How do I plan trial series in the pilot plant to optimise mixing time, speed and liquid addition for a new spice recipe?
Trial planning in the pilot plant (DoE)
The optimisation of a spice formulation is carried out in a structured manner with the help of statistical design of experiments (DoE). The aim is to obtain robust statements on mixing time, rotational speed and liquid addition from a small number of trials. Interactions are specifically taken into account. All trials take place under reproducible conditions.
At the outset, clear target variables and assessment criteria are defined. Typical criteria are:
- Homogeneity (coefficient of variation CV below 5 %)
- Particle size distribution and agglomerates
- Residual moisture and water activity (aw)
- Bulk density and free-flowing behaviour
- Sensory properties (colour, aroma, taste)
- Dust content and visual quality
In addition, the following process limits are recorded:
- Energy input
- Product temperature
- Wall build-up
- Cleaning effort
The most important influencing factors and ranges are defined and delimited.
- Mixing time: typically 30–600 s.
- Circumferential speed: approximately 1–6 m/s at the main mixing tool, approximately 10–30 m/s at the cutting rotor.
- Liquid addition: 0–15 % (quantity, timing, type of input).
Further variables such as fill level, raw material sequence and nozzle type are kept constant or taken into account systematically.
Trial design (DoE structure): planning takes place in three stages.
- Screening: identification of the main effects (e.g. fractional factorial).
- Optimisation: modelling of interactions (e.g. CCD, Box-Behnken).
- Verification: confirmation of the optima by repeat trials.
The sequence is randomised. Centre points serve to assess scatter and curvature.
Conduct of the trials
The mixing time is determined via time series. Samples are taken at several positions. The optimum point is reached when the CV value attains a stable plateau. Excessively long mixing times should be avoided, since otherwise abrasion, dust and loss of aroma can occur.
The rotational speed determines the mechanical load. What is assessed is the relationship between rapid blending and product protection. High rotational speeds improve the break-up of agglomerates but increase breakage and fine dust.
In the case of liquid addition, the timing, the type of dosing and the nozzle characteristics are examined. The aim is to achieve uniform wetting without the formation of lumps. The post-mixing time is adjusted specifically.
Data acquisition and evaluation: all relevant process data are recorded.
- Power and torque.
- Product temperature, addition quantities and times.
- Samples and analytical data.
The evaluation is carried out by means of regression and ANOVA. Significant effects and interactions are identified. The results are presented as response surfaces. Several target variables can be combined via desirability functions.
Scale-up into production
Scale-up is taken into account at an early stage. The following serve as transfer criteria:
- constant circumferential speed or Froude number,
- constant specific power input P/V,
- comparable spray characteristics for liquids.
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.
The aim is the direct transferability of the optimum parameters.
The results are statistically secured and validated. Samples from various positions are evaluated (CV). The analysis is carried out according to the product (e.g. sieving, colorimetry, active ingredient analysis).
How amixon® plans trial and scale-up programmes in the pilot plant
At the outset comes the definition of objectives, in which the operator's clear acceptance criteria are established. These include mixing quality (e.g. RSD), discharge rate, product protection (particle integrity, no heat input), batch size and cycle time. amixon® transfers these requirements into a structured trial programme. Testing takes place on one of 35 test units in a size that matches the available product quantity.
The scale-up-relevant parameters are investigated.
- Mixing time,
- Circumferential speed (typically 0.8–3.5 m/s),
- Fill level (high mixing quality already from approximately 10 %),
- Liquid addition (quantity, dosing rate, input system such as a lance or two-fluid nozzle, optionally with a cutting rotor).
Samples are taken at every operating point and the mixing quality is determined. The discharge rates are verified gravimetrically. Trial cascades deliver robust data.
Certain product properties cannot, however, be reliably calculated. These include particle breakage, dust generation, preservation of the bulk density, instantisability and the discharge behaviour. For this purpose amixon® conducts staged trial series with different batch sizes, e.g. 1 l, 100 l, 400 l, 2,000 l and 3,000 l. Large pilot plant mixers up to 3 m³ are also available. This produces robust data for the scale-up to approximately 15,000 litres.
All trials are evaluated and documented jointly, in order to create a well-founded basis for investment. The trial report confirms the optimum process parameters under real conditions. The trial report defines the suitable apparatus and serves as the basis for the URS. Target variables such as RSD, discharge rate and cycle time are established precisely. Investment risks are reduced in advance.
amixon® mixers work at low speed and gently, so that gentle processing of sensitive products is assured. Fragile particles such as herbs, tea or spices remain intact. Almost complete discharge and rapid cleaning make frequent product changes possible.
Spices are natural products. Their quality fluctuates according to origin, harvest time and freshness. Fluctuations in quality can frequently be evened out by larger, homogenised batches. The Gyraton® mixing silo homogenises batches of up to 100 m³, both batchwise and continuously.
At the end of a trial campaign there is a works tour at amixon®.
Manufacture and quality assurance
amixon® develops and manufactures exclusively in Paderborn. All components come from Germany. As a certified welding company with international qualifications, amixon® manufactures every mixer as a one-off on the basis of the URS. Quality control is carried out entirely in-house. Every specification is traceable down to component level. Spare parts remain reproducibly available in the long term.