Once-through thermosyphon

Thermosyphon Reboilers Once-through thermosyphon reboilers... 

Figure 5.2 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. 

The once-through thermosyphon reboiler, shown in Fig. 5.2, operates as follows ... 

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. 

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

Figure 5.3 shows a once-through thermosyphon reboiler with a vertical baffle. This looks quite a bit different from Fig. 5.2, 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. 

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 ... 

Circulating Versus Once-Through Thermosyphon Rebollers... 

A once-through thermosyphon reboiler not only functions as the bottom theoretical stage of a tower, it also functions as if it is a superfractionation stage. This means that it is more important than any individual tray in the tower. This is especially true with a multi-component tower bottoms product. 

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. 

These temperatures indicated that most of the bottoms gasoline product was coming from the bottom tray of the tower, rather than from the reboiler effluent. Or the once-through thermosyphon reboiler was no longer functioning as the bottom theoretical fractionation stage of my debutanizer. 

Side reboilers and intercoolers also function as super-fractionation separation stages. They are just a subcase of once-through thermosyphon reboilers and partial overhead condensers. 

Once-Through Thermosyphon Sulfur Plant Start-Up... 

Big can plus fire—once-through thermosyphon reboiler... 

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