Solids Suspension and Mixing in Subsurface
Aeration Systems Using Static Mixer^ Aerators
GEORGE R. FISETTE, Senior Process Engineer
Kenics Corporation
Denvers, Massachusetts 01923
INTRODUCTION TO KENICS ENGINEERING
ST A TIC MIXER Unit Concept
The STATIC MIXER unit is a mixer with no moving parts. It resulted from research
for a more effective mixer for blending two component reactive resins. The mixer developed
was a significant and radical change in mixing technology. The concept has subsequently
been applied to such areas as two phase dispersion, mass transfer, in-line chemical
reactions, viscous mixing and gas-liquid contacting. The mixer, itself, operates as a plug
flow device utilizing the interactions of flow division and radial mixing. As the flowing
material contacts the leading edge of a helical element, the fluid is split into the two streams
and forced to following the geometric path created by the element shape. At the succeeding
element, the two flows are split into two again, thus creating a mathematical progression of
division of the flowing stream.
In either laminar or turbulent flow, rotational circulation of a processed material
around its hydraulic center in each half of the helix region, causes radial mixing. The
velocity components of the flow in the unit are shifted, creating a new series of velocity
vectors and thus forcing the materials from the center outward to the outer wall of the tube.
The overall effect of radial mixing is to cause the stream to be continuously and
completely inverted radially, such that particles entering at the center of the stream are
forced to the outer wall and vice versa, on a continuous basis.
It is the interaction of flow division and radial mixing which produces the turbulent
and eddy currents that result in the mass transfer of the STATIC MIXER unit. These
currents bring about a renewal of the interfacial surfaces and continual contacting and
mixing of the two portions. Thus, transverse gradients in temperature, velocity and
composition are eliminated. Precise control over inline processing is assured in numerous
blending, dispersion and mass transfer applications.
Kenics Aeration System
The application of the STATIC MIXER unit concept to aeration, has resulted in a subsurface system offering the advantages of diffused and mechanical systems without the
problems of either. The STATIC AERATOR system incorporates large non-clogging
orifices with a five element STATIC MIXER unit all mounted on the basin bottom to
obtain the increased oxygen transfer. Operating on the air-lift pump principle the STATIC
AERATOR circulates water from the bottom to the top of the basin. Surface flow then
radiates outward until the flow from adjacent aerators is balanced. Within the volume of
influence (unit cell) of each aerator a completely mixed condition exists since the water is set
into a vertical circulation path by the pumping action of the aerator. This circulation
maintains a uniform solids distribution and allows for maximizing the oxygen transfer. An
interface is formed around each aerator unit cell across which mixing takes place due to
unsteady state turbulence and eddy currents. It is this interfacial mixing which allows the
aeration basin to be completely mixed.
The action of the aerator elements on the air-water stream is the combined effect of
fluid shear, radial mixing, and flow inversion, to produce the turbulence and eddy currents
that heighten the mass transfer. The air bubbles after leaving the orifice are split and re-split
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