BTG partners with Nalco and Norske Skog on innovative closed loop control system
Increased production speeds, demand for brighter, top quality paper grades, more complex printing technologies and ever-present cost pressures calls for expert application of the latest wet end instruments and technologies.
Norske Skog Albury
Located in Albury, New South Wales, the mill commenced production in 1981 and ranks among the best newsprint production facilities in the world.
Annual production capacity is some 235,000 tonnes, representing around 30% of newsprint and related grades used in Australia each year.
The mill uses up to 55% recycled fiber, which is produced in Australia’s first large scale newsprint deinking plant. Commissioned in 1993, this facility recycles around 160,000 tonnes of old newspapers and magazines each year. The other source of fiber used at the mill is plantation radiata pine.
For papermakers, producing high quality newsprint from machines using TMP and deinked pulp (DIP) always represents a complex challenge because of the inherent high levels of process variability.
Faced with chronic process instability, Norske Skog’s large Albury newsprint mill in southeastern Australia recognized the need for better control of white water solids, particularly after shutdowns, during grade changes and when there were significant changes in broke content. While manual tests used to control solids levels were adequate under stable conditions, they simply couldn’t cope with the levels of variability that were a regular part of the mill’s day-to-day operation.
To address the problem, Norske Skog embarked on a joint project with Nalco Australia involving the installation of a state-of-the-art BTG Mütek RET-20 Retention Analyzer and a move to closed loop control.
As the world’s leading water treatment and process improvement company, Nalco combines an extensive background in papermaking technologies with a strong commitment to improving its customers’ productivity. Through an innovative project involving close cooperation between the mill’s production and technical staff, BTG’s own consistency control experts, and Norske Skog’s regular chemical suppliers, the Albury mill in now enjoying a host of cost-saving benefits, including greater efficiency, better overall product quality, and substantially reduced chemical costs.
Operational challenges
Brightness is a major determinant of the value of today’s paper products. With variable quality TMP and DIP, achieving desired brightness levels requires frequent changes to bleaching chemical dosage – so that the bleaching process itself becomes another contributor to process variability.
The Norske Skog Albury Mill is a fast, modern facility that uses mixed TMP/DIP furnish and both hydrogen peroxide and hydrosulfite bleaching. In addition to high potential system instability, the mill has to cope with two sources of hydrophobic contaminants – wood pitch components, and stickies originating from deinking furnish. The blending of two pulps of different chemical characteristics represents a complicated process creating considerable operational challenges, from differences in pH and the possibility of slow equilibration processes involving internal fiber structure, to the generation of hardness with calcium carbonate fillers, and more.
In 2006, the Albury mill undertook a significant rebuild with the aim of increasing machine speed from 1,300 to 1,550 m/min. The mill’s process is based around a two-component retention program composed of a Hybrid Coagulant (HC) and a Cationic Flocculant (FC) which is premixed before being added into the machine furnish.
As part of the rebuild, the addition of this premixed retention program was improved by installation of a Trump-Jet feeding system in the post screen position. But while this modification contributed significantly to fresh water savings, it had little effect on the consumption of HC/FP. When the machine had reached its operating speed of 1,550 m/min, consumption of HC/FP was in the range of 400-600 g/t, with white water (WW) solids levels fluctuating between 0.4% to 0.6%.
Competitive pressures
The Mütek RET-20 Retention Analyzer is the industry benchmark, delivering the highest accuracy in ash measurement
In an increasingly competitive market, the importance of process stability in newsprint applications simply can’t be over-estimated; with online increased speeds of paper machine value, real time stabilization of the process increases, and the costs of non-conformance are higher. Uncontrolled changes in WW solids – indicative of changes in retention, drainage and colloidal control levels – negatively impact machine efficiency and machine deposition. The Albury mill’s technicians observed such problems in the first weeks after the rebuild – clearly indicating a need for an improved retention program control strategy.
Prior to the rebuild, white water consistency control was based on making defined HC/FP dosage changes based on results of manual tests performed by operators three times during every 12-hour shift. Typical problems with such tests – low frequency, human error in consistency determination, and delays before response – took on even more importance at higher machine speeds.
Limited access to operator services during start-ups, when control of the retention program is most critical, also made machine start-up difficult, leading to production losses. And the mill found that after start-up it took only 2-3 days for a clean system to gradually become contaminated with white water consistency exceeding the defined target. Measures taken to correct that excessive white water consistency would then often lead to increased destabilization of colloidal materials, prompting runnability and machine deposit issues.
System set-up
Changes in furnish characteristics and water chemistry impact wet end performance – retention, drainage and deposit control. And that impact is felt from the forming section all the way to the reel.
The initial objectives of closed loop control were to maintain stable machine operation through better white water consistency, primarily after maintenance shuts and grade changes. Promised added benefits included the ability to determine correct chemical dosage levels (to avoid costly overdosing and excessively large dosage changes) and improved sheet properties (formation, strength, porosity, printability). A specific target of 0.36%± .02% white water consistency was set, which was to be maintained during normal operation, machine start-up and grade changes.
The mill chose a Mütek BTG RET-20 Retention Analyzer to measure solids and ash consistency on the headbox and whitewater streams. Installed at the beginning of 2007, the model was selected on the basis of a set of performance capabilities which are well-suited to typical newsprint operation, along with solid industry references and experienced local support.
BTG’s Mütek RET-20 Retention Analyzer
RET-20 measuring principle
The Mütek RET-20 employs two light beams and uses BTG’s patented Peak Method based on optical scattering effects. The first light beam (LED) measures the total and fine particle consistency of paper suspension, while the second (laser) determines the actual ash/filler content of the sample by determining Peak value of light transmission.
Peak Method
The Peak Method operates on the assumption that light interaction with the paper suspension is defined by two primary component particles – large (fibers) and small (fines and fillers). When the light with a narrow diameter (0.6mm) passes, the suspension intensity of the transmitted light will vary between three extreme cases: VCW( clear water value), Vp(peak value) and VDC (direct current).
The maximum value observed in the real stock measurement, VP, is recorded when only fines and fillers interact with the beam of light. Calibration allows the correlation of this value with small particle consistency.
Figure 1
Bump test showing three consecutive 50g/t dosage increases.
The first test involved increasing retention program dosage from 500 to 550 g/t under stable machine running conditions, with constant basis weight and furnish composition. The next test included a series of three changes in HC/FP pre-mix dosage, each by 50g/t (see Figure 1).
The first step from 450 to 500 g/t decreased WW solids from 0.48 to 0.465%. The next step from 500 to 550 g/t decreased WW solids further, to 0.45%, and the final step, from 550 to 600 g/t, decreased WW solids to 0.43% initially, and to 0.42% after three hours. Response time on each step was about 40 minutes. Reducing the retention program dosage to original levels resulted in a gradual WW consistency increase.
Current value ranges built into the control logic have a delay time interval between 15-45 minutes and a retention program dosage step ranging between 10-40 g/t. If actual consistency is more than 0.01% under the target, the dosage of retention program steps down by 10g/t; if it is higher than the target value, it steps up.
First results
Stability in WW consistency significantly improved after closed loop control was initiated. Standard deviation based on one-hour average improved by 62%, and retention program consumption began to decrease about two weeks before the system was moved to closed loop control, most likely because of changes in recycled fiber (RCF) chemistry.
The positive impact of the online control strategy has also been reflected in other machine operating and quality parameters.
Key results include:
- Reduced fluctuation in headbox pressure and secondary fan pump of 25-40%.
- Dewatering in the former has improved, cutting steam consumption by 6%.
- Improved MD direction variability of 6%.
Ongoing benefits
Figure 2
Improvements in white water consistency control before and after the implementation of closed loop control.
Installation of the Mütek RET-20, combined with the mill’s very responsive retention program, has delivered substantial, measurable improvements to production stability and chemical dosage costs. In addition to a 62% measured improvement in standard deviation of white water fluctuations, the headbox pressure and secondary fan pump are more stable, with a 25-40% improvement in standard deviation of fluctuations. The mill has also observed a 6% improvement in steam usage and a 6% improvement in paper caliper standard deviation. HC/FP addition to the post screen addition point has decreased by 10-20%, resulting in considerable chemical savings.
White water fluctuations due to changing broke levels, changing grades and start-ups after scheduled maintenance periods can also now be much more easily controlled without close operator involvement – for better ease of operation.