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preheat train until last, if they consider the HEN at all.
Packie4
and Watkins5
included correlations based on
empirical data to relate the number of stages, the re¯ ux
and the sharpness of separation in a column section.
Watkins5
also presented empirical correlations for the
amount of stripping steam to use.
The interactions between a crude oil distillation tower
and the HEN associated with it are signi
cant; therefore the
design of the column and the HEN should be considered in
the context of the available utilities. The heat exchanger
network includes the preheat train, furnace, reboilers and
stream strippers, condensers and product coolers. Theutilities typically used are hot ¯ ue gas (exhaust gases from
a red heater), steam at various pressures and temperatures
and cooling water or air. Various design modi
cations can
improve the separation ef
ciency of a distillation column;
i.e. they can reduce the utilities required to perform a
speci
ed separation in a xed number of stages. These
modi
cations often have an adverse effect on opportunities
for heat recovery in the process. The order in which
modi
cations should be considered is derived from an
analysis of the effect of the modi
cations on the heat
recovery potential.
2. CORNERSTONES OF THE NEW DESIGN
PROCEDURE
Four design principles will be presented and used to
develop the new design procedure. The rst of these is the
decomposition of a complex column into an equivalent
sequence of simple columns. The second and third are the
use of a reboiler, rather than stripping steam, and the
introduction of thermal coupling to improve the separation
ef
ciency. The last is the use of pinch technology to analyse
the impact of proposed modi
cations on the utility
requirements of the process. These four design principles
are presented below.
2.1 Decomposition of Complex Columns
A complex column can be decomposed into a sequence of
simple columns (Hengstebeck6
, Glinos and Malone7
).
Figure 1 shows how a column with a side-stripper is
equivalent, in terms of its heat and material balances, to a
thermally-coupled indirect sequence of two columns.
Longer sequences are obtained for columns with more
side-strippers (Carlberg and Westerberg8
). The focus here is
on the indirect sequence of columns since it is the
decomposition of a typical crude oil distillation tower.
Other complex arrangements can also be decomposed in an
analogousmanner (Liebmann9
), but are not commonly used
for crude oil distillation.
There are several advantages to using these decomposi-
tion techniques to model a crude oil distillation column.
Firstly, it is easier to understand simple columns than a
complex column, and the impact of design modi
cations is
more clearly observed. Secondly, it is easier to simulate a
sequence of simple columns than a complex column.
Thirdly, better design techniques and short-cut methods
exist for simple columns than for complex columns; these
techniques relate the number of stages, re¯ ux ratio, feed
stage location and product purity.The equivalence between a complex column and a
sequence of simple columns works in both directions, so a
complex column may be decomposed to a sequence of
columns and any sequence of columns may be `merged’ to
form a complex column. Complex columns are traditionally
used for crude oil distillation, as they are generally more
cost-effective and require less space than a train of columns.
2.2 Alternative Vaporization Mechanisms: Steam
Stripping and Reboiling
Light components are removed from the liquid in a