Objetivo de la estimulación

A tubular photobioreactor was built by Massey University technical staff specifically for this

project (Figures 3.8–3.10). The reactor had a working volume of 75–78 L. It consisted of a

vertical bubble column (152 mm internal diameter, 6.3 mm wall thickness, 1.5 m tall) made

of clear Plexiglas (the ‘degassing column’ in Figure 3.8) as the broth reservoir and a

serpentine continuous loop of borosilicate glass as the light harvester (‘tubular loop’ in

Figure 3.8). The bubble column had a stainless steel bottom zone that was supported on a stand bolted to the floor. The steel zone included a stainless steel cooling coil, a carbon

dioxide sparging frit (200 Pm; GKN Filters GmbH, Radevormwald, Germany), a perforated

pipe ring sparger (12 holes, 1.5 mm in diameter) for air, and a drain valve. On its side, the bubble column had a valve for sampling. The top of the bubble column was covered by a bolted-on PVC headplate that had ports for a dissolved oxygen sensor, a pH sensor, gas exhaust, addition of media components, feed and harvest. Near the top of the bubble column,

Chapter 3 Materials and Methods

close to the operating level of the culture broth, a connection was provided for the broth returning from the light harvest loop (Figure 3.8). Close to the top of the bubble column, a Pt100 temperature sensor (Omega Engineering Inc., Stamford, CT, USA; platinum RTD

Sensor PRTF19-2-100-1/8-6-E) was installed (‘temperature sensor’ in Figure 3.8).

Figure 3.8 Tubular photobioreactor.

LED array LED controller pH sensor Oxygen sensor Temperature sensor Degassing column Tubular loop Circulation pump Protective fence Main control panel LED array LED controller pH sensor Oxygen sensor Temperature sensor Degassing column Tubular loop Circulation pump Protective fence Main control panel

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56

Figure 3.9 Broth recirculation in the tubular loop photobioreactor.

Figure 3.10 Main control panel of the tubular photobioreactor.

CO₂filter

CO₂supply CO₂flowmeter Algal culture flow

CO₂filter

CO₂supply CO₂flowmeter Algal culture flow

Control mode selection switches Dissolved oxygen controller Temperature controller pH controller

Mains power switch

LED power switch

Control mode selection switches Dissolved oxygen controller Temperature controller pH controller

Mains power switch

Chapter 3 Materials and Methods

The light capture loop was constructed of QVF borosilicate glass 3.3 fittings (QVF

Engineering GmbH, Mainz, Germany). At thicknesses in the range of 2–5 mm, borosilicate

glass 3.3 has negligible absorption (d10%) of light in the visible range (i.e. 400–800 nm)

(QVF, 2002), or photosynthetically active range (i.e. 400–700 nm). All glass tubes were

horizontal to the ground (Figure 3.8 and Figure 3.9). The tubes were arranged in two parallel arrays, mounted perpendicular to the ground (Figure 3.9). Each array had six tubes (50 mm internal diameter, 1500 mm long). The wall-to-wall distance between adjacent tubes in an array was the same as the outer diameter of a tube. The two parallel arrays were offset vertically by a distance equal to the outer diameter of the solar tubes. Thus, each tube was fully exposed to light coming horizontally from a wall-mounted panel located parallel to the

tubular array (‘LED array’ in Figure 3.8). The open ends of the tubes were connected using U-bends of borosilicate glass to obtain a single continuous serpentine flow channel. The various joints were sealed with PTFE (poly (tetrafluoroethylene), or Teflon) gaskets that

were flush with the internal walls and GMP-compliant (GMP – good manufacturing

practices).

The light collection loop of the photobioreactor was placed parallel to a wall a0.3 m

from the face of a light emission diode (LED) array that was supported on the said wall (Figure 3.9). The lighting array illuminated the entire projected area of the light collection loop with monochromatic light of 660 nm. The LED array (Figure 3.9) could be adjusted to

provide a photosynthetically active radiation (PAR) level of up to a1540 Pmol˜m2s1, or

nearly full tropical sunlight.

The flow entered the light collector loop at the bottom and exited at the top (Figure 3.9). The bottom exit of the bubble column was connected to the entrance of the glass loop

via a centrifugal pump (‘circulation pump’ in Figure 3.8) and paddlewheel flow meter. The flow exiting the top of the light collection loop was returned to the bubble column via a clear

Chapter 3 Materials and Methods

58

polymer tubing (Figure 3.9). The recirculation flow rate through the light loop was set at the pump and measured by the flow meter.

Temperature, dissolved oxygen concentration and pH were controlled in the bubble column. The main control panel is shown in Figure 3.10. Temperature was controlled automatically (Omega Engineering Inc., Stamford, CT, USA; 1/4 DIN Compact Temperature Controller model CN2110-R20) by on/off switching (solenoid valve) of the mains cooling water. The water flowed through the cooling coil at a preset rate. The temperature measured by the above mentioned sensor provided the control information. Only a cooling capability was provided in the photobioreactor to compensate for the heat absorbed in the light capture loop. The photobioreactor was placed within a temperature controlled room.

Concentration of dissolved oxygen (DO) was controlled automatically using the signal from the DO sensor (Omega Engineering Inc., Stamford, CT, USA; DOE-601 dissolved oxygen sensor, DOE-601-SC sensor cartridge, and DOE-600-SMK submersion

mounting kit) (‘Oxygen Sensor’ in Figure 3.8). The controller used was the model

DOCN602 (Omega Engineering Inc., Stamford, CT, USA). Control was achieved by

sparging air at a preset flow rate of 5 L min1 through the bubble column to strip out the

oxygen produced by photosynthesis. The air sparging was switched on (solenoid valve) once the measured oxygen concentration rose above the setpoint concentration. Air injected into the photobioreactor had passed through a humidifier and been prefiltered through a

sterilizing grade filter (0.2 Pm Acropak 1500 SuPar“ membrane; Pall Corporation,

Portsmouth, UK). Prehumidification eliminated evaporative loss of water from the photobioreactor.

pH was controlled by injecting carbon dioxide in response to a signal from a pH sensor (Cole-Parmer 3-ft submersible double-junction pH electrode incorporating a

Chapter 3 Materials and Methods

temperature sensor (100 ohm RTD) for automatic temperature compensation; KH-27001-83)

(‘pH sensor’ in Figure 3.8). Carbon dioxide was injected at a preset flow rate of 1 L min1 whenever the pH value rose above a specified value and injection was continued until the set point pH had been re-established. Control of pH ensured that carbon dioxide was provided as needed so that the rate of photosynthesis would not be limited by a lack of carbon dioxide. An on/off controller was used (Eutech Instruments 1/4-DIN pH 800 on/off controller KH- 56705-05; Cole-Parmer Corp., Vernon Hills, IL, USA) to switch on the carbon dioxide supply. Carbon dioxide injected into the photobioreactor had passed through a sterilizing

grade filter (0.2 Pm Acropak 800 PTFE membrane; Pall Corporation, Portsmouth, UK). The

pH sensor was calibrated every 6-months, or prior to commencing a continuous culture run, using standard buffers of pH 7.0 and 4.0.

The photobioreactor that had been filled with the medium was inoculated using

cultures that had been grown in Duran bottles (Section 3.4) to a density of around 2–3 g L1.

The volume of the inoculum was generally 10% (v/v) of the initial working volume of the photobioreactor.