Knudsen Number Sensitivity to Pressure Drop in Composite Ceramic Membranes
DOI:
https://doi.org/10.11113/jamst.v30n1.335Keywords:
Knudsen number; ceramic nanomaterials; mean free path; hydrogen separation; contact angleAbstract
Gas transport in nanoscale ceramic membranes is fundamentally governed by the Knudsen number (Kn), which differentiates between molecular dominated and continuum dominated flow regimes. This study experimentally evaluates the sensitivity of Kn to transmembrane pressure across multilayer alumina membranes comprising a 15 nm selective layer, a 200 nm intermediate layer, and a 6000 nm macroporous support. Hydrogen (H₂), carbon dioxide (CO₂), and air were investigated at 100 °C over a pressure range of 20–300 kPa. The results consistently show an inverse relationship between Kn and pressure for all gases and pore sizes, as predicted by kinetic gas theory due to the pressure dependent decrease in molecular mean free path. Among the gases studied, hydrogen exhibits the strongest pressure sensitivity in the 15 nm layer, followed by CO₂ and air, reflecting differences in molecular size and diffusivity. Linear regression applied to the experimental Kn–ΔP trends yields coefficients of determination of R² ≈ 0.8875 across all gases and pore diameters, confirming high linearity and strong internal consistency in the measurements. Although the R² values remain constant, each gas exhibits a distinct regression slope and intercept, as shown in Figures 4–6, indicating differing Kn pressure response characteristics. Complementary SEM and dynamic wettability measurements further support the mechanistic interpretation. The 15 nm top layer shows the highest hydrophilicity (equilibrium contact angle ≈ 60°), promoting enhanced gas–wall interactions. The 200 nm layer exhibits intermediate wettability (≈ 70°), while the 6000 nm support is weakly hydrophobic (≈ 93°). These structural and surface properties help explain the observed trends: smaller and more hydrophilic pores intensify molecule–wall collisions, amplifying Knudsen-dominated transport. Overall, the findings provide validated experimental benchmarks for modelling rarefied gas transport in composite ceramic membranes, with implications for hydrogen purification, CO₂ separation, and catalytic membrane reactor design.
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