Horizontal thermosiphon

This method may also be used for the thermal design of horizontal thermosiphon rehoilers. The recirculation rate and pressure profile of the thermosiphon loop can be calculated by the methods of Fair [Pet Refiner, 39(2), I05-I23 (I960)]. 

Collins, Gerald K., Horizontal-Thermosiphon Reboiler Design, Chemical Engineering, July 19, 1976. 

For horizontal thermosiphon/natural units the boiling fluid is almost always on the shell side, with the heating medium in the tubes. In the vertical units the reboiling of the fluid is in the tubes. For kettle units, the boiling is in the shell. ColUns suggests a rule of thumb that if the viscosity of the reboUer is less than 0.5 centipoise (cp), the vertical thermosiphon should be considered, but when the viscosity is more than 0.5 cp, the horizontal reboUer is probably more economical. 

Horizontal thermosiphon Capable of moderately high heat transfer rates. Low residence time in heated zone. Not easily fouled. Good controllability. Easy maintenance and cleaning. Extra piping and space required. Equivalent to theoretical plate only at high recycle. 

Figure 10-96A. Horizontal thermosiphon reboiler, a. Recirculating feed system, b. Once-through feed system. Both are natural circulation. (Used by permission Yilmaz, S. B. Chemical Engineering Progress, V. 83, No. 11, 1987. American Institute of Chemical Engineers. All rights reserved.)...

Figure 10-96B. Horizontal thermosiphon reboiler on distillation column shell and tube design, not kettle. Boiling in shell.

Yilmaz recommends that the maximum velocity in the exit from the horizontal thermosiphon reboiler be the work ofCollinsd ... 

The kettle unit used in the reboiling service usually has an internal weir to maintain a fixed liquid level and tube coverage. The bottoms draw-off is from the weir section. The reboiling handled in horizontal thermosiphon units omits the disengaging space because the liquid-vapor mixture should enter the distillation tower where disengaging takes place. The chiller often keeps the kettle design but does not use the weir because no liquid bottoms draw off when a refrigerant is vaporized. 

The article of Fair and Klip presents a detailed analysis of the necessary design features and equations for horizontal kettle reboilers, horizontal thermosiphon reboilers, and vertical thermosiphon reboilers. Other useful references on reboilers are 185,186,188,190,192,194,195,196,197, and 201. 

COLLINS, G. K. (1976) Chem. Eng., NY 83 (July 19th) 149. Horizontal-thermosiphon reboiler design. 

Critical heat flux in kettle and horizontal thermosiphon reboilers... 

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

G and H shells are used primarily for vaporizing the shell-side fluid, especially as horizontal thermosiphon reboilers. Partial longitudinal baffles force the entering liquid to flow across the entire length of the tubes and minimize the possibility of dry-out or stagnant vapor pockets. Leakage... 

FIGURE 15.77 Horizontal thermosiphon reboiler (from Hewitt et al. [13], with permission. Copy-right CRC Press, Boca Raton, FL). 

The common types of reboilers (Fig. 15.1) are vertical thermosiphon, horizontal thermosiphon, forced-circulation, kettle, and internal. The advantages and disadvantages of each type are compared in Table... 

Rgure 15.1 Reboiler tjrpes. (a) Vertical thermosiphon (b) horizontal thermosiphon (c) forced circulation id) kettle (e) internal. 

Note that the second and third considerations make horizontal thermosiphon reboilers attractive in many superfractionators (e.g., propane-propylene splitters), where high heat transfer areas are required and where increasing column height to supply additional reboiler head is costly. 

Problems described for vertical thermosiphon reboilers (see previous section) are shared by their horizontal counterparts. Some may even be more potent in horizontal thermosiphon reboilers. Operating problems experienced with horizontal thermosiphon reboilers and not experienced in vertical thermosiphon reboilers are ... 

The following techniques are commonly used to prevent liquid maldistribution is horizontal thermosiphon rehoilers ... 

Liquid distribution. The liquid distribution requirement of kettle reboilers is similar to that of horizontal thermosiphon reboilers. The techniques for overcoming liquid maldistribution in horizontal thermosiphon reboilers (Sec. 15.5) also apply to kettle reboilers. The author is familiar with one case in which channeling in a kettle reboiler was evidenced by the shell surface being much warmer in the center (above the inlet) than near the shell ends. The problem was eliminated, and heat transfer largely improved, after a horizontal baffle (which directed liquid toward the sides) was installed above the inlet nozzle.. The baffle arrangement must not restrict inlet liquid flow, as this may back liquid up in the column. In one reported case (237), such liquid backup caused column flooding. 

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