How do you scale a process from the pilot plant to production without losing mixing quality?
In the scale-up of powder mixtures, what counts above all is that the mixing movement, the rotational frequency of the mixing tool and the mechanical stress are transferred sensibly from the pilot plant into production. What is decisive here are not the characteristic values typical of liquids, but the rheological behaviour of the bulk material itself.
Most important points
- Geometric similarity: mixing chamber, tools and fill level must stand in a comparable relation.
- Circumferential speed and rotational speed: they determine the particle movement and the mechanical stress.
- Mixing time: must not merely be adopted formally at large scale, but must be matched to homogeneity and product protection.
- Energy input: too much energy can cause abrasion, segregation or the development of heat.
- Segregation: differences in the density, size and shape of the particles must be taken into account in the scale-up.
Practical consequence
A good scale-up for powders means: as little change as possible to the mixing kinematics, as much adaptation as necessary to product and size. Verification takes place through pilot-plant trials, piloting and sampling of the resulting mixing quality.
How amixon® transfers a process step by step from the pilot plant into production
Loss of mixing quality during scale-up rarely originates in the mixer itself, but in the gaps of the procedure: unclear target values, trials with a substitute product, parameters that are reinterpreted along the way. amixon® therefore works to a fixed sequence, at the end of which the production machine delivers the same values as the trial mixer.
Step 1: Define acceptance criteria before the first trial
Together with the operator, the target values are defined in writing before the first batch runs: the required mixing quality as coefficient of variation, degree of discharge, cycle time, smallest and largest batch mass, and the permissible stress on the product. Without these values, it is impossible to judge later whether a scale-up has succeeded.
Step 2: Trial with the original product
The trials are run with the original product, not with model substances, and with real fill levels, batch sizes and the intended temperature and pressure range. The Paderborn pilot plant has 35 test units available for this, supplemented by pilot plants in Japan, India, Thailand, China, South Korea and the USA. The unit size is chosen to match the available product quantity and the later target batch.
Step 3: Systematically vary scale-up-relevant parameters
The parameters varied are specifically those that can later be transferred to the production machine: mixing time, circumferential speed within a window of approximately 0.8 to 3.5 m/s, fill level and, where relevant, dosing rate and the method of liquid addition. Rotational speed itself is not a transfer variable, because it varies with the diameter of the mixing tool.
Step 4: Measure results instead of estimating them
Mixing quality is determined analytically via a defined sampling plan, and the degree of discharge gravimetrically. Product protection, energy input, cleanability and reproducibility across several batches are also assessed. All process data are recorded electronically and documented in a trial report.
Step 5: Transfer without a scale jump
Because trial and production mixers work on the same SinConcave®/SinConvex® mixing principle, and mixing quality is maintained across a fill-level range of approximately 10 to 100 %, the parameters determined are carried unchanged into the design. The size grid in 100-litre increments from 100 to 50,000 litres makes it possible to select exactly the target size, instead of moving up to the next catalogue size. For large batches, the Gyraton® silo mixer takes over from 10 up to approximately 100 m³.
Step 6: From the trial report into the URS
The documented parameters become part of the User Requirement Specification and are thus contractually binding. Every apparatus is manufactured as a one-off on this basis. The factory acceptance test in Paderborn and the on-site acceptance are checked against the same acceptance criteria that were defined before the first trial.
Step 7: Fine-tuning during start-up
During commissioning, the mixing programmes – with speed, duration and, where applicable, rotor use – are stored in the control system, so that every batch is run reproducibly. amixon® accompanies the start-up and remains a point of contact afterwards for product changes and further process optimisation.
What remains to be considered outside the mixer
The homogeneity achieved in the mixer can be lost again in downstream steps. Conveying, intermediate storage and filling favour segregation, particularly where there are large differences in density, particle size or flow behaviour. amixon® therefore assesses not only the mixing quality at the outlet during the trial, but also the discharge behaviour – residual discharge without a bulk cone and without segregation is part of scale-up verification, not a downstream issue.