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151 shastri

  1. 1. Optimization of a Novel Photobioreactor Design using Computational Fluid Dynamics Abhinav Soman Department of Biotechnology VIT University, Vellore Yogendra Shastri Department of Chemical Engineering Indian Institute of Technology Bombay
  2. 2. Algal biofuels: Potential fuel option with many challenges  Advantages:  Higher yield per unit area  Productive land not required  Compatible with fresh and saline water  Challenges:  High cost of cultivation and harvesting  Low yield requiring significant drying
  3. 3. Closed Photobioreactor (PBR) cultivation shows promise Attribute Open-pond Photobioreactor Cost Low High Yield Low High Contamination High Low  Target yield: 2-3 gm/liters
  4. 4. Several bottlenecks exist in using a photobioreactor (PBR) Expensive to build and maintain High power consumption Overall efficiency is low Limitations to scalability
  5. 5. PBR design plays a crucial role in its performance  Performance parameters of interest:  Light penetration  Hydrodynamics  Mixing  and settling Proposal:  Develop a novel PBR design  Develop a computational fluid dynamics (CFD) model  Compare the performance of the novel design with conventional design using CFD
  6. 6. Two promising designs – Air lift and Flat plate PBR • Less photoinhibition, even under high light intensity • Better liquid flow • Enhanced gas exchange • Better irradiation cycle • Suitable for shear sensitive strains Courtesy Xu et al. (2009) • Low surface area to volume ratio • Strong self shading for poorly circulated PBRs • Scale up not economical
  7. 7. Two promising designs – Air lift and Flat plate PBR • High surface area to volume ration • Lower accumulation of dissolved oxygen • Low power consumption • Good mass transfer capability Courtesy Singh and Sharma (2012) • Photoinhibition likely for high intensity radiations • Low photosynthetic efficiency • Damage to cells due to high stress
  8. 8. Proposed design combines the airlift and flat plate PBR designs
  9. 9. Proposed design combines the airlift and flat plate PBR designs Outer body with flat plates for better light intake Central draft tube from an air-lift reactor
  10. 10. Circular air/CO2 sparger located at the base of the draft tube Central draft tube Sparger
  11. 11. Baffles were added to maintain flow distribution and consistency Baffles
  12. 12. Top-view of the proposed PBR design Outer reactor with flat panel surface Baffles for flow distribution Inner draft tube Sparger
  13. 13. Rounded corners for the outer body are used
  14. 14. Computational Fluid Dynamics (CFD) simulations  Develop a CFD model for a air-lift PBR design published in literature  Validate the conventional model results using the CFD model  Adapt the CFD model for the proposed novel design  Compare the simulation results of the conventional and novel design
  15. 15. CFD Model Development and Validation  Air-lift model studied by Luo and AL-Dahan (2011) Outer tube: • Height: 1.13 m • Diameter: 0.13 m Inner tube: • Height: 1.05 m • Diameter: 0.09 m Boundary Conditions: • Inlet : Superficial gas velocity – 1cm/s • Incident radiation : 50 W/m2 • Outlet : Pressure outlet Courtesy : Luo and AlDahan (2011) Geometry used for FLUENT simulation
  16. 16. CFD Model Details • 3D steady state simulation with coarse griding • Flow Modelling: Eulerian –Eulerian multiphase modelling • Turbulence Modelling : Standard k-ɛ model with mixture multiphase model • Drag Force : Schiller-Naumann drag correlation • Irradiation simulation: Discrete Ordinates Model (DO) • Particle trajectory tracking : Discrete Phase Modelling (DPM)
  17. 17. The observations reported in Luo and AlDahan (2011) were reproduced reasonably Slight deviations were probably due to a different drag model
  18. 18. The CFD model was used to simulate the novel design
  19. 19. Dimensions of the novel design were varied to determine the “optimal” set of dimensions No. Value 1 Outer cuboid height (m) 1.13 2 Parameters varied: • Width • Height • Top clearance • Bottom clearance Property Inner draft tube height (m) “Width” (m) 1.05 4 Draft tube inner diameter (m) 0.09 5 Side length of the 0.10 outer cuboid casing (m) 3 0.18
  20. 20. “Width” had a significant impact on the gas holdup
  21. 21. “Width” had a significant impact on the irradiance Good irradiance history for small width (2 cm) Particle trapped in the concentric space due to higher width (6 cm)
  22. 22. Optimized design was determined based on the simulation results Property Value Outer cuboid height (m) 1.15 Inner draft tube height (m) “Width” (m) 1.05 Draft tube inner diameter (m) 0.09 Top and bottom clearance (m) 0.05 0.02
  23. 23. Irradiance history and gas hold-up showed desirable values for the optimized design
  24. 24. Contours of gas holdup Contours of incident radiation Contours of axial liquid velocity Velocity vectors for Air
  25. 25. Conclusions  The novel design has better light/dark cycling patterns for algal cells.  A higher superficial gas velocity must be to achieve higher gas holdup and turbulent kinetic energy  Additional simulations with algal growth kinetics needed
  26. 26. References  Hu-Ping Luo, Muthanna H. Al-Dahhan.(2011). Verification and validation of CFD simulations for local flow dynamics in a draft tube airlift bioreactor, Chemical Engineering Science. 66: 907-923.  Ling Xu, Pamela J. Weathers, Xue-Rong Xiong, Chun-Zhao Liu. (2009) Microalgal bioreactors: Challenges and opportunities, Eng. Life Sci. 3: 178– 189.  O. Pulz. (2001). Photobioreactors: production systems for phototrophic microorganisms, Appl Microbiol Biotechnol. 57: 287–293.  Aditya M. Kunjapur and R. Bruce Eldridge.( 2010), Photobioreactor Design for Commercial Biofuel Production from Microalgae, Ind. Eng. Chem. Res. 49: 3516–3526.  R.N. Singh, Shaishav Sharma.(2012). Development of suitable photobioreactor for algae production – A review, Renewable and Sustainable Energy Reviews, Volume 16, Issue 4, 2347-2353
  27. 27. Thank You ! yshastri@iitb.ac.in
  28. 28. Why Microalgae ? Courtesy: Chisti, 2007
  29. 29. Comparison between the two systems Courtesy : Xu et al., 2009
  30. 30. Air-lift model studied by Luo and ALDahan (2011) • Unstructured tetrahedral mesh was generated comprising 68864 elements • The bubble diameter was set 0.005 m (Simcik et al., 2011) • Air inlet velocity of 0.38 m/s in the axial direction • Air volume fraction was set to 0.5 • Simulation was initialized with 0.01 volume fraction and 0.01 m/s air inlet velocity • Single wavelength region (400 nm to 700 nm) for DO model • Water refractive index was set to 1.34 and absorbtion coefficient was set to 0.3 • Surface injection was used to inject 44 particles from the inlet surface that mimic the microalgae size (5 µm) and neutral buoyancy (998 g/m3)
  31. 31. Simulations performed:
  32. 32. “Width” had a significant impact on the axial liquid velocity
  33. 33. Height variations did not impact the performance much
  34. 34. Top clearance did not impact the performance much
  35. 35. Unstructured mesh
  36. 36. Outer walls that receive radiation from light source.
  37. 37. Velocity Vectors of liquid

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