Q & A > Question Details
Im working in diesel hydro desulphurization unit. We have one stripper-stabilizer section with common O/H system. Direct stripping steam is used here. We used to get product diesel Cu strip result as 1b. Recently we are not able to meet product copper strip of 1 (we are getting it as >3).
The following are attempted to normalize the section:
a. Checked the steam line for condensation. Line temperature is at 300 °C and there is no water
b. Upstream & downstream exchangers were checked for leaks. No leaks
c. Inlet temperature increased gradually from 235 to 254 °C
d. Stripping steam increased gradually from 3 MT to 4.5 MT
e. Withdrawn more distillate from reflux drum.
After doing the above, we could reduce the result from > 3 to 2.
Simulated the column with earlier conditions & with present conditions. I could not find any H2S slip in bottoms for both the conditions. Even i tried to simulate the column by taking out few trays. There is no improvement.
Please provide your valuable suggestions to improve it further. Also please provide reply for the queries given below:
1. Cu strip result definers with color. But how much H2S will be there in product if the result is 1, 2, 3?
2. How to find out whether there is flooding in the column. DP across the column is at 0.25 Kg/cm2. Will it vary severely if column is flooding?
 
Answers
22/02/2013 A: Satish Angadi, Haldor topsoe, satish.angadi@gmail.com
How is the stabiliser performance? normal? if yes, then this seems case for stripper tray dis-lodgement in stripper.
late answer ... would like to know what was the outcome.
15/01/2013 A: Lindsay McRae, Pall Corporation, Lindsay_McRae@pall.com
You might want to check if there is a problem with any sour water carying over from LP separator to your stripper. If there is, you might consider to install a high efficiency Liquid Liquid coalescer (and pre-filter) after the LP separator to break the emulsion and remove the sour water. Pall Corp has supplied such Liquid Liquid coalescers to refiners to tackle corrosion.
15/01/2013 A: keith bowers, B and B Consulting, kebowers47@gmail.com
H2S is REAL BAD for Cu strip corrosion. Fractional ppm will drive it from 1A to 3+.
Sampling for trace H2S is difficult as it tends to escape container. Immediate 'lead acetate paper' testing MAY detect it. A 'Drager Tube' might also. A quick and sure qualiative only test is to shake sample up with a few drops of clean murcury. The mercury will form a skim/tarnish with even trace H2S. BE CAREFUL with the mercury and properly dispose of it.
This situation (Bad CU strip corrosion) usually results when overhead naphtha draw is reduced too much. The H2S MUST go overhead--. Reducing overhead make (to maximise diesel ) is risky because of the probability of trace H2S remaining. 'Simulation' is not precise enough with vevry low H2S loadings to be useful. H2S is very non-ideal
15/01/2013 A: Ralph Ragsdale, Ragsdale Refining Courses, ralph.ragsdale@att.net
It is unfortunate that the overhead systems are common. The H2S should be stripped out in the 1st column and REMOVED. But you have what you have to work with. Is there a reboiler on the stabilizer, and did you use the observed (calculated from flow meters) duty in your simulation? Or, if it is “live” steam to the bottom of the stabilizer, did you calibrate the steam flow meter? Sometimes something as simple as a faulty flow meter can be the problem. Yes, flooding will cause an increase in column pressure drop. A faulty flow meter can cause flooding as well as too little heat input. The reflux is an indirect measure of the extent the column is loaded. Has that meter been calibrated?
15/01/2013 A: Eric Vetters, ProCorr Consulting Services, ewvetters@yahoo.com
How did you check for leaks? They can be hard to detect. If you know the pressure balance you can at least tell whether a leak would possibly cause a problem. If you can find a place to catch a sample upstream of such an exchanger, you can check copper strip on the upstream sample to see if there is a change across the exchanger. Trying to find leaks by taking exchangers off line and pressure testing sometimes works and sometimes doesn't. In addition to upstream and downstream testing you can use tracer studies to look for leaks.
Have you done a tower scan to verify that your trays are intact. Something like a slug of water in the steam could have blown out or severely damaged the trays, causing the stripper to no longer perform properly. Typical tray pressure drop is about .1 psi/tray if the DP gets up around .15 psi/tray you are likely flooding. The tower scan would also tell you if you are flooding the column. The same people that scan towers can also do tracer studies.
This at least gives you a couple of possibilities to check out. Good luck.