Two-stage drying of wheat and barley
The results of a theoretical and experimental investigation into the drying of wheat and barley in two stages with an intervening rest period are presented. The reduction in drying time, excluding rest period, has been determined in comparison with the conventional continuous drying for various drying requirements. The effect of airflow rate and the temperature difference between grain and air on the reduction in moisture content and the time required to cool the grain during dryeration is also included. The moisture diffusion equation was solved numerically assuming a spherical grain. The variable grid spacing, Crank-Nicolson approximation technique and the Gauss-Seidel iterative procedure was employed. The theoretical predictions were compared with experimental results. The drying and resting was performed on a thin layer at a temperature of 60°C. An automatic micro-computer based system was developed to record and store the experimental data. The results indicate that the moisture redistribution during resting is well advanced after a period of two hours for wheat and one hour for barley. The extent of redistribution was measured by the increase in drying rate observed as the rest period was extended. An optimum moisture content for commencing resting is specified, which is a function of initial, final and equilibrium moisture contents. This optimum was chosen to minimise the actual drying time. There is good agreement between the theoreticaaand experimental predictions. It was found that the incorporation of a surface resistance into the diffusion model improves the description of the experimental results. The results enable a drying strategy to be specified that reduces the actual drying time by as much as 39%. - iv For dryeration experiments, the grains pre-heated to different temperatures were put into a well insulated aluminium cylinder and aerated at various airflow rates. An airflow rate of about 60-120 m3/hr/m3 of grain was found to be optimum. The moisture reduction during cooling was observed to be 0.65 to 0.78% (db) per 10°C temperature difference. It was noticed that moisture reduction also depends on initial moisture content of the grain. The practical implications of two-stage drying are discussed.