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Effect of Kinetic Hydrate Inhibitor Polyvinylcaprolactam on Cyclopentane Hydrate Cohesion Forces and Growth Reuben Wu,†,‡ Zachary M. Aman,† Eric F. May,† Karen A. Kozielski,§ Patrick G. Hartley,‡ Nobuo Maeda,‡ and Amadeu K. Sum*,∥ †

Centre for Energy, School of Mechanical and Chemical Engineering, The University of Western Australia, M050, Crawley, Western Australia 6009, Australia ‡ Commonwealth Scientific and Industrial Research Organisation (CSIRO) Materials Science and Engineering, and §Commonwealth Scientific and Industrial Research Organisation (CSIRO) Earth Science and Resource Engineering, Ian Wark Laboratory, Bayview Avenue, Clayton, Victoria 3168, Australia ∥ Center for Hydrate Research, Chemical and Biological Engineering Department, Colorado School of Mines, Golden, Colorado 80401, United States ABSTRACT: The effect of Polyvinylcaprolactam (PVCap), a commonly used kinetic hydrate inhibitor (KHI), on the cohesion force between cyclopentane hydrate particles was measured using a micromechanical force apparatus. The presence of PVCap in the aqueous bulk phase reduced the average hydrate cohesive force by 54% (from 1.49 to 0.69 mN/m). However, the cohesion forces did not vary significantly as a function of either the PVCap concentration (0.005−0.5 wt %) or the temperature (from 1.1 to 7.2 °C). When a layer of PVCap solution was applied to the surface of a pure cyclopentane hydrate particle in a bulk liquid cyclopentane phase, the interparticle cohesive force was reduced by 45% (from 4.3 to 2.4 mN/m). Hydrate growth on droplets of PVCap solutions was also studied by contacting a water droplet with a cyclopentane hydrate particle in a bulk cyclopentane phase. In cases where PVCap was absent, complete conversion of the water droplet to hydrate occurred within 30 s. However, when a water droplet of PVCap solution was brought into contact with a hydrate particle, hydrate film growth was significantly slowed, requiring over 2 h for complete conversion.

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INTRODUCTION Gas hydrates are non-stoichiometric crystalline compounds formed when certain gas molecules (e.g., methane, ethane, and carbon dioxide) are trapped in cages consisting of a hydrogenbonded network of water molecules.1 Gas hydrates typically form under high-pressure and low-temperature conditions, such as those commonly encountered in deep-sea oil and gas pipelines. Hydrate formation in flowlines can cause plugging, leading to major safety and economic concerns. Conventional methods of preventing hydrate formation involve the use of thermodynamic hydrate inhibitors (THIs), such as methanol or monoethylene glycol, which work by shifting the hydrate equilibrium condition to higher pressures and lower temperatures. However, THIs are used in high concentrations, as much as 70−80 vol % relative to the amount of water,2 and can be a significant operating cost, especially as oil and gas exploration moves to increasingly deeper water. More recently, a class of chemicals termed kinetic hydrate inhibitors (KHIs) has been demonstrated to delay the onset of hydrate growth at low dosages (about 1 wt %).3 The mechanism by which KHIs work is still unclear, but one common hypothesis is that the lactam rings on the KHI molecules adsorb to the hydrate crystal and sterically hinder further hydrate growth.4 Turner et al.5 (in collaboration with Abrahamson) proposed a four-step conceptual mechanism for hydrate plug formation in oil-dominated pipelines consisting of (1) entrainment of water droplets in the oil phase, (2) growth of the hydrate shell on the water droplets at the water−oil interface, (3) agglomeration/ deposition of hydrate particles, and (4) complete plugging of © 2014 American Chemical Society

the pipeline. One of the key phenomena in the process leading to plugging is the agglomeration/deposition of the hydrate particles. Using a micromechanical force (MMF) apparatus, direct measurements of cohesion/adhesion forces between hydrate particles and surfaces can be performed. Taylor et al.6 performed a scoping study on the effects of contact time, applied load, temperature, and anti-agglomerants on the cohesion forces between tetrahydrofuran (THF) hydrate particles. Because the solubility of THF in decane led to various issues (such as causing the initially stoichiometric droplet to become non-stoichiometric, because THF is soluble in decane and THF in the THF/H2O droplet can be depleted with time), subsequent studies were then performed using cyclopentane to form hydrates. Dieker et al.7 studied the effect of crude oil and its components on the cohesion forces between cyclopentane hydrate particles. Nicholas et al.8 measured the adhesion force between cyclopentane hydrate and a carbon steel surface. Aspenes et al.9 extended this study to include different surface materials and the presence of water and also petroleum acids in the oil phase. Aman et al.10 then studied the effect of sorbitan monooleate (an anti-agglomerant), polypropylene glycol, and naphthenic acid on cyclopentane hydrate cohesion forces using a model oil system. The experimental scope of cyclopentane hydrate cohesive measurements was then extended to include the temperature, contact time, and applied Received: January 27, 2014 Revised: April 23, 2014 Published: April 28, 2014 3632

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load effects.11 Aman et al.11 showed that the cohesive force dependence upon the temperature, contact time, and applied load could be attributed to different cohesive mechanisms present depending upon the bulk solution. To investigate the cohesive mechanisms present in our system and the effect of the addition of polyvinylcaprolactam (PVCap), we also performed measurements over a range of temperatures. Although KHIs are primarily used to inhibit hydrate growth, it is also important to know their effect on the interaction of hydrate particles in the event of hydrate formation. In this work, we first investigate the effect of PVCap, a commonly used KHI, on the cohesion forces between cyclopentane hydrate particles. Second, we report qualitative observations of PVCap on hydrate formation on a liquid droplet.

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PVCap solution and liquid cyclopentane were placed in a vial and vigorously shaken for 10 min to ensure that the PVCap solution was saturated with cyclopentane. This was to ensure that the hydrate particles did not dissociate because of solubility differences. If this solution was not sufficiently saturated (shaken for less than 10 min), the hydrate particles immersed in the solution were subsequently observed to dissociate. The cyclopentane-saturated PVCap solution was precooled prior to injection into the cell. The higher density PVCap solution displaced the cyclopentane layer at the bottom of the cell and became the bulk solution surrounding the hydrate particles. Therefore, injection of the PVCap solution effectively changed the bulk phase surrounding the formed hydrate particles from liquid cyclopentane to cyclopentane-saturated PVCap aqueous solution. The particles were left for 15 min before the pull-off measurements were carried out to avoid adverse effects from the injection procedure, which would have introduced small convection currents and/or agitations in the solution. Also, this time allowed for possible PVCap adsorption on the hydrate particles. Figure 2 shows an illustration of a pull-off measurement. From the initial starting position, the right-hand cantilever (top particle) was

MATERIALS AND METHODS

The MMF apparatus consists of a Carl Zeiss inverted light Axiovert S100 microscope connected to a recording camera (Cohu, model 4915-2030). An aluminum cell was placed on the microscope stage and was surrounded by an aluminum cooling jacket connected to a propylene glycol/water cooling bath. About 20 mL of liquid cyclopentane (purity of >99%, Acros Organics) was placed in the cell and cooled to −3.2 °C. The temperature in the bulk solution of the cell was monitored with a calibrated thermocouple. Figure 1 shows

Figure 2. Schematic illustration of the experimental pull-off procedure. (a) Particles prior to contact. (b) Top particle is then lowered to contact the bottom particle by a distance p and held for approximately 10 s. (c) Top particle is then slowly raised (d) until the particles detach. The displacement d is measured at the point just before the particles detach.

moved to contact the left-hand cantilever (bottom particle) with a constant applied load, p, (about 1.5 μN) and maintained at this position for 10 s. We use a contact time of 10 s, consistent with previous measurements using the same apparatus.10,11 The top particle was then raised at a constant velocity. The maximum displacement prior to the particles detaching, d, was measured using the software ImageJ.12 The cohesion force was calculated by multiplying d with the spring constant of the left-hand cantilever. The spring constants of the cantilevers used ranged from 0.009 to 0.017 N/m, while the values for d ranged from 10 to 50 μm. The calculated cohesion force was then divided by the harmonic mean radius of the particles. The harmonic mean radius is defined as R* = 1/2(1/R1 + 1/R2), where R1 and R2 are the radii of particles 1 and 2, respectively. The R* values for the particles used were in the range of 320−420 μm. For each experiment, between 50 and 100 pull-off measurements were performed and both the average and standard deviation of each data set were reported. Cohesion Force Measurements between Cyclopentane Hydrate Particles with PVCap in Liquid Cyclopentane. After 10 min, the PVCap solution was removed with a syringe, so that the hydrate particles were immersed in liquid cyclopentane once again. However, because the PVCap solution strongly wetted the hydrate particles, it was observed that a thin film of PVCap solution remained on the hydrate particles after the bulk PVCap solution was removed. To completely convert the PVCap layer into cyclopentane hydrate before performing the pull-off measurements, the system was cooled to −1 °C, where it was held for 10 min before increasing to the desired experimental temperature, where the particles were then left for about 1 h. In the subsequent experiments with these particles, we did not find any evidence of a PVCap-rich aqueous layer on them; therefore,

Figure 1. Top-view photo of the experimental cell of the MMF apparatus. a top-view image of the experimental setup. To reduce the evaporation rate of cyclopentane from the cell, nitrogen gas was first bubbled through liquid cyclopentane (85% purity, OmniSolv) and then into the drybox, where the MMF was contained. As shown in Figure 1, the cell contained two cantilevers that hold the hydrate particles. The lefthand cantilever, which had a known spring constant, was held stationary throughout the experiment. The right-hand cantilever was connected to a remotely operated Eppendorf Patchman micromanipulator. Cohesion Force Measurements for Hydrate Particles Immersed in PVCap Solution. To form the hydrate particles, a water droplet was placed on the tip of the cantilevers. The water droplet on the cantilever was then immersed in liquid nitrogen for 15 s to convert it into ice. The cantilevers were quickly transferred into the cell, so that the ice particles were immersed in liquid cyclopentane. The cell temperature was gradually increased to about 3.2 °C at 0.1 °C/min to melt the ice particle and form hydrate. The hydrate particles were left to anneal for 30 min. The temperature range at which measurements could be performed reliably was restricted to 0− 7.7 °C. This was to avoid ice formation (at temperatures below 0 °C) and hydrate dissociation (at temperatures above 7.7 °C). PVCap (BASF, molecular weight of 3000) solutions of the desired concentration were first prepared (ranging from 0.005 to 0.5 wt %) with deionized water. The maximum PVCap concentration used was 0.5 wt %, similar to dosages used in field conditions.3 Equal volumes of 3633

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we assume that this procedure was sufficient to completely convert the layer to hydrate. Hydrate Growth on a Droplet of PVCap Solution. On the right-hand cantilever, we placed a droplet of PVCap solution that was immersed in liquid cyclopentane in the cell. The droplet was then manipulated to gently contact the hydrate particle.

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RESULTS In each experiment, between 50 and 100 pull-off measurements were performed with a unique pair of hydrate particles. Figure 3

Figure 4. Average cohesion forces between cyclopentane hydrate particles for 14 experiments. The dashed and dotted lines show the average and uncertainty in the cohesive force, respectively, in the absence of PVCap, as measured by Joshi.14 The diamonds show the two experiments in which the PVCap concentration was 0.005 wt %. The triangle shows the single experiment in which we used 0.1 wt % PVCap. The rectangles represent 0.01 wt % PVCap, while the circles represent 0.5 wt % PVCap. The error bars show the standard deviation in each data set.

error are represented for each experiment (that uses the same pair of hydrate particles), it can be seen that the results are relatively consistent and reproducible (within the experimental uncertainty). We also studied the cohesion forces between hydrate particles as a function of the temperature, from 1.1 to 7.2 °C. As shown in Figure 5, the cohesion forces were independent of the temperature in the range considered.

Figure 3. Distribution of cohesive forces for a total of 55 pull-off measurements with 0.5 wt % PVCap at 3.2 °C, average applied load of 1.49 μN, and contact time of 10 s. We present the cohesive forces in units of mN/m instead of mN because we divide the cohesive forces by the harmonic mean radii of the particles. The particle sizes were typically in the range of 300−500 μm.

provides an example of the distribution in cohesion forces for one particular experiment with 0.5 wt % PVCap at 3.2 °C. The distribution in cohesion forces is due to the slight variations in the alignment between the particles. Changes in particle alignment could cause changes in the effective contact area between the particles and, therefore, different cohesion forces. Our apparatus currently does not allow us to quantify the variation in the particle alignment. No general trend with time was observed in the cohesion forces in any of the experiments, suggesting that particle “history” did not have an effect on the results. In all of the experiments, an average applied load of 1.5 μN and a contact time of 10 s were used. Figure 4 shows a summary of the results for the measurements performed with PVCap concentrations ranging from 0.005 to 0.5 wt %. The horizontal dashed line shows the average cohesion force (1.49 ± 0.27 mN/m) between cyclopentane hydrate particles in a cyclopentane-saturated water solution at 3.2 °C, while the horizontal dotted lines represent the error range, as reported by Aman et al.13 and Joshi.14 In all of these experiments, the temperature was maintained at 3.2 °C. Figure 4 shows that cohesion forces were reduced in the presence of PVCap solution, but no significant variation in the cohesion forces was observed in the concentration range considered. On the basis of the 14 experiments performed, the average cohesion force between cyclopentane hydrate particles in PVCap solution was 0.69 ± 0.23 mN/m at 3.2 °C. When the average cohesion force and

Figure 5. Average cohesion force between cyclopentane hydrate particles immersed in 0.1 wt % PVCap solution at temperatures ranging from 1.1 to 7.2 °C.

When a layer of PVCap solution was introduced on the surface of the particles and allowed to convert to hydrate, we found that the cohesion forces between the particles were further lowered. Figure 6 shows that the average cohesion forces between cyclopentane hydrate particles “coated” with PVCap fall between 1.8 and 2.7 mN/m (average of 2.4 mN/ m). The average cohesion force between pure cyclopentane hydrate particles immersed in liquid cyclopentane is about 4.3 ± 0.4 mN/m.11 At the end of the experiment, we raised the 3634

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Figure 7. Sequence of images for hydrate formation on the water droplet without PVCap. (a) Water droplet (top particle) and cyclopentane hydrate particle (bottom particle) before the particles are brought into contact. The particles are immersed in liquid cyclopentane at 0.5 °C. (b) At 1 s after the droplet is brought into contact with the hydrate particle, hydrate growth appears on the surface of the droplet. (c) At 10 s after initial contact. (d) Hydrate completely covers the water droplet after 30 s.

Figure 6. Average cohesion force between cyclopentane hydrate particles “coated” with a layer of PVCap solution. The horizontal dashed line represents the average cohesion force between cyclopentane hydrate particles without PVCap present, while the horizontal dotted lines represent the uncertainty range, as measured by Aman et al.11 The rectangles show the results for 0.005 wt % PVCap, and the diamonds represent 0.1 wt % PVCap.

temperature to determine if the presence of the PVCap on the hydrate surface had an effect on the hydrate equilibrium dissociation temperature. We found that the cyclopentane hydrate particle could be raised to 8.8 °C (at 0.1 °C/min) before it began to dissociate (equilibrium dissociation temperature for cyclopentane hydrate = 7.7 °C). This was likely a kinetic effect because KHIs do not shift the hydrate equilibrium temperatures.3 This was consistent with observations by Rider,15 who found that THF crystals placed in PVCap solution could be raised to a temperature of 5.0 °C, which is above the equilibrium temperature of 4.4 °C. Hydrate Growth on a PVCap Droplet. To study how the presence of PVCap affects hydrate growth on a water droplet, a droplet of PVCap solution was brought into contact with a formed hydrate particle at 0.5 °C. In the absence of PVCap, hydrate growth was observed to occur on the water droplet immediately upon contact with the hydrate particle (Figure 7). Hydrate growth was observed to occur along the water− cyclopentane interface, covering the entire droplet surface after 30 s. When a droplet of 0.5 wt % PVCap solution was contacted with a formed hydrate particle, a change in the growth mechanism was observed (Figure 8). The droplet gradually spread onto the hydrate particle, eventually surrounding the entire particle after 50 min (Figure 8e). The outer PVCap solution layer then converted into hydrate after 125 min (Figure 8f).

Figure 8. Sequence of images for hydrate formation on the water droplet with PVCap. (a) Droplet of 0.5 wt % PVCap solution (top particle) brought into contact with a formed hydrate particle (bottom particle) suspended in liquid cyclopentane at 0.5 °C. (b) At 2 min after contact. (c) At 10 min after contact. (d) At 20 min after contact. (e) At 50 min after contact. (f) At 125 min after contact.

DISCUSSION There are three different types of cohesive mechanisms present that are relevant to the length scale in this study: capillary attraction, solid−solid cohesion, and sintering.16 In an oil continuous phase, the temperature dependence of the measured cohesion forces was attributed to the presence of a “quasi-liquid layer” (QLL) forming an aqueous capillary bridge between two particles. Aman et al.11 provide a complete discussion of the capillary bridge theory. The higher the surface temperature, the thicker the QLL layer (because of surface

melting), which resulted in a larger capillary bridge and, therefore, a larger capillary force. In addition to the temperature, the cohesion force was also found to depend upon the contact time between two particles. Above a contact time of 30 s, the cohesive forces were found to increase by an order of magnitude. This increase was attributed to hydrate growth along the capillary bridge, which converted it into a hydrate bridge. The cohesive force mechanism shifts from being

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surface of hydrate crystals. Wu et al.23 showed that a KHI solution containing PVCap significantly decreased the growth rate of hydrate films at a gas−water interface. The KHI caused the film growth to transition from a heat-transfer-limited rate to a mass-transfer-limited rate. Using molecular dynamics simulation, Anderson et al.24 showed that the PVCap molecule has a favorable free energy of binding to the surface of the hydrate crystal. It is possible that the presence of the PVCap molecules with favorable free energy of binding to the hydrate particle contributed to the stronger wetting behavior. The longer time delay until complete conversion of the PVCap solution to hydrate (2 h, as compared to 30 s) could indicate that, in addition, the PVCap inhibited growth. Alternatively, it is possible that a very thin hydrate film was forming on the droplet as the PVCap solution spread over the particle, reducing the rate at which it could do so.

capillary-dominated to sintering-dominated above a contact time of 30 s. For hydrate particles suspended in water, Aman et al.13 showed that the formation of a capillary bridge was unlikely and proposed that the cohesion force was the result of solid−solid cohesion. They found that the average cohesion force between cyclopentane hydrate particles suspended in water was about 1.49 mN/m. In this work, we measured the average cohesion force between cyclopentane hydrate particles suspended in dilute PVCap solution to be 0.69 ± 0.23 mN/m at 3.2 °C. When the PVCap solution was introduced in the form of a layer on the cyclopentane hydrate surface, the cohesion forces were reduced from an average of 4.3 mN/m to 2.4 mN/m at 3.2 °C. The addition of PVCap reduced average cohesion forces by 54% when introduced in the bulk solution and 44% when directly applied to the hydrate surface in bulk cyclopentane. Studies have shown that hydrate crystals grown in bulk solutions containing KHIs exhibited significant morphological changes, as compared to hydrate crystals formed in the absence of KHIs, and it has been hypothesized that these changes are due to the KHI molecules adsorbing to the hydrate surface.17,18 It follows that such changes to the hydrate surfaces will also result in changes to the cohesive forces between hydrate particles. The lack of temperature dependence in the cohesion forces (Figure 4) was also observed by Joshi14 for cyclopentane hydrate particles suspended in water and supports the solid− solid cohesion hypothesis. The PVCap concentration was not found to have an effect on the measured cohesion forces over the range of 0.005−0.5 wt %. This suggests that the adsorption onset point of PVCap is below 0.005 wt %. These results are also consistent with observations reported by Song et al.,19 who found that the cohesion energies in cyclopentane hydrate− PVCap systems showed little variation for PVCap concentrations between 100 and 5000 ppm. Hydrate Growth on the PVCap Droplet. When a water droplet was brought into contact with a formed hydrate particle at 0.5 °C, hydrate growth was observed on the droplet, which then converted into a hydrate particle within 30 s, as shown in Figure 7. However, when a droplet of PVCap solution was brought into contact with a formed hydrate particle, the droplet was observed to wet the hydrate particle more strongly. These observations provide indirect evidence that the PVCap molecules are “hydrate-philic” and are adsorbing to the hydrate−water interface. Freer,20 who studied a water−decane system, showed that the addition of 0.5 wt % PVCap lowered the surface tension of water from 70.2 to 50.8 mN/m. They also hypothesized that, at the hydrate−water interface, these surfactants were likely to be as or more surface-active than at the water−decane interface. In Figure 7, it is likely that the water droplet could not spread onto the hydrate particle because hydrate growth over the water droplet surface occurred immediately upon contact. However, in Figure 8, the presence of PVCap inhibited hydrate formation/growth, therefore allowing the droplet to spread onto the particle. The overall surface energy could be reduced by spreading, because the interfacial area was significantly reduced. The fact that PVCap slowed hydrate growth on a water droplet demonstrates explicitly that it is interfering with both the water−oil and hydrate−oil interfaces. It must be rejected from the interface before growth can continue, and this energetic penalty delays the growth rate. Makogon et al.21 and King et al.22 have shown that PVCap molecules can suppress growth by adsorbing to the

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CONCLUSION In this study, we found that PVCap reduced cohesion forces between cyclopentane hydrate particles by an average of 54% when introduced to the bulk phase and 45% when directly applied to the particle surface. These cohesion forces did not vary significantly as a function of the temperature or PVCap concentration. The presence of PVCap in a water droplet visibly changed the hydrate growth mechanism, causing a stronger interaction between the droplet and hydrate particle and significantly slower conversion rate. These results indicate that, in the event of hydrate formation in systems where KHIs are present, plug formation may not occur as rapidly as in uninhibited systems.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The visit by Reuben Wu to the Colorado School of Mines (CSM) was supported in part by the Australian Research Council (ARC) Future fellowship scheme of Nobuo Maeda (FT0991892) and the Petroleum and Geothermal Research Portfolio of CSIRO. This work was partially funded by the CSM Hydrate Consortium (current and past members): BP, Chevron, ConocoPhillips, ExxonMobil, Halliburton, Multichem, Nalco, Petrobras, SPT Group, Schlumberger, Shell, Statoil, and Total.

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