(119a) Troubleshooting and Rectification of a Giant C3 Splitter Tower Problem Part 1: Troubleshooting

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.

 Our troubleshooting investigation combined hydraulic analysis and detailed multipass distribution calculations with the specialized technique of multichordal gamma scanning with quantitative analysis¹. The hydraulic analysis and multipass calculations did not identify a reason for the low tray efficiencies, but confirmed that the trays are prone to channeling and maldistribution due to their large open areas. The gamma scans showed a maldistributed pattern on the trays, with high L/V ratios on the inside panels and low L/V ratios on the outside panels. The scans showed vapor cross flow channeling (VCFC) on the outside panels and reverse vapor cross flow channeling (RVCFC) on the inside panels. Flooding was observed on the inside panels well below the calculated flood point. The scans pointed at a combination of VCFC and multipass maldistribution as the root cause.

 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. The high ratios of flow path length to tray spacing (2.4 to 3.7), high weir loads, and integral trusses projecting a significant depth (4”) into the vapor space were other conditions that promoted the channeling. Correlating liquid distribution with the effective weir lengths of the mod arc side downcomers (MOAD’s) proved to be a challenge but our hydraulic calculations showed that the distribution ratios were quite robust to variation in the effective length of the MOADS.

 To the best of our knowledge, this is the very first time that field measurements demonstrated interaction between VCFC with the inside-to-outside-pass maldistribution. A lesson learnt is that this interaction must be considered when designing and operating large-diameter towers. Finally, our investigation highlights that excessive open areas render trays prone to channeling and maldistribution, especially in large diameter towers containing multipass trays.

 1. Kister, H. Z., “Use Quantitative Gamma Scans to Troubleshoot Maldistribution on Trays”, Chem. Eng. Progr., February 2013.