(21f) Drag Force Flow Sensors for in-Line Monitoring of Powder Processes

In-line Process Analytical Technology (PAT) tools can produce data which allows for real-time monitoring and control of material without the need to disrupt the manufacturing process. As well as providing minimal disruption, in-line measurements have several advantages when compared with off-line and at line techniques. Most importantly the data represents a greater proportion of the in-process material and the measurements are made under process conditions – this is especially important for powder and granular systems whose behaviour is dependent on localised stress and strain rates.

Several PAT tools exist for in-line measurement, typically based on particle size analysis or spectrographic techniques, however, there are some limitations associated with these methods. The application of a Drag Force Flow (DFF) sensor overcomes these limitations and provides robust, highly sensitive and high (temporal) resolution in-line measurement of the local flow forces within process equipment. A DFF sensor is a thin, hollow pin, whose deflection in the flow is measured by two optical strain gauges affixed to the inner surface of the pin. The sensor is calibrated to measure DFF, this is then converted to a Force Pulse Magnitude (FPM) which is influenced by fundamental parameters of the material such as density and shear viscosity. In addition to force, the DFF sensor also outputs temperature information.

The fast measurement rate (500 Hz), sensitivity (0.4 Pa to 100 MPa) and stainless steel construction mean that the DFF can be used in a diverse range of applications from High Shear Wet Granulation through to mixing and blending and flow in powder transport systems

To evaluate the applicability of the DFF sensor, the flow behaviour of several types of granular material was investigated during a simple blending process. The DFF probe was able to detect clear and repeatable differences between samples and identify variations in in-process behaviour.

In a second study, the ability of the DFF sensor to track the addition of a liquid binder and consequently inform on blend uniformity was investigated. A short chain soluble carbohydrate was mixed with two binders with notably different viscosities (Binder A = ~0.001 Pa.s and Binder B = 100 Pa.s) at 1% w.w. and 3% w.w concentration. Clear differences were observed in the FPM profiles for the four blends. Both blends containing Binder A exhibited a sharp increase in FPM followed by a decrease until a steady state was achieved, suggesting uniform mixing. The 3% formulation required approximately 60 s less to reach steady state. Binder B (3%) generated a similar profile however the peak in the FPM was notably broader suggesting significant differences in the dispersion mode for the two binders.

The DFF sensor was demonstrated to be a highly sensitive device, capable of detecting minor variations in powder and granulate properties. As such, this technique methodology may be suitable for real-time in-line continuous monitoring of typical pharmaceutical unit operations such as blending, mixing and High Shear Wet Granulation (detection of end point/blend uniformity) and in powder transport systems.