Which process sensors for torque, temperature and moisture are useful for detecting granulation end points in a continuous mixer?
The granulation end point in a continuous-flow mixer is detected most reliably with a combination of torque, temperature and moisture sensors. Each variable describes a different aspect of the process state and should not be considered in isolation.
Torque / power consumption
Measuring the shaft torque or the drive power is the most important direct indicator of granulation progress. As binder liquid and grain growth increase, the cohesion, viscosity and flow resistance of the moist material rise. The torque transmitted increases and forms a characteristic curve. A plateau or a defined threshold value of this curve frequently marks the optimum granulation state in practice. The transition from wetting to the growth and compaction phase shows itself in a marked rise in torque followed by stabilisation. The measurement can be made via torque measuring shafts, strain gauges or indirectly via motor current and power consumption, and is suitable for trend analyses and statistical process control. In continuous granulation processes the interpretation is more complex than in batch operation, because steady-state operating conditions and material flows have to be assessed; trend analyses and soft sensor models are used here.
Temperature
The product temperature describes the thermal and energetic situation in the mixing chamber. Friction and shear forces in the compacted granulate bed generate frictional heat, while wetting and evaporation additionally influence the temperature profile. A rise in temperature frequently correlates with increased mechanical loading and compaction; drops in temperature can indicate stronger evaporation or over-wetting. Measurement is typically made with resistance thermometers in product contact or without contact using IR sensors at the outlet or at sight glasses. Temperature provides early indications of over-granulation, over-wetting or thermal overload and is important with thermolabile active ingredients in order to avoid degradation. As an end point indicator it is generally used to supplement torque and moisture rather than as the sole target variable.
Moisture
Inline recording of the water content secures the material state. Moisture governs capillary and adhesive forces, the degree of wetting, grain enlargement and the mechanical stability of the granules, and is decisive for downstream steps such as drying, tabletting and shelf life. Typical methods are near-infrared spectroscopy as a fast, contact-free inline method with real-time control potential, which requires chemometric calibration, and microwave resonance sensors for volumetric moisture measurement with low sensitivity to soiling. Capacitive sensors are inexpensive and easy to integrate but more strongly matrix-dependent and often less precise. Moisture sensors make it possible to define a target range in which the desired granule structure and flow properties are achieved, and serve as a reference signal for validating the mechanical and thermal measured variables and for controlling binder addition.
A sensible combination and extensions
None of the variables named is sufficient on its own to detect the granulation end point robustly in a continuous-flow mixer. In practice, multivariate sensor fusion is pursued: torque or power describes the mechanical state, temperature the thermal and energetic state, and moisture the material state. In modern PAT concepts these signals are brought together in multivariate models in order to detect end points reproducibly, build closed control strategies, compensate for raw material fluctuations and reduce rejects. Acoustic emission measurement or optical methods for particle size analysis can optionally be integrated, in order to link granule morphology directly with the process signals.
How amixon® equips the RMG continuous mixer with process instrumentation
Instrumentation as a supplied component – integration as an amixon® competence
amixon® does not develop or manufacture sensors of its own. The apparatus can, however, be equipped with a wide range of suitable measuring systems for process monitoring, for automation and for safeguarding occupational and product safety. Which measuring principles, sensor designs and makes are used is always determined with reference to the application.
amixon® handles the process-engineering and design integration of the instrumentation into the RMG continuous mixer. Together with the operator and the operator’s instrumentation supplier, the selection, installation position, hygienic design, signal processing and the connection to the higher-level plant control system are defined. What matters is which measured variable correlates reliably with the desired granulate quality under the actual process conditions.
Imaging online measurement methods can be particularly informative. They make it possible to record particle size and, where applicable, particle shape continuously. The data obtained can be used to monitor the granulation process and – provided the process design allows for it – to control it in a targeted manner. The key manipulated variables are in particular:
- the quantity added, or the mass flow, of the granulating liquid,
- the rotational frequency of the mixing tool,
- the mass flow of the solid,
- the specific energy input and
- where applicable, the product temperature.
Torque, power consumption, temperature and moisture also provide valuable indications of the current process state. They can make changes in wetting, agglomerate formation, product consistency or adhesion behaviour visible. Whether a single measured variable is sufficient or whether several signals have to be evaluated in combination depends on the formulation, the raw material quality and the process management.
Process understanding is gained in the pilot plant
For a specific formulation, amixon® can investigate in the pilot plant with the original product which measured variables best represent the desired granulation state. Relevant process data are recorded for this purpose and compared with analytically determined quality characteristics of the granulate produced. The results are documented traceably in the trial report. These investigations create a sound basis for the design and subsequent commissioning of a production plant. They help to assess at an early stage the interactions between formulation, raw material quality, initial moisture, liquid addition, energy input, circumferential speed of the mixing tool, residence time and dosing accuracy. In this way suitable operating windows can be defined and process parameters established for a stable, reproducible granulation process.
RMG continuous mixer: experience, service and spare parts
The RMG continuous mixer has proved itself very well for selected product classes and for continuous mixing and agglomeration tasks. amixon® manufactures such apparatus on request, provides support with maintenance tasks and ensures a long-term supply of spare parts.
Further development of this particular mixing and granulating system has, however, not been a focus of amixon®’s development work since 2010. Today the focus lies on precision mixers for powders, solids and pasty products with vertically or obliquely arranged mixing shafts. These systems are particularly suitable for applications with high requirements regarding mixing quality, residual discharge, gentle product handling, vacuum processes, thermal treatment as well as hygienic design and containment.