Effect of product bed height and air velocity on the drying rate of extruded maize pellets

Abstract

In the food processing industry, specifically food drying and cooling, there is still a lack of available information that is based on experiments and reliable data. A specific product that has not been investigated is pure maize extruded pellets used for porridge made for human consumption. In order to use these pellets as porridge, it is finely milled. To ensure that the pellets are properly milled, the pellets must be dried and cooled to ensure that the mill has a good efficiency and that the product has an acceptable shelf life. Therefore, by investigating the drying and cooling kinetics of the product, an improved and more efficient process can be obtained. Two factors that have an influence on the drying kinetics, is the ambient air velocity through the product bed and the product bed height. Through the optimization of the performance of a counterflow bed dryer and cooler, energy costs and time can be saved. The purpose of this project is to acquire experimental data by investigating the effect that the product bed height and the air velocity through the bed has on the drying and cooling performance in order to ease the design process of a counterflow dryer/cooler with optimized performance. This exploration will include experiments on an experimental drying test bed. In these experiments, ambient air will be used at different air velocities and product bed heights. Performance parameters such as the total moisture loss, the drying rate, the moisture loss rate and the moisture loss per kilowatt of fan power (kW) will be evaluated in terms of the bed height and the air velocity. Conclusions can be then reached as to what bed height and air velocity deliver an optimum cooling/drying performance. This information will then be presented to ease the design process of the cooler/dryer. A mathematical model is also created to estimate the drying rate at certain specified parameters. Using the drying rate value can aid the designing process by estimating the ideal size of the cooler/dryer for a specified rate of product flow through the cooler/dryer. The model is validated by comparing it to the experimental results. Research has been done on the mechanical design of a counterflow dryer/cooler to see what factors are involved in drying and cooling. By evaluating the effect of these factors, the researcher concluded that increased air velocity in a counterflow dryer/cooler increases the drying rate; this is due to the mass transfer rate that is increased. However, the air velocity maximum in a continuous counterflow cooler must not exceed the minimum fluidization velocity, as the product will start to mix and will prevent even drying and cooling. The increase in product bed height also increases the drying rate that is caused by a decrease in cooling rate. A decrease in cooling rate results in a longer time for evaporation and mass transfer from the product, due to the difference in partial pressure between the water in the air and the water in the product. By evaluating the performance, the researcher concluded that the optimum parameters in which to operate the counterflow dryer/cooler, is at a bed depth of 0.4 m and at an air velocity of 1.8 m/s. The best drying rate is obtained at an air velocity of 2.2 m/s, but this velocity causes fluidization and will not fit the application of this dryer. Furthermore the information presented can thus be used to design a counterflow cooler/dryer with minimum inputs.