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International audience; Two preliminary experiments show that a non-equilibrium situation can be easily encountered during a natural drying process. This leads us to reconsider the thermodynamic local equilibrium assumption, and propose a macroscopic two-equation model that takes into account mass exchange kinetics between the liquid and vapour phase. Numerical simulation of this theoretical model is then compared to experimental drying kinetics of soil columns. The discrepancies observed between the theoretical prediction and the experimental results are discussed. This contribution emphasizes the importance of such non-equilibrium phenomenon when modelling water transport in hygroscopic porous media.

This anick concerns the convective drying of natural rubber in granule form and aims at improving production. Parts of the research have already been published. With a validated mathematical model, il is possible to know the properties of the medium at any moment and for any point in the tayer. The research led to a simulation program for a zone-type industrial dnjir. The simulation program was used in an exhaustive study of the influence of the drying pifttfmeters for the design of a prototype industrial drier.

A model taking into account the simultaneous influence of mechanical and hygrometric actions (Mrani and al., 1995a, 1995b) was applied to study of the dehydration of a cylinder of highly deformable biphase gel (agar gel) (Mrani and al., 1995b). Numerical solution of the water transport equation and the mechanical equilibrium equation provided access to the water content and stresses and strains in the cylinder. The numerical results were validated for water content and overall deformation of gel cylinders. The model clearly shows the inversion of the curve of the faces of the cylinder observed experimentally. Three deformation phases were observed. In the first phase, when the concaveness of the faces of the cylinder was turned inwards, tractive stress appeared at the surface of the sample. More uniform distribution of the water content became established in a second phase and the cylinder recovered its initial shape corresponding to the relaxation of the stress state. In the third phase, the con...

An analysis of convective drying of natural rubber in granule form. Exploitation of the mathematical model developed from analysis of internal transfers requires determination of the ‘drying rate’ and the ‘energy exchange coefficient’ between the material and the drying air. This was performed using a portable laboratory drier at the production site, avoiding the problem of changes in the material during transport. Analysis of the drying kinetics of material in thin layers led to modelling the drying rate at the temperatures, rates and relative humidity values encountered in industrial driers. The energy exchange coefficient was found by measurement of air and granule temperatures. Thick layer trials showed the suitability of the model for simulation of changes in moisture content and temperature of air and granules in a granular environment. The study as a whole led to the development of a computer program for simulating a zone drier. This software can be used to optimise the management of an in...

The mathematical model proposed enables the description of mass and energy transfers during convection drying of granular materials. It shows evolution of the water content of the material in time and space, air and grain temperature, relative humidity of intergranular air, settling of the layer. mass energy and mass power input. Operation of the model requires experimenral determination of the water exchange coefficient between the material and drying air using thin layer drying kinetics. The software proposed enables the simulation of thick layers of granular material, as in an industrial drier. It is applied to the drying of natural rubber, taking into account the characteristics of the drying air and the flux direction in each drier zone. The aim of simulation is to optimise drying panmeters for improvement of the management and possibly the design or industrial driers.

ABSTRACT A numerical model of isothermal drying of deformable substances is proposed. The model is applied in a uni-directional case to analysis of water transport in a sphere of agar ge. The simulation program makes it possible to calculate the water content fields and def rmations and stresses in the sphere during drying. Validation of the water content profile results is proposed by the experimental study of the dehydration of gel spheres in a sugar solution. The sensitivity of internal stresses to drying conditions is then analysed.