MAJOR FINDINGS

1
Microalgae production in closed-system
bioreactors based on internal mixing and
residence times.
Mason, M.J., Cuello, J.L. and Fitzsimmons, K.M.
University of Arizona Department of Agricultural
and Biosystems Engineering, Tucson AZ 85721
210038 U.S.A.
SUMMARY This study focused on a key
challenge in achieving the large-scale production
of microalgae in photobioreactors. The aim of the
study was to correlate quantitatively defined
hydrodynamic conditions within pneumatic-type
photobioreactors with their corresponding algae
growth rates, and use the defined hydrodynamic
conditions as a basis for scaling up the
photobioreactors. The photobioreactors
investigated included the bubble column and the
external air-lift reactor, while the hydrodynamic
parameters determined included mean residence
time, axial dispersion coefficient, vessel
dispersion number, Bodenstein Number, Reynolds
Number, VVM (measure of gas flow per aerated
reactor volume), and mixing time. Two sizes of
each reactor type were tested 5 cm diameter
with a 4.4 L volume and 7.5 cm diameter with a
10.2 L volume. There were three levels of gas
flow rate, 2.8, 5.7 and 8.5 L/min, and either a 3
mm or 6 mm bubble size. The results showed that
(1) For the same reactor size, bubble size and
gas flow rate, the bubble column had the lowest
vessel dispersion number among the two reactor
types. This corresponded with the bubble column
also having the longest mixing time (2)
Botryococcus braunii growth at the low gas flow
rate significantly exceeded that at the medium
gas flow rate for either the bubble column or the
external air-lift reactor, indicating that the
hydrodynamic conditions generated at the low gas
flow rate were more optimal to the growth of B.
braunii in both the bubble column and the
external air-left reactor than those generated at
the medium gas flow rate (3) The bubble column
(representing low VVM, low coefficient of
longitudinal dispersion, low vessel dispersion
number and long mixing time) overall proved to be
the better photobioreactor for B. braunii growth
at hydrodynamic conditions generated at either
low or medium gas flow rate (4) B. braunii in
the large bubble column at low gas flow rate had
a productivity of about 0.25 kg/m3-day,
representing a successful scale up of B.
brauniis productivity of 0.22 kg/m3-day from the
flask scale of 0.050 L to the benchtop scale of
10.2 L and (5) Replication of the set of
hydrodynamic conditions corresponding to the
large bubble column at low gas flow rate (2.8
L/min) -- low coefficient of axial dispersion
(0.005 m2/s), low vessel dispersion number
(0.011), high Bodenstein Number (90.0), low VVM
(0.275/min), and slow mixing time (265 s) --
constituted a promising basis for scaling up the
production of B. braunii in a bubble column
reactor.

• METHODS
• Residence Time Distributions
• Reactors operated as convective flow units with
  water flushed through and sodium chloride tracer
  injected and monitored every second with an
  electrical conductivity sensor using a datalogger
  (figure 1)
• Residence time distribution data used to
  determine vessel dispersion number (degree of
  axial dispersion within reactor) at different gas
  flow rates using
• ??2 ?2/ t2 2(D/uL)
  2(D/uL)2 (1-e-uL/D)

• METHODS
• Mixing Times
• Reactors operated as closed bubble column and
  external loop, with water recirculated and
  injected pulse of sodium chloride tracer is
  monitored every second with an electrical
  conductivity sensor using a datalogger (figure 2)

Table 1. Summary of fluid flow characteristics
for the bubble column, convective-flow column,
and external air-lift reactor with fine (3 mm)
and coarse (6 mm) gas bubbles. The volume (4.4.
L) of the small reactor (diameter 5 cm) was
roughly half the volume (10.2 L) of the large
reactor (diameter 7.5 cm). The red circles
indicate treatments chosen for the succeeding
growth experiments.
Figure 3. Algae density in the two reactor types
tested at low flow rate, estimated from
absorbance readings of culture optical density at
540 nm.

• METHODS
• Growth Trial
• Microalgae grown in photobioreactors representing
  select hydrodynamic conditions the bubble-column
  reactor (large size, coarse bubbles) --
  representing low coefficient of longitudinal
  dispersion, low vessel dispersion number, low VVM
  and long mixing time, and the air-lift reactor
  (large size, coarse bubbles) representing high
  coefficient of longitudinal dispersion, high
  vessel dispersion number, high VVM and short
  mixing time.
• Treatments grown out for 20 days- beyond
  exponential growth phase
• Nutrient solution added at sampling (200ml) to
  replace evaporation and ensure nutrient
  availability
• Absorbance data collected from 540nm wavelength
  and compared to pre-determined standard curve
  from weighed and dried samples
• Treatment means (a 0.05) differentiated using
  post hoc t-tests

• OBJECTIVES
• Define hydrodynamic conditions within two
  photobioreactor types in terms of mean residence
  time, vessel dispersion number, Bodenstein
  Number, Reynolds Number, VVM, and mixing time
• Correlate algae productivity with hydrodynamic
  conditions in the photobioreactors

Figure 4. Algae density in the two reactor types
tested at medium flow rate, estimated from
absorbance readings of culture optical density
at 540 nm.
Figure 5. Comparison of performance of various
treatments on maximum specific growth of
microalgae

• MAJOR FINDINGS
• For the same reactor size, bubble size and
  volumetric flow rate, the bubble column had the
  lowest vessel dispersion number among the two
  reactor types of bubble column and external
  air-lift reactor. This corresponded with the
  bubble column also having the longest mixing time
• B. braunii growth at the low gas flow rate
  significantly exceeded that at the medium gas
  flow rate for either the bubble column or the
  external air-lift reactor (plt0.05 at all
  measurement days), indicating that the
  hydrodynamic conditions generated at the low gas
  flow rate were more optimal to the growth of B.
  braunii in both the bubble column and the
  external air-left reactor than those generated at
  the medium gas flow rate
• With fine bubbles, the large bubble column and
  external air-lift reactors showed no significant
  difference in vessel dispersion numbers. With
  coarse bubbles, the large bubble column and
  external air-lift reactors had significantly
  different vessel dispersion numbers at low flow
  rate (p0.007) and at medium flow rate (p.0005).
  
• The bubble column reactor overall proved to be
  the better photobioreactor for B. braunii growth
  under the hydrodynamic conditions generated at
  either low or medium gas flow rate, compared to
  the external air-lift reactor

Figure 1. Schematic of residence time
distribution experimental setup. Water is flushed
through reactors at known rate, with gas flow, as
a pulse of NaCl solution is injected at inlet and
measured at outlet.
Figure 2. Schematic of mixing time experimental
setup. Water is recirculated through reactors at
known gas flow rates, as a pulse of NaCl solution
is injected measured until completely mixed.
CONCLUSIONS In summary, this study showed
that the growth of microalgae can be greatly
enhanced by optimizing the hydrodynamic
conditions within the culture vessel in terms of
axial dispersion and mixing time, and that
successful scale-up from small flasks to large
reactors is possible through duplicating these
optimal hydrodynamic conditions.