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PHOTOBIOREACTORS FOR MICROALGAL
CULTIVATION: DESIGN CONSIDERATIONS
& COMPLICATIONS
Ramkrishna Sen
Department of Biotechnology
IIT Kharagpur
E-mail: [email protected]
SALIENT FEATURES
Photobioreactors – What & Why? Design Considerations – Purpose & Target
Parameters
Critical inputs
Steps & Requirements
Outcome and validation
Design Complications Current knowledge and lacuna
Process maintenance
Dependence on culture and conditions
Steady state operations
Special requirements
Benchmarking
CONCLUSIONS
PHOTOBIOREACTORS (PBR) – WHAT AND WHY?
Cultivation under defined/controlled conditions
Prevent contamination with undesirable microorganism
The main benefits of closed bioreactor systems include higher areal productivities
The prevention of water loss by evaporation.
More appropriate for sensitive strains (which grow in non-extreme environments) or when the final product is of high value
Offers higher level of control • pH and Temperature
• Species selection
• Aeration and Mixing
• Evaporation losses
• BUT,
Higher capital, operational and maintenance costs
A typical photo-bioreactor is a three phase closed reactor
system with culture medium as the liquid phase; cells as the
solid phase, and mostly, air as the gas phase.
TYPES OF PHOTOBIOREACTORS
Open
Raceway pond
Circular pond
Closed
Tubular
Bubble column
Air-lift
Flat panel
And others (pyrimidal, hybrid)
[Posten, 2009]
PBR – TECHNICAL ISSUES & BOTTLENECKS
CHALLENGES
Low productivity of algae
Expensive for algal biomass production for low
value – high volume products (Biofuels)
Contamination by other species
Scale-up
High fossil-fuel energy input
Hence, proper choice and design of
reactor is of paramount importance.
(iii)
(i)
TUBULAR PHOTOBIOREACTORS
(i)Vertical Tubular Reactors (VTR): The airlift and bubble column reactors are composed of vertical tubing
(ii) Horizontal Tubular Reactors:
Suitable alternative to VTR
Handle large working volumes
(iii) Helical tubular reactor: Flexible plastic tube coiled in a circular framework.
• Composed of polyethylene or glass tubes, Polyethylene bags, Plexiglas etc.
[Carvalho et al., 2006; Chisti, 2007]
(ii)
(iii)
PBR–DESIGN: CRITICAL PARAMETERS & INPUTS
Re-usability (Easy to clean and reuse)
Material of construction (Strong; Inert; pH-Temp-Salinity tolerant)
Lighting (Light penetration, intensity, photoperiod and flashing)
Mixing (Poor mixing causes unsteady state; biofouling & oxygen hold up)
Aeration – Sparger design (Bubble size/number; mass transfer; feed gas pressure > pressure drop)
pH (CO2 solubilization; culture ageing; medium composition)
Temperature (Lighting effect; Removal of excess heat)
BUBBLE-COLUMN/AIRLIFT REACTOR (BCR)
ADVANTAGES
•High mass transfer
•Good mixing with low shear stress,
•High potentials for scalability,
• Easy to sterilize,
•Low fouling,
• Reduced photoinhibition / photo-oxidation
DISADVANTAGES
•Small illumination surface area
• High energy usage
• Their construction require sophisticated
materials
•Decrease of illumination surface area upon
scale-up.
Source: Chisti, 2009
where kL= mass-transfer coefficient
where UG= superficial gas velocity,
Ub= Bubble-rise velocity Now &
Therefore
where aL= the specific gas-liquid interfacial area
ε= the overall gas holdup and
dB =the mean bubble diameter
DESIGNING A BUBBLE COLUMN REACTOR FOR
BETTER OXYGEN REMOVAL: A CASE STUDY
Finding relation between overall mass transfer coefficient of
oxygen and superficial gas velocity for BCR
Also &
Hence
Divide
by UG
The parameter c ≈ 1 in the bubble flow
regime
Equation 1
Calculating mass transfer coefficient using DO probe
where C* =saturation conc. of DO,
Co =initial conc. of DO at time to
C =DO conc. at any time t
Gas Holdup = where ht =vertical distance between
the manometer taps,
Dhm =manometer reading
Equation 3
Equation 2
Specific power input =
where P =power input due to aeration,
VL = culture volume,
g = gravitational acceleration,
UG =superficial gas velocity based on the
entire cross-sectional area of the reactor tube.
ρL= Liquid density
Equation 4
Equation 5
From Chisti (1989)
EXPERIMENTAL SETUP
Reactor used ID of reactor = 0.193 m
Gas-free liquid height =2 m.
Volume = 0.06 m3
CASE STUDY:
Species = Phaeodactylum tricornutum
Light: The mean outdoor irradiance = 200 ± 69
mE/m2/s in morning and 1056 ± 278 mE/m2/s at
noon
Inoculum conc= 0.07 g/l
UG = 0.011 m/s
Specific power input 109 W/ m3
Temp = 20oC
Temperature control with cooling coils
Figure 2. Comparison of the measured
gas holdup inthe bubble column with the
correlations ofChisti 1989 for sea water
assas
Figure 3. Correlation of the measured
kLaL with the superficial aeration
velocity UG
USING LITERATURE TO DESIGN REACTOR
From Fig. 2 we know that at P/VL= 300 W/m3 ε=10%
From Equation 4 with ρL= 1030 kg/m3 for sea water UG= 0.03 m/s
From Fig 3. and Equation 1 kLaL = 0.036 sec-1
With known P from compressor rating. we can determine culture volume. Gas hold-up should be considered while designing reactor volume. Generally reactor volume = 1.1 to 1.3 times culture volume
From Fig. 3 6z = 2.222 . Therefore z= 0.37. Therefore bubble diameter 0.37*dB= kL
We know aL= n*4/24 *3.141 *dB3= 0.036/kL
Therefore n*dB= 0.66 m where number of bubbles (n)
This helps design sparger holes (quantity and diameter) and sparger area.
From Sparger area and light restrictions one designs cross-section area.
From cross-section area and reactor volume reactor height is designed
REFERENCES
A.S. Miron, F. G. Camacho, A. C. Gomez, E.M.
Grima and Y.Chisti (2009) Bubble-Column and Airlift
Photobioreactors for Algal Culture. AIChE 48(9)-
1872-1887.
E. Molina, J. Fernandez, F.G. Acien, Y. Chisti
(2001) Tubular photobioreactor design for algal
cultures. Journal of Biotechnology 92: 113–131.
A.P. Carvalho, L.A. Meireles, F. X. Malcata
(2006)Microalgal Reactors: A Review of Enclosed
System Designs and Performances Biotechnol. Prog.
22, 1490−1506.
www.oilgae.com
www.fao.org
SHORTCOMINGS & COMPLICATIONS:
Inadequate literature and contradicting data
Scale up challenges; Unsteady state operation
Maintenance of same velocity profile for multiple runs
Self shading / flashing effect and Biofouling
Limiting nutrients
Control of critical process parameters
Removal of oxygen from the growth system
Assessing water requirements (source, recycle, chemistries and evaporation issues)
Determining CO2 availability and delivery methods,
Algae cultivation systems need to cost-effectively and evenly distribute light within the algae culture.
Efficiency of use of solar energy and carbon dioxide.
Prone to contamination with non-target algae
High capital, operating and maintenance costs
CONCLUDING REMARKS
No single prescription for PBR Design
Energy input minimization
Optimization
Scalability
ACKNLOWLEDGEMENT
My research students – Ganeshan, Ankush & Vikrama
Prof. Ruma Pal, Calcutta University
CSIR – NMITLI Program