AN ANALYSIS OF HEAT AND MASS TRANSFER IN A COLUMN TYPE GRAIN DRYER

Abstract

For the purpose of storage, food grains are required to be kept within a certain range of temperatures and moisture contents to avoid the growth of fungi. In this study, the effects of air temperature, initial moisture content, bed height, and gas velocity through a grain column during drying were investigated. Wheat was selected and conditioned to simulate high moisture contents normally found after harvest. Air was heated to simulate the ambient conditions found prevalent in tropical countries. A mathematical model has been developed to describe the simultaneous heat and mass transfer that take place in the column of a food grain to be dried. Isotherm equations have been incorporated in the mass transfer equation to reduce number of dependent variables. Both heat and mass transfer equations are solved using a numerical method. A parametric study has been made to identify the effect of some key parameters that affect the performance of the dryer. Effect of moisture dependent transport properties has also been discussed in the parametric study. Experiments were conducted under controlled condition to gain a better understanding of the moisture movement. It has been found from experimental results that the movement of drying front is a combined effect of sorption and desorption phenomenon, especially when the air is close to ambient temperature (30-35°C). To attain a local equilibrium with the surrounding air, grain in the warmer region loses moisture while grain in the cooler region gain moisture. This drying and wetting phenomenon is continued until the whole grain column attains a thermodynamic equilibrium. It was found that the temperature and the airflow controlled the rate of moisture removal, and vary with drying air temperature, air velocity, drying time and bed height. For the conditions considered, a decrease in air velocity of about 10% due to change in bed resistance to airflow was also observed during the whole drying process. A comparison of the experimental results with the theoretical predictions shows good agreement, except at low air temperatures, velocities and high bed depth. The discrepancies between analytical and predicted results could be attributed to condensation and re-evaporation in the upper layers. This phenomenon was not considered in the present mathematical method. However, it is concluded, that the model can be used satisfactorily in practical drying situation as the slower drying conditions would not be used commercially because of the likelihood of microbial spoilage.