Chemical Engineering August 2014 - 42

Julius Montz
Cover Story
for instance, if a four-product DWC
is considered in conjunction with the
separation of a wide boiling feed.
Regarding the practitioners reluctance
to consider design and
implementation of four-product
DWCs, there are certain process
and mechanical design and operation-related
concerns, which require
a thorough consideration. By applying
adequate solutions, a DWC
could become a cost-effective alternative
for conventional sequences,
both as a new design or a retrofit
option. What we aim at here is to
address the potential for further expansion
of the application window,
and convince the readers that the
implementation and versatility of a
DWC is not as limited as generally
believed (that is, as suggested in extensive
literature, which contains
mainly redundant information).
New openings
Upon reading the most recent publication
by Staak and others [15],
which describes a highly successful
application of a multi-purpose DWC
in a Lonza chemicals processing
plant in Visp, Switzerland, we may
say that a technology breakthrough
has occurred that will mark another
milestone in the development
of DWC technology. Among other
periodic operations, this DWC has
also taken over the function of a
batch distillation column, enabling
both higher yield and higher bottom
product purity, thanks to a large
reduction of bottoms temperature.
The added benefits, as experienced
in this case, are expected to be more
than appealing to move others to
look for similar applications.
The first four-product DWCs reflect
the simple, single-partition
wall configuration proposed by Kaibel
in 1987 [9], which is shown schematically
in Figure 4. As described
elsewhere [8], this configuration
appeared to be practical and within
the range of existing know-how and
experience, but, thermodynamically
it is not optimal. In the space in between
the two side-product drawoffs,
a certain amount of remixing
of the medium-boiling components
occurs, resulting in entropy forma100
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1985
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Figure
3. This graph shows the number of DWCs delivered by a German vendor
over the years
2005
2010
2015
D (C5-C6)
A
C2
F
B
C
Feed
C1
S1 (BRC*)
S2 (Toluene)
C3
D
Figure 4. A single-partition
wall, four-product DWC, also
known as a Kaibel column, is
shown here
* Benzene-rich
component
B (Heavies)
Figure 5. Shown here is a typical sequence of
three simple-distillation columns for the separation
of an aromatics mixture into four product streams,
considered in the present study
tion, that is, a less energy-efficient
operation than is achievable with a
configuration that employs full thermal
coupling.
A logical next step would be to consider
implementation of more complex,
fully thermally coupled DWC
configurations for obtaining four
products, to maximize potential energy,
capital and plot area savings.
However, the chemical process industries
(CPI), particularly the largescale
sectors, like petroleum refining,
gas processing, petrochemicals and
chemicals manufacturing industries,
42 ChemiCal engineering www.Che.Com august 2014
although under heavy pressure to
comply in the short term with legislation
for the reduction of CO2
emissions - which can effectively
be reached by implementing DWC
and other energy saving distillation
technologies - seem to be reluctant
to make a decisive step in that direction.
The reason for this may be a
lack of established design procedures
and fear of unstable operation.
Realizing that this may be a
major burden, we have recently
undertaken, in a cooperative effort
with I.J. Halvorsen of Sintef ICT
Number of delivered DWCs
http://www.Che.Com
Chemical Engineering August 2014

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