1. MAJOR FINDINGS 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 (p<0.05 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 (p=0.007) 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 OBJECTIVES OBJECTIVES 1.Define hydrodynamic conditions within two photobioreactor types in terms of mean residence time, vessel dispersion number, Bodenstein Number, Reynold’s Number, VVM, and mixing time; 2.Correlate algae productivity with hydrodynamic conditions in the photobioreactors Mason, M.J., Cuello, J.L. and Fitzsimmons, K.M. Microalgae production in closed-system bioreactors based on internal mixing and residence times. Microalgae production in closed-system bioreactors based on internal mixing and residence times. SUMMARY 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, Reynold’s 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/m 3 -day, representing a successful scale up of B. braunii’s productivity of 0.22 kg/m 3 -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 m 2 /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 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 (α = 0.05) differentiated using post hoc t-tests CONCLUSIONS 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. METHODS 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 / t *2 = 2(D/uL) – 2(D/uL) 2 (1-e-uL/D) University of Arizona Department of Agricultural and Biosystems Engineering, Tucson AZ 85721 210038 U.S.A. EC Measurement NaCl Pulse Input EC Measurement NaCl Pulse Input EC Measurement EC Measurement 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. Specific growth (kg/m^3-day) flask, mixing, 5% CO2 P tricorn., 1% CO2 lg bbl col low, no CO2 flask, mixing, no CO2 lg air lft low, no CO2 lg bbl col med, no CO2 lg air lft med, no CO2 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 Figure 5. Comparison of performance of various treatments on maximum specific growth of microalgae 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. 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. METHODS 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)