Chemical Engineering August 2014 - 44

Table 2. Dimensions, inTernals, operaTing pressures anD
Cover Story
with complex tray and structured
packing arrangements should be
able to arrive at best solutions in
given situations.
A non-welded wall, where appropriate,
enables easy and precise
assembly of packed beds of various
shapes separated by partition
walls [7]. Namely, the installation
progresses as in a conventional column,
and the partition wall is also
assembled progressively by adding
new elements. These are in dimensions
that are easy to handle and
quite light, because the thickness of
the non-welded partition wall can be
as low as practical - usually 1.5-2
mm. Packing elements adjacent
to the partition wall are equipped
with robust wall wipers that, in addition
to scrapping the liquid from
the walls, also serve to fix the partition
wall in place. This implies that
installed beds can also be easily removed,
if required in case of troubleshooting
or a revamp.
An important advantage of a
non-welded partition wall is avoidance
of potential welding-related
problems (thermal stresses, flatness
of the wall), which may become
pronounced when a partition wall
needs to be welded in an off-center
position. On the process side, the
main concern is the possibility of
product(s) contamination, by allowing
small amounts of vapor or liquid
(or both) on the wrong side of the
partition wall. This could be avoided
by using sealing strips of appropriate
material to fill the gap between
the partition wall and the column
wall, which additionally can compensate
for common shell-diameter
deviations. In critical sections, like
feed and product draw-off zones,
welding a short section of partition
wall could be considered as a safe
measure. However, without enough
experience in this respect, separations
involving parts-per-million
(ppm) and parts-per-billion (ppb)
purity requirements may require
welding of the partition wall over
its entire length, which is more
demanding and costly than no- or
partial-welding approaches. One
should note that even in case of a
non-welded partition-wall instalpressure
Drops of convenTional Three-column sequence (c1/
c2/c3), single-parTiTion, Three-parTiTions anD Two-parTiTions
Dwcs consiDereD in This sTuDy
C1/C2/C3
Top pressure, bar
Reboiler duty, MW
Energy saving, %
Stage requirement, No.
Sieve trays, No.
Shell height, m
Shell diameter, m
Packed beds
Shell(s) volume, m3
Shell volume saving, %
Pressure drop, bar
*Main column stage count
lation, fixing liquid collectors and
distributors will require a certain
amount of welding activities, including
the partition wall in redistribution
sections, as well as
rings and other local points on the
column walls needed for auxiliaryequipment-fixing
purposes.
Figure 9 shows a drawing illustrating
the pressure drop situation
in partitioned sections of the DWC
shown in Figure 7. The pressure
drop balances according to Equations
(1) and (2) need to be arranged
for the most representative operating
condition during the design
phase. The inevitable differences in
individual pressure drops of packed
beds in parallel sections are balanced
by adjusting the free area of
liquid collectors accordingly. As seen
in Table 2, the pressure drop associated
with operating packed singleand
multi-partition wall DWCs is
rather low. Details on hydraulic design
of these configurations can be
found elsewhere [20].
For those reluctant to consider the
four-product DWC shown in Figure
7 as a new design, the associated
uncertainties and potential risks
could be lessened if the required arrangement
would first be tested in
a revamp of an existing column sequence.
Where appropriate, transforming
an existing three-column
sequence into a DWC would allow
energy savings equivalent to that
achievable in new designs [21]. Indeed
the economic and environmental
incentives are so strong that a
four-product DWC should definitely
44 ChemiCal engineering www.Che.Com august 2014
be considered as a retrofit option for
existing three-column sequences.
Revamp and retrofit options
As previously mentioned, revamps
of conventional distillation columns
into a DWC have been realized in
practice; however this was in natural
situations, namely, with conventional
side-product columns [10,
13, 14]. However, this could also be
done with common two-column sequences
[21] with the same effects,
that is, expected energy savings accompanied
by a considerable capacity
increase. One should not forget
that the reduced energy requirement
is equivalent to the reduction
in boil-up rate - in other words, a
correspondingly reduced vapor flowrate.
This implies a reduced shell
diameter in new designs, and in
case of existing columns, ensures a
corresponding capacity increase. If
original columns contain trays and
these could be replaced with structured
packings, then the chance
is large that all required stages
will be accommodated in one shell.
This means that one of the original
columns transformed into a DWC
would replace two conventional columns,
leaving one column and auxiliaries
available for other purposes.
If the same top pressure is used, a
packed DWC will have a much lower
bottom pressure and temperature;
that is, an increased vapor volume,
which means that the bottom stage
will be limiting and will dictate the
extent of the potential capacity increase.
However, this concern is the
1.7/2.7/1.013
3.8/3.1/3.1
-
40/38/38
61/59/59
40.5/39.5/39.5
2/2/1.8
-
352
-
0.31/0.27/0.24
Single
partition
2.5
5.7
43
Three
partitions
2.5
4.81
52
-
Two
partitions
2.5
4.81
52
169/129* 202/130* 174/130*
-
-
68.6
2.2
10
261
26
0.114
69
2
13
216
39
0.117
69
2
11
216
39
0.105
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Chemical Engineering August 2014

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