To control a paper machine wet end, you must first measure and understand the process. Then you can take effective actions that will result in quality and runnability improvements," says Stephane Deshaies, coordinator of technical services at Abitibi-Consolidated Inc., Belgo Div., in Shawinigan, Que. Deshaies was describing the Belgo newsprint mill's continuing program to improve the stability of its white water systems using online retention and white water consistency measurements.
By concentrating first on stabilizing the white water system of one of its four paper machines, the Belgo mill has learned a great deal about how upstream pulp quality and white water management strategies can affect wet end stability and paper quality. By making process changes and by better control of furnish quality, the mill has improved wet end stability and has seen improvements in paper quality and efficiency.
Figure 1.Wet end instrumentation on the Belgo mill's No. 6 paper machine includes the RM-200, RMi and LC-100-different generations of sensors that measure white water solids in the headbox, tray water, and common white water, respectively.
In a similar continuous improvement program at the company's Laurentide Div. in Grande-Mere, Que., online white water consistency and retention measuring instruments have been used to stabilize white water consistency. This mill produces groundwood-containing printing grades with clay in the furnish. The mill has implemented a unique control strategy that uses paper machine disc filters to stabilize the solids content in paper machine surplus white water going back to the groundwood mill. Most significantly, it has been able to control tray water consistency at much higher solids levels and, consequently, has been able to reduce the costs of retention polymers.
In both cases, the mills started their improvement programs by installing online instruments, carefully studying the results, and then implementing stepwise improvements that still continue today.
LEARNING THE PROCESS. The Belgo mill selected the No. 6 paper machine as the machine that would benefit most from wet end stabilization. The machine produces standard newsprint from 80% thermomechanical pulp (TMP) and 20% deinked pulp (DIP) furnish. This machine shares a common white water system with three other machines in the 1,000-tpd mill. All four machines are equipped with top wire formers.
Starting in 1997, wet end instrumentation on the No. 6 paper machine was upgraded to include two Kajaani RM-200 sensors from Neles Automation that measure consistency in the headbox and the top wire tray water. Also, a new Kajaani RMi was installed to continuously measure the consistency of the tray water from the bottom wire. This latest measurement is used to continuously control the tray water consistency by regulating polymer flow. Deshaies says that several of this instrument's new features make it suitable for stable online control. These include zero calibration with fresh water, intelligent alarms, and sample flow measurements. A previous-generation LC-100 sensor measures the consistency of the common white water used to dilute the TMP, DIP, and broke furnishes. Figure 1 shows the wet end instrumentation on the No. 6 paper machine at the Belgo mill.
When tray water consistency was first put on control, the polymer control valve often ran out of control range, cycling between the maximum and minimum flows imposed by the limits set in the control strategy. Often, this undesirable "bang-bang" type of control is not the fault of the individual control loop. In many cases, the polymer dosage control cannot compensate for uncontrolled variations in upstream furnish conditions that affect tray water consistency. If this happens, the process is basically out of control range until the upstream conditions are leveled. This was the challenge for the Belgo mill's process improvement program.
"Before the wet end measurements, we knew there were some things happening on the paper mach-ine, but we didn't know the impact," explains Deshaies. "Once we had the instrumentation, we could see the impact, and then the solutions became clear."
Stéphane Deshaies, coordinator of technical services at the Belgo mill, reports that wet end sensors have led to better overall white water management.
CHANGED STRATEGIES, IMPROVED QUALITY. The Belgo mill next made several changes to improve the stability of the pulp furnish and the white water management strategy. Most significantly, Deshaies notes that paper machine white water was segregated from the TMP dilution water. Prior to the changes, variations in the flow of low consistency TMP mill dilution water, at 0.05%, were causing instabilities in the consistency of the common paper machine white water, at about 0.4% consistency. Now, the TMP dilution flow to the paper mill has been shut off and less excess white water from the paper mill is being used in the TMP mill. This has resulted in a more stable control over furnish dilution and more stable tray water consistency
Ken Godin, senior process engineer at the Laurentide mill, says that “chemical savings alone have justified the purchase of the online sensors.”
Upstream furnish quality was also enhanced. Broke consistency control was improved by mechanical changes and piping changes in the broke thickener and also by updating the control tuning. The mill also implemented a closed loop control on TMP freeness using an online pulp quality measurement.
The changes made so far in the Belgo mill's program have resulted in better control over the wet end stability on the No. 6 paper machine (Figure 2) and have reduced variations in paper quality (Table 1). During the program, mill efficiency and production levels steadily increased. At the same time, the linting tendency of the paper has been reduced.
"The paper from the No. 6 machine rates as the best amongst several suppliers to an important newspaper printer," describes Deshaies. "To reduce linting, we eliminated the sources of variability in the process, including the TMP, white water systems, and in the broke systems."
The Belgo mill will continue to use the online measurements to further improve wet end operations. The RMi sensor includes measurements of conductivity, pH, and temperature. These measurements will help the mill to optimize the No. 6 paper machine wet end operation in response to changes in DIP percentage in the furnish and DIP quality. Also, online fiber length measurement included in the RM-200 headbox stock measurement will be used to optimize the furnish blend of TMP, DIP, and broke.
SMALL STEPS ADD UP AT LAURENTIDE. Ken Godin, senior process engineer at the Laurentide Div. in Grande-Mere, Que., describes the continuous improvement program as "a series of small steps that add up to progress when they are implemented." To accomplish this progress, Godin says that "perseverance, discipline, and follow-up" were crucial.
The Laurentide mill's program was aimed at improving stability of the No. 11 paper machine's wet end and the surplus white water going back to the groundwood mill. The program started with the installation of three Kajaani RM-200 consistency sensors from Neles Automation that measure total solids and ash content on the headbox stock, the bottom wire tray white water, and the surplus white water used to dilute the pulp furnish. The No. 11 paper machine produces 600 tpd of specialty printing papers from furnish containing stone groundwood, ultra high-yield sulfite, kraft, and clay. The move to higher sheet ash levels was the driving force behind the white water stabilization program.
"Initially, we were more interested in using online measurement systems to obtain information on wet end characteristics than in using these systems in closed-loop control stategies," says Godin. "But, the No. 11 paper machine used the most retention chemicals in the mill, so there was a significant economic benefit available by cutting back on retention aid use."
The mill evaluated the RM-200 sensors by a detailed statistical evaluation study using a technique called "evaluating the measurement system" (EMS). The study defined the precision of the solids measurements and their suitability for control purposes. This study also benchmarked the paper machine's process variability for comparison to future improvements. Based on this study, Godin says that the machine "was less stable than we thought."
TABLE 1. The furnish and wet end improvement programs have reduced paper quality variability at the Belgo mill.
UNIQUE DISC FILTER CONTROL. Using the RM-200, the Laurentide mill found important variations in the total solids and ash content in the surplus white water going back to the groundwood mill. These variations were also linked to variations measured at the paper machine headbox and in the tray water. Not all the white water on the No. 11 paper machine could be filtered using the tandem disc filters, so when white water flows and solids levels fluctuated during speed or grade changes, the ratio of untreated white water to treated white water also changed. This destabilized the screen room solids levels.
FIGURE 2. Common white water variations have improved considerably by changing the white water handling strategy.
To stabilize the surplus white water solids, the mill implemented a strategy to alter the ratio of treated white water to untreated white water by adjusting the rotation speed of the filters (Figure 3). This strategy, which is now on automatic control, takes out the longer-term variations in white water solids by compensating for paper machine speed and grade changes. The mill is now investigating ways to reduce short-term variations.
FIGURE 3. A strategy to control the disc filters stabilizes solids levels of surplus white water transferred to the groundwood pulp mill. The RM–200 online solids sensor controls rotation speed of the filters to adjust the ratio of treated to untreated white water.
This change in control strategy, plus changes in broke handling strategies, have produced significant improvements in white water stability. Godin reports that the variability of surplus white water solids has been reduced by 54%, and tray bottom wire tray water solids are 27% more stable.
MANAGING RETENTION TO SAVE CHEMICALS. Tray water consistency is now under control using modified control charting procedures. The improved stability has allowed the Laurentide mill to intentionally increase tray water solids by reducing polymer addition rates. With the online measurement, Godin reports that the mill is now able to confidently run at total solids levels of 7.0 g/l compared to 4.0 to 5.0 g/l before the change to lower retention levels. This major change in white water solids levels has resulted in significant chemical savings on the No. 11 paper machine. Also, as a result of other machine improvements, speeds have been increased from 980 mpm to more than 1,100 mpm on the average.
"The chemical savings alone justify the purchase of the online sensors," says Godin. "We have had no apparent effects on paper quality or runnability, even though the ash content is higher and we are running at higher speeds."
The mill is continuing the program to reduce process variability. During the summer of 2000, they also implemented a continuous headbox ash control loop to stabilize sheet ash levels.
Mark Williamson is a freelance writer based in Thornhill, Ont.
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