Mixing Science and Practice

The Purpose of Mixing

Let us start out by asking the question: what is the purpose of mixing? Then, we can understand why we need chemical mixing simulation. We carry out mixing in order to affect a process in a desired manner. For example, we blend miscible fluids to make concentrations and temperature more homogeneous in the vessel, but at the same time, we may do this to enhance the chemical reactions and also control their reaction selectivity. We can also carry out mixing to suspend solids in the liquid to create a uniform suspension that we then transfer into another tank for the next stage.

We may also carry out the suspension of solids in a liquid. For example, if you have a catalyst, then you want the catalyst uniformly distributed in the reactor; or, if you are feeding a centrifuge, then you want to suspend the solid so that you can feed a uniform concentration of solids to a centrifuge. We may also want to create a dispersion of the liquid, one liquid and another, oil and water; or disperse the gas in the liquid. In some cases, there will be multiples of these events or desired outcomes for the process to be successful and we need to pay attention to that.

Let us define mixing: Mixing is a process operation where mechanical energy induces motion in a volume of fluid to achieve a desired process result. This is the working definition that we should use all the time. In addition to the word “mix” we need to describe what occurs in the process vessel, in the process, and in the fluid. Imagine that you are Maxwell Demon, attached to a molecule in the fluid and experiencing various phenomena. If you are able to describe the phenomena that are occurring while floating around in the vessel, then you achieve a much better insight as to what is happening and what you need to do to be successful in your process.

With that definition, we see that a mixer converts mechanical energy into kinetic energy to induce a hydrodynamic environment in the vessel. There is bulk or macro-flow carrying in the vessel as well as micro-flow turbulent eddies. As a result of the different velocity, shear stresses and strains are also occurring in the vessel.

The challenge is to understand the hydrodynamic environment and select, design, and mix it to create the environment that is best suited for the success of the process. The goal then is to use the energy efficient, to compare the mechanical energy efficiently, and to create the appropriate hydrodynamic environment.

When you have information about the distribution of energies of different impellers, the challenge is to select the impellers and operate them in a manner that results in the hydrodynamic environment best suited for your process needs. This would be the optimal result of the correct chemical process calculations.  

The late Professor Octave Levenspiel’s message was, “One cannot design an effective reactor without paying close attention to the hydrodynamics flow patterns and contacting.” A lack of knowledge of the complete kinetics may not be as consequential. This is, indeed, very true. One can have the complete kinetics but if the necessary hydrodynamics are not realized, then the results can be disastrous. Therefore, effective mixing that generates the required hydrodynamics is required for success.

Question: What is “good” mixing and a “good” liquid mixing process?

Answer: The most common answer to this question relates to greater intensity of mixing, greater homogenization, and better suspension and distribution of the suspended solids and so on.

However, this is not always the case and acting in this way can cause the process carried out in the stirrer reactor to lead to unsuccessful results such as increased impurities and unwanted products.

Good mixing is when the mixing induces a hydrodynamic environment such that the results are as desired.

Question: How does VisiMix help identify good mixing?

Answer: VisiMix allows you to determine the main mixing parameters of the process that give rise to the desired results. If you want to scale up/down or transfer the process to another reactor, then these parameters should be similar in the new reactor. This knowledge helps reduce the number of experiments and, thus, saves time and money.

This article is based on Dr. Victor Atiemo-Obeng’s lecture on VisiMix Forum.