Thermosyphon Circulation 4

[EmmanuelTop]

Dear members of Eng-Tips,

I have noticed cyclic behaviour of naphtha stabilizer during the last few days. The main cause of tower unstable operation is erratic heat input, delivered by bottoms reboiler. The column is operating at 80% of design capacity, with regular feed and regular temperature level of reboiler heating fluid (250C). The reboiler is horizontal H-type, recirculating without baffle (bottoms product has the same composition as reboiler feed).

The symptoms were:

1) Initial state: Tower bottoms temperature was 165C, while the reboiler outlet temperature was 170C. These parameters indicate normal reboiler operation.

2) After increasing heating fluid flowrate through the reboiler, reboiler outlet temperature increased to 175C and tower bottoms temperature dropped to 160C, indicating that process fluid flow through reboiler was reduced. Moreover, bottoms product flowrate started to oscillate between 0 and 60m3/h, followed by column overhead receiver pressure swinging (from 5.5 to 6.0 barg). Column behaviour has confirmed very erratic heat input, resulting in some sort of cycling. Cycle frequency is 5-10 minutes.
After decreasing heating fluid flowrate, everything came back and the tower operated again in its initial state. This phenomenon does not occur at lower feed rates (70% of design tower capacity and lower). Actually, this was the first time operating the tower at maximum bottoms temperature and feed rate higher than 70% of design.

Since reboiler is designed to achieve tower bottoms temperature of 175C at 100% tower capacity, what are possible explanations for diminished thermosyphon circulation?

- Undersized reboiler?
- Slug flow in reboiler outlet piping (cycling in tower operation)?
- Changes in boiling regime, or something else?
- Composition induced cycling?

According to A. Sloley, vaporization blanketing (binding) and critical flux limitation are the main causes of vaporization limits in reboilers. Since we are facing vapor phase superheating in reboiler (while net circulation rate drops in cycles, followed by light components in bottoms product), blanketing could be an explanation. Calculated (operating) heat flux is well below maximum design practice of 45kW/m2.

 

[EmmanuelTop]

You are absolutely right, 25362!

 

[EmmanuelTop]

Switching level cascade control to bigger bottoms product flowrate eliminated the problem, although it didn't look that way at the first moment.

Do you have any ideas what was happening in the reboiler, with level cascade on smaller product flow from the bottoms? How does it affect circulation rates?

 

[reena1957]

Dear Emmanueltop,
Read my reply above dated 1st Dec 2006 above and see your answer there. best wishes

 

[DBenson78]

In my experience of dealing with tower problems where it has been operating normally and then "something" has changed to upset the process it's better to try and get an understanding of it prior to taking a shutdown, i.e. get a full understanding of the problem before acting.

Tower scan's are invaluable from the likes of QTT / Tracerco to understand what is happening throughout the tower.

I have included of a case study for your info

http://www.tracerco.com/bulletins-casestudies-005.htm

http://www.tracerco.com/products-processdiagnostics-towerscan.htm

These techniques will help you undetrstand the circulation of the product around the tower, let us all know how you get on