Insights into the Behavior of Biological Clathrate ... - ACS Publications

Apr 25, 2014 - *Address: 2360 East Mall, Vancouver, Canada, V6T 1Z3. .... Influence of natural gas production chemicals on scale production in MEG reg...
0 downloads 0 Views 1MB Size
Download PDF
Article pubs.acs.org/crystal

Insights into the Behavior of Biological Clathrate Hydrate Inhibitors in Aqueous Saline Solutions Hassan Sharifi,† Virginia K. Walker,∥ John Ripmeester,‡ and Peter Englezos*,† †

Department of Chemical and Biological Engineering, University of British Columbia, Vancouver, British Columbia Canada Department of Biology, and School of Environmental Studies, Queen’s University, Kingston, Ontario, Canada ‡ Steacie Institute for Molecular Sciences, National Research Council Canada, Ottawa, Ontario, Canada ∥

ABSTRACT: The performance of two antifreeze proteins (AFPs), type I and III (AFP I and III), as kinetic natural gas hydrate inhibitors in saline solutions was evaluated using a high pressure micro differential scanning calorimeter and a high pressure apparatus consisting of two crystallizers. Although AFP I and III were found to prolong crystallization time and reduce initial gas hydrate growth in saline solutions, the growth rate increased when hydrate started to form in the gas phase. Circular dichroism experiments suggested that the saline solution did not perturb AFP I and III structures. However, in the presence of saline, the inhibitory activity of AFP I to prolong induction time decreased while AFP III was more active. As a consequence, we propose that a decrease in hydrophobic forces and the neutralization of ion charges could explain AFP adsorption to the surface of hydrate crystals. Once the hydrate formed, melting was delayed, and consequently hydrate decomposition took longer in the presence of AFPs. We suggest conditions whereby the properties of AFPs could be harnessed for petrochemical recovery and transport.

■

to evaluate the performance of these inhibitors,27−32 their ability to alter gas hydrate crystal nucleation and/or growth is still not understood. Despite these difficulties, we consider it essential to assess the performance of these biological inhibitors under simulated field conditions in the laboratory, in order to address concerns of the transferability of kinetic inhibition results between laboratory scale experiments and field tests.33,34 In this regard, the performance of different AFPs in the presence of gas mixtures has been assessed using a number of techniques and instruments, in the hopes of more effectively moving to field testing.24,27−32 This is especially relevant as hydrocarbon exploration shifts to deeper wells offshore and to sensitive aquatic ecosystems. Ironically then, to our knowledge, only a single report has documented the utility of one AFP (type III; AFP III) to inhibit gas hydrate nucleation in a saline solution.24 These authors found that the activity of this AFP was weakened in the presence of saline and silica gel, but they did not examine the subsequent impact on hydrate growth and dissociation. With respect to ice formation, AFP III was a more effective inhibitor in tested saline solutions.35 The effect of saline on the performance of AFPs (AFP I and III) on nucleation, growth, and dissociation of gas hydrate crystals has hitherto not been examined. Here we have established a hydrate formation system that imitates off-shore conditions. These conditions were accom-

INTRODUCTION Formation of natural gas hydrate crystals during prospecting and in oil and gas pipelines has been recognized as a major cause of transmission pipelines blockage.1−6 Conventionally, thermodynamic hydrate inhibitors (THIs) such as methanol or glycols are injected to oil and gas pipelines to impede gas hydrate formation. THIs shift the thermodynamic hydrate phase boundary outside normal operating conditions. However, for environmental as well as economic reasons,7 it may be preferable to employ small quantities (less than 1 wt %) of low dosage hydrate inhibitors (LDHIs) to manage the risk of hydrate formation, if not complete inhibition.8 LDHIs include kinetic hydrate inhibitors (KHIs) and antiagglomerates (AAs).9−11 The active components of KHIs are generally water-soluble polymers that significantly delay gas hydrate nucleation and usually also alter post nucleation crystal growth.8,12 Nonetheless, environmental restrictions have limited the use of some KHIs because of their poor biodegradibility.13 Antifreeze proteins (AFPs) found in a few fish, insects, and plants among other organisms can lower the freezing point of ice in a noncolligative manner.14−18 An adsorption−inhibition mechanism has been proposed to explain the action of AFPs in inhibiting ice crystal growth.19 These biological inhibitors also delay gas hydrate nucleation points in both single and multicomponent gas mixtures, and in some cases they increased hydrate induction time more than the synthesized KHIs, polyvinylpyrrolidone (PVP), and polyvinylcaprolactam (PVCap).20−26 Although various techniques have been used © 2014 American Chemical Society

Received: February 12, 2014 Revised: April 15, 2014 Published: April 25, 2014 2923

dx.doi.org/10.1021/cg500218q | Cryst. Growth Des. 2014, 14, 2923−2930

Crystal Growth & Design

Article

K) and 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. Experiments were terminated after 48 h when hydrates had formed in the crystallizers. As hydrate nucleation is an exothermic process,4 the nucleation point was identified by the increase in the aqueous phase temperature accompanied by a sudden pressure drop in the supply reservoirs. The number of moles of gas consumed to form hydrates and dissolve in liquid phase at any given time was calculated as described.40 Gas Hydrate Crystal Dissociation. Hydrate dissociation experiments were undertaken 48 h after the formation of gas hydrates in the crystallizers, simulating pipeline blockage. Hydrate dissociation was initiated by no longer stirring and increasing the water bath temperature from 274.15 K to 301.15 at a rate of 11 K/h. The start of heating was considered as the zero time in the dissociation experiments. Data were recorded every 5 s until the experiments were terminated coincident with a pressure plateau at 301.15 K. Since dissociation experiments were carried out under constant volume, crystallizer pressure increased due to the thermal expansion, the evolution of dissolved gas in the liquid phase, as well as gas hydrate dissociation. The number of moles of released gas attributed to the hydrate dissociation, and the reduction of gas solubility in the liquid phase at any given time was calculated as described.41 In order to facilitate comparison between experiments, the amount of released gas was normalized (divided by the total recovered gas).27 Once gas hydrate dissociation experiments were terminated, the crystallizers were depressurized, drained, washed with deionized water, and subsequently dried with air in preparation for the next experiment. High Pressure Micro Differential Scanning Calorimetry. A high pressure micro differential scanning calorimeter (HP-μDSC 7 Evo; Setaram Inc.) was used to determine the influence of the inhibitors on gas hydrate formation and dissociation. The calorimeter was equipped with a double-stage temperature control and Peltier coolers allowing operational temperatures between 228.15 and 393.15 K and a programmable temperature scanning rate of 0.001−2 K/min. Two stainless steel high pressure cells (1 mL), designed to tolerate up to 40.0 MPa, were used to keep two customized stainless steel sample holders consisting of a base with four depressions for experimental solutions (as described37). Samples (1 μL of the appropriate solution) were injected into the depressions using a microsyringe, placed in the high pressure cell, and pressurized to 7.0 MPa with the gas mixture, using an empty sample holder in the reference cell as a reference control. Once the pressure and temperature of both cells achieved 7.0 MPa and 303.15 K, an isothermal protocol was used for hydrate formation and subsequent dissociation. Briefly, temperature was decreased from 303.15 to 259.15 K at a rate of 1 K/min and then kept constant at 259.15 K for 8 h in order to form gas hydrates. Decomposition was achieved by increasing the temperature to 303.15 K at a rate of 0.2 K/min. The starting time for the isothermal protocol was considered as the zero time in the DSC experiments. Gas hydrate nucleation and decomposition are exothermic and endothermic processes,4 respectively. Hence, exothermic peaks represent gas hydrate nucleation and endothermic peaks illustrate hydrate decomposition.42,43

plished by using a multicomponent gas mixture; applying high driving forces (over pressure or subcooling); and increasing water salinity to model seawater conditions. Our apparatus consisted of two high pressure crystallizers and a high pressure micro differential scanning calorimeter, which were used to assess the impact of two representative AFPs on natural gas hydrate formation and dissociation processes under these more realistic conditions.

■

EXPERIMENTAL SECTION

Materials. Saline solutions (a mass fraction of 3.5% NaCl for most experiments, but also 2% or 5 wt % NaCl) were prepared in distilled, deionized water (ddH 2O). Two biological KHIs were used: synthesized desalted type I AFP (AFP I; α-helical protein of ∼3.2 kDa; Shanghai Apeptide, Swiss-Prot Database accession number P04002) and type III AFP (AFP III; globular protein of ∼7 kDa; A/F Protein Canada Inc., Swiss-Prot Database accession number P19414). The KHIs were diluted to 0.1 mM in the saline solution. A natural gas mixture (UHP grade) consisting of methane (93%)/ethane (5%)/ propane (2%) was supplied by Praxair Technology Inc. Structure II (sII) hydrate forms under the conditions used here. Circular Dichroism (CD). CD was performed for AFPs in ddH2O and saline solution. AFP I and AFP III were diluted to ∼40 μM (or 100 μM for AFP III), in either ddH2O or the 3.5% saline solution and examined for CD using an Olis Rapid Scanning Monochromator 1000 spectrometer (Olis Inc.) with the cuvette temperature maintained at 277 K. A minimum of five scans were averaged, and the curves were corrected using the appropriate blank. High Pressure Crystallizer Apparatus. A high pressure apparatus was used to conduct gas uptake and decomposition experiments under constant pressure and volume, respectively. This apparatus is described in detail elsewhere.36 Briefly, it consisted of two 211 mL high pressure crystallizers surrounded by tubing and fitted with two circular Lexan viewing windows in front and back. The crystallizers were equipped with four baffles to control vortex formation during mixing that was accomplished by a gas induced impeller, which in turn was coupled with a hollow shaft and rotated with a magnetic driven motor. One additional 300 mL stainless steel vessel acted as a supply gas reservoir and was connected to each crystallizer in order to provide constant pressure, utilizing a high pressure control valve coupled with a PID controller. Both crystallizers and supply reservoirs were immersed in a temperature-controlled circulating bath with the temperature regulated by an external refrigerating/heating programmable circulator. A specified Labview program was written to receive, convert, and record pressure and temperature signals including: temperature of the gas and liquid phases in the crystallizers, temperature and pressure of supply reservoirs, pressure of crystallizers and also to communicate with the high pressure control valve. Gas Hydrate Crystal Formation. A constant cooling rate under conditions of constant pressure37,38 was applied to imitate pipeline field conditions.36 The crystallizers were loaded with 80 mL of experimental aqueous solutions (either saline or AFPs in saline solutions). Once the water bath temperature was stabilized at 293.15 K, the crystallizers were pressurized with the gas mixture until they were just lower than the equilibrium hydrate formation point. They were subsequently depressurized three times to purge air from the system. The crystallizers were then allowed to rise to the experimental pressure (7.0 MPa). In order to provide gas from supply reservoirs to the crystallizers, the supply reservoirs were pressurized to 9.0−10.0 MPa. A constant stirring rate of 500 rpm was applied to contact gas and liquid phases, and the PID controllers were set to 7.0 MPa in order to retain a constant pressure in the crystallizers throughout gas hydrate formation. Under these conditions, the crystallizer contents lie outside the hydrate stable zone (293.15 K at 7.0 MPa with Teq = 288.8 K at 7.0 MPa, as calculated by CSMGem39), and no hydrate was formed initially. Once the temperature and pressure in the crystallizers and supply reservoirs stabilized, the temperature of the water bath was reduced (at a rate of 1 K/h) to achieve to the targeted value (274.15

■

RESULTS

Circular Dichroism. AFP I showed a spectral minimum at 219 nm and a maximum at 190 nm, characteristic of an α helix irrespective of the solvent (Figure 1a). Similarly, the AFP III spectrum was consistent in both water and saline with a less regular globular protein structure (Figure 1b). Gas Hydrate Nucleation. Since gas hydrate formation is exothermic,4 nucleation in the crystallizers was easily monitored by the temperature spikes in the aqueous phase accompanied by a sudden reduction in pressure. Under programmed cooling rates and with gas provided by supply reservoirs to prevent pressure loss, a sudden decrease in supply reservoir pressure represented the onset of gas hydrate nucleation. To ensure that 2924

dx.doi.org/10.1021/cg500218q | Cryst. Growth Des. 2014, 14, 2923−2930

Crystal Growth & Design

Article

AFP I and III approach this level of effectiveness by delaying nucleation by 1.3 and 1.6 times, respectively. Interestingly, these observations indicated that AFP III in the presence of 3.5% saline is as effective as a synthesized kinetic inhibitor. In order to complement the results obtained with the crystallizers, differential scanning calorimetry was also used to determine the influence of the inhibitors on gas hydrate formation. Similar to the stirred reactor observations, the addition of AFPs was found to delay nucleation. Compared to control conditions, gas hydrate nucleation was delayed in the presence of AFP I and III an average of 16 and 45 min, respectively (Figure 4 and Table 1). Although the two different experimental assessments of nucleation time were clearly distinct, the average delay in nucleation time was strikingly similar, and with the HP-μDSC, mean hydrate nucleation was delayed by a factor of 1.2 and 1.6 for AFP I and III, respectively. Both gas uptake and DSC experiments were in concordance; they indicated that the biological inhibitors increased gas hydrate induction time in the presence of 3.5 wt % NaCl and that AFP III showed more inhibitory activity than AFP I. This conclusion contrasts with the previously reported performance of these AFPs in small crystallizers maintained under constant temperature and pressure and filled with the pure water.27 Since direct comparisons with different equipment and experimental design are difficult, we repeated our crystallizer experiments using solutions as described by previous experimental parameters.27 In water, AFP I had a longer induction time than AFP III; nucleation in water was delayed 359 min in the presence of AFP I and 119 min in the presence of AFP III. Thus, the presence of 3.5% NaCl promoted the performance of AFP III by increasing the average induction time 173 min (from 763 to 936 min). However, the inhibitory activity of AFP I was reduced 232 min (from 1003 to 771 min) in saline solution. These observations prompted us to investigate the impact of saline concentration on induction time. In 5 wt % NaCl and AFP I the induction time was reduced 91 min (from 771 to 680 min; not shown). In 2 wt % NaCl and AFP III, the induction time was decreased 132 min (from 936 to 804 min). Gas Hydrate Crystal Growth. The cumulative gas consumed during gas hydrate formation was different in the presence and absence of inhibitors (Figure 5). The addition of AFPs increased gas hydrate induction time, as discussed above, but also had an impact on gas hydrate growth. In the presence of AFPs, the hydrate growth period appeared to be divided into two stages (Figure 5a) until catastrophic growth. This behavior was not observed in control experiments. The interface of these two stages is shown more clearly in Figure 5b. In the first section (772−1020 min, and 937−1110 min for AFP I and III, respectively) gas hydrates grew with an average rate of 0.0131 and 0.0139 mmol/min for AFP I and III, respectively (Figure 5b; Table 2). Both of these rates are lower than the rate of hydrate formation in control experiments (0.0214 mmol/min). In the second portion of the growth period (1020−1400 min and 1110−1550 min for AFP I and III, respectively), however, the AFPs appeared to increase hydrate growth rates (0.0354 and 0.0370 mmol/min for AFP I and III, respectively) compared to control experiments (0.0214 mmol/min; Table 2). Similar behavior has also been previously reported in the presence of synthesized inhibitors.36,45 Thus, we suggest that it is characteristic of KHIs, including AFPs, to control hydrate growth only up to a critical point, after which hydrates grow faster. This is not a desirable commercial attribute.

Figure 1. Circular dichroism analysis of the biological KHIs in ddH2O (blue solid line) and 3.5% saline (red dashed line) plotted as millidegrees vs wavelength for AFP I (a) and AFP III (b). Note that the depicted scans are the average of a minimum of five scans subtracted from the average of five control solutions and normalized to 40 μM (AFP I) or 100 μM (AFP III). Scans in saline were terminated at 193 nm due to the high background generated by NaCl.

the gas phase composition did not change during gas hydrate formation, this was also monitored; no significant change in the ethane or propane composition was noted for 350 min (not shown). Crystallizer temperature profiles of 3.5% saline solutions with and without inhibitors (AFP I and III) show that induction times for the gas hydrate formation compared to control solutions were delayed with AFP I and III an average of 170 and 333 min, respectively (Figure 2, and Table 1).

Figure 2. Temperature profiles of 3.5% saline solutions with and without inhibitors in the experiments conducted at a cooling rate of 1 K/h and Pexp = 7.0 MPa. The average values of induction times from two independent experiments are shown. Exothermal peaks are marked with arrows for controls (black line), AFP I (blue line), and AFP III (red line).

Confirmation that the temperature increases (Figure 2) coincided with gas hydrate formation was obtained by monitoring the simultaneous change in pressure reduction rates in the supply vessels (Figure 3). Previously, it has been reported that onset of gas hydrate nucleation in saline solution can be delayed by as much as 1.7 and 1.6 times in the presence of PVP and PVCap, respectively,36,44 and here we show that 2925

dx.doi.org/10.1021/cg500218q | Cryst. Growth Des. 2014, 14, 2923−2930

Crystal Growth & Design

Article

Table 1. Experimental Conditions, Showing Induction Times and Nucleation Temperature in Both HP-μDSC and Autoclave Experiments gas uptake experiments (Pexp = 7.0 MPa) induction time (min) experiment

solution

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

control AFP I AFP III

average 596.5 610.5 756.2 786.6 928.8 943.4

DSC experiments (Pexp = 7.0 MPa)

nucleation temperature (K)

induction time (min)

average

603.5

283.8 284.0 280.7 281.3 278.4 278.6

771.4 936.1

283.9 281.0 278.5

average 75.8 80.8 92.3 95.5 122.1 124.7

78.3 93.9 123.4

Figure 3. Pressure profiles of the supply reservoirs during gas hydrate formation experiments conducted 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 3.5% saline solutions. The average values of induction times from two independent experiments are shown. Arrows indicate inflections in the pressure reduction rate curves.

Figure 5. Cumulative gas consumption during hydrate formation in 3.5% saline solution, controls without inhibitors (black line) and with AFP I (blue dotted line) or AFP III (red dashed line) under a cooling rate of 1 K/h and Pexp = 7.0 MPa. (a) Induction times are shown by arrows: 603.5 min (control), 771.4 min (AFP I), and 936.1 min (AFP III). (b) Gas consumption is shown from 550 to 1150 min with induction times shown as arrows. Stars indicate the onset of catastrophic growth.

form due to extreme conditions, it is important that they be easily melted so that production and transport can resume. The increase in pressure of the crystallizers (Figure 7) is due to the decrease in gas solubility (evolution of dissolved gas), the gas hydrate dissociation and thermal expansion. The effect of temperature on gas phase thermal expansion is compensated in the calculated numbers of moles of released gas. Hence, the number of released moles increased due to hydrate decomposition and gas desorption. Crystallizer pressure profiles started to rise at ∼25 min for all cases (Figure 7). However, the normalized released gas concentration remained at zero for ∼45 min. Therefore, the increase in the crystallizer pressure before ∼70 min was due to the thermal expansion only. The increase in calculated gas release started at ∼78 min in control experiments, while addition of AFP I and III decreased this time to ∼71.5 and ∼68.5 min, respectively. We attribute the increases in released gas to gas hydrate dissociation and gas

Figure 4. HP-μDSC experiments showing hydrate nucleation in the 3.5% saline solution controls (black line) and with AFP I (blue dotted line) or AFP III (red dashed line) at Pexp = 7.0 MPa, and Texp = 259.15 K.

Gas phase temperature was monitored throughout gas hydrate growth (Figure 6). Since gas hydrate formation is exothermic, elevation of the gas phase temperature is indicative of hydrate formation. Gas hydrate-mediated increases in the gas phase started at ∼1020 and 1110 min in the presence of AFP I and III, respectively, which are coincident with the increases in gas hydrate growth rate in the presence of AFPs (Figure 6). Therefore, once hydrate formation had initiated in the presence of AFPs in the gas phase, gas hydrate grew faster compared to the experiments without inhibitors. Gas Hydrate Crystal Dissociation. Industry not only requires KHIs to inhibit nucleation, but in the event hydrates 2926

dx.doi.org/10.1021/cg500218q | Cryst. Growth Des. 2014, 14, 2923−2930

Crystal Growth & Design

Article

Table 2. Calculated Gas Hydrate Growth Rates in Different Growth Periods and Gas Hydrate Dissociation Rates with Standard Errors in Parentheses growth period 1a solution control AFP I AFP III

period occurrence (min) 605−1100 772−1020 937−1110

growth period 2a

growth rate (mol/min) −4

period occurrence (min)

growth rate (mol/min)

−7

2.14 × 10 (3.27 × 10 ) 1.31 × 10−4 (3.2 × 10−7) 1.39 × 10−4 (2.79 × 10−7)

1020−1400 1110−1550

−4

−7

3.54 × 10 (3.26 × 10 ) 3.70 × 10−4 (6.84 × 10−7)

dissociation rateb (mol/min) 0.015 (2.75 × 10−5) 0.013 (2.15 × 10−5) 0.011 (2.03 × 10−5)

a

Gas hydrate formed under constant pressure (7.0 MPa) and a cooling rate of 1 K/h. bGas hydrate dissociation was carried out at a heating rate of 11 K/h.

Figure 6. Gas phase temperature profiles during gas hydrate formation for the control (black line) and the presence of AFP I (blue dotted line) or AFP III (red dashed line) under a cooling rate of 1 K/h and Pexp = 7.0 MPa.

Figure 8. Hydrate dissociation profiles in HP-μDSC experiments for the 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 = 7.0 MPa (equilibrium hydrate formation temperature at experimental pressure is 288.8 K; calculated by CSMGem39).

of hydrate formed by the sudden freezing of subcooled solution. In the presence of AFP I and III hydrate (sII) also dissociated at 289.1 K, but in addition to the expected hydrate dissociation peak, two other peaks at 290.2 and 292.5 K were observed (Figure 8). Remarkably, AFP addition obviously reduced the amount of formed hydrate corresponding to the first peak, suggesting that different crystalline structures formed. Overall, hydrate dissociation took longer in the presence of AFPs, consistent with the results obtained by the autoclave analysis.

■

DISCUSSION AFPs likely inhibit gas hydrate growth by adsorption− inhibition,21 but the exact mechanism is still not understood. The two fish AFPs used here are structurally distinct; AFP I has an α-helical conformation and AFP III is globular, but both have relatively “flat” ice adsorption sites. AFPs appear to adsorb to ice crystals by hydrogen bonding of clathrate-arranged water molecules present on their hydrophobic ice-binding sites.46 The principle electrolyte in teleost fish serum is NaCl, and serum osmolarity increases under cold conditions to about half of the surrounding seawater, and thus we reasoned that these proteins would retain their active structures in saline. CD analysis confirmed that both AFPs retained their characteristic structure in either solvent (Figure 1). Theoretically, however, the addition of NaCl to the solution should decrease hydrophobic forces,47 and as a result it is possible that there could be reduced hydrate inhibition in 3.5% saline. Such a reduction was observed for AFP I. To confirm this, the effect of a higher salt concentration (5 wt % NaCl) was investigated. Induction time

Figure 7. Calculated normalized released gas (upper panel) and pressure profiles (lower panel) during gas hydrate dissociation in the control experiments (black line) and in the presence of AFP I (blue dotted line) and AFP III (red dashed line) in 3.5% saline solutions.

desorption. Therefore, gas hydrate dissociation started sooner in the presence of AFPs, as has been reported in the presence of synthesized inhibitors.36 Nevertheless, complete gas hydrate dissociation took modestly longer in the presence of AFP I and III compared to the control experiments by a factor of 1.26 and 1.31, respectively, since the hydrate dissociation rate was decreased compared to the control solution (Figure 7; Table 2). Gas hydrate dissociation can be identified by endothermic peaks in HP-μDSC experiments. Heat flow profiles (Figure 8) showed that in control experiments hydrate dissociated at 289.1 K, very close to the equilibrium value for sII hydrate (288.8 K at Pexp = 7.0 MPa, calculated by CSMGem39). These profiles show “tails”, which presumably reflect the variable composition 2927

dx.doi.org/10.1021/cg500218q | Cryst. Growth Des. 2014, 14, 2923−2930

Crystal Growth & Design

Article

crystals and stabilize them so that at temperatures close to melting, ice recrystallization is effectively prevented. If this property of AFPs could be retained in the presence of commercial AAs, it is possible that a solution to flow assurance might be found in allowing hydrate crystals to form in pipelines with AA preventing aggregation of these crystals and AFPs preventing their recrystallization.

was further reduced 91 min, showing that the effectiveness of AFP I to inhibit gas hydrate formation was reduced by addition of NaCl, which we suggest is related to the reduction of hydrophobic interactions. In contrast with the AFP I experiments, the addition of 3.5% NaCl increased AFP III activity as a gas hydrate inhibitor. Previously, it was reported that addition of positive ions such as Ca2+ or Na1+ to an aqueous solution of AFP III enhanced its ice inhibition activity.35 This effect was attributed to positive ions being attracted to the negatively charged AFP III protein surface,48 resulting in a reduction in repulsive electrostatic forces between AFP III molecules with the consequent increase in the number of proteins on the ice crystal surface.35 Thus, although a higher salt concentration could reduce hydrophobic forces, this may be offset by an increase in the effective concentration of the inhibitor. To investigate the association of AFP III hydrate inhibition and saline concentration, the performance of type III AFP was then tested at lower solute concentration (2 wt % NaCl). In this case, the gas hydrate induction time was further decreased 132 min compared that in 3.5% NaCl, and as predicted. By adsorption to embryonic hydrate crystals, AFPs would reduce the available hydrate surface area,23,25 increasing the intrinsic reaction resistance and therefore decreasing the rate of hydrate formation. Evidence of such an effect was seen in the initial phase of gas hydrate growth where AFPs adsorbed to the hydrate surface reduced growth, but in the second phase, the gas consumption rate was similar to growth shown by controls (Figure 5a). Faster hydrate formation was also seen when the increase in gas temperature was monitored (Figure 6). Similar observations have been reported in the presence of chemical inhibitors,36,45,49 but the mechanism has not been explained. Possibly, the higher porosity of the hydrate crystal structure in the presence of kinetic inhibitors50 could facilitate water transport by capillary action, thereby enhancing gas hydrate formation. For AFPs, this increasing porosity would presumably allow the surface area to expand faster than the AFPs could orient themselves on the crystal. Not only was the induction time longer in the presence of AFPs, but hydrate dissociation also took longer, with more than one dissociation peak observed (Figure 8). Previously, it has been suggested that hydrates formed in the presence of biological KHIs have two different hydrate structures32 and compositional changes.29 Since the equilibrium temperature to form sI hydrate with our gas mixture at 7.0 MPa was 283 K (calculated by CSMGem39), as previously noted,32 the observed additional peaks at 290 and 292 in the presence of AFPs would not correspond to different known structures of hydrate. Although earlier work examined the dissociation of hydrates formed in methane in the presence of PVCap and may not be relevant here, of two endothermic peaks in DSC experiments, the first one was sI.42 An increase in PVCap concentration reduced the area of the first peak and increased the second. We suggest that hydrate compositional changes can indeed contribute to the multistep dissociation curve (Figure 8), but additionally, AFP adsorption on the gas hydrate surface results in more stabilized hydrate crystals, enhancing the crystalline nature of the hydrate particles.50 Such an influence on hydrate crystals has been previously noted in morphological experiments in which hydrates appeared to be “harder” in the presence of KHIs compared to the control experiments.50 We suggest that this is consistent with a property of AFPs with respect to ice recrystallization inhibition; AFPs adsorb to ice

■

CONCLUSIONS The performance of two AFPs (type I and III) was evaluated under the mimicked field conditions consisting of water salinity and high driving forces in terms of temperature subcooling using a set of two identical high pressure crystallizers and with a calorimeter. Although saline did not appear to influence the AFP structures, as evidenced by circular dichroism assessments, the inhibitory activity of AFP I decreased while AFP III performed more efficiently. Addition of AFPs regulated gas hydrate growth initially but only until the point when the rate of crystal growth accelerated rapidly. The formed gas hydrates in the presence of AFPs dissociated at higher temperatures compared to the control solutions. We suggest that initially, adsorption of AFPs stabilizes hydrate crystals, but later, incorporation of AFPs into the crystal structure may lead to changes in the hydrate composition as suggested by additional dissociation peaks upon melting.

■

AUTHOR INFORMATION

Corresponding Author

*Address: 2360 East Mall, Vancouver, Canada, V6T 1Z3. Phone: +1 604-822-6184. Fax: +1 604-822-6003. E-mail: peter. [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The financial support from the Natural Sciences and Engineering Research Council of Canada (NSERC) is greatly appreciated. A Queen’s University Research Chair and the Protein Function and Discovery Research and Training Facility at Queen’s are also acknowledged.

■

REFERENCES

(1) Hammerschmidt, E. G. Formation of Gas Hydrates in Natural Gas Transmission Lines. Ind. Eng. Chem. 1934, 26, 851−855. (2) Ellison, B. T.; Gallagher, C. T.; Frostman, L. M.; Lorimer, S. E. The Physical Chemistry of Wax, Hydrates, and Asphaltene. In Offshore Technology Conference, OTC 11963, Houston, TX, 2000. (3) Mehta, A. P.; Klomp, U. C. An Industry Perspective on the State of the Art of Hydrates Management. In Fifth International Conference on Gas Hydrates, Trondheim, Norway, 2005. (4) Sloan, E. D.; Koh, C. A. Clathrate Hydrates of Natural Gases; CRC Press LLc: Boca Raton, FL, 2008. (5) Davidson, D. W. Clathrate Hydrates. In Water in Crystalline Hydrates Aqueous Solutions of Simple Nonelectrolytes; Springer: New York, 1973; pp 115−234. (6) Englezos, P. Clathrate Hydrates. Ind. Eng. Chem. Res. 1993, 32, 1251−1274. (7) Sloan, E. D. Fundamental Principles and Applications of Natural Gas Hydrates. Nature 2003, 426, 353−363. (8) Kelland, M. A. History of the Development of Low Dosage Hydrate Inhibitors. Energy Fuels 2006, 20, 825−847. (9) Creek, J. L. Efficient Hydrate Plug Prevention. Energy Fuels 2012, 26, 4112−4116.

2928

dx.doi.org/10.1021/cg500218q | Cryst. Growth Des. 2014, 14, 2923−2930

Crystal Growth & Design

Article

(31) Ohno, H.; Moudrakovski, I.; Gordienko, R.; Ripmeester, J.; Walker, V. K. Structures of Hydrocarbon Hydrates during Formation with and without Inhibitors. J. Phys. Chem. A 2012, 116, 1337−1343. (32) 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. (33) Lederhos, J. P.; Sloan, E. D. Transferability of Kinetic Inhibitors between Laboratory and Pilot Plant. In SPE Annual Technical Conference and Exhibition, Denver, Colorado, SPE 36588; Society of Petroleum Engineers: Richardson, TX, 1996. (34) Notz, P. K.; Bumgardner, S. B.; Schaneman, B. D.; Todd, J. L. Application of Kinetic Inhibitors to Gas Hydrate Problems. Old Prod. Facil. 1996, 11, 256−260. (35) Du, N.; Liu, X. Y. Enhanced Antifreeze Effect of Antifreeze Protein on Ice Nucleation by Electrolyte. Cryst. Growth Des. 2008, 8, 3290−3294. (36) 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. (37) Ajiro, H.; Takemoto, Y.; Akashi, M.; Chua, P. C.; Kelland, M. A. Study of the Kinetic Hydrate Inhibitor Performance of a Series of Poly (N-Alkyl-N-Vinylacetamide) S. Energy Fuels 2010, 24, 6400−6410. (38) O’Reilly, R.; Ieong, N. S.; Chua, P. C.; Kelland, M. A. Missing Poly (N-Vinyl Lactam) Kinetic Hydrate Inhibitor: High-Pressure Kinetic Hydrate Inhibition of Structure II Gas Hydrates with Poly (NVinyl Piperidone) and Other Poly (N-Vinyl Lactam) Homopolymers. Energy Fuels 2011, 25, 4595−4599. (39) 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. (40) 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. (41) 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. (42) 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. (43) Dalmazzone, D.; Kharrat, M.; Lachet, V.; Fouconnier, B.; Clausse, D. DSC and PVT Measurements. J. Therm. Anal. Calorim. 2002, 70, 493−505. (44) 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. (45) Yang, J.; Tohidi, B. Characterization of Inhibition Mechanisms of Kinetic Hydrate Inhibitors Using Ultrasonic Test Technique. Chem. Eng. Sci. 2011, 66, 278−283. (46) Garnham, C. P.; Campbell, R. L.; Davies, P. L. Anchored Clathrate Waters Bind Antifreeze Proteins to Ice. Proc. Natl. Acad. Sci. U. S. A. 2011, 108, 7363−7367. (47) Wang, L.; Yoon, R.-H. Hydrophobic Forces in the Foam Films Stabilized by Sodium Dodecyl Sulfate: Effect of Electrolyte. Langmuir 2004, 20, 11457−11464. (48) Baardsnes, J.; Kuiper, M. J.; Davies, P. L. Antifreeze Protein Dimer When Two Ice-Binding Faces Are Better than one. J. Biol. Chem. 2003, 278, 38942−38947. (49) Kumar, R.; Lee, J. D.; Song, M.; Englezos, P. Kinetic Inhibitor Effects on Methane/propane Clathrate Hydrate-Crystal Growth at the Gas/water and Water/n-Heptane Interfaces. J. Cryst. Growth 2008, 310, 1154−1166. (50) Lee, J. D.; Englezos, P. Unusual Kinetic Inhibitor Effects on Gas Hydrate Formation. Chem. Eng. Sci. 2006, 61, 1368−1376. (51) Habetinova, E.; Lund, A.; Larsen, R. Hydrate Dissociation under the Influence of Low-Dosage Kinetic Inhibitors. In Proceedings of the 4th International Conference on Gas Hydrates, Yokohama, Japan, Domestic Organizing Committee: Japan, May, 2002. (52) Gulbrandsen, A. C.; Svartaas, T. M. Influence of Formation Temperature and Inhibitor Concentration on the Dissociation

(10) Lederhos, J. P.; Long, J. P.; Sum, A.; Christiansen, R. L.; Sloan, E. D., Jr Effective Kinetic Inhibitors for Natural Gas Hydrates. Chem. Eng. Sci. 1996, 51, 1221−1229. (11) Fu, B.; Neff, S.; Mathur, A.; Bakeev, K. Application of LowDosage Hydrate Inhibitors in Deepwater Operations. Old Prod. Facil. 2002, 17, 133−137. (12) Fu, S. B.; Cenegy, L. M.; Neff, C. S. A Summary of Successful Field Applications of a Kinetic Hydrate Inhibitor. In SPE International Symposium on Oilfield Chemistry, SPE 65022; Society of Petroleum Engineers: Richardson, TX, 2001. (13) Del Villano, L.; Kommedal, R.; Kelland, M. A. Class of Kinetic Hydrate Inhibitors with Good Biodegradability. Energy Fuels 2008, 22, 3143−3149. (14) Davies, P. L.; Baardsnes, J.; Kuiper, M. J.; Walker, V. K. Structure and Function of Antifreeze Proteins. Philos. Trans. R. Soc. London B. Biol. Sci. 2002, 357, 927−935. (15) Ewart, K. V.; Lin, Q.; Hew, C. L. Structure, Function and Evolution of Antifreeze Proteins. Cell. Mol. Life Sci. CMLS 1999, 55, 271−283. (16) Venketesh, S.; Dayananda, C. Properties, Potentials, and Prospects of Antifreeze Proteins. Crit. Rev. Biotechnol. 2008, 28, 57− 82. (17) Wathen, B.; Kuiper, M.; Walker, V.; Jia, Z. New Simulation Model of Multicomponent Crystal Growth and Inhibition. Chem. Eur. J. 2004, 10, 1598−1605. (18) Yeh, Y.; Feeney, R. E. Antifreeze Proteins: Structures and Mechanisms of Function. Chem. Rev. 1996, 96, 601−618. (19) 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. (20) 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. (21) 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. (22) Zeng, H.; Wilson, L. D.; Walker, V. K.; Ripmeester, J. A. The Inhibition of Tetrahydrofuran Clathrate-Hydrate Formation with Antifreeze Protein. Can. J. Phys. 2003, 81, 17−24. (23) Jensen, L.; Ramløv, H.; Thomsen, K.; von Solms, N. Inhibition of Methane Hydrate Formation by Ice-Structuring Proteins. Ind. Eng. Chem. Res. 2010, 49, 1486−1492. (24) 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. (25) 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. (26) Zeng, H.; Walker, V. K.; Ripmeester, J. A. Approaches to the Design of Better Low-Dosage Gas Hydrate Inhibitors. Angew. Chem. 2007, 119, 5498−5500. (27) 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. (28) 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. (29) 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. (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. 2929

dx.doi.org/10.1021/cg500218q | Cryst. Growth Des. 2014, 14, 2923−2930

Crystal Growth & Design

Article

Temperature for Hydrates Formed with Poly Vinyl Caprolactam. In Proceedings of the 6th International Conference on Gas Hydrates, Vancouver, Canada, 2008.

2930

dx.doi.org/10.1021/cg500218q | Cryst. Growth Des. 2014, 14, 2923−2930