(672e) Study of Carrier Saturation Phenomenon in Facilitated Transport Membrane for CO2 Capture from Low CO2 Concentration Sources

Amine-containing facilitated transport membranes exhibit superior CO₂ separation performance because the CO₂ transport is enhanced by the reversible reaction between CO₂ and the active carrier, amine, incorporated in the membrane. Continuously increasing the CO₂ partial pressure leads to the constant CO₂ flux, which does not increase with CO₂ partial pressure, i.e., the carrier saturation phenomenon. At the carrier saturation, all available carrier molecules have been reacted with the high pressure CO₂. However, this carrier saturation phenomenon can be beneficial for CO₂ capture from low CO₂ concentration (~1% CO₂) sources, e.g., the residual flue gas after the primary CO₂ capture system (with 90% CO₂ removal) and coal mine gas streams. For CO₂ separation from low concentration sources, an improved membrane separation performance can be obtained. With reducing the CO₂ concentration in the feed gas, both CO₂ permeance and CO₂/N₂ selectivity increased. These were mainly due to more available amine carriers for CO₂ molecular transport at lower CO₂ concentration conditions. In this study of CO₂ separation from low concentration feed, the membrane used consisted of polyvinylamine serving as the fixed-site carrier and piperazine glycinate acting as the mobile carrier. A CO₂ permeance of 785 GPU (1 GPU = 10^-6 cm³ (STP)·cm^-2·s^-1·cmHg^-1) and a CO₂/N₂ selectivity of 150 were obtained with a feed of 20% CO₂ and 80% N₂ on a dry basis (16.6% CO₂ and 66.2% N₂ along with the saturation water vapor at 17.2%) at the typical flue gas temperature of 57°C. By reducing the CO₂ concentration in the feed, a significant CO₂ permeance increase was observed when the CO₂ concentration was below 5%. For instance, the membrane showed a CO₂ permeance of 950 GPU and a CO₂/N₂ selectivity of 190 with a feed of 1% CO₂ and 99% N₂ on a dry basis. The observed performance increase was due to the carrier saturation phenomenon as more carrier molecules were available for CO₂ transport at a lower CO₂ concentration. Additionally, it was found that the effect of SO₂ was less significant when the CO₂ concentration was low. The CO₂ permeance dropped by approximately 2.5% with an introduction of 3 ppm SO₂ in the feed gas of 20% CO₂ and 80% N₂ on a 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 a dry basis at the same temperature. Therefore, the studied membrane can be applied for CO₂ separation from low concentration sources with an exceptional separation performance.