Correct Problem Diagnosis Debottlenecks a Gas Plant with No Hardware Changes

This paper describes a case study of a gas plant (designed by others) that was bottlenecked by the reboil system of a deethanizer stripper. The reboiler system that consisted of two parallel once-through thermosiphon circuits experienced two different steady states. When operated at low rates, an undesirable steady state was established with maldistribution between the two circuits, in which one was circulating and the other remained liquid-full. This steady state persisted when the tower feed rates were raised until the circulating reboiler reached its heat transfer limitation, still with the other circuit liquid-full.

Pressure drop calculations, close design reviews, examination of operating charts, and gamma scans, led to the correct diagnosis that enabled debottlenecking the reboiler circuits and the entire gas plant with no hardware modifications. The desired steady state was established by an operation that temporarily generated higher liquid circulation and pressure drop through the reboiler circuits.

This case teaches a number of very important key lessons:

• With a parallel reboiler system, maldistribution between parallel circuits does occur and can cause severe bottlenecks. To the best of our knowledge, this is the first time that such maldistribution is reported in the literature.

• Pressure balances need to be considered at the design not only for the maximum design rates, but also at the minimum expected operation rates.

• It is important to provide flexibility in once-through reboiler systems that will allow operating as a circulating thermosiphon systems, at least at low rates.

• Finally, good troubleshooting can provide low cost (in this case, no cost) debottleneck of an entire gas plant.