(690g) A Fundamental Comparison of the Stability and Dissociation Aspects of Structure I and Structure II Hydrates from the Perspective of Feasible Natural Gas Storage

Authors: 
Bhattacharjee, G., National University of Singapore
Veluswamy, H. P., National University of Singapore
Kumar, R., Indian Institute of Technology Madras
Linga, P., National University of Singapore
The hydrate based approach to storing natural gas (known as Solidified Natural Gas (SNG) technology) has been gaining popularity in recent times owing to its environmentally benign operation, large volumetric capacity and non-explosive character. By transitioning from sI to sII hydrates through the introduction of thermodynamic promoters like tetrahydrofuran (THF) to the system, researchers have not only been able to favourably tune the thermodynamic operating conditions for hydrate formation (significantly higher temperature and lower pressure for hydrate formation), but also demonstrated remarkable kinetic performance during hydrate formation at the revised operating conditions. However, the true feasibility of this approach lies in the long term stability and thus storability of mixed CH4-THF sII hydrates at atmospheric pressure and how it compares with that for pure CH4 sI hydrate under similar storage conditions.

According to thermodynamic data, the equilibrium temperature for mixed CH4-THF hydrates (considering 6 mol % of THF in the system) at 0.12 MPa is 277.7K, while the same for pure CH4 hydrate at 0.1 MPa is 193 K. Thus ideally, for storage at atmospheric pressure and moderately low temperatures (268-271 K), mixed CH4-THF sII hydrate should be able to demonstrate complete stability while the pure CH4 sI counterpart should dissociate rather rapidly. In the current study - a first of its kind, a like for like comparison was made for the stability and dissociation characteristics of methodologically identical pure CH4 (sI) and mixed CH4-THF (sII) hydrates. Cylindrical sI and sII hydrate pellets were synthesized using a one of a kind SNG technology prototype. Synthesized hydrate pellets were transferred to storage units for seven to ten day stability tests at atmospheric pressure and moderately low temperatures (~268-271 K). Rate of hydrate dissociation via thermal stimulation using room temperature water stream was separately studied for the synthesised sI and sII pellets. Results indicate that while sII pellets were completely stable at the investigated storage conditions, sI pellets started dissociating as soon as the tests were initiated. This demonstrates clearly the remarkable stability of sII hydrates when stored at atmospheric pressure and moderately low temperatures, thus highlighting a major downstream advantage of opting for the mixed sII hydrate route as far as SNG technology is concerned. The results obtained from this study are of great importance from both technical and economic standpoints and should go a long way in ensuring SNG technology as an attractive alternative for mid to long term storage of natural gas.