Chemical Engineering August 2014 - 41

Julius Montz
Chronology of
DWC teChnology
F
Figure 2. These photographs show (a) a reflux splitter, (b) an installed semi-circular
narrow trough liquid distributor, and (c) the layout of the top layer of structured
packing installed in a partitioned section of the column
tor above. The insert beside Figure
2b shows a drawing of a narrow
trough with drip tubes containing
drip point increasing legs (Montz
type S). Such distributors are used
for very low specific liquid loads (<1
m3/m2h) as encountered in deep
vacuum applications, and have been
found to perform well.
One should note that in a DWC
there are at least six sections, which
may differ considerably in liquid or
vapor (or both) loads. A ring welded
to the column wall is used to position
and fix the distributor, and an
inspection manway is placed in the
partition wall. The liquid leaving
the lower beds from the prefractionator
and main column sides is collected,
mixed and delivered to the
bed in the conventional bottom section
of the column.
Figure 2c shows the top view of
a semicircular packing layer consisting
of tightly packed segments.
Depending on the nature and operating
conditions of separation, both
gauze and sheet-metal packings
are used. The former, usually with
a specific geometric area of 500
or even 750 m2/m3, are preferred
in demanding separations under
deep vacuum, while the common
choice for moderate vacuum and
near atmospheric applications are
corrugated-sheet-metal structured
packings with surface areas of 200
to 350 m2/m3, in both conventional
and high-performance versions.
Regarding design and operation,
a distinguishing feature of a DWC
is the so-called " vapor split, " that is,
the distribution of vapor ascending
from the conventional bottom part
of the bed into two streams, one
entering the prefractionator and
another entering the main column
side. This occurs spontaneously,
and the resulting vapor flowrates
correspond to those required to arrive
at the same pressure drop on
both sides of the partition wall. A
prerequisite for good functioning
of a DWC is that the vapor split arranged
by hydraulic design in conjunction
with fixed liquid flowrates
provides vapor flowrates that will
comply with the liquid-to-vapor ratios
required to accomplish the desired
separation at the prefractionator
and main column sides.
Proven advantages
As proven in many industrial applications,
a three-product DWC
enables, on average, 30% saving in
energy and an equivalent saving in
capital, as well as a considerable reduction
of required plot area compared
to conventional two column
sequences [4-6]. Other potential
benefits include reduced thermal
degradation of sensitive products,
often increased product quality and
recovery in case of specialty chemicals,
reduced number of equipment
to control and maintain, and more.
Knowing all of the potential benefits,
it is strange to see that nearly
thirty years since the first industrial
application of a DWC (see box,
right), the number of installations
is still relatively quite small - approximately
200, which is practically
negligible compared to the
number of distillation columns in
operation worldwide.
The applications described in
the box, and many others - even
with highest, electronics-grade purity
requirements - have proven
that a DWC, although atypical, is
just a distillation column, arranged
in a more compact and direct way
than is the case with two- or threecolumn
sequences used throughout
the process industries to obtain
three products of desired purity.
Full thermal coupling, as employed
in a DWC, will always ensure an enigure
3 shows the number of DWCs
delivered over the years by a German
vendor. One can see in Figure 3
that the first two DWCs were delivered in
1985, and from 1996 the number of applications
is growing faster steadily. The
point of onset of a stronger increase in the
number of deliveries coincides with the
adoption and implementation of DWCs
with non-welded partition walls. This can
be considered as the first milestone in the
development of this technology.
Indeed, by adopting non-welded wall
technology, it became possible to place
the partition in an off-center position,
which allowed accommodation of separations
with much bigger variations
in composition and relative volatilities
of components as well as two phase
feeds than before [7]. Even more, it
enabled cost-effective design and construction
of the first DWC for obtaining
four products in one shell [8]. Such a
DWC configuration, known generally
as " Kaibel column " , was introduced
in 1987 by G. Kaibel [9] and installed
for the first time in a BASF SE plant in
2002 [8]. These were equipped with
structured packings.
The first and very successful revamp
of a conventional side-product pyrolysis
gasoline fractionation column using
structured packings was reported by
Uhde (now ThyssenKrupp Industrial Solutions)
in 2000 [10, 11]. A 7-min video
showing this project can be found on
YouTube, under " The Divided Wall Column " .
Another application success story
from Uhde is the first DWC accommodating
Morphylane extractive distillation
process [11].
The first tray DWC was put into operation
at a Sasol plant in South Africa in
2000 [12]. With this another milestone
was reached, and the DWC made an
inroad into petroleum refining world
dominated by large scale applications.
Interestingly, the second tray DWC installed
at Sasol, with an internal diameter
of 5.2 m and a tangent-to-tangent
height of 100 m, is one of tallest distillation
columns ever built. Other refiners
followed soon, reporting successful revamps
of typical side-product columns
including in one case off-center position
of a non-welded partition wall [14]. ❏
ergy saving that is approximately
equivalent to the energy required
by the smaller of two reboilers employed
in the conventional sequence,
provided the nature and conditions
of the separation being considered
will not make it unfeasible. Indeed,
this may appear so in certain cases;
ChemiCal engineering www.Che.Com august 2014 41
http://www.Che.Com
Chemical Engineering August 2014

Table of Contents for the Digital Edition of Chemical Engineering August 2014
