On-line moisture detection for a microwave vacuum dryer.

A method has been developed to determine the moisture end point in a microwave vacuum dryer using near-infrared spectroscopy (NIR). Dryer studies were conducted in two T.K. Fielder microwave dryers, a Spectrum 65 and a Spectrum 1200. On-line spectra were collected using a NIRSystems Model 5000 Analyzer with a fiber-optic probe. The calibration equation uses absorbances measured at 1410, 1930, and 1630 nm. Moisture values determined on-line with this equation correlate to within 1% of the Karl Fischer results for samples below 6% moisture. The absolute SEP is 0.6% and the average residual is -0.2%. At levels above 6% moisture, the NIR results do not correlate well with Karl Fischer but the moisture curve can be used to assess the homogeneity of the water in the granulation. The combination of qualitative and quantitative information makes it possible to monitor the drying process and to predict consistently the moisture end point in the microwave dryer.