Wet end controls: part two of a three part series
Wet end measurements and controls can help papermakers to stay in that window where sheet quality and runnability are at their best.
As explained in part one, establishing a stable furnish flow to the machine with feedforward weight control is a vital first step for machine stability. However, more destabilizing influences can crop up in the short circulation and forming section and these instabilities can disrupt the paper web quality and lead to poor runnability, machine deposits and sheet flaws.
Papermaking furnish and chemistry has become decidedly more complex in recent years. Today, machine speeds are higher while basis weights are lower, the use of RCF and DIP is now commonplace, ash is present in many previously virgin furnishes, and ash levels are generally higher in the sheet and white water. To add to the complexity, water systems are now more closed and so-called anionic trash circulates throughout.
With these changes and complications, it is more difficult to control the stock delivery and paper forming processes compared to previous virgin fiber machines. However, wet end measurements and controls have stepped up and provided the needed stability. They can help papermakers to stay in that window of stability where sheet quality and runnability are at their best.
Stability during breaks
Headbox and white water consistency measurements are used to calculate total retention and ash retention. Retention can be optimized within a certain window but stabilizing white water consistency is the main objective, and that is accomplished through retention aid dosage control. In particular, these measurements and controls are used to stabilize the wet end during startups and web break periods when more broke containing ash is fed into the furnish. The control automatically regulates fresh filler flow to compensate for changes in broke flow. Retention aid flows are similarly adjusted to stabilize the white water consistency so the sheet is threaded easily since it is close to its quality target even before it reaches the reel. Repeated breaks are thus avoided and the sheet is on the reel more quickly.
Figure 1 shows that a copy paper manufacturer was able to improve the stability of the white water consistency by more than 60%. This was accomplished with multi-variable controls which included the feedforward weight control referred to in part one. Figure 2 shows a significant reduction in break duration since the wet web quality was never far off target even during breaks and the sheet rethreaded easily. Repeated breaks during the recovery period were averted.
Figure 1: Wet end multi-variable controls stabilized the short circulation system of this copy paper machine by more than 60%.
Figure 2: Break lost time on this machine was reduced with multi-variable wet end controls
With wet end controls papermakers now have the opportunity to control retention or white water consistency at the right level for good sheet quality and runnability. The highest retention is not necessary the best operating point so it is quite common for papermakers to adjust the targets for white water consistency within a certain window which is appropriate for the grade. This degree of selective control allows the chemicals to be used precisely and not over-used as they might have been in the past. One producer of LWC estimates 10% savings in chemical consumption; other estimates range up to 25%.
Anionic trash is neutralized
The measurement and management of anionic trash (dissolved and colloidal substances) has taken on a new importance since many paper, board and tissue furnishes contain up to 100% recycled fiber. Coated broke is another uncontrolled source. These troublesome charged substances can destabilize the electrochemistry of the machine wet end, resulting in unstable retention, machine deposits and sheet defects. Moreover, the effectiveness of sheet additives can be reduced.
Online sampling analyzers, which are essentially automated chemistry labs, measure the charge density of these fine materials and express it as cationic charge demand. That is an indication of how much fixative chemical or other additive is required to neutralize that charge. Papermakers can then decide the optimum charge balance-point where the process runs best.
The charge demand and the short circulation consistency measurements often work hand in hand to stabilize retention, sheet drainage and the efficiency of paper additives. For instance, the charge demand measurement can indicate a carryover of anionic trash from a DIP plant or from coated broke. Applying just the right amount of fixative chemical to get the right charge balance will create the electrochemical environment where starches, wet strength resins, retention aids and drainage aids will work effectively.
One Canadian newsprint producer reports that the combination of charge and white water consistency control has made a two-component retention aid program much more stable, resulting in even drainage, an even release point from the center press roll and 50% fewer sheet breaks. These benefits, plus a significant saving in chemicals, resulted in a six-month investment payback.
Closed loop control of paper additives using charge level has produced some impressive results for a European producer of multi-ply coated board. By measuring charge at four locations and by automatically controlling the flow of fixative the use of poly-aluminum chloride (PAC) was reduced by 40%, the use of sizing agents and starch in the top sheet layer was eliminated and the break occurrences were reduced by 45%. All told, the investment was paid back in 2 months.
A more stable wet end also helps to maintain a cleaner papermaking process with fewer deposits, sheet flaws and related breaks. A free-sheet producer reports a significant reduction in sheet defects caused by the deposition of hydrolyzed ASA sizing agent.
Stabilizing tissue making
Tissue machines are an important and growing application for wet end chemistry and consistency measurements and controls. They can have a positive impact on machine stability, creping performance, sheet quality and the cost-effective use of additives such as wet strength resins.
By controlling retention and immobilizing fines in the sheet the stability of creping performance and its direct impact on product quality can be improved. The Yankee coating can be stabilized and relatively free of fines contamination. Thus, the sheet quality can be optimized and the crepe ratio altered to achieve higher quality production. Blade life can also be extended. Figure 3 shows how more stable wet end charge and machine operation can be achieved with fixative chemical control.
Figure 3: Fixative control on a tissue machine stabilized charge demand resulting in a more stable wet end.
Proved their worth
Wet end measurements and controls have proved their worth over the past twenty years, however there is still potential for new applications that are aimed at optimum product quality with minimum furnish or additive cost. Substituting lower cost furnish makes sense economically but may introduce more instabilities that wet end controls can help to smooth out. Multi-ply board machines are a new challenge and applications on tissue machines are still developing.
Many papermakers want to add more ash to the sheet to reduce costs. In fact, it has been reported recently that a European free-sheet mill has successfully made quality paper at 35 % ash. Excellent wet end stability is absolutely required for that, as well as a good cooperation between chemical and automation suppliers.
In part three of this series, we will look at where multi-variable wet end and comprehensive multi-dimensional paper machine controls may be headed in the future. How can the window of stability be refined even further?
Some data and figures are courtesy of BTG and Metso