Natural gas (NG) has been recognized as the cleanest burning fossil fuel and a vital resource to alleviate anthropogenic CO2 emissions to enable a transition into a carbon-constrained world. There is a necessity to develop safe, reliable, and efficient technology for large-scale NG storage.
Fiery ice (Burning Natural Gas Hydrate). Figure courtesy Praveen Linga.
Solidified natural gas (SNG) technology via clathrate hydrates offers a high NG storage capacity at temperate conditions (atmospheric pressure and moderate temperatures) compared to compressed natural gas (CNG) or adsorbed natural gas (ANG). Though Liquefied Natural Gas (LNG) facilities are devoted to transporting NG, the cryogenic temperature requirement (-162 °C) and the continuous boil-off issues limit its adoptability for long-term storage application. Clathrate hydrates or gas hydrates are crystalline ice-like compounds formed by guest molecules (such as CH4 gas and THF liquid) and host water molecules at suitable pressure and temperature conditions. Clathrate hydrates classically crystalize in three different structures or geometries, named structure I (sI, cubic), structure II (sII, cubic), and structure H (sH, hexagonal).
SNG is nonexplosive, environmentally compatible, and economical. A major concern for the SNG technology is that the use of sI hydrates requires low storage temperature (-20 °C) for storage, and the stability depends on the anomalous self-preservation effect at this temperature. Methane (sI) hydrates are thermodynamically stable at a temperature of -80 °C at atmospheric pressure.
In order to overcome this issue, it is desirable to move away from sI hydrates. In this direction, mixed methane-tetrahydrofuran (CH4-THF) hydrates (sII) offer a great promise to shift the thermodynamics to very mild conditions, as the mixed CH4-THF hydrate is thermodynamically more stable than pure methane hydrate (sI). However, molecular-level understanding of these thermodynamically or kinetically controlled hydrate structures is elusive in the open literature. Thus, through a series of carefully planned experimental work utilizing state-of-the-art analytical techniques like high-pressure differential scanning calorimetry (HP μ-DSC) and an in-situ Raman spectroscopy, we investigate the mixed CH4-THF hydrate formation (with 5.56 mol% THF, stoichiometric amount) in the presence of a surfactant, sodium dodecyl sulfate (SDS, as kinetic promoter).
Fig.1 Co-occurrence of pure methane (sI) & mixed CH4-THF hydrates (sII) evidenced through DSC thermogram and Raman spectra (Republished with permission from iScience)
Through HP μ-DSC analysis, we found that the presence of SDS in water-THF solution promotes the nucleation and growth of sI hydrate (pure methane hydrates) crystals. In other words, we can say that pure methane (sI) and mixed CH4– THF hydrates (sII) both co-exist in the presence of SDS (Refer to Fig.1). However, in the absence of SDS, we found that instead of sI hydrates, pure THF (sII) and mixed CH4– THF hydrates (sII) coincide during hydrate formation. Moreover, if thermodynamics restrict the formation of sI hydrates, the presence of SDS in the water-THF system may enhance the formation of mixed CH4-THF (sII) hydrate with significantly high methane uptake.
Our findings present a significant enhancement in the methane storage capacity through mixed CH4-THF hydrates in the presence of SDS, possibly due to (i) the sharing of large cages by methane molecules along with THF, and (ii) improving the methane enclathration in the small cages of mixed CH4-THF hydrates. In summary, we present a kinetically and thermodynamically controlled encaging of methane molecules in small and large cages of sI and sII hydrates. Our findings offer new insights for the development of an efficient process for large-scale methane storage at temperate conditions in mixed CH4-THF hydrates (sII) through solidified natural gas technology.
These findings are described in the article recently published in iScience, entitled Sodium Dodecyl Sulfate Preferentially Promotes Enclathration of Methane in Mixed Methane-Tetrahydrofuran Hydrates, authored by Dr. Asheesh Kumar (now at The University of Western Australia) and Prof. Praveen Linga from the National University of Singapore, and Prof. Rajnish Kumar from the Indian Institute of Technology, Madras, India.