Elsevier

Chemical Engineering Journal

Volume 290, 15 April 2016, Pages 161-173
Chemical Engineering Journal

Rapid methane hydrate formation to develop a cost effective large scale energy storage system

https://doi.org/10.1016/j.cej.2016.01.026Get rights and content

Highlights

  • A new method for producing methane hydrates rapidly is presented.

  • Tetrahydrofuran acts both as a thermodynamic and kinetic promoter in an unstirred tank reactor.

  • A cost effective, large scale solidified natural gas (SNG) storage system is proposed.

Abstract

Natural gas (NG) is the cleanest burning fossil fuel and its usage can significantly reduce CO2 emissions from power plants. With its widespread use, there is an ever increasing need to develop technologies to store NG on a large scale. NG storage via clathrate hydrates is the best option for a large scale storage system because of its non-explosive nature, mild storage conditions, high volumetric capacity and being an environmentally benign process. In this work, we demonstrate a new method to achieve rapid methane hydrate formation in an unstirred tank reactor configuration (UTR) at moderate temperature and pressure conditions employing tetrahydrofuran (THF) as a promoter. For the first time, THF is reported to act both as a thermodynamic and an excellent kinetic promoter for methane hydrate formation. We demonstrate a multi-scale experimental validation of our method to a volumetric sample scale-up factor of 120 and internal reactor diameter scale-up factor of 10. Further, new insights on the dissociation behavior of the hydrates are reported. There is a competitive edge for storing NG via clathrate hydrates compared to compressed natural gas storage both in terms of cost and safety.

Introduction

NG is the cleanest burning fossil fuel and can meet stringent environmental norms, including reduced CO2 emissions. This century is the golden age for natural gas (NG) according to International Energy Agency (IEA) [1]. With the inevitable global shift to a NG based economy, there is an ever increasing need to develop technologies to store NG on a large scale. Storage options for NG include LNG (Liquefied NG), CNG (Compressed NG), SNG (Solidified NG) via clathrate hydrates or as ANG (Adsorbed NG) via sorbents. LNG has a volumetric capacity of about 600 v/v and is a very effective mode to transport NG from source to market [2] but not suitable for storage due to the continuous gas boil-off issues. CNG (∼200 v/v at 200 atm) as mode of storage requires high pressure for both production and storage. The inherent flammable/explosive nature of natural gas, practically makes CNG hazardous for large scale NG storage. ANG requires adsorbing NG onto sorbents like activated carbon, carbon nanotubes (CNTs), graphene, metal organic frameworks (MOFs) etc. [3], [4], [5], [6] Given the history of the development of CNTs for NG storage and known scale-up issues, there is a need to demonstrate process development for these materials.

Storage of natural gas as SNG via clathrate hydrates offers an excellent opportunity to store NG on a large scale. The term SNG (solidified natural gas) is coined similar to LNG, CNG or ANG and is defined as natural gas locked in the solid cages formed by water molecules. There are several advantages of SNG technology: high volumetric storage capacity, being environmentally benign, non-explosive nature, extremely safe to handle and cost effective [7], [8]. Another highlight of SNG technology is that it is not sensitive to the presence of higher hydrocarbons like traces of ethane, propane etc. This is due to the fact that trace hydrocarbons can also be captured in the water cages if present and essentially they can 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 7.25 MPa pressure for pure methane gas. ‘Self-preservation’ is another phenomenon that is a characteristic advantage to SNG technology [9], [10]. Maintaining subzero storage conditions at atmospheric pressure ensures extremely slow hydrate dissociation rates and thus offers kinetic stability due to self-preservation effect. Mimachi et al. [11] had recently demonstrated successful storage of NGH pellets for about 3 months under atmospheric pressure at 253 K without any considerable change in hydrate mass fraction and the stored gas composition. Moreover, energy recovery from SNG is relatively simple just like melting ice that can be achieved using low waste heat or seawater. Research on SNG has seen a progressive development in the past decade with demonstration plants in Japan and South Korea. Apart from NG storage, gas hydrates are promising candidates in storing hydrogen [12], [13], [14], to develop an environmentally benign technology for carbon dioxide capture and sequestration [15], [16], [17], [18], desalination of seawater [19], [20], cold storage [21], [22] and gas separation applications [23], [24]. Gas hydrates are also considered as a huge energy resource of the future due their presence in nature as either permafrost or marine deposits distributed across the world [25], [26], [27].

Despite several advantages highlighted for the SNG technology, there are a few challenges that impede the commercial deployment of this technology. There are four steps involved in SNG technology: hydrate formation step, dewatering step, pelletizing step, and storage step (involves depressurization to reach the storage condition of 1 atm). Major research challenges for the SNG technology are in the formation step (slow kinetics of hydrate formation and severe operating process conditions) and the storage step (cost of refrigeration during the storage of SNG at 248 K). Generally, the kinetics of hydrate formation in traditional stirred tank reactors is affected by mass transfer limitation at the gas/liquid interface due to the inability to continuously mix the gas/liquid/solid system for sustaining the hydrate growth. Also, due to high process costs involved, stirred tank reactors are energy intensive for a large scale deployment of SNG technology. On the other hand, in a quiescent unstirred system, methane hydrates are observed to form a hydrate film commonly referred to as “skin” at the gas–liquid interface offering resistance to further hydrate growth [28]. Two possible approaches to overcome the slow kinetics are: to employ an effective and innovative reactor configuration to enhance the kinetics but at the same time, the configuration should not be energy intensive for large scale deployment. The second approach is to choose promoters that can enhance the kinetics without compromising the storage capacity and operate at moderate experimental conditions.

Recent literature studies highlight the advantage of using fixed bed reactor configuration for achieving improved kinetics of methane hydrate formation. Materials like silica gel, aluminium foam, activated carbon, carbon tubes and hollow nano-silica were studied as fixed bed supports for improving the methane hydrate formation kinetics [29], [30], [31], [32]. Further, novel materials like dry water [33] (free flowing hydrophobic silica powder with dispersed water), hollow silica [34], [35] (very low bulk density) and dry gels [36] (gelling agent along with dry water for improving recyclability) were experimented for studying the methane hydrate formation kinetics. While recent literature works have shown enhanced kinetics in the presence of porous materials, in the overall SNG production and storage process chain, the use of porous material is less practical due to the challenges involved in pelletizing the hydrates along with the porous materials for effective storage and transport application. Another challenge is the increase in the volume of storage due to the presence of porous materials along with hydrates. Other reactor configurations [37] for providing improved gas/liquid contact for methane hydrate formation include bubble column [38], spray reactors [39], [40], [41], [42] and eject type loop reactor [43]. Though it has been demonstrated that it is feasible to employ spray type configuration for methane hydrate production on a large scale, it will be cost intensive compared to the classic unstirred reactor configuration employed in the current study.

Apart from different reactor configurations, different thermodynamic and kinetic promoters are being evaluated for improving the hydrate formation conditions and the kinetics respectively [44], [45], [46], [47], [48]. Thermodynamic promoters are chemicals that facilitate hydrate formation at moderate temperature and pressure conditions but at the cost of storage capacity. This happens due to the fact that promoter molecules occupy the hydrate cages along with guest methane/natural gas. Methane forms a standard sI hydrate structure on its own. However, in the presence of thermodynamic promoter it is possible for methane to be present in other caged structures like sII or sH depending on the guest molecules [48], [49], [50]. With the assumption of complete occupancy of methane in small and large cages of sI hydrate structure, the theoretical methane storage capacity is 172 v/v (based on the gas release at STP). For sII and sH hydrate structures, large cages are occupied by added promoters (large guest molecules) and methane occupies small cages of sII and both small and medium cages of sH structure. With the assumption of complete occupancy of methane in aforesaid cages of sII and sH, the volumetric storage capacities are computed to be 115 and 143 v/v respectively [49]. Despite the 33% and 17% reduction in storage capacity considering sII and sH hydrate structures, the flexibility of hydrate formation at much moderate pressures and high temperatures than pure sI hydrates will offer higher incentives in the reduction of compression and refrigeration costs which will offer greater benefit for commercial scale SNG production. Several studies have been reported in the literature on sH methane hydrate formation with the presence of large molecules like neohexane, tert-butyl methyl ether (TBME), methylcyclohexane, etc. as guest. Lee et al. [48] investigated the kinetics of sH hydrate for different sH hydrate formers and methane in a stirred tank reactor and reported a highest methane uptake of 0.0261 mol/mol of water using TBME promoter. Later, several works were conducted starting with ice instead of liquid water and a thermal ramping method was reported to work effectively in achieving higher conversion. In a practical situation, considering the scale of production, it will be an energy intensive to form hydrates at 253 K. In addition, heat and mass transfer effects during the scale up remains to be addressed and so far none of the studies have attempted to scale up the use of solid ice as a raw material for SNG technology. On the other hand, the function of kinetic promoter (surfactant) is to act like a catalyst that will enhance the kinetics of hydrate formation. Surfactants reduce the surface tension of the water and several studies have employed surfactants to promote the hydrate formation kinetics in the literature [46], [51], [52], [53]. While surfactants have been reported to work effectively to enhance the kinetics of methane hydrate formation, they do not have any effect on the high operating conditions. Moreover, decomposition challenge due to the foam formation is an issue for the use of surfactants [36], [54]. A reduction in the severity of operating conditions during hydrate formation step and reduction in the refrigeration cost can only be achieved by the choice of a suitable thermodynamic promoter that can also promote hydrate formation kinetics.

In the current study, we use tetrahydrofuran (THF), a well-known sII hydrate former in an unstirred tank reactor configuration and investigate the formation of mixed methane/THF hydrates at mild operating conditions. THF is well known to be an effective thermodynamic promoter [12], [55], [56]. Hydrate phase equilibrium data for methane + THF + water system [57], [58], [59], [60] along with kinetic studies on bubbling reactor [38] and stirred tank reactor [61] are reported in the literature. However to the best of our knowledge, investigation of this system in quiescent unstirred setup under experimented conditions is not available in the literature.

Section snippets

Materials

99.9% pure methane gas procured from Singapore Oxygen Air Liquide Private Ltd (SOXAL), Tetrahydrofuran (THF) of 99.99% purity (AR grade) procured from Fisher Chemicals and the de-ionized water obtained from Elga micromeg deionizaton apparatus were used for the conduction of experiments

Experimental methods

Morphology of mixed methane/THF hydrates were studied using the experimental setup detailed in the study by Veluswamy et al. [62]. Preliminary experiments revealed that the acrylic column was being etched by the

Rapid methane hydrate formation in unstirred tank reactor (UTR)

Fig. 1 presents the rapid methane hydrate formation kinetic data along with visual images that we observed through the microscope. 2 ml of stoichiometric 5.6 mol% THF solution at 283.2 K was subjected to a methane pressure of 7.2 MPa in a transparent crystallizer. Due to use of THF in water which also participates in hydrate formation, equilibrium pressure for methane hydrate formation is achieved at 283.2 K and 0.5 MPa [58]. We observed drastic and extremely fast hydrate formation kinetics (as shown

Discussion

A schematic of the process chain of SNG technology is presented in Fig. 8. There are four steps involved in SNG technology: hydrate formation step, dewatering step to remove the unconverted water, pelletizing step, and finally cooling & depressurizing step to reach the desired storage conditions (248 K and 1 atm). As highlighted the major bottlenecks are the process and energy intensive formation step and the energy intensive storage step. Earlier works in literature have attempted to address the

Conclusion

SNG technology provides a viable alternative to store natural gas in molecular form by locking them in clathrate cages formed by water. Rapid formation of mixed methane/THF hydrate in UTR configuration was observed and reported for the first time. The reason for rapid kinetics and high methane gas uptake is the distinct hollow crater like formation aided by well-connected channels for fluid flow in the upward direction (along the reactor walls) and simultaneous dendritic crystal growth (within

Acknowledgements

The financial support from the National University of Singapore (R-279-000-420-750, R-261-508-001-646 and R-261-508-001-733) is greatly appreciated. Rajnish Kumar acknowledges financial support from CSIR project CSC 0102 and Pramoch Rangsunvigit acknowledges the support from the Thailand Research Fund.

References (81)

  • Y. Xie et al.

    Experimental study on a small scale of gas hydrate cold storage apparatus

    Appl. Energy

    (2010)
  • A. Eslamimanesh et al.

    Application of gas hydrate formation in separation processes: a review of experimental studies

    J. Chem. Thermodyn.

    (2012)
  • T.E. Rufford et al.

    The removal of CO2 and N2 from natural gas: a review of conventional and emerging process technologies

    J. Petrol. Sci. Eng.

    (2012)
  • Z.R. Chong et al.

    Review of natural gas hydrates as an energy resource: prospects and challenges

    Appl. Energy

    (2016)
  • J. Pasieka et al.

    Investigating the effects of hydrophobic and hydrophilic multi-wall carbon nanotubes on methane hydrate growth kinetics

    Chem. Eng. Sci.

    (2013)
  • V.D. Chari et al.

    Methane hydrates formation and dissociation in nano silica suspension

    J. Nat. Gas Sci. Eng.

    (2013)
  • X. Lang et al.

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

    J. Nat. Gas Chem.

    (2010)
  • Y.T. Luo et al.

    Study on the kinetics of hydrate formation in a bubble column

    Chem. Eng. Sci.

    (2007)
  • N.-J. Kim et al.

    Formation enhancement of methane hydrate for natural gas transport and storage

    Energy

    (2010)
  • F. Rossi et al.

    Investigation on a novel reactor for gas hydrate production

    Appl. Energy

    (2012)
  • K. Okutani et al.

    Surfactant effects on hydrate formation in an unstirred gas/liquid system: an experimental study using methane and sodium alkyl sulfates

    Chem. Eng. Sci.

    (2008)
  • H. Ganji et al.

    Effect of different surfactants on methane hydrate formation rate, stability and storage capacity

    Fuel

    (2007)
  • N. Ando et al.

    Surfactant effects on hydrate formation in an unstirred gas/liquid system: an experimental study using methane and micelle-forming surfactants

    Chem. Eng. Sci.

    (2012)
  • W. Lin et al.

    Effect of surfactant on the formation and dissociation kinetic behavior of methane hydrate

    Chem. Eng. Sci.

    (2004)
  • Y. Zhong et al.

    Surfactant effects on gas hydrate formation

    Chem. Eng. Sci.

    (2000)
  • T.A. Strobel et al.

    Thermodynamic predictions of various tetrahydrofuran and hydrogen clathrate hydrates

    Fluid Phase Equilib.

    (2009)
  • Y.T. Seo et al.

    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)
  • H.P. Veluswamy et al.

    Influence of cationic and non-ionic surfactants on the kinetics of mixed hydrogen/tetrahydrofuran hydrates

    Chem. Eng. Sci.

    (2015)
  • T. Nakamura et al.

    Stability boundaries of gas hydrates helped by methane—structure-H hydrates of methylcyclohexane and cis-1,2-dimethylcyclohexane

    Chem. Eng. Sci.

    (2003)
  • R. Susilo et al.

    Tuning methane content in gas hydrates via thermodynamic modeling and molecular dynamics simulation

    Fluid Phase Equilib.

    (2008)
  • H.J. Lee et al.

    Gas hydrate formation process for pre-combustion capture of carbon dioxide

    Energy

    (2010)
  • P. Linga et al.

    A new apparatus to enhance the rate of gas hydrate formation: application to capture of carbon dioxide

    Int. J. Greenhouse Gas Control

    (2010)
  • P. Linga et al.

    Gas hydrate formation from hydrogen/carbon dioxide and nitrogen/carbon dioxide gas mixtures

    Chem. Eng. Sci.

    (2007)
  • J. Du et al.

    Effects of ionic surfactants on methane hydrate formation kinetics in a static system

    Adv. Powder Technol.

    (2014)
  • S. Takeya et al.

    Particle size effect of hydrate for self-preservation

    Chem. Eng. Sci.

    (2005)
  • F. Birol, J. Corben, M. Argiri, M. Baroni, A. Corbeau, L. Cozzi, A. Yangisawa, Are we entering a golden age of gas, IEA...
  • E. Bekyarova et al.

    Single-wall nanostructured carbon for methane storage

    J. Phys. Chem. B

    (2003)
  • Y. He et al.

    Methane storage in metal-organic frameworks

    Chem. Soc. Rev.

    (2014)
  • T. Düren et al.

    Design of new materials for methane storage

    Langmuir

    (2004)
  • M. Eddaoudi et al.

    Systematic design of pore size and functionality in isoreticular MOFs and their application in methane storage

    Science

    (2002)
  • Cited by (269)

    View all citing articles on Scopus
    View full text