Use of a microwave sensor to monitor bulk density during grain drying

Authors: Lewis, Micah; Trabelsi, Samir

Submitted to: Applied Engineering in Agriculture
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/20/2023
Publication Date: 6/12/2023
Citation: Lewis, M.A., & Trabelsi, S. (2023). Use of a microwave sensor to monitor bulk density during grain drying. Applied Engineering in Agriculture, 39(3):359-366. https://doi.org/10.13031/aea.15452.
DOI: https://doi.org/10.13031/aea.15452

Interpretive Summary: As agricultural commodities are dried while stored in silos after harvest, the removal of moisture causes the product to reduce in mass and volume. Density is an important parameter because it can indicate certain quality parameters about the crop. It is also helpful in determining pressure within the silo, grain mass, and airflow resistance. Despite its usefulness, density is difficult to measure in silo as it varies throughout. Therefore, a microwave sensor has shown promise in determining density in large volumes, and it is being evaluated. An eighth-scale grain drying system using a microwave sensor, developed within USDA, was developed to observe and control the drying process of cereal grain and oilseed products. The microwave sensor along with 8 temperature sensors and 4 relative humidity sensors are utilized within the system to measure parameters in real-time during the drying process. For this experiment, the eighth-scale grain drying bin was filled with wheat to a height of 60 cm. As the wheat dried, temperature and relative humidity were measured in real-time at different heights within the drying bed. The microwave sensor was used to determine moisture content and density in real-time at the 30-cm height within the bed of wheat. The data collected at the 30-cm height was used to model the density at other locations by using heat and mass transfer principles. Results from the simulation compared well with the empirical data. The observed root mean squared error (RMSE) between the two was 0.0054 g/cm3. 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 pressure and airflow within the silo.

Technical 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, a simultaneous occurrence of heat and mass transfer fuel the removal of moisture from the product over time. As the removal of moisture persists, the product reduces in mass and volume. Such losses produce shrinkage, and thus, density changes and varies throughout the drying product for the duration of the drying process. Density is an important parameter because it is indicative of other parameters such as meat content, test weight, and crop yield. 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 “local” locations within a bed of grain or seed. It can be calculated for small quantities and estimated for large quantities via 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 the same measured empirically at the center of the volume of wheat resulted in a root mean squared 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.