(84a) Can Kettle Reboilers Turn Into Thermosiphons?

The depropanizer in the Hess lean oil gas plant in Tioga, ND experienced premature flooding at rates well below the trays and downcomer limits. The flooding was initiated by a rise in tower base level above the reboiler return inlet, originating from a bottleneck in the kettle reboiler circuit. The rising base level filled the tower base with liquid and/or froth which propagated upwards to flood the lower trays in the tower.

An extensive troubleshooting investigation combined gamma scans, lab analyses, temperature surveys, neutron backscatter of the reboiler and its inlet/outlet lines, plant tests, and plant monitoring at steady running at high and low rates, to investigate the nature of the restriction in the reboiler circuit. These techniques were backed by simulation and process and hydraulic calculations. The variety of the techniques afforded verification and cross checks of the findings by entirely independent techniques.

Several techniques showed independently and conclusively that the kettle was experiencing severe entrainment from its inlet region into the disengagement drum above. Some of this entrainment was captured by the disengagement drum, the rest continued to the tower base. At the higher rates test, just before the tower flooded, the entrainment captured by the drum exceeded 40% of the total kettle liquid product.

The tests conclusively identified two different regions of operation in the kettle. One region stretched from the reboiler feed to the second vapor line, characterized by temperatures close to the feed liquid, intense boiling action, and high entrainment. The second region stretched from the second vapor line to the kettle liquid draw, characterized by rising and hot temperatures, and low or no entrainment. These regions were previously reported by Rubbers et al (3) for a kettle in extractive distillation service.

Entrainment measurements and calculations based on the reboiler force balance showed that the entrainment from the intense boiling action in the first region raised the liquid level in the tower, converting the kettle into a full blown thermosiphon. This represents the first time where a kettle reboiler was monitored and demonstrated by comprehensive field tests to behave like a thermosiphon.