August 24, 2007

Acetic Acid Heat Exchanger Failure

We have a shell & tube heat exchanger as feed preheater in our glycol ether acetate plant where mixed feed is preheated to ~130-140°C using steam on shell side. It has failed recently & we observed few strange things.





Background

In the process of manufacturing of Glycol ether acetate (Or Cellosolve acetate) we conventionally use Cellosolve & Acetic acid and esterification reaction occurs with the water formation in CSTR reactors. Toluene is used as entrainer for water removal.


After reaction step, the product mix is sent to mix fraction column separation where All unreacted components and toluene are removed as overhead & recycled back to the reactor.

Feed is mixed with this recycle stream & sent to a preheater upstream of the reactor. The mixture is preheated to ~130-140°C using steam.

Incident

On 17 Aug when we stopped the plant for some modification, we found the organic compounds in the condensate line for the reactors. So initially we thought about some leakages in the internal coils of the reactor but while opening up the flanges in the system we found that organics are not coming from the reactor side rather coming from the header side. So we isolated entire system & traced it back to this exchanger.

After confirmation from the drain point at max elevation, we decided to open it in the morning of 18 Aug and found this scene at site.




This is a 6 pass exchanger with organics on tube side & heating steam on shell side.





The tube failure was found on the inlet side of process fluid which is at left bottom in actual orientation also, as shown in the pic above.

It is clear from the pics that other tubes dont have even a single spot of corrsoion or leakge as shown above & below.



So what is the situation of failure at the time of opening of exchanger & why it is so?



Just another enlarged view of failed tubes at tubesheet end.........




We have inspected the tubes using boroscopy from inside & found that the actual leakage is inside the tubes at the end side of first pass (outlet side) not on the side shown (Inlet of first pass).



The picture above is showing a hole at the top left and severe pitting inside the tubes.

Our mechanical analysis is under progress & I will update this one as soon as I get those reports.

Till then I invite your comments.

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1 comments:

profmaster said...

Sorry I had to push publish button few times due to large size of the post containing big images. This might have caused several mails to you regarding the same.

So I apologize & will try to avoid in future.

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