Inhibition Activity of Antifreeze Proteins with Natural Gas Hydrates in

May 8, 2014 - Department of Chemical and Biological Engineering, University of British Columbia, Vancouver, British Columbia V6T 1Z3, Canada. ‡ Depa...
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Inhibition Activity of Antifreeze Proteins with Natural Gas Hydrates in Saline and the Light Crude Oil Mimic, Heptane Hassan Sharifi,† Virginia K. Walker,‡ John Ripmeester,§ and Peter Englezos*,† †

Department of Chemical and Biological Engineering, University of British Columbia, Vancouver, British Columbia V6T 1Z3, Canada Department of Biology, and School of Environmental Studies, Queen’s University, Kingston, Ontario K7L 3N6, Canada § Steacie Institute for Molecular Sciences, National Research Council Canada, Ottawa, Ontario K1A OR6, Canada ‡

ABSTRACT: For practical purposes, kinetic hydrate inhibitors must perform predictably in the presence of oil as well as saline and high driving forces, but such deterministic behavior is rarely achieved. Here, we evaluated two biological inhibitors, type I and type III antifreeze proteins (AFPs I and III), under these exacting conditions using a double high-pressure crystallizer apparatus and additionally assayed using high-pressure micro-differential scanning calorimetry. The two AFP types behaved somewhat differently under these environmental conditions. The addition of AFP I reduced natural gas hydrate induction time, whereas AFP III had no impact on hydrate crystal nucleation. Nonetheless, for both AFPs, gas hydrate growth was significantly inhibited to ∼50% of that found in control experiments. Once hydrate had formed, decomposition was slower and started later. Thus, gas hydrates formed in the presence of APF I and III appeared to remain stable outside the hydrate stable zone, an observation that has also been noted for other inhibitors. Our observations have potential implications for the use of biological inhibitors under subsea pipeline conditions.

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INTRODUCTION Gas hydrate crystals form under appropriate thermodynamic conditions,1 such as those found in subsea pipelines.2,3 For the past 20 years, the risk of such gas hydrate blockage, has been progressively managed with the addition of kinetic hydrate inhibitors (KHIs).2,4 Although KHIs are typically water-soluble polymers, their poor biodegradability5 and their risk to the environment have motivated research on biological inhibitors, including antifreeze proteins (AFPs) as natural kinetic inhibitors to hinder gas hydrate formation. AFPs best known from certain fish, plants, and insects inhibit ice formation in a non-colligative manner by adsorption to the surface of ice crystals.6−10 As well, two distinct fish AFPs, types I and III (AFPs I and III) impede the formation of gas hydrates (both structures I and II) in demineralized water systems.11−15 They are also able to retard gas hydrate formation in saline solutions.14,16 Interactions between hydrophobic residues in AFPs and their associated clathrate waters and subsequent hydrogen bonding to specific planes of ice have been proposed to govern the adsorption−inhibition mechanism for ice growth inhibition. However, the ability of AFPs to alter gas hydrate crystal nucleation and/or growth is still not understood. Notwithstanding an academic interest in the adsorptive mechanism to an alternative crystal substrate, there are practical reasons to evaluate the performance of these additives under circumstances that are closer to actual field conditions. The performance of AFPs under situations that mimic as far as possible the operating conditions of subsea pipelines, including high driving force, saline conditions, and more than one hydrate former, has been evaluated by ourselves and others14−19 using different experimental approaches. To date, however, only one published report has assayed the performance of one of these inhibitors, AFP III, in the presence of heptane.14 Those authors reported the formation of initial © 2014 American Chemical Society

hydrate slurry but did not examine subsequent hydrate growth and dissociation. However, it is well-known from field reports in pipelines as well as laboratory experiments that the presence of liquid hydrocarbon can impact the performance of KHIs.20,21 Here, we have made an effort to establish a system to model actual off-shore pipeline conditions in laboratory-scale experiments to evaluate the performance of AFPs I and III. We have used a multi-component gas mixture instead of a single gas, saline solutions to mimic seawater, high driving force (over pressure or subcooling) as models for the rigors of pipeline conditions, and the addition of n-heptane to simulate the presence of light crude oil. A high-pressure calorimeter and an apparatus consisting of double-stirred crystallizers were used to examine the performance of these AFPs under these defined and exacting conditions.

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EXPERIMENTAL SECTION

Materials. A mass fraction of 3.5% NaCl (Fisher Scientific) in deionized water was used as the saline solution and to mimic seawater salinity. Two biological KHIs were employed: synthesized desalted winter flounder fish type I AFP (AFP I; α-helical protein of ∼3.2 kDa; Shanghai Apeptide) and ocean pout fish type III AFP (AFP III; globular protein of ∼7 kDa; A/F Protein Canada, Inc., Swiss-Prot Database accession number P19414). These proteins were diluted to 0.1 mM in the saline solution. A natural gas mixture [ultra-high-purity (UHP) grade] consisting of methane (93%), ethane (5%), and propane (2%) supplied by Praxair Technology, Inc. was used as a gas hydrate former. The liquid hydrocarbon phase was n-heptane (Fisher Scientific). High-Pressure Apparatus. The apparatus consisted of two 211 mL high-pressure crystallizers (described elsewhere20), which allowed Received: March 5, 2014 Revised: May 8, 2014 Published: May 8, 2014 3712

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endothermic processes,1 exothermic peaks represent gas hydrate nucleation and endothermic peaks depict hydrate dissociation.26,27 The appropriate identification of ice formation (at ∼250 K), salt eutectic point (at ∼252 K), and melted ice (at ∼271 K) was facilitated by conducting control experiments with saline solutions in the presence of heptane under atmospheric pressure.

for the formation and dissociation of gas hydrates under constant pressure and volume, respectively. A constant pressure was facilitated by connecting a supply gas vessel to each crystallizer through a control valve coupled with a proportional−integral−derivative (PID) controller. The temperature was regulated by the immersion of both crystallizers and supply vessels in a water bath connected to a programmable temperature-controlled refrigerator/heater. Gas Hydrate Formation. The contents of actual sea pipelines usually cool from outside the hydrate stable zone to inside the hydrate region by surrounding seawater under a constant pressure.21 Subsequently, the pipeline temperature is likely to be constant and close to the seawater temperature. Therefore, a constant cooling rate under a constant pressure was used to mimic as much as possible the conditions in pipelines. The crystallizers were loaded with 80 mL of the appropriate solution (either saline or each of the AFPs in saline) and 40 mL of n-heptane. Once the temperature in the crystallizers had stabilized at 293.15 K, the crystallizers were pressurized with the desired gas mixture to a pressure lower than the equilibrium hydrate formation pressure and, subsequently, depressurized 3 times to displace air from the crystallizers. The crystallizers were then pressurized to 7.0 MPa. When the supply vessels were kept pressurized to 9.0−10.0 MPa, the pressure in the crystallizers could be kept constant using the pressure control valve coupled with a PID controller. The gas and liquid phases were mixed by agitation at a steady stirring rate of 500 rpm. As a result, the crystallizer contents were maintained initially inside the hydrate-free region (293.15 K and 7.0 MPa; Teq = 285.6 K at 7.0 MPa, as calculated by CSMGem22), and no hydrate formed. Subsequently, the bath temperature was decreased (at a rate of 1 K/h) to 274.15 K and then kept constant at 274.15 K for 48 h. The start of cooling was considered as the zero time, and data were recorded every 5 s. Because hydrate nucleation is exothermic,1 the nucleation point was identified by the rise in the aqueous phase temperature, measured by an installed thermocouple and the accompanying sudden pressure drop in the supply vessels. The number of moles of gas consumed to form gas hydrate and dissolved in the liquid phase at any given time was calculated by measuring the gas-phase temperature and pressure in the crystallizers and supply vessels and as described elsewhere.23,24 Gas Hydrate Dissociation. Hydrate decomposition was initiated 48 h after formation by discontinuing stirring in the crystallizers, thereby simulating pipeline blockage conditions. The bath temperature was increased from 274.15 to 301.15 K at a rate of 11 K/h under constant volume conditions. The start of heating was considered as the zero time in the decomposition process. Once the crystallizer pressure reached a plateau at a final temperature of 301.15 K (for both the liquid and gas phases), the experiments were terminated. Data were recorded every 5 s. Thermal expansion, desorption of dissolved gas in the liquid phase, and gas hydrate dissociation are all sources of crystallizer pressure built-up under constant volume conditions. The number of moles of released gas attributed to the hydrate dissociation and gas desorption at any given time was calculated as described.25 The calculated released gas was normalized among different experiments to facilitate comparisons.15 High-Pressure Micro-Differential Scanning Calorimetry (DSC). A high-pressure micro-differential scanning calorimeter (HPμDSC 7 Evo; Setaram, Inc.) was used to investigate the effect of the inhibitors on the energetics of gas hydrate formation and decomposition. Samples (1 μL of the appropriate solution and 1 μL of n-heptane) were injected into the sample pits using a microsyringe and placed in the high-pressure cell using an empty sample holder in the other cell as a reference control. Both cells were pressurized to 2.0 MPa with the gas mixture and subsequently depressurized 3 times to remove air from the system. Once the pressure and temperature of both cells reached 8.0 MPa and 303.15 K, a temperature ramping method was used to assess hydrate formation and dissociation. The starting time for temperature ramping was considered as the zero time in the DSC experiments. In the temperature-ramping protocol, the temperature was decreased from 303.15 to 243.15 K (at a rate of 0.2 K/min) and subsequently increased at the same rate to its initial value. Because gas hydrate nucleation and decomposition are exothermic and

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RESULTS AND DISCUSSION Gas Hydrate Nucleation and Growth. Gas hydrate nucleation was indicated by a rise in the aqueous phase temperature in the crystallizers1 (Figure 1), coinciding with a

Figure 1. Temperature in the aqueous phase in the crystallizer with and without inhibitors at a cooling rate of 1 K/h and Pexp = 7.0 MPa. The average values of induction times are shown. Exothermal peaks are marked with circles for control (black line), AFP I (blue line), and AFP III (red line).

sudden reduction in the pressure of the related supply vessel (Figure 2). The addition of AFP I to the saline solution in the

Figure 2. Pressure profiles of the supply vessels during gas hydrate formation experiments at a cooling rate of 1 K/h without inhibitors (black control line) and with inhibitors AFP I (blue dotted line) and AFP III (red dashed line) in saline. The average values of induction times are shown. Arrows indicate inflections in the pressure reduction rate curves.

presence of n-heptane substantially reduced the time for gas hydrate induction compared to the control (from 1205 to 977 min). This represents an 18% reduction in the nucleation time (Figure 1 and Table 1). Similar observations for other KHIs in the presence of liquid hydrocarbon have been previously reported.14,20 This reduction in the nucleation time is a worrisome problem for the industry. In contrast, the addition of AFP III in saline solution in the presence of n-heptane did not change the induction time (Figure 1 and Table 1). Recently, it was observed that, in the absence of heptane, AFP III increased the time to nucleation more than AFP I.16 Here, in the presence of n-heptane, AFP III not only outperformed AFP I 3713

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Table 1. Experimental Conditions, Showing Induction Times and Nucleation Temperatures in both HP-μDSC and Autoclave Experiments DSC experiments (Pexp = 8.0 MPa)a

gas uptake experiments (Pexp = 7.0 MPa) induction time (min) experiment

solution

1A 1B 1C 2A 2B 2C 3A 3B 3C

control

AFP I

AFP III

nucleation temperature (K)

averaged 1212.0 1204.0 1199.0 990.3 963.2

1205.0 ± 5.4

1223.6 1193.8

1208.7 ± 15

976.8 ± 14

ΔTsubcooling (K) b

nucleation temperature (K)

averaged

ΔTsubcoolingc (K)

averaged

274.0 274.2 274.1 277.3 277.8

274.1 ± 0.1

11.5

277.6 ± 0.3

8.1

274.0 273.9

274.0 ± 0.1

11.7

257.5 257.4 257.2 258.1 258.0 258.2 256.7 257.5 257.6

257.4 ± 0.2

29.1

258.1 ± 0.1

28.4

257.3 ± 0.4

29.2

a Ice was formed at ∼250 K in DSC experiments. bΔTsubcooling = Teq − Tnucleation, where Teq = 285.6 K at Pexp = 7.0 MPa. cΔTsubcooling = Teq − Tnucleation, where Teq = 286.5 K at Pexp = 8.0 MPa. d±Standard error. It should be noted that less stochastic behavior was observed because of the available high driving forces and applied temperature-ramping method.

also reduce interfacial surface tension, possibly because of its amphipathic α-helix conformation.7,8 Consequently, the masstransfer barrier for gas hydrate nucleation might be decreased. It would then be expected to reduce the nucleation time. This was not the case for AFP III, however, which is a globular protein, and because the addition of this KHI did not change the induction time from that seen in controls, it is possible that some protein was extracted into the heptane phase. Neither protein structure was influenced by saline, as assessed by circular dichroism;16 therefore, the differences in nucleation times for these two biological inhibitors are due to the presence of the hydrocarbon phase. After nucleation, both AFPs inhibited natural gas hydrate growth (Figure 4). The addition of AFPs I and III in saline

with respect to the induction time but also was superior than previously reported KHIs, which also reduced gas hydrate induction time in the presence of a light crude mimic.20 Figure 3 shows heat flow profiles, with exothermic peaks indicating gas hydrate nucleation. The effects of AFPs I and III

Figure 3. HP-μDSC experiments showing hydrate nucleation peaks in the control (black line) and with AFP I (blue dotted line) or AFP III (red dashed line) additives at Pexp = 8.0 MPa and a cooling rate at 0.2 K/min.

on gas hydrate nucleation were concordant with the highpressure crystallizer experiments. The gas hydrate nucleation temperature was increased by the addition of AFP I to the saline solutions in the presence of n-heptane (from 257.4 to 258.1 K); however, AFP III did not alter the nucleation temperature (Figure 3 and Table 1). Because the subcooling temperature (Teq − Tnucleation) in the presence of AFP III (29.1 K) was higher than that shown by AFP I (28.4 K), again AFP III performed better than AFP I. Indeed, at 29.1 K, AFP III performed more efficiently than some commercial inhibitors, as evidenced by previously reported subcooling temperatures of 28.3 and 28.9 K for gas hydrate nucleation in the presence of polyvinylpyrrolidone (PVP) and polyvinylcaprolactam (PVCap), respectively.20 It is still not understood how the addition of KHIs in the presence of liquid hydrocarbon influences gas hydrate nucleation. However, a decrease in the interfacial surface tension between the aqueous and liquid hydrocarbon phases28 and the consequent decrease in mass-transfer resistance were reported as a possible reason for the reduction in the induction time for synthesized KHIs.20 By this reasoning, AFP I might

Figure 4. Cumulative gas consumption during hydrate formation in control (black line) and with AFP I (blue dotted line) or AFP III (red dashed line) in saline solution at a cooling rate of 1 K/h and Pexp = 7.0 MPa. Arrows show the nucleation points for each experiment.

solutions in the presence of n-heptane decreased the growth of post-nucleation hydrate crystals seen at 0.073 mmol/min (calculated on the basis of the shown trend in Figure 4) for control experiments. The addition of AFPs I and III reduced the growth rate to 0.017 and 0.019 mmol/min, respectively. As a consequence, the total moles of formed hydrate in 48 h was reduced from 0.168 mol in control experiments to 0.081 and 0.098 mol in the presence of AFPs I and III, respectively. In addition, this suggests that AFP was still effective in heptane. Overall, the reduction in the hydrate growth rate is impressive because, in similar conditions, the commercial inhibitors PVP and PVCap have been reported20 to reduce the formed hydrate 3714

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this case, gas hydrate dissociation started at ∼51 min in the control experiment; however, this time was modestly prolonged to ∼53 and ∼59 min in the presence of AFPs I and III, respectively. Consequently, the gas hydrate dissociation took longer in the presence of AFPs. Such an influence was reported for the impact of AFPs on gas hydrate dissociation in demineralized water and saline solutions without liquid hydrocarbon.15,16 The formed gas hydrate dissociated in two steps in the presence of AFPs. Similarly, commercial KHIs also delay gas hydrate dissociation.20 To augment the results obtained with the crystallizers, DSC was also used to determine the influence of the inhibitors on gas hydrate dissociation, with endothermic peaks representing gas hydrate melting (Figure 6). The calculated hydrate

to 0.036 and 0.014 mmol/min, respectively. By this measure, then AFPs had superior control of gas hydrate growth compared to that of PVP and were close to that seen for PVCap. Table 2 shows the summary of the comparison between the performance of PVP and PVCap as commercial inhibitors and Table 2. Comparison between the Performance (Ability) of PVP, PVCap, and AFPs I and III in Gas Hydrate Formation under Different Conditionsa hydrate formation mixture to prolong the induction time to decrease the growth rate to reduce the extent of catastrophic growth a

saline solution

saline + n-heptane

PVP20 PVCap20 AFP I16

AFP III (this work) PVCap20 no catastrophic growth was observed

The most effective inhibitor is shown.

AFPs I and III as biological inhibitors in the inhibition of gas hydrate nucleation and growth under different circumstances. The mechanism of gas hydrate inhibition by AFPs is still not understood. If the amphipathic AFP I remained at the interface of the aqueous and hydrocarbon phases, then these molecules would presumably be readily available for adsorption on forming gas hydrate crystals and, thus, would retard hydrate growth.10 It is curious, however, that AFP III without negative impact on nucleation, as other KHIs, was a very effective hydrate growth inhibitor. We speculate that, although the inhibitor entered the hydrocarbon phase and did not impact nucleation, once crystallization was initiated, the mixing of the phases and capillary action then allowed AFP III to adsorb and incorporate into the growing hydrate and, thus, inhibit the growth rate. Gas Hydrate Dissociation. In cases where KHIs are deployed in the field, it is important for the oil and gas industry to avoid lengthy shutdown times; once formed, hydrates should be readily decomposed, so that production can be resumed. In this regard, the high-pressure crystallizers and differential scanning calorimeter were used to evaluate the influence of the biological inhibitors on gas hydrate dissociation in the presence of heptane. Figure 5 depicts the normalized released gas profiles for different KHIs during gas hydrate dissociation in the crystallizers. In the presence of AFPs, dissociation started later, as compared to control (saline + heptane) experiments. In

Figure 6. Hydrate dissociation profiles in HP-μDSC experiments for control (black line) and the presence of AFP I (blue dotted line) or AFP III (red dashed line) under a heating rate of 0.2 K/min and Pexp = 8.0 MPa.

equilibrium temperature at 8.0 MPa for the natural gas mixture in the presence of saline solution was 289.7 K.22 The equilibrium temperature in the presence of heptane is calculated to be 286.5 K.22 This is because the presence of heptane in a well-mixed system changes the gas-phase composition.29 It is notable, however, that, because of the small amount of n-heptane (1 μL) in the DSC experiments compared to the large amount of gas phase and the absence of stirring, the gas composition was not affected by the addition of the liquid hydrocarbon. The equilibrium dissociation temperature was 289.7 K in saline solution with or without n-heptane. In control experiments, an endothermic peak was observed at 289.4 K, quite close to the equilibrium temperature at 8.0 MPa, as discussed above. The peak was followed by a curve that presumably reflects the variable composition of hydrate formed by the sudden freezing of subcooled solution. However, in the presence of AFPs, multi-peak dissociation was observed. The first peak at 289.6 K likely corresponded to the later gas hydrate dissociation, which was also observed in the autoclave analysis. Endothermic peaks at higher temperatures were also observed in the presence of AFPs, one peak at 291.3 K for APF III and two peaks at 290.6 and 292.25 K for AFP I (Figure 6). Multipeak dissociation has been previously reported for AFPs in saline solution in the absence of liquid hydrocarbon.16 Therefore, these additional peaks do not reflect an impact of n-heptane on hydrate dissociation. Finally, hydrate dissociation took longer in the presence of AFPs I and III, consistent with the results gained by the high-pressure crystallizer analysis. The reason for the appearance of endothermic peaks in the presence of KHIs at temperatures higher than the equilibrium

Figure 5. Calculated normalized released gas during gas hydrate dissociation in control experiments (black line) and the presence of AFP I (blue dotted line) and AFP III (red dashed line) in saline solutions. 3715

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(4) Kelland, M. A. History of the development of low dosage hydrate inhibitors. Energy Fuels 2006, 20, 825−847. (5) Del Villano, L.; Kommedal, R.; Kelland, M. A. Class of kinetic hydrate inhibitors with good biodegradability. Energy Fuels 2008, 22, 3143−3149. (6) Raymond, J. A.; DeVries, A. L. Adsorption inhibition as a mechanism of freezing resistance in polar fishes. Proc. Natl. Acad. Sci. U. S. A. 1977, 74, 2589−2593. (7) Davies, P. L.; Baardsnes, J.; Kuiper, M. J.; Walker, V. K. Structure and function of antifreeze proteins. Philos. Trans. R. Soc., B 2002, 357, 927−935. (8) Ewart, K. V.; Lin, Q.; Hew, C. L. Structure, function and evolution of antifreeze proteins. Cell. Mol. Life Sci. 1999, 55, 271−283. (9) Yeh, Y.; Feeney, R. E. Antifreeze proteins: Structures and mechanisms of function. Chem. Rev. 1996, 96, 601−618. (10) Wathen, B.; Kuiper, M.; Walker, V.; Jia, Z. New simulation model of multicomponent crystal growth and inhibition. Chem.Eur. J. 2004, 10, 1598−1605. (11) Zeng, H.; Moudrakovski, I. L.; Ripmeester, J. A.; Walker, V. K. Effect of antifreeze protein on nucleation, growth and memory of gas hydrates. AIChE J. 2006, 52, 3304−3309. (12) Zeng, H.; Wilson, L. D.; Walker, V. K.; Ripmeester, J. A. Effect of antifreeze proteins on the nucleation, growth, and the memory effect during tetrahydrofuran clathrate hydrate formation. J. Am. Chem. Soc. 2006, 128, 2844−2850. (13) Al-Adel, S.; Dick, J. A.; El-Ghafari, R.; Servio, P. The effect of biological and polymeric inhibitors on methane gas hydrate growth kinetics. Fluid Phase Equilib. 2008, 267, 92−98. (14) Jensen, L.; Thomsen, K.; von Solms, N. Inhibition of structure I and II gas hydrates using synthetic and biological kinetic inhibitors. Energy Fuels 2010, 25, 17−23. (15) Daraboina, N.; Linga, P.; Ripmeester, J.; Walker, V. K.; Englezos, P. Natural gas hydrate formation and decomposition in the presence of kinetic inhibitors. 2. Stirred reactor experiments. Energy Fuels 2011, 25, 4384−4391. (16) Sharifi, H.; Walker, V. K.; Ripmeester, J. A.; Englezos, P. Insights into the behaviour of biological clathrate hydrate inhibitors in aqueous saline solutions. Cryst. Growth Des. 2014, DOI: 10.1021/cg500218q. (17) Daraboina, N.; Ripmeester, J.; Walker, V. K.; Englezos, P. Natural gas hydrate formation and decomposition in the presence of kinetic inhibitors. 1. High pressure calorimetry. Energy Fuels 2011, 25, 4392−4397. (18) Daraboina, N.; Moudrakovski, I. L.; Ripmeester, J. A.; Walker, V. K.; Englezos, P. Assessing the performance of commercial and biological gas hydrate inhibitors using nuclear magnetic resonance microscopy and a stirred autoclave. Fuel 2013, 105, 630−635. (19) Ohno, H.; Susilo, R.; Gordienko, R.; Ripmeester, J.; Walker, V. K. Interaction of antifreeze proteins with hydrocarbon hydrates. Chem.Eur. J. 2010, 16, 10409−10417. (20) Sharifi, H.; Ripmeester, J.; Walker, V. K.; Englezos, P. Kinetic inhibition of natural gas hydrates in saline solutions and heptane. Fuel 2014, 117, 109−117. (21) Glenat, P.; Peytavy, J.-L.; Holland-Jones, N.; Grainger, M. South-pars phases 2 and 3: The kinetic hydrate inhibitor (KHI) experience applied at field start-up. Proceedings of the Abu Dhabi International Conference and Exhibition; Abu Dhabi, United Arab Emirates, Oct 10−13, 2004. (22) Ballard, A. L.; Sloan, E. D., Jr. The next generation of hydrate prediction: I. Hydrate standard states and incorporation of spectroscopy. Fluid Phase Equilib. 2002, 194, 371−383. (23) Linga, P.; Kumar, R.; Englezos, P. Gas hydrate formation from hydrogen/carbon dioxide and nitrogen/carbon dioxide gas mixtures. Chem. Eng. Sci. 2007, 62, 4268−4276. (24) Sharifi, H.; Hatzikiriakos, S. G.; Englezos, P. Rheological evaluation of kinetic hydrate inhibitors in NaCl/n-heptane solutions. AIChE J. 2014, DOI: 10.1002/aic.14433. (25) Haligva, C.; Linga, P.; Ripmeester, J. A.; Englezos, P. Recovery of methane from a variable-volume bed of silica sand/hydrate by depressurization. Energy Fuels 2010, 24, 2947−2955.

temperature is still not understood. However, it is clear that some portion of the hydrates formed in the presence of AFPs was stable outside the hydrate stability region, implying a heterogeneous hydrate structure. Previously, it was suggested that adsorption of inhibitor molecules on the surface of natural gas hydrate crystals could dictate compositional changes in the formed hydrate,19,30 and this proposal may be applicable here. In addition, if AFPs adsorb to hydrate crystals at the phase interface, they may allow for the hydrates to remain outside the stability field, a phenomenon termed anomalous or selfpreservation.1 Perhaps analogously, AFPs adsorb to ice crystals and prevent the melting of small crystals or ice recrystallization, which is normally observed at temperatures just below the equilibrium freezing point. Nonetheless, it must be noted that, in comparison to controls, only half the volume of hydrates formed in the presence of AFPs, suggesting to us that, if AFPs were used in practical situations, it would be better from a flow assurance point of view.

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CONCLUSION The performance of two biological KHIs, AFPs I and III, were evaluated under conditions that simulated as much as possible subsea pipeline conditions. These include the presence of a multi-component gas mixture, liquid hydrocarbon, and saline water in high driving forces desired to form hydrate. Under these conditions, AFP I was similar to commercial KHIs, in that the induction time was reduced. Meanwhile, there was a significant reduction in hydrate growth. However, gas hydrate dissociation was prolonged and took longer. AFP III was superior, in that it did not reduce nucleation time and hydrate growth was decreased to 50%, a level comparable to that seen with commercial KHIs. A delay in dissociation of a portion of the formed hydrate was seen with AFP III, suggestive of a hydrate self-preservation phenomenon. Taken together, AFPs, in particular AFP III, show promise as nucleation-neutral hydrate growth inhibitors that are active in the presence of a light crude model, making it an attractive candidate for industrial consideration.

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AUTHOR INFORMATION

Corresponding Author

*Telephone: +1-604-822-6184. Fax: +1-604-822-6003. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The financial support from the Natural Sciences and Engineering Research Council of Canada (NSERC) is greatly appreciated. Hassan Sharifi thanks the University of British Columbia (UBC) for a Four Year Fellowship.

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REFERENCES

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(26) Lachance, J. W.; Dendy Sloan, E.; Koh, C. A. Effect of hydrate formation/dissociation on emulsion stability using DSC and visual techniques. Chem. Eng. Sci. 2008, 63, 3942−3947. (27) Dalmazzone, D.; Kharrat, M.; Lachet, V.; Fouconnier, B.; Clausse, D. DSC and PVT measurements. J. Therm. Anal. Calorim. 2002, 70, 493−505. (28) Lou, A.; Pethica, B. A.; Somasundaran, P. Interfacial and monolayer properties of poly(vinylcaprolactam). Langmuir 2000, 16, 7691−7693. (29) Peter, B.; Dagobert, K.; Iradj, R. Influence of liquid hydrocarbons on gas hydrate equilibrium. Proceedings of the European Petroleum Conference; Cannes, France, Nov 16−18, 1992. (30) Daraboina, N.; Ripmeester, J.; Walker, V. K.; Englezos, P. Natural gas hydrate formation and decomposition in the presence of kinetic inhibitors. 3. Structural and compositional changes. Energy Fuels 2011, 25, 4398−4404.

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