# Once-Through Thermosyphon Reboilers

Figure 7.2 shows a once-through thermosyphon reboiler. The driving force to promote flow through this reboiler is the density difference between the reboiler feed line and the froth filled reboiler return line. For example:

• The specific gravity of the liquid in the reboiler feed line is 0.600.
• The height of liquid above the reboiler inlet is 20 ft.
• The mixed-phase specific gravity of the froth leaving the reboiler is 0.061.
• The height of the return line is 15 ft.
• Feet of water per psi =2.31.

The differential pressure driving force is then

What happens to this differential pressure of 4.7 psig? It is consumed in overcoming the frictional losses, due to the flow in the

• Reboiler
• Inlet line
• Outlet line
• Nozzles

If these frictional losses are less than the 4.7 psig given above, then the inlet line does not run liquid full. If the frictional losses are more than the 4.7 psig, the reboiler draw-off pan overflows, and flow to the reboiler is reduced until such time as the frictional losses drop to the available thermosyphon driving force.

The once-through thermosyphon reboiler, shown in Fig. 7.2,
operates as follows:

• All the liquid from the bottom tray flows to the reboiler.
• None of the liquid from the bottom of the tower flows to the reboiler.
• All the bottoms product comes from the liquid portion of the reboiler effluent.
• None of the liquid from the bottom tray flows to the bottom of the tower.

This means that when the once-through thermosyphon reboiler is working correctly, the reboiler outlet temperature and the towerbottom temperature are identical. If the tower-bottom temperature is cooler than the reboiler outlet temperature, something has gone wrong with the thermosyphon circulation.