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    Articles techniques évalués par des pairs PAPTAC – Novembre / Décembre 2010

    LOW CONSISTENCY REFINING OF OXALIC ACID PRETREATED WOOD SHAVINGS
    RAFFINAGE BASSE CONCENTRATION DE ROGNURES DE BOIS PRÉTRAITÉ AVEC DE L’ACIDE OXALIQUE

    Taegeun Kang, George Soong, Xue Feng Chang, Rodger Beatson, James A. Olson, and D. Mark Martinez

    The objective of this study was to evaluate the potential use of wood shavings as a raw material for the low consistency (LC) refining at the primary refining stage to significantly reduce electrical energy consumption. It was possible to produce wood shavings with longer fibre length than wood chips. Oxalic acid, followed by alkaline peroxide, were applied to both wood shavings and wood chips before LC refining. LC refined wood shavings were found to be a low energy raw material. The refining energy was reduced by about 33% using wood shavings compared to wood chips at a given freeness, and oxalic acid pretreatment of wood shavings further reduced the refining energy by approximately 57%. Tensile strength and brightness of LC refined wood shavings was found to be higher than high consistency refined wood chips. Oxalic acid pretreatment further improved the tensile strength of LC refined wood shavings, but not the brightness. This study demonstrates the potential to develop a novel mechanical pulping process that produces high tensile, high brightness pulp with half the electrical energy consumption.

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    MULTIVARIABLE CONTROL AND ENERGY OPTIMIZATION OF TISSUE MACHINES
    SYSTÈME MULTIVARIABLE ET OPTIMISATION DE L'ÉNERGIE DES MACHINES À PAPIER MINCE

    Stephen Chu, Ross MacHattie, and Johan Backström

    The desire to increase profits by minimizing operating costs without sacrificing paper quality and runability is a goal all papermakers strive for. Modern tissue machines are typically equipped with more than twenty low-level control loops and multiple sheet property measurements at various locations along the machine. It is a large and strongly coupled process that can be difficult for control engineers to optimize without advanced multivariable control techniques.
    This paper examines the process interactions and energy cost reductions using model predictive control (MPC) technology with an optimization layer that automatically drives the process towards the lowest cost while honoring hard process and quality constraints. The studied paper machine was equipped with a fast scanning moisture measurement before the Yankee dryer in addition to the measurements of a traditional reel scanner.

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    WHOLE LOG CHIPPING OF MOUNTAIN PINE BEETLE KILLED WOOD FOR THE PULP AND PAPER INDUSTRY

    Barbara Dalpke, and Paul Bicho

    To alleviate the current chip supply shortage in the Province of British Columbia, and to make use of mountain pine-beetle-killed wood that is unsuitable for lumber production, whole log chipping of heavily checked, grey-stage beetle-killed wood to provide an additional chip supply for the pulp and paper industry is explored. Chipping productivity, hog fuel and fibre recovery, and chip quality of a satellite chipping yard are evaluated. Whole log chipping of heavily checked grey-stage trees resulted in high ratios of pins and fines, but otherwise good quality chips. Kraft and TMP pulps made from such whole log chips did not show any quality concerns. Heavily-checked, grey-stage mountain pine beetle-killed wood thus provides a suitable source of raw material for whole log chipping operations and the resulting chips can be used to supplement the chip feed of kraft and TMP mills.

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    AUTOMATION OF A MICROCONTAMINANTS DEPOSITION TESTER RELATING THE RATE OF DEPOSITION TO THE PRESSURE DROP ACROSS A COLLECTOR

    Gilles Dorris, and Carlos Castro

    The importance of organic microcontaminants (i.e. pitch and microstickies) as contributors to deposition problems in paper machines is recognized but there is no consensus in the industry on a microcontaminants definition and on monitoring method. Rather than measuring the quantity of pitch/microstickies in pulp or whitewater samples, we previously described a means of monitoring their deposition rate in forming fabrics because this is one of the most common deposit locations in paper mills using recycled pulps. In this report, we describe the automation of this dynamic deposition test, whose key feature is the continuous measurement of pressure across a fabric rather than the measurement of deposited material. The sigmoid shape of the deposition curve is explained by a change in flow regime as the pores of the fabric are progressively plugged. Capture of the P100 whitewater elements in the fabric is caused by a combination of overlapping phenomena such as surface pore deposition, pore bridging by fibrillar elements, and filtration of flowing elements when the pores start to be constricted. To obtain a measurable rate of deposition, the presence of lypophilic extractives is required both on the surface of the fines and also in the water phase, as colloidal particles.

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    ON-LINE MEASUREMENT OF AIR CONTENT IN PULP AND PAPERMAKING SYSTEMS. PART II: APPLICATIONS TO DEFOAMER ADDITION AND TO THE CORRECTION OF A CONSISTENCY TRANSMITTER

    Gilles Dorris, Sylvain Gendron, Natalie Pagé, Terry Murray, Yuxia Ben, and Henry Peters

    We have previously described the use of a pressure-based, on-line air content probe in various commercial flotation cells used for the removal of ink and other hydrophobic contaminants from recycled pulps. In this second report, other uses of the air content probe are described. For example, the probe was installed in the foam tank of a recycling mill to correct the signal of a microwave consistency transmitter. Without this correction, predicted consistencies of flotation rejects were highly erroneous. We also describe how the air content measurement was successfully used to automatically control defoamer addition in the foam tank of flotation rejects. Finally, on-line measurement of air content in the vat of two kraft brownstock washers is shown to provide useful information on the effect of air on pulp drainage and washer effectiveness.

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    DECONTAMINATION OF WHITEWATER FINES BY LABORATORY-SCALE FLOTATION

    Michelle Ricard, Gilles Dorris, Christine Lapointe, Natalie Pagé, and Yuxia Ben

    To our knowledge, mills have not yet developed strategies to selectively remove contaminated or aged fines from the wet end of the machine. To alleviate some of the drawbacks associated with non-selective means of cleaning whitewater, we have explored the use of dispersed air flotation. We carried out flotation tests on nine process water samples that were obtained from six different mills producing various types of paper or board. We found that laboratory flotation improved the quality of mill process water solids and increased the brightness of the handsheets made from whitewater fines by up to 2.4 units while reducing their extractives and ink content by up to 59% and 48%, respectively. Flotation selectively removed the smallest and most contaminated solids from paper machine waters. Decontamination of whitewater by flotation may lead to improved sheet properties, reduction in the use of papermaking and bleaching chemicals, improved paper machine runnability, and increased water system closure

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    REFINING OF LONG FIBRE FRACTIONS AFTER FRACTIONATION

    Quanqing Zha, Robert Lanouettem, and Kwei-Nam Law

    In this work, a primary-stage black spruce TMP was fractionated into long and short fibre fractions using two-stage screening. The long fraction (secondary reject) was further fractionated with a two-stage slotted screen to obtain earlywood and latewood enriched fractions. These fractions were refined to evaluate the efficiency of fractionation in order to improve the overall pulp quality. We observed that refining consistency, number of refining stages and the fraction itself influenced the refining behavior of the long fibre fractions. The latewood- enriched fraction had a better fibre development potential than the earlywood-enriched fraction but it required higher energy consumption. Low consistency refining consumed less refining energy but it led to lower mechanical properties. Separate refining could reduce refining energy and increase handsheet’s light scattering coefficient. Recombination of these refined fractions indicated that energy saving without too much strength loss is possible.

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    UNDERSTANDING THE EFFECT OF CHEMICAL PRETREATMENT ON PROPERTIES OF ASPEN CTMP

    Zhirun Yuan, Yang Gao and Kecheng Li

    In this study, the effect of sulfite and sodium hydroxide pretreatment on aspen CTMP fiber properties was examined in comparison with the impregnation with water only, aiming to understand how the impregnation chemicals affect fibre morphology and fibre surface chemistry and how these properties relate to fiber separation, refining, and pulp properties.
    Impregnation of wood chips with sulfite resulted in more long fibers, less fines and shives, and a higher brightness compared with impregnation of wood chips with water only. It is observed that sulphonated pulps have more middle lamella coated fibers after the first-stage refining and more flake-like fines than the pulp produced from water impregnation. This is mainly attributed to the difference in fiber separation. The pulp produced from water impregnation has a lower light scattering coefficient than the sulphonated pulp although the former has more fines.

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    THE IMPACT OF WOOD SPECIES ON THE TREATMENT OF BROWNSTOCK WITH HYDROGEN PEROXIDE

    Dan Davies, Gerry Pageau, and Dr. H.U. Süss

    The Howe Sound mill uses an ODEopDD sequence to produce fully bleached Kraft pulp from northern softwood. Tests of ClO2 consumption by the liquid portion of the 3.5% consistency stock feeding the Do stage were generally under 10% of total Do stage usage but did reach that level on occasion. Hydrogen peroxide (H2O2) was added to the bleach feed just minutes prior to ClO2 addition in the hopes of reducing overall bleaching cost. Key findings of the trial were that costs can indeed be reduced as long as there is only trace ClO2 residual in the filtrate used for dilution and peroxide dosage is kept low enough to ensure it is completely consumed before ClO2 is added.

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    MEASURING UNIFORMITY IN KRAFT DIGESTERS USING FLOW-FOLLOWING SENSORS

    E. Albadvi, T. C. M. Graham, E. Liu, M. Alaqqad, C. P. J. Bennington, R. J. Kerekes, M. Martinez and S. Mirabbasi

    Variability studies in kraft pulp digesters have been the subject of intense interest for many years. This paper reviews the various past approaches to measure this variability and then introduces a new method of its measurement: the SmartChip. This device measures and records the temperature directly within the digester as it flows with the wood chips during the kraft cook. Multiple SmartChips deployed in a single cook will then provide insight on the temperature variability and heat transfer mechanisms occurring within the digester. Moreover, by taking pulp samples in the vicinity of the SmartChip sensors, correlations between nonuniformity and temperature gradients can be established. The SmartChips have been tested in two laboratory batch digesters and as expected, little variability was observed in these small scale devices. These findings suggest that the SmartChip works well under the harsh conditions of the kraft cook and further work is warranted to develop the instrument for its use at the industrial scale.

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