Separation membranes selectively separate gases, and the separation coefficient of gases is a key performance index in measuring separation effect. In this paper, by testing the permeability of nitrogen and carbon dioxide to a certain membrane sample, the separation ability of the sample to nitrogen and carbon dioxide is evaluated. The test principle, the parameters, application of the pressure differential method testing equipment, and the test process are introduced. Reference for the analysis and test of the separation effect of the membrane is provided.


Separation membrane is characterized by a selective permeability which is based on the permeability of different gases in the membrane to achieve gas separation. Compared with other gas separation technologies, the advantages of gas separation membranes include operation in ambient temperature, low energy consumption, environment friendly, no phase change, and small size. Therefore, gas separation membranes are used in nitrogen preparation, oxygen enrichment, hydrogen purification and recovery, and removal of acidic gases. It has also been widely used in recovery of organic steam and other fields. The factors affecting the performance of gas separation membranes include its permeability and selectivity. The permeability affects the separation efficiency of gas separation membranes. The selectivity affects the separation effectiveness of gas separation membranes. Effective gas separation membranes should have high permeability and selectivity. Separation coefficient is a key performance index for evaluating the selectivity of separation membranes. In this paper, the separation coefficient of a separation membrane sample was measured.

Test Sample:

In this experiment, a gas separation membrane was used to test the separation effect of the sample on nitrogen (N2) and carbon dioxide (CO2).

The separation coefficient can be expressed by the ratio of permeability coefficient of separated gas. The gas permeability coefficients of N2 and CO2 can be measured by pressure differential method. The test process is based on ASTM D1434 (Standard Test Method for Determining Gas Permeability Characteristics of Plastic Film and Sheeting).

The sample was tested on the VAC-VBS differential pressure gas permeability tester developed and manufactured by Labthink Instruments.

The VAC-VBS is equipped with pressure a sensor. Pressure differential enables gas permeation through the sample. Pressure sensor can monitor the change of gas pressure. The specific test process is as follows: the clamped sample divides the test chambers into and upper and lower chamber. The upper chamber is filled with a test gas under a certain pressure and the lower chamber is vacuum pumped to form a low-pressure environment. The test gas permeates through the upper chamber to the lower chamber. The pressure in the lower chamber varies with the increase of the amount of gas permeated. The gas permeability of the sample can be obtained by real-time monitoring of the pressure change in the low-pressure chamber with a pressure sensor. The gas permeability coefficient is the product of the gas permeability and the thickness of the sample.

Test Process:

Three specimens with a diameter of 97 mm are cut from the surface of the sample and the thickness of the specimens is measured.

Apply a thin layer of vacuum grease evenly around the test area of the three test chambers and place a supporting filter paper on each side. Paste the samples into the test chamber, press the samples gently, and remove the bubbles in the contact area between the test chamber and the test chamber. Close the top cover of the test chamber.

The equipment is connected to a nitrogen gas source. The parameters such as sample name, thickness, test temperature, humidity and test mode are set on the control software of the equipment. Click on the test options and the test begins. The equipment is tested according to the set parameters, and the test results are displayed after the test.

After the nitrogen permeability test is completed, the equipment is reconnected to the carbon dioxide gas source to test the carbon dioxide gas permeability coefficient of the sample.

Test Result:

The permeability coefficient of nitrogen and carbon dioxide is 6.3289 × 10-11 cm3/cm/cm-2/s-1?/mHg-1 and 1.6455×10-9cm3/cm/cm-2/s-1/cmHg-1 respectively. The separation coefficient of CO2/N2 is 26.0.


Gas separation coefficient is key performance index for evaluating the separation effect of separation membrane. The permeability coefficients of nitrogen and carbon dioxide were tested respectively. The CO2/N2 separation coefficient of the tested separation membrane samples was obtained. The higher the separation coefficient, the better the effect of the separation membrane on gas. To view all of Labthink’s gas permeability equipment, visit