Spheroid-Encapsulated Ionic Liquids for Gas Separation

Combustion of fossil fuels produces carbon dioxide (CO₂), a greenhouse gas contributing to global climate change. As the demand for energy continues to increase, atmospheric levels of CO₂ will continue to rise. There is thus a growing demand to mitigate CO₂ emissions, particularly from industrial sources. Currently, CO₂ capture with aqueous amines dominates industrial processes. Although effective, amines are corrosive and require significant energy for regeneration. Indeed, amine scrubbing of flue gas consumes an estimated 30% of the power generated by coal-fired power plants. Thus, there exists a significant need for technologies that are highly efficient at CO₂ capture/separation and do so with low cost and energy penalties.

A class of compounds known as ionic liquids (ILs) represents a highly promising CO₂ capture/separation technology. Ionic liquids are molten salts, typically containing an organic cation and either an organic or inorganic anion. Significantly, ILs readily absorb CO₂, with low affinity for other gases such as CH4, H2 and N2. In addition to their selectiveness for CO₂, ILs possess a number of other desirable properties that make them ideal candidates for CO₂ capture. ILs are non-volatile and thermally stable. They are inexpensive and more energy efficient than competing technologies, requiring little energy for regeneration. Ionic liquids do have one negative feature, however. Upon binding CO₂, many ILs undergo a drastic increase in viscosity. This high viscosity severely limits the use of ILs in liquid absorbers (i.e., scrubbers) and is thus a major obstacle to industrial application of ILs. Modification of ILs is therefore necessary before these agents can be widely accepted as a CO₂ capture/separation technology.

NETL researchers have designed a means of overcoming the high viscosity problems associated with ILs. Methods have been developed to fabricate novel porous, core-shell type polymer spheroids in which ILs are subsequently encapsulated. The 1 to 3 mm diameter spheroids possess a high surface area to volume ratio, ideal for gas absorption. The spheroids also offer the advantage of a large internal payload, allowing 70-80 wt.% of IL to be ‘loaded’, resulting in maximal CO₂ capture capacity. Encapsulation in polymer spheroids permits the use of ILs not previously compatible with traditional gas-liquid contactors, as the CO₂-dependent viscosity increase is now localized to the spheroidal interior. The design of the spheroids allows close packing, enabling a high density of spheroids in any capture vessel, again maximizing CO₂ absorption. Significantly, the spheroids can easily be ‘tuned’ for specific applications by modifying or varying the component polymers, ILs, or spheroidal architecture.