Staffan Haugwitz

Anti-windup in mid-ranging control

The implementation of anti-windup methods in mid-ranging control needs further attention. It is demonstrated how use of standard anti-windup schemes may give unnecessary performance degradation during saturation. The problem is illustrated for two separate systems, control of oxygen concentration in a bio-reactor and temperature control of a cooling system. In the paper, guidelines are derived for how to design the standard anti-windup scheme to recover performance. As an alternative a modified anti-windup scheme for mid-ranging control is presented that minimizes the performance degradation during saturation.

Analysis and design of startup control of a chemical plate reactor with uncertainties; a hybrid approach

The startup of a chemical plate reactor is investigated. A hybrid startup control scheme is presented and a discussion is made on how hybrid transition points may be chosen to allow safe startup despite actuator limitations, process nonlinearities and model uncertainties. The event-driven startup improves the robustness to uncertainties and disturbances, since each part of the startup is associated with a feedback in the form of a transition condition that needs to be satisfied to allow the startup to continue.

Challenges in start-up control of a heat exchange reactor with exothermic reactions; a hybrid approach

In this paper, the control of a continuous heat exchange reactor is investigated from a hybrid perspective with focus on the start-up phase and the transition to the optimal operating point. The temperature sensitive exothermic reaction leads to the possibility of multiple steady states and in combination with safety constraints forms an interesting challenge for a safe and efficient start-up. A series of MPC controllers are developed with a switching logic that transfers the process from initial rest to continuous optimal operation mode. The control procedure is verified in simulations with a full nonlinear model of the Open Plate Reactor, an improved heat exchange reactor being developed by Alfa Laval AB. The case study can be seen as a benchmark problem for start-up control of exothermic reactions (Less)

DYNAMIC OPTIMIZATION OF A PLATE REACTOR START-UP SUPPORTED BY MODELICA-BASED CODE GENERATION SOFTWARE

Abstract In this paper, start-up of a plate reactor is considered. Dynamic optimization is used to obtain start-up trajectories, and a feedback control structure for on-line control. The problem is challenging, since the process model is large and highly non-linear. In addition, the plant is subject to uncertainty. Special attention is given to the problem of formulating an optimal control problem based on physical insight. The robustness properties of the optimal solution are explored in simulation, by introducing parameter perturbations into the model. Automatic computer tools which greatly simplifies the task of formulating complex dynamic optimization problems are briefly discussed.

Process control of an Open Plate Reactor

A new chemical reactor, the Open Plate Reactor, is being developed by Alfa Laval AB. It combines good mixing with high heat transfer capacity. With the new concept, highly exothermic reactions can be produced using more concentrated reactants. In the paper, the reactor type is presented and a process control system is developed.A utility system to provide the reactor with cooling water is designed and experimentally verified. The utility temperature controller is based on a mid-ranging control structure to increase the operating range of the hydraulic equipment.A Model Predictive Controller is proposed to maximize the reaction yield under hard input and state constraints. An extended Kalman filter is implemented to estimate unmeasured concentrations, disturbances and unknown model parameters. Simulations show that the designed process control system gives high reaction yield and ensures that the temperature inside the reactor does not exceed a pre-defined safety limit. (Less)

Modeling and Optimization of a Grade Change for Multistage Polyethylene Reactors

Abstract Grade changes in polyethylene reactors, i.e., changes of operating conditions, are performed on a regular basis to adapt to market demands. In this paper, a dynamic optimization procedure is presented built upon the Modelica language extended with Optimica constructs for formulation of optimization problems. A Modelica library for the Borstar® multistage polyethylene reactors at Borealis AB, consisting of two slurry and one gas phase reactor, has been constructed. Using http://JModelica.org , a framework to translate dynamic optimization problems to NLP problems, optimal grade transitions between grades currently used at Borealis AB, can be calculated. Optimal inflows and grade key variables are shown.

Modelica Based Grade Change Optimization for a Polyethylene Reactor

This paper presents a dynamic optimization procedure of grade changes of polyethylene production. The optimization is built upon a novel modular Modelica library containing e.g., non-linear DAE models for polyethylene reactors based on models currently used in nonlinear MPC of industrial reactors at Borealis AB. Using Optimica, which extends the Modelica language with constructs for optimization problems, and JModelica.org, a novel framework to translate such optimization problems into NLP problems, grade transition ptimization problems can be solved. The solution procedure and a transition xample with optimal inputs and outputs are given in the paper showing promising results.

Calibration of a polyethylene plant for grade change optimisations

A polyethylene plant model coded in Modelica and based on a nonlinear MPC model currently used at Borealis AB is considered for calibration. A case study of model calibration at steady-state for four different operating points are analysed, both when looking at one operating point separately, but also to calibrate several simultaneously. Both model parameters and reactor inputs are calibrated for true plant measurement data. To solve the parameter estimation problem, the JModelica.org platform is used, offering tools to express and solve calibration problems. Calibration was obtained with narrow confidence intervals and shows a potential to improve the model accuracy by changing the parameter values. The results will be used for dynamic optimisations of grade changes. *