Page 199 - Applied Process Design For Chemical And Petrochemical Plants Volume III

66131_Ludwig_CH10F 5/30/2001 4:35 PM Page 162

162 Applied Process Design for Chemical and Petrochemical Plants

Vaporizers, usually termed reboilers for chemical or petro- • High surface area.
chemical plant operations can be of several types: • Process is on the shell side, with possibly less fouling, or
easy access for cleaning the outside tubes.
• Horizontal • Tubes have easy access for cleaning on tube side, when
• Vertical fouled.
• Horizontal or vertical as shell and tube units operated by • Greater flexibility for operator handling high liquid rates.
• Natural circulation, which includes thermosiphon • Lower boiling point elevation than vertical units.
action
• Forced circulation or pump-through Figures 10-96A—E illustrate horizontal and vertical ther-
• Horizontal kettle-type units mosiphon reboiler flow arrangements.
For a distillation column bottoms heating, such as shown
Table 10-25 provides a helpful breakdown of the major in Figures 10-96A and 10-96B, the bottoms liquid from the
types and characteristics of vaporizers and reboilers used in column flows under system pressure and liquid head into
the industry. and through the shell side of the horizontal thermosiphon
For horizontal thermosiphon/natural units the boiling fluid reboiler. The two-phase (liquid vapor) mixture flows from
is almost always on the shell side, with the heating medium in the reboiler back into the distillation column either on the
the tubes. In the vertical units the reboiling of the fluid is in the bottom tray or just under this tray into the column vapor
tubes. For kettle units, the boiling is in the shell. Collins 185 sug- space above the bottoms liquid, with the vapor passing
gests a “rule of thumb” that if the viscosity of the reboiler is less upward into/through the bottom or first tray. The density
than 0.5 centipoise (cp), the vertical thermosiphon should be difference between the liquid in the column and the two-
considered, but when the viscosity is more than 0.5 cp, the hor- phase mixture in the heat exchanger (reboiler) and the riser
izontal reboiler is probably more economical. (outlet piping from the shell side of reboiler) cause the ther-
Because reboilers are used extensively with bottoms boil- mosiphon circulation through the reboiler. 186 According to
ing of distillation columns, the horizontal units have some Yilmaz, 186 the horizontal thermosiphon reboilers compared
advantages. 185

Table 10-25
Types and Characteristics of Process Reboilers

Type Advantages Disadvantages
Vertical thermosiphon Capable of very high heat transfer rates. Maintenance and cleaning can be awkward.
Compact; simple piping required. Additional column skirt required.
Low residence time in heated zone. Equivalent to theoretical plate only at high
Not easily fouled. recycle.
Good controllability.
Horizontal thermosiphon Capable of moderately high heat transfer rates. Extra piping and space required.
Low residence time in heated zone. Equivalent to theoretical plate only at high
Not easily fouled. recycle.
Good controllability.
Easy maintenance and cleaning.
Once-through natural Capable of moderately high heat transfer rates. Maintenance and cleaning can be awkward.
circulation Compact; simple piping required. Additional column skirt height required.
Low residence time in heated zone. No control over circulation rate.
Not easily fouled. Danger of backup in column.
Equivalent to theoretical plate. Danger of excessive per-pass vaporization.
Flooded-bundle (kettle) Easy maintenance and cleaning. Lower heat transfer rates.
Convenient when heating medium is dirty. Extra piping and space required.
Equivalent to theoretical plate. High residence time in heated zone.
Contains vapor disengaging space. Easily fouled.
Forced circulation* Viscous and solid-containing liquids can be Relatively expensive due to extra shell volume.
circulated. Cost of pump and pumping.
Enables an erosion-fouling balance. Leakage of material at stuffing box.
Circulation rate can be controlled.
*Advantages and disadvantages will in general correspond to the type of reboiler to which forced circulation is applied. The advantages and
disadvantages shown are in addition.
Used by permission: Fair, J. R. Petroleum Refiner, Feb. 1960, p. 105. ©Gulf Publishing Company. All rights reserved.