Spherical porous particle drying using BEM approach

Abstract

The paper covers a porous particle drying problem, which can be divided into two or three stages. The first stage is drying of surface moisture, the second stage is drying inside the particle, and the third, one which is relevant only to hygroscopic material, represents the change of particle moisture due to the change of environmental moisture. The second stage is the most relevant for the porous materials, and is, therefore, covered in more detail in this paper, with the main focus on the heat transfer inside the particle which affects the drying kinetics. The heat transfer problem inside the spherical particle has been solved using BEM, by transforming a 3D problem into a quasi 1D case by assuming uniform boundary conditions all around the particle, resulting in the solution depending only on the radial direction. The solution of the heat transfer needs to be calculated accurately as it directly affects the evaporation rate of the liquid on the interface between the dry crust and the wet core, which dictates the drying speed and affects the drying time. An in-depth analysis of space and time discretisation was performed on a typical spray drying example, where it is shown that a choice of a correct time step is cruicial for achieving good computational accuracy of the drying kinetics. The proposed numerical approach has also been tested on various drying conditions, with changing the particle size and the temperature of the drying gas, which have a largest effect on the drying kinetics. Finally, an analysis of the computed drying times is made as this is the most important parameter from the practical point of view, especially when designing the drying chambers.