Kinetics of Aqueous Extraction of Hemicelluloses from Spruce in an Intensified Reactor System

Abstract

The aqueous extraction of hemicelluloses has gained increasing interest with new emerging applications for hemicelluloses in the modern forest-based biorefinery concept. The extraction kinetics play a key role in their industrial utilization. The traditional kinetic studies and models for the selective dissolution of softwoods, however, always incorporate high concentrations of sodium hydroxide and sodium sulfide relevant to pulping, and the kinetics in pure water is left outside the scope of these investigations. Aqueous extraction of hemicelluloses from spruce sapwood was investigated with a new cascade reactor setup, which was developed for intensified investigation of solid-liquid dissolution kinetics. The experiments were performed at 150-170 °C with a particle size of 1.25-2 mm and solid loads of about 6.25 g of dry wood/L in the kinetic regime of intrinsic kinetics. The pH of the liquid phase was measured during the reaction. The selectivity of the dissolution and degradation of hemicelluloses was examined qualitatively. The total concentrations of arabinose (Ara), xylose (Xyl), galactose (Gal), glucose (Glc), mannose (Man), rhamnose (Rha), glucuronic acid (GlcA), 4-O-methylglucuronic acid (4-O-MeGlcA), and galacturonic acid (GalA) were analyzed from the liquid and solid phases during the reaction. The dissolution was observed to be highly temperature dependent, and degradation of the compounds was observed. A kinetic model was developed for the overall extraction of the hemicelluloses. The activation energy was determined to be 135 kJ/mol. No significant influence of the pH on the actual extraction kinetics was observed, even though it influences strongly the degradation of the compounds through hydrolysis.

Obtaining Spruce Hemicelluloses of Desired Molar Mass by using Pressurized Hot Water Extraction

“…The activation energies found in the literature (E a = 100-135 kJ mol À1 ), [24,[38][39][40] corresponded well with the results in our own work (120-122 kJ mol À1 ). The activation energies found in the literature (E a = 100-135 kJ mol À1 ), [24,[38][39][40] corresponded well with the results in our own work (120-122 kJ mol À1 ).…”

“…After predetermined points in time, the reactors were bypassed one by one and cooled rapidly by quenching to collect the liquid and solid samples (Table 1) according to GrØnman et al [24] The samples from both phases were stored at À18 8C prior to analysis. This was done to avoid that water penetration into the chips would influence the extraction kinetics.…”

“…In the literature, the pH values during the extraction process have been studied in the 150-170 8C temperature range; [24,29,35,43,44] however, the 120-140 8C range has obtained substantially less attention. The behavior of pH as a function of time at 120-170 8C is displayed in Figure 4.…”

“…[22][23][24][25][26][27][28][29][30][31][32][33][34] The solvent is nontoxic, and if further processing and utilization involves water, [31] no costly separation steps are needed. Several different extraction methods have been investigated, and pressurized hot water extraction has proven to be a very efficient method for extracting the hemicelluloses in an environmentally friendly way.…”

Chemithermomechanical and kraft pulping of Pinus radiata wood chips after the hydrothermal extraction of hemicelluloses

“…In both CTMP and kraft processes, a part of the labile hemicelluloses is solubilized during the cooking process, with minor increases in the heating value of the black liquors or with increased biological oxygen demand of the CTMP effluents (Helmerius et al 2010). The attempts to extract hemicelluloses prior to pulping are well documented by means of diluted acids (Al-Dajani et al 2009;Houtman andHorn 2011), alkaline solutions (Al-Dajani andTschirner 2008;Johakimu and Andrew 2013), steam explosion (Cara et al 2006;Martin-Sampedro et al 2011;Schütt et al 2011), pressurized hot water (Song et al 2008(Song et al , 2011Yoon et al 2008;Grénman et al 2011), and other methods (Mao et al 2008;Jun et al 2012;Rodríguez-López et al 2012;Vila et al 2013). The resulting hemicelluloses could be hydrolyzed to monomers and used as precursors in chemical and biochemical synthesis for the food, pharmaceutical, and chemical industries.…”