The Scottish Environment Protection Agency (SEPA) says it's disappointed flaring happened at the Fife Ethylene Plant yesterday, less than a fortnight after normal production resumed there.
The regulator was informed just before 3pm of the unplanned flaring at the plant.
ExxonMobil said yesterday the elevated flare was needed to manage a 'process upset'.
But in a further update this morning, the firm states the compressor was safely re-started last night.
Work to bring the distillation towers back to normal production is being carried out.
Measures have also been taken to cut the size of the flare.
Another update is expected later today.
SEPA's Head of Environmental Performance Chris Dailly said: "Having been clear that flaring must become the exception rather than the routine, we're disappointed that flaring by ExxonMobil Chemical Limited has occurred again so soon after the restart.
"Officers made immediate contact with the company and we understand that the flaring relates to a trip of a major compressor."
Mr Dailly says the flaring is a reminder of the importance of the steps needed to reduce the severity of it.
He added: "Whilst recognising that flaring is an important safety mechanism, this is a further reminder of why the short and medium term steps being taken by SEPA to mitigate flaring is so critical, including noise-reducing flare tips in 2020/21 and bringing forward plans for fully enclosed ground flares."
Councillor Linda Holt from Mossmorran Action Group said: "Process upset and compressor trip explain nothing.
"Once again ExxonMobil is forced to resort to emergency flaring because something has gone badly wrong."
Fife Ethylene Plant Manager Jacob McAlister said: "Our team safely re-started the compressor early yesterday evening, and are now working to bring our distillation towers back to normal production.
"We have also taken steps to reduce the required elevated flare size."
This afternoon, ExxonMobil has confirmed operations are back to normal, with the elevated flare no longer in use.
Image from Richard Webb released under Attribution-ShareAlike 2.0 Generic
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