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Recently we have a corrosion in the DCU main tower side near the heavy naphtha section. We are thinking that is because chloride that produced acid. This chloride are from the crude desalter mainly. Is it common this kind of corrosion in the main tower side near heavy naphtha section ? What are the process conditions to keep under control this corrosion ? The upper section has lining and did not have problem. Is it possible this kind of corrosion be ammonium chloride salts ?
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Even though I have had my experience with the FCC Main Tower, we were careful not to allow the temperature in our naphtha section carefully as salts could form, lead to corrosion, and possibly plug up the packing.
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When there are measurable chlorides in the Coker fractionator overhead accumulator water, there is a risk of ammonium chloride salts. The Coker generates ammonia as one of the decomposition products from the nitrogen in the feed. Keeping the main fractionator overhead temperature over 250 F (121 C) is a way to prevent the formation of salts in the fractionator.
Ammonium chloride salts can be very corrosive. There have been many examples of poor desalter performance resulting in high chlorides in the Coker overhead system that has deposited salts within the fractionator that has resulted in severe corrosion. Lowering the chlorides and increasing the overhead temperature are the two actions that can prevent the salt deposition. Operating with a tower top temperature at 25 F (15 C) higher than the calculated salt point temperature should prevent the deposition of salts within the fractionator. If this provides a Coker naphtha with a distillation that is too heavy, then focus on removing chlorides from the feed to the unit.
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