In 2019, the mill was looking to solve issues with its recausticizing plant that was causing variation in white liquor quality. In particular, the mill was looking to install an advisory platform for its recausticizing process to provide enhanced visibility for operators in the white liquor plant.
Matti Toivonen, Metsä Fibre’s Vice President of Technology says, “At the outset and start-up of the mill the decision was taken not to have any advanced tools to help the operators so that they learned how to run the process themselves. In the beginning, apart from a few basic controls, the white liquor plant was essentially run manually.
“However, we soon noticed that we were getting a lot of variation in burnt lime and white liquor quality, so it was decided to implement a system to help us solve these issues, as well as give us a good overview of what was happening across the white liquor plant. We turned to ANDRITZ as they delivered the plant and had deep knowledge of the processes, as well as working references using the Digital Twin solution.”
DIGITAL TWIN – OPTIMIZING THE WHITE LIQUOR PROCESS TO THE FULL
Metris Digital Twin is a spearhead online tool, powered by IDEAS technology, that provides comprehensive process performance insights for process improvement, operational visibility, and proactive decision-making. The Digital Twin technology can be applied across the mill and in all process islands.
The Digital Twin works in the white liquor plant by directing the automatic process control, a task usually carried out by a human operator. The technology has the ability to look around and decide on smart setpoints, optimizing the process to the full. The technology also has the ability to look ahead and decide on the next setpoints according to the data gathered from the process.
Jani Honga, Manager, ANDRITZ Automation explains, “With the Digital Twin implemented in the white liquor plant, we can predict the behavior of the process in real time by using a combination of virtual transmitters and soft sensors.
“The ultimate goal of the Digital Twin is to stabilize the process and maintain the optimal level of production. The most important factor here is monitoring and managing the causticizing efficiency (CE%), taking out the manual part of the action, and automating the process. Other advantages that the Digital Twin brings are providing visibility of the process, and advising on factors such as production levels and lime mud quality.”
The contract for implementing Digital Twin in the white liquor plant at Äänekoski was signed in March 2019 and work began on building the model offline. This was a first not only for the mill, but also for ANDRITZ. By August 2019 live communication with the Digital Twin application was set up and starting to advise the operators by providing predictions and process information. By March 2021 the mill had optimal closed loop control of the white liquor plant, allowing full visibility, monitoring, and maintenance of the process.
The ANDRITZ scope of supply to the mill included a Digital Twin of the recausticizing plant to optimize white liquor quality, along with virtual transmitters.
The outputs now being obtained from the Digital Twin include:
- Early (3 hours) predictions for lime, lime milk, and white liquor quality
- Under and over liming predictions for scheduling white liquor filter washing
- Optimum delta temperature and lime/GL ratio RSP for slaker Digital Twin control (DCS control logic)
- Calculator for production and inventory planning
- Virtual densitometer (GL, lime mud) and white liquor flowmeters for white liquor and lime mud