Corrosion assignment Failure in distillation column Production schedule The column normally operates on full production for 6 days with the 7th day used for cleaning and plant maintenance. The lower sections of the column and the reboiler are steam cleaned during the weekly shutdown to remove deposits and residues which appear to build-up on the reboiler tube surfaces and column trays. Recent history The column had been in safe operation for 7 years separating acetone. One year ago it was decided to change the column to make it suitable for separation of a more complex mixture; this would involve removing the base of the column and replacing it with a modified, and taller, bottom section. Six months ago this change was made and operation commenced on the separation of dimethly formamide (DMF) and tri-ethylamine (TEA). Plant flow chart During the last 6 months, a weld failure occurred in the reboiler/column pipework after about 3 months' operation. This pipework was replaced. Also a couple of small leaks developed, and were rewelded, in the new bottom section after a further month's operation. During the initial pilot plant operating period there had been problems with the liquid levels on the bottom trays with violent surges of liquid and vapour resulting in the eventual collapse of the bottom tray which was subsequently removed. Bubble caps on tray In order to improve the process, a new bottom was ordered and is about to be installed during the annual shutdown and some modifications are to be carried out on the column over the next week to allow full production to start. The column internals are to be changed and the bottom section will be replaced; this will allow the unit to operate on full production instead of as a semi-pilot plant. In order to meet the agreed production schedules the column must be capable of continuous (i.e. 6 day) operation during the next year. Analyze Corrosion Failure causes, suggest corrosion assessment and propose corrosion prevention for this failure structure