A set of partial differential equations describing the steam drying of a fixed bed of brewers’ spent grain was developed based on energy and mass balances in a similar manner to the derivation of equations for air drying models. Instead of air humidity as an independent quantity in air drying models, the mass flow rate of steam was used as one of the independent variables in the steam drying model. The simplified model was solved using a finite-difference method, which was accomplished by the computer program written in FORTRAN language. The simulation program was developed based on the finite-difference equations formed from the model, the known initial and boundary conditions, and the division of three possible zones in the fixed bed. The validation of the drying model was made by comparing the predicated results with the experimental data. There existed a good agreement between the calculated and measured average moisture contents in the spent grain samples during the drying process.

Determination of equilibrium moisture content (EMC)for brewers' spent grain (BSG) and distillers' spent grain (DSG) in superheated steam involved drying samples of the spent grains for a sufficient time in superheated steam, completely drying the steam-dried samples in a vacuum oven dryer and finally calculating the EMC of the samples. Under atmospheric pressure, the EMC decreased substantially, from 9.3 to 1.7 kg/100 kg dry, solids for BSG and from 6.7 to 1.4 kg/100 kg dry solids for DSG, with an increase in the steam temperature from 105 degreesC to 140 degreesC. On average, the EMC changed by, only 1.0 kg/100 kg dry solids beyond the 140 degreesC steam temperature and was close to zero at temperatures above 180 degreesC The sorption relationships for the tested spent grains in superheated steam were well represented by isobars by, plotting the EMC against the steam temperature or the relative pressure of steam. Two types of equations with regression-determined coefficients were proposed for describing the isobars.

Tang, Z. and Cenkowski, S. 2000. Dehydration dynamics of potatoes in superheated steam and hot air. Can. Agric. Eng. 42:043-049. Superheated-steam at atmospheric pressure is an alternative drying medium for dehydrating materials insensitive to temperature equal to or above 100°C. This research compared the dehydration characteristics, temperature histories, drying rates, and overall moisture diffusivities of cylindrical potato samples exposed to superheated steam and hot air at 125, 145, and 165°C. A small amount of moisture (0.18 to 0.47 kg/kg db, dry basis) dependent on the steam temperature was gained from steam condensation on the sample surface during the warm-up period from the superheated-steam. The temperature of the drying medium had a greater effect on the drying rate, overall moisture diffusivity, and consequently dehydration time for the superheatedsteam dehydration than for the hot-air dehydration. Increasing the temperature from 125 to 165°C decreased the dehydration time by 60 and 24% for the superheated-steam and hot-air dehydration, respectively. A constant-rate drying period was only observed with superheated steam at 125 and 145°C. There existed an inversion temperature point between 145 to 165°C for the first dehydration stage

The drying characteristics of sugar-beet pulp in superheated steam and hot air at three temperatures (130, 157, and 183 degrees C) and three velocities (0.24, 0.32, and 0.37 m/s) of drying media (air or steam) under atmospheric pressure were studied. The drying rate was higher and the drying time as shorter in superheated steam than in hot air of rite same temperature. The drying rate increased and the drying time decreased with increased temperature for both superheated-steam drying and hot-air drying; however; the effect of temperature on superheated-steam drying was greater than on hot-air drying. At 157 degrees C, increasing the velocity of the drying medium did not affect the drying characteristics of the samples dehydrated with hot air bur increased the drying rate at moisture contents greater than 0.5 kg/kg dry basis (d.b.) for superheated-steam drying. Sugar-beet pulp dehydrated with either superheated steam or hot air under the same drying conditions (157 degrees C and 0.32 m/s) had the same water activities.