Accelerated Hydrate Crystal Growth in the Presence of Low Dosage

Article pubs.acs.org/jced

Accelerated Hydrate Crystal Growth in the Presence of Low Dosage Additives Known as Kinetic Hydrate Inhibitors Hassan Sharifi and Peter Englezos* Department of Chemical and Biological Engineering, University of British Columbia, 2360 East Mall, Vancouver, British Columbia V6T 1Z3, Canada ABSTRACT: Kinetic hydrate inhibitors (KHIs) or low dosage hydrate inhibitors (LDHIs) are known as additives employed to delay the onset of gas hydrate nucleation time in hydrocarbon pipelines. It has been observed, however, that in laboratory experiments accelerated hydrate growth called catastrophic growth can occur. This may be a serious problem if it occurs in a field application of kinetic inhibitors. The mechanism of such accelerated hydrate growth in the presence of KHIs is still not understood. A highpressure microdifferential scanning calorimeter was employed to study the accelerated hydrate growth in the presence of chemical and biological inhibitors. It is hypothesized that capillary action facilitates the transport of water molecules across the formed hydrate layer from the bulk of the liquid water phase to the gas−liquid interface. This in turn might be the governing mechanism for catastrophic hydrate growth in the presence of KHIs. In addition, the hydrate catastrophic index is introduced in this work as a parameter to quantify the phenomenon based on the laboratory data and the type of experiment conducted. The HCI may then serve as a measure of the pipeline hydrate plugging potential.

■

INTRODUCTION The formation of solid crystals by hydrocarbon molecules and water (known as gas or clathrate hydrates) remains a serious flow assurance problem in hydrocarbon pipelines and related processing facilities since Hammershcmidt first reported on the issue.1−4 The oil and gas industry has been dealing with the issue of unwanted hydrate formation in pipelines through the injection of inhibiting chemicals known as thermodynamic hydrate inhibitors (THIs). Motivated by environmental restrictions and economic incentives (high consumption rate of THIs especially in far remote and deep resources), kinetic hydrate inhibitors (KHIs) or low dosage hydrate inhibitors (LDHIs) have also been employed in recent years.5−13 Chemical (polyvinylpyrrolidone, PVP; and polyvinylcaprolactam, PVCap) and biological (antifreeze proteins known as AFPs) additives have been employed as kinetic hydrate inhibitors. These additives have been studied in the lab by various laboratory techniques. KHIs are able to postpone the onset of gas hydrate crystallization and to control the growth of postnucleation crystals. The adsorption of the inhibitors on gas hydrate crystals has been proposed as a plausible mechanism to explain the performance of KHIs.14,15 Recently, it has been reported that even though the addition of kinetic hydrate inhibitors (both chemical and biological ones) reduced the growth rate of gas hydrate crystals up to a certain point this was followed by an acceleration in gas hydrate growth that has been called catastrophic hydrate growth.16−20 More specifically in the experiments conducted in the stirred vessel type crystallizers, KHIs were found to reduce the gas hydrate formation rate in the aqueous liquid phase. However, once hydrate crystals started to form in the bulk gas phase, catastrophic hydrate growth was detected.16,17 It is noteworthy © XXXX American Chemical Society

that such unusual kinetic inhibitor effects on gas hydrate formation leading to accelerated hydrate growth called catastrophic hydrate crystal growth was first observed and reported by Lee and Englezos and Kumar et al.21,22 The reason for such accelerated hydrate formation is still not clear. However, it has been proposed that the hydrate crystals formed in the presence of KHIs might have a morphology that facilitates capillary movement of water molecules to the gas/ liquid interface.21,23 Since accelerated hydrate growth is not desirable in a field application of kinetic hydrate inhibitors, it is crucial to understand the governing reasons for the occurrence of this phenomenon. The accelerated hydrate crystal growth has so far been described qualitatively. While this is inevitable for the experiments examining the macroscopic morphology of hydrates, a quantitative operational definition will be proposed in this work. Specifically the hydrate catastrophic index (HCI) will be defined for different experimental settings that showed such hydrate growth. The introduction of the HCI is intended to serve as a measure of the pipeline hydrate plugging potential (PHPP). Previously reported data on the viscosity of the hydrate slurry will be employed. This choice is based on the tacit assumption that the hydrate slurry viscosity is the best variable to represent the tendency of the hydrate mass to plug the pipeline (plugging potential). It has been shown that Special Issue: In Honor of E. Dendy Sloan on the Occasion of His 70th Birthday Received: June 30, 2014 Accepted: October 9, 2014

A

dx.doi.org/10.1021/je500591q | J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Journal of Chemical & Engineering Data

Article

Table 1. List of the Chemicals Used in This Work

a

chemical

source

initial purity

molecular weight

final concentration

PVCapa PVPb AFP IIIc NaCl gas mixture

BASF Acros Organics A/F Protein Canada Fisher Scientific Praxair Technology Inc.

mass fraction of 0.4 in ethanol mole fraction of 1.0

23.3 kDa 3.5 kDa 7 kDa 58.4 g·mol−1 17.2

0.1 mmol·kg−1 in saline solution

mole fraction: CH4, 0.93 C2H6, 0.05 C3H8, 0.02

0.6 mol·kg−1

Poly(vinylcaprolactam). bPoly(vinylpyrrolidone). cType III antifreeze protein.

hydrate was formed from aqueous solutions, while in the second one, ice formed prior to the formation of gas hydrate. On the basis of the preliminary experiments (not shown), it was found that ice would form once the sample temperature was lowered below 250 K while cooling. Hence, the temperature of 258 K was chosen as the minimum temperature to avoid ice formation during the experiments. Experiments without Ice Formation. To form a hydrate at 258 K the system was pressurized to 7.0 MPa with the gas mixture to provide enough driving force (over pressure or subcooling) for hydrate formation. Once the pressure in the sample and reference cell reached 7.0 MPa and was stabilized, an isothermal program was started. Figure 1 depicts the heat

calorimetry could be applied to study the mass transfer resistance across formed hydrate films.24 Therefore, a highpressure microdifferential scanning calorimeter was also used to observe the hydrate formation in the presence of three kinetic hydrate inhibitors (PVP, PVCap, and AFP type III) to assess the HCI through the energetic effects.

■

EXPERIMENTAL SECTION Materials. Saline solutions with the molality of 0.6 mol·kg−1 were prepared by dissolving NaCl in distilled and deionized water. Two chemical and one biological KHIs were used: poly(vinylpyrrolidone) (PVP, average molecular mass of 3.5 kDa; Acros Organics), a solution (with the mass fraction of 0.40 in ethanol) of poly(vinylcaprolactam) (PVCap, average molecular mass of ∼23.3 kDa; BASF) 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 mmol·kg−1 in the saline solutions. A natural gas mixture (UHP grade) consisting of methane/ethane/ propane with the mole fractions of 0.93/0.05/0.02 supplied by Praxair Technology, Inc. was used as a gas hydrate former. Table 1 shows the list of the chemicals used in this work. High-Pressure Microdifferential Scanning Calorimetry (HP-μDSC). A HP-μDSC (μDSC 7 Evo; Setaram, Inc.) was used to conduct gas hydrate formation and dissociation experiments by measuring the released and required heat flow, respectively. The uncertainty of the measured heat flow is 0.02 μW. The high-pressure calorimeter and its use in hydrate formation and dissociation is described in detail elsewhere.16 Briefly, the calorimeter uses double-stage temperature control with Peltier coolers allowing operation between 228 K and 393 K with a programmable temperature scanning rate (heating and cooling) of 0.001 K·min−1 to 2 K·min−1. The DSC has two high-pressure cells (up to 40 MPa) with a volume of 1 mL called sample and reference cells. A customized stainless steel sample holder was used to hold the experimental solutions. The sample holder has a base with four pits (diameter of 1.5 mm, depth of 2.6 mm), with support from a rod (diameter of 1.6 mm, length of 7 mm).16 Gas Hydrate Formation and Dissociation in the Calorimeter. Samples (4 μL of the experimental solution) were injected into the allocated pits (1 μL in each depression) drilled on the sample holder using a microsyringe, and the holder was placed in the high-pressure sample cell. Both sample and reference cells were pressurized and depressurized with the experimental gas mixture three times to remove air from the system. To determine the impact of water permeation caused by the capillary actions across the formed gas hydrate layer on the performance of inhibitors, two different methods were employed to form gas hydrate. In the first procedure, gas

Figure 1. Typical temperature (red dashed line) and heat flow (blue continuous line) trends during hydrate formation (Pexp = 7.0 MPa) from a saline solution and subsequent dissociation of the formed hydrate. Hydrate nucleation is shown by a circle around the exothermic peaks and hydrate dissociation is indicated by an arrow pointing to the endothermic peak. Ice did not form during these experiments.

flow and temperature profiles during an experiment. Using this experimental protocol, after 1 h stabilization at 303 K, the temperature was decreased from 303 K to 258 K at a rate of 1 K·min−1, and then was kept constant at 258 K for a period of time. The list of the 32 hydrate formation experiments at 7.0 MPa without ice formation and relevant data is shown in Table 2. For each solution (KHI-free, PVP, PVCap, and AFP III) two experiments were conducted that lasted for 5 h, another two for 10 h, and a third set of two experiments lasted for 20 h. After the period of 5 or 10 or 20 h, the formed gas hydrate was decomposed by increasing the temperature to 303 K at a rate of 0.25 K·min−1 (Figure 1). The heat released during the dissociation process was used as an indicator of the total amount of hydrate formed. Finally, an experiment was B



Article

Experiments with Ice Formation. The temperature was reduced to 243 K in order to ensure a temperature well below the ice nucleation point (250 K). The pressure was selected to be 5.0 MPa. This pressure was chosen so that hydrate forms after ice formation as seen in Figure 2. The system was

Table 2. List of Hydrate Formation Experiments at 7.0 MPa without Ice Formation and Relevant Data Including Isothermal Period (t), Released/Absorbed Heat for Hydrate Formation/Dissociation (H) and Calculated Hydrate Catastrophic Index (HCI) for Each Experiment H/mJ expt 1A 1B 2A 2B 3A 3B 4A 4B 5A 5B 6A 6B 7A 7B 8A 8B 9A 9B 10A 10B 11A 11B 12A 12B 13A 13B 14A 14B 15A 15B 16A 16B

t/h 2.5 5 10 20

2.5 5 10 20

2.5 5 10 20

2.5 5 10 20

average KHI-free Solution 23.4a 23.1a 58.5b 60.2b 68.8b 71.2b 86.8b 88.9b PVP Solution 19.9a 20.4a 82.5b 86.6b 89.2b 93.2b 104.7b 110.6b PVCap Solution 20.3a 19.1a 66.5b 69.1b 97.5b 102.2b 105.8b 110.0b AFP III Solution 22.7a 21.0a 58.8b 60.8b 74.2b 77.0b 95.3b 99.1b

HCI

23.3 59.4

1.0

70.0

1.0

87.8

1.0

20.2 84.5

1.6

91.2

1.5

107.6

1.4

Figure 2. Typical temperature (red dashed line) and heat flow (blue continuous line) trends in the second experimental method (Pexp = 5.0 MPa) to form gas hydrate from ice and subsequently dissociate the formed hydrate. Hydrate and ice nucleation are shown by a horizontal and vertical circle, respectively. Arrows show the eutectic temperature, ice melting, and hydrate dissociation temperature from left to right, respectively.

pressurized to 5.0 MPa with the gas mixture. The list of hydrate formation experiments at 5.0 MPa with ice formation and relevant data are seen in Table 3. It is noted that once the pressure of the sample and reference cells was stabilized at the desired value, the isothermal program was started. Figure 2 shows the typical temperature and heat flow trends. Using this experimental protocol, after 1 h stabilization at 303 K, the temperature was decreased from 303 K to 243 K at a rate of 1 K·min−1, and then kept constant at 243 K for 5 h, 10 h, and 20 h in three individual experiments similar to the experiments without ice formation. Subsequently, the temperature was increased to 303 K at a rate of 0.25 K·min−1 in order to dissociate the formed hydrate (Figure 2). Hydrate dissociation is indicated by an endothermic peak located at ∼286 K. It is noted that the hydrate equilibrium temperature at 5.0 MPa as calculated by CSMGem2 is 286 K. The endothermic peak in Figure 2 that was detected at ∼252 K represents the eutectic temperature for the NaCl solution. Again, a blank experiment was also performed by using a saline solution (no inhibitors present) at atmospheric pressure. In the blank experiment, ice melting was observed as an endothermic peak at ∼271 K. Finally, the amount of required heat to dissociate the formed gas hydrate was calculated and represents the amount of formed gas hydrate crystals. The Hydrate Catastrophic Index (HCI). The hydrate catastrophic index (HCI) is introduced as a parameter to quantify the observed accelerated or enhanced gas hydrate crystal growth in the presence of KHIs. Since high-pressure stirred crystallizers, a high-pressure microdifferential scanning calorimeter, and a high-pressure rheometer were employed previously to evaluate the performance of KHIs,16−18,25 three individual indexes are defined for each set of experimental setups. Table 4 shows the three indexes. If the HCI has a value that is more than 1.0 then the hydrate growth would be labeled as catastrophic growth. The deviation of the value of the HCI

19.7 67.8

1.3

99.8

1.7

107.9

1.5

21.8 59.8

1.1

75.6

1.2

97.2

1.2

a

Refers to heat calculated from the exothermic peaks in Figure 3 during the first 2.5 h. bRefers to heat calculated from the endothermic peaks corresponding to hydrate dissociation at 5 h, 10 h, and 20 h. Combined expanded uncertainty Uc is Uc(H) = 0.05 mJ.

performed by following the same protocol and using a saline solution (no inhibitors present) and at atmospheric pressure (blank experiment). There was no exothermic peak observed during the cooling. In addition, no endothermic peak was detected at ∼271 K that would represent ice melting for the 0.6 mol·kg−1 of NaCl solution. This experiment serves as evidence that ice did not form and thus, the observed exothermic peaks during the hydrate formation represent hydrate phase transitions. Also, hydrate dissociation was detected by endothermic peaks located at ∼289 K (Figure 1). It is noted that the equilibrium dissociation temperature at 7.0 MPa is calculated to be 289 K by using CSMGem.2 It is also noted that these endothermic peaks did not appear in the blank experiment as expected. C



Article

Table 3. List of Hydrate Formation Experiments at 5.0 MPa with Ice Formation and Relevant Data Including Isothermal Period (t), Released/Adsorbed Heat for Hydrate Formation/Dissociation (H), Calculated Hydrate Catastrophic Index (HCI), Required Heat for Ice Melting (Hice), Amount of Formed Ice (Mi) and Remaining Water (Mw) for Each Experiment H/mJ expt

t/h

1A 1B 2A 2B 3A 3B 4A 4B

2.5

5A 5B 6A 6B 7A 7B 8A 8B

HCI

Hicec/mJ

Mi/mg

Mwd/μL

70.4

1.00

87.7

1.00

98.9

1.00

974.2 978.4 991.8 966.4 989.2 992.7

2.9 2.9 3.0 2.9 3.0 3.0

1.1 1.1 1.0 1.1 1.0 1.0

1060.3 955.1 1037.6 950.0 978.4 965.7

3.2 2.9 3.1 2.8 2.9 2.9

0.8 1.1 0.9 1.2 1.1 1.1

980.9 940.8 961.7 946.1 949.2 954.0

2.9 2.8 2.9 2.8 2.8 2.9

1.1 1.2 1.1 1.2 1.2 1.1

1077.9 994.1 1047.6 990.3 1009.5 996.2

3.2 3.0 3.1 3.0 3.0 3.0

0.8 1.0 0.9 1.0 1.0 1.0

average 61.0a 63.2a 67.0b 73.7b 84.5b 90.8b 96.1b 101.7b

5 10 20

55.3a 56.2a 62.6b 65.7b 84.6b 85.8b 96.1b 97.2b

2.5 5 10 20

KHI-free Solution 62.1

PVP Solution 55.8 64.2

1.02

85.2

1.08

96.7

1.09

PVCap Solution 9A 9B 10A 10B 11A 11B 12A 12B 13A 13B 14A 14B 15A 15B 16A 16B

2.5

not observed

5 10 20

2.5 5 10 20

46.7b 48.3b 60.8b 63.8b 66.5b 69.8b 51.1a 52.3a 57.4b 59.4b 73.1b 81.3b 88.8b 82.2b

47.5 62.3 68.2 AFP III Solution 51.7 58.4

0.99

77.2

1.06

85.5

1.04

Refers to heat calculated from the exothermic peaks in Figure 4 during the first 2.5 h. bRefers to heat calculated from the endothermic peaks corresponding to hydrate dissociation at 5 h, 10 h, and 20 h. cHeat of fusion of ice: 334 mJ·mg−1. Combined expanded uncertainties Uc are Uc(H) = 0.05 mJ, Uc(Hice) = 0.1 mJ, Uc(Mi) = 0.2 μg. dWater density was assumed 1 mg·μL−1. a

The experiments were continued for 5 h, 10 h, and 20 h. Although no exothermic peaks were observed after 2.5 h in all the experiments, this does not mean that hydrate formation stopped after the initial formation. The experiments were run for 5 h, 10 h, and 20 h, and then the hydrate was dissociated. The amount of heat required for hydrate dissociation at 5, 10, and 20 h for the experiments without ice formation was determined from the corresponding endothermic peaks, and the results are shown in Table 2. The heat data indicate that hydrate growth occurs during the entire period of observation. The data suggest that the first 2.5 h of isothermal operation corresponded to the nucleation and most likely some hydrate growth. During this period the amount of formed hydrate in the presence of inhibitors is less compared to that in the KHI-free solution. Alternately, the heat data at 5 h, 10 h, and 20 h suggest that more hydrate formed in the inhibited systems. Therefore, although the addition of KHIs

above 1 is considered as an indicator of the degree to which the hydrate growth is accelerated or enhanced.

■

RESULTS AND DISCUSSION

Gas Hydrate Formation without Ice Formation. Figure 3 shows the heat flow trends during gas hydrate formation in a typical experiment (with and without inhibitors) in the first 150 min. It is noted that after 150 min exothermic peaks were not detected. The area under the peaks is used to calculate the amount of heat released during the phase change. The amount of heat is used as an indicator of the amount of the hydrate formed during a period of 2.5 h (150 min) of isothermal operation. Table 2 shows the amounts of heat for the first 2.5 h. Four peaks were observed for each solution because there are four droplets in the calorimeter (see Experimental Section). However, the AFP III peaks are not as distinct. D



Article

experiments performed in a stirred vessel using chemical and biological inhibitors.16−19 In the calorimeter experiments, it is expected that a hydrate layer forms on the surface of the aqueous droplets and creates a mass transfer barrier to gas diffusion. Our experiments cannot confirm whether water permeation or gas diffusion is governing the gas hydrate growth in the presence of the hydrate layer between the hydrate forming gas and the liquid water phase. The water permeation model26,27 invokes capillary action governing the transport water molecules in the formed hydrate layer. Recently, this model has been used to explain that hydrate growth is controlled by the movement of water within the hydrate film.28 It has also been reported that polycrystalline gas hydrate films are generally porous23 and it is plausible that the addition of kinetic inhibitors likely increases the porosity and or permeability of the formed gas hydrate. If the water permeation model is true, it means that the longer the experiment lasts, the more amounts of hydrate will form through the continuous contact of water with the hydrate forming gas at the hydrate/gas interface. The HCI for the calorimeter experiments without ice formation are also shown in Table 2. Clearly, the addition of kinetic inhibitors accelerated hydrate growth after a certain point as the calculated HCIs were higher than 1.0. Gas hydrate formation data obtained in a high-pressure vessel and a highpressure rheometer that were reported elsewhere,16−18,25 were also used to calculate the corresponding HCI. Table 5 shows the values of these calculated HCIs which also indicate accelerated hydrate growth in the presence of inhibitors. Formation of Gas Hydrate with Ice Formation. As discussed in the Experimental Section the experiments were conducted at 5.0 MPa and 243 K. Ice formed prior to the formation of hydrate crystals as seen in Figure 2. The formation of ice is expected to surround the water droplet and thus create a barrier between water and gas molecules. To determine if the entire water droplet was converted to ice, the heat of fusion of ice is employed along with the information from the endothermic peaks corresponding to ice melting (Figure 2). Table 3 shows the amount of required heat to melt the formed ice in each experiment. For example, in experiment 2A, 974.2 mJ was required for ice melting. The amount of heat associated with hydrate formation and dissociation is also shown in Table 3. For example, in experiment 1A, 61 mJ was released during hydrate formation in the first 2.5 h of isothermal operation. On the other hand, the heat required for dissociation after 5 h, 10 h, and 20 h was determined from the corresponding

Table 4. Defined Hydrate Catastrophic Indexes (HCI) for Different Experimental Setups experimental apparatus

hydrate catastrophic index (HCI)

high-pressure stirred crystallizer (HP-SC)

(Rch−KHI)/Ri

high-pressure rheometer (HP-R)

(ήch−KHI)/ήi

high-pressure microdifferential scanning calorimeter (HP-μDSC)

(HtKHI/ HiKHI)/ (Ht/Hi)

parameter definition Rch−KHI/mol·min−1: hydrate growth rate in the presence of KHI at the on-set of catastrophic growth. Ri/mol·min−1: hydrate growth rate in the absence of KHI. ήch−KHI/Pa·s·s−1: rate of increase in viscosity in the presence of KHIs at the on-set of catastrophic growth. ήi/Pa·s·s−1: rate of increase in viscosity in the absence of KHI. HtKHI/mJ: total absorbed heat in the presence of KHI HiKHI/mJ: released heat in the presence of KHI in the nucleation part Ht/mJ: total absorbed heat in the absence of KHI Hi/mJ: released heat in the absence of KHI in the nucleation part

Figure 3. Typical heat flow trends during gas hydrate nucleation in different saline solutions including KHI-free (black continuous line), PVP (blue dotted line), PVCap (red dashed line), and AFP III (blue continuous line) at 7.0 MPa and 258 K. Ice did not form during these experiments.

may reduce the amount of hydrate formed initially, more hydrate would form over a longer period of time. Such behavior was previously observed and reported in morphology21,22 experiments conducted in the presence of PVP and in the

Table 5. Calculated Hydrate Catastrophic Index (HCI) Based on the Rate of Increase in the Slurry Viscosity (ή) and the Gas Hydrate Growth Rate (R) for the Experiments Conducted in the High-Pressure Rheometer and the High-Pressure Stirred Crystallizers, Respectively experimental method high-pressure rheometer

18

high-pressure stirred crystallizer16,17

ή/mPa·s·s−1

solutiona

−2

KHI-free PVP PVCap KHI-free PVP PVCap AFP I AFP III

1.2 (1·10 ) 4.3 (8·10−2) 8.1 (2·10−2) N/A

R/mmol·min−1

HCI

N/A

1.00 3.58 6.75 1.00 1.67 1.69 1.65 1.73

0.214 0.358 0.362 0.354 0.370

(3·10−4) (2·10−4) (1·10−4) (3·10−4) (6·10−4)

a

In the presence of liquid hydrocarbon, no catastrophic growth was observed16,25 and therefore HCI might be considered as 1. Combined expanded uncertainties Uc are reported in the parentheses. E



Article

HCI was calculated for this inhibitor. It is reasonable to assume that the liquid water trapped within the layer of formed ice could not migrate to the interface of hydrate-ice by capillary action through the ice layer. Therefore, the main mechanism to continue gas hydrate growth in the presence of ice might be the gas diffusion across the formed hydrate layer and also the formation of hydrate from ice.

endothermic peaks. For example, in experiment 2A, 67 mJ was required for the hydrate dissociation. Since 334 mJ is required to melt 1 mg of ice, the amount of formed ice in each experiment can be calculated and the results are presented in Table 3. As 4 μL of water was used in total, it is concluded that almost 75% of the initial water was converted to ice. Under these circumstances, gas molecules must diffuse across the formed ice layer to reach the water molecules in the core to form hydrate. Gas molecules may also form hydrate with the ice. However, water molecules are not likely to migrate within the ice layer. Finally, it is not clear what the fate of the KHI molecules is during freezing of the water molecules. Figure 4 illustrates the heat flow during the gas hydrate formation in a typical experiment with and without inhibitors in

■

CONCLUSIONS Kinetic hydrate inhibitors are injected to subsea oil and gas transmission pipelines to delay gas hydrate formation. This practice may involve a serious risk because in the presence of these additives accelerated gas hydrate growth may occur. Such growth could have catastrophic consequences in the operation (blockage). Although this phenomenon has been identified in laboratory settings, the likelihood of such occurrence in a field application must be considered to avoid any unwanted safety and economic consequences. The hydrate catastrophic index (HCI) was introduced in this work as a parameter to quantify the phenomenon. The HCI is defined based on the laboratory data on the viscosity of the hydrate slurry (rheometry), gas uptake data, or the energetics of the hydrate crystallization process (calorimetry). It is expected that the HCI may serve as a measure of the pipeline hydrate plugging potential (PHPP). In addition, this work presents experimental data in which it is shown that the addition of PVP, PVCap, and AFP III as kinetic gas hydrate inhibitor to aqueous saline solutions accelerates the growth of hydrate crystals. Remarkably, such hydrate growth was not detected when gas hydrates stated to be formed from ice instead of aqueous droplets. It was suggested that water permeation through the hydrate might be considered as the probable reason for the growth of gas hydrates after the initial formation. In addition, kinetic inhibitors may increase hydrate porosity which then facilitates the permeation of the water molecules through the hydrate layer from the aqueous liquid phase to the gas−liquid interface due to the capillary forces. The above observations suggest that extreme caution should be exercised to ensure the safe application of kinetic inhibitors in the field.

Figure 4. Typical heat flow trends during gas hydrate nucleation in different saline solutions including KHI-free (black continuous line), PVP (dotted blue line), PVCap (red dashed line), and AFP III (blue continuous line) at 5.0 MPa and 243 K. Ice was formed during these experiments.

the first 360 min (6 h) to show all the observed exothermic peaks corresponding to hydrate nucleation/formation. Four exothermic peaks were detected in KHI-free experiments as four droplets have been placed in the sample holder. However, in the presence of PVP and AFP III only two peaks were detected, which might be related to the nucleation of the two droplets. Interestingly, there is no peak associated with the presence of PVCap. This might be considered as an indicator of no hydrate formation in the system. However, endothermic peaks were observed during the dissociation of the formed hydrate. The experiments with ice were continued for 10 h and 20 h. Although no exothermic peaks were observed after 6 h in all the experiments, this does not mean that hydrate formation stopped after the initial formation. The amount of heat required for hydrate dissociation at 5 h, 10 h, and 20 h for the experiments with ice formation (experiments 2A to 8B) were determined from the corresponding endothermic peaks, and the results are shown in Table 3. On the basis of the heat data, it is seen that in all experiments containing kinetic inhibitors (especially PVCap and AFP III) the amount of formed hydrate was found to be less than the amount formed in the KHI-free experiments. According to the data presented in Table 3, the calculated HCIs in the presence of KHIs with ice are close to 1.0, which shows the lack of hydrate acceleration growth. It must be noted that the total released heat during hydrate nucleation (Figure 2) is required to calculate the HCIs. As no nucleation peaks were observed in the presence of PVCap, no

■

AUTHOR INFORMATION

Corresponding Author

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

The financial support from the Natural Sciences and Engineering Research Council of Canada (NSERC) is greatly appreciated. Hassan Sharifi would like to thank UBC for a Four Year Fellowship. Notes

The authors declare no competing financial interest.

■

REFERENCES

(1) Hammerschmidt, E. G. Formation of Gas Hydrates in Natural Gas Transmission Lines. Ind. Eng. Chem. 1934, 26, 851−855. (2) Sloan, E. D.; Koh, C. A. Clathrate Hydrates of Natural Gases; CRC Press, LLC: Boca Raton, FL, 2008. (3) Englezos, P. Clathrate Hydrates. Ind. Eng. Chem. Res. 1993, 32, 1251−1274. (4) Sloan, E. D. Fundamental Principles and Applications of Natural Gas Hydrates. Nature 2003, 426, 353−363.

F



Article

(5) Bloys, B.; Lacey, C. LaboratoryTesting and Field Trial of a New Kinetic Hydrate Inhibitor. In Offshore Technology Conference; Offshore Technology Conference: Houston TX, May 1, 1995. (6) Christiansen, R. L.; Sloan, E. D. Mechanisms and Kinetics of Hydrate Formation. Ann. N.Y. Acad. Sci. 1994, 715, 283−305. (7) Long, J.; Lederhose, J.; Sum, A.; Christiansen, R.; Sloan, E. D. Kinetic Inhibitors of Natural Gas Hydrates. In Proceedings of the Annual Convention-Gas Processors Association; Gas Processors Association: Tulsa, OK, 1994; pp 85−85. (8) 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. (9) Englezos, P. Nucleation and Growth of Gas Hydrate Crystals in Relation to Kinetic Inhibition. Oil Gas Sci. Technol. 1996, 51, 789− 795. (10) Sloan, E. D. A Changing Hydrate ParadigmFrom Apprehension to Avoidance to Risk Management. Fluid Phase Equilib. 2005, 228, 67−74. (11) Kelland, M. A. History of the Development of Low Dosage Hydrate Inhibitors. Energy Fuels 2006, 20, 825−847. (12) Fu, B.; Neff, S.; Mathur, A.; Bakeev, K. Application of LowDosage Hydrate Inhibitors in Deepwater Operations. Old Prod. Facil. 2002, 17, 133−137. (13) Fu, B. The Development of Advanced Kinetic Hydrate Inhibitors. Spec. Publ.-R. Soc. Chem. 2002, 280, 264−276. (14) Anderson, B. J.; Tester, J. W.; Borghi, G. P.; Trout, B. L. Properties of Inhibitors of Methane Hydrate Formation via Molecular Dynamics Simulations. J. Am. Chem. Soc. 2005, 127, 17852−17862. (15) 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. (16) 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. (17) 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, 14, 2923−2930. (18) Sharifi, H.; Hatzikiriakos, S. G.; Englezos, P. Rheological Evaluation of Kinetic Hydrate Inhibitors in NaCl/n-Heptane Solutions. AIChE J. 2014, 60, 2654−2659. (19) Yang, J.; Tohidi, B. Characterization of Inhibition Mechanisms of Kinetic Hydrate Inhibitors Using Ultrasonic Test Technique. Chem. Eng. Sci. 2011, 66, 278−283. (20) Cha, M.; Shin, K.; Seo, Y.; Shin, J.-Y.; Kang, S.-P. Catastrophic Growth of Gas Hydrates in the Presence of Kinetic Hydrate Inhibitors. J. Phys. Chem. A 2013, 117, 13988−13995. (21) Lee, J. D.; Englezos, P. Unusual Kinetic Inhibitor Effects on Gas Hydrate Formation. Chem. Eng. Sci. 2006, 61, 1368−1376. (22) 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. (23) Austvik, T.; Li, X.; Gjertsen, L. H. Hydrate Plug Properties: Formation and Removal of Plugs. Ann. N.Y. Acad. Sci. 2000, 912, 294− 303. (24) Davies, S. R.; Lachance, J. W.; Sloan, E. D.; Koh, C. A. HighPressure Differential Scanning Calorimetry Measurements of the Mass Transfer Resistance across a Methane Hydrate Film as a Function of Time and Subcooling. Ind. Eng. Chem. Res. 2010, 49, 12319−12326. (25) Sharifi, H.; Walker, V. K.; Ripmeester, J. A.; Englezos, P. Inhibition Activity of Antifreeze Proteins with Natural Gas Hydrates in Saline and the Light Crude Oil Mimic, Heptane. Energy Fuels 2014, 28, 3712−3717. (26) Mori, Y. H.; Mochizuki, T. Mass Transport across Clathrate Hydrate FilmsA Capillary Permeation Model. Chem. Eng. Sci. 1997, 52, 3613−3616.

(27) Mori, Y.; Mochizuki, T. Modeling of Mass Transport across a Hydrate Layer Intervening between Liquid Water and “Guest” Fluid Phases. 2nd Int. Symp. Gas Hydrates 1996, 267−274. (28) Davies, S. R.; Sloan, E. D.; Sum, A. K.; Koh, C. A. In Situ Studies of the Mass Transfer Mechanism across a Methane Hydrate Film Using High-Resolution Confocal Raman Spectroscopy. J. Phys. Chem. C 2010, 114, 1173−1180.

G


Accelerated Hydrate Crystal Growth in the Presence of Low Dosage

Recommend Documents