Mathematical Modelling of Oxygen and Carbon Dioxide Composition in the Interstitial Atmosphere of Silo-Bags

Abstract

The CO2 and O2 concentration in hermetic storage systems depends on the balance between respiration of the ecosystem (consumption of O2 and generation of CO2), the entrance of external O2 to the system, and the loss of CO2 to the ambient air. A preliminary model was developed to simulate the gas exchange in silo-bags, considering the aerobic grains and microflora CO2 production and neglecting the contribution by insect respiration. Dependence of the rate of CO2 production on grain temperature, moisture content and storage time was taken into account and evaluated by use of the correlation developed by White et al. (1982) for wheat. The influence of initial grain temperature (25, 30 and 43ºC) and moisture content (12 -16 % w.b.) as well as degree of gas-tightness of the silo-bag was investigated. The model was run for the climatic conditions of the south east of Buenos Aires province, Argentina, for a storage period of 6 months, from January to July. The gas concentration predicted by the model under different storage conditions was compared with data available from a series of field experiments developed by INTA-EEA Balcarce. Globally, predicted values were in good accordance with the measured levels of gas concentrations. For all temperatures, at 12% w.b, the CO2 concentration remained below 4%. At 30ºC and 14% w.b., CO2 level increased to 12%. At 15% and 16% w.b, O2 was consumed after 80 and 40 days, respectively. CO2 reached 21% and then, because of the loss to ambient air (differential permeability for O2 and CO2), it fell below 16%. At 43ºC, O2 was consumed after 80 and 25 and 10 days, for 14, 15 and 16% w.b. respectively. Under these conditions, CO2 reached 21% and then fell below 15%.