Global nonlinear optimization and optimal control for applications in pulp and paper industry

Abstract

This work covers current problems of process simulation concerning energy efficiency in pulp and paper industry, discusses the numerical problems that arise from modeling with sate-of-the-art methods and embeds modeling, simulation, sensitivity analysis and optimization into a new framework with extended capabilities. Mathematical models for the drying process in a paper machine and for the dynamic process in the wet-end of a paper machine are developed and simulations as well as optimizations are carried out. A new tool for the complete parametric sensitivity analysis of stationary process models developed in gPROMS is presented and used with the drying process model. The results are used to derive an optimization problem whose results lead to a new conceptional layout of paper machine drying sections which is currently applied for patent by Voith Paper, Heidenheim. A new method of solving time-optimal control problems for grade change problems is derived and implemented for use with gPROMS and successfully applied to exemplary problems of optimal wet-end control and a refinement procedure for optimal control structures is tested. To confirm the results of the optimizations, a global optimization framework is developed. It is based on a generalization and an adaptive extension of the so-called tunneling algorithm for global minimization of smooth functions. The use of numerical methods for box-constrained optimization problems based on the solution of initial value problems is discussed with regard on the use with a tunneling-type algorithm. Numerical benchmarks with multi-dimensional non-convex test functions of the presented algorithm for global optimization are performed and compared by means of statistics to analyze the effect of the suggested extensions.