Circulating thermosyphon reboilers Circulation, forcing

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. 

The total condensation of a vapor to a liquid is best illustrated by the condensation of steam to water. Figure 13.1 is a rather accurate reproduction of the radiator that heated my apartment in Brooklyn. Steam flowed from the boiler in the basement. The steam condensed inside the radiator, and flowed back into the boiler, through the condensate drain line. This is a form of thermosyphon circulation. The driving force for the circulation is the differential density between the water in the condensate drain line and the steam supply line to the radiator (see Chap. 5, discussion of thermosyphon reboilers). 

Kettle-type reboilers are sometimes used with vacuum columns. They eiiminace the need for the circulating pump required with a forced-circulation reboiler, and avoid the temperature elevation cncoimtered in the lower end of a thermosyphon reboiler. The usual arrangement is that of the flooded-bundle type... 

A total condenser is used to produce liquid reflux and distillate product. The reboiler is a partial reboiler (vapor is boiled off a liquid pool). The composition of this liquid pool is the same as the bottom product ODmposition. Thermosyphon, ketde, internal, and forced-circulation reboilers ate aU usually partial reboilers. 

Forced-circulation reboilers are widely used for vacuum columns because of their lower pressure drop, for applications where a thermosyphon would be expected to foul, and for applications where thermally sensitive materials are being distilled. 

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