Four types of reboilers are :
• Once-through thermosyphon reboilers
• Circulating thermosyphon reboilers
• Forced-circulation reboilers
• Kettle or gravity-fed reboilers
There are dozens of other types of reboilers, but these four represent the majority of applications. Regardless of the type of reboiler used, the following statement is correct: Almost as many towers flood because of reboiler problems as because of tray problems.
The theory of thermosyphon, or natural circulation, can be illustrated by the airlift pump shown in Fig. 7.1. This system is being used to recover gold bearing gravel from the Magdalena River in Colombia, South America. Compressed air is forced to the bottom of the river through the air line. The air is injected into the bottom of the riser tube. The aerated water in the riser tube is less dense than the water in the river. This creates a pressure imbalance between points A and B. Since the pressure at point B is less than that at point A, water (as well as the gold and gravel) is sucked off the bottom of the river and up into the riser tube. We can calculate the pressure difference between points A and B as follows:
HRW = height of water above the bottom of the riser, ft
DRW = specific gravity of fluid in the riser; in this case 1.0
HRT = height of the aerated water in the riser tube, ft
DRT = specific gravity of aerated water in the riser tube (this number can be obtained only by a trial-anderror calculation procedure)
AP = differential pressure between points A and B, psi
In a thermosyphon or natural-circulation reboiler, there is, of course, no source of air. The aerated liquid is a froth or foam produced by the vaporization of the reboiler feed. Without a source of heat, there can be no vaporization. And without vaporization, there will be no circulation. So we can say that the source of energy that drives the circulation in a thermosyphon reboiler is the heating medium to the reboiler.