Chemical Engineering March 2014 - 45

Guidelines for pilot plant testinG
Feature Report
ligible axial mixing of the liquid.
This type of mixing in an actual
fermenter (Figure 2) would serve to
move dissolved oxygen that is near
the bottom to upper levels where
it is needed, and to move oxygendeleted
liquid that is near the top
to flow downward. This movement
increases the oxygen-transfer driving
force near the bottom of the
fermenter. However, as shown in
Figure 3B, the improvement in the
oxygen transfer rate near the bottom
of the fermenter causes the oxygen
in the gas to run out sooner.
The fermenter should not be designed
as tall as 36 m, because of
the very poor oxygen transfer in
the upper part of the fermenter at
such heights. Thus, huge fermenters
need to grow fat, not tall.
High gas flowrates to the fermenter
increase the number of
bubbles, which increases the bubble
surface area and thereby increases
kL×a. In addition, with more airflow
the oxygen concentration depletes
more slowly, thereby increasing
the overall oxygen-transfer driving
force. However, since the bubbles
rise only so fast, the increasing airflow
will decrease the liquid volume
in the tank. An increase in gas flowrate
will also increase the agitator
size. The more air there is, the more
the impellers will have to disperse,
and the higher the mixer motor
power will be. This presents an interesting
optimization problem.
What is the optimum air flowrate?
Demonstration scale
Scaleup is about business risk. In
order to evaluate the risk involved,
it is important to determine what elements
of the design involve performance
uncertainty. An intermediate-scale
demonstration plant might
be required to prove that scaleup
considerations are well understood.
Thanks to the use of external heat
exchangers, the heat transfer coefficients
(U), the effective heat-transfer
area (A), and the temperature
driving forces (∆T) are all known, so
that heat (Q) can be calculated:
Q = (U)(A)(∆Tlog mean)
(2)
The above analysis shows that,
1. The minimum volume should be 250 gal (950 L) for scalable mass-transfer testing.
A 20-gal tank can be used to evaluate blending and impeller placement
2. Liquid-level-to-tank-height ratio, and tank geometry should be similar to full scale
3. Baffles and heating coils on pilot scale should be similar to full-scale tank
4. Test the fluid with the organism, if possible. If not, use water, knowing the oxygen
transfer rate results will be different
5. The gas and sparging system should be similar (the same would be better) as the
one to be used on full scale
6. Make sure the sparge location is under the main gas-dispersing impeller
7. Use a rotameter with capabilities to fluctuate the gas flowrate over a range (use at
least four different flowrates)
8. One flowrate should be the same vessel volumes per minute as the full scale -
achieving the same superficial gas velocity will be difficult
9. Different styles and diameter of impellers should be tested. Include the ability to
adjust location of the impellers
10. Variable-speed drive should be used to alter speed to test four different power levels
11. Use a tachometer to measure the operating speed of the shaft and impeller
12. Use a torque sensor to record mixer horsepower while the test is running
13. Dissolved oxygen probe locations should be at the top and the bottom of the tank.
Keep them away from baffles and any other dead spots
14. Take note of how important the location of the lower impeller is in relation to the
sparger
15. Make sure the tank will be tall enough to account for the gas hold-up. The hold-up
will increase the liquid level, sometimes significantly, if the mixer has produced a
well-dispersed system
16. Acid/base indicator or conductivity probes can be used for qualitative blend-time
evaluation
for heat transfer, the design factors
are already well understood
and predictable, and thus present a
low risk to the project. In the case
of mass transfer, however, the mass
transfer conductance used in Equation
(1) is not well known for fermenters
above about 100,000 gallons.
Pilot testing is required.
Pilot-scale testing
Pilot work is critical for any new
process. For fermentation applications,
pilot work is required to understand
how the organism will
behave under specific process conditions.
The information studied on
the pilot scale for a fermenter must
include the following: mass transfer,
gas dispersion and blending. All
three are of equal importance.
If the mass transfer requirements
are not met, the organisms in the
fermenter will die because there is
not enough power available to force
the liquid/gas boundary layer transfer
to take place.
If the gas dispersion requirements
are not met, the air is not properly
distributed throughout the vessel
and again, the organisms will die. If
the tank is not well blended, the nutrients
that are added to the vessel,
the heat transfer and the pH will
not be uniform. The organism will
not survive in this environment.
46 ChemiCal engineering www.Che.Com marCh 2014
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All of these are undesirable results.
The information gleaned from the
pilot work is used to successfully
model the full-scale operation. Pilot
plant work will determine what
impeller style(s), diameter(s) and
power levels are required for the
agitator to successfully perform.
Proper experiment set-up and
execution will make sure repeatable
results are achieved on the full
scale. The specific parameters that
must be examined are: tank geometry,
baffle and coil arrangement
and gas-sparging system. The tank
geometry ratios, and baffle and coil
arrangements should be similar
between full scale and pilot scale.
Pilot testing should be done with
the exact process fluid to be used on
the full scale, or a fluid with very
similar properties. The liquid-levelto-tank-diameter
ratio should be
constant in scaleup, as should the
type of gas and sparge system. The
lower impeller should be located at
a specific distance above the sparger
and that ratio should remain unchanged
between scales.
Traditional laboratory-scale testing
is performed at a minimum volume
range between 20 to 250 gal.
When considering pilot scale work,
a tank with a minimum volume of
750 gal, or a 4-ft-dia. × 8-ft tank
should be considered.

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Chemical Engineering March 2014

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