Reboiler types thermosyphon

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. 

REBOILER TYPE (KETTLE OR THERMOSYPHON). TRAYS NUMBERED FROM TOP OR BOTTOM TRAY NUMBER ... 

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. 

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). 

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 Horizontal exchangers are more easily maintained than vertical, as tube bundle can be more easily withdrawn.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. 

Reboilers. Figure 8.3 was used to find the reboiler base cost. Please observe that each fractionation unit (three units required) has a reboiler section. These are thermosyphon-type reboilers, shell/tube units, heated by hot circulated oil. 

As an exercise, the reader is invited to demonstrate that both for condenser and reboiler, the degrees of freedom are (A +4), identical with a flash. Typically, the specifications are input stream N. + 2) variables plus two others. Outlet pressure is usually imposed. The remaining variable may be liquid or vapour fraction, including bubble-point liquid (1=1), dew-point vapour (1=0), or sub-cooled liquid or superheated vapour (unusual). The above specifications enable to compute the duty Q, but this may be given also as specification. Note also that in steady state flowsheeting the reflux drum is included in the simulation of condenser. The type of condenser (partial, total, or sub-cooled liquid), as well as the type of reboiler (kettle or thermosyphon) does not change the analysis. 

Two types of thermosyphon reboilers are used vertical and horizontal. 

Kettle-type reboilers are sometimes used with vacuum columns. They eiiminace the need for the circulating pump required with a forced-circulation reboiler, and avoid the temperature elevation cncoimtered in the lower end of a thermosyphon reboiler. The usual arrangement is that of the flooded-bundle type... 

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