Numerical modelling of non-Newtonian fluid in a rotational cross-flow MBR

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Presentation given at 6th IWA Specialist Conference on Membrane Technology for Water & Wastewater Treatment
4-7 October 2011, Aachen, Germany

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Numerical modelling of non-Newtonian fluid in a rotational cross-flow MBR

  1. 1. Numerical modelling of non-Newtonian fluid in a rotational cross-flow MBRT. R. Bentzen1, N. Ratkovich1, S. Madsen2, J. C. Jensen2, S. N. Bak2 & M.R. Rasmussen1 1Department of Civil Engineering, Aalborg University – Denmark 2Grundfos BioBooster – Denmark6th IWA Specialist Conference on Membrane Technology for Water & Wastewater Treatment October 5, 2011, Aachen - Germany No. 1 of 17
  2. 2. OutlineIntroduction & Objectives• Fouling• Rotational Cross-Flow MBRBackground• Non-Newtonian behavior (viscosity)• Wall shear stress in rotating systemsMethodology• Experimental measurements & CFD modelResults & discussion• Wall shear stressConclusions No. 2 of 17
  3. 3. IntroductionMembrane fouling• Fouling is the main bottleneck of the widespread of MBR systems.• Decrease permeate flux• Increase trans-membrane pressureControl/reduction of fouling• Process hydrodynamics can decrease and/or control fouling… • by increasing liquid cross-flow velocity.• Increase permeate flux• Surface shear stress → scouring effect• Increase mass transfer (cake layer → bulk region) No. 3 of 17
  4. 4. Rotational cross-flow (RCF) MBR (GrundfosBioBooster®)• It operates… • between 20 – 40 lmh • pressurize system (~5 bar) • up to 5 times higher sludge concentration than in conventional MBR systems (TSS up to 50 g l-1).• Rotating impellers between filtration membrane discs prevent fouling.• Impellers ensures low viscosity in the reactor biomass due to the non-Newtonian behaviour of activated sludge (AS).• energy consumption and flux. No. 4 of 17
  5. 5. Background (I) No. 5 of 17
  6. 6. Background (II) No. 6 of 17
  7. 7. Methodology (I)Tangential velocity measurements• RCF MBR operates between 50 to 350 rpm• Experimental tangential velocity measured at 59, 119 and 177 rpm with water• Measured with Laser Doppler Anemometry (LDA)• LDA is an optical technique to measure velocity field in transparent media and cannot be used with activated sludge. No. 7 of 17
  8. 8. Methodology (II)CFD model• Star CCM+ v6.04• Single phase• Rigid body motion• Turbulent model • k- SST (water) - validation • Laminar (AS)• Enhanced wall treatment (y+ < 1) No. 8 of 17
  9. 9. Results (I)Reynolds number for waterReynolds number for AS No. 9 of 17
  10. 10. Results (II)Tangential velocity measurements for water• A good agreement between the experimental measurements and the CFD simulation results, with an error up to 8 %. No. 10 of 17
  11. 11. Results (III)Shear stress (for water) 50 rpm 150 rpm250 rpm 350 rpm No. 11 of 17
  12. 12. Results (IV) Disk type kShear stress for water Infinite smooth 0.313 disks• Shear stress at the membrane Smooth disks 0.450 • Theoretical solution: Disk with vanes 0.650 – 0.840 • k: velocity coefficient which depends on disk geometry (vanes) Grundfos (14 0.795 vanes) 2000 rpm *Torras et al, 2009 No. 12 of 17
  13. 13. Results (V)Area-weighted average shear stress at the membrane No. 13 of 17
  14. 14. Results (VI) 250 rpmShear stress for AS 30 g/l 40 g/l 50 g/l No. 14 of 17
  15. 15. Results (VII) 250 rpm No. 15 of 17
  16. 16. Results (VIII) Area-weighted average shear stress (Pa) AS AS AS Water (rpm) (30 g l-1) (40 g l-1) (50 g l-1) 50 1.8 13.5 22.6 34.3 150 8.2 19.9 29.0 40.7 250 13.8 25.5 34.6 46.3 350 19.2 30.9 40.0 51.7 No. 16 of 17
  17. 17. Conclusions• A proper validation of the CFD model was made in terms of tangential velocity measurements using a LDA system with water.• RCF MBR operates with AS and LDA measurements cannot be made.• CFD model was modified to account for the viscosity of AS.• Local shear stress at any place of the membrane surface and area- weighted average shear stress was determined.• An empirical relationship was made, to determine the area- weighted average shear stress in function of the angular velocity (in rpm) and the TSS. No. 17 of 17
  18. 18. Thank you for your attention No. 18 of 17
  19. 19. Consultancy services on CFD for MBR• CFD has become a frontline tool for virtual simulations (conceptual design & performance prediction)• CFD not only gives qualitative data very exact quantitative predictions.• CFD tools and techniques are extensively validated with experimental and analytical results allowing more robust models for R&D.• Software expertise: • CAD: Rhino • Meshing: GAMBIT, ICEM, Star CCM+ • Solver: Star CCM+, CFX, Fluent• Domain expertise • CAD repair for meshing • Moving mesh • Turbulence modelling • Multiphase modelling No. 19 of 17

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