Kettle Reboiler Duty Swings
Kettle Reboiler Duty Swings
(OP)
The kettle reboiler in our Deethanizer tower has a pretty consistent swing that has been a mystery for the past 20 years. The steam to the reboiler is controlled by Tray 2 of the tower. Every ten minutes this temperature cycles up and down. This wave is strong enough to increase the overhead gas make by 12% and decrease the bottoms make by 25%. This cycle occurs even when we break the temperature control loop and we place the steam valve in AUTO or in Manual. I have measured the condensate pressure before the trap and it is steady at 50 psig. Does anyone has any ideas of what can cause this increase/decrease in duty?
Could a be a drastic change in tower level? There is a 4'5" height differential between the reboiler return line and the reboiler baffle. I thought this was enough.
Could a be a drastic change in tower level? There is a 4'5" height differential between the reboiler return line and the reboiler baffle. I thought this was enough.





RE: Kettle Reboiler Duty Swings
Perhaps condensate level swings because the traps are not working properly? Perhaps control-application issue?
RE: Kettle Reboiler Duty Swings
RE: Kettle Reboiler Duty Swings
Or perhaps condensate is flashing downstream of traps, causing condensate flow restriction...
RE: Kettle Reboiler Duty Swings
RE: Kettle Reboiler Duty Swings
RE: Kettle Reboiler Duty Swings
How is your level control arranged? It should be on the kettle side, not in the tower. I'm asking this because I've noticed serious design errors in several high pressure fractionators - puttinf LC in the tower, instead on the baffle oferflow side.
Did you notice any changes in temperature of condensate being drained? Steam traps usually present weak spots when not designed properly.
RE: Kettle Reboiler Duty Swings
Yes, the deethanizer is a lean oil absorber and a stripper in one. We control the lean oil temperature and the lean oil rate very well and there is no overhead accumulator. The gas goes straight to the FG system. The tower pressure is constant at 150 psig. The feed to it comes from the Low line FG compressor and other units (reformer stab ovh liquid, alky stab ovh liquid, etc). It fluctuates according to the performance of the other units. The level control in the deethanizer is in the reboiler and is very small (2'x3'). The operators say that we used to use 80-90 BPH of lean oil. Now, we are using only 40 BPH. The reason is that the downstream tower level(depentanizer) goes 100%. Of course, it may fill up becuase the rate to it (deethanizer bottoms) fluctuates between 90BPH and 150+ BPH and its level control valve can't keep up. They were about to solve the problem by installing a pump in the depentanizer tower until...I confused everyone up, that is.
I will monitor the condensate temperature to see if it varies much. Should I increase the lean oil rate back to 80 BPH even though we can't get the flow out in the Depentanizer? The operators are going to think I am crazy.
RE: Kettle Reboiler Duty Swings
RE: Kettle Reboiler Duty Swings
I did a survey on the depentanizer bottoms and the exchanger's (an air fin and a water cooler) pressure drop is less than designed. AFter the coolers, the depentanizer bottoms goes thru a level controller (LV-2)and a three way valve that directs the flow to tankage and to a recycle drum (that send the lean oil back to the deethanizer (LV-4)). LV-2 has a Cv of 35 (max flow 220 BPH). LV-4 has a Cv of 22 (max flow 140 BPH) and it had no bypass. We built a bypass around LV-4; but, the level in the depentanizer continues to reach 100% periodically. WE opened the bypass around LV-2 and we saw no improvement, actually we loose level if we leave the bypass open for a long period of time. The average bottoms flow is 140 BPH but when LV-502 opens 100% the flow exceeds 150 BPH. We even built another jumpover and lined up the reboiler pump to LV-2 and the level still goes 100%. That is why I started to focus on the changes in the feed to the depentanizer and I noticed that the feed to the depent changes from 90 - 150+ BPH. If I can level this flow maybe I can eliminate the periods of time that I fill up.
Thank you all for your interest and support.
RE: Kettle Reboiler Duty Swings
Using the same formula, Pd = (liquid height @ tower - weir height)*(density/2.31), the liquid height @ the tower is only 4'11". The problem is that the tower T-T line is 7'4" from the ground. Does this mean that I have no liquid level in the tower but only liquid in the line to the reboiler. Is this possible?
Any help is appreciated.
RE: Kettle Reboiler Duty Swings
RE: Kettle Reboiler Duty Swings
We have seen this problem before in a complex recovery section before. The problem was ethane/ethylene recycle which caused big swings in liquid levels in the plant. Temperatures did not change at all, but the liquid levels and heat loads would swing dramatically. Do you have a sketch of your recovery section? 713-204-3615