Towards a novel generation of dissolving pulp
The demand for high-purity cellulose pulps, also known as dissolving pulps, has substantially increased during the last few years. The upturn of dissolving pulps in the market may be attributed to a consistent growth of regenerated cellulose fibers production, largely initiated by an increasing demand in China and other Asian countries. Market studies indicate that this trend of increasing demand of regenerated cellulose fibers and thus dissolving pulps will prevail during the next decades.
It is estimated that in 2050 the annual demand of textile fibers ranges between 120 and 130 million tons owing to the growth of population and living standards. Because of the environmental and agricultural restrictions for cotton production, cotton cannot keep the present share of 31% from global fiber production. Thus, it can be predicted that in 2050 the gap of cellulose-based fibers is ranging between 7 and 10 million tons. However, the growing demand for highly purified cellulose pulps is not only limited to textile applications, but also concerns the manufacture of cellulose acetate for high value-added films, plastics and coatings as well as cellulose mixed ethers for lacquers and printing, cellulose ethers and cellulose powder which have found important applications in food and pharmaceutical industries. Moreover, dissolving pulps seem to be the preferred substrate for the manufacture of nanofibrillated cellulose (NFC), a future precursor of advanced materials. Currently, dissolving wood pulps are produced by the acid sulfite and the vapor-phase prehydrolysis kraft processes which were both developed in the 1950s. While the former remained technically largely unchanged, a modern displacement cooking procedure was adopted to the vapor-phase prehydrolysis kraft process, known as Visbatch®, VisCBC processes as well as other displacement cooking techniques developed by Metso and GL&V, respectively.
The growing demand of high purity dissolving pulps, however, requires the development of novel process concepts which allow both the realization of advanced biorefinery concepts and the manufacture of pure cellulose pulps, revealing a quality profile comparable to that of cotton linters.
Our innovation strategy with regard to dissolving pulp comprises both the further development of existing technologies as well as radical innovations.
Principally, acid sulfite pulping offers a good basis for the realization of the biorefinery concept in that the three lignocellulosic polymers, cellulose, hemicellulose and lignin may be recovered as dissolving pulp, monomeric sugars and lignosulfonate in economically attractive quantities. However, a lot of drawbacks are associated with the present acid sulfite pulping technology: low flexibility in the selection of raw material sources, long overall cooking time due to very slow impregnation of wood chips, inefficient and costly recovery of cooking chemicals, conversion of substantial amounts of released monomeric sugars to aldonic acids which reduces and hinders the recovery of the former and finally the low cellulose purity of the resulting dissolving pulps. SO2-ethanol-water pulping (SEW) has the potential to be a viable alternative to metal-based acid sulfite cooking. Yet the former process has certain distinct advantages over the latter in terms of rational biomass use and operational efficiency. The ethanol present in the cooking liquor allows fast transport of the pulping agents to the reaction sites inside the wood, which eliminates the need for a separate impregnation step and decreases substantially the overall cooking duration. Further, ethanol is known to be a better solvent for lignin and lignosulfonate than water. The absence of a base in the process reduces the recovery cycle to simple distillation of ethanol and unreacted SO2. Ethanol does not participate in the reactions and can thus be recovered almost quantitatively. The presence of ethanol and the absence of a base reduce the amount of hydrosulfite anions, which are responsible for the oxidation of monosaccharides to aldonic acids, a wasteful pathway seen in conventional acid sulfite pulping.
Together with the pioneers of SEW pulping, Professor Adriaan van Heiningen and Dr. Mikhail Iakovlev, the SEW fractionation concept is currently investigated with regard to the manufacture of dissolving pulp from spruce wood. The experimental work is largely done by You Xiang within the framework of her Master’s thesis. The preliminary results confirm the above mentioned advantages of the SEW process over the Mg-based process. However, the yield and the purity of the resulting dissolving pulp are comparable to those of the Mg-based acid sulfite process.
The limitations in terms of cellulose purity as revealed for the SEW process may be overcome by the following two developments: The first concept envisions hydrothermolysis as a pre-fractionation process where hemicelluloses may be removed quantitatively, if required. At the same time large amounts of lignin are degraded, presumably by the cleavage of LCC bonds and, moreover, by homolytic cleavage of arylether bonds. The final delignification may be accomplished by subsequent mild alkaline delignification or by dissolution in an appropriate solvent. To minimize secondary side reactions, e.g. recondensation reactions, hydrothermolysis will be carried out in a flow-through reactor system. As a first reactor system we pursue a flow-through reactor with combined recirculation and percolation mode as shown in Figure 1.
Figure 1: Equipment for hydrothermal pretreatment and Soda anthraquinone cooking (W = water; SAQ = Soda-AQ cooking; H = hydrolysate)
Following this concept, the sugar concentration reaches a level which ensures a cost-effective conversion to value-added products. By applying appropriate conditions, the hemicellulose content of the wood may be lowered to 1-2% on odw, while the cellulose content remains basically unchanged.
The second technical concept of hydrothermal treatment constitutes a shrinking-bed reactor. It is a percolation-type reactor, capable of reaching temperatures up to 300 °C and pressures up to 130 bars. The reactor is characterized by a piston that moves downwards and compresses the woody biomass along with its degradation and dissolution. This allows for a constant packing density within the reactor chamber, as well as for a reduction of the liquid retention time at a given flow-rate. This novel technology is expected to be very efficient and selective in the removal of hemicelluloses. Figure 2 shows an image of the core part of the reactor system.
Figure 2: Lab-scale shrinking-bed reactor
As already mentioned, the residual lignin remaining after the hydrothermal treatment is removed either by mild Soda-Anthraquinone (SAQ) treatment or by dissolution in a selective lignin solvent. The addition of an appropriate stabilizer during SAQ cooking has shown to preserve pulp yield efficiently. The sulfur-free lignin released during SAQ cooking or solvent extraction is isolated and purified following known protocols.
The second approach for the manufacture of a high-purity cellulose pulp envisages pre-alkaline extraction followed by SAQ cooking and post-alkaline extraction treatment. This fractionation scheme aims at producing cellulose pulp and hemicelluloses both of high molar mass and purity. Quite recently we could show that pre-alkaline extraction of hardwood allows the removal of about one third of the wood xylan as a high molar mass polymer while only small amounts of lignin are removed concomitantly which cheapens subsequent xylan isolation and purification procedures. Pre-alkaline extraction of wood does not significantly lower the hemicellulose content of the unbleached pulp. However, it contributes to a higher lignin purity of the black liquor which in turn simplifies the recovery of sulfur-free lignin. Thus, the final pulp purity is accomplished by cold caustic extraction (CCE), typically applied after oxygen delignification. This treatment is particularly suited for the removal of xylan, while it is less efficient for the separation of glucomannan, the main hemicellulose component of a softwood pulp. For the refining of the latter we suggested to add borates to the alkaline extraction solution which is known to facilitate the removal of glucomannans. The final bleached pulp is subjected to an endoglucanase (EG) treatment for the adjustment of pulp viscosity to the desired level and to improve the accessibility of the individual cellulose molecules. The latter is important for subsequent dissolution or derivatization reactions of a dissolving pulp. The technical concept is visualized in Figure 2.
Figure 3: Scheme of the manufacture of high molar mass, high-purity cellulose pulp and the separation of hemicelluloses and sulfur-free lignin.
All the mentioned concepts of dissolving pulp manufacture are based on existing pulping technologies. However, a completely new concept for the preparation of a pure cellulose pulp is currently under investigation. Unfortunately, it is too early to disclose this ground-breaking technology due to a pending patent application.
Last but not least it is my pleasure to introduce my dissolving pulp research team:
From left to right: Dr. Marc Borrega, MSc. Lidia Testova, Xiang You and Dr. Mikhail Iakovlev.
Herbert Sixta, Biorefineries Research, Department of Forest Products Technology, School of Chemical Technology