Reboilers thermosyphon

Thermosyphon Reboilers Once-through thermosyphon reboilers 

What happens to this differential pressure of 4.7 psig It is consumed in overcoming the frictional losses, due to the flow in the 

If these frictional losses are less than the 4.7 psig given above, then the inlet line does not run liquid full. If the frictional losses are more than the 4.7 psig, the reboiler draw-off pan overflows, and flow to the reboiler is reduced until such time as the frictional losses drop to the available thermosyphon driving force. 

The once-through thermosyphon reboiler, shown in Fig. 5.2, operates as follows  

This means that when the once-through thermosyphon reboiler is working correctly, the reboiler outlet temperature, and the tower bottoms temperature, are identical. If the tower-bottom temperature is cooler then the reboiler outlet temperature, then something has gone wrong with the thermosyphon circulation. 

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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. ... 

The upward flow of gas and Hquid in a pipe is subject to an interesting and potentially important instabiHty. As gas flow increases, Hquid holdup decreases and frictional losses rise. At low gas velocity the decrease in Hquid holdup and gravity head more than compensates for the increase in frictional losses. Thus an increase in gas velocity is accompanied by a decrease in pressure drop along the pipe, a potentially unstable situation if the flows of gas and Hquid are sensitive to the pressure drop in the pipe. Such a situation can arise in a thermosyphon reboiler, which depends on the difference in density between the Hquid and a Hquid—vapor mixture to produce circulation. The instabiHty is manifested as cycHc surging of the Hquid flow entering the boiler and of the vapor flow leaving it. 

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.

Fair, J. R., What You Need to Design Thermosyphon Reboilers, Petroleum Refiner, February 1960. 

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

Kern, Robert, Thermosyphon Reboiler Piping Simplified, Hydrocarbon Processing, December 1968. 

Sloley, A. W., Properly Design Thermosyphon Reboilers, Chemical Engineering Progress, March 1997. 

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

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). 

When laying out the diagram, it is only necessary to show the relative elevation of the process connections to the equipment where these affect the process operation for example, the net positive suction head (NPSH) of pumps, barometric legs, syphons and the operation of thermosyphon reboilers. 

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...

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. 

Thermosyphon reboilers can suffer from flow instabilities if too high a heat flux is used. The liquid and vapour flow in the tubes is not smooth but tends to pulsate, and at high heat fluxes the pulsations can become large enough to cause vapour locking. A good practice is to install a flow restriction in the inlet line, a valve or orifice plate, so that the flow resistance can be adjusted should vapour locking occur in operation. 

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. 

An example of the need to take into account process considerations is the need to elevate the base of columns to provide the necessary net positive suction head to a pump (see Chapter 5) or the operating head for a thermosyphon reboiler (see Chapter 12). 

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