Macro level mixing - Physical processing
Macro-level mixing topics covered here describe how the flow pattern generated by the impeller or impellers
affects the suspension and incorporation of solid particles and their distribution within the vessel. Most often
people think of solids suspension from the bottom of the vessel because solids are usually heavier
than the displaced
liquid, but the incorporation of floating solids is also described here. Parameters that affect
are the shape of solids, solid size distribution, solid concentration, solid density, and liquid density and viscosity.
Solid dispersions (slurries) and pastes are described here, too. The quality of the solids distribution is discussed,
which includes the description of fillets, on bottom motion, off bottom motion, and uniform solids suspension.
Reactors include batch reactors and continuous flow reactors (CSTR). Some mixing application examples are agitated
leaching in the mining industry, rubber crumb, crystallization, precipitations, etc. Abrasion and impeller wear
are important factors to consider in solid-liquid mixing.
The more you look at solids suspension, the more you realize that we are not even close to comprehending
what is going on, even with the help of CFD. If you would like to contribute your ideas to this forum,
please email us with your story or experience.
We are currently working on a difficult solids suspension project in a continuous reactor, with inlets and
risers. These tanks are huge, so if you guessed mining projects you are absolutely correct. It seems as though
many papers on solids suspension miss the scale-up issue because "measurements are made in small tanks with
relatively large particles." Well, take the same particles and put them into a large tank and you get a
different story. Here is another quote. "Applying Equation 1 for scale-up with x= -0.55 (Zwietering, 1958;
Nienow, 1968) results in just 40% of the critical power input in full-scale, based on the model measurements.
This is equivalent to an underestimation of the critical stirrer speed of about 30%, which is totally
unacceptable." And this was just for a 21 m3 tank. Can you imagine what Zwietering would do for a tank
with 1000 m3? And it is the most quoted paper in solids suspension! It is even endorsed by the North
American Mixing Forum (NAMF) in their new book "Handbook of Industrial Mixing" (2003) edited by Ed Paul,
Victor Atiemo-Obeng and Suzanne Kresta.
The Germans are addressing this and it seems as though it is not being considered in North America.
We plan on discussing those correlations here as soon as we wrap up this project.
And then there is the settling velocity issue. There are as many solutions to solving it as authors.
And you need the settling velocity first before solving the solids suspension issues. Zwietering and
Lightnin use the terminal settling velocity of a single particle. The Germans use the swarm settling
velocity. And of course the Germans are not in unison in extrapolating the swarm settling velocity
from the single particle velocity. The Germans are also describing the flow number of their impellers
in the presence of solids and they do affect Nq dramatically as the volume concentration of the solids
Solids are usually added at the surface of a liquid from the tank top. The description of solids in a mixing
tank depends on if the solids float or sink. Just because the solids density is heavier that the fluid around it,
solids can float, especially fine powders. Also, solids with a density less than the surrounding fluid can sink
in a well aerated vessel. The first question to ask is: Do the solids float?
We are currently in the process of updating the mixing forum. Check back here at a later date for more information.