Intensification of methane and hydrogen storage in clathrate hydrate and future prospect

Journal of Natural Gas Chemistry 19(2010)203–209

Review

Intensification of methane and hydrogen storage in clathrate hydrate and future prospect Xuemei Lang∗ , Shuanshi Fan,

Yanhong Wang

School of Chemistry and Chemical Engineering, The Key Lab of Enhanced Heat Transfer and Energy Conservation, Ministry of Education, South China University of Technology, 510640 Guangzhou, Guangdong, China [ Manuscript received January 26, 2010; revised March 25, 2010 ]

Abstract

Dr. Xuemei Lang was born in 1968. She received the M. Eng. Degree in environmental chemical engineering and the D. Eng. Degree in materials science from South China University of Technology in 1993 and 2005. Now she works in the South China University of Technology and focuses on the clathrate hydrate and hydrogen storage technology.

Gas hydrate is a new technology for energy gas (methane/hydrogen) storage due to its large capacity of gas storage and safe. But industrial application of hydrate storage process was hindered by some problems. For methane, the main problems are low formation rate and storage capacity, which can be solved by strengthening mass and heat transfer, such as adding additives, stirring, bubbling, etc. One kind of additives can change the equilibrium curve to reduce the formation pressure of methane hydrate, and the other kind of additives is surfactant, which can form micelle with water and increase the interface of water-gas. Dry water has the similar effects on the methane hydrate as surfactant. Additionally, stirring, bubbling, and spraying can increase formation rate and storage capacity due to mass transfer strengthened. Inserting internal or external heat exchange also can improve formation rate because of good heat transfer. For hydrogen, the main difficulties are very high pressure for hydrate formed. Tetrahydrofuran (THF), tetrabutylammonium bromide (TBAB) and tetrabutylammonium fluoride (TBAF) have been proved to be able to decrease the hydrogen hydrate formation pressure significantly. Key words clathrate hydrate; methane; hydrogen; formation rate; storage capacity

1. Introduction

Dr. Shuanshi Fan was born in 1965. He received the M. Eng. and the D. Eng. Degrees in chemical engineering from Dalian University of Technology in 1993 and 1996. He finished Postdoctoral research at China University of Petroleum in 1998. Then he worked for 7 years as a Research Fellow in Guangzhou Institute of Energy, Chinese Academy of Sciences. He became a professor and PhD supervisor of China University of Technology in 2001; He worked as a visiting scholar at Keio University in Japan and Colorado School of Mines in United States, in 2002 and 2005. Now he is a Chair Professor of South China University of Technology and the Director of the Key Laboratory of Enhanced Heat Transfer & Energy Conservation, Ministry of Education. Since 1998 he has been engaged in the research work of clathrate hydrate and has now focused on the nature gas hydrate preparation/inhibition, energy storage materials and technology, energy-saving airconditioning technology.

Gas hydrates belong to a general class of inclusion compounds commonly known as clathrates, which are crystalline solids composed of water and gas [1]. The gas molecules (guests) are trapped in water cavities (host) that are composed of hydrogen-bonded water molecules. Typical natural gas molecules include methane, ethane, propane, and carbon dioxide. It is generally acknowledged that clathrate hydrates have properties for large capacity gas storage, fractionation of guest mixtures, and a high heat of formation/decomposition. ∗

Dr. Yanhong Wang was born in 1979. She received the M. Eng. and the D. Eng. Degrees in chemical engineering from Tianjin University in 2005 and 2008. Now she works in the South China University of Technology and focused on the clathrate hydrate.

Corresponding author. Tel: +86-20-22236581; Fax: +86-20-22236581; E-mail: [email protected] This work was supported by the National 863 Program (2007AA03Z229) and the Fundamental Research Funds for the Central Universities (2009ZM0185).

Copyright©2010, Dalian Institute of Chemical Physics, Chinese Academy of Sciences. All rights reserved. doi:10.1016/S1003-9953(09)60079-7

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Gas hydrate formation depends on four factors: gas, water, temperature and pressure. Under appropriate thermodynamic conditions (usually high pressure and/or low temperature), water will form a stable hydrogen bonded lattice consisting of cages around the small molecules (gas molecules). Methane and hydrogen are two kinds of clean energy. But as energy density and boiling point of two gases are very low, it is difficult to store and transport, which limits the methane usage and development of hydrogen energy. Thus, safe and economic method for methane storage and transport should be studied and found.

hexane, might be introduced to methane/water system in order to lower the hydrate formation pressure without reducing the storage capacity of gas to a certain extent in the case of sH hydrate. The pressure to form a sH hydrate is usually 30%∼80% of that of the sI methane hydrate which depends on the nature of liquid hydrocarbons used as an additive. Additives also can improve the hydrate formation rate and storage capacity. These additives contain salts, surfactants and liquid hydrocarbons.

2. Methane storage in hydrate Methane hydrate is crystalline inclusion compounds of water and methane. Methane hydrate has high storage capacity, namely 180 volumes methane can be stored in per volume methane hydrate under standard conditions (180 V/V). And methane hydrate can be maintained at about −20 ◦ C and atmospheric pressure [2]. However, industrial applications of hydrate storage processes have been hindered by some problems, such as slow formation rates, unreacted interstitial water as a large percentage of the hydrate mass, reliability of hydrate storage capacity and economy of process scale-up. There are two approaches to solve these problems: by chemical and mechanical means. 2.1. Chemical means Gas hydrate formation is generally divided into two consecutive steps. The first step involves the nuclei formation and the second step is growth process. Gas dissolution, formation of nuclei and growth of new nucleus are mainly affected by pressure and temperature conditions of the system. Additionally, gas hydrate formation is also affected by mass transfer and heat transfer. Increasing the water-gas interfacial area and strengthening heat exchange can improve hydrate formation. And the key of storage capacity is increasing the cage occupancies of hydrate and reducing liquid water in hydrate. The phase equilibrium curve of methane hydrate formation is shown in Figure 1. Upon the equilibrium (lower temperature and higher pressure than formation equilibrium curve), the hydrate can form. And if the system deviated from the curve farther, the formation rate is higher. Generally, the additives were often used to change the phase equilibrium conditions of methane hydrate. Some additives such as methanol, ethylene glycol, methycyclohexane can make the equilibrium curve moving to left, which was often used as the hydrate inhibitor in industrial pipeline [3]. Other additives such as tetrahydrofuran (THF) are added to the water/methane system, and a sII hydrate may form at a pressure that is 30%, or even less, of the pressure required for the formation of sI methane hydrate. However, the methane storage capacity in the sII hydrate is to be less than 60% of that of the sI methane hydrate. Mostly liquid hydrocarbons of large molecule such as 1,3-dimethylcyclohexane and methylcyclo-

Figure 1. Methane hydrate formation phase equilibrium curve

Guo et al. [4] investigated the calcium hypochlorite use for methane hydrate additives, and proved that it has two evident effects on methane hydrate formation. Firstly, it reduces the degree of supercooling. Secondly, calcium hypochlorite helps most of the water to form hydrate in relatively short time under moderate conditions. Methane molecules occupy 79% of the large and small cavities in the lattices, and the most storage capacity can reach 163 V/V. Zhang et al. studied the influence of potassium oxalate monohydrate (POM) as additives on hydrate formation rate at 4.5 MPa and 275.4 K without agitation [5]. When POM concentration was 1000 ppm, the storage capacity of natural gas hydrate (NGH) was as much as 137.2 V/V. Surfactant is another kind of additives used as promoter for methane hydrate formation, which can change the emulsifiability and surface tension of solution, including nonionic, anionic and cationic surfactant. Cationic surfactant has a little effect on the hydrate formation. When the surfactant concentration reaches a critical micelle concentration (CMC) and surfactant molecules associate as micelles, the presence of micelles containing solubilized hydrocarbon gas could account for the observed phenomenon of subsurface hydrate formation, which was previously considered to occur only at the bulk water-gas interface (Figure 2). Fan and his co-workers studied non-ionic surfactant alkylpolyglucoside (APG), dodecyl polysaccharide glycoside (DPG); anionic surfactant sodium dodecyl sulfate (SDS), sodium dodecyl benzene sulfonate (SDBS) [5], and their mixture as promoter of methane hydrate in a quiescent system [5−10]. All results showed that these surfactants can enhance hydrate storage capacity and reduce the processing time of hydrate formation.

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2.2. Mechanical means

Figure 2. Principle of micellar surfactant to increase the gas hydrate formation rate

The effect of nonionic surfactant (DPG) on hydrate storage capacity is less pronounced compared to that of anionic surfactant (SDS, SDBS). The influence of SDBS on hydrate formation rate is more than that of APG and SDS. The mixed surfactants of SDS and DPG could reduce the time of hydrate formation, but also reduced the hydrate storage capacity at the same time compared to the surfactant SDS.

The key of surfactant improving the gas hydrate formation rate is that surfactant molecules associate as micelles, which enclosed the gas and water in micelles and increase the water-gas interface. In present, a new method, dry water, was introduced for methane hydrate formation. Dry water is a free-flowing powder prepared by mixing water, hydrophobic silica particles and air at high speeds. Dry water is a water-inair inverse foam consisting of water droplets surrounded by a network of hydrophobic fumed silica which prevents droplets coalescence (Figure 3). Using dry water store methane, the storage capacity can reach 175 V/V, and methane hydrate formation rate can be increased [12]. Water dispersed by silica increased the water-gas interface and made the bulk water react with methane to form hydrate. But the dry water reused after methane hydrate dissociation, the storage capacity and kinetics degraded significantly.

Figure 3. Conceptual illustration of dry water and result of methane storage [12]

Besides mass transfer, heat transfer also affects the hydrate formation rate. However, surfactant or dry water only strengthen the mass transfer through dispersed water, while heat transfer of system is not modified. Base on the dry water concept, Fan et al. proposed the concepts of frozen dry wa-

ter (Figure 4). The frozen dry water, combining the dry water with ice particles, can greatly promote the hydrate formation since the former can enhance the mass transfer and the later strengthen the heat exchange. Furthermore, frozen dry water can combine with surfactant (large-molecular guest substance,

Figure 4. The concept of new dry water

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LMGS), and nano-copper powder to form new frozen dry water. Dry water and LMGS both disperse water and strengthen the mass transfer, while nano-copper powder and ice particles can enhance thermal conductivity and heat exchange of new dry water. The gas hydrate formation reaction is an interfacial phenomenon, and the rates of hydrate formation have been shown to be inversely proportional to the thickness of the hydrate zone and the surface area of the growing methane hydrate particles. Besides above methods (the use of high pressures, surfactants, micron-sized water particles), vigorous mechanical mixing is another method for increasing clathrate formation kinetics. Stirring is the most common method using in methane hydrate formation to improve mass transfer performance and

heat transfer performance of the methane hydration process. Induction time was shortened effectively and storage capacity was increased when stirring was applied to the methane hydrate process. Furthermore, hydrate formation is influenced by stirring velocity (Vs ) and time (ts ) (Figure 5). Hao et al. found that an appropriate stirring velocity and stirring time were 320 rpm and 30 min, respectively, under the pressure of 5.0 MPa, for which the storage capacity and reaction time were 159.1 V/V and 370 min, respectively [13,14]. Stirring reactors for hydrate formation have functioned for accelerating the gas dissolved in water, and strengthening heat transfer. When gas hydrate formed in gas-water interface, the stirring speeded up the thermal diffusion, the latent heat of hydrate formation released in time and did not hinder the growth of hydrate.

Figure 5. Effect of stirring velocity and time on methane hydrate formation [14]. (a) p = 5.0 MPa, ts = 30 min; (b) p = 5.0 MPa, Vs = 320 rpm

The bubble tower is a traditional industrial reactor type suitable for the gas-liquid reactant system. As hydrate formation is also a gas-liquid reaction, a bubble tower might be a suitable choice for the industrial hydration process. Actually, the hydrate formation rate on the surface of the moving bubble was high. But the formed hydrate shell was not very easy to be broken up. The bubbles with hydrate shells tended to agglomerate rather than merge into bigger bubble. This kind of characteristic of hydrate shell hindered the further formation of hydrate and led to the lower consumption rate of methane [15]. In order to increase hydrate formation rate, we recommend using tiny bubbles to increase the specific gasliquid interfacial area, increasing the fluid turbulence to peel off the hydrate shell and therefore keeping the methane bubble directly contact with liquid reactant. The increase of methane flux also results in the increase of the hydrate formation rate because higher methane flux produces more bubbles and larger total gas-liquid interface [15]. Based on the principle of the ejector-type loop reactor (ELP), which utilize the kinetic energy of a high-velocity liquid jet to entrain the gas phase and create a fine dispersion of the two phases, a methane gas hydrate formation system has been built by Tang [16]. Their study found three different bubble regimes without a static mixer by adjusting the gas entrain-

ment rate (Figure 6), and the flow in the free suction state of the ejector occured in the jet regime. In the case with a static mixer, microbubbles can form. At the same experimental condition, the static mixer shortened the induction time due to microbubbles. But it did not have a positive effect on the hydrate formation rate. The gas consumption rate from ELR is fairly comparable with the formation rate of water stirring reactor. Spraying reactor is another reactor type for gas-liquid reactant system. Differing from bubble tower, in spraying reactor the liquid is in dispersion state, that is, to introduce water droplets in gas continuous phases. Mori and his coworkers designed and modified spraying reactor and used it for methane hydrate formation [17−23]. In their latest design (Figure 7), two precooled liquids, water and a LMGS, such as methylcyclohexane, are injected in the form of co-planar cylindrical jets into a hydrate-forming gas phase confined in a high-pressure chamber, where LMGS provides guest molecules to fit into the (512 68 ) cages of a structure-H hydrate. The liquid jets collide, thereby forming a radially expanding sheet, which, in turn, sprinkles from its rim tiny water/LMGS compound droplets into the gas phase. The jet-impingement technique has a productivity of hydrate almost equivalent or even superior to that of the water spraying technique under equivalent operational conditions.

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Figure 6. Jet patterns of the ejector-type loop reactor. (a) single bubble regime, (b) intermediate regime, (c) jet regime, (d) using static mixer [16]

Figure 7. Conceptual illustration of a twin-jet hydrate-forming system by Mori et al. [23]

One of our patents also introduced a static-mix jet reactor based on the principle of static mixer and spraying reactor (Figure 8) [24]. Gas and water intensively mixed at the throat tube, and then sprayed into cavity of hydrate formation. Mixed water and gas collide in high speed jet, forming micro droplet with a structure of gas in water. This structure guarantees the ample water-gas interface and improves the hydrate formation rate. Sometimes, whether the heat of hydrate formation remove effectively or not will depend on many factors such as stirring, bubble, spraying, and therefore, the hydrate formation rate could be limited. An effective method for removing the heat from the inside of the reactor is inserting the

Figure 8. The structure of static-mix jet reactor [24]. (A) Nuzzle, (B) Entrainment chamber, (C) Venturi, (D) Diffusion tube, (E) Windows, (F) Water bath jacket; (1) Valve needle, (2) Working fluid inlet, (3) Ejecting fluid inlet, (4) Working fluid outlet, (5) Gas outlet, (6) Waters bath inlet, (7) Water bath outlet, (8,9,10) Temperature sensor interface, (11,12,13) Pressure sensor interface

external or internal heat exchanger into the reactor system. JFE Engineering Corp. built a set of tube reactor for hydrate formation, which size is length 5 m × width 1.5 m × height 1.8 m. Hydrate formed in the tube reactor was equal to hydrate formed in reactor with a large tubular exchanger. The velocity of hydrate formation enhanced. If the capacity and velocity of gas storage can reach a good level, the scale-up will be a problem. Many researches demonstrated that the scale-up effect was very obvious, that is, the specific hydrate formation rate, the moles of gas consumed per unit mass of water and time, decreased rapidly with the increasing mass of water loaded in the reactor. A multideck cell-type vessel (Figure 9) as the internals of the reactor can eliminate the scale-up effect [25]. This vessel basically consists of a number of stacked boxes, which divided into a series of uniform cells by metal plates. The metal plates are the cool solid surface during the hydrate formation for they are welded on the heat transfer tubes. The SDS aqueous solution is loaded in these cells with the same level. Between two neighboring boxes, there are interspaces for the hydrate forming gas flowing into each deck of the vessel easily. The multi-deck cell type vessel is placed in the high pressure reactor. In this case, the reaction rate depends mainly on the cell volume and the quantity of water loaded and little on the total volume of the vessel and total quantity of water loaded. Thus the scale-up effect can be eliminated to a large extent [25].

Figure 9. Schematic of the multi-deck cell-type vessel [25]

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3. Hydrogen storage in hydrate The technology of using hydrogen as an environmentally clean and efficient fuel is an active research area worldwide. One of the major challenges in establishing a hydrogen-based economy is effective storage and delivery of hydrogen. At 180 MPa and at 249 K, hydrogen and water will form sII clathrate hydrate [26]. Because the diameter of hydrogen molecule (0.23 nm) compared to that of methane molecule (0.44 nm) is obviously less than the size of sII clathrate hydrate cages (D(512) = 0.39 nm), it is easily for hydrogen to enter into the cage of hydrate under appropriate thermodynamic conditions. Additionally, the formation heat of hydrogen hydrate is also lower than that of methane hydrate. So, the difficulties of methane hydrate formation, i.e. mass transfer and heat transfer, did not occur in hydrogen hydrate formation. This is the advantages of hydrogen hydrate with the fast formation and decomposition kinetics. However, there are other problems in hydrogen hydrate. That is a very high pressure required for its formation (∼200 MPa at 273 K) and low storage capacity. Thus, the topic of hydrogen hydrate is how to decrease the pressure for its production, and to improve storage capacity. The addition of a promoter molecule, such as tetrahydrofuran (THF), was found to significantly reduce the formation pressure (from 200 to 7 MPa at 273 K) by filling the large cages and stabilizing the sII structure; the small cages can then be filled by hydrogen molecules. This scheme provides a trade-off between improving synthesis or storage conditions and the reduction in hydrogen storage capacity resulting from the addition of the second guest. Florusse et al. demonstrated that the binary H2 /THF hydrate will form readily at conditions of near ambient temperature and pressure (∼5 MPa and 280 K), but single hydrogen molecule occupancy of the small cage gives the lowest energy [27]. Lee et al. [28] improved the pressure (12 MPa) and reduced the temperature (270 K) of THF-H2 hydrate formation found that double occupancy of hydrogen in the small cages and hydrogen storage reached near 4 wt%. TBAB is a promotor for hydrogen hydrate formation. H2 +TBAB can form a semi-clathrate, which stability is even greater than that of THF+H2 hydrate. And if TBAF is used instead of TBAB, the stability can be increased even further [29,30]. Hydrogen hydrate also can combine with some hydrogen storage materials, such as metal organic frameworks (MOFs) or covalent organic frameworks (COFs) materials, hydride, fuel cell, to improve the storage capacity. 4. Conclusions Methane and hydrogen are good, clean energy in the present and future. Gas hydrate is an effective method for methane and hydrogen storage due to their high storage capacity, environmental friendly and safety. However, gas storage in hydrate still has a distance away from application. The main

difficulties are low formation rate of methane hydrate, very high formation pressure of hydrogen hydrate and low storage capacity for both gases. The researches on methane and hydrogen hydrate in the present and future will focus on the following aspects: (a) Milder hydrate forming conditions. To lower the pressure and to increase the temperature of formation, make the hydrate formed under near atmosphere conditions. (b) Fast formation. It is the key point of the industrial application of gas storage in hydrate. (c) Higher storage capacity. Methane hydrate should reach the storage ratio of 175 V/V. For the hydrogen, International Energy Agency to determine the storage capacity of the new hydrogen-storage materials should be more than 5 wt%. (d) Reversible. The process of hydrate formation should be reversible. The materials or additives used in hydrate formation can store and release gas repeatedly. (e) To improve the productivity of continuous, miniaturization reactors. Last but not least, base on the above researches, to scale-up the processes of hydrate formation and gas storage for their industrial application. Acknowledgements Financial support is gratefully acknowledged from the National 863 Program (2007AA03Z229) and the Fundamental Research Funds for the Central Universities (2009ZM0185).

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