solution in these cases.
I. INTRODUCTION While the problem of hydrate formation in hydrocarbon pipelines has been known since the 1930s,1 conventional methods of mitigation, such as the use of methanol and glycols, have become increasingly expensive as production moves to deeper water and more remote fields.2 5 In addition, the use of these chemicals may be restricted because of environmental concerns and the need to avoid downstream problems with catalysts in oil refining or with sour gas reservoirs. During the past 15 20 years, the industry has moved toward an interest in the use of synthetic polymers such as polyvinylpyrrolidone (PVP) or polyvinylcaprolactam (PVCap). These polymers are known as low dosage hydrate inhibitors (LDHIs) because they are used at low concentrations to control hydrate, as opposed to the high concentrations required of thermodynamic hydrate inhibitors. There is also an interest in the future use of “green” or biological inhibitors,3,6 such as antifreeze proteins (AFPs). They have been engineered by evolutionary processes to facilitate the overwintering survival of certain organisms. AFPs lower the freezing point relative to the melting point by adsorbing to the ice surface.7 In contrast, thermodynamic inhibitors, decrease the freezing point colligatively. AFPs can inhibit tetrahydrofuran (THF), propane, methane, and CO2 hydrates.6,8 11 Gordienko et al.12 observed that AFP adsorption could modify THF hydrate morphology, therefore indicating that AFPs could inhibit hydrate growth, likely by an adsorption inhibition mechanism. Despite this progress, however, the reasons behind the activities of either chemical or biological inhibitors toward hydrate crystal nucleation and growth are not well understood. High pressure-microdifferential scanning calorimetry (HP-μDSC) has proved useful for the study of hydrate phase transitions and to investigate the PVCap-mediated delay of hydrate nucleation.13 r 2011 American Chemical Society
Ohno et al.14 used HP-μDSC with a silica gel medium to examine the effect of chemical and biological kinetic inhibitors on natural gas hydrate formation. The strength of this work is that it allowed the collection of sufficient data for statistical analysis without any serious effects of pore size on the thermodynamics of hydrate formation (pore size >1 μm). Admittedly, the matrix changes nucleation and growth conditions from those observed in industrial pipelines, but neither is a stirred reactor an ideal simulation. The formation equilibria of mixed hydrates are complex and have been observed to shift.9,15 Indeed, in the presence of small (
