must be measured regularly. These points might cast some minds back to the horrific ASU explosion that took place at the GTL plant in Bintulu, Malaysia in 1997. At that time, there were forest fires on the Island of Sarawak, where Bintulu is located. The smoke contained soot particles which accumulated in the cryogenic liquid oxygen reboiler on one of the ASUs. By coincidence, in the same year in China, an ASU at the Fushun ethylene complex, which produced oxygen to convert the ethylene to ethylene-oxide suffered a similar explosion. In this case, the root cause was abnormally high ethylene levels in the ambient air due to venting on the ethylene plant during a shutdown. In both Bintulu and Fushun, the combination of liquid oxygen, combustible hydrocarbon material, and aluminium, which is used to construct the reboiler, led to a massive explosion and fire. Process safety lessons have been captured in the EIGA doc 65/13: ‘Safe operation of reboilers/condensers in air separation units’. The principal precautions against hydrocarbon contaminant build-up in the cryogenic liquid in the ASU are well established and focus on CO 2 analysis at the warm end of the ASU and hydrocarbons analysis in the cryogenic liquid oxygen sump. Analysis of CO 2 break-through from the pre-purifier unit (PPU) is used to warn about the possibility that hydrocarbons such as methane or ethane are not being removed by the PPU and are also entering the ASU. Analysis of hydrocarbons in the cryogenic liquid, generally by extraction of liquid oxygen from the main reboiler sump, is the second line of defence. This can be achieved using a total hydrocarbon analyser. Additionally, for ASUs that use reversing heat exchangers at their warm-end, routine analysis of acetylene in the liquid oxygen from the main reboiler sump is also required. Reflecting on these issues is a stark reminder that ASU design and operation relies on engineering expertise to ensure that the Giga- scale production of oxygen to make low carbon hydrogen is done safely.
deliver CO 2 captured from a range of industries based in Victoria’s Latrobe Valley, such as the HESC project and existing fertiliser plants. The main CO 2 transmission pipeline will be more than 100 km long with a 10 km offshore leg extending into the Bass Strait. CarbonNet has the potential to capture 5 million tonnes of CO 2 per year, giving it a similar scale to the existing Gorgon CCS project off the coast of Western Australia. The CarbonNet CO 2 collection network is a complex ‘hub and cluster’ scheme where CO 2 will be captured from several plants and fed into a feeder network connected to a long- distance transmission pipeline. This concept mirrors existing natural gas pipeline grids. Similar CCS networks are proposed for the Zero Carbon Humber project in the UK and the PORTHOS scheme in Rotterdam, NL. The goal is to reduce the unit costs of CCS to enable affordable decarbonisation in support of the energy transition. Hazards of oxygen production require careful mitigation The hazards of hydrogen are well known. Furthermore, it is worth reflecting on the challenge of ensuring safe ASU operations to produce oxygen: there are inherent hazards, and the challenge is to minimise the risk. The devastated ASU site at the Henan Gas Group Yima coal gasification plant in Sanmenxia in China reminds us that ASU operations have the potential to lead to tragedy. Fifteen people lost their lives and 16 others were seriously injured in the explosion in July 2019. The investigation at Yima concluded that “the direct cause of the accident was that the leakage into the cold box of the air separation device was not handled in time and ’sand explosion’ occurred”. The cold box then collapsed onto a 500 cubic metre liquid oxygen storage tank. Further explosions and fires followed. The sequence of events after the cryogenic liquid leak into the cold box is understood but the events which caused that leakage are still not determined. However, the accident investigation report makes some pertinent recommendations such as controlling organic matter entering the main air compressor. It also recommends that the hydrocarbon content of the liquid oxygen system
Stephen B Harrison sbh@sbh4.de
www.decarbonisationtechnology.com
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