Chemical Engineering January 2011 - 35

Shaft
Alignment
cuit (including return piping), which
gives the required liquid height above
the exchanger necessary to drive the
desired reboiler flow.
Kettle systems. Properly designed
kettle systems do not usually require
significant liquid head for reboiler
flow, so the skirt height is typically
governed by another factor such as
bottoms-pump NPSH requirement.
The elevation, however, must be high
enough to satisfy the force balance
without having the liquid level in the
sump approach the reboiler return
inlet. In the case of a trapout, the
trapout must be at an elevation high
enough so that the force balance does
not lead to liquid overflow into the
chimneys [23]. In the case of a tower
whose pressure is sufficiently above
downstream equipment to drive bottoms
product onward without a pump,
the skirt height may be quite low.
Conversely, if bottoms evacuation or
drainage requirements dictate a significant
skirt height, and it is not desirable
to have the liquid level in the
kettle inlet pipe, the kettle exchanger
itself may need to be situated on a
high foundation because of the elevation
relationship between kettle overspill
weir elevation and tower bottom
liquid level. This elevation difference
is given by the kettle pressure balance
as described previously.
Forced circulation systems. The liquid
head necessary for a forced circulation
system is based on the NPSH
requirement of the reboiler circulation
pump. Typically, the tower bottom tangent
line is elevated about 15 ft (4.5
Authors
Current members of Fractionation Research Inc.
Design Practices Committee
Jeffery A. Bell, UOP LLC, Des Plains, Ill.
Doug Bouck, Separations Solutions, Ltd., Aurora,
Ohio
John P. Farone, Dow Chemical (retired), Cedar
Park, Tex.
Joseph Flowers, E.I. DuPont de Nemours &
Company, Wilmington, Del.
Ed Grave, ExxonMobil Upstream Research
Company, Houston
Henry Z. Kister, Fluor Corporation, Aliso Viejo,
Calif.
Todd Marut, ExxonMobil Research and Engineering
Company, Fairfax, Va.
Paul Morehead, Koch-Glitsch, LP, Dallas, Tex.
Ron Olsson, Celanese Ltd., Bishop, Tex.
Joseph Parker, Eastman Chemical Company,
Kingsport, Tenn.
m) [11] to provide sufficient NPSH. If
a separate product pump is used, its
NPSH requirement may govern.
Things to avoid
In Figure 5A, we have an internal reboiler.
Note that the Design Practice
Committee generally recommends
against using internal reboilers because
they are known to have caused
numerous operating and capacity
problems in previous applications.
In Figure 5B, this two pass arrangement
is not recommended because the
reboiler vapor must pass through the
downcomer's liquid curtain in an attempt
to distribute itself evenly to both
passes. The bottom tray should have
two side downcomers in this case.
In Figure 5C, the reboiler return
fluid is directed toward the bottom
tray downcomer and/or seal pan. This
design can fail in a number of ways,
including: (1) backup of the bottom
tray downcomer; (2) entrainment of
seal pan overflow liquid by the returning
vapor; (3) mechanical failure of the
bottom tray downcomer from fluid impingement;
or (4) heat transfer from
reboiler return fluid to the liquid in
the downcomer, causing vaporization
and choking inside the downcomer.
In Figure 5D, the reboiler return
pipe has been routed through the
downcomer. Again, this can fail by vaporizing
liquid in the downcomer and
choking it. Also, if the bottom section
trays are heavily liquid loaded, this
design might block enough downcomer
area to cause backup flooding.
■
Edited by Gerald Ondrey
Daniel R. Summers, Chair, Sulzer Chemtech
USA, Inc., Tulsa, Okla.
Waldo E. de Villiers, Shell Global Solutions
(US) Inc., Houston
Larry Wilder, Dow Chemical Company, South
Charleston, W.V.
Simon Xu, Process Technology, The Shaw Group,
Houston
Neil Yeoman, LNCK Associates, Merrick, N.Y.
Any correspondence regarding this paper should
be directed to one of the following members:
Daniel R. Summers Sulzer Chemtech USA, Inc.
(P.O. Box 700480 Tulsa, OK 74170-0480
Phone: 918-447-7654; Email: dan.summers@
sulzer.com)
Ed Grave, ExxonMobil Upstream Research Co.
(3120 Buffalo Speedway
Houston, TX 77098; Phone: 713-431-7262; Email:
edward.j.grave@exxonmobil.com)
Henry Kister, Fluor Corp. (3 Polaris Way, Aliso
Viejo, CA 92698. Phone: 949-349-4679; Email:
henry.kister@fluor.com)
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Chemical Engineering January 2011

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