Suspended solid abatement in a conical fluidized bed flocculator

Abstract

With the random movement of silica gel beads in a conical fluidized bed, micro-vortices resulting from the fluidization promoted the collision and aggregation of suspended fine kaolin powders. The abatement efficiencies of the suspended fine solids under several hydrodynamic conditions were studied, and a suitable control strategy for operating the conical fluidized bed flocculators was identified. The suspended solids abatement efficiency was found to increase with increasing Camp Number and flocculation time (T), but decreased with the increase of velocity gradient (G) within the range studied in this research (165.1–189.6 s^-1). The abatement efficiencies were all more than 60% at the range of G = 165–180 s^-1 and T = 15–33 s at an initial kaolin solid concentration of 150 mg·L^-1, polymer aluminum chloride dosage of 60 mg·L^-1 and sedimentation time of 20 min. However, the formation of flocs was influenced by the liquid backmixing. Excessive backmixing caused the breakup of flocs and resulted in difficulty for the fine powders to aggregate and sediment to the reactor bottom. The results of the calculated fractal dimension and measured free sedimentation velocity of flocs obtained at different runs showed similar flocs properties, and indicated an easy control strategy for sedimentation of the flocs.

Keywords

conical fluidized bed / flocculation / velocity gradient

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Acknowledgements

The authors are grateful for the financial support from the Natural Sciences Foundation of China (Grant Nos. 50908096 and 50908097) and Research Fund for the Doctoral Program of Higher Education of China (No. 20090061120035).

Symbols

A	surface area of floc, m2
CD	drag force coefficient
D2	two-dimensional fractal dimension
E	abatement efficiency
G	velocity gradient, s-1
H0	initial bed height, m or mm
H	expanded bed height, m or mm
L	flocculation loading, g·m-3
m	treatment quantity of particles/colloids, g
P	circumference of the floc, m
Re	reynolds number
T	flocculation time, s
U	superficial liquid velocity, m·s-1 or mm·s-1
Ut	terminal settling velocity, m·s-1 or mm·s-1
Vs	initial volume of solid phase, m3
a	major diameter of the flocs, μm or m
b	minor diameter of the flocs, μm or m
dp	particle diameter, μm or m
dv	equivalent volume diameter, μm or m
g	gravitational acceleration, m·s-2
n	RZ exponent
α	constant in dimensional fractal dimension equation
ϵ	bed voidage
μ	fluid viscosity, pa·s
ρl	density of liquid phase, kg·m-3
ρs	density of solid phase, kg·m-3

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