Q & A > Question Details
What are the best preventative measures for avoiding coking in wash bed section of vacuum tower and corrosion of top section of CDU?
06/08/2018 A: Amish Ajmeri, Bahrain Petrolium company, amish_ajmeri@yahoo.com
To avoiding coking in wash bed section of vacuum tower 1) maintain optimum heater outlet temperature if heater temperature high more then requirement hydrocarbon crack and become coke formation which is chock the bed and due to high temperature crack gas produced so vacuum column load will increased and vacuum will be less drop. 2) over flash flow maintain optimum approx 3-4% so total bed become wet.
To avoid corrosion of top section of CDU 1) crude column top temperature maintain above HCL dew point minimum +10 deg C 2) crude column over head water ph maintain between 6.2 to 6.8 . 3) used chemical on over head section for making film between over head section inner surface metal and process fluid
4) crude oil salt contain maintain below 6 PTB. 5) used NaOH before hot pre heat train for suppress salt formation 6) maintain desalter efficiency that means after desalter salt contain less then 6 PTB and BS&W less then 0.5% .7) if used high TAN crude always used HAC corrosion inhibitor.
09/02/2009 A: Eric Vetters, ProCorr Consulting Services, ewvetters@yahoo.com
Key is getting adequate true overflash. You need to make sure that you are not hitting a dry point in the middle of the wash bed where coke will start to form. True overflash can be calculated by various methods or you can estimate it from process data. You want the dirty wash oil draw temperature to be at least 15 -20F cooler than the flash zone temperature. You can also do asphaltene and/or metals balances on the dirty wash oil vs. vacuum residue to estimate how much of your wash oil is actually making it all the way through the wash bed.
To prevent corrosion in the top of the tower, you need to maintain overhead temperatures high enough to avoid water and/or salt condensation in the top of the tower. Current diesel economy is pushing a lot of refineries to lower overhead temperatures, thus creating a corrosive environment in the top of the crude tower. You also need to maintain top pumparound return and reflux temperatures as hot as possible to avoid localized cold spots in the top of the tower that could allow shock condensation. You also need to avoid putting wet reflux back to the column. If you can't consistently do all of these things, you may need to put a higher metallurgy cladding or weld overlay in the top of the column.
07/02/2009 A: keith bowers, B and B Consulting, kebowers47@gmail.com
Wash bed coking is caused by two different mechanisms. Too high a heater outlet temperature will cause cracking and severe coking in the bottoms and wash oil section. Another cause is too little 'overflash' in the pursuit of energy savings. To 'work right', the wash zone MUST have net downward flow of liquid. The wash liquid has to be well distributed so all surfaces are well wetted and flushed. A 'dry' spot WILL coke up and grow.
CDU top section and overhead corrosion may have several causes. Different solutions are required. Inadequate desalting will result in chloride salts which decompose in the heater and cause hydrochloric acid attack in the water wet top sections. Excessive cracking will increase H2S, which will be a very aggressive corrosion agent where any water is present. You need to consult with the anti-foulants vendor for the plant and conduct analytical testing to assess the cause, then trials of candidate corrosion inhibitors.