Exposed-layer barley drying: Three models fitted to new data up to 150°C☆

Abstract Apparatus described in a previous paper was used to obtain a set of exposed layer drying curves for naturally wet spring barley harvested at 0·410, 0·350, 0·269 and o·215 moisture content (m.c.) dry basis, and for 11 drying air temperatures from 50 to 150°C. Sample weight and surface temperature were recorded most frequently during the initial phase of rapid heating and high drying rates. Equilibrium m.c.'s were determined by continuing drying for 24 h in an oven. Three models were fitted to the data. The simple exponential Newton model described the effect of air temperature on drying rate, but did not predict well the shape of the drying curves. The Page model gave a much better prediction of the observed drying curve, but was still poor at describing initial drying behaviour. The third model was a numerical solution to Fick's law of diffusion for a sphere with moisture-dependent diffusivity. The model, which used a time-varying boundary condition, predicted the complex shape of the drying curve well. Of the three parameters of the model, two were correlated with grain temperature and one with the more usual parameter, air temperature. In simulations of deep-bed drying the simpler models are expected to be useful where economy of computation is paramount, but the diffusion model is more accurate and allows intra-kernel moisture movement to be modelled.