What is a Reboiler?
Reboilers are a typical heat exchanger that produces vapor to drive fractional distillation separation. In classical fractional distillation services, all the vapor to drive the separation comes from the reboiler. (Alternatively, externally generated vapor, feed preheat, or inter-reboiler systems may be used). Proper reboiler operation is vital to effective distillation.
Types of Reboilers
Reboilers are classified depending on the orientation of the reboiler and the type of circulation used.
Depending on the reboiler orientation there are two types of reboilers:
- Vertical Reboiler (Fig. 2) and
- Horizontal Reboiler (Fig. 1 and 3)
Depending on the types of circulation, the following five types of reboilers are widely used
- Fired Reboilers
- Force Circulation Reboilers
- Kettle Reboilers
- Thermosyphon Reboilers
- Internal reboilers
The selection of the proper type of reboiler for any specific service is the most difficult job of reboiler design. Reboilers normally are shell-and-tube exchangers that heat up the fractionation column fluid utilizing the heat transfer from steam. Specific services may use other specialized designs including stab-ins, plate-fins, spiral-plate, and others. The purpose here is not to go into the design details of each specific type but rather to examine the selection criteria that favor one configuration over another. Since shell-and-tubes are so common, most of the discussion focuses on them but some factors favoring other designs are covered as well.
Factors influencing Reboiler Type Selection
Many factors influence reboiler type selection. In the end, all these factors reduce economics. Every plant will weigh the trade-off between these factors differently. No one-size-fits-all selection exists. Major factors include:
- Plot space available
- Total duty required
- The fraction of tower liquid traffic vaporized
- Fouling tendency
- Temperature approach available
- Temperature approach required
- Corrosion of the fluid.
- Design temperature and Pressure.
- Characteristics of the reboiler boiling fluid.
- Heating medium requirements.
All of the above-mentioned factors affect the desired configuration of the Reboiler. The major configuration selections include:
- Forced reboiler versus natural circulation reboiler
- Tube side versus shell side vaporization inside reboiler
- Once-through versus process recirculation reboiler
- Single-shell versus multiple-shell systems
- Vertical versus horizontal reboiler
- Stab-in bundles
- Other types
Figures 1 to 4 shows common types of reboilers. Figure 1 shows two horizontal, shell-side boiling configurations. Figure 1A is a recirculating thermosyphon reboiler. Figure 2A is a once-through thermosyphon reboiler. Figure 2 shows vertical configurations. Figure 2C is a tube-side boiling configuration with the once-through flow and Figure 2D is a shell-side boiling recirculating thermosyphon. Figure 3E shows a kettle reboiler and Figure 3F shows a forced-circulation reboiler. Figure 4G illustrates a stab-in bundle and figure 4H shows a forced-circulation, fired heater.
Reboiler Type Selection Table
Table 1 includes the major factors in making a choice for the reboiler type selection.
|Factor||Favored types||Disfavored types|
|Low bottoms product fraction compared to boil-up||Recirculating|
|High bottoms product fraction compared to boil-up||Once-through reboiler||Recirculating|
|Low relative volatility systems||Recirculating|
|High relative volatility systems||Once-through||Recirculating|
|Large exchanger size or high duty requirements||Horizontal reboiler||Vertical reboiler|
|Leaks hazardous or difficult to deal with||Stab-in|
|Tight temperature approach||Spiral-plate|
|Thermally unstable products||Recirculating (no baffle)||Kettle|
|Tight plot plan||Vertical||Horizontal|
|Ample plot plan||Horizontal|
|High temperatures||Fired heaters|
|High heat fluxes||Forced circulation|
Other systems in addition to the ones shown here are also possible. Of course, every reboiler system’s final choice will depend upon specific design details involved. Many reboiler systems have specific characteristics that favor designs that might not be immediately apparent.
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