Understanding the heat and mass transfer of hygroscopic porous materials.

摘要

Drying of hygroscopic porous material is a simultaneous heat and mass transfer process involving energy input to vaporize the moisture within the sample and vapor diffusion out of the material. This research has shown that temperature gradients existed throughout drying of bread samples under convective hot air conditions (50–90°C oven temperature and 1–3 m/s air velocity). To properly analyze drying behavior, an apparatus was constructed using microwave energy and convective hot air which obtained constant temperature conditions with respect to time and position within the bread sample (40–70°C sample temperature). Drying curves were analyzed to compare effective diffusivities calculated from convective hot air conditions (2.35–4.21 × 10 −5 cm2/s) isothermal conditions (7.6–18.3 × 10−5 cm2/s), and a significant difference was shown. However, Fick's diffusion equation was shown only to predict drying time and not drying rate.; Constant temperature was observed at the center of the bread samples during convective hot air drying. This constant temperature occurring within a sample is known as a pseudo-wet-bulb temperature and indicates phase change occurring. A first-order irreversible kinetic model was developed to predict moisture loss during isothermal drying by modeling evaporation during drying. The drying curves obtained from the isothermal drying experiments were analyzed to determine the first-order rate constant. The rate equation was shown to properly predict drying of bread samples when isothermal conditions exists throughout the entire moisture range, from 0.9 to 0.002 g/g dry solid.; Temperature and moisture profiles were measured for 1.8, 4.0, and 6.2 cm diameter bread samples dried in convective hot air. The center moisture profiles showed constant moisture content. The constant moisture content and the constant temperature at the center region indicate vapor re-condensation occurring. Effective moisture diffusivity is term which lumps together all of the internal moisture transfer, and thus the diffusion equation was analyzed as a predictive model during convective drying. The simulations show that the diffusion model was unable to predict moisture profiles during drying. The kinetic model was also analyzed as a predictive model during convective drying. However with re-condensation occurring within the bread samples during drying, the irreversible kinetic model grossly over-predicted moisture loss throughout the entire sample. These results show that a model should be developed which would model evaporation and re-condensation to properly predict moisture profiles during drying of hygroscopic porous materials.