Advanced Process Control (APC) is regarded by many as a secret weapon in this because it can help make better use of automation systems. It empowers operators to apply their experience to improve quality and profitability rather than perform mundane tasks. It also serves as a link between low-level automation systems and higher-level management to help close the production loop efficiently,” said Abhay Anand, APC, Optimization & Data Analytics Specialist at ABB Pulp & Paper.
ABB states that one of the fundamental tools in an APC solution is model predictive control (MPC), a set of algorithms for feedback and feedforward control based on a process model. While around since the 1980s, it is still relatively new for the pulp and paper industry. The intricacy of the pulping process with its plethora of interacting parameters, varying dynamics and conflicting objectives has made it extremely difficult to model and control. Moreover, the process is easily disturbed by many factors such as the quality of raw material, age of the equipment and ambient weather. This has necessitated the evolution of MPC into a more robust and adaptable form before it could be adopted in the pulp and paper industry.
According to Anand, ABB’s APC solutions have been successfully implemented in pulp mill areas such as digesters, washers, bleaching plants, lime kilns, causticizers, and multi-effect evaporators, as well as paper machines. Having APC modules in an integrated pulp and paper mill can help reduce variability to ensure a constant stream of high-quality pulp. This makes the work of the paper machine controls easier and allows the mill to focus on cost reduction and efficiency improvements without having to worry about disturbances from the pulp quality.
ABB considers there to be three success factors when implementing APC:
- Having a solid foundation of accurate sensors, robust actuators, and a reliable distributed control system (DCS) – upon which the APC is delivered – in place. If the measurement system or DCS is not up to scratch, the APC will not be able to deliver the expected benefits.
- Ensuring that the APC is not simply a black box but can incorporate both operational data and process knowledge into its models to achieve the targets with minimal efforts.
- Transparency and ease of use is important so that the operators can relate to the decisions that the APC makes, visualize the predictions, and use the system intuitively for fast and efficient start-ups.
“Once APC is implemented, it’s equally important to continuously monitor performance and share key performance indicators to enable ongoing improvements. We believe that performance can be sustained, and in fact improved, over many years through close collaboration with a service-based model for long-term successes,” added Anand.
One such customer of ABB’s commissioned their first APC in their integrated mill 10 years ago. Thanks to continuous improvements since implementation, the pulp mill has recently been ranked top performer in their global Group in terms of continuous digester production load according to Anand. And that’s after the initial APC implementation helped the mill reduce Kappa variations by 56%. Process optimization is a never-ending journey; this case illustrates that a long-term commitment to adapt to mill changes and conditions will continue to bear fruit.
According to ABB, additional solutions can be integrated with APC to help further optimize the process automation. A Pulp Tracking feature, for example, provides a way to chart the movement of key pulp properties throughout the process, with the insights used to build models for various APC modules across the mill. Constraint Management is a concept patented by ABB to dynamically calculate high and low limits for the APC variables.
“The APC can be further supplemented with virtual measurements when physical measurements are infrequent or not available. Also known as soft sensors, these calculated measurements are particularly valuable in the pulp and paper industry where many processes are notoriously difficult to measure. They allow the operators to model and control parameters that cannot directly be measured and ensure that the APC can adapt to varying process conditions and maintain tight control, which otherwise would not be possible,” Anand said.
ABB believes that driven by the need for better control over variability and reduced production costs, many more developments will likely come in short order to help keep mills competitive. APC solutions today are already proving far more versatile with an extended functionality that can adapt not only to a target but to a range as well, allowing it to find the best operating regime. This ultimately leads to significant economic savings without compromising on quality.
Text Leif Lindberg