ABB Australia Pty Ltd sulfur
hexafluoride recycling plant wins Banksia Foundation Award
The plant, recently commissioned by ABB (Aust.) Pty Ltd takes impure sulfur hexafluoride (SF6) from high-voltage switch gear on power stations and purifies it to as-new quality. The gas introduced by Solvay over 50 years ago, acts as a plasma quencher by virtue of its self repairing qualities. Nevertheless in prolonged use it breaks down eventually forming toxic and/or undesirable products such as HF and metal fluorides as well as oxidised derivative compounds. Contaminants include the non-condensibles air, nitrogen, as well as oil and water. Because new SF6 is expensive and one of the most potent greenhouse gases (ca 23,000 times CO2) it makes good sense to recycle it.
The technology on the new plant is based on cryogenic separation for which ABB Aust. hold a patent. The original manual plant used liquid nitrogen in open vessels to freeze out the SF6 such that non-condensibles could be released. It must have been spectacular to watch clouds of nitrogen vapour emerging as in a classic science fiction film while control valves let out a piercing scream as they operated under choked flow at critical velocity. The new plant does away with the hazards and drama operating quietly and methodically to a programmed logic controller. Liquid nitrogen is still used for cryogenic separation but instead of open-topped tanks, the refrigerant is contained within a heat exchanger and its flow is regulated for maximum economy. Other contaminants are removed by solid-state filters. A vacuum pump scavenges the last trace of SF6 from the feed cylinder. New cylinders are filled from the large liquid storage vessel (seen in the photos). The entire plant can purge itself when it needs to be relocated.
After some years of contemplating a fully automated replacement for the manual system, ABB Aust. contacted SN2 in June 2007. A feasibility study, reports on design alternatives and a series of conceptual designs followed. With the project pace speeding up and increasing confidence developing, ABB employed Brett Alexander to manage the project. Now Dr Alexander, Brett was at that time nearing the end of his PhD in chemical engineering.
SN2 did the conceptual design and much of the detailed design for ABB following design constraints provided by ABB from 2008 to 2010, which involved, amongst other things, designing the process flow diagram, calculations of equations of state and selection of equipment (compressor, valves, instruments, materials of construction etc.), HAZOP and assisting the PLC programming as well a solving the unique problems that arise with any new project.