Dividing-wall columns (DWCs)also called
partitioned-wall columnshave been introduced as an
attractive option to reduce energy consumption and capital
costs in distillation processes. However,
risks and worries associated with the implementation of DWCs
hamper the expansion of this technology in
conservative process industries. This article introduces a real
implementation case in which a conventional column was upgraded
to a DWC. It also discusses how the risks in DWC implementation
can be mitigated by establishing contingencies and predicting
the column performance via modeling.
Since Wright first introduced the concept of the
DWC1 in 1949, over 90 commercial-scale applications
have been reported.2 However, the industrial
implementation of DWCs has been slow, despite the potential
benefits and promising results of this technology. Most reported DWC
applications have been attributed to a few leading companies,
namely BASF, Dow and UOP.3 Recently, other companies
have begun to consider this technology more seriously, although
only large companies with sufficient research and development
(R&D) capabilities can evaluate and benefit from such
complex, energy-saving technologies.
The slow acceptance of DWCs is also due to industries
entrenched preference for proven methods over potentially risky
new technologies. Hydrocarbon plant operators generally
prioritize stable production over energy savings. Such caution
is necessary, and it is important that any new system be able
to demonstrate its suitability before it is implemented.
This article examines the implementation of a DWC in an LG
Chem Ltd. plant, with emphasis on the practical risks of the
installation. Extensive modeling studies were used to mitigate
implementation risks, check for human error, and demonstrate
the industrial suitability of DWCs.
Plant specifications prior to DWC installation.
Fig. 1 outlines a 2-ethylhexanol (2-EH) production plant, in
which butyraldehyde (BAL) is synthesized from propylene and
synthesis gas by the oxo reaction. Normal butyraldehyde (NBAL)
and isobutyraldehyde (IBAL) isomers are then separated through
an isomer process, and the NBAL is converted to 2-EH by aldol
condensation. Crude 2-EH is then generated from EPA and
hydrogen by hydrogenation and purified to a final 2-EH product
in an alcohol purification unit.
1. 2-EH production.
The alcohol purification unit consists of two sequential simple
distillation columns: a heavies-cut
column and a lights-cut column. Column-targeting studies were
performed to improve the energy efficiency of the alcohol
purification unit. The studies showed that energy consumption
could be reduced by 5% with a preheater that recovers heat from
the second columns bottom stream. Modification of the
columns interior with high-performance packing did not
yield significant energy savings.
A preliminary feasibility study for installing the DWC was
conducted. The DWC configuration shown in Fig. 2a
retrofit of the existing columnwas proposed. Simulation
of this DWC configuration showed that considerable energy
savings could be achieved at a reasonable installation cost.
Field engineers and plant operators reviewed the feasibility
study, but some were skeptical about the DWCs performance
and reliability. Further technical
evidence was needed to convince the field engineers of the
DWCs merits and to ensure safe operation before the
installation of the DWC.
2. Process flowsheet for 2-EH
purification before (left) and after (right) DWC
DWC with conventional operation.
DWCs involve simple concepts that should be familiar to
chemical engineers. However, plant operators are more familiar
with conventional, two-product column systems. A DWC column may
also be required to switch between DWC operating mode and
conventional operating mode, due to unforeseen circumstances or
maintenance. Therefore, dual
operation was incorporated into the DWC to allow for
conventional operation as a contingency (Fig. 3).
3. A DWC capable of two operating
The operating mode of this DWC can be switched by varying the
valve positions (Table 1). Additional features required for the
contingency mode are vapor-equalizing lines, feed nozzles and
several block valves that are also useful for maintenance.
In DWC mode, the side product is withdrawn from the middle
of the main section. Liquid flow from the overhead is directed
to a liquid-splitting device (i.e., a reflux splitter), and the
two liquid streams are then introduced into the prefractionator
section and main section. Vapor- and liquid-splitting ratios in
the dividing-wall section are the most important variables in
DWC operation.4 Liquid splitting can be easily
adjusted using the reflux splitter, making it suitable as a
manipulated operating variable. Vapor splitting, however, is
determined by the columns interior and the pressure drop
of each dividing-wall section, making it a design variable that
cannot be freely adjusted after column installation. Therefore,
the columns internal design must be based on an accurate
prediction of the hydraulics in the partition section to ensure
that the vapor-splitting ratio follows the design. Ensuring the
integrity of the column requires close collaboration with
internal suppliers and other vendors at every stage of the DWC
During contingency operation, the side draw is not used and
liquid flow from the column overhead directly enters each
partitioned section. A specially designed distributor can allow
dual operation by evenly distributing liquid into each
partitioned section. The difference between the pressure drops
of the prefractionator and main sections should be minimized,
and the two sections should act as one in the contingency mode.
An equalizing vapor line must be present to alleviate pressure
differences. It is possible to predict pressure drops in the
partitioned section with hydraulic correlations from the column
internal suppliers and through rigorous computational fluid
dynamics (CFD) analysis. Simple correlation is generally
sufficient to calculate the pressure drop of each section to
Also, as the arrangements of nozzles and pipes are
complicated and potentially confusing, a 3D visual model was
created to help reduce human error in construction (Fig. 4).
4. Three-dimensional models of the
Predicting column performance.
Installing a partitioning wall does not guarantee energy
savings. The benefits are system specific and depend on the
properties of the separation system and the required products.
Potential benefits should, therefore, be rigorously estimated
through an extensive case study. Simulation software can be
used to design a DWC and to predict its performance using
built-in, thermodynamic physical properties. For an existing
column, plant operations data can be used to reassure the reliability of the physical and
thermodynamic property methods selected. Design
variablessuch as total number of trays, feed and product
locations, and liquid- and vapor-splitting ratios in the
partitioned sectioncan be determined by simulation. Construction costs should be
estimated with regard to site-specific concerns.
The concept of a DWC with dual operating modes can be
realized without greatly increasing the installation cost.
Before implementing the concept, the performance of each
operational mode of the column was predicted through rigorous
modeling. Fig. 5 shows column temperature profiles for the two
operational modes. In DWC mode, the temperature profile of the
prefractionator section differs from that of the main section,
while they are identical in contingency mode.
5. Temperature profiles for DWC
and contingency modes.
The vapor-splitting ratio in the partitioned section is
uncertain and cannot be adjusted freely once the column is
installed, while the liquid-splitting ratio can be easily
manipulated during operation. Therefore, to assess the effects
of unexpected performance deteriorations, sensitivity studies
are required that examine possible variations of the actual
vapor-splitting ratio. Temperature profiles are shown for
vapor-split ratios in DWC mode (Fig. 6) and contingency mode
(Fig. 7). In the example column in Fig. 6, performance was not
greatly affected by changing the vapor-splitting ratio. Product
quality remained within the product specifications for the
vapor-splitting ratios tested.
6. Temperature profiles for different
vapor-split ratios in DWC mode.
7. Temperature profiles for different
vapor-split ratios in contingency
The effects of changes in feed composition, the
liquid-splitting ratio and other operating variables were
modeled to ensure that performance and product quality could be
maintained within acceptable ranges. Nominal operating
conditions were set and reviewed before startup. The
columns temperature profile during operation was
well-matched to the simulation results.
DWCs generally lack operational flexibility. If a plant is
not operating with a normal load during startup or shutdown,
the column should be operated with a considerably lower
feedrate. DWCs use less energy, implying that vapor and liquid
loadings are lower than in conventional columns. With lower
loads, internal flows can be increased by raising the boil-up
and reflux rates, at the cost of increased energy use. The
example column was successfully operated at below half of the
normal load without showing problems during startup.
Foundation reinforcement, instrumentation.
In this installation, the bottom section of the
existing column could be used. The only problem related to the
implementation was ensuring sufficient structural strength for
the increased height and weight of the column after
retrofitting. To evaluate the possibility of reusing the
original column, its flooding factor was checked to analyze its
hydraulic performance. Since the vapor load is generally
proportional to the required reboiler duty, the existing column
section, including the internals, could potentially be used in
the retrofit. If modification is necessary, the first option is
modifying the internals, which does not require major changes
to the columns bottom section, including the reboiler and piping. To reduce
downtime, the rest of the column section was produced in the
shop and welded to the bottom section at the plant site. The
installation work was completed within 12 days.
The increased number of temperature sensors helped engineers
monitor the column operation and detect any abnormal behavior.
The retrofitted column consisted of seven packing sections,
with two to four temperature sensors installed in each. The
Plant Information System (PIS) and the Distributed Control
System (DCS) screens were modified, along with additional
instruments and control loops.
Operator training and troubleshooting.
The concepts of the DWC, its operational modes, and
the new equipment were explained to the plant operators, as
were the startup and shutdown procedures. The operators
understood the concept and benefits of DWCs, and their
questions were clarified with 3D models and simulation results.
The help of operators and plant engineers was essential to the
projects success at all
stages, from the preliminary study to the actual
implementation. Thorough and clear communication reduces
unnecessary time losses and risks during implementation. In
most cases, process engineers who design DWC columns are not
aware of site-specific concerns.
A DWC with two operating modes was designed and studied to
minimize operational risks and to ensure a safe retrofit to an
existing column. The DWC was then installed in an LG Chem Ltd.
plant, and it has been operating successfully since startup.
Computer modeling was an important factor in establishing the
DWC, as these columns require extensive analysis.
The existing conventional column was easily upgraded to a
DWC through the addition of a partition wall. This upgrade was
achieved in an acceptable period of time. The proposed DWC
arrangement can be used in the revamp of other single
conventional columns, allowing for significant reductions of
energy expenses at a reasonably low capital cost.
The idea of a dual-mode DWC alleviates plant
personnels concerns about the potential risks of DWC
implementation and thus contributes to the greater application
of DWCs in the hydrocarbon industry. HP
This research was supported by an Energy Efficiency and
Resources grant from the Korea Institute of Energy Technology Evaluation and Planning
(KETEP), funded by the Korean Government Ministry of Knowledge
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