What additional physicochemical properties do N₂-selective membranes exploit to achieve separation of nitrogen from methane, beyond the differences in molecular size, shape (linear N₂ vs. tetrahedral CH₄), and quadrupole moment (4.08 cm² ×10⁴⁰ for N₂ vs. 0 for CH₄)?
Other parameters:
1) Solubility (in the case of dense polymeric membranes with the solution-diffusion mechanism where, however, the solubility is usually in favour of methane). If the membrane does not have significant size-selectivity (e.g. in rubbery membranes) you will accumulate N2 in the retentate.
2) Molar mass (in the case of sufficiently tight porous membranes with the Knudsen diffusion mechanism). Again, the lower molar mass is in favour of methane permeation and you will concentrate N2 in the retentate)
Johannes Carolus (John) Jansen Thank you for the insights! I am specifically working with N2-selective membranes, and my goal is to have methane concentrated in the high-pressure retentate.
Article N2/CH4 Separation via Mixed Matrix Composite Membranes: Crys...
Based on your explanation, it seems that solubility and molar mass could play a role in this process. In my case, I’m more focused on ensuring methane retention rather than N2 accumulation in the retentate.
Dear Siraj Ahmad this separation is so poor (selectivity roughly 0.25-4) that you will need multiple stages and recycle streams to get sufficient enrichment of methane and low methane losses.
You may enhance efficiency if you add at least one step where methane is selectively permeated (from a methane poor stream) and recycled back to the feed.
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