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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