Research progress of promoters and promoting mechanisms for hydrate-based gas separation

Unconventional natural gas is a type of high-quality clean energy, which often contains some gases as impurities that cause reductions in its combustion heat value and utilization efficiency. Therefore, developing gas separation technologies to remove or separate these impurity gases and concentrate methane content is necessary. As a newly emerging gas separation technology, hydrate-based gas separation technology currently requires exploration on ways to greatly increase hydration rate to promote its industrial application. Screening green and environmentally-friendly promoters has become a research hotspot in recent decades. Amino acids, starch, and other biological substances have attracted much attention owing to their wide accessibility and environmental protection. Ionic liquids (ILs), which are a new type of lowly volatile and recyclable solvents, exhibit excellent performance in promoting gas hydrates formation and growth. Furthermore, ILs exhibit adjustable and controllable structures, which make them potential promoters in hydrate-based gas separation. Presently, no universally recognized theory on hydrate formation mechanism exists for various promoters. In this paper, different promotion mechanisms of gas hydration were described and discussed in detail, which included surface tension reduction theory, critical micelle theory, capillary effect theory, template effect theory, and surface hydrophobic effect theory. Both traditional promoters (e.g., THF, CP, and SDS) and bio-environmental promoters (e.g., amino acids and starch) were reviewed in the terms of gas hydration equilibrium condition, dynamic regularity, and hydrates-acceleration mechanisms. Particularly, the application of ILs, which are a type of semi-clathrate promoter, in gas hydration was elaborated. Study on the structural properties of promoters, their aggregation morphology in water, and intermolecular interactions between ILs, gases, and water is necessary for the establishment of a promoter-screening system for various gas hydrations.