How Advanced Process Control moves pulp and paper mills closer towards autonomous operations

Process Optimization
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Pulp and paper producers are under constant pressure to reduce production and maintenance costs, maximize uptime and maintain high quality.

As such, there is a greater appetite – and demand – for the many ways in which producers can benefit from digital technologies by bridging the gap that has traditionally existed between information technology (IT) and operations technology (OT).


Digitalization should be a part of every company’s strategy to move the pulp and paper industry towards autonomous operations, driving productivity and profitability improvements. Advanced Process Control (APC) is increasingly being used by mills as an enabler in their digital transformation. It serves as a link between low-level automation systems and higher-level management such as production planning systems.

APC has become indispensable to many operators today. The implementation of flexible APC solutions allows for the integration of economic optimization strategies to help lower production costs and greater adaptation to changes in targets or seasonal conditions, for example. Many more developments are likely to follow in the next few years to help mills stay competitive.

APC solutions today are already proving to be versatile with an extended functionality that can adapt not only to a target but to a range as well, allowing the best operating regime to be found. This ultimately leads to significant savings without compromising on quality.

Adapting APC for the complexity of pulp and paper processes

One of the fundamental tools in any APC is model predictive control (MPC), which is a set of algorithms for feedback and feed-forward control based on a process model. While MPC has been around since the 1980s, it is relatively new for the pulp and paper industry -  in large part due to the complexities of the process. Kraft pulping, for example, has many interacting processes, varying dynamics, and conflicting objectives, and is extremely difficult to model and control. It can also be easily disturbed by many factors such as raw material quality, equipment age and the ambient weather. 

Over the years, ABB has developed additional tools to further optimize process automation and make APC into a more robust and adaptable form, leading to easier and more successful adoption for pulp and paper processes. Areas where ABB APC solutions have been successfully implemented include pulp digesters, brown stock washers, bleach plants, lime kilns, causticizers, and multi-effect evaporators as well as paper machines. Process variability can be significantly reduced, creating savings in energy and chemicals, and often reducing downtime and maintenance costs. In integrated pulp and paper mills, having APC modules in the pulp mill ensures a continuous stream of high-quality pulp to the paper machine. This makes the work of the paper machine controls easier and allows the mill to focus on cost reduction and efficiency improvement without having to worry about disturbances due to changes in pulp quality.

abb 1july22 2 minHow the various advanced process control systems are applied in a pulp mill by ABB One critical development has been the inclusion of 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. Take for example our Kappa Virtual Measurement, which estimates Kappa in the cooking zone of a digester, where it is impossible to physically measure Kappa, providing more visibility into the pulp quality across the cooking plant. With more frequent Kappa measurements, the APC can make immediate corrections to create more even cooking. The results of these changes are also immediately visible in the calculations.

ABB offers additional tools to further optimize process automation, such as Pulp Tracking and Constraint Management. Pulp Tracking is a way to chart the movement of key pulp properties throughout the process, such as moisture content in wood chips, pulp conductivity in brown stock washers, or pulp brightness in bleaching. These variables are tracked in space and time up to the location of interest, with the insights used to build models for various APC modules across the mill. If off-spec pulp is produced for a short while at the digester, for example, the system can predict when  this slug of pulp will reach the various bleaching stages and adjust bleaching parameters accordingly.

Constraint Management is a concept patented by ABB which dynamically calculates high and low limits for the APC variables. It uses  tracking in coordination with predictive controls to help processes that are typically characterized by non-linearity, slow dynamics, and high interactions amongst key process variables. This approach ensures that the APC can adapt to varying process conditions and maintain tight control which otherwise wouldn’t be possible.

APC pulp and paper applications

ABB has found that there are three critical success factors when implementing APC into any process:

  1. Having accurate sensors, robust actuators, and a reliable distributed control system (DCS) in place is critical. These are the foundation on which the APC is delivered and if the measurement system or DCS is not up to scratch, the APC will not be able to deliver the expected benefits.
  2. Ability to combine operational data with process knowledge is paramount. Ensuring that the APC is not a black box but can incorporate the operators experience into its models along with the data-based models is key to achieving the targets with minimal efforts.
  3. Transparency and ease of use is critical to get the operators’ buy-in. It is very important that the operators can relate with 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 is equally important to continuously monitor performance and share key performance indicators so it can be sustained. ABB believes that performance can be sustained, and even improved, over many years through close collaboration with a service-based model for long-term successes. 

Following the evolution of these critical factors has resulted in  APC being applied to many different processes. Take for example the lime kiln operation, which is one of the major consumers of energy in the kraft process and exhibits a lot of variability in energy use, quality and productivity. Although the lime kiln process is straightforward in principle, kiln operators still face difficult issues such as low thermal efficiency, high fuel consumption, process delays, build-up of rings and dust, overheating of refractory, poor quality of lime and increased emissions. OPT800 Lime manages these challenges to achieve optimum control and efficient lime production. 

The solution works to optimize lime production rates, reduce energy consumption and emissions, increase re-burned lime availability, decrease residual carbonate variations, and improve overall operation with increased visibility of the process. It combines laboratory results by operators with MPC, thus getting the best of both approaches in a fully product-specific solution, with minimum need for on-site development. This enables tighter control of the lime production process, leading to more stable pulp mill operations.

Looking to the fiberline side of pulp mill operations, mills can achieve optimum pulp quality leveraging APC in the cooking process. When ABB’s OPT800 Cook/C APC module is applied in a continuous pulp digester, it controls an estimated Kappa number in the cooking zone, residual alkali at the extraction stage, chip level in the digester and blow consistency. Typical results include reduced variability in the final Kappa number and reduced steam and alkali consumption, resulting in lower energy and bleaching costs.

Another example of APC implementation is on the wet end of a paper machine. Producing paper on a paper machine involves a series of highly complex processes and variation in any of these processes could lead to the formation of a defect in the sheet or a weakness in the paper web, eventually leading to a sheet break or rejection due to poor quality. Wet End Control helps to monitor and analyze the wet-end processes and can significantly reduce downtime, the usage of chemical additives and production costs, while maximizing production.

Delivering tangible results for customers

Many mills recognize that implementing more than one APC module in a pulp mill can result in synergy and smoother operations, and as such are using multiple modules to get the maximum value from their process. One pulp mill in Asia installed APC modules in both the lime kiln and causticizing operation, which are intimately linked in the kraft process, since lime from the kiln is added to the causticizer to prepare white liquor. In the lime kiln, variability in temperature and excess oxygen was reduced by 40% along with an 8% reduction in energy use. In the causticizing operation, variability in causticizing efficiency was reduced by 31%, with an 11% savings in lime consumption.

In one European pulp mill expansion project - which doubled the capacity of the mill - the owners chose to implement Pulp Tracking throughout as well as five APC modules, in the oxygen delignification, bleaching, brown stock washing, causticizing and evaporation processes.

Another European customer first installed an ABB APC pulping module in their continuous digester 10 years ago and has hit new production and quality milestones with every update and adjustment since. Most recently, the mill achieved the best continuous digester performance across the organization — proof that long-term commitment to adapt to mill changes will continue to bear fruit. However, it doesn’t end there, and the mill is about to start on its next project to automate start-ups and shutdowns, avoiding the off-spec quality that can often result from handling these tasks manually. This shows that process optimization is a never-ending journey.


Focusing on digitalization, and embracing APC as a key part of this, is a major opportunity for pulp and paper mills aiming to remain competitive and efficient. Not only can it help mills achieve smoother processes, but APC can also make dynamic adjustments to changing variables to keep quality consistently high. This can help to truly shift pulp and paper towards autonomous operations, driving improvement of both productivity and profitability within the industry.

ABB (ABBN: SIX Swiss Ex) is a leading global technology company that energizes the transformation of society and industry to achieve a more productive, sustainable future. By connecting software to its electrification, robotics, automation and motion portfolio, ABB pushes the boundaries of technology to drive performance to new levels. With a history of excellence stretching back more than 130 years, ABB’s success is driven by about 105,000 talented employees in over 100 countries.

ABB’s Process Automation business is a leader in automation, electrification and digitalization for the process and hybrid industries. We serve our customers with a broad portfolio of products, systems, and end-to-end solutions, including our # 1 distributed control system, software, and lifecycle services, industry-specific products as well as measurement and analytics, marine and turbocharging offerings. As the global #2 in the market, we build on our deep domain expertise, diverse team and global footprint, and are dedicated to helping our customers increase competitiveness, improve their return on investment and run safe, smart, and sustainable operations.

Source: ABB