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temperature allows an appropriate amount of the feed to
vaporize at column pressure; however it should not be so
high as to incur excessive amounts of cracking in the
furnace.
The hot crude oil is fed to the lower part of a distillation
tower, the condenser of which is at approximately atmos-
pheric pressure. The feed is partially vaporized at the
column pressure and is further vaporized by the injection of
steam into the bottom of the column. The column, which
usually has 30 to 50 stages and four to eight products, serves
to separate the feed into products with relatively narrow
boiling ranges. The overhead condenser condenses the
stripping steam (as a separate phase) and any condensable
hydrocarbons; some of this stream is returned to the column
as re¯ ux and some is withdrawn as the top product. Liquid
side-streams are withdrawn from the column at variouspoints and are stripped in side-strippers before being
removed as products. Stripping steam, reboiling or, less
commonly, vacuum, may be used to strip the liquid side-
streams; the volatile material is returned to the main
column. The stripping steam is at a low pressure; to prevent
it from condensing in the stripper, it is superheated.
The cooling requirements of the column consist of
product cooling before storage (to reduce the risk of
explosion), vapour condensation and intermediate cooling
in the column. Without intermediate cooling, the vapour
traf
c in the column would increase up the column; this
could lead to loading problems or would increase the cost of
the column. These intermediate coolers also enhance
control of the column and provide sources of heat at
higher temperatures than the top condenser.
It is complicated and costly to withdraw and condense an
intermediate vapour stream, so a common method of
providing intermediate cooling is to withdraw and cool a
liquid stream. The liquid may be withdrawn from a stage,
cooled and returned to just below the stage (this is known as
a `pump-back’ ) or may be withdrawn from a stage, cooled
and returned to a stage above in a pump-around loop. In both
cases, vapour condensation on the three to four stages below
the liquid return provides heat to the subcooled liquid. On
these stages, since no liquid is vaporized, no separation
takes place. The cooling duty of a pump-back scheme is
limited by the nite ¯ ow of liquid in the column.
1.2 Objectives of the New Design Procedure
A design procedure for a crude oil distillation system
should determine the distribution of stages in the column,
the optimum pump-around ¯ ows and duties and the
optimum ¯ ows of live steam for stripping and duties for
reboiling. This work develops a systematic procedure for
the design of crude oil distillation towers.We shall focus on
towers with side-strippers, since these are typically found in
the re
ning industry. The extension of the principles to
consider any sequence of columns is discussed brie¯ y at the
end of the paper.
The literature concerning the design and retro
t of crude
oil distillation systems is relatively scarce and mostly fairly
old. One of the reasons for its scarcity is that much of the
knowledge in this area is kept in-house by the big re
ning
and contracting companies. The literature is heuristic in
nature and often based on case studies. These shortcomings
arise because crude oil distillation is a highly complex
system, which is dif
cult to model and analyse.
Design procedures for the crude oil distillation system
were proposed by Nelson3
, Packie4
and Watkins5
. These
design procedures leave the design of the HEN and feed