Miller Magazine Issue 108 / December 2018

80 MAKALE MILLER / DECEMBER 2018 of fumigation doses to sublethal levels before grain has been disinfected. A model to predict fumigant losses due to sorp- tion is implemented for the calculations which asserts that phosphine is absorbed into the grain and at the same time also degrades in air. Insect mortality It is known that the effect of phosphine on the mortality of grain insects is due to both the level of the phosphine concentration and the time of exposure. An insect mortality indicator function is also included in the model with a de- pendency on the species and strain of insect (here set for the Rhyzopertha Dominica species) Results The simulation model yielded, among other results, the de- velopment of phosphine concentration for the entire duration of the fumigation treatment (9 days). In Figure 5, the time evolution of phosphine at the 4 locations is presented. Speci- fically, sensor data are compared against model predictions. The best correlation occurs for A and B positions which are lo- cated on the silo side where the recirculation system was ins- talled. Their maximum concentration is reached at the end of the 4th day, followed by a decrease due to diffusion, losses, and sorption by the stored product. Concerning, locations C and D, sensor data reveal lower concentration values as the model also predicts. A small discrepancy is observed at the time that the maximum value is reached. According to sensor data, phosphine concentration has an upward trend until the end of the 7th day, whereas the CFD model underestimates to the end of the 5th day. Minor fluctuations, with hourly ti- mescales, occur due to natural convection currents which are the result of temperature differences imposed by the unstea- diness of ambient conditions. The currents create upward and downward air movements that transport phosphine along. The overall performance of the CFD model is considered satisfactory ensuring the validity of the phosphine concentra- tion predictions for the entire silo space as the ones presen- ted in Figure 6. Particularly, Figure 6 shows the spatial distri- bution of phosphine at four time instances. The advantages of using a recirculation system can be clearly seen since on the second day, phosphine has reached every position insi- de the silo. Until the 6th day, higher concentration values are observed on the top regions of the silo, near the alumi- num phosphide bags but as their degassification completes a more uniform phosphine distribution is reached (Figure 6, 8th day). A video showing the model predictions for the entire fumigation process could be found here: https://youtu.be/ iISBS7eoWb8 A useful augmentation of the phosphine concentration pro- files is the prediction of the insect extinction. Figure 7 shows the areas (red color) in which the Rhyzopertha Dominica spe- cies could not survive the fumigation process. As expected, areas near the Aluminum Phosphide bags and at the piping outlet are the first ones that reach lethal levels. According to the simulation, at the end of the 7th day, there are still some areas that insects could be still alive. A video showing the insect extinction predictions for the entire fumigation process could be found here: https://youtu.be/54uJ1ZJIkrk THE BENEFITS Since the precision fumigation method is widely applicable on any type of commodity, storage or phosphine formulati- on, users from all over the world could experience significant benefits such as the minimization of failed fumigations, cost reduction of the pest control application, traceability and de- fensibility against quality claims from retailers. Figure 5: Phosphine concentration [ppm] comparison of sensor data (solid li- nes) vs. simulation predictions (dashed lines) at 4 locations inside the storage. Figure 6: Phosphine concentration profiles at 4 time instances. A video of the simulation is also available in the following link: https://youtu.be/iISBS7eoWb8 Figure 7: Extinction of insects at 3 time instances. Red color indicates zones with 99.9% insect mortality. A video presenting insect mortality during the entire process can be found in the following link: https:// youtu.be/54uJ1ZJIkrk