(628d) Novel Facilitated Transport Membrane for Post-Combustion Carbon Capture: From Membrane Synthesis to Process Design

Novel Facilitated Transport Membrane for Post-combustion Carbon Capture: From Membrane Synthesis to Process Design

Yang Han and W.S. Winston Ho

William G. Lowrie Department of Chemical and Biomolecular Engineering, The Ohio State University

Large-scale application of membrane in post-combustion carbon capture has been limited by the trade-off between CO₂ permeance and CO₂/N₂ selectivity of most polymeric membrane materials. In order to overcome this limitation, research efforts on facilitated transport membrane (FTM) have been devised with the objectives of (1) developing carriers with high CO₂ loading capacity and reactive diffusivity, and (2) designing membrane processes that can capitalize on the outstanding selectivities shown by FTM. In this presentation, novel FTM was synthesized in a composite membrane configuration with an 170 nm selective layer coated on a polyethersulfone nanoporous substrate. In the selective layer, polyvinylamine with amine-sites covalently bound to the polymer backbone was used as fixed-site carrier and an amino acid salt, synthesized by deprotonating sarcosine with 2-(1-piperazinyl)ethylamine, was blended as mobile carrier. The membrane demonstrated a CO₂ permeance of 1500 GPU (1 GPU = 10^-6 cm³ (STP)·cm^-2·s^-1·cmHg^-1) and a CO₂/N₂ selectivity >140 at 67 °C with 1 atm feed and permeate pressures. Two membrane processes were designed for the FTM to decarbonizing coal derived flue gas with 90% CO₂ recovery. In the first membrane process, i.e., air sweep process, the flue gas flowed to a vacuum membrane stage to produce a >95% pure CO₂ permeate; the remaining CO₂ was removed by an air-sweep membrane stage. The CO₂-laden sweep air was fed to a power plant boiler as combustion air. In the second membrane process, i.e., retentate recycle process, the flue gas was passed to two membrane stages in an enriching cascade. A portion of the CO₂-depleted retentate of the first membrane stage was recycled as its own sweep gas, while a vacuum was pulled for the second membrane stage. Both membrane processes yielded a capture cost <40/tonne CO₂ in the 2007 dollars, which met the target set by the Department of Energy (DOE) for 2025. A common element in these two processes was a vacuum membrane stage with a permeate pressure as low as 0.3 atm. Under this testing condition, however, the elastic selective layer of the synthesized FTM sunk into the nanoporous substrate, which resulted in a drastically reduced CO₂ permeance. To address this issue, multi-walled carbon nanotubes (MWNTs) wrapped by a copolymer poly(vinylpyrrolidone-co-vinyl acetate) were dispersed in the selective layer as reinforcement fillers. The gas permeation measurements showed that the incorporation of MWNTs strengthened the polymer matrix and the selective layer penetration was refrained by a 3 wt.% MWNTs loading. The presence of MWNTs also mitigated the polymer compaction if the feed gas was compressed. The reinforced membrane demonstrated stable separation performance for a feed pressure up to 4 atm with a vacuum of 0.3 atm pull on the permeate side.