Study of the supercritical drying of wet okara

This study investigated the drying of okara (wet soybean pulp) using supercritical carbon dioxide at a low temperature of 40˚C. The okara used in this work was obtained directly from a soy bean milk processing factory in Singapore and the samples were dried using supercritical carbon dioxide at supercritical pressure of 103 bars at a temperature of 40˚C, with a CO2 flow-rate of 20ml/min. The drying time varied from 10 minutes to 60 minutes to investigate the drying kinetics and water extraction efficiency of supercritical carbon dioxide at the conditions used. The drying tests were carried for low sample mass loading (<500mg) and high sample mass loading (>1000mg) to determine the effect of sample loading on the drying. The results showed that the water uptake efficiency of supercritical CO2 was higher at high mass loading (36.9%) compared to a lower efficiency at low mass loading (18.6%). This suggested that a packed bed configuration where the extraction vessel was filled with wet okara would be favourable for the extraction. A pilot scale supercritical drying unit was proposed using the experimental results obtained in this study and calculations for pressure drop using Ergun’s equation showed that the pressure drop from a 4m tall extraction column would be approximately 7 bars. The low pressure drop was attributed to the liquid-like density (638 kg/m³) and gas-like low viscosity (0.049 mPa.s) of carbon dioxide at supercritical conditions. This study demonstrated the feasibility of drying high moisture particulates such as okara using supercritical carbon dioxide even at low temperatures.