Comparison of hot-water extraction and steam treatment for production of high purity-grade dissolving pulp from green bamboo

The performance of hot-water extraction (HWE) and steam treatment (ST), followed by kraft pulping were compared for production of high purity-grade dissolving pulp from green bamboo. With the same prehydrolysis intensity (represented by the P-factor), the fractionation efficiency of HWE is far lower than that of ST. Because of lower removal of non-cellulosic components, the solid residue from HWE (even at approximately double the prehydrolysis intensity, P-factor = 1,379) required more active alkali (AA) during kraft pulping to obtain a cellulose purity equivalent to that achieved by the ST (P-factor = 756)-kraft process. To reach equivalent hemicellulose removal, HWE required more severe intensity than ST. However, FTIR and SEM characterizations of solid residue confirmed that intensified HWE resulted in significant lignin condensation. Antagonistic effects of hemicellulose removal and lignin condensation extent on subsequent kraft pulping were therefore more apparent in HWE than that in ST. Under the same kraft pulping conditions, lignin condensation from a severely intensified HWE process (P-factor = 2,020) caused greater cellulose yield and viscosity loss than that found for ST. Finally, at a given residual pentosan or lignin content, the cellulose yields from all HWE-kraft pulps were about 3 % lower than those from ST-kraft pulps. Consequently, based on an optimally setup chlorine dioxide bleaching stage, a cellulosic pulp with alpha-cellulose content of 97.6 % and viscosity of 927 mL/g was successfully produced from a ST-kraft pulp (P-factor = 756, AA = 19 %).     

Sulfur-free dissolving pulps and their application for viscose and lyocell

In this study, the concept of multifunctional alkaline pulping has been approved to produce high-purity and high-yield dissolving pulps. The selective removal of hemicelluloses was achieved by either water autohydrolysis (PH) or alkaline extraction (E) both applied as pre-treatments prior to cooking. Alternatively, hemicelluloses were isolated after oxygen delignification in a process step denoted as cold caustic extraction (CCE). Eucalyptus globulus wood chips were used as the raw material for kraft and soda-AQ pulping. In all process modifications sulfur was successfully replaced by anthraquinone. By these modifications purified dissolving pulps were subjected to TCF bleaching and comprehensive viscose and lyocell application tests. All pulps met the specifications for dissolving pulps. Further more, CCE-pulps showed a significantly higher yield after final bleaching. Morphological changes such as ultrastructure of the preserved outer cell wall layers, specific surface area and lateral fibril aggregate dimension correlated with the reduced reactivity towards regular viscose processing. The residual xylan after alkali purification depicted a lower content of functional groups and a higher molecular weight and was obviously entrapped in the cellulose fibril aggregates which render the hemicelluloses more resistant to steeping in the standard viscose process. Simultaneously, the supramolecular structure of the cellulose is partly converted from cellulose I to cellulose II by the alkaline purification step which did not influence the pulps reactivity significantly. Nevertheless, these differences in pulp parameters did not affect the lyocell process due to the outstanding solubility of the pulps in NMMO. Laboratory spinning revealed good fiber strength for both, regular viscose and lyocell fibers. The high molecular weight xylan of the CCE-treated pulps even took part in fiber forming.     

Ionic liquid extraction method for upgrading eucalyptus kraft pulp to high purity dissolving pulp

High purity cellulose from wood is an important raw material for many applications such as cellulosic fibers, films or the manufacture of various cellulose acetate products. Hitherto, multi-step refining processes are needed for an efficient hemicellulose removal, most of them suffering from severe cellulose losses. Recently, a novel method for producing high purity cellulose from bleached paper grade birch kraft pulp was presented. In this so called IONCELL process, hemicelluloses are extracted by an ionic liquid–water mixture and both fractions can be recovered without yield losses or polymer degradation. Herein, it is demonstrated that bleached Eucalyptus urograndis kraft pulp can be refined to high purity acetate grade pulp via the IONCELL process. The hemicellulose content could be reduced from initial 16.6 to 2.4 wt% while persevering the cellulose I crystal form by using an optimized 1-ethyl-3-methylimidazolium dimethylphosphate-water mixture as the extraction medium. The degree of polymerization was then reduced by a sulfuric acid treatment for subsequent acetylation of the pulp, resulting in a final hemicellulose content of 2.2 wt%. When pre-treating the pulp enzymatically with endoxylanase, the final hemicellulose content could be reduced even to 1.7 wt%. For comparison, the eucalyptus kraft pulp was also subjected to cold caustic extraction and the same subsequent acid treatment which led to 3.9 wt% of residual hemicelluloses. The performance in acetylation of all produced pulps was tested and compared to commercial acetate grade pulp. The endoxylanase-IONCELL-treated pulp showed superior properties. Thus, an ecologically and economically efficient alternative for the production of highest value cellulose pulp is presented.     

Evaluation of new organosolv dissolving pulps. Part I: Preparation, analytical characterization and viscose processability

New acidic organosolv pulping processes, such as Acetosolv, Formacell and Milox, promise to have superior potential in terms of purification selectivity and specific investment costs. Consequently, a thorough investigation of these new acidic pulping processes in comparison to state-of-the-art acidic magnesium sulfite technology was conducted. The impact of pulping and bleaching parameters on the physical and chemical characteristics was studied to compare process efficiency and selectivity for each type of pulp made from Eucalypt wood. In addition to a detailed analysis of the chemical composition and physical properties on a molecular and supramolecular level, the TCF-bleached dissolving pulps were tested for their applicability in viscose fiber production. The influence of pulp properties as determined by standard and advanced analytical methods on process performance and selected fiber properties is emphasized.     

Purification of cellulosic pulp by hot water extraction

Hot water extraction (HWE) of pulp in a flow-through reactor was evaluated as a method to purify paper-grade pulps. About 50–80 % of the xylan and up to 50 % of the lignin in unbleached birch Kraft pulp was extracted by the HWE without losses in cellulose yield. The residual xylan content in the extracted pulps was predominantly too high for dissolving-grade applications, but some of the pulps with a xylan content of 5–7 % might still be suitable as rayon-grade pulps. Increasing extraction temperature lowered the xylan content at which cellulose yield started to decrease. Furthermore, at any given xylan content, increasing extraction temperature resulted in cellulosic pulp with higher degree of polymerization. The extracted xylan was recovered almost quantitatively as xylo-oligosaccharides. The results suggest that HWEs at elevated temperatures may be applied to purify cellulosic pulps, preferably containing a low xylan content, and to recover the extracted sugars.     

Hydrogen peroxide bleaching of cellulose pulps obtained from brewer’s spent grain

Brewer’s spent grain (BSG) was evaluated for bleached pulp production. Two cellulose pulps with different chemical compositions were produced by soda pulping: one from the original raw material and the other from material pretreated by dilute acid. Both of them were bleached by a totally chlorine-free sequence performed in three stages, using 5% hydrogen peroxide in the two initial, and a 0.25 N NaOH solution in the last one. Chemical composition, kappa number, viscosity, brightness and yield of bleached and unbleached pulps were evaluated. The high hemicellulose (28.4% w/w) and extractives (5.8% w/w) contents in original BSG affected the pulping and bleaching processes. However, soda pulping of acid pretreated BSG gave a cellulose-rich pulp (90.4% w/w) with low hemicellulose and extractives contents (7.9% w/w and <3.4% w/w, respectively), which was easily bleached achieving a kappa number of 11.21, viscosity of 3.12 cp, brightness of 71.3%, cellulose content of 95.7% w/w, and residual lignin of 3.4% w/w. Alkaline and oxidative delignification of acid pretreated BSG was found as an attractive approach for producing high-purity, chlorine-free cellulose pulp.     

Reactivity of dissolving pulp: characterisation using chemical properties, NMR spectroscopy and multivariate data analysis

The reactivity of dissolving pulp was experimentally determined in termsof residual cellulose in viscose. The correlations between 11 chemicalproperties of pulp and filter values and residual cellulose contents of viscosewere then investigated by multivariate data analysis. Both the viscose filtervalue and the residual cellulose were well modelled from the 11 propertiesby partial least squares regression. The results show that pulps with highacetone extractable fractions, high magnesium contents, low alkali resistanceand low viscosity, gave low viscose filter values and low residual cellulosecontents. Pulps with low residual cellulose contents also had low carboxylgroupcontents and low polydispersity. The results are interpreted as that in pulpwith high reactivity, the hemicellulose content is low and that the cellulosechains are shorter and more soluble in alkali. An explanation of the positiveeffect from the high extractive content is that the extractives facilitate thediffusion of carbon disulfide. A principal component analysis of CP/MAS¹³C-NMR spectral data of six pulp samples showed that differences inreactivity between the pulps could be explained by variations in the hydrogenbonds in the cellulose and/or changes in the glucosidic bonds. In a separatestudy electron beam processing enhanced the reactivity, i.e. lowered theresidual cellulose content, of the investigated pulps. The magnitude of theelectron dose, within the tested range (5.4–23.7 kGy), didnotseem to be important, but the reactivity within pulp sheets tended to be ratherinhomogeneous.     

Reactivity of dissolving pulps modified by TEMPO-mediated oxidation

The reactivity of dissolving pulps towards derivatization or dissolution is a crucial quality parameter and is mainly determined by the accessibility of the hydroxyl groups. When dissolving pulps are produced from paper-grade pulps by cold caustic extraction (CCE), their reactivity is often inferior as compared to commercial prehydrolysis kraft dissolving pulps. It was hypothesized that pulp reactivity can be enhanced by the introduction of small amounts of substituents to facilitate interchain accessibility. In this study, CCE-treated Eucalyptus globulus kraft paper pulp was subjected to TEMPO-mediated oxidation to initiate partial oxidation of the C₆-hydroxyl groups to carboxyl groups. The effect of this pulp modification on the reactivity towards xanthation and the subsequent dissolution in diluted aqueous alkali solution (viscose process) as well as the dissolution in complexing and non-complexing solvents, respectively, was thoroughly examined. The results revealed that the oxidized pulps rich in C₆-carboxylate groups impeded the xanthation reaction obviously because of the reduced availability of hydroxyl groups. When N-methylmorpholine-N-oxide monohydrate was used as a direct solvent, a very high content of C₆-carboxylate groups was found to reduce the solubility of the pulp fibers as less hydrogen bonds can be formed with NMMO·H₂O. In the case of dissolution in the complexing solvent cupriethylenediamine, the dissolution mechanism of cellulose was not deteriorated by the high content of C₆-carboxylate groups. Instead, the oxidation procedure increased the hydrophilic character and the swelling capacity of the outer cell wall layers allowed homogeneous dissolution.     

Possibilities for Optimization of Industrial Alkaline Steeping of Wood-Based Cellulose Fibers

Steeping of cellulosic materials in aqueous solution of NaOH is a common pre-treatment in several industrial processes for production of cellulose-based products, including viscose fibers. This study investigated whether the span of commonly applied process settings has the potential for process optimization regarding purity, yield, and degree of transformation to alkali cellulose. A hardwood kraft dissolving pulp was extracted with 17–20 wt% aq. NaOH at 40−50 °C. The regenerated residue of the pulp was characterized regarding its chemical composition, molecular structure, and cellulose conformation. Yield was shown to be favored primarily by low temperature and secondly by high alkali concentration. Purity of xylan developed inversely. Both purity of xylan and yield varied over the applied span of settings to an extent which makes case-adapted process optimization meaningful. Decreasing the steeping temperature by 2 °C increased xylan content in the residue with 0.13%-units over the whole span of applied alkali concentrations, while yield increased by 0.15%-units when extracting with 17 wt% aq. NaOH, and by 0.20%-units when extracting with 20 wt%. Moreover, the yield-favoring conditions resulted in a narrower molecular weight distribution. The degree of transformation via alkali cellulose to cellulose II, as determined with Raman spectroscopy, was found to be high at all extraction settings applied.     

Properties of cellulose pulps from acidic and basic processes

Research has intensified in recent years on organic solvent pulping processes to supplement or replace conventional pulping processes. One of the main problems with organosolv pulps is the inferior tear strength compared to kraft pulps. An investigation of the properties of two acidic (acetic acid organosolv and acid sulfite) and one basic white spruce pulp (kraft) was carried out to determine factors affecting differences in tear strength. Properties evaluated were lignin and sugar content, mineral composition, ESCA oxygen-to-carbon ratios, acid-base characteristics, water wettabilities, degree of polymerization and crystallinity of cellulose, fiber length and coarseness, and physical properties of the various pulps. Differences in tear strength have been attributed to degradation and changes in the cellulose structure, the hemicellulose-lignin matrix in which the degree of polymerization of hemicelluloses plays the most important role in low yield pulps, and finally, the bonding capacity of the fiber surfaces.     

13C CPMAS NMR investigations of cellulose polymorphs in different pulps

In order to obtain information about the crystallinity and polymorphs of cellulose, and the occurrence of hemicelluloses in pulp fibers, wood cellulose, bacterial cellulose, cotton linters, viscose, and celluloses in different pulps were investigated by solid state ¹³C CPMAS NMR spectroscopy. A mixed softwood kraft pulp and a dissolving-grade pulp were treated under strongly alkaline and acidic conditions and the effect on cellulose crystallinity was studied. The presence of different crystalline polymorphs of cellulose and the amounts of hemicelluloses are considered.     

Catalyzed sulphite and semichemical pulping

Very high yield sulphite pulps were produced by cooking black spruce wafers in pulping liquors at pH 7 or 10, containing 0.1% (on O.D. wood) of soluble anthraquinone (SAQ). These pulps had better strength properties relative to controls prepared without SAQ, breaking length and burst index being greater, on average, by 20%. Other improvements included: increased pulping rate, lower lignin contents at comparable pulp yields, and higher carbohydrate content at the same level of residual lignin in pulp (this resulted in an increase of total pulp yield by 2%). Results of cooks in liquors ranging in pH from 4 to 10, and under variable conditions of time (20–60 min) and temperature (120–160°C) suggested that: firstly, AQ does not act as a pulping catalyst at pH 4, and secondly, the sulphonate contents of AQ-catalyzed pulps are lower than those of the uncatalyzed controls. In the light of the lower sulphonate content, the higher strength is unexpected.     

Hemicellulose in Dissolving Pulp and its Behaviour during its Processing to Viscose

The amount of “hemicellulose” in pulps varies according to wood species, and the pulping processes including their bleaching agents. Making viscose cellulosic and non-cellulosic material is removed during mercerisation which is the first processing step. Low molecular weight material is also formed during the reduction of the degree of polymerisation in order to fit the alkali cellulose for xanthation and dissolving, respectively. In this work commercially available dissolving pulps with respect to their behaviour during the preparation of viscose fibres shall be discussed. For these investigations a Eucalyptus sulphite and a Eucalyptus pre- hydrolysed sulphate pulps were selected.     

A precise study on the feasibility of enzyme treatments of a kraft pulp for viscose application

The development of efficient process steps to convert paper-grade to dissolving pulps was investigated as part of the work programme to improve the process economics. The challenge of pulp refinement comprises the selective removal of hemicelluloses and the precise adjustment of the pulp viscosity, while maintaining the reactivity of the pulp as required for viscose application. The purpose of this study was to investigate the effects of various enzyme treatments on a commercial oxygen-delignified Eucalyptus globulus paper-grade kraft pulp in the course of a total chlorine free bleaching sequence in combination with refining techniques following the principle of Modified Kraft Cooking (Sixta et al. 2007). The objectives were to assess its applicability as viscose pulp besides the reduction of chemical consumption in alkaline and ozone bleaching steps by means of xylanase pre-treatment and the controlled adjustment of final pulp viscosity utilizing endoglucanase post-treatment. Xylanase pre-treatment combined with cold caustic extraction at reduced alkalinity efficiently removed the hemicelluloses from the pulp and clearly increased the pulp brightness by extensive removal of hexenuronic acid side chains. The xylanase pre-treated pulp showed increased reactivity towards xanthation and high viscose dope quality in terms of particle content. The dependence of cellulose chain scission on the applied endoglucanase concentration was analyzed in detail, and this allowed precise viscosity reduction as well as reactivity increase. The differently treated pulps, with and without xylanase pre-treatment, were of very narrow molecular weight distribution and the quality of the spun fibers were very similar to those viscose fibers from commercial dissolving pulps.     

Influence of xylan on the degradability of laboratory kraft pulps from hardwood and reed canary grass in acid hydrolysis

The depolymerisation of laboratory-prepared kraft pulps from birch, eucalyptus and reed canary grass during acid hydrolysis was studied. The intention was to study especially the influence of xylan content on the levelling-off degree of polymerisation (LODP) and on the dissolution of carbohydrates during the acid hydrolysis. The xylan content in the pulps was varied by prehydrolysis prior to the kraft pulping or by alkali-extraction of the bleached pulps, and the levelling-off degree of polymerisation was compared with the amount of xylan left in the pulps at LODP. The dimensions of the original fibers in the pulps and of the fiber fragments after hydrolysis were also measured. It was found that the fiber fragments after hydrolysis were longer in the pulps containing a higher amount of xylan. Xylan thus appears to prevent degradation during acid hydrolysis, both on the fiber level and on the individual cellulose fibril level.     

Reactivity of Dissolving Pulp for Processing Viscose

Summary : Pulp reactivity is a kinetic term and is always connected with a certain derivatization process. The quality and hence the market value of the pulp is determined by such characteristics as α- cellulose content, solubility, brightness, ash content, as well as the amount of soluble material in dichloromethane. However, solubility data, especially S₁₈ and S₁₀ values do not characterise dissolving pulp reactivity. These are indicative of pulp solubility and provide some information regarding losses of material during pulp processing. One way by which the pulp reactivity for viscose making can be characterised is the investigation of the mercerisation step. Following the mercerisation kinetics by help of the molecular weight distribution of cellulose II the behaviour especially of the high molecular weight cellulose gives information regarding the accessibility and therefore, about the reactivity of the pulp aside from losses in low molecular weight cellulose. This behaviour will be shown on different pulps and the physicochemical background will be discussed in relation to results obtained from wide angle X-ray scattering and Raman investigations. The influence of the behaviour of the pulp during mercerising on the viscose process, and the molecular weight distribution of the viscose including the distribution of the xanthogenate groups along the chain was investigated and will also be discussed.     

Effect of Glucuronoxylan on the Hornification of Eucalyptus globulus Bleached Pulps

The effect of 4-O-methylglucuronoxylan (GX) on the hornification of bleached kraft and acid sulphite Eucalyptus globulus chemical pulps has been investigated. Almost straight-line dependence of kraft pulp hornification from GX content was explained through the diminishing of fibrils aggregation and better accessibility of amorphous cellulose regions to water in GX enriched pulps. The higher hornification of sulphite than kraft pulp was assigned to lower GX content in the former and to unfavourable rearrangement of cellulose molecules in crystalline and amorphous regions during acid sulphite pulping.     

SO2–ethanol–water (SEW) fractionation process: production of dissolving pulp from spruce

SO₂–ethanol–water (SEW) fractionation process is a highly attractive platform for future lignocellulosic Biorefineries. Its governing advantages include high flexibility in the selection of the raw material, simple and efficient recovery of fractionation chemicals, absence of carbohydrate degradation (both cellulose and hemicelluloses), and high reaction rates. The process is suitable for production of various carbohydrate- and lignin-based products including papermaking pulp, glucose, bioalcohols and lignosulfonates. The present paper addresses the possibility of producing dissolving pulp from spruce using SEW fractionation followed by ECF bleaching with and without hot caustic extraction. Comprehensive characterisation of chemical and macromolecular properties of the SEW dissolving pulps was complemented by determining the quality of viscose. The comparison with conventional viscose-grade acid sulfite pulps revealed close proximity in all properties. Therefore, considering the advantages of SEW process, it is suggested as a possible replacement for acid sulfite process in dissolving pulp manufacturing.     

Nypa fruticans Frond Waste for Pure Cellulose Utilizing Sulphur-Free and Totally Chlorine-Free Processes

The search for alternative methods for the production of new materials or fuel from renewable and sustainable biomass feedstocks has gained increasing attention. In this study, Nypa fruticans (nipa palm) fronds from agricultural residues were evaluated to produce pure cellulose by combining prehydrolysis for 1–3 h at 150 °C, sulfur-free soda cooking for 1–1.5 h at 160 °C with 13–25% active alkali (AA), 0.1% soluble anthraquinone (SAQ) catalyst, and three-stage totally chlorine-free (TCF) bleaching, namely oxygen, peroxymonosulfuric acid, and alkaline hydrogen peroxide stages. The optimal conditions were 3 h prehydrolysis and 1.5 h cooking with 20% AA. Soda cooking with SAQ was better than the kraft and soda process without SAQ. The method decreased the kappa number as a residual lignin content index of pulp from 13.4 to 9.9–10.2 and improved the yields by approximately 6%. The TCF bleaching application produced pure cellulose with a brightness of 92.2% ISO, 94.8% α-cellulose, viscosity of 7.9 cP, and 0.2% ash content. These findings show that nipa palm fronds can be used to produce pure cellulose, serving as a dissolving pulp grade for viscose rayon and cellulose derivatives.     

Thermogravimetric measurement of amorphous cellulose content in flax fibre and flax pulp

Usually the raw material for flax pulp production is a blend which contains fibres and shives. In order to better understanding the structure of these materials and the effects of flax pulping, X-ray diffraction and thermogravimetry analysis under air atmosphere have been used. There was a significant effect of the fibre size on the composition, crystallinity, and thermal behaviour of the flax pulps. On the other hand, data obtained from thermogravimetric analysis have been modelled on the basis of two cellulose types characterized by different crystallinity levels, using kinetics equations based on the nucleation concept. As a result of these simulations, composition of the samples, pulp crystallinity and the proportion of amorphous cellulose are calculated.