Crystal Growth of Clathrate Hydrate in a Flowing Liquid Water System

Article pubs.acs.org/crystal

Crystal Growth of Clathrate Hydrate in a Flowing Liquid Water System with Methane Gas Muhammad Aifaa, Takehide Kodama, and Ryo Ohmura* Department of Mechanical Engineering, Keio University, Yokohama 223-8522, Japan ABSTRACT: This paper reports the visual observation of the growth and formation of clathrate hydrate crystals in a stream of flowing liquid water presaturated with methane gas. The experimental apparatus used in this study was devised by inserting a porous pipe inside the reactor, thereby allowing methane hydrate crystals to form and grow in the liquid water stream. The morphology of methane hydrate crystals grown in a liquid water stream varied depending on the system subcooling temperature, ΔTsub. ΔTsub is defined as the difference between the equilibrium temperature of the hydrate, Teq, and the system temperature, Tex. The crystal growth in the bulk of liquid water was observed when ΔTsub > 4.5 K. When 4.5 K < ΔTsub < 9.0 K, polygonal flat plate crystals were observed growing from the porous pipe surface. At 9.0−10.0 K, the crystal morphology enters the transition phase, where the polygonal flat plate crystals started to change into dendritic crystals. When the subcooling temperature, ΔTsub, is greater than 10.0 K, only dendritic crystals were seen to grow into the liquid water. On the basis of these observations, a continuous supply of the guest substance will enhance the growth of methane hydrate as the particle size of the crystals formed in this study was larger than that of the crystals observed in the previous study.

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INTRODUCTION Clathrate hydrates are crystalline water-based solid compounds resembling ice crystals, in which molecules called “guests” are trapped inside cages of hydrogen-bonded water molecules. Guest substances that can form clathrate hydrates are mainly hydrocarbons and noble gases. Clathrate hydrates have several unique properties such as high gas storage capacities, guest substance selectivities, and large formation/dissociation heats. Hence, hydrate-based technologies have been proposed by utilizing these properties. Typical examples of those technologies are transportation and storage of natural gas or hydrogen,1,2 development of highly efficient heat pump/ refrigeration systems,3 etc. The necessity of clathrate hydrate research is undeniable, especially in the energy and environmental fields to avoid problems concerning hydrates such as clogging of oil or gas pipelines caused by hydrate formation.4 Thus, our understanding of the mechanistic nature of clathrate hydrates is important not only for dealing with industrial issues but also for developing the technologies mentioned above. This study of the crystal morphology of the hydrate crystals that focuses on the crystal growth behavior and mechanism of clathrate hydrate formation based on visual observation. The crystal morphology term used in this study is defined as the geometric configuration of the crystal such as crystal size and shape. Natural gas hydrates have attracted the attention of researchers and industries around the world as natural gas hydrates can be a promising future energy resource5 and also can be used for transportation and storage of natural gas. For the efficient exploitation of natural gas from naturally occurring hydrate-bearing sediments at the sea bottom without causing any geohazards, the mechanical properties of the hydrate© XXXX American Chemical Society

bearing sediments must be evaluated carefully. The growth behavior of the hydrates formed in the pore spaces between sediment particles should depend on the crystal morphology that later affects the mechanical properties of the hydratebearing sediments.5,6 Meanwhile, Japanese industrial groups have demonstrated the transportation and storage of natural gas in the form of hydrate pellets,7 which would improve the stability of natural gas hydrates and thus increase the transportation and storage efficiency. They have recently completed the construction and test operation of a pilot plant8 and will be ready to commercialize their projects in the near future. The pellet production process mainly consisted of hydrate production, dehydration, and pelletization. The slurry hydrate that is generated in the production stage is injected into the dehydrator where the hydrate particles consolidate and form a porous bed at the drainage screen of the dehydrator.9 The water passed through the screen leaving the powdered hydrate, which is later compressed in the pelletizer into pillow-shaped pellets. During dehydration, the slurry conveyance and dehydrating efficiency would be improved with an increased size of hydrate particles. Accordingly, our understanding of the crystal morphology of the hydrate is vital and necessary for better plant design, resulting in more products and a lower cost. It is generally known that hydrate crystals preferentially form at the gas−liquid interface; that is, a polycrystalline thin film of hydrate would form, intervening between the guest gas and liquid water phases. After the hydrate film completely had Received: July 3, 2014 Revised: December 24, 2014

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DOI: 10.1021/cg500992c Cryst. Growth Des. XXXX, XXX, XXX−XXX

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covered the interface, hydrate crystals would then grow into the liquid water presaturated with guest substances. Several studies of the crystal growth of hydrate at the gas−liquid water interface10−14 have been reported, including reports on systems with a kinetic inhibitor15 and seawater.16 Hence, it seems that a comprehensive understanding of the morphology of hydrates at the gas−liquid interface has been developed in recent years. Nevertheless, our knowledge on the morphology of hydrate crystals grown into liquid water is limited as there are only a few previous studies and reports on that field. Smelik and King17 have reported a study of the morphology of single hydrate crystals. Using a pressurized optical cell, they observed that several cubic forms of single methane (CH4) hydrate crystals were formed, the most common being the rhombic dodecahedron and in some cases the trapezohedron. They also suggested that the construction of [512] cages would be a limiting factor for hydrate crystal surface growth and the growth rate. Ohmura et al.18,19 have conducted studies on the macroscopic morphology of the hydrate crystals grown in liquid water presaturated with a guest substance. They reported that the crystal morphologies of the single CH4 and CO2 hydrates tend to depend on the thermodynamic conditions (pressure and subcooling temperature). Via observation in a methane/liquid water system, polygon-shaped crystals of CH4 hydrate were observed when the driving force was low but the crystals became dendritic as the driving force increased. They also presented an analysis using a dimensionless parameter that is configured by mass transfer of the guest substance dissolved in liquid water to predict the crystal morphology of single gas hydrates grown in liquid water. The observations of CH4 hydrate crystal growth that was systematized using the dimensionless index agreed with the prediction made by Ohmura et al.19 on the basis of the results previously reported for the system with CO2. As reviewed above, Ohmura et al.18,19 have shown that the strength of the driving force for the diffusive mass transfer of the guest molecules could be a determining factor for the morphology of hydrate crystals grown in liquid water presaturated with guest substances. However, our understanding of the morphology of hydrate crystals grown into liquid water is inadequate because of the limited number of studies. Further study in this field could greatly contribute to the development of hydrate-based technologies as well as to our understanding of the properties of hydrate-bearing sediments. As mentioned above, producing larger hydrate crystals would increase the dewatering efficiency, which is also relevant for the longer storage of hydrate pellets. In addition, the abundant amounts of guest substances, mainly methane, that were continuously supplied to the hydrate growth’s site due to the flow of water and concentration gradient needed to be taken into consideration when evaluating the naturally occurring hydrates at the sea bottom,20 yet there has been no study of the morphology of hydrate crystals grown in a system with a continuous flow of liquid water and of how the mass transfer of guest molecules would affect the crystal morphology. Hence, we decided to conduct a study of the formation and growth of hydrate crystals in a flowing liquid water system presaturated with methane gas. The main objective of this study was to investigate the effects of driving force of the mass transfer of the guest molecules on the hydrate crystal growth, and the observed results of hydrate crystal morphology were systematized using a dimensionless index expressing the driving force for mass transfer.

Article

EXPERIMENTAL APPARATUS AND PROCEDURE

The sample fluids used in the experiment to form a hydrate were methane [certified purity of 99.9 vol % (Sumitomo Seika Chemicals, Co., Ltd.)] and liquid water, which was distilled and deionized before use using a laboratory water distiller (Yamato Scientific Co., Ltd., model WG222). The techniques used to purify the liquid water were the ion exchange process and distillation. The experimental setup that was used for the hydrate crystal growth study is presented schematically in Figure 1, while Figure 2 is a cross

Figure 1. Schematic diagram of the experimental apparatus.

Figure 2. Cross section of the hydrate formation reactor. section of the hydrate formation reactor. The experimental setup mainly consisted of a 206 cm3 capacity dissolution bath that is equipped with a stirring device, a plunger pump, and a hydrate formation reactor. These parts were connected to form a cycle as shown in Figure 1, and a methane-saturated aqueous solution is circulated in this cycle. The hydrate formation reactor was made of a stainless steel cylinder with a pair of flange-type glass windows. The inner space of the reactor was 25 mm in diameter and 20 mm in axial length. The temperature inside the reactor, Tex, was controlled by B

DOI: 10.1021/cg500992c Cryst. Growth Des. XXXX, XXX, XXX−XXX

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barrier to prevent further contact of the bulk fluid phase, which also prevents the further supply of the guest substance from the free gas phase. Hydrate crystals grew in the liquid water phase afterward. In this system, these crystals were seen to grow in two patterns. One of them was crystal growth directly from the porous pipe surface. Most of the previous observations of hydrate crystal growth in liquid water showed that the crystals grew from the film into the liquid water bulk. However, the hydrate crystal in this system started to grow from the surface of the porous pipe because the pipe created a gas−liquid interface as shown in Figure 4, which is a favorable condition

circulating a temperature-controlled ethylene glycol solution through a brass jacket covering the reactor while the pressure inside the reactor, P, was controlled by supplying the sample gas from a gas cylinder through a pressure-regulating valve. A porous pipe was inserted inside the reactor, and the sample gas was injected into the pipe to enhance hydrate crystal nucleation. The purpose of the porous pipe was to form the hydrate and maintain the growth of crystal in flowing water. The reactor was the test section where the hydrate is formed, and observation was conducted. A thermocouple (ϕ1.0 mm, Ichimura Metal Co., Ltd.) was inserted inside the porous pipe, and Tex was measured with an uncertainty of ±0.2 K. Meanwhile, P was measured by a strain-gauge pressure transducer with an uncertainty of ±0.05 MPa. The temperature and pressure inside the dissolution bath were also measured using these instruments. The necessary quantity of deionized water was injected into the system (∼220 cm3), and then methane gas was injected in excess. The air in the experiment system was replaced with methane gas by repeating the pressurization of the system with methane and evacuating it from the system. P was then set at a prescribed level. The stirrer in the dissolution bath was turned on, and the mixture in the system was maintained under constant conditions for 1−2 days to allow the methane gas to saturate into the water. The temperature in the dissolution bath was set 1−2 K higher than Teq, the triple methane/hydrate/liquid water equilibrium temperature corresponding to P, while the temperature in the reactor, Tex, was first set to ∼264 K to form hydrate (and also ice) and then increased to a temperature 3− 4 K higher than Teq. These steps were taken, first, to prevent hydrate formation in the dissolution bath and, second, to exert the memory effect on the mixture in the reactor as it can shorten the induction time for hydrate re-formation. After the dissociation of the formed hydrate was visually confirmed, Tex was then reduced to a temperature lower than Teq by approximately 4−12 K to observe hydrate crystal formation and growth in the reactor. The plunger pump was started up when hydrate formation in the reactor was visually confirmed, and the aqueous saturated methane solution was circulated throughout the system with a flow rate of ∼1.5 cm3/min. The formation and growth of hydrate crystals were observed and recorded every minute using a CMOS camera (Fortissimo Corp., model CMOS130-USB2) and a microscope (Edmund Optics Co., Ltd.). The system subcooling temperature, ΔTsub, is defined as the index of the driving force for the crystal growth that is used throughout this study. ΔTsub is the difference between system temperature Tex and equilibrium temperature Teq that corresponds to the prescribed pressure P. Teq was calculated with CSMGem.21

Figure 4. Gas−liquid interface formed by the porous pipe.

for the growth of hydrate crystals. Floating crystals also formed in the liquid water bulk, which grew after the crystals attached to the pipe or to the nearby crystals. Hydrate crystal formation and growth in this system were observed as the occurrence of both patterns in most of the experimental runs, regardless of the pressure and system subcooling temperature. Figure 5 shows a series of hydrate formation and growth data observed in a methane/liquid water system at several subcooling temperatures (ΔTsub values of 6.4, 9.1, and 10.4

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RESULTS AND DISCUSSION Figure 3 shows the general growth behavior of methane hydrate crystals in flowing liquid water. Hydrate crystals initially were nucleated at the gas−liquid interface and eventually formed a thin layer of hydrate intervening between the gas−liquid interfaces. The solid hydrate film at the interface then acts as a

Figure 3. General process of the hydrate crystal growth behavior observed with a methane/flowing water system. Water flows from the right.

Figure 5. Difference in hydrate crystal growth that depends on ΔTsub. C

DOI: 10.1021/cg500992c Cryst. Growth Des. XXXX, XXX, XXX−XXX

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Figure 6. Arrangement of hydrate crystal observations systemized with Mcr. The images in the top part were reproduced with permission from ref 25. Copyright 2014 John Wiley & Sons Ltd.

K) at 7.5 ± 0.1 MPa. We observed that hydrate crystals that formed at the gas−liquid water interface grew into a thin hydrate film covering the interface within several seconds (subjected to subcooling temperature variations). At a ΔTsub of 6.4 K, a few minutes after complete coverage had been achieved, the micrometer size of hydrate crystals started to germinate (Figure 5a). After 10 h, the crystals were seen to grow into a polygon-like solid toward the liquid water bulk. Nineteen hours after the initial hydrate formation, the crystals had grown to 1.1−1.2 mm in axial length. At a ΔTsub of 9.1 K, the growth of dendritic crystals was observed after ∼4 h (Figure 5b). At the initial stage, polygonal flat plate crystals were seen growing from the porous pipe surface. Those polygonal crystals continued to grow for several hours. After 3 h, dendritic-like crystals started to bud from the tip of the polygonal plate, and eventually, only dendritic crystals were seen growing in the axial direction into the liquid water bulk. Subsequently, a change in morphology from polygonal to dendrites was observed. The size of the crystal was 0.3−0.7 mm on a polygonal side and 1.0−1.5 mm in axial length. When the subcooling temperature was increased to 10.4 K, polygonal hydrate crystals were no longer observed growing into liquid water bulk as they were completely replaced by dendritic crystals (Figure 5c). The main branch of the crystals grew in the axial direction to approximately 1.1−2.6 mm. At the same time, the growth of crystals perpendicular to the main branch, approximately 0.2− 0.5 mm, was also observed. The crystal growth of methane hydrate presented above supposedly depends on the subcooling temperature as the morphology of crystals varied as the subcooling temperature increased. The crystal shape changed from polygonal to dendritic with an increase in ΔTsub. However, analysis of hydrate crystal growth must carefully account for not only heat but also mass transfer limitations. The relation of crystal morphology and driving force can also be clarified using the concept of mass transfer in water. In this study, the flow of an

aqueous saturated methane solution to the test cell is kept constant, which means that the hydrate crystals grow in a steady flow environment. Therefore, we can readily assume that the mass transfer of the guest substance from the water phase to the hydrate growth site should be promoted because of the steady convection, leading to an enhancement of hydrate crystal growth. The observations of hydrate crystal growth based on the mass transfer mechanism were previously presented by Ohmura et al.22 for hydrate crystal growth in an HCFC-141b/ water system and a methane/water system. They reported that mass transfer occurred because of the difference in methane concentration in the liquid water equilibrium with methane hydrate and in liquid water bulk. They arranged the observations using a nondimensional index, Mcr, which represents the mass transfer of the guest substance as the driving force instead of subcooling temperature. The nondimensional index for the mass transfer-controlled hydrate crystal growth mentioned above was reported in detail by Ohmura et al.18 The Mcr index can be defined as follows: n[xgs̃ (Tpri) − xgs(h) ̃ (Tex )] ≡ Mcr

(1)

where n is the hydration number, x̃gs is the solubility, expressed as the mole fraction of the guest substance in liquid water at Tpri, and Tpri is the temperature at which the guest gas, liquid water, and hydrate phases are in equilibrium. The values of the Mcr index corresponding to these experiments were calculated by referring to the experimental data for x̃gs and x̃gs(h).23,24 The hydrate crystal morphologies observed in this study and in the system reported by Ohmura et al.19 are summarized in Figure 6 by plotting the Mcr index on the horizontal axis. In our study, methane hydrate crystal growth was not observed when the Mcr index was smaller than 0.0027. When the Mcr index is in the range of 0.0027−0.0055, polygon-shaped crystals were observed. The Mcr range from 0.0055 to 0.0060 is the range in which polygonal crystals started to change to dendrites, but D

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when Mcr > 0.0060, only dendritic crystals were seen to grow into the liquid bulk. From the observations made by Ohmura et al.18,19 for carbon dioxide and methane hydrate, the dendritic crystals started to grow when the Mcr index was ∼0.09 or above, whereas the dendritic crystals were observed in a methane/steady water flow system when Mcr > 0.0055. The definition of Mcr reported by Ohmura et al.18 (eq 1) was derived without considering the forced convection, and the mass transfer coefficient is thought to be simply proportional to the diffusion coefficient. However, the mass transfer coefficient will be large enough to affect the crystal growth rate in the presence of a liquid flow. The rate of mass transfer of the guest substance dissolved in liquid water to the crystal surfaces affects the crystal’s volumetric growth rate, which later influenced the crystal morphology of hydrate crystals. The volumetric growth rate per unit crystal (v̇h) may be expressed as vḣ ∝ hm,g n[xgs̃ (Tpri) − xgs(h) ̃ (Tex )]

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This study was supported by Keirin-Racing-based research promotion funds from the JKA Foundation and by JPS KAKENHI Grant 25289045. We express our gratitude to Mr. K. Matsushita (HBM Co., Hokkaido, Japan) for his help in the experimental work.

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(2)

where hm,g is the mass transfer coefficient for the guest substance. From eq 2, the relationship among the volumetric growth rate, mass transfer coefficient, and Mcr can be expressed as follows. vḣ ∝ hm,g Mcr

(3)

From this, the volumetric growth rate is proportional to the product of mass transfer coefficient and Mcr. In this study, hm,g obviously became larger because of the steady flow environment, thus resulting in the growth of dendritic crystals even when the Mcr index is small. Besides, the size of the hydrate crystals grown in this study noticeably increased compare to the size of the hydrate crystals previously observed in the quiescent system.

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CONCLUSIONS Visual observations of clathrate hydrate crystal formation and growth in flowing liquid water presaturated with methane gas were conducted at a ΔTsub of 4.0−12.0 K and 7.5 ± 0.1 MPa. The morphology of methane hydrate crystals changed from polygonal to dendritic with an increase in ΔTsub. We also present the final result of observations systemized with dimensionless parameter Mcr, an index that is relevant to the driving force for hydrate crystal growth. As a result, we noticed that the magnitude of Mcr in this study is different from those of other studies. This is a result of the continuous supply of the guest substance that promoted the growth of hydrate crystals. Besides, the hydrate crystals observed in this study showed the same habit that was observed in a previous study; crystal morphology changed with an increase in Mcr. However, crystals grown in flowing liquid water seem to be larger than those in the quiescent system. The knowledge obtained in this study provides new insight into the process design of the production of hydrate for natural gas storage and transportation and the analyses of the recovered naturally occurring natural gas hydrate samples.

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REFERENCES

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AUTHOR INFORMATION

Corresponding Author

*Department of Mechanical Engineering, Keio University, 314-1 Hiyoshi, Kohoku-ku, Yokohama 223-8522, Japan. Phone: +81-45-566-1813. E-mail: [email protected]. E

DOI: 10.1021/cg500992c Cryst. Growth Des. XXXX, XXX, XXX−XXX