Moisture transport in shrinking gels during drying
Abstract
The goal of the current work is to develop a mechanistic and mathematical tool to predict quality changes in viscoelastic biopolymers during drying. Initially a mechanistic model of drying that accounts for shrinkage during drying is proposed. It is postulated that shrinkage equals the amount of moisture lost during drying of high moisture materials if the glass-transition temperature (T$\sb{g})$ of the material is below the temperature of drying. When T$\sb{g}$ of the material is above the temperature of drying, little shrinkage takes place and vapor-spaces form during drying. Moisture transport occurs predominantly in the liquid phase in the former case while it could occur in both the liquid and vapor phases in the latter case. Further since T$\sb{g}$ increases as drying proceeds, the moisture transport mechanisms may change from a liquid-dominated regime to a liquid and vapor-dominated regime in the former case. The proposed mechanistic model of drying is then incorporated into a novel thermomechanical theory of multiphase transport in deforming porous media to develop a moisture transport equation of drying. This equation is valid for saturated drying where shrinkage compensates for loss of water. Dimensional analysis of the transport equation reveals that the Deborah number determines if the shrinkage viscosity or the moisture diffusivity control the drying kinetics. A new dimensionless parameter, termed the Shell number (ratio of Biot and Deborah numbers), is proposed to describe the rate of surface drying. Numerical simulation of the drying model shows that at high Deborah numbers drying is non-Fickian and shell formation occurs causing a shut-off of further drying. At low Deborah numbers drying is Fickian and drying shut-off is absent. The proposed drying model is verified by comparing model predictions with data from drying experiments at different temperatures with starch-gluten gels of high moistures (greater than 150% (dry basis)). Material properties measured include moisture self-diffusion coefficient, bending modulus and isotherms at different temperatures and moistures. These properties were used as input parameters in the drying model. The model predictions agree reasonably well with experimental drying data, and the predicted moisture profiles agree qualitatively with published moisture profiles in literature.
Degree
Ph.D.
Advisors
Okos, Purdue University.
Subject Area
Chemical engineering
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