Kinetic study of methane storage in hydrophobic ZIF-8 by adsorption-hydration hybrid technology

Adsorption-hydration hybrid (AHH) technology combining methane adsorption and hydrate formation has attracted extensive attention for methane storage/transportation. This work explored the AHH kinetic properties in the hydrophobic metal-organic framework ZIF-8, using a visualization platform, to elucidate the relationship between adsorption and hydrate formation. A two-stage adsorption process of methane physisorption and hydrate formation-adsorption occurred, in which hydrate fibers and hydrate chunks were formed successively. To investigate the effects of water content (Rw) and pressure on AHH kinetic properties, the mass transfer coefficient of methane and kinetic constant of hydrate formation in two stages were calculated, respectively. Increasing Rw from 0.3 to 1.0 resulted in a significant decrease of 68.2% and 89.8% in the mass transfer coefficient of methane in the first stage and the kinetic constant of hydrate formation in the second stage, respectively. The higher Rw hindered hydrate fiber formation by impeding methane diffusion within the pore space, while raising the initial pressure promoted methane diffusion. Specifically, when the pressure was increased from 5.3 MPa to 7.0 MPa, the methane physisorption rate and hydrate formation rate could increase by up to 1.77 and 1.35 times, respectively. Furthermore, it was found that the decrease of physisorption rate further reduced the hydrate formation rate, revealing the relationship between physisorption and hydrate formation. A combined analysis of initial pressure and water content suggested that the inhibitory effect of Rw on mass transfer outweighed the promoting effect of initial pressure.