Reboiler Exchanger and System Type Selection

How do you select the basic reboiler system for your service?

To: asloley@distillationgroup.com
Subject: Question - reboilers.
Date: Sat, 09 Jun 2001 03:54:48 -0000

Andrew,

What are some of the differences between reboiler types? How can we select a reboiler for a new column?

B., Asian Chemical Plant

To: B.
Subject: Reboilers.
Date: Wed, 18 July 2001 15:34 -0500

B.,

Reboilers generate vapor to drive fractional distillation separation. In classical fractional distillation services all the vapor to drive the separation comes from the reboiler. (Alternate systems may use externally generated vapor, feed preheat, or inter-reboiler systems). Proper reboiler operation is vital to effective distillation.

The most critical element of reboiler design is the selection of the proper type of reboiler for a service. Most reboilers are shell-and-tube exchangers. Specific services may use other specialized designs including stab-ins, plate-fins, spiral-plate and others. The purpose here is not to go into 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.

Many factors influence reboiler type selection. In the end, all these factors reduce to economics. Every plant will weight the trade-off between these factors differently. No one-size fits all selection exists. Major factors include:

• Plot space available
• Total duty required
• Fraction of tower liquid traffic vaporized
• Fouling tendency
• Temperature approach available
• Temperature approach required

All these affect the configuration desired. The major configuration selections include:

• Forced versus natural circulation
• Tube side versus shell side vaporization
• Once-through versus process recirculation
• Single-shell versus multiple-shell systems
• Vertical versus horizontal orientation
• Stab-in bundles
• Other types

Figures 1 to 4 show common types of reboilers. Figure 1 shows two horizontal, shell-side boiling configurations. Figure 1A is a recirculating thermosyphon. Figure 2A is a once-through thermosyphon. Figure 2 shows vertical configurations. Figure 2C is a tube-side boiling configuration with 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. Table 1 includes the major factors in making an exchanger system choice.

Other systems in addition to the ones shown here are also possible. Of course, every system's final choice will depend upon specific design details involved. Many systems have specific characteristics that favor designs that might not be immediately apparent.

Andrew Sloley
DGI