Thermosyphon reboiler design

For reboilers, especially thermosyphon types, the devil is in the details. The information presented herein is intended for preliminary work. Final design is performed by experienced engineers using detailed design techniques. 

Fair, J. R., What You Need to Design Thermosyphon Reboilers, Petroleum Refiner, February 1960. 

Sloley, A. W., Properly Design Thermosyphon Reboilers, Chemical Engineering Progress, March 1997. 

Reboilers are energy-balance devices attached to distillation columns to help control the temperature. Reboilers have two basic designs thermosyphon and kettle. 

The column inventory also can be reducdd by the use of low-holdup column internals, including the holdup in the column base. As the design progresses, other features can be included to reduce the inventory. Thermosyphon reboilers have a lower inventory than kettle reboilers. Peripheral equipment such as reboilers can be located inside the column. ... 

Reboilers need to be located next to the tower they serve, except for the pump-through types, which can be located elsewhere. Fired heater reboilers are always located away from the associated tower and use a pump to circulate the bottoms. Ketde-type reboders are preferred from an operational and hydraulic standpoint because they can be designed without the worry of having to ensure sufficient head for circulation required by thermosyphon reboders. However, ketde reboders require a larger-diameter shed that is more cosdy, and the reboder must be supported at a sufficient elevation to get the product to the bottoms pump with adequate NPSH. 

For thermosyphon reboilers, the hydraulic aspects are as important as the heat transfer aspects. The design of thermosyphon reboiler piping is too broad a subject for this handbook. Some good articles on the subject can be found in References 2-14. Reference 3 is particularly good for horizontal thermosyphon reboilers. Table 1 has typical vertical thermosyphon design standards. 

Frank, O. and Prickett, R. D., Designing Vertical Thermosyphon Reboilers, Chemical Engineering, September 3, 1973. 

High Pressures. Thermosyphon reboilers present design problems at the two extremes of the pressure scale. Near the critical pressure, the maximum allowable flux drops. 

Orrell, W. H., Physical Considerations in Designing Vertical Thermosyphon Reboilers, Chem. Eng, Sept. 17, (1973). 

Recover column reboiler thermosyphon, shell and tube, fixed tube sheets, heat transfer area 4 m2, design pressure 2 bar, materials carbon steel. 

Extensive work on the performance and design of thermosyphon reboilers has been carried out by HTFS and HTRI, and proprietary design programs are available from these organisations. 

Make a preliminary design for a vertical thermosyphon reboiler for the column specified in Example 11.9. Take the vapour rate required to be 36 kmol/h. 

Make a preliminary mechanical design for the vertical thermosyphon reboiler for which the thermal design was done as Example 12.9 in Chapter 12. The inlet liquid nozzle and the steam connections will be 50 mm inside diameter. Flat plate end closures will be used on both headers. The reboiler will be hung from four bracket supports, positioned 0.5 m down from the top tube plate. The shell and tubes will be of semi-killed carbon steel. 

Thermosyphon reboilers are usually cheapest but not suitable for high-viscosity liquids or vacuum operation. Further discussion of reboilers will be deferred until Chapter 15. In the preliminary phases of a design, it is best to assume that enough stages are available in the column itself to achieve the required separation. 

This usually lies between 0.25 and 6. The greater the value of recirculation ratio, the less fouling there is in the reboiler. Lower values tend to be used in horizontal thermosyphons and higher values (greater than 4) used in vertical thermosyphons. The recirculation ratio is a degree of freedom at the discretion of the designer. This should be fixed later when the detailed design is carried out. 

Given these arguments, it is not surprising that the most common design of reboiler is the vertical thermosyphon. 

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