10 replies to this topic

[Venkat @89]

In our debutanizer column most of the time, the overhead vapor stream gets completely condensed, the average LPG temperature downstream of Cooling water condenser is 40degC. It is often observed, even after condensation, the pressure upstream and downstream of the condenser, that is in reflux drum remaining same, at 14.3kgf/cm2g. why is it so?

 

Why does condensation process is not reducing the pressure in reflux drum? How to understand this scenario? 

 

What should be the ideal pressure profile along the column overhead in total condensation case?

 

Can we consider the pressure in reflux drum is due to reflux liquid vapor pressure? Kindly help me.

 

Composition of overhead in Molar% is H2S 11.37% C2 - 0.27% C3- 33.94% IC4 -20.88% & NC4 - 32.93 and 0.61% material whose boiling is 27degC.

Edited by Venkat1989, 18 January 2023 - 10:49 PM.

[Bobby Strain]

I recall that a pressure increase can occur in a condenser. You won't know until you measure both locations with the same calibrated gauge. Or a DP cell. But, I wouldn't waste time on any effort since all seems to be operating well.

 

Bobby

[Venkat @89]

Dear Bobby sir,

Thanks for your response, my question is not about whether the column has got any issue or not?

I just want to understand, why even after LPG condensation, why there is no reduction in pressure in reflux drum?. Guide me please

Edited by Venkat1989, 15 January 2023 - 07:08 AM.

[latexman]

There is a backpressure control scheme on reflux drum for non-condensables to fuel gas/LP flare.  Both CVs show closed.  Could there be enough flow/leaking to keep the pressure stable?  To test, open one or both CVs.

[johneyissac98]

What is the top tray temperature and the accumulator temperature I believe it is almost the same because in overhead condenser only phase change takes place sensible heat is not removed therefore at that same temperature it will exert same vapour pressure. Of course there will be some pressure drop but we can neglect for our topic.

[Venkat @89]

Top tray temperature is 60degC and condenser outlet temperature is average 40degC.

[horatorres]

}The pressure drop must be minimal,, if you measurement with a calibrated manometer will not see any difference  a low DP diaphragm maybe could see any

 

horacio

[Venkat @89]

Hi everyone, thanks for your responses, got the answer for my question, why there is not much pressure reduction in reflux drum even after condensation of LPG, in the downstream of cooling water condenser, I would like to elaborate, it may help anyone who has got stuck in same confusion that I have been in.

 

It is absolutely true, complete condensation of LPG vapor should cause drop in pressure due to phase change and the resultant pressure in the reflux drum after complete condensation should be the vapor pressure of the reflux liquid at the existing temperature.

 

The most important reason why the maximum pressure difference across condenser and reflux drum is not completely reflected even after complete condensation of LPG is because the reflux drum is in floating condition with condenser outlet line, so the resultant pressure in reflux drum due to vapor pressure will be the back pressure to the incoming vapors to the condensers and the maximum pressure difference across the condenser upstream and downstream will be the pressure loss due to flow of vapor.

 

In case if the condenser downstream has got any control valve, where such configuration is normally seen in Naphtha Splitters, to control column top pressure, in such cases the observed the pressure difference across the condenser and reflux drum would be slightly higher and the difference would be much more higher in such configurations if the hot-bypass valve is 100% closed condition. In such configuration you can feel the drastic drop in pressure in reflux drum due to complete condensation of vapor.

 

This is what I have understood. Any corrections to this answer can be given, if I had gone wrong anywhere in my observation. Thank you all. Enjoy the subject.

[Bobby Strain]

I can't agree that you got the correct answer to your query. But, if you are satisfied, you can now move on to resolve pressing issues.

 

Bobby

[Venkat @89]

I can't agree that you got the correct answer to your query. But, if you are satisfied, you can now move on to resolve pressing issues.

 

Bobby

You can correct me sir. 

[SilverShaded]

There should be some vapour in the drum maintaining pressure.  Either uncondensed vapours if there are incondensibles, or, vapour from a vapour by-pass line that is not in equilibrium with the liquid, if the drum is meant to be sub-cooled.  Both situations will maintain pressure in the drum.  What you probably don't have is a drum which condenses everything as that could pull a vacuum (and difficult to do level control is there is no level).