The Three Directional
Components of Mixing:
SWIRL VS. VERTICAL & RADIAL MIXING
There are three directional components to mixing in a vertical
cylindrical tank. The same is true for a plug flow device, such as a static
mixer, it is just the orientation that changes (generally static mixers are
viewed using a sideward flow).
The vertical component is motion from the tank bottom to the liquid surface
and from the liquid surface to the tank bottom (the later is referred to as
recirculation). The radial component is motion from the tank centerline to the
tank wall and the tank wall to the tank centerline.
The angular component is commonly referred to as swirl or vortexing,
which is defined as angular mixing. Once again, the same is true for a plug flow design, except the view of the vertical cylinder
would be turned on its side.
By far, the worst or least desired component of mixing is angular or swirl.
Generally speaking, the objective of mixing is to have component
"A" to interact with component "B" to form product
"C" or the desired end result. Angular mixing or swirl is analogous to, or can be compared
to, two (2) horses on a merry-go-round. They
never meet and as such demonstrate poor mixing. Swirl is actually a bit
more dynamic but you get the general idea as to why it's not the preferred
method of mixing, especially when you factor in other mechanical design
consideration that further penalize a mixing design.
So at this point, you might be asking
yourself why is angular mixing considered poor mixing. The answer may not
be quite so obvious, but when swirl is present, it becomes the dominant flow
pattern. Imagine a bunch of kids in a 24' diameter pool all walking, or
rather jump-walking, in the same "angular-direction" thereby creating
a massive angular flow pattern (whirlpool). Now try to stop and imagine
those fluid forces on your body. Those forces are significant and must be
accounted for. Now look where all the pool solids accumulated (at
the bottom in the very center of the pool). Alas, the very principles of a
cyclone separator system. The vertical and radial components of mixing do
exist but are negligible compared to swirl or radial mixing. The solids do
not suspend because the vertical component of mixing is non-existent. Now
imagine trying to suspend solids or to add a reactant at the liquid surface ...
there are no compelling vertical forces to either incorporate or suspend the
material. You could argue that the sucking action of a vortex will
incorporate solids, but on it's own, will that same vortex suspend the solids or
better yet, disperse them. To redirect the the angular flow pattern either
an angular offset mounting arrangement could be used or anti-swirl baffles must
be incorporated to redirect the flow
pattern.
So why
is this mixing discussion not just a simple matter of common sense. Mixing applications are typically viewed from the liquids surface
or from above a vertical cylindrical tank. A
common example of this view is someone stirring a cup of coffee using a spoon to
angularly rotate the contents of the mug. From this view, the surface motion generated by a swirl pattern indicates good mixing.
If we were to observe that same application from the side of the tank, it
would indicate something very different. Solids
added, such as sugar, would initially be caught in the swirl pattern, they would eventually fall
vertically downward, where the heavier solids drop due to the effects of
gravity. This flow pattern is
actually the design concept of the operation of a cyclone separator
(heavy components fall due to the effects of gravity).
Since the mixing is predominantly dominated by the angular component of
mixing (swirl), there is
virtually no vertical component to suspend the solids off the tank bottom.
The same could be said of coffee creamer, which just clumps and lies on the
surface. Again, there is no vertical component of mixing to incorporate
the solids using just a dominant angular flow pattern. As such, the solids will accumulate and may even form a quasi-boundary
layer on the tank bottom if the solids are sticky or if they tend to pack.
Most mixing application, such as blending, suffer from the same flow
scheme. Imagine if you will, stratified vertical layers of either varying
viscosities and/or densities. So what is not clear common knowledge, and
is not commonly understood and perceived, is
that although there is some (actually very little) vertical and radial mixing,
once swirl is present, it dominates the flow scheme. In other words, the
horses of the merry-go-round no only don't get the chance to meet radially, they
don't get to interact vertically either, which are the prime objectives of mixing.
There are two (2) primary methods of overcoming the angular
component of mixing in a vertical cylindrical tank. In the case
where the mixer is mounted vertically on the tanks
centerline, multiple
anti-swirl baffle plates
{typically running the full straight side of the tank) are vertically mounted to
the tanks walls. When the angular
motion encounters the baffle, it has no choice but to be redirected either
upward or downward. Since the
liquid level is somewhat constant, the flow directed up the baffle will then
re-circulate back toward the centerline of the tank. Thereby achieving good top to bottom motion or the preferred
vertical and radial components of mixing. The
second method is to use an angular offset mounting
arrangement.
In this case the impeller rotates clockwise in the liquid which sets up a
clockwise motion. The shaft & impeller are oriented in such a way to
provide a downward discharge that is directed counter clockwise.
These two motions counter each other in such a way as to eliminate
angular motion to achieve the preferred vertical and radial components of
mixing.
Static or plug flow mixers
operate using internal baffles. Some of
these internal are designed to create swirl or angular mixing. Others
create a combination of effects but in all cases, the residence time
distribution, or the actual time within the mixed zone, is very small (just a
few seconds or less) where downstream mixing, or entrained residual flow patterns are
relied upon to complete the mixing. Static mixing is exceptional for
instantaneous reactions but suffers greatly when the reactions are either are
time dependent or are somewhat time dependent. Mixing problems arise when
the assumption that the reaction is instantaneous, such as for water treatment
flocculation, where further downstream processing and upset conditions are
apparent, which affects both chemical usage and water quality.
09.21.23