Mixing and Eddie Currents in a Modified Bubble Column Reactor

Paper Number: RRV03-0043 An ASAE Meeting Presentation

Mixing and Eddie Currents in a Modified Bubble Column Reactor Anil Kommareddy, Research Associate South Dakota State University, P.O.Box 7033, Brookings, SD 57007, [email protected].

Dr. Gary Anderson, Professor South Dakota State University, AE 2120, Brookings, SD 57007, [email protected].

Written for presentation at the 2003 CSAE/ASAE Annual Intersectional Meeting Sponsored by the Red River Section of ASAE Quality Inn & Suites 301 3rd Avenue North Fargo, North Dakota, USA October 3-4, 2003

Abstract. A modified bubble column reactor is introduced and compared with existing photobioreactors. The modified bubble column reactor uses a porous membrane as the interface between the air and the liquid. The characteristics of the porous membrane are investigated. Gas transfer characteristics of carbon dioxide and oxygen where investigated for the modified bubble column reactor. Bubble size and gas holdup relationships are provided for the modified bubble column reactor. General mixing characteristics of the modified bubble column is qualitatively investigated. Factors that would affect the algae in a bubble column and eddy current length in relation to algae damage are discussed. Keywords. Photobioreactor, bubble column reactor, porous membrane, gas holdup, eddy current length, microalgae.

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Introduction Mixing in a bubble column photobioreactor (PBR) is one of the most important factors for large-scale growth of high value microalgae. Microalgae have greater capacity to fix carbon dioxide at a higher rate than higher order plants (Morita et. al., 2000; The National Academics, 2000). Microalgae fixation/conversion of carbon dioxide into useful livestock fodder using pollutants like ammonium, potassium, phosphorous, ammonia, calcium, magnesium, sulfate, bicarbonate, carbonate etc. from the livestock industry could help in reducing greenhouse gasses in the atmosphere (Morita et. al., 2000; Kommareddy and Anderson, 2003). In recent times microalgae’s ability to produce hydrogen is being investigated (Matsunaga et. al., 1999; Tsygankov et. al., 2002; Tsygankov et. al., 1999; Kosourov et. al., 2002; ORNL, 2000). Potential high-value photosynthetic microalgae that can be used in food and pharmaceuticals (Grima et. al., 1998; Miron et. al., 1998; Rubio et. al., 1999) are also being investigated for high volume production. There are numerous other applications where algae can be used as dyes, pigments etc. Hence there is a requirement for large-scale production using a photobioreactor. Broadly, bioreactors can be classified as open systems or closed systems. Open systems are ponds, constructed on the large open areas, in rows with growth medium exposed to environment and sunlight. Closed systems are those where growth medium is in enclosed in way from the environment. Open systems have many disadvantages over closed system, like they are hard to control and, contamination from external environment is high and could cause the microalgae mutate (Rubio et. al., 1998). Closed systems are easy to monitor, less chances of contamination, better mass transfer (varies based on the type of bioreactor) and occupy less space for the same algal growth. Closed systems can be classified as tubular bioreactors, stirred bioreactors, airlift bioreactors, and sparged bubble column reactors. Modified sparged bubble column reactors use a porous membrane instead of fixed hole openings used in sparged bubble column reactors. The porous membrane has small pores (orifices) which allow air to pass from plenum into the growth medium (Kleinjan and Anderson, 1999; Poulsen and Iversen, 1997). Tubular photobioreactors consist of long thin tubes arranged in different geometrical patterns (helical, straight tubes) to optimize irradiance from a point light source (sun). Generally liquid growth medium is circulated in these tubes by air bubbling and by injection of air into one end of the system and degassed at the other end. Several research papers discuss construction, light regime, mass transfer and scale up issues of these photobioreactors (Grima, 1998; Miron, 1998). Experiments have showed (Miron et. al., 1998) that a large-scale tubular photobioreactor has failed and the main reason attributed for its failure was the large dissolved oxygen in the system. It was also reported that they are difficult to build and maintain, and have limited scalability. Mechanically stirred bioreactors use baffles to stir the growth medium to attain a mass transfer of air into liquid. A drawback of the stirred medium is if stirred vigorously the algae cell wall would be damaged by the high fluid shear forces (Molina et. al., 1997). If it is stirred slow, eddy currents will not be established that move the algae toward the light source thereby decreasing the efficiency of light available for the photosynthetic process and also reducing mass transfer of nutrients from the air to the liquid in the systems. Airlift bioreactors are basically a column divided into two parts and air is bubble through only one side of the partition which causes a liquid current pattern to develop with the air bubble side called the riser and other part called the down comer. For a more detailed explanation of airlift bioreactors please refer Chisti, 1989, Miron et. al., 2000. Gas holdups (entrapment of air in liquid medium) rates are close to bubble column reactor holdup rates, but these bioreactors

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develop a shear stress on the algal cells as they circulate in the reactor at high speeds and could eventually damage the algae. These bioreactors are extensively investigated for fermentation process but have not been looked at as a replacement for the popular tubular photobioreactors until recent times (Miron et. al., 2000). Bubble column bioreactors are vertical columns either cylindrical or rectangular filled with growth medium and air is bubble through a sparged system installed at the bottom. These system have the highest gas hold ups rates which means they have the best mass transfer compared to other systems (Miron et. al., 2000). A modified version of these bubble column bioreactors is porous membrane reactors, which have efficient aeration, give smaller bubbles, and pressure drop across the membrane is low compared with other rigid sparged bubble column reactors. These characteristics are achievable at high gas flow rates, with low energy costs (Poulsen et. al., 1997). Information about these types of bioreactors is limited. This paper attempts to compare the performance of porous membrane based bioreactors with bubble column reactors and attempts to solve for the mass transfer equations and provide a general understand the mixing process in these types of bioreactors. It also looks at the eddy current lengths created by bubbles as they move along the length of the bioreactor and hence discuss the implication of these eddy current lengths in the production of algae.

Porous membrane resistance to air flow with varying water depth and air velocity An acrylic cylindrical tube was chosen for testing. The test system is comprised of a top and bottom chamber, each 3 in (0.0762 m) diameter, see Figure 1 and 2. A disk shaped piece of porous material (Porex 7896, 35µ, manufacturer: Interstate Specialty Products) was sandwiched between the compartments with a gasket to demonstrate the ability of the porous membrane to diffuse air into a water solution. The cross-sectional area of porous membrane was 7.0686 in2 (4.56 x 10-3 m2). Compressed air from air compressor was fed through the air regulator and air flow meter (F-4001, manufacturer: Gilmont Instruments) into the plenum. Gauge pressure was measured by the Air Data Multimeter (ADM – 860, manufacturer: Shortridge Instruments Inc.) in inches of water. Air then moves from the plenum to the water through the porous membrane. The water depth was varied from 0-9 inches (0 - 0.2286 m) in 1inch (0.0254 m) increments. The airflow through the test apparatus was varied from 2-14 CFH (0.05663 – 0.2548 cubic meter per hour) in increments of 2 CFH (0.05663 cubic meter per hour) for each water depth. Superficial gas velocity is calculated by dividing volumetric flow rate with crosssectional area of the porous membrane. The variation in volumetric flow rate results with superficial gas velocity increased in the increments of 0.9297 m/s units. The system was given 5 minutes to equilibrate before the resistance was recorded for a given depth and airflow rate. Seventy measurements were made for each porous membrane sample, the mean of the results are tabulated and are provided in Appendix A, Table 1. Six different porous membrane samples were cut randomly from 42” X 44” (1.0668 – 1.1176 m) sheet so that average properties of the sheet could be evaluated.

Results from the test: The following can be deduced from the data presented in Figures 3, 4 and 5. 1. Figures 3 and 4 show that the resistance offered due to that porous membrane is closed to zero at all the measured superficial gas velocities, which proves that there is no loss of energy due to porous membrane interface.

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2. When liquid is initially added to the bubble column there is a sudden change in the resistance and is not linear with change in superficial gas velocity, see Figure 3. 3. As the depth of the liquid increased, the system reaches linearity of resistance with change in depth, see Figure 4. It can be seen from the Figure 4, that at depths of 0.127 m and higher, there is a steady increase in resistance with change in the depth of the liquid suggesting that pressure (resistance) change relative to superficial gas velocity is constant. 4. Figure 4 suggests that at low superficial gas velocities (i.e.) less 2 m/s the resistance is non linear and may be hard to predict. 5. It can also be seen from Figure 4, that up to 0.0508 m (2 in) depth the resistance change is large. This could be due to surface tension at air-liquid interface. Generally this should not be of great concern, as the column liquid heights would be much larger than 0.0508 m. 6. At low superficial gas velocity 0.9297 m/s the change in resistance with depth is not as linear as with higher superficial gas velocities see Figure 5. 7. It can be seen from Figures 4 and 5 that both depth and superficial gas velocity affects the resistance to airflow offered by the porous membrane. 8. The separation between curves in Figure 4 is more than the separation between curves in Figure 5, suggesting that resistance is more dependent on water depth than superficial gas velocity. It can be concluded from the experiment that unless the system is running at very low superficial gas velocities (< 2 m/s) or/and very low depths (< 0.05 m) the resistance is linearly dependent on airflow velocity and water depth.

Air Bubbles in solution

Air Flow meter

Porous Membrane

Air Regulator Value Air Data Multimeter (Pressure Sensor)

Plenum

Figure 1: Experiment setup to test the porous membrane diffuser.

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Figure 2: Experimental setup for testing resistance of porous membrane

Superficial gas velocity vs Pressure (at different water depths) 1600

Pressure (Pa)

1400 1200 1000 0m 0.0254 m

800 600 400 200 0 0.00

1.00

2.00

3.00

4.00

5.00

6.00

7.00

Superficial gas velocity (m/s)

Figure 3: Graph showing increase in resistance when water is added to the bubble column

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Superficial gas velocity (vs) Pressure at different water depths 4500 4000 Pressure (Pa)

3500 3000 2500 2000 1500 1000 500 0 0

1

2

3

4

5

6

7

0m 0.0254 m 0.0508 m 0.0762 m 0.1016 m 0.127 m 0.1524 m 0.1778 m 0.2032 m 0.2286 m

Superficial gas velocity (m/s)

Figure 4: Graph showing change in resistance due to change in superficial gas velocity at constant height.

Depth vs Pressure at different superficial gas velocities 4500.00 4000.00

Pressure (Pa)

3500.00

0.9297 m/s 1.8594 m/s 2.7891 m/s 3.7188 m/s 4.6485 m/s 5.5782 m/s 6.5079 m/s

3000.00 2500.00 2000.00 1500.00 1000.00 500.00 0.00 0

0.05

0.1

0.15

0.2

0.25

Depth (m) Figure 5: Graph showing change in resistance with change in depth at constant superficial gas velocity.

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Factors influencing the growth of algae in bubble column reactors Some of the factors influencing the growth of microalgae in bubble columns can be listed as follows. 1. Length or duration of exposure of algae to light due to mixing. 2. Mass transfer in modified bubble column. 3. Bubble size / diameter. 4. Gas holdup in the system and bubble velocity. 5. Factors associated with microalgae damage. 6. Cell size in relation to eddy current lengths.

1. Length or duration of exposure of algae to light due to mixing. Experimental evidence suggests that algae need some dark periods, but due to limited literature, this cannot be verified at this stage (Miron et. al., 1999). Huang and Rorrer (2002), point out that marine red algae show light inhibition when the algae is exposed to light more than 20 hours in a 24 hour period, they also gave the minimum time light is needed 10 hours and optimum illumination time of 16 hours. Apart from this literature all other photobioreactors used sunlight as the source of energy, so there is a dark period (due to night time) every day. The duration of the dark period is defined by the geographic location and month in the year Grima et. al., 1998; Miron et. al., 1998. In the bubble columns when bubbles are injected in to the system they create a mixing pattern which would cause the algae to mix in the system and hence move it in and out from the light source enabling more algae to be exposed to light and hence increase the growth. Bubble mixing also causes more gas to be entraped in the growth medium (also known as gas holdup) than any other type of bioreactor (Chisti, 1989, Miron, et. al. 1998). Gas holdup is also dependent upon the temperature and rate of aeration. Higher aeration causes higher gas holdups (Miron, et. al., 1998). Another important aspect to be considered in bubble columns is diameter of the bubbles, if the bubbles are small and dense they would cause the light to reflect back out of the system and hence penetration of light into the system would be limited. Bubbles less than 6mm diameter are considered small and are to be avoided (Miron, et. al., 1998). Sparged bubble columns give out relatively uniform diameter bubbles, compared to porous membrane bubble column reactors. Another important aspect of membrane based bubble column reactors is that it is hard to predict which orifices of porous membrane would be open and which wouldn’t be as observed by Talbot et. al., 1991. The main disadvantage of using sparged bubble column reactors is that there is a large resistance to air flow and a lot of energy is wasted to over come it affecting, cost effectiveness if used in large a scale system. A membrane base bubble column used for large-scale systems, which is closely related to sparged bubble column but with less resistance to air flow in the system would be an economical alternative. More experiments need to be conducted to truly understand all the mixing characteristics of the porous membrane based bubble columns and their influence in moving algae into dark and light regions of the bioreactor. The same experimental setup used for finding resistance of the porous membrane was used to understand the mixing characteristics of a membrane based bubble column. Water is filled to a height of 0.2286m (9 in) in the column and saw dust was added to the water. Initially the system pressure was set in equilibrium with the pressure of the

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water on the porous membrane, i.e. no bubbling was allowed (see Figure 6). At this time some of the heavy saw dust settled down to the bottom of the column and the lighter sawdust floated on the top of the water column. The airflow rate was increased and bubbles started to form and move up. As the airflow rate is increased more and more bubbles are emitted from the porous membrane and mixing started (see Figure 7). It can be seen from Figure, 7 that the sawdust is uniformly mixed in the system. Qualitatively it can be concluded that uniform mixing takes place in membrane based bubble column reactors. The mixing observed should be able to move the algae through the system, into and out of light regions.

Figure 6: Bubble column with no bubbles in the system hence the sawdust is accumulated either at the top or at the bottom depending up on the size of the sawdust.

Figure 7: Bubble column mixing the sawdust in the system

2. Mass transfer in modified bubble column. One of the main parameters of concern with phototropic algae is transfer of carbon dioxide from air to algae and the removal of oxygen from the bioreactor. The main failure of tubular photobioreactors is attributed to this factor, as there was build up of oxygen in the system inhibiting the growth of algae (Miron et. al., 1998), hence a system should not at any stage be over saturated with oxygen as this would cause algae to shutdown photosynthesis and growth (Rubio et. al., 1999). Another important factor of concern is the availability of carbon dioxide in the photobioreactor. Carbon dioxide should not reach the higher concentration limit for a species of algae. High concentrations can trigger inhibition and if it is too low then the algae would be starved. The optimum values are highly dependent on the species of algae (Sobczuk et. al., 2000). The major limiting factor for a stirred type photobioreactor is the limitation of carbon dioxide in the system as the addition of carbon dioxide is by a small inlet and mixed using a stirrer (Gunter, 2001). As algae consist of 40 - 50% carbon by mass, it needs to be available in abundance for its photosynthesis and cell growth (Rubio et. al., 1999; Sobczuk et. al., 2000).

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Solubility and diffusivity of carbon dioxide and oxygen in water under normal atmospheric conditions is very low (Talbot et. al., 1991). Carbon dioxide is slightly more soluble than oxygen in water. When carbon dioxide is dissolved in water it comes into equilibrium with carbonates and bicarbonates. So all of the carbon dioxide, which is dissolved in water doesn’t exist as dissolved carbon dioxide, only a part of it is available for algae to use. Chisti 1989, showed that out of the eight types of resistance between transfer of mass from air to algae the only resistance which is significant is the mass transfer across gas-liquid interface (KLa). Talbot et. al., 1991 showed that the amount of the dissolved carbon dioxide is independent of other ingredients in the liquid solution such as ammonia. The paper also showed that the relationship between volumetric overall mass transfer coefficient of oxygen and carbon dioxide are related by their diffusivities in a liquid. The relationship is:

D  K L a (CO2 ) = K L a (O2 )  CO 2   DO 2 

0.5

1

where KLa(CO2) = Volumetric overall mass transfer coefficient based on liquid concentration of carbon dioxide (s-1). KLa(O2) = Volumetric overall mass transfer coefficient based on liquid concentration of oxygen (s-1). DCO2 = Diffusivity of carbon dioxide in liquid phase (m2/s). DO2 = Diffusivity of oxygen in liquid phase (m2/s). Diffusivity of carbon dioxide in water and diffusivity of oxygen in water are related by equation 2 (Regnier et. al., 2000).

DCO 2  MWO 2  =  DO 2  MWCO 2 

0.5

2

where MWO2 = Molecular weight of oxygen. MWCO2 = Molecular weight of carbon dioxide. Substituting equation 2 in to equation 1 results in equation 3.

 MWO 2  K L a (CO2 ) = K L a (O2 )    MWCO 2 

0.25

3

Terms previously defined. Equation 3 relates KLa(CO2) with KLa(O2) accounting for the diffusivity of the two gases in water by a factor of 0.923 which is in agreement with 0.93 published by Rubio et. al., 1999. There are many ways KLa(O2) can be found for a given photobioreactor they are well documented (Chisti 1989; Rubio et. al., 1989; Grima et. al., 1998). Availability of carbon dioxide in air is only 0.03% by volume when compared 20.93% by volume for oxygen. The above results shows that higher aeration rates or an external source of carbon dioxide added to the aerating air is needed for better mass transfer of carbon dioxide in to the

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growth medium. This observation is in agreement with Rubio et. al., 1999 and Poulsen and Iversen, 1998. As long as the pressure in the modified bubble column is kept close to atmospheric pressure, there wouldn’t be any dissolved oxygen build up. Due to heavy aeration rates oxygen in the growth solution generally never exceeds saturation for a given temperature, so photobioreactors would not have oxygen as growth inhibiting factor in the system.

3. Bubble size / diameter. Bubble size plays a major role in the hydrodynamics of bubble column reactors. Bubble diameters are measured by photographic measurements for modified bubble column reactors (Talbot et. al., 1991). Krishna and Baten, 2003 describe homogenous bubbly flow regime as that regime during which gas holdup ε increases linearly with the superficial velocity U, provided it stays below a certain value called Utrans see Figure8. Above Utrans is called heterogeneous or churn-turbulent flow regime. Generally uniform bubbles with a narrow a distribution of 1-7mm are found in this homogenous regime where as large bubble sizes of 20-70 mm are formed in heterogeneous regimes see Figure 8. As superficial velocities reach Utrans, coalescence of the bubble takes place to produce large bubbles.

Figure 8: Experimental data on gas holdup in a 0.1m diameter bubble column operating with the air-water system spanning both the homogeneous and heterogeneous flow regimes (Krishna and Baten, 2003)

4. Gas holdup in the system and bubble velocity. Gas holdup is the amount of gas entrapped in the liquid as bubbles at a given time in the system. Gas holdup in a bubble column in terms of superficial gas-liquid interface area a and mean diameter of the bubble is given by equation 4 (Miron et. al., 2000; Poulsen and Iversen, 1998).

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a=

6ε d B (1 − ε )

4

where, dB = is mean bubble diameter Gas holdup in terms of superficial gas velocity U and mean bubble rise velocity Ub is given by equation 5 (Miron et. al., 2000).

ε=

U Ub

5

Terms previously defined. Bubble rise velocity is a function of bubble diameter.

5. Factors associated with microalgae damage. Bubble columns are not used as photobioreactors; there is very little experimental evidence dealing with the factors associated with damage to microalgae (Miron et. al., 2000). Like animal cells, plant cells are affected by shear rate in the surrounding liquid, but have a much less dramatic responses, such as a decrease in growth rate or changes in metabolite synthesis that may take place before shear rate reaches a lethal limit (Wu and Merchuk, 2002). Nevertheless, shear stress needs to be checked since algae are living organisms and can be affected by the external stresses induced by the hydrodynamics of the bubble column. The causes for animal cell death given by Meier .et al., 1999 associates bubble bursts (1) cells that are in suspension near a bursting bubble may be killed by the shear associated with the burst, (2) cells that are trapped in the bubble lamella may be killed by the bubble burst, and (3) the bubble burst may kill cells that attached to the bubble during its rise. These factors should be investigated for algae.

6. Cell Size in relation to eddy current lengths. Eddy currents associated with a rising bubble could be detrimental if the cell size is close to or larger than to the eddy current lengths (Miron et. al. 1998). Generally the size of the algae is considered to be around 45µm. More research is need to develop a relationship with the bubble size, eddy current lengths associate with it and column diameter. At present very little literature is available for algae growth in bubble column reactors.

Summary Basic resistance characteristics of porous membrane were investigated at different superficial gas velocities and depths. For large-scale photobioreactors, using porous membrane based bubble column economical due to low resistance offered to the superficial gas flow. Mass transfer coefficient for the modified bubble column reactor is compared with a traditional bubble column reactor and a method to find mass transfer coefficient of carbon dioxide was presented. Bubble diameter, gas holdup, mean bubble rise velocity for modified column reactors are discussed. Factors affecting cell damage were presented. Mixing characteristics of modified bubble column reactor was presented. Important characteristics like eddy current lengths in relation to cell size are discussed.

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References Chisti, M. Y. 1989. Airlift Bioreactors. New York, N.Y. : Elsevier Science Publishers LTD. Grima, E. M., F. G. A. Fernandez, F. G. Camacho, and Y. Chisti. 1998. Photobioreactors: light regime, mass transfer, and scaleup. Journal of Biotechnology. 70: 231-247. Gunther W. S. 2001. A Photo-Bioreactor on-Line biomass and growth rate estimations for optimization of light intensity in cultures of phototropic microorganisms. M.S. thesis. Sohngaardsholmsvej, Aalborg, Denmark: Aalborg University, Department of life Science. Available at: http://www.sitecenter.dk/wsg/nss-folder/mappe/Thesis.pdf. Accessed on: June 18, 2003. Huang, Y., and G. L. Rorrer. 2002. Optimal Temperature and Photoperiod for the Cultivation of Agardhiella subulate Microplantlets in a Bubble-Column Photobioreactor. Biotechnology and Bioengineering. 79(2): 135-144. Kleinjan, N., and G. Anderson. 1999. Design and Testing of a Prototype Photobioreactor. Unpublished U. G. Thesis. Brookings, S.D.: South Dakota State University, Department of Agricultural Engineering. Kommareddy, A. and G. Anderson. 2003. Study of light as a parameter in the growth of algae in Photo-Bio Reactor. ASAE Paper No. 034057. Las Vegas, Nevada: ASAE. Kosourov, S., A. Tsygonkov, M. Seibert, and M. L. Ghirardi. 2002. Sustained Hydrogen Photoproduction by Chlamydomonas reinhardtii: Effects of Culture Parameters. Biotechnology and Bioengineering. 78(7): 731-740. Krishna, R. and J. M. V. Baten. 2003. Mass transfer in bubble columns. Catalysis Today. 79-80: 67-75. Matsunaga, T., T. Hatano, and A. Yamada. 2000. Microaerobic Hydrogen Production by Photosynthetic Bacteria in a Double-Phase Photobioreactor. Biotechnology and Bioengineering. 68(6): 647-651. Miron, A. S., A. C. Gomez, F. G. Camacho, E. M. Grima, and Y. Chisti. 1998. Comparative evaluation of compact photobioreactors for large-scale monoculture of microalgae. Journal of Biotechnology. 70: 249-270. Miron, A. S., F. G. Camacho, A. C. Gomez, E. M. Grima, and Y. Chisti. 2000. Bubble-Column and Airlift Photobioreactors for Algal Culture. American Institute of Chemical Engineers Journal. 46(9): 1872-1887. Morita, M., Y. Watanabe, and H. Saiki. 2000. Investigation of Photobioreactor Design for Enhancing the Photosynthetic Productivity of Microalgae. Biotechnology and Bioengineering. 69(6): 693-698. ORNL (Oak Ridge National Laboratory) 2000. Producing and Detecting Hydrogen. http://www.ornl.gov/ORNLReview/v33_2_00/hydrogen Accessed on January 6, 2003. Poulsen, B. R. and J. J. L. Iversen. 1997. Characterization of Gas Transfer and Mixing in a Bubble Column Equipped with a Rubber Membrane Diffuser. Biotechnology and Bioengineering. 58(6): 633-641. Regnier, P., J. P. Vanderborght, J. P. O’Kane, and C. I. Steefel. 2000. Carbon dioxide transfer across the interface. Available at: http://www.geo.uu.nl/Research/Geochemistry/kb/Knowledgebook/CO2_transfer.pdf Accessed on: September 19, 2003.

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Rubio, F. C., F. G. A. Fernandez, J. A. S. Perez, F. G. Camacho, and E. M. Grima. 1998. Prediction of Dissolved Oxygen and Carbon Dioxide Concentration Profiles in Tubular Photobioreactor for Microalgal Culture. Biotechnology and Bioengineering. 62(1): 71-86. Sobczuk, T. M., F. G. Camacho, F. C. Rubio, F. G. A. Fernandez and E. M. Grima. 2000. Carbon dioxide Uptake Efficiency by Outdoor Microalgal Cultures in Tubular Airlift Photobioreactors. Biotechnology and Bioengineering. 67(4): 465-475. Talbot, P., M. P. Gortares, R. W. Lencki, and J. D. L. Noue. 1991. Absorption of CO2 in Algal Mass Culture Systems: A Different Characterization Approach. Biotechnology and Bioengineering. 37: 834-842. The National Academics, 2000. Report of the Workshop on Biology-based Technology to Enchance Human Well-being and Function in Extended Space Exploration. http://www.nap.edu/ssb/wbsech2.htm Accessed on April 23, 2003. Tsygonkov, A. A., V. B. Borodin, K. K. Rao, and D. O. Hall. 1999. H2 Photoproduction by Batch Culture of Anabena variabilis ATCC 29413 and Its Mutant PK84 in a Photobioreactor. Biotechnology and Bioengineering. 64(4): 709-715. Tsygonkov, A. A., A. S. Fedorov, S.N. Kosourov, and K. K. Rao. 2002. Hydrogen Production by Cyanobacteria in an Automated Outdoor Photobioreactor under Aerobic Conditions. Biotechnology and Bioengineering. 80(7): 777-783. Wu, X., and J. C. Merchuk. 2002. Simulation of Algae Growth in a Bench-Scale Bubble Column Reactor. Biotechnology and Bioengineering. 80(2): 156-168.

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Appendix A Table 1: Mean pressure of the samples for particular depth and superficial gas velocity Water depth (m) 0 0.0254 0.0508 0.0762 0.1016 0.127 0.1524 Superficial gas velocity(m/s) 0.9297 3.41 601.72 1343.93 1755.12 2159.92 2447.65 2707.77 1.8594 7.27 985.66 1595.60 1911.16 2188.28 2570.85 2828.44 2.7891 10.48 1144.19 1660.91 1978.50 2257.49 2632.47 2887.74 3.7188 14.11 1251.46 1715.23 2030.91 2311.35 2680.41 2944.96 4.6485 16.94 1334.44 1753.00 2067.57 2347.84 2721.04 2980.21 5.5782 20.24 1388.26 1783.52 2103.77 2384.46 2752.97 3017.94 6.5079 23.98 1437.90 1824.95 2139.31 2420.66 2786.15 3049.04

0.1778

0.2032

0.2286

2949.94 3086.36 3139.86 3201.64 3238.13 3277.11 3316.09

3265.50 3376.63 3430.54 3491.49 3526.33 3562.40 3596.40

3464.13 3620.87 3672.29 3732.83 3803.32 3811.20 3846.03

Pressure in pascals

2