Desidratação osmótica e secagem convectiva de maçã cortada em forma de paralelepípedo descritas por solução numérica tridimensional da equação de difusão.

Abstract

In the present work, a study was conducted on the osmotic dehydration and convective drying of apple, sliced in parallelepiped shape, described by diffusion models, considering fruit shrinkage and variable mass diffusivity. The numerical solution of three -dimensional diffusion equation in Cartesian coordinates has been obtained by the method of finite volumes with fully implicit formulation and first kind boundary condition. The experiments on osmotic dehydration (sucrose solution) and convective drying were performed a t several operating conditions. A computer code was developed in FORTRAN, comprising an optimizer based on the inverse method, coupled with the numerical solution of the diffusion equation , with a graphical user interface. The program concerning the numerical solution provides the kinetics of the mass transfer of water or sucrose, when the process parameters are known. The process parameters for optimization are determined by using a set of experimental data, through minimizing an objective function, known as chi-square. Several validation tests have been performed on the program. These tests produced satisfactory and consistent results when compared to other experiments cited in the literature. For convective drying, the boundary condition of the third kind has been considered more appropriate. Results are presented on the kinetics of osmotic dehydration (water and sucrose) and convective drying. The process temperature and osmotic solution concentration had an impact on the two phenomena, but the temperature was most predominant. A study on mass distribution of water and sucrose during the osmotic dehydration and on water distribution during convective drying was accomplished. The results obtained through mathematical models , considering variable mass diffusivities and shrinkage proved to be a better match to the experimental data.