Modelling the drying of spent grains in superheated steam

Spent grains are usually required to be dried to about 0.11 kg/kg db for prolonging the storage time and reducing the mass and transportation cost of the products. Superheated steam can be used for the direct drying of spent grains for saving energy compared with conventional rotary-drum drying. The drying dynamics of spent grains in superheated steam can be studied by simulating the drying process with mathematical models. The principal objective of this research was, therefore, to develop the mathematical models describing the drying of thin layers and deep beds of spent grains in superheated steam. To achieve this objective, three aspects of work were done in this research: (1) determination of the equilibrium moisture content (EMC) of spent grains in superheated steam, which is the important parameter for the modelling of drying, (2) development of the mathematical models for drying thin layers of spent grains in superheated steam, which also provide the base for modelling deep-bed drying, and (3) development of the mathematical model for drying fixed beds of spent grains in superheated steam and validation of the drying model.;Two equations with regression-determined coefficients were developed for describing the EMC of spent grains in superheated steam. The steam temperature had a significant effect on the EMC. Under atmospheric pressure, the EMC decreased substantially from 9.3 to 1.7 kg/100 kg dry solids for brewers' spent grain (BSG) and from 6.7 to 1.4 kg/100 kg dry solids for distillers' spent grain (DSG) with an increase in the steam temperature from 105 to 140°C. The EMC reduced slightly beyond 140°C and approached zero at 200°C.;A mathematical model was proposed for describing the drying of thin-layer spent grains in superheated steam. Not only the temperature but also the velocity of steam had a significant effect on the steam drying process. Increasing steam temperature from 110 to 180°C decreased the drying time by 80% for drying of BSG and by 85% when drying DSG at a steam velocity of 0.7 m/s. At 145°C, an increase in steam velocity from 0.3 to 1.1 m/s cut the drying time by half for both spent grains.;A partial-differential-equation (p.d.e.) model describing the steam drying of spent grains in a fixed bed was developed using the mass flow rate of steam as one of the important variables. The model was solved with a finite-difference method, which was accomplished by a computer program written in the FORTRAN language. A good agreement was obtained between the calculated and measured average moisture contents in BSG samples during the drying process.