Developing Suitable Gas Separation Membrane for Breathing Apparatus

Advanced coal mine practices have greatly improved working conditions and reduced the risk for hazardous gas exposure in the underground environment. In increasingly rare but still dangerous conditions where gas exposure is unavoidable, self-contained self-rescuer (SCSR) respirators continue to be essential supporting devices. The traditional approach used to filter air in such respirators carries two major disadvantages including:

• The energy (i.e. heat) released having the potential to damage the sensitive tissues in the respiratory system; and
• The short usable time and shelf life for the catalyst.

The main objective of this project was to develop a gas separation membrane that can help to increase the oxygen concentration and reduce other unwanted gases under certain challenging conditions. The team has achieved this by taking a systematic approach that includes four tasks:

• Create the baseline of gas mixture via on-site monitoring and literature review;

• Explore various techniques to fabricate the gas separation membrane with high permeability and selectivity;

• Design a proper lab set-up for testing the membrane performance and durability in the well-controlled lab environment; and

• Create a respirator prototype by using the Drager BG4+ as the platform.

The literature review and on-site data analysis indicated that CO₂/O₂ is the most important gas pair in the closed-loop SCSR application, while the CH₄/O₂ pair is critical for the open-loop scenario. The challenging gas compositions have been investigated and confirmed to suit both the closed-loop and open-loop conditions. Among the various types of membrane fabrications in the preliminary trials, the testing results indicated that polyimide (PI) and polymers of intrinsic microporosity (PIMs) are the two most promising materials for CO₂/O₂ separation under typical ambient temperature and pressure. The selectivity of O₂/CH₄ and CO₂/O₂ gas pairs could reach high levels of ~10 and ~5 at 25 C and 1 bar, which confirmed the suitability of using these membranes for SCSR applications.

After evaluating several SCSRs on the market, the team has decided to use Drager BG4+ as the prototype platform, mainly due to it having both a clear layout and a large space to work with. The first prototype of membrane module has been designed and fabricated to replace the CO₂ absorber in Drager BG4+. The positive results showed that the PI/PIMs hybrid membrane can effectively separate the targeted gas pairs with satisfactory selectivity. However, the team also discovered a major challenge during the prolonged operation. The water vapour in human breath can block the membrane pores and further deactivate the gas separation capability after extended operation (>30 mins).