Design of thermosyphon reboilers

To calculate the circulation rate it is necessary to make a pressure balance round the system. 

A typical design procedure will include the following steps  

Calculate the vaporisation rate required from the specified duty. 

Estimate the exchanger area from an assumed value for the overall heat-transfer coefficient. Decide the exchanger layout and piping dimensions. 

Assume a value for the circulation rate through the exchanger. 

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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. 

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. 

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. ... 

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

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. 

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. 

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. 

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. Kettle-type reboilers 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 reboilers. However, kettle reboilers require a larger-diameter shell that is more cosdy, and the reboiler must be supported at a sufficient elevation to get the product to the bottoms pump with adequate NPSH. 

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

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