In the scale-up of ring-layer granulators, geometry, fill level, tool speed, binder addition, granulation time and energy input must above all be transferred to the production scale. The aim is to keep granule size, strength, moisture distribution and homogeneity reproducible at large scale.

1. Geometric similarity

The vessel and tool geometry should remain as comparable as possible. Wall clearances, internals, nozzle positions and the effective working zone of the granulator are equally important, because they directly influence the formation of the ring layer.

Even small geometric deviations can change the flow and wetting behaviour considerably.

2. Kinematic scaling

For RMG, the circumferential speed or tip speed of the tools is frequently used as the central scaling variable. It is usually more meaningful than a mere statement of rotational speed, because it better describes the mechanical stress on the powder bed.

The Froude number is likewise frequently used as a guide, because it represents the ratio of centrifugal to gravitational forces. In practice, however, not just one characteristic value but a scaling package adapted to the product is usually used.

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.

3. Specific energy input and end point

The granulation process depends strongly on the specific energy input and on the process end point. Too little energy leads to weak, fine-grained granules, too much energy to oversize grain, unwanted compaction and increased abrasion. The end point should therefore be defined not by a fixed time but by a measurable criterion.

4. Liquid addition and wetting

The binder or spray rate must be adjusted proportionally and reproducibly in the scale-up. Droplet size, nozzle arrangement, spray pattern and point of introduction are equally important, because they influence local over-moistening and thereby lump formation.

With larger plants, several nozzles or adapted spray geometries are often necessary in order to ensure an even distribution.

5. Residence time and fill level

Fill level and throughput must be chosen so that the ring layer remains stable and granulation proceeds evenly. With continuous plants the residence time distribution is particularly important, because it directly influences granule quality.

A similar fill level range between pilot-plant and production plant is therefore usually advisable.

6. Thermal behaviour

At production scale, heat removal often deteriorates because the ratio of surface area to volume falls. Product temperature, binder behaviour and drying can therefore change more strongly than in the pilot plant.

Temperature monitoring and, where appropriate, cooling or adapted process times are therefore part of a robust scale-up.

7. Mechanical stress and wear

As size increases, torque, bearing load and wear requirements rise. Tools, seals and walls must therefore be designed for the higher load.

With abrasive formulations, wear-resistant surfaces and a readily accessible construction are particularly important.

8. Measurement technology and PAT

For a reliable transfer, torque, power, product temperature and moisture should be recorded continuously. PAT tools and other online measurement systems can help to control the course of granulation and the end point better.

Scaling is thereby secured not only by calculation but also by process analytics.

9. Cleaning and safety

With a larger scale, the requirements for residual discharge, cleanability, dust tightness and explosion protection increase.

With dusty products, earthing, inerting and suitable relief concepts are to be planned in as well.