Faced with a difficult to control process and high bleaching chemical consumption Burgo's Ardennes mill in Belgium needed to update its measurements and controls to achieve better process stability, consistent quality and lower costs. And that has been accomplished through Metso's expert Bleaching Optimization services. The mill lists its capacity as 360,000 tonnes/yr, but that has recently been increased to 400,000 tonnes/yr, all of which is for short fiber papermaking applications, either used in the integrated coated paper mill or sold to external customers. The final brightness targets range between 88 and 90.
Burgo Ardennes bleach plant
The wood species used at the Ardennes mill are poplar, beech, birch and oak and this mixture presents a challenge for bleach plant operations. The bleaching of mixed hardwood pulps is a very difficult task as the differing delignification and brightening characteristics of various wood species are added complications to the naturally complex chemical reaction process. Effective control implementation requires some in depth knowledge of the process and how to deal with disturbances. Bleaching process stability and efficiency is influenced by many process variables that can affect the reaction rate, chemical consumption and final pulp brightness endpoint. Therefore, the measurements and the controls have to be in top notch working order and tuned to perfection to get process stability and the target quality with minimum chemical dosage.
The improvements at the Ardennes mill have been achieved by a successful measurement and control implementation using new Metso analyzers and Bleaching Optimization services that have demonstrated a substantial saving in bleaching chemicals with a more stable process. Most importantly, the stability has been maintained after the project was completed in the summer of 2013 by a continuing Performance Service Agreement (PSA). After the initial commissioning, the controls have been adapted to significantly higher production rates later in the later part of 2013. Remote diagnostic reporting has pointed out some process improvements that were implemented with positive results. The improvements in chemical utilization are continuing.
Tailor made controls
Eric Bazzoni, fiber line production manager, sums up the mill's objectives and the results achieved: "Before the new controls it was difficult to achieve higher brightness targets for some customers. We were having problems with process stability and high chemical consumption. Our goal with the new measurements and controls was to stabilize the process and reduce costs. Now, we have a big advantage as we can achieve the right brightness even at the higher production rates. Before, it was difficult to achieve stability from shift to shift as operators would manually change parameters. Now, the control system adjusts by itself and the parameters are the same. We do not have any off-specification pulp."
He points out that chemical consumption is kept to a minimum since the chemical residual of chlorine dioxide after the bleaching towers is at a very low level. He says there are no problems with off-gassing that might cause health, safety and environmental problems.
Pierre Carnevali, mill projects manager, adds his thoughts on the controls implementation, "I am impressed that the Metso engineers have adapted the controls to our own process. For us, it is a tailor made control for our own mill."
Project scope defined by audit
The project scope was defined by a Metso measurement and controls audit requested by the mill. The process study defined the measurement and control requirements and the agreed control guarantees for final brightness stability and reduction in chlorine dioxide consumption. The study concluded that many of the process measurements were obsolete and needed replacement with modern versions. The Metso engineers also recommended some changes in the measurement scheme that would improve process controllability. François Zune, Burgo process engineer, says the most significant recommendation was to change the pulp Kappa sampling point from after the B0 (or D0) stage chemical mixer to before the mixer. This would provide a more responsive feedforward control.
The new measurements included a Metso Kappa Q for online Kappa measurement at five sampling locations, three new Metso Cormec5 inline brightness measurements and three new pH measurement points that include a Metso Filtrate sampler. The new filtrate sampler provides a clean sample to the pH probe and thus avoids fouling and measurement instabilities that are common to the previous inline measurements. Metso engineers implemented the Bleaching Optimization using advanced process control (APC) tools in order to optimize the stability and costs of the bleaching operation. The process measurement points are shown in Fig.1
Fig. 1 - Bleaching process measurement points
The control strategy automatically manages delignification (as measured by Kappa) and brightness to achieve the final brightness target in a cost-effective way. This is done by avoiding over consumption of bleaching chemical. In 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 to ensure that the brightness target will be achieved. This is typically what operators do in a manually controlled operation as Burgo had. When the standard deviation of brightness is reduced due to more effective process control, this "safety margin" can be cut. This results in significant chemical savings.
The control guarantees were tested in the summer of 2013 and the results were better than the guarantees. Brightness variability was reduced by 47.9% and chlorine dioxide consumption was reduced by 8.7%. The project was accepted.
But this was not the end of the task to maintain and improve the controls. The mill planned to increase production from 1,015 tonnes/day to 1,100 tonnes/day. This would affect the residence time in the bleaching towers hence change the dynamics of the bleaching chemical reaction. With a shorter residence time the consumption of chemical went up as expected. Metso retuned the controls for the shorter residence time and different control dynamics .The controls were retuned to give the original and expected process stability. This was done under a continuing PSA that includes an annual site visit for control issues. To keep the important measurements in good working order a Preventive Maintenance Agreement contract for Kappa Q and other fiber line measurement includes several visits per year.
The PSA includes remote system and process monitoring though a link to Metso and 24/7 telephone help services. The service monitors the bleaching process performance stage by stage and issues daily and monthly control performance reports. Bazzoni finds this a useful tool as it gives a concise report on key control objectives and uncovers any problems. Carnevali adds: "The reports tell us if we are in a good control range or not and if we are consuming the right amount of chemicals."
Even after a short time this service proved its worth by detecting a problem, notes Zune. He says that a process temperature regulation problem was detected by the remote link to Metso. Temperature has a significant impact on reaction rate so this problem had to be fixed to improve stability. This was done and the bleaching is more stable, reports Zune.
The optimization process continues. Metso has recommended that B1 stage pH could be reduced to save bleaching chemical further. The preliminary results look promising. FRP piping has been installed to avoid corrosion at the lower pH levels. Control tuning was ongoing at the end of March, 2014.
Zune says that the mill people have learned a lot about bleaching control from Metso. The remote and telephone help services help to detect and correct problems quickly. "It has opened our eyes," he concludes.
The mill will continue to improve mill operations with the installation later this year of Metso's Causticizing Optimization along with a Metso Recovery Analyzer that will measure key chemical concentrations in the process. The goals are to stabilize and increase causticizing efficiency, stabilize residual carbonate levels and to reduce fresh lime consumption.
Mark Williamson is a journalist/engineer based in Thornhill, ON, Canada PPI
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