Kettle Reboiler Duty Swings

[lgmarti]

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.

 

[tkdhwjd]

Besides checking for the chest pressure, have you checked the performance of steam traps?
Perhaps condensate level swings because the traps are not working properly? Perhaps control-application issue?

 

[lgmarti]

I checked the condensate header pressure to the steam trap and it is steady at 50 psig. Should this be an indication that the trap is working properly? It is maintaining a steady pressure in the steam.

 

[tkdhwjd]

If the traps are sticking and do not open or close properly, the condensate level will swing, affecting the heat exchanger surface area, and hence reboiler duty.

Or perhaps condensate is flashing downstream of traps, causing condensate flow restriction...

 

[25362]

Is the tower pressure controlled ? How is the partial overhead condenser performing ? Is the reflux ratio constant ?

 

[quadtracker]

I am assuming that this is a rich oil deethanizer. Is your feed to the tower steady? Is your flow rate high enough for the size of the system? Maybe the tower level is not being controlled properly, are you noticing major changes in the R.O. fractionator feed flow rate from the deethanizer, temp or level swings? Its sounds like similiar problems you get when you first start-up the system, not enough feed product and the level is bouncing around. Can you re-run off spec product back into the deethanizer? The reboiler probably can't keep up with the changes in level in the tower, try to work the tower harder, flood it as much as you can.

 

[MaNaTMoS]

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.

 

[lgmarti]

Thank you so much for all your responses.

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.

 

[quadtracker]

The bottleneck in the system appears to be the depentanizer high level problem. What has changed over time with the tower, why is the level remaining high now? Has the tower pressure been lowered? What about the valve trim size on the level controller, has someone reduced the size? Is there a manual by-pass valve around the level control valve, so that you can change out the trim (plug and seat) to a bigger size? Is there a downstream heat exchanger that could be dirty and partly plugged? I would check out a few things before adding a pump, it may not be neccessary.

 

[lgmarti]

The operators have been fantasazing about the depentanizer bottoms pump for at least 10 years. So the problem is not new. They were really excited about putting the pump. I may say, I am not a very popular person now since I stopped the pump idea.

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.

 

[lgmarti]

I took the pressure in the reboiler inlet line. The pressure is 167 psig. I calculated close a pressure of 215 psig based on the difference in height between the weir, a 50% level in the tower and a tower top pressure of 150 psig.

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.

 

[lastone]

You stated that the condensate pressure before the trap is steady at 50 psig. What is the downstream pressure in the condensate return header? Second, what pressure steam are you using for heat. Third, how much steam do you use? Every time you see the tower temperature rise do you see a corresponding increase in the steam flow to the reboiler?

 

[steveen]

IGmarti,

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