Evaluation of the superposition method for predicting gas leakage rates during fumigations in empty model silos

•Prediction of fumigation half-loss times by superposition method was studied.•Fumigant concentration curves followed first-order kinetic characteristics.•Concentration decay rates can be satisfactorily predicted by superposition method.

In precision fumigation practices, half-loss time (HLT) is used for forecasting the success of a fumigation job. The objective of this study was to explore the feasibility of using the superposition method for predicting HLTs. The superposition equation describes the total gas leakage rate from a structure in terms of prevailing wind speeds, temperature differences between inside and outside of the structure, and the effective leakage area (ELA) of the structure. Two fumigation experiments were conducted in 228.5 and 2160 l cylindrical polypropylene model silos using carbon dioxide as a tracer gas. The pressure-flow rate pressurization test was performed before each fumigation in order to determine the ELA of the silo. The resulting fumigant concentration curves followed first-order kinetic characteristics. The purpose of the first experiment was to determine the wind and stack coefficients of the silos. In order to evaluate the accuracy of the superposition method, in the second experiment three replicates of fumigation trials were conducted for each model silo at combinations of three constant wind speeds and four constant temperature differences. The recorded concentration decay curve of each fumigation trial was fitted with the first-order kinetic equation to determine the actual HLT. The actual HLTs of the small and large silos ranged from 3.21 ± 0.47 to 6.71 ± 0.62 h and from 4.49 ± 0.67 to 18.74 ± 0.24 h, respectively. Then, these actual HLTs were compared with the HLTs predicted by the superposition equation. The previously determined coefficients were incorporated in the prediction of HLTs. The percentage prediction errors ranged from 1.65 ± 0.50 to 15.70 ± 1.10% for the small silo and from 1.76 ± 1.21 to 10.43 ± 1.36% for the larger one. The actual and predicted HLTs were linearly correlated with slopes close to unity (R2 > 0.894). The results of this research illustrated that fumigant concentration decay rates can be satisfactorily predicted by the superposition method.