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- The effect of pre-hydrolysis Kraft pulping conditions on the production of dissolving pulp and valorization of by-productsPublication . Cunha, André Eduardo Palos; Simões, Rogério Manuel dos SantosThis study evaluated the production of dissolving pulps from Eucalyptus globulus through the pre-hydrolysis Kraft process. While the pulp and paper industries employ the best available technologies, there is potential to enhance their overall environmental and economic performance by valorizing secondary products generated from current processes. This is also true for dissolving pulp production, which requires extensive removal of hemicelluloses; their valorization can help make the process more cost-effective. Given that 30-45% of the wood used is converted into dissolving pulp, the by-products account for a sizable portion (55-70%) of the raw material utilized. To address these issues, the work comprises two complementary parts. The first part focuses on the main product, the dissolving pulp. The second part examines the valorization of by-products, specifically the hemicelluloses removed during pre-hydrolysis and the lignin extracted during Kraft/Soda cooking. Dissolving pulp production has been extensively researched over the past 50 years, with continuous studies exploring processes and raw materials. These efforts aim to optimize pre-hydrolysis and cooking conditions to maximize yield and improve product quality. To explore this topic further, an experimental system was developed to conduct the pre-hydrolysis stage under non-conventional conditions using a continuous flow system. The impact of the liquid renewal rate in pre-hydrolysis, manipulated by the flow rate, was investigated, maintaining temperature and time at standard conditions. The laboratory-developed system enabled the assessment of how pre-hydrolysis conditions affect the properties of the dissolving pulp. The quality of the dissolving pulp was assessed based on various properties, such as porosity, cellulose degree of polymerization, crystallinity, chemical composition, and Fock reactivity, which are crucial parameters for control. The dissolving pulp produced using the liquid renewal in pre-hydrolysis provided a pulp with a lower cellulose degree of polymerization (800 cm3/g vs. 1158 cm3/g), lower xylan content (3.0 % vs 6.8 %) and higher Fock reactivity (71.2 % vs. 52.2 %). In addition, chemical, enzymatic, and mechanical treatments were investigated to enhance the understanding of the relative importance of the pulp properties in Fock reactivity. The objective was to examine each property individually and its influence on pulp reactivity, which is a limiting factor for its dissolution. Although the various approaches facilitate the individual exploration of different properties, intrinsic interdependencies—such as the relationship between the Segal crystallinity index and degree of polymerization—add complexity to the investigation. The expected negative correlation between cellulose degree of polymerization and Fock reactivity was confirmed. Mechanical grinding simultaneously provoked a decrease in crystal size, the Segal crystallinity index, and the cellulose degree of polymerization, as well as an increase in Fock reactivity, illustrating the difficulty in deconvolving the effects of pulp properties. The endoglucanase treatment resulted in a significant decrease in the cellulose degree of polymerization of Kraft pulp (from 873 cm³/g to 558 cm³/g) and a corresponding increase in Fock reactivity (from 52% to 65%), without significantly affecting crystal size and crystallinity index, thereby reinforcing the key role of cellulose degree of polymerization. In the second part of this thesis, considering dissolving pulp production, where hemicellulose and lignin must be removed from the wood, valorizing by-products is crucial for enhancing the industry's sustainability and economic viability. Through advanced extraction, fractionation, and conversion techniques, hemicelluloses and lignin can be transformed into various value-added materials derived from sustainable sources, enabling them to compete directly with products derived from fossil sources. The continuous flow system was used to examine the impact of the residence time of the hemicelluloses extracted from wood inside the pre-hydrolysis reactor on the degree of polymerization of xylooligosaccharides, high-value compounds, and dissolving pulp properties. When the pre-hydrolysis batch mode was changed to the liquid renewal mode, the X2-X3 yield in the hydrolysate increased from 6.1% to 25.0%, representing an almost 5-fold increase in added-value compounds. Additionally, a wide range of methods—chemical, enzymatic, and mechanical—were explored to produce xylan-based oligosaccharides (X1-X6) following the extraction of hemicellulose from various sources. Among the evaluated methods, the enzymatic reaction of hemicelluloses with xylanases emerged as a more environmentally friendly alternative, and it is also selective for compounds of interest, such as xylobiose and xylotriose. However, this method encounters limitations related to substrate-enzyme accessibility, for which several optimizations have been proposed. After pre-hydrolysis, most part of the lignin remains in the solid residue, which now has low hemicellulose content (2-5%). During the subsequent Kraft/Soda cooking stage, lignin was removed along with any remaining hemicellulose by applying heat, pressure, and a highly alkaline medium (Na₂S and NaOH in the Kraft process and NaOH in the Soda process). These processes generated black liquor with a high lignin content and minimal impurities. After the black liquor's acidification, lignin with specific characteristics and high purity was obtained. Lignin can be utilized in two primary ways: first, after depolymerization, it serves as a sustainable source of basic chemical compounds; second, after polymerization, it functions as a polymer with unique mechanical properties. In the present work, the investigation focused on the second approach, emphasizing the valorization of lignin through polymerization to examine its distinctive mechanical characteristics. Since degradation and depolymerization occur during the extraction of lignin from wood, resulting in the loss of some of its properties, especially mechanical characteristics, repolymerization becomes essential to produce compounds with high mechanical strength. As an alternative to traditional chemical reactions, a bacterial alkaline laccase was used to catalyze lignin polymerization/cross-linking in solution and after imbibition in a cellulose porous medium. The lignin polymerization/cross-linking in solution was directly demonstrated; the average molecular weight increased, and the alkaline soluble lignin fraction decreased by 51.0 % after being exposed to alkaline laccase, suggesting strong lignin cross-linking in solution. Lignin imbibition in paper and in-situ polymerization was explored to maximize the formation of bonds within paper-based composites. Among the properties influenced by this cross-linking, the significant increase in tensile strength and decreased wettability of the composite are particularly noteworthy. In summary, this study examined the impact of pre-hydrolysis conditions on the properties of dissolving pulp and correlated the significance of these properties with the pulp's reactivity. Additionally, the valorization of by-products from the pre-hydrolysis cooking process, such as the production of xylooligosaccharides from hemicelluloses and the extraction and integration of lignin in composites, enhances the principles of biorefinery and circular economy within the paper and pulp industry. When subjected to effective valorization strategies, both by-products have the potential to be transformed from mere by-products into valuable co-products of the cooking process. This approach promotes improved resource utilization and reduces waste, aligning with more sustainable practices from both environmental and economic perspectives.