Description and explanation of a FCC overhead system unexpected corrosion

A leak occurred in a FCC unit main fractionnator overhead line elbow in June 2006. Following this leak, investigations showed acidic corrosion-erosion in the overhead circuit. This type of corrosion was unexpected in this circuit. Process water analysis revealed that during normal process operations, acidic corrosion wasn't possible. 2002-2006 unit run analysis highlighted the fact that this unit has been frequently in transient phase called catalyst recirculation. Water sampled during those periods exhibited a very low pH due to the presence of sulfuric acid. During catalyst recirculation operation sulfur trioxide is formed. SO3 goes up in the main fractionnator overhead and condensed as H2SO4 with the water vapor. This media has corroded carbon steel equipments located after the water dew point. A way to mitigate this corrosion is a condensed water pH control during those periods. This paper covers the description and the explanation of this leak Keyword: Refining, Corrosion Failure, FCC unit, Acidic dew point corrosion, Sulfuric acid corrosion