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PULP BLEACHING
To reduce bleaching costs and to stabilize fiberline operations, the Oglethorpe, Ga., mill replaced the digester top and upgraded the inside
By Kelly H. Ferguson, Editor
Weyerhaeuser Modernizes Flint River with Continuous Digester Overhaul
Pulp and paper mills in North America have a unique challenge these days when it comes to modernizing operations—make it count. While it’s easy for mill engineers and managers to recognize the need for upgrading older equipment, the task of justifying a major capital outlay is much more difficult.
Managers and operators at Weyerhaeuser Co.’s Flint River fluff pulp mill in Oglethorpe, Ga., had long recognized that the mill’s pulping operation would require upgrading to satisfy continuously improving product quality and environmental performance requirements. However, business pressures dictated that the mill develop an upgrade plan that would fit into a long-term strategy that includes, among other criteria, good return on investment and meeting Weyerhaeuser’s overall “minimum impact manufacturing” strategy required for every company facility.
In mid-1996, after almost two years of planning and work, the mill’s “brown side” optimization program was complete and started up. The result includes more stable operation of the mill’s continuous digester, improved pulp properties leaving the digester, better yield, more efficient use of bleaching chemicals, and significant environmental improvements.
In addition, as an outgrowth of this project, the Flint River mill has become involved in Project XL, a Clinton administration program aimed at “developing and implementing alternative strategies that replace or modify specific regulatory requirements, produce superior environmental performance, and promote greater accountability to stakeholders.” In the July issue, Pulp & Paper will profile the Flint River mill’s involvement in Project XL and discuss how this fiberline modernization laid the groundwork.
DRIVE FOR OPERATIONAL STABILITY. The Flint River mill was built by Procter & Gamble in 1980 (operated as Buckeye Cellulose Corp.) to produce fluff pulp and was one of the facilities Weyerhaeuser purchased in 1993 when P&G decided to divest its pulp operations. The mill has traditionally been on the leading edge of pulping and bleaching technology, installing an oxygen delignification system as part of the original startup and converting to 100% chlorine dioxide substitution in its bleach plant in 1989.
Dick Martiny, a long-time Flint River employee who led site optimization for the mill and who is now retired, said mill management was aware of the “evolution” going on in the paper industry. “We recognized that some of the practices that our industry used in the late 1980s would not be acceptable in the 1990s,” he says. “We had to look at where we were economically, where we were environmentally, and where we were in our development of people systems to prepare for the coming years.”
To address environmental issues, the mill began examining many of the new bleaching technologies—for example, ozone, peroxide, and enzymes—that have been widely discussed in the industry. But, as Martiny puts it, the discussion of improved bleaching sequences always pointed to the importance of a well-operating pulp mill. Thus, the mill began focusing attention on digester operation.
The mill’s original fiberline, as designed, had performed satisfactorily since startup. The line included a two-vessel steam/liquor phase digester designed to produce pulp at 65 kappa. Pulp was blown from the digester to two blowline refiners and then washed in a two-stage diffusion washer. Washed pulp was sent through a three-stage cleaning system, thickened, and then fed into the mill’s high-consistency oxygen delignification system. The demand on the O2 delignification system was to take pulp at kappa in the mid-60s and drop it into the mid-20s.
The problem with the original design, however, was that the system was very sensitive to variations in both raw material and operating parameters all the way through the fiberline. This created problems with operating consistency. In addition, pulp cleanliness (based on dirt-count reduction) was not as effective as laboratory trials had shown it would be.
The first step was to remove the blowline refiners. This meant that the demand on the digester was to cook to lower kappa numbers—first into the high 40s, and then into the mid-30s just prior to the digester modification project—before pulp entered the O2 delig system. In 1990, the mill added a pressure diffuser to the digester blowline. Meanwhile, the mill continued forcing the digester to operate at a higher throughput.
Martiny says the product would not have been widely accepted by other customers serving markets beyond the branded absorbent products served by P&G. When Weyerhaeuser expanded the mill’s customer base, the system had to be modified to improve product quality.
One of the most affected areas of the digester was the wash zone. Martiny comments that the original digester included an efficient wash zone, but one that was designed for high-consistency, high-kappa pulp. Studying the problems that occurred in the digester bottom revealed that they actually originated in the top of the digester.
The major issue was how the chips were first distributed and then flowed from the top of the digester to the bottom. The dome-topped original design caused uneven distribution of chips, sometimes creating higher chip piles on one side or even producing a donut shape. This uneven shape then caused channeling, with some chips moving through the steam and liquor phases too quickly and resulting in improper cooking. Thus, the mill experienced an increased amount of rejects.
Gary Dees, Flint River’s pulping technology manager, says the digester problems manifested themselves in a number of ways. “We were seeing variability in kappa and variability in rejects,” he says. “Also, to get this digester to operate at the level of reliability that would support the rest of the plant, we had to lower the pins target to the digester below what we felt was most economical.”
The key to much of these problems was at the top of the digester. “We believed that if we could get good control of that feature at the top, we could do a much better job of washing at the bottom,” Martiny says. “But if you have a variation in the top of a vapor-phase digester, it multiplies all the way down.”
At the same time the chip feed problem was being examined, new digester developments were being introduced by the major equipment suppliers. The suppliers claimed that significant improvements could be made in a digester’s ability to cook to lower kappa numbers. The mill spent almost a year analyzing these options and running models to determine what the best approach would be.
A RADICAL STEP. The chosen approach was to cut the top off the digester and add a different top that would even out chip distribution. At one point, the project engineering team had even put together a proposal for a low-cost project that would have focused solely on that goal. Martiny points out that such a project would have only been a short-term solution, without satisfying greater long-term business or environmental goals, such as the minimum impact manufacturing (MIM) strategy.
“Doing the project the right way meant doing more than just changing the digester top,” he says. “We needed the ability to serve the needs of all stakeholders. These business requirements were driving us to a project with broader scope. And from an environmental standpoint, we were committed to minimizing our impact while utilizing sound business practices.”
Working with Kvaerner Pulping to analyze the various process models, the mill decided to modify the digester top and convert the unit to the supplier’s IsoThermal Cooking (ITC) process. This would provide an extended cook at more moderate conditions. While the digester width could not be changed, the new top added an extra 60 ft to give a full hour of cooking in the digester (Figure 1).
Dees points out that such modifications, based on laboratory tests and process modeling, would yield a number of benefits. “The initial expectations of the modified digester were that we could lower the kappa number by 37% without sacrificing wood yield or digester rate. That would help us get even better performance in the O2 stage, improve viscosity, and send a lower kappa pulp into the bleach plant.
“Combined with improved washing in the digester, we expected that our bleaching efficiency would improve and overall bleaching chemical costs would decrease. As a secondary goal, we felt the digester modification would allow us to improve our wood yield, by returning to more normal Weyerhaeuser levels of pins and fines fed to the digester.”
Another interesting aspect of the project were the structural studies that went into adding the new digester top. The structure’s foundation was an issue from the beginning, according to Lee Fink, the modernization’s project engineer.
“We completed three different foundation studies—one by Kvaerner and two by independent parties—before we were convinced that our foundation would support the extra mass of this digester,” Fink says. “But the foundation was extremely crucial not only for the mass, but even more for the increased height and the torsional movement that would occur in a strong wind.”
PROJECT TIMELINE. The mill chose Kvaerner as its engineer, procure, and construct (EPC) contractor for the project. Parts of the project were performed while the digester was in operation and during three outages spaced over almost two years—Nov. 1994, Sept. 1995, and Apr. 1996.
Fink says that part of the project was done using pre-appropriation funds. After the first shutdown, the project team began putting together a full appropriation request for the entire project and negotiating the contract with Kvaerner. The full project was approved by the company’s board of directors in June 1995.
The first shutdown included installation of a complete set of new wall screens to prepare for isothermal cooking circulation. The digester’s existing trim screens were also replaced by a new set of screens. However, the digester continued to operate based on its original design.
The second shutdown, approximately ten days long, was scheduled primarily to perform minor work, but also included the installation of a new secondary knotter to replace two original secondary knotters (not part of the Kvaerner project scope). Fink says that, in scoping the project, it was determined that the existing knotting and screening system could not operate effectively at the new targets; so the system was modernized.
The final shutdown, which lasted about 28 days, was used to cut the top off the digester and replace it with the new top. The new top included a new set of trim screens, new strain-gauge mechanical chip level indicators, new central pipe, and a new inverted top separator. Also during this shutdown, new extraction screens were installed, and the digester’s original washing screens were replaced with box screens. This gives the new digester a total of five digester screening areas, which include trim circulation screens, extraction screens, upper ITC circulation screens, lower ITC circulation screens, and wash circulation screens. A number of heaters and liquor pumps were also installed as part of this outage.
As the name implies, the isothermal cooking technology provides an even temperature throughout the digester. The technology uses higher circulation flows to produce even alkali and temperature profiles, resulting in an even cook at the center for the digester as well as at the digester wall. To achieve this even cook, the digester must operate at a positive dilution factor; thus, there was the need to modify the Flint River mill’s digester’s screening and circulation system.
Beyond the digester modification to isothermal cooking, the mill also:
• Changed the impregnation vessel and digester outlet devices from constant-speed drive units to variable speed AC units
• Installed an injection system that sends knots back into the chip bin
• Modified the condensate collection system with the addition of three new heat exchangers and reuse of two existing heat exchangers; also included a new in-house designed condensate return system for all of the new heat exchangers
• Performed a major upgrade to the fiberline’s motor control center room, adding about 25% more capacity.
The fiberline’s control system was also modernized. Dees comments that a Honeywell-Measurex advanced control package had been installed several years prior to the modification project. However, due to the inconsistent operation of the digester the usefulness of the package suffered, especially in the impregnation vessel and for digester-chip level control and dilution.
“It’s very hard to control the dilution factor when you’re running negative dilution factors at times,” Dees says. After the mill started up the digester in 1996, the mill worked with Honeywell-Measurex to design the advanced control package for the ITC process. The control package has been online since early 1997, and Dees says the mill is averaging about 97% utilization on all programs within the package. The mill also added some automatic analyzers, such as a Kajanni online kappa analyzer, to reduce the amount of manual sampling the mill had previously performed.
In all, Fink says, the mill project came in on time during all three outages and was under budget, due in part to the fact that some equipment—e.g., a white liquor oxidation system and a foam control system—was not needed and not installed.
THE PEOPLE ASPECT. From his perspective, Martiny acknowledges that this project allowed the mill’s staff to stretch its capabilities. “The staff at this site had been operating basically the same flow sheet for almost 12 years, and it was time for them to take on the next tough challenge,” he says. “With this project, operators began getting better control of digester parameters and the liquor cycles.”
John Andrews, a process engineer at the mill who headed up the training effort, says the original thinking was that training for this project would be easy. “We already had trained the 12 digester operators,” he says. But during a team meeting one day, members began examining how the operating philosophy would be changing once the modified digester started up.
“I started listing all the additional people that really needed to be trained,” Andrews says. “Eventually, the list had more than 100 names on it. Besides the basic operating changes, there were so many items that, at first, seemed minor. But after further consideration, the need for more training was clear.” Recognizing the entire realm of modifications made in such areas as different types of valves, the change in drive systems, etc., Andrews’ list grew even more. It now included digester operators, bleach plant operators, steam and power operators, screening operators, mechanical and electrical maintenance, production coordinators, and some of the mill’s management team.
Training took several forms, including both traditional and “high-tech.” Besides the familiar classroom and hands-on training, the mill also used a PC-based digester simulator, developed by Kvaerner. In addition, Weyerhaeuser worked with Training Systems Inc. and the project’s major suppliers to develop an interactive, multi-level CD-ROM-based training package that included proficiency testing of trainees before they could proceed to the next level.
BENEFICIAL RESULTS. As already mentioned, the project was completed on time, under budget, and with an outstanding safety record. The project team now indicates that the digester is meeting all performance expectations. The project has significantly reduced the operating variability of the digester and improved overall fiberline performance.
Dees points out that the mill is operating the modified digester somewhat differently than originally designed. “Through some testing and trial and error, we find that we don’t need to run the upper ITC circulation system,” he says. “So, we have elected to leave that part of the system offline, using it as a spare set of extraction screens. We still achieve the targeted benefits and have exceeded everything we hoped for this project.”
Dees adds that with the use of the advanced control package, digester runnability problems are “virtually nonexistent.” “We use advanced control totally, with less need for operator interface at the basic control level,” he says.
In looking at the three primary drivers of the project—better economic performance, improved product quality, and reduced environmental impact—all areas were affected positively. The optimization project, as mentioned earlier, played a major role in the Flint River mill’s Project XL performance. The improved cooking process, combined with the mill’s existing oxygen delignification system and 100% chlorine-dioxide bleaching process (the bleaching sequence is D-Eo-D), enables the mill to exceed the EPA’s Cluster Rule effluent guidelines and evolve toward a 10 m3/day bleach-plant discharge. The project offset increased steam use and prevented the need for a Prevention of Significant Deterioration (PSD) air permit.
Other benefits noted for each major project driver include:
Production improvements
• Pulp production target has been increased by an average of 1.5%
• Chlorine dioxide use has decreased by more than 20%
• Caustic use has decreased by more than 30%
• Defoamer use has decreased
• Cost for trucking knots to landfill has been eliminated
• Maintenance materials have been significantly decreased
• The operation requires fewer pulp mill operators
• Condensate return has been increased by 38%.
Process/product quality improvements
• Digester has proven the capability to produce at new kappa targets
• Uncooked chips at the digester blowline have been reduced to 0.4%
• Digester dilution factor has increased to 1.1
• Blowline kappa standard deviation has been reduced to 1.45
• Brightness standard deviation from the D2 tower has been reduced by 26%.
Environmental improvements
• Color (lb/a.d. mt) from the bleach plant has been reduced by 55%
• Biological oxygen demand (lb/a.d. mt) in the bleach plant effluent has been reduced by 48%
• Adsorbable organic halide level (kg/a.d. mt) has been reduced by 9%
• Landfill life has been extended by 9.5%
• Total suspended solids (lb/a.d. mt) has been reduced by 40%
• Chemical oxygen demand (lb/a.d. mt) has been reduced by 30%
• Total mill water use has decreased by 6%.
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