Roman Peczalski

Vacuum contact drying of small spherical beads in a rotating drum with a hot wall was studied using the discrete element method (DEM). Both the mechanical and thermal interactions between particles were described. The drying rate was supposed to be entirely controlled by heat transfer, either between the particles and the hot wall or between adjacent particles. For modelling purposes, the drying process was divided in three steps: 1) initial heating up of the particles, 2) free liquid water vaporization within the particles at constant temperature, 3) bounded water desorption. The standard EDEM software routines were customized by the authors in order to take into account the coupling between the mass and heat balance of the particle and to handle its water content evolution. The test product was a bed of several hundred, millimetric size, spherical beads made of micro-crystalline cellulose, initially water saturated and frozen. The test dryer was a small scale horizontal rotary drum with a hot peripheral wall. Overall (averaged over all beads) and individual (one singular bead) water content and temperature profiles were simulated. The overall drying rate curve was derived from the average water content time profile. By extracting and plotting the time evolution of the minimal, maximal and average beads water content, the dispersion of beads’ water contents within the bed was globally assessed. For given operating conditions, these simulations can provide the processing time required to obtain a batch where every single particle has a water content below the required limit.

Mechanical flow and segregation phenomena within a bed composed of milli-metric size spherical beads rotated in a horizontal drum were investigated. The beads population was bi-dispersed, with two kinds of binary (half by half) compositions: a bi-size bed with two different sizes and a bi-density bed with two different densities. The distributions of the beads were observed optically on the front side of the bed by means of a lab-scale drum prototype. Different numbers and lengths of peripheral straight baffles were tested as well as different drum filling ratios. The photographical data were processed to obtain the front layer mechanical segregation index. This experimental index was compared to the simulated one, obtained by means of commercial discrete element software EDEM. The simulations were corroborated by the experiments provided that the friction coefficients of the discrete elements method (DEM) model were correctly adjusted. The global segregation index was also calculated from simulation data for all considered cases and its values were lower and less sensitive to baffles’ configurations than those for the front layer.

This electronic document is a “live” template and already defines the components of your paper Simulation and optimization of the drying process may require the calculation of the water diffusivity of the product. In this work we have described the estimation of the coefficients of diffusion of the moisture in the henna leaves from the drying curves obtained with DVS device. Two different methods can be used to evaluate this coefficient (analytical method and numerical method). In this application note, we discussed the method based on the analytical solution of a Fick diffusion model which allowed the determination of the diffusion coefficient by simple linear regression on the experimental data. The results obtained showed that the diffusivity is not strongly dependent on the water content of the product. On the other hand, it increases well with the increase in the temperature of the product.

The aim of this work was to simulate in two-dimensions the spatio-temporal evolution of the moisture content, the temperature and the mechanical stress within a highly deformable and water saturated product during convective drying. The material under study was an elongated potato sample with a square section placed in hot air flow. A comprehensive hydro-thermal model had been merged with a mechanical model, assuming a viscoelastic material, a plane deformation and an isotropic linear hydric-shrinkage of the sample. This model was validated on the basis of the average water content and core temperature curves for drying trials under different operating conditions. The material viscoelastic properties were measured by means of stress relaxation tests at different water contents. The viscoelastic behavior was described by a generalized Maxwell model whose parameters were correlated to water content. The simulations of the spatio-temporal distributions of mechanical stress were performed and interpreted in terms of product potential damage. The sample shape was also predicted all aver the drying process with reasonable accuracy.