Which inline sensors (torque, temperature, moisture) are useful for process monitoring in mixers?
For process monitoring in powder mixing, torque or power consumption, temperature and moisture are the most important measured variables. However, they provide different information and must be interpreted on a product-specific basis. None of these variables proves adequate mixing quality on its own. Validated, product-specific correlations or additional PAT methods are required in particular for determining homogeneity.
The torque or power consumption of the mixing drive is a practical, indirect signal. It can indicate changes in flow behaviour, compaction, the tendency to build up, or fill level. In wetting, granulation or de-agglomeration processes, a characteristic torque profile can point to a defined process state. A plateau in torque, however, does not automatically signal that every powder component is homogeneously distributed. The signals are particularly suitable for plant protection and for detecting blockages, overloads or unusual product states. Measurement can be made directly at the shaft or indirectly via the motor's active power consumption.
For dry powders, temperature measurement is relevant above all for monitoring frictional heat, temperature-control processes, drying and safety-critical reactions. Pt100 sensors or thermocouples can be installed in the vessel wall, in the product area or in the temperature-control jacket. The measuring point must be chosen carefully: a wall temperature is not automatically identical to the temperature of the entire product bed. Several measuring points can be useful where temperature gradients, local heating or uneven heat transfer are expected.
Moisture measurement is particularly important where moisture influences flowability, dust behaviour, agglomeration, storage stability or the quality of the end product. This applies, for example, to granulation, wetting, drying and coating. Systems for spectroscopic inline moisture measurement (NIR) can capture moisture and, with suitable calibration, further quality characteristics quickly and without contact. The measurement relates, however, to the area the sensor reaches optically. The measuring window, build-up, particle movement and the representativeness of the measuring point must therefore be taken into account. NIR sensor technology is also used for monitoring powder mixtures and for determining a mixing endpoint.
Microwave-based or capacitive moisture sensors can be an alternative depending on the application. Microwave measurements capture larger product volumes than optical surface measurements, but likewise require product-specific calibration. Capacitive sensors are economical and robust, but often react more strongly to changes in recipe, density or temperature.
Useful supplementary measured variables are fill level, mixing-tool speed, pressure, vacuum, residual oxygen content, gas flow, vibration, and bearing and seal temperatures. For products at risk of dust explosion, sensors, cable glands and process connections must be executed to ATEX or IECEx compliance. For abrasive powders, wear-resistant sensor windows and a protected installation position are also important.
Direct inline particle-size measurement in the mixing chamber is technically demanding for dry powders and not standard. It tends to be used in special measuring sections or downstream conveying lines instead. Conductivity and pH value, by contrast, are typical measured variables for liquid systems and are not relevant for pure dry-mixing processes.
Process data is captured via the machine control system and can be passed to a process control system or to higher-level control and production systems. Common interfaces are, for example, 4–20 mA, HART, IO-Link or standardised communication and fieldbus standards. This makes it possible to realise batch records, trend curves, limit-value monitoring, alarms and condition-based maintenance.
In summary, the combination of torque or power consumption, temperature and moisture gives a solid picture of the powder process. Torque shows mechanical changes, temperature captures heat development, and moisture describes a major influence on flow behaviour, agglomeration and product stability. For robust endpoint detection, these signals must be checked with the original product, calibrated and validated against suitable quality analyses.
How amixon® integrates sensor technology into powder processes
amixon® designs process sensor technology and automation on a project-specific basis. The focus is on measured variables that make product state, energy input and batch reproducibility traceable. Typical signals are the drive's torque or power consumption, product temperature, moisture and fill level. In vacuum processes, pressure, gas atmosphere, vapour profile and condensate quantity are added.
Torque and power consumption provide indications of flow behaviour, compaction, build-up, wetting and changes in product state. In granulation, wetting or de-agglomeration processes, a characteristic signal profile can point to a defined process state. Torque alone, however, is not a universally valid proof of mixing homogeneity. Its informative value is examined and calibrated for each recipe with the original product.
Temperature monitoring serves to control frictional heat, temperature management, drying and thermally sensitive products. Measuring points can be arranged in the product area, in the vessel wall or in the temperature-control jacket. In vacuum mixer-dryers and reactors of the VMT and AMT series, system pressure, product temperature and vapour profile supplement process monitoring. The combination of these measured variables supports the assessment of drying progress and the definition of a reproducible drying endpoint.
Moisture measurement is particularly relevant for wetting, granulation, coating and drying. Depending on the product, NIR, microwave or capacitive measuring methods can be used. The measuring points are designed so that they deliver reliable signals under real conditions of particle movement, dust, abrasion and possible build-up. An integrated sampler can supplement the inline data with targeted product samples.
Sensor technology in the pilot plant
According to the company, more than 35 test machines are available in the amixon® pilot plant for mixing, finishing, vacuum and drying trials. Some of these test mixers have special sensor technology in the mixing tool, in the vessel side wall and in the base area.
This allows local mechanical stresses in the product bed to be captured at different positions and evaluated spatially. The measurement data shows how contact forces, shear stresses and energy input are distributed within the mixing chamber. They help to better understand the product movement and identify critical stress zones.
These findings are particularly valuable for the design of large powder mixers. They support the selection of mixing tool, speed, fill level and mixing time. This allows the scale-up from the pilot-plant machine to the production plant to be verified with the original product.
Automation and endpoint
amixon® apparatus is designed on the basis of the User Requirement Specification. Sensor technology, measuring points, PLC functions, batch logging and interfaces to the control system are defined within the project. Mixing programmes can be stored in the PLC and run reproducibly for each batch.
Data is passed to SCADA, MES or ERP systems via the agreed interfaces. Barcode scanners can supplement batch-related documentation. In regulated environments, requirements for data integrity, user rights, audit trail and electronic batch documentation can be taken into account in the automation.
A reliable endpoint does not result from a single sensor value. amixon® determines in the pilot plant which combination of torque, temperature, moisture, pressure profile, mixing time and product analysis is informative for the particular recipe. This trial data forms the basis for the sensor design and the mixing programme of the later production plant.