The papers summarized here are from the TAPPI Journal June and July 2018 issues. TAPPI Journal is an online publication of relevant and timely peer-reviewed research delivered via email and free to all TAPPI members. To receive TAPPI Journal, join TAPPI at www.tappi.org.
Using online bubble size and total dissolved solids measurements to investigate the performance of oxygen delignification
Riku Kopra, Heikki Mutikainen, and Olli Dahl
ABSTRACT: The main target of oxygen delignification is to continue delignification that started in cooking in a more selective manner than occurs in the digester (i.e., remove a substantial fraction of the residual lignin using oxygen and alkali at a moderate temperature). Delignification with oxygen is a gentler way of reducing the kappa number than extended cooking. Lowering inlet kappa to bleaching also decreases bleaching chemicals consumption and, because of this, reduces organic wastewater load from bleaching.
The researchers studied the performance of oxygen delignification by installing bubble size imaging systems and refractometers, which measure dissolved dry solids in the oxygen stage feed. Based on these measurements, they gathered information about gas dispersion (bubble size distribution) and the behavior of dissolved matter in the hardwood mill’s oxygen delignification stage. Gas dispersion improved (i.e., average bubble size decreased) when the chemical mixer speed increased. Increasing the mixer speed and the amount of oxygen yielded higher kappa number reductions and increased the amount of dissolved organic matter.
The methods described in this study could help mills adjust the oxygen charge and gas dispersion in delignification processes and allow for future monitoring and controlling of the oxygen process.
Evaluation of the out-of-plane response of fiber networks with a representative volume element model
Yujun Li, Zengzhi Yu, Stefanie Reese, and Jaan-Willem Simon
Many natural and synthetic materials have fibrous microstructures, including nonwoven fabrics, paper, and fiberboard. Experimentally evaluating their out-of-plane mechanical behavior can be difficult because of the small thickness compared with the in-plane dimension. To properly predict such properties, network-scale models are required to obtain homogenized material mechanics by considering fiber-scale mechanisms.
The researchers demonstrate a three-dimensional representative volume element (RVE) for fiber networks using the finite element method. They first adopted the classical deposition procedure to generate fiber networks with random or preferential fiber orientations and then an artificial compression to achieve the practical fiber volume fraction. The hollow fibers, described with elastic-plastic brick elements, were joined by interface-based cohesive zone elements in all fiber-fiber contact areas. Thereafter, the fiber networks were subjected to displacement boundary conditions, and their apparent mechanical response was evaluated by a homogenized stress.
To determine the RVE dimension, the researchers conducted an RVE size convergence study for the out-of-plane compression and tension. Finally, they evaluated the apparent out-of-plane response of the obtained RVE for four loading cases: out-of-plane compression, tension, simple shear, and pure shear. The results show a quite different mechanical behavior of fiber networks between all these out-of-plane loading cases, particularly between tension and compression. This research demonstrates that virtual fiber networks can be generated to evaluate the out-of-plane mechanical properties.
Factors affecting the free shrinkage of handsheets: apparent density, fines content, water retention value, and grammage
Dewatering in the paper machine occurs mechanically in the forming and pressing sections; however, most remaining water, the removal of which requires applying high temperatures, evaporates in the dryer section. As a result, the paper web shrinks due to shrinkage of individual fibers in the paper web. On the paper machine, the paper web is under restraint in the machine direction (MD), whereas it can shrink in the cross-machine direction (CD). The edges of the web shrink more than the center of the web. A shrinkage profile is therefore created in the CD of the web.
All machine-made papers exhibit a CD shrinkage profile. The CD shrinkage profile is significant because it affects the final product quality and manufacturing efficiency. The prime cause of the CD shrinkage profile during drying is free shrinkage. Researchers investigated the effects of several wood pulp fibers on the free shrinkage of handsheets to better understand the mechanism of paper shrinkage during drying processes.
Through this study, mills can better understand the role of different fiber types, density, fines content, water retention value, and grammage on the free shrinkage of handsheets formed from a selection of softwood and hardwood pulps.
Influence of tensile straining and fibril angle on the stiffness and strength of previously dried kraft pulp fibers
John M. Long and Warren J. Batchelor
It has long been known that in individual wood fibers, the tensile mechanical properties are heavily determined by the nature of the cellulose chains in the fibers. Both elastic tensile modulus and strength are directly related to the fibril angle. Classic work from the 1970s measured modulus and strength in wood fibers as a function of the fibril angle and compared the results with theoretical models. The measurements were hampered by the presence of defects in the fibers, some that occurred naturally and others that resulted from pulp processing.
In this study, we performed more accurate measurements of strength and modulus in single fibers of radiata pine by loading the fibers in cycles, gradually pulling some of the defects out of the fibers in an attempt to obtain defect-free values of modulus and strength. We then plotted these properties against measured fibril angle and compared our results with theoretical models. The results show that even when the fiber had reached maximum load before fracture, at a given value of fibril angle, it still had a measured modulus that is around half the theoretically expected value. The results suggest that the load required to fully remove defects from the fibers may be larger than the fibers can bear before fracturing.
Information contained in this investigation will help paper researchers and engineers better understand the relationship between processing defects and mechanical properties of single wood fibers.
Effects of lignin chemistry on oxygen delignification performance
Valeria Juste Gomes, Hasan Jameel, Hou-min Chang, Robert Narron, Jorge Colodette, and Peter W. Hart
This study focused on characterizing the chemical and structural properties of isolated lignin from six hardwoods and their kraft pulps in an attempt to better understand the relationship between lignin’s chemical properties and resultant oxygen delignification performance.
Several hardwood samples were cooked under the same conditions with varying alkali charges to obtain unbleached pulps with kappa numbers between 19 and 20. These pulps were then subjected to an oxygen delignification stage. Both processes were evaluated for pulp quality, residual lignin, and O-stage delignification efficiency. The oxygen delignification stage was carried out under fixed conditions and evaluated with regards to kappa number, which was corrected for hexenuronic acid (HexA) contributions. Results revealed that different hardwood species exhibited differing oxygen delignification efficiencies. A high correlation was found between the O-stage delignification efficiency and the content of phenolic groups in the unbleached lignin, which confirmed that free phenolic groups are the reactive site for molecular oxygen attack. When different hardwood species were compared, the HexA contents were not found to affect O-stage delignification efficiencies.
The researchers are attempting to understand how the structure of different hardwood species impacts oxygen delignification performance. If they can develop this understanding, the performance of the oxygen stage can be tailored to specific seasonal changes in the wood hardwood species being harvested.