PHOTOBIOREACTORS FOR MICROALGAL CULTIVATION DESIGN ...?PHOTOBIOREACTORS FOR MICROALGAL CULTIVATION: DESIGN CONSIDERATIONS COMPLICATIONS ... The main benefits of closed bioreactor systems

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  • PHOTOBIOREACTORS FOR MICROALGAL

    CULTIVATION: DESIGN CONSIDERATIONS

    & COMPLICATIONS

    Ramkrishna Sen

    Department of Biotechnology

    IIT Kharagpur

    E-mail: rksen@yahoo.com

    mailto:rksen@yahoo.com

  • 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)

  • PBRDESIGN: 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

    = 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.

    L= 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 =10%

    From Equation 4 with L= 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: 113131.

    A.P. Carvalho, L.A. Meireles, F. X. Malcata

    (2006)Microalgal Reactors: A Review of Enclosed

    System Designs and Performances Biotechnol. Prog.

    22, 14901506.

    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