Chemical Engineering January 2011 - 34

Cover Story
5A. Internal reboiler
NOT RECOMMENDED
Heating
fluid in
From
Heating
fluid out
reboiler
5B. 2-pass preferential baffle
NOT RECOMMENDED
5C. Poor vapor return orientation
NOT RECOMMENDED
5C. Vapor return through
downcommer
NOT RECOMMENDED
Bottoms
product
To
reboiler
Bottoms
product
FIGURE 5. These four conigurations are NOT RECOMMENDED: A. Insert reboiler; B. Two-pass preferential baffle; C. Poor vapor
return orientation; and D. Vapor return through downcomer
* Reboiler circulation (thermosyphon
driving force or pump NPSH)
* Bottoms product pump NPSH
* Tower or reboiler drainage to downstream
equipment, if required
The sections below discuss head considerations
for various reboiler types
in more detail to allow an assessment
of their contribution to required tower
skirt height. Note that when an unbaffled
bottoms arrangement is specified,
the liquid head used in the reboiler
flow calculations should be based on
the lowest operating liquid level allowed
(typically designated LLL). But
the thermal and hydraulic design of
the reboiler circuit should comprehend
both HLL (highest operating liquid
level allowed) and LLL process limits,
and the reboiler inlet and outlet lines
References
1. Kister, H. Z., What Caused Tower Malfunctions
in the Last 50 Years, Trans. IChemE,
Vol. 81, Part A, January 2003.
2. Kister, op. cit., p. 442.
3. Kister, H. Z., " Distillation Operation " ,
McGraw Hill, Inc., New York, 1990, p. 436
(adapted from Table 15.1).
4. Jacobs, J. K., Reboiler Selection Simplified,
Hydrocarbon Processing and Petroleum Refiner
40:70 (1961), p. 190.
5. Kister, op. cit., p. 434.
6. Collins, G. K., Horizontal-thermosiphon-reboiler
design, Chem. Eng., July 19, 1976, p. 149.
7. Fair, J. R., " Reboilers / A General Review of
Predictive Models and Design Practices, " December
7, 1992, p. 1 (prepared for " The Encyclopedia
of Chemical Processing and Design, "
J. J. McKetta, Editor).
should be sized to handle circulation
rates at HLL operating conditions. If
a constant-head baffle arrangement
is used for a thermosyphon system,
there will be different liquid levels to
consider on the reboiler and product
sides of the baffle, and the designer
should use LLL on the product side for
all product hydraulic calculations.
Vertical thermosyphon systems.
Generally this type of exchanger is
hung off the tower itself, and the
height of the system is determined by
the selected length of the exchanger
tubes. Common tube lengths are from
6 to 20 ft (2 to 6 m), with the longer
lengths applicable to designs that require
large heat transfer areas [11].
Reboiler tube length is often shorter
at lower column process pressure (for
8. Sloley, A. W., Properly Design Thermosyphon
Reboilers, Chem. Eng. Prog., March 1997, pp.
54-55.
9. Shah, G. C., Troubleshooting reboiler systems,
Chem. Eng. Prog., July 1979, p. 53.
10. Mukherjee, R., Effectively Design Shell-andTube
Heat Exchangers, Chem. Eng. Prog.,
February 1998.
11. Design guideline reported by one or more
FRI Design Practices Committee members.
12. Kern, R., How to design piping for reboiler
systems, Chem. Eng., August 4, 1975, pp.
107-113.
13. Fair, op. cit., entire document (pp. 1-34).
14. Kister, H. Z., and M. Chaves, Kettle Troubleshooting,
Chem. Eng., p. 26, February, 2010.
15. Collins, op. cit., entire article (pp. 149-152).
DISCLAIMER - Fractionation Research Inc. (FRI) and its Design Practices
Committee (DPC) do not represent, warrant, or otherwise guarantee,
expressly or implicitly, that following the procedures and recommendations
outlined in this document will satisfy any specific need the reader has or
the suitability, accuracy, reliability, or completeness of the information or
procedures contained herein.
The users of the procedures and recommendations apply them at their
own election and at their own risk. FRI and its DPC and the contribu34
CHEMICAL ENGINEERING WWW.CHE.COM JANUARY 2011
example, near atmospheric pressure)
to minimize liquid hydrostatic head,
which maximizes LMTD (log mean
temperature difference) because vaporization
can start at a lower temperature.
If the reboiler feed piping
enters the exchanger channel from
below, additional skirt height may be
required for this as well.
Horizontal thermosyphon systems.
In this case, the reboiler exchanger is
typically located at a minimum practical
distance above grade to allow
for piping clearances, ease of maintenance,
or condensate drainage if necessary
(the reboiler tubeside outlet
nozzle is usually located above the top
of the condensate drum for this purpose).
Then a pressure balance calculation
is performed for the reboiler cir16.
Orrell, W. H., Physical Considerations in Designing
Vertical Thermosyphon Reboilers,
Chem. Eng., September 17, 1973, entire article
(pp. 120-122).
17. Kister, H. Z., " Distillation Troubleshooting " ,
p. 155, Wiley, 2006.
18. Bu-Naiyan, A., and K. Kamali, Troubleshooting
Debutanizer at Ras Tanura Refinery's Hydrocracker
Unit, PETEM, October 4, 2004.
19. Jacobs, op. cit., p. 190.
20. Kister, op. cit., p. 94
21. Jacobs, op. cit., p. 194.
22. Hepp, P. S., Internal column reboilers - liquid
level measurement, Chem. Eng. Prog.
59:2, February 1963, pp. 66-69.
23. Rubbers, E., others, Distillation Reboiler
Startup Can Pose Challenges, Chem. Eng., p.
55, February 2004.
tors to this paper each expressly disclaims liability for any loss, damage
or injury suffered or incurred as a result of or related to anyone using or
relying on any of the procedures and recommendations in this document.
The information and recommended practices included in this document
are not intended to replace individual company standards or sound judgment
in any circumstances. The information and recommendations in this
document are offered as guidelines only for the development of individual
company standards and procedures.

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