UNDERSTANDING THE EFFECTS OF KINETIC ADDITIVES ON GAS HYDRATE GROWTH

Transcription

1 Proceedings of the 8th International Conference on Gas Hydrates (ICGH8-2014), Beijing, China, 28 July - 1 August, 2014 UNDERSTANDING THE EFFECTS OF KINETIC ADDITIVES ON GAS HYDRATE GROWTH Jonathan VERRETT, Dany POSTERARO, Jason IVALL, Spencer BRENNAN and Phillip SERVIO Department of Chemical Engineering, McGill University, Room 3060, Wong Building, 3610 University Street, Montreal, H3A 0C5, CANADA ABSTRACT This paper presents a review of recent work performed to understand the mechanism of kinetic additives (KAs) on hydrate growth. Specific focus has been placed on the promoter sodium dodecyl sulfate (SDS) and the inhibitor polyvinylpyrrolidone (PVP). The effects of these KAs on gas consumption and mole fraction during hydrate growth in a stirred tank crystallizer are assessed. KA concentrations under 1000 ppmw are used so as to not affect bulk properties such as solubility and viscosity. Both compounds are found to have significant effects on growth over the concentration range with SDS increasing growth rate by a factor of 4.9 and PVP decreasing growth rate by a factor of 3.9. The presence of both compounds increased the mole fraction of the hydrate former in the liquid phase during growth. The Bergeron and Servio model is used for analysis of KA activity with a specific focus on the vapour-liquid (V-L) and liquid-hydrate (L-H) interfaces. SDS is thought to promote growth by two mechanisms; increasing mass transfer at the V-L interface as well as increasing hydrate surface area at the L-H interface. PVP appeared to have no effect on mass-transfer at the V-L interface, instead decreasing growth by either disrupting heterogeneous nucleation or blocking growth sites at the L-H interface. Keywords: methane gas hydrates, kinetic inhibitors, growth promoters, solubility, mole fraction, PVP, SDS INTRODUCTION Gas hydrates are hailed as many things, a cleaner energy source, a menace to pipelines and deep-sea oil operations, and a gas storage and separation technology to name a few. Throughout all these applications, the study of formation kinetics is essential to understand how to prevent or promote hydrate growth. Additives affecting growth can be grouped into two broad categories: thermodynamic and kinetic. Thermodynamic additives (TAs) affect phase equilibrium by strongly interacting with the crystal structure and are generally used in large quantities (over 10 wt%); whereas kinetic additives (KAs) affect growth at lower concentrations (less than 2 wt%) and do not principally affect phase equilibrium. Both TAs and KAs can be used to promote or inhibit hydrate growth. In flow assurance and promotion applications focus has shifted from TAs to KAs, due to simplified processing, smaller environmental impact and overall cost savings [1]. However, the mechanisms of KAs are not well understood compared to TAs, hindering their implementation in industrial processes. KAs that promote growth include a variety of surfactants, the most notable of which is sodium dodecyl sulfate (SDS). Extensively documented in literature, it has been shown to be very effective at promoting hydrate growth at concentrations under 1000 ppmw. Initial reports on SDS showed that above 242 ppmw it would increase growth by a factor of over 700 in quiescent systems [2]. Micellization, causing an increase in hydrateformer solubility, was initially proposed to explain the promotion behavior because of the sudden growth increase above a certain SDS concentration. Conductivity tests later showed that SDS does not micellize at the low temperatures

2 under which hydrate formation was tested [3]. Focus then shifted to the effects of SDS at interfaces. Measurements at the vapour-liquid (V- L) interface show SDS reduces surface tension [4]. At the crystal surface, SDS adsorption has been shown to reduce zeta-potential [5] and also alter crystal morphology [6]. KAs used to inhibit hydrate growth generally have two methods of action: nucleation prevention and growth reduction. Nucleation prevention focuses on ensuring that hydrate clusters of a critical size are not formed and thus hydrates do not nucleate and grow. Growth reduction focuses on limiting hydrate growth following nucleation. A large number of substances have been screened for their ability to inhibit gas hydrates [7]. Notable inhibitors that have been identified and extensively studied include polyvinylpyrrolidone (PVP) and polyvinylcaprolactam (PVCap) [1]. Both these polymers contain a lactam ring that interacts with the hydrate crystal structure and is thought to insert itself into open cages at the surface, thereby limiting growth [8]. The polymers have also been found to prevent and delay nucleation [9]. Many mechanisms have been proposed, such as water perturbation or disruption of crystal nuclei before they reach a critical size. Adsorption to surfaces upon which hydrates heterogeneously nucleate, thereby blocking hydrate nucleation, has also been proposed as a possible inhibition mechanism [10]. Activity at interfaces appears to be the key to understanding gas hydrate promotion and inhibition by KAs. This paper presents an overview of recent studies performed by our group to characterize the effects of PVP and SDS concentration on hydrate growth kinetics. Analysis is done using a kinetic model coupled with growth and mole fraction data. Results help to identify which interfacial promotion mechanisms play a role in affecting growth rate. THEORY regions have been indicated on the curve. Gas initially dissolves within the liquid phase to reach saturation, followed by supersaturation. This period of supersaturation is generally know as induction and ends with the formation of stable hydrate nuclei at the turbidity point. Turbidity is marked by a sharp change in gas consumption and an increase in temperature in the system due to the exothermic nature of hydrate crystallization. Gas Consumption (moles) n eq Dissolution t eq Induction Turbidity Point Saturation Time (min) t tb Growth 15 Minutes Degree of Supersaturation Growth rate Figure 1: Typical gas consumption curve obtained during hydrate growth in a stirred tank crystallizer. Nucleation in a system can occur either heterogeneously or homogeneously. Heterogeneous nucleation occurs on impurities in the water or on reactor surfaces, which reduce the energy barrier for the formation of a new phase. Homogeneous nucleation occurs within the bulk liquid and requires larger nuclei to overcome the energy of creating a new interface. Studies on THF hydrates have found that the temperatures at which homogenous nucleation occur to be far lower than hydrate formation temperatures generally tested [11]. It is thus believed that most hydrate nucleation events seen in the literature, including those analyzed in the current study, occur heterogeneously. Vapour y i V Liquid Hydrate z i H Hydrate kinetics have been studied in a variety of reaction conditions. The current paper focuses on a stirred three-phase, vapour-liquid-hydrate (V-L- H), slurry reactor system. Well-stirred systems have been shown to simplify hydrate growth kinetic analysis by reducing mass transfer resistances. A typical gas consumption curve within this system can be seen in Figure 1. Notable Gas Mole Fraction x i V-L R V-L vapour IF V-L R V-L Liquid x i L Driving force x i L-H Gas Transfer R L-H Liquid R H Reaction IF L-H

3 Figure 2: Gas transfer and resistances in a V-L-H system during growth. Following turbidity, hydrate growth can proceed in the system. This involves gas dissolving from the vapour into the liquid to eventually be incorporated into the gas hydrate. Figure 2 presents the gas transfer between phases during growth and shows there are four resistances to growth in the system. Two resistances are at the V-L interface (IF V-L ): one on the vapour side (R V- L Vapour ) and another on the liquid side (R V-L Liquid). Studies have shown the resistance on the vapour side to be negligible compared to the liquid-side resistance to mass transfer [12]. Since the main resistance at this interface is on the liquid side, the diffusion layer in the diagram (denoted by the dashed line) has been shown to be smaller on the vapour side. The other two important resistances are at the liquid-hydrate (L-H) interface (IF L-H ). These are the mass transfer resistance of the gas to the surface of the hydrate particle (R L-H Liquid) and the enclathration of the gas particles in the crystal lattice, which we call the reactive resistance (R H Reaction). An order-of-magnitude analysis previously performed by Bergeron and Servio has shown that in stirred systems, the reactive resistance is estimated to be much greater than the mass transfer resistance to the surface [12]. This analysis leaves the liquid mass transfer resistance at the V-L interface and the reaction resistance at the L-H interface as the two main resistances in the system. Equations 1 and 2 are the governing equations for transfer of the guest gas (dn i /dt) at each of these interfaces. dn i dt = C w0k L V LA V L (x i V L x i L ) (1) dn i dt = C w0k r A L H (x i L x i L H ) (2) C w0 is the molar concentration of water initially present. During the start of the growth phase, little water is consumed compared to the total amount in the reactor and thus C w0 is assumed to be constant. k L V-L is the mass transfer coefficient for the V-L interface on the liquid side, and k r is the intrinsic reaction rate. A V-L and A L-H are the respective V-L and L-H surface areas. x i V-L is the hypothetical mole fraction at the V-L interface, x i L is the bulk liquid mole fraction, and x i L-H is the mole fraction at equilibrium (solubility) of the guest hydrate former, i. A variety of kinetic models have been developed based on assumptions and equations similar to the ones above and can be found in a review by Ribeiro and Lage [13]. Previous models used driving forces (DFs) based on gas phase measurements and thus included terms describing the mass transfer at the V-L surface. These terms are difficult to estimate due to changes in hydrodynamic parameters such as stirring rate, surface tension and viscosity. A recent model developed by Bergeron and Servio avoids characterization of the V-L interface by instead using measurements of the gas mole fraction in the bulk (x i L) - the advantage being that the direct measurement x i L takes into account any mass transfer changes in the system [12]. The model is a simple rearrangement of Equation 2 and can be found as Equation 3. The difference between the guest gas mole fraction in the bulk liquid phase (x i L) and the guest gas solubility (x i L-H) is used as the DF. Previous studies have found gas solubility to be a weak function of pressure and thus the solubility is experimentally determined at the experimental temperature (T exp ) and equilibrium pressure (P eqm ) under the V-L-H equilibrium [14]. The surface reaction term is represented by the second moment of particle size (µ 2 ), which can be estimated from particle size analysis, and the intrinsic reaction rate constant (k r ) calculated in previous reports for a variety of compounds [12, 15]. dn i dt = C (x i L(T exp,p exp ) xi ) L H (T exp,p eqm ) w0 1 (πµ 2 k r ) (3) Initial growth shows a linear gas consumption profile in a well-stirred crystallizer. The gas consumption is thus averaged over the first 15 minutes of growth to give the initial growth rate (dn i /dt). The gas consumption rate can deviate from linearity if growth is very rapid due to changes in solution temperature from the exothermic crystallization reaction. Such deviations occur with SDS and thus a smaller timeframe of 7.5 minutes is used for analysis [16]. When large amounts of inhibitors are added, the rate may follow a quadratic trend. Surface area is found to be a quadratic function of time and thus a highly limited surface area is thought to cause

4 such a behavior [17]. Within the studies analyzed, the growth rate was sufficiently linear, or so close to no growth, that a linear fit was used to analyze gas consumption data. Previous studies have found that guest mole fraction and second moment do not vary significantly over this timeframe [15, 18]. Since the system is at a quasi-steady state, the gas transfer at both interfaces must be equal. If one resistance or surface area term is affected, the guest gas mole fraction will adjust accordingly. For example, if the resistance to mass transfer at the V-L interface decreases, the mole fraction in the bulk will increase. This decreases the DF for mass transfer at the V-L surface and increases the reaction rate at the L-H surface, re-equilibrating gas transfer at both interfaces. Bulk mole fraction measurements thus provide a powerful analytical tool to characterize the quasi-steady state found in stirred tank crystallizers and assess changes in growth kinetics. Specific details of the experimental apparatus and procedure can be found in previous publications [16, 18]. Briefly, gas hydrates are grown in an isothermal, isobaric stirred tank crystallizer where guest gas is added so as to maintain pressure. Gas mole fraction during growth is assessed by stopping stirring and taking a 10-mL bulk liquid sample from the system for analysis with a gasometer. DISCUSSION In this section we review the effects of KAs on hydrate growth using the Bergeron and Servio model as an analytical framework. The first section details the effect of SDS on growth promotion; the second section looks at PVP effects on growth inhibition. These analyses focus on methane as the guest gas but, as will be shown by theories on KAs mechanism of action, it is likely the results can be extrapolated for other KAs and guest compounds. Both sections examine the effects of KAs on solubility, growth rate and mole fraction as well as KA mechanisms. KINETIC GROWTH PROMOTION: SDS Solubility Previous results have shown that SDS has a very minor effect on solubility near hydrate forming conditions in a V-L system. Peng et al. reported that SDS does affect solubility, with data showing maximum solubility differences of 5% at concentrations below 4mM [19]. Similar results were obtained under V-L-H equilibrium, showing a maximal deviation of solubility of 5%, which was attributed to experimental error [16]. Since concentrations of SDS used to promote hydrate growth are generally smaller than 4 mm or roughly 1150 ppmw, it is not expected that their presence would affect gas solubility. This is especially true given that there is no micelle formation in the system [3]. Though SDS may slightly affect solubility, it is unlikely such a small change would lead to the increases in methane growth rate of over 400% observed with stirred systems. As discussed later, observed changes in mole fraction in the presence of SDS are more significant than any changes in solubility. The effect of mole fraction on DF is thus much more pronounced than the effect of solubility. For these reasons, the solubility is taken to be constant at the SDS concentrations investigated. Relative growth rate SDS Concentration (ppmw) Figure 3: Hydrate growth rates observed at various SDS concentrations at K and 4645kPa. Growth rates are relative to pure water [20]. Growth A detailed study on the effect of SDS concentration ranging from 28 to 1150 ppmw on hydrate growth in a stirred system was previously undertaken [20]. Figure 3 presents the results from the study with growth rates normalized to the growth rate of water in the reactor system (14.4 µmol/s). Also shown are 95% error bars based on replicates performed at each condition are also shown. At 57 ppmw, growth was similar to that of pure water, however it rapidly increased to a maximum of 4.9 times the growth rate of water at 575 ppmw. The overall trend was sigmoidal,

5 starting out at the same growth rate as water and then increasing rapidly after 150 ppmw to plateau above concentrations of 360 ppmw. Mole Fraction Increases of up to 20% in methane mole fraction during growth were observed with the addition of SDS [16]. This decreases the DF for mass transfer at the V-L interface. However, higher gas consumption indicates elevated gas transfer at both the V-L and L-H interfaces. This indicates the surface area and/or mass transfer coefficient must rise to account for both the higher gas consumption and smaller DF. Previous studies by Watanabe et al. have shown that under hydrate forming conditions, SDS can reduce interfacial surface tension [4]. A roughly linear decrease in surface tension from 51.9 mn/m at 500 ppmw to 30.9 mn/m at 2250 ppmw was observed for metastable water at 275 K and 3.9 MPa. Concentrations below 500 ppmw were not measured, but the trend indicates that SDS is having significant physical effects at the V-L interface, possibly increasing the mass transfer coefficient. To further investigate this, dissolution studies were undertaken in a stirred V-L system. The system was pressurized from 200 kpa to the hydrate equilibrium conditions of 3145 kpa at K [21]. Puzzlingly, methane uptake was found to be slower as SDS concentration increased, indicating a decreased mass transfer coefficient. One plausible theory is that the presence of hydrates in the crystallizer alters the V-L mass transfer in such a way that it cannot be compared to dissolution without hydrate crystals. Increased dissolution rates have been found in systems with nanoparticles, with their main effect being an increase in vapour-liquid contact area [22, 23]. The nanoparticles also reduced surface tension from 70 mn/m to 40 mn/m, not unlike the addition of SDS in hydrate systems. If SDS is increasing the number of small hydrate nuclei, this may increase surface area at the interface and allow greater mass transfer. Promotion mechanisms The DF for growth, being the difference between the mole fraction and the solubility, was found to increase 2.4 fold at 575 ppmw SDS compared to pure water. At this same SDS concentration, the gas consumption increased 4.9 fold, meaning that another factor must also be increasing to account for this difference in growth. As mentioned previously, during initial growth stages the amount of liquid water available for hydrate growth does not change significantly. The reaction rate constant is also unlikely to change because the surfactant shows no thermodynamic effects and it is unlikely that it acts as a catalyst. Catalysts generally have specific structures that allow them to affect reaction kinetics, but similar increases in growth can be found with other surfactants with different structures [20]. This leaves the second moment of particle size, which is proportional to surface area, to account for differences in growth. This would indicate a mechanism containing interactions involving SDS at the L-H interface. SDS has been found to adsorb to the hydrate surface and affect zeta potential. Lo et al. investigated SDS adsorption on cyclopentane (sii) hydrates and found a two plateau type adsorption with zeta potential being reduced from -60 mv with no SDS to -120mV at 1000 ppmw [5]. Multiple theories have been proposed for possible effects of this adsorption on the hydrate surface. The adsorbed layer may reduce agglomeration of hydrate particles either sterically or by increasing charge and electrostatic repulsion at the hydrate surface. This could increase the available surface area for growth, or contribute to greater L-H contact time, leading to higher gas consumption. Should this, or another direct effect on particle size be present, it would be observable using particle size analysis during growth. If particle size is not affected, theories concerning molecular interactions at the surface have been suggested. One such theory is that SDS near the hydrate particle surface creates hydrophobic micro-domains due to their non-polar tails. These regions preferentially concentrate methane near the hydrate surface and thus help to incorporate it into the crystal lattice. Whether one, or a combination of these theories is correct, a fundamental understanding of promoter mechanism can help guide future development and selection of promoters for hydrates and other crystalline compounds. KINETIC GROWTH INHIBITION: PVP Solubility Similar to SDS, PVP has no appreciable effect on methane solubility at concentrations below 1000 ppmw. Methane solubility measurements

6 performed at 700 ppmw showed a 2% deviation [24]. Methane mole fraction results show a much larger change with PVP concentration than any effect on solubility. Note that PVP concentration in other studies can be as high as 2 wt% in order to greatly reduce hydrate growth. With increasing PVP concentration, thermodynamic effects, and notably effects on solubility, will become more prominent and may alter inhibition mechanisms. The current study analyzed growth mechanisms with PVP solutions below 1000 ppmw and thus assumed methane solubility to be constant. Growth The effect of PVP concentration on hydrate growth was recently evaluated over the range of 0.7 to 20,000 ppmw [25]. A variety of temperature and pressure conditions were tested ranging from to K and 4645 to 7183 kpa. The same general trend of growth rate with concentration was found at all conditions tested. A selection of growth rates at K and 5645 kpa can be seen in Figure 4. Though the trend appears linear, the logarithmic scale shows that for each successive decrease in growth, a 10-fold higher PVP concentration is required. PVP affects hydrate growth at less than 1 pppw and shows effects over a very broad concentration range. Continuing the trend, near zero growth was obtained at roughly 10,000 ppmw (not shown). As found in previous reports, gas consumption following nucleation was never zero; though hydrate growth was slow, it was never completely inhibited [26]. Pressure increases, while holding temperature constant, made PVP significantly less effective at growth inhibition at high PVP concentrations. A higher pressure DF for growth requires more PVP for an equivalent growth inhibition. The opposite trend is found at low PVP concentrations where increases in formation pressure caused PVP to begin impacting growth at lower concentrations. This is most likely due to increases in pure water growth rate, making it easier to determine changes due to PVP. Effects of changes in temperature, while keeping a constant pressure DF, were difficult to analyze. The complex effects of temperature on kinetics and solubility make the cause of changes hard to determine. Rela%ve'growth'rate'' 1" 0.9" 0.8" 0.7" 0.6" 0.5" 0.4" 0.3" 0.2" 0.1" Rela1ve"growth" Driving"force" 0" 1" 0.1" 1" 10" 100" 1000" PVP'Concentra%on'(ppmw)' Figure 4: Hydrate growth inhibition and DFs observed at various PVP concentrations at K and 5645 kpa. Concentration and relative DFs are plotted on a logarithmic scale. All values are relative to pure water runs [24]. Mole fraction The addition of inhibitor was found to have a drastic effect on mole fraction. Figure 4 shows the DFs found from mole fraction tests performed at K and 5645 kpa at a variety of PVP concentrations. At PVP concentrations near 1 ppmw, the mole fraction during tests is similar to that with pure water. As PVP loading rises, the mole fraction rises and reaches a maximum increase of 80% at a concentration of 70 ppmw before plateauing. This represents a 42-fold increase in DF compared to pure water runs. It is unlikely that these effects can be attributed to bulk changes in solution. Hydrodynamic properties such as density and viscosity were not found to vary significantly below 1000 ppmw [27]. Measurements of surface tension show that there is a slight decrease (~2%) upon the addition of 100 ppmw of PVP, but any PVP added above this amount has little effect. Dissolution studies reveal that PVP reduces gas consumption in a manner similar to SDS. The higher mole fraction in the liquid increases the reaction DF but also decreases mass transfer DF at the V-L interface. This behavior indicates the system is becoming more reaction-limited, with the reaction requiring a higher DF. The small changes in interfacial properties indicate that PVP is not having a significant effect on the interface, and that V-L mass-transfer is not limiting the overall gas consumption in the system. 100" 10" Rela%ve'Driving'Force'

7 Inhibition mechanisms With PVP, the same assumptions as with SDS can be used for analysis with the growth model. There is evidence that PVP has no effect on solubility, and we assume that it does not affect the intrinsic kinetics and that the density of water stays constant. The only term remaining is the L-H surface area. A decrease in this variable must compensate not only for the decrease in growth, but also the increase in DF with PVP. For example, at 700 ppmw PVP, growth decreases by a factor of 3.9 and DF increases by a factor of 34, meaning that the surface area for growth must decrease by a factor of 131. Previous studies have shown PVP s ability to adsorb to the hydrate surface [28]. One proposed mechanism for growth inhibition is that PVP adheres to impurities in the solution and other surfaces, reducing heterogeneous nucleation sites [10]. This would limit the number of hydrate particles initially present for growth and thus reduce surface area. Supporting evidence of this theory is presented in a study by Pic et al. where PVP was injected in a stirred crystallizer following nucleation in the absence of inhibitors [29]. The injection brought the total PVP concentration to 1 wt%, more than enough to significantly inhibit hydrate growth. However hydrate growth was observed to continued at the previous rate, as if no inhibitor had been injected. Within the same system if PVP at the same concentration was injected before hydrate nucleation, it had an appreciable effect on growth. The presence of PVP in solution before nucleation was deemed important for PVP s functioning as an inhibitor. This study appears to contradict evidence from quiescent single crystal studies where hydrate growth was inhibited by the addition of surfactants following crystal formation [26]. Analysis of PVP shows that it does not form as dense a layer at the hydrate surface as other inhibitors such as PVCap, which can completely inhibit growth [28]. PVP adsorption may block growth sites or may also function to severely limit mass transfer to the hydrate surface. Whether either or both of these are occurring, we can think of this effect as limiting effective surface area for growth following hydrate nucleation. Results from the stirred crystallizer and quiescent single crystal studies fit into a theory whereby the foremost effect of PVP in stirred systems is inhibiting nucleation sites, whereas crystal adsorption dominates in quiescent systems. In the case of single crystals, the surface area is small and thus a sufficient amount of PVP can adsorb at high concentrations to prevent growth. However within a stirred tank crystallizer, a large number of particles are formed during uninhibited nucleation, creating too large a surface area for PVP to inhibit. Further studies with particle size analysis will shed light on whether PVP affects the number or size of particles present in the crystallizer. CONCLUSION Both kinetic promoters and inhibitors dramatically affect hydrate growth at concentrations below 1000 ppmw. Insignificant changes to bulk properties at such concentrations indicate that activity at the V-L and L-H interfaces are the cause of kinetic additive effects. A variety of mechanisms have been proposed to explain these effects on growth. To the best of our knowledge, SDS promotes growth by increasing mass transfer at the V-L interface as well as increasing hydrate surface area for growth at the L-H interface. PVP does not appear to have an appreciable effect at the V-L interface, instead decreasing growth due to its effects disrupting heterogeneous nucleation or attachment to the L-H interface. Further kinetic studies, specifically with particle size analysis, will help shed light on kinetic growth promotion and inhibition mechanisms. This information will provide opportunities to improve upon existing kinetic additives and continue to expand their applications. ACKNOWLEDGEMENTS The authors are grateful to the Natural Sciences and Engineering Research Council of Canada (NSERC), McGill University and specifically the McGill Engineering Doctoral Award (MEDA) and Vadasz fellowship, le Fonds Québécois de la Recherche sur la Nature et les Technologies (FQRNT) and the Canada Research Chair Program (CRC) for financial funding and support.

8 REFERENCES [1] Kelland MA. History of the development of low dosage hydrate inhibitors. Energy Fuels 2006;20(3): [2] Zhong Y, Rogers RE. Surfactant effects on gas hydrate formation. Chem. Eng. Sci. 2000;55(19): [3] Di Profio P, Arca S, Germani R, Savelli G. Surfactant promoting effects on clathrate hydrate formation: Are micelles really involved? Chem. Eng. Sci. 2005;60(15): [4] Watanabe K, Niwa S, Mori YH. Surface Tensions of Aqueous Solutions of Sodium Alkyl Sulfates in Contact with Methane under Hydrate- Forming Conditions. J. Chem. Eng. Data 2005;50: [5] Lo C, Zhang JS, Couzis A, Somasundaran P, Lee JW. Adsorption of Cationic and Anionic Surfactants on Cyclopentane Hydrates. J. Phys. Chem. C 2010;114: [6] Yoslim J, Linga P, Englezos P. Enhanced growth of methane-propane clathrate hydrate crystals with sodium dodecyl sulfate, sodium tetradecyl sulfate, and sodium hexadecyl sulfate surfactants. J. Cryst. Growth 2010;313(1): [7] Long JP, Lederhos JP, Sum AK, Christiansen RL, Sloan ED. Kinetic Inhibitors of Natural Gas Hydrates. In: Proceedings of the 73rd Gas Processors Association Annual Convention, New Orleans, LA, 1994: [8] Ohno H, Moudrakovski I, Gordienko R, Ripmeester J, Walker VK. Structures of hydrocarbon hydrates during formation with and without inhibitors. J Phys. Chem. A 2012;116(5): [9] May EF, Wu R, Kelland MA, Aman ZM, Kozielski KA, Hartley PG, Maeda N. Quantitative kinetic inhibitor comparisons and memory effect measurements from hydrate formation probability distributions. Chem. Eng. Sci. 2014;107:1-12. [10] Zeng H, Walker VK, Ripmeester JA. Approaches to the design of better low-dosage gas hydrate inhibitors. Ange. Chem. Int. Ed. 2007;46(28): [11] Zeng H, Wilson LD, Walker VK, Ripmeester JA. Effect of antifreeze proteins on the nucleation, growth, and the memory effect during tetrahydrofuran clathrate hydrate formation. J. Am. Chem. Soc. 2006;128(9): [12] Bergeron S, Servio P. Reaction rate constant of propane hydrate formation. Fluid Phase Equilibr. 2008;265(1-2): [13] Ribeiro CP, Lage PLC. Modelling of hydrate formation kinetics: State-of-the-art and future directions. Chem. Eng. Sci. 2008;63(8): [14] Servio P, Englezos P. Measurement of Dissolved Methane in Water in Equilibrium with Its Hydrate. J. Chem. Eng. Data 2002;47(1): [15] Bergeron S, Beltran JG, Servio P. Reaction rate constant of methane clathrate formation. Fuel 2009;89(2): [16] Verrett J, Posteraro D, Servio P. Surfactant effects on methane solubility and mole fraction during hydrate growth. Chem. Eng. Sci. 2012;84(0): [17] Al-Adel S, Dick JAG, El-Ghafari R, Servio P. The effect of biological and polymeric inhibitors on methane gas hydrate growth kinetics. Fluid Phase Equilibr. 2008;267(1): [18] Bergeron S, Servio P. CO2 and CH4 mole fraction measurements during hydrate growth in a semi-batch stirred tank reactor and its significance to kinetic modeling. Fluid Phase Equilibr. 2009;276(2): [19] Peng B-Z, Chen G-J, Luo H, Sun C-Y. Solubility measurement of methane in aqueous solution of sodium dodecyl sulfate at ambient temperature and near hydrate conditions. J. Colloid Interface Sci. 2006;304(2): [20] Verrett J, Servio P. Evaluating surfactants and their effect on methane mole fraction during hydrate growth. Ind. Eng. Chem. Res. 2012;51(40): [21] Posteraro D, Brennan S, Milan M, Servio P. The Effect of Sodium Dodecyl Sulphate and Polyvinylpyrrolidone on The Dissolution of Methane Gas. Private Communication [22] Olle B, Bucak S, Holmes TC, Bromberg L, Hatton TA, Wang DIC. Enhancement of Oxygen Mass Transfer Using Functionalized Magnetic Nanoparticles. Ind. Eng. Chem. Res. 2006;45(12): [23] Pasieka J, Coulombe S, Servio P. The Effect of Hydrophilic and Hydrophobic Multi-Wall Carbon Nanotubes on Methane Dissolution Rates in Water at Three Phase Equilibrium (V-Lw-H) Conditions Private Communication [24] Posteraro D, Ivall J, Servio P. Evaluating the Effect of the Kinetic Inhibitor Polyvinylpyrrolidone (PVP) on Methane Mole Fraction during Hydrate Growth. Private Communication [25] Posteraro D, Verrett J, Servio P. A detailed study on the effects of Polyvinylpyrrolidone (PVP)

9 concentration on methane hydrate growth. Private Communication [26] Larsen R, Knight CA, Sloan Jr ED. Clathrate hydrate growth and inhibition. Fluid Phase Equilibr. 1998; : [27] Águila-Hernández J, Trejo A, García-Flores BE. Volumetric and Surface Tension Behavior of Aqueous Solutions of Polyvinylpyrrolidone in the Range (288 to 303) K. J. Chem. Eng. Data 2011;56(5): [28] Zeng H, Lu H, Huva E, Walker VK, Ripmeester JA. Differences in nucleator adsorption may explain distinct inhibition activities of two gas hydrate kinetic inhibitors. Chem. Eng. Sci. 2008;63(15): [29] Pic JS, Herri JM, Cournil M. Experimental influence of kinetic inhibitors on methane hydrate particle size distribution during batch crystallization in water. Can. J. Chem. Eng. 2001;79(3):