Since its introduction 20 years ago Metso Automation’s fibre line and recovery line optimisation controls have been installed in more than 120 pulp mill processes around the world.
These supervisory controls, which are mainly based on adaptive and predictive process running models, achieve a high level of stability in the unit processes in the fibre and recovery lines. In addition, the operation of the entire fibre or recovery line has been stabilised and optimised. Control actions are, in many cases, based on Metso Automation’s Kajaani online process analysers which measure the process condition, chemical reaction rates and product quality. Optimisation controls have produced a good return on investment by significantly increasing process productivity and efficiency, stabilising product quality, increasing energy generation and reducing energy and chemical consumption. Emissions levels have been lowered as well.
Batch and continuous pulping processes are stabilised during steady state operation and, most importantly, during process upsets and grade transition periods. Changes in production rate, pulp grade and species changes are managed smoothly with minimum disruption to quality and productivity. The controls determine the best course of action required for process stability.
By stabilising the cooking processes Kappa number variation has been significantly reduced, pulp yield has been increased, and white liquor consumption and wood consumption have been reduced. Production increases in the cooking and washing processes have been made possible by higher yields and fewer disturbances. Steam energy savings are also a major part of the return on investment. Pulp washing efficiency has been improved and bleach pulp quality has been enhanced by these model-based controls. Significant bleaching chemical savings have been documented.
Thermal efficiencies have been improved by implementing optimising controls in kraft recovery lines. In the back liquor combustion process higher reduction degrees have been achieved and extra steam has been generated. By optimising the soot blowing process, the use of boiler tube cleaning steam has been significantly reduced. Improved control over heat transfer in the evaporation process has resulted in reduced steam consumption as well as more stable solids content. Stabilised burning conditions in the lime kiln have led to lower specific energy consumption and have lowered the use of externally purchased fuel. Causticising plant throughput, efficiencies and liquor strength stability are improved by the lime quality controls and model-based controls of the causticising process.
There are further energy savings available by optimisation of the entire recovery line process by managing the energy balance and the storage capacity. These whole-line controls stabilise the operation and, by taking advantage of the enhanced stability of operation, optimise energy consumption. This optimisation control adapts the chemical circulation to the production rates of the digester and washing lines according to liquor capacities and energy consumption. The recovery line management application employs simulated operating models of the chemical recirculation system. This coordinated control makes it possible to change production rates with minimal disturbances and improved controllability.
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