In-situ Monitoring of Moisture Content in Powder Processing Using Dielectric Probe Measurements with Multivariate Calibration

Presented is a study of application of open-ended coaxial probe for in-line measurements of moisture content of pharmaceutical powders during mixing processes. The procedure utilizes a multivariate calibration technique relating the moisture content to the spectrum (1-19 GHz) of a density independent function derived from the permittivity epsi\´ and the loss factor epsi" of the powder samples. The conducted calibrations and experiments utilized powder mixes, where micro crystalline cellulose was the dominant component. Moisture levels from 1.5% to 8.2% were studied. Two different mixer impeller speeds were also tested. The achieved accuracy had a relative error in the interval 27% for the 1.5% moisture level and below 10% for the rest of the levels studied, the latter within the theoretical method uncertainty. The density independence of the utilized ratio was confirmed by its insensitivity to the mixer impeller speed