Temperature-Driven Transition from Knudsen Diffusion to Viscous Flow in a Macroporous Ceramic Membrane

Ceramic membranes show potential for high-temperature CO2 extraction from flue gas; nevertheless, their performance under simultaneous heat and pressure stress is not well comprehended. This research addresses the temperature-dependent CO2/N2 separation characteristics of a commercial ceramic membrane (pore size ~0.1–1 µm) utilizing simulated flue gas (11.8% CO2, 74.2% N2, 2.5% O2, remainder CH4) at temperatures ranging from 60 to 140 °C and pressures between 4 and 6 bar. Calibrated GC-TCD was used to quantify permeate compositions across multiple operating valve openings. With a CO2/N2 selectivity (α) of 0.75 at 4 bars, the maximum CO2 enrichment peaked at 80 °C (10.8 mol%), getting close to the Knudsen diffusion limit (0.80). Selectivity decreased dramatically beyond 100 °C—α = 0.61 (100 °C), 0.45 (140 °C)—and CO2 dropped to 5.8% at 4 bar and 2.2% at 6 bars. Viscous flow dominance was shown by the strong pressure amplification—α decreased by more than 60% from 4 to 6 bar at all temperatures. These findings emphasize the possibility of performance collapse in hot, pressured flue streams and identify the limited operating window under which Knudsen-controlled transport can be maintained. The study provides quantitative evidence of a transition in transport regime under mixed flue-gas conditions.