Complicated hydrate formation kinetics induced by ion specific effect in porous surface

Abstract

It was recently found that halogen salts can facilitate hydrate formation at low concentrations, and this finding provides new ideas on hydrate-based adsorption-hydration natural gas (AHNG) technology by using diluted seawater so as to save fresh water. However, complicated hydrate formation kinetics was commonly anticipated due to ion specific effect, while porous surface further enhances the effect. To improve the understanding of the synergy of ion specific effect and porous surface on hydrate formation kinetics, hydrate formation loaded by activated carbon (AC) in the presence of 4 halogen salts was carried out. It was found that both of promotion and antagonism effect appears in the ion-porous surface system, KI performs the best on promoting hydrate nucleation, and hydrate induction period even disappears when 0.05 wt% KI was applied. In contrast, 0.1 wt% NaCl enhances methane storage capacity and performs excellent at high water contents, and an average increase of 9.6 V/V in methane storage capacity was obtained for different water contents. The promotion mechanisms of specific ions on hydrate formation were speculated, and I− and Na+ were found to perform better on enhancing hydrate nucleation and conversion, respectively. I− can induce interfacial hydration shells to promote hydrate nucleation, while the porous surface significantly increases gas–liquid contact area and enhance the promotion, and Na+ improves water-to-hydrate conversion in confined porespace due to ion selection induced by confinement.