Once-through thermosyphon circulating

Loss of once-through thermosyphon circulation. There are several common causes of loss of circulation. The common symptoms of this problem are... 

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

This last statement requires some clarification. But to understand our explanation, an understanding of the important differences between a once-through thermosyphon reboiler, and a circulating thermosyphon reboiler, is critical. Figure 5.4 shows a circulating reboiler. In this reboiler... 

We said before that it was wrong to return the effluent, from a once-through reboiler, with a vertical baffle, to the cold side of the tower s bottom. Doing so would actually make the once-through thermosyphon reboiler work more like a circulating reboiler. But if this is bad, then the once-through reboiler, must be better than the circulating reboiler. But why ... 

Figure 5.6 shows a once-through forced-circulation reboiler. Such a reboiler differs from a thermosyphon reboiler in that it has a pump to force circulation, rather than rely on natural or thermosyphon circulation. This seems rather wasteful—and it is. 

Circulating Versus Once-Through Thermosyphon Rebollers... 

One helpful suggestion is to avoid the use of kettle reboilers or once-through thermosyphon reboilers, in favor of circulating thermosyphon reboilers. That is, feed the reboiler directly from the bottom of the stripper and design for about a 10% to 20% vaporization rate (see Fig. 47.3). This will reduce, but unfortunately not eliminate, the reboiler tube failures by maintaining more liquid flow across the tubes. 

For most once-through and many circulating thermosyphon reboilers (see Figs. 18-1 and 18-2), a high liquid level covering the reboiler vapor return nozzle will retard thermosyphon circulation and cause a precipitous loss in reboiler duty, instead of tower flooding. 

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