Chemical Engineering August 2014 - 43

Table 1. base case for figure 5
Feed
C5-C6
(F)
Flowrate (ton/h)
Mass fractions
n-Hexane & lighter
Benzene
3-Methylhexane
Toluene
Ethilbenzene & heavier
*BRC = Benzene-rich component
31.7
0.2517
0.0855
0.0204
0.2474
0.3950
(A)
7.4
0.9869
0.0131
-
-
-
BRC*
(B)
3.9
0.1642
0.6750
0.1608
-
-
Toluene
(C)
8.0
-
-
0.0026
0.9718
0.0256
Heavies
(D)
12.4
-
-
-
0.0061
0.9939
of a two-partition-wall DWC in this
and similar situations.
Figure 7 shows a detailed drawand
S. Skogestad of the Norwegian
University of Science and Technology
(NTNU; Trondheim, Norway), a
joint research effort to thoroughly
evaluate the design and operation
of various feasible configurations of
a packed four-product DWC using
an industrially relevant aromatics
plant as a base case [16-18]. Figure
5 shows the base case configuration
considered, and Table 1 contains a
summary of feed and product compositions.
The hydraulic design
and packed-column dimensioning
methods for three- and four-product
DWCs used in these studies are described
in detail elsewhere [19, 20].
According to detailed simulation
studies summarized in Ref. 18, both
energy and capital savings in excess
of 50% appear to be achievable.
Such a high potential for reduction
of CO2 emissions and increased
competitiveness creates a strong incentive
to consider implementation
of four-product DWCs in practice.
Coupled four-product DWC
Minimization of energy requirements
in the case of a four-product
separation implies employing a
Petlyuk or full thermal-coupling
arrangement, which requires an internal
configuration with three sections
in parallel as shown in Figure
6a. Such a complex configuration
with three liquid and three vapor
splits has not yet been attempted
in practice.
As mentioned before, for a given
liquid split, vapor splits are set by
the amount of flow resistance arranged
during the design, and the
flowrates leading to pressure-drop
equalization should comply with
those required by separation (a
fixed L/V ratio for each of sections in
parallel). These could be influenced
to a certain extent by manipulation
of liquid flowrates, but proper control
would be possible only if provisions
could be made to influence
vapor flows during operation. Such
devices are not yet available commercially
and some indication on
developments in that direction can
be found in the patent literature.
Confronted with this, the previously
mentioned research consortium
has considered various options
and arrived at a considerably simpler
internal configuration, shown
schematically in Figure 6b, which
is equivalent to the fully thermally
coupled one (Figure 6a), but includes
only two vapor splits. Table 2
contains basic performance data as
obtained for conventional and three
four-product DWC configurations
shown in Figures 4, 5 and 6. The
energy and column-volume saving
numbers speak for themselves, indicating
that even a non-optimal, but
proven single-partition DWC will
bring impressive gains compared to
the conventional sequence.
More competitive in this respect is
a fully thermally coupled, multi-partition
DWC, which requires 15.5%
less energy and 16.5% less volume
than a single-partition, four-product
DWC. So in the present case, there
is no doubt whether to go for a DWC,
but the question is for which one. Although
significant, the difference in
related capital cost savings (for details,
see Ref. 18) may appear to be
the less important argument here
than financial benefits for years
to come based on total cooling and
heat-input savings. These are more
than appealing and should justify at
least a serious consideration of design
and practical implementation
ing of this column, including auxiliary
equipment, indicating that
the zone above the feed containing
three sections in parallel is rather
short, with bottoms of three beds at
the same level. The middle one is
a narrow bed, which is taller than
that on the prefractionator side and
shorter than that on the main column
side. Such a demanding configuration
could be assembled as a
packed column, using existing nonwelded
technology know-how and
means utilized during construction
of single-partition-wall, four-product
DWCs [7, 8].
Figure 8a shows the top view of
cross sectional areas at three characteristic
elevations. In case of offcenter
positioning of the partition
wall, these sections can be smaller
and/or larger than a half-circle,
while in the column segment with
three sections in parallel the crosssectional
area of the middle bed is
practically rectangular. Those feeling
uncomfortable with this layout
could consider, for this segment of
the DWC, a concentric column arrangement
(see Figure 8b) with the
middle column bed placed in the
inner column, and prefractionator
and main column sections placed
in the annular spaces of the outer
ring. Both packings and trays can
be made to fit into the given form,
but special attention needs to be
paid such that flow patterns of
phases are arranged to resemble
that associated with common practices.
In the case of trayed DWCs,
layout of the tray (that is, placing
downcomers and arranging favorable
flow paths) may become a serious
challenge. In the case of packed
DWCs, the partition walls introduce
additional wall-zone area. To
avoid potential performance-deteriorating
wall effects, structured
packings need to be equipped with
effective wall wipers, and for trays,
downcomers need to be placed in
dead zones and so on. However, all
this belongs to established distillation
column know-how and design
practices, and designers involved
ChemiCal engineering www.Che.Com august 2014 43
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Chemical Engineering August 2014

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