Thermosiphon Reboilers are often used in industry, this article will briefly touch on the available type of reboilers in industry. then look into thermosiphon reboilers types, their advantages, disadvantages, compare them to other types and attempt to answer some common questions. The article will also attempt to highlight any industry guidelines available for thermosiphon reboilers.
Reboilers Versus Vaporizers
Reboilers differ from vaporizers. Reboilers are used with distillation columns to vaporise a fraction of the bottom product where in vaporizers all the feed is essentially vaporized.
Types of reboilers
Thermosiphon reboilers are the main topic of this article, but below are a short list of other popular types for reboilers;
- Forced Circulation Reboilers
- Kettle Reboilers
- Internal Reboilers
- Vertical Thermosiphon Reboilers
- Horizontal Thermosiphon Reboilers
Normally steam is used as a heating source however hot oil can also be used. Most of the reboilers are shell and tube type heat exchangers.
Forced circulation reboilers
The bottoms fluid is pumped through the exchanger, the vapor formed is separated in the base of the column
Forced circulation reboilers are suitable for handling viscous and heavy fouling fluids, low vacuum operations and low rates of vaporizations. The main disadvantage for forced circulation reboilers is that a pump is required also pumping costs will be high and not to dismiss that there is a risk of leakage of hot fluid that can occur at the pump seal. To avoid the risk of leakage canned rotor pump types are usually specified for this purpose.
Kettle Reboilers
In kettle reboilers, the tube bundle is submerged in the shell side liquid with provision of an overflow weir located at the end of the bundle, the weir is mainly to ensure that the tube bundle remain submerged in shell side liquid. The liquid product overflows into a reservoir section of the reboiler from which it is withdrawn.
Suitable for vacuum operation and often used as vaporizers as a separate vapor liquid disengagement vessel is not needed. Kettle reboilers have high rates of vaporization as up to 80% of the feed is vaporized and can handle higher viscosity liquids. They also have a lower heat transfer coefficients comparing to other types of reboilers due to lack of liquid circulation. Kettle reboilers are not suitable for fouling materials and have high residence time and hence not suitable for sensitive liquids. In general, they are more expensive than thermosiphon reboilers as they have larger shells and difficult to clean the shell side.
If the kettle reboiler suffers from excessive pressure drop then its likely to malfunction causing the liquid to backup in the column base beyond the reboiler return elevation leading to premature flooding and capacity loss.
Internal reboilers
This type is mainly a U-Tube bundle inserted into the side of the column. The bundle doesn’t have a shell or connecting pipe, instead it requires a large flange and internal supports. The internal reboiler is a good option for space saving however it has a limited heat transfer area. This is due to the size of the column as tubes. The column needs to be designed specifically to accommodate this type of reboilers. This type can be expensive and if the it needs servicing the column has to be shut down as there is no alternative operation is possible.
Thermosiphon reboilers
Vertical Thermosiphon Reboilers
Thermosiphon reboilers have two main orientations, vertical and horizontal.
Vertical thermosiphon reboilers rely on natural circulation with vaporization taking place in tube side. The liquid circulation through the exchanger is maintained by the difference in density between the two-phase mixture of vapor and liquid in the exchanger and the single-phase liquid in the base of the column.
Heating medium: Shell side
Process Fluid: Tube side
Vertical thermosiphon reboilers are usually attached directly to the distillation column. The heating fluid used is typically condensing steam on the shell side of the exchanger.
This type of reboilers have higher heat transfer coefficients and Less susceptible to fouling issues, this is due to the high circulation that can be achieved. Vertical Thermosiphon reboilers also require less space and piping and has low maintenance costs.
The performance of this type of reboiler can be poor if fluids are close to critical conditions. This can happen when liquid and vapor have similar densities. This can give a poor driving force for recirculation hence poorer performance.
Horizontal Thermosiphon Reboilers
Thermosiphon reboilers have two main orientations, vertical and horizontal.
horizontal thermosiphon reboilers rely on natural circulation with vaporization taking place in shell side. The liquid circulation through the exchanger is maintained by the difference in density between the two-phase mixture of vapor and liquid in the exchanger and the single-phase liquid in the base of the column.
This type has a lower headroom than the vertical type but has a more pipework and plot area. Horizontal thermosiphon reboilers also have higher circulation rates, this provides a higher temperature driving force. Also, high velocities and low exit vapour factions, reduced fouling.
Disadvantages of this type includes difficult cleaning on the shell side, and as mentioned before more complicated piping and large plot area.
Heating medium: Tube side
Process Fluid: Shell side
Why do we have a difference in fluid placement in vertical versus horizontal thermosiphon types?
As we understand that the placement of process fluid is in the shell side for horizontal thermosiphon reboilers. The reason behind this is that it’s impossible to establish a vertical density gradient in the tubes as they are aligned horizontally. Where in the vertical type this is much easier to establish due to its vertical orientation.
Some general recommendations
Thermosiphon reboilers have guidelines that are followed by engineers during the design process, below is a short list:
- Recommended maximum heat flux
For natural circulation reboilers
37 kW/m2 (12000 Btu/h ft2) - For forced circulation reboilers
94.64 kW/m2 (30000 Btu/h ft2) - Normal recommended ratio of circulated liquid to generated vapor in a circulation reboiler. The ratio is 4:1
- Maximum recommended %Feed vaporized in vertical thermosiphon reboiler is 30% for higher fouling fluids, a lower %Feed vaporized should be used to avoid fouling deposits
Advantages and disadvantages of different types of reboilers
The table below provides a summary:
| Type | Advantages | Disadvantages |
|---|---|---|
| Forced circulation | + Higher heat transfer coefficients + Can handle viscous and solid containing liquids + Circulation rate can be controlled | + Relatively expensive due to extra shell volume, pump and pumping costs. + Leakage of material at stuffing box |
| Kettle | + Low skirt height + Easy control + Easy to maintain and clean | + Lower heat transfer rates + High footprint requiring extra piping and space + Easily fouled + High residence time in heated zone leading to degradation of heat sensitive fluids |
| Once-through natural circulation | + Compact with simple piping requirement + Can produce high heat transfer rates. + Low residence time in heated zone. + Not easily fouled. | + No control over the circulation rate +Danger of excessive vaporization per pass + Danger of backup in column + Need additional column skirt height |
| Horizontal Thermosiphon | + Higher heat transfer rates + Low residence time in heated zone + Not easily fouled + Reduced fouling due to higher circulation rates | + High footprint requiring extra piping and spacing + Difficult cleaning on shell side |
| Vertical Thermosiphon | + Higher heat transfer rates + Compact with simple piping requirement + Low residence time in heated zone + Not easily fouled | + Need additional column skirt height + Poor performance if fluids are close to critical conditions. |
| Internal Reboiler | + Good for space saving + Doesn’t require a shell | + Limited heat transfer area as it need to be designed specifically for the column that accommodate it + Maintenance can be inconvenient |
Useful readings
- S. N. Raju, Fluid mechanics, Heat transfer, and Mass transfer chemical engineering practice, AIChE, 2011, John Wiley & Son, Inc., Publication
- K Sinnott, Coulson & Richardson’s Chemical Engineering, Volume 6, 3rd Edition, Butterworth Heinemann
Useful software
Kettle Reboiler Design (KRD)
Shell and Tube Heat Exchanger Design (S&THex)
