Convective-diffusive transport of carbon dioxide through stored-grain bulks

A finite element model was developed to predict the convective-diffusive transport of introduced carbon dioxide (CO2) through stored-grain bulks. The CO2 concentrations predicted by the developed convective-diffusive model were compared with measured CO2 concentrations from three wheat-filled 1.42-m-diameter x 1.37-m-tall bins. The bins were equipped with three different partially perforated floor openings (circular near the center, rectangular, and circular near the wall) to simulate true three-dimensional movement of CO2. Dry ice was used to create high CO2 concentrations in the wheat bulks. In the model, effective diffusivities in the longitudinal and lateral directions were used in place of a diffusion coefficient during the dry ice sublimation period and a diffusion coefficient was used thereafter. The effective diffusivities incorporated the effects of both the bulk movement caused by the dry ice sublimation and the diffusion caused by the concentration gradient. The predicted concentrations were close to the measured concentrations at 12 h and thereafter. In the initial sampling times, however, the errors were high (mean relative percent errors ranged from 30 to 60% in all the experiments). The need for additional data on the flow characteristics of CO2 through wheat bulks, CO2 sorption by wheat at low initial concentrations, and the effect of including the gravity term in the governing differential equation for accurate model predictions is discussed.