Reboiler horizontal 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. 

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). 

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. 

Shell-and-tube heat exchangers can be broken down into (1) pipe-coil (2) double-pipe (3) fixed-head, single-pass (4) fixed-head, multipass (5) floating-head, multipass (U-tube) (6) kettle reboiler (7) thermosyphon reboiler and (8) shell nomenclature. These devices can be mounted vertically or horizontally. 

Particularly in the pettolcum industry, horizontal-thermosyphon reboilers are sometimes encountered. 

Rgure 3-13. Recommended tube removal pattern to reduce vapor blinding in kettle or horizontal thermosyphon reboilers. Dingman etaL, 196 ... 

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. 

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

When considering the steam side of steam heated reboilers, it is best to think about the reboiler as a steam condenser. The steam, at least for a conventional horizontal reboiler, is usually on the tube side of the exchanger, as shown in Fig. 8.1. The steam is on the tube side, because the shell side was selected for the process fluid. If the reboiler is a thermosyphon, or natural-circulation reboiler, then low-process-side pressure drop is important. For a horizontal reboiler, it is easiest to obtain a low pressure drop for the fluid being vaporized by placing it on the shell side. 

Downstream of the reaction zone, the lower static pressure permits the reactor content to boil and applies the thermosyphon effect for circulation. EDC vapor leaves the horizontal vessel and either enters the reboiler of a column (e.g., reboiler of high-boil-heads and/or vacuum column) or a heat exchanger, which condenses the EDC vapor. The reaction heat is transferred to the column indirectly. A fraction of the condensed EDC is fed back to the reactor and the rest is directly sent to the EDC cracker without further distillation. 

Two types of thermosyphon reboilers are used vertical and horizontal. 

I worked as a process design engineer for Amoco Oil in Chicago until 1980. Likely, I designed about 50 distillation columns, 90 percent of which had horizontal, natural thermosyphon circulation reboilers. 

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