Drying of Coffee (Coffea arabica L.) in a Fixed Bed with a Simulated Periodic Airflow Reversal

Abstract

Reversing the airflow at regular intervals throughout the drying period of coffee in a fixed bed has been proposed as a feasible method of reducing the final moisture content difference throughout the bed depth. The drying curves of six individual sections and the time-temperature history of the drying air at seven evenly spaced points in a total bed depth of 0·90 m were investigated. Tests were conducted using two levels of reversing-time interval, 3 h and 6 h, two levels of drying air temperature, 50°C and 70°C, and an airflow rate of 15 m3 min-1 m-2. Comparative tests were performed using the conventional one-direction airflow drying method. Reductions in final moisture content difference ranging from 65 to 73% were observed by reversing the airflow direction every 3 or 6 h for drying air temperature of 50°C as compared to the conventional drying method. However, for drying air temperature of 70°C and 3 h reversing-time interval, despite reductions of up to 67% in the final moisture content difference, non-uniformity of the moisture distribution was still quite severe. The reversed direction airflow drying method was used to reduce moisture content from 0·33, decimal d.b. to 0·16 d.b., by increasing the inlet drying air temperature from 50 to 70°C. It was found that the final moisture content difference was reduced on average from 0·044 to 0·013 d.b. versus 0·078 to 0·026 d.b., respectively, when compared to the conventional one-direction airflow method.