The market for paper and paperboard grades is highly competitive, leading many pulp and paper mills to diversify their product lines with new paper grades. For other mills, the focus is on cost reduction or on lightweighting. Market diversification can benefit mills’ bottom lines, yet paper-machine efficiency is usually worse at lower basis weights (BW), which are increasingly in demand.
Paper-machine efficiency issues, such as web breaks and rejection by the converter, arise because the non-uniformity and variability of key properties, such as strength and moisture profiles, are amplified on lower BW papers.
“Conventional ways of improving paper-machine efficiency include maintaining or increasing wet- and dry-web strength or upgrading sections of a paper machine,” says FPInnovations paper and consumer products manager, Frédéric Parent. “Maintaining or increasing strength entails the use of chemicals, stronger pulps, and or strength additives, while upgrading can be costly, leaving many mills to work with their existing paper machines.”
Parent says research and development centres like FPInnovations are interested in gaining a better understanding of the fundamental concerns affecting efficiency rather than replacing equipment or adding chemicals, which don’t always solve the problem.
Infrared moisture-sensor analysis
FPInnovations’ research demonstrates that moisture and drying non-uniformity are key criteria affecting web performance on paper machines and at the end-user stage. FPInnovations researchers use a portable infrared moisture sensor to measure the moisture content of the paper, as well as to evaluate machine and cross directions for moisture profiles at different sections of paper machines.
A mill experiencing several wrinkles in the dryer section increased its kraft content to eight per cent to avoid paper breaks but wanted to eventually reduce its kraft use while maintaining paper-machine efficiency.
Moisture profile measurements were carried out. The profiles were not uniform at the couch or after the press section. A slope found at the couch was present after pressing, indicating the bulk of the issue originated before the couch. After drying just before the reel, the slope completely disappeared and the cross-directional profile of the solid content was uniform. The mill’s online scanner also showed uniform moisture content at the reel.
Says Parent: “A non-uniform moisture profile entering the dryer section created non-uniform drying and tension profiles in the cross direction. Sections of the web with higher moisture content took longer to dry and led to longer sheets in the machine direction.”
The mill made changes and was able to reduce its kraft use by half, representing an annual savings of $200,000. Understanding the underlying causes of paper-machine efficiency issues could allow mills to implement low-cost and efficient solutions using existing equipment.