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(Oil pumps) 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.

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.

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.

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