Multistage Impinging Stream Drying for Okara

In this study, a multistage drying concept (one-stage and two-stage drying) was applied in an attempt to enhance the performance and energy efficiency of an impinging stream dryer for okara, which is a soy residue obtained from soymilk production. The volumetric water evaporation rate, the volumetric heat transfer coefficient and the specific energy consumption of the drying system at various drying conditions were evaluated. For one-stage drying, the volumetric water evaporation rate increased with an increase in the okara feed rate and the drying temperature. The effect of the drying temperature was insignificant at the minimum okara feed rate, however. For two-stage drying, the drying temperature insignificantly affected the volumetric water evaporation rate. However, an increase in the okara feed rate led to an increase in the volumetric water evaporation rate. The effects of the drying temperature and the okara feed rate on the volumetric heat transfer coefficient were similar to those of the volumetric water evaporation rate, for both one-stage and two-stage drying. The maximum volumetric water evaporation rate and the maximum volumetric heat transfer coefficient of 1784 kgwater m-3 h-1 and 3385 W m-3 K-1, respectively, were noted during one-stage drying at the drying temperature of 150°C and the okara feed rate of 45 kgdrysolid h-1. Regarding the energy efficiency of the drying process expressed in terms of the specific energy consumption (SEC), an increase in the drying temperature and the okara feed rate led to lower values of the SEC for one-stage drying. The effect of the okara feed rate on the SEC for two-stage drying was similar to that for one-stage drying. However, the opposite trend was observed for the effect of the drying temperature. When a multistage drying concept was applied to the impinging stream dryer, the percentage of moisture content reduction of the okara were substantially decreased. The results obtained from this study implied that an impinging stream dryer with a multistage drying concept may be used to dry okara or other high-moisture particulate materials. Keywords: Impinging stream dryer, Multistage drying, Okara, Specific energy consumption, Volumetric heat transfer coefficient, Volumetric water evaporation rate.