We were using spent caustic from merichem unit in net gas scrubber instead of fresh caustic to treat off gas in our isomerization unit. It has caused heavy corrosion in monel distributor inside the scrubber. What may be possible reasons for corrosion? Has anybody else faced the same issue?
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17/03/2016
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Peter Marsh, XBP Refining Consultants Ltd, peter.marsh@xbprefining.co.uk
What is the nature of the corrosion damage to the net gas distributor? Are you certain the damage is caused by changes to the quality of the makeup caustic solution? Corrosion/erosion damage to the distributor can occur even when using fresh caustic and maintaining the caustic strength within the desired range. Damage can include “keyholing” (enlarging) of the distributor holes, thinning of the header pipe and laterals and corrosion/erosion of plate flanges and bolts on the laterals. A common cause is ingress of caustic solution into the distributor either due to low gas exit velocities at turndown or inadequate sealing of the ungasketted plate flanges on the laterals.
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17/03/2016
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Prakash Pimparkar, Environmental Consulting Services , prakash.pimparkar@yahoo.in
It is not clear the waste caustic is from pre wash or oxidation, pre wash section caustic should be OK oxidation waste may have proprietory oxidises enhancing corrosion.Filtration and GAC treatment should work in both cases.
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17/03/2016
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NS Murthy, Suez, murthy.ns.ext@suez.com
It is pretty interesting problem that you have stated. Process and Operation team generally look at left over caustic strength in waste stream (here in this case from Merichem unit) for recycling / reuse rather than looking at other contaminants that could be carried along. We undertake routinely such process reliability concerns for refiners and guide them with appropriate strategies to optimize caustic use. Did you check SEM observation on the failed Monel distributor to get tell-take indications on cause for such corrosion?. Also, indicate the pr and temp profiles in the Isom unit net gas scrubber.
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