Investigating the influence of grain drying with ambient air versus heated air within an eighth-scale grain drying bin

Authors: Lewis, Micah; Trabelsi, Samir

Submitted to: Applied Engineering in Agriculture
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/12/2022
Publication Date: 6/1/2022
Citation: Lewis, M.A., Trabelsi, S. 2022. Investigating the influence of grain drying with ambient air versus heated air within an eighth-scale grain drying bin. Applied Engineering in Agriculture. 38(3):523-533. https://doi.org/10.13031/aea.14890.
DOI: https://doi.org/10.13031/aea.14890

Interpretive Summary: Cereal grain and oilseed products such as wheat and soybean are stored in large bins after harvest where they are dried from the bottom up. Drying is essential to lower such products to a safe level of moisture content to ensure safe storage. The temperature of the drying air is important and has a direct impact on the rate at which the product dries. Using heated air offers a risk of overdrying the bottom layers while trying to completely dry the upper layers. Using ambient air removes the risk of overdrying the bottom layers; however, it creates a risk of underdrying the upper layers, making them susceptible to mold and spoilage. An eighth-scale grain drying system, developed within USDA ARS, was equipped with temperature, moisture, and relative humidity sensors at different positions to observe change in temperature and moisture as the soybean and wheat dried. For both products, drying trials were run using heated and ambient air. The drying process was also simulated by using differential equations for the same conditions. Drying parameters such as absolute humidity, air temperature, grain temperature, and moisture content were modeled with respect to location and time. Simulation results agreed well with the laboratory trials using the eighth-scale drying system. A 30% and 72% increase in drying time were observed when ambient instead of heated air was used to dry wheat and soybeans, respectively. Depending on climate, these increases in drying time could lead to spoilage.

Technical Abstract: The choice of whether to use heated or ambient air during grain and oilseed drying is often complex, and it has a great effect on the overall quality of the product. A significant problem when drying with heated air is overdrying the bottom layer in attempting to dry the top layer. This is possible because grains or oilseeds within a storage bin can be more than 15.2 m deep. However, while the bottom layer is less likely to be overdried when using ambient air, there is a risk that the upper layers could be subject to mold and degradation if moisture is not removed quickly enough. An eighth-scale grain drying bin was equipped with temperature, moisture, and relative humidity sensors at different heights to observe temperature and moisture migration throughout the 60-cm deep bed as corn and wheat were dried. For both commodities, trials were run where they were dried with heated air and ambient air. Likewise, thin-layer drying models were applied to the deep bed, and the drying process was simulated using the finite difference method for both commodities using ambient and heated air. The absolute humidity, air temperature, grain temperature, and moisture content were modeled throughout the drying bed with respect to location and time. Data obtained from the simulations compared well with the experimental data. When comparing the moisture content determined from simulation with the same measured with the microwave moisture sensor within the drying system, root mean square error (RMSE) values for corn and wheat were 0.61 (heated air), and 0.45% and 0.50% (ambient air) moisture content wet basis (w.b.), respectively. A 30% increase in drying time was observed when ambient air was used to dry wheat. A 2-% increase in ending moisture content was observed when drying with ambient air for both commodities. The empirical results obtained from the eighth-scale drying system confirmed the results obtained from simulation for each trial. While results obtained may be unique to the current study, the effectiveness of the models was demonstrated.