(119b) Troubleshooting and Rectification of a Giant C3 Splitter Tower Part 2: Rectification

The PetroLogistics Propylene dehydrogenation (PDH) 28 feet ID C3 Splitter started up in October 2010 and experienced low tray efficiencies and premature flood in its first year of operation. Due to the low tray efficiency it could not produce on-spec polymer grade propylene. PetroLogistics, Fluor (who was not involved in the tower design), and Sulzer formed a taskforce to conduct a troubleshooting investigation to determine the root cause of the poor performance and to propose a fix.

 In Part 1, our troubleshooting investigation identified maldistribution pattern on the trays, with high L/V ratios on the inside panels and low L/V ratios on the outside panels, to be the root cause of the poor performance. Our investigation identified the open slot area (15% of the active area) of the fixed valves to be the prime factor inducing the channeling and maldistribution. A likely initiator of the multipass maldistribution was liquid preferentially flowing to the inside panels from the false downcomers distributing the flashing reflux to the panels of the top tray.

 An unplanned shutdown of the PDH unit in June 2011 provided less than 30 days to engineer, fabricate, and install the fix.  The most important and also most challenging modification was to blank the open slot area of the fixed valves from 15% to 11% of the active area. This was necessary to eliminate the channeling and the maldistribution between the passes.

 Achieving an adequate blanking pattern turned out to be a unique challenge. The fixed valves are staggered, making it impossible to fully blank about half of the openings. Short flow path lengths and presence of support members created the risk of forming stagnant regions, where the froth may collapse. Downtime was to be minimized. It took innovative engineering to beat these challenges in the time frame given. The paper describes how this was achieved.

 The task force also modified the piping entrance into the reflux false downcomers and increased the false downcomer height to prevent liquid pouring out from the false downcomers and preferentially flowing into the inside panels. Other less important modifications included adding anti-jump baffles to prevent cross movement of liquid across downcomers and adding downcomer blocks on the inboard side of the off-center downcomers to improve liquid distribution to the passes. Even though these were considered less important, they were easy to implement and therefore incorporated.

Upon restart, the modified tower achieved tray efficiencies comparable to those obtained in a well-operated, smaller-diameter, low pressure C3 Splitter. Flooding and instability were fully eliminated. The tower successfully produces on-spec polymer grade propylene.