A review of solidified natural gas (SNG) technology for gas storage via clathrate hydrates

Applied Energy 216 (2018) 262–285

Contents lists available at ScienceDirect

Applied Energy journal homepage: www.elsevier.com/locate/apenergy

A review of solidified natural gas (SNG) technology for gas storage via clathrate hydrates Hari Prakash Veluswamya, Asheesh Kumara, Yutaek Seob, Ju Dong Leec, Praveen Lingaa,

T

⁎

a

Department of Chemical and Biomolecular Engineering, National University of Singapore, Singapore 117585, Singapore Department of Naval Architecture and Ocean Engineering, RIMSE, Seoul National University, Seoul 08826, Republic of Korea c Offshore Plant Resources R&D Center, Korea Institute of Industrial Technology, Busan, Republic of Korea b

H I G H L I G H T S review on Solidified Natural Gas (SNG) Technology via clathrate hydrates. • First for improving the kinetics and storage capacity is presented. • Prospects examination of ‘self-preservation’ and ‘tuning’ effect in hydrates is presented. • Critical • Challenges and future directives for commercial deployment of SNG technology are outlined.

A R T I C L E I N F O

A B S T R A C T

Keywords: Gas hydrates SNG technology Natural gas Energy storage Storage capacity Methane storage

Natural gas (NG), the cleanest burning fossil fuel, plays a crucial role in meeting the global energy demand, contributing to 24% and is projected to grow at a rate of about 2% until 2040. Natural gas is also considered as the bridging fuel to transition into a carbon-constrained world with reduced carbon dioxide emissions whilst catering to the huge energy demand. Efficient and effective modes of NG storage/transport are dire need in the current golden era of natural gas. A plethora of advantages offered by storing NG in the form of hydrates carve a niche for this novel technology. Termed as solidified natural gas (SNG) technology, it has remarkable potential to store multi-fold volumes of natural gas in compact hydrate crystals offering the safest and the most environmental friendly mode of NG storage. This review provides an account on the research efforts put forth in this technology. Hydrate formation and storage aspects have been examined thoroughly with a subtle account on the gas recovery. The review encompasses studies conducted using different promoters (thermodynamic, kinetic or a combination of both) in different reactor configurations, novel/innovative approaches and hybrid processes adopted to improve the kinetics of hydrate formation and to increase the gas storage capacity. Detailed sections on the ‘self-preservation’ and ‘tuning’ effect in hydrates have been included due to their significance in SNG technology. Process chain of the SNG technology, underlying challenges and measures adopted to deploy the SNG technology for large-scale NG storage applications are included in this review.

1. Introduction Natural gas (NG) is the cleanest burning fossil fuel and is abundantly available in nature both in conventional and in unconventional forms (Shale gas, natural gas hydrates, tight gas, etc.). Natural gas predominantly contains methane (approximately 90% and above in most cases) along with a small percentage of higher hydrocarbon gases like ethane, propane and butane. Natural gas may also contain small amounts of carbon dioxide, nitrogen, hydrogen sulfide and trace amounts of water vapour [1]. Amongst fossil fuels, the share of natural gas towards primary energy will continue to increase for at least

⁎

another twenty years until 2040 [2]. With this inevitable shift to a natural gas based economy globally, there is an ever-increasing need to develop effective technologies to store and transport natural gas efficiently. Improved power generation efficiency, high calorific value and low carbon emission (compared to coal and gasoline) favor the increased utilization of natural gas for power generation application. In fact, using natural gas for power generation results in about 50% and 33% reduction in CO2 emission in comparison to using coal and oil, respectively [3]. Natural gas is also used extensively for other industrial and manufacturing applications. Several approaches have been considered worldwide for the

Corresponding author. E-mail address: [email protected] (P. Linga).

https://doi.org/10.1016/j.apenergy.2018.02.059 Received 12 September 2017; Received in revised form 14 January 2018; Accepted 8 February 2018 0306-2619/ © 2018 Elsevier Ltd. All rights reserved.

Applied Energy 216 (2018) 262–285

H.P. Veluswamy et al.

calculations for MOFs reported in literature were performed based on density of a single crystal. However, when calculated on the basis of bulk powder (when used in large scale), there will be a substantial reduction in volumetric storage capacity. Further, other factors like mechanical stability, thermal conductivity, presence of impurities and most importantly the high cost of the material will impede the deployment of MOFs for large-scale NG storage [5]. Storing and transporting natural gas in clathrate hydrates, referred as Solidified Natural Gas (SNG) from now on, is a promising alternative due to several characteristic advantages that include

transportation of natural gas. The most common approach is transportation through gas pipeline which is not always practical considering the distance, cost, feasibility and accessibility of the delivery location. Another approach for NG transportation and storage at a much smaller volume is through compressed natural gas (CNG). However, safety concern and the poor volumetric storage capacity are the characteristic drawbacks for this approach. Transportation of NG in liquid form, Liquefied Natural Gas (LNG) has been considered the most suitable approach for large scale and long distance transportation due to the high volumetric storage capacity (600 v/v compared to STP conditions). Currently, LNG tankers are used to transport natural gas from source to the areas of demand. Adsorbed natural gas (ANG) is another possible approach to transport NG by adsorbing on to sorbents like carbon nanotubes (CNTs), graphene, metal organic frameworks (MOFs), etc. US Department of Energy (DOE) target for on-board vehicular methane transport/storage requirement is 263 v/v at STP and 0.5 g/g of adsorbent gravimetric storage capacity. Few MOFs are reported to have higher gravimetric and considerable volumetric storage capacities with the potential to meet DOE target for methane storage/ transport [4]. It is noted that a number of materials for ANG are under active research and development phase for usage in automobiles as an alternative to CNG powered vehicles. Natural gas can be stored either in gaseous, liquid or solid forms. Available pathways for NG storage and transport are provided in Fig. 1. Underground inventory is one of the most common methods that includes storing natural gas in depleted reservoirs of oil and/or natural gas fields, aquifers, and salt cavern formations. Characteristic aspects of these storage modes include the capacity and the ease of deliverability of NG. For achieving a volume reduction of about 200 times, it is noted that CNG has a very high pressure requirement (200 bars and above), practically making it not suitable for large scale NG storage due to the extremely high cost involved in the design of high pressure, large volume storage tanks and the inherent explosive nature of CNG. Though considered the best mode of NG transport, the extreme low temperature requirement (−162 °C) to keep LNG stable as well as the continuous boil-off issues associated with LNG deters its use for largescale, long term storage applications. Gravimetrically, the storage capacity for ANG is higher due to the relatively larger surface area and higher porosity. However, volumetric storage capacity observed in these materials is lower and this volumetric storage capacity is a key driver for practical applications [4]. Volumetric storage capacity

(i) Clathrate hydrate formation process is environmentally benign as it uses only water and very low concentration of promoters (used when required to improve the operating conditions of storage) (ii) Guest gas (methane) is stored in its respective molecular form, almost complete recovery or utilization is possible just by simple depressurization or minimal thermal stimulation (iii) Moderate temperature and pressure conditions required during formation and storage (in the presence of low concentration of promoters) (iv) Highly compact mode of storage with relatively high energy content per unit volume and (v) It is extremely safe mode of storage due to its non-explosive nature. Table 1 presents the comparison of different technologies available for storage of natural gas in molecular form. Formation, storage conditions along with advantages and disadvantages of each of these methods have been included. From the table, it is clear that the proposed SNG method cannot meet the DOE targets envisaged for on-board vehicular transportation. Thus, SNG technology can cater only to stationary applications that require natural gas and it can also enable natural gas storage suited for power generation applications. 2. Scope of the review Despite the substantial research in storing methane (natural gas) in hydrates for past three decades, there is no comprehensive review paper summarizing the hydrate-based technology for storing and transporting methane/natural gas. This review attempts to summarize the experimental work conducted till date to store methane /natural gas in the form of hydrates and presents the current status of solidified natural gas

Fig. 1. Pathways for natural gas storage and transportation (Images on UGS obtained from http://www.energyinfrastructure.org/energy-101/natural-gas-storage).

263

264

High safety risk; expensive high pressure storage vessels and costly multistage compression

Disadvantages

Poor mechanical stability; cost and contamination of adsorbent; thermal effect on adsorbent

Boil-off gas; high cooling energy requirement with specialized insulation

Non-explosive (low safety risk); environmentally benign; compact mode of storage; moderate conditions of formation and storage Further lower volumetric storage capacity (may not be able to meet needs of vehicular transport)

0.1 MPa and 277.2 K$ Low heat stimulation

0.1 MPa and 193 K# Low heat stimulation Non-explosive (low safety risk); environmentally benign; compact mode of storage Slightly lower volumetric storage capacity (may not be able to meet needs of vehicular transport)

3–5 MPa and 283.2 K

0.08

115^

With promoter

10 MPa and 274.2 K

0.134

170

Without promoter

SNG

* Assuming HKUST-1 metal organic framework as the adsorbent material [6,7]. ^ Maximum theoretical storage capacity calculated assuming 5.6 mol% THF as the promoter. # Dissociation temperature for pure sI methane hydrate at atmospheric pressure is 193 K [8]; due to the anomalous self-preservation effect observed, hydrates were found to be quite stable (with minimal dissociation rate) even at 253.2 K under atmospheric pressure. This temperature was envisaged for storing and transporting hydrate pellets in the first demonstration plant put forth by Japan at Yanai Power plant [9]. $ Temperature of 277.2 K is listed as equilibrium temperature of pure THF hydrate at 1 atm is 277.6 K [10].

Advantages

6.5 MPa and 298.2 K Desorption

6.5 MPa and 298.2 K

Low pressure storage; High gravimetric capacity; light weight

0.1 MPa and 113.2 K with huge energy input for cooling 0.20 MPa and 113.2 K Regasification

Multistage compression

0.22

267*

ANG

Highest volumetric and energy density

NA

NA

20 MPa and 293.2 K Direct use - pressure adjustment required Can be used readily without any additional recovery step

600

230

Storage conditions Recovery of gas

Volumetric capacity (v/v) Gravimetric capacity (g/g) Formation conditions

LNG

CNG

Table 1 Comparison of different technologies available for natural gas storage and transport.

H.P. Veluswamy et al.

Applied Energy 216 (2018) 262–285

Applied Energy 216 (2018) 262–285

H.P. Veluswamy et al.

water desalination and gas separations [44–47] have been studied and researched intensively. Early review papers focused on overall beneficial aspects of hydrates and their potential applications. Following this, detailed review papers on the role of hydrates in each of the listed applications were published in the literature [48–51]. Babu et al. [52] reviewed hydrate based gas separation processes for pre-combustion CO2 capture. Ma et al. [53] recently published a review on the fundamental properties of CO2 hydrates along with CO2 capture and separation using hydration method. Storing hydrogen gas in hydrates has been reviewed and documented amply in the literature [54–57]. Eslamimanesh et al. [58] documented experimental studies focusing on the application of gas hydrates in separation processes. Harnessing clathrate hydrate technology for cold storage in air conditioning systems was elaborately discussed by Wang et al. [59]. Natural gas hydrate deposits in reserves spread across the world have been documented by Boswell and Collett [60] and their potential as future energy source have been highlighted by many researchers [61,62]. Different methods/measures adopted to recover natural gas safely from reserves have also been investigated in detail [14–16]. A recent review by Linga and Clarke presents a detailed account of different materials reported in the literature to enhance the kinetics of hydrate formation for gas hydrate applications [63]. Khurana et al. [64] have reviewed critically the hydrate nucleation mechanisms and pathways.

(SNG) technology suited for storing and transporting natural gas. Future directives in this area of research are also put forth. Pure sI methane hydrates formation starting from ice or water, other different mixed methane hydrate structures (sII, sH and semi-clathrates) formed in presence of different thermodynamic promoters and the improvement in hydrate formation kinetics in the presence of kinetic promoters (surfactants) are discussed elaborately. Further, two salient phenomena observed in methane (natural gas) hydrates namely ‘self-preservation effect’ and ‘tuning effect’ are included in separate sections. Available patents on storing methane/natural gas in hydrates have been compiled and presented. Novel materials, hybrid methods and approaches adopted for improving the gas storage capacity and enhancing the hydrate formation kinetics are also covered in this review. However, studies on molecular dynamic simulations (MDS) and modelling aspects of methane hydrate formation/dissociation are excluded from the scope of this review. Ribeiro and Lage [11] presented a comprehensive review of different models for natural gas hydrate formation (including hydrate nucleation and hydrate growth behavior) and limitations of each of these models were discussed along with future directions. English and MacElroy [12] have summarized in detail the exhaustive efforts put forth in molecular simulations of clathrate hydrates during the nucleation, growth and dissociation of hydrates along with the future prospects and challenges for this research arena in their recent review paper. Dissociation/recovery of gas from hydrates has not been discussed in detail due to the similarities that exist in the gas recovery from natural gas hydrate deposits that have been amply reviewed in the literature [13–17].

4. sI methane hydrates Methane hydrates were first reported by Villard as early as 1888 [65] yet they did not get attention until 1934, when Hammerschmidt reported that gas hydrates were found to plug the natural gas transmission lines [21]. Thus, initially natural gas hydrates were considered a nuisance causing plugging of underground oil and gas pipelines. In 1942, Benesh Matthew patented a novel method to store fuel gases (hydrocarbon rich gas/natural gas) in the form of solid hydrates using water [66]. Further investigations on methane storage in hydrates were performed by Miller and Strong [67] and Parent [68]. Kobayashi and Kutz [69] studied methane hydrate formation at high pressures in the range of 27–77 MPa (4000–11200 psi). sI structure of methane hydrate was first reported in 1952 [70,71]. Gas hydrate deposits were first found naturally in Siberian permafrost region during 1964. Phase equilibrium data of pure methane hydrate has been examined exhaustively and documented in literature [72–76]. Sloan and Koh provide a compilation of the methane hydrate phase equilibrium data available in literature [20]. Investigation of kinetics of methane hydrate formation was first performed in 1983 by Vysniauskas and Bishnoi [77]. Semi-batch experiments were performed in a stirred tank reactor in the pressure range of 3–10 MPa and temperature range of 274–284 K. Kinetics was found to be prominently dependent on water-gas interfacial area, the degree of super cooling and temperature/pressure conditions. The history of water sample was found to influence the induction/nucleation time but had no effect on the kinetics of hydrate formation. A semi-empirical model was formulated to correlate the kinetics of methane hydrate formation observed during the experimental study. Increase in pressure was found to increase the methane gas consumption whereas increase in temperature was observed to decrease the methane consumption rate. Further investigation on methane hydrate formation kinetics was performed by Englezos et al. [78]. Crystallization theory along with mass transfer phenomena occurring at gas-liquid interface was considered in the development of a mechanistic model with one adjustable parameter that represented the rate constant for hydrate growth. The driving force for hydrate growth was reported to be the difference between the three phase equilibrium fugacity and the fugacity of the dissolved gas. Investigation of hydrate decomposition kinetics was performed by depressurization (reducing the pressure below threephase equilibrium pressure at experimental temperature). An intrinsic kinetic model for decomposition was proposed and the decomposition

3. Gas hydrates Clathrate hydrates are non-stoichiometric crystalline compounds formed when guest molecules of suitable size and shape are incorporated in well-defined cages of the host lattice made of hydrogen bonded water molecules [18–20]. Clathrates are not chemical compounds; guest molecules inside the cage interact with water molecules by Van der Waals forces (physical bonding) and hence these guest gas molecules retain their inherent properties inside the host cage structures. This also translates to an easy way of recovering or releasing the stored gas from clathrates by disturbing the weak Van der Waals forces. There are three commonly known hydrate structures namely, sI, sII and sH. Each of these structures has characteristic cage size and shape, lattice parameters and the number of water molecules forming a unit structure [19,20]. The type of hydrate structure formed depends on the molecular size of the guest involved and the conditions of hydrate formation. Molecules having diameter 4–6 Å typically form sI (e.g., CH4 and C2H6), molecules which are 6–7 Å form sII (e.g., C3H8) structure and molecules which are greater > 7 Å form sH hydrate structures. Exceptions include hydrogen and argon which are less than 4 Å but form sII hydrates. Gas hydrate research has progressed over several decades from a mere academic curiosity to being regarded as pertinent for flow assurance in the oil and gas industry. Hydrates have been regarded as a nuisance in deep offshore oil and gas production due to their occurrence in pipelines and offshore facilities disrupting the operation and resulting in huge losses [20,21]. Later in 1960’s, after the discovery of naturally occurring hydrate deposits, there has been a considerable interest among the research community spanning across disciplines of chemical engineering, mechanical engineering, chemistry, civil and environmental engineering to understand the energy and environmental impact of these hydrate deposits [22–29]. The industry interest of these clathrates kicked off when it was found that clathrate hydrates serve as a huge potential energy source [22,30–32]. Over the past two decades, several applications of gas hydrates for potential technological applications like carbon dioxide capture [33–37], carbon dioxide sequestration [38,39], storage & transport of natural gas/hydrogen [40–43] and other novel applications like cold thermal storage, sea 265

Applied Energy 216 (2018) 262–285

H.P. Veluswamy et al.

was similar to the methane hydrate formation enhancement in presence of ethanol observed by Chen et al. [89] Recently, Faizullin et al. [91,92] observed hydrate formation in layers of amorphous ice by condensation of molecular beams of gas and water on the substrate cooled with liquid nitrogen. Though it is possible to achieve a high yield (close to 100%) of methane hydrate starting from ice, energy requirement for achieving so is quite high. Scalability of this approach with ice as a starting material is yet to be demonstrated. Further, ice reformation (obtaining ice grains of size about 0.2 mm) is required for repeated cycles of methane storage, which would attribute to additional cost and necessitates additional maintenance. In addition, considerable time (about 8–10 h) is required for hydrate formation accompanied by single or multiple heating/cooling cycles. Thus, considering the NG storage process on a large scale, methane hydrate formation starting from ice may not be a viable option owing to the high cost and longer hydrate formation times required. To enhance the dynamics of the hydrate formation process starting from bulk water, it has been proposed to add chemical additives commonly referred to as ‘promoters’ discussed more in detail in following sections.

rate was found to depend on the particle surface area, temperature and pressure (difference between fugacity of gas at decomposition pressure and three phase equilibrium) [79]. Freer et al. [80] studied hydrate film growth at gas/liquid interface with experiments performed at 3.55–9.10 MPa and temperatures between 274.2 and 277.2 K. They also put forth a model for hydrate formation accounting both heat transfer and kinetics. Ohmura et al. [81] studied formation and growth of methane hydrate in a quiescent system using water saturated with methane. It was reported that hydrate film was first formed at the gas-liquid interface followed by further growth of hydrate from the hydrate film into the liquid water. The hydrate crystals were either columnar or dendritic depending on the driving force available for hydrate formation. 4.1. Methane hydrate formation starting from ice Falabella [82] studied the kinetics of methane hydrate formation using ice crystals in the temperature range of 148–191 K under pressures of 0.088 MPa–0.125 MPa. Based on observed kinetics, the optimal condition of methane hydrate formation was at 183 K and 0.125 MPa with about 80% hydrate conversion along with highest rate of methane uptake. Hwang et al. [83] investigated methane hydrate formation starting with ice from temperature of 258.2 K rising to 273.2 K and above. They reported that melting ice had a profound impact in forming methane hydrates rather than ice or bulk water and it was advantageous as the energy produced during hydrate formation was adsorbed by the melting ice. Stern and co-workers reported nearly complete conversion of metastable ice grains (0.2 mm diameter) at 250 K to methane hydrates by warming the ice sample to temperature of 290 K under high methane pressure of 25–30 MPa for about 8 h followed by cooling to 250 K and later quenching to 77 K [84–86]. Wang et al. [87] studied the kinetics of methane hydrate formation from deuterated ice particles in isothermal conditions (253, 263, 270 or 273 K) at 6.9 MPa and non-isothermal conditions of temperature ramping from 263 K to 280 K at the rate of 1.4 K/h at 10.3 MPa. Isothermal mode of formation resulted in 42–71% of ice to hydrate conversion. Shrinking core model with diffusion controlled formation of hydrates from ice was proposed to explain the mechanism of hydrate formation under isothermal conditions. On the other hand, complete conversion of ice to hydrates was achieved by the non-isothermal procedure (characterized by neutron diffraction). Further investigation on the kinetics of hydrate formation was done by Kuhs et al. [88] using hydrogenated and deuterated polydisperse ice powders in the temperature range of 245–270 K at about 6 MPa pressure. Two stages of methane hydrate growth from ice were reported that involved initial hydrate film spreading over the ice (stage I) and the hydrate shell growth phase (stage II) which further included two steps, i.e., clathration reaction at the gas/ice interface and the diffusion of gas through the hydrate layer surrounding the shrinking ice cores. Chen et al. [89] reported the significant promotion effect of ethanol (small volume of about 2 ml) on methane hydrate formation rate starting from ice. About 90% hydrate conversion was achieved in 10 h starting from initial temperature of 270.1 K at about 16.5 MPa. Faster hydrate formation rates and increased yields of methane hydrate were observed in trials conducted with slower rates of pressurization and lower initial temperatures of ice samples. Further, the effect of ice grain size, degree of compaction and mixing with sand during the hydrate formation were also documented in this study. McLaurin et al. [90] revealed that methanol, well-known antifreeze acted as a catalyst promoting methane hydrate formation in ice (increase by two orders of magnitude) when used in the concentration range of 0.6–10 wt% at 253 K and 12.5 MPa. Similar catalytic effect was observed in presence of ammonia in concentration range of 0.3–2.7 wt%. Further, simulations performed by McLaurin et al. [90] showed that methanol from the surface aqueous layer slowly migrated into ice lattice and methane gas preferentially adsorbed onto the aqueous methanol surface layer. This observation

5. Kinetic promoters for methane hydrate formation Kinetic promoters are additives that help to enhance the rate of hydrate formation without influencing the thermodynamics. Thus, hydrate formation conditions (T and P) and hydrate structure will not be affected by the inclusion of kinetic promoters. Predominantly, a number of surfactants have been reported to function as effective kinetic promoters for hydrate formation 5.1. Surfactants The first study investigating the effect of surfactants on the kinetics of methane hydrate formation was performed by Kalogerakis et al. [93]. The effect of an anionic, sodium dodecyl sulfate (SDS) surfactant along with three non-ionic surfactants in influencing the kinetics of methane hydrate formation was studied. SDS had a pronounced effect in increasing the rate of hydrate formation in comparison to non-ionic surfactants. Detailed investigation on the effect of SDS surfactant on ethane and natural gas (90% methane, 6% ethane and 4% propane gas mixture) hydrate formation kinetics was performed by Zhong and Rogers [94]. SDS surfactant closer to critical micelle concentration (CMC) resulted in a multifold increase in the rate of natural gas hydrate formation in a quiescent unstirred system. The reason for the same was attributed to the formation of micelles that solubilized hydrocarbon gases to initiate a subsurface resulting in kinetic promotion. With SDS addition, hydrate particles adsorbed on the metal walls of the reactor and packed symmetrically as they grew, with more interstitial water converting to hydrates reaching closer to theoretical hydrocarbon storage capacities in hydrates. Gas uptake of 156 vol/vol was achieved at 3.89 MPa and 275.4 K in about 150 min from the hydrate nucleation. High hydrate formation rate was achieved in a simple quiescent system in the presence of surfactant, minimizing the formation cost making it suitable for large-scale applications. Sun et al. [95] also report a similar characteristic promotion by SDS anionic surfactant better than dodecyl polysaccharide glycoside (non-ionic surfactant) under similar experimental conditions. Gas uptake of about 159 vol/vol was achieved at 274.05 K at 3.9 MPa using 300 ppm SDS, the natural gas mixture used in their study had composition of 92% methane, 5% ethane and 3% propane. Experiments were performed to evaluate the performance of different classes of surfactants (anionic, cationic and non-ionic) in promoting the kinetics of methane hydrate formation and determining the optimal concentration of surfactant that effectively improved the rate of methane hydrate formation under selected operating conditions (P and 266

Applied Energy 216 (2018) 262–285

H.P. Veluswamy et al.

It has been reported in most of these studies that a substantial reduction in induction time and improvement in the dissociation rate of hydrates was observed in presence of added surfactants (kinetic promoters). However, few studies have highlighted that despite the application of conventional surfactants even in low concentrations (order of 100–1000 ppm), foam formation is observed during the dissociation (degassing) or the gas recovery phase [124–126]. This foam impedes the hydrate dissociation lowering the gas release rate, thus undesirable for large scale deployment for methane storage application. Also, significant environmental concerns exist for the degradation of conventional surfactants [127]. Hence, research on alternative kinetic promoters to overcome the drawbacks of conventional surfactants without compromising on the performance (kinetic promotion) was pursued. Amino acids and starches (discussed in following sections) are promising options as they are bio-molecules (easy bio-degradation) and do not result in foam formation during the gas recovery.

T) [96–104]. Anionic SDS surfactant was shown to demonstrate a higher degree of rate improvement in unstirred and stirred configurations in comparison to other surfactants. Verrett et al. [105] observed that SDS surfactant had no effect on the bulk solubility of methane but resulted in significant increase of methane mole fraction in bulk liquid during hydrate growth. Zhang et al. [106] record that SDS accelerated hydrate growth rate not only by reducing the interfacial tension between hydrate and liquid but also increasing the total surface area of hydrate particles and the gas-liquid interfacial area. Further, they documented that at temperatures below the ice melting point, hydrate formation is significantly affected by the concentration of SDS. Lucia et al. [107] performed a kinetic study in forming methane hydrates on a large scale spray reactor having an internal volume of 25 L in presence of SDS surfactant. Methane hydrate formation occurred quickly within minutes in this reactor due to the application of SDS surfactant. Other prominent anionic surfactants including linear alkyl benzene sulfonate (LABS), dodecylbenzene sulfonic acid (DBSA), sodium oleate (SO), lithium dodecyl sulfate (LDS), sodium dodecyl sulfonate (SDSN), sodium dodecyl benzene sulfonate (SDBS), other sodium alkyl sulfates like sodium butyl sulfate, sodium tetradecyl sulfate (STS), sodium hexadecyl sulfate (SHS) and sodium octadecyl sulfate were reported to promote methane hydrate formation [96–98,102,108,109]. Sodium alkyl sulfates exhibited a similar behavior in promoting methane hydrate formation like SDS. Amongst the studied surfactants, those having longer carbon chain lengths were found to be required in much lesser concentrations than SDS to exhibit similar promotion effect under similar conditions of hydrate formation [102]. Further, cationic surfactants like cetyl trimethyl ammonium bromide (CTAB), dodecylamine hydrochloride (DAH), N-dodecylpropane-1,3-diamine hydrochloride (DN2Cl), hexadecyl-trimethyl-ammonium bromide (HTABr) and nonionic surfactants like ethoxylated nonylphenol (ENP), polyoxyethylene (20) cetyl ether (Brij-58), tergitol have also been reported to promote methane hydrate formation rates [98,99,101,110,111]. Apart from conventional surfactants, other chemicals including hydrotopes, bio surfactants and gemini surfactants were also studied for improving the rate of methane hydrate formation [112–116]. Gnanendran and Amin [112] reported an improvement in natural gas hydrate formation kinetics in presence of para-toluene sulfonic acid (pTSA) additive. p-TSA belongs to the class of hydrotopes that are different from classical surfactants as they exhibit amphiphilic character with short hydrophobic regions and have substantial ability to solubilize nonpolar compounds in water. Optimum concentration of p-TSA for promoting methane hydrate formation was reported to be 3500 ppm at 263 K and 10.2 MPa. Rogers et al. [114] investigated the prospect of using five different classifications of bio-surfactants in promoting natural gas hydrate formation in sand/clay pack. Surfactin bio-surfactant obtained from Bacillus subtilis was reported to exhibit the highest rate of hydrate formation of 50 mmol/h at 2.3 MPa and 274.2 K. Recent study using the rhamnolipid obtained from Pseudomonas aeruginosa strain A11 showed characteristic methane hydrate promotion [116]. Further new category of surfactants namely Gemini surfactants/dimeric surfactants were also explored for their promoting ability of hydrate formation. These surfactants have two surfactant molecules linked by a spacer and thus are different from classical surfactants. They are characterized by increased surface activity and solubilization capacities with only low concentration required to result in the promoting effect than their corresponding monomeric counterparts [113]. Mechanism by which the surfactant promotes hydrate formation was examined by different researchers and had been a highly debated topic. Different mechanisms by which surfactants promote the rate of hydrate formation have been discussed and documented in the review by Kumar et al. [117]. Capillary suction leading the growth of hydrate front in upward direction along the reactor walls resulting in improved rates [118,119], adsorption of surfactant on hydrates [120–122] and micelle formation [94,123] are predominant mechanisms attributing to the improvement in kinetics of hydrate formation in presence of surfactants.

5.2. Amino acids Protein building blocks, amino acids consist of an amine (eNH2) and carboxylic (eCOOH) functional group along with a characteristic side chain. These amino acids also exhibit the potential to improve the kinetics of methane hydrate formation. Initially, amino acids were reported to inhibit the hydrate formation systems using CO2 and tetrahydrofuran guests [128,129]. However, study by Liu et al. [126] documented the promotion effect of selected natural amino acids on the kinetics of methane hydrate formation. Amongst the amino acids studied, they reported a high methane uptake of 143 mg methane/g of solution (equivalent to 161.7 mmol of gas/mol of water) with t90 (time taken for 90% completion) of approximately 20 min for the experiment conducted with starting methane pressure of 9.5 MPa and cooling to 273 K in presence of 0.5 wt% l-leucine amino acid. Morphology study by Veluswamy et al. [124] revealed a characteristic ‘methane bubble’ in the bulk solution aided with ‘breathing effect’ that attributed to the enhanced methane hydrate formation kinetics observed in presence of leucine amino acid. Investigation on hydrate formation kinetics in presence of histidine amino acid was performed by Bhattacharjee et al. [130]. In addition to experimental investigations, molecular dynamic simulations were also performed in presence of 1 wt% histidine. Though the rate of hydrate formation with 1 wt% histidine was lower in comparison to 1 wt% SDS at 5 MPa and 274.15 K, final methane gas uptake was comparable using both SDS and histidine. Further, recent study by Veluswamy et al. [131] reported a striking similarity in methane gas uptake and kinetics of hydrate formation in presence of 0.3 wt% leucine at 10 MPa and 275.2 K in both stirred and unstirred reactor configurations. Methane uptake of 133 mmol gas/mol of water was achieved in 1 h predominantly due to the flexible and porous nature of methane hydrates formed in the presence of amino acid. This study presented a combinatorial approach of methane hydrate formation to enable a deterministic and fast nucleation coupled with faster crystallization kinetics amalgamating the positive aspects of stirred and quiescent reactor configurations. In addition, using amino acid for hydrate formation ensured the absence of foam formation unlike that observed when using surfactant based kinetic promoters. Veluswamy et al. [132] also studied the effect of three different type of amino acids – tryptophan (non-polar hydrophobic with aromatic side chain), histidine (polar basic amino acid with aromatic side chain) and arginine (polar basic amino acid with aliphatic side chain) on the kinetics of methane hydrate formation. From experiments performed, it was inferred that the presence of an aromatic side chain and hydrophobic nature of amino acid resulted in a better enhancement of methane hydrate formation. Though higher concentration of amino acid would be required to achieve the similar kinetic promotion effect as that of conventional surfactant, the environmental benign nature and lack of foam formation during gas recovery enable the favorable application of amino acids for methane hydrate formation. 267

Applied Energy 216 (2018) 262–285

H.P. Veluswamy et al.

5.3. Polymers and starches Taheri et al. [133] reported that addition of 5000 ppm of water soluble hydroxy ethyl cellulose polymer (Mw of 90,000) resulted in improved formation kinetics and increased methane uptake at 275.2 K and 10 MPa. Two polymers - poly(2-acrylamido-2-methylpropane sulfonic acid), sodium salt and poly(acrylic acid), sodium salt were observed to promote methane hydrate formation kinetics [134]. In this study, lower molecular weight (Mw - 2100) poly (acrylic acid), sodium salt at 0.5 wt% concentration was reported to exhibit the best promotion effect at 6.5 MPa and 277.2 K. Similarly, 1 wt% of sodium salt of poly acrylic acid was also found to promote methane hydrate formation kinetics at 274.5 K and 6 MPa by Kumar et al. [135]. Poly vinyl alcohol (PVA) accelerated the methane hydrate formation kinetics at 276.2 K and 4.6 MPa [136]. Initially, cationic starches including tapioca starch and polyethylene oxide were found to inhibit the hydrate formation by delaying the nucleation time using methane gas and gas mixtures comprising of methane/ethane and methane/propane [137]. However, later it was found that potato starch in concentrations of 500 ppm was found to demonstrate kinetics similar to that of SDS surfactant and maximum methane uptake of 163 v/v was achieved using 300 ppm water soluble potato starch [138]. Further, the dissociation rates of methane hydrates were substantially lowered in presence of these starches in comparison to SDS surfactant. Ganji et al. [139] also report similar lowering of methane hydrate dissociation rates in presence of xanthan gum and starch. Maize starch at concentrations > 400 ppm was found to improve the kinetics of hydrate formation and an optimal concentration of 800 ppm was reported at 8 MPa and 275.2 K [140]. Apart from these promoters reported for enhancing kinetics, there are few studies that document the effect of ultrasonic waves on methane hydrate formation. Park et al. [141] reported that increased methane uptake with less induction time was possible with the application of 150 W ultrasonic waves during methane hydrate formation at subcooling temperatures of 0.5 K and 5.7 K.

Fig. 2. Hydrate equilibrium methane + THF + water systems.

data

for

methane + water

system

and

formation using 6 mol% THF was investigated in a bubble column reactor [148] in temperature range of 277.7–280.2 K and pressures of 0.2–1.0 MPa. When methane gas was bubbled through THF solution maintained at experimental temperature, a thin hydrate film covering the gas bubble initially formed gradually turning into hydrate shell thereby reducing the rise velocity of the bubble. Bubbles with the formed hydrate shell tend to agglomerate when collided and remained in the solution offering resistance to the growth of hydrates from further incoming gas bubbles. Methane consumption rate was found to increase with decrease of experimental temperature, increase of experimental pressure and increase of methane flux. Zhengfu et al. [149] studied the mixed methane hydrate formation kinetics in spray reactor with 6 mol% THF solution in temperature range of 277.3–278.6 K and pressures of 0.8–2.0 MPa. Liquid flow rate and experimental pressure were found to influence the rate of methane consumption. Experimental pressures in both the discussed studies were maintained in the range of 0.2–2 MPa; it is not possible to form pure methane hydrates at such low pressures and at temperatures above 274.2 K. This shows the characteristic advantage of using the thermodynamic promoter THF resulting in quite moderate conditions of hydrate formation. Total methane gas uptake achieved was not reported in these studies. Sharma et al. [150] studied kinetics of mixed methane hydrate formation and dissociation in presence of 6 mol% THF in a stirred tank reactor configuration. Starting methane pressure was varied between 0.88 and 8.2 MPa at 288 K (cooling from 303 K to experimental temperature after pressurization with methane gas). It was reported that the kinetics of mixed methane hydrate formation with THF was significantly better than that observed for methane hydrate formation in the same experimental setup with similar experimental procedure resulting in reduced hydrate formation times at relatively higher temperatures. Maximum methane uptake reported to be achieved was 88 ( ± 2.6) mmol. Induction time for the mixed methane/THF hydrate formation was found to be faster compared to pure methane hydrate formation. It was also reported that methane hydrate yield using THF was always higher than that observed for pure methane hydrates at same experimental pressures in the same experimental setup studied.

6. Thermodynamic promoters for methane hydrate formation Thermodynamic promoters are compounds that alter/shift the equilibrium conditions of methane hydrate formation. Addition of thermodynamic promoters for hydrate formation results in more moderate formation conditions (lower pressure and high temperature), leading to lower energy requirement during hydrate formation. The drawback of application of thermodynamic promoters is the obvious reduction in methane storage capacity due to the occupation and stabilization of large hydrate cages by promoter molecules themselves. However, the multi fold reduction of pressure at more ambient temperatures offset the reduction in storage capacity. Search for the optimal promoter concentration to achieve the maximum storage capacity at more moderate conditions of formation has been the research direction in this domain. Veluswamy presented a brief review on the thermodynamic and kinetic promoters studied for methane hydrate formation in his thesis work [142]. 6.1. sII structure forming promoters The most well-known sII forming promoter is tetrahydrofuran. Phase equilibrium studies conducted on methane + THF + water system with different concentrations of THF are available in the literature [143–147]. Fig. 2 presents the hydrate phase equilibrium data of methane + THF + water system at different THF concentrations along with hydrate phase equilibrium of methane + water system. It can be clearly seen from Fig. 2 that a profound reduction in equilibrium pressures at higher temperatures was achieved with the application of THF thermodynamic promoter. Kinetics of mixed methane hydrate 268

Applied Energy 216 (2018) 262–285

H.P. Veluswamy et al.

Another study also re-instates the similar formation kinetics of mixed methane hydrate formation in presence of THF [144]. Dissociation study on mixed methane/THF hydrates [151] showed that amongst the temperature studied between 268.2 and 284.2 K, 272.2 K was optimal. It took about 66 days for complete dissociation of mixed methane/THF hydrates maintained under pressure of 0.3 MPa in comparison to just 16 days for pure sI methane hydrates, establishing the fact that mixed sII methane hydrates are more stable than pure sI methane hydrate and thus suited for long-term storage applications. Recently, Veluswamy et al. [152] reported rapid methane hydrate formation in presence of stoichiometric THF solution in a simple unstirred reactor configuration at 7.2 MPa and 283.2 K. THF was reported to behave as a thermodynamic and a kinetic promoter resulting in methane uptake of about 63 mmol gas/mol of water. This uptake was about 12 times higher than the experiment without THF in similar unstirred configuration under comparable pressure driving force. Further, the feasibility to scale up the process without significant deterioration in methane uptake and kinetics was demonstrated in this study. Veluswamy et al. [153] studied the effect of pressure and temperature on the mixed methane/THF hydrate formation kinetics. Experiments were performed at three pressures of 3.0, 5.0 and 7.2 MPa with temperatures of 283.2, 288.2 and 293.2 K. Substantial methane uptake of about 63 mmol gas/mol of water was realized even at 3.0 MPa and 283.2 K in 2 h. In addition, considerable methane uptake with sluggish kinetics was observed closer to ambient temperature (293.2 K) and 7.2 MPa highlighting the potential of this process for commercial applications. Such an uptake would not be possible using pure methane under stated experimental conditions. Cyclopentane (CP) is another sII forming promoter that has been studied for the formation of mixed methane hydrates. Equilibrium data for methane/CP mixed hydrate formation is available in literature [154]. Lv et al. [155] investigated methane hydrate formation kinetics in presence of cyclopentane in a novel large size bubble column reactor. Methane and cyclopentane at different independent flow rates were simultaneously bubbled through the water taken in bubble reactor column in the temperature range of 276.45–280.15 K under 1.0–2.0 MPa pressures. Experimental T and P were varied along with flow rates of methane and cyclopentane to study the hydrate formation kinetics. Formation of thin hydrate shell was observed at CP/water phase boundary. The flow rate of CP did not have much influence on the methane gas consumption/kinetics of hydrate formation. However, increasing the flow rate of CP reduced the induction time for the mixed methane hydrate formation. Improved gas uptake was observed at lower experimental temperatures and higher experimental pressures. Cai et al. [156] investigated methane hydrate formation using cyclopentane promoter in a stirred tank reactor configuration at temperatures > 288 K and pressure < 3 MPa where neither pure methane nor pure CP hydrates can form. Small cage occupancy of methane was analyzed using Raman/NMR and it was found to be independent of methane pressure implying that methane mass transfer, characteristic of a stirred tank reactor configuration, limited the hydrate growth. Other prominent sII forming promoters studied for methane hydrate formation include propane [157], iodomethane [158], propanone (acetone) [147,159,160], 1,4 dioxane [145], 1,3 dioxolane [147], propanol [161], isopropanol [162], tertiary butyl alcohol [163] and propylene oxide [145]. Equilibrium plots reported in literature for different sII promoters excluding THF is provided in Fig. 3.

Fig. 3. Hydrate phase equilibrium data for methane + water system with different sII promoters.

dimethyl butane (neohexane) as the promoter in a Jerguson rocking cell. It was reported with the addition of 0.1 wt% polyvinylpyrrolidone (PVP), increased water to sH hydrate conversion was achieved. Probable reason for the increased water to hydrate conversion was attributed to slow hydrate formation and the occurrence of dendritic shaped hydrate crystals that reduced the occlusion of water during hydrate formation in presence of PVP. Ohmura et al. [166] investigated sH methane hydrate formation using methyl cyclohexane (MCH) promoter in a spray reactor configuration. Water was sprayed downward through methane gas phase onto the liquid-MCH layer lying on the pool of water at 2.8 MPa and 275.2 K. MCH/water ratio was varied during the experimental trials and sH hydrates were observed to form predominantly at promoter/water interface than gas/promoter interface. Tsuji et al. [167] evaluated the performance of 5 sH hydrate promoters – neohexane, 2-methylcyclohexane, pinacolone, isoamyl alcohol and tertiary butyl methyl ether (TBME) in the spray reactor setup used by Ohmura et al. [166] under same experimental conditions. They observed that TBME was the best sH promoter for sH methane hydrate formation owing to higher solubility in water and it was cheaper than other tested sH promoters. Further, Tsuji et al. [168] performed similar study using TBME, MCH and neohexane promoters with natural gas mixture (90% methane + 7% ethane + 3% propane) under similar experimental conditions. They report significant performance of MCH and neohexane in promoting sH hydrate formation better than TBME that had shown the best performance with pure methane gas in their earlier study [167]. MCH that showed weakest promotion effect in presence of pure methane was observed to be the best promoter using natural gas mixture (90% methane + 7% ethane + 3% propane). Hydrate phase equilibrium plots of different sH promoters reported in the literature [165,169–172] is included in Fig. 4.

6.2. sH structure forming promoters sH hydrates were first reported by Ripmeester et al. [164] in 1987, these hydrates were typically formed by very large size guest molecules. Theoretically the volume of methane occupying small and medium cages formed using sH promoter is shown to have the highest volume than sI and sII hydrate structures. Khokar et al. [165] studied methane storage in sH hydrate using 2,2 269

Applied Energy 216 (2018) 262–285

H.P. Veluswamy et al.

significantly affected the induction time rather than the rate of hydrate formation. Recently, Mazraeno et al. [170] experimentally studied the formation kinetics of sH methane hydrate formation in presence of MCH, methylcyclopentane (MCP) and TBME at temperatures in the range of 274.15–277.15 K and at pressures 2.5 MPa above sI equilibrium at each experimental temperature in a stirred tank configuration. Authors also modelled the kinetics of hydrate formation using chemical affinity. Two stage hydrate growth phase obtained during experiments were satisfactorily predicted by the proposed model. From the listed experimental studies using different sH promoters for methane hydrate formation, it is observed that the performance of sH promoter is highly variant depending on the method of hydrate formation (starting from ice or water), gas mixture composition (pure methane or natural gas mixture) and the conditions of hydrate formation (driving force at different T and P). Thus, despite the higher methane storage capacity offered due to cages in sH hydrate structure compared to sII hydrates, the application of these promoters for large scale applications has not yet been advocated primarily due to potential environmental hazards of these immiscible or partially miscible promoters in addition to the high cost incurred for the process. Further, the regeneration ability of sH promoters (use for multiple hydrate formation cycles with associated solvent loss) has not yet been documented in the literature. 6.3. Other hydrate structure forming promoters – semi-clathrates Fig. 4. Hydrate phase equilibrium data for methane + water + sH promoter systems.

Apart from well-known standard structures of hydrates namely sI, sII and sH, methane is also able to occupy and stabilize other hydrate structures like semi-clathrates. Semi-clathrates are a class of promoters in which a part of the compound (anionic) is involved in hydrate cage formation whereas the remaining part (cationic) occupies the cages of the hydrate structure. Characteristic features of these compounds are their stability and ease of hydrate formation close to room temperature and at nominal pressures compared to sI, sII and sH hydrates. Tetraalkylammonium/phosphonium compounds are typical semiclathrate forming compounds. Fowler et al. [177] first reported that quaternary ammonium salts dissolved in water have the ability to form hydrates. The key advantage of the application of semi-clathrates is the ability to store methane at ambient temperatures (without any cooling requirement). Wang et al. [178] studied methane hydrate formation using tetra-iso-amylammonium bromide (TiAAB) semiclathrate promoter (2.6 mol% and 3.7 mol% compositions) in presence of porous emulsion template polymer support. Mixed semiclathrates were formed at 8.6 MPa pressure and 303.2 K. These hydrates were found to be stable at ambient P (1 bar) even at 293 K for at least 4 h. High stability of mixed methane/TiAAB semi-clathrates with no deterioration of methane storage capacity was observed in about 20 cycles of methane hydrate formation and decomposition. However, only 35–40 v/v storage capacity of methane could be achieved against approximately 170 v/v capacity observed in pure methane hydrates. There are also hydrate phase equilibrium studies of methane with other semiclathrates like tetrabutyl ammonium bromide (TBAB) [179–181], tetrabutyl ammonium chloride (TBAC) [182], terabutyl ammonium fluoride (TBAF) [183] and tetraisopentyl ammonium fluoride [184] available in the literature and they are presented in Fig. 5. Thus, despite the possibility of most moderate hydrate formation conditions for mixed methane semi-clathrates, the methane storage is not quite promising for methane/natural gas storage applications.

Lee et al. [173] performed detailed investigation of the kinetics of methane hydrate formation in presence of three different sH forming promoters – neohexane, TBME, and MCH in a semi-batch stirred tank reactor configuration at pressures of 0.63–1.5 MPa above the equilibrium at 273.65 and 275.5 K. The hydrate formation rate was reported to be the fastest for tert-butyl methyl ether (TBME) promoter, slower with neohexane and the slowest using methylcyclohexane (MCH) promoter. At the same driving force, the rate of methane gas consumption during hydrate formation with the TBME was almost 3 times faster than that of the pure methane-water system. The decomposition rate of methane hydrates formed with TBME promoter was also faster than that of the other two promoters studied. Susilo et al. [174,175] also investigated the performance of these three promoters on methane hydrate formation starting from ice. The hydrate formation/crystallization was allowed to proceed for 20 h at 253 K before increasing the temperature to 274 K at either low (4.3 MPa) or high (8.1 MPa) starting pressures. This method of hydrate formation was adopted due to the higher percentage (> 90%) of hydrate formation in a short time without the requirement of mixing. Three distinct stages of hydrate growth were observed depending on the hydrate formation procedure adopted. The first stage involved the nucleation with 20–30% ice to hydrate conversion at ice/liquid promoter interface followed by reduced ice to hydrate conversion rate in the second stage due to the resistance offered by hydrate film formation on the ice. The third stage involved melting of ice due to temperature ramp from 253 to 274 K. It was found that TBME promoter demonstrated the highest rate of hydrate formation and yield compared to other two sH forming promoters in the first two stages of hydrate formation. Yet, it showed the least rate of hydrate conversion during the ice melting stage (third stage) probably due to the increased interaction of TBME with water molecules preventing them from participating in hydrate cage formation. Neohexane was found to be the best sH hydrate former with methane under the experimental conditions studied manifesting complete ice to hydrate conversion in short time with increased methane storage capacity in the hydrate. Servio and Englezos [176] also report the morphology of sH methane hydrate in presence of neohexane at 274.65 K and pressures between 1.8 and 5.6 MPa. It was reported that the driving force

7. Methane storage capacities in different hydrate structures Theoretically computed methane storage capacities (in terms of mmol gas/g of water taken and mg gas/g of water) for sI, sII and sH hydrates is presented in Fig. 6. This calculation was performed with the assumption of complete (100%) cage occupancy in both small and large 270

Applied Energy 216 (2018) 262–285

H.P. Veluswamy et al.

Fig. 7. Volumetric methane storage capacities in different hydrate structures.

7.1. Tuning effect in presence of thermodynamic promoters Kim et al. [185] report a characteristic ‘tuning effect’ during mixed methane/THF hydrate formation wherein THF concentration was varied from stoichiometric concentration of 5.56 mol% (the maximum concentration of THF to occupy large 51262 cages of sII hydrate structure completely) to very low concentration of 0.05 mol% during which they observed a significant increase in occupation of methane in large cages. At stoichiometric composition, there is no room (space) for methane molecules to occupy large cages of sII structure as THF completely occupied the cages. NMR spectroscopy showed a gradual increase in the intensity of signal from large cage of sII structure as the concentration was lowered from 5.56 mol% to 0.2 mol%. However, further lowering the concentration, the NMR signal intensity reduced at 0.1 mol% and no signal could be observed at 0.05 mol%. Thus, the authors refer to 0.2 mol% as the critical guest concentration (CGC) below which the promoter solution is too dilute and methane does not increasingly occupy the large cages as observed for concentrations above CGC. Experiments were performed at 2 MPa pressure and 268 K to remain in sII hydrate formation domain. Authors also observed a similar tuning effect using tertiary butyl amine, another sII forming hydrate promoter with methane at 7 MPa and 268 K with CGC of 1 mol %. From observed results, they postulated that CGC was characteristic of the chemical nature of promoter involving in mixed hydrate formation. In this direction, recently Moryama et al. [186] had confirmed the formation of structure-II hydrates in presence of deuterated tetrahydrofuran and found that the small-cage occupancy of CH4 in structure-II mixed hydrate increased with increasing pressure. A recent molecular dynamics study by Wu et al. [187] reported that the presence of methane and THF had a synergistic effect on the formation of mixed THF-CH4 hydrates. Seo et al. [188] presented the absolute cage occupancies for these mixed THF-CH4 (sII hydrates) in presence of stoichiometric amount of THF to be θs,CH4 = 0.3684, θL,CH4 = 0 and θL,THF = 0.9948 (no methane in large cage). With the aid of Raman spectrum, Prasad et al. [189] verified that methane molecules did not occupy the large cage using 5.88 mol% THF and there was no co-existence of sI hydrates (pure methane hydrates) and sII hydrates at stoichiometric amount of THF under the experimental conditions employed. However, the co-existence of sI and sII hydrates was confirmed by Seo et al. [188] using 3.0 mol% THF with absolute cage occupancies of θs,CH4 = 0.9272, θL,CH4 = 0 and θL,THF = 0.8041 for mixed THFCH4 (sII) hydrates and θs,CH4 = 0.9282, θL,CH4 = 0.9599 for pure methane (sI) hydrates. Experimental procedure adopted by Seo et al. [188] was by reducing the temperature from 298.2 K to 270 K in a reactor that was pressurized with methane at 5 MPa (50 bar). Two different hydrate formation zones were observed during cooling, the first one for the mixed methane-THF sII hydrate (about 298 K) formation followed by pure sI methane hydrate (about 276 K) formation. Chari et al.[190] also observed a similar coexistence of sI and sII hydrates

Fig. 5. Hydrate phase equilibrium data for methane + water + tetralalkylammonium salt systems.

Fig. 6. Gravimetric methane storage capacities in different hydrate structures.

cages for all structures. 1 molecule of methane was assumed to occupy each of the large and small cages in sI structure. For sII and sH, 1 molecule of methane was assumed to occupy each of the small cage and 1 molecule of the promoter was assumed to occupy each of the large cage. Additionally, in case of sH hydrate, 1 molecule of methane was found to occupy each of the medium sized cage as well. Gravimetric capacities presented were computed for sII and sH structures considering representatives of THF and Methylcyclohexane (MCH) respectively. Though the gravimetric weight may vary slightly depending on the promoter chosen (molecular weight of the promoter changes), the gas uptake predominantly lies in the range presented. Fig. 7 presents the volumetric storage capacities computed on similar assumptions as detailed for gravimetric calculations. sI hydrates present the highest 171.8 vol/vol storage capacity at STP. About 30% drop in the volumetric storage capacity is observed for sII hydrates and 15% capacity drop for sH hydrates in comparison to pure sI hydrates. It has to be noted that these capacities were computed on the 100% cage occupancy basis whereas in actual cases, the occupancy might be lower yielding lower volumetric storage capacities. 271

Applied Energy 216 (2018) 262–285

H.P. Veluswamy et al.

It was shown in the study by Veluswamy et al. [153] that presence of small concentration of kinetic promoter, SDS (100 ppm) along with stoichiometric THF (5.6 mol%) is able to significantly promote mixed methane hydrate formation kinetics with completion of hydrate formation in just 1 h with methane uptake of 3.5 kmol/m3 of water even at near ambient temperature of 293.2 K and 7.2 MPa. It was stated that the observed kinetics in presence of SDS at 293.2 K was similar to that at 283.2 K using stoichiometric THF at same starting pressure of 7.2 MPa. Synergistic effect of SDS and THF was envisaged only at high temperatures during mixed methane/THF hydrate formation. Inclusion of SDS kinetic promoter at 283.2 K resulted in about 20% and 60% reduction in methane uptake capacity at starting pressure of 7.2 MPa and 3.0 MPa respectively. Kang et al. [158] report rapid mixed methane hydrate formation in presence of iodomethane and 500 ppm SDS in an unstirred reactor system at 260 K and 1.5 MPa. It was reported that it took just 1.5 min to convert into mixed sII hydrates in quiescent system at aforesaid experimental conditions. Partoon and Javarmardi [159] investigated the kinetics in presence of propanone (acetone) and SDS promoters and inferred that synergistic presence of both promoters enhanced the rate and improved the methane gas storage capacity in hydrates. Experiments were performed with starting pressure of 5.0 MPa and 288 K with slow rate of cooling to 275 K. The stability of mixed hydrates was evaluated at atmospheric pressure and temperature of 268 K. Kumar et al. [196] also experimented methane hydrate formation using combination of THF (at two concentrations of 2.78 mol% and 5.6 mol%) and 0.1 wt% SDS (1000 ppm) starting from 5.0 MPa pressure and cooling in steps to 274.15 K and then to 265.15 K. Though the stoichiometric THF along with SDS resulted in highest rate of hydrate formation, the total gas consumption was less. 0.1 wt% SDS kinetic promoter when used alone resulted in the highest methane storage capacity under experimented conditions. Stability of mixed methane/ THF hydrates was high with slow dissociation rates which was apt for energy storage applications. Lim et al. [197] report enhancement of kinetics in presence of stoichiometric THF solution and 0.003 wt% oxidised multi walled carbon nanotubes (OMWCNT) at 3.0 MPa and 274.15 K with 5.2 times higher gas uptake than with pure water. Mech et al. [198] studied methane hydrate formation in presence of varying concentrations of two thermodynamic promoters -THF and TBAB along with SDS being the kinetic promoter at 276.15 K and three different pressures of 3.0, 5.5 and 7.5 MPa in a stirred tank reactor. It was reported that at 7.5 MPa, synergistic combination of thermodynamic promoter, THF (0.05 mass fraction) and kinetic promoter, SDS (600 ppm) yielded the highest methane uptake and formation kinetics. Whereas, at lower pressure of 3.0 MPa, synergistic combination of two thermodynamic promoters, THF (0.01 mass fraction) and TBAB (0.1 mass fraction) promoters yielded the better uptake and kinetics. Interesting report of employing cyclopentane (sII) hydrate seeds for improving the kinetics of sI methane hydrate formation was put forth by Baek et al. [199]. Erfani et al. [200] performed methane hydrate formation in presence of three sH promoters namely – MCH, MCP and TBME. They evaluated the effect of five non-ionic surfactants - Nonyl Phenol Ethoxylates (NPE), Lauryl Alcohol Ethoxylates (LAE), Ethylene Oxide/ Propylene Oxide copolymer (EO/PO), polyoxyethylene sorbitan mono palmitate (Tween®), Butyl Phenol ethoxylates (Triton™X100) during the sH hydrate formation at 275.2 K and driving force of 3.9 MPa. For MCH based sH hydrates, synergistic effect was observed using the mixture of 0.5 wt% NPE6O and 0.5 wt% Triton with minimal induction time, faster rate coupled with high gas uptake. Best surfactants for promoting sH hydrate formation using TBME and MCP were 1 wt% of TritonX100 and NPE6O respectively. Further, synergistic hydrate formation was also observed by mixing two sH promoters – TBME + MCP and TBME + MCH. The best promotion effect of the entire study was reported to be envisaging combination of TBME + MCH promoters along with the combination of NPE6O + TritonX100 surfactants.

using 0.033 mol fractions of THF promoter and Tertiary butyl amine promoter individually adopting a similar experimental procedure. Another study by Seo et al. [191] examined the large cage occupancy of methane at different THF concentrations of 5.6, 3.0, 1.0, 0.5, 0.2 and 0.1 mol% during the mixed hydrate formation. NMR characterization coupled with in-situ Raman analysis highlight the experimental evidence of methane in large cages of sII hydrates at THF concentration ≤ 1 mol%. Cage occupancies were found to fluctuate with time and were not homogeneous throughout the sample, thus highly dependent on the synthesis procedure adopted for mixed hydrate formation. Recently, Kumar et al. [192] employing a high pressure micro-DSC shed further insights into the formation of mixed THF-CH4 hydrates (sII) without any signature of sI hydrates (pure methane hydrates) using stoichiometric amount of THF. Experiments were performed in a unstirred reactor configuration using 5.56 mol% THF solution that was cooled under atmospheric pressure to ∼275.2 K to form pure THF hydrate (sII structure). This was followed by pressurization with methane to 3.2 MPa at 275.2 K (This pressure was below the equilibrium pressure of methane at 275.2 K to avoid sI hydrate formation). Next step involved the melting of pure THF hydrates when the temperature was increased to 283.2 K that simultaneously resulted in the formation of mixed THF–CH4 hydrates. The final step involved the dissociation of mixed THF–CH4 hydrates by combination of depressurization to 0.5 MPa followed by thermal stimulation to 298.2 K. Choi et al. [193] report the tuning effect with methane occupying large sII cages in presence of lower concentration of tetramethylammonium hydroxide, a unique behavior considering the ionic nature of tetramethyl ammonium hydroxide promoter during methane hydrate formation. Further, mixed hydrates formed were found to be very stable in comparison to pure hydrates formed using teramethylammonium hydroxide and coexistence of sI and sII hydrate structures was observed at higher water concentrations. Susilo et al. [194] present tuning effect of mixed methane hydrates in presence of propane and THF using thermodynamic modelling and molecular dynamic simulations. Kim et al. [195] report a tuning effect observed during the double hydrate formation of methane and tertiary butyl amine (tBuNH2) by varying tBuNH2 concentration between 1.0 and 9.3 mol%. It should be understood that though it is possible to increase the methane occupancy in large cages of hydrate structure (sII) at lower concentrations of promoter, the water to hydrate conversion is significantly lowered at lower concentrations of the promoter. Considering the same volume of 0.5 mol% and 5.56 mol% (stoichiometric) THF solutions, it is reasonable to assume that the water to hydrate conversion employing 0.5 mol% THF solution will be only about 10% of the conversion that is achieved employing 5.56 mol% THF solution in the sII hydrate forming region (as the THF available for mixed hydrate formation is reduced at least by 10 times). Further, hydrate formation conditions will not be quite moderate with only minor deviation from pure sI hydrate equilibrium due to the lower promoter concentration used. Moreover, the stability of mixed hydrates formed at such low concentrations has to be investigated for usage in large scale storage applications. 8. Measures to improve the hydrate formation kinetics using promoters 8.1. Combination of promoters for methane hydrate formation There have been studies in the literature that document the methane/natural gas hydrate formation kinetics with mixture of two kinetic promoters or two thermodynamic promoters or combination of thermodynamic and kinetic promoters. The mixed promoters mainly aim to provide a synergistic effect in improving the kinetics as well as storage capacity (gas uptake). 272

Applied Energy 216 (2018) 262–285

H.P. Veluswamy et al.

Fig. 8. Timeline for the report of different materials employed to enhance the kinetics of methane hydrate formation.

high pressure methane environment to form methane hydrates was also carried out [204]. Optimization of flow rates of liquid streams is important in such reactor configurations or else it may lead to clogging of spray nozzles due to increased hydrate formation in the spray zone. Other engineering challenges of removing hydrate from the reactor coupled with efficient measures to recycle/pump water and promoter streams have to be addressed before scaling these reactor configurations for continuous hydrate production suited for commercial applications. Recently, Xiao et al. [205] outlined the use of vertical reciprocating impacts to improve the hydrate formation rates with improved gas uptake. A study by Wang et al. [47] showed the possibility of storing methane as renewable bio-clathrates. Kinetics of methane hydrate formation in presence of biologically pre-structured materials like mushroom, eggplant, tomato, etc. was studied. Naturally available water content in these materials formed hydrates with maximum methane uptake of 123 v/v reported using mushroom (Agaricus bisporus) at 273.2 K and 9.5 MPa in 500 min. However, methane storage capacity and kinetics was found to deteriorate when these biological materials were subjected to second cycle of methane hydrate formation probably due to the destabilization of biological structures/loss of morphology during the freeze/thaw cycle. A recent review by Linga and Clarke [63] documents and discusses in detail different materials employed for increasing the rate of hydrate formation handling different gases including methane and CO2. Different materials like silica gel, activated carbon, dry water, dry gel, sand, zeolite, hollow silica and aluminum foam were used as fixed bed support to study the methane hydrate formation kinetics [31,125,206–218]. Fig. 8 presents the timeline for the first report on different materials in literature for enhancing the kinetics of methane hydrate formation. Zhou et al. [219] were the first to report an improvement in methane uptake using activated carbon due to hydrate formation when experimenting at pressures of 0–11 MPa and temperatures of 275–283 K. They determined that an optimal water to activated carbon ratio is necessary for achieving an increased methane uptake. Perrin et al. [220] also observed similar increased methane uptake in wet activated carbon in comparison to dry activated carbon predominantly due to hydrate formation at 275.2 K and pressures up to 8 MPa. Yan et al. [221] proposed a mechanism and developed a kinetic model of methane hydrate formation in presence of wet activated carbon. Babu et al. [222] observed the formation of methane hydrates in presence of activated carbon at 277.15 K and 8.0 MPa. Transient hydrate crystal formation/dissociation in the stable hydrate region was observed in the presence of activated carbon under experimental conditions. Siangsai et al. [208] studied the effect of particle size on the kinetics of methane hydrate formation at 8 MPa and 277.2 K. They

Though by combination of different promoters, enhanced methane hydrate formation kinetics and increased gas uptake could be achieved in comparison to the individual promoters, there is an undeniable requirement of optimization of concentration of promoters at hydrate formation conditions (T and P) by using different reactor configurations. The optimization of concentration might be tedious at times, considering the variation in gas phase composition (for practical applications), hence the applicability of the synergistic promotion for large scale hydrate formation has to be explored and documented with the associated limitation. 8.2. Reactor configurations and materials for methane hydrate formation The simplest configuration of providing gas/liquid contact for hydrate formation is the quiescent unstirred system. However, methane hydrate forms as a film commonly referred as “skin” at the interface imposing a mass transfer resistance for further hydrate growth thereby resulting in very low methane gas uptake [81,87]. Employment of mechanical stirring or agitation is one way to improve the gas liquid contact as the agitation renews the gas/liquid interface for improved hydrate formation [77,78]. Effect of kinetic promoters on methane hydrate formation in stirred tank reactor configuration is well documented in the literature [93,101,104,110]. Despite the significant reduction in the induction time using stirred tank reactor, the presence of agitator demands high energy requirement. Thus, it might be difficult to use a stirred tank reactor configuration for large-scale deployment in forming methane hydrates. Other reactor configurations for improved gas/liquid contact during hydrate formation include bubble column [148,155] and spray reactors [107,166,168,201]. Lucia et al. [107] studied methane hydrate formation in a large-scale spray reactor of 25 L internal volume. Authors studied the effect of spraying time, gas pressure, water amount loaded, and differential pressure on gas nozzles on methane uptake and water to hydrate conversion. Best kinetic performance was observed at 8 MPa and 276.6 K using 9 L of 300 ppm surfactant solution. Eject type loop reactor was designed by Tang et al. [202] utilizing the kinetic energy of a high-velocity liquid jet to entrain the gas phase and to create a fine dispersion of two phases for improved methane hydrate formation. Adjustment of gas entrainment rate resulted in three different regimes namely single bubble, intermediate and jet regime for gas/liquid dispersion in the reactor. In presence of a static mixer, micro bubbles were observed to form, shortening induction time of hydrate formation, yet the formation rate was lower in this arrangement. Lang et al. [203] provide an elaborate discussion of different modes of gas/liquid contact available for hydrate formation in the literature. A novel method of impinging jets of water and large guest molecule promoter streams in 273

Applied Energy 216 (2018) 262–285

H.P. Veluswamy et al.

for storing methane as hydrates despite positive attributes of low bulk density and high porosity. There will be considerable cost of hollow silica when used for large-scale storage. Chari et al. [215] report about 80% of water conversion to hydrates by employing nanosilica suspensions with t90 of about 300 min using silica to water ratios of 1:4 and 1:8. Methane uptake was found to be steady for multiple freezing/ thawing cycles signifying improved stability of this method for methane gas storage. Multiwalled carbon nanotubes and oxidised multiwalled carbon nanotubes added in distilled water were reported to enhance methane hydrate formation at optimal concentrations of 0.004 wt% and 0.003 wt % respectively [228]. Methane gas uptake with the use of carbon nanotubes was found to be higher than that observed in water in presence of conventional surfactant like SDS [229]. Further, physically shorter (10–20 µm in length) C95 multiwalled carbon nanotubes resulted in about 3 times higher gas uptake than water (without carbon nanotubes) and performed better than longer C95 multiwalled carbon nanotubes (∼200 µm in length) [230]. Kinetics of methane hydrate formation in presence of hydrophobic and hydrophilic multiwalled carbon nanotubes was put forth by Pasieka et al. [231] with experiments performed at 275.2 K and 4.6 MPa. Hydrophobic multiwalled carbon nanotubes showed better promotion effect of about 6% (in comparison to water without nanotubes) at loading of 5–10 ppm. However, hydrophilic multiwalled carbon nanotubes showed better promotion of about 16% achieved at both 0.1 ppm and 10 ppm loading. 1 wt% hummers graphene nano-fluid functioned efficiently in promoting natural gas hydrate formation reducing the induction time along with the improvement in gas uptake at 277.2 K and 6.9 MPa [232]. Effect of single walled carbon nanotubes of 1 wt% stabilized with 1.5 wt% SDS on methane hydrate formation kinetics was studied by Ghozatloo et al. [233]. Metallic or non-metallic nanoparticles when added to water result in high heat transfer co-efficient nano-fluid that aid in the acceleration of methane hydrate formation. Arjang et al. [234] reported an improvement in methane gas uptake with considerable reduction in induction time in the presence of silver nanoparticles synthesized (using silver nitrate, sodium borohydride and trisodium citrate) at 275.2 K and starting pressures of 4.7 MPa and 5.7 MPa. Najibi et al. [235] reported enhanced methane hydrate formation kinetics, higher water to hydrate conversion and reduction in induction time by employing copper oxide nanoparticles (0.05, 0.1 and 1 wt%) along with 0.035 wt% SDS at 5/6 MPa and 274.65/276.65 K. Aliabadi et al. [236] also observed a similar synergistic promotion effect using copper oxide nanoparticles (optimal 10 ppm concentration) stabilized with 500 ppm SDS at 275.65 K and 5.5 MPa. Pahlavanzadeh et al. [237] studied the effect of copper nanofluids and CTAB surfactant on methane hydrate formation. They report that CTAB surfactant at higher concentrations is effective in enhancing kinetics better than copper nanoparticles. However, high concentration of copper nanofluids exhibited an improved gas uptake. A synergistic effect was observed using optimized concentrations of copper nanoparticles and CTAB resulting in enhanced kinetics and improved gas uptake at 5.5 MPa and 275.2 K. Kakati et al. [238] recently reported kinetic enhancement of natural gas hydrate formation with improved gas uptake using 0.8 wt% of ZnO and Al2O3 nanoparticles (individually) stabilized with 0.03 wt% SDS at 282 K. Recently, Casco et al. [239] studied methane hydrate formation using metal organic frameworks (MOF) for the first time. Hydrophobic (ZIF-8) and hydrophilic (MIL-100 (Fe)) MOFs at 275.2 K and 3–5 MPa were used to form methane hydrates. This is a hybrid method of storing methane as adsorption occurs in addition to methane hydrate formation. It was found that hydrophobic MOFs resulted in a high yield in comparison to hydrophilic MOFs due to the prevention of water accessing the interior cavities (that can effectively adsorb methane in contrast to hydrophilic MOFs where water fills interior pore spaces as well) and predominant hydrate formation in interparticle spaces. Though proof of concept has been established, there are several factors

observed that high water to hydrate conversion of about 96.5% was achieved for activated carbon in the size range of 841–1680 µm (larger size particles). The recovery was found to be higher for small size range, 250–420 µm (about 98.1%) of activated carbon studied. Wang et al. [212] reported short induction times of about 5–10 min with 175 v/v of methane gas uptake at 273.2 K starting from pressure of 8.6 MPa using dry water. Dry water was a mixture prepared using water, hydrophobic silica and air at very high mixing speeds of about 19,000 rpm resulting in free flowing powder. However, considerable reduction in methane uptake and degradation of kinetics were observed when dry water was reused for successive hydrate formation cycles due to the agglomeration of water droplets during the decomposition cycle. Re-blending of dry water was able to reinstate the high methane uptake and improved rate of hydrate formation. Yet, additional energy and reactor modification requirement restricts the usage of dry water for commercial applications. To overcome this problem, dry gel system was proposed which had an additional gelling agent, gellan gum added to the dry water that yielded better recyclability of the material for successive hydrate formation cycles [125]. Addition of gelling agent lowered the methane gas uptake and the rate of hydrate formation was not as high as the application of SDS kinetic promoter in water. Another method of improving the recyclability of dry water suggested by Ding et al. [223], lists the application of hydrogel microspheres along with dry water. This addition aids in the co-stabilizing effect between the hydrogel microspheres and the dry-water droplets suitable for multiple hydrate formation cycles without considerable reduction in methane uptake capacity. Study by Fan et al. [224] documents the application of surfactant dry solution (surfactant solution along with hydrophobic silica particles dispersed in air) that yielded high methane storage capacity with increased formation rate of hydrate formation than dry water [212] due to the presence of kinetic promoter, the SDS surfactant. Recent study by Shi et al. [225] examined the reversible nature of porous hydrogel particles of two polymers for methane hydrate formation and presented the salient criteria for achieving enhanced methane storage capacity. Silica gel of different particles sizes (6 nm, 30 nm and 100 nm) were employed by Kang et al. [226] for methane hydrate formation with driving force in the range of 0.5–4.85 MPa and temperatures in the range of 271.15–277.15 K. Even at lower driving forces, 100 nm silica gels demonstrated higher rates of methane hydrate formation. Type 3A and 5A zeolites were also examined for methane hydrate formation with and without the presence of SDS surfactant at 8.3 MPa and 273.5 K [216]. Aluminum foam employed for methane hydrate formation enhanced the thermal conductivity resulting in improved heat transfer during hydrate formation at 4.2, 6.0 and 8.3 MPa at an experimental temperature of 273.15 K [207]. Linga et al. [218] investigated hydrate formation in presence of methane and hydrocarbon gas mixtures using silica sand. They reported higher water to hydrate conversion along with faster formation rates in presence of sand unlike that of stirred tank reactor under same experimental conditions. Recently, hollow silica was proposed as an alternate porous media material for enhancing the kinetics of methane hydrate formation [213,214]. Hollow silica structure consists of an inner void surrounded by a thin solid outer shell that has several advantageous properties including extremely low bulk density, high porosity, high surface area, etc. Veluswamy et al. [227] studied the morphology of methane hydrate formation and dissociation in presence of various ratios of hollow silica to water. Critical hollow silica to water ratio of 1:6 was reported. Beyond this ratio, hydrates were observed to preferentially crystallize on the top of the hollow silica bed by drawing water from the interstitial pores, whereas below this critical ratio the hydrate formation occurred within the bed between inter-particular spaces of hollow silica. A small fraction of hollow silica was observed to be displaced from the bed during the hydrate formation above the critical hollow silica to water ratio due to the low bulk density of hollow silica. This highlighted the engineering challenge in the application of lightweight hollow silica 274

Applied Energy 216 (2018) 262–285

H.P. Veluswamy et al.

different reactor configurations, different kinetic and thermodynamic promoters (Sections 5 and 6) either individually or as a synergistic combination are evaluated for improving the hydrate formation conditions and the kinetics as discussed in previous sections. Methane is the major component in natural gas (> 90%) and most of the discussed research had used pure methane for studying the hydrate formation kinetics. It is well-known that methane forms sI hydrate structure at rigorous conditions of hydrate formation (hydrate forming pressures of methane are high). Ethane also results in sI hydrate formation at more moderate conditions than methane sI hydrate. For instance, at 278.2 K, formation pressure of ethane hydrate (equilibrium pressure) is just 0.87 MPa as against 7.28 MPa for pure methane. With the addition of higher hydrocarbon, propane the hydrate structure changes to sII due the large size of propane and the hydrate formation conditions are further moderate with only 0.49 MPa required for pure propane hydrate formation at 278.2 K. Thus, another highlight of the SNG technology is that it is not sensitive to the presence of higher hydrocarbons like traces of ethane, propane, etc. due to the fact that they can also be captured in the water cages and essentially result in a milder operating and storage conditions for SNG technology. At 283.2 K, presence of 5% propane (remaining methane) in natural gas results in hydrate formation pressure of just 2.17 MPa in comparison to 7.28 MPa pressure for pure methane gas. However, the composition of gas stored in hydrates might be different from the feed natural gas due to the selective enrichment of one gas over the other. Selective enrichment of higher hydrocarbon gases like ethane and propane in comparison to methane in the hydrate phase has been documented by Seo et al. [240] with the aid of NMR spectroscopy. Also, there are many studies available in literature investigating the hydrate formation kinetics using the mixture of methane, ethane and propane at different compositions mimicking the commercial natural gas [218,241–243]. So far, this review has discussed different methods, promoters and novel materials that were investigated to enhance the kinetics of hydrate formation to store methane/natural gas for the commercial deployment of SNG technology. Another important stage in the SNG process cycle is the storage of hydrates. It is necessary to maintain the pressure and temperature of hydrates at or above equilibrium in order to enable the stable and prolonged storage of hydrates and prevent dissociation. Gudmundsson et al. [244] investigated the stability of a natural gas hydrate (formed from natural gas having 92 mol% methane, 5 mol% ethane, and 3 mol% propane in a STR at pressures of 2–6 MPa in temperature range of 273.15–293.15 K) by storing hydrates in deep freezers at temperature of 268.2 K, 263. 2 K and 255.2 K under atmospheric pressures and report that hydrates synthesized were stable up to 10 days under these storage conditions. The dissociation temperature for pure methane hydrate at 1 atm is 193 K. Thus, based on thermodynamics, hydrates will not be stable at any temperature higher than 193 K at atmospheric pressure. In other words, hydrates have to be

that have to be examined in order to utilize MOFs for hydrate formation suitable for commercial applications. Despite the improved methane uptake capacity and faster hydrate formation kinetics due to the presence of higher surface area of contact in presence of porous media (materials), specific engineering challenges exist when applied for natural gas storage on a large scale. Additional material cost and handling incurred in the employment of porous media also impose hurdle for large-scale deployment of these materials for methane storage. Further, there is a considerable lowering of gravimetric storage capacity and regenerability issues for using the porous media for repeated hydrate formation/dissociation cycles when employing porous medium for hydrate formation. 9. SNG technology – pathway to commercialization 9.1. Process chain and technology status SNG technology is a cost effective gas storage (due to milder production and storage conditions) option, resulting in high volumetric storage capacity, being environmentally benign [47], with the relative ease in gas recovery, being non-explosive and very safe to handle. Moreover, being a physical process, energy recovery from SNG is very easy (using low waste heat or seawater) and the energy loss during energy recovery is non-existent. Fig. 9 presents the schematic of the process chain involved in storing natural gas in the form of hydrates (SNG) technology. The four steps involved include - natural gas hydrates formation; dewatering step to remove the un-reacted (unconverted) water; pelletizing step to form hydrate pellets; finally cooling & depressurizing step to reach the necessitated storage conditions. Major research challenges for SNG production and storage system are in the formation step (slow kinetics of hydrate formation, severe operating process conditions) and during the storage step, (refrigeration required to maintain low temperature of about −20 °C relying on the anomalous ‘self-preservation’ of hydrates). Dewatering and pelleting step are established engineering processes and can be readily applied for SNG technology. Possible approaches to overcome the slow kinetics are: to employ an effective and innovative reactor configuration for enhancing kinetics with improved gas/liquid contact. The prospective reactor configuration should not be energy intensive for large-scale deployment. Another approach is to choose suitable promoters that can enhance the formation kinetics with the ability to operate at moderate experimental conditions without significant compromise on the storage capacity. While recent literature works have shown enhanced kinetics in the presence of porous materials, in the overall SNG production and storage process chain (Fig. 9), the use of porous material is less practical due to the inability to pelletize the hydrates along with the porous materials for effective storage and transport applications. Apart from

Fig. 9. Process flow sheet of a natural gas production and storage chain via hydrate technology (modified from Veluswamy et al. [152]).

275

Applied Energy 216 (2018) 262–285

H.P. Veluswamy et al.

Continuous Pelletizing system (TPCP), suited for continuous production of NGH pellets. Hydrate pelletization process was conducted inside the 253 K (−20 deg C) freezer in order to prevent dissociation of the NGH banking on the self-preservation. rectangular parallelepipeds of 10 mm × 10 mm × 11 mm dimensions was cut from pellet strip of NGH extruded from the TPCP. The effect of feeding pressure, rotational speed, and pressure ratio of TPCP were evaluated for making NGH pellets. A 1-Ton/day capacity NGH pellet production has been demonstrated by South Korean researchers at Korea Institute of Industrial Technology (KITECH) in 2012. The NGH pellet plant has 5 main process consisting of NGH formation & dewatering, cooling and depressurizing along with pelletizing. In addition NGH storage and regasification process (5 m3/day) were also included in order to investigate all segments of NGH supply chain (production - transportation - storage regasification). Actual plant images are provided in Fig. 11. In order to enhance hydrate formation and effectively remove the un-reacted water, they used a double helix hydrate reactor which includes cooling jacket with different winding-direction of helix blades (US 8,936,759 B2) [248]. A summary table of patents associated to SNG technology for innovative methods and apparatus is presented in the supplementary information (Table TS1). In 1970 s Exxon Research Engineering Company and Chevron Research Company invented an approach for transportation of natural gas in the form of hydrates (US3,514,274 [249], US 3,975,167 A [250]). From 1990s, number of inventions began to increase in the SNG technology, Jon Steinar Gudmundsson invented the method for the production of gas hydrates for storage and transportation. Hydrates synthesized in a reactor were withdrawn and

stored at 193 K (-80 deg C) or lesser under atmospheric pressure to prevent dissociation. Such a temperature is too low and storage under such drastic conditions incurs huge cost that may affect the scale up of SNG technology. However, a characteristic phenomenon termed as “Self-preservation effect”, is observed due to which methane hydrates are kinetically stable in 240–271 K temperature regime, despite being in unstable thermodynamic zone. This anomaly aids in the preservation of the hydrate structure for duration up to few days or weeks, which is promising for the storage/transport of natural gas. However, such an anomalous behavior is not yet understood and ongoing research aims to study and decipher the self-preservation behavior. This has been discussed in detail in the next section considering its importance to the storage of hydrates. At present, state of the art of SNG technology for transportation purposes is dominated by Japan, in particular by Mitsui Engineering & Shipbuilding (MES) and partners. In 2003, MES constructed a Process Development Unit (PDU) along with Japan Oil, Gas and Metals National Corporation (JOGMEC) to demonstrate the continuous NGH production, pelletization, storage and re-gasification. The production capacity of PDU was 600 kg per day of methane hydrate. Successively in 2005, MES accomplished an NGH experimental production plant supported by JOGMEC again to develop new and competitive process of NGH with natural gas mixture where dewatering and high-pressure pelletizer were implemented. This second-stage NGH production plant called Bench Scale Unit (BSU) was at Chiba works in MES [9]. Following this 5-Ton/day hydrate pellet producing pilot plant was set up at Yanai Power Plant in 2008. Fig. 10 presents the hydrate pellet machine along with produced hydrate pellets developed by MES. Lee et al. [247] designed an extrusion-type Twin-roll Press for

Fig. 10. Hydrate Pellet Machine (HPM) to pelletize NGH developed by Mitsui Engineering & Shipbuilding, Japan and the corresponding NGH pellets. (Images adopted and modified from Murayama et al. [245], and Mimachi et al. [246]).

276

Applied Energy 216 (2018) 262–285

H.P. Veluswamy et al.

Fig. 11. NGH production plant setup by South Korean researchers at Korean Institute of Industrial Technology (KITECH). Images courtesy of KITECH.

hydrate equilibrium of 193 K at 1 atm. Authors report characteristic three regimes during the dissociation of methane hydrates by their formulated pressure release method. Temperature regime between 241 and 271 K was distinct in retaining > 90 vol% methane hydrates for about 10 h when dissociating rather than temperature regimes of 195–240 K and > 272 K in which > 90 vol% of similar methane hydrate sample dissociated in just 10 s. Further, in the reported anomalous hydrate preservation zone (241–271 K), the rate of hydrate dissociation observed was not uniform, there existed variation in hydrate dissociation rates. Hydrate dissociation observed in between 256 and 264 K displayed rates higher than the domains between 245 and 255 K or 265–271 K. Takeya et al. [269] measured dissociate rates of methane hydrates using X-ray diffraction at atmospheric pressure and temperatures between 148 K and 253 K. Authors attribute slow dissociation rates observed between 168 and 198 K due to the ice shielding effect. Giavarani and Maccioni [270] further investigated the self-preservation effect by varying the ice content in hydrates, at low pressures between 0.1 and 0.3 MPa in the temperature range of 269–272 K and characterized the hydrates using modulated differential scanning calorimetry (MDSC). They reported a reduced dissociation rate for samples with lower concentration of gas in hydrates (high ice composition) at slightly higher pressures of 0.3 MPa than 0.1 MPa (atmospheric pressure) stating it to be more feasible and economical for practical applications. Takeya et al. [271] investigated the effect of particle size on selfpreservation of methane hydrates using X-ray diffraction at atmospheric pressure and temperatures between 135 K and 263 K. Different size ranges of hydrate particles between 20 and 1400 µm were employed in the study. Large sized particles were demonstrated to preserve methane gas in hydrates in comparison to small particles at equal volumes and same experimental temperature. From X-ray diffraction study, they were able to compute the thickness of ice observed for different hydrate particle sizes and they state that the ice layer thickness is similar for all hydrate particle size ranges employed when

passed to an agglomeration step in order to increase the density of the hydrate and to embed more gas in the interstices between hydrate particles. Agglomerated hydrate particles were then transported to a transportation unit or storage container under adiabatic conditions at atmosphere pressure or at a slightly high pressure at sub-zero temperatures (−10 °C to −15 °C) (WO 1,993,001,153 A1 [251], US 5,536,893 A (1996) [252], WO 1,996,041,096 A1 [253]). In 1999, Mobil Oil Corporation invented a gas hydrate storage reservoir where heat energy from the sun can be used to dissociate hydrates whenever gas is required by the user (US 5,964,093 A [254]). Later in the 21st century, innovations in SNG technology were dominated by Mitsui Engineering & Shipbuilding (MES) and partners. They invented various apparatus for production of hydrates, dewatering and pelletizing devices (EP 2006362 A1 [255], EP 2218766 A1 [256], EP 2130896 A1 [257], WO 2003006589 A1 [258], WO 2003019068 A2 [259], US 8096798 B2 [260], US 9039949 B2 [261], US 20140203471 A1 [262], US 20050107648 A1 [263]). Korea has also played a remarkable role in inventions of SNG technology, Ju Dong Lee and his co-workers invented an apparatus for continuously producing and pelletizing gas hydrates using dual cylinder (US 8486340 B2 [264]). Kang et al. [265] invented a container for storing, transporting, and dissociating hydrate pellets. This container comprised an outer container made up of plurality of frames; an inner container that was rotatable, storing hydrate pellets within that had an internal insulated surface. A refrigerating machine was installed in the outer container and the inner container was equipped with a heating wire to dissociate the hydrate pellets (EP 2781468 B1 [265]). 9.2. Self-preservation effect and challenges The term “self-preservation’ was first coined by Yakushev and Istomin [266] and later investigated in detail by Stern and his coworkers [244,267,268]. Stern et al. [8] demonstrated the bulk preservation of porous pure methane hydrate about 50–75 K above the 277

Applied Energy 216 (2018) 262–285

H.P. Veluswamy et al.

when subjected to dissociation by depressurization at 268 K and 0.1 MPa (1 atm). Despite the well-known stability of sII hydrates at higher temperatures and lower pressures in comparison to sI hydrates, about 96% of the sII hydrate formed using 91%methane-9% ethane gas mixture dissociated in just 3 min when depressurized to 1 atm at 268 K. This temperature was demonstrated to be the most optimal for preserving sI hydrates in their early study. Further, Takeya and Ripmeester [280] reported that the guest gas molecule rather than the structure of hydrates significantly influenced the self-preservation effect. Interaction of a number of guest gas molecules including CH4, C2H6, CH2F2, CHF3, Xe, H2S, CH3F, CF4, CO2, O2, N2, Ar, and Kr with water and the influence of the dissociating pressure on anomalous preservation behavior of hydrates formed using these guests were examined in their study. Also, another study by Takeya and Ripmeester [281] presented plate-like ice crystal growth during the dissociation of CH4 (sI) hydrates whereas no such crystals were observed during the dissociation of C2H6, CH4 + C2H6 and C3H8 hydrates postulating that morphology of hexagonal ice during the hydrate formation played a prominent role in aiding the anomalous self-preservation. Wen et al. [282] presented a review on the progress of research work in understanding the self-preservation effect. They highlight that though there is an increased understanding of the anomalous behavior, further studies on hydrate formation mechanisms and parameters influencing the self-preservation effect are necessary for establishing the commercial storage and transportation of natural gas via clathrate hydrates. Zhang and Rogers [283] reported the first time observation of self-preservation effect in sII hydrates formed using natural gas. They reported ultrastability of sI hydrates from pure methane as well as sII hydrates formed from C1 (90%)–C2 (6%) –C3 (4%) gas mixture in presence of sodium dodecyl sulfate surfactant at 268.2 K under atmospheric pressure. Ultrastability was attributed predominantly to the difference of hydrate formation mechanism in presence of the surfactant. Small size of hydrates with large surface area formed in presence of the surfactant contain small voids that have the ability to contain the equilibrium pressure with minimal layer of ice thickness. High heat and mass transfer rates with minimum exterior surface area of hydrate mass along with better symmetry was observed during hydrate formation in presence of surfactant. This resulted in ultra-stability of sII natural gas hydrates for at least 10 days at temperatures as high as 270.2 K under atmospheric pressures with only 0.04% of gas released from total gas stored in hydrates. Mimachi et al. [246] evaluated the long-term storage of natural gas hydrate pellets containing CH4, C2H6 and C3H8 for envisaging hydrate based process for natural gas storage and transportation. They demonstrated the successful storage of hydrate pellets produced from bench scale unit for 3 months at 253 K and atmospheric pressure. It was found > 70% mass fraction of hydrates remained even after three months of pellet storage at listed storage conditions holding the promise of hydrates for storing natural gas. In absence of any thermodynamic promoter, hydrates must rely on the ‘self-preservation’ effect for utilizing natural gas hydrates for practical NG storage and transport applications. Though there is an increased understanding of the anomalous preservation in recent years, the exact underlying mechanism is not yet established. Further, though the hydrate dissociation rate is very less under temperature conditions for utilizing the self-preservation effect, there is always a small percentage of natural gas loss during the storage. In addition, for large scale storage facilities, the ice formation on the shell of the hydrate pellets cannot be controlled that may result in deformation of hydrate pellets resulting in increased gas release due to dissociation. Thus, a paradigm shift from persisting with pure methane hydrate (sI hydrate) to other structures (like sII or sH or semi-clathrate) that require the presence of a thermodynamic promoter is required in order to realize the potential of commercializing SNG technology. In presence of a thermodynamic promoter like tetrahydrofuran, for instance, SNG storage can be envisaged without the dependence on self-preservation at more moderate temperatures (268–273 K). In addition, mixed hydrates

dissociated at temperatures less than 190 K. However, when dissociated at higher temperatures, larger size particles were observed to have a thicker layer of ice than smaller size hydrate particles. Mimachi et al. [272] also studied the particle size effect on the self-preservation of methane hydrate and report that hydrates of particle size > 0.5 mm are able to be preserved for two weeks at 253 K and 1 atm. Stoporev et al. [273] demonstrated for the first time the self-preservation behavior using small size hydrate particle (< 42 µm) suspension in oil and established that effect of hydrate sample size on self-preservation was less pronounced in comparison to conventional hydrate samples (without oil). Further investigation revealed the formation of a dense ice shell on the hydrate particles facilitated by heavy hydrophobic–hydrophilic oil components that adsorb on hydrate particles from oil suspensions. This study highlighted the possibility of controlling the stability of hydrate particles by adsorbing selected favorable components on the hydrate particle surfaces. ‘Ice shielding’ effect i.e. the presence of thin film of ice on hydrate sample or presence of significant amount of ice in hydrate sample was proposed as the most plausible reason for the self-preservation effect observed in the temperature domain of 241–271 K. However, at the most optimal temperature for the observation of self-preservation effect, 269 ± 1 K, wherein the dissociation rate was the least resulted in only a very thin layer of ice of 4 µm on each hydrate grain. Ability of such a thin ice layer able to contain the high methane pressure (about 2 MPa) of hydrate was not convincing [267]. Also, the ice fraction did not occur uniformly on the hydrate grain surface, thus the actual reason for such anomalous preservation is not yet conclusively known. However, the so-called ‘ice-shielding’ does play a significant role in preserving the gas hydrates but may not be the only reason to explain the anomalous behavior. Other factors that attribute to the self-preservation of hydrates include microstructure of ice, the number and type of ice defects and the annealing rates of ice crystals as reported in the literature [274,275]. Fig. 12 presents the main factors that influence the self-preservation effect exhibited by hydrates. This figure has been adapted from the study by Rehder et al. [276]. Bai et al. [277] performed molecular dynamics simulations to reveal the coupling between mass transfer resistance and heat transfer resistance as the driving mechanism for self-preservation effect in hydrates. Vlasov [278] presented a diffusion model of gas hydrate dissociation into ice and gas providing an explaintion for self-preservation in terms of the developed model. Self-preservation effect in sI hydrates (formed from methane gas) is predominantly available in literature. However, the same effect in presence of other guest gases (resulting in other hydrate structures than sI) was not observed. Stern et al. [279] reported that sII hydrates formed from 91%methane-9% ethane gas mixture did not preserve

Fig. 12. Main components influencing the self-preservation effect. Fields written in italics are recognized to be important for the anomaly, but the underlying mechanisms for the same are still unclear. Adapted from Rehder et al. [276].

278

Applied Energy 216 (2018) 262–285

H.P. Veluswamy et al.

media on the total cost were evaluated. Total capital investment, operation/maintenance costs and total cost for production of natural gas hydrates (NGH) along with the marine transportation (shipping) costs were calculated and presented. The capital cost for the LNG as an established method was about 48% higher than the proposed NGH method for production, storage and transportation of natural gas. LNG requiring huge capital investment was found to be the best option to transport NG from huge gas reserves whereas for stranded gas reserves, hydrate based NG storage technology was the most suitable option considering the economics and feasibility. Favorable economics for hydrate based process for NG transport were reinforced by similar simulation and case studies [292–294]. Najebi et al. [295] evaluated piped natural gas (PNG), CNG, LNG and NGH (natural gas hydrates) technologies for transporting 100 × 106 standard m3/d natural gas from Assaluyeh port in south of Iran to potential markets. They report that PNG is the best option for NG transport for distance up to 2700 km and LNG was the best NG transportation mode for distance above 2700 km. They also state that hydrate based transport system was not so effective considering the huge capacity and high temperature of Persian Gulf that was envisaged at 300 K. Further, Khalilpour and Karimi [296] evaluated CNG, LNG, Gas to liquids (GTL) and NGH (natural gas hydrates) as options for transporting NG from stranded gas resources. Each of these options were evaluated based on the reservoir capacity, distance to market, safety, process CAPEX and OPEX and they present a plot with the sweet spot for each of the evaluated technologies as a function of associated capacity and distance to market. It was found that NG transport vide hydrates is the apt selection for handling lower capacity for longer distances. Rehder et al. [276] as a part of the German project, Submarine Gas Hydrate Resources (SUGAR) re-investigated the gas transport by gas hydrate pellets. Economic evaluation of methane hydrate based transport technology in comparison to pipeline, LNG and CNG transportation was performed. Capital investment as well as operational costs were considered for the analysis. Wide set of scenarios with production rates from 20 to 800 103 Nm3 h−1 and transport distances from 200 to 10,000 km were employed. Model calculations performed manifest that no economic benefit of methane hydrate transportation exist versus competing technologies. They also outlined the conceptual design of methane hydrate pellet carrier ship with a specialized containment system. This system comprised of eight cylindrical cargo tanks, that were arranged horizontally and pivot-mounted. This arrangement ensured tank rotation, so that it would avoid sintering of methane hydrate pellets during ship transport without which the unloading process would be complicated (arising from sintered hydrate pellets). Specialized cooled cargo holds and conveying system were proposed to load/unload hydrate pellets from the cylindrical pellet storage tanks. The entire containment system including the conveyor was designed for a pressure of up to 2 bar and temperature of −20 °C to stabilize the hydrate pellets in the self-preservation during transportation. Earlier report on concept and features of carriers by Ota et al. [297] presented the storage and transport of hydrate pellets for inland transportation which was later demonstrated in the 5-Ton/day hydrate pellet producing pilot plant put forth in Japan in 2008. Further, a detailed report on the conceptual design of hydrate pellet carrying system in Korea was provided by Kim et al. [298] in 2014. Despite contradicting reports on the economics of natural gas hydrate based process for transporting natural gas, it has to be noted that all listed studies consider only sI methane hydrate formation in a conventional stirred tank reactor in the absence of thermodynamic promoter. Further, storage conditions considered were in the ‘self-preservation’ regime. Also, there is substantial amount of hydrate slurry to be handled during the process of hydrate pellet production. Thus, with inclusion of thermodynamic promoter, both the hydrate formation and storage conditions will be more moderate with minimal handling/ maintenance cost involved in handling hydrate slurry resulting in improved overall cost economics (both capital and operating costs) in

formed are completely stable at these moderate temperatures resulting in no loss of natural gas making it suitable for long-term NG storage. 9.3. Dissociation and recovery of stored gas from hydrate pellets In SNG technology, it has been demonstrated to store and transport gas (methane or natural gas) vide hydrate pellets. Pure methane as well as natural gas hydrate pellets have to be stored at lower temperatures (in the range of 241–271 K) under ambient pressure to ensure the stability (banking on the self-preservation effect). However, in presence of thermodynamic promoter, the storage can be envisaged at much more moderate temperatures even above 273 K without relying on the less understood self-preservation. Recovery of the stored gas is also salient aspect of the SNG technology, it is expected that close to 100% recovery of the stored gas is possible vide SNG technology. Thus it is highly advantageous than ANG (adsorbed natural gas) where the complete recovery is not feasible. To recover the stored methane/natural gas from hydrate pellets (with and without thermodynamic promoters), a simple thermal stimulation is proposed. Low waste heat or minimal heat corresponding to the enthalpy of dissociation is required to melt hydrates and release the gas. Also, this approach is simple and has been amply discussed in the literature [16,284,285]. These listed works on thermal stimulation are based on the perspective of recovering gas from hydrate reservoirs. There are only few studies that investigate the dissociation of hydrate from synethesized pellets from the laboratory. Lee et al. [286] studied the effect of hot water injection on the hydrate pellet dissociation by observing the change in the shape of the hydrate pellet with varying parameters that include water temperature, flow rate and hydrate conversion (during the hydrate formation). The recovered gas bubbling from the pellet resulted in a secondary flow that resulted in enhanced dissociation of hydrate pellet. Kawamura et al. [287,288] studied the dissociation characteristics of mixed hydrate pellets (prepared using C1eC2, C1eC3 and C1eC2eC3 gas mixtures) under different isobaric/isothermal conditions in presence of water and aqueous xanthan gum solutions (mimicking the drilling fluid) and have presented an analytical model for mixed gas hydrate pellet dissociation. More focused studies on the dissociation of pure and mixed methane hydrates (in presence of thermodynamic promoters) pellets are essential in order to scaleup the dissociation process to recover the stored gas from hydrate pellets suited for SNG technology. Unlike the drastic release of gas from conventional NG storage methodologies, controlled release of gas from hydrates is feasible and is achieved by supplying the necessary heat at any desired location suiting the desired application. It has to be noted that the enthalpy of dissociation for pure sI methane hydrates and mixed methane/THF hydrates (sII hydrates) is reported to be 54.44 ± 1.45 kJ/mol [289] and 144 ± 0.5 kJ/mol [63] respectively. Though the dissociation enthalpy is significantly higher for mixed hydrates, the offset of storage/formation temperature conditions offered by mixed sII hydrates outweigh the energy required to dissociate hydrate pellets. Further as outlined earlier, low waste heat can be utilized for the dissociation of hydrate pellets. 9.4. Cost analysis of hydrates based technology for storing/transporting natural gas Gudmundsson and Borehang [290] were the first to propose and evaluate the prospects of employing natural gas hydrates for transporting natural gas in comparison to LNG. They calculated capital cost required for transporting 400 MMscf/day of natural gas for a distance of 5500 km using both LNG and gas hydrates. They reported at least 25% lower capital cost for hydrate based natural gas transport in comparison to conventional LNG process. Javanmardi et al. [291] performed a detailed economic analysis for envisaging natural gas hydrates for natural gas transportation. The effects of operational conditions like hydrate storage temperature and seawater temperature as cooling 279

Applied Energy 216 (2018) 262–285

H.P. Veluswamy et al.

the highest stability with the dissociation temperature of 4.5 deg C (84.7 deg C above sI hydrate dissociation temperature). Thus, the temperature requirement during storage of sII hydrates will be very moderate in comparison to sI and sH hydrates. Optimization of the promoter concentration will be a critical step and it requires the need for molecular level investigations (for understanding the cage occupancy that results in the tuning effect) coupled with the reliability of using the optimized concentration during scale-up of the process. The presence of thermodynamic promoter does offset the gas storage capacity due to the inclusion of thermodynamic promoter into hydrate structure. However, the hydrate formation at reduced pressures (reduced compression cost) and higher temperature (reduced cooling cost) is more attractive considering the overall process economics. Lucia et al. [107] reported that compression cost attributed to 70–80% of the total cost for methane hydrate formation in a large-scale spray reactor of 25 L internal volume. Thus, in order to minimize the overall process cost on a large scale, it is required to form hydrates only at moderate pressures. Hence, addition of thermodynamic promoter is quintessential for optimizing the overall process cost despite a minor compromise in the methane/natural gas storage capacity. Further, hydrates formed in presence of thermodynamic promoters are very stable, so the hydrate storage is possible at temperatures even close to 273.2 K at atmospheric pressures and there is no need to rely on the anomalous ‘self-preservation’ effect. Research on promising thermodynamic promoters that are environmentally benign with the ability to form hydrates rapidly in simple reactor configurations demonstrating substantial gas storage capacity is required. Also, the effect of thermodynamic promoter in presence of higher hydrocarbons (like ethane and propane) and other impurities (like CO2) of natural gas in influencing the hydrate formation kinetics must be studied. Hydrate storage and stability studies for longer duration are necessary for the commercial deployment of the SNG technology. Thus, SNG technology requires hydrate formation at moderate pressures and close to ambient temperatures with higher rates of hydrate formation in a simple inexpensive reactor configuration. Storage of hydrates at atmospheric pressure and ambient temperatures must be the target to make SNG technology the most promising and competing technology for storing natural gas. SNG technology is well suited for stationary applications handling lower volumes of methane (natural gas). A promising option will be to employ the SNG technology for storing and transporting methane produced from biomass gasification plants. Predominantly, the output of the biogas plant comprises of CH4 (about 60%) and CO2 (about 40%); the methane from the biogas can be separated and stored vide SNG technology. As reported recently by Veluswamy et al. [153], it is possbile to rapidly form mixed methane/THF hydrates (sII hydrates) at moderate pressures (3.0–5.0 MPa) and at temperature close to room temperatures by synergistically combining with small quantities of surfactants. Volume reduction of about 110 times compared to conditions at STP is attractive for such small scale storage and transport. A schematic of the proposed process is provided in Fig. 14. Economic feasibility of SNG technology could be further improved by using seawater as a feed instead of pure water. The major constituents of seawater are dissolved salts, particularly sodium chloride (NaCl). Kumar et al. [63] report that the presence of 3.0 wt% NaCl had no significant effect on the equilibrium conditions of mixed CH4-THF hydrates at low pressure (less than 1.2 MPa) for a temperature range of 285.9–290.5 K. However, slight deviation in equilibrium curves occured at higher pressure (> 2.0 MPa). Further kinetic studies need to be performed to elucidate the kinetic feasibility of employing seawater or saline water for the use of SNG technology.

comparison to the competing conventional NG storage technologies. However, the environmental impact of the added thermodynamic promoter has to be studied for employing the process commercially for NG storage/transport applications. 9.5. Safety aspects of SNG technology As outlined in the introduction section, hydrate based SNG technology is the safest option to store and transport natural gas. Methane or natural gas stored in hydrates can be released in a non-explosive manner and can be easily contained even when ignited unlike the conventional modes of NG storage. As hydrates dissociate to yield water (with or without promoter) and gas, the generated water curtails the explosive nature of NG unlike that for LNG or CNG that is prone to explosion on ignition. Kim et al. [299] present a detailed Hazard Identification (HAZID) study for employing natural gas hydrate (NGH) carriers (akin to LNG carriers) for transporting hydrate pellets. They concluded that majority of hazards to the NGH carrier fall in the acceptable risk region and indicated the necessity of HAZOP (Hazard and operatability) study to be performed. This clearly demonstrates the employability of hydrate based technology for practical applications. 9.6. Future directions SNG technology holds a significant promise for long-term largescale natural gas storage. There are two routes for the SNG technology, the one in which pure sI hydrates are formed (in presence of methane) and sII hydrates (in presence of ethane, propane and higher hydrocarbons) stored in the temperature regime relying on the ‘self-preservation’ effect. This necessitates further understanding of the anomalous behavior at different temperatures, for different size and shapes of hydrate pellets. This also mandates the demonstration of reliable longterm storage (in terms of few months to years) of hydrate pellets under optimized temperature suited for commercial applications. The alternative and the most promising route is by the addition of thermodynamic promoter at optimal concentrations for effective and efficient hydrate formation/storage. Fig. 13 shows the dissociation temperature for the different structures sI (methane), sII (methane + stoichiometric THF) and sH (methane + stoichiometric methyl cyclohexane). It can be seen that sI methane hydrate has the lowest dissociation temperature, implying the least stability amongst the three structures. In presence of thermodynamic promoters, the stability is improved resulting in higher dissociation temperatures. sH hydrate formed using stoichiometric methyl cyclohexane has dissociation temperature of −56.7 deg C (obtained from CSMGem), 23.5 deg C higher than sI methane hydrate without any promoter. sII hydrate in presence of stoichiometric THF demonstrates

10. Conclusion This review comprehensively summarizes the current status of storing natural gas in the form of hydrates. Different experimental

Fig. 13. Dissociation temperatures for different hydrate structures at 1 atm.

280

Applied Energy 216 (2018) 262–285

H.P. Veluswamy et al.

challenges. J Am Chem Soc 2013;135:11887–94. [8] Stern LA, Circone S, Kirby SH, Durham WB. Preservation of methane hydrate at 1 atm. Energy Fuels 2001;15:499–501. [9] Watanabe S, Takahashi S, Mizubayashi H, Murata S, Murakami H. A demonstration project of NGH land transportation system. In: Proceedings of the 6th international conference on gas hydrates; 2008. p. 6–10. [10] Strobel TA, Koh CA, Sloan ED. Thermodynamic predictions of various tetrahydrofuran and hydrogen clathrate hydrates. Fluid Phase Equilib 2009;280:61–7. [11] Ribeiro Jr CP, Lage PLC. Modelling of hydrate formation kinetics: State-of-the-art and future directions. Chem Eng Sci 2008;63:2007–34. [12] English NJ, MacElroy JMD. Perspectives on molecular simulation of clathrate hydrates: Progress, prospects and challenges. Chem Eng Sci 2015;121:133–56. [13] Yin Z, Chong ZR, Tan HK, Linga P. Review of gas hydrate dissociation kinetic models for energy recovery. J Nat Gas Sci Eng 2016;35, Part B:1362–87. [14] Komatsu H, Ota M, Smith Jr RL, Inomata H. Review of CO2–CH4 clathrate hydrate replacement reaction laboratory studies – Properties and kinetics. J Taiwan Inst Chem Eng 2013;44:517–37. [15] Sun Y, Lü X, Guo W. A review on simulation models for exploration and exploitation of natural gas hydrate. Arabian J Geosci 2014;7:2199–214. [16] Chong ZR, Yang SHB, Babu P, Linga P, Li X-S. Review of natural gas hydrates as an energy resource: Prospects and challenges. Appl Energy 2016;162:1633–52. [17] Demirbas A. Processes for methane production from gas hydrates. Methane gas hydrate. London: Springer; 2010. p. 161–81. [18] Davidson DW. Gas hydrates. In: Frank F, editor. Water: a comprehensive treatise. New York: Plenum Press; 1973. [19] Englezos P. Clathrate hydrates. Ind Eng Chem Res 1993;32:1251–74. [20] Sloan ED, Koh CA. Clathrate hydrates of natural gases. CRC Press; 2008. [21] Hammerschmidt EG. Formation of gas hydrates in natural gas transmission lines. Ind Eng Chem 1934;26:851–5. [22] Makogon YF, Trebin FA, Trofimuk AA, Tsarev VP, Cherskii N. Detection of a pool of natural gas in a solid (hydrated gas) state. Doklady Akademii Nauk SSSR. 1971;196:203–6. [23] Makogon YF. Hydrates of natural gas. Tulsa: Penn Well Books; 1981. [24] Kvenvolden K. Potential effects of gas hydrate on human welfare. Proc Natl Acad Sci US 1999;96:3420. [25] Rempel AW, Buffett BA. Formation and accumulation of gas hydrate in porous media. J Geophys Res B: Solid Earth 1997;102:10151–64. [26] Zatsepina OY, Buffett BA. Phase equilibrium of gas hydrate: Implications for the formation of hydrate in the deep sea floor. Geophys Res Lett 1997;24:1567–70. [27] Wang Y, Li XS, Li G, Zhang Y, Li B, Feng JC. A three-dimensional study on methane hydrate decomposition with different methods using five-spot well. Appl Energy 2013;112:83–92. [28] Wang Y, Li XS, Li G, Zhang Y, Li B, Chen ZY. Experimental investigation into methane hydrate production during three-dimensional thermal stimulation with five-spot well system. Appl Energy 2013;110:90–7. [29] Li XS, Yang B, Duan LP, Li G, Huang NS, Zhang Y. Experimental study on gas production from methane hydrate in porous media by SAGD method. Appl Energy 2013;112:1233–40. [30] Klauda JB, Sandler SI. Global distribution of methane hydrate in ocean sediment. Energy Fuels 2005;19:459–70. [31] Linga P, Haligva C, Nam SC, Ripmeester JA, Englezos P. Gas hydrate formation in a variable volume bed of silica sand particles. Energy Fuels 2009;23:5496–507. [32] Makogon YF, Holditch SA, Makogon TY. Natural gas-hydrates - A potential energy source for the 21st Century. J Petrol Sci Eng 2007;56:14–31. [33] Kang SP, Lee H. Recovery of CO2 from flue gas using gas hydrate: Thermodynamic verification through phase equilibrium measurements. Environ Sci Technol 2000;34:4397–400. [34] Linga P, Kumar R, Englezos P. The clathrate hydrate process for post and precombustion capture of carbon dioxide. J Hazard Mater. 2007;149:625–9. [35] Babu P, Kumar R, Linga P. Pre-combustion capture of carbon dioxide in a fixed bed reactor using the clathrate hydrate process. Energy 2013;50:364–73. [36] Lee HJ, Lee JD, Linga P, Englezos P, Kim YS, Lee MS, et al. Gas hydrate formation process for pre-combustion capture of carbon dioxide. Energy 2010;35:2729–33. [37] Li X-S, Xu C-G, Chen Z-Y, Wu H-J. Hydrate-based pre-combustion carbon dioxide capture process in the system with tetra-n-butyl ammonium bromide solution in the presence of cyclopentane. Energy 2011;36:1394–403. [38] Koh DY, Kang H, Kim DO, Park J, Cha M, Lee H. Recovery of methane from gas hydrates intercalated within natural sediments using CO 2 and a CO 2/N 2 gas mixture. ChemSusChem 2012;5:1443–8. [39] Lee S, Liang LY, Riestenberg D, West OR, Tsouris C, Adams E. CO2 hydrate composite for ocean carbon sequestration. Environ Sci Technol 2003;37:3701–8. [40] Lee H, Lee JW, Kim DY, Park J, Seo YT, Zeng H, et al. Tuning clathrate hydrates for hydrogen storage. Nature 2005;434:743–6. [41] Gudmundsson JS, Parlaktuna M, Khokhar AA. Storing natural-gas as frozen hydrate. SPE Prod Fac 1994;9:69–73. [42] Nakayama T, Tomura S, Ozaki M, Ohmura R, Mori YH. Engineering investigation of hydrogen storage in the form of clathrate hydrates: conceptual design of hydrate production plants. Energy Fuels 2010;24:2576–88. [43] Kumar R, Klug DD, Ratcliffe CI, Tulk CA, Ripmeester JA. Low-pressure synthesis and charecterization of hydrogen-filled ice Ic. Angew Chem-Int Ed 2012; 51. [44] Kamata Y, Oyama H, Shimada W, Ebinuma T, Takeya S, Uchida T, et al. Gas separation method using tetra-n-butyl ammonium bromide semi-clathrate hydrate. Jpn J Appl Phys 2004;43:362–5. [45] Nagata T, Tajima H, Yamasaki A, Kiyono F, Abe Y. An analysis of gas separation processes of HFC-134a from gaseous mixtures with nitrogen-Comparison of two types of gas separation methods, liquefaction and hydrate-based methods, in terms

Fig. 14. Application of SNG technology for storing methane generated from Bio gas plant.

approaches envisaged for enhancing the hydrate formation kinetics and increasing the gas storage capacity were discussed. Though predominant focus was on experimental studies performed on a macro level, sections on tuning effect and self-preservation addressed the molecular level aspects as well. Available patents for storing natural gas in hydrates were compiled and presented. Future directives for further improvement on the SNG (solidified natural gas) technology suited for commercial deployment were also outlined. There is a need for a paradigm shift to the use of sII instead of sI hydrate structure to realize the potential of commercializing the SNG technology for the development of large scale storage systems and also to facilitate storage at very mild conditions. This review further highlights the existing challenges that needs to be mitigated or overcome in order to propel the SNG technology for NG storage and transport in the near future. Acknowledgements The work was funded in part under the Energy Innovation Research Programme (EIRP, Award No. NRF2015EWTEIRP002-002), administrated by the Energy Market Authority (EMA), Singapore. Authors also acknowledge Lloyd’s Register Global Technology Center Singapore for the funding support. Authors thank Mr. Sharad Kumar for his help in plotting the hydrate phase equilibrium figures presented in the review. Ju Dong Lee acknowledges the support by Korea Institute of Industrial Technology (EO170039, EO177040). Appendix A. Supplementary material Compilation of patents and their invention summary associated with SNG technology is provided in Table TS1. Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.apenergy.2018.02.059. References [1] Demirbas A. Natural gas. Methane gas hydrate. London: Springer; 2010. p. 57–76. [2] Sieminski A. International energy outlook. Energy Information Administration (EIA); 2014. [3] Leveraging natural gas to reduce greenhouse gas emissions. Center for climate and energy solutions; June 2013. [4] Casco ME, Martínez-Escandell M, Gadea-Ramos E, Kaneko K, Silvestre-Albero J, Rodríguez-Reinoso F. High-pressure methane storage in porous materials: are carbon materials in the pole position? Chem Mater 2015;27:959–64. [5] Mason JA, Veenstra M, Long JR. Evaluating metal-organic frameworks for natural gas storage. Chem Sci 2014;5:32–51. [6] He Y, Zhou W, Qian G, Chen B. Methane storage in metal-organic frameworks. Chem Soc Rev 2014;43:5657–78. [7] Peng Y, Krungleviciute V, Eryazici I, Hupp JT, Farha OK, Yildirim T. Methane storage in metal-organic frameworks: current records, surprise findings, and

281

Applied Energy 216 (2018) 262–285

H.P. Veluswamy et al.

Macrocycl Chem 1990;8:103–16. [84] Stern LA, Kirby SH, Durham WB. Peculiarities of methane clathrate hydrate formation and solid-state deformation, including possible superheating of water ice. Science 1996;273:1843–8. [85] Stern LA, Kirby SH, Durham WB. Polycrystalline methane hydrate: synthesis from superheated ice, and low-temperature mechanical properties. Energy Fuels 1998;12:201–11. [86] Stern LA, Hogenboom DL, Durham WB, Kirby SH, Chou IM. Optical-cell evidence for superheated ice under gas-hydrate-forming conditions. J Phys Chem B 1998;102:2627–32. [87] Wang X, Schultz AJ, Halpern Y. Kinetics of methane hydrate formation from polycrystalline deuterated ice. J Phys Chem A 2002;106:7304–9. [88] Kuhs WF, Staykova DK, Salamatin AN. Formation of methane hydrate from polydisperse ice powders. J Phys Chem B 2006;110:13283–95. [89] Chen P-C, Huang W-L, Stern LA. Methane hydrate synthesis from ice: influence of pressurization and ethanol on optimizing formation rates and hydrate yield. Energy Fuels 2010;24:2390–403. [90] McLaurin G, Shin K, Alavi S, Ripmeester JA. Antifreezes act as catalysts for methane hydrate formation from ice. Angew Chem 2014;126:10597–601. [91] Faizullin MZ, Vinogradov AV, Koverda VP. Hydrate formation in layers of gassaturated amorphous ice. Chem Eng Sci 2015;130:135–43. [92] Faizullin MZ, Vinogradov AV, Koverda VP. Formation of clathrate hydrates under crystallization of gas-saturated amorphous ice. Int J Heat Mass Transf 2013;65:649–54. [93] Kalogerakis N, Jamaluddin AKM, Dholabhai PD, Bishnoi PR. Effect of surfactants on hydrate formation kinetics. Society of Petroleum Engineers; 1993. [94] Zhong Y, Rogers RE. Surfactant effects on gas hydrate formation. Chem Eng Sci 2000;55:4175–87. [95] Sun Z-g, Wang R, Ma R, Guo K, Fan S. Natural gas storage in hydrates with the presence of promoters. Energy Convers Manage 2003;44:2733–42. [96] Ando N, Kuwabara Y, Mori YH. Surfactant effects on hydrate formation in an unstirred gas/liquid system: An experimental study using methane and micelleforming surfactants. Chem Eng Sci 2012;73:79–85. [97] Daimaru T, Yamasaki A, Yanagisawa Y. Effect of surfactant carbon chain length on hydrate formation kinetics. J Petrol Sci Eng 2007;56:89–96. [98] Di Profio P, Arca S, Germani R, Savelli G. Novel nanostructured media for gas storage and transport: clathrate hydrates of methane and hydrogen. J Fuel Cell Sci Technol 2007;4:49–55. [99] Du J, Li H, Wang L. Effects of ionic surfactants on methane hydrate formation kinetics in a static system. Adv Powder Technol 2014;25:1227–33. [100] Ganji H, Manteghian M, Sadaghiani Zadeh K, Omidkhah MR, Rahimi Mofrad H. Effect of different surfactants on methane hydrate formation rate, stability and storage capacity. Fuel 2007;86:434–41. [101] Fazlali A, Kazemi S-A, Keshavarz-Moraveji M, Mohammadi AH. Impact of different surfactants and their mixtures on methane-hydrate formation. Energy Technol 2013;1:471–7. [102] Okutani K, Kuwabara Y, Mori YH. Surfactant effects on hydrate formation in an unstirred gas/liquid system: An experimental study using methane and sodium alkyl sulfates. Chem Eng Sci 2008;63:183–94. [103] Lee S, Zhang J, Mehta R, Woo TK, Lee JW. Methane hydrate equilibrium and formation kinetics in the presence of an anionic surfactant. J Phys Chem C 2007;111:4734–9. [104] Lin W, Chen GJ, Sun CY, Guo XQ, Wu ZK, Liang MY, et al. Effect of surfactant on the formation and dissociation kinetic behavior of methane hydrate. Chem Eng Sci 2004;59:4449–55. [105] Verrett J, Posteraro D, Servio P. Surfactant effects on methane solubility and mole fraction during hydrate growth. Chem Eng Sci 2012;84:80–4. [106] Zhang JS, Lee S, Lee JW. Kinetics of methane hydrate formation from SDS solution. Ind Eng Chem Res 2007;46:6353–9. [107] Lucia B, Castellani B, Rossi F, Cotana F, Morini E, Nicolini A, et al. Experimental investigations on scaled-up methane hydrate production with surfactant promotion: Energy considerations. J Petrol Sci Eng 2014;120:187–93. [108] Dicharry C, Diaz J, Torré J-P, Ricaurte M. Influence of the carbon chain length of a sulfate-based surfactant on the formation of CO2, CH4 and CO2–CH4 gas hydrates. Chem Eng Sci 2016;152:736–45. [109] Wang F, Jia Z-Z, Luo S-J, Fu S-F, Wang L, Shi X-S, et al. Effects of different anionic surfactants on methane hydrate formation. Chem Eng Sci 2015;137:896–903. [110] Link DD, Ladner EP, Elsen HA, Taylor CE. Formation and dissociation studies for optimizing the uptake of methane by methane hydrates. Fluid Phase Equilib 2003;211:1–10. [111] Saw VK, Gudala M, Udayabhanu G, Mandal A, Laik S. Kinetics of methane hydrate formation and its dissociation in presence of non-ionic surfactant Tergitol. J Unconventional Oil Gas Resources 2014;6:54–9. [112] Gnanendran N, Amin R. The effect of hydrotropes on gas hydrate formation. J Petrol Sci Eng 2003;40:37–46. [113] Mishal Y. Effect of Gemini surfactant on the formation kinetic behavior of methane hydrate. Canada: McGill University; 2008. [114] Rogers RE, Kothapalli C, Lee MS, Woolsey JR. Catalysis of gas hydrates by biosurfactants in seawater-saturated sand/clay. Can J Chem Eng 2003;81:973–80. [115] Rogers R, Zhang G, Dearman J, Woods C. Investigations into surfactant/gas hydrate relationship. J Petrol Sci Eng 2007;56:82–8. [116] Arora A, Cameotra SS, Kumar R, Balomajumder C, Singh AK, Santhakumari B, et al. Biosurfactant as a promoter of methane hydrate formation: thermodynamic and kinetic studies. Sci Rep 2016;6:20893. [117] Kumar A, Bhattacharjee G, Kulkarni BD, Kumar R. Role of surfactants in promoting gas hydrate formation. Ind Eng Chem Res 2015;54:12217–32.

of the equilibrium recovery ratio. Sep Purif Technol 2009;64:351–6. [46] K-n Park, Hong SY, Lee JW, Kang KC, Lee YC, Ha M-G, et al. A new apparatus for seawater desalination by gas hydrate process and removal characteristics of dissolved minerals (Na+, Mg2+, Ca2+, K+, B3+). Desalination 2011;274:91–6. [47] Wang W, Ma C, Lin P, Sun L, Cooper AI. Gas storage in renewable bioclathrates. Energy Environ Sci 2013;6:105–7. [48] Englezos P, Lee JD. Gas hydrates: A cleaner source of energy and opportunity for innovative technologies. Korean J Chem Eng 2005;22:671–81. [49] Chatti I, Delahaye A, Fournaison L, Petitet J-P. Benefits and drawbacks of clathrate hydrates: a review of their areas of interest. Energy Convers Manage 2005;46:1333–43. [50] Sloan ED. Fundamental principles and applications of natural gas hydrates. Nature 2003;426:353–63. [51] Koh CA, Sloan ED, Sum AK, Wu DT. Fundamentals and applications of gas hydrates. Ann Rev Chem Biomol Eng 2011;2:237–57. [52] Babu P, Linga P, Kumar R, Englezos P. A review of the hydrate based gas separation (HBGS) process for carbon dioxide pre-combustion capture. Energy 2015;85:261–79. [53] Ma ZW, Zhang P, Bao HS, Deng S. Review of fundamental properties of CO2 hydrates and CO2 capture and separation using hydration method. Renew Sustain Energy Rev 2016;53:1273–302. [54] Strobel TA, Hester KC, Koh CA, Sum AK, Sloan ED. Properties of the clathrates of hydrogen and developments in their applicability for hydrogen storage. Chem Phys Lett 2009;478:97–109. [55] Strobel TA, Koh CA, Sloan ED. Hydrogen storage properties of clathrate hydrate materials. Fluid Phase Equilib 2007;261:382–9. [56] Struzhkin VV, Militzer B, Mao WL, Mao HK, Hemley RJ. Hydrogen storage in molecular clathrates. Chem Rev 2007;107:4133–51. [57] Veluswamy HP, Chin WI, Linga P. Clathrate hydrates for hydrogen storage: The impact of tetrahydrofuran, tetra-n-butylammonium bromide and cyclopentane as promoters on the macroscopic kinetics. Int J Hydrogen Energy 2014;39:16234–43. [58] Eslamimanesh A, Mohammadi AH, Richon D, Naidoo P, Ramjugernath D. Application of gas hydrate formation in separation processes: A review of experimental studies. J Chem Thermodyn 2012;46:62–71. [59] Wang X, Dennis M, Hou L. Clathrate hydrate technology for cold storage in air conditioning systems. Renew Sustain Energy Rev 2014;36:34–51. [60] Boswell R, Collett TS. Current perspectives on gas hydrate resources. Energy Environ Sci 2011;4:1206–15. [61] Makogon YF, Holditch SA, Makogon TY. Natural gas-hydrates — A potential energy source for the 21st century. J Petrol Sci Eng 2007;56:14–31. [62] Seol J, Lee H. Natural gas hydrate as a potential energy resource: From occurrence to production. Korean J Chem Eng 2013;30:771–86. [63] Kumar A, Vedula SS, Kumar R, Linga P. Hydrate phase equilibrium data of mixed methane-tetrahydrofuran hydrates in saline water. J Chem Thermodyn 2018;117:2–8. [64] Khurana M, Yin Z, Linga P. A review of clathrate hydrate nucleation. ACS Sustain Chem Eng 2017;5:11176–203. [65] Villard P. Sur quelques nouveaux hydrates de gaz. Compt Rend 1888;106:1602–3. [66] Matthew B. The use of gas hydrates in improving the load factor of gas supply systems. Google Patents; 1942. [67] Miller B, Strong E. Hydrate storage of natural gas. Am Gas Assoc Monthly 1946;28:63–7. [68] Parent J. The storage of natural gas as hydrate. Inst Gas Techn Res Bull 1948;1. [69] Kobayashi R, Katz DL. Methane hydrate at high pressure. J Petrol Technol 1949;01. [70] Mv Stackelberg, Müller HR. On the structure of gas hydrates. J Chem Phys 1951;19:1319–20. [71] Claussen WF, Polglase MF. Solubilities and structures in aqueous aliphatic hydrocarbon solutions. J Am Chem Soc 1952;74:4817–9. [72] Makogon TY, Sloan Jr. ED. Phase equilibrium for methane hydrate from 190 to 262 K. J Chem Eng Data 1994;39:351–3. [73] De Roo JL, Peters CJ, Lichtenthaler RN, Diepen GAM. Occurrence of methane hydrate in saturated and unsaturated solutions of sodium chloride and water in dependence of temperature and pressure. AIChE J 1983;29:651–7. [74] Thakore JL, Holder GD. Solid vapor azeotropes in hydrate-forming systems. Ind Eng Chem Res 1987;26:462–9. [75] Adisasmito S, Frank RJ, Sloan ED. Hydrates of carbon dioxide and methane mixtures. J Chem Eng Data 1991;36:68–71. [76] Nakamura T, Makino T, Sugahara T, Ohgaki K. Stability boundaries of gas hydrates helped by methane—structure-H hydrates of methylcyclohexane and cis-1,2-dimethylcyclohexane. Chem Eng Sci 2003;58:269–73. [77] Vysniauskas A, Bishnoi PR. A kinetic study of methane hydrate formation. Chem Eng Sci 1983;38:1061–72. [78] Englezos P, Kalogerakis N, Dholabhai PD, Bishnoi PR. Kinetics of gas hydrate formation from mixtures of methane and ethane. Chem Eng Sci 1987;42:2659–66. [79] Kim HC, Bishnoi PR, Heidemann RA, Rizvi SSH. Kinetics of methane hydrate decomposition. Chem Eng Sci 1987;42:1645–53. [80] Freer EM, Sami Selim M, Dendy Sloan Jr E. Methane hydrate film growth kinetics. Fluid Phase Equilib 2001;185:65–75. [81] Ohmura R, Matsuda S, Uchida T, Ebinuma T, Narita H. Clathrate hydrate crystal growth in liquid water saturated with a guest substance: observations in a methane + water system. Cryst Growth Des 2005;5:953–7. [82] Falabella BJ. A study of natural gas hydrates. University of Massachusetts Amherst; 1975. [83] Hwang MJ, Wright DA, Kapur A, Holder GD. An experimental study of crystallization and crystal growth of methane hydrates from melting ice. J Incl Phenom

282

Applied Energy 216 (2018) 262–285

H.P. Veluswamy et al.

[151] Giavarini C, Maccioni F, Santarelli ML. Dissociation rate of THF-methane hydrates. Pet Sci Technol 2008;26:2147–58. [152] Veluswamy HP, Wong AJH, Babu P, Kumar R, Kulprathipanja S, Rangsunvigit P, et al. Rapid methane hydrate formation to develop a cost effective large scale energy storage system. Chem Eng J 2016;290:161–73. [153] Veluswamy HP, Kumar S, Kumar R, Rangsunvigit P, Linga P. Enhanced clathrate hydrate formation kinetics at near ambient temperatures and moderate pressures: Application to natural gas storage. Fuel 2016;182:907–19. [154] Tohidi B, Danesh A, Todd AC, Burgass RW, Østergaard KK. Equilibrium data and thermodynamic modelling of cyclopentane and neopentane hydrates. Fluid Phase Equilib 1997;138:241–50. [155] Lv Q-N, Li X-S, Xu C-G, Chen Z-Y. Experimental investigation of the formation of cyclopentane-methane hydrate in a novel and large-size bubble column reactor. Ind Eng Chem Res 2012;51:5967–75. [156] Cai L, Pethica BA, Debenedetti PG, Sundaresan S. Formation kinetics of cyclopentane–methane binary clathrate hydrate. Chem Eng Sci 2014;119:147–57. [157] Kini RA, Dec SF, Sloan ED. Methane + propane structure II hydrate formation kinetics. J Phys Chem A 2004;108:9550–6. [158] Kang H, Ahn Y-H, Koh D-Y, Baek S, Lee JW, Lee H. Rapid clathrate hydrate formation using a heavy guest molecule with sodium dodecyl sulfate. Ind Eng Chem Res 2016;55:6079–84. [159] Partoon B, Javanmardi J. Effect of mixed thermodynamic and kinetic hydrate promoters on methane hydrate phase boundary and formation kinetics. J Chem Eng Data 2013;58:501–9. [160] Mainusch S, Peters CJ, de Swaan Arons J, Javanmardi J, Moshfeghian M. Experimental determination and modeling of methane hydrates in mixtures of acetone and water. J Chem Eng Data 1997;42:948–50. [161] Maekawa T. Equilibrium conditions for clathrate hydrates formed from methane and aqueous propanol solutions. Fluid Phase Equilib 2008;267:1–5. [162] Sizikov AA, Manakov AY, Aladko EY. Pressure dependence of gas hydrate formation in triple systems water – 2-Propanol–methane and water – 2Propanol–hydrogen. Fluid Phase Equilib 2016;425:351–7. [163] Park Y, Cha M, Shin W, Cha J-H, Lee H, Ripmeester JA. Thermodynamic and spectroscopic analysis of tertbutyl alcohol hydrate: application for the methane gas storage and transportation. In: Proceedings of the 6th international conference on gas hydrates (ICGH 2008), Vancouver, British Columbia, CANADA2008. [164] Ripmeester JA, Tse JS, Ratcliffe CI, Powell BM. A new clathrate hydrate structure. Nature 1987;325:135–6. [165] Khokhar AA, Gudmundsson JS, Sloan ED. Gas storage in structure H hydrates. Fluid Phase Equilib 1998;150–151:383–92. [166] Ohmura R, Kashiwazaki S, Shiota S, Tsuji H, Mori YH. Structure-I and structure-H hydrate formation using water spraying. Energy Fuels 2002;16:1141–7. [167] Tsuji H, Ohmura R, Mori YH. Forming structure-H hydrates using water spraying in methane gas: effects of chemical species of large-molecule guest substances. Energy Fuels 2004;18:418–24. [168] Tsuji H, Kobayashi T, Ohmura R, Mori YH. Hydrate formation by water spraying in a methane + ethane + propane gas mixture: an attempt at promoting hydrate formation utilizing large-molecule guest substances for structure-H hydrates. Energy Fuels 2005;19:869–76. [169] Hütz U, Englezos P. Measurement of structure H hydrate phase equilibrium and the effect of electrolytes. Fluid Phase Equilib 1996;117:178–85. [170] Mazraeno MS, Varaminian F, Vafaie sefti M. Experimental and modeling investigation on structure H hydrate formation kinetics. Energy Convers Manage 2013;76:1–7. [171] Mehta AP, Sloan ED. Structure H hydrate phase equilibria of methane + liquid hydrocarbon mixtures. J Chem Eng Data 1993;38:580–2. [172] Ohmura R, Uchida T, Takeya S, Nagao J, Minagawa H, Ebinuma T, et al. Phase equilibrium for structure-H hydrates formed with methane and either pinacolone (3,3-dimethyl-2-butanone) or pinacolyl alcohol (3,3-dimethyl-2-butanol). J Chem Eng Data 2003;48:1337–40. [173] Lee JD, Susilo R, Englezos P. Kinetics of structure H gas hydrate. Energy Fuels 2005;19:1008–15. [174] Susilo R, Ripmeester JA, Englezos P. Methane conversion rate into structure H hydrate crystals from ice. AIChE J 2007;53:2451–60. [175] Susilo R. Methane storage and transport via structure H clathrate hydrate. University of British Columbia; 2008. [176] Servio P, Englezos P. Morphology study of structure H hydrate formation from water droplets. Cryst Growth Des 2003;3:61–6. [177] Fowler DL, Loebenstein WV, Pall DB, Kraus CA. Some unusual hydrates of quaternary ammonium salts. J Am Chem Soc 1940;62:1140–2. [178] Wang W, Carter BO, Bray CL, Steiner A, Bacsa J, Jones JTA, et al. Reversible methane storage in a polymer-supported semi-clathrate hydrate at ambient temperature and pressure. Chem Mater 2009;21:3810–5. [179] Li D-L, Du J-W, Fan S-S, Liang D-Q, Li X-S, Huang N-S. Clathrate dissociation conditions for methane + tetra-n-butyl ammonium bromide (TBAB) + water. J Chem Eng Data 2007;52:1916–8. [180] Arjmandi M, Chapoy A, Tohidi B. Equilibrium data of hydrogen, methane, nitrogen, carbon dioxide, and natural gas in semi-clathrate hydrates of tetrabutyl ammonium bromide. J Chem Eng Data 2007;52:2153–8. [181] Long X, Wang Y, Lang X, Fan S, Chen J. Hydrate equilibrium measurements for CH4, CO2, and CH4 + CO2 in the presence of tetra-n-butyl ammonium bromide. J Chem Eng Data 2016;61:3897–901. [182] Makino T, Yamamoto T, Nagata K, Sakamoto H, Hashimoto S, Sugahara T, et al. Thermodynamic stabilities of tetra-n-butyl ammonium chloride + H2, N2, CH4, CO2, or C2H6 semiclathrate hydrate systems. J Chem Eng Data 2010;55:839–41. [183] Mohammadi A, Manteghian M, Mohammadi AH. Dissociation data of

[118] Gayet P, Dicharry C, Marion G, Graciaa A, Lachaise J, Nesterov A. Experimental determination of methane hydrate dissociation curve up to 55MPa by using a small amount of surfactant as hydrate promoter. Chem Eng Sci 2005;60:5751–8. [119] Yoslim J, Linga P, Englezos P. Enhanced growth of methane-propane clathrate hydrate crystals with sodium dodecyl sulfate, sodium tetradecyl sulfate, and sodium hexadecyl sulfate surfactants. J Cryst Growth 2010;313:68–80. [120] Zhang JS, Lo C, Somasundaran P, Lu S, Couzis A, Lee JW. Adsorption of sodium dodecyl sulfate at THF hydrate/liquid interface. J Phys Chem C 2008;112:12381–5. [121] Lo C, Zhang JS, Couzis A, Somasundaran P, Lee JW. Adsorption of cationic and anionic surfactants on cyclopentane hydrates. J Phys Chem C 2010;114:13385–9. [122] Lo C, Zhang J, Somasundaran P, Lee JW. Raman Spectroscopic Studies of Surfactant Effect on the Water Structure around Hydrate Guest Molecules. The Journal of Physical Chemistry Letters. 2010;1:2676–9. [123] Bhattacharjee G, Kushwaha OS, Kumar A, Khan MY, Patel JN, Kumar R. Effects of micellization on growth kinetics of methane hydrate. Ind Eng Chem Res 2017;56:3687–98. [124] Veluswamy HP, Hong QW, Linga P. Morphology study of methane hydrate formation and dissociation in the presence of amino acid. Cryst Growth Des 2016;16:5932–45. [125] Carter BO, Wang W, Adams DJ, Cooper AI. Gas storage in “dry water” and “dry gel” clathrates. Langmuir 2010;26:3186–93. [126] Liu Y, Chen B, Chen Y, Zhang S, Guo W, Cai Y, et al. Methane storage in a hydrated form as promoted by leucines for possible application to natural gas transportation and storage. Energy Technol 2015. [127] Scott MJ, Jones MN. The biodegradation of surfactants in the environment. Biochim Biophys Acta (BBA) - Biomembr 2000;1508:235–51. [128] Naeiji P, Arjomandi A, Varaminian F. Amino acids as kinetic inhibitors for tetrahydrofuran hydrate formation: Experimental study and kinetic modeling. J Nat Gas Sci Eng 2014;21:64–70. [129] Sa J-H, Lee BR, Park D-H, Han K, Chun HD, Lee K-H. Amino acids as natural inhibitors for hydrate formation in CO2 sequestration. Environ Sci Technol 2011;45:5885–91. [130] Bhattacharjee G, Choudhary N, Kumar A, Chakrabarty S, Kumar R. Effect of the amino acid l-histidine on methane hydrate growth kinetics. J Nat Gas Sci Eng 2016;35, Part B:1453–62. [131] Veluswamy HP, Kumar A, Kumar R, Linga P. An innovative approach to enhance methane hydrate formation kinetics with leucine for energy storage application. Appl Energy 2017;188:190–9. [132] Veluswamy HP, Lee PY, Premasinghe K, Linga P. Effect of biofriendly amino acids on the kinetics of methane hydrate formation and dissociation. Ind Eng Chem Res 2017;56:6145–54. [133] Mohammad-Taheri M, Moghaddam AZ, Nazari K, Zanjani NG. Methane hydrate stability in the presence of water-soluble hydroxyalkyl cellulose. J Nat Gas Chem 2012;21:119–25. [134] Al-Adel S. The effect of biological and polymeric inhibitors on methane gas hydrate growth kinetics. Canada: McGill University; 2007. [135] Kumar A, Sakpal T, Kumar R. Influence of low-dosage hydrate inhibitors on methane clathrate hydrate formation and dissociation kinetics. Energy Technol 2015;3:717–25. [136] Karaaslan U, Parlaktuna M. Promotion effect of polymers and surfactants on hydrate formation rate. Energy Fuels 2002;16:1413–6. [137] Dong Lee J, Wu H, Englezos P. Cationic starches as gas hydrate kinetic inhibitors. Chem Eng Sci 2007;62:6548–55. [138] Fakharian H, Ganji H, Naderi Far A, Kameli M. Potato starch as methane hydrate promoter. Fuel 2012;94:356–60. [139] Ganji H, Manteghian M, Rahimi Mofrad H. Effect of mixed compounds on methane hydrate formation and dissociation rates and storage capacity. Fuel Process Technol 2007;88:891–5. [140] Maghsoodloo Babakhani S, Alamdari A. Effect of maize starch on methane hydrate formation/dissociation rates and stability. J Nat Gas Sci Eng 2015;26:1–5. [141] Park S-S, Kim N-J. Study on methane hydrate formation using ultrasonic waves. J Ind Eng Chem 2013;19:1668–72. [142] Veluswamy HP. Energy storage in clathrate hydrates. Singapore: National University of Singapore; 2015. [143] Zhang Q, Chen G-J, Huang Q, Sun C-Y, Guo X-Q, Ma Q-L. Hydrate formation conditions of a hydrogen + methane gas mixture in tetrahydrofuran + water. J Chem Eng Data 2005;50:234–6. [144] Sowjanya Y, Prasad PSR. Formation kinetics & phase stability of double hydrates of C4H8O and CO2/CH4: A comparison with pure systems. J Nat Gas Sci Eng 2014;18:58–63. [145] Seo YT, Kang SP, Lee H. Experimental determination and thermodynamic modeling of methane and nitrogen hydrates in the presence of THF, propylene oxide, 1,4-dioxane and acetone. Fluid Phase Equilib 2001;189:99–110. [146] Mohammadi AH, Richon D. Phase Equilibria of Clathrate Hydrates of Tetrahydrofuran + Hydrogen Sulfide and Tetrahydrofuran + Methane. Ind Eng Chem Res. 2009;48:7838–41. [147] De Deugd R, Jager M, de Swaan Arons J. Mixed hydrates of methane and watersoluble hydrocarbons modeling of empirical results. AIChE J 2001;47:693–704. [148] Luo YT, Zhu JH, Fan SS, Chen GJ. Study on the kinetics of hydrate formation in a bubble column. Chem Eng Sci 2007;62:1000–9. [149] Zhengfu N, Shixi Z, Qin Z, Shuangyi Z, Guangjin C. Experimental and modeling study of kinetics for methane hydrate formation with tetrahydrofuran as promoter. Pet Sci 2007;4:61–5. [150] Sharma D, Sowjanya Y, Chari VD, Prasad P. Methane storage in mixed hydrates with tetrahydrofuran. Indian J Chem Technol 2014;21:114–9.

283

Applied Energy 216 (2018) 262–285

H.P. Veluswamy et al.

[184] [185]

[186]

[187] [188]

[189] [190]

[191]

[192]

[193]

[194]

[195] [196]

[197]

[198]

[199] [200]

[201] [202] [203] [204]

[205]

[206] [207] [208]

[209]

[210]

[211] [212] [213]

[214] [215]

semiclathrate hydrates for the systems of tetra-n-butylammonium fluoride (TBAF) + methane + water, TBAF + carbon dioxide + water, and TBAF + nitrogen + water. J Chem Eng Data 2013;58:3545–50. Hughes TJ, Marsh KN. Methane semi-clathrate hydrate phase equilibria with tetraisopentylammonium fluoride. J Chem Eng Data 2011;56:4597–603. Kim D-Y, Park J, Lee J-w, Ripmeester JA, Lee H. Critical guest concentration and complete tuning pattern appearing in the binary clathrate hydrates. J Am Chem Soc 2006;128:15360–1. Moryama CT, Sugahara T, Yatabe Franco DY, Mimachi H. In situ raman spectroscopic studies on small-cage occupancy of methane in the simple methane and methane + deuterated tetrahydrofuran mixed hydrates. J Chem Eng Data 2015;60:3581–7. Wu J-Y, Chen L-J, Chen Y-P, Lin S-T. Molecular dynamics study on the nucleation of methane + tetrahydrofuran mixed guest hydrate. PCCP 2016;18:9935–47. Seo Y-T, Lee H, Moudrakovski I, Ripmeester JA. Phase behavior and structural characterization of coexisting pure and mixed clathrate hydrates. ChemPhysChem 2003;4:379–82. Prasad PSR, Sowjanya Y, Shiva Prasad K. Micro-Raman investigations of mixed gas hydrates. Vib Spectrosc 2009;50:319–23. Dhanunjana Chari V, Sharma DVSGK, Prasad PSR. Methane hydrate phase stability with lower mole fractions of tetrahydrofuran (THF) and tert-butylamine (tBuNH2). Fluid Phase Equilib 2012;315:126–30. Seo Y, Lee J-W, Kumar R, Moudrakovski IL, Lee H, Ripmeester JA. Tuning the composition of guest molecules in clathrate hydrates: NMR identification and its significance to gas storage. Chem – Asian J 2009;4:1266–74. Kumar A, Daraboina N, Kumar R, Linga P. Experimental investigation to elucidate why tetrahydrofuran rapidly promotes methane hydrate formation kinetics: applicable to energy storage. J Phys Chem C 2016;120:29062–8. Choi S, Shin K, Lee H. Structure transition and tuning pattern in the double (tetramethylammonium hydroxide + gaseous guests) clathrate hydrates. J Phys Chem B 2007;111:10224–30. Susilo R, Alavi S, Ripmeester J, Englezos P. Tuning methane content in gas hydrates via thermodynamic modeling and molecular dynamics simulation. Fluid Phase Equilib 2008;263:6–17. Kim D-Y, Lee J-w, Seo Y-T, Ripmeester JA, Lee H. Structural transition and tuning of tert-butylamine hydrate. Angew Chem Int Ed 2005;44:7749–52. Kumar A, Kushwaha OS, Rangsunvigit P, Linga P, Kumar R. Effect of additives on formation and decomposition kinetics of methane clathrate hydrates: Application in energy storage and transportation. Can J Chem Eng 2016;94:2160–7. Lim S-H, Riffat SB, Park S-S, Oh S-J, Chun W, Kim N-J. Enhancement of methane hydrate formation using a mixture of tetrahydrofuran and oxidized multi-wall carbon nanotubes. Int J Energy Res 2014;38:374–9. Mech D, Gupta P, Sangwai JS. Kinetics of methane hydrate formation in an aqueous solution of thermodynamic promoters (THF and TBAB) with and without kinetic promoter (SDS). J Nat Gas Sci Eng 2016;35, Part B:1519–34. Baek S, Ahn Y-H, Zhang J, Min J, Lee H, Lee JW. Enhanced methane hydrate formation with cyclopentane hydrate seeds. Appl Energy 2017;202:32–41. Erfani A, Fallah-Jokandan E, Varaminian F. Effects of non-ionic surfactants on formation kinetics of structure H hydrate regarding transportation and storage of natural gas. J Nat Gas Sci Eng 2017;37:397–408. Rossi F, Filipponi M, Castellani B. Investigation on a novel reactor for gas hydrate production. Appl Energy 2012;99:167–72. Tang L-G, Li X-S, Feng Z-P, Lin Y-L, Fan S-S. Natural gas hydrate formation in an ejector loop reactor: preliminary study. Ind Eng Chem Res 2006;45:7934–40. Lang X, Fan S, Wang Y. Intensification of methane and hydrogen storage in clathrate hydrate and future prospect. J Nat Gas Chem 2010;19:203–9. Murakami T, Kuritsuka H, Fujii H, Mori YH. Forming a structure-H hydrate using water and methylcyclohexane jets impinging on each other in a methane atmosphere. Energy Fuels 2009;23:1619–25. Xiao P, Yang X-M, Sun C-Y, Cui J-L, Li N, Chen G-J. Enhancing methane hydrate formation in bulk water using vertical reciprocating impact. Chem Eng J 2018;336:649–58. Kang S-P, Seo Y, Jang W. Kinetics of methane and carbon dioxide hydrate formation in silica gel pores. Energy Fuels 2009;23:3711–5. Yang L, Fan S, Wang Y, Lang X, Xie D. Accelerated formation of methane hydrate in aluminum foam. Ind Eng Chem Res 2011;50:11563–9. Siangsai A, Rangsunvigit P, Kitiyanan B, Kulprathipanja S, Linga P. Investigation on the roles of activated carbon particle sizes on methane hydrate formation and dissociation. Chem Eng Sci 2015;126:383–9. Siangsai A, Rangsunvigit P, Kitiyanan B, Kulprathipanja S. Improved methane hydrate formation rate using treated activated carbon and tetrahydrofuran 2014;47:352–7. Siangsai A, Rangsunvigit P, Kitiyanan B, Kulprathipanja S. Roles of activated carbon and tetrahydrofuran on methane hydrate phase equilibrium. 2014:1195–200. Park J, Shin K, Kim J, Lee H, Seo Y, Maeda N, et al. Effect of hydrate shell formation on the stability of dry water. J Phys Chem C 2015;119:1690–9. Wang W, Bray CL, Adams DJ, Cooper AI. Methane storage in dry water gas hydrates. J Am Chem Soc 2008;130:11608–9. Prasad PSR, Sowjanya Y, Dhanunjana Chari V. Enhancement in methane storage capacity in gas hydrates formed in hollow silica. J Phys Chem C 2014;118:7759–64. Prasad PSR. Methane hydrate formation and dissociation in the presence of hollow silica. J Chem Eng Data 2015;60:304–10. Chari VD, Sharma DVSGK, Prasad PSR, Murthy SR. Methane hydrates formation and dissociation in nano silica suspension. J Nat Gas Sci Eng 2013;11:7–11.

[216] Zang X, Du J, Liang D, Fan S, Tang C. Influence of a-type zeolite on methane hydrate formation. Chin J Chem Eng 2009;17:854–9. [217] Haligva C, Linga P, Ripmeester JA, Englezos P. Recovery of methane from a variable-volume bed of silica sand/hydrate by depressurization. Energy Fuels 2010;24:2947–55. [218] Linga P, Daraboina N, Ripmeester JA, Englezos P. Enhanced rate of gas hydrate formation in a fixed bed column filled with sand compared to a stirred vessel. Chem Eng Sci 2012;68:617–23. [219] Zhou L, Sun Y, Zhou Y. Enhancement of the methane storage on activated carbon by preadsorbed water. AIChE J 2002;48:2412–6. [220] Perrin A, Celzard A, Marêché JF, Furdin G. Methane storage within dry and wet active carbons: a comparative study. Energy Fuels 2003;17:1283–91. [221] Yan L, Chen G, Pang W, Liu J. Experimental and modeling study on hydrate formation in wet activated carbon. J Phys Chem B 2005;109:6025–30. [222] Babu P, Yee D, Linga P, Palmer A, Khoo BC, Tan TS, et al. Morphology of methane hydrate formation in porous media. Energy Fuels 2013;27:3364–72. [223] Ding A, Yang L, Fan S, Lou X. Reversible methane storage in porous hydrogel supported clathrates. Chem Eng Sci 2013;96:124–30. [224] Fan S, Yang L, Wang Y, Lang X, Wen Y, Lou X. Rapid and high capacity methane storage in clathrate hydrates using surfactant dry solution. Chem Eng Sci 2014;106:53–9. [225] Shi B-H, Yang L, Fan S-S, Lou X. An investigation on repeated methane hydrates formation in porous hydrogel particles. Fuel 2017;194:395–405. [226] Kang S-P, Lee J-W. Formation characteristics of synthesized natural gas hydrates in meso- and macroporous silica gels. J Phys Chem B 2010;114:6973–8. [227] Veluswamy HP, Prasad PSR, Linga P. Mechanism of methane hydrate formation in the presence of hollow silica. Korean J Chem Eng 2016;33:2050–62. [228] Park S-S, An E-J, Lee S-B, Chun W-g, Kim N-J. Characteristics of methane hydrate formation in carbon nanofluids. J Ind Eng Chem 2012;18:443–8. [229] Park S-S, Lee S-B, Kim N-J. Effect of multi-walled carbon nanotubes on methane hydrate formation. J Ind Eng Chem 2010;16:551–5. [230] Kim N-J, Park S-S, Kim HT, Chun W. A comparative study on the enhanced formation of methane hydrate using CM-95 and CM-100 MWCNTs. Int Commun Heat Mass Transfer 2011;38:31–6. [231] Pasieka J, Coulombe S, Servio P. Investigating the effects of hydrophobic and hydrophilic multi-wall carbon nanotubes on methane hydrate growth kinetics. Chem Eng Sci 2013;104:998–1002. [232] Ghozatloo A, Hosseini M, Shariaty-Niassar M. Improvement and enhancement of natural gas hydrate formation process by Hummers' graphene. J Nat Gas Sci Eng 2015;27, Part 2:1229–33. [233] Ghozatloo A, Shariaty-Niassar M, Hassanisadi M. Effect of single walled carbon nanotubes on natural gas hydrate formation. Iranian J Chem Eng 2014;11. [234] Arjang S, Manteghian M, Mohammadi A. Effect of synthesized silver nanoparticles in promoting methane hydrate formation at 4.7 MPa and 5.7 MPa. Chem Eng Res Des 2013;91:1050–4. [235] Najibi H, Mirzaee Shayegan M, Heidary H. Experimental investigation of methane hydrate formation in the presence of copper oxide nanoparticles and SDS. J Nat Gas Sci Eng 2015;23:315–23. [236] Aliabadi M, Rasoolzadeh A, Esmaeilzadeh F, Alamdari A. Experimental study of using CuO nanoparticles as a methane hydrate promoter. J Nat Gas Sci Eng 2015;27, Part 3:1518–22. [237] Pahlavanzadeh H, Rezaei S, Khanlarkhani M, Manteghian M, Mohammadi AH. Kinetic study of methane hydrate formation in the presence of copper nanoparticles and CTAB. J Nat Gas Sci Eng 2016;34:803–10. [238] Kakati H, Mandal A, Laik S. Promoting effect of Al2O3/ZnO-based nanofluids stabilized by SDS surfactant on CH4+C2H6+C3H8 hydrate formation. J Ind Eng Chem 2016;35:357–68. [239] Casco ME, Rey F, Jorda JL, Rudic S, Fauth F, Martinez-Escandell M, et al. Paving the way for methane hydrate formation on metal-organic frameworks (MOFs). Chem Sci 2016;7:3658–66. [240] Seo Y, Kang S-P, Jang W. Structure and composition analysis of natural gas hydrates: 13C NMR spectroscopic and gas uptake measurements of mixed gas hydrates. J Phys Chem A 2009;113:9641–9. [241] Mohebbi V, Behbahani RM. Experimental study on gas hydrate formation from natural gas mixture. J Nat Gas Sci Eng 2014;18:47–52. [242] Kumar R, Linga P, Moudrakovski I, Ripmeester JA, Englezos P. Structure and kinetics of gas hydrates from methane/ethane/propane mixtures relevant to the design of natural gas hydrate storage and transport facilities. AIChE J 2008;54:2132–44. [243] Kondo W, Ohtsuka K, Ohmura R, Takeya S, Mori YH. Clathrate-hydrate formation from a hydrocarbon gas mixture: Compositional evolution of formed hydrate during an isobaric semi-batch hydrate-forming operation. Appl Energy 2014;113:864–71. [244] Gudmundsson JS, Parlaktuna M, Khokhar A. Storing natural gas as frozen hydrate. SPE Prod Facil 1994;9:69–73. [245] Murayama T, Iwabuchi W, Ito M, Takahashi M. Effects of guest gas on pelletizing performance of natural gas hydrate (NGH) pellets. In: Proceedings of the 7th international conference on gas hydrates; 2011. [246] Mimachi H, Takahashi M, Takeya S, Gotoh Y, Yoneyama A, Hyodo K, et al. Effect of long-term storage and thermal history on the gas content of natural gas hydrate pellets under ambient pressure. Energy Fuels 2015;29:4827–34. [247] Lee Y-H, Koh B-H, Kim HS, Song MH. Compressive strength properties of natural gas hydrate pellet by continuous extrusion from a twin-roll system. Adv Mater Sci Eng 2013;2013:6. [248] Lee JD, Lee, Jin Woo, Park, Kyeong Nam, Kim, Joung Ha. Double helix type gas hydrate reactor. United States: Korea Institute of Industrial Technology

284

Applied Energy 216 (2018) 262–285

H.P. Veluswamy et al.

[270] Giavarini C, Maccioni F. Self-preservation at low pressures of methane hydrates with various gas contents. Ind Eng Chem Res 2004;43:6616–21. [271] Takeya S, Uchida T, Nagao J, Ohmura R, Shimada W, Kamata Y, et al. Particle size effect of hydrate for self-preservation. Chem Eng Sci 2005;60:1383–7. [272] Mimachi H, Takeya S, Yoneyama A, Hyodo K, Takeda T, Gotoh Y, et al. Natural gas storage and transportation within gas hydrate of smaller particle: Size dependence of self-preservation phenomenon of natural gas hydrate. Chem Eng Sci 2014;118:208–13. [273] Stoporev AS, Manakov AY, Altunina LK, Bogoslovsky AV. Self-Preservation behaviour of methane hydrate particles in oil suspensions. Mendeleev Commun 2012;22:336–7. [274] Falenty A, Kuhs WF, Glockzin M, Rehder G. “Self-preservation” of CH4 hydrates for gas transport technology: pressure-temperature dependence and ice microstructures. Energy Fuels 2014;28:6275–83. [275] Istomin V, Yakushev V, Makhonina N, Kwon V, Chuvilin E. Self-preservation phenomenon of gas hydrates. Gas Ind Russia 2006:16–27. [276] Rehder G, Eckl R, Elfgen M, Falenty A, Hamann R, Kähler N, et al. Methane hydrate pellet transport using the self-preservation effect: a techno-economic analysis. Energies 2012;5:2499. [277] Bai D, Zhang D, Zhang X, Chen G. Origin of self-preservation effect for hydrate decomposition: coupling of mass and heat transfer resistances 2015;5:14599. [278] Vlasov VA. Diffusion model of gas hydrate dissociation into ice and gas: Simulation of the self-preservation effect. Int J Heat Mass Transf 2016;102:631–6. [279] Stern LA, Circone S, Kirby SH, Durham WB. Temperature, pressure, and compositional effects on anomalous or “self” preservation of gas hydrates. Can J Phys 2003;81:271–83. [280] Takeya S, Ripmeester JA. Dissociation behavior of clathrate hydrates to ice and dependence on guest molecules. Angew Chem Int Ed 2008;47:1276–9. [281] Takeya S, Ripmeester JA. Anomalous preservation of CH4 hydrate and its dependence on the morphology of hexagonal ice. ChemPhysChem 2010;11:70–3. [282] Wen YG, Chen QX, Chen YW, Fan SS. Research progress on hydrate self-preservation effect applied to storage and transportation of natural gas. Advanced materials research. Trans Tech Publ; 2013. p. 795–801. [283] Zhang G, Rogers RE. Ultra-stability of gas hydrates at 1 atm and 268.2 K. Chem Eng Sci 2008;63:2066–74. [284] Cranganu C. In-situ thermal stimulation of gas hydrates. J Petrol Sci Eng 2009;65:76–80. [285] Tang LG, Xiao R, Huang C, Feng ZP, Fan SS. Experimental investigation of production behavior of gas hydrate under thermal stimulation in unconsolidated sediment. Energy Fuels 2005;19:2402–7. [286] Lee S-H, Yoon Y-S, Seong K-J. Experimental study on the dissociation characteristics of methane hydrate pellet by hot water injection. Trans Korean Soc Mech Eng B 2011;35:1177–84. [287] Kawamura T, Sakamoto Y, Ohtake M, Yamamoto Y, Komai T, Haneda H, et al. Dissociation behavior of pellet shaped mixed gas hydrate samples that contain propane as a guest. Energy Convers Manage 2006;47:2491–8. [288] Kawamura T, Ohga K, Higuchi K, Yoon JH, Yamamoto Y, Komai T, et al. Dissociation behavior of pellet-shaped methane−ethane mixed gas hydrate samples. Energy Fuels 2003;17:614–8. [289] Gupta A, Lachance J, Sloan ED, Koh CA. Measurements of methane hydrate heat of dissociation using high pressure differential scanning calorimetry. Chem Eng Sci 2008;63:5848–53. [290] Gudmundsson J, Borrehaug A. Natural gas hydrate-an alternative to liquefied natural gas? Pet Rev 1996;50:232–5. [291] Javanmardi J, Nasrifar K, Najibi SH, Moshfeghian M. Economic evaluation of natural gas hydrate as an alternative for natural gas transportation. Appl Therm Eng 2005;25:1708–23. [292] Taheri Z, Shabani MR, Nazari K, Mehdizaheh A. Natural gas transportation and storage by hydrate technology: Iran case study. J Nat Gas Sci Eng 2014;21:846–9. [293] Abdalla BK, Abdullatef NA. Simulation and economic evaluation of natural gas hydrates [NGH] as an alternative to liquefied natural gas [LNG]. Catal Today 2005;106:256–8. [294] Bortnowska M. Development of new technologies for shipping natural gas by sea. Polish Maritime Res 2009:70. [295] Najibi H, Rezaei R, Javanmardi J, Nasrifar K, Moshfeghian M. Economic evaluation of natural gas transportation from Iran’s South-Pars gas field to market. Appl Therm Eng 2009;29:2009–15. [296] Khalilpour R, Karimi IA. Evaluation of LNG, CNG, GTL and NGH for monetization of stranded associated gas with the incentive of carbon credit. IPTC-14083-MS. International petroleum technology conference. doi:10.2523/IPTC-14083-MS. [297] Ota S, Kawano H, Hirai K, Kamei M. Concept and features of natural gas hydrate pellet carriers. OCEANS'04 MTTS/IEEE TECHNO-OCEAN'04: IEEE; 2004. p. 1115–20. [298] Kim K, Kim Y, Kang H. Recent advances in natural gas hydrate carriers for gas transportation. J Korean Soc Mar Eng 2014;38:589–601. [299] Kim K, Kang H, Kim Y. Risk assessment for natural gas hydrate carriers: a hazard identification (HAZID) study. Energies 2015;8:3142–64.

(Chungcheongnam-do, KR); 2015. [249] Cahn RP, Johnston, Robert H., Plumstead Jr., James A. Transportation of natural gas as a hydrate. United States: Exxon Research Engineering Co; 1970. [250] Nierman AJ. Transportation of natural gas as a hydrate. United States: Chevron Research Company (San Francisco, CA); 1976. [251] Gudmundsson J, Steinar. Method for production of gas hydrates for transportation and storage. Gudmundsson, Jon, Steinar; 1993 [252] Gudmundsson JS. Method for production of gas hydrates for transportation and storage. United States: Gudmundsson; Jon S.; 1996. [253] Gudmundsson J, Steinar. Method of oil and gas transportation. Gudmundsson, Jon, Steinar; 1996. [254] Heinemann RF, Huang David Da-teh, Long Jinping, Saeger Roland Bernard. Gas hydrate storage reservoir. United States: Mobil Oil Corporation (Fairfax, VA); 1999. [255] Katoh Y, Horiguchi, Kiyoshi, Iwasaki, Toru, Nagamori, Shigeru. Process for producing gas hydrate pellet. European: Mitsui Engineering and Shipbuilding Co, Ltd. (6-4 Tsukiji 5-chome, Chuo-ku, Tokyo 104-8439, JP); 2008. [256] Iwasaki T, Takahashi M, Arai T, Takahashi S, Takamoto K, Ogawa K, et al. Process and Apparatus for Producing Gas Hydrate Pellet. European: Mitsui Engineering & Shipbuilding Co., Ltd. (6-4 Tsukiji 5-chome, Chuo-ku; 2010. [257] Kanda N, Takahashi M, Iwasaki T. Process for producing mixed gas hydrate. European: Mitsui Engineering and Shipbuilding Co, Ltd. (6-4 Tsukiji 5-chome Chuo-ku, Tokyo 104-8439, JP); 2009. [258] Takaoki T, Nagamori, Shigeru, Kato, Yuichi, Arai, Takashi. Method of pelletizing, handling and transporting gas hydrate. Mitsui engineering & Shipbuilding Co., Ltd. (6-4 Tsukiji 5-chome, Chuo-ku, Tokyo, 104-8439, JP),Takaoki, Tatsuya c/o Mitsui Engineering & Shipbuilding Co. Ltd. (6-4, Tsukiji 5-chom, Chuo-ku Tokyo, 1048439, JP), Nagamori, Shigeru c/o Mitsui Engineering & Shipbuilding Co. Ltd. (6-4, Tsukiji 5-chom, Chuo-ku Tokyo, 104–8439, JP), Kato, Yuichi c/o Mitsui Engineering & Shipbuilding Co. (LTD. CHIBA WORKS 1, Yawatakaigandor, Ichihara-shi Chiba, 290-8601, JP), Arai, Takashi c/o Mitsui Engineering & Shipbuilding Co. (LTD. CHIBA WORKS 1, Yawatakaigandor, Ichihara-shi Chiba, 290-8601, JP); 2003. [259] Kimura T, Iwasaki S, Itoh K, Kondo Y, Yoshikawa K, Nagayasu H, et al. Dewatering device and method for gas hydrate slurrys. Mitsubishi Heavy Industries, Ltd. (5-1 Marunouchi 2-chome, Chiyoda-ku, Tokyo, 110-8315, JP), Kimura, Takahiro c/o Mitsubishi Heavy Industries Ltd. (Kobe Shipyard & Machinery Works; 2003. p. 1–1. [260] Watanabe K, Suganoya Kiyoaki, Yoshida Takahiro, Ogawa Kenji, Nanbara Shigeru, Imai Shinji. Gas hydrate compression molding machine. United States: Mitsui Engineering & Shipbuilding Co., Ltd. (Tokyo, JP),The Chugoku Electric Power Co., Inc. (Hiroshima, JP); 2012. [261] Iwabuchi WC, JP, Egami, Tomoaki (Chiba, JP), Narita, Hideo (Hokkaido, JP), Nagao, Jiro (Hokkaido, JP), Suzuki, Kiyofumi (Hokkaido, JP). Method of molding gas hydrate pellet. United States: Mitsui Engineering and Shipbuilding Co., Ltd (Tokyo, JP), National Institute of Advanced Industrial Science and Technology (Tokyo, JP); 2015. [262] Iwabuchi WC, JP, Egami, Tomoaki (Chiba, JP), Narita, Hideo (Hokkaido, JP), Nagao, Jiro (Hokkaido, JP), Suzuki, Kiyofumi (Hokkaido, JP). Method of molding gas hydrate pellet. United States: Mitsui Engineering and Shipbuilding Co., Ltd. (Tokyo, JP); 2014. [263] Kimura T, Iwasaki, Shojiro, Itoh, Katsuo, Uehara, Satoru, Yoshikawa, Kozo, Nagayasu, Hiromitsu, Ema, Haruhiko, Watabe, Masaharu, Kondo, Yuichi, Fujita, Hisayoshi, Endo, Hitoshi, Kita, Yoshihiro. Gas hydrate production device and gas hydrate dehydrating device. United States: Kimura Takahiro, Iwasaki shojiro, Itoh Katsuo, Uehara Satoru, Yoshikawa Kozo, Nagayasu Hiromitsu, Ema Haruhiko, Watabe Masaharu, Kondo Yuichi, Fujita Hisayoshi, Endo Hitoshi, Kita Yoshihiro; 2005. [264] Lee JD, Kim, Hyoung Jae, Kim, Sung Ryul, Hong, Sang Yeon, Park, Hye Ok, Ha, Mun Keun, Jeon, Seok Ku, Ahn, Hoon, Woo, Ta Kwan. Apparatus and method for continuously producing and pelletizing gas hydrates using dual cylinder. United States: Korea Institute of Industrial Technology (Seoul, KR), Samsung Heavy Industries Co., Ltd. (Gyeongsangnam-Do, KR), Hyundai Engineering Co., Ltd. (Seoul, KR), Daewoo Engineering & Construction Co., Ltd. (Seoul, KR), Sungilturbine Co., Ltd. (Busan, KR); 2013. [265] Kang HJ, Lee DK, Choi J. Container for storing, transporting, and disassociating hydrate pellets and method for storing, transporting, and disassociating hydrate pellets by using same. European: Korea Institute of Ocean Science and Technology (787 Haean-ro Sangnok-gu Ansan-si, Gyeonggi-do 426-744, KR); 2016. [266] Yakushev V, Istomin VI. Gas-hydrates self-preservation effect. Phys Chem Ice 1992:136–9. [267] Stern LA, Circone S, Kirby SH, Durham WB. Anomalous preservation of pure methane hydrate at 1 atm. J Phys Chem B 2001;105:1756–62. [268] Prasad PSR, Chari VD. Preservation of methane gas in the form of hydrates: Use of mixed hydrates. J Nat Gas Sci Eng 2015;25:10–4. [269] Takeya S, Ebinuma T, Uchida T, Nagao J, Narita H. Self-preservation effect and dissociation rates of CH4 hydrate. J Cryst Growth 2002;237–239, Part 1:379–82.

285