2.6 Gas separation performance test
All tests to assess the gas separation performance of membranes were accomplished in a home-made membrane module in the classic Wicke-Kallenbach test method. As shown in Figure 2, the tubular membrane was sealed with high-temperature resistant O-rings to ensure good gastight. The gas separation performance evaluation were divided into two types: single gas permeation test and mixed gas separation test. The feed gas was introduced into the shell side of the tubular membrane module. The feed gas flow (H2, CO2, N2, CH4, C2H4) for the single gas permeation test was 50 ml/min, while the feed gas flow (H2, CO2) for the mixed gas separation test was 100 ml/min with the volume ratio of 1:1 and the sweep gas was argon (50 ml/min). During the test, the pressure on both sides was kept at 1 atm, and the temperature was adjusted as needed. Most experiments were carried on at room temperature unless specified. The flow rate of all gases was measured with a mass flow controller and calibrated with a soap bubble flowmeter. The gas on the permeate side was swept into the gas chromatograph (GC Agilent-7890B) with a TCD detector for analysis. The experimental data was obtained by taking the average value of at least three data points after the gas separation performance was stable to ensure accuracy. The gas permeance expressed the gas separation performance of the single or mixture gas\(P_{i}\)\(mol\cdot m^{-2}\cdot s^{-1}\cdot\mathrm{p}\mathrm{a}^{\mathrm{-1}}\)), the ideal selectivity of the single gas \(S_{i/j}\) and the gas mixture separation factor \(\alpha_{i/j}\), respectively, as defined by the following formula,
\begin{equation} P_{i}=\frac{N_{i}}{\left(\Delta P_{i}\cdot S\right)}\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ (1)\nonumber \\ \end{equation}
where \(N_{i}\) was the permeation rate \(\text{mol}\cdot s^{-1}\)of the gas component \(i\), \(\Delta P_{i}\) was the transmembrane pressure difference of the gas component \(i\), and \(S\) was the effective utilization area of the membrane. Considering gas permeance was usually reported in a more widely used unit of GPUs, thus it can be converted from the standard unit through the following equation.54
\(\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ 1\ GPU=3.35\times 10^{-10}\text{mol}\cdot m^{-2}\cdot s^{-1}\cdot pa^{-1}\text{\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ }\)(2)
The ideal selectivity of a single gas \(S_{i/j}\) referred to the ratio of the gas permeance of different gas components,
\begin{equation} S_{\mathrm{i/j}}=\frac{P_{i}}{P_{j}}\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ (3)\nonumber \\ \end{equation}
The separation factor \(\alpha_{i/j}\) of the mixture can be calculated by the following formula,
\begin{equation} \alpha_{i/j}=\frac{\mathrm{\ }{x_{i}}_{[\text{perm}\mathrm{]}}/{x_{j}}_{[\text{perm}]}}{y_{i[\text{feed}]}/y_{j[\text{feed}]}}\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ (4)\nonumber \\ \end{equation}
where \(x_{i[perm]}\) and \(x_{j[perm]}\)refer to the molar fraction of gas component \(i\) and gas component\(j\) on the permeation side, while \(y_{i[feed]}\) and\(y_{j[feed]}\) refer to the molar fraction of gas component \(i\) and gas componentin the feed gas, respectively.