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    stripper or side-stripper by vaporization. In a crude oil
    distillation column,material is commonly vaporized using a
    reboiler or by injecting live steam (steam stripping). The
    fundamental principles of vaporization are quite different in
    the two cases, so the choice of stripping medium has a
    signi
    cant impact on the utility requirements of the column.
    In a steam-stripped column, superheated low-pressure
    steam is used. At these conditions the steam stays in the
    vapour phase. The presence of the steam reduces the partial
    pressure of the hydrocarbons in the vapour phase and
    therefore reduces the vaporization temperature of the
    mixture (Gary and Handwerk10
    ). Energy for vaporization
    is obtained not from the steam, but from the liquid to be
    vaporized. This results in an inverse temperature pro
    le in
    the stripper, where the liquid on the bottom stage is cooler
    than the liquid on the stage above it, and the bottoms
    product is below its boiling point.
    Steam stripping changes the partial pressure of the
    hydrocarbons in the mixture, as approximated by Dalton’ s
    law:where NHC is the molar ¯ ow rate of hydrocarbons in the
    vapour phase, NSt
    is the molar ¯ ow rate of steam (which is
    assumed to be entirely in the vapour phase), pHC is the
    partial pressure of the hydrocarbon mixture, and pT is the
    total pressure of the system.
    From equation (1) it can be seen that the effectiveness of
    the steam in reducing the partial pressure of the hydro-
    carbons diminishes as more steam is added to the column. In
    a reboiler, on the other hand, the amount of crude oil
    vaporized is roughly proportional to the reboiler duty.
    Figure 2 compares the fraction of crude oil vaporized by
    steam stripping and a steam-heated reboiler for a range of
    steam ¯ ow rates, showing how steam stripping is only more
    ef
    cient than reboiling where a small fraction of the
    material is required to vaporize.
    Reboiling is generally more energy-ef
    cient than steam
    stripping, but its impact on heat recovery opportunities,
    product quality and investment costs is also a factor. Since
    the temperature of the heat source required is signi
    cantly
    higher when using a reboiler than when stripping steam is
    used, this reduces heat recovery opportunities. The higher
    product temperature has a positive side-effect for heat
    recovery, since cooling of the product provides a larger heat
    source. Table 1 summarizes the effects of using a reboiler,
    rather than steam stripping, on the potential for heat
    recovery. Furthermore, since there is less steam to condense
    when using a reboiler rather than stripping steam, the
    condenser duty is decreased.
    When a reboiler is used, rather than stripping steam, a
    smaller column may be required, since a reboiler results in
    less vapour traf
    c in the column; however a reboiler is more
    costly to install. The product quality may be compromised
    when a reboiler is used; more coke formation, cracking and
    product decomposition may be incurred as a result of the
    higher temperature at the bottom of the column.
    Reboiling and steam stripping can be combined, for
    example by using a steam-agitated reboiler. The relative
    amounts of live steam and reboiling should be selected
    according to heat recovery opportunities, utility costs,
    column capacity constraints and investment costs.
    2.3 Thermal Coupling
    In thermally coupled columns, the ¯ ow of material from a
    downstream column to an adjacent upstream column
    provides some of the necessary heat transfer by direct
    contact. Thermal coupling can reduce signi
    cantly heat
    duties in a sequence of columns (Tedder and Rudd11
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