distillation tower trays damaging

[rezaeb]

Hi guys,
Has anyone experienced with damaging of distillation tower trays during initial start up. This tower is desigend to separate methanol from water while for intial start up only water is used for warming up. A gas and a steam reboiler (total two separate reboilers)supply heat to tower which has 5600 mm dia. Some fluctuations were seen from upstream front end though is not a resonable justification for mchanical damage. In fact 11 first trays out of 85 got damaged.
It would be highly appreciated if somebody can give me some guidlines or similar experiences.
Good luck

 

[reena1957]

Dear Rezaeb, It is not uncommon to have damaged trays in start-up, as the instruments are just getting commissioned and operators are in the learning mode. Probable reasons are:
1. High levels of liquid above the feed inlet/reboiler vapor outlet(level indicator malfunction)
2. Sudden pulling of vacuum when the teperatures are high to cause a pressure surge.
3.Introduction of water into the system at a time when the temperature and pressure will lead to sudden evaporation.
4. Poor workmanship in fitting trays so that they cannot withstand even small pressure surges.
5. Explosion in some tray section where the conditions are just right.
These are just a few that I have come across and the causes obviously tell you where you have to be careful.
best wishes

 

[EmmanuelTop]

Hello Rezaeb,

If trays have been assembled correctly, high liquid level during initial start-up might be the cause of mechanical damage. Poor level control (which is always an issue after unit turnaround) would lead to liquid splashing against the bottom trays, when heat is introduced into the tower.

Top trays damage is also possible when significant ammount of water is pumped back to the column through reflux system.

Here is one interesting article about high liquid levels:

http://www.revamps.com/Revamps/documents/198.pdf

 

[rezaeb]

Dear Reena1957, Thank you very much for you reply. Kindly note the followings:
1. There are two separate LI and LT and both showed almost same level well below feed inlet/reboiler vap. outlet nozzle. On the contrary, we faced with low level, showing a sudden evaporation.
2. Tower was operating on a total reflux condition with a pressure of around 2 bar.g(during water operation) compared to it's normal operating pressure of 8 bar.g (with methanol-water). This may lead to a pressure surge during some certain front end instabilities.
3. Trays were damaged badly (totally bended) from the center (down comers) and upward, showing a localized upward sudden force and lack of sufficient liquid load.
4. You addressed to explosion. Would you please explain more. What could be seen without expanation was damaged trays were totally dark in color while originally were ss410. I mean that evidences of some kind of explosion were noticed but nobody knows why.

With very best wishes
rezaeb

 

[reena1957]

Dear Rezaeb, I do not know exactly how an explosive mixture can form in your case, especially with water in the system. But my experience was in a Refinery Vacuunm column which was a bit hot after shutdown anf blocked up. Air was leaking into the column from the vacuum system (which was shutdown without the final outlet valve properly closed). There was pyroforic iron(iron sulfide which ignites on exposure to air)in the system and which triggered the explosion at the correct place. Yes, there was black dust all over the damaged trays.I cannot explain your case - it is you to do the sherlock Homes job yourself!

 

[rezaeb]

Dear Emmanuel,
Thank you for the article. I downloaded that and am busy with it. But as I explained for Reena there are two separate LI and LT showed almost same reading.
Since the plant is currently stopped, both of them will be checked. But I believe that tower was operating below dump point due to very low liquid rate, resulting an unsealing downcomers and severe hydrulic instability. This followed by a pressure surge due to noncontrolled heat input coming from upstream, leading to a sudden evaporation and subsequently followed by introduction of cold water injection to compensate liquid level loss caused damaging in bottom trays.
Unfortunately, startup stability diagram is not available so it's asked from vendor to supply it for next startup.
It would be appreciated if someone can evaluate this reasoning.
Thanx and Regards