Reboiler types

Figure 7 provides an overview of reboiler selection choices. The accompanying notes provide information for a quick or first cut estimate of the appropriate type for a given application. Tables 2 - and 3 provide additional, more detailed, selection data. Table 2 gives advantages and disadvantages for all the major reboiler types. Table 3 is limited to thermosyphon types. 

Feed to the unit may be all liquid, all vapor, or a mixture of liquid and vapor. Use of a reboiler causes the heavier liquid product to leave the system at a higher temperature than if steam stripping alone were used. For this reason, the use of reboilers is restricted to those systems in which the temperature is well below the level at which cracking takes place. In practice, materials boiling above about 400°F are seldom distilled in a reboiler type unit. 

The mechanism of boiling is essentially nucleate pool hoiling. In hoth styles of rehoiler the liquid velocity is relatively low compared to thermosiphon units. Jacobs provides an extensive comparison of advantages and disadvantages of essentially all the reboiler types used in industrial plants. Palen and Taborek conducted extensive studies of available data and proposed nucleate boiling equations to correlate various data from the available 14 equations down to a selected 6 for detailed study. The study was limited to various hydrocarbons and hydrocarbon mixtures. Their conclusions after computer correlations of the results from several equations were as follows. 

REBOILER TYPE (KETTLE OR THERMOSYPHON). TRAYS NUMBERED FROM TOP OR BOTTOM TRAY NUMBER ... 

Reboiler type = Vertical steam-heated thermosiphon... 

FIG. 11 -36 (Continued) Heat-exchanger-component nomenclature, (c) Outside-packed floating-head exchanger. Type AER (d) U-tube heat exchanger. Type CFU. (e) Kettle-type floating-head reboiler. Type AKT. (Standard of Tubular Exchanger Manufacturers Association, 6th ed., 1978.)... 

Where sufficient liquid head is unavailable, or additional head is expensive to provide. Alternative reboiler types generally have a lower liquid head requirement. 

Where a large surface area is required. Alternative reboiler types can generally offer more area per shell and more shells per column. 

Sump level. Usually, the bottom level manipulates the bottom flow or boilup. If the bottom sump is unbaffled, it is commonly used as a liquid source for both the reboiler and bottom product. In this case, for certain reboiler types (e.g., thermosiphon), the level must be controlled within a narrow range in order to supply a constant head to the reboiler. Level fluctuations may lead to unsteady reboiler action and column upsets (Sec. 15.3). If a baffle is installed, tight level control is normally not required, particularly if bottom product goes to storage. 

Rigorous Module The number of theoretical plates must be specified, along with the condenser and reboiler type, column pressure and pressure drop, feed tray locations, and side product locations (if side stream products are desired). In addition, the total number of specifications given must be equal to the number of products (top, bottom, and side streams) produced. These product specifications are often a source of problems, and this is illustrated in Example 13.2. 

It is easy to include a superheat section by adding hei t to the bundle (added surface) sufficient to achieve the desired superheat. This is impossible in either jacketed shell or reboiler-type vaporizers without the addition of a separate superheating element at substantial added cost. 

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