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I have a question regarding the corrosion issue in fractionator on CDU. However, during the turnaround we have noticed very severe corrosion on the first three trays of column. The third tray has mostly corroded. These trays are made out of Monel. The top pumparound goes from the third to the first tray. The corrosion coupon that are placed on the suction of pump-around pump have shown different corrosion rates during last year - from low to severe corrosion rate. The top temperature is lowered to about 120 oC to maximize middle distillate yield. Did anybody face with similar problem?
I know that ammonia, amines, nitric acid are corrosive to monel. I am suspicious about the presence of tramp (or even neutralizing) amines on the first three trays. However, in open literature on internet I have found also different opinions the resistance of monel on hydrochloric acid.
Did anybody face similar problem? Could it be the issue due to change in the crude blend? Any help would be helpful. Thank you in advance.
 
Answers
05/12/2018 A: Muhammad Faisal, Oman Oil Refineries & Petroleum Industry (Orpic)-Oman, engr.faisalfraz@gmail.com
few things to be ensure:
(1) Monel is prone to amine-salts/TN2 in desalted crude. ensure desalter performance is satisfactory. use proprietary acid treatment (desalting water PH = 7 max) to ensure knocking down all amines, TN2 going to Brine (don't allow to carry with desalted crude).
(2)Ensure TPA return temp > dew point/desublimation temp.
(3). ensure OVHD drum SW qaulity is well control. minimize use of cold reflux. also ensure reflux moisture contents < 200ppm or even less. (this will cause corrosion).
(4). always use "stripped water for desalting".
(5). TPA flow to be kept maximum and column top temp also higher than dew point/desublimation.
20/04/2017 A: NS Murthy, Suez, murthy.ns.ext@suez.com
Couple of points one has to look into so as to derive a holistic solution to this issue.
1. Dew point and salt point margins between column top temp and prevailing other ambience in the tower.
2. Possibility of different ionic species which depends on the wash water being used as make up to u/s desalters.
3. Cavitation corrosion possibility
4. Water condensation / ingress from stand by top pump around loop and cold reflux being used.
5. Tracking of boot water for tell tale indications during operation, etc.
Suggest you contact nearest GE Water and Process Technology team members to take 'right' approach and mitigate this concern.
24/09/2016 A: Leonardo Leite Garcia de Souza, Petrobras SA / Duque de Caxias Refinery, llgs394041@yahoo.com.br
We have experienced the same problem in our lubes distillation units and, lately, I believe the same thing is happening in our fuels distillation unit too. I don't believe the problem is crude related, since we use very different crudes for making lubes (middle east) and for fuels (brazilian). The section of the towers is also the same, above the naphta pump-around return tray, next to the top. We have found that the corrosion mechanism that literally "eats" our monel (too) trays is corrosion under deposit. It seems that the neutralizing and filmic amine used (the middle east crudes used have too much H2S) degrades into amonium salts that deposit on the trays, since they are not hydrocarbon soluble. Brazil starvs for middle distillates and we also operate our atmospheric crude towers top temperature very low, limited to what we believe is the dew point of the salts. There are a couple of things I believe you could do and I will try to implement it for future turn-arounds: if your trays don't have fixed valves yet, consider replacing them with fixed valve trays, which are less prone to corrosion and fouling; consider increasing the thickness of the trays and the gage of the valves (there will be more material to be "eaten" before it fails under corrosion); depending on the design of your feed device for the cold pump-around return, there could be "cold" regions in the inlet tray (consider changing it, if it's the case); consider increasing the pump-around return temperature (you will have to compensate the heat drawn decrease increasing the flowrate, if possible) and finally, consider calling your amine supplier and see if he can deliver a more "engineered" product meeting your needs (we did that too).
23/09/2016 A: Ralph Ragsdale, Ragsdale Refining Courses, ralph.ragsdale@att.net
If you are taking light naphtha overhead and heavy naphtha as the first side draw, water is a liquid in the top section of the column. With most of today’s crudes and their impurities, severe corrosion will occur where the water is a liquid. The preferred design is to yield full range naphtha overhead and split it in the next column, where there is no stripping steam.
22/09/2016 A: Eric Vetters, ProCorr Consulting Services, ewvetters@yahoo.com
Problems in the top trays of crude towers are common and monel does not always hold up well. Amine or ammonia alts are most likely cause of the problem. Source could be the crude, slops or stripped sour water going to the desalter, or neutralizing amines. Operating conditions and design of the top of the tower are also parameters that need to be evaluated. I have experience with these sorts of problems and can help you analyze your system to identify the root cause should you need technical support.