Taking wet end chemistry to the ext level: constant improvements to popular chemical strategies can take pulp and paper mills to new levels of wet end control and efficiency.
* Hew increased used of deaerators is changing wet end management
* Which developing coagulant strategies are most effective
* Which micro particulate programs hold the most promise
* "Wet end chemistry: Paring down, pairing up," Janice Bottiglieri, Solutions!, Dec. 2002, p. 26.
* To learn more about TAPPI's wet end operations short course, go to: http://www.tappi.org/index.asp?rc =1&pid=23875&ch=4&ip=
* To view previous Solutions! articles on this topic, search for "wet end chemistry" in the category "all journals" at www.tappi.org
The science of chemistry was once the realm of alchemists trying to transform base metal into gold. Paper scientists today also seek to harness chemical properties to create "gold"--paper that performs better, runs better, and sells better than the competition. The biggest difference is that chemistry today actually works.
Good paper chemistry is not merely following the latest trend. Solutions! asked a panel of industry experts to evaluate the latest developments in three specific categories of wet end chemistry:
* Deaerators in place of defoamers
* New coagulant strategies
* Micro particulate programs
In any of its several forms, air in paper stock can cause various problems. It can hinder chemical efficiencies, impede proper drainage, and negatively affect runnability. More complete system closure--currently a goal tot many mills--can increase the problem of air in a system.
Three types of air are typical to paper stock according to Al McCoy, Jr., technical marketing manager-defoamers for Vulcan Performance Chemicals. They are dissolved air that forms in liquid exposed to the atmosphere, entrained air that forms due to agitation, and foam that is air in the form of large bubbles that generally float on top of a liquid carrier. Deaerators work by applying a vacuum to white water to lower the boiling point. McCoy said he has seen deaerators effectively remove all three types of air.
"Dissolved air does not present problems in a stock system because the concentrations are very low. The micro-bubble size is so minute that drainage impairment is not generally an issue," McCoy stated. "With proper agitation and viscosity, the dissolved air can form larger bubbles and become entrained air. Entrained air has larger micro bubbles with an affinity for stock and fines. These micro-bubbles are larger than dissolved air and block drainage." The stable nature of entrained air makes relieving it from a system difficult. Foam can cause problems ranging from drainage impedance to exacerbating deposits and cavitation of pumps. This is the easiest form of air to destroy.
"The advantage of a deaerator is that it very effectively relieves virtually 100% of all air to improve drainage characteristics from the slice," McCoy continued. "The disadvantages include the capital expenditures for the system installation, the maintenance expense, and the fact that when these systems have an upset and lose the proper vacuum they generate more entrained air into the system due to agitation within the vessel. An airless liquid is 'hungry' for atmospheric equilibrium and will quickly receive air at the first point of availability. Systems using a deaerator alone therefore still have foam in the wire pit," he explained.
According to Garry Beswetherick, technical manager, Raisio Chemicals Canada Inc., a good rule of thumb is to have less than 0.5% entrained air in the headbox. "This will provide better formation of the sheet and faster drainage on the wire," he said. "Mills can use a deaerator, defoamers, or a combination of both."
Understanding synergies and incompatibilities with other wet end additives and the paper machine system is vital. "The benefits and perils of defoamers vs. deaerators are excellent examples," said Gary A. Headrick, product manager, Buckman Laboratories, Memphis, Tennessee, USA.
"Deaerator use allows the removal or reduction of defoamers that can interfere with bonding," Headrick said. "Deaerators allow for a reduction in chemical costs, more consistent test values, and reduced cull. Deaerator use can reduce deposits and sheet defects, and improve runnability."
If used responsibly or even with deaerators, chemical defoamers or antifoams often carry through the system and remain effective throughout the various sources of air entrainment according to Headrick. "Also, certain types of defoamer chemistries improve drainage not only by the release of the interfering and entrained air but also by reducing surface tension and bound water--capabilities not possessed by mechanical deaerators."
According to Jerker Nilsson, who directs system application development research and development with Eka Chemicals, "Entrained air bubbles can harm runnability by reducing equipment efficiency, leading to lower sheet quality. Surface activity from deaerators can cause an increase in dirt accumulation and negatively impact sizing.
"Not using deaerators can slow machine speed, cause more frequent breaks, reduce drainage efficiency, and hurt paper quality because of less smoothness and gloss and imperfections such as pin-holes," Nilsson noted. "One clear advantage of deaerators is that they effectively eliminate defoamers. This means one less chemical is in the wet end to cause interferences or block effectiveness of high performance chemistry. On balance, they are a trend in the right direction."
Kasy King of Papermaking Process Consulting LLC, Appleton Wisconsin, USA, agrees. "The move toward increased use of deaerators vs. defoamers provides a papermaker with one more tool to becoming a proactive manager of systems instead of a reactive fire fighter. Deaerators prevent entrained air from forming while defoamers destroy entrained air after formation. Deaerators help papermakers prevent upsets and therefore cut operating costs. Perhaps a more significant advantage is in the time saved from the fire fighting that engineers and technical personnel can then use to plan and think ahead."
Deaeration technology can easily connect to on-line sensors for air measurement and closed loop control according to King. "Once controlled, the system usually demonstrates reduced chemical costs and improved operating efficiencies. By controlling entrained air, the papermaker has an easier path for retention control, deposit control, sizing efficiency, formation, and strength."
COAGULANTS PROMOTE CLEANLINESS
Some consider alum as the first coagulant to make paper. It has been used since the development of papermaking to promote sizing, control pH, and retain colloidal substances. The trend toward alkaline conversions that began in the early 1990s fostered the development of organic coagulants or highly charged organic polymers to replace aluminum salts. Since then, low molecular weight, highly charged density polymers (coagulants) have become popular for neutralizing contaminants not only in coated paper production but also in all paper grades.
Coagulant use falls into two broad areas explained Xavier Cardoso, marketing manager, Ondeo Nalco Europe. The first area is the treatment of contaminants coming from virgin pulp (pitch) or recycled fiber (stickies). The second use is to neutralize excess anionic charge to increase the effectiveness of a retention program. "In the first case, the best strategy is to treat contaminants at the source before they agglomerate and create deposits. In the second case, coagulants find use in the short loop to reinforce the retention program."
The most effective strategies eliminate or reduce the source of contaminants originating from various fiber species--both virgin and recycled--said Mark Woiceshyn of Raisio Canada. "I favor 'coagulants' that pacify organic contaminants in virgin and recycled pulp. The addition point should be close to where these organic contaminants are released from the fibers."
The three most common areas where papermakers use coagulants for deposit control are coated broke for white pitch, mechanical pulps for natural pitch and bleaching residuals, and recycled pulps for stickles and a host of other contaminants according to Ian Thorburn, technical manager, Raisio Chemicals U.S., Inc. "Coagulants used for controlling these contaminants collectively have low molecular weight and high charge density, but they have significant differences in molecular weight, structure, charge contribution, and relative hydrophilicity," he said. "They each have very different performance characteristics. In mixed pulp furnishes, identification of the origin of a deposit and eliminating any external factors such as pH or temperature shocks are the initial steps."
"These coagulants are very selective," Easy King noted. "There are many to choose from. For any given stock stream or white water flow, one coagulant may work but others will not. Do not forget you are looking for coagulation and retention--not simply charge neutralization. Laboratory screening of these coagulants is essential to a successful machine trial. Turbidity is a very quick and easy test to use for this screening and for monitoring success on a machine.
"One other trick that has proven very successful is to treat with the coagulant and then follow with about 5-10 pounds per ton of an adsorber such as talc," King continued. "This helps in the retention process, renders stickies less sticky, and is sufficiently low in dose to avoid upsetting the process in any way. It is also cheap filler that reduces the cost of fiber."
OTHER FACTORS TO CONSIDER
According to Dean Gudlauski, technical marketing manager, retention and drainage aids, Vulcan Performance Chemicals, the most effective coagulant strategy depends not only on grade structure and furnish type but also on the level of fines, organic and inorganic contaminants, conductivity, pH, and where the problem occurs. "A coagulant may require use with a dispersant or detackifier. In most cases, you will want to retain those fines, fillers, and colloidal materials to the fibers as strongly as possible to send out with the paper sheet. Simultaneously, you want to balance the level and way these materials are being retained so they do not deposit at points of high shear.
"Grades where coagulant is a neutralizing agent to prevent deposition include LWC, alkaline printing and writing, newsprint, paperboard, and tissue," he added.
No guarantee exists that traditional coagulants such as polyamines or poly-dadmac polymers alone will effectively neutralize contaminants in coated broke and wet end operations advised Steve Tremont, director of marketing, Ciba Specialty Chemicals, Suffolk, Virginia, USA. "One effective strategy is to identify the real source of the bad actors' causing machine deposits and sheet defects clearly. After identifying the real source of the contamination such as wood pitch or stickies, a combined approach of next generation ultrahigh molecular weight organic fixatives and inorganic adsorbing agents such as bentonite or talc can find effective use to minimize costly machine downtime and off-quality paper or paperboard."
Arno de Beer, an Eka Chemicals retention specialist for Europe, agreed that coagulants have their place but said that other and often better ways exist to handle contaminants. "Today, anionic trash catchers (ATC) can be tailor-made to eliminate contaminants like wood extractires, lignin derivatives, undissolved white pitch, and stickles. As a result, system performance of retention aids, sizing agents, and other additives can substantially increase."
MICRO PARTICULATES HOLD PROMISE
Over the past several years, micro particulates have been replacing high molecular weight retention aids. Initially used for alkaline fine paper machines, micro particulate programs will likely see wider use as costs begin to drop.
"The major compelling event that drove the introduction and acceptance of microparticle retention and drainage aid systems was the alkaline conversion of the fine paper market with PCC and alkaline size to replace acid papermaking with rosin size and filler clays," said Ciba's Tremont. "The three major microparticle systems developed involved silica, bentonite, and micropolymers with cationic flocculants, wet end starch, or both." While these systems helped papermakers achieve high retention and drainage, he noted that paper formation could be jeopardized--especially on high speed top wire and twin wire formers.
According to Greg Bengtson, who leads Eka Chemicals global Compozil program, "Some years ago microparticulates often replaced polymer systems in fine paper, most notably starch/silica combinations. However, synthetic PAMs have endured as a key component of high performance retention systems. They show great potential for being a critical component of advanced nanoparticle systems for the future."
Bengtson noted that recycled board, LWC, SC and newsprint will gain significantly from advanced PAMs and silica nanotechnology combinations. He added, "New polymers are being developed to heighten silica nanoparticle effectiveness, opening new opportunities with new and evolving grades."
The number of microparticulates has increased to offer papermakers more options added Buckman's Gary Headrick. "The early micro particulate technologies using bentonite and silica led the industry to new and improved levels of drainage, formation, and retention. While the original silica system was first used with cationic starch in the absence of a synthetic flocculant, later years have seen wider use with synthetic coagulants and polyacrylamide flocculants to take advantage of the synergy.
"Today, a number of other microparticulates also find use including some new generation silicas, synthetic anionic and cationic sols, and uniquely engineered flocculants that provide similar and additional unique advantages," Headrick continued. "The most notable advantage is the improvement in filler and fines distribution. Some new programs find use in combination to provide exceptional control of drainage, formation, and other sheet properties such as opacity, printability, and formation."
How can a mill choose which microparticulate program is best? "Starch can be consumed quickly by dirty furnishes, I would suggest that microparticulates using starch will find use in grades of paper where the furnish is relatively clean--free of anionic trash," said Kasy King. "Some recent developments in high charge starches can overcome these dirty furnishes, but they are not in widespread use. Some microparticulates use polymers, not starches. These polymers are more likely to maintain their charge better than starch would in a dirty system."
According to Ron Richmond, marketing group leader for Kemira, mills initially used micro particulate systems in fine paper grades for increased retention and drainage. New high-speed dual wire machines make drainage less of an issue, "but high retention with good formation becomes more important. Microparticulate systems have an edge in giving a sheet with small, even floes that result in good formation. Grades where drainage still is vital such as heavyweight liner and corrugated medium are now benefiting from the application of micro particulate retention systems."
SEEK OUT NEW TECHNOLOGY
Said Xavier Cardoso at Ondeo Nalco Europe, "microparticles give papermakers the possibility to upgrade retention using combinations of flocculants and microparticles to obtain wet end flexibility. This allows them to increase productivity and paper quality.
"Microparticles move the compromise between retention, drainage, and formation to a higher level," he continued. Papermakers can now increase retention to keep good runnability without sacrificing formation. They can also improve the retention of cationic additives."
Ciba's Tremont agrees. "The next generation of advanced microparticle systems will allow papermakers to "de-couple" the effects of high retention and rapid drainage without compromising paper formation." He adds, "To be on the front line of papermaking and mill technology management, papermakers are encouraged to attend industry events such as the recent TAPPI Papermakers Conference in Chicago and the PIMA International Management Conference, to be held in New York in June." S!
THE HIDDEN POWER OF WET-END SENSORS: ACCURATE MD DRAINAGE PROFILING AND MEASUREMENT
By Brad Garnett, Honeywell Industry Solutions
Over the past year, Honeywell has installed a number of wet end measurement applications using a series of non-scanning sensors installed on the former to continuously measure drainage. These sensors measure the water weight of the sheet at the wet end and generate an on-line MD drainage profile, which is used to monitor wet end stability and improve sheet formation.
In one lightweight coated (LWC) application, five sensors were installed in the forming elements between the headbox and the dryline. The resulting on-line MD drainage profile shows not only the absolute water weight, but also the rate of drainage down the machine. Figure 1 shows the drainage curve on the LWC machine in terms of distance from the headbox against water weight value as measured from the five MD sensors.
[FUGURE 1 OMITTED]
This kind of on-line knowledge of the MD drainage profile allows papermakers to rapidly set up and maintain the optimum drainage rate profile to optimize formation, first pass retention and water removal on the former. The effect of changes in stock characteristics, including freeness and chemical additives, are immediately visible. The sensors provide a useful tool for both setting up the former and identifying machine process changes.
Figure 2 shows an example of the MD drainage contour display that relates to changes in the drainage profile in Figure 1. The trends show how the water weight moves around on the forming table over time. The distance between the trends identifies the rate of drainage down the table and how this varies with stock composition and process conditions. The continuous on-line measurement of the MD drainage profile provides a unique insight into complex interactions of wet end chemistry, refining, stock blending, and the former setup that have such a significant impact on both sheet quality and production efficiency.
[FIGURE 2 OMITTED]
Janice Bottiglieri is senior editor of Solutions! and editor of TAPPI JOURNAL. Contact her at firstname.lastname@example.org.
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|Title Annotation:||Paper Chemistry|
|Publication:||Solutions - for People, Processes and Paper|
|Date:||Jun 1, 2003|
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