Strange Ethyelen fractionator reboilers performance 1

[Fahad 90]

Hi everyone,

I am a Sr. process Engineer working in Ethylene plant and recently we had a strange challenging issue in the Ethylene fractionator reboilers where the reboiler thermosyphon circulation stopped suddenly without any previous signs of duty decline.

the Ethylene fractionator reboilers are using Propylene refrigerant as a heating medium with inlet conditions ( P= 6.2 Kg/cm2.g, T=6-7 C). normally, condensation of C3R outlet will take place in C3 accumulator drum where the latent heat is been given to the Ethylene bottom side. However, the level in C3R accumulator drum went to zero and never returned back to normal since 20th of April.

what we have done so far:
1- we have injected methanol just in case hydrate formation is there and it didn't help.
2- we checked the C3R supply parameters and there was no major change before this issue reported.
3- we checked and calibrated all related instrumentations.
4- we have flared and vented from both Ethylene and C3R sides many times to ensure no lighter materials are trapped limiting the heat transfer.
5- we have checked both side pressure drop with normal readings.
6- we brought a specialized company for Gamma scanning where the column and reboilers have been scanned without any major finding or internal damages.

Note: we have eliminated the fact those reboilers can be fouled as it is a clean surface.

we have simulated the C3R outlet (T= -10 C, P=6.15 Kg/cm2.g) where it should be subcooled. however, it is vapor.

we have tried once to reduce the valve outlet of C3 accumulator to give it more residence time for heat transfer and we have succeeded to increase the level up to 60%, and when we returned the valve output to normal opening, the level went to zero again.

sorry for the long and detailed post. however, I am seeking a fresh brain looking from different angle and if any of you have experienced this before, please share the results accordingly.

thanks
Fahad

 

[chicopee]

I have never been involved eventho I worked at Monsanto during my younger years and we had a lot of reactors at the time. Nonetheless, doesn't the circulation within a reboiler thermosyphon work on the principle of temperature gradient affecting the density of the fluid?

 

[georgeverghese]

You say thermosyphon circulation has stopped or reduced a lot at this C2 splitter reboiler. Since this is on natural flow with static head from reboiler feed bottoms sump, a few more things to check would be :
a) any changes made to control level in the sump feeding this reboiler. A lower level can mean less recirculation. Low level in this sump can also be due to some leak in the partition plate separating the reboiler feed from the product sump.
b) The pressure drop reading on the C2 splitter reboiler side will not change ( as you have found out) even when there is some restriction in the feed or exit paths to / from the reboiler. When there is a restriction, since this side is operating on static head from reboiler feed sump, feed flow will just reduce to match the restriction, while pressure drop remains unchanged. What can this restriction be which is decreasing recirculation flow?

You have excluded the possibility of tube leak at this reboiler affecting reboiler performance by light vapor migration - blanketing. Which is at higher pressure - the C2 splitter side or the C3 side? This conclusion you have arrived at may be valid only if C2 splitter side pressure is higher. Info I have says splitter pressure is approx 1900kpag.

Level in the C3 accumulator has risen to 60% merely because you have restricted the exit flow at this exit valve. It hasn't improved the situation.

 

[Fahad 90]

Thanks guys for your reply, the issue has been resolved by flaring C3R from different locations in the loop. it seems that some lighters were trapped somewhere in the system.

thanks again

 

[georgeverghese]

Okay, it just dawned on me yesterday that feed C3= refrigerant might have C2/ C2= contaminant in it, which then accumulated on the tubeside of this reboiler. But then you said that there was no improvement in cooling even after flaring the tubeside of this reboiler?

 

[Latexman]

I think I would suspect this valve and it's control - "we have tried once to reduce the valve outlet of C3 accumulator to give it more residence time for heat transfer and we have succeeded to increase the level up to 60%, and when we returned the valve output to normal opening, the level went to zero again.

Good luck,
Latexman

To a ChE, the glass is always full - 1/2 air and 1/2 water