How does scale-up from the pilot plant to production work so that mixing quality and particle integrity are preserved?
In the scale-up of powder mixtures, the parameters of geometry, fill level, rotational speed, energy input and mixing time are transferred above all from the pilot plant into production. The aim is to keep homogeneity and protection of particles reproducible at the larger scale.
Basic principle
Mere enlargement is not sufficient, however, since flow, energy distribution and stress also change with the scale. Alongside geometric similarity, kinematic and dynamic similarity must therefore be taken into account.
Important control variables:
- Fill level: keep as constant as possible within the validated range.
- Froude number: with solids mixers whose mixing tool is mounted horizontally, it is often important in order to obtain similar movement patterns.
- Circumferential speed: particularly relevant where particle breakage or coating abrasion is to be limited.
- Specific power input (P/V): helpful with dispersive tasks, but critical because of heating and possible particle damage.
Securing mixing quality
Mixing homogeneity is secured through sampling and statistical evaluation. With batch processes this means that mixing time and rotational speed must be chosen so that the desired uniformity is achieved without mixing for unnecessarily long. With continuous processes the residence time distribution is additionally decisive.
Securing particle integrity
For sensitive products, the shortest possible process time with sufficient mixing quality is ideal. An excessive circumferential speed or too great an energy input can cause abrasion, grain breakage or damage to coatings, while too low an intensity leads to insufficient homogeneity.
In practice a compromise is therefore usually chosen between the Froude number (where the mixing tool is mounted horizontally), circumferential speed and energy input, matched to product and mixer type.
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.
How amixon® secures the scale-up from the pilot plant into production
The mixing principle is identical across all scales.
The core of the amixon® approach: trial and production mixers work with the same mixing system. The SinConcave®/SinConvex® helical ribbon mixing tool conveys the mix upwards at the periphery and downwards in the centre. This three-dimensional total flow has the same characteristics irrespective of the size.
The product flow I_V can be calculated conveniently.
I_V = A · v_ax = (π/4) · (D² − d²) · Φ · S · n · ζ
- I_V: conveying capacity of a vertical mixing helix
- D/d: outer/inner diameter of the helix
- Φ: fill level
- S: pitch of the helix
- n: rotational frequency
- ζ: velocity coefficient of rotational frequency
In this respect congruence can always be established across the most varied of sizes.
Scaling takes place via the restratification formula, that is via the conveying volume flow of the helical mixing tool and the restratification frequency derived from it – not via the rotational speed alone: all sizes are operated within the same window of approximately 0.8 to 3.5 m/s, generally at low speed. Flow, mechanical stress on particles and conditions of similarity between the scales are thereby preserved.
Mixing quality independent of fill level
amixon® mixers achieve the highest mixing qualities even at a low fill level (approximately 10 %), namely the technically ideal random mixture, which cannot be improved in practice. Batch sizes can vary within the same mixer without homogeneity having to be revalidated. This defuses the most critical moment in a scale-up: the first small production batches in the large machine.
A fine size range instead of jumps in scale
On request, amixon® manufactures mixer sizes in 100-litre increments from 100 to 50,000 litres (VM, HM, AM). The Gyraton® GM continues from 10 to approximately 100 m³ in 1 m³ increments. The step from the validated trial scale to the target size can be chosen as finely as desired, since there is no forced jump to the next catalogue size that would violate the similarity.
Validation before the investment
The amixon® pilot plant has 35 test units of various sizes available. Trials run with the original product, at real fill levels and within the planned temperature and pressure range. They are carried out, evaluated and documented together with amixon® experts. The parameters determined (circumferential speed, mixing time and fill level) flow one to one into the design of the production machine.
Manufacture in Paderborn as the quality foundation
amixon® develops and manufactures exclusively at the Paderborn works, with the greatest depth of fabrication and all components from Germany. As a certified welding company with European, Japanese, Korean and American qualifications, amixon® designs every apparatus as a one-off on the basis of the operator's URS. Quality control remains entirely in-house and every specification is verifiable down to component level. This manufacturing sovereignty also secures long-term supply. Every component can still be reproduced decades later.
Service across the entire life cycle
After commissioning, amixon® continues to stand at the operator's side: regular inspections and preventive maintenance secure availability, on request with predictive maintenance. Selected wear parts are supplied together with the initial delivery, and most spare parts are held at the Paderborn site as well as at the service bases in Japan and the USA. A lifetime spare parts service is thereby guaranteed, even where original suppliers cease to exist. Many amixon® machines have been in daily use for more than 30 years. Through modernisation and retrofitting they remain at the latest state of the art.