Gas permeation properties of NCMs

Using an indigenously built gas permeation unit and adopting a constant volume and variable pressure method, the NCMs’ single gas permeability was assessed. The membranes (9.4 cm²) were fixed on top of the gas permeation cell when the permeation pressure reached a steady state on the permeate side. Figure 1 shows the image. A gas permeameter was used to measure the permeabilities of N₂ and CO₂. Stainless steel permeation cells, which make up the permeameter (permeate side), separate upstream (feed side) and downstream (permeate side). A transducer measures the change in permeate pressure over time on the downstream side. A 13.302 cm² area of the membrane is exposed to the gas in this cell. Pure gas permeation was measured using a constant volume variable pressure. An increase in pressure was plotted over time based on the raw data. The following equation can be used to calculate gas permeability: $$P=\frac{Vl}{A}\frac{T_0}{P_f{P}_{0}T}\left[{\left(\frac{dp}{dt}\right)}_{ss}-{\left(\frac{dp}{dt}\right)}_{leak}\right]$$
where:-

P is the permeability of the gas through the membrane (barrer);

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(1 Barrer = 10⁻¹⁰ cm³ (STP) cm cm⁻² s⁻¹ cmHg⁻¹);

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V is the permeate volume (cm³);

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l is the thickness of the membrane layer (cm);

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A is the effective area of the membrane (cm²);

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P_f is the feed pressure (cmHg);

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P₀ is the pressure at the standard state (76 cmHg);

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T is the absolute operating temperature (K);

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T₀ is the temperature at the standard state (273.15 K);

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(dp/dt)ss is the steady-state pressure increase in the permeate side (cmHg s^-1) under the feed pressure;

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(dp/dt)leak is the pressure increase in the permeate side under vacuum (leakage pressure increase).

Gas pairs A and B were calculated to determine their ideal selectivity, α_A/B. The ratio of their permeability is defined as follows: $${\mathsf{\alpha }}_{A/B}=\frac{P_A}{P_B}.$$