2.2. Reboilers
2.2.1. Classification of reboilers
There are three major types of reboilers:
i. Thermosyphon natural circulation reboiler: The boiling occurs inside the tubes in vertical thermosyphon reboiler and inside shell in horizontal thermosyphon reboiler ( Figure 1.10) . In vertical thermosyphon reboiler, the liquid circulation occurs due to density difference between vapor-liquid mixture (two phase) in the exchanger from the reboiler and the liquid through the downcomer to the reboiler.
Advantages: most economical because no pump is required.
Limitations: not suitable for heavily viscous fluid; high construction cost for the installation of the column base at suitable elevation to get thermosyphon effect; not suitable for low temperature difference processes due to boiling point elevation imposed by static head.
(a) |
(b) |
Figure 1.10. Thermosyphon reboiler [5] . (a) Horizontal thermosyphon reboiler. (b) Vertical thermosyphon reboiler |
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ii. Forced circulation reboiler : The liquid is fed by means of a pump. Forced circulation reboilers with vertical or horizontal tubes boiling may be designed. Forced circulation reboilers are similar to vertical thermosiphon reboilers, except the pump is used for the circulation of the liquid and the hot liquid flows inside column. To calculate the heat transfer coefficient it is generally assumed that, heat is transferred only by forced convection. The usual method of shell and tube exchanger design can be used.
Advantage: suitable for viscous and highly fouling fluids.
Disadvantage: high pumping and maintenance cost; pump is required to circulate the boiling liquid through the tubes and back into the column.
iii. Kettle reboiler : The tube bundle is immerged in a pool of liquid at the base of the column in an oversize shell ( Figure 1.11 ). Kettle reboiler is also called a “submerged bundle reboiler”. The height of the tube bundle is usually 40-60% of the shell ID. The submergence of the tube bundle is assured by an overflow weir at height of typically 5-15 cm from the upper surface of topmost tubes.
Advantage: suitable for vacuum operation and high vaporization rate up to about 80% of the feed.
Limitations: low heat transfer rate than other types as there is no liquid circulation (low velocity); not appropriate for fouling fluids; kettle reboiler is not suitable for heat sensitive materials as it has higher residence time.
The bundle diameter Db, can be obtained from the empirical equation ([2] page 647-649 ) :
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(1.16) |
where, Db = bundle diameter [mm], nt = number of tubes, do = tube outside diameter [mm]. The values of the constants k1 and n1 are in Table 1.5.