What is a Reboiler? Types and Selection of Reboilers

Reboilers: Definition and Importance in Distillation

A reboiler is a crucial component of a distillation column used for generating vapor that drives the fractional distillation separation process. In traditional fractional distillation applications, the reboiler supplies all the vapor required for the separation. Proper operation of the reboiler is critical for effective distillation.

Types of Reboilers

Reboilers are classified based on their orientation and the type of circulation used.

Vertical and Horizontal Reboilers: Overview and Differences

There are two main types of reboilers based on orientation: vertical and horizontal. A vertical reboiler is installed vertically, while a horizontal reboiler is positioned horizontally. In comparison to vertical reboilers, horizontal reboilers are generally used for larger columns and can handle higher heat loads.

Types of Circulation in Reboilers: Fired, Force, Kettle, Thermosyphon, and Internal

Reboilers are also classified based on the type of circulation they use. The following five types of reboilers are commonly used:

1. Fired Reboilers
2. Force Circulation Reboilers
3. Kettle Reboilers
4. Thermosyphon Reboilers
5. Internal Reboilers

Factors to Consider When Selecting a Reboiler

Choosing the right type of reboiler for a specific application can be challenging. Reboilers are typically shell-and-tube exchangers that heat up the fractionation column fluid using steam heat transfer. However, specific applications may require specialized designs such as stab-ins, plate-fins, spiral-plate, and others. When selecting a reboiler, the following factors must be considered:

• Thermal performance
• Fouling tendency
• Material compatibility
• Maintenance and cleaning requirements
• Capital and operating costs

Factors Influencing Reboiler Type Selection

The selection of the appropriate type of reboiler for a specific application is a challenging task, as it involves balancing various factors that impact the economics of the distillation process. Different plants may prioritize these factors differently based on their specific requirements. Here are some of the major factors that influence the selection of a reboiler:

Plot Space Availability

The amount of plot space available for the reboiler can influence the selection of its type. Vertical reboilers are generally preferred when space is limited, while horizontal reboilers are used when more space is available.

Total Duty Required

The total duty required for the reboiler depends on the amount of liquid traffic that needs to be vaporized to achieve the desired level of separation. This factor plays a crucial role in determining the type of reboiler required for the application.

Fouling Tendency

Fouling is a common problem in reboilers, and the type of reboiler selected must have a design that can withstand the fouling tendency of the fluid. Factors such as the physical and chemical properties of the fluid, as well as the operating temperature and pressure, can impact the fouling tendency.

Temperature Approach Available and Required

The temperature approach available and required is a significant factor when selecting a reboiler. A large temperature approach can be favorable for some applications, while others require a smaller temperature approach. The size and type of the reboiler will depend on the temperature approach requirements.

Corrosion of the Fluid

The corrosive nature of the fluid can also impact the selection of the type of reboiler. Special materials may be required to prevent corrosion of the equipment, which can impact the overall cost of the reboiler.

Design Temperature and Pressure

The design temperature and pressure of the reboiler are critical factors in the selection process, as they impact the type of materials used for construction and the overall cost of the equipment.

Characteristics of the Reboiler Boiling Fluid

The characteristics of the reboiler boiling fluid, such as its viscosity, thermal conductivity, and specific heat capacity, must also be considered when selecting a reboiler.

Heating Medium Requirements

Finally, the heating medium requirements of the reboiler, such as the type of fuel, steam, or electricity required, will impact the selection of the type of reboiler.

Reboiler Configuration Factors

The selection of a specific reboiler configuration is influenced by several factors, which include the required duties, the available space, and the desired temperature approach. The major configuration selections include:

Forced Versus Natural Circulation Reboilers
A forced reboiler is used when there is a large duty requirement or when the fluid has a high fouling tendency. A natural circulation reboiler, on the other hand, uses gravity-induced circulation to promote boiling and is often used when the heat transfer requirements are moderate.

Tube-Side Versus Shell-Side Vaporization
In tube-side vaporization, the fluid is boiled inside the tubes, while in shell-side vaporization, the fluid is boiled outside the tubes. Tube-side vaporization is often used when the heat transfer coefficients on the tube side are high, and shell-side vaporization is used when the fluid has a high fouling tendency.

Once-Through Versus Process Recirculation Reboilers
A once-through reboiler has a single pass of the heating medium through the heat exchanger, while a process recirculation reboiler recirculates the fluid to be heated through the heat exchanger. Once-through reboilers are often used when the heating medium is expensive, while recirculation reboilers are used when the fouling tendency is high.

Single-Shell Versus Multiple-Shell Systems
A single-shell system has a single shell and tube bundle, while a multiple-shell system has two or more shells and tube bundles. Multiple-shell systems are often used when the fluid has a high fouling tendency or when there are space constraints.

Vertical Versus Horizontal Reboilers
A vertical reboiler has a vertical shell, while a horizontal reboiler has a horizontal shell. Vertical reboilers are often used when there is limited plot space, while horizontal reboilers are used when the fouling tendency is high.

Stab-in Bundles and Other Types
Stab-in bundles are often used when the fluid has a high fouling tendency or when the plot space is limited. Other types of reboilers, such as plate-fins and spiral-plate, are used when the fluid has a high fouling tendency or when there are space constraints.

Common Reboiler Configurations
Figure 1 to 4 show common types of reboilers. Figure 1 shows two horizontal, shell-side boiling configurations. Figure 1A is a recirculating thermosyphon reboiler, while 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. Finally, figure 4G illustrates a stab-in bundle, and figure 4H shows a forced-circulation, fired heater.

Reboiler Type Selection Table

Factor	Favored types	Disfavored types
High heat fluxes	Forced circulation Flooded bundles	Natural circulation
Ample plot plan	Horizontal
Thermally unstable products	Recirculating (no baffle)	Kettle Once through
Tight temperature approach	Spiral-plate Plate-fin	Shell-and-tube
Leaks are hazardous or difficult to deal with	Stab-in
Large exchanger size or high-duty requirements	Horizontal reboiler	Vertical reboiler
Low relative volatility systems	Recirculating
Low bottoms product fraction compared to boil-up	Recirculating Kettle	Once-through
Tight plot plan	Vertical	Horizontal
Solids present	Kettle Spiral-plate	Plate-fin
Exotic materials	Stab-in
Small exchangers	Vertical Stab-in
High relative volatility systems	Once-through	Recirculating
Recirculating	Once-through reboiler	Recirculating
High temperatures	Fired heaters

Table 1 outlines the various types of reboilers and their associated characteristics, which can then be used to select the most suitable system for a specific design. When deciding on a reboiler system, there are several factors to consider, such as the cost, efficiency, size, and other design details. Depending on the specifics of the design, one type of reboiler may be better suited than another.