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INTRODUCTION ABSTRACT: An Atmospheric Residue De-sulphurizer Unit was revamped in year 2004 with a new online catalyst replacement technology process for increasing the operating run length, as well as capacity, of the unit. This paper describes three corrosion case studies which were a direct result of the revamp of the unit and inadequate understanding of the corrosion mechanisms / degradation expected due to process / capacity changes. The first case history is an exchanger tube sheet and components which experienced corrosion due to high temperature sulfidation1. The second case history addresses erosion of an inlet nozzle elbow and bulging / thinning of the impingement plate in the high pressure cold separator vessel (HPCS Vessel). The third case history documents high temperature sulphidation1 corrosion on diesel stripper inlet piping and a connected tower liquid inlet nozzle which was caused by an increase in operating fluid temperatures resulting in high corrosion rates. Based on these case histories, a thorough re-evaluation of the unit has been under taken using RBI methodology and is expected to result in revised inspection plans and intervals.
Atmospheric Residue Desulfurization (ARDS) is a well established hydro treating process, operated primarily to desulfurize atmospheric residues from Crude units and to prepare feed stocks for downstream conversion units like Hydrocrackers and Delayed Coker units. The product, desulfurized residue, is not only low in sulfur but has improved pour points and lower viscosities as well. In addition to existing ARDS technology, the unit was revamped with OCR® (On-Stream Catalyst Replacement) Technology in September 2004 to achieve a more efficient desulfurization. After the revamp of ARDS Unit with On-Stream Catalyst Replacement facility, operating capacity of the unit has now increased to 84,000 BPOD (˜ 27% increase) against original capacity of 66,000 BPOD. Revamped ARDS Unit consists of five principal systems:
? Reduced Crude/Atmospheric Residum (AR) Feed.
? Make Up Hydrogen Compression
? Reaction
? On-Stream Catalyst Handling
? Distillation
Brief History of Exchanger 1989: Case History 1 Corrosion of Exchanger Tube Sheet & Components due to High Temperature H2S/H2 Corrosion : Fractionator Bottoms / Feed Heat Exchanger was replaced with a new, resized heat exchanger as part of ARDS-OCR revamp and commissioned on 30th September 2004. In the original exchanger, prior to the revamp, no significant corrosion of tube bundle or other components had occurred. No tube bundle replacement was needed within the 14 years of operation. However, after revamp in 2004, severe corrosion and damage was noticed, mainly on the shell side (OD) of tube bundle which prompted major repairs, component replacement and bundle re-tubing.
Installed and commissioned as part of original ARDS Unit.
2006: 2004: Replaced with a larger capacity exchanger (no change in materials)
After opening the exchanger covers, heavy deposits and scales were noticed on the shell side tube OD surface and tube bundle was found stuck inside the shell. When finally removed, it was found completely covered with thick deposits. Perforations and severe corrosion of baffles & bypass strips was observed during initial inspection.
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