Kraft process variability reduction in pulping, washing and bleaching: 3 factors for success

Download PDF - First published in Fall 2019 issue of Pulp & Paper Canada: Vol. 120 No.4. Author: Martin Fairbank Ph.D. based on the interview with Dr. Abhijit Badwe, advanced process control expert at ABB

How to prevent off-spec pulp quality, save energy costs, reduce chemical usage and even lower maintenance downtime in pulp mills.

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Take corrective actions before pulp variability increases by applying ABB's patented advanced process control approach. Plus, 3 critical success factors observed from over 100 APC applications.

Taking control of variability in the pulp mill

In today’s pulp and paper industry, especially for commodity products in a stable or declining market, it is difficult for producers to improve profitability by increasing output, since there is no guarantee that extra product would find a buyer.

One avenue that can be explored to improve the profitability of kraft pulp mills is to reduce their process variability, which can lead to cost savings from reductions in energy usage, chemical usage, off-spec tonnage and even maintenance downtime.

Process variability can have a number of causes, including raw material variation, sensor drift, control loop tuning, grade changes and human factors. The implementation of advanced process control strategies can minimize this variability and many producers are implementing these strategies in pulp and paper mills globally.

Process variability can have a number of causes, including raw material variation, sensor drift, control loop tuning, grade changes and human factors.

Main pulp mill areas to benefit from variability reduction

The main areas of the fiber line in a pulp mill that can benefit from variability reduction are pulping, brown stock washing and bleaching. For example, APC can be applied in a continuous digester. In this case, the following variables are controlled:

  • Kappa number (as estimated in the cooking zone)
  • Residual alkali at extraction
  • Chip level in the digester
  • Blow consistency

Typical results of APC implementation in the digester include reduced variability in Kappa number and reduced steam consumption, resulting in lower energy and bleaching costs.

The implementation of advanced process control strategies can minimize variability and reduce costs.
Stabilized production rate in a continuous digester reduces variability

Typical applications and results

Brown stock washing

OPT800 Wash module

In the operation of brown stock washing, the main objective is to achieve removal of impurities in the pulp with a minimum of fresh water consumption. Poor performance at the brown stock washers can significantly affect the stability of operation and result in increased operating costs due to higher energy requirements at the evaporators and higher bleach use.

The control strategy used in the OPT800 Wash module involves computing optimal setpoints for (a) the dilution factor in each washing stage and (b) the defoamer charge to maintain the alkali loss at a specified target while respecting constraints on wash liquor inventory, i.e. the filtrate tank levels. This results in reduced usage of fresh water, a decrease in alkali losses and an increase in dry solids in the evaporation liquor, thereby reducing steam consumption in the evaporation plant.

Recent APC results at European mill
Alkali losses
  - 30%
Fresh water consumption
  - 20%
Steam consumption
in evaporation plant
  - 12%

Recovery area

OPT800 modules for evaporation, causticizing and the lime kiln

APC techniques can also be applied in the recovery area of the pulp mill. ABB has developed OPT800 modules for evaporation, causticizing and the lime kiln. A British Columbia pulp mill wanted to reduce fuel consumption while also improving the quality of lime produced in the kiln. By applying OPT800 Lime, they were able to achieve a five per cent reduction in energy consumption and a more consistent residual carbonate in the re-burnt lime.

Similarly, another Canadian mill in New Brunswick was aiming to produce consistent quality of thick black liquor while minimizing steam usage in the evaporation plant. With the implementation of OPT800 Vapor, the mill was able to reduce black liquor density variation by 40 percent by optimizing steam usage.

Recent APC results at Canadian mills
Fuel consumption
  - 5%
Black liquor density variation
  - 40%

3 factors for APC project success

At ABB, we have observed there are three critical success factors.

1. Good quality of both historical and live data

First, it’s important to have good quality of both historical and live data. This will depend on the age of the underlying distributed control system (DCS); it can be difficult to extract data from older DSCs and to interface the optimization system with the DCS.

2. Good sensors and accurate measurements

Secondly, success depends on having good sensors and accurate measurements. Some measurements are must-have items for success and others are good-to-have. Assessment of the measurement system is included in the Diagnose step of the project. If the measurement system is not up to par, the APC won’t deliver the anticipated results.

3. Willingness to explore the effects of changing variables

Finally, the mill that implements an APC project must be open to not only changing the way they operate their process, but also to allowing the process to be deliberately perturbed, or “bumped.” This latter step is necessary for a certain period during the Implement step, as it provides information to build the process model and understand how the variables interact. Willingness to explore the effects of changing each variable independently builds a better-quality model, which leads to a better quality of control achievable.

How ABB combines predictive controls and pulp-tracking

ABB has developed advanced process control (APC) modules called OPT800 series, which fall under ABB Ability™ Performance Optimization solutions for pulp mills. ABB has installed over 100 APC applications in the pulp and paper industry, including 20 in the last four years. With our approach to APC implementation, control is accomplished by applying a combination of predictive controls and pulp-tracking techniques to reduce variability.

This patented approach helps in taking corrective actions before the damage is done (a variability increase) as opposed to conventional controls that act after off-spec quality is detected / measured.

ABB has installed over 100 APC applications in the pulp and paper industry
Pulp tracking

Pulp tracking is a concept that can track the performance of various key pulp properties through the process, such as moisture content in wood chips, pulp conductivity in brown stock washers, or chemical dosage in the first stage of bleaching.

These process variables are tracked in space and time up to the location of interest. These transformed variables help build a soft sensor model that is then used in the appropriate APC module for online prediction of controlled variables. If off-spec pulp is produced for ten minutes at the digester, for example, the system can predict when this slug of pulp will reach the various bleaching stages and adjusts bleaching parameters accordingly.

Diagnostic and prediction

The pulp-tracking module also serves as a diagnostic and visualization tool for the operators. For example, if off-spec pulp or increased variability is observed at an intermediate stage in bleaching, the operator can drill down and determine the possible source(s) of this variability: off-spec process parameters in an upstream operation or even issues with wood chip quality.

Furthermore, in the case of grade changes, the tracking function aids in accurate prediction of when the new grade would arrive at a given location along the fiber line. This helps in appropriate adjustment of recipe parameters to minimize production of mixed/intermediate grade pulp.
Constraint management

ABB’s OPT800 modules are designed for specific stages of the pulp process that combine a number of techniques, including traditional model predictive control (MPC), soft sensors, pulp tracking, and a patented novel “constraint management” module, which calculates new low and high constraints for manipulated variables (MVs) dynamically.

The constraint management module enables the use of the tracking function in coordination with predictive controls to help pulp process controls that are typically characterized by nonlinearities, slow dynamics and highly interacting multivariable relationships amongst key process variables.

APC project implementation steps

In order to estimate the potential benefits of APC, an initial study of the pulp mill and its behavior must be carried out. A structured approach is used to work with clients through three stages of each project: Diagnose, Implement and Sustain.
Diagnose
In the Diagnose step, the first objective is an audit of the process along with its measurement and basic control system. Bottlenecks are determined, as well as what can be improved without adding any new controls, for instance by changing operating parameters or existing control loops. Secondly, based on expected variability improvement results, an action plan with an estimate of results obtainable from APC implementation is made with a predicted return on investment.

Implement
In the Implement step, first the appropriate OPT module is installed, complete with pulp tracking, MPC, soft sensors and expert controls. The system is then tuned and the mill personnel are trained in using the system.


Sustain
In the Sustain step, performance is monitored continuously and maintained at the agreed level, working with mill operators and staff. Payment is on a monthly basis and is performance linked, i.e. payment occurs only when the performance reaches the agreed level. This can be either until the end of a negotiated contract period or on a perpetual basis, depending on the needs of the mill and how the project is structured.

Conclusion

Advanced process control has evolved over the last 30 years with the availability of better sensors, better control strategies and cheaper and faster computing power. Driven by the opportunity to have better control over variability and reduce production costs, many more applications of advanced process control are likely to be installed to help pulp and paper manufacturers increase visibility and gain a competitive advantage as overseas competition increases.

Explore potential benefits of implementing APC at your mill.

Original article | Pulp and Paper Canada
Taking Control of Variability in the Pulp Mill
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Pulp and paper process optimization opportunities with digital technologies
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APC for Pulp Mills
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