In situ monitoring and control of moisture content in pharmaceutical powder processes using an open-ended coaxial probe

An analysis of microwave measurements of the complex dielectric constant of different mixtures of pharmaceutical materials using an open-ended coaxial probe is presented. Using the probe in combination with a network analyser, measurements in the frequency range of 1–19 GHz were conducted. Calibration measurements on conditioned samples were first acquired in a controlled laboratory environment, and then in situ measurements, taken in a small-scale high-shear mixer, were also obtained. The dominating material in the investigated mixtures was microcrystalline cellulose. By using the suggested microwave method, a novel possibility for in situ measurements of the initial moisture content of the powder mixture before and at the beginning of the water addition stage is demonstrated. In situ density-independent estimation of the moisture content having a relative error of below 10% for the moisture interval of 2–14% is demonstrated. The possibility of performing an adaptive control of the evolution of the mixing process by utilizing the microwave sensor information is also presented.

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