Extraction of hemicelluloses from fiberized spruce wood
Introduction
Norway Spruce (Picea abies) contains 25–30% hemicelluloses, of which about 18% is present in the compound middle lamella, 19% in the S1 layer and 63% in the S2 and S3 layers of the secondary cell wall (Fengel and Wegener, 1983, Sjöström, 1993). The extractability of hemicelluloses from wood is limited, probably due to that lignin crosslink the different polysaccharides which decreases their solubility (Lawoko, Henriksson, & Gellerstedt, 2006), but a portion of these hemicelluloses is soluble in hot water or even water at room temperature (Casebier et al., 1969, Örså et al., 1997, Willför and Holmbom, 2004). The amount of water soluble hemicelluloses is large at high temperatures and long treatment times, however, hydrolytic cleavage degrades the polysaccharides at elevated conditions (Lundqvist et al., 2002, Song et al., 2008). A considerably large amount of hemicelluloses can be extracted from ground wood compared to wood chips (Song et al., 2008) on the cost of low molecular weight hemicelluloses and a wet solid residue having a value corresponding to the fuel value. The dissolution is even more extensive from thermomechanical pulp (Thornton et al., 1994, Willför et al., 2003b). The refining process in thermomechanical pulping (TMP) vigorously cracks S1/S2 fiber walls, which prompts the dissolution of hemicelluloses from cell walls into process water. Hemicellulose from TMP process water can be fractionated with high purity by combining different techniques; for example, ultrafiltration and chromatography (Westerberg et al., 2012, Willför et al., 2003a). Water soluble polysaccharides mainly consists of O-acetyl-galactoglucomannans (AcGGM) (Lundqvist et al., 2003, Thornton et al., 1994) with molecular weight ranging up to as high as 60 kDa (Lundqvist et al., 2002, Willför et al., 2003b). Other water soluble substances include mainly acidic arabinogalactans, some xylans and lignin (Lundqvist et al., 2002, Willför and Holmbom, 2004).
Potential applications of hemicelluloses are still exploring. To date hemicelluloses have been tested for making films to be used as gas barriers with promising mechanical properties (Edlund et al., 2010, Höije et al., 2005, Mikkonen et al., 2012), as an additive in paper making to improve paper strength properties (Hannuksela et al., 2003, Hartmans et al., 2009) and as hydrogels (Gabrielii et al., 2000, Lindblad et al., 2001).
TMP is an energy intensive process and most of the energy is converted to heat while refining. This is a large economical and environmental problem both when TMP is used for classical pulping for paper properties and also for the method as a pre-treatment stage in biorefinery concepts. However, in recent years much work has been done to understand the mechanism of mechanical pulping on the wood structure in order to reduce the energy demand during refining. Mechanical pre-treatment of chips combined with increased refining intensity has been demonstrated to considerably reduce the energy demand in a novel process called advanced thermomechanical pulping (ATMP) (Johansson, Hill, Gorski, & Axelsson, 2011). The pre-treatment stage in ATMP consists of an impressafiner and a fiberizer. The impressafiner is a screw-press where chips are compressed at a high strain in a pressurized environment. A fiberizer is essentially a small moderately pressurized refiner which defibrates the wood chips into fiber bundles called as fiberized wood (Hill et al., 2009, Hill et al., 2010). The compressive pre-treatment separates the wood fibers and creates cracks in S1 and S2 fiber walls which increases the surface area and decrease the mass transport length (Kure, Dahlqvist, Sabourin, & Helle, 1999). Compressive pre-treatment prior to mainline refining saves up to 20% energy compared to conventionally produced TMP pulps (Sabourin, Aichinger, & Wiseman, 2003). Integrating a hot water extraction system together with mechanical pre-treatment in the primary stages of either mechanical or chemical pulping processes will, in addition to saving energy, produce valuable products from isolated hemicelluloses which otherwise are degraded and/or dissolved in the processing liquors (Heiningen, 2006, Lisboa et al., 2005).
In this study, fiberized spruce wood has been subjected to hot water extractions with the goal to investigate its possible role in a biorefinery context by extracting high molecular weight galactoglucomannan (GGM) and that the solid residue could be used as a raw material in the pulping processes. Different extraction conditions of time and temperature were used in order to investigate the effect on the amount and molar mass of isolated polysaccharides. Since the aim was to isolate relatively native hemicelluloses therefore elevated treatment conditions were not studied due to the risk of polymer degradation. The extracted material was investigated for lignocellulosic composition, molar mass distribution and acetyl content. An attempt to reuse the extraction liquor containing dissolved solids from the wood has also been made in order to foresee the potential in a mill application to increase the concentration of hemicelluloses in the extraction liquor.
Section snippets
Material and mechanical pre-treatment
Chips of Norway spruce (P. abies) were obtained directly after the impressafiner from Braviken paper mill, Holmen Paper AB in Norrköping, Sweden. Impressafined chips were steamed for 5 min with atmospheric steam and fiberized using a pilot scale 12″ disc refiner (Sprout-Waldron) at Chalmers University of Technology, Sweden. The net energy input of the refiner was adjusted to 300 kWh/odt wood by adjusting the speed of the conveyer belt delivering wood to the refiner in a single pass mode.
Effects of mechanical pre-treatment
Mechanical pre-treatment stage was comprised of macerating the wood chips in an impressafiner followed by defibration in a fiberizer. In the impressafiner (a plug screw-press), chips were compressed at high strain in a pressurized environment that squeezed out water, extractives and entrained air. Defibration was performed in the refiner at a net energy input of 300 kWh/odt wood, considerably lower than the energy demand of a primary refiner in a TMP mill. Impressafined chips retain the basic
Conclusions
This study showed that
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The fiberizer, with a considerably low energy input of 300 kWh/odt wood, cracked the fiber walls to the extent which enabled a partial dissolution of the wood polymers into water.
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A fraction of hemicelluloses was readily dissolved in water from the fiberized wood at all temperatures. However, the extraction yield increased with the increase in temperature and time reaching to the maximum at 90 °C and 120 min.
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The extract obtained at 90 °C/120 min contained about 6 mg/g wood of
Acknowledgments
Holmen Paper AB is acknowledged for providing us with the impressafined spruce chips used in this study. Tommy Friberg and Liming Zhang are thanked for their help with refining and translating NMR data, respectively. Knut and Alice Wallenberg foundation is gratefully recognised for the financial support within Wallenberg Wood Science Center.
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