Reboiler vertical thermosyphon

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. 

Figure 3. Vertical thermosyphon reboiler connected to tower.

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

Inlet Line. Unstable circulation can result if the inlet line to a vertical theimosyphon reboiler is too large. The tubes of a vertical thermosyphon reboiler fire individually. The tubes can backfire excessively if the liquid inlet line is too large. They don t have to backfire all the way into the tower to cause problems, just to the inlet tubesheet. It is common to put flanges in the inlet liquid line so an orifice can be added later, if required, to provide proper dampening effect. 

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

Figure 12.58. Vertical thermosyphon reboiler, liquid and vapour flows...

Kern recommends that the heat flux in thermosyphon reboilers, based on the total heat-transfer area, should not exceed 37,900 W/m2 (12,000 Btu/ft2h). For horizontal thermosyphon reboilers, Collins recommends a maximum flux ranging from 47,300 W/m2 for 20-mm tubes to 56,800 W/m2 for 25-mm tubes (15,000 to 18,000 Btu/ft2h). These rule of thumb values are now thought to be too conservative see Skellence el al. (1968) and Furzer (1990). Correlations for determining the maximum heat flux for vertical thermosyphons are given by Lee et al. (1956) and Palen et al. (1974) and for horizontal thermosyphons by Yilmaz (1987). 

The tube lengths used for vertical thermosyphon reboilers vary from 1.83 m (6 ft) for vacuum service to 3.66 m (12 ft) for pressure operation. A good size for general applications is 2.44 m (8 ft) by 25 mm internal diameter. Larger tube diameters, up to 50 mm, are used for fouling systems. 

A fixed tube sheet will be used for a vertical thermosyphon reboiler. From Figure 12.10, shell diametrical clearance = 14 mm,... 

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. 

Furzer, I. A. (1990) Ind. Eng. Chem. Res. 29, 1396. Vertical thermosyphon reboilers. Maximum heat flux and separation efficiency. 

A vertical thermosyphon reboiler is required for a column. The liquid at the base of the column is essentially pure n-butane. A vapour rate of 5 kg/s is required. The pressure at the base of the column is 20.9 bar. Saturated steam at 5 bar will be used for heating. 

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. 

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. 

The design of vertical thermosyphon reboilers requires iterative calculations in which the exchanger needs to be divided into zones. The energy and pressure balances need to be performed simultaneously. Frank and Prickett17 performed a range of detailed simulations and presented the results graphically. This can be used as the basis of preliminary design. 

A distillation operation separating a low-viscosity hydrocarbon mixture requires three shell-and-tube heat exchangers. The liquid feed is to be preheated to saturated liquid by heat recovery from another low-viscosity hydrocarbon stream. The reboiler is to be a vertical thermosyphon using steam heating. 

Frank O and Prickett RD (1973) Design of Vertical Thermosyphon Reboilers, Chem Eng, 3 107. 

VAN Edmonds, S. (1994) Masters Thesis, University of Wales Swansea. A short-cut design procedure for vertical thermosyphon reboilers. 

This process differs from others in several engineering details. The severe duty placed on the reboiler of the aluminum chloride recovery tower due to fouling was a problem which was solved by a vertical thermosyphon reboiler operating ivith sufficient head to provide a high rate of liquid circulation. Another problem was suitable pumps to recycle aluminum chloride slurry to the top of the reactor. Centrifugal pumps with mechanical seals using butane flush were most satisfactory. 

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