Short-cut calculation for non-isothermal drying of shrinking and non-shrinking particles and of hollow spheres containing an expanding central gas core

\u3cp\u3eA procedure is presented for the calculation of non-isothermal drying rates of shrinking and non-shrinking particles of different geometries including rectangular bodies, cylinders, spheres and hollow cylinders and spheres. The implication on drying rate of changes in volume of the gas bubble in hollow spheres is taken into account. Starting point are the experimentally observed isothermal drying rates versus fraction water removed of a slab of the material of interest at two temperature levels. With the aid of a short cut calculation method based on the exact numerical computer solutions of the diffusion equation valid for concentration dependent diffusion coefficients and complex boundary conditions the information obtained from the isothermal slab drying experiments is transformed to other geometries and non-isothermal conditions.\u3c/p\u3

Estimation of effective diffusivity in drying of heterogeneous porous media

This work presents a new method to evaluate the effective diffusivity in porous solids, considering the mass-transfer mechanisms occurring in the pores and on the surface. The binary friction model was used to evaluate the effective gas-phase diffusivity and the Maxwell-Stefan surface diffusion equation to the transport of bound water. Different expressions for the effective diffusivity for porous solids were derived considering parallel diffusion, series diffusion, and the Clausius-Mossotti model. The numerical results were compared to experimental data obtained for Son clay. In the pore diffusivity, when both are present, capillary flow showed dominance over gas diffusion. For the entire moisture content range, only a combination of the models presented good agreement with experimental data, showing changes in the interdependence of mechanisms during the drying process

Determination of diffusion coefficients from experimental moisture concentration profiles in clay

In order to measure moisture-concn. profiles, a method based on neutron transmission was applied. Drying curves and diffusion coeffs. were calcd. from these profiles. The diffusion coeffs. appeared to be similar to the diffusion coeffs. detd. by comparing the numerical soln. of the diffusion equation with exptl. drying curves. The accuracy of both methods is about the same. Because the moisture concn. profiles are flat at high moisture concns., the accuracy of the calcd. diffusion coeffs. at high moisture concns. is not as high as is obtained at lower moisture concns. The neutron-transmission-based method is reproducible and has the advantage that it is not necessary to assume a certain relation between the diffusion coeff. and the moisture content. [on SciFinder (R)

On the risk of cracking in clay drying

Based on the assumptions related to porous media, the governing equations of mass transfer and static equilibrium are presented. The mechanical stresses generated by the drying strains are expressed according to the linear-elastic model. The von Mises cracking criterion is introduced in order to locate the area where risk for cracking occurs. The model is applied to the drying of Kaolin clay. Moisture and solid displacements results as well as evolutions of the criterion are exposed. The danger of cracking is the highest at the beginning of the drying, since the yield stress is low. The criterion reaches its peak value during the first hour and at a particular point, located on the surface, exactly in-between two corners. Moisture evolution has been measured by means of nuclear magnetic resonance (NMR) imaging, during the drying of a piece of Kaolin clay. The diffusion coefficient is evaluated from these experimental results. Finally, the model is used to reproduce them