Apparent diffusion coefficients of carbon dioxide through grain bulks

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Ganapathy, Shunmugam
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The diffusion coefficient of carbon dioxide (CO2) through grain bulks was determined with the following variables: (1) grain bulk (wheat, barley, and canola); (2) moisture content (a dry, a wet, and a damp condition for each grain bulk); (3) temperature (5, 15, 25, and 40C); (4) direction of flow of gas (upward, horizontal, and downward); (5) porosity (two levels for each grain bulk); (6) grain kernel orientation (vertical and horizontal); (7) initial concentration in the gas-chamber (20, 40, and 60%); and (8) dockage (0, 4, 8, and 12%). Experiments on transient diffusion of CO2 through grain bulks were conducted using a diffusion cell placed inside a temperature- and humidity-controlled chamber. The CO2 concentrations along the grain column were measured at 0.5 h, 1h, and every hour until 6 h from the start of diffusion. The diffusion process was modeled as one-dimensional transient diffusion solved using Crank-Nicholson's implicit finite difference method. The CO2 concentrations along the grain column predicted by the model were compared with the measured values for different diffusion coefficients. The diffusion coefficient which gave the least mean relative error between the predicted and measured CO2 concentrations was considered the diffusion coefficient for the given conditions. A sink term 'q' was introduced in the diffusion equation to account for the sorption of CO2 in the grain bulk. For canola and barley, experiments were conducted at 15 and 25C and at three moisture levels (dry, damp, and wet conditions) to determine the sorption of CO 2. Sorption values reported in the literature were used for wheat. The diffusion coefficients of CO2 through wheat bulks ranged from 5.9 * 10-6 to 7.6 * 10-6 m2s-1; through barley bulks ranged from 5.1 * 10-6 to 8.4 * 10-6 m2s-1; and through canola, bulks ranged from 3.7 * 10-6 to 5.3 * 10 -6 m2s-1 for the test conditions studied. Increasing the moisture content from the dry to the damp condition decreased diffusion coefficients for all three grain bulks. A further increase in moisture content did not affect the diffusion coefficient in wheat and canola whereas in barley a decrease in the diffusion coefficient was observed as the moisture content was increased from the damp to the wet condition. An increase in temperature generally increased the diffusion coefficient of CO2 in all three grain bulks. Diffusion in the downward direction resulted in higher diffusion coefficients in all the grain bulks. No significant difference in diffusion coefficients was observed between upward and horizontal directions of flow. An increase in porosity resulted in a higher diffusion coefficient in all the grain bulks tested. The diffusion coefficient of CO 2 was higher for vertical grain kernel orientation than horizontal grain kernel orientation for all the three grain bulks for upward gas flow, although the difference between the two kernel orientations for canola bulks was not significant because the seeds are round. No significant change in diffusion coefficients through grain bulks was detected as the initial CO2 concentration was varied between 20 and 60%. The diffusion coefficient of CO2 increased linearly as the foreign material content was increased from 0 to 12% in all the grain bulks.