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Micromechanical Interactions between Clathrate Hydrate Particles and Water Droplets: Experiment and Modeling Chenwei Liu, Mingzhong Li, Chunting Liu, Kaili Geng, and Yuxing Li Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.6b00668 • Publication Date (Web): 25 Jul 2016 Downloaded from http://pubs.acs.org on August 2, 2016
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Micromechanical Interactions between Clathrate Hydrate Particles and Water Droplets: Experiment and Modeling Chenwei Liu1,2, Mingzhong Li1*, Chunting Liu1, Kaili Geng1, Yuxing Li2* 1
2
College of Petroleum Engineering, China University of Petroleum, Qingdao 266580, China
Shandong Provincial Key Laboratory of oil and gas storage and transportation, China University of Petroleum, Qingdao 266580, China
ABSTRACT The micromechanical interactions between hydrate particles and unconverted water droplets play an important role in determining hydrate agglomeration, which is a key cause of hydrate blockages. In this study, the interaction behaviors between cyclopentane (CyC5) hydrate particles and water droplets in different conditions were directly investigated using a micromechanical force (MMF) apparatus. For a smaller extent of subcooling, no hydrate was visibly converted from the water droplet during the measurement. A modified theoretical model was proposed to predict the corresponding interaction behavior. A parabolic approximation was found to be adequate for describing the liquid bridge shape. The insignificant change in the interfacial area between the liquid and the hydrate as the separation distance varied, suggesting the presence of a strong wetting hysteresis between liquid bridges and hydrate particles. The capillary force model can predict the interaction force with satisfactory accuracy. At a higher level of subcooling, the amount of hydrate converted from water droplet during the interaction led to a reduction in liquid volume and to dynamic changes in the boundary. The theoretical model presented here is not adequate for this specific case. Furthermore, a lower temperature induces more hydrate formation during the measurement, which can increase the adhesion force. Compared with cohesion forces between a hydrate particle and a particle, adhesion forces between a hydrate particle and a droplet should dominate hydrate agglomeration. The present experiment and modeling contributes an improvement in the current 1
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understanding of hydrate agglomeration, leading to new potential strategies to control this process. Keywords: cyclopentane hydrate particle, Laplace pressure, capillary force, parabolic approximation, rupture distance
1. INTRODUCTION Gas hydrates are solid inclusion compounds, where molecular cages of water surround small hydrocarbon species, such as methane, ethane, and carbon dioxide, under specific pressures and temperatures.1 In oil and gas transportation, hydrates can form due to relatively low temperature and high pressure conditions, thereby leading to complete pipeline blockage. With the gradual increase in demand for energy resources, exploration for recovering hydrocarbon resources from new reservoirs has shifted into deep and ultra-deep water. These new reservoirs often have environmental factors (i.e., low temperature and high pressure) sufficient to create hydrate blockages in oil-gas transportation lines, thereby presenting a major reliability of flow issue.1 For an oil-dominated system, a conceptual model with four successive steps has been proposed for describing the hydrate plug formation in pipelines.2 The model suggests that hydrate agglomeration is the limiting factor in plug formation.1 Furthermore, after acquiring detailed insight into the agglomeration process (Figure 1), Fidel-Dufour suggested that two basic interactions exist: (i) between a hydrate particle and another particle, and (ii) between a hydrate particle and an unconverted water droplet.3 One effective method to evaluate the interactions is to measure the interaction forces/behaviors between them directly. Multiple experimental and theoretical works to date have focused primarily on hydrate/hydrate particle interaction. Yang et al. presented initial measurements of cohesive force between tetrahydrofuran hydrate particles.4 The observed forces and trends were explained by the capillary cohesion of rough surfaces, with the capillary bridging liquid stabilized by the negative curvature of the bridging liquid/n-decane interface when below its freezing point. Further work by Taylor et al.5 and Dieker et al.6 suggested that the operating requirements for tetrahydrofuran hydrate can lead to suspected ice contamination in cohesive 2
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hydrate
water
(a)
(b)
(c)
(d)
Figure 1. Schematic of the agglomeration process of hydrate particles with droplets as proposed by Fidel-Dufour.3
force measurements, providing motivation to study cyclopentane hydrate. Dieker et al.7 measured interparticle cohesive force between cyclopentane hydrates and hydrates as a function of subcooling and also evaluated the effect of crude oil. They suggested that crude oils with high acid or asphaltene content are more likely to exhibit nonplugging tendencies in oil and gas flowlines. On the basis of experimental observations, Aman et al.8 presented a comprehensive model to estimate hydrate interparticle forces as a function of the following variables: contact time, amount of unconverted water (available to form liquid bridges), temperature, and water-oil interfacial tension. This model included a capillary bridge component that dominates the interparticle forces over short contact times (below approximately 30 s) and a sintering component, which accounts for fractures between hydrate bridges. Recently, Lee et al.9 compared the cohesive forces of cyclopentane hydrates with and without thermodynamic inhibitors. They suggested that the mechanism for the cohesive force between hydrate particles depends on many factors, including temperature, amount of unconverted water, and particle roughness. The interaction and adhesion force between hydrate particles and unconverted water droplets, another important micromechanical interaction force, is considered to play an important role in the early stages of agglomeration. However, only a few papers in the literature discuss this, and experimental work accounts for most cited research.10–12 Song et al.10, 11 measured the interaction behavior of CyC5 hydrate particles and water droplets using a z-directional microbalance, and then investigated the effects of substrate morphology and chemical additives on interfacial dynamics. Furthermore, with the same microbalance system, Cha et al.12 investigated the effects of
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water droplet volume on the interfacial dynamic behavior between the CP hydrates and water droplets in a CP/n-decane oil mixture. In this paper, the interaction behaviors between cyclopentane (CyC5) hydrate particles and water droplets was investigated using a micromechanical force (MMF) apparatus developed by the Colorado School of Mines Center for Hydrate Research. As a result of our experimental observations, we propose a modified pendular liquid bridge model to quantitatively predict interaction behaviors between hydrate particles and water droplets with constant liquid volume. The experiments and modeling work presented here improve on our current understanding of hydrate particle-water droplet interaction behavior, which suggests new potential strategies to control hydrate agglomeration.
2. EXPERIMENTAL SECTION CyC5 hydrate particle-water droplet interactions were measured using a micromechanical force apparatus (MMF), which has been widely used to measure cohesion forces between hydrate-hydrate and hydrate-surface.4,9,13,14 Details about the MMF apparatus can be found elsewhere.15 Before the measurement, a CyC5 hydrate particle with a radius of approximately 342±27 µm was formed at the end of a glass fiber,4–9 and a water droplet with an equivalent radius of approximately 267±16 µm was placed on a polished aluminum plate, all in an aluminum cell (Figure 2) filled with CyC5 (98% purity, Fisher Scientific) or a mixture of CyC5 and mineral oil 70T (STE Oil Company, Inc.).16 The aluminum cell was surrounded by a cooling jacket connected to an external refrigeration unit with a precision of ±0.1°C to control the cell temperature. As illustrated in Figures 2(a) and 2(b), during the approaching period, the droplet was slowly brought into contact with the hydrate particle, creating a preload force on the hydrate particle as a result of the bending of the glass fiber. While “retracting,” the droplet slowly moved away from the hydrate particle until the droplet/hydrate particle was detached (see Figures 2(c) and 2(d)). The approaching and retracting velocities of the plate were approximately 9±3 µm/s, effectively 4
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Droplet
(a)
Hydrate
CyC5 Aluminum Rod
Microcapillary Tube
Flat Plate Glass Fiber
(b)
(c)
Glycol Cooling Jacket
(d) Figure 2. Experimental cell setup for interaction measurements between CyC5 hydrate particles and water droplets (left) and the sequence of steps in the measurements (right), with (a) and (b) representing the "approaching" phase, and (c) and (d) represent the "retracting" phase. The movement of the droplet/flat plate is controlled by a remotely operated micromanipulator (Eppendorf Patchman 5173).
reducing fluctuations of the observation targets (e.g., hydrate particle, flat plate and so on). The entire measurement process is captured in real-time at 30 frames per second through digital video microscopy. The relatively slow operating speed and high frame frequency of the image capture offer satisfactory images for further processing. The sizes of the particle and droplet, the liquid bridge shapes/outlines, the liquid-solid contact angles, and the displacement of the glass fibers were obtained with image processing software (Image J).17 For the measurements in this study, the contact angles of the liquid bridge between the hydrate particle and the plate were determined with a standard deviation less than 2° and a coefficient of variation (i.e., standard deviation/mean×100%) less than 3%. The standard deviations of the bridge profile and the displacement of the hydrate particles were found to be less than 4 µm. The interaction force between the hydrate particle and the water droplet was determined using Hooke’s law, where the force is directly proportional to the spring constant of the glass fiber cantilever and the displacement of the hydrate particle. The spring constant of the stationary cantilever was indirectly calibrated using a tungsten wire
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of a known spring constant.15 The uncertainty of the displacement of the hydrate particle (
