Oxygen Delignification Stage Research Articles

On the role of carbohydrates in oxygen delignification

The long-term trend regarding wood is an increase in price. Because wood contributes to a large part of production costs, the efficient utilisation of wood is greatly desired to reduce production costs for kraft pulp producers. During the 1990s, the development of improved modified kraft cooking began, which led to higher yields. There was also a trend of terminating kraft cooking at a higher kappa number to maximise the overall yield. For hardwood, the defibration point became a critical setback in allowing this termination at a high kappa number. This thesis discusses how this issue has been tackled in the laboratory by using improved modified kraft cooking combined with extended impregnation to enable a decrease in reject content and shift the defibration point towards a higher kappa number for hardwood. This lab concept is referred to as extended impregnation kraft cooking (EIC), and this thesis reveals that EIC cooking efficiently reduces the reject content for both birch and eucalypt. By using EIC cooking, the defibration point was shifted to a kappa number of ca. 30 from ca. 20 using conventional kraft cooking. This study demonstrates the great potential for achieving a higher overall yield for eucalypt by terminating the EIC cooking at a high kappa number, but with the conditions used in this thesis, no improvement in yield was observed for birch. An important issue is that the termination of kraft cooking at high kappa number increases the demand for extended oxygen delignification to reach a similar kappa number into bleaching, i.e., due to cost and environmental reasons. Extended oxygen delignification was shown to be possible for both birch and eucalypt EIC pulps (i.e., from kappa number 27 to 10) with an acceptable pulp viscosity number. The other part of this thesis addresses aspects regarding the limitations in oxygen delignification. It has previously been shown in the literature that a high xylan yield of kraft cooking could negatively affect the efficiency of subsequent oxygen delignification. In this work, the increased xylan content in eucalypt kraft pulp within the range of 8–18% had only a marginally negative impact on the oxygen delignification efficiency after correcting for the HexA contribution to the kappa number. It is also desired to extend the oxygen delignification towards lower kappa number, i.e., below kappa number 10 to decrease the bleaching chemical requirement. In this study, the hypothesis that the reduced efficiency of oxygen delignification at low kappa numbers could partly be due to the formation of oxidisable carbohydrate-related structures (i.e., HexA and/or other non-lignin structures) was also tested. No formation was established. On the other hand, a final oxygen delignification stage in the bleaching could be an attractive alternative for reducing yellowing and enhancing brightness; in fact, this has led to the development of a patent (SE 528066).

Structural Differences between the Lignin−Carbohydrate Complexes Present in Wood and in Chemical Pulps

Lignin-carbohydrate complexes (LCCs) were prepared in quantitative yield from spruce wood and from the corresponding kraft and oxygen-delignified pulps and were separated into different fractions on the basis of their carbohydrate composition. To obtain an understanding of the differences in lignin structure and reactivity within the various LCC fractions, thioacidolysis in combination with gas chromatography was used to quantify the content of beta-O-4 structures in the lignin. Periodate oxidation followed by determination of methanol was used to quantify the phenolic hydroxyl groups. Furthermore, size exclusion chromatography (SEC) of the thioacidolysis fractions was used to monitor any differences between the original molecular size distribution and that after the delignification processes. Characteristic differences between the various LCC fractions were observed, clearly indicating that two different forms of lignin are present in the wood fiber wall. These forms are linked to glucomannan and xylan, respectively. On pulping, the different LCCs have different reactivities. The xylan-linked lignin is to a large extent degraded, whereas the glucomannan-linked lignin undergoes a partial condensation to form more high molecular mass material. The latter seems to be rather unchanged during a subsequent oxygen-delignification stage. On the basis of these findings, a modified arrangement of the fiber wall polymers is suggested.

Mechanistic Insights into the Oxidation of Veratryl Alcohol with Co(salen) and Oxygen in Aqueous Media: An in‐situ Spectroscopic Study (Eur. J. Inorg. Chem. 13/2005)

AbstractThe cover picture shows the proposed mechanism of the Co(salen)‐catalyzed oxidation of a lignin model compound in water, as elucidated by several in‐situ spectroscopic techniques. Catalytic oxygen activation can help to prevent the formation of hazardous side products in organic oxidations, especially if harmless solvents are used. The target in this work is lignin, which is found in wood and must be removed as selectively as possible before paper production. In view of avoiding chlorine as the oxidant, the idea is to catalytically enhance the oxygen delignification stage in the pulp bleaching sequence. Details are discussed in the article by T. Repo, B. M. Weckhuysen et al. on p. 2591 ff.

Monoethanolamine (MEA) pulping of beech and spruce wood for production of dissolving pulp

Abstract Delignification with the use of monoethanolamine (MEA) is an innovative, environmentally friendly chemical pulping process that, in contrast to conventional pulping methods, works without the use of sulphur compounds. Of particular benefit is the simple MEA recovery by distillation, allowing black liquor combustion to be dispensed with and the dissolved lignin to be gained. The MEA process is suitable for pulping both hardwood (e.g. beech) and softwood (e.g. spruce). Compared with conventional processes, the MEA process has the advantage of achieving a high degree of delignification through an increase in temperature during pulping without any appreciable damage to the cellulose. When manufacturing dissolving pulps, however, this specific selectivity of MEA requires the removal of the hemicelluloses in an additional stage, preferably by acid prehydrolysis. The conditions of this prehydrolysis step must be selected specifically to be in keeping with the ultimate use of the dissolving pulp, as this stage involves the cellulose chains being chemically attacked. The residual lignin is removed after pulping in a sequence of different bleaching stages. Substituting MEA for the sodium hydroxide solution required as a bleaching alkali in the oxygen delignification stage proves to be especially beneficial, as in this case the COD after bleaching can be considerably reduced by treating the bleaching filtrate from this stage jointly with the effluents from the MEA pulping process.

Organic matter balance in ECF kraft mill fiberline

Organic matter balances of an ECF kraft mill fiberline were studied in three different operational conditions of the oxygen delignification stage by implementing mill measurements, collecting routine mill data and combining them in a modified PROCELL steady state model. Dissolved volatile solids was the basic measurement for organic matter in liquid streams. Normal operation of the O2D0E(O)D1E(OP)D2 fiberline was described successfully. It was also possible to describe exceptional operating situations in the oxygen delignification stage and their effects on other parts of the production process reasonably well in order to focus studies on aspects requiring further attention. The existence of organic matter lost through complete degradation and volatile organic compounds in unit processes as well as its sensitivity to yield and operational situations is shown. The different perspectives of pulp production and wastewater treatment can be brought closer to each other using the approach in this study. The variation in the fractions discharged to wastewater treatment, although a relatively small share of the overall organic matter balance, will continue to become more important also for pulp production when effluent streams are increasingly turned back to the production line through further closure. Studies like the one presented here, can contribute to the evaluation of this development.

Precipitation of lignin and extractives on kraft pulp: effect on surface chemistry, surface morphology and paper strength

The effect of pH on the formation of precipitates (lignin, extractives and metals) on kraft pulp surfaces was examined by electron spectroscopy for chemical analysis, time-of-flight secondary ion mass spectrometry and atomic force microscopy (AFM). A softwood kraft pulp slurry from an oxygen delignification stage was diluted to 3% consistency with water or an acidic Z filtrate. After heating to 70 °C the pH was lowered from 11 to 2–5, using sulphuric acid. Lignin and extractives precipitated at pH values below 6, and their amounts increased with decreasing pH. Most of the precipitated lignin was found on the pulp surface after sheet forming, whereas the main part of the precipitated extractives could be easily washed away with water. The layer of precipitated lignin was apparently thicker than the layer of extractives. AFM showed the precipitated material as a granular phase. Neither surface morphology nor surface coverage depended on the addition of Z filtrate. The amount of metals ID the pulp and on the pulp surface decreased when pH was lowered to 2. More metals, such as Ca and Mg, were detected ID the pulps as well as on the sheet surfaces when the pulp was diluted with Z filtrate. Strength and bonding properties of the pulp sheets were slightly impaired by the precipitated material. Acidification appears to be the main reason for the precipitation of both lignin and extractives on the pulp surfaces. This should be taken into account when filtrates are recycled ID the bleaching or washing of pulps.

Nitrogen-Centered Activators of Peroxide-Reinforced Oxygen Delignification

In an effort to augment the efficiency of an oxygen delignification stage, we examined the effectiveness of various heterocyclic aromatic nitrogen compounds as possible activators of such a stage. It was shown that a variety of such compounds may act as activators in the presence of hydrogen peroxide, improving the delignification efficiency of oxygen delignification. Among the group of activators examined, 1,10-phenanthroline was found to be the most potent. Injecting the activator in the middle of the delignification stage (simulating a two-stage process) was shown to provide most of the benefits. The optimum injection time was found to at 30 min from the onset of the delignification. At the optimal charge of 1,10-phenanthroline (0.3%), the delignification was improved by 18%. While the presence of copper ions was found to further improve the efficiency of the activating system, this was accompanied by an extensive carbohydrate degradation.

Surface Characterization of Softwood and Hardwood Kraft Pulp Fibers from Different Stages in a Bleaching Sequence

Pulp samples from a softwood kraft pulp line and a hardwood kraft pulp line were used to study how the chemical surface composition of different kraft pulp fibers are modified as the fibers are processed in the fiber line. The pulps were bleached according to the following sequence: OQ(OP)(ZQ)(OP). Three different surface analyzing techniques were used and compared to each other in order to determine the content of lignin and extractives on the fiber surfaces. The three techniques studied were electron spectroscopy for chemical analysis (ESCA), ESCA in combination with mercurization of the lignin, and mechanical peeling of the outer part of the fiber surface followed by standard chemical analyzing techniques. All analyzing techniques show that the surface coverage by lignin and extractives is higher compared with that of the bulk of the fibers. Standard ESCA, that uses both the theoretical and the measured values of the O/C ratio and C1 carbon content, in most cases gives the highest values on the surface coverage by lignin on both softwood and hardwood kraft pulp fibers, followed by the mercurization and mechanical peeling techniques. More extractives are detected on all kraft pulp fiber surfaces when standard ESCA is used as compared to the mechanical peeling technique. The obtained results indicate that the mercurization technique in combination with ESCA analysis is a good alternative for surface analysis of kraft pulp fibers. However, further investigations are needed to support this recommendation. Standard ESCA indicates that the surface lignin on both unbleached softwood and hardwood kraft pulp fibers has lower reactivity in an oxygen delignification stage as compared to the bulk lignin. In opposite, the mercurization technique and the mechanical peeling technique indicate similar reactivity of bulk and surface lignin towards oxygen. Furthermore, all ESCA techniques show that the decrease in surface coverage by lignin and total lignin content of both softwood and hardwood pulp fibers is low in the last hydrogen peroxide bleaching stage. Oxygen delignification is most efficient in removing surface extractives on both softwood and hardwood pulps and adsorption of surface extractives occurs after the ozone bleaching stage on all pulp samples.

Determination of the Surface Layer of Kraft Pulp Fibers by Field Emission Scanning Electron Microscopy (FE-SEM)

The surfaces of commercial softwood kraft pulp fibers were observed by FE-SEM to determine the surface layers of the fibers. Unbleached pulp fibers were covered by fibrils running in random fashion, and the fibrils sometimes extended to a pit area, which were characteristics of the primary wall. The existence of primary wall on the surfaces of unbleached kraft pulp fibers would cause the higher lignin content of the surfaces of the fibers than that of the whole fibers. Large parts of the primary wall were removed during bleaching process, and the fibrils running densely perpendicular to the fiber axis were observed on full bleached pulp fibers, which was a characteristic of the outer layer of secondary walL The degree of the primary wall removal was different between the pulp samples. In one pulp sample, large amount of the primary wall was removed during oxygen delignification stage, although, in another pulp sample, some removal took place during oxygen delignification stage.

From Hypochlorite Bleaching to an ECF Sequence

Today. the Japanese Pulp and Paper Industry is facing more stringent regulations on chloroform emission. Therefore. most of the pulp mills are looking for a way of decreasing chloroform emission formed during their conventional pulp bleaching. It is well known that chloroform is mainly generated during the hypochlorite stage. With this background. it was decided, in this first part, to modify the existing bleaching sequence O C/D Ep H D used in a Japanese pulp mill to eliminate the H stage. The second part deals with the total replacement of Cl2 by ClO2 in order to prepare the Japanese pulp mills to the ECF bleaching considered in many other countries as the Best Available Technology (BAT). Finally the last optimization steps concerned the modification of the conventional oxygen delignification stage and the change of the operating conditions during the final D stage.

Effects of xylanase treatment on fibre morphology in totally chlorine free bleaching (TCF) of Eucalyptus pulp

The effects of xylanase on pulp properties and fibre morphology in a TCF (totally chlorine free) bleaching sequence has been studied. Enzymatic treatment is usually applied after oxygen delignification, but in this paper the effect of xylanase treatment before the oxygen delignification bleaching stage was studied. The application of xylanases in TCF bleaching of Eucalyptus kraft pulps enables the production of fully bleached pulps with high brightness (90% ISO) and viscosity (above 800 mL/g). The optimal NaOH charge in the oxygen delignification stage was studied. Xylanase treatment improves the accessibility of the bleaching chemicals to the fibres. The improvement of ‘bleachability’ produced by the enzyme reduces the quantity of ozone needed in subsequent bleaching stages. SEM (scanning electron microscopy) performed after oxygen delignification showed notable changes on fibre surfaces.

Reduced brightness variations by extended oxygen delignification

The kappa number reduction for softwood kraft pulps in an alkaline environment was studied with and without oxygen present. It was shown that the major part of the kappa number reduction in oxygen delignification stages is due to the reaction chemistry but the importance of effective extraction washing was also proved in experiments with subsequent leaching in alkaline environments. During repetitive oxygen stages the kappa number reduction levelled off. Process parameters for extended oxygen delignification (OO) were chosen according to results from an investigation, using RSM (Response Surface Modelling) with a CCF (Central Composite Face-centred) design. The kappa number reduction and the brightness increase were mainly affected by the variations of pressure, pulp consistency and alkali charge. The extended oxygen delignification was shown to eliminate differences in the chromophore content of the pulp. Thus the conditions necessary for controlled charges of bleaching chemicals and stable bleaching conditions can be reached.

Analysis of pitch deposits produced in Kraft pulp mills using a totally chlorine free bleaching sequence

Two organic deposits accumulated in a Kraft pulp mill during pulping of Eucalyptus globulus wood and throughout a TCF (totally chlorine free) bleaching sequence were characterized. One deposit was collected after cooking and an oxygen delignification stage while the other was collected after bleaching with hydrogen peroxide. The deposits were Soxhlet extracted with acetone, and the extracts redissolved in chloroform and subsequently analyzed by gas chromatography (GC) and GC–mass spectrometry (MS) using short and medium length high temperature capillary columns, respectively. On the other hand, the insoluble residues left after the acetone extraction were analyzed by Curie-point flash pyrolysis–GC–MS and by pyrolysis–methylation–GC–MS. The compounds identified in the deposits arise from the E. globulus wood lipophilic extractives that survive the pulping and bleaching processes. Triglycerides were completely hydrolyzed during the Kraft cooking and the fatty acids dissolved. Steroids (alcohols, hydrocarbons, ketones and esters) and waxes were the main components in the deposit collected after the oxygen delignification stage. After the bleaching with hydrogen peroxide, content of the waxes were reduced and fatty acids appeared. High amounts of fatty acids salts were also identified in the deposit collected after the oxygen stage, and in minor amounts in the deposit collected after hydrogen peroxide bleaching. In contrast, this deposit was mainly made up of high amounts of lignin-derived phenolic moieties.

Operating Data of Kraft Pulp Plant in Japan (1998)

Pulping Technical Committee of J. TAPPI made questionnaires about 1998 operating data of kraft pulp mills in Japan. This paper was the second report of that questionnaires and the report about bleaching process, the manufacturing of bleaching chemicals and the treatment after bleaching (screening. cleaning, etc). There are 50 BKP lines (30mills) and 6 semi-bleaching KP lines in Japan. Only 4 lines (3mills) have not Oxygen delignification stage. There are 3 ECF bleaching lines and all of them are first-D sequence. Ozone is not used as pulp bleaching chemicals in Japan.First bleaching stage of major BKP lines is C (23 lines) or C/D (24 lines. C/D=87/13). About 75% of alkaline extraction stage are reinforced by oxygen and/or hypochlorite or hydrogen peroxide. 46 lines use hypochlorite and 11 lines use hydrogen peroxide and 21 lines use DnD as last stage and 1 line use xylanase as pre-bleaching stage. Average addition of chlorine and total available chlorine is 18.0kg /t and 29.3kg/t. Average kappa number of UKP is 11.4 (10.2, LUKP) and the brightness and the viscosity 85.8 and 15. 8 cp. Average COD discharge and effluent water from bleaching process is 15kg/t and 31m3/t.

計装 ＫＰ ＥＣＦ操業での制御性改善

Kraft Pulp plant of Kushiro Mill, Nippon Paper Co., Ltd. was originally started in 1975. Then this plant was set up the oxygen delignification stage, and had been operated at one Hypochlorite stage (only reached 63 brightness, NYKP).In June 1996, our Mill introduced a new cooking technology, Lo-SolidsTM Cooking and Elementary Chlorine Free equipment (86 brightness, NBKP) not suing the chlorine and Hypochlorite at all for the first time in Japan. Now this plant is oprerated at the threestage, D 1-EOP-D 2 bleach sequence.In this paper, let us introduce the control details of adding chemicals at the previous Hypochlorite plant, applied control to new ECF plant and a viewpoint of selection DCS (Distributed Control System) for all over the Kraft Pulp plant. Accepting SI unit and features of the host computer are also described.