Three case studies show how an innovative cleaning technology from Nalco has helped mills improve quality, increase productivity and eliminate excess energy use
March 2008
By Todd Pickering
An old papermaking adage states: “Quality is made in the forming section. Money is made in the press section. Energy is used in the dryer section.” Indeed, in non-tissue grades, once the sheet is formed, the purpose of the rest of the machine is to consolidate and dry the sheet without destroying what was “built” in the forming section. The press section is a key unit operation as it starts the process and can seriously impact the evaporative load in the dryer section.
Pressing is a highly dynamic process that is only somewhat understood. Much of what is known about press section optimization is learned through empirical studies. Papermakers are constantly searching for ways to improve pressing efficiency. There is much activity in the industry focused on improving pressing efficiency, including new fabric design, roll construction, press rebuilds (shoe presses, etc), and tailored cleaning and maintenance programs.
Relevance of Press Roll Condition
The predominant pressing configuration uses a hard center roll against which two pressing rolls apply pressure to squeeze the water out of the sheet. In addition, to press fabrics, the condition of the press rolls can play a critical role in press efficiency and sheet quality. Yet, roll treatment is often forgotten in efforts to improve press operation and machine speed. Contamination of hard rolls can lead to significant losses in machine efficiency: sheet breaks, shortened doctor blade life and reduced machine speeds. Losses can manifest themselves in sheet quality problems as well – holes, loss of strength, reduced smoothness and increased shrinkage.
Compromised roll condition is not always obvious. Due to the highly dynamic nature of the pressing operation, seemingly insignificant changes in roll condition can have a large impact on press section efficiency. Indications of poor roll conditioning include picking/crumbing at the doctor blades, short doctor blade life, apparent film on the roll (observe the sheet edge on the roll), higher size consumption (implies low retention which can lead to deposits on the rolls), high number of holes or wet-end breaks and stock build-up on face-side rolls).
The single most important indicator is draw. Draw is defined as the speed difference between two machine sections that is required to maintain a stable, wrinkle-free sheet transfer. Draw is directly correlated to overall machine speed and is one of the primary tools used in managing the machine. Myriad forces are in play at the point at which the sheet releases from the hard roll, including adhesion, pressure and centrifugal forces. A high draw indicates that a stronger force is required to overcome these forces and “pull” the sheet off the hard roll. When the center roll becomes contaminated, the sheet will stick to the roll, increasing adhesion, requiring higher draw and forcing the papermaker to reduce speed or risk increased sheet defects and breaks. Sheet release problems increase with higher machine speeds, increased felt life, age of the rolls, increased closure of mill water loops and decreased furnish quality. All of these factors are common realities of a modern paper mill and, therefore, the opportunity for roll treatment programs continues to grow in the industry.
Benefits of Roll Treatment
An effective roll treatment program should strive to improve sheet release by reducing the surface tension between the sheet and the roll surface. This is accomplished by a number of mechanisms, including one or more of the following:
- Dissolving or emulsifying contaminants
- Minimizing temperature and pH shock
- Applying a barrier layer to the roll surface.
A roll treatment program is applied continuously to the roll at low dosages. Nalco best practice applies the product through a fan shower placed between the double doctor blades on the center roll, Figure 1.
There are various types of chemistry solutions that can be used, including barrier, barrier hybrid, wetting agent, and solvent chemistries. Typically, the most effective chemistries are those that both dissolve the contaminants as well as provide some modification of surface tension of the water film on the roll. But each machine can be different and requires a full understanding of the relevant parameters, including furnish, grade mix, press configuration, roll cover composition, etc.
A good roll treatment program also requires appropriate showering and good water quality. Also beneficial is a release-monitoring tool, such as a laser distance-measuring device, so that the release point can be tracked on the machine’s distributed control system (DCS). A laser-monitoring tool can be very useful in proactively managing your roll treatment program by identifying and fixing changes in the machine draw before they lead to runnability problems.
It should be noted that roll treatment trials are quick to evaluate and optimize. Typically, the effect of a roll treatment program can be detected within minutes of feeding the product, making it a very easy trial to perform, Figure 2. With the proper feed equipment it can also be a very flexible program, allowing the machine superintendent the ability to tweak the treatment dosing to counter any process upsets such as changes in furnish quality, retention, and press fabric condition.
Although this article focuses on the benefits of multi-nip, center roll treatment programs, the technology can be applied to other rolls on the machine as well. Nalco has delivered gains in the form of a reduction in wet-end breaks by two per week by treating a lumpbreaker roll. Treating a third press top roll on a newsprint machine reduced third press breaks by 85% in one case. A roll treatment program on a granite roll in a straight-thru press delivered an increase of 8% in the internal bond strength of the sheet.
In this article three center roll case studies that delivered significant gains to the papermaker through the use of NAL-TEX® Cleaning Technologies for roll treatment will be studied closely.
Case Study 1
In this example, a mill with a 750-tonne/day, high-speed newsprint machine using a predominantly thermomechanical pulp (TMP) furnish experienced increased variability in its chip supply to the TMP plant, which resulted in excessive pitch deposits onto the tri-vent center roll. A previous center roll program used a surfactant base product with good surface tension reduction, but with no capable pitch solubility. As a result of pitch deposition, the mill could not achieve its speed goal of 1,400 m/min due to excessive 3rd to 4th press draw, leading to increased sheet breaks. Machine speed was limited to 1,380 m/min.
After completing a press section audit, the mill switched to a Nalco solvent-based roll treatment product. Within one hour of initiating the program, the 3rd to 4th press draw had been reduced by 50%, from 2.4 to 1.2% as the pitch buildup was removed from the center roll. The mill then reduced the dose to a maintenance level and continued to recognize gains in the form of increased machine speed and reduced sheet breaks. Speed was increased to the budgeted 1,400 m/min and sheet breaks were reduced by 60%. Furthermore, the technology was flexible, meaning product flow rates were adjusted in times of high pitch loading, allowing Nalco and the mill to work together to adapt to the reality of variable quality in chip supply. In total, Nalco’s Mechanical-Operational-Chemical (MOC) approach delivered a 5% gain in on-machine efficiency (OME), one of the key business drivers for the mill.
Case Study 2
In this example, a 1,200-tonne/day uncoated freesheet machine using blended hardwood and softwood kraft and recycled fiber expressed an interest in increasing production. Nalco performed a machine audit and identified the tri-nip center roll as a cost-effective opportunity to increase machine speed. The limitation on the machine was directly related to the increase in draw that occurred whenever the mill tried to increase machine speed. When the mill tried to increase machine speed, edge wrinkles appeared and sheet shrinkage exceeded targets, forcing the mill to limit their speed. Increased draw also created picking problems and increased wet-end breaks. There were no obvious runnability problems that had previously been attributed to the center roll. Nalco proposed a proactive approach to roll treatment that could deliver the gains the mill was seeking.
Nalco implemented a program that included a wetting agent and modifications in roll showering practices. Results of the program were immediate and dramatic. Draw was reduced, which the mill chose to take in the form of an increase of 50 ft/min in machine speed. Furthermore, sheet shrinkage was controlled and the wrinkling observed at higher speeds immediately went away with the new program, allowing the mill to maintain the higher speed. Defect survivability increased and wet end sheet breaks were reduced. The mill validated another benefit of roll treatment programs by confirming that the new program gave it the added flexibility to run higher levels of recycled fiber, thereby reducing mill costs further. Nalco best practices delivered machine efficiency gains to the customer along with higher sheet quality, improving mill profitability.
Case Study 3
In this final example, a high-speed newsprint machine running predominantly recycled fiber was experiencing problems with center roll breaks and threading up after breaks because of adhesion of the sheet to the center roll. Downtime due to breaks averaged more than 70 minutes daily. Nalco’s press audit identified a stickies buildup on the center roll coming from the recycled fiber that the current roll treatment program could not control.
Nalco implemented a new solvent-based program complete with best practice shower application. The product was able to completely dissolve the stickies contaminant on the center roll. It also acted to reduce the surface tension of the water layer on the roll, reducing the draw required to transfer the sheet from the center roll. Sheet breaks, having been caused in part by the excessive draw, dropped by more than 50%. In total, the mill realized an improvement in on-machine efficiency of more than 1%. For a high-speed machine, this was a significant improvement in profitability. In addition, Figure 3 indicates additional gains in materials costs due to an extension in doctor blade life.
Deposits in pulping and papermaking systems result in lost operating efficiency, excess energy use, off-spec product, increased material costs, and potentially hazardous working conditions. Nalco’s NAL-TEX® Cleaning Technologies was designed to address these challenges. By properly implementing NAL-TEX Cleaning Technologies, pulp and papermaker producers can improve the safety of their operations, increase on-machine efficiency, improve product quality, and optimize material costs – contributing to sustainability goals and their bottom line.
Special thanks to Pat Sturtevant, Principal Consultant and Ollie O’Neal, Jr., Staff Scientist, Nalco Company, for their contributions to this article.
Todd Pickering is product manager, Nalco Company, Paper Services Division.
