Scale-up

Definition

Scale-up refers to the transfer of a process developed on a laboratory or pilot plant scale to production scale without compromising product quality or reproducibility. In mixing technology, this means that the mixing quality, particle structure, residual moisture and discharge behaviour should achieve the same values in the large batch as in the laboratory test.

Principles of similarity

The transfer is based on considerations of similarity. Geometric similarity means that the mixing chamber and the tool geometry scale up in the same ratio. Kinematic similarity refers to comparable flow patterns, whilst dynamic similarity refers to a constant ratio of the forces acting on the system.

In mixing systems with horizontally mounted mixing tools, as well as in rotating free-fall, drum and toss mixers, the Froude number – that is, the ratio of inertial forces to gravitational forces – is frequently kept constant. In practical terms, this means that as the tool radius increases, the rotational frequency decreases, whilst the peripheral speed remains within a comparable range.

In vertical mixers with forced recirculation, the Froude number is not a design or scale-up criterion. The key factor here is that the mixing principle remains the same across all sizes and that the recirculation covers the entire mixing chamber regardless of the fill level; transferability is verified in the pilot plant using the original product.

What cannot be scaled up

Not all effects can be quantified by a single parameter. Particle breakage, abrasion, dust formation, segregation during discharge, the loss of instantaneous properties and heat dissipation via the vessel wall behave differently on a large scale because the surface-to-volume ratio decreases and residence times increase. These effects only become apparent at a pilot scale where the batch size is close to that of the subsequent production batch.

Procedure

A robust scale-up begins with a mixing trial using the original product. In a parameter study, mixing time, rotational speed, fill level, the point of addition and sequence of addition of the components, as well as the type and quantity of liquid additives, are systematically varied. The mixing quality is assessed using the coefficient of variation, along with the degree of discharge, particle integrity, energy input and product temperature.

The validated values are documented and transferred to the plant control system as a recipe record. It is only this documentation that makes the risk of the scale-up calculable and underpins the investment decision.

Distinction

Scale-up is not the same as capacity expansion. If merely a second plant of the same size is procured, the process parameters remain unchanged. A scale-up occurs when the batch size itself increases and the operating procedure must therefore be adapted.