Storage capacities of CO2 hydrates in packed silica bed in multiple formation processes

Hydrate-based gas storage is known to be an economical and eco-friendly method. In this work, the growth of CO2 hydrate was repeated 5 times in the silica gels with certain water saturation at an initial pressure of 3.7 MPa, 278.15 K. The influence of sodium dodecyl sulfate (SDS) and water saturation ranging from 30 to 60% on CO2 storage efficiency was measured. Results showed that silica gels with 30% water saturation got the highest CO2 storage capacity with the water conversion of 91% within 24 h and did not have evident decay in repeated tests. The volume ratio of adsorbed CO2 to silica gels was found to decrease from 46 to 19 as the initial water saturation increased from 30 to 60%. In repeated runs, the volume ratio descended evidently in the initial 3 runs. Water film covered on the silica gel particles was assumed to be the major obstacle for hydrate growth. SDS was found to enhance the volume ratio in the silica gel with high water saturations and reduce the decay of volume ratio in repeated test. However, the unexpected long hydrate nucleation period ranging from 5 to 8 h would occur after 3 repeated test. SEM images revealed that the hydrate crystals attached with each other like pomegranate seeds in pure water system, while they changed into flakes which adhere on the silica gel particles. Neither of these morphologies provide a path for CO2 diffusion. Therefore, avoiding the formation water film on silica gel particles and increasing the porosity of the porous particles are the key to improve gas storage efficiency.