PHOTOBIOREACTORS FOR MICROALGAL

CULTIVATION: DESIGN CONSIDERATIONS

& COMPLICATIONS

Ramkrishna Sen

Department of Biotechnology

IIT Kharagpur

E-mail: 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)

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