By Mark Williamson, Journalist Engineer
BRUSSELS,
Jan. 31, 2009
(Viewpoint) -
Brazil is home to some of the largest pulp mills in the world. Naturally, those mills have a correspondingly large bill for chemicals which produce the bright white pulp used in the world’s papers. It therefore makes good sense - both environmentally and economically - to provide just the right amount of brightness with a minimum amount of brightening chemicals. Less chemical use means more profitability and a better environment. It’s a win-win-win result.
Online bleaching process analyzers and process controls have played a major role in reducing the use of expensive chemicals. Maximilian Furley, South American business manager for the Kajaani product line of Metso Automation, Brazil explains how: “To achieve the right quality for the lowest cost many Brazilian pulp mills, and many others around the world, are using Metso Automation technology - notably Kajaani analyzers and bleaching controls - in their fiber lines. With online analyses and stable controls, pulp mill processes can be run very close to their optimum conditions. Pulp producers are therefore achieving better quality and, at the same time, lowering chemical usage. As good examples, Aracruz, Veracel and Lwarcel are leading the way with this advanced technology,” he says.
Aracruz pioneers
Aracruz, a pioneer producer of bleached eucalyptus pulp, has used Kajaani bleach plant measurements since the late 1970s. In recent years, the 2-million tonne/yr Barra do Riacho mill has upgraded its bleach plant control technology with the newest online analyzers and optimizing controls. Bleach plants B and C were equipped with kajaaniBRITEi automated kappa sampling analyzers and kajaaniCORMECi and kajaaniPOLAROXi inline brightness and chemical residual analyzers. Using these online measurements, the optimizing controls, called Compensated Kappa Factor (CKF) controls, provided a significant reduction in chlorine dioxide consumption per tonne of pulp. This optimization control was developed by Metso Automation in the early 1990s in response to the changed chemical reaction dynamics in Do bleaching stages compared with older chlorine-only stages. Since ECF bleaching is now the industry norm, Compensated Kappa Factor Control has become the industry standard method of control.
In chlorine dioxide stages, adding more chemical past a certain optimum point no longer produces a corresponding brightness response. In many cases, the chemical dosage is kept above this optimum limit in order to ensure the brightness target will be achieved. This is typically what operators do in a manually controlled operation. When the standard deviation of brightness is reduced due to more effective process control, this “safety margin” can be cut. Thus, the Aracruz mill saved chemical costs because the operators were not overdosing. The full report can be seen in the October 2005 issue of Pulp & Paper International.
The news spread
The news of the success at Aracruz spread within the industry. So, with similar expectations, the new Veracel Cellulose pulp mill, a joint venture between Aracruz and Stora Enso, invested in comparable online measurements for its new 900,000-tonne/yr fiber line, which was started up in May 2005.
For pulp lignin content (Kappa) analysis the mill purchased a kajaaniKAPPAi which samples before the bleach plant A stage (after O2 delignification) and a kajaaniBRITEi which samples from four points in the ADoEopD1P beaching sequence. In addition, one kajaaniCORMECi inline brightness analyzer, three kajaaniPOLAROX-Fi and one kajaaniPOLAROXi chemical residual analyzers and three Filtrate Pistons for pH analysis were included in the initial scope of supply. To check the final baled pulp production, a PulpExpert DCD regularly samples pulp sheets from the baling line and measures brightness and dirt count.
Ronaldo Morales Aguilar, fiber line coordinator at Veracel, reports that the Kajaani analyzers were up and running and calibrated very quickly and those measurements contributed to a fast mill startup. “We had an excellent mill startup. Kajaani people did a great job during this phase and that helped us a lot. The analyzers gave us trustworthy information from the beginning,” he says.
Stabilize brightness then reduce ClO2
With trusty online measurements and a good startup behind them the mill then implemented the bleach plant optimization controls. The objective was to stabilize final product brightness variability and then reduce chlorine dioxide consumption. The Compensated Kappa Factor (CKF) controls were implemented in stages Do and D1.
The primary controls are based on automated measurements of pulp lignin content by the kajaaniBRITEi. The lignin content in the incoming pulp defines the bleaching demand and the amount of chlorine dioxide required to remove the lignin in the pulp. This feedforward control is trimmed by the continuous, fast feedback measurement of pulp brightness and chemical residual using the kajaaniCORMECi and kajaaniPOLAROXi analyzers. The Kappa Factor for the stage is therefore continuously adapted to the incoming pulp quality and the resulting bleached pulp quality.
Of course, the first priority in any pulp mill is to produce an even quality product that satisfies customers’ expectations. With CKF control, the post D1 brightness variability was reduced and that carried through to reduced variability in the final product. Aguilar says the standard deviation of the final pulp brightness is less than 0.5% ISO.
With that quality stability established chlorine dioxide consumption could then be reduced in a controlled way. Initial control evaluation tests confirmed that the chlorine dioxide consumption was reduced by 7.4% in the Do stage. Those initial results have been improved even further to at least 11% savings from startup. “Day by day we have achieved better chemical consumption numbers,” says Aguilar.
The Veracel mill claims it has the lowest consumption of chlorine dioxide of any pulp mill in the world. Furthermore the quality control is just as impressive. The mill says it can guarantee 100% of its product is higher than the minimum and brightness downgrades are now zero.
The mill has installed new kajaaniCORMECi and kajaaniPOLAROXi analyzers in the D1 and P stages and looks forward to trimming costs even further. With these additional analyzers Aguilar believes Veracel will have the most complete line of analyzers of any pulp mill in the world. “We can check all chemical consumptions step-by-step in the bleaching process,” he adds.
Lwarcel steps up learning curve
The Lwarcel Celulose E Papel bleached eucalyptus pulp mill in Lençóis Paulista in São Paulo state is another pulp producer who has seen the twin benefits of better quality and reduced costs. Fast-responding online analyzers and advanced controls of the oxygen delignification and bleaching operations have permitted the mill to optimize its product quality and reduce chlorine dioxide consumption dramatically since the final phase of project P500 was inaugurated in 2006.The new fiber line raised the production of the mill from 250 air dried (ad) tonnes/day to 600 ad tonnes/day (210,000 ad tonnes/yr).
“In a short sequence bleach plant you can’t make mistakes. That is why Metso Automation’s Kajaani analyzers were very important us in the startup of our new fiberline. They were a valuable aid in the normal learning curve associated with new processes,” states Dalton Manzi Júnior, production manager.
Learning the characteristics of new fiberline processes while a mill is still producing pulp is a challenging task for a startup team. The mill reasoned they needed the fast online measurements provided by Kajaani analyzers to define quickly how the new processes responded and to make well-informed decisions to modify and control them better.
In a presentation to Metso Automation users, João Ismael Freschi Fiorese, electrical engineer, listed several objectives of the high level of instrumentation:
• Better control and stability of the new processes: the continuous digester, the oxygen delignification and the bleach plant br/> • Management and reduction of chemical consumption
• Quality improvement
• Reduced laboratory analysis
• Decreased errors in testing repeatability
The mill staff evaluated Metso Automation’s analyzer and bleaching control references at other eucalyptus pulp mills in Brazil. Lwarcel had positive experiences with previous generation Cormec and Polarox analyzers in the old line A at the Lençóis Paulista mill.
The online analyzers include a kajaaniKAPPAi with five sampling points for measuring pulp lignin content from the digester to before the D1 stage, 2 Cormeci for measuring inline pulp brightness and 2 kajaaniPOLAROXi and 1 kajaaniPOLAROXfi for measuring chemical residual content in the bleaching process. Six Kajaani filtrate pistons were provided for pH analysis.
With the analyzers’ performance well established right from the startup the mill was learning more and more about their process, product quality and how to manage chemicals. Next, the advanced control stage of the automation program was implemented.
Because of the fast chemical reaction dynamics in the D0 stage and the short, 15-minute retention time, the control of the pulp delignification and bleaching processes needs to be very responsive to upstream changes in pulp lignin content and other variable factors such as alkali carryover from pulp washing, temperature, pH, reactor retention time and pulp consistency. Moreover, a short bleaching sequence like Lwarcel’s (ADoEopD1) is not so tolerant of large swings in bleaching chemical demand; sudden changes can adversely affect the final pulp’s brightness and shive content.
To provide responsive control of the post-digester processes the kajaaniKAPPAi samples from five locations in the fiber line:
• After the digester
• Before and after the oxygen delignification plant
• After the A (acid) stage outlet, before the Do stage
• After the Eop stage, before the D1 stage
In the two-stage oxygen plant, which has a total retention time of 1.5 hours, oxygen and alkali dosages are regulated by feedforward control from the digester outlet Kappa measurement and by feedback control from the post O2 stage Kappa measurement. The Kappa number from this stage is controlled to a target of 11.
The Do stage is controlled by Compensated Kappa Factor Control. The D1 stage is controlled in a similar feedforward and feedback manner. Kappa and pulp brightness measurements are made prior to the ClO2 addition point. The kajaaniPOLAROXi analyzer, located after the ClO2 addition point, provides fast feedback control to trim the feedforward control in case any other disturbance such as pH, washing carryover, or the ClO2 concentration have altered the control’s response. A final pulp brightness measurement after the D1 stage ensures final pulp brightness targets (from 90.5 to 91.0 ISO) are maintained within specifications.
Major chemical savings
Júnior reports that, since the project startup, bleaching chemical consumption has been reduced significantly – from more than 4 kg of ClO2 per tonne of pulp to the present level of 10.3. This reduction has been accomplished in several ways, including process changes initiated by the mill and the automated controls. Júnior credits the optimization controls with a 0.5 kg/tonne reduction.
With the online information provided by the Kajaani analyses several improvements were made, notably in pulp washing efficiency. Also, with the aid of the online analyses the balance of chemical addition between the Do and the D1 stages has been modified to ensure quality objectives are met.
In conclusion, Júnior is convinced of the value of online analyses and controls. “The Metso equipment helped us to learn about our processes and adjust them. It is impossible to run the short sequence bleach plant without Kappa analyses. Otherwise, you pay a higher cost using expensive chemicals,” he states.
Mark Williamson is a journalist engineer from Thornhill, Ontario, Canada
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