Dynamic optimization of multiple-zone air impingement drying processes

This article introduces a mathematical programming model for the rigorous optimization of multiple-zone air impingement dryers for drying thin liquid films on continuous substrates. The formulation is based on a modular representation of the drying process and includes a non-linear Partial Differential Equation (PDE) governing the mass transport in the film. A simultaneous approach is selected to solve the PDE dynamic optimization problem. By the full discretization of all variables, a Non-Linear Programming (NLP) model is obtained and the system of differential equations is then solved simultaneously with the optimization, resulting in a large-scale optimization problem. This approach allows an easy implementation of the process operation limits as inequality constraints, e.g. constraints to avoid bubble formation in the film, risk of solvent vapor explosion in the exhaust air. Various drying scenarios are investigated by considering different objective functions in the formulation, e.g. minimization of heat consumption, maximization of the production rate. Numerical computations illustrate the proposed optimization method for different drying scenarios for which optimal operation conditions with respect to the objective function and constraints are determined. Trends for the optimal operation of air impingement drying processes are extracted from the optimization results obtained for the drying scenarios.