Upgrade of Low-Concentration Oxygen-Bearing Coal Bed Methane by a Vacuum Pressure Swing Adsorption Process: Performance Study and Safety Analysis

The recovery of methane from oxygen-bearing coal bed methane (CBM) (25% CH₄/65% N₂/10% O₂) by vacuum pressure swing adsorption (VPSA) was studied systematically in this work. Nevertheless, it is difficult to implement systematic safety investigations of the process just by bench-scale experiments and pilot plants. Emphasis was placed on the construction of a new way to analyze process safety based on a developed explosive triangle theory that could be able to provide guidance for comprehensive safety evaluation in the VPSA process for upgrading of oxygen-bearing CBM. The simulation results revealed that a two-column seven-step VPSA process not only could generate 50.40% CH₄ purity with 86.3% product recovery but also could achieve higher product productivity of 1.494 (mol of CH₄)·kg_ads^–1·h^–1 and lower energy consumption of 0.179 kWh·Nm^–3 feed as well. The results of the safety analysis procedure according to the simulation results demonstrated that the flammability zone distribution mainly exists in the adsorption, pressure equalization, replacement, and pressurization steps and varied regularly with each step time and axial distance. A two-stage VPSA process with nitrogen dilution of the feed gas that is able to achieve safe operation in a further separation system was well-verified through simulative investigation