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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
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