Powder blending is one of the most critical unit operations in the pharmaceutical industry; however, it receives relatively little consideration in comparison to the more common unit operations such as distillation and heat exchange, writes Liam Meagher.

The characterisation of powder blending is not as easily quantifiable and cannot be explained through standard models. The goal of this research was to identify the fundamentals of powder blender design, the powder properties which affect blending and test these fundamental principles in a laboratory setting.

Furthermore, an undergraduate teaching software application was developed which enables blender equipment selection based on the input of some common powder characteristics such as, flowability and particle segregation.

With the input of design constraints such as production capacity and the number of batches to be produced, the app will calculate the size of the mixer needed, the dimension of the mixer, optimum mixing speed, power consumption and provide suggestions for mixing time.

The teaching application is online:-https://liammeagher.shinyapps.io/blender_selector/

Overview of powder blending and the technology

There are three principal types of mixing for dry powder blending: diffusive mixing, convective mixing and shear mixing. There are designs of industrial mixers available and each blender is based off one or a combination of these principles.

  1. In diffusive mixing, the particles of solids move randomly in a rolling motion, usually caused by the rotary motion of the container. There is no forced pattern in the mixer, as such than the one that could be done by an agitator. This type of mixing is typically the longest and can be prone to segregation. 
  2. For convective mixing, the movement of solids is forced due to a mixing tool or agitator that defines a pattern within the convective mixer. The mixing tool moves large groups of particles, agitating and splitting the bulk of the particles and thereby achieving mixing. Mixing is shorter than for diffusive mixers, although the actual mixing time will depend on the design of the agitator, process parameters such as mixing speeds and nature of the solids to be mixed. 
  3. In shear mixing, a mixing tool at high speed is used to make groups of particles slip in between each other to mix. The high shearing force can be particularly efficient when the mixture has some agglomerates that must be broken to achieve a good mix. 

Froude number

The Froude number is discussed in the table below and is a dimensionless number used to characterise how particles will be mixed in a dry mixer. If Fr < 1, gravity forces are higher than the inertial forces due to agitation or movement of the mixer, thus the mixing is done in dense phase, with particles rolling over each other. If Fr = 1, that is the point where the inertial forces and gravity forces balance.

It is defined as:

Fr =u2rg


u = tip speed of mixing element (m/s)

r = Radius of mixing element (m)

g = acceleration due to gravity (m/s^2)

Table 1.Mixer Types: Advantages and Disadvantages 

Powder characteristics which affect blending 

The aim of this research was to create a logical pathway from defining the type of powder to be blended and the selection of equipment needed. The first consideration is the characteristics of a powder which affect its behaviour.

While there are numerous variables which affect powder blending such as particle size distribution, particle shape, particle size ratio and bulk density, this research found that these could be summarised in whether a powder blend was prone to segregation and its flowability.

What is segregation?

Segregation of powders is when a blend de-mixes from a previously homogenous bulk solid. Adverse effects of segregation include reduced blend quality and flow problems.

Segregation is a function of shape, density and size of the particles; however, size is the primary factor. As a rule of thumb, segregation occurs when the size ratio of the particles is greater than 1.3.

What makes a powder free flowing?

Powders are free flowing when the particles do not stick together. If particles are cohesive, they stick to one another to form aggregates. Cohesion has an inverse relationship with particle size, so the smaller the particle to more likely it will not be free flowing. Particles smaller than 100 micrometers are usually cohesive.


Experimentation was conducted to validate some of the literature review findings with respect to how powders would perform under differing conditions. Different particulate material like nuts, sugar strands, sugar balls along with plastic beads were blended in binary mixtures using a simple single shaft agitator.

The experiments conducted yielded results with respect to how powder particle properties affect powder characteristics. The particle properties tested were particle size, the particle size ratio and the brittleness of the material, i.e., how likely it is to degrade under high shear force.

From the experiments it can be concluded that:

  • Non-segregating mixtures will achieve homogeneity quickly and so all mixing mechanisms should be effective. (Note: the black line indicates the actual percentage of the material in the blend at a given time, with the blue line indicated what was measured. So, the closer the blue line is to the orange, the more homogenous the blend.)

  • Powders which tend to segregate are difficult to mix and will provide problems, however the best chance at achieved a good mixture is to use a high shear mixing mechanism, or if physical degradation is acceptable, a milling mechanism. As can be observed homogeneity improves with mixer speed. 

  • If segregation is an issue and the addition of water is possible then it will make it easier to mix and so could be mixed using a convective mechanism;
  • As segregation appears to be the major issue when mixing powders, and segregation is large based on particle size ratio, particle size ratio is the major characteristic to consider when selecting a blender;
  • Addition of water can reduce the effects of segregation;
  • A blend will require multiple stages if you have a component which is a very small percentage of the overall blend.

From those assumptions a decision tree was constructed which identifies the mixing mechanism most effective for a given powder blend. As a powder’s tendency towards segregation and its flowability are the main factors to consider when selecting a powder blender, they are the first questions. 

Figure 3. Dry Powder Blending Selection Decision Tree

Powder blender selector web tool 

With the construction of the decision tree, a bespoke powder blender selection tool was created. It was developed using the coding application R studio. RStudio is a free, open-source IDE (integrated development environment) for R.

The app works by sequentially asking the user questions on the powder characteristics and outputs:

  • Whether it should be mixed in multiple stages or single stage;
  • The optimum powder blender from the options of Orbiting screw mixer, Double shaft paddle mixer, Ribbon Blender, High speed impeller, Tumble blender and hammer mill.

The user then goes to the corresponding mixer tab and inputs production Capacity, desired batch size, powder density and the required fill level of the mixer and the app outputs:

  • Vessel size;
  • Vessel dimensions (depends on the mixer but will be a combination of radius, mixing tool radius, height, width and length);
  • Mixing speed;
  • Froude number and tip speed of the mixing tool;
  • Recommended mixing time;
  • Power consumption.

The app also provides explanation for powder segregation, flowability and Froude number.

Figure 4. Screenshot of the apps main page


While the experimentation and sampling practices were rudimentary and may not be scientifically reliable, it does follow the accepted trends of particle behaviour during mixing.

Thus, the decision tree can be considered more of an approximation or a rough guide to interpreting particle behaviour.

The teaching application itself is a mainly educational tool. However, it may also have a use in industry by giving design engineers an idea of the equipment and utility requirements for a given project.

This application would be extremely useful to other undergraduate student engineers and could be incorporated into the design projects of other students as it would hopefully give them a much broader understanding of particle behaviour and dry powder blending. 

Author: Liam Meagher; academic supervisors: Kevin Cronin and Denis Ring, Process & Chemical Engineering, University College Cork