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 componentj in the feed gas, respectively.