NONDESTRUCTIVE TESTING: Optical process control reduces papermaking waste and cost

Researchers at Lawrence Berkeley National Laboratory (LBNL; Berkeley, CA) and the Georgia Institute of Technology (Atlanta, GA) have successfully completed the first full-scale trial of a prototype noncontact laser ultrasound system that monitors paper quality during manufacture.

Currently, paper quality is determined after the fact. A 15- to 30-ton roll of paper is manufactured and then a few samples analyzed at the end of the roll. If the sample measurements don’t meet specifications, the entire roll is recycled into pulp or sold as an inferior grade of paper. To avoid such substantial losses, manufacturers tend to overengineer their paper by adding excess pulp, erring on the side of strength rather than weakness. This practice wastes energy and raw materials, however.

By using a noncontact laser system to monitor paper-quality parameters during manufacture and to provide process-control feedback on the fly, the LBNL-Georgia Tech team hopes to optimize the process and ultimately save about $200 million in energy costs and $330 million in fiber costs annually in the U.S. alone.

“This is the first full-scale demonstration of the sensor on a commercial papermaking machine while it’s in operation,” said Paul Ridgway in describing a two-week test conducted last winter at a Boise Cascade paper mill in Jackson, AL. “Boise Cascade engineers considered the trial to be quite successful and are hopeful that a six-month trial will be conducted at the same mill.”

Ridgway developed the laser-sensing system along with principal investigator Rick Russo at the LBNL Environmental Energy Technologies Division, and also in collaboration with researchers at the Georgia Tech Institute of Paper Science and Technology. Pilot testing of the system, in preparation for this year’s full-scale demonstration, was conducted two years ago at the Mead Paper research center in Chillicothe, OH.

Shock waves

The sensing system consists of a pulsed Nd:YAG laser to generate ultrasonic waves, a Mach-Zehnder interferometer to measure wave propagation, a rotating-mirror system to compensate for paper motion, and software for mechanical-system control and data collection. The actual sensing interface, along with temperature and position-control systems, was housed in an aluminum enclosure and mounted above the paper web on a scanning platform (see figure). Cables and optical fiber connected the remote instrument platform and laser, respectively, to the mounted sensing interface.

The pulsed laser delivered a 1.06-µm, 5-ns pulse to the paper web, traveling initially through the optical fiber and then through a 10-mm-focal-length aspheric lens. The low-power HeNe interferometer beam was focused onto the paper web between 5 and 15 mm away from the Nd:YAG beam. Because the paper moves through the scanner at about 20 m/s, the rotating mirror was placed in the HeNe beam path to move the detection point along with the paper during each 5-ns measurement. An optical encoder caused the Nd:YAG laser to fire only when the sensing beam was perpendicular to the paper ribbon, which kept the distance between excitation and sensing beams constant. The sensor measured the time it took for ultrasonic shock waves to travel from excitation to detection points, and the resulting velocity measurement enabled the researchers to calculate the bending stiffness and the out-of-plane shear rigidity of the paper.

“Out-of-plane shear rigidity is a sensitive indicator of fiber bonding and is an important contributor to in-plane compressive strength,” wrote Ridgway and colleagues in a paper presented last year at the 16th World Conference on Nondestructive Testing (Montreal, Que., Canada). “And the ability to monitor bending stiffness during manufacture and implement the corresponding feedback process control is expected to reduce production costs by reducing the basis weight needed to reach stiffness targets and reducing the amount of off-standard or low stiffness product.”

Additional efficiency savings upon implementation of process monitoring are expected to come from a reduction in paper breaks due to increased uniformity of strength throughout the paper web and a significant reduction in the generation of off-standard paper that ultimately must be recycled.