A comparative analysis of theoretical models and experimental research for spray drying

Abstract

Spraying is very often applied in the chemical and food-processing industries. It includes the spraying of a suspension - spraying with a content of about 30 % to 65 % of a dry substance. The drying time is in most cases only several seconds. In this work the convective drying principle in warm air is presented. A mathematical model of momentum, heat-and-mass transfer in the atomization-swirler zone was proposed. An 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 for different parameters of the atomization and initial particle size distribution. Changes in the air temperature inside the stream of sprayed material, material temperature, evaporation rate, and changes in the Sauter diameter and the distribution of particle diameter vs. their distance from the atomizer were determined experimentally. Good agreement between the experimental results and the 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. In order to consider the 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 the drying of a 55 % mass solution of starch and water. It was found that our model described the process of drying in the analyzed system. An extensive literature survey on spray drying, including unsteady-state phenomena, is presented in the paper. The main sources of errors occurring in the modeling 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.