The market for paper and paperboard grades is highly competitive leading many pulp and paper mills to diversify their product lines, including producing new paper grades with growth potential, while for other mills, the focus is on cost reduction or on lightweighting.
Market diversification can positively impact 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. Many mills are restricted to using their existing paper machines with minimal capital investment on equipment and rebuilds.”
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. FPInnovations’ focus has been on mapping out uniformity in papermaking and at the end-user stage. The long-term goal is to reduce production costs while maintaining efficiency and quality.
Infrared moisture-sensor analysis
FPInnovations’ research demonstrates that moisture and drying non-uniformity are key parameters affecting web performance on paper machines and at the end-user stage. A portable infrared moisture sensor is used 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, where the paper is accessible. The sensor is a reflecting type in the Z-direction and is usually installed at a distance of 1.5 inches from the web surface. The ratio between the two main wavelengths is proportional to the moisture content in the paper. In sending these signals onto the moving web, the altered and non-altered wavelengths are reflected back to the sensor. When compared, the moisture content of the web can be determined.
A case in point outlines how tackling the root cause of disturbances can be in a mill’s best interest.
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.
“A possible link was made between the non-uniform solid-content profile and wrinkle formation in the dryer,” 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 improved dewatering uniformity in the cross section, as well as corrected the front-to-back vacuum difference at suction boxes. The mill was able to reduce kraft use by half, representing an annual savings of $200,000.
A large part of paper-machine efficiency lies in a mill’s knowledge of operations and uniform profiles at the wet-end, which can improve paper-machine runnability. Understanding the underlying causes of paper-machine efficiency issues could allow mills to implement low-cost and efficient solutions using existing equipment.