Comparison of Drying Rate and Temperature Convergence in Grains and Seed with an Eighth-scale Grain Drying System

Authors: Lewis, Micah; Trabelsi, Samir

Submitted to: ASABE Annual International Meeting
Publication Type: Proceedings
Publication Acceptance Date: 3/20/2020
Publication Date: 7/15/2020
Citation: Lewis, M.A., Trabelsi, S. 2020. Comparison of Drying Rate and Temperature Convergence in Grains and Seed with an Eighth-scale Grain Drying System. ASABE Annual International Meeting. ASABE Paper No. 2001378.

Interpretive Summary: Annually, several thousand tons of cereal grain (including corn and wheat) and oilseed (such as soybean) are stored in large cylindrical grain bins all over the world. While in storage, these commodities must be dried and maintained at suitable moisture content, usually from 11 to 13 %, to provide safe storage and prevent spoilage. Drying these commodities can be difficult since they can be stored with a bed depth greater than 15.2 m. They are dried from the bottom up by aeration, a process in which heated or non-heated air is forced up through the material. It is important to know moisture content and other drying parameters such as temperature and relative humidity at different heights throughout the bin. Lack of this information often leads to overdrying of the bottom layers and underdrying of the upper layers. Existing techniques use relative humidity to predict the moisture content within grain bins. However, research has shown that such approaches can be ambiguous in settings with high moisture and high temperature, normal conditions during drying. Microwave moisture sensing has proven to be a more accurate alternative to obtain real-time moisture content with low error. A microwave moisture sensor, developed within USDA ARS, was used in the development of an eighth-scale grain drying system. The system was also equipped with 8 temperature sensors and 4 relative humidity sensors at different heights within the grain bin. Using custom software, the system can facilitate and monitor the drying process of goods such as cereal grains, oilseeds, and nuts within an eighth-scale drying bin without user interaction. The microwave moisture sensor was calibrated using soybean and wheat samples ranging from 8 to 19 % moisture content and from 0 to 50 °C. The 57-cm diameter bin was filled to a height of 60 cm with approximately 110 kg of soybean and wheat in separate trials. The soybean and wheat were dried until the equilibrium moisture content was met, a level dependent upon the temperature and relative humidity of the surrounding air. As the goods dried, moisture content, temperature, and relative humidity at different heights within the grain bin were measured and recorded every 12 seconds. The resulting data showed the difference in drying rate between the soybean and wheat although conditions were similar. The wheat dried 20 hours slower than the soybean despite having similar initial temperature and moisture content. It also took the bed of wheat 10 more hours than the bed of soybean to converge to the temperature of the air being blown in for drying. This research showed that real-time monitoring of moisture content is achievable in cereal grains and oilseed using microwave sensing. Data from different locations throughout the drying bin allow the user to see how moisture moves through and out of the product. The data also allow the user to observe differences in drying behavior among commodities, resulting in more commodity-specific approaches. This continued research is useful to improve efficiency in drying in grain bins and ensure that moisture content is ideal throughout the bed.

Technical Abstract: Several thousand tons of cereal grain and oilseed are stored annually in large cylindrical grain bins after harvest. It is important that such commodities be dried and maintained at suitable moisture content to provide safe storage and avoid product degradation. While in the grain bin, they are dried from the bottom up by aeration, a process in which heated or ambient air is forced up through the material. As the bed of material dries from the bottom, the upper layers initially gain moisture before they begin to dry. This phenomenon occurs as the drying front traverses the bed from bottom to top. The movement of heat and moisture through the drying bed are influenced by many factors, including the thermal properties of the grain or seed. Therefore, a method to accurately monitor moisture content in real-time at different levels is needed. Existing techniques such as moisture cables are not as accurate in situations of high humidity and high moisture content. They also only monitor within a small area around the sensor. By using a microwave moisture sensor operating at 5.8 GHz, developed within USDA ARS, the moisture content of grain or oilseed can be measured continuously, providing real-time moisture content during drying. The microwave moisture sensor was implemented within an eighth-scale grain drying system equipped with eight temperature sensors and four relative humidity sensors to observe drying parameters and moisture migration throughout as grain or oilseed dried. The drying rate for soybean and wheat dried under the same conditions was observed. The time it took the bed to converge to the temperature of the drying air was also observed and compared among the commodities.