Simulation study of a hybrid cryogenic and membrane separation system for SF₆ recovery from aged gas mixture in electrical power apparatus

Despite its wide use in the electrical industry, sulphur hexafluoride (SF₆) is a potent greenhouse gas with high global warming potential (GWP) and atmospheric lifetime. Therefore, there is a growing interest in recycling, reconditioning, and reusing aged SF₆. In addition, the upcoming F-gas regulation with increasing restrictions imposed on SF₆ could result in banning the production of virgin SF₆, consequently putting pressure on end-users to recycle aged SF₆ for further reuse. Currently, the cryogenic separation process is often utilised to recondition aged SF₆ in electrical power apparatus. However, this method is not suitable to treat mixtures with SF₆ content lower than 40 mol.% and produces a waste gas containing up to 15 mol.% of SF₆. This study aims to tackle the aforementioned limitations of the existing SF₆ reconditioning process by implementing membrane units. For this purpose, a mathematical model of the membrane gas separation process is developed and integrated into an Aspen Plus simulation via CAPE-OPEN interface. A set of cost model equations collected from the open literature is also used to study the cost of the SF₆ recovery process. The simulation for a feed mixture of 20 mol.% SF₆ and 80 mol.% N₂ with a flow rate of 10,000 kg/day demonstrated the system's capability to recover SF₆ efficiently, achieving the product purity of 98.3 mol.% and SF₆ recovery of 97.5 %. Further optimisation reduced the specific recovery cost and specific energy consumption by 3.1 % and 10.5 %, respectively. The SF₆ content in the waste gases was also reduced to less than 1 mol.%.