Mixing processes

Why should you perform a simulation of mixing process?

The mixing of bulk material is present as a procedural step in countless process chains and is often of high importance for the resulting product quality.

In addition to drum and trough mixers, there are other types such as cone or screw mixers, whereby the type is selected depending on the substances to be mixed.

Common to all mixing processes is that homogeneity is always used as the main criterion for the quality of the mixing process. In addition to this parameter, however, other parameters such as local shear rates are of particular importance, depending on the mechanical load capacity.

If materials with comparatively low mechanical strength, such as polymers, are to be mixed with ceramics, for example, excessive shearing can lead to comminution of the mechanically less stable material, which means that the necessary product qualities can no longer be achieved in subsequent process steps.

Mixing of bulk material

Velocity distribution in the mixing process

What are the benefits?

The trial-based investigation or adjustment of mixing processes can be carried out well with regard to homogeneity. However, if quantities such as the shear rate mentioned are of high importance, this can only be done experimentally at great expense in terms of material and time. Here, simulative analysis offers significant advantages, as listed below.

  • Real modeling of the mixing process with rapid variation of material properties in order to quantify process influences
  • Determination of material stress (including shear) at every position of the mixer
  • no or only very low material input and lower energy consumption
  • build-up of process knowledge with each further simulation
  • independent of scheduling limitiations

What is the simulation capable of?

All mixer kinematics and geometries can be mapped, from very simple drum mixers to mixers with agitators or even screw mixers.

The mixing process can be modeled as a pure dry process or wet process. The underlying material behavior of the individual fractions and their interaction is taken into account.

This is particularly important for bulk materials with a strong tendency to agglomeration, in order to reproduce real behavior. For this purpose, the cohesion/adhesion behavior of the material is taken into account by appropriate models. Likewise, the friction of the particles with each other and with the mixer represents an important influencing variable for the mixing result, which is also taken into account.

Mixing of three bulk materials in fast motion (dry process)

Behavior of the individual fractions in the mixing process (time lapse)

Since dry mixing processes, in which only solids are mixed, are difficult to assess during the process, a trial-based process definition is carried out in most companies.

Mixing tests are carried out and then the mixing result is determined. If problems occur, such as insufficient mixing or decomposition of the substances, it is very difficult to identify possible solutions.

By simulating the mixing process, the distribution of the individual fractions can be assessed over the course of the mixing process in order to derive process adjustments. This includes, for example, the adjustment of the agitator movement or the design optimization of the mixer.

In addition to looking at the individual fraction, the process can be analyzed and thus understood in a variety of ways.

By means of a section through the mixing tank, for example, the condition of the mixed material can be observed over time in order to identify dead water zones in which there is no mixing.

In this case, it becomes clear that there is insufficient movement of the bulk material in the lower area of the container.

Cut through the mixing process (time lapse)

Wet mixing process to produce a dispersion

Wet mixing processes allow in principle a better observability compared to dry mixing processes, but the mixing is strongly dependent on the viscosity of the liquid and existing density differences.

In the production of dispersions, an unfavorable combination of these properties can result in low or insufficient distribution of the bulk material in the fluid.

By simulating the wet mixing process by coupling the Discrete Element Method (DEM) and Smoothed Particle Hydrodynamics (SPH), this multiphase problem can be modeled.

The proportion of solids in the simulated mixing process can also be taken into account on a case-specific basis and even the smallest quantities of solids can be simulated with comparatively large quantities of liquid. For example, the mixing process of 2000 litres of liquid with 5 g of powder can be simulated.

Mixing of 2000 l fluid and 5 g powder

Fluid movement in 2000 l mixing tank