Chemical Engineering August 2014 - 46

Cover Story
one partition wall. In other words,
in the present situation, it appeared
possible to avoid having three sections
in parallel, which reduces construction
and operation (one vapor
split eliminated) effort to the level
required when dealing with conventional
three-product DWCs.
This was achieved by simply placing
the prefractionator rectification
section bed, which is in the new design
on the level of the middle and
main column sections, into the top
of the high pressure column. This,
in turn, allows significant enlarging
of the cross-sectional area of the
middle and main column sections
that are placed in the bottom of the
low pressure column. This appeared
to be sufficient to compensate for expansion
in vapor volume associated
with a drop of 0.7 bar in operating
pressure. A more detailed elaboration
of this revamp case, indicating
a rather short payback time (around
1 year), can be found elsewhere [21].
With energy savings turning into
gains in the second year, a significant
increase in profitability of this and
similar plants could be expected.
Although the above gains are
the results of preliminary evaluations,
they are highly encouraging
and provide incentive and motivation
for a more detailed engineering
step, namely, to identify and consider
additional complexities and
constraints associated with such a
revamp, which could emerge as a
reason to avoid it. First of all, the
time available for such a revamp
may be insufficient to arrange
modifications of two existing tray
columns in order to accommodate
a packed DWC. Trays and tray
support rings need to be removed,
which is a routine but demanding
and time-consuming activity inside
the shell. Also, to accommodate such
a complex configuration with packings
and auxiliary equipment, a lot
of local welding inside the shell will
be needed to provide the required
structural strength and support for
numerous packed beds, liquid collectors
and distributors. Utilizing
a non-welded partition wall would
minimize effort and time needed to
install partition walls and packed
(a)
(b)
Figure 8. Presented here are the cross-sectional areas of partitioned sections as
encountered in the four-product DWC shown in Figure 7 (a), and an alternative layout
for column segment with three sections in parallel (b)
beds in a quite confined space.
Also it is certain that a number
of additional manholes will be required
to allow installation and access
to redistribution sections for
future inspection and maintenance,
as well as nozzles for feed, side
draw-off, and for liquid and vapor
traffic between the two columns and
instrumentation. Also, the pressure
drop associated with the transport
of vapor from the first into the second
column has not been accounted
for as a part of pressure-drop balance
determining the vapor split
in the first column. This and other
potential uncertainties need to be
evaluated properly and accounted
for during detailed process and mechanical
design phase.
Indeed, the methods developed to
allow conceptual design and detailed
dimensioning of packed three- and
four-product DWCs can be used to
get a quite realistic picture of a DWC.
However, each case is different and a
thorough detailed technical analysis
is required to see whether a new design
or a revamp is a good option.
Closing thoughts and outlook
As mentioned before, a DWC is an
unusual distillation column that
uses conventional equipment in a
complex arrangement, which introduces
additional difficulties and uncertainties
related to design and operation.
Going from well-established
three- to less known four-product
separations makes everything more
difficult, and potential constraints
need to be evaluated properly. These
constraints, however, may differ for
new designs and revamps, as well as
for packed and tray columns.
If we consider a new design, then
a DWC will always be taller than
any of the individual columns from
the original sequence. In demanding
separations, a rather tall column
may be required, which, in conjunction
with a small diameter (capac46
ChemiCal engineering www.Che.Com august 2014
ity), becomes a mechanical design
and installation concern. On the
process side, this means a larger
pressure drop, which dictates the
temperature spread between the top
and bottom. This will be much more
pronounced in a trayed compared to
a packed DWC, and will depend on
the spread of boiling points of the
feed mixture. This implies that closeboiling
mixtures, similar to that considered
in our base case, lend themselves
better to separation in a DWC
than wide boiling mixtures. C1 to
C5+ components, as encountered in
natural-gas-liquids (NGL) treating
and ethylene manufacturing plants
[22], are good examples.
If single-pressure operation is not
feasible, there are a number of alternative
energy-saving options to be
considered. In a four-product case, a
common, three-product DWC could
be connected in series with a conventional
column. In three-product
situations, the prefractionator and
main column side could be accommodated
in two thermally coupled
shells operated at different pressures.
This will ensure the same energy
savings, but in a less cost-effective
way. Another, less efficient but
still highly rewarding alternative
could be a pressure cascade, with a
high-pressure condenser serving as
a low-pressure column reboiler.
The temperature spread between
the top and bottom becomes a problem
if this requires a switch from
cheaper to more expensive cooling
or heating media. Unlike conventional
columns, DWCs also exhibit
a lateral temperature gradient in
partitioned sections, which if excessive
(say above 30K) may become a
problem. Heat transfer across the
partition wall may adversely affect
the performance of both trayed and
packed columns. In trayed columns,
excessive vaporization can occur on
the tray deck and in the downcomers.
The only remedy is to consider
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Chemical Engineering August 2014

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