The effects of tank size and impeller type on turbulent flocculation

Ducoste, Joel Jean

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https://hdl.handle.net/2142/20407 Copy

Description

Title

The effects of tank size and impeller type on turbulent flocculation

Author(s)

Ducoste, Joel Jean

Issue Date

1996

Doctoral Committee Chair(s)

Clark, Mark M.

Department of Study

Civil and Environmental Engineering

Discipline

• Engineering, Mechanical
• Civil Engineering

Degree Granting Institution

University of Illinois at Urbana-Champaign

Degree Name

Ph.D.

Degree Level

Dissertation

Keyword(s)

• Engineering, Civil
• Engineering, Mechanical
• Engineering, Environmental

Language

eng

Abstract

• A basic conceptual and experimental study of the flocculation process is described. The turbulence in three square flocculation reactors of size (6.7in)$\sp3$ ($\approx$5L), 1ft$\sp3$ ($\approx$28L), and (2.7 ft)$\sp3$ ($\approx$560L) with two impeller types (Rushton turbine and A310 fluid foil impeller) were evaluated experimentally using a laser Doppler velocimeter and numerically using a computational fluid dynamic software called FIDAP. The results of both the experimental and numerical characterization of the fluid mechanics in the flocculation basin indicate that when G$\rm\sb{m}$ = constant, the rms turbulent fluctuating velocity was proportional to the impeller tip speed and increased with increasing tank size regardless of impeller type. The fluid mechanics results also indicate that the ratio of the rms fluctuating velocity divided by ND, the tip speed, was higher for the Rushton turbine than for the A310 foil impeller especially in the impeller discharge zone. Furthermore, the rms value in the impeller discharge zone was found to be proportional to N$\rm\sb{p}\ \sp{0.5}$ND regardless of tank size and impeller type where N$\rm\sb{p}$ represents the dimensionless impeller power number. $\rm N\sb{p}\ \sp{0.5}$ND represents the magnitude of the rms fluctuating velocity in a region where the turbulent flow is anisotropic. These turbulence results suggest that G$\rm\sb{m}$ does not adequately describe the turbulence produced by the Rushton turbine or the A310 foil impeller in the 5L, 28L, and 560L square tanks.
• The steady state floc size distribution was measured in situ using a photographic technique. The results of the photographic measurements show a shift in the cumulative particle size distribution from larger particles to smaller particles with increasing tank size regardless of impeller type. These photographic results also indicate a shift in the distribution from larger particles to smaller particles moving from the A310 foil impeller to the Rushton turbine. As a measure of the steady state floc size distribution, the volume mean particle size, standard deviation, and maximum particle size were evaluated for each tank size and impeller type setup. These three particle distribution statistics were found to be a function of $\rm N\sb{p}\ \sp{0.5}$ND. The experimental results showed that by increasing the quantity $\rm N\sb{p}\ \sp{0.5}$ND, the volume mean particle size, standard deviation, and maximum particle size all decreased.
• A population balance model was developed that included the turbulence intensity in the impeller discharge zone in the breakup part of the population balance rate equation. Reinforcing the photographic results of the floc size distribution, the population balance model predicts the shift in the particle size distribution from larger particles to smaller particles with increasing tank size. The model is also effective at predicting the shift in the particle size distribution from larger particles to smaller particles moving from the A310 foil impeller to the Rushton turbine. The results from the flocculation experiments and model simulations suggest that the steady state floc size distribution is limited by the turbulence intensity in the impeller discharge zone. These results clearly show that steady state floc size distribution is not a function of the average intensity of the turbulent motion throughout the reactor volume as suggested in the models based on G$\rm\sb{m}.$

Type of Resource

text

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http://hdl.handle.net/2142/20407

Copyright and License Information

Copyright 1996 Ducoste, Joel Jean

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