Development and experimental verification of momentum, heat and mass transfer model in spray drying

Abstract

A mathematical model of momentum, heat and mass transfer in the atomization zone was proposed. Uneven distribution of particles and entrainment effects were taken into account in the model. To verify the model an extensive experimental investigation was performed on water evaporation at different initial air temperatures, feed rates, flow rates of the drying agent and different parameters of atomization: spray cone angle and initial particle size distribution. Changes in air temperature inside the stream of sprayed material, material temperature, evaporation rate, and changes in Sauter diameter and distribution of particle diameters vs. their distance from the atomizer were determined experimentally. Damping screens allowed a flat profile of air flow rate in the tunnel to be obtained. Good agreement between experimental results and theoretical data was achieved. In the paper an attempt was made to apply the model to the calculation of spray drying in a pilot plant dryer. The air supplied to the dryer tangentially to the axis was characterized by a high swirl. The model of air flow in the dryer was determined theoretically using the Computational Fluid Dynamics CFX program. Results obtained in this way were used in our own model to determine particle trajectories in the dryer and to solve heat and mass balances for the continuous and dispersed phases. In order to consider swirl air flow pattern in the dryer, changes in heat and mass balance were made for the particle—drying agent system. The model was verified experimentally on the basis of results of investigations on drying of a 20% solution of sodium chloride. Additionally, our own results were compared with those obtained from the particle source in the cell type model for the drying system being studied. It was found that our model well described the process of drying in the analyzed system. Results obtained from our model were also very similar to the results obtained from the particle source in the cell type model. An extensive literature survey on spray drying, including unsteady state phenomena, is presented in the paper. Main sources of errors occurring in modelling of the spray drying process were discussed. A critical estimation of the existing mathematical models of the process was made and some of them were described in detail.