(159g) Diagnosis Key to Rectification of Premature Flood and Instability in a Debutanizer Tower | AIChE

(159g) Diagnosis Key to Rectification of Premature Flood and Instability in a Debutanizer Tower

Authors 

Clancy-Jundt, B., PetroLogistics
The Flint Hills Resources (FHR) Debutanizer tower in their Propane Dehydrogenation (PDH) Unit in Houston, Texas, experienced premature flood at high loadings. A sharp pressure drop rise was encountered when the tower internal vapor rate through the top section exceeded 13,000 to 15,000 pph. This pressure drop rise was accompanied with an increase in the bottom C4 content. Instability and fluctuations of key process variables such as tower differential pressure and bottom flow rate occurred at all rates, intensifying when the tower flooded.

To diagnose and solve the problem, and to debottleneck the tower, Fluor, who was not involved in the original design but has extensive distillation expertise, was invited to join the FHR troubleshooting task force.

Plots of tower differential pressure (dP) versus vapor loads were prepared. These confirmed the flooding, and suggested that it was most likely occurring above the feed. Hydraulic calculations did not show proximity of any of the major flood limits. The percent jet flood for all trays was below 50%, downcomer liquid backups were very low, and so were downcomer inlet velocities. So no major flood limit should have been encountered.

Three gamma scans were performed at different rates. The latest of these, on February 17th, 2015, was performed at high vapor loads and high dP. The scans looked similar to each other. No flooding was observed in any of the gamma scans.

Closely reviewing the hydraulics revealed extremely low head losses at the downcomer exits of the trays above the feed, about 0.11 inches of liquid. This raised the suspicion of a possible loss of downcomer seal. Applying the Fluor downcomer unsealing model confirmed that vapor was likely to break the seal on these downcomers, and to generate downcomer unsealing flood. Unsealing of downcomers may have also occurred below the feed, but our model showed that there it is most likely that the unsealing only led to tray efficiency reduction but not to flooding.

The downcomer unsealing flood was unable to explain the fluctuations, as they also occurred when the tower was not flooded. Further investigation traced the root cause of these fluctuations to the unvented vapor space above the baffled bottoms draw compartment. Trapped unvented gas generated backpressure on the reboiler compartment liquid with intermittent geysering and fluctuations, as detailed in this paper.

To solve the problem, and with economics favoring reducing tray spacing to increase the number of trays, the tower was retrayed. Based on the above diagnosis, the new trays had effectively lower downcomer clearances. A gooseneck vent was added to vent the bottom draw compartment, and the downcomer from the bottom tray was shortened, reducing the submergence of the bottom downcomer in the sump liquid.

The tower is now operated with better separation than ever at 10% higher loads than previously with no flooding, no fluctuations, and at low pressure drops, attesting to the value of good diagnosis for economically solving tower problems.

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