(45d) How Does a Collector Tray Leak Impact Column Operation?

Collector trays are commonly used in distillation columns to provide positive material balance control. They are designed with utmost care so that liquid can collect and flow without hydraulic restriction and without leaking. Controlling liquid leakage from a collector tray is paramount to ensuring adequate column heat and material balance. Many heavy oil fractionators, including FCC Main Fractionators, use collectors for material balance.

Most operating and engineering companies require tight leakage criteria for collector trays during installation. Engineers often envision that a leak would result in a discreet, predictable stream of liquid from the collector tray, falling neatly onto the fractionation device below. The lost product would instead contribute additional reflux to the section below – a neat and convenient story – while slightly un-optimized, would result in limited operational difficulty. Along with many in industry, the authors held this belief before troubleshooting the two case studies discussed in this paper.

The authors will present the results from two FCC Main Column troubleshooting cases. The first case is from a 100 MBD FCC Main Column that was having trouble yielding LCO at design rates. The second case is from a 30 MBD FCC Main Column that had severe instability above a certain collector tray liquid level where leakage commenced. Both towers exhibited leaking LCO collector trays, but drastically different resultant unit performances due to the type of internals below the collector tray (trays vs packing).