Troubleshooting Once-Through Reboilers

Once-through reboilers can offer greater thermal and separation efficiencies than conventional circulating-thermosiphon reboilers, but are subject to different modes of failure.

Reboilers are an important component of distillation systems. Reboilers generate the vapor that flows up the distillation column. Common types of reboilers include circulating-thermosiphon, forced-circulation, and once-through reboilers.

Circulating-thermosiphon reboilers draw liquid from the tower bottoms, partially vaporize it, and deliver the two-phase fluid back to the tower bottoms above the liquid level. The difference in the static head pressure enables the circulation, as the partially vaporized fluid has a much lower density than the liquid entering the reboiler. Forced-circulation reboilers work the same way as circulating thermosiphon reboilers, but they use a pump to ensure circulation.

Once-through reboilers draw liquid from the bottoms tray of the column, partially vaporize it, and deliver the two-phase fluid back to the tower bottoms above the liquid level (Figure 1). No circulation occurs; all of the liquid phase exiting the reboiler and falling to the tower bottoms leaves with the bottoms product.

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▲Figure 1. Once-through reboilers draw liquid from the bottom tray, whereas circulating reboilers draw liquid from the tower bottoms.

This article provides design and operation recommendations for once-through reboilers and reviews troubleshooting steps for a few operating problems.