Hybrid Membrane/Absorption Process for Post-Combustion CO2 Capture

Gas Technology Institute (GTI) and PoroGen Corporation are jointly developing a novel hybrid membrane – absorption process for post-combustion CO₂ capture. The novel membrane process combines advantageous features of both absorption and membrane processes to generate cost-effective separation and capture of CO₂ from flue gases. In this process, CO₂-containing gas passes through small-diameter membrane tubes (hollow fibers with porous walls) while a CO₂ selective solvent (typically an amine solution) flows on the shell side of the membrane module. CO₂ permeates through the membrane and is absorbed in the selective solvent. The CO₂ rich solvent can be regenerated in a second membrane module operated in a reverse process.

We are making extensive laboratory and bench-scale measurements to bring this technology development to commercialization. A membrane contactor skid for post-combustion CO₂ capture has been constructed at GTI. This skid was designed to a scale representing 25 kW equivalents of CO₂ capture (0.5 tons/day). We have established the feasibility of utilizing super-hydrophobic polyether ether ketone (PEEK) hollow fiber membrane contactor in combination with chemical solvents to separate and capture CO₂ from simulated flue gases. With  aMDEA and activated K₂CO₃ solvents, we have achieved at least 90% CO₂ capture in one stage. The measured volumetric mass transfer coefficient of the hybrid membrane absorption process is as high as 1.8 (sec)^-1, which is more than 20 time greater than the maximum mass transfer coefficient of a packed column. The large mass transfer coefficient translates to much reduced equipment size. The reduced size requirements, in turn, translate to lower solvent inventories, and smaller footprint (lower visual impact) for siting at congested power plants. The system operates at less than 2 psi gas-side pressure drop, which is comparable to those achieved with very large diameter columns, and much lower than those of conventional membrane systems.

Initial test results also showed that the CO₂ capture performance was not affected by flue gas contaminants O₂, NO₂, and SO₂ with aMDEA solvent. The test demonstrated that 15% less of the inlet SO₂ and 9.2% less of the inlet NO₂ were absorbed by the solvent in a membrane contactor as compared to a conventional amine scrubber during CO₂ capture. It is thus expected that the formation of heat-stable salts in the contactor process will be reduced. Preliminary economic evaluation based on lab absorber only testing data indicates a 55% increase in the cost of electricity (COE), which is better than DOE’s benchmark amine absorption technology which results in a 85% increase in COE. Regeneration testing this year is expected to further decrease the COE to less than 35%, achieving DOE’s target of increase in COE.