Use of a Microwave Sensor to Monitor Bulk Density during Grain Drying

Highlights An eighth-scale grain drying system was used to monitor real-time change of bulk density in a bed of wheat during drying. Density was determined from dielectric properties measured by a microwave sensor at the center of the bed. Density was simulated at other locations by modeling heat and mass transfer. Root mean square error (RMSE) when comparing the simulated results to the empirical results at the center of the bed was 0.0054 g/cm3. Abstract. Agricultural commodities such as cereal grains, oilseeds, and nuts are dried before and during storage to minimize, if not prevent, degradation of quality. While drying, heat and mass transfer occur simultaneously resulting in the removal of moisture from the product over time. As the moisture is removed, mass and volume are reduced. Such losses produce shrinkage, and thus, density changes throughout the drying product for the duration of the drying process. Density is an important parameter because it is indicative of other quality parameters such as test weight. It is also critical for operators because it can be used to estimate grain mass in silos, determine resistance to airflow, and predict grain pressure in silos. Despite its usefulness, bulk density is difficult to measure at specific locations within a bed of grain or seed. It can be calculated for small quantities and estimated for large quantities by gravimetric means; however, it has been shown that bulk density varies throughout a bed of grain or seed. Such variances are caused by compaction from upper layers and differences in moisture content. Therefore, an eighth-scale grain drying system was used to observe the real-time change in bulk density within a 153,000-cm3 volume bed of wheat. Bulk density was determined empirically at the center of the volume from dielectric properties measured with a microwave sensor, and it was simulated at other locations by modeling heat and mass transfer. Comparison between the bulk density determined from simulation and that measured empirically at the center of the volume of wheat resulted in a root mean square error (RMSE) of 0.0054 g/cm3; thus, the accuracy of the models was confirmed. Real-time knowledge of bulk density at various locations can aid in monitoring the quality of the product being dried and provide essential information concerning the pressure and airflow throughout the entire volume. Keywords: Bulk density, Dielectric properties, Drying modeling, Grain drying, Microwave sensing, Real-time monitoring.