February 13, 2017 | General

Digester Mixing Fundamentals: Q&A With Gene Smith

Gene Smith

Gene Smith is an Applications Engineer with SUMA America, Inc., a technology provider for digester mixers and agitators.

February 2017

BioCycle: What is meant by “non-Newtonian behavior”?
Gene Smith: A Newtonian liquid, like water, has the same viscosity, no matter how you beat on it. It might change a bit with temperature but very little.
There are several non-Newtonian behaviors. A favorite with children is to make a slurry of corn starch in water. It thickens with shear. This means if you put your finger on the surface and press lightly, your finger will sink in it, as it would in any thick liquid. If you ram your finger into it fast and hard, you might break your finger, or at least your nail.
Then there are shear-thinning liquids. Ah, ketchup. Not only shear thinning but has a yield stress. This means you have to shake the bottle to get the ketchup to flow in the first place. If you stir it vigorously, it will get thinner (lower viscosity). Thick manure is like this as well but I don’t suggest trying it on your French fries.
In a digester, if you have a feed of thick manure and it has a significant yield stress, then you must apply sufficient force (thrust, shear, whatever you want to call it) to overcome the yield stress in the total volume. If you don’t apply sufficient force, you only fluidize the digestate around the propeller, in essence creating a cavern of flowing liquid in a body of gelled muck. So here, you need more energy.
Non-Newtonian behavior in itself does not mean you need more energy to mix such a fluid. You need a viscosity of several thousand cP to affect the required energy. The power required is directly proportional to the specific gravity, though.
BioCycle: What is cP short for and what exactly does it measure?
Smith: A cP is short for centipoise, a measure of dynamic viscosity. In a simplistic way to explain it, it relates the force needed to move one layer of liquid across another layer of the same liquid. In essence, it is a measure of the resistance of a liquid to flow.
BioCycle: What is the translation of a typical velocity gradient of 50 to 80 sec-1?
Smith: A long time ago, two engineers tried to come up with some quantified way to size mixers. They looked at a lot of applications and correlated the volume, viscosity and power consumed for those cases where the desired results were obtained. So, velocity gradient is basically power per unit volume with the viscosity factored in.

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