The GPSA GPSA Engi Engine neer erin ing g Data Data Book Book 1 has an excellent section on reboilers. The most common types are the following:
• Forced circulation (Figure 1) • Natural circulation (Figure 2) • • • •
Once-through Recirculating Vertical thermosyphon (Figure 3) Horizontal thermosyphon (Figure 4) Flooded bundle (Kettle) (Figure 5) Recirculating—Baffled bottom (Figure 6)
Figure 1. Forced-circulation reboiler arrangement.
For thermosyphon reboilers, the hydraulic aspects are as important as the heat transfer aspects. The design of thermosyphon reboiler piping is too broad a subject for this handbook. Some good articles on the subject can be found in References 2–14. Reference 3 is particularly good for horizontal thermosyphon reboilers. Table 1 has typical vertical thermosyphon design standards.
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 12 and 313 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. For reboilers, especially thermosyphon types, “the devil is in the details.” The information presented herein is intended for preliminary work. Final design is performed by experienced engineers using detailed design techniques. General notes for thermosyphon reboilers:
Figure 2. Natural-circulation Natural-circulation reboiler arrangements.
1. Never use inclined inclined piping for two-phase flow in a process plant. This is particularly true for reboiler return piping. Use only horizontal or vertical runs. 2. If the reboiler heating heating medium is condensing condensing steam, provide a desuperheater if the superheat is is more than 40–60°F. (text continued on page 88)
Figure 3. Vertical thermosyphon reboiler connected to tower.
Figure 4. Horizontal thermosyphon reboiler.
Figure 6. Recirculating—Baffled bottom.13 (Reproduced with permission of the American Institute of Chemical Engineers, copyright 1997 AIChE, all rights reserved.)
Figure 5. Kettle reboiler arrangement.
Table 1 Typical Thermosiphon Reboiler Design Standards*
*By permission, D. C. Lee, J. W. Dorsey, G. Z. Moore and F. D. Mayfield , Chem. Eng. Prog., Vol. 52, No. 4., p. 160 (1956). **Cross Section area of vapor nozzle off to channel must be minimum of 1.25 times total flow area of all tubes.
Figure 7. Quick Selection Guide. 1
Preferable to recirculating where acceptable vaporization rates can be maintained (less than 25–30%). This type is chosen when there is a need to minimize exposure of degradable and/or fouling substances to high temperatures. 2 Used for large duties, dirty process, and frequent cleaning required. The process is usually in the shell side. This type is used in 95% of oil refinery thermosyphon applications. 3 Used for small duties, clean process, and only infrequent cleaning required. Vaporization is usually less than 30%, but less than 15% if the fractionator pressure is below 50 psig. The viscosity of the reboiler feed should be less than 0.5 cp. Put a butterfly valve in the reboiler inlet piping. This type is used in nearly 100% of chemical plant thermosyphon applications (70% of petrochemical). 4 Greater stability than unbaffled. 5 Usually used where piping pressure drop is high and therefore natural circulation is impractical. 6 Very stable and easy to control. Has no two-phase flow. Allows low tower skirt height. This type is expensive, however.
Table 2 Reboiler Comparison12
(table continued on next page)
Table 2 (continued) Reboiler Comparison12
Reproduced with permission from Hydrocarbon Processing, Oct. 1992, copyright Gulf Publishing Co., Houston, Texas, all rights reserved.
Table 3 Thermosyphon Selection Criteria 13
Reproduced with permission of the American Institute of Chemical Engineers, copyright 1997 AIChE, all rights reserved.
