Chemical Engineering December 2022 - 38

FIGURE 2. This reaction graph shows the hydrogen uptake rate in
relation to the reaction time. The blue curve describes an ideal behavior
(non-linear), and the red curve gives an example of a linear
hydrogen uptake. The formula in the diagram gives an example of a
typical second-order reaction
similar when moving the process to
larger scales. Moreover, reactions
are usually not conducted under
well-defined mixing conditions.
Often, laboratory reactors or other
reaction vessels are not equipped
with baffles, so the contribution of
vortex gassing will be much higher
compared to larger scales. Thus, it
is recommended to follow up a successful
feasibility study with pilot
testing at the multi-liter scale, which
is described in the next section using
a 5-L pilot testing example (Figure 1).
Pilot testing
Frequent mistakes that are encountered
at the milliliter scale, such as the
incorrect assumption of the impeller
flow regime or Reynolds number,
which result in over-estimation of the
viscosity, excessive volume-specific
power inputs or not accounting for
the importance of the ratio of vessel
wall area to reactor volume, can be
adjusted during pilot testing at the
5-L scale.
Hence, the key aim of pilot trials
is to reproduce the results obtained
under well-defined mixing conditions.
Typically, only the process data collected
under these conditions will
provide useful information for a safe
and reliable scaleup and de-risking.
Moreover,
scale allow process-parameter optimizations,
such as the improvement
of the chemical conversion or finetuning
of the selectivity.
A major challenge during the pilot
trials is to identify possible limitations
to the production scale in terms of
mixing time, mass transfer and particle
suspension. These limitations can
then be considered in the agitator, re38
actor
and plant design.
For this reason, it is
advantageous to have
the possibility of investigating
chemical reactions
starting from intermediate
sizes of 5 L up
to approximately 60 L.
A constant hydrogen
uptake rate over the
batch time clearly indicates
that the hydrogenation
is carried out
in mass-transfer-limited
conditions. Hence, it is
advantageous that pilot
reactors are equipped
with mass-flow controllers for an
accurate measurement of both the
hydrogen uptake rate and the cumulative
total hydrogen uptake. A linear
hydrogen uptake usually shows that
the reaction is limited when compared
to the typical S-type behavior
of a reaction (Figure 2).
In
hydrogenation
reactions,
the
pilot studies at the 5-L
limitations may be related to mass
transfer - usually due to the transfer
of the gas into the liquid. This means
that the reaction is slowed down by
an insufficient availability of hydrogen.
If this is the case, the utilization
of a self-inducing gassing turbine
helps to overcome this problem by
increasing the hydrogen availability
via constant, secondary hydrogengas
recirculation. Even with these
modifications, reactions may still be
limited, necessitating additional adjustments.
Here, a possible solution
can be found by increasing the volume-specific
power input by the agitator.
Another solution, which
may favor the reaction rate, is
to increase the hydrogen pressure.
The best results can be
obtained by increasing both parameters,
but there are also reactions
that do not respond to
these modifications, since they
are not limited by the availability
of hydrogen.
If the hydrogen uptake rate
remains linear, the reaction obviously
was not limited by the
availability of hydrogen, but possibly
by a limitation in the total
solid-liquid interfacial area of
the catalyst in the reactor. This
can be investigated in extended
tests to see if the linear behavior
can be improved by increased
catalyst concentrations.
In summary, by observing the
hydrogen uptake behavior, certain
limitations related to mass transfer
can be deduced. Based on the
individual analysis of the source of
the limitation, tailored solutions to
best address these limitations can
be developed.
Catalyst reusability study
Feedstocks derived from biomass
may have more complex side-products
and impurities when compared
to traditional,
industrially produced
raw materials. Additionally, a feedstock's
quality may vary due to natural
fluctuations, requiring a more
demanding process-control strategy
and the need for a broader, more
robust process window. Due to
the presence of a potentially higher
degree of catalyst poisons, the aspect
of catalyst reusability becomes
more important.
To determine the total cost of ownership,
engineers should not only
consider the costs for the raw materials,
catalyst manufacturing and
utilities - they should also evaluate
catalyst reusability. The more cycles
the catalyst can be reused until replacement
with fresh catalyst, the
more economical the entire process
becomes. A determination of the
possible number of catalyst reuses
can be evaluated during pilot testing.
One key piece of information derived
from such a study is the evaluation
of the number of reuses until the reaction
time becomes too long due
to catalyst deactivation. Here, " too
FIGURE 3. The Penney diagram is a useful tool for scaling
up stirred reactors, wherein multiple process criterion can
be visualized
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