Chemical Engineering January 2022 - 25

northern location, large flows of cold
water will require treatment seasonally.
This necessitates a robust system
with predictable operation and
minimal upsets that can offer quick
ramp-up following the spring freshet
and quick turn-down prior to the
freeze-up of water in fall.
Construction of the 1,000 gal/
min capacity Selen-IX plant was
substantially completed in fall 2019
but freezing temperatures delayed
commissioning to spring 2020. Performance
testing of the plant concluded
in September 2020 and from
there, the plant operated continuously
at the maximum design flow,
where it met performance expectations
and produced effluent containing
<2 ppb selenium.
HONOR ACHIEVEMENT
Dow Deutschland:
Walocel™ M120-01
Cellulose ethers are vital additives
for dry mortar cementitious-tile
adhesives, because they help ensure
mortars retain water, which is
required for durable bonding between
tile and substrate. The waterretention
efficiency improves with
increasing solution viscosity, which
scales with polymer molecular
weight. To be economical, the cellulose
used to synthesize the polymer
is preferably pulp from wood
sources and thus the polymer chain
length is limited by nature.
Launched in 2020, Walocel M12001
Cellulose Ether is the first scaled
commercial product obtained by
long-chain branching. Overcoming
nature's limitations in molecular
weight is a step-change innovation
compared to standard cellulose
ethers, enabling dosage reduction in
mortars by up to 25% without compromising
performance.
Production process. Walocel cellulose-ether
production involves
cellulose grinding, batch-wise
etherification, purification with hot
water, drying and milling (Figure 3).
Etherification of the cellulose is a
three-phase heterogeneous reaction
of cellulose, where surface area,
solid-liquid volume ratio, and mixing
speed are critically important. As
the target was to long chain branch
the high-molecular-weight cellulose
chains without causing
significant gelation,
new process technology
was needed. The challenge
during the product
development was to
introduce the crosslinker
into a well-established
process and achieve its
homogenous distribution
resulting in high reaction
yield and optimized
batch run rates.
From polymer science
literature it was known
that long-chain branching reactions
are probabilistic processes dependent
on crosslinker concentration,
crosslinker end-group reaction kinetics,
and the parent polymer molecular
weight distribution. The key is to
promote the probability of a single
graft between two parent chains versus
other outcomes. While the simplest
modeling conclusion is to use a
very dilute crosslinker concentration,
this does neither comprehend the
myriad possible outcomes nor ensure
a successful process or a viable
production window.
Scaleup. After successful testing
of first laboratory-scale high-molecular-weight
prototypes, systematic
process investigation focused on
key reaction engineering variables
started. At laboratory scale, dilute
additions can be supported by high
shear mixing rates; at production
scale, this option is not possible. At
laboratory scale, heats of reaction
are more easily managed as the vessel
surface area to volume ratio is
large compared to production scale.
Managing reaction heat and reaction
pathways, while not significantly reducing
run rates, is much more challenging.
Other approaches to maximize
crosslinker dispersion needed
to be developed using analytical
tools that enabled following postreaction
outcome versus process
parameters. These efforts demonstrated
a viable production process
window that delivers water-soluble
cellulose ethers confirmed by systematic
in-house and users' mortars
application tests.
Production-plant-scale
implementation
of Walocel M120-01 Cellulose
Ether required design and
CHEMICAL ENGINEERING WWW.CHEMENGONLINE.COM
JANUARY 2022
Dow
FIGURE 3. The production steps for producing Walocel celluloseether
are shown here
installation of a tailor-made dosage
system and changes to mixing
speed and reactor solid-liquid volume
ratios. The need for homogenous
distribution of reactants in
the heterogeneous reaction mixture
and process safety requirements
also impacted the design.
The combination of spray-nozzle
design, understanding of the mixing
behavior inside the ploughshare
reactor depending on the filling level
and swelling behavior of cellulose
raw material during the process,
process automation
strategy and
the measurement instrumentation
were key engineering parameters for
a successful and reproducible process
development on plant scale.
Without negative impact on the reaction
itself, as well as the reaction
temperature profile, the controlled
crosslinking step using a bifunctional
diglycidyl ether could be successfully
added into the high-quality and
highly cost-optimized cellulose ether
reaction process.
The production of Walocel M12001
Cellulose Ether required addition
of a new reactive substance to an
existing reaction process. Because
cellulose ethers are produced under
pressure and the final product is
water washed to remove byproducts
from etherification, any new reactant
must be thoroughly studied
for both reaction and environmental
safety. To ensure the highest level
of process safety, a stepwise process
development combined with a
structured management of change
(MOC) process was applied. Once
identified, suitable substance
classes with low chemical hazards
were evaluated in cooperation with
Dow's Reactive Chemicals group
25
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Chemical Engineering January 2022

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