Promoting methane hydrate formation with expanded graphite additives: Application to solidified natural gas storage

Abstract Solidified natural gas storage (SNG) using hydrate can provide environmentally friendly, highly compact and non explosive mode of NG storage at mild storage conditions. The efficient and rapid formation of natural gas hydrate is of great significance to the industrial application of SNG. In this work, in order to improve the gas storage capacity and absorption rate of CH4 hydrate, experiments were carried by adding expanded graphite (EG) with sodium dodecyl sulfate (SDS) with a pressure range of 5.0–8.0 MPa and a temperature of 273.2 K. The results indicate that EG can not only promote the formation of hydrate by promoting the nucleation of hydrate, but also accelerated the growth of hydrate by enhancing hydration heat transfer. EG system (0.05 wt%, 0.08 wt%, and 0.10 wt%) enabled faster hydrate formation than a liquid water system, with a content of 0.08 wt% optimal for the enhancement of hydrate formation. In the combination of EG (0.08 wt%) and SDS solution, EG + 0.03 wt% SDS system can improve the kinetics of hydrate formation to the greatest extent; the maximum gas storage capacity reached 190.4 STP/g H2O, and the maximum gas storage speed reached 20.84 cm3·g−1·min−1 at 273.2 K and 6 MPa. Compared with the SDS system, the maximum methane uptake and rates of EG + 0.03 wt% SDS system increased by 11.90% ~ 26.18% and 15.09% ~ 51.67% under different pressures respectively. At relatively low pressure, the hydrate can achieve a saturation of ~90% in EG + 0.03 wt% SDS system. On the other hand, compared with other carbon materials such as graphite and carbon nanotubes, the gas storage capacity and average gas storage rate of EG system were 1.12–1.37 times and 1.67–30.17 times higher, respectively, under similar temperature and pressure conditions. Therefore, a collaborative mode of enhancing gas concentration, nucleation site and heat transfer efficiency at the gas–liquid interface of hydrate reaction with only one material (EG) was proposed to promote the formation of hydrate. Further study on the promotion mechanism of EG and amplification experiments are expected to provide an effective way for the commercial application of SNG process in future work.