Thermosyphon reboilers vertical

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). 

Thermosyphon reboilers are the most economical type for most applications, but are not suitable for high viscosity fluids or high vacuum operation. They would not normally be specified for pressures below 0.3 bar. A disadvantage of this type is that the column base must be elevated to provide the hydrostatic head required for the thermosyphon effect. This will increase the cost of the column supporting-structure. Horizontal reboilers require less headroom than vertical, but have more complex pipework. Horizontal exchangers are more easily maintained than vertical, as tube bundle can be more easily withdrawn. 

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. 

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. 

Under what circumstances might a thermosyphon reboiler be orientated vertically or horizontally ... 

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

Figure 5.3 shows a once-through thermosyphon reboiler with a vertical baffle. This looks quite a bit different from Fig. 5.2, but processwise, it is the same. Note that the reboiler return liquid goes only to the hot side of the tower bottoms. Putting the reboiler return liquid to the colder side of the tower bottoms represents poor design practice. 

We said before that it was wrong to return the effluent, from a once-through reboiler, with a vertical baffle, to the cold side of the tower s bottom. Doing so would actually make the once-through thermosyphon reboiler work more like a circulating reboiler. But if this is bad, then the once-through reboiler, must be better than the circulating reboiler. But why ... 

Evaluation of the boiling heat transfer coefficient in vertical tubes, as in thermosyphon reboilers, is based on a group of equations, (42)-(48), of Table 8.10. A suitable procedure is listed following these equations in that table. 

---