Thermosyphons

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. ... 

This problem is solved in the reactor shown in Fig. 10.6. Ethylene and chlorine are introduced into circulating liquid dichloroethane. They dissolve and react to form more dichloroethane. No boiling takes place in the zone where the reactants are introduced or in the zone of reaction. As shown in Fig. 10.6, the reactor has a U-leg in which dichloroethane circulates as a result of gas lift and thermosyphon effects. Ethylene and chlorine are introduced at the bottom of the up-leg, which is under sufficient hydrostatic head to prevent boiling. 

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. 

When alloy piping or large bore piping is required, the associated equipment is located together as much as possible to keep the pipe mns short, preferably nozzle-to-nozzle by avoiding the piperack. Items such as elevated overhead condensers are located near the source and destination. Similarly, thermosyphon reboHers need to be placed adjacent to the column they reboH. Where gravity flow is required, these lines must be kept short and sloped. Space allocation for future additions must also be considered. 

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. Ketde-type reboders 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 reboders. However, ketde reboders require a larger-diameter shed that is more cosdy, and the reboder must be supported at a sufficient elevation to get the product to the bottoms pump with adequate NPSH. 

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 7 provides an overview of reboiler selection choices. The accompanying notes provide information for a quick or first cut estimate of the appropriate type for a given application. Tables 2 - and 3 provide additional, more detailed, selection data. Table 2 gives advantages and disadvantages for all the major reboiler types. Table 3 is limited to thermosyphon types. 

For reboilers, especially thermosyphon types, the devil is in the details. The information presented herein is intended for preliminary work. Final design is performed by experienced engineers using detailed design techniques. 

Figure 3. Vertical thermosyphon reboiler connected to tower.

Preferable to recirculating where acceptable vaporization rates can be maintained (less than 25-30%). This type is chosen when there is a need to minimize exposure of degradable and/or fouling substances to high temperatures. Used for large duties, dirty process, and frequent cleaning required. The process is usually in the shell side. This type is used in 95% of oil refinery thermosyphon applications. 

Used for small duties, clean process, and only infrequent cleaning required. Vaporization is usually less than 30%, but less than 15% if the fractionator pressure is below 50psig. The viscosity of the reboiler feed should be less than 0.5 cp. Put a butterfly valve in the reboiler inlet piping. This type is used in nearly 100% of chemical plant thermosyphon applications (70% of petrochemical). 

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. 

Tliiee methods of cooling are in common use, the pressurized cooling Ihiid system, the thermosyphon, and the static system. The static system IS used on smaller compressors and is probably the least common. Cool-... 

The thermosyphon is a good system for remote areas where utilities are limited, but requires some careful design to ensure proper operation. This is a circulating system with the motive force derived from the change in densiiy of the cooling fluid from the hot to the cold sections of the sysieni. API dl8 pennits this system for discharge gas temperatures below 210 F or a temperature rise across the compressor of 150°F or less. 

In 1909 one company began selling solar water heaters with separate collectors and insulated storage tanks. The collectors were made of copper tubing soldered to a copper plate in a glass covered box. The hot water was transferred to the storage tank by thermosyphon action, so the insulated tank had to be installed above the collector, typically in the attic or on the roof It was often connected to an auxiliary heater on the stove or furnace to supplement solar... 

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. 

Recover column reboiler thermosyphon, shell and tube, fixed tube sheets, heat transfer area 4 m2, design pressure 2 bar, materials carbon steel. 

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. 

Kettle reboilers have lower heat-transfer coefficients than the other types, as there is no liquid circulation. They are not suitable for fouling materials, and have a high residence time. They will generally be more expensive than an equivalent thermosyphon type as a larger shell is needed, but if the duty is such that the bundle can be installed in the column base, the cost will be competitive with the other types. They are often used as vaporisers, as a separate vapour-liquid disengagement vessel is not needed. They are suitable for vacuum operation, and for high rates of vaporisation, up to 80 per cent of the feed. 

Figure 12.58. Vertical thermosyphon reboiler, liquid and vapour flows...

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