Minimum Fluidizing Velocities for Various Bed Packings

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Minimum Fluidizing Velocities for Various Bed
Packings

• By
• Andrew Maycock

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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

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Fluidization Apparatus
Figure 1 Example of fluidization bed
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Overview

• Theoretical Approach
• Experimental Approach
• Results
• Method Summaries
• Conclusions
• QA

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Theoretical Approach

• Bernoullis Equation
• Correlations for friction loss terms through
  porous media

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Ergun Equation

• Sphericity term included
• Composed of known or obtainable parameters

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Minimum Fluidizing Velocity

• Ergun Equation solved simultaneously with force
  balance.
• May assume that flow is laminar (NRe lt 20)
  (Equation reduces to laminar friction term)

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Experimental Approach
Figure 2 Example of fluidization bed
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Determining MFV

• Change occurs in slope of pressure drop plot

Figure 3 Plot of pressure drop vs. Fluid Velocity
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Particle Properties

• Graduated cylinder for bed density
• Displaced volume for particle density
• Microscopic photos for sphericity

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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

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Experimental Problems

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

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Results
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Pulverized Coal Results
Figure 3 Microscopic photo of pulverized coal
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Pulverized Coal Results (cont.)
Figure 4 Pressure drop data and Ergun Equation
for pulverized coal at 26.2 C
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Pulverized Coal Results (cont.)
Figure 5 Pressure drop data and Ergun Equation
for pulverized coal at 32.8 C
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Pulverized Coal Results (cont.)
Figure 6 Pressure drop data and Ergun Equation
for pulverized coal at 39.9 C
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Pulverized Coal Results (cont.)
Example of results for pulverized coal
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Glass Bead Results
Figure 7 Microscopic photo of glass beads
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Glass Bead Results (cont.)
Figure 8 Pressure drop data and Ergun Equation
for glass beads at 30.0 C
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Glass Bead Results (cont.)
Figure 9 Pressure drop data and Ergun Equation
for glass beads at 37.8 C
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Glass Bead Results (cont.)
Figure 10 Pressure drop data and Ergun Equation
for glass beads at 42.1 C
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Glass Bead Results (cont.)
Example of results for glass beads
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The Laminar Assumption
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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

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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

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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).

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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

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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/wik
  i/Sphericity

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Questions

• 5 Minute Question Period