The transfer of heat to and from process fluids is an essential part of most chemical processes. A reboiler is a heat exchanger that is used to generate the vapor supplied to the bottom tray of a distillation column. The liquid from the bottom of the column is partially vaporized in the exchanger, which is usually of the shell-and-tube type. The
heating medium is most often condensing steam, but commercial heat-transfer fluids and other process streams are also used. Boiling takes place either in the tubes or in the shell, depending on the type of reboiler
The following design guidelines should be considered when the selection of a reboiler type is made.
1. Fouling -Tube-side is easier to clean than shell-side.
2. Corrosion – corrosion or process cleanliness may dictate the use of expensive alloys; therefore, these fluids are placed inside tubes in order to save the cost of an alloy shell.
3. Pressure – high pressure fluids are placed on tube side to avoid the expense of thick walled shells. For very low pressures (vacuum) other factors involved in the selection of reboiler type determines the tube-side fluid.
4. Temperatures – very hot fluids are placed inside tube to reduce shell costs. The lower stress limits at high temperatures affect shell design the same as high pressures.
5. Heating medium requirements may be more important than the boiling liquid requirements.
6. Boiling fluid characteristics: Temperature sensitive liquids require low holdup design. Boiling range and mixture concentration together with available ΔT affect circulation requirements to avoid stagnation. Foaming can be better handled inside tubes.
7. Temperature difference and type of boiling (film or nucleate) affects the selection.
8. Space constraints; e.g., if head room is limited then vertical units would be inappropriate or the limitation of space for internal reboilers.
9. Enhanced surfaces are suitable only for some types.
Types of Reboilers
Reboilers are classified according to their orientation and the type of circulation employed. The most commonly used types are described below.
A. Kettle Reboiler
Kettle reboilers are commonly applied when a wide range of process operations (high turndown capability), large heat exchange surface, or high vapor quality is required.
Kettle reboiler is also called a “submerged bundle reboiler”. Installations include column bottom reboilers, side reboilers, or vaporizers. Kettles are generally more costly than other reboiler types due to shell size, surge volume size, and uncertainty in the TMTD.
They are often used as vaporizers, as a separate vapor-liquid disengagement vessel is not needed. They are suitable for vacuum operation and for high rates of vaporization up to 80% of the feed.
B. Horizontal Thermosyphon
This is a very common type of reboiler. Horizontal thermosiphon reboilers are the preferred reboiler type in refining applications. The process side is on the shell side, and the heating medium is on the tube side. The boiling occurs inside shell in horizontal thermosyphon. There is recirculation around the base of the column. A mixture of vapor and liquid leaves the reboiler and enters the base of the column where it separates.
Compared to the vertical thermosiphon reboiler, the horizontal thermosiphon reboiler generally requires less headroom but have more complex pipework and plot space making it more expensive to install and has a higher fouling tendency which leads to a slightly lower availability (because of outages for cleaning). Horizontal exchangers are more easily maintained than vertical, as tube bundles can be more easily withdrawn.
They are generally better suited than vertical thermosyphons for services with very large duties.
C. Vertical Thermosyphon
This is a very common type of reboiler in the chemical and petrochemical industries.
Vertical thermosiphon reboilers are used almost exclusively in chemical applications, while the petrochemical industry is about 70% vertical and 30% horizontal. In vertical thermosyphon reboiler, the liquid circulation occurs due to density difference between vapor-liquid mixture (two phase) in the exchanger from the reboiler and the liquid through the downcomer to the reboiler.
Read also the Basics of Shell and Tube Heat Exchangers
D. Forced-Circulation Reboilers
Forced circulation reboilers are similar to vertical thermosiphon reboilers, except the pump is used for the circulation of the liquid and the hot liquid flows inside column.
Usually arranged in a Unbaffled Recirculating Circuit unless there is a critical temperature level beyond which the process material undergoes decomposition or polymerization. If this is the case then a preferential type column draw-off design would be recommended over the Unbaffled Recirculation design.
For sensitive materials, precautions should be taken in the design of fired reboilers, such that the pressure drop is reasonably low and the heat rate in the heater is such that the film temperatures in the furnace tubes does not approach a temperature where excess fouling, product decomposition, or polymerization can initiate. The main use of forced flow reboilers is in services with severe fouling problems and/or highly viscous (greater than 25 cp) liquids for which kettle and thermosyphon reboilers are not well suited. Pumping costs render forced flow units uneconomical for routine services.
E. Internal Reboilers
Also known as stab-in reboilers or stab-in bundles, internal reboilers are another special application of the horizontal reboiler design. The internal reboiler is usually used where the process can be on the shell side and the reboiler surface area is small enough to fit into the distillation column bottom sump. The process side is on the shell side and the heating medium is on the tube side.
Boiling takes place in the pool of liquid at the bottom of the tower, the heating fluid being inside the bundle of tubes. Since the boiling liquid forms froth, which may vary in density, controlling bottom level can be difficult. This fact can makes this type of reboiler less attractive, particularly in foaming and vacuum services. Applications where internal
reboilers are sometimes used include:
• Batch distillation: where the tube bundle can easily be fitted into the batch drum, and periodic cleaning can be easily accommodated.
• Very low heat duty clean services: where column diameter is large due to other considerations, and where the reboiler tube bundle is small.
Kettle Type Reboiler:
Kettle reboilers are commonly applied when a wide range of process operations (high turndown capability), large heat exchange surface, or high vapor quality is required.
Kettle reboiler is also called a “submerged bundle reboiler”. Installations include column bottom reboilers, side reboilers, or vaporizers. Kettles are generally more costly than other reboiler types due to shell size, surge volume size, and uncertainty in the TMTD.
They are often used as vaporizers, as a separate vapor-liquid disengagement vessel is not needed. They are suitable for vacuum operation and for high rates of vaporization up to 80% of the feed.
The kettle reboiler is an exchanger that has a tube bundle immersed in a liquid bath, with substantial vapor disengaging space above the vapor. Vapor and liquid are separated in the reboiler’s disengaging space, so the return line carries essentially vapor. Kettle arrangements are once-through systems; reboiler effluent liquid does
neither recirculate nor back-mix with bottom tray liquid.
Vaporization takes place on the outside of tubes immersed in a pool of liquid. The bottom product is taken from an overflow from the liquid pool and there is no recirculation between the reboiler and the column. In some designs, the tube bundle can be installed in the base of the column as an internal reboiler. The kettle reboiler incorporates a volume above the liquid pool and tube bundle for vapor and liquid disengagement.
Here some of design for kettle reboiler:
1. The shell diameter is typically 40% greater than the bundle diameter.
2. The height of the tube bundle is usually 40-60% of the shell ID.
3. Kettle reboilers are typically designed with an overflow weir, which creates a
separate liquid product compartment within the exchanger shell.
Resources:
1. Reboiler Design Guidelines – KLM Group.
2. Heat Transfer Equipment.