The important differences between a once-through thermosyphon reboiler and a circulating thermosyphon reboiler is critical. Figure 7.4 shows a circulating reboiler. In this reboiler
• The reboiler outlet temperature is always higher than the tower-bottom temperature.
• Some of the liquid from the reboiler outlet will always recirculate back into the reboiler feed.
• Some of the liquid from the bottom tray drops into the bottoms product.
• The tower-bottom product temperature and composition are the same as the temperature and composition of the feed to the reboiler.
The liquid feed rate to the once-through thermosyphon reboiler is limited to the amount of liquid overflowing the bottom tray. The liquid feed rate to the circulating thermosyphon reboiler can be quite high—limited only by the available liquid head thermosyphon driving force. However, we should note that the liquid head thermosyphon driving force for a circulating thermosyphon reboiler is proportional to the height of the liquid level in the bottom of the tower above the reboiler inlet nozzle, whereas with a once-through thermosyphon reboiler, as described previously, the corresponding height is the elevation of the floor of the draw-off pan sump above the reboiler inlet nozzle.
For a circulating thermosyphon reboiler, the rate of circulation can be increased by
• Increasing the steam or hot-oil flow through the reboiler. This reduces the specific gravity or density of the froth or foam in the reboiler effluent line.
• Increasing the tower bottoms liquid level. However, should this level reach the reboiler return nozzle, thermosyphon flow will be restricted or even stop. Then the reboiler heat duty will be reduced, and the tower pressure will drop. Sometimes this may cause the tower to flood.