design, construction and validation of an internally-lit airlift photobioreactor

7/28/2019 Design, Construction and Validation of an Internally-Lit Airlift Photobioreactor

1/116

Design, Construction and Validation of an Internally-Lit Airlift Photobioreactor

A thesis presented to

the faculty of

the Russ College of Engineering and Technology of Ohio University

In partial fulfillment

of the requirements for the degree

Master of Science

Esteban Hincapie

August 2010


2/116


3/116

3

ABSTRACT

HINCAPIE, ESTEBAN, M.S., August 2010, Mechanical Engineering

Design, Construction and Validation of an Internally-Lit Airlift Photobioreactor (116 pp.)

Director of Thesis: Ben J. Stuart

A novel photobioreactor for growing algae was developed from a previous Ohio

University patent. The proposed design uses the air lift principle to enhance the culture

circulation and induce light/dark cycles to the microorganisms. Optical fibers were used

to distribute photons inside the culture media providing an opportunity to control both

light cycle and light intensity. The fibers were coupled to an artificial light source,

however the development of this approach aims for the future use of natural light

collected through parabolic solar collector. This idea could also allow the use of non-

clear materials for photobioreactor construction diminishing costs and increasing

durability.

A 30-liter laboratory scale unit was designed and constructed using inexpensive

plastic fiber optic cables. Materials were selected to assure fast construction and

maximize light use. As a research tool, the device should meet the criteria of removable,

replaceable, and accessible for maintenance. All the pieces of the device were planned to

be able to be removed and replaced by ones of alternative design, assuring flexibility for

future researchers using or modifying the device. Internal light levels were determined in


4/116

4

The reactor was inoculated with the algal strain Chlorella sp. and sparged with air.

The reactor was operated in batch mode and daily monitored for pH, temperature, and

biomass concentration and activity. The productivity of the novel device was determined,

0.011 h-1

, suggesting the proposed design can be effectively and economically used in

carbon dioxide mitigation technologies and in the production of algal biomass for biofuel

and other bioproducts.

Approved: _____________________________________________________________

Ben J. Stuart

Associate Professor of Civil Engineering


5/116

5

TABLE OF CONTENTS

Page

ABSTRACT ........................................................................................................................ 3

TABLE OF CONTENTS .................................................................................................... 5

LIST OF TABLES .............................................................................................................. 8

LIST OF FIGURES ............................................................................................................ 9

1 INTRODUCTION .................................................................................................... 11

1.1 Energy Consumption ........................................................................................ 11

1.2 Fossil Fuels ....................................................................................................... 11

1.3 Importance of Coal Energy ............................................................................... 12

1.4 Biofuels ............................................................................................................. 121.5 Photobioreactors ............................................................................................... 13

1.6 Significance of Research ................................................................................... 19

1.7 Project Objectives ............................................................................................. 19

2 LITERATURE REVIEW ......................................................................................... 21

2.1 Algae as a Source for Biofuels.......................................................................... 21

2.2 Carbon Mitigation and Sequestration ............................................................... 22

2.3 Photobioreactors Types ..................................................................................... 25

2.4 Bubble Column and Air Lift Reactor................................................................ 27

2.5 Hydrodynamics ................................................................................................. 28

2.6 Gas Flow ........................................................................................................... 31

2.7 Light Availability .............................................................................................. 31

2.8 Productivity ....................................................................................................... 32

3 DESIGN APPROACH .............................................................................................. 34

3.1 Design Constraints ............................................................................................ 34


6/116

6

3.4.2 Determination of the Minimum Number of Fibers ............................... 433.4.3 Light Distribution along the Fiber Optic .............................................. 46

3.4.4 Selection of the Fibers and Separation between Levels ........................ 48

3.4.5 Fixation of the Fiber Optic .................................................................... 49

3.4.6 Flow Levels ........................................................................................... 51

3.4.7 Sparger .................................................................................................. 53

3.4.8 Draft Tube Support and Pipe Centering................................................ 55

3.4.9 Fittings and Flow Measurement............................................................ 56

3.4.10 Mobility of the System.......................................................................... 59

3.4.11 Cost ....................................................................................................... 60

3.5 Analytical Methods ........................................................................................... 61

3.5.1 Fluorescence ......................................................................................... 62

3.5.2 Temperature .......................................................................................... 62

3.5.3 pH .......................................................................................................... 63

3.5.4 Cell counting ......................................................................................... 63

3.5.5 Light ...................................................................................................... 63

3.5.6 Air Flow ................................................................................................ 64

3.5.7 Media .................................................................................................... 64

4 RESULTS AND DISCUSSION ............................................................................... 65

4.1 Introduction ....................................................................................................... 65

4.2 Product Performance and Maintenance ............................................................ 66

4.2.1 Sparger Problems .................................................................................. 69

4.2.2 Light Intensity ....................................................................................... 70

4.3 Flow Modeling .................................................................................................. 72

4.4 Mixing Characterization ................................................................................... 74

4.5 Cell Calibration Curve ...................................................................................... 76


7/116

7

5.2 Design Approach .............................................................................................. 965.3 Light Intensity ................................................................................................... 97

5.4 Mixing Characterization ................................................................................... 98

5.5 Operation and Maintenance .............................................................................. 98

5.6 Specific Growth Rate ........................................................................................ 99

6 Recommendations ................................................................................................... 100

WORKS CITED ..............................................................................................................106

APPENDICES .................................................................................................................110

Appendix A Additional Results ..................................................................................110

Appendix B Media Recipes .........................................................................................114

Appendix C Solid Red Standard Check for the Turner Fluorometer ..........................115

Appendix D Composition of Bristol Media ................................................................116

Appendix E Cell counting results ................................................................................117


8/116

8

LIST OF TABLES

Page

Table 1. Analysis of the pipes available ........................................................................... 40

Table 2. Characteristics of the light bulb .......................................................................... 43

Table 3. Determination of the amount of fibers ................................................................ 44

Table 4. Light intensity readings for different distances .................................................. 46

Table 5. Comparison of side and tip light emission for the fiber optic ............................ 47

Table 6. Characteristics of the selected fiber optic ........................................................... 48

Table 7. Different superficial gas velocities reported in the literature ............................. 52

Table 8. Cost of the unit .................................................................................................... 61

Table 9. Summary of tests performed ............................................................................... 66

Table 10. Light intensity inside the operating reactor ...................................................... 72

Table 11. Calculated hydrodynamic properties for the air lift unit ................................... 73

Table 12. Measurements and calculations of Test 6 ......................................................... 83

Table 13. Mean maximum specific growth constant for Test 6 ....................................... 84

Table 14. Measurements and calculations of Test 7 ......................................................... 85

Table 15. Mean maximum specific growth constant for Test 7 ....................................... 86

Table 16. Measurements and calculations of Test 10 ....................................................... 91

Table 17. Test 10 ............................................................................................................... 92

Table 18. Measurements and calculations of Test 11 ....................................................... 93

Table 19. Test 11 ............................................................................................................... 94Table 20. Summary of the most representative intervals of Test 6, 7, 10 and 11 ............. 95

Table 21. Results Test 2 .................................................................................................. 109

Table 22. Results Test 3 .................................................................................................. 110


9/116

9

LIST OF FIGURES

Page

Figure 1. Bubble column photobioreactor (Bayless, 2007) .............................................. 16

Figure 2. Air lift reactor concept ....................................................................................... 17

Figure 3. Pattern flow in the annular sparged air lift ........................................................ 18

Figure 4. Operating range of air lift and bubble columns ................................................. 18

Figure 5. Carbon capture and sequestration technologies ................................................ 23

Figure 6. Carbon capture and sequestration with algae facility ........................................ 24

Figure 7. Near horizontal and loop tube horizontal reactors ............................................ 26

Figure 8. Flat V shaped reactor and fermenter ................................................................. 27

Figure 9. Free area between the downcomer and the riser ............................................... 29

Figure 10. Iteration process to determine the hydrodynamics parameters ....................... 30

Figure 11. Air lift photobioreactor conceptual design ...................................................... 38

Figure 12. Basic design of the internally lit air lift photobioreactor ................................. 41

Figure 13. Mock up for determining the light intensity .................................................... 45

Figure 14. Demonstration of side discharge light loss from fiber optic ........................... 46

Figure 15. Mock up for determining the distance between levels .................................... 49

Figure 16. Mesh as a fixation mechanism for the fiber optics .......................................... 50

Figure 17. Final design for the fixation mechanism for the fiber optics ........................... 51

Figure 18. Induced flow profiles ....................................................................................... 53

Figure 19. Different mufflers considered for sparging ..................................................... 54Figure 20. Final design of the bottom of the reactor showing the sparger ....................... 54

Figure 21. Mock up showing the alignment problem ....................................................... 55

Figure 22. Final design of the draft tube support system .................................................. 56


10/116

10

Figure 28. Operating sparger of the reactor ...................................................................... 70Figure 29. Underwater light sensor in the illuminated riser ............................................. 71

Figure 30. Rig for measuring the mixing characteristics .................................................. 74

Figure 31. Concentration of ion Cl- vs time ...................................................................... 75

Figure 32. Correlation between cell number and chlorophyll a ........................................ 76

Figure 33. pH changes in Test 1 ....................................................................................... 78

Figure 34. pH changes in Test 8 showing the addition of NaOH ..................................... 78

Figure 35. Biomass increase during Test 6 ....................................................................... 79

Figure 36. Biomass increase during Test 6 (log plot) ....................................................... 80

Figure 37. Biomass increase during Test 7 ....................................................................... 81

Figure 38. Biomass increase during Test 7 (log plot) ....................................................... 82

Figure 39. Biomass increase during Test 10 ..................................................................... 88

Figure 40. Biomass increase during Test 10 (log plot) ..................................................... 88

Figure 41. Biomass increase during Test 11 ..................................................................... 90

Figure 42. Biomass increase during Test 11 (log plot) ..................................................... 90


11/116

11

1 INTRODUCTION1.1 Energy Consumption

Forecast studies suggest that the world energy demand will increase by 50% by the

year 2025. Developing countries would be responsible for 74% of the increase (EIA,

2008). This phenomenon will be driven by an increase in the population in developing

countries and their consumption per capita as shown in Table 1.

Table 1. Power consumption and population trends 1992-2025 (Andrews, 2007)

There are several national and international bodies that research and publish future

energy forecasts. Most of the projections predict that fossil fuels will continue to be the

primary energy source until the year 2050 (IPCC, 2005).

1.2 Fossil FuelsFossil fuels are responsible for more than 85% of the worlds current primary

energy production (Miller 2005) The reliance on fossil fuels raises two important

Population(billions)

Powerpercapita(kW) Totalpowerconsumption(TW) Increase

DevelopedCountries 1.2 7.5 9DevelopingCountries 4.1 1.1 4.5Total 5.3 13.5

DevelopedCountries 1.4 3.8 5.3 41%DevelopingCountries 6.8 2.2 15 233%Total 8.2 20.3 50%

1992

2025


12/116

12

carbon dioxide (CO2) as a byproduct. CO2 is a green house gas (GHG). GHGs retain

heat inside the earth atmosphere, causing an increase in the temperature of the earths

surface over time. This effect is known as global warming/climate change (Katzer, 2007).

1.3 Importance of Coal EnergyCoal is the most abundant fossil fuel in the world (Hoffert et al., 2002). Coal will

supply one third of the new energy consumption (EIA, 2008). By 1998, coal had the

highest reserve to production ratio (R/P) among the fossil fuels with 218 years, while

natural gas was 63 years and oil 43 years (Miller, 2005). The coal reserves are distributed

around the world, but it is important to note that United States has the largest reserves

with more than 270 billion short tons (Miller, 2005).

Coal is also the primary source for power generation throughout the world and in

the United States. Coal accounts for more than the 49% of U.S. electricity production.

However, coal power plants also have higher CO2 emission rates compared with natural

gas power plants which increase the concern over the use of this fuel due to the CO2

green house effect.

1.4 BiofuelsBiofuels are high hydrocarbon content compounds that come from biological

organisms such as algae, plants or animals. Biofuels are distinguished from fossil fuels in

that their use is within a short time after the death of the biological organisms, while


13/116

13

Biofuels can be used instead of fossil fuels in some applications due to their

hydrocarbon content. Current technology development for biofuels is demonstrated in the

use of ethanol and biodiesel instead of gasoline and conventional diesel in the

transportation industry. Wood chips as a replacement for coal is also an example in the

power generation sector. The emission of fewer pollutants such as particulate matter,

carbon monoxide and unburned hydrocarbons is another advantage of biofuels over

conventional fossil fuels (Demirbas, 2009).

Algae have several advantages over other biological sources as a feedstock for

biofuels (Danquah et al., 2009). Algae have a higher ratio of oil production to required

cultivated area compared with more common sources such as corn, soybean, rapeseed,

jatropha and others (Chisti, 2008). Another characteristic of algae is that it can double its

biomass at an exponential rate (Chisti, 2007).

World annual oil consumption is expected to grow from the current level of 82

million barrels per day (Mb/d) to more than 104 Mb/d by 2030. Taking into consideration

the decline in production of the important oil fields, the world will require finding new oil

sources equivalent to six times the current oil production of Saudi Arabia. Recent studies

suggest that biofuels will have an important role in meeting the future oil shortage (IEA,

2009).

1.5 Photobioreactors


14/116

14

antioxidants, and other substances, many bioreactors have already been designed and

constructed (Chisti, 2008). One of the most important design and performance parameters

for a bioreactor is the amount of CO2 captured, or converted into algae, through the

photosynthetic process.

The first type of bioreactor, and the simplest one, is a natural (or artificial) lagoon

where paddle wheel agitation systems are used (Dutil, 2002). These designs are known in

the literature as open bioreactors (Tredici, 1999). Open systems require large amounts of

land area and have temperature and light fluctuations due to seasonal variations. Another

problem with open bioreactors is in the exposure to foreign microorganism contaminants

that usually compete with the target cultured strain, diminishing or neutralizing the

growth rate of the organism of interest (Tredici, 1999).

Efforts in design and construction of other types of bioreactors have been

completed, and several existing models units are closed systems. Generally, a closed

bioreactor consists of a container (tube, box, serpentine, etc.) in which the algal slurry is

recirculated. Light and heat are provided by various methods inside the system, resulting

in an optimum environment for growing the algae. Closed bioreactors are often referred

to as photobioreactors.

There are several advantages of photobioreactors over conventional open systems.

The closed systems allow more precise control over critical algal growth parameters


15/116

15

reactors require less land for the same biomass production. Finally, photobioreactors

could extend the growth potential from a few current microorganism strains used in open

systems to more than thousands of strains of phototropic algae (Tredici, 1999). Since

bioreactors require carbon dioxide to produce the algae, they could be used in the future

to address CO2 emissions while producing a source of biological matter for biofuels and

other bioproducts.

The design and scale up of photobioreactors still require further development. The

most important areas have been identified as 1) having a efficient lighting process, 2)

inefficient supply of carbon dioxide and oxygen removal, 3) absence of photobioreactor

engineering and scale up (Grima, 1999).

During the past decade, the Ohio Coal Research Center (OCRC) at Ohio University

has been working on the design and construction of different types of bioreactors. In

particular, the researchers at the OCRC developed a new patented design of a bubble

column bioreactor (Bayless, 2007). Bubble column photobioreactors are common in

biotechnology industries and laboratories, however new design concepts continue to

improve their performance.

The patented bioreactor is comprised of a cylindrical container in which a mixture

of air and carbon dioxide is introduced through a sparger at the bottom of the reactor

(Figure 1). The column is filled with a culture of water and algae. The gas flows in the


16/116

16

fiber optic cables distribute photons around the volume of the liquid, providing the

required light for growing the algae.

Figure 1. Bubble column photobioreactor (Bayless, 2007)

Another type of reactor also common in the process industry is the air lift

bioreactor. This type of reactor is usually composed of an internal pipe or baffle that

induces a liquid flow pattern as shown in Figure 2. Air lift systems are composed of a

riser and a downcomer. This reactor has been used extensively in the past for growing

microalgae (Grima, 1999; Barbosa, 2003). However, an extensive literature review was

unable to identify the development of an internally lit air lift reactor in previous work.


17/116

17

Figure 2. Air lift reactor concept

Air lift reactors differ from bubble column reactors in several aspects. The pattern

of flow in the bubble column devices follow the path of multiple flow cells inside the

column, while the air lift system generates a continuous upward flow in the draft tube and

a continuous downward flow in the annular space (Figure 3). Furthermore, the liquid

velocity is determined by the gas flow in the air lift system, while in the bubble column

reactor, velocity is independent of gas flow. This feature allows for high liquid linear

velocities inside the air lift reactor without the use of an external recirculation mechanism

(Chisti, 1989). This difference is important due to the shearing sensitivity of the algae,

and the fact that a hydrodynamic environment capable of cell shear is easily achieved

i id bi t (Mi t l 1999) Additi ll th i lift t h hi h


18/116

18

Figure 3. Pattern flow in the annular sparged air lift (left) and bubble column (right)reactors (Chisti, 1989)


19/116

19

1.6

Significance of Research

The proposed work was the development of a device that would help to address

two of the most important energy concerns of the world today. The design and

construction of the bench scale air lift photobioreactor would contribute to the

development of biofuels which are able to replace petroleum for liquid transportation

fuels. Further, this device could also be used for CO2 remediation in diverse industries

such as power plants, cement plants and others as explained in Section 2.2.

This work also intended to extend the development of the original OCRC bubble

column design idea to use fiber optics to deliver light for microalgae growth. The

contribution of the proposed work is that the concept was applied to the air lift reactor

design, providing new knowledge to the photobioreactor field. Additionally, this work

planned to determine the productivity of the reactor and thus provided a comparison point

with other technologies.

1.7 Project ObjectivesThe objective of this thesis was to design, construct, and validate the productivity

of an internally lit air lift photobioreactor for growing algae. The airlift photobioreactor

embodied several design features to facilitate its operation and maximize algal

production. An extensive literature review was done in order to identify different criteria

for the design process. The goal was to gain an understanding of current photobioreactors


20/116

20

robust construction. The final device should be easy to move, maintain and modify for

future research.

The final result of the project was the successful validation of the designed and

constructed internal lit air lift bioreactor.

Successful operation validation was defined by the following characteristics:

1. A description of the reactor hydrodynamics: A mathematical model of the

reactors main operational parameters was calculated using the equations presented in

Sec. 2.5. These calculations were used to assure the reactor was operated under the

ranges established in the literature. Specifically, the superficial gas velocity and

superficial liquid velocity were within the values previously reported for other air lift

units.

2. Verification of the superficial liquid velocity and circulation time: These two

parameters were experimentally determined as a part of an Undergraduate Independent

Research Project associated with this thesis. The reported values were compared with the

analytical model.

3. Measurement of internal light intensity: The light intensity was measured in

one point of the system providing an approximate value of the internal light levels

achieved in the lit chamber of the photobioreactor.

4. Determination of the algal specific growth rate constant under the following


21/116

21

2

LITERATURE REVIEW

2.1 Algae as a Source for BiofuelsAlgae can be defined from the economic point of view as microorganisms able to

harvest the sun and transform its energy to high value products using relatively

inexpensive resources (i.e. CO2 and H2O). Algae were first studied by German scientists

during World War II as they were trying to find alternate sources of proteins (Skjanes, 2007).

Algae have been historically considered as a source of chemical and food

components more than an energy supplier. The first concept of using algae as a biofuel

was developed circa 1955 with the growing of microorganisms in ponds that were

digested to produce combustible gas (Hu et al., 2008). During the oil embargo of the

1970s, the concept of algae as biofuel source emerged (Hu et al., 2008). The most

important work over that time was the Aquatic Species Program conducted by the U.S.

Department of Energy to identify and extensively research over 3,000 algal strains.

Today, algae are recognized in the forefront of biofuels production potential with

advantages such as high lipid accumulation, CO2 mitigation, and suitability to grow in a

variety of systems including bioreactors (Hu et al., 2008).

When compared with other renewable biofuels, biodiesel from microalgae seems to

be the only fuel with the potential to replace all the world oil transportation consumption

(Chisti, 2008). Theoretical calculations show that annual oil production of algae could be


22/116

22

consumption. In contrast, microalgae would require only 3% of the U.S. agricultural area

to substitute the oil transportation consumption of the U.S. (Chisti, 2008).

Algae production for biofuels could also help in the mitigation of CO2 emissions.

Approximately 50% of the algal dry biomass is carbon, which is primarily derived from

atmospheric CO2. That means that the CO2 produced by power plants, which is available

at little or no cost, could be used for growing algae (Chisti, 2008). Another source of CO 2

could be the dry algae itself. Once the oil is extracted, the dry algal biomass can be co-

fired in a power plant with its carbon emissions feeding the photobioreactors. All of the

energy required by the facility will be provided by the consumption of the dry biomass

making the whole production carbon neutral (Chisti, 2008).

However, biofuel production from algae is still a laboratory-scale process and

critical engineering developments are required in areas such as algal strain selection,

culture mass production, and lipid extraction to bring it to the industrial scale (Hu et al.,

2008). Specifically, there is a lack of developed photobioreactor engineering that

prevents accurate predictions of productivity, performance, scale up and the design of

industrial devices. The absence of that information undermines a reliable investment in

large-scale microalgal biodiesel production facilities (Chisti, 2008).

2.2 Carbon Mitigation and SequestrationWorld CO2 emissions are expected to increase from current levels of 29 gigatonnes


23/116

23

predicted, will require investments of at least 0.6% of the worlds total GDP per year. On

the other hand, there has been important research activity in the last several years related

to the removal of carbon dioxide and recycling of carbon using microalgae. Microalgae

are an ideal choice for this function since they have a higher photosynthetic efficiency

than other plants (Yun et al., 2001).

Among the different CO2 mitigation options, CO2 Capture and Sequestration (CCS)

is considered an important alternative for the stabilization of green house gas emissions

(IPCC, 2005). CCS can be applied to large point sources of green house gases, such as

power plants, cement plants, refineries and other industrial facilities. CCS generally

consists of a CO2 separation process and the subsequent compression of CO2 (Figure 5).

Then, the CO2 is pumped through pipelines and buried underground or in the deep ocean.


24/116

24

CCS can reduce green house gas emissions up to 90% in power plants when

compared with current cases without any carbon mitigation technology. Research shows

that, by the year 2050, CCS will be technically feasible for around 20-40% of CO2

emissions (IPCC, 2005). Carbon capture and sequestration is important for this project

since its deployment will create a new CO2 compressed source. Air lift photobioreactor

technology requires pressurized CO2 for growing the algae and CCS could be an

important supplier in the future, as depicted in Figure 6. This research was focused in the

internal air lift development, in anticipation of a potential for broader, extensive use of

this system in CCS facilities.


25/116

25

2.3 Photobioreactors TypesThe term photobioreactor has been used in the past to also refer to open systems

like ponds and water channels. However, current use of this term is usually reserved for

closed systems that are isolated from the environment, avoiding possible contamination

(Grima, 1999). Another advantage of closed systems is a lower cost of the harvesting

stage due to the possibility of achieving higher cell densities. Nevertheless, open ponds

are less capital intensive and closed systems only recently appeared in industrial

applications (Carvalho, 2006).

There are several designs of photobioreactors and most of them can be classified

into three categories: tubular, flat plate and fermentors. The former two are the only ones

that can take advantage of the outdoor free light while fermentors cannot due to their

stainless steel construction (Carvalho, 2006). Fermentors are characterized for being able

to achieve high culture densities (>50 g/L) while the other types of photobioreactors can

handle concentrations lower than 10 g/L (Andersen, 2005). However, tubular and flat

plate systems are currently the most popular type of reactors due to their lighting

characteristics (Carvalho, 2006).

Tubular reactors can be categorized as vertical or horizontal, as well as other

configurations. The bubble column and the air lift system are considered within the first

category. Examples of horizontal tubes are the near horizontal and loop reactors depicted


26/116

26

Figure 7. Near horizontal (a) and loop tube horizontal (b) reactors (Carvalho, 2006)

Flat plate reactors are characterized by their high surface to volume ratio, similar to

the ponds, hence providing efficient light use. However, these systems have been

considered expensive and some problems in culture flow have been reported. An example

of a novel flat plate system is shown in Figure 8.

Fermentors are the last type of photobioreactors and have been criticized for their

low area to volume ratio which decreases the sunlight capture. These systems often

require the use of artificial light, increasing their operating costs. Fermentors also use a

mechanical stirrer that homogenizes the broth (Figure 8), and they are characterized by a

high level of control of culture conditions like pH and temperature. Fermentors have been

widely used in the food and pharmaceutical industries leading to a significant quantity of


27/116

27

Figure 8. Flat V shaped reactor (left) and fermentor (right) (Carvalho, 2006)

2.4 Bubble Column and Air Lift ReactorBubble column and air lift reactors are relatively simple devices which are widely

used among the chemical, petrochemical, and wastewater industries as reactors, absorbers

and strippers for gas-liquid systems. These reactors offer many advantages due to their

large gas-liquid interfacial areas. However, past research has focused mostly on their use

in non phototropic applications (Miron et al., 2000).

Air lift and bubble column reactors are frequently used as bioreactors on a research

scale. However, there is little or no development of techniques to scale them up to

industrial size where they could become an important design option taking into

consideration their advantages over other types of bioreactors (Miron et al., 2000).


28/116

28

2.5 HydrodynamicsImportant hydrodynamic considerations for the design and performance of bubble

column and air lift reactors include gas hold up, and superficial gas and liquid velocities.

The superficial liquid velocity equation can be derived from an energy balance inside the

reactor. The details of its derivation are not shown here and can be found in other

references (Chisti, 1989). The superficial liquid velocity ULris determined by:

.

Equation 1

where is the percentage of gas in the riser respective to the liquid, also known as gashold up, and is the downcomer hold up. and are the areas of the riser and thedowncomer, and is the gas liquid dispersion height. The parameter is a function ofthe unaerated liquid height (

) and the overall gas hold up inside the reactor (

), as

defined by Equation 2.

Equation 2The factor, also known as the frictional loss coefficient, can be determined

using Equation 3:

11.40. Equation 3where is the free area between the riser and the downcomer as shown in Figure 9


29/116

29

Figure 9. Free area between the downcomer and the riser (shaded) (Chisti, 1989)

The gas hold up can be calculated using the governing equation of the bubble

column reactor as defined in Equation 4 (Chisti, 1989).

... Equation 4Then, the hold up in the downcomer induced by the liquid circulation is determined

by:

0.89 Equation 5Equations 1 to 6 can be solved through an iteration process guessing an initial valueforULras explained in Figure 10. This was the process used to determine the

hydrodynamics results discussed in Section 4.3.


30/116

30


31/116

31

2.6 Gas FlowThe shearing action of the bubbles in the liquid has been previously studied. The

shearing action has been considered as a positive variable since it induces slurry mixing

and light transfer. However, excessive shearing can cause cell damage and death. Cell

damage due to shearing actions has been considered in the past as a key problem for the

development of algal photobioreactors (Barbosa, 2003).

There are constraints in the flow rate regarding the maximum and minimum levels.

High flow rates can induce excessive levels of turbulence inside the reactor, a potentially

dangerous shearing condition for microalgae cells. Additionally, elevated gas flow can

reduce the light distribution, diminishing a critical parameter of bioreactor performance.

Past research has identified the presence of micro-bubbles in the reactor which may

reduce light penetration in the fluid. The small diameter bubbles create a cloudy

environment that induces shading of the algae by the bubbles (Miron et al., 2000).

However, low gas flow rates decrease the circulation velocity in the reactor creating

stagnant zones in the system. Algal cells can accumulate in those areas away from light

and CO2 supply (Miron et al., 2000).

2.7 Light AvailabilityThe light subsystem is a key component of the design process since phototropic

growing processes are considered light limited (Tredici, 1999). Furthermore, light cannot


32/116

32

Certain parameters like nutrients and temperature are relatively easy to regulate while

illumination is more difficult to control (Grima, 1999). A faster development of algal

biotechnology depends on the design of new photobioreactors in which light is efficiently

used (Barbosa, 2003). In the past, nearly all bioreactor designs have relied on a high

surface area to volume ratio in order to achieve light penetration and high photosynthetic

efficiencies.

Light availability and intensity are the most important limitations for the growing

of photosynthetic cultures (Grima, 1999). Previous research efforts have studied light

penetration through an algal suspension in a quantitative manner. Those studies have

concluded that the photon flux density (PFD) of a light ray decays with the fluid depth

according to Equation 6 (Yun et al., 2001):

Equation 6

whereA is the polychromatic light attenuation coefficient in m-1, and PTand PTo are the

transmitted and incident photon flux densities in mol/(m2s). This equation is important

to understand that light has an exponential decay with distance, the reason why the

distance between light sources for this photobioreactor is an important design criterion.

2.8 ProductivityThe increase in a population of cells can be described by the following differential

equation:


33/116

33

2002). The determination of the specific growth rate constant is an important method for

measuring the productivity of microalgal cultures (Andersen, 2005). Equation 7 can be

integrated to obtain its linear form:

Equation 8where 1 and 0 are different moments in time. Equation 8 can be used to find the specific

growth rate constant if theNvalues are plotted on a logarithmic chart. Additionally, the

Least Squares Regression can be applied to several time intervals of the algal growing

phase in order to determine the maximum mean specific growth rate constant. This

approach is used in Section 4.6.


34/116

34

3 DESIGN APPROACH3.1 Design Constraints

There are limits for everything and the design process is not an exception (Dym et

al., 2004). There were several constraints with regard to the design process for this

system that were identified through past experience and review of the recent literature.

The design approach addressed the identified constraints and implemented suitable

solutions.

The dimension and characteristics of the fiber optic cables used in the reactor were

defined previously. It was anticipated that a parabolic solar collector (Sunlight Direct,

2008) that concentrates the light in a cable composed of 127 optical fibers would be

acquired. Each fiber has a diameter of 3 millimeters. Therefore, the design had to

accommodate the same dimensions of the fibers in such a way that the air lift device

could operate with the sunlight collector once installed. As the collector was not available

prior to the completion of the project, this research used an artificial source to provide the

light for growing the microalgae (Section 3.4.1). An additional benefit of the artificial

light source was that the use of the natural system would affect the repeatability of

growth rate measurements due to the variability of the natural sunlight throughout the day

and from day to day.

Another important constraint related to size was that air lift systems should follow a


35/116

35

1989). The design accommodated this geometric restriction through the selection of pipe

sizes as explained in Section 3.3.

The gas flow should be maintained within certain limits as discussed earlier

(Section 2.6). Extremely low or high gas flow rates could impose growth inhibiting

conditions for the algae. This project identified the limits based on previous work and

defined adequate levels of flow for the proposed reactor as explained in Section 3.4.6.

As a research tool, the device should meet the criteria of being removable,

replaceable, and accessible for maintenance. All the pieces of the device were able to be

removed and replaced by ones of an alternative design, assuring flexibility for future

researchers using the device. The amount of permanent fixtures was minimized so there

will be easy access for future modifications.

3.1.1 Subsystem DesignsThe first subsystem designed was the light distribution system. This subsystem was

comprised of a light source, the fiber optic cables, any necessary fixtures, and the

distribution subsystem. The source provided the required light levels inside the reactor

through the use of the fiber optics. The source was selected from commercially available

illumination systems. The unit was adapted to fulfill the required function taking into

consideration the safety and reliability of the system, and targeted light intensity (Section

3.4.1).


36/116

36

Plastic Optic Fibers (POF) are stronger, supporting more stress and pressure than glass

ones. Further, plastic fibers have more flexibility and a smaller bending radius than glass

systems, a primary reason they are broadly used in architectural and automobile

applications. Plastic fibers are also usually less expensive than traditional ones made of

glass (Bailey, 2003).

The optical fibers were carried inside the reactor volume using a central core pipe

acting as a header that minimized interference with the algal growth and provided

sufficient space for the required number of fibers (Section 3.2). Once the fibers were

directed from the light source into the header, a subsystem was developed that distributed

the photons within the algal slurry (Section 3.4.5).

The containment vessel was comprised of materials that offer ease of construction

and may be made leak proof at a reasonable cost. Material homogeneity was desired to

keep compatibility and constructability of the sub-components (Section 3.3). The size of

the containment vessel was determined by a combination of two criteria. The first

consideration was the availability of the materials in the commercial market. Second, the

material was selected according to geometry restrictions. The diameters of the pipes

obeyed the general characteristic of air lift reactors regarding the ratio of the downcomer

and riser areas as explained earlier in Section 3.1. The size of the device also took into

consideration that this was a prototype system at the bench-scale. The size constraints


37/116


38/116

38

Figure 11. Air lift photobioreactor conceptual design

The riser was illuminated with fiber optics that were directed through the header

pipe placed in the center of the reactor. The header served as the conduit to carry the fiber

optic cables inside the reactor volume, as well as the gas supply lines. This component

also provided the structural support required to hold the fiber optics in place while in the

flow path of the growth media that carried the algae. The draft tube was the required

baffle to allow circulation of the growth media inside the air lift device. The liquid

circulated upwards in the area contained between the header and draft tube, while the

media moved downward between the draft tube and the containment vessel. Both header


39/116

39

3.3 Basic DesignThe size of the reactor was determined based on the original purpose of the project

of building a laboratory-scale unit. The existence of a previous patent on the bubble

column system and an existing bench scale unit were important in the decision to build a

larger unit. This process is how a one meter height unit was proposed. The diameter was

also modified from the original patent because the header increased the dead space inside

the system. Therefore, to keep the same working volume, the height of the unit was

established at one meter (Bayless, 2007).

A commercial material easy to obtain and work with was chosen for the

construction. It was necessary at the same time to take into consideration important

constraints of the material application such as; permanent immersion under water,

tolerance with algae, and stabilization properties. It was quickly decided to eliminate steel

pipes from consideration because of the difficulty to manufacture (i.e. to open holes and

cutting) and the corrosion potential in aqueous environments. Also, stainless steel was

expensive and not easy to manufacture. Acrylic was considered as it has been used in the

past in photobioreactors applications (Hsieh & Wu, 2009). However, the acrylic bonding

process is more complicated than PVC and requires more expensive products. The PVC

bonding process is well known and is used every day in plumbing applications. This is

why PVC was chosen as the best commercial material for the laboratory unit.


40/116

40

Taking into consideration the conceptual design defined first using a header in the

core of the reactor, it was necessary to analyze different commercial pipe sizes available

in the market that met the previous criteria. This process was combined with the design

constraint of the size of the header, taking into consideration that the header pipe would

carry all the fiber optic cables and the gas line. It was determined that the minimum

diameter of the header to carry 44 optical cables along with the gas line to be at least 2

inches. Based on the previous analysis, Table 1 was produced.

Table 1. Analysis of the pipes available

Header/Draft/Containment Diameter Ratio

in/in/in Ar/Ad

Ad 10,036Ar 15,781

Ad 12,256

Ar 12,432

Ad 12,256

Ar 8,378

Ad 28,634

Ar 15,781

Ad 13,179

Ar 28,289

2/6/10

3/8/10

0.6

2.1

4/6/8

1.0

0.7

Areas

mm2

1.62/6/8

3/6/8


41/116

41

The best combination was 3 inches for the header, 6 inches for the draft tube and 8

inches for the containment vessel. A basic drawing of the system was proposed in Figure

12, based on the discussion of the separation between levels in Section 3.4.4.

Additionally, the light loss in a fiber optic cable is proportional to the number of curves

(Abdul, 2007). This was another design criterion for the system. To minimize the bending

of the cable, a 45 inclined system of holes in the header was proposed to enhance a

smooth transition from the inside to the outside of the pipe. This is also indicated in

Figure 12.


42/116

42

3.4 Detailed DesignIn this section, several considerations are analyzed to define the final characteristics

of the novel device.

3.4.1 Light Source SelectionTwo sources were analyzed for providing the light for the air lift photobioreactor.

The first option considered was an old Kodak slide projector. The projector was powered

by a 300W light bulb working at 82 Volts and had an integrated fan to dissipate the heat.

This projector was considered due to its light focusing hardware and high power and

voltage.

The second light source option was a microscope illuminator powered with a EKE

light bulb of 21 volts and 150 watts (GE lighting, Cleveland USA). This equipment was

considered since these devices are designed to provide light through cables for

microscope applications. The light intensity was measured in both sources using a dry

Li-cor LI190 quantum sensor on four different points of the light exit hole (LI-COR,

Lincoln, NE, USA). The light photosynthetically active radiation (PAR) (Biosciences,

2008) readings for the slide projector were 6,000 500 mol/(m2s) while for the

microscope illuminator they were 11,000 2,000 mol/(m2s). The microscope

illuminator was chosen as the light source for the project based on its higher intensity,

and the information of the bulb used is listed in Table 2.


43/116

43

Table 2. Characteristics of the light bulb

Consideration/Property Value

Bulb type High intensity halogen

Bulb temperature 3,350 Kelvin

Base GX5.3 Bipin

Color Rendering Index 100

Average life 200 hrs

3.4.2 Determination of the Minimum Number of FibersIn order to achieve the required light intensity inside the photobioreactor it was

necessary to determine the minimum number of optical fibers at each level in the system.

This approach was based on the theoretical treatment of light as a particle (photon). First,

the intensity provided by the light source was measured at approximately the same

distance from the optical fibers tips. A Li-cor LI190 quantum sensor was faced against

the light bulb and the readings recorded. The average light intensity obtained is reported

in Table 3. It was then possible to determine the number of micromoles of photons per

fiber based on the area of each cable. Finally, the number of fibers required was

computed taking into consideration the area of the riser previously defined.


44/116

44

Table 3. Determination of the amount of fibers

To confirm that the target light intensity was achieved with 11 fibers per level as

suggested by the previous calculations, a column mock-up was developed. The rig was

constructed using the same header dimension (3 inches) as defined previously with two

mounted optical fibers. The cables were attached according to the final design using PVC

bolts as explained subsequently in Section 3.4.5. The angular distance between the two

fibers was calculated based on the number of fibers and determined to be 32. The light

sensor was placed at a defined distance (35, 95 and 120 mm) facing the optical fibers as

shown in Figure 13 in a dark room. The light sensor was displaced in the tangential axis

at seven positions and each reading was separated 8, as explained in Figure 13.

1 PFD provided by light source (mol/(sm2))

a11000

2 Area of fiber (m2) 7.06E-06

3 Photons per fiber (mol/(sf)) 0.08

4 Number of fibers per level (f) 11

5 Total incident light per level (mol/s) 0.91

6 Average Irradiance per level (mol/(sm2)) 73

PFD: Photon flux density

a = measured at 10 mm distance from the light bulb

LIGHT CALCULATION


45/116


46/116

46

Table 4. Light intensity readings for different distances inside the photobioreactor

3.4.3 Light Distribution along the Fiber OpticOne of the main advantages of the proposed reactor is to transport the light from

the light source to the algal culture. As explained before, this was achieved with the use

of the fiber optics, but there was a concern about light losses through the side of the fiber

along its length; an issue that was not considered in Section 0. A small mock-up was

developed to determine the magnitude of light loss (Figure 14).

Distance

mm

1 2 3 4 5 6 7 Average

35 55 437 544 215 599 603 98 364

95 124 170 206 213 211 185 144 179

120 63 84 99 103 105 98 69 89

Readings

mol/(s*m2)


47/116


48/116

48

the amount of light discharged from the side surface against the amount leaving the tip. It

was found that the side emission was approximately 7% of the intensity at the tip.

3.4.4 Selection of the Fibers and Separation between LevelsThe fiber optic selected was the PGR-FB3000

supplied by Moritex USA. The

fiber was composed of the polymer polymethyl methacrylate and the average diameter of

the fiber was 3 mm. The fibers could operate up to a maximum temperature of 70oC,

which is higher than the reactor operation temperature (room temperature). The selected

fiber optic had a maximum bending radius of 20 mm. The fiber characteristics are listed

in Table 6.

Table 6. Characteristics of the selected fiber optic

dB/m is the common unit for light losses

Characteristic

Spool Price $280

Length Spool 492 ft

Bending Radius >20mm

Temperature Range up to 70 C

Core Material PolymethylmethacrylateCladding Material Fluorinated polymer

Attenuation (dB/m) 0.2


49/116

49

It was found that for a separation of 8 cm, the fiber optics where under significant strain

which induced possible risk of rupture. This was a key constraint because during the

cleaning of the system, the fibers could be impacted which could cause premature

rupture.

A separation of 12 cm would generate larger dark zones in the illuminated area and

additionally would increase the height of the reactor. These were the main reasons for the

decision to select a distance between levels of 10 cm.

Figure 15. Mock up for determining the distance between levels

3.4.5 Fixation of the Fiber Optic


50/116

50

had the disadvantage of imposing significant resistance to the liquid flow. Additionally,

the mesh would require support both inside and outside of the pipes to provide sufficient

stiffness. That stated, the mesh would encompass all of the area of the riser as shown in

Figure 16. Another significant problem would be the potential of clogging and difficulty

cleaning the system due to the intricate configuration of the mesh. Furthermore, it was

not possible to assure that the fiber optics would maintain a position since the mesh could

not provide a reliable fixation mechanism.

Figure 16. Mesh as a fixation mechanism for the fiber optics

A different design was then proposed based on individual supports rather than a

combined mesh (Figure 17). This provided the necessary fixation required while

maintaining a structurally sound point for the fiber optic cables. Since the material of the

pipes was PVC, PVC bolts were chosen as the element to provide the support as shown in


51/116

52


52/116

52

whole area of the reactor as if it had no water. Table 7 summarizes different values forUg

for reactors reported in the literature.

Table 7. Different superficial gas velocities reported in the literature

Table 7 shows that previous studies have been successful growing algae in the

superficial gas velocity range of 0.07 to 2.50 cm/s. However, an upper limit of 3.40 cm/s

was reported where some concern regarding cell damage due to shear stress arose. Since

the superficial gas velocity () is the driver of gas circulation, it was necessary to

Reactor Diameter Ug Fg Total

(cm) (cm/s) (LPM)

1 Ranjbar1 4.6 ALR Yes 0.07 0.1

2 Ranjbar2 4.6 ALR Yes 0.13 0.1

3 Vasconcelos1 3.5 BCR Yes 0.50 0.3

4 Vasconcelos2 21 BCR Yes 0.60 12.5

5 Camacho1 8 BCR Yes 1.00 3.0

6 Pilot Air lift 20.3 ALR Yes 1.80 13.3

8 Vasconcelos 3 21 BCR Yes 2.50 52.0

9 Vasconcelos 4a 3.5 BCR Yes 3.40 2.0

10 Camacho1 8 BCR Yes 5.00 15.1

Fg = Gas flow

ALR = Air lift reactor

BCR = Bubble column reactor

a= Limit before detecting lethal conditions

# Study TypeAlgal

growth


53/116

54


54/116

54

Figure 19. Different mufflers considered for sparging

A Swagelok piping arrangement was selected to supply the gas flow to the

spargers and provide the necessary stiffness to keep all of its components in place. The

system was composed of the required elbows and tees as seen in Figure 20. Additionally,

a test plug was inserted into the bottom of the header pipe in order to isolate the header

internal volume from the working volume of the photobioreactor (Figure 20). A hole was

drilled in the test plug to connect the gas supply hose with the sparger arrangement.

55


55/116

55

3.4.8 Draft Tube Support and Pipe CenteringThe air lift reactor differs from the bubble column system due to its internal draft

tube. Additionally, the proposed design had an additional pipe to deliver and fix in place

the fiber optics and the sparger system. The development of a mock up at the beginning

of the construction was used to discern any alignment problems between the pipes. This

was a key requirement since the lack of alignment would distort the uniform air lift flow

of the media. A set of PVC bolts and rods were selected to eliminate any alignment

problems. Six bolts, three at the top and three at the bottom, spaced at 120intervals,

were inserted into the header providing the necessary centering between the header and

the draft tube. The same solution was applied in order to align the draft tube with the

containment vessel (Figure 21).


56/116

57


57/116

the bottom flange. However, it was necessary to analyze its final position in order to

determine whether it should be in the bottom or in the wall of the flange as presented in

Figure 23.

Placing the drain a few inches above the bottom prevented the collection of

samples that could contain algal sediments that were not representative of the reactor

culture.

58


58/116

59


59/116

3.4.10 Mobility of the SystemIt was predetermined that all reactor testing would be done in the ISEE facility,

however it was necessary to transport the reactor to multiple rooms within the laboratory.

For example, it was essential to take it from the testing room where the compressed air

system was, to the cleaning room where the floor drain was located. Additionally, several

auxiliary pieces of equipment were attached to the reactor, such as the light source, flow

rotameters, and future CO2 tank. It was decided to develop a mobile rack system that was

able to easily transport all of the reactor components to different rooms of the research

facility.

The design of the mobility cart was also based on the assumption that a future

additional unit (either of the same type or different reactor) would be built. Space was

therefore reserved so the mobility cart was able to carry two research reactors along with

their required sub-systems. The dimensions of the cart were also constrained by the size

of the doorways (width and height) of the research building. A conceptual design of the

cart was developed based on the considerations discussed above (Figure 24, left). The

detailed design and construction of the cart was completed by the ISEE shop department

and a picture of the final cart is shown in Figure 24 (right).

60


60/116

Figure 24. Basic design of the stand (left) and its final construction

3.4.11 CostThe costs of all components of the reactor are shown in Table 8. They represent

only direct cost of materials and do not include labor costs for manufacturing or the

design of the unit. Labor costs are estimated to be about 250 man hours. Assuming an

hourly cost of $10 per hour for fabrication time these costs can be estimated to be about

61


61/116

Table 8. Cost of the unit

3.5 Analytical MethodsThe analytical methods used for the design process of Chapter 3 and productivity

Item Quantity Units Unit cost Total Cost

$ $

Pipe

8" Clear PVC 4 ft 50 150

6" White PVC 3 ft 7

3" White PVC 4 ft 4

Flange

Flange 2 u 47 95

Blind Flange 2 u 80 160

Bolts & Gaskets 2 u 46 92

Bolts & Nuts

Light Fixation Bolts 44 u 1 44

Centering Bolts 6 u 1 6

Suspension Rods 12 u 3 36

Suspension Nuts 5 u 5 25

Sparger

Mufflers 4 u 6 24

Swage Connections 4 u 6 24

Plug 1 u 6 6

Fiber Optic 300 ft 300Light

Light Source 1 u 150 150

Controller 1 u 10 10

Stand 1 u 350 350

Rotameters & Valves 6 - 23

Subtotal 1506


62/116


63/116

64


64/116

underwater sensors, as well as the light meter, were fabricated by LI-COR (Lincoln, NE,

USA).

3.5.6 Air FlowThe flow rate of air going into the reactor was controlled by a FL 3440ST rotameter

(OMEGA, Connecticut, USA). Daily readings of the rotameter were taken and recorded

at the time of collecting samples for the fluorescence and pH analysis.

3.5.7 MediaDifferent media recipes and nutrient supply products were used during the

productivity tests of the reactor. All the media were based on the use of Reverse Osmosis

water (RO water) supplied by the filtration units in the laboratory.

Botanicare and Calmag products were used as explained in Section 4.6 and its

composition is listed in Appendix B. Bristol was the media suggested by the Culture

Collection of Algae at the University of Texas at Austin (UTEX), the origin of the

cultures. Bristol was used as described in Section 4.7 and its composition is also listed in

Appendix B.

65


65/116

4 RESULTS AND DISCUSSION4.1 Introduction

Twelve tests were performed to determine the specific growth rate constant in the

photobioreactor and determine the operation and maintenance procedures of the novel

device as presented in Table 9. The alga that was used for all the tests was Chlorella sp.

(UTEX 2714). The experiments focused on gaining an understanding of the system more

than a rigorous statistical determination of the rate constant. The first four experiments

(Tests 1-4) examined the reactor behavior and some problems were encountered with the

sparger and cleaning procedures as described in Section 4.2. All experiments compared

performance using different growth media in the reactor. The first eight experiments

(Tests 1-8) used a combination of RO water and daily additions of Botanicare with two

different buffers as described in Section 4.6. The last three experiments (Tests 10-12)

used Bristol media as recommended by UTEX. Selected results are presented in the

sections below and data collected from each test can be found in Appendix A. All the

productivity tests were performed under the same air flow rate of 13.3 LPM.

66


66/116

Table 9. Summary of tests performed

4.2 Product Performance and MaintenanceIn this section, select isolated results from the tests are presented that are indicative

of maintenance and operating findings A more detailed discussion about the productivity

Test # Duration Media pH Buffer

Initial Final

days g/L g/L h-1

1 17 33.0 51.0 0.001 Botanicare1 Phosphate

2 5 4.6 8.2 0.005 Botanicare Phosphate

3 10 15.6 54.9 0.005 Botanicare Phosphate

4 13 31.0 116.0 0.004 Botanicare2 Phosphate

5 16 12.0 93.0 0.005 Botanicare NaOH

6 9 21.0 78.0 0.006 Botanicare NaOH

7 8 26.0 69.0 0.005 Botanicare NaOH

8 8 22.0 6.0 -0.007 Botanicare NaOH

9 4 10.1 19.3 0.007 BG-11 HEPES

10 10 7.6 85.3 0.010 Bristol None

11 12 8.4 86.7 0.008 Bristol None

12 3 14.6 5.5 -0.014 Bristol + PP3 None

1 = Sparger problems, low productivity

2 = Insufficient data

3 = Proteose peptone (PP)

Chlorophyll Growth rate

constant


67/116

68

b f h di d di f fl i h di f i


68/116

be a reason for the distorted readings ofin vivo fluorescence in the media of Test 8 since

the biomass was growing on the surfaces of the reactor rather than in suspension.

However, the light source (the optical fiber tips), always remained clean and provided the

required luminescence to the culture.

Figure 26. Chlorophyll a readings for Test 8

22.6

29.027.6

33.9

28.9

19.4

9.16.0

0.0

5.0

10.0

15.0

20.0

25.0

30.0

35.0

40.0

0 50 100 150 200

Chlo

rophylla(ppb)

Time (hours)


69/116

70

set of spargers for each test was laborious and costly therefore it was decided to remove


70/116

set of spargers for each test was laborious and costly, therefore it was decided to remove

them and only operate with free jet streams for subsequent tests as shown in Figure 28.

Figure 28. Operating sparger in the reactor

4.2.2 Light IntensityAn underwater LI-192 light sensor was used to measure the approximate light

intensity in the operating system (LI-COR Lincoln, NE, USA). The light sensor was

connected to a LI-250A Light Meter (LI-COR Lincoln, NE, USA). The underwater

sensor was placed inside the lit chamber as presented in Figure 29.

71


71/116

Figure 29. Underwater light sensor in the illuminated riser

The light intensity was recorded during a productivity shake down test for 5 days.

A 10 second average was determined automatically by the light meter and the results are

presented in Table 10. The intensities presented in Table 10 were recorded while the

reactor was in operation, thus they include the action of the bubbles. It was noticed that

changes in the flow rate altered the amount of light captured by the light sensor. The flow

rate used during the test was 13.3 LPM. This implies that it is not possible to determine a

direct correlation between light intensity and Chlorophyll a since the action of the

bubbles was present as well. However, some conclusions can be found after analyzing the

data. The first measurement indicates that the light levels inside the illuminated area are

above the specified target of 80 mol/(m2s). In this case, the measured intensity doubles


72/116


73/116


74/116

75

The change in concentration with time can be seen in Figure 31. The oscillation in


75/116

g g

the concentration is caused by the upward and downward circulation in the air lift system

and can be analyzed to indicate circulation time. Figure 31 shows the circulation time in

the system for an operating air flow rate of 13.3 LPM. These results show that the actual

circulation time of the system is somewhere between 20 and 25 seconds. This is slightly

longer than the predicted circulation time of 16.4 seconds. However, this still indicates

that an algal cell takes an average of 23 seconds to circulate around the system.

Figure 31. Concentration of Cl- ion vs. time for the operating airlift photobioreactor

5

10

15

20

25

30

35

4045

50

55

60

65

70

75

0

2

4

6

8

10

12

0 10 20 30 40 50 60 70 80

Cl-(mg/L)

Time (s)

25 s 20 s 25 s

25 s20 s


76/116


77/116

78

10.00


78/116

Figure 33. pH changes during Test 1

Due to increase of Na+ in the media, it was decided to use NaOH to neutralize the

acidic nature of the nutrients source without including NaH2PO4H2O. The next four tests

(Tests 5-8) were run using NaOH to control the pH and an example of the pH profile

during Test 8 is shown in Figure 34. Overall, no advantage was observed using NaOH

instead of Sodium Phosphate monobasic when the average specific growth rate constants

are compared (Table 9).

7.36 7.636.88 6.71

6.355.90 5.73 5.63 5.55 5.41

6.476.80 6.67 6.61 6.53 6.38 6.15

0.00

5.00

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

pH(-)

Time (Days)

7.1

6.7 6.7

6.56.7

7.47.2

6.9

5 5

6.0

6.5

7.0

7.5

pH

79

Two of the tests, the most relevant in terms of productivity and quality of data,


79/116

were evaluated in more detail. Table 12 presents the data in the following order; date and

time of the sampling, accumulated time in hours, and average chlorophyll a concentration

based on three readings (R1, R2 and R3). The standard deviation of the readings was also

calculated and presented along with the instantaneous specific growth rate constant as

determined for each sample point. The number of algal cells was determined based on the

calibration curve and that productivity also presented. Finally, the pH, temperature and

daily addition of Botanicare are reported.

In Test 6, the biomass increased continuously for 91 hours, and then decreased

slightly until the 135th hour (Figure 35). There is no a clear explanation for the decrease

in biomass, however it is hypothesized that a drop in pH could have interfered with

optimal growth conditions.

21.2 24.4

35.7

54.2

63.3 63.7 60.7

80.9 81.9 78.1

0.95 1.10

1.61

2.442.85 2.87 2.74

3.65 3.69 3.52

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

0

10

2030

40

50

60

70

80

90

CellDensity(106/mL)

Chlor

ophylla(ppb)

80

The specific growth rate constant () for different intervals was calculated for the


80/116

test. The growth constant estimated between points 0-91 hours was 0.013 h-1 with an R2

coefficient of 0.955 (Table 13). This indicates that exponential growth was experienced

during that period. Exponential growth at a higher rate was achieved between hours 26-

67, with =0.019 h-1 and R2=0.994 (Table 13). The other intervals were under lag or

stagnant phase as can be seen in Figure 36. A maximum instantaneous growth rate of

0.022 h-1 was achieved during this test.

Figure 36. Biomass increase during Test 6 (log plot)

In Test 7, the amount of Botanicare added each day was increased as can be seen in

Table 14 It was hypothesized that increasing the availability of nutrient would enhance

21.224.4

35.7

54.263.3 63.7 60.7

80.9 81.9 78.1

0.951.10

1.61

2.442.85 2.87 2.74

3.65 3.69 3.52

0.1

1.0

10.0

1

10

100

0 50 100 150 200

CellDensity(106/mL

)

Chlorophylla(ppb)

Time (hours)

Average Cell Density

81

38) indicates that the Chlorella in Test 7 did not experience exponential growth. This


81/116

result can also be confirmed by analyzing the R2 coefficients lower than 0.9 provided in

Table 15. The highest instantaneous growth rate was 0.027 h-1 at 72 hours.

It is unclear if the erratic behavior of the algal activity was caused by a pH drop or

was simply a Chl a measurement error. It can be seen that there was a consistent decrease

of 1.3 pH units in the three days prior to the Chl a reading of 37.3 g/L at 94 hours.

However, an error in the measurement of chlorophyll a cannot be ruled out. The use of

continuous monitoring systems could avoid errors such as this in the future as discussed

in the recommendations section.

26.122.3 22.7

52.6

37.3

56.6

63.1

69.0

1.21.0 1.0

2.4

1.7

2.62.8

3.1

0.00.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

0

10

20

30

40

50

60

70

80

0 50 100 150 200

CellDensity(106/mL)

Chlorophylla(p

pb)

Time (hours)


82/116


83/116


84/116


85/116


86/116


87/116


88/116


89/116


90/116


91/116

92


92/116

Table 17. Test 10

Interval Time Productivity Square

h h-1

1-9 190 0.011 0.994

2-8 142 0.011 0.992

2-6 103 0.013 0.999

6-9 69 0.010 0.965

93

Table 18. Measurements and calculations of Test 11


93/116

Time Chl a Average R1 R2 R3 STD Productivity Cell dens ity Productivity pH Temperature

h g/L g/L g/L g/L g/L h-1

*106/mL h

-1 C

0 5/25/2010 18:00 0 7.1 22.1

1 5/26/2010 23:00 29 10.4 10.6 10.2 10.4 0.2 0.47 7.2 29.2

2 5/27/2010 17:00 47 13.3 12.9 12.7 14.4 0.9 0.014 0.60 0.014 7.3 29.1

3 5/29/2010 14:18 92 19.7 19.9 19.8 19.5 0.2 0.009 0.89 0.009 7.4 29.5

4 5/30/2010 18:00 120 28.1 28.4 28.0 28.0 0.2 0.013 1.27 0.013 7.4 29.5

5 5/31/2010 20:50 146 46.8 47.0 46.8 46.5 0.3 0.020 2.11 0.020 7.5 29.3

6 6/1/2010 16:50 166 56.2 56.9 56.7 55.0 1.0 0.009 2.53 0.009 7.5 29.3

7 6/2/2010 16:00 190 68.7 68.8 69.1 68.3 0.4 0.008 3.10 0.008 7.5 29.5

8 6/3/2010 16:00 214 54.21 54.6 54.1 53.9 0.4 -0.010 2.44 -0.010 7.5 29.3

9 6/4/2010 19:35 241 55.8 56.1 56 55.3 0.4 0.001 2.52 0.001 7.7 26.0

10 6/6/2010 13:00 283 75.9 76.3 76.3 75.2 0.6 0.007 3.42 0.007 7.7 21.5

11 6/7/2010 13:30 307 86.7 83 83.9 93.3 5.7 0.006 3.91 0.006 7.8 21.4

1= Light bulb burned

Date/Sampling timeInterval


94/116


95/116


96/116

97

bolts and supports rods of the concentric cylinders provided an easy and safe manner to

maintain the draft tube alignment during operation of the system. A geometric

7/28/2019 Design, Construction and Validation of an Int

design, construction and validation of an internally-lit airlift photobioreactor

Documents