Article Cite This: Energy Fuels 2019, 33, 5081−5090
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Effect of Wax Crystals on Nucleation during Gas Hydrate Formation Dongxu Zhang,† Qiyu Huang,*,† Haimin Zheng,†,‡ Wei Wang,† Xianwen Cheng,† Rongbin Li,† and Weidong Li† †
Beijing Key Laboratory of Urban Oil and Gas Distribution Technology, China University of Petroleum, Beijing 102249, P. R. China CNOOC Research Institute Co., Ltd., Beijing 100027, P. R. China
‡
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S Supporting Information *
ABSTRACT: Hydrate formation and wax deposition pose great flow assurance challenges to subsea oil pipes, especially when the two phenomena co-occur. Wax crystals can have a significant impact on hydrate nucleation and growth kinetics, but this phenomenon has not been studied in great detail. Here, the effect of wax crystals on hydrate nucleation was investigated using both molecular dynamics simulation methods and experiments conducted using a custom-designed high-pressure autoclave equipped with an on-line viscometer. Both the simulation and the experimental results demonstrated that the presence of wax crystals inhibits hydrate nucleation. The simulations showed that water droplets tend to approach and adsorb on wax crystals prior to nucleation, thus inhibiting the formation of hydrate cages. The experiments demonstrated that water cut and stirring rate play a significant role in determining the hydrate nucleation rate. In addition, adding more wax increased the viscosity of the emulsion, which limits mass transfer of gas to the oil−water interface. and gas mass transfer. Sloan et al.19 monitored the process of water droplets converting to hydrates in water-in-oil emulsions and found that every water droplet acted as an individual reactor and supported a hydrate shell formation model. A number of studies have been performed to understand the characteristics of wax deposition in water-oil two-phase flows using either flow loop or cold finger apparatus.21−27 Effects of the droplet size and distribution of the dispersed phase and water cut on wax deposition in water-in-oil emulsions have been studied by Zhang et al.21 Zheng et al.24 characterized the droplet size in the deposited and bulk phases using NMR techniques and found that the incorporation of droplets led to slough-off of the deposit in the pipe flow. Bruno28 proposed that dispersion of water droplets in the emulsion reduced the wax diffusion coefficient and made the diffusion path more tortuous due to the incompatibility of the two phases. Couto and Bruno also proposed oil−water wax deposition models.22,23 In most of the studies in the field till date, hydrate formation and wax deposition have been investigated independently, effectively being treated as unrelated problems. Only a select few studies investigating the effects of wax formation on the hydrate phase boundary have presented thermodynamic models combining wax precipitation with hydrate formation.29,30 In particular, Mahabadian’s30 work analyzed the component effects of wax precipitation on hydrate formation and found that the amount of heavy alkanes in the liquid decreased after the wax separated out, promoting hydrate formation by leaving more light hydrate-forming components in the mixture. However, the effect of wax crystals on hydrate formation from a kinetic standpoint is still short of
1. INTRODUCTION Gas hydrates are solid crystalline compounds wherein water molecules form cage-like structures around guest gas molecules at low temperatures and high pressures.1,2 Gas hydrates have been extensively studied in the oil and gas industry as their formation in transmission lines may result in oil pipeline blockage and economic hazards.3−6 Another concern in the oil and gas industry is a phenomenon known as wax deposition,7 wherein wax molecules dissolved in crude oil can crystallize out and deposit on the inner pipe wall below the wax appearance temperature (WAT). With the exploration and development of oil fields moving into even deeper waters, multiphase pipelines containing water, oil, and gas are widely used to deliver petroleum fluids from offshore to onshore processing facilities. The fluids are usually present in the form of water-in-oil emulsions in the multiphase pipelines because of the presence of natural surfactants and low water cut (i.e., the volume fraction of water in the liquid). However, deep water multiphase pipelines at high pressures and low temperatures can provide favorable conditions for gas hydrate formation and wax deposition. The mechanisms of hydrate formation and wax deposition in water-in-oil emulsions have been widely studied, which has greatly improved our scientific understanding of these phenomena. One of the methods commonly used to study hydrate formation includes the use of differential scanning calorimetry (DSC), autoclaves, and high-pressure flow loops.8−12 A series of studies on the rheology of hydrate slurries formed from emulsions have been also reported in the literature.13−17 In addition, several notable studies have focused on hydrate particle formation kinetics and mechanisms.18−20 Li et al.18 found that the rate of hydrate formation increased with increasing total water droplet surface area and established a mathematical model of hydrate formation kinetics in water-in-oil emulsions based on the crystal growth theory © 2019 American Chemical Society
Received: March 18, 2019 Revised: May 20, 2019 Published: May 22, 2019 5081
DOI: 10.1021/acs.energyfuels.9b00815 Energy Fuels 2019, 33, 5081−5090
Article
Energy & Fuels investigation. Gao31 investigated the interactions between gas hydrates and wax precipitation using a hydrate rocking cell apparatus and concluded that hydrate particle formation could assist in the precipitation and subsequent deposition of wax out of the solution. However, in most real-world scenarios in deep water multiphase pipelines, the WAT is above the hydrate formation temperature. Therefore, it is of key importance to study the effect of wax crystals on hydrate formation and investigate the kinetics of the process. Molecular dynamics (MD) simulations are a helpful and efficient tool for obtaining a deep understanding of hydrate formation processes and mechanisms at the molecular level.32,33 Information on the kinetic and structural properties of hydrate systems may be more easily accessible through simulations rather than experimental techniques as the required conditions of high pressure and low temperature are challenging to recreate in a laboratory setting. Jacobson et al.,34,35 along with Vatamanu and Kusalik,36 proposed a twostep nucleation mechanism based on MD simulations, where amorphous hydrate-like solids are formed first as intermediates and later grow or transform into crystalline hydrates. Bai et al.37 successfully employed MD simulations of kinetic pathways in the process of CO2 hydrate formation on the silica surface and proposed a three-step nucleation process. Recently, Liang and Kusalik32 conducted MD studies of hydrate nucleation near the silica surface with the results indicating that nucleation can initiate from the surfaces of model hydroxylated silica. Zhang et al.38 found that guest’s hydration shell was more ordered, and that the system entropy was lowered with increasing guest concentration, causing the system structure to tend toward the solid phase. However, in simulations, the nucleation of CH4 or CO2 hydrates can take quite long even under strong driving forces.39,40 It has been shown that H2S can nucleate more easily than other natural gas components,41 and in addition, CH4 and H2S have similar molecular diameters (4.36 and 4.58 Å, respectively) and are both thought to form structure I (sI) hydrates.42 As a result of this, the present study used H2S hydrates to investigate the effect of wax on gas hydrate nucleation in the MD simulations. In the present study, the effect of precipitated wax crystals on the nucleation process during hydrate formation in waterin-oil emulsions was investigated by means of MD simulations and experiments. In the simulation work, three systems were considered, wherein the initial distance between the water droplet and the wax crystal was varied. In the experimental work, hydrate formation was carried out in water-in-oil emulsions with different wax contents. A high-pressure autoclave with an on-line viscometer was employed for this purpose. Furthermore, the hydrate nucleation rates in water-inwaxy oil emulsions with various water cuts and various stirring rates were also investigated.
initial configuration setup is shown in Figure 1. The LennardJones sites were arranged in an fcc [111] structure and placed
Figure 1. Initial simulation structure of the systems with wax. The yellow points represent H2S molecules, blue and white balls represent oxygen and hydrogen atoms, and pink balls represent wax molecules, respectively.
at specific locations away from the spherical water droplet. Five-layer wax molecules are distributed symmetrically on the intermediate layer, as shown in pink in Figure 1. As the figure shows, three simulation systems of water droplets were established, considering of droplets (1) far away from, (2) near, and (3) on the wax crystal surface; these are referred to in the text as system-1, system-2, and system-3, respectively. It can be seen that the spherical water droplet in system-1 is in the middle of the box in the initial state, and the lengths a and b are equal, but in system-3, a ≫ b, such that b is effectively zero. In addition, a system without wax was built as a contrast, and the initial structure of which is shown in Figure 2.
Figure 2. Initial simulation structure of the system without wax. The yellow points represent H2S molecules, and pink and white balls represent oxygen and hydrogen atoms, respectively.
Parameters of these systems, the potential equation, and the values of the water and H2S models are presented in the simulation detail section of the Supporting Information included with this work. The MD package Gromacs47 was used for MD simulations, with Lorentz−Berthelot mixing rules48 used for the interactions between different molecules. A cut off of 1.0 nm was utilized for van der Waals forces whereas the long-ranged coulomb forces were calculated using particle-mesh Ewald.49 The truncation radius of adjacent atomic search was set as 1.0 nm. MD simulations were performed at the NPT ensemble,50,51 wherein constant pressure and temperature were maintained using the Parrinello−Rahman and Nose−Hoover algorithms, respectively. The hydrate formation process simulations run and the simulations of water droplet trajectory before nucleation were performed at 220 and 250 K, respectively, with all runs performed at a pressure of 50 MPa. The segment times used in this work were 50 ns in the
2. SIMULATION METHODOLOGY In the present study, the TIP4P/2005 model43 was employed for calculating the intermolecular potential of water. This model has been widely used in molecular simulations of hydrate formation and is suitable over the temperature range from 123 to 573 K.44,45 For H2S molecules, a four-site potential model put forward by Forester et al.46 was chosen, which has been demonstrated to describe the properties of H2S well in both the solid and the bulk liquid phases. The solidphase wax crystal composed of Lennard-Jones interaction sites was added at the boundary of the system. A schematic of the 5082
DOI: 10.1021/acs.energyfuels.9b00815 Energy Fuels 2019, 33, 5081−5090
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Supporting Information. More details of the apparatus can be found elsewhere.52 3.2. Materials. The continuous oil phase used throughout the experiments was −10# diesel, provided by SINOPEC filling station (China). All water used in the experiments was de-ionized using a water purification machine, and the CO2 gas used was 99.99% pure (Shaanxi Hongwei Gas Technology Co., Ltd., China). Wax (Fushun Petrochemical Company, China) employed in the experiments was a paraffin mixture from C17 to C35. The carbon number distribution of the wax is listed in Table S2 of the Supporting Information. 3.3. Wax Dissolution and Emulsion Preparation. The first step in the experiments was dissolving the wax in diesel, with the resulting mixture referred to as waxy oil. A certain mass of wax was weighed and added into diesel, then agitated at 500 rpm in a 60 °C temperature-controlled water bath for 15 min to ensure that the wax was completely dissolved. Once mixing was complete, the WAT of the waxy oil was measured using DSC.53 Following this, the water-in-oil emulsion was prepared by agitating a mixture of waxy oil and deionized water at 60 °C using an IKARW 20 Digital stirrer at 800 rpm for 30 min. The sample container was thoroughly sealed to prevent evaporation of volatile hydrocarbons during the stirring process. 3.4. Hydrate Formation. Prior to the experiments, the autoclave was cleaned using petroleum ether and de-ionized water, and the cleaning procedure was repeated twice.54 The temperature of the autoclave was set to the predetermined value and, once stabilized, the prepared emulsion was poured into the autoclave. Following this, the autoclave was sealed and any air remaining inside was removed by displacement with the experimental gas (CO2). Gas was injected while stirring the tank until the target pressure was reached. The autoclave was kept under pressure for a specified time period55 and then allowed to cool. The start of the cooling process was defined as the time-zero for the hydrate formation experiment. During the cooling process, gas was added to the autoclave at regular intervals to maintain the pressure at the desired value until the target temperature was reached. The nucleation point was identified as the moment when the temperature spiked suddenly because hydrate formation is exothermic reaction.55,56
simulation runs of the trajectory of the spherical water droplet and 600 ns for the hydrate formation process, with a time step of 2 fs. The initial systems were equilibrated for 10 000 ps, following which five independent simulation runs (denoted as A to E) were performed that were extracted from the last 200 ps of the equilibrium trajectory.
3. EXPERIMENTAL SECTION 3.1. Apparatus. A schematic diagram of the experimental apparatus employed in this work is shown in Figure 3. The apparatus
Figure 3. Schematic drawing of the high-pressure autoclave apparatus. mainly consisted of an autoclave, a pressure control system, two temperature control systems, a gas flowmeter, a stirring device, an online viscometer, and a data logger. The autoclave was constructed from 316 stainless steel with an effective volume of 4.5 L and is rated for pressures up to 15 MPa. It was connected to a CO2 gas tank via a pressure-regulated valve. A gas flowmeter (with a range of 0−1000 mL/min) was installed at the autoclave inlet to measure gas consumption. Two temperature control systems (thermostatic baths) were used to control the autoclave temperature: one on the outside and one on the cold finger in the middle of the autoclave. The temperature range of both thermostatic baths was −20 to 100 °C. During the experimental process, the pressure and temperature in the autoclave were displayed and recorded using a pressure transducer and a thermal resistance detector, respectively. The contents of the autoclave were stirred by a magnetically driven anchor agitator on the lid of the autoclave with a range of 0−800 rpm. An on-line viscometer (VISCOpro 2000, Cambridge, USA) was used in the autoclave, with a range of 0.1−1000 cP and a pressure rating up to 12 MPa. The operating principles of the viscometer are introduced in the
4. RESULTS AND DISCUSSION 4.1. Effect of Wax on Nucleation of H2S Hydrate. To deconvolute the effect of wax crystals on gas hydrate nucleation, simulations of the hydrate cage formation rate and the trajectory of the water droplet before nucleation in the presence of wax were performed using Gromacs. The degree of liquid water into hydrate conversion obtained from the simulations was reflected by the evolution of the total number of different cages,57 on account of temperature invariability in the NPT ensemble. The various hydrate cage types (i.e., 512, 51262, 51263, 51264, 4151062, 51268, and 435663) were identified by
Figure 4. Evolution of seven cage types in one simulation run: (a) run D in the system without wax; (b) run A in the system with the water droplet far away from the wax crystal surface. 5083
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Figure 5. Evolution of the total number of cages over the five runs in: (a) the system with the water droplet far away from the wax crystal surface; (b) the system with the droplet near the wax crystal surface; (c) the system with the droplet on the wax crystal surface; and (d) the system without wax. (e) Average time requirement for different total numbers of cages over the five runs in the three systems with wax. All the runs were performed at 220 K, 50 MPa.
were conducted at the same scenarios for investigation, which were widely used in MD simulations.6,41 The time evolution of the total number of cages indicates that the wax crystal has a significant hindering effect on hydrate cage formation. For the systems with wax, shown in Figure 5a−c, the hydrate cage formation is fastest in system-1 and slowest in system-3. This is quantified in Figure 5e, which shows that the average times over five simulation runs for the 10 cages are 276, 287, and 387 ns in systems-1, -2, and -3, respectively. Hence, with the distance between the droplet and wax crystal decreasing, the rate of cage formation decreased. On the other hand, the average time requirement for 10 cages is 74 ns in the system without wax (Figure 5d), which is 202 ns less than that of system-1. Moreover, Figure 5d shows that the maximum time requirement for 50 cages is less than 150 ns in the system without wax, which is far less than that for the systems with wax. In the wax-free system, no sticking of the water droplet with the surface of wax, and the whole surface acts as a hydrate nucleation space; hence, the hydrate cages formed faster. In the presence of wax, however, the wax crystal may impact the trajectory of the water droplet and adsorb on the droplet surface, hence decreasing the cage formation rate and hindering the nucleation process. Furthermore, the time needed for cage formation appeared to be positively correlated to the distance between the water droplet and the wax. In actual deep water pipelines, the consequence of this is that in water-in-oil emulsions with higher wax content, droplets are closer to wax crystal surfaces as more crystals are precipitated, and the chances of contact and adsorption increase accordingly; hence, the rate of hydrate nucleation may be much slower than that of wax-free condition.
the topological structure of hydrogen bonds between water molecules.58 Water molecules were considered to be hydrogenbonded when the distance between the corresponding oxygen atoms was less than 0.35 nm (i.e., Roo < 0.35 nm) and the angle between the two oxygen atoms and the hydrogen atom was less than 30° (i.e., ∠OOH < 30°).59 Figure 4 shows typical curves for the evolution of cage types over one simulation run in the system without wax (Figure 4a) and in system-1 (Figure 4b). As the Figure 4a shows, 512 cages have the highest number of cages, followed by the 51262 cages, which are in accordance with the results of Lauricella et al.60 As we know, H2S is observed mainly to form sI hydrates under moderate pressures, where the guests occupy both the 512 and 51262 cages.1 Recently, it has been demonstrated that H2S hydrate can form various types including cages of crystalline sI, structure II (sII), and HS-I hydrates, as well as other irregular cages in NPT simulations,6,41 wherein sI and sII are the two dominants. Both sI and sII crystals have 512 cages. In addition, previous have found that typically mall 512 cages dominate the system in the initial stages.6,71 Therefore, 512 cages are first in abundance. The next in abundance, large 51262 cages, reflects the thermodynamic preference for sI. Similar to the evolution of various cage types without wax, the 512 cages and 51262 cages are observed to dominate in the system-1 (Figure 4b), which indicates that wax has little effect on cage types and distribution proportion in hydrate formation. 4.1.1. Effect of Wax Crystal on the Time Evolution of the Total Number of Cages. Figure 5 shows the time evolution of the total number of cages. It should be noted that the evolution in each simulation run appeared somewhat different as the molecular motion is a stochastic process within certain limits. In order to confirm the reproducibility and to achieve estimates of average value, five independent simulation runs 5084
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Energy & Fuels 4.1.2. Trajectory of Water Droplet. To gain a deeper understanding of the effects of wax on hydrate nucleation, simulations of the droplet movement trajectory before nucleation in the three systems with wax were performed. The probability distribution of the water droplet in the zdirection at different times was then calculated according to the values over the five independent simulation runs, with the location of the droplet represented by its center of mass. The origin of the coordinate system was chosen as the center of mass of the spherical water droplet in system-1. As shown in Figure 6, the probability distribution of the water droplet
available due to wax occupation. In real-life deep water multiphase pipelines, a mass of wax crystals is dispersed around the water droplets at temperatures below the WAT. In such case, water droplets and wax crystal particles can easily adsorb on each other in water-in-waxy oil emulsions and move together in the bulk, especially in the high wax content conditions. As a result, the wax crystals can impact the trajectory of water droplets and adsorb on the water surface, thus inhibiting hydrate nucleation. 4.2. Effect of Wax on Nucleation of CO2 Hydrates. To obtain more concrete evidence of the impact of wax crystals on hydrate nucleation, gas hydrate formation experiments in water-in-waxy oil emulsions with various wax contents were conducted using the high-pressure autoclave apparatus described in section 3.1. CO2 was selected as the gas phase for use in the experiments due to operational safety considerations and considering its increasing application in enhanced oil recovery in the oil and gas industry.61 In addition, CO2 is a common component in natural gas,62 with CH4 and CO2 both being known to form structure I hydrates. WAT of the samples was measured using DSC (Auto Q20, TA Instruments, USA). During testing, the reference and sample were heated to 60 °C63 and then cooled to −20 °C at a rate of 5 °C/min. The relationship between heat flow and temperature was recorded to determine the WAT (Figure 7a).64,65 The measured WATs in these experiment were all above 10 °C, which are higher than the hydrate formation temperature ( Swater−oil,30% > Swater−oil,20% > Swater−oil,10% (4)
where the number in the subscript refers to the water cut. All of the emulsions were prepared with the same stirring rate and total stirring time. Thus, the energy provided by the stirring process to disperse water droplets was equal for all of the emulsions. Hence, the size of water droplets increased somewhat with higher water cut, as less energy was available per water droplet. The average water droplet diameters were 30.4, 37.7, 48.8, and 58.7 μm in emulsions with 10, 20, 30, and
(3)
where Seff refers to the available water−oil interfacial area for hydrate nucleation and Swater−oil refers to the total water−oil interfacial area. The total water−oil interfacial area increases with water cut as follows 5087
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Figure 14. (a) Temperature variation during the hydrate formation process at different stirring rates for an emulsion with 20 vol % water cut and 7 wt % wax at the target conditions of 4 °C and 2.3 MPa; (b) schematic drawing of the effect of the stirring rate on wax particles in the emulsion.
and may even strengthen the motion and collision of the two. However, higher stirring rates may cause the crystals to separate from the water−oil interface, thus increasing Seff and promoting hydrate nucleation.
40 vol % water cut, respectively. As a result, the increase in the available water−oil interfacial area, which determines the hydration rate, was relatively small compared to the increase in water cut. In water-in-waxy oil emulsions, the effect of water cut on the hydrate nucleation rate was larger than that in emulsions without wax, as Figure 13 shows. Here, hydrates were formed at 380, 636, and 1072 min in emulsions with 40, 30, and 20 vol % water cut, respectively. For the emulsion with 10% water cut, hydrates did not form in the period of 1500 min. The discrepancy between the two sets of experiments (without and with wax) is due to the wax crystals being adsorbed or encased on the water−oil surface, thus reducing the available water−oil interfacial area for nucleation. Here, the relationship between the effective and real water−oil interfacial area is given by eq 5 Seff = Swater − oil − Swax
5. CONCLUSIONS MD simulations and experiments were performed to investigate the effect of wax crystals on hydrate nucleation in water-in-waxy oil emulsions. The simulation results showed that wax crystals limit the number of hydrate cages in nucleation of H2S. In addition, the experimental results indicated that the hydrate nucleation rate of CO2 decreases with increasing wax content. Thus, the inhibition hydrate nucleation by wax crystals was observed in both simulations and experiments. As shown in the simulations, water droplets tended to approach and adsorb on wax crystals before nucleation, which is thought to be the primary mechanism behind the inhibition effect, as this decreases the available surface area for hydrate nucleation and lowers the probability of cross nucleation. Moreover, wax crystals encased at the water−oil interface from a diffusive boundary layer, thereby limiting the mass transfer of gas molecules to the water droplet surface. It was also observed that increasing the amount of wax in the emulsion increased its viscosity, thus further lowering the mass transfer rate. The experimental results from this study also showed that in emulsions both with and without wax the hydrate nucleation rate increased with increasing water cut; however, this effect was more pronounced in the emulsions with wax. In addition, a threshold water cut was identified below which hydrates cannot form because of the limited interfacial surface area for wax crystal adsorption and encasement. Finally, it was also found that increasing the stirring rate enhanced hydrate nucleation. Future studies will include experiments with field wax deposits and crude oil and will further investigate the effect of wax on the hydrate growth stage and formation rate.
(5)
where Swax refers to the surface encased or occupied by wax crystals. For waxy emulsions with 10% water cut, hydrates did not form in the experimental time of 1500 min, which indicated that the effective water−oil interfacial area was almost zero in this case. In these experiments, it was shown that in the presence of wax, the minimum water cut required for hydrate formation is larger than that for emulsions without wax. In addition, the effect of water cut on hydrate nucleation is much more pronounced than that for wax-free emulsions. 4.4. Stirring Rate Effect on Hydrate Nucleation in the Presence of Wax. Temperature−time curves during the hydrate formation process at different stirring rates in the presence of wax are shown in Figure 14a. As the figure shows, the hydrate nucleation rate increased with an increase in stirring rate. One reason for this is that the higher stirring rate increases the degree of dispersion of the water in the emulsion, providing more nucleation sites. Moreover, at higher stirring rates, the larger water−oil interfacial area and higher shear force also promote cross nucleation, and the increased gas dissolution and dispersion can enhance the mass transfer rate and further promote hydrate nucleation. In addition, in water-in-waxy oil emulsions, stirring intensity can impact the distribution of wax crystals in the bulk, as these have a tendency to distribute on the water surface after precipitation and move together with droplets, as illustrated in Figure 14b. When the stirring rate is low, the shear force is not large enough to separate wax crystals from the water droplets
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.energyfuels.9b00815. Specific details of the simulation systems, schematics and parameters of the TIP/2005 water model and H2S 5088
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model, carbon number distribution of wax, the measured WAT with various wax contents in waxy oils, and the operation principles of the on-line viscometer (PDF)
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Dongxu Zhang: 0000-0003-4240-636X Wei Wang: 0000-0001-9885-3092 Rongbin Li: 0000-0003-4584-9799 Weidong Li: 0000-0002-3145-4190 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by grants from the National Natural Science Foundation of China (nos 51534007 and 51374224), which are gratefully acknowledged.
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