Effects of Contaminants on Separation Performance of Amine-Containing Facilitated Transport Membrane for CO2 Separation

Amine-containing facilitated transport membrane has been suggested as a promising approach for post-combustion carbon capture (PCC) due to its superior CO₂ separation performance. Treating real flue gas, or residual flue gas, may be problematic as it contains not simply just CO₂ and N₂. The minor contaminants, such as SO₂ and O₂, may be the major factors that limit the long-term membrane stability. With the presence of SO₂ in the feed gas, SO₂ could preferably react with amine carriers and hinder the permeation of CO₂ in amine-containing facilitated transport membranes. The typical coal derived flue gas after flue gas desulfurization treatment is at its discharge temperature of 57°C with 40 – 50 ppm SO₂. The SO₂ level can be further reduced to even 1 ppm by a caustic scrubber, i.e., 20 wt.% NaOH aqueous solution. Hence, possible amine degradation and competitive reactions raise the concerns on the membranes for use in PCC. In this paper, an amine-containing facilitated transport membrane, containing polyvinylamine (PVAm) as fixed-site carrier and amino acid salts as mobile carrier, was tested at 57°C. SO₂ concentrations of < 10 ppm were employed in our transport study. Interestingly, with a feed of 20% CO₂ and 80% N₂ on dry basis, this membrane showed stable separation performance in the presence of 1 – 3 ppm SO₂ at 57°C. The infrared (IR) spectra of the membrane components exposed to SO₂ with respect to exposure time were collected. When exposed to SO₂ at 57°C, no significant change was found in IR spectra for PVAm fixed-site carrier, while only a weak physisorption and a very small amount of sulfite species were observed in the mobile carrier. The spectra results were consistent with the observed membrane separation performance, which was an affirmation of the weak SO₂ interference of the amines in the membrane. Moreover, an O₂ concentration of 3% was also added in the feed gas to simulate the real flue gas condition. The membrane also showed a stable separation performance upon exposure to 3 ppm SO₂ and 3% O₂ for > 150 hours.

In addition, the membrane was employed for CO₂ capture from the residual flue gas after the primary CO₂ capture system (with 90% CO₂ removal), which contained a low CO₂ concentration of ~1%. It was found that the effect of SO₂ was less significant when the CO₂ concentration was low. The CO₂ permeance dropped by approximately 2 – 3% with an introduction of 3 ppm SO₂ in the feed gas of 20% CO₂ and 80% N₂ on dry basis at 57°C. However, no performance reduction was observed after exposing the membrane to 3 ppm SO₂ with the feed gas of 1% CO₂ and 99% N₂ on dry basis at the same temperature. The stable membrane performance at 57°C with SO₂ at ppm levels and O₂ at 3% indicates the applicability of the developed amine-containing membrane for use in PCC.