A robust scale-up of mixing processes for powdered goods calls for systematic trial planning that takes geometric, kinematic and dynamic similarity between laboratory, pilot and production scale into account. The aim is the reproducible transfer of mixing time, fill level and tool speed, so that mixing quality, homogeneity and product protection are preserved.

Basic principles and scale-up criteria

At the outset, the critical process parameters and the dominant mixing objective are established, for instance homogenisation, freedom from segregation, product protection or residual discharge. On this basis it is decided which characteristic value is preferably kept constant in the scale-up, for example the circumferential speed, the Froude number or, in suitable cases, an energy-based approach.

The Froude number relates to rotating free-fall, drum and throw mixers as well as to systems with a horizontally mounted mixing tool. In vertical mixers with forced restratification it is not a design or scale-up criterion. What is decisive there is that the mixing principle remains the same across all sizes and that the restratification covers the entire mixing chamber independently of the fill level; transferability is secured in the pilot plant with the original product.

Since not all characteristic numbers can be constant at the same time, the similarity criterion has to be prioritised according to the dominant mixing mechanism. A robust test plan also includes trials close to the process limits, in order to assess the stability of the process at large scale.

Geometric similarity and fill level

The fill level is of great importance in powder mixers with a horizontally mounted mixing shaft.

For reliable transfer, geometric similarity between the scales is decisive. Ratios such as H/D and d/D as well as comparable wall clearances and tool geometries should remain as constant as possible.

The fill level markedly influences the flow pattern, dead zones and power consumption. Overfilling encourages dead zones and can lengthen the mixing time, whereas underfilling can lead to unfavourable circulation behaviour and a reduced mixing effect. It is therefore advisable to investigate several fill levels at small scale and then to transfer the optimum range.

Determining and scaling the mixing time

The mixing time is not a fixed constant but a function of rotational speed, geometry and material properties. At small scale, the mixing kinetics are recorded via mixing quality curves and a target homogeneity level is defined, for instance via the coefficient of variation or a comparable characteristic value.

For transfer to the larger scale, the mixing time is secured experimentally and not scaled up by calculation alone. With powdered goods in particular, bulk behaviour, cohesion, density differences and particle size distribution are so strongly product-dependent that trial values are markedly more robust than rules of thumb.

Deriving the tool speed

The tool speed at the target scale results from the chosen scale-up rule. With a constant circumferential speed it follows that the rotational speed falls as the diameter grows. This approach is often particularly sensible for powdered goods, because it keeps the movement pattern of the bulk material comparable and avoids excessive loading.

The calculated rotational speed always has to be checked against mechanical limits, product-specific sensitivities and practical process stability. A choice of rotational speed based purely on calculated values is generally not sufficient with powders.

Trial planning, measured variables and validation

For a robust scale-up, a structured trial plan with the factors rotational speed, fill level and mixing time suggests itself. Important measured variables are power consumption, torque, mixing quality, residual discharge and product-related characteristics such as moisture distribution, homogeneity or particle build-up.

After transfer to production scale, validation trials follow in order to capture real fluctuations in raw materials, the edges of the fill level range and the actual movement behaviour at large scale. Deviations are documented and used to refine the scaling model.