What solutions do manufacturers offer for integrated moisture measurement and endpoint determination when drying in the mixer?
Several measuring methods are available for integrated moisture measurement and endpoint determination when drying in the mixer. In practice, the drying endpoint is usually not determined from a single signal, but through the combination of moisture, product temperature, pressure, vapour or condensate profile, mixing-tool power, and validated reference measurements.
NIR spectroscopy is a fast, contact-free method for moisture measurement. The sensor measures the reflectance of the mix in the near-infrared range through a sight glass or a flush-mounted measuring point. With suitable calibration, NIR can capture moisture in real time and additionally give indications of further product properties. Because NIR mainly captures the visible area of the product bed, a representative mixing movement, a clean measuring window and product-related calibration are decisive.
Microwave-based methods capture the dielectric properties of the mix. Depending on the installation type and product, they can capture a larger product volume than optical methods and are less dependent on colour or surface appearance. However, recipe, bulk density, temperature and material distribution also influence the signal. Calibration with the original product and suitable reference samples is therefore required here too.
Capacitive sensors are a robust and comparatively economical solution for trend monitoring of moisture. Their readings, however, depend more strongly on recipe, temperature, density and particle structure. Conductive methods are only useful for products with sufficient electrical conductivity and are therefore not suitable for many dry powders.
Torque and power consumption of the mixing tool supplement direct moisture measurement. During drying they can indicate changes in flow behaviour, stickiness, compaction, agglomeration or wall build-up. They do not measure moisture directly, but they provide valuable information on process stability, energy input and plant protection.
The most direct endpoint is a validated residual-moisture limit. The drying process ends automatically once the moisture sensor reaches the defined target value. This target value is always product-specific. It depends, for example, on storage stability, flowability, further processing or regulatory requirements. A generic target value such as 0.5% residual moisture is therefore not transferable to all products.
Product temperature provides additional indications of the drying progress. As long as relevant quantities of a volatile component are evaporating, part of the heat supplied is needed as enthalpy of evaporation. With constant heating power, a rise in temperature can indicate a decreasing moisture content. This signal, however, depends on the heating medium, vacuum, heat transfer and product properties, and should not be used as the sole endpoint criterion.
In vacuum mixer-dryers, process pressure, product temperature, vapour profile and condensate quantity supplement the monitoring. If the quantity of condensable vapour falls to a low level at a defined temperature and pressure level, this can indicate an advanced drying state. The most reliable endpoint determination results from combining this process data with an analytically confirmed residual moisture value.
Sensors are integrated as flush as possible into the vessel wall or installed via hygienic connections. This limits dead spaces, mechanical damage and build-up. With dusting, sticky or condensing products, optical measuring windows can require flushing, a wiper or a suitable cleaning method. With large mixers, several measuring points can be useful, provided they capture representative areas of the moving product bed.
The measurement data is processed in the PLC and can be passed to higher-level control and production systems. This enables trend recording, batch logging, alarm limits and automatic endpoint detection. In regulated areas, data integrity, audit trail, user rights and traceable calibration of the measuring systems are additionally important.
How amixon® determines the drying endpoint
For mixing, granulation and drying processes, amixon® combines several process signals. The focus is on product temperature, pressure profile, moisture, mixing-tool power and, for vacuum processes, vapour and condensate profile. Only the product-related combination of these measured variables enables a robust and reproducible endpoint determination.
In vacuum mixer-dryers and reactors of the VMT and AMT series, product temperature and system pressure are monitored continuously. Condensate quantity, vapour flow and the profile of drive power can also be evaluated. If the quantity of vapour removed drops significantly at a defined temperature and pressure level, this points to an advancing drying state. A reliable endpoint, however, is not derived from pressure or temperature alone, but is secured through combination with the defined moisture target value and, where applicable, an analytical reference sample.
Depending on the product, moisture measurement can be carried out using NIR, microwave or capacitive sensor technology. NIR sensors capture the moisture profile in the visible product area without contact. Microwave and capacitive methods can be a useful alternative where optical measuring windows are restricted by dust, build-up or product properties. All methods require calibration with the original product and a measuring point that delivers data representative of the particular product movement.
Torque and power consumption of the mixing tool can also usefully supplement the process. They show changes in flow behaviour, stickiness, build-up or compaction. During drying they do not measure residual moisture directly, but can serve as an additional signal for product state, process stability and plant protection.
An integrated sampler supplements the inline sensor technology with targeted product checks. Laboratory analysis then serves as a reference for calibrating and validating the process signals. This allows moisture profile, temperature, pressure, mixing-tool power and analytically determined residual moisture to be linked together.
Project-specific sensor technology
amixon® apparatus is designed on the basis of the User Requirement Specification. Sensor technology, measuring points, the depth of automation, PLC functions and the desired batch documentation are established within the project. The design takes into account temperature, pressure, vacuum, abrasion, the tendency to build up, hygienic design and, where applicable, ATEX requirements.
Mixing and drying programmes can be stored in the PLC. Data is passed to SCADA, MES or ERP systems via the agreed interfaces. Barcode scanners can supplement batch-related documentation. In regulated applications, requirements for data integrity, user rights, audit trail and electronic batch documentation can be taken into account.
Determining the endpoint in the pilot plant
In the amixon® pilot plant, it is determined with the original product which combination of moisture, product temperature, pressure profile, condensate quantity, mixing-tool power and mixing time indicates the drying endpoint most reliably. The data obtained from this forms the basis for the sensor design, the drying programme and the later production plant.