Managing retention, drainage and formation.

Retention, drainage, and formation are almost always critical to good papermaking, but usually are opposed to each other in their control. How do papermakers and technical engineers control retention, drainage, and formation to optimize all three without harming any one of the three? What are the control "knobs" and what are the criteria used to move each control? Also, with the increasing number of wet end additives, there are fewer locations available to add these materials. How do you get around this problem and identify the optimum addition point for each chemical? Solutions! asked several wet end chemistry experts to tackle these questions.

Managing tradeoffs is vital, said Sal Mirza global marketing manager, Buckman Laboratories, Memphis, Tennessee, USA. "The key is to get a balance of the three, or at least focus on optimizing two out of three to meet paper quality parameters," he said.

Jim Davis, business manager, retention, drainage, clarification at Hercules Pulp and Paper Division Wilmington, Delaware, noted that the properties of retention, drainage and formation are related and can be influenced by many factors--including the machine set-up, furnish characteristics, and retention program.

"Each of these areas must be used to balance properties to avoid harming one while improving one of the others," he said. "The use of advanced retention and drainage technologies, such as structured organic particulates, allows a wider operating window to work within by providing higher levels of retention and drainage at the same formation level compared to traditional flocculent retention programs. This allows for higher drainage capabilities when needed to improve formation through adjustment of the machine set up or furnish characteristics."

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Jaime Millan, team leader, retention aids group for Bayer Chemicals, noted that the differences in paper machine configurations and paper grades offer different challenges to everyone in the paper industry. Since there is not an absolute answer, Millan suggests focusing on the following:

* The specific mill's needs, whether they are creating new paper grades or improving an existing grade for a new market.

* The optimization of the paper machine retention, drainage and formation properties so that they are cost/performance effective, rather than striving for a general number without consideration of the paper machine and furnish limitations.

"Drainage is particularly important because it is related to machine productivity. Retention helps to reduce manufacturing costs, and formation offers the opportunity to differentiate your paper from others through its optical and print properties (quality)," said Millan. "The combination of the papermaker's machine experience, wet-end chemistry supplier expertise, and machine fabric supplier expertise will lead to the best solution for that specific machine.

"First, know your paper machine capabilities and match them with the desired final paper grade quality," Millan continued. "Second, create a team with specific objectives (mill technical/engineering, mill production, chemical supplier, fabric supplier). Finally, generate an action plan with the recommendations and data generated from different sources (laboratory/pilot machine studies, analytical evaluations, paper machine audits and benchmarking comparisons)."

Steve Tremont, Marketing Director for Ciba Specialty Chemicals, Water and Paper Treatment, Suffolk, Virginia, noted that, for papermakers, optimizing retention, drainage, and formation (RDF) is a balancing act of controlling wet end mechanical and chemical variables.

"The next generation of advanced microparticle RDF systems combine the synergies of inorganic microparticles and organic micropolymers to 'de-couple' the effects of high retention and rapid drainage to fine tune the wet end and optimize formation," he said. "For continuous monitoring and control of the microparticle and micropolymer systems, an 'on-line' closed loop control system measuring first pass and ash retention are commonly applied for maximum machine efficiency."

Laura Schmitt, product specialist for Nalco Company, Naperville, Illinois, said that controlling RDF helps papermakers achieve desired final sheet properties. She suggested using a "Mechanical, Operational, and Chemical" (MOC) approach to balance critical wet end interactions.

"This approach uses all tools available in the manufacturing process to optimize the wet end for maximum performance, and ultimately helps the papermaker meet his or her goals," she said.

Greg Bengtson, global leader for Compozil, Eka Chemicals' nanoparticle system for the wet end, said that retention is best controlled by closing the loop with on-line monitoring and controlling whitewater solids with C-PAM dosage.

Said Bengtson, "Drainage should be a combination of mechanical and chemical control strategies optimized by using nanoparticle silica chemistry, the 'toolbox and tools' to work toward optimal formation." Noting that peak formation is really about in-depth understanding of a particular machine's idiosyncrasies and fine-tuned chemistry, Bengtson sees recent advances in chemistry capable of allowing "decoupling" of the three parameters, but complete decoupling as impossible.

Added Patrik Simonson, Eka Chemicals' senior application specialist, retention and dewatering, "The balancing act between retention and drainage critically impacts formation. Better retention produces stronger and larger floc size, and sometimes overly fast locking of the floc structure. Papermakers can compensate for this condition by reducing flow box concentration, increasing machine speed, or doing more refining.

"Simply stated, a robust retention and drainage system allows papermakers room to optimize, according to their objectives," Simonson said. "In most applications, a late application of C-PAM should be optimized at minimum dosages for best formation. This also depends on dynamic conditions in the wire section."

TWEAKING THE CONTROL KNOBS

The next key question in managing RDF is knowing what the control "knobs" are and the criteria used to move each control. According to Mirza, sheet quality and production are the main criteria. "Each paper machine or process will have its own key control parameter and sweet spot for each parameter. Some of the 'control knobs' include couch solids, whitewater solids, press solids, headbox consistency, filler loading in whitewater or headbox, short fiber, broke usage, mechanical refining, headbox parameters (J/W ratio), headbox head, and slice opening," he said.

Davis noted that while the control points can be specific to each machine, furnish and mechanical adjustments are typically made first to balance the machine operation and the sheet properties. "The introduction or adjustment of the retention program can then be used to increase retention and drainage," he said. "Finally, a flexible program will allow the retention and drainage features to be converted through mechanical optimization to improved formation benefits."

Millan categorized the control knobs as follows:

* Furnish parameters (composition, refining level and properties)

* Wet-end additive program (type, dose, sequence and expertise)

* Headbox/former operation parameters (J/W ratio, L/b ratio, former turbulence/activity, dewatering devices/measurements).

"The criteria to move each one will depend on the paper grade quality targets," said Millan. "A chart with operational parameters versus the effect on paper properties is very useful for the paper machine crew."

Bengtson pointed out that with retention, C-PAM is usually measured by whitewater solids. On the drainage front, nanoparticle is measured by couch vacuum, couch solids, steam pressure, former flowboxes, or some combination.

"In addition to traditional on-line monitoring and control, we are now exploring on-line NIR (near infrared) technology to help a papermaker improve paper properties, chemical/filler content, and achieve a more precise understanding of what is happening in the sheet while it's being made," he said.

Tremont noted that, in applying the recently patented "de-coupling" RDF technology, inorganic microparticles are applied to control the drainage while organic micropolymers control the fiber and filler retention to achieve maximum retention, drainage, and formation.

Schmitt said that, when looking at the different chemistries, several control knobs can be used. "First, coagulants and macro-coagulants help balance the charge of the wet end," she said. "This is essential in the retention of detrimental colloidal substances in recycled and closed systems. Managing this charge allows for a cleaner, more efficient process that produces better drainage and retention without hurting formation. Second, flocculants are high molecular weight polymers that are used to retain fiber, fillers, and fines."

The addition of a flocculant allows papermakers to substitute lower quality furnish and increase filler loadings because of its high bridging mechanism, Schmitt added. "The addition location and the dilution ratio of the flocculant are critical to understanding its impact on sheet structure," said Schmitt. "Finally, microparticles are used to increase ash retention and drainage. Microparticle use can have a significant impact on final sheet properties because of its ability to re-floc a fiber after a shear point. The management of all three components gives the papermaker the tools to adjust his or her mechanical and operational knobs."

OPTIMUM ADDITION POINTS

With the number of wet end additives growing, placement of addition points for paper chemicals has become a key issue. According to Buckman's Mirza, each additive must have a clear purpose and perform effectively and efficiently. To do this, each additive should have sufficient contact and mixing time for it to react with the furnish. "Papermakers should carry out a system profile to match product performance needs with wet parameters such as temperature, pH, conductivity, hardness, DCS, SCS, and with mechanical issues such as mixing and shear conditions."

Hercules' Davis noted that, although there are many potential wet end additives, the key is to understand additive interaction and design compatible programs to achieve the needed result with the least number of additives and the lowest operating cost.

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"Retention programs are often one of the most complex programs in the wet end. Choosing one that is flexible and has broad compatibility with other additives can help simplify the wet end and avoid running out of places to add more additives," said Davis. "The optimum additive feed point is selected through an understanding of all of the chemistries, experience on that particular machine and other machines, and--in some cases--through lab testing. It is also critical to inject the additive properly to achieve good mixing and optimum performance."

Papermakers should also consider alternative strategies for reducing the number of additives in the wet end. For example, a new surface size allows the papermaker to take size additives out of the wet end, maximize retention and drainage, enhance wet end cleanliness, and deliver sizing at the size press, said Davis.

Bayer's Millan believes that minimum chemical usage is a key factor. "We believe that 'less is better' in terms of wet-end additive application," said Millan. "The challenge is to select the proper chemical additives to perform and achieve the retention, drainage and formation objectives while setting up for a specific paper grade. The wet end is a system and there are interactions between all the components (chemicals, fibers and fillers). This cannot be forgotten at the time the wet end chemical program is evaluated. Paper machine audits (fiber charge and cationic demand profiles, retention/drainage evaluations) help to select the proper chemicals and addition points. However, paper machine trial results determine and help to optimize the chemical wet-end program."

Eka's Simonson suggests that, even with the increased number of wet end additives, there are many available addition points. Said Simonson, "The more limiting issue is that available points are often inadequate for good mixing of chemicals and furnish, leading to dosage ineffectiveness--especially on paper machines with high water closure. In most cases, one does not identify the optimum dosage point but optimizes the product(s) properties to the available points. This relates mostly to molecular weight and charge densities of cationic chemicals, such as ATC, C-PAM, and starch."

Eka's Bengtson puts it this way: "As a provider of productivity and end product solutions for papermakers--applying high performance chemistry tools--we must proceed with a dosage plan only after precisely understanding goals and objectives on a grade and machine specific basis. Smart chemistry also means the use of fewer functional chemicals and carefully selected high performance chemicals at lower dosages."

Nalco's Schmitt pointed out that understanding the chemistry and charge of the different additives when designing or adding to a wet end program is critical. "Some products require more contact time with the fiber (or another chemistry) while others should be fed close to the headbox to provide maximum effectiveness," she said. "Products of different charge should be fed apart from each other, but co-addition of certain chemistries has been shown to provide benefits over feeding them through separate feed points. Chemical suppliers can and should help identify the optimum point for each chemical," she concluded.

RELATED ARTICLE

WHAT YOU WILL LEARN

* How to optimize retention, drainage and formation.

* What are the key "control knobs" in wet end chemistry.

* Optimum chemical addition points.

ADDITIONAL RESOURCES

* "The ups and downs of specialty chemicals," by Kasy King, Solutions!, December 2002, Product Code: 02DECS03 (Enter product code on www.tappi.org in the search engine).

* "Taking wet end chemistry to the next level," by Jan Bottiglieri, Solutions!, June 2003, Product Code: 03JUNS048.

* "A marriage for performance sake," by Kasy King, Solutions!, April 2003: Product Code: 03APRS029.

EDITED BY ALAN ROOKS, EDITORIAL DIRECTOR