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Fuel 259 (2020) 116201
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Full Length Article
Particle size dependence of clathrate hydrate particle cohesion in liquid/ gaseous hydrocarbons
T
Shenglong Wanga, Shuanshi Fana, , Xuemei Langa, Yanhong Wanga, Pengfei Wangb ⁎
a
Key Laboratory of Heat Transfer Enhancement and Energy Conservation of Education Ministry, School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510640, China b Academy for Advanced Interdisciplinary Studies, Southern University of Science and Technology, Shenzhen 518055, China
ARTICLE INFO
ABSTRACT
Keywords: Hydrate particle Cohesion Flow assurance Agglomeration
Clathrate hydrate particle cohesion dominates agglomeration of hydrate particles in gas/oil pipelines, and the existence of a capillary bridge between hydrate particles was assumed to be the reason of hydrate particle cohesion. This assumption implied the cohesive force between hydrate particles were radius-dependent but without any experimental validations. The cohesive force between cyclopentane (CP) hydrate particles in liquid CP phase and the cohesive force between CH4/C2H6 gas hydrate particles in the gas phase were measured respectively by using micromechanical force apparatus, and the effects of particle size and annealing time on hydrate particle cohesion in different phases were investigated. The cohesive force between hydrate particles was found independent of particle size in liquid CP phase but was linearly correlated with the effective radius of the hydrate particle pair in the gas phase. Long annealing time decreased cohesive force measured in liquid CP phase but had no significant effect on the cohesion of hydrate particle in the gas phase. A possible mechanism was therefore proposed to interpret hydrate particle cohesion in different phases: The cohesion of hydrate particles in liquid hydrocarbon was dominated by the sintering of asperities, but hydrate particle cohesion in gaseous hydrocarbons was still caused by the existence of capillary bridge.
1. Introduction Clathrate hydrates are crystalline solids formed by water and guest molecules under low-temperature and high-pressure conditions [1]. Natural gas hydrates bearing in deep-sea sediments or permafrost are known as potential fossil fuel resource with huge reserves [2–7]. However, in gas/oil pipelines, hydrate particles can form, agglomerate and adhere to the wall, which may cause blockages and other safety issues during gas/oil transportation [8,9]. The force between hydrate particles, also known as the cohesive force, has been considered to be one of the dominant parameters controlling hydrate agglomeration in pipelines [8]. In order to study the mechanism of hydrate particle agglomeration, the micromechanical force apparatus (MMF) was developed and used to directly measure cohesive forces between hydrate particles [10,11]. Cyclopentane (CP), tetrahydrofuran (THF), CH4, C2H6, and CO2 were used as hydrate formers, and effects of temperature, contact time, and the existence of additives on the cohesive force were investigated in previous publications [10,12–14]. It was inferred from experimental results and the analogy between ice and CP hydrate that the capillary force generated by a thin liquid
⁎
layer between hydrate particles (also known as a capillary bridge) caused hydrate particles to cohere [15,16]. This assumption implies a correlation between hydrate particle size and the cohesive force because the capillary force between two solid particles with the existence of a liquid bridge has been proved to be particle-size-dependent [17–20]. The average particle diameter mentioned in most of the previous publications is ~800 μm, which is approximately 1–2 orders of magnitude larger than the diameter of hydrate particles formed in gas/oil pipelines (1–100 μm) according to focus beam reflectance measurements (FBRM) [21–23]. This discrepancy suggests that a nondimensionalization method is needed to make cohesive forces measured under laboratory conditions quantitatively comparable to the cohesive force between hydrate particles in gas/oil pipelines. Whereas, the relationship between cohesive force and particle size of clathrate hydrate particles was rarely reported, which made the feasibility of nondimensionalization of hydrate particle cohesive force doubtful. Therefore, the particle size dependence of cohesive forces between hydrate particles needs to be specifically discussed, which can not only help to understand the hydrate particle cohesion mechanism but also clarify the reliability of cohesive force results measured in the laboratory as guidance for
Corresponding author. E-mail addresses: [email protected] (S. Wang), [email protected] (S. Fan).
https://doi.org/10.1016/j.fuel.2019.116201 Received 20 June 2019; Received in revised form 12 August 2019; Accepted 10 September 2019 0016-2361/ © 2019 Elsevier Ltd. All rights reserved.
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hydrate agglomeration simulation and prediction in pipelines. In this study, cohesive forces between CP hydrate particles and CH4/ C2H6 gas mixture hydrate particles with different sizes were measured, and effects of particle size and annealing time on hydrate particle cohesion in liquid and gaseous hydrocarbons were thoroughly discussed. A possible mechanism was proposed based on experimental results to interpret hydrate particle cohesion in liquid and gaseous hydrocarbons. 2. Materials and methods 2.1. Preparation of CP and CH4/C2H6 hydrate particles Fig. 1. Schematic illustration of cohesive force measurement procedures (adapted from Wang et al. [14]).
The cohesive force between hydrate particles was measured by using atmospheric- pressure and high-pressure MMF apparatus. The atmospheric-pressure and high-pressure apparatus both contain two cantilevers, one is stationary and the other is movable. The displacement of the moveable cantilever was controlled by a nano-manipulator with a minimum step of ~10 μm. The difference is that for the atmospheric-pressure setup, two cantilevers are open to the atmosphere of CP saturated air, but for the high-pressure setup, the two cantilevers are sealed in a pressurized cell to maintain the high-pressure condition for CH4/C2H6 hydrate formation. The configuration of the two systems is shown in Fig. S1 of the Supplementary Information. CP hydrate particles and CH4/C2H6 hydrate particles were both formed from pre-formed ice particles on the tips of two glass fibers. One glass fiber was fixed on the movable cantilever, and the other was fixed on the stationary cantilever. The ice particle was formed by quenching a deionized water droplet into liquid nitrogen for ~10 s. For the preparation of CP hydrate particles, the pre-formed ice particle was submerged in liquid CP (98% purity, Sigma-Aldrich, USA) at 3.0 °C, atmospheric pressure to form a CP hydrate particle. Once hydrate conversion started, the particle was annealed to ensure the hydrate shell was solid enough to withstand at least 40 pull-off tests, and this time interval was defined as the annealing time. The annealing time in this research was 30 min for CP hydrate. A typical CP hydrate pair formed following the aforementioned procedure is shown in Fig. S2. A gas mixture of 74.3 mol% CH4 & 25.7 mol% C2H6 (General Air, USA) was used to form gas hydrate particles. Ice particles were firstly formed from deionized water droplets to provide seeds for gas hydrate formation, which was similar to CP hydrate particle formation. After the ice particle was formed, it was transferred to a sealed cell. The CH4/ C2H6 gas mixture was then injected to pressurize the cell to 2.2 MPa, and the temperature was cooled down to 1 °C. The gas hydrate particle was also annealed for at least 30 min to ensure the hydrate shell strength. A typical gas hydrate particle pair formed following the aforementioned procedure is shown in Fig. S3. CP and CH4/C2H6 hydrate particles with different sizes were formed by controlling the size of the pre-formed ice particle. The size of the hydrate particle would be slightly different from the ice particle due to the morphology change during hydrate conversion. Therefore, the radius of a specific hydrate particle reported in this research was measured at the beginning of the cohesive force measurement.
2.2. Cohesive force measurement procedures The procedure of cohesive force measurement is shown in Fig. 1: (1) The particle fixed on the moveable cantilever is brought to contact with the particle fixed on the stationary cantilever; (2) A preload force is then applied on two particles with a displacement of D1 and a contact time of 10 s. D1 was set to ~20 μm for both CP hydrate and gas hydrate cohesion force measurements, and preload force distributions for CP hydrate particles and CH4/C2H6 hydrate particles are shown in Figs. S4 and S5 respectively; (3) The moveable cantilever is moved away from the stationary cantilever with a constant velocity until the particle pair broke apart; (4) The maximum displacement of the particle fixed on the stationary cantilever was measured as D2. The pull-off procedure was performed in the horizontal direction. Therefore the effect of gravity on the value of measured force was negligible. The whole process of force measurement was recorded by a CCD camera mounted on the microscope at 10 frames per second. The cohesive force between two particles was then calculated by Hooke’s law: (1)
F = KD2
where F is the measured cohesive force between particles, K is the spring constant of the glass fiber fixed on the stationary cantilever, and D2 is the maximum displacement of the tip of the glass fiber fixed on the stationary cantilever. D1 and D2 were determined by analyzing the recorded video with the aid of ImageJ. For each particle pair, the cohesive force was measured at least 40 times, and the average cohesive force from multiple measurements was reported as the result. The measured cohesive force was thought to be correlated with the particle size in previous publications [17,19,20]. Therefore the cohesive force should be nondimensionalized by the harmonic mean radius of the particle pair to eliminate the effect of particle size on the cohesive force, as shown in Eqs. (2) and (3): [14,24]
F =
F R
(2)
R =
2R1 R2 R1 + R2
(3)
where F is the directly measured cohesive force, F* is the dimensionless cohesive force; R1 is the radius of the particle fixed on the movable cantilever (the top particle in Fig. 1), R2 is the radius of the particle
Table 1 Parameters of different particles. Hydrate former
Temperature (°C)
Subcoolinga (°C)
Pressure (MPa)
R* (μm)
F* (mN/m)
F (μN)
rb
CP CH4/C2H6
3.0 1.0
5.6 5.8
0.1 2.2
134–578 298–552
2.42–14.7 25.0–44.1
1.24–2.47 7.94–26.7
−0.149 0.971
a
[1]. b
Subcooling is the difference between phase equilibrium temperature and cell temperature. Hydrate phase equilibrium temperature was calculated by CSMGem r is the Pearson’s correlation coefficient of R* and F. 2
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fixed on the stationary cantilever (the bottom particle in Fig. 1); R* is the effective radius of two particles. The uncertainty boundary of F is defined as the 95% confidence interval of the t-distribution of results collected from at least 40 independent pull-off tests, and error bars of data reported stand for the 95% confidence interval. In this study, to clarify the relationship between the cohesive force and the hydrate particle size, both F and F* were presented as results of cohesive force measurements for comparison. Formation conditions and other detailed parameters of hydrate particles used in this study are shown in Table 1. 3. Results and discussion 3.1. Particle size dependence of CP hydrate cohesion in liquid CP phase To investigate the relationship between the effective radius R* and cohesive force F, 27 pairs of CP hydrate particles with R* ranging from 134 μm to 578 μm were formed at 3 °C with a 30 min annealing time, and the cohesive force was measured. Cohesive force F is shown in Fig. 2 as a function of the effective radius R*. It is shown that the measured cohesive force fluctuates around ~1.8 μN, and the minimum and maximum forces are 1.24 μN and 2.47 μN respectively. Pearson’s correlation coefficient (PCC) r for F and R* equals to −0.149 considering the error (as shown in Table 1), which indicated no apparent linear relationship between the cohesive force F and the effective radius R*. This result indicated that the cohesive force between CP hydrate particles was nearly constant when particle size was the only variable considering the experimental error.
Fig. 3. F* of CP hydrate particles in liquid CP phase measured in this study (solid squares) and the value reported in literature (grid area).
not applicable for the quantification of the cohesive force between hydrate particles in liquid CP, especially when the particle is relatively small (radius < 325 μm). Data reported in Figs. 2 and 3 use effective radius R* to characterize the size of a specific CP hydrate particle pair. Whereas, two CP hydrate particles in one test were usually unequal sized. Thus the effect of the size difference of particles in each particle pair on the measured cohesive force was also investigated. The particle size difference in one pair was defined by the ratio of R1 to R2. Cohesive force F as a function of R1/R2 is shown in Fig. 4. 22 out of 27 CP hydrate particle pairs had a close R1/R2 ratio around 1.0. For comparison, 5 out of 27 particle pairs had a relatively significant difference in particle size (R1/R2 ratio > 1.5), which was manipulated intentionally during particle preparation. As shown in the Figure, the variation of particle size ratio has no significant influence on the measured cohesive force. However, in the modeling of capillary force between to unequal sized particles presented by Chen et al., the capillary force significantly depending on the particle size difference, and the cohesive force was supposed to increase when R1/R2 was close to 1 [17].
Fig. 2. The cohesive force between CP hydrate particles in liquid CP phase as a function of the effective radius.
To compare the cohesive force F measured in this study with reported values in the literature, F was nondimensionalized by the effective radius R* as F*. F* were compared with the widely cited dimensionless cohesive force of CP hydrate particle [16]: 4.3 ± 0.4 mN/ m, which was measured at 3.2 °C with an annealing time of 30 min (as shown in Fig. 3). It is shown that the dimensionless cohesive force F* decreases with the increasing effective radius R*, and F* measured in this study is close to the reported value when the effective radius of CP hydrate particle pair was between a narrow range of 325 μm–475 μm. When the effective radius is close to 150 μm, the cohesive force can be ~3 times higher than the value reported in the literature. Although the radius of CP hydrate particle used in literature was not given, it was mentioned that the minimum particle radius was between 300 μm and 500 μm. This result indicates that the nondimensionalization method is
Fig. 4. The cohesive force F between CP hydrate particles in liquid CP phase as a function of particle radius ratio R1/R2. 3
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It can be concluded from data presented in Figs. 2, 3, and 4 that the cohesive force between CP hydrate particles is not relevant to either the particle size or the particle size difference. This may lead to several new understandings of MMF results of CP hydrate particles: 1) The dimensionless cohesive force F* is not a precise enough parameter to characterize the cohesion mechanism of CP hydrate particles of different sizes; 2) The cohesive force of hydrate particles formed in pipelines with the existence of oil phase could be significantly underestimated by using nondimensionalization method when taken 4.3 mN/ m as the reference, because hydrate particles formed in pipeline conditions (diameter: ~10 μm) are much smaller than CP hydrate particles (diameter: ~800 μm) formed in MMF tests, but hydrate particle cohesive forces in the two scenarios can be reasonably close; 3) Capillary force or the existence of a liquid bridge between particles is probably not the dominant mechanism of hydrate particle cohesion in liquid hydrocarbon phase, which is because in theoretical models describing
3.2. Particle size dependence of CH4/C2H6 gas hydrate particle cohesion in the gas phase 74.3 mol% CH4 & 25.7 mol% C2H6 gas mixture was used to form sII hydrate particles, and the cohesive force between gas hydrate particles of different sizes was measured. The effective radius of gas hydrate particle pairs was between 298 μm and 552 μm, and the measured cohesive force F as a function of R* was shown in Fig. 5 (a). PCC is calculated to test the correlation between F and R*, and the value equals to 0.971 including the error (as shown in Table 1). This result indicates a significant linear correlation between F and R* for gas hydrate particles in contrast to CP hydrate results (r = −0.149). The dimensionless cohesive force F* between gas hydrate particles is plotted in Fig. 5 (b) as a function of R*. The data suggest that the nondimensionalization method is necessary when comparing cohesive forces between hydrate particles in the gaseous hydrocarbon phase.
Fig. 5. The cohesive force F (a) and the dimensionless cohesive force F* (b) between CH4/C2H6 hydrate particles in the gas phase as a function of the effective radius R*
capillary force between two spheres proposed by different researchers, particle radius is always one of the controlling parameters of the cohesive force [17,25–28]. The possible mechanism of CP hydrate particle cohesion in liquid CP phase was considered to be the sintering of asperities, which was discussed in Section 3.4.
It can be inferred from results presented previously that the cohesive force between CH4/C2H6 hydrate particles in the gas phase is possibly dominated by the capillary bridge mechanism. To verify this hypothesis, the cohesive force of CH4/C2H6 hydrate particles measured in this research (see Fig. 6) were compared with cohesive forces of CP hydrate measured in CPsaturated N2 (gas phase) and liquid CP at 3 °C in literature [16,29]. It needs
Fig. 6. The cohesive force between hydrate particles measured in gas and liquid phases. 4
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to be clarified that the cohesive force between CP hydrate particles was reported in the dimensionless form F* with a radius range of ~400 μm in literature. Therefore, to avoid the influence of particle-size difference on the comparison, dimensionless cohesive forces of hydrate particle pairs with similar R* (as close to 400 μm as possible) measured in this research were presented. As shown in the Figure, the cohesive force of CP hydrate particles in the gas phase is close to the cohesive force of CH4/C2H6 hydrate particles in the gas phase considering the error. Additionally, as reported in literature, a liquid layer on the surface of CP hydrate particle was observed during the measurement of cohesive force after leaving the particle in the gas phase for 465 s [29]. These results suggest that cohesion mechanisms of CP hydrate particles and CH4/C2H6 hydrate particles in the gas phase are similar, and are possibly both dominated by the capillary force generated by the liquid bridge between particles. In contrast, the cohesive force between hydrate particles measured in liquid CP phase is far lower than the cohesive force measured in the gas phase, which suggests that hydrate particle cohesion in liquid phases is possible dominated by a different mechanism instead of the capillary bridge theory. It can also be assumed that the discrepancy between CP hydrate particle cohesion and CH4/ C2H6 hydrate particle cohesion in the aspect of particle size dependence is due to the different phases where the cohesive force is measured.
Cohesive forces between CH4/C2H6 hydrate particles with annealing times between 30 min and 1380 min are shown in Fig. 8. For comparison, cohesive forces of CH4/C2H6 hydrate measured at 1 °C, 3.45 MPa in liquid hydrocarbon with different annealing times by Hu et al. is also cited in Fig. 8 [13]. It is shown that the cohesive force of gas hydrate decreases with the increasing annealing time in liquid hydrocarbons, which is similar to the cohesive force of CP hydrate in liquid CP phase. In contrast, the cohesive force of gas hydrate measured in gas phase shows no significant correlation with the change of annealing time. Moreover, when the annealing time is increased to ~1300 min, the cohesive force measured in the gas phase remains at ~30 mN/m, but the cohesive force measured in the liquid hydrocarbon phase is only ~4.7 mN/m, which is quite comparable with the cohesive force between CP hydrate particles in liquid CP phase measured in literature [16] and this research (nondimensionalized with R* of ~400 μm) considering the similar driving force and particle size.
3.3. Effect of particle size on the cohesive force between hydrate particles with different annealing times The annealing time is a parameter defined as the time interval between the beginning of hydrate conversion observed on the water droplet and the beginning of cohesive force measurement. The annealing time is usually used to estimate the conversion of hydrate particle when the driving force of hydrate formation (subcooling, pressure, etc.) is constant. To further understand the mechanism of clathrate hydrate particle cohesion, and investigate the possible reason for the discrepancy of particle size dependence between CP hydrate particles and CH4/C2H6 hydrate particles, the effect of annealing time on cohesive forces was studied. Cohesive forces of 6 pairs of CP hydrate particles with annealing time varied from 30 min to 150 min were measured as shown in Fig. 7. It is shown that long annealing time causes the cohesive force to decrease, especially when comparing the cohesive force measured at 150 min with the cohesive force measured at 30 min. Additionally, Cohesive force is still independent of particle size when the annealing time is longer than 30 min. It is also shown that the deviation of cohesive forces between particles with different radii decreases with the increasing annealing time, which suggests the surface condition of CP hydrate particles becomes stable (better repeatability for force measurements) with relatively long annealing time.
Fig. 8. The cohesive force between CH4/C2H6 hydrate particles in the gas phase and liquid hydrocarbon phase with different annealing times.
It can be concluded from results shown in Figs. 7 and 8 that the annealing time has significant effects on the cohesion of hydrate particles in liquid hydrocarbon phase but not in the gas phase, which suggests that mechanisms of hydrate particle cohesion in liquid and gaseous hydrocarbons are possibly different. 3.4. Possible mechanisms for hydrate particle cohesion in liquid and gaseous hydrocarbons The cohesive force between CP hydrate particles in liquid CP phase has been proved to be uncorrelated with the particle size. In contrast, the cohesive force between CH4/C2H6 hydrate particles has been found to be linearly correlated with the effective particle radius in the gas phase. Therefore, a possible mechanism is proposed to interpret hydrate particle cohesion in liquid and gaseous hydrocarbons. The cohesion of hydrate particles in liquid CP phase is supposed to be dominated by hydrate sintering between asperities on the particle surface as shown in Fig. 9 (a). Reasons are listed as follows: 1) the contact angle of water on CP hydrate surface in liquid CP phase is close to 90° (92.5° ± 8.5° at 0 °C to7.7 °C, 1 atm. measured by Brown et al. [30]), and the solubility of water in cyclopentane is very low (0.0046 wt% at 0 °C, 1 atm. measured by Englin et al. [31]). Even if there is unconverted water or pre-melted water on the surface of hydrate particles from the thermodynamic point of view, a liquid bridge is unlikely to form between particles due to the very small curvature of meniscus according to the Young–Laplace equation [32]. However, this
Fig. 7. The cohesive force between CP hydrate particles in liquid CP phase with different annealing times. 5
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Based on the proposed hypothesis of hydrate particle cohesion mechanisms, the reason for the discrepancy of the particle size dependence of hydrate particle cohesion in liquid and gaseous hydrocarbons can be explained. Moreover, it indicates that when applying cohesive force results measured in the laboratory into theoretical modeling of hydrate agglomeration or the evaluation of hydrate inhibitor, the validity of nondimensionalization should be considered, especially for the scenario of hydrate particle cohesion in liquid hydrocarbons. 4. Conclusions Cohesive forces between clathrate hydrate particles in liquid and gaseous hydrocarbons were measured by using micromechanical force apparatus, and effects of particle size and annealing time on cohesive forces were discussed. It has been found that CP hydrate particle cohesive forces in liquid CP phase were independent of the effective radius of particle pairs, but cohesive forces between CH4/C2H6 hydrate particles were linearly correlated with the particle effective radius in the gas phase. This result suggested that the nondimensionalization was not always feasible when applying values of directly measured cohesive force into the simulation of hydrate agglomeration in gas/oil pipelines or the evaluation of hydrate inhibitors. A hypothesis for possible mechanisms dominating hydrate particle cohesion in different phases was proposed based on experimental observations and results. It was considered that the cohesion of hydrate particle in liquid hydrocarbon phase was due to the sintering of asperities on the particle surface, and the cohesion of hydrate particle in gaseous hydrocarbon was dominated by the capillary bridge formed between particles. This hypothesis explained experimental discrepancies in particle size and annealing time dependence between hydrate particle cohesion in liquid CP phase and CH4/C2H6 gas phase. Results presented in this research provide insights on differences and connections between cohesive force measured under laboratory conditions and hydrate particle agglomeration in gas/oil pipelines.
Fig. 9. Illustrations of possible mechanisms dominate hydrate particle cohesion in liquid and gas hydrocarbons: (a) Sintering of asperities dominates hydrate particle cohesion in liquid CP; (b) Capillary bridge dominates hydrate cohesion in CH4/C2H6.
trace amount of water on the surface of the particle may promote hydrate reformation (sintering) upon contact. 2) It has been indicated by different researchers that long contact time would lead to high cohesive forces between hydrate particles in liquid hydrocarbons, which was possibly due to the sintering of hydrate particles [13,16,33]. Therefore, it can be assumed that cohesive forces between hydrate particles are mainly related to the dimension of asperities involved in sintering (surface roughness) instead of the size of the particle, which explains the reason why cohesive forces measured between CP hydrate particles in liquid CP phase showed no dependence on particle sizes. 3) It has been found that with the increase of annealing time, the surface of hydrate particle became rougher (images of CP hydrate particles in liquid CP phase with annealing times between 30 min and 150 min are shown in Fig. S2 of the supplementary information), and the cohesive force decreased. This result suggests that for hydrate particles with a relatively long annealing time (rough surface), fewer asperities (only outmost ones) are involved in the sintering process, and a low cohesive force is expected. For the cohesion of CH4/C2H6 hydrate particles in the gas phase, it has been found that the cohesive force F was linearly correlated with the effective particle radius R*, and longer annealing time had no significant effect on the force. Therefore, it is supposed that the cohesion of hydrate particles in gaseous hydrocarbon is dominated by the capillary bridge between particles with the following reasons (as shown in Fig. 9 (b)): 1) Cohesive force in gas phase is significantly higher than the force measured in liquid hydrocarbon phase under similar condition according to results reported in literature and this research (see Figs. 6 and 8), which implies that mechanisms dominate hydrate cohesion in gas and liquid phases are different. 2) The existence of a liquid film on the surface of a hydrate particle in the gas phase is thermodynamically favorable, which has been validated by theoretical analysis, direct observations and Raman spectroscopy results [29,34,35]. 2) The surface of hydrate was found to be hydrophilic in the gas phase by Hirata and Mori [36]. Therefore the contact angle of water on hydrate particle surface in the gas phase is smaller than 90°. 3) The cohesive force between hydrate particles in the gas phase has been proved to be dependent on the particle size, which is consistent with the capillary bridge theory that the volume of the liquid bridge is correlated with the particle size [20,26,37]. 4) The annealing time has no significant effect on cohesive forces between CH4/C2H6 hydrate particles in the gas phase, which suggests that a thermodynamically stable capillary bridge which exceeds the average asperity size may exist on the particle surface (see Fig. S3 in the supplementary information for details of hydrate particles with different annealing times in the gas phase) [32,38].
Acknowledgements The authors would like to acknowledge following foundations: China Postdoctoral Science Foundation (2019M652891, 2019TQ0302). Key Program of National Natural Science Foundation of China (21736005). SW would like to thank Dr. Carolyn A. Koh for her guidance and suggestions on micromechanical force measurements. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.fuel.2019.116201. References [1] Sloan ED, Koh CA. Clathrate hydrates of natural gases. 3 ed. Boca Raton: CRC Press; 2008. [2] Yang L, Liu Y, Zhang H, Xiao B, Guo X, Wei R, et al. The status of exploitation techniques of natural gas hydrate. Chin J Chem Eng 2019. [3] Kuang Y, Yang L, Li Q, Lv X, Li Y, Yu B, et al. Physical characteristic analysis of unconsolidated sediments containing gas hydrate recovered from the Shenhu Area of the South China sea. J Petrol Sci Eng 2019;181:106173. [4] Wang B, Fan Z, Wang P, Liu Y, Zhao J, Song Y. Analysis of depressurization mode on gas recovery from methane hydrate deposits and the concomitant ice generation. Appl Energ 2018;227:624–33. [5] Li X-S, Yang B, Zhang Y, Li G, Duan L-P, Wang Y, et al. Experimental investigation into gas production from methane hydrate in sediment by depressurization in a novel pilot-scale hydrate simulator. Appl Energ 2012;93:722–32. [6] Wang Y, Feng J-C, Li X-S, Zhang Y, Li G. Large scale experimental evaluation to methane hydrate dissociation below quadruple point in sandy sediment. Appl Energ 2016;162:372–81. [7] Wang Y, Feng J-C, Li X-S, Zhang Y, Chen Z-Y. Fluid flow mechanisms and heat transfer characteristics of gas recovery from gas-saturated and water-saturated hydrate reservoirs. Int J Heat Mass Transfer 2018;118:1115–27.
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agglomeration at high Watercut by new chemical formulation. Energ Fuel 2015;29(5):2901–5. Akhfash M, Aman ZM, Du J, Pickering PF, Johns ML, Koh CA, et al. Microscale detection of hydrate blockage onset in high-pressure gas-water systems. Energ Fuel 2017;31(5):4875–85. Brown E, Khan MN, Salmin D, Wells J, Wang S, Peters CJ, et al. Cyclopentane hydrate cohesion measurements and phase equilibrium predictions. J Nat Gas Sci Eng 2016;35:1435–40. Lievano D, Velankar S, McCarthy JJ. The rupture force of liquid bridges in two and three particle systems. Powder Technol 2017;313:18–26. Wang L, Su F, Xu H, Rong W, Xie H. Capillary bridges and capillary forces between two axisymmetric power–law particles. Particuology 2016;27:122–7. Dörmann M, Schmid H-J. Simulation of capillary bridges between particles. Procedia Eng 2015;102:14–23. Payam AF, Fathipour M. A capillary force model for interactions between two spheres. Particuology 2011;9(4):381–6. Aman ZM, Joshi SE, Sloan ED, Sum AK, Koh CA. Micromechanical cohesion force measurements to determine cyclopentane hydrate interfacial properties. J Colloid Interface Sci 2012;376(1):283–8. Brown EP, Hu S, Wells J, Wang X, Koh CA. Direct Measurements of Contact Angles on Cyclopentane Hydrates. Energ Fuel 2018;32(6):6619–26. Énglin BA, Platé AF, Tugolukov VM, Pryanishnikova MA. Solubility of water in individual hydrocarbons. Chem Technol Fuels Oils 1965;1(9):722–6. Butt H-J, Kappl M. Normal capillary forces. Adv Colloid Interface Sci 2009;146(1):48–60. Taylor CJ, Dieker LE, Miller KT, Koh CA, Sloan Jr ED. Micromechanical adhesion force measurements between tetrahydrofuran hydrate particles. J Colloid Interface Sci 2007;306(2):255–61. Maeda N. Is the Surface of Gas Hydrates Dry? Energies 2015;8(6):5361–9. Davies SR, Sloan ED, Sum AK, Koh CA. 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(2):1173–80. Hirata A, Mori YH. How liquids wet clathrate hydrates: some macroscopic observations. Chem Eng Sci 1998;53(14):2641–3. Dörmann M, Schmid H-J. Simulation of Capillary Bridges between Nanoscale Particles. Langmuir 2014;30(4):1055–62. Jones R, Pollock HM, Cleaver JAS, Hodges CS. Adhesion forces between glass and silicon surfaces in air studied by AFM: effects of relative humidity, particle size, roughness, and surface treatment. Langmuir 2002;18(21):8045–55.
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Full Length Article
Particle size dependence of clathrate hydrate particle cohesion in liquid/ gaseous hydrocarbons
T
Shenglong Wanga, Shuanshi Fana, , Xuemei Langa, Yanhong Wanga, Pengfei Wangb ⁎
a
Key Laboratory of Heat Transfer Enhancement and Energy Conservation of Education Ministry, School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510640, China b Academy for Advanced Interdisciplinary Studies, Southern University of Science and Technology, Shenzhen 518055, China
ARTICLE INFO
ABSTRACT
Keywords: Hydrate particle Cohesion Flow assurance Agglomeration
Clathrate hydrate particle cohesion dominates agglomeration of hydrate particles in gas/oil pipelines, and the existence of a capillary bridge between hydrate particles was assumed to be the reason of hydrate particle cohesion. This assumption implied the cohesive force between hydrate particles were radius-dependent but without any experimental validations. The cohesive force between cyclopentane (CP) hydrate particles in liquid CP phase and the cohesive force between CH4/C2H6 gas hydrate particles in the gas phase were measured respectively by using micromechanical force apparatus, and the effects of particle size and annealing time on hydrate particle cohesion in different phases were investigated. The cohesive force between hydrate particles was found independent of particle size in liquid CP phase but was linearly correlated with the effective radius of the hydrate particle pair in the gas phase. Long annealing time decreased cohesive force measured in liquid CP phase but had no significant effect on the cohesion of hydrate particle in the gas phase. A possible mechanism was therefore proposed to interpret hydrate particle cohesion in different phases: The cohesion of hydrate particles in liquid hydrocarbon was dominated by the sintering of asperities, but hydrate particle cohesion in gaseous hydrocarbons was still caused by the existence of capillary bridge.
1. Introduction Clathrate hydrates are crystalline solids formed by water and guest molecules under low-temperature and high-pressure conditions [1]. Natural gas hydrates bearing in deep-sea sediments or permafrost are known as potential fossil fuel resource with huge reserves [2–7]. However, in gas/oil pipelines, hydrate particles can form, agglomerate and adhere to the wall, which may cause blockages and other safety issues during gas/oil transportation [8,9]. The force between hydrate particles, also known as the cohesive force, has been considered to be one of the dominant parameters controlling hydrate agglomeration in pipelines [8]. In order to study the mechanism of hydrate particle agglomeration, the micromechanical force apparatus (MMF) was developed and used to directly measure cohesive forces between hydrate particles [10,11]. Cyclopentane (CP), tetrahydrofuran (THF), CH4, C2H6, and CO2 were used as hydrate formers, and effects of temperature, contact time, and the existence of additives on the cohesive force were investigated in previous publications [10,12–14]. It was inferred from experimental results and the analogy between ice and CP hydrate that the capillary force generated by a thin liquid
⁎
layer between hydrate particles (also known as a capillary bridge) caused hydrate particles to cohere [15,16]. This assumption implies a correlation between hydrate particle size and the cohesive force because the capillary force between two solid particles with the existence of a liquid bridge has been proved to be particle-size-dependent [17–20]. The average particle diameter mentioned in most of the previous publications is ~800 μm, which is approximately 1–2 orders of magnitude larger than the diameter of hydrate particles formed in gas/oil pipelines (1–100 μm) according to focus beam reflectance measurements (FBRM) [21–23]. This discrepancy suggests that a nondimensionalization method is needed to make cohesive forces measured under laboratory conditions quantitatively comparable to the cohesive force between hydrate particles in gas/oil pipelines. Whereas, the relationship between cohesive force and particle size of clathrate hydrate particles was rarely reported, which made the feasibility of nondimensionalization of hydrate particle cohesive force doubtful. Therefore, the particle size dependence of cohesive forces between hydrate particles needs to be specifically discussed, which can not only help to understand the hydrate particle cohesion mechanism but also clarify the reliability of cohesive force results measured in the laboratory as guidance for
Corresponding author. E-mail addresses: [email protected] (S. Wang), [email protected] (S. Fan).
https://doi.org/10.1016/j.fuel.2019.116201 Received 20 June 2019; Received in revised form 12 August 2019; Accepted 10 September 2019 0016-2361/ © 2019 Elsevier Ltd. All rights reserved.
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hydrate agglomeration simulation and prediction in pipelines. In this study, cohesive forces between CP hydrate particles and CH4/ C2H6 gas mixture hydrate particles with different sizes were measured, and effects of particle size and annealing time on hydrate particle cohesion in liquid and gaseous hydrocarbons were thoroughly discussed. A possible mechanism was proposed based on experimental results to interpret hydrate particle cohesion in liquid and gaseous hydrocarbons. 2. Materials and methods 2.1. Preparation of CP and CH4/C2H6 hydrate particles Fig. 1. Schematic illustration of cohesive force measurement procedures (adapted from Wang et al. [14]).
The cohesive force between hydrate particles was measured by using atmospheric- pressure and high-pressure MMF apparatus. The atmospheric-pressure and high-pressure apparatus both contain two cantilevers, one is stationary and the other is movable. The displacement of the moveable cantilever was controlled by a nano-manipulator with a minimum step of ~10 μm. The difference is that for the atmospheric-pressure setup, two cantilevers are open to the atmosphere of CP saturated air, but for the high-pressure setup, the two cantilevers are sealed in a pressurized cell to maintain the high-pressure condition for CH4/C2H6 hydrate formation. The configuration of the two systems is shown in Fig. S1 of the Supplementary Information. CP hydrate particles and CH4/C2H6 hydrate particles were both formed from pre-formed ice particles on the tips of two glass fibers. One glass fiber was fixed on the movable cantilever, and the other was fixed on the stationary cantilever. The ice particle was formed by quenching a deionized water droplet into liquid nitrogen for ~10 s. For the preparation of CP hydrate particles, the pre-formed ice particle was submerged in liquid CP (98% purity, Sigma-Aldrich, USA) at 3.0 °C, atmospheric pressure to form a CP hydrate particle. Once hydrate conversion started, the particle was annealed to ensure the hydrate shell was solid enough to withstand at least 40 pull-off tests, and this time interval was defined as the annealing time. The annealing time in this research was 30 min for CP hydrate. A typical CP hydrate pair formed following the aforementioned procedure is shown in Fig. S2. A gas mixture of 74.3 mol% CH4 & 25.7 mol% C2H6 (General Air, USA) was used to form gas hydrate particles. Ice particles were firstly formed from deionized water droplets to provide seeds for gas hydrate formation, which was similar to CP hydrate particle formation. After the ice particle was formed, it was transferred to a sealed cell. The CH4/ C2H6 gas mixture was then injected to pressurize the cell to 2.2 MPa, and the temperature was cooled down to 1 °C. The gas hydrate particle was also annealed for at least 30 min to ensure the hydrate shell strength. A typical gas hydrate particle pair formed following the aforementioned procedure is shown in Fig. S3. CP and CH4/C2H6 hydrate particles with different sizes were formed by controlling the size of the pre-formed ice particle. The size of the hydrate particle would be slightly different from the ice particle due to the morphology change during hydrate conversion. Therefore, the radius of a specific hydrate particle reported in this research was measured at the beginning of the cohesive force measurement.
2.2. Cohesive force measurement procedures The procedure of cohesive force measurement is shown in Fig. 1: (1) The particle fixed on the moveable cantilever is brought to contact with the particle fixed on the stationary cantilever; (2) A preload force is then applied on two particles with a displacement of D1 and a contact time of 10 s. D1 was set to ~20 μm for both CP hydrate and gas hydrate cohesion force measurements, and preload force distributions for CP hydrate particles and CH4/C2H6 hydrate particles are shown in Figs. S4 and S5 respectively; (3) The moveable cantilever is moved away from the stationary cantilever with a constant velocity until the particle pair broke apart; (4) The maximum displacement of the particle fixed on the stationary cantilever was measured as D2. The pull-off procedure was performed in the horizontal direction. Therefore the effect of gravity on the value of measured force was negligible. The whole process of force measurement was recorded by a CCD camera mounted on the microscope at 10 frames per second. The cohesive force between two particles was then calculated by Hooke’s law: (1)
F = KD2
where F is the measured cohesive force between particles, K is the spring constant of the glass fiber fixed on the stationary cantilever, and D2 is the maximum displacement of the tip of the glass fiber fixed on the stationary cantilever. D1 and D2 were determined by analyzing the recorded video with the aid of ImageJ. For each particle pair, the cohesive force was measured at least 40 times, and the average cohesive force from multiple measurements was reported as the result. The measured cohesive force was thought to be correlated with the particle size in previous publications [17,19,20]. Therefore the cohesive force should be nondimensionalized by the harmonic mean radius of the particle pair to eliminate the effect of particle size on the cohesive force, as shown in Eqs. (2) and (3): [14,24]
F =
F R
(2)
R =
2R1 R2 R1 + R2
(3)
where F is the directly measured cohesive force, F* is the dimensionless cohesive force; R1 is the radius of the particle fixed on the movable cantilever (the top particle in Fig. 1), R2 is the radius of the particle
Table 1 Parameters of different particles. Hydrate former
Temperature (°C)
Subcoolinga (°C)
Pressure (MPa)
R* (μm)
F* (mN/m)
F (μN)
rb
CP CH4/C2H6
3.0 1.0
5.6 5.8
0.1 2.2
134–578 298–552
2.42–14.7 25.0–44.1
1.24–2.47 7.94–26.7
−0.149 0.971
a
[1]. b
Subcooling is the difference between phase equilibrium temperature and cell temperature. Hydrate phase equilibrium temperature was calculated by CSMGem r is the Pearson’s correlation coefficient of R* and F. 2
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fixed on the stationary cantilever (the bottom particle in Fig. 1); R* is the effective radius of two particles. The uncertainty boundary of F is defined as the 95% confidence interval of the t-distribution of results collected from at least 40 independent pull-off tests, and error bars of data reported stand for the 95% confidence interval. In this study, to clarify the relationship between the cohesive force and the hydrate particle size, both F and F* were presented as results of cohesive force measurements for comparison. Formation conditions and other detailed parameters of hydrate particles used in this study are shown in Table 1. 3. Results and discussion 3.1. Particle size dependence of CP hydrate cohesion in liquid CP phase To investigate the relationship between the effective radius R* and cohesive force F, 27 pairs of CP hydrate particles with R* ranging from 134 μm to 578 μm were formed at 3 °C with a 30 min annealing time, and the cohesive force was measured. Cohesive force F is shown in Fig. 2 as a function of the effective radius R*. It is shown that the measured cohesive force fluctuates around ~1.8 μN, and the minimum and maximum forces are 1.24 μN and 2.47 μN respectively. Pearson’s correlation coefficient (PCC) r for F and R* equals to −0.149 considering the error (as shown in Table 1), which indicated no apparent linear relationship between the cohesive force F and the effective radius R*. This result indicated that the cohesive force between CP hydrate particles was nearly constant when particle size was the only variable considering the experimental error.
Fig. 3. F* of CP hydrate particles in liquid CP phase measured in this study (solid squares) and the value reported in literature (grid area).
not applicable for the quantification of the cohesive force between hydrate particles in liquid CP, especially when the particle is relatively small (radius < 325 μm). Data reported in Figs. 2 and 3 use effective radius R* to characterize the size of a specific CP hydrate particle pair. Whereas, two CP hydrate particles in one test were usually unequal sized. Thus the effect of the size difference of particles in each particle pair on the measured cohesive force was also investigated. The particle size difference in one pair was defined by the ratio of R1 to R2. Cohesive force F as a function of R1/R2 is shown in Fig. 4. 22 out of 27 CP hydrate particle pairs had a close R1/R2 ratio around 1.0. For comparison, 5 out of 27 particle pairs had a relatively significant difference in particle size (R1/R2 ratio > 1.5), which was manipulated intentionally during particle preparation. As shown in the Figure, the variation of particle size ratio has no significant influence on the measured cohesive force. However, in the modeling of capillary force between to unequal sized particles presented by Chen et al., the capillary force significantly depending on the particle size difference, and the cohesive force was supposed to increase when R1/R2 was close to 1 [17].
Fig. 2. The cohesive force between CP hydrate particles in liquid CP phase as a function of the effective radius.
To compare the cohesive force F measured in this study with reported values in the literature, F was nondimensionalized by the effective radius R* as F*. F* were compared with the widely cited dimensionless cohesive force of CP hydrate particle [16]: 4.3 ± 0.4 mN/ m, which was measured at 3.2 °C with an annealing time of 30 min (as shown in Fig. 3). It is shown that the dimensionless cohesive force F* decreases with the increasing effective radius R*, and F* measured in this study is close to the reported value when the effective radius of CP hydrate particle pair was between a narrow range of 325 μm–475 μm. When the effective radius is close to 150 μm, the cohesive force can be ~3 times higher than the value reported in the literature. Although the radius of CP hydrate particle used in literature was not given, it was mentioned that the minimum particle radius was between 300 μm and 500 μm. This result indicates that the nondimensionalization method is
Fig. 4. The cohesive force F between CP hydrate particles in liquid CP phase as a function of particle radius ratio R1/R2. 3
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It can be concluded from data presented in Figs. 2, 3, and 4 that the cohesive force between CP hydrate particles is not relevant to either the particle size or the particle size difference. This may lead to several new understandings of MMF results of CP hydrate particles: 1) The dimensionless cohesive force F* is not a precise enough parameter to characterize the cohesion mechanism of CP hydrate particles of different sizes; 2) The cohesive force of hydrate particles formed in pipelines with the existence of oil phase could be significantly underestimated by using nondimensionalization method when taken 4.3 mN/ m as the reference, because hydrate particles formed in pipeline conditions (diameter: ~10 μm) are much smaller than CP hydrate particles (diameter: ~800 μm) formed in MMF tests, but hydrate particle cohesive forces in the two scenarios can be reasonably close; 3) Capillary force or the existence of a liquid bridge between particles is probably not the dominant mechanism of hydrate particle cohesion in liquid hydrocarbon phase, which is because in theoretical models describing
3.2. Particle size dependence of CH4/C2H6 gas hydrate particle cohesion in the gas phase 74.3 mol% CH4 & 25.7 mol% C2H6 gas mixture was used to form sII hydrate particles, and the cohesive force between gas hydrate particles of different sizes was measured. The effective radius of gas hydrate particle pairs was between 298 μm and 552 μm, and the measured cohesive force F as a function of R* was shown in Fig. 5 (a). PCC is calculated to test the correlation between F and R*, and the value equals to 0.971 including the error (as shown in Table 1). This result indicates a significant linear correlation between F and R* for gas hydrate particles in contrast to CP hydrate results (r = −0.149). The dimensionless cohesive force F* between gas hydrate particles is plotted in Fig. 5 (b) as a function of R*. The data suggest that the nondimensionalization method is necessary when comparing cohesive forces between hydrate particles in the gaseous hydrocarbon phase.
Fig. 5. The cohesive force F (a) and the dimensionless cohesive force F* (b) between CH4/C2H6 hydrate particles in the gas phase as a function of the effective radius R*
capillary force between two spheres proposed by different researchers, particle radius is always one of the controlling parameters of the cohesive force [17,25–28]. The possible mechanism of CP hydrate particle cohesion in liquid CP phase was considered to be the sintering of asperities, which was discussed in Section 3.4.
It can be inferred from results presented previously that the cohesive force between CH4/C2H6 hydrate particles in the gas phase is possibly dominated by the capillary bridge mechanism. To verify this hypothesis, the cohesive force of CH4/C2H6 hydrate particles measured in this research (see Fig. 6) were compared with cohesive forces of CP hydrate measured in CPsaturated N2 (gas phase) and liquid CP at 3 °C in literature [16,29]. It needs
Fig. 6. The cohesive force between hydrate particles measured in gas and liquid phases. 4
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to be clarified that the cohesive force between CP hydrate particles was reported in the dimensionless form F* with a radius range of ~400 μm in literature. Therefore, to avoid the influence of particle-size difference on the comparison, dimensionless cohesive forces of hydrate particle pairs with similar R* (as close to 400 μm as possible) measured in this research were presented. As shown in the Figure, the cohesive force of CP hydrate particles in the gas phase is close to the cohesive force of CH4/C2H6 hydrate particles in the gas phase considering the error. Additionally, as reported in literature, a liquid layer on the surface of CP hydrate particle was observed during the measurement of cohesive force after leaving the particle in the gas phase for 465 s [29]. These results suggest that cohesion mechanisms of CP hydrate particles and CH4/C2H6 hydrate particles in the gas phase are similar, and are possibly both dominated by the capillary force generated by the liquid bridge between particles. In contrast, the cohesive force between hydrate particles measured in liquid CP phase is far lower than the cohesive force measured in the gas phase, which suggests that hydrate particle cohesion in liquid phases is possible dominated by a different mechanism instead of the capillary bridge theory. It can also be assumed that the discrepancy between CP hydrate particle cohesion and CH4/ C2H6 hydrate particle cohesion in the aspect of particle size dependence is due to the different phases where the cohesive force is measured.
Cohesive forces between CH4/C2H6 hydrate particles with annealing times between 30 min and 1380 min are shown in Fig. 8. For comparison, cohesive forces of CH4/C2H6 hydrate measured at 1 °C, 3.45 MPa in liquid hydrocarbon with different annealing times by Hu et al. is also cited in Fig. 8 [13]. It is shown that the cohesive force of gas hydrate decreases with the increasing annealing time in liquid hydrocarbons, which is similar to the cohesive force of CP hydrate in liquid CP phase. In contrast, the cohesive force of gas hydrate measured in gas phase shows no significant correlation with the change of annealing time. Moreover, when the annealing time is increased to ~1300 min, the cohesive force measured in the gas phase remains at ~30 mN/m, but the cohesive force measured in the liquid hydrocarbon phase is only ~4.7 mN/m, which is quite comparable with the cohesive force between CP hydrate particles in liquid CP phase measured in literature [16] and this research (nondimensionalized with R* of ~400 μm) considering the similar driving force and particle size.
3.3. Effect of particle size on the cohesive force between hydrate particles with different annealing times The annealing time is a parameter defined as the time interval between the beginning of hydrate conversion observed on the water droplet and the beginning of cohesive force measurement. The annealing time is usually used to estimate the conversion of hydrate particle when the driving force of hydrate formation (subcooling, pressure, etc.) is constant. To further understand the mechanism of clathrate hydrate particle cohesion, and investigate the possible reason for the discrepancy of particle size dependence between CP hydrate particles and CH4/C2H6 hydrate particles, the effect of annealing time on cohesive forces was studied. Cohesive forces of 6 pairs of CP hydrate particles with annealing time varied from 30 min to 150 min were measured as shown in Fig. 7. It is shown that long annealing time causes the cohesive force to decrease, especially when comparing the cohesive force measured at 150 min with the cohesive force measured at 30 min. Additionally, Cohesive force is still independent of particle size when the annealing time is longer than 30 min. It is also shown that the deviation of cohesive forces between particles with different radii decreases with the increasing annealing time, which suggests the surface condition of CP hydrate particles becomes stable (better repeatability for force measurements) with relatively long annealing time.
Fig. 8. The cohesive force between CH4/C2H6 hydrate particles in the gas phase and liquid hydrocarbon phase with different annealing times.
It can be concluded from results shown in Figs. 7 and 8 that the annealing time has significant effects on the cohesion of hydrate particles in liquid hydrocarbon phase but not in the gas phase, which suggests that mechanisms of hydrate particle cohesion in liquid and gaseous hydrocarbons are possibly different. 3.4. Possible mechanisms for hydrate particle cohesion in liquid and gaseous hydrocarbons The cohesive force between CP hydrate particles in liquid CP phase has been proved to be uncorrelated with the particle size. In contrast, the cohesive force between CH4/C2H6 hydrate particles has been found to be linearly correlated with the effective particle radius in the gas phase. Therefore, a possible mechanism is proposed to interpret hydrate particle cohesion in liquid and gaseous hydrocarbons. The cohesion of hydrate particles in liquid CP phase is supposed to be dominated by hydrate sintering between asperities on the particle surface as shown in Fig. 9 (a). Reasons are listed as follows: 1) the contact angle of water on CP hydrate surface in liquid CP phase is close to 90° (92.5° ± 8.5° at 0 °C to7.7 °C, 1 atm. measured by Brown et al. [30]), and the solubility of water in cyclopentane is very low (0.0046 wt% at 0 °C, 1 atm. measured by Englin et al. [31]). Even if there is unconverted water or pre-melted water on the surface of hydrate particles from the thermodynamic point of view, a liquid bridge is unlikely to form between particles due to the very small curvature of meniscus according to the Young–Laplace equation [32]. However, this
Fig. 7. The cohesive force between CP hydrate particles in liquid CP phase with different annealing times. 5
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Based on the proposed hypothesis of hydrate particle cohesion mechanisms, the reason for the discrepancy of the particle size dependence of hydrate particle cohesion in liquid and gaseous hydrocarbons can be explained. Moreover, it indicates that when applying cohesive force results measured in the laboratory into theoretical modeling of hydrate agglomeration or the evaluation of hydrate inhibitor, the validity of nondimensionalization should be considered, especially for the scenario of hydrate particle cohesion in liquid hydrocarbons. 4. Conclusions Cohesive forces between clathrate hydrate particles in liquid and gaseous hydrocarbons were measured by using micromechanical force apparatus, and effects of particle size and annealing time on cohesive forces were discussed. It has been found that CP hydrate particle cohesive forces in liquid CP phase were independent of the effective radius of particle pairs, but cohesive forces between CH4/C2H6 hydrate particles were linearly correlated with the particle effective radius in the gas phase. This result suggested that the nondimensionalization was not always feasible when applying values of directly measured cohesive force into the simulation of hydrate agglomeration in gas/oil pipelines or the evaluation of hydrate inhibitors. A hypothesis for possible mechanisms dominating hydrate particle cohesion in different phases was proposed based on experimental observations and results. It was considered that the cohesion of hydrate particle in liquid hydrocarbon phase was due to the sintering of asperities on the particle surface, and the cohesion of hydrate particle in gaseous hydrocarbon was dominated by the capillary bridge formed between particles. This hypothesis explained experimental discrepancies in particle size and annealing time dependence between hydrate particle cohesion in liquid CP phase and CH4/C2H6 gas phase. Results presented in this research provide insights on differences and connections between cohesive force measured under laboratory conditions and hydrate particle agglomeration in gas/oil pipelines.
Fig. 9. Illustrations of possible mechanisms dominate hydrate particle cohesion in liquid and gas hydrocarbons: (a) Sintering of asperities dominates hydrate particle cohesion in liquid CP; (b) Capillary bridge dominates hydrate cohesion in CH4/C2H6.
trace amount of water on the surface of the particle may promote hydrate reformation (sintering) upon contact. 2) It has been indicated by different researchers that long contact time would lead to high cohesive forces between hydrate particles in liquid hydrocarbons, which was possibly due to the sintering of hydrate particles [13,16,33]. Therefore, it can be assumed that cohesive forces between hydrate particles are mainly related to the dimension of asperities involved in sintering (surface roughness) instead of the size of the particle, which explains the reason why cohesive forces measured between CP hydrate particles in liquid CP phase showed no dependence on particle sizes. 3) It has been found that with the increase of annealing time, the surface of hydrate particle became rougher (images of CP hydrate particles in liquid CP phase with annealing times between 30 min and 150 min are shown in Fig. S2 of the supplementary information), and the cohesive force decreased. This result suggests that for hydrate particles with a relatively long annealing time (rough surface), fewer asperities (only outmost ones) are involved in the sintering process, and a low cohesive force is expected. For the cohesion of CH4/C2H6 hydrate particles in the gas phase, it has been found that the cohesive force F was linearly correlated with the effective particle radius R*, and longer annealing time had no significant effect on the force. Therefore, it is supposed that the cohesion of hydrate particles in gaseous hydrocarbon is dominated by the capillary bridge between particles with the following reasons (as shown in Fig. 9 (b)): 1) Cohesive force in gas phase is significantly higher than the force measured in liquid hydrocarbon phase under similar condition according to results reported in literature and this research (see Figs. 6 and 8), which implies that mechanisms dominate hydrate cohesion in gas and liquid phases are different. 2) The existence of a liquid film on the surface of a hydrate particle in the gas phase is thermodynamically favorable, which has been validated by theoretical analysis, direct observations and Raman spectroscopy results [29,34,35]. 2) The surface of hydrate was found to be hydrophilic in the gas phase by Hirata and Mori [36]. Therefore the contact angle of water on hydrate particle surface in the gas phase is smaller than 90°. 3) The cohesive force between hydrate particles in the gas phase has been proved to be dependent on the particle size, which is consistent with the capillary bridge theory that the volume of the liquid bridge is correlated with the particle size [20,26,37]. 4) The annealing time has no significant effect on cohesive forces between CH4/C2H6 hydrate particles in the gas phase, which suggests that a thermodynamically stable capillary bridge which exceeds the average asperity size may exist on the particle surface (see Fig. S3 in the supplementary information for details of hydrate particles with different annealing times in the gas phase) [32,38].
Acknowledgements The authors would like to acknowledge following foundations: China Postdoctoral Science Foundation (2019M652891, 2019TQ0302). Key Program of National Natural Science Foundation of China (21736005). SW would like to thank Dr. Carolyn A. Koh for her guidance and suggestions on micromechanical force measurements. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.fuel.2019.116201. References [1] Sloan ED, Koh CA. Clathrate hydrates of natural gases. 3 ed. Boca Raton: CRC Press; 2008. [2] Yang L, Liu Y, Zhang H, Xiao B, Guo X, Wei R, et al. The status of exploitation techniques of natural gas hydrate. Chin J Chem Eng 2019. [3] Kuang Y, Yang L, Li Q, Lv X, Li Y, Yu B, et al. Physical characteristic analysis of unconsolidated sediments containing gas hydrate recovered from the Shenhu Area of the South China sea. J Petrol Sci Eng 2019;181:106173. [4] Wang B, Fan Z, Wang P, Liu Y, Zhao J, Song Y. Analysis of depressurization mode on gas recovery from methane hydrate deposits and the concomitant ice generation. Appl Energ 2018;227:624–33. [5] Li X-S, Yang B, Zhang Y, Li G, Duan L-P, Wang Y, et al. Experimental investigation into gas production from methane hydrate in sediment by depressurization in a novel pilot-scale hydrate simulator. Appl Energ 2012;93:722–32. [6] Wang Y, Feng J-C, Li X-S, Zhang Y, Li G. Large scale experimental evaluation to methane hydrate dissociation below quadruple point in sandy sediment. Appl Energ 2016;162:372–81. [7] Wang Y, Feng J-C, Li X-S, Zhang Y, Chen Z-Y. Fluid flow mechanisms and heat transfer characteristics of gas recovery from gas-saturated and water-saturated hydrate reservoirs. Int J Heat Mass Transfer 2018;118:1115–27.
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