Loss of Downcomer Seal on a Depropanizer During a Plant Start up

The bottom C4’s product specification could not be met on the Depropanizer tower during a plant start up. The olefins unit had been started with a new start up procedure, which involved significant reduction in feed rates to the unit until achieving on spec ethylene. C3 & C4’s product were to be stored in off spec storage facilities. The business plan was calling for a light feed slate, which would cause low rates in the C3/C4 area. As feed rates reached targets, the depropanizer bottom C4’s was off spec. The tower overhead product was on spec and responsive to reflux, but the stripping section was not responsive to reboil rates. Simulation of the operating conditions suggested that the tray load was identical as the pre shut down loads. The pre & post shut down tower pressure drop was relatively identical. Gamma scan of the tower did not show any mechanical anomalies, but reveal an unloaded tower. Suspecting a loss of downcomer seal, the liquid load to the tower was increase through external liquid feed and reflux. The reboiler rate was also dropped for a few hours. The tower regained efficiency and product met specification shortly after the reboil rate was re-established. This paper describes the sequence of event and the symptoms observed. The corrective actions are also discussed.