Numerical understanding of two-phase flow and transport during spray drying using reaction engineering approach

Accurately reproducing and predicting transport behaviors inside spray towers still remain as a challenging issue. Most existing numerical results are validated only against outlet measurements. However, disagreements in internal multiphysics can prevent the results from accurately reflecting two-phase flow and transport within towers. In this work, a three-dimensional spray drying model was constructed using the Euler-Lagrange framework and validated in a pilot-scale centrifugal spray drying system with maltodextrin solution. The reaction engineering approach was adopted to describe the convection mass transfer via a user-defined function. Eighteen internal temperatures and outlet parameters were experimentally measured in validating numerical results. The results showed that the average relative error between the calculated temperatures and the experimental data was 1.76%. The relative errors between the predicted outlet air temperature, product water content, and average particle size and the experimental values were 0.06, 3.48, and 0.68%, respectively. The high-precision prediction of internal temperatures and outlet parameters demonstrated the model's reliability. The results also revealed a stronger evaporation around the tower centerline, where higher air velocity and temperature, as well as lower water vapor content, were observed. Droplet trajectories showed that large-size droplets required longer drying time, whereas small-size droplets dried rapidly but had longer residence time due to swirl and backflow. The effects of inlet air temperature, feed rate, disk rotational speed, and inlet air angle on droplet drying behaviors were further investigated. The research results are expected to provide industrialized guidance for designing a spray dryer and optimizing operating conditions.