A comparative study on the effect of superheated steam and hot air drying on microstructure of distillers' spent grain pellets using X-ray micro-computed tomography

The spatially correlated microstructure of a material during any processing operation such as drying is very important as it influences the thermo-physical properties of the material. The heterogeneity of the distillers' spent grain (DSG) pellets' microstructure was evaluated using the X-ray micro-computed tomography. The effect of two drying methods, i.e. hot air and superheated steam (SS) drying on the microstructure of this porous material was compared. The DSG pellets were prepared by mixing different amounts of distillers' solubles (10, 30, and 50% w/w) with the coarse grain fraction of the spent grains. The effect of solubles on the microstructure (total porosity, open porosity, closed porosity, and connectivity) and stability (dimensional/volumetric changes and change in density) of the pellet prior to and post drying (hot air and SS drying) was analyzed statistically. The results showed that SS drying caused an overall expansion of the pellet in the range of 90-133%. The increase in the open porosity of the pellet during SS drying enhanced the drying process resulting in a decrease in the drying time by about 81% when compared to hot air drying. Also, by increasing the concentration of solubles above 30% w/w, the dimensional stability of pellets improved by 50% for both drying methods thereby improving the mechanical strength of pellets. These results are crucial in optimizing the composition of pellets from the point of view of the pellet's dimensional stability and drying efficiency.