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Minimum Fluidizing Velocities for Various Bed Packings

Minimum Fluidizing Velocities for Various Bed Packings. By Andrew Maycock. Introduction to Fluidization. Fluid flowed through bottom of a fixed bed Fluidization is the balance of gravity, drag and buoyant forces Suspended particles have larger effective surface area than a packed fixed bed

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Minimum Fluidizing Velocities for Various Bed Packings

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  1. Minimum Fluidizing Velocities for Various Bed Packings By Andrew Maycock

  2. Introduction to Fluidization • Fluid flowed through bottom of a fixed bed • Fluidization is the balance of gravity, drag and buoyant forces • Suspended particles have larger effective surface area than a packed fixed bed • The smallest velocity at which fluidization occurs is the minimum fluidization velocity

  3. Fluidization Apparatus Figure 1: Example of fluidization bed

  4. Overview • Theoretical Approach • Experimental Approach • Results • Method Summaries • Conclusions • Q&A

  5. Theoretical Approach • Bernoulli’s Equation • Correlations for friction loss terms through porous media

  6. Ergun Equation • Sphericity term included • Composed of known or obtainable parameters

  7. Minimum Fluidizing Velocity • Ergun Equation solved simultaneously with force balance. • May assume that flow is laminar (NRe < 20) (Equation reduces to laminar friction term)

  8. Experimental Approach Figure 2: Example of fluidization bed

  9. Determining MFV • Change occurs in slope of pressure drop plot Figure 3: Plot of pressure drop vs. Fluid Velocity

  10. Particle Properties • Graduated cylinder for bed density • Displaced volume for particle density • Microscopic photos for sphericity

  11. Experimental Procedure • Glass Beads and Pulverized Coal • Increase mass flowrate • Measure pressure drop across bed • Change temperature and repeat • Determine fluid properties using correlations and equations of state

  12. Experimental Problems • Poor Distribution • Faulty or imprecise pressure gauges • Difficulty in determining when fluidization has been reached

  13. Results

  14. Pulverized Coal Results Figure 3: Microscopic photo of pulverized coal

  15. Pulverized Coal Results (cont.) Figure 4: Pressure drop data and Ergun Equation for pulverized coal at 26.2 °C

  16. Pulverized Coal Results (cont.) Figure 5: Pressure drop data and Ergun Equation for pulverized coal at 32.8 °C

  17. Pulverized Coal Results (cont.) Figure 6: Pressure drop data and Ergun Equation for pulverized coal at 39.9 °C

  18. Pulverized Coal Results (cont.) Example of results for pulverized coal

  19. Glass Bead Results Figure 7: Microscopic photo of glass beads

  20. Glass Bead Results (cont.) Figure 8: Pressure drop data and Ergun Equation for glass beads at 30.0 °C

  21. Glass Bead Results (cont.) Figure 9: Pressure drop data and Ergun Equation for glass beads at 37.8 °C

  22. Glass Bead Results (cont.) Figure 10: Pressure drop data and Ergun Equation for glass beads at 42.1 °C

  23. Glass Bead Results (cont.) Example of results for glass beads

  24. The Laminar Assumption

  25. The Laminar Assumption (cont.) • Reported to be accurate for Particle Reynolds Numbers under 20 • More accurate as Reynolds Numbers get smaller • Typical values within 15-30% of Ergun Equation • Has no consistent relation to experimental value

  26. Experimental Summary • Experimental determination is accurate and necessary • Difficult to determining exact value for minimum fluidizing velocity • Error in minimum fluidizing velocity measurement based on test interval

  27. Correlation Summary • Provide a good estimate for actual fluidizing velocity. • Require difficult estimation of bed height and void fraction for operation above minimum fluidizing velocity. • Ergun Equation can show unrealistic results, as in this case. • Decent estimation requires accurate particle property values (void fraction and particle density are difficult to determine due to adsorption).

  28. Conclusions • Correlations are useful, but not substitute for actual experimentation • Experimentation necessary because of inaccurate and imprecise instrumentation • Correlations are useful for industrial processes which are usually operated at two to three times the minimum fluidizing velocity

  29. References • de Nevers, Noel, Air Pollution Control Engineering, 2nd ed. Mc-Graw Hill, New York (2005). • de Nevers, Noel, Fluid Mechanics for Chemical Engineers, 3rd ed. Mc-Graw Hill, New York (2005). • Seader, J.D. and Henley, Ernest J., Separation Process Principles, 2nd ed.Wiley, Danver, Massachusetts (2006). • Wikipedia, Sphericity, http://en.wikipedia.org/wiki/Sphericity

  30. Questions • 5 Minute Question Period

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