In the chemical and pharmaceutical industries, it is very useful to know both the density and the moisture content of bulk powders. Real-time knowledge of these values can help optimize a process. Traditionally, the techniques used to measure these parameters require intrusion into the system, which makes real-time measurements nearly impossible. However, they can be estimated noninvasively from the dielectric properties of the materials using a microwave sensor. While many studies have focused on measuring the dielectric properties of static powders, this research has investigated the feasibility of using a microwave sensor in the presence of rapidly flowing powders.
An experimental apparatus was fabricated to uniformly stir powders above the microwave sensor at high velocities, around 800 RPM, while measurements of the complex dielectric properties were taken in real time. Calibration curves of the calculated real versus imaginary components of the complex permittivity were constructed along lines of constant moisture content. Similar to the results generated for static powders, the slopes of the calibration curves from the flowing powder increased with increasing moisture content. Both the bulk density and the moisture content of unknown samples can be determined by utilizing the calibration curves. An additional experiment was performed to further confirm the accuracy of the calibration curves.
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