Thermosyphon circulation

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 5.6 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). 

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. 

I worked as a process design engineer for Amoco Oil in Chicago until 1980. Likely, I designed about 50 distillation columns, 90 percent of which had horizontal, natural thermosyphon circulation reboilers. 

Thermosyphon circulation (or natural circulation) Partially vaporizing a liquid causes it to flow to a higher elevation. The driving force is the density difference between the denser liquid feed and the less dense, partly vaporized effluent. 

Forced circulation This is when a liquid is circulated through a heater or exchanger by means of a pump. A more complex and costly option as compared to thermosyphon circulation. 

I hope that understanding these concepts and terms will help you in understanding the remainder of our book. Some concepts reappear in different forms. For example, check my descriptions of draft and thermosyphon circulation. They are really the same concept—heat that causes a density difference causes a flow. The application is different, but the concept is the same. 

Figures 22.1, 22.2, and 22.3 illustrate the three types of steamgenerating equipment that are used in process plants. Most process imits do not have the boiler shown in Fig. 22.1. Water flows via thermosyphon circulation (i.e., the density difference between the steam drum and the mud drum). This is the kind of design used for a large boiler of the type found in the Utilities Department of a refinery or power plant.

Figure 22.1 Fired boiler water circulates by thermosyphon circulation through tubes.

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