[gas_treater]

Dear all,

Nowaday we face a problem in our LPG Recovery Unit. At the first time we saw that the Deethanizer overhead temperature is much more higher than the designed temperature (60+ deg C vs designed 4 deg C) even when the condenser outlet temperature can be maintained at -23 deg C (designed condenser outlet Temperature). So we suspected that either the reflux cannot reach the top column or the reflux can reach the top column but at a temperature higher than -23 deg C.

By observation we saw that the pressure indicator in the reflux pump's discharge pressure indicated 35 kgf/cm2 - high enough to pump the liquid up to top Deethanizer column (which pressure is 29 kgf/cm2) and the pump is running well. So it looked like there is no problem related to pumping performance.

But after we made some window cutting in the insulated pipeline and check the pipe surface temperature at some points with IR thermometer we saw that there is a gradual temperature increase. At the first point - right below the condenser outlet nozzle - we found that the temperature is -23 deg C or matched with the temperature shown in the DCS. But the temperature reached 0 deg C at the second point - the pump suction - despite the temperature betwee those two point is only 15 meter (with an insulated receiver vessel in the middle). And next we found that the temperature at about 20 meter downstream of the pump's discharge is equal with ambient temperature - about 29 deg C. Finally, we check the temperature at about 10 meter below the reflux inlet nozzle and found that the temperature is still at near ambient temperature 29-30 deg C.

Through above description, does anyone think that there is another possible cause for that kind of phenomenon except "insulation problem"? I'm a little bit curious because of anyone thought that we have followed the standard when installing the insulation.

Thank you very much for any helpful information and experience sharing.

[ARGOS++]

Dear Gas_treater;

Please allow me a stupid Question:

overhead ... temperature (60+ deg C vs designed 4 deg C)

If I re-translated this correctly using the scheme in the attachment:
  Overhead temp. =  +60°C instead of  +4°C with an ambient temp. of  +30°C
How cut such be possible without an additional source of energy?
Have you tried to do a mass/energy balancing of the flows?
Is the feeder mixture of the composition you expected?
What is the flow-out of the head of the column, if there is one?

Sorry if I should have misunderstood your problem.

Good Luck!
                   ARGOS⁺⁺                   

[gas_treater]

Hi, ARGOS++.

Thank you for replying. Sorry if there is an information I forgot to tell you. The Deethanizer is equipped with a cryogenic condenser to condense the overhead vapor just like I told before - and a Bottom Reboiler which is designed to heat the bottom liquid to 125 deg C. So - I think - as long as the reboiler can supply enough heat to the bottom Deethanizer as much as designed (lets say the bottom temperature can reach 125 deg C), the vapor will flow upward the column and without enough cooling from reflux (which is designed to be -23 deg C but actually about 30 deg C) the temperature at top column will only between 125 deg C and 30 deg C. That would be the 60-70 deg C I told you before.

Best regards.

[ARGOS++]

Dear Gas_treater;

Your last thought was my first Idea, but I was not able to identify the required additional energy-source.

In my opinion you have not only a push-effect from the bottom of the rectification column; you have at the same time also a pull-effect from the cryogenic condenser at the top.
I think there is a quite hot “Sahara-Wind” through your rectification column bringing all out of the balance and the taken design.
I don’t know if it would be already enough, if you “dim” the Reboilor to a certain degree.
But I don’t think there is anything wrong with “your” isolation of the rectification column.
(I agree with your diagnose - as good as I know.)

I hope I was anyway of some help to you.

Good Luck!
                    ARGOS⁺⁺