Carbon molecular sieve membranes for CO2/N2 separations: Evaluating subambient temperature performance

Flue gas CO2 capture has received significant attention in recent years to mitigate the environmental impact of greenhouse gases. In spite of excellent performance by polymer membranes, there is still a need for more robust membranes to exceed the polymer upper bound at subambient temperatures, and carbon molecular sieve (CMS) membranes could offer a potential solution. In the present study, as a proof of concept, CMS hollow fiber membranes derived from defect-free 6FDA/BPDA-DAM polymer precursors at a pyrolysis temperature of 550 °C were investigated for CO2/N2 separation. Both ambient (35 °C) and subambient (−20 °C) temperature performance for these CMS membranes have been studied using a combination of pressure decay sorption and permeation techniques. Permeation results using CO2/N2 (20:80) mixed gas CMS showed CO2/N2 selectivity of ~109 and CO2 permeance of ~ 107 GPU at −20 °C. Most interestingly, a decrease of only ~ 33% in CO2 permeance with a large increase in CO2/N2 selectivity (~ 4.5 times) was observed for CMS membranes on decreasing temperature from 35 °C to −20 °C. Gas sorption measurements indicated that the Langmuir hole filling capacity (C'H) for CO2 is very high compared to N2 at both ambient and subambient temperatures. This trend was attributed to higher sorbed density of CO2 at saturation in the micropores due to its transition from gas phase to the liquid phase at subambient temperatures. Significant increases in sorption selectivity as well as diffusion selectivity for CO2/N2 going from ambient to subambient temperatures make CMS membranes a potential substitute for polymer membranes for CO2/N2 separation.