Volume 25, Issue 2

The sorption kinetics for birch black liquor xylan onto unbleached softwood kraft fibers at industrial relevant process conditions was investigated. The impact of process conditions on total sorption and rate was studied. It was found that an unbleached fiber can increase its weight up to 40% by sorption of xylan if process conditions are favorable and sufficient xylan is present in the liquor. A decrease in pH, an increase in temperature and an increase in ionic strength enhances both the initial sorption rate and the maximum sorption, as expected from the literature. Moreover, an increased xylan charge from 5 to 10 g/l increased sorption by about 20-30% on average. At the highest temperature tested some deviations from the expected results were obtained, which indicates that more than one mechanism affect sorption. Analysis of the surface carbohydrate composition suggests that xylan is preferably sorbed on the fiber surfaces. This is especially obvious at higher sorption temperatures. SEM pictographs of the cellulose fibers indicate that xylan sorbs as single xylan chains at 87 and 127°C. In contrast, at 167°C globular structures are observed on the fibers, which probably is due to precipitation of globular xylan aggregates from the liquor.

A model describing the sorption kinetics for black liquor birch xylan onto unbleached softwood kraft fibers at industrially relevant process conditions has been developed. The experimental results support the suggestion that xylan sorption is a physical process. Our model is based on the assumption that the mass transfer between bulk liquor and fiber surfaces is the rate determining step. The effects of changes in process conditions like temperature, NaOH concentration and ion strength are included in the model. The model fits the experimental data quite well for sorption temperatures between 87 and 127°C. Other mechanisms affect the observed sorption at higher sorption temperatures. Consequently, more data is needed to produce a model for sorption at elevated temperatures. Deviations from the experimental data at temperatures up to 127°C are proposed to be a result of competing mechanisms not accounted for in our model. These can, for instance, be the transportation of xylan from the surface into the cell wall, the degradation of xylan in solution, the removal of substituents from the xylan backbone, or a decrease in NaOH concentration in the solution.

This review covers the effect of mechanical pre-treatment of wood chips on the energy consumption in refining and the quality of pulp. To understand the mechanisms of mechanical pre-treatment, a short description of relevant refining theory and reported effects of pre-treatment on wood morphology is given. Mechanical pre-treatment offers a chance to utilize the energy needed to defibrate chips in a more efficient way, minimizing the cyclic elastic deformations which are the main defibration mechanism in refining. Studies of fibre morphology indicate that compressive pretreatment mechanically introduces favorable weak points in the S1 and S2 fibre walls where defibration proceeds easier upon subsequent refining. Published results which cover the effect of the pretreatment on energy consumption and pulp properties are reviewed. Energy reduction of between 10% and 30% is reported in the literature. High ratio of volumetric compression is necessary. Pressurized conditions are required to ensure that the fibres are not damaged during the pre-treatment. Other effects of compressive pretreatment include a more uniform chip size and moisture content, better penetration of chemicals and removal of extractives from the chips. A list of equipment used for chip pre-compression is provided together with published results of pilot-scale and mill-scale operation.

Removal of extractives from the pulp furnish is of great importance for the improvement of paper machine efficiency and also for reducing the energy consumption during the thermomechanical pulp refining process. Extractives can exist in many different forms in the process water; as colloidal particles, dissolved or attached to fines and fibres. It is therefore important to know in which form they exist in order to fully understand their behaviour. In this paper, we report on an evaluation of the removal of extractives released from chips of different raw materials pre-treated in an Impressafiner, in pilot and in mill scale. In pilot trial the raw materials used were loblolly pine and white spruce and in the mil scale trial the raw material used was norway spruce. The colloidal stability of extractives present in the pressate water from the Impressafiner and their flocculation behaviour by cationic polymers (CPAM and Poly-DADMAC) under different conditions was also investigated. Calculations of mass balances around an Impressafiner showed that it was possible to remove up to 40% of extractives before the refining process. The reduction in total extractives content was mainly due to released resin acids while fatty acids, triglycerides, steryl esters and sterols to a large extent remained in the wood chips after pre-treatment. The removal of extractives from pine was four times higher than from spruce chips. The results can be explained in terms of the extractive composition in the raw material and the morphological differences in the wood structure.

Magnesium hydroxide, which is a weak alkali, can be used as the alkali source for peroxide bleaching of mechanical pulps. This magnesium hydroxide-based peroxide bleaching process has been commercialized in the paper industry. In this paper, we review the literature results regarding the process chemistry of the Mg(OH) 2 -based peroxide bleaching process, including the kinetics, peroxide decomposition, anionic trash/COD/oxalate formation, the effect on pulp properties. The benefits associated with the Mg(OH) 2 - based peroxide bleaching process are discussed. We also discuss the mill implementation of the Mg(OH) 2 - based peroxide bleaching process in the pulp and paper industry. A case is presented where magnesium hydroxide completely replaces sodium hydroxide as the alkali source. A partial magnesium substitution for sodium hydroxide can also be an option.

A compilation of the applied experimental conditions when studying metal ion adsorption onto kraft fibres, and the resulting conclusion, revealed that the ionic strength conditions used during the experiments were an important dividing factor. At low ionic strengths, the conclusion has regularly been that the Donnan ion-exchange model could correctly predict the adsorption while, at higher ionic strengths, it has often been concluded that the formation of specific metal-ion fibre complexes must be assumed. To study this apparent influence from the presence of monovalent sodium ions, Cu K-edge EXAFS spectra of Cu 2+ ions adsorbed to kraft fibres were collected in media of “0” to 100 mM NaCl. Combined with previous data, these measurements confirmed that at very low ionic strength, the importance of specific interactions between the chemically modified cellulose fibres and the Cu(II) ions significantly decreased. For a detailed description of the adsorption phenomenon, both types of interactions must be considered simultaneously. For most technical and engineering applications, however, the Donnan model can be used at low ionic strength conditions, i.e. I ≲ 10 mM. At higher ionic strengths, though, the inclusion of specific complexes in the model is necessary for correctly describing the adsorption of di- and trivalent cations with strong complex forming properties.

A pilot-scale fourdrinier former has been developed for the purpose of investigating the relationship between retention and paper formation (features, retention aids, dosage points, etc.). The main objective of this publication was to present the R-F (Retention and formation)-machine and demonstrate some of its fields of applications. For a fine paper stock (90% hardwood and 10% softwood) with addition of 25% filler (based on total solids content), the relationship between retention and formation was investigated for a microparticulate retention aid (cationic polyacrylamide together with anionic montmorillonite clay). The retention-formation relationship of the retention aid system was investigated after choosing standardized machine operating conditions (e.g. the jet-to-wire speed ratio). As expected, the formation was impaired when the retention was increased. Since good reproducibility was attained, the R-F (Retention and formation)-machine was found to be a useful tool for studying the relationship between retention and paper formation.

The interdependency between retention and paper formation was studied using a previously designed R-F (Retention and formation)-machine. The objective of the work was to investigate the retentionformation relationship for different types of retention aid systems. Both single-component cationic polyacrylamides with varying molecular weights and polyacrylamide- based microparticulate retention aids were investigated on the R-F-machine, for a fine paper stock with addition of 25% filler (based on total solids content). Results showed that the retention-formation relationship was not dependent on the retention aid systems used, since all systems provided similar relationships. In terms of the retention performance, significant differences were observed, depending on the choice of microparticle system and molecular weight of the used cationic polyacrylamide.

Experimental results on the fibre orientation in a laboratory scale headbox are reported. Images containing fibres in approximately 1 mm thick slices parallel to the wall were captured at different wall distances. A steerable filter was used to determine the orientation of bleached and unbeaten birch fibres, suspended in water, at different distances from one of the inclined walls of the headbox contraction. Due to optical limitations only dilute suspensions were studied. It is shown that the fibre orientation distribution varies with the distance from the wall. Sufficiently far upstream in the headbox a more anisotropic distribution is found closer to the wall.

Dynamic modeling and simulation software can be used to simulate the industrial processes such as papermaking in order to improve the efficiency of the process. A dynamic simulation model can be combined with an optimization method to maximize the advantages which simulation tools can offer. In this paper, we combine dynamic process simulation and optimization to see how it can benefit a papermaking process which was envisaged to consist of a TMP mill, stock preparation, short circulation, a paper machine, and a reel. The aim is to increase the efficiency and minimize the costs involved in papermaking.

The L&W Elrepho d/0 and the Spectrolino 45/0 instruments are examined using paper samples with different properties. External factors that influence the measurements - such as the sample background, the instrument calibration and the sample inhomogeneity - are studied, and a methodology for their minimization is presented. Experimental measurements show that such external factors, if not minimized by proper routines, affect the inter-instrument differences far more (up to 4-5 ΔE* ab ) than the instrument geometry (the effect of which is small and of order 0.1 ΔE* ab ). The DORT2002 radiative transfer model is used to simulate differences caused by instrument geometry. The simulated and measured differences are found to agree in magnitude, and the differences are mapped against sample properties. It is observed that the 45/0 instrument detects higher reflectance from paper samples with negligible absorption and transmittance. When there is considerable absorption (dyed samples) or transmittance (thin samples), the d/0 instrument detects higher reflectance. The physical mechanism behind this behavior is studied and explained using DORT2002, and the instrument differences are shown to depend on the anisotropy of the reflected light. The model/measurement agreement is satisfactory as the characteristic behavior is captured in almost all cases studied. This new understanding is important for facilitating accurate data exchange between the paper and graphic arts industries, but also for interpretation of reflectance measurements in general.

The integration of an ethanol production process based on prehydrolysis of softwood chips to a kraft pulp mill is discussed. A simulation model of this biorefinery has been created with WinGEMS simulation software to calculate the mass and energy balances of the biorefinery and to examine the effects of the integration on the operation of the pulp mill. Integrating the prehydrolysis process to the modeled pulp mill producing 1000 Adt/day of pine pulp would increase the wood consumption by 16%. With the increased wood consumption, 40 t/day of ethanol could be produced, and the excess power production could be increased by 460 MWh/day. The integration of the ethanol production process to a pulp mill would have considerable effects on the operation of the pulp mill. The coupling of hydrolyzate evaporation with black liquor evaporation would require major modifications in the evaporation plant. Due to the burning of organic residue from the ethanol process and to the increase in the heating value of black liquor, the load on the recovery boiler would increase by 31%.

The aim of this study was to find conditions under which it is possible to produce hemicelluloses-rich extract from Siberian larch in addition to pre-extracted pulp with a yield comparable to normal kraft pulp. The study was conducted in order to determine proof of concept of a process utilizing Siberian larch for production of papermaking pulp and biochemically obtained value-added products, i.e. biofuels or lactic acid. By pre-extraction at 160 ºC for 60 minutes approximately 17% of the wood material could be dissolved. At kappa number 30, the pre-extracted kraft pulp gave 5% lower total yield than a conventional kraft pulp. Addition of 4% polysulfide (PS) and 0.1% anthraquinone (AQ) on wood increased the pre-extracted kraft pulp yield nearly to the same level as that of regular kraft pulp. Pre-extraction at 160 ºC for 60 minutes allowed reduction of EA charge by 3% on wood and Hfactor by 300-450. The viscosity of the pre-extracted PSAQ kraft pulps was higher than that of conventional larch kraft pulps and had a somewhat inferior strength and optical properties. The results indicate that under optimized conditions it is possible to produce preextracted PSAQ larch pulp without significant losses in pulp yield and quality, and in addition generate a new feed stock of carbohydrates for further utilization for value-added products.