Economic nonlinear model predictive control for mechanical pulping processes

In this paper we present an economic model predictive control (econ MPC) strategy for a two-stage (primary and secondary refining) Mechanical Pulping (MP) process. The MP process is a complex multi-input multi-output (MIMO) nonlinear process with strong interactions among the variables. In order to guarantee both stability and convergence of the closed-loop nonlinear MP process, two different econ MPC schemes are proposed: one with penalty on the increment of the input and one with penalty on the offset of the input from its steady-state target. We demonstrate that both econ MPC schemes achieve significant amount of energy reduction in terms of the specific energy consumed by the process while ensuring closed-loop stability and convergence to a nearby steady-state. In addition, we show that more energy reduction can be achieved by using the econ MPC with penalty on the input increment compared with the other scheme. Simulation results also demonstrate the potential benefits of using econ MPC over the standard MPC technique.

I. Introduction

The mechanical pulping (MP) process plays an important role in the pulp and paper industry. In recent years, this industry has faced a number of new challenges due to a.) the increasing electrical energy prices. About 1/3 the cost of mechanical pulp is electrical energy which is rapidly increasing in recent years. This increase is driving the industry into new process and controller designs that make it significantly low energy. b.) the changing of the demand markets and products. In recently years, the demands for newsprint and other printing and writing paper are rapidly declining at a rate of more than 10% a year, while the demand for packaging and absorbent products continues to increase globally. This results in MP mills moving into products that have dramatically different quality requirements than conventional products. With the aforementioned reasons, it is imperative for the MP mills to develop advanced process control techniques to reduce electrical energy consumption and adapt to the new market demands. In the MP process, high consistency wood chip refiners, which are extensively used to break wood chips into individual fibres, play a central role in producing high-quality pulp. Extensive research efforts have been made over the past two decades towards refiner optimization, cost reduction and product quality improvement in the MP plant [1]–[4]. However, the advanced control of today's mechanical pulping processes is still relatively basic [5].