Predicting drying in multiple-zone ovens
In the coating industry, the drying of solvent-coated polymeric films takes place in convected heated dryers, which usually consist of a series of zones. The operating conditions of airflow, solvent partial pressure(s), and temperature at the entrance of each zone are chosen to minimize the drying time while maintaining an acceptable product quality. In this work, the drying behavior of polymer solutions in such oven configurations is predicted from binary and multicomponent drying models. Both models involve coupled heat- and mass-transfer equations that describe the changes in the concentration of each solvent, the temperature, and the thickness of the film throughout the drying. The model equations become highly nonlinear because of the strong and complicated concentration and temperature dependencies of the thermodynamic and transport properties of polymer solutions. These nonlinear equations are solved numerically using the finite difference approximation. The solutions show that multiple-zone ovens can be used to eliminate bubble formation and to minimize the residual solvent content by controlling the operating conditions individually or simultaneously.