Hemicelluloses removal in autohydrolysis pretreatment enhances the subsequent alkali impregnation effectiveness of poplar sapwood

doi:10.1016/j.biortech.2016.10.017

Bioresource Technology

Volume 222, December 2016, Pages 361-366

https://doi.org/10.1016/j.biortech.2016.10.017 Get rights and content

Highlights

•
Poplar sapwood with different hemicelluloses content by autohydrolysis was got.
•
The autohydrolyzed poplar sapwood was subjected to alkali impregnation.
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Porosities of the fiber cell walls were increased due to the hemicelluloses removal.
•
Hemicelluloses removal raised alkali impregnation effectiveness of poplar sapwood.

Abstract

This work is aimed at investigating the influence of changes in the content of hemicelluloses of the autohydrolyzed poplar sapwood on the subsequent alkali impregnation for chemi-mechanical pulping (CMP). An alkali impregnation process was conducted using the autohydrolyzed poplar sapwood with different content of hemicelluloses as raw materials. The results showed that both the amount of NaOH consumption and swelling degree of poplar sapwood increased with the removal of hemicelluloses, thus enhancing the alkali impregnation effectiveness. The hemicelluloses removal can also shorten the alkali impregnation time for the autohydrolyzed poplar sapwood to achieve the similar impregnation effectiveness of unautohydrolyzed poplar sapwood. All of these can be attributed to the fact that the hemicelluloses removal would result in the exposure of more free hydroxyl groups on the cellulose and an increase in the porosity of the fiber cell walls.

Introduction

As a fractionation method of biomass chemical composition, autohydrolysis pretreatment has been widely studied by many researchers due to its cost-effectiveness in the removal of hemicelluloses. Yoon et al. (2008) found that autohydrolysis of loblolly pine chips could lead to the removal of a great part of hemicelluloses, with a minor dissolution of lignin and celluloses. Duarte et al. (2011) studied the influence of autohydrolysis on the mechanical properties of sugar maple kraft pulp fibers. They noticed that the fiber lengths had little change, while the fines content was reduced, and the tensile and burst strengths of the corresponding handsheets also had significant losses after autohydrolysis pretreatment. When autohydrolysis pretreatment was applied to chemi-thermomechanical pulping (CTMP) (i.e., one kind of chemi-mechanical pulping (CMP)), it could be beneficial not only to the high-value utilization of hemicelluloses in wood raw materials but also to the reduction of refining energy consumption with acceptable variation of pulp properties (Hou et al., 2014).

During autohydrolysis pretreatment, hydronium ions can react with the acetyl groups on the side chains of the xylan in hardwood, and the resultant acetic acid will act as the catalytic agent to further promote more removal of the acetyl groups (Gütsch et al., 2012, Li et al., 2014). The content of hemicelluloses in the autohydrolyzed wood chips will be reduced significantly, and thus inevitably affect the subsequent alkali impregnation in CMP using the autohydrolyzed wood chips as raw materials.

Alkali impregnation is an important process commonly used in CMP. A homogeneous impregnation determines the chemical distribution in the interior of wood matrix, which affects the properties of resultant pulp (Bengtsson and Simonson, 1984, Malkov et al., 2003, Zanuttini and Marzocchi, 2003). Zanuttini and Marzocchi (2003) confirmed that the wood alkali swelling, which increased with the reinforcement of the alkaline action, could determine the strength and optical properties of chemi-mechanical pulp. In addition, the energy consumption of further refining in CTMP was also drastically reduced by alkali impregnation (Stationwala, 1994, Shen et al., 2002).

An alkali impregnation process involves both mass transfer and chemical reaction. There are three factors influence the mass transfer in wood chips (Malkov, 2002; Bao and Lv, 1992), which relate to the properties of wood itself, impregnation liquor properties and the process conditions. Among them, the most important factors are the structure of wood capillaries and wood chemical composition (Malkov, 2002). While for the chemical reaction, the alkali added would react mainly with the acetyl groups and other weak acidic groups of wood (Katz et al., 1981). These reactions, which are expected in the alkali impregnation process, are favorable for the subsequent refining and defibrillation in CTMP (Zanuttini and Marzocchi, 1997, Sixta, 2006). The direct relationship between wood fiber swelling and the content of acetyl groups has been established by Zanuttini and his co-workers (Zanuttini et al., 1998, Zanuttini et al., 2003). They also found that the deacetylation reaction accounted for the most of the alkali consumption in the alkali impregnation, and that the swelling caused by the alkali impregnation was related to the acetyl groups, which mainly come from the hemicelluloses chains, rather than acidic groups. Evidently, the content of hemicelluloses, in great part, determines the alkali impregnation effectiveness.

In Xu’s study (Xu et al., 2016), the effect of autohydrolysis pretreatment on the subsequent alkali impregnation of poplar wood chips was explored. However, how did the content of hemicelluloses remained in the autohydrolyzed wood chips affect the subsequent alkali impregnation effectiveness was not studied in detail. In this paper, the autohydrolyzed poplar sapwood with different content of hemicelluloses was selected as raw materials to perform an alkali impregnation. Both the amount of NaOH consumption and alkali swelling degree of the sapwood were determined to characterize the change in the alkali impregnation effectiveness. The mechanism of hemicelluloses removal by autohydrolysis pretreatment affecting the subsequent alkali impregnation effectiveness was then investigated. The objective of this work is to improve the performance in CMP using the autohydrolyzed poplar wood chips as raw materials.

Section snippets

Materials

A 100 mm length (longitudinal direction) poplar stem wood disc with a diameter of 28.8 cm was obtained from a freshly felled 7-year-old poplar in Tangshan, China. After being debarked, the disc was then air-dried at room temperature until the moisture content was reduced to approximately 10%. The sapwood portion was selected and then cut in the form of 30 mm (longitudinal) × 30 mm (radial) × 10 mm (tangential) pieces. After being picked out the knots, the poplar sapwood chips were washed with deionized

Proposed concept for enhancing alkali impregnation effectiveness with increasing removal of hemicelluloses by autohydrolysis pretreatment

Under the premise of keeping pulping yield, the objective of alkali impregnation of wood chips in CMP is to create a favorable condition for further refining by improving the swelling of fibers. Improving the accessibility of wood fiber cell walls is an effective way to enhance the alkali impregnation effectiveness. To our knowledge, two different methods could be used to improve the accessibility: (1) breaking the hydrogen bonding between the acetyl groups on hemicelluloses and the

Conclusions

The effect of hemicelluloses removal caused by autohydrolysis pretreatment on the alkali impregnation effectiveness of the autohydrolyzed poplar sapwood was studied. Removing a certain amount of hemicelluloses in the poplar sapwood by autohydrolysis pretreatment led to an increase in porosity of the fiber cell walls and in the exposure of more free hydroxyl groups on the cellulose, enhancing the subsequent alkali impregnation effectiveness of the autohydrolyzed poplar sapwood. The

Acknowledgements

This work was financially supported by the Natural Science Foundation of China (Grants 31270630, 31300491 and 31570574) and Project of Tianjin University of Science and Technology “13th Five-Year” Comprehensive Investment Innovation Team.

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Hemicelluloses removal in autohydrolysis pretreatment enhances the subsequent alkali impregnation effectiveness of poplar sapwood

Highlights

Abstract

Introduction

Section snippets

Materials

Proposed concept for enhancing alkali impregnation effectiveness with increasing removal of hemicelluloses by autohydrolysis pretreatment

Conclusions

Acknowledgements

Bioresour. Technol.

Bioresour. Technol.

Bioresour. Technol.

Chem. Eng. Sci.

Process Biochem.

A study on the principle of permeability control of wood

Sci. Silvae Sin.

Sulphite treatment of aspen. Factors affecting the formation of carboxylate and sulphonate groups

Pap. Puu

Chemimechanical pulping of birch wood chips. 1. Studies on impregnation of wood blocks using radioactive sulfur

Pap. Puu Pap. Tim.

Determination of organic acids in extraction liquor from hot-water prehydrolysis of eucalyptus by ion chromatography

China Pulp Pap.

Effect of hot water extraction on hardwood kraft pulp fibers (Acer saccharum, sugar maple)

Ind. Eng. Chem. Res.

Achieving refining energy savings and pulp properties for poplar chemi-thermomechanical pulp improvement through optimized autohydrolysis pretreatment

Ind. Eng. Chem. Res.

A mechanism for the alkali strengthening of mechanical pulps

Tappi J.

The role of hemicelluloses in the delignification of wood

Can. J. Chem.

Release of acetic acid and its effect on the dissolution of carbohydrates in the autohydrolysis pretreatment of poplar prior to chemi-thermomechanical pulping

Ind. Eng. Chem. Res.