Pulping/Bleaching

Thunder Bay, Ont., producer of softwood and hardwood kraft pulp upgrades its two ECF fiberlines for more consistent products

Advanced Fiberline Controls Yield Return for Avenor Market Pulp Mill

By Mark Williamson

Pulping/
Bleaching

Modern digital process control systems have played an important role in improving an operator's ability to efficiently regulate, sequence, and coordinate process operations. But, as high-tech replacements for pneumatic controls and relays, have digital systems really produced the expected leap in process performance and bottom line returns? The answer is sometimes unclear. There have been clearly defined successes and some cases that are difficult to assess from the bottom line, even though the technology is working well.

Success stories truly stand out, so when a major pulp producer such as Avenor Inc. attributes major productivity increases, more consistent product quality, and cost reductions to process control, the reasons for their success are worth documenting. Avenor's bleached kraft pulp mill in Thunder Bay, Ont., recently upgraded the process controls on its two elemental chlorine-free (ECF) fiberlines. The mill's "A" line produces 600 tpd of bleached softwood pulp, while the "B" line produces 900 tpd of bleached softwood pulp and bleached hardwood pulp.

ADVANCED CONTROLS. Rather than simply upgrading pneumatics and relay logic to their digital equivalents, Avenor decided to go a step further by implementing advanced digester controls based on process models, coupled with measurements of the liquor strength, the progress of chemical reactions, and the degree of fiber delignification. This higher level of control was considered essential to stabilize process conditions and make smooth transitions so that the operators could then concentrate on optimizing quality and costs of production.

Prior to the upgrade of the control technology, the operators would frequently be required to intervene when an upset occurred and control the operation themselves. Fiberline A, which dates back to 1966, had mainly pneumatic controls. The 1976-vintage B line was equipped with electronic controllers and some pneumatics. A and B digesters and B bleach line were equipped with a Foxboro Spec 200 computer system which was linked to some loops and performed limited supervisory functions such as ratio and cascade controls.

The mill selected the Foxboro I/A system as the distributed control system (DCS) platform for both lines from chip feeding, through the two Kamyr continuous digesters, the ECF bleach plants, and the ClO2 chemical preparation plant. Foxboro partnered with Modo Chemetics (now Kvaerner Chemetics) to implement advanced digester controls configured entirely within the I/A system.

Since no two fiberlines are run in exactly the same way, control systems are not usually standard off-the-shelf solutions. The implementation of mill-specific strategies was therefore done with the involvement of operations and mill engineering personnel and kraft mill process control and information technology personnel. The software's flexibility and the DCS-based configuration was an attractive combination.

Cooking strategies employed in each digester include Kappa number control through H-factor, effective alkali control, level control, and production rate control. Blow consistency control is implemented on A digester, while dilution factor control is used on B digester. B digester, which runs hardwood and softwood, also has species change control.

NEW INFRARED EA ANALYZER. The existing Kajaani Kappa analyzer, which samples from each digester, was integrated into the control strategies. In addition, the mill considered it important to measure effective alkali (EA) to assess liquor strength, EA-to-wood ratios, and reaction progress in the cooking zone. Avenor installed a promising new EA analyzer jointly developed by Kvaerner Chemetics and the Pulp and Paper Research Institute of Canada (Paprican). The Avenor installation is the first commercial installation of the FTIR liquor analyzer.

FTIR is an acronym describing the method of measurement which uses Fourier Transform Infrared Spectroscopy. The optical sensor with a flow-through design eliminates much of the mechanical complexity of previous EA samplers, which used titrations to determine liquor concentrations. With the FTIR, no reagents are required.

The analyzer is set up to measure nine points in the two digesters, at about 5 min/measurement cycle. The scheduling and priority of the sampling sequence can be adjusted through the Foxboro system. Operators can also request an additional test for confirmation of a test result. The analyzer programming also includes automatic standardization and service diagnostics.

With an EA profile across the digesters, liquor charging, the cooking process, and the Kappa number can be more precisely controlled even during production rate changes and species changes on B digester.

OPERATOR INVOLVEMENT. But with all this high-tech gadgetry and new, unfamiliar control strategies, the process operators could not be left in the background. Avenor considered that the success of the new controls would be largely determined by operator participation in the design and operation of the controls.

"Let the computer control and then the operators can manage the process," is the Avenor philosophy expressed by Dale Smith, process development superintendent. This process management philosophy, which was a different way of thinking for the operators, was considered essential to achieve their productivity and quality objectives. "The operators could make or break the success of the project," according to Pat Falcigno, mill manager, kraft.

The operators had limited computer experience. Furthermore, they had to face another new challenge by changing from English to metric units. Neither A or B fiberline operators had exposure to the concept of H-factor or advanced controls. The operators' natural apprehension had to be addressed by promoting their involvement in the design and implementation process.

In addition to the usual training and practice on simulators, the operators took leading roles in the design of controls, video screen graphics, and interlocking sequences in cooperation with the mill's engineers, plus Foxboro and Kvaerner Chemetics personnel. The video screens are customized according to the operators' preference. The digester control pages are graphically intensive with many flow loops, while the bleach plant operators prefer a more traditional control panel layout. As a continuing program, operators initiate software change orders to improve controls, procedural functions, or displays.

With two groups of operating staff and different operating procedures, the A and B lines were implemented sequentially. A bleach plant was commissioned first, followed by A digester. With more predictable delignification in the digester, the bleach plant sees a more consistent product with reduced "hard shots and soft shots" which upset chemical application and product quality. With this more consistent level of performance, Falcigno feels that A bleach plant is "one of the best running in North America."

B line automation was completed in the spring of 1997. Both lines are now operated through a centralized control room. The ClO2 chemical plant controls were moved into this room in late 1997.

PRODUCTION BOOSTS, CHEMICAL SAVINGS. With better management of chemical application, cooking conditions, and delignification, the Avenor mill can produce a pulp with more consistent residual lignin and a pulp that washes well with low black liquor carryover. The bleach plant operators need not be concerned about having to use excess chemicals in anticipation of uncontrolled high Kappa numbers. Kappa number targets can therefore be managed to optimize pulp quality and bleaching chemical consumption.

The other pulp mill product is black liquor, which must be processed in the recovery cycle. Targets can now be managed to alleviate limitations in recovery operations which might cause a reduction in production rate.

The mill reports that Kappa number variability has been significantly reduced. For a period of three to four months before and after commissioning of the controls on A digester, variability was reduced by more than 40%. Before commissioning, the standard deviations of Kappa number ranged from 4.0 to 5.6. After commissioning, the range was from 2.3 to 3.0.

Bleaching chemical consumption on A line has been reduced by 10%. There may be some seasonal factors involved in this evaluation. Most significantly, overall production is up by a consistent 10%. The results achieved have exceeded mill expectations. The mill also sees that it is now in a better position to deal with customer needs by custom-designing products with specific functional properties.