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? 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.
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
TYPES OF PHOTOBIOREACTORS
Open Raceway pond Circular pond
Closed
Tubular Bubble column Air-lift Flat panel And others (pyrimidal, hybrid)
PBR – TECHNICAL ISSUES & BOTTLENECKS
[Posten, 2009]
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.
TUBULAR PHOTOBIOREACTORS (ii)
(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
(i) (iii)
(iii) Helical tubular reactor: Flexible plastic tube coiled in a circular framework. Composed of polyethylene or glass tubes, Polyethylene bags, Plexiglas etc. •
(iii)
[Carvalho et al., 2006; Chisti, 2007]
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
Temperature (Lighting effect; Removal of excess heat)
(CO2 solubilization; culture ageing; medium composition)
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
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 where aL= the specific gas-liquid interfacial area ε= the overall gas holdup and dB =the mean bubble diameter where kL= mass-transfer coefficient
&
Now
Therefore
where UG= superficial gas velocity, Ub= Bubble-rise velocity
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
Equation 2
Gas Holdup =
where
Equation 3
ht =vertical distance between the manometer taps, Dhm =manometer reading
Specific power input =
where
Equation 4
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 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
assas
Figure 2. Comparison of the measured gas holdup inthe bubble column with the correlations ofChisti 1989 for sea water
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 costeffectively 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
