Scandinavian Working Papers in Economics

Discussion Papers,
Norwegian School of Economics, Department of Business and Management Science

No 2007/9: Optimized on-line process control of bleaching operations with OptCab

Patrik Flisberg (), Stefan Nilsson () and Mikael Rönnqvist ()
Additional contact information
Patrik Flisberg: Department of Mathematics, Linköping University, Postal: Linköping University, Department of Mathematics, 581 83 Linköping, Sweden
Stefan Nilsson: Billerud Skärblacka AB, Sweden, Postal: Billerud Skärblacka AB, Box 703, SE-169 27 Solna, Sweden
Mikael Rönnqvist: Dept. of Finance and Management Science, Norwegian School of Economics and Business Administration, Postal: NHH , Department of Finance and Management Science, Helleveien 30, N-5045 Bergen, Norway

Abstract: To produce pulp for paper production or as market pulp is a complicated on-line process with many integrated stages that impact the final quality. In the bleaching plant which is at the end of pulp production, the main objective is to increase pulp brightness within specified limits. Here chemical treatments are applied in sequential stages to achieve the right brightness while striving to maintain the pulp strength as unaffected as possible. The raw material, i.e. pulp logs and wood chips from saw mills, differ in quality and properties. Due to this, it is important to continuously update the amount of chemicals added to the pulp in real-time. This is typically done by experienced operators. In this paper, we describe an on-line optimization based decision support system called OptCab that controls the bleaching process at Billerud AB's paper mill in Skärblacka. The solution approach is based on two phases. In phase one, we establish approximations of each of the processes based on process data collected on-line. These approximations are found by solving a set of constrained least square problems and are updated every 15 minutes. In phase two, we formulate an overall nonlinear control problem that links all stages together and aims to minimize the cost of chemicals. This is solved on-line every five minutes. The system has been in operation during the last three years providing a 10% reduction in the use of chemicals. Additional benefits include a more stable brightness quality.

Keywords: Pulp production; on-line optimization based decision support system; nonlinear control problem

JEL-codes: C61

19 pages, March 27, 2007

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