Loss of Once-Through Thermosyphon Circulation

There are several common causes of loss of circulation. The common symptoms of this problem are

• Inability to achieve normal reboiler duty.
• Low reflux drum level, accompanied by low tower pressure, even at a low reflux rate.
• Bottoms product too light.
• Reboiler outlet temperature hotter than the tower-bottom temperature.
• Opening the steam or hot-oil inlet heat supply valve does not seem to get more heat into the tower.

The typical causes of this problem are

• Bottom tray in tower leaking due to a low dry tray pressure drop
• Bottom tray, seal pan, or draw-off pan damaged
• Reboiler partially plugged
• Reboiler feed line restricted
• Reboiler design pressure drop excessive
• Tower-bottom liquid level covering the reboiler vapor return

If the loss of circulation is due to damage or leakage inside the tower, one can restore flow by opening the start-up line (valve A shown in Fig. 7.2), and raising the liquid level. But if the reboiler is fouled, this will not help.

Figure 7.3 shows a once-through thermosyphon reboiler with a vertical baffle. This looks quite a bit different from Fig. 7.2, but processwise, it is the same. Note that the reboiler return liquid goes only to the hot side of the tower bottoms. Putting the reboiler return liquid to the colder side of the tower bottoms represents poor design practice. While most designers do it this way, it is still wrong.

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