CHEMICAL REACTION ENGINEERING LABORATORY Characterization of Flow Patterns in Stirred Tank Reactors (STR) Aravind R. Rammohan Chemical Reaction Engineering Laboratory (CREL) Advisor: Professor M. P. Dudukovic’ (CREL) Co-Advisor: Dr. V. V. Ranade (NCL, India) CREL Annual Meeting November 15 th, 2001

CHEMICAL REACTION ENGINEERING LABORATORY Motivation 1)For Single Phase Study - Techniques like LDA, DPIV etc. : Eulerian measurements - CARPT :Lagrangian information, complement the available information in stirred tanks 2)For Multiphase Study -LDA, DPIV etc. limited to non-opaque multiphase systems with low holdup of dispersed phase -Only ‘non’ optical techniques like CARPT & CT can probe into such flows

CHEMICAL REACTION ENGINEERING LABORATORY Outline – Define Objectives – Experimental Setup for Single Phase Study – CARPT Technique – Results and Discussions – Identify the Issues that need to be Addressed

CHEMICAL REACTION ENGINEERING LABORATORY Objectives –Verify mass balance –Qualitative Validation Compare Qualitative features captured by CARPT with visualization studies (Kemoun, 1995) –Quantitative Validation Compare Radial Pumping numbers from CARPT with Experimental data Compare Mean Velocities in Impeller Stream Compare Turbulent Kinetic Energy in impeller Stream –Model Single Phase flows in stirred tanks –Identify the issues that need to be addressed

CHEMICAL REACTION ENGINEERING LABORATORY Stirred Tank for Experimental Study D I /4 D I /5 Blade D I = D T /3 3D I /4 Rushton Turbine Fluid Used for Experiment - Water (density: 1 gm/c.c.) Reynolds Number range = (N=150 rpm, Re imp =12,345); H T =D T D T =200mm D T /3 D T /10  = D T /31.5

CHEMICAL REACTION ENGINEERING LABORATORY The Details of CARPT Z1Z1 Z2Z2 Z4Z4 Z3Z3 Octagonal Base Sc46, Radioactive strength (80  Ci) 16 Na I detectors Al Supports for detectors Z 1 =2.86cms Z 2 =7.72cms Z 3 =12.59cms Z 4 =17.45cms Details of Setup Data Processing of Radiation Intensity Received by N detectors from a Single Radioactive Sc-46 Particle Intensity “I” for N detectors (Photon Counts) Calibration Curves “I vs D(distance)” Distance “ D” from Particle to N Detectors Weighted Least Squares Regression Particle Position P(t) Filter Noise Due to Statistical Fluctuation Instantaneous Lagrangian Velocities Time Averaged Velocities Turbulence Parameters

CHEMICAL REACTION ENGINEERING LABORATORY Calibration & Reconstruction Grid for Calibration R=0, 1.9, 5.7 & 9.5 cms (4 locations) Z=0-20 cms (11 locations)  =0-360 o (12 locations) N calib =11+3x11x12=407

CHEMICAL REACTION ENGINEERING LABORATORY CARPT Details Details of Setup Z1Z1 Z2Z2 Z4Z4 Z3Z3 Octagonal Base Sc46, Radioactive strength (80  Ci) 16 Na I detectors Al Supports for detectors Z 1 =2.86cms Z 2 =7.72cms Z 3 =12.59cms Z 4 =17.45cms Data Processing of Radiation Intensity Received by N detectors from a Single Radioactive Sc-46 Particle Intensity “I” for N detectors (Photon Counts) Calibration Curves “I vs D(distance)” Distance “ D” from Particle to N Detectors Weighted Least Squares Regression Particle Position P(t) Filter Noise Due to Statistical Fluctuation Instantaneous Lagrangian Velocities Time Averaged Velocities Turbulence Parameters

CHEMICAL REACTION ENGINEERING LABORATORY Lagrangian Eulerian Lagrangian Particle Trajectories & Lagrangian Velocities available 3-D grid in STR for CARPT Shaft Baffles Disc N  = 72 cells, N R = 20 cells and N Z = 40 cells, total= 57600,  =5 o,  r=5.0 mms,  z=5.0 mms

CHEMICAL REACTION ENGINEERING LABORATORY Verification of Mass Balance Verification of Mass Balance DCDC 2b Surface S 1 Surface S 2 Blade D I = D T /3 3D I /4 Surface S 3 Control Volume for Mass Balance Calculations Compute Flow In and Flow Out Along Every Surface. Mass Balance Check ? Q totin =Q totout

CHEMICAL REACTION ENGINEERING LABORATORY

Qualitative Validation Qualitative Validation - Location of Eye of Recirculation Loops - Azimuthally Averaged Velocity vector plot V tip =0.53 m/s Eye of Loop

CHEMICAL REACTION ENGINEERING LABORATORY Location of Eye of Recirculation Loops

CHEMICAL REACTION ENGINEERING LABORATORY Qualitative Validation - Shape and Location of Dead Zones Qualitative Validation - Shape and Location of Dead Zones Dead zones Star Fish Pattern R cm  V V r. Disc Blades Baffles Plane at the bottom of the tank CARPT Visualization

CHEMICAL REACTION ENGINEERING LABORATORY Grid Independence of CARPT measurements

CHEMICAL REACTION ENGINEERING LABORATORY Grid Independence of CARPT measurements

CHEMICAL REACTION ENGINEERING LABORATORY Comparison of Radial Pumping Numbers from CARPT with Data from the literature with Data from the literature

CHEMICAL REACTION ENGINEERING LABORATORY Axial Profile of Radial Velocity in the Impeller Stream

CHEMICAL REACTION ENGINEERING LABORATORY Axial Profile of Radial Velocity in the Impeller Stream

CHEMICAL REACTION ENGINEERING LABORATORY Turbulent Kinetic Energy Profile in the Impeller Stream

CHEMICAL REACTION ENGINEERING LABORATORY Findings CARPT measured Mean Velocities compare well (within 4-8%) with Mahoust (1987) and Kemoun (1991) whose tank dimensions are exactly same as current setup. However, CARPT measurements are lower (10-20%) than the other reported data. CARPT measured rms velocities are lower than those obtained from other techniques. CARPT measured turbulent kinetic energy lower (30-50%) than that obtained from other techniques.

CHEMICAL REACTION ENGINEERING LABORATORY Where are we losing this information ? Flow considerations –Sampling frequency (50 Hz) too low ? –Is the tracer too big to follow the fluid closely ? Nature of the experimental technique –Statistical nature of photon emission –Reconstruction based on calibration map where solid angle effects are not accounted for (Roy et al., 1999) –Wavelet-based filtering - Are we filtering off fluctuations of the fluid in addition to noise ?

CHEMICAL REACTION ENGINEERING LABORATORY Current Work Evaluated flow followability of tracers of different size and density ratio Probed two phase flows (gas -liquid) in STR using CARPT and CT (Computed Tomography) Obtained extensive local gas holdup and liquid velocity information

CHEMICAL REACTION ENGINEERING LABORATORY Single Phase CFD Approaches Black Box Approach (Needs experimental input) Unsteady Approaches (Computationally very intensive) äDeforming mesh äSliding mesh Quasi steady approaches äMultiple reference frames äSnapshot approach Grids used for current work N  =94, N R =57 & N z =78 (N tot ~410000)

CHEMICAL REACTION ENGINEERING LABORATORY Predictions of Radial Profile of Tangential Velocity Single phase CFD simulations predictions are comparable to LDA values but over predict the CARPT data

CHEMICAL REACTION ENGINEERING LABORATORY Current CFD Work Lagrangian particle tracking simulations 2 phase simulations in STR using two fluid model + snapshot approach

CHEMICAL REACTION ENGINEERING LABORATORY Acknowledgements - CREL Sponsors - Colleagues at CREL