Failure analysis on abnormal damage and corrosion of methanol distillation column in MTBE production plant

Methyl tert-butyl ether (MTBE) is a commonly used gasoline additive that improves combustion and reduces pollutant emissions. The methanol distillation column is an integral part of the recovery system in MTBE production plants, used to separate and recycle unreacted methanol from water through distillation. During regular maintenance, severe damage to the support ring and abnormal perforations in the column were discovered. Despite repair welding, the column still leaked. Various investigations were conducted to determine the root cause of the failure. The chemical composition, metallurgical structure, and hardness of the support ring and column materials were examined, and the medium in the column was characterized. The medium at the bottom of the column was unexpectedly subjected to an undesirable excessive acidity (with a pH of approximately 3.8) dominated by the upstream etherification reaction deacidification, and provided common basic conditions for localized pitting and uniform corrosion of low carbon steel. What's more, the appearance of clean and smooth corrosion pits and scour traces on the perforated column wall, as well as honeycomb pits and arc-shaped layered traces on the support ring, revealed that the corrosion failure was intensified due to the interaction of selective erosive corrosion and medium concentration accelerated corrosion caused by multiple corrosion types of the acidic medium. In addition, the unpolished surface from incomplete repair welding was another significant factor. Based on these findings, specific countermeasures were implemented and proved effective.