International Conference on Gas Hydrates (ICGH) (6th : 2008)


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  HYDRATE PARTICLES ADHESION FORCE MEASUREMENTS: EFFECTS OF TEMPERATURE, LOW DOSAGE INHIBITORS, AND INTERFACIAL ENERGY   Craig J. Taylor, Laura E. Dieker, Kelly T. Miller, Carolyn A. Koh, E. Dendy Sloan   Department of Chemical Engineering Colorado School of Mines 1600 Illinois St, Golden, CO, 80401 USA    ABSTRACT Micromechanical adhesion force measurements were performed on tetrahydrofuran (THF) hydrate particles in n-decane.  The experiments were performed at atmospheric pressure over the temperature range 261–275 K.  A scoping study characterized the effects of temperature, anti-agglomerants, and interfacial energy on the particle adhesion forces. The adhesion force between hydrate particles was found to increase with temperature and the interfacial energy of the surrounding liquid.  The adhesion force of hydrates was directly proportional to the contact time and contact force. Both sorbitan monolaurate (Span20) and poly-N- vinyl caprolactam (PVCap) decreased the adhesion force between the hydrate particles. The measured forces and trends were explained by a capillary bridge between the particles.  Keywords: micromechanical, adhesion force, tetrahydrofuran, sorbitan monolaurate, poly-N- vinyl caprolactam    Corresponding author: Phone: +1 (303) 273-3723 Fax +1 (303) 273-3730 E-mail: NOMENCLATURE F      Adhesion Force [N] k   Spring constant [N/m] R   Radius [m] R*    Harmonic mean radius [m] δ   Displacement [m] γLL    Interfacial Energy [mN/m] θ   Contact Angle [°]  INTRODUCTION Natural gas hydrates commonly form at high pressures and low temperatures in offshore oil and gas pipelines [1, 2]. According to a proposed mechanism for hydrate formation in oil dominated pipelines [3], water droplets become entrained in the continuous oil phase and form hydrate particles; the hydrate particles can then agglomerate, increasing the hydrate suspension viscosity, and form a hydrate plug [4]. In order to avoid hydrate plugs, traditional thermodynamic inhibitors, such as methanol or monoethylene glycol (MEG), are often employed to shift the system operation out of the stable hydrate formation conditions. Recent risk management techniques to prevent hydrate plugs include the addition of surface active chemicals, including anti-agglomerants and kinetic crystal growth inhibitors, or the use of cold flow technology to maintain hydrate slurry flow by preventing small hydrated particles from agglomerating.  The objective of this work was to measure the adhesion forces between hydrate particles to determine what parameters reduce hydrate adhesion and thus prevent hydrated particles from agglomerating. A scoping study of the parameters affecting the adhesive forces between sII THF hydrates particles was performed [5]. Specifically, Proceedings of the 6th International Conference on Gas Hydrates (ICGH 2008), Vancouver, British Columbia, CANADA, July 6-10, 2008. we studied the effect of contact force, contact time, temperature, interfacial energy of the immersed medium, and the addition of anti- agglomerants on adhesive forces of hydrates. MATERIALS AND METHODS In 2004, Yang et al. [6], from this laboratory, used a novel micromechanical testing technique, adapted from work by Eccleston and Miller [7] and Yeung et al. [8], to measure particle-particle adhesion forces of ice and tetrahydrofuran (THF) clathrate hydrates.  Although the previous work showed a clear trend of decreasing adhesive force with decreasing temperature below the hydrate formation point, there was unfortunately significant variability between measurements on different pairs of particles [6]. In this work, we have refined the experimental technique in order to produce highly repeatable force distributions between different particle pairs. As in many studies of gas hydrates [2], THF hydrate is used as a model system because of its ease of formation and stability. THF, which stabilizes the large cages of hydrate structure II, is completely miscible in water, making hydrate formation straightforward. Unlike most gas hydrates, THF hydrate is also stable at atmospheric pressure at temperatures near the ice point (melting temperature = 4.4 °C), which allows adhesion measurements of hydrate particles submersed in a cooled liquid to be performed using a simple cooling cell.  Stoichiometric solutions (where THF occupies all sII large cages) of 19.06 wt% THF (Mallinckrodt, 99.8%) in water (Mallinckrodt, HPLC grade) were prepared gravimetrically; THF hydrate of this composition melts at 4.4 °C. The THF hydrate particles used in these adhesion force measurements were prepared in two steps. Firstly, a droplet of stoichiometric THF/water solution was placed on the end of a glass fiber cantilever beam. The THF/water droplet (~500 µm in diameter) was then quenched in liquid nitrogen to form a THF hydrate particle. Secondly, the THF hydrate particle was annealed to produce a spherically smooth particle. The absence of ice in the THF hydrate particles (particularly at high subcooling temperatures) was confirmed by increasing the temperature of the system above the ice point and verifying that the THF hydrate particles remained intact with no visual evidence of decomposition. The particles were typically immersed in n-decane (pure); however some measurements were also performed in toluene (HPLC) and Conroe crude oil [3, 9-10].  The micromechanical technique as illustrated in Figure 1 was used to measure the adhesive force between hydrate particles. Digital video microscopy was used to track the movement of particles attached to low spring constant cantilever beams, as the beams were displaced with micromanipulators [6]. The adhesive force (F) was determined from the displacement δ and the spring constant k of the cantilever beam via Hooke’s Law.   kF        (1)   Displacement A B C THF Hydrate Particles Glass Fiber Cantilever Microcapillary Tube   Figure 1: Schematic illustration of the micromechanical testing apparatus. (A) Particles being brought into contact. (B) Particles being pulled apart. (C) Particles after they have broken apart. The hydrate particles are completely submerged in a cooled bath of liquid. The microcapillary tubes are attached to micromanipulators.  Four major improvements were made to the original apparatus used in the work of Yang et al. [6]: (1) the installation of a Plexiglas chamber around the apparatus, filled with continuously flowing dry air, to eliminate condensation on the cell and blurring of the recorded images, (2) a new aluminum cooling cell (Figure 2) which allowed bent micro-capillary tubes to be top-loaded into the cell for more accurate manipulation control of the hydrate particles, (3) a thermocouple was positioned within 1 mm of the hydrate particles for more accurate temperature measurement, and (4) the particle contact geometry was changed from parallel to perpendicular as shown in Figure 1, to reduce slippage between the particles [5].  Cantilever Thermocouple Cooling bath Manipulator   Figure 2: New aluminum cooling cell  The glass fiber cantilever spring constant was determined via a two step calibration method [11]. The first step was an absolute calibration of the spring constant of tungsten wire standards. The tungsten wire was placed on top of a steel level arm hanging from the bottom of an analytical balance. The wire was displaced using a micromanipulator and the resulting force was measured. A series of ~20 displacement increments were used to generate a plot of force versus displacement; the slope of this plot was the wire spring constant.  The tungsten wires were then used as calibration standards in the second step of the calibration process, which determined the relative spring constant of the glass fibers. In this step, the wire and fiber are brought into contact and placed in compression using the micromanipulators; the fiber is then pulled away from the wire, and each cantilever returned to its unstressed position. Since the force on each cantilever must be equal, the ratio of measured displacements is equal to the inverse of the ratio of spring constants:  2 1 1 2 k k           (2)  where δ1 and δ2 are the displacements and k1 and k2 are the spring constants. This process was repeated multiple times for a range of applied forces, and the spring constants were determined. For each hydrate adhesion experiment, 40 pull-off measurements were performed to fully characterize the adhesion force distribution. Additionally, to account for different particle diameters, all force measurements were normalized by the harmonic mean radius (R*) of the particle pair, where           21 * 11 2 11 RRR                                        (3)  RESULTS AND DISCUSSION Figure 3 shows the adhesion force distributions for three different cantilever beams and particle pairs using the new apparatus geometry and improved experimental technique. These distributions of measured forces were narrow and right skewed, with an s-shaped curve. The three distributions were exceptionally similar, and the averages of the three distributions were within 8% of one other. The consistency among different particle pairs and cantilevers allowed quantitative examination of the effects of contact force, contact time, temperature, surface energy of the immersed liquid with water, and the addition of surface active components.    Figure 3: Cumulative force distributions with the new geometry and technique.  Effect of contact force In order to investigate the effect of initial contact force (preload), different compressive preloads were applied to pairs of particles at -3.2 °C (7.6 °C subcooling) with a contact time of 5 seconds. As shown in Figure 4, an increase in contact force resulted in an increase in adhesion force. This result implies that particle compression could play a significant role in the rheological behavior of hydrate slurries.    Figure 4: Average adhesion force for different contact forces, for 8 different particle pairs, and 3 different cantilevers. Data range bars represent the high and low values for 40 different pull-off experiments at a given contact force. The line represents an exponential trend through the data points. The average standard deviation of the force measurements was 0.037 N/m.  Effect of contact time The effect of preload contact time was investigated by applying a constant contact force of 5.5 µN at - 12.5 °C and varying the contact time from 1 second to 15 hours. The adhesion force increased linearly with contact time as shown in Figure 5. The data range bars in Figure 5 for contact times of 1 second, 5 seconds and 1 minute (60 seconds) represent the high and low values of the force distribution for 40 pull off experiments. Only one pull off measurement was performed for a 15 hour contact time, due to the longevity of each experiment.  Particles held in contact for many hours sintered together, and additional force was then required to pull the particles apart. Figure 6 shows a sequence of video images of two hydrate particles compressed together with a contact force of 5.5 µN at -12.5 °C for 11 hours. The images were processed to enhance the edges of the hydrate particles, using a Sobel edge detection filter in the image analysis program ImageJ [12]; illumination, image acquisition, and image processing steps were kept constant in this image sequence. Figure 6A clearly shows two distinct edges after the particles were held in contact for 20 seconds. Over the course of hours, the particle edges in Figures 6B&C begin to merge together. Finally, in Figure 6D, the two particles were almost completely connected together and a neck between the particles was formed.    Figure 5: Average adhesion force for long contact times. Data range bars represent the high and low values for multiple different pull-off experiments at a given contact force. The line represents a logarithmic trend line through the data. The average standard deviation of the force measurements was 0.007 N/m.  A B C D 100 µm   Figure 6: Sequence of pictures displaying two THF hydrates compressed together over a period of hours at -12.5 °C. The pictures were modified using a Sobel edge detection filter to enhance the edges of the hydrate particles. Hydrates held in compression with a contact force of 6 µN for (A) 20 seconds, (B) 32 minutes, (C) 7 hours, and (D) 11 hours. Image acquisition parameters (illumination, focus) were held constant over time. Effect of temperature Yang et al. [6] showed that the adhesion force increased with temperature for both THF hydrates and ice. This effect was further investigated with the improved technique and apparatus. The adhesion force increased as the temperature was increased (subcooling decreased) to near the particle melting temperature. The variability of the data was significantly reduced in these new measurements (i.e. standard deviation for Yang et al.’s measurements was 0.063 N/m, while standard deviation for improved measurements was 0.028 N/m). Figure 7 shows the average adhesion force for multiple particle pairs as a function of temperature. The particles were held in contact for 5 seconds with a contact force of 7 µN.    Figure 7: Average adhesion force versus temperature for various THF hydrate pairs. Data range bars represent the width of each force distribution from the above Figure. The standard deviation of the force measurements was 0.028 N/m.  Yang et al. [6] proposed that a capillary bridging mechanism, in conjunction with surface roughness, was responsible for the increase in adhesion force as the temperature was raised toward the melting temperature, with the bridging aqueous phase arising from either (1) a quasiliquid layer on the hydrate particle surfaces [13] or (2) liquid at the particle-particle contact being stabilized below its freezing point by the negative curvature of the n-decane/bridging liquid interface. In either case, the amount of liquid at the particle contact increases with temperature, thus increasing the bridging area and increasing the adhesion force. The current measurements of the effect of temperature on adhesion force support the hypothesis by Yang et al. [6] that capillary bridging is the dominant mechanism of adhesion between hydrate particles.  Effect of interfacial energy In the capillary bridge theory, liquid water wets the surface of the particles and creates a liquid neck/bridge between the particles when brought into contact. In capillary bridge theory, the maximum adhesion force between two spherical bodies is approximately given by the relationship [14]   cos2 * LL CBT R F                                            (4)  where R* is the harmonic mean radius of the two particles, γLL is liquid-liquid interfacial energy and θ is the contact angle of the capillary liquid on the solid particle [13]. The contact angle, θ, of liquid water on hydrates is assumed to be 0° due to the strongly hydrophilic nature of the surface [15].    Figure 8: Adhesion force versus surface tension with water. The dotted line represents a linear trend through experimental data points. The solid line represents the capillary bridge theory model for hydrophilic surfaces.  To test the capillary bridge model, adhesion force measurements of THF hydrates were performed by immersing the particles in three different liquids (n-decane, toluene, and Conroe crude oil), with results shown in Figure 8. Measurements were performed using THF hydrate particles with a contact force of 8 µN, contact time of 5 seconds, and temperature of -3.2 °C (subcooling = 7.6 °C). The hydrate particle adhesion force increased with increasing surface tension (water/decane = 50.4 mN/m [16], water/toluene = 34.5 mN/m [17], and water/Conroe oil = 25.0 mN/m [3]). The dashed line represents a linear trend through experimental data points.  The maximum adhesion force (θ = 0°, cos (0°) = 1) predicted by the capillary bridge theory is also plotted as a function of liquid-liquid interfacial energy in Figure 8, indicated by the solid line. Since the capillary bridge theory predicts the maximum adhesion force, this calculation is expected to be higher than the measured forces.  These results show that the interfacial energy of the surrounding fluid substantially affects the adhesion force of THF hydrate particles. We therefore suggest oil/water interfacial energy is an important factor in hydrate agglomeration in a pipeline. Due to natural surfactants, interfacial energies in crude oils tend to be ~25 mN/m, although they can range from 15 to 40 mN/m [3]. The adhesion force between the particles can thus change substantially, depending upon the oil in the pipeline. One might consider reducing the interfacial energy of the oil and water to prevent plug formation.  Addition of anti-agglomerants The effects of two additives were examined to determine their anti-agglomerant potential for preventing hydrate plugs in pipelines. Sorbitan monolaurate (Span20), a previously reported anti- agglomerant [18], and poly-N-vinyl caprolactam (PVCap), a known kinetic hydrate inhibitor [19], were tested for their effect on the hydrate-hydrate adhesion force.  Adhesive force measurements were performed on THF hydrate particles immersed in 3 wt% Span 20 / 97 wt% n-decane  solution, at a temperature of - 3.2 °C (subcooling = 7.6 °C), a contact time of 5 seconds, and a contact force of ~ 8.3 µN. Figure 9 shows the average adhesion force between the two hydrate particles was reduced by 85 % with Span 20 present in the n-decane phase, from 0.100 N/m to 0.015 N/m. Adhesive force measurements were also performed on THF hydrate particles in which a 97 wt% decane and 3 wt % mixture of 1 wt% PVCap (in 99 wt % ethylene glycol) was used as the cooling fluid. These adhesion force measurements were performed at a temperature of -3.2 °C, with a contact force of ~ 6.1 µN for 5 seconds. The addition of PVCap to the n-decane phase reduced the average adhesion force by 75 %, from 0.051 N/m to 0.013 N/m (Figure 9). From these initial investigations of THF hydrates with Span 20 and PVCap in solution with n-decane, the adhesion force was reduced the most substantially, compared to the other parameters (temperature or subcooling, contact force, and contact time).    Figure 9: Force measurements of THF hydrates with Span 20, PVCap, and no anti-agglomerants present in the n-decane phase. Data range bars represent the width of each force distribution. The standard deviations of the force measurements with Span 20 and PVCap were 0.0046 and 0.0032 N/m respectively. The standard deviation for the non-agglomerant measurements was 0.026 N/m.  From our preliminary investigation on the effect of additives exhibiting anti-agglomerant activity, the mechanisms of the force reduction remain uncertain. In the case of PVCap, we hypothesize that the polymer provided a short-range steric barrier by absorbing to the hydrate surfaces. PVCap, a hydrate crystal growth inhibitor, is known to have an affinity for hydrate surfaces [20, 21] and it is reasonable to assume that the absorbed layer provides a steric repulsion. Additionally, Freer [22] showed the addition of 0.5 wt% PVCap in water lowered the surface tension of water from 70.18 mN/m to 51.82 mN/m; thus, PVCap could be expected to lower the interfacial energy between n-decane and water, which would also decrease the strength of the capillary bridge. In the case of Span 20, an extremely long-range repulsive force, extending to ~60 µm, was observed between the particles (Figure 10). A dark-field image of the THF hydrate surface, in the presence of 3 wt% Span 20 dissolved in the n- decane phase, reveals tiny hair-like features, ~25 µm in length, extending from the hydrate surface (Figure 11). While the nature of these features is unknown, we believe that they cause the extremely long-range steric repulsion seen in Figure 10. Additionally, previous work has shown that the addition of Span 20 to n-alkanes decreases the interfacial energy with water [23], which would also decrease the strength of any capillary bridge acting between the particles at contact.    Figure 10: Repulsive force of THF hydrate particles in n-decane/Span 20 solution versus separation distance, for four different approaches/contacts between the THF hydrate particles.     Figure 11: Micrograph of THF hydrate particle in n-decane with 3 wt% Span 20 solution. Implication for hydrate slurry flow Hydrate particles are believed to generally flow as sticky wet particles, which can readily agglomerate to plug pipelines. However a novel technology referred to as “cold flow” or “slurry flow” has been developed for long-distance transportation of oil, gas, and water by converting the water to a very stable and transportable gas hydrate slurry [24-26]. In “cold flow” all excess water is converted to hydrate particles which flow as dispersed “dry” particles in a pipeline.  Adhesive force measurements of THF hydrate particles provide new physical insight into the parameters which facilitate hydrate slurry flow. Because hydrate surfaces are strongly hydrophilic, the presence of free water in the system would be expected to promote formation of very strong capillary bridges. Removing free water will thus reduce agglomeration. The cold temperature reduces the adhesion force between hydrate particles, again reducing agglomeration of the hydrates. If both of these requirements are fulfilled, the hydrate particles will flow as small dispersed particles in the pipeline.  SUMMARY The adhesion force between THF hydrate particles was found to be directly proportional to the contact time, contact force, and interfacial energy of the surrounding medium with water, but inversely proportional to the temperature. Both Span 20 and PVCap reduce the adhesion force between hydrate particles. Adhesive force measurements between THF hydrate particles provide physical insight into cold flow technology.   REFERENCES [1] Sloan, ED. Clathrate Hydrates of Natural Gases, 2nd ed. New York, Marcel Dekker, 1998. [2] Koh, C. Towards a Fundamental Understanding of Natural Gas Hydrates. Chem. Soc. Rev. 2002; 31: 157-167 [3] Turner DJ. Clathrate Hydrate Formation in Water-in-Oil Dispersions. Thesis, Colorado School of Mines, Golden, CO, 2005. [4] Austvik T, Li X, Gjertsen LH. Hydrate Plug Properties Formation and Removal of Plugs. NYAS 912, 2000. p. 294. [5] Taylor C.J. et al. Micromechanical adhesion force measurements between tetrahydrofuran hydrate particles.  Journal of Colloid and Interface Science 2007; 206: 255-261 [6] Yang SO, Kleehammer DM, Huo Z, Sloan E.D. Jr., Miller KT, Temperature Dependence of Particle-Particle Adherence Forces in Ice and Clathrate Hydrates. J. 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[19] Kelland M.A. History of the Development of Low Dosage Hydrate Inhibitors, Energy & Fuels 2006; 20: 825. [20] Perktold K, Hofer M, Rappitsch G, Loew M, Kuban B.D, Friedman M.H, Makogon T.Y, Larsen R, Knight C.A, Sloan E.D Jr. Melt growth of tetrahydrofuran clathrate hydrate and its inhibition: Method and first results. J. Crystal. Growth 1997; 179 (1):  258. [21] King HE Jr., Hutter JL, Lin MY, Sun T, Polymer Conformations of Gas-Hydrate Kinetic Inhibitors: A Small-Angle Neutron Scattering Study. J. Chem. Phys 2000; 112: 2523. [22] Freer E. Methane Hydrate Growth Kinetics. Thesis, Colorado School of Mines, Golden, CO, 2002. [23] Peltonen L, Hirvonen J, Yliruusi J. The Behavior of Sorbitan Surfactants at the Water-Oil Interface: Straight Chained Hydrocarbons from Pentane to Dodecane as an Oil Phase. J. Colloid Interface Science 2001; 240: 272. [24] Austvik T. Hydrate Formation and Behavior in Pipes. Thesis, Norwegian University of Science and Technology, Trondheim, (1992). 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