Accelerated Hydrate Crystal Growth in the Presence of Low Dosage Additives Known as Kinetic Hydrate Inhibitors Hassan Sharifi and Peter Englezos* Department of Chemical and Biological Engineering, University of British Columbia, 2360 East Mall, Vancouver, British Columbia V6T 1Z3, Canada ABSTRACT: Kinetic hydrate inhibitors (KHIs) or low dosage hydrate inhibitors (LDHIs) are known as additives employed to delay the onset of gas hydrate nucleation time in hydrocarbon pipelines. It has been observed, however, that in laboratory experiments accelerated hydrate growth called catastrophic growth can occur. This may be a serious problem if it occurs in a field application of kinetic inhibitors. The mechanism of such accelerated hydrate growth in the presence of KHIs is still not understood. A high- pressure microdifferential scanning calorimeter was employed to study the accelerated hydrate growth in the presence of chemical and biological inhibitors. It is hypothesized that capillary action facilitates the transport of water molecules across the formed hydrate layer from the bulk of the liquid water phase to the gas-liquid interface. This in turn might be the governing mechanism for catastrophic hydrate growth in the presence of KHIs. In addition, the hydrate catastrophic index is introduced in this work as a parameter to quantify the phenomenon based on the laboratory data and the type of experiment conducted. The HCI may then serve as a measure of the pipeline hydrate plugging potential. ■ INTRODUCTION The formation of solid crystals by hydrocarbon molecules and water (known as gas or clathrate hydrates) remains a serious flow assurance problem in hydrocarbon pipelines and related processing facilities since Hammershcmidt first reported on the issue. 1-4 The oil and gas industry has been dealing with the issue of unwanted hydrate formation in pipelines through the injection of inhibiting chemicals known as thermodynamic hydrate inhibitors (THIs). Motivated by environmental restrictions and economic incentives (high consumption rate of THIs especially in far remote and deep resources), kinetic hydrate inhibitors (KHIs) or low dosage hydrate inhibitors (LDHIs) have also been employed in recent years. 5-13 Chemical (polyvinylpyrrolidone, PVP; and polyvinylcapro- lactam, PVCap) and biological (antifreeze proteins known as AFPs) additives have been employed as kinetic hydrate inhibitors. These additives have been studied in the lab by various laboratory techniques. KHIs are able to postpone the onset of gas hydrate crystallization and to control the growth of postnucleation crystals. The adsorption of the inhibitors on gas hydrate crystals has been proposed as a plausible mechanism to explain the performance of KHIs. 14,15 Recently, it has been reported that even though the addition of kinetic hydrate inhibitors (both chemical and biological ones) reduced the growth rate of gas hydrate crystals up to a certain point this was followed by an acceleration in gas hydrate growth that has been called catastrophic hydrate growth. 16-20 More specifically in the experiments conducted in the stirred vessel type crystallizers, KHIs were found to reduce the gas hydrate formation rate in the aqueous liquid phase. However, once hydrate crystals started to form in the bulk gas phase, catastrophic hydrate growth was detected. 16,17 It is noteworthy that such unusual kinetic inhibitor effects on gas hydrate formation leading to accelerated hydrate growth called catastrophic hydrate crystal growth was first observed and reported by Lee and Englezos and Kumar et al. 21,22 The reason for such accelerated hydrate formation is still not clear. However, it has been proposed that the hydrate crystals formed in the presence of KHIs might have a morphology that facilitates capillary movement of water molecules to the gas/ liquid interface. 21,23 Since accelerated hydrate growth is not desirable in a field application of kinetic hydrate inhibitors, it is crucial to understand the governing reasons for the occurrence of this phenomenon. The accelerated hydrate crystal growth has so far been described qualitatively. While this is inevitable for the experiments examining the macroscopic morphology of hydrates, a quantitative operational definition will be proposed in this work. Specifically the hydrate catastrophic index (HCI) will be defined for different experimental settings that showed such hydrate growth. The introduction of the HCI is intended to serve as a measure of the pipeline hydrate plugging potential (PHPP). Previously reported data on the viscosity of the hydrate slurry will be employed. This choice is based on the tacit assumption that the hydrate slurry viscosity is the best variable to represent the tendency of the hydrate mass to plug the pipeline (plugging potential). It has been shown that Special Issue: In Honor of E. Dendy Sloan on the Occasion of His 70th Birthday Received: June 30, 2014 Accepted: October 9, 2014 Article pubs.acs.org/jced © XXXX American Chemical Society A dx.doi.org/10.1021/je500591q | J. Chem. Eng. Data XXXX, XXX, XXX-XXX