Avaliação da cinética de secagem de uma única partícula esférica: um estudo de modelagem e simulação utilizando coeficiente de difusão variável

Abstract

Spray dryers have a wide range of applications in the industry for producing powdered products. Precise control of the process is essential because atomization can result in particles with various sizes, shapes, and properties. Particle morphology and shrinkage can be influenced by several factors, such as temperature, solution concentration, and drying fluid characteristics. A dynamic mathematical model was proposed to study the drying of a particle subjected to convective drying. The particle is considered spherical and symmetrical throughout the drying process, neglecting rotation and internal circulation of water and solids. The model is divided into two drying stages and comprises a system of algebraic-differential equations. In the first stage, heating, evaporation, and shrinkage occur simultaneously. In the second stage, heating, core shrinkage, interface evaporation, and water vapor diffusion through the crust occur. The simulated results were compared to experimental data from the literature using the R² coefficient as a performance parameter. The first stage is strictly influenced by the characteristics of the drying air, while the second stage mainly depends on the formed porous crust. There is a rapid heating ((∆𝑡 ≅ 7𝑠) of the droplets to the wet-bulb temperature, at which point constant-rate evaporation occurs. The results suggested a modification in the constant diffusion coefficient, which was subsequently re-equated. The proposed model did not match the experimental drying data of colloidal silica even when using a variable diffusion coefficient. With a variable coefficient, at the end of drying, the simulation indicated that approximately 91% of the skim milk particle's mass corresponds to the dry crust. The proposed model, evaluated for skim milk drying, showed good agreement with experimental mass data (𝑅2 > 0.9953) and temperature data (𝑅2 = 0.9879). Sensitivity analysis assessed ±5% variations in six factors and their influence on four output variables. It was noted that the total energy absorbed by the particle was the most sensitive output variable to variations in all analyzed factors. The drying air temperature directly influences the final temperature of the particle but does not change its final diameter. The sensitivity analysis of particle size highlighted the importance of atomization control in a spray dryer.