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Improved permeability of autohydrolyzed poplar sapwood against sodium hydroxide for CMP production

  • Xiaoya Jiang , Qingxi Hou EMAIL logo , Wei Liu , Honglei Zhang , Yudi Cui and Xiaodi Wang
Published/Copyright: January 24, 2018
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Holzforschung
From the journal Holzforschung Volume 72 Issue 5

Abstract

Poplar sapwood chips were autohydrolyzed at three severity levels and subsequently treated with alkali for the production of chemi-mechanical pulp (CMP). The aim was to study the variation of the principal pathways for sodium hydroxide solution and the alkali concentration and distribution inside the autohydrolyzed chips as a function of autohydrolysis (AH) severity. The tyloses in the vessels and some pit membranes were partly dissolved after AH, while the skeletal structure of the fiber and vessel cell walls remained intact. AH had a positive influence on the diffusion and the uniform distribution of NaOH solution both in the axial and radial direction of the chips, but the axial direction was more favored than the radial one.

Keywords: alkali impregnation; autohydrolysis (AH); Na+ concentration and distribution; permeability of poplar; poplar sapwood (sW); scanning electron microscopy and energy dispersive X-ray analysis (SEM-EDXA); severity of autohydrolysis

Introduction

Extracting hemicelluloses before pulping is one of the approaches for developing the biorefinery concept, in which the obtained hemicelluloses can be used to produce value-added bioproducts (Rivas et al. 2013, 2016; Krogell et al. 2014; Lehto and Alén 2015; Vallejos et al. 2015; Deloule et al. 2017). Autohydrolysis (AH) has a good selectivity for hemicelluloses removal (Yu et al. 2010), and its application was proposed prior to chemical pulping (Testova et al. 2011; Sanglard et al. 2013), chemi-mechanical pulping (CMP), and enzymatic hydrolysis (Duarte et al. 2011; Hou et al. 2014; Tunc and van Heiningen 2014; Deloule at al. 2017). However, such pretreatment changes the wood properties ineluctably (Lu et al. 1994; Leschinsky et al. 2008, 2009; Tunc and van Heiningen 2008). Paredes et al. (2009) investigated the influence of AH on surface properties, porosity and the ultrastructure of red maple wood chips and found more pores with larger size in the cell walls. In this context, Xu et al. (2016) observed a better wettability and higher volume porosity after AH and more fluid absorption as a function of AH intensity.

Alkali impregnation is an essential parameter for CMP production (Malkov et al. 2001; Kolavali 2013) as it is related to mass transfer and chemical reactions, in the course of which the esters of acetyl and other weak acidic groups are saponified (Katz et al. 1981; Zanuttini et al. 1998). The chemical reactions influence the subsequent alkali distribution inside the chips (Egas et al. 2002). In other words, the effectiveness of the alkali impregnation relies on the homogeneity of the alkali treatment, which in turn, influences the pulp quality (Stone and Green 1958; Bengtsson and Simonson 1984; Malkov et al. 2003). Jiang et al. (2016) explored the removal of hemicelluloses by AH of poplar sapwood (sW). However, the alkali distribution after such a pretreatment is not well investigated.

To study this effect in hardwoods, the vessels as the main transport channels should be the focus (Bao et al. 1999). The vessels of some trees may be blocked by tyloses (Tomoyuki et al. 2001). The fiber cell lumens of hardwood, which are connected to each other by pits, also serve for fluid transport (Thomas 1976), while the pits consist of a pit cavity and a pit membrane. The latter is an obstacle to fluid transport. Removing the tyloses and making the pit membranes permeable are key steps for a better impregnation of hardwood chips.

The working hypothesis in the present work is that AH pretreatment could contribute to the better penetrability of wood for alkaline solutions. For checking this, both the axial and radial penetrability of poplar sW after AH was investigated via scanning electron microscopy and dispersive X-ray analysis (SEM-EDXA). The morphological effects of AH were observed. A homogeneous model based on alkali concentrations in the autohydrolyzed poplar sW chips was established. The expectation was that AH would improve the effectivity of CMP production and that this treatment could contribute to application of the biorefinery concept for effective utilization of forest resources.

Materials and methods

Materials:

Discs with a diameter of 28.8 cm were cut from 1.3 m height from freshly felled poplar trees (7-year-old trees grown in Tangshan, China). After debarking, the discs were air-dried at room temperature (rT) until the moisture content (MC) was ca. 10%. The sW portion was selected and cut into blocks along the annual ring, from which samples (30×30×10 mm3) (L×R×T) were prepared (Figure 1). The knots were removed and the chips were washed with deionized water and air-dried.

Figure 1: Schematic diagram of poplar sapwood (sW) chip preparation and the sampling methods of the test specimens for alkali-impregnation (all distances in the drawing are in mm).
Figure 1:

Schematic diagram of poplar sapwood (sW) chip preparation and the sampling methods of the test specimens for alkali-impregnation (all distances in the drawing are in mm).

Autohydrolysis:

Five equal parts of 200 g samples were prepared for AH pretreatment, which was performed in a 6 l digester (M/K systems Inc., MA, USA) equipped with a centrifugal pump for liquor circulation. For more details see Jiang et al. (2016). The combined hydrolysis factor (CHF) served to quantify the intensity of AH (Zhu et al. 2012):

             CHF = t × exp (25.6 − 11000 / T),

where t is the AH time in min, and T is the AH temperature in Kelvin.

Three CHFs were generated (2.78, 10.77 and 73.63) by the AH parameters 393.15 K/30 min, 413.15 K/30 min, and 433.15 K/60 min, respectively. The wood samples treated at the three CHF levels will be referred to in the following as WCHF3, WCHF11 and WCHF74. After AH, the chips were washed thoroughly with deionized water until the filtrate was colorless and its pH was neutral. After being air-dried at rT the chips were kept in plastic bags.

SEM analysis:

The handmade micro slices were gold coated and then observed using SEM (JSM-IT300LV, JEOL, Tokyo, Japan).

Alkali impregnation:

The MCs of the control and autohydrolyzed poplar wood chips were firstly adjusted to 100%. The alkali impregnation was carried out at a liquid/solid ratio of 10:1 (ml g−1) in a polyethylene bag at 80°C. The test chips and the corresponding NaOH (Sinopharm Group Co. Ltd., Shanghai, China) solution (0.5 mol l−1) were separately preheated in polyethylene bags in the same hot water bath. About 20 min later, as the temperatures of the NaOH solution reached 80°C, the chips were immediately immersed in the NaOH solution. Then the bag loaded with the NaOH solution and the chips remained in the water bath, and the impregnation was continued for 60 min. Then the immersed chips were rapidly separated from the alkali and immersed into liquid nitrogen for about 5 min to stop the reaction. The obtained chips were stored in a freezer (DW-86L728J, Haier Co. Ltd., Qingdao, China) at −70°C.

Sampling methods of the test slices:

Every six frozen chips with the same CHF were selected and dried in a freeze dryer (Alpha 1–2 LDplus, Christ, Osterode, Germany) to avoid any secondary thermal diffusion. Then each dried chip was cut into four pieces with a dimension of 15×15×10 mm3 (L×R×T) with a fine power saw and only the lower right specimen for the chip was used (Figure 1). Before Na+ analysis in the L direction, each specimen was further cut into three slices equally along the L direction (Figure 1), i.e. the dimension of the slices was 15×5×10 mm3. Consequently, three different L test layers were obtained, namely at 0, 5 and 10 mm distance from the L axis. A similar cutting scheme was applied for the analysis in the R direction (Figure 1). The dimension of the each specimen was 5×15×10 mm3 with the L distances from 0, 5 and 10 mm. The testing points at an interval of 1.5 mm on the central line, both in the L direction (for the L test) and in the R direction (for the R test), were chosen to determine the Na+ concentration by means of SEM-EDXA (Bengtsson et al. 1988; Sharareh et al. 1996; Kazi et al. 1997). SEM parameters: 10 kV and 10 mm working distance.

NaOH distribution:

Assumptions: (1) the sW chip matrix is axisymmetric, i.e. the characteristics of the whole poplar wood is represented by one quarter of the whole chip (Figure 1); (2) the distribution of NaOH solution inside the chip is represented by that of Na+ in the poplar sW chip; (3) the NaOH concentration surrounding the chips in the bulk solution remains constant; (4) heat-transfer limitations can be considered as negligible, i.e. the temperature throughout the test sW chips was supposed to be uniform. Accordingly, the extent of an alkali impregnation can be determined by SEM-EDXA, with Na+ as the tracer being linearly proportional to the NaOH concentration. Therefore, CNa=κINa and Ce,Na=κIe,Na, where κ is the proportionality constant; CNa and Ce,Na are the Na+ concentrations at a measuring point and the edge of a test chip, respectively; and INa and Ie,Na are the X-ray intensities of Na at the measuring point and the edge of the test chip, respectively. As κ is a constant, CNa/Ce,Na=INa/Ie,Na. Ideally, the Na+ concentration on the outside surface of the chips is the same as that of the bulk solution (Ce,Na=Cbulk,Na). The CNa/Ce,Na ratio was selected to characterize the variation of Na+ inside the chips and Ie,Na was determined via extrapolation. The X-ray intensities of Na+ at four measuring points vs. the distances from the edge of the chip were plotted, while the X-ray intensity and the distance are denominated as Y and X axes, respectively. The X-ray intensity at the intersection between the trend line and Y axis is defined as Ie,Na. On the test layer of the R or L distances of 0 mm, the area with CNa/Ce,Na≥0.2 was defined as impregnation zone, because in the original chips the resultant CNa/Ce,Na ratio was less than 0.2.

Results and discussion

Vessels, pits and cell walls after AH

Figure 2 shows the cross-section status of the original and autohydrolyzed poplar sW chips (for brevity: WAH). As visible in Figure 2a and c that the skeletal structure of vessels and cell walls are intact in WAH. Neither the fiber nor the vessel cell walls are collapsed which is in agreement with the results of Kristensen et al. (2008). However, in a larger magnification, some pores and/or cracks could be found on the cross-section of the cell wall (Figure 2b and d).

Figure 2: The skeletal structure of vessels and cell walls of poplar sapwood chips are intact, but some pores and/or cracks can be found on the cross-section of the cell wall after autohydrolysis.SEM images of the cross-section status of poplar sapwood (sW) chips: (a) and (b) Wcontr; and (c) and d) WCHF74.
Figure 2:

The skeletal structure of vessels and cell walls of poplar sapwood chips are intact, but some pores and/or cracks can be found on the cross-section of the cell wall after autohydrolysis.

SEM images of the cross-section status of poplar sapwood (sW) chips: (a) and (b) Wcontr; and (c) and d) WCHF74.

Only a small amount of tyloses (see the arrows in Figure 3a) could be observed in the vessels of the original poplar sW chips (for brevity: Wcontr). The amount of tyloses, as sac-like membranes that enter the vessels from adjacent parenchyma cells through pit pairs, increases with increasing age of the poplar trees. The vessel interior of the WAH is quite clean (Figure 3b–d), even though the intensity of AH treatment was weak (Figure 3b). This is due to the accelerated mass transfer of alkali at higher temperatures and pressures and not to the effect of AH pretreatment.

Figure 3: The tyloses in the vessels of poplar sapwood chips disappeared after autohydrolysis.SEM images of the vessels in poplar sapwood (sW) chips: (a) Wcontr; (b) WCHF3; (c) WCHF11; and (d) WCHF74.
Figure 3:

The tyloses in the vessels of poplar sapwood chips disappeared after autohydrolysis.

SEM images of the vessels in poplar sapwood (sW) chips: (a) Wcontr; (b) WCHF3; (c) WCHF11; and (d) WCHF74.

Figure 4a shows the intact membranes in the pits of the Wcontr. In the case of WCHF11, some crevices or pores appeared on the pit membranes (Figure 4b). WCHF74 shows many disrupted membranes (Figure 4c). But a few of the pit membranes, which are probably much thicker, are still intact (Figure 4d). It can be concluded that most pit membranes are dissolved and/or damaged during AH. A pit membrane consists of the primary wall and middle lamella, and is sustained by a randomly interwoven microfibril (MF) network (Siau 2012). Hemicelluloses are still the main chemical components of pit membranes. After a part removal of hemicelluloses by AH, the MFs on a pit membrane could be disturbed and showed some voids (Kerr and Goring 1975), which became larger with increasing AH intensity. As a result, the pit membranes became penetrable.

Figure 4: The pit membranes of poplar sapwood chips were partially dissolved and/or disrupted after autohydrolysis.SEM images of the pits in poplar sapwood (sW) chips: (a) Wcontr; (b) WCHF11; and (c) and (d) WCHF74.
Figure 4:

The pit membranes of poplar sapwood chips were partially dissolved and/or disrupted after autohydrolysis.

SEM images of the pits in poplar sapwood (sW) chips: (a) Wcontr; (b) WCHF11; and (c) and (d) WCHF74.

Distribution of Na+ in the L direction

The concentration ratios of Na+ (CNa/Ce,Na) in R distances of 0, 5, and 10 mm (Figure 1) vs. an L distance are plotted in Figure 5. Independently from the AH degree, the diffusion was more intense at the edges than in the center of the chip which was not otherwise expected (Zanuttini et al. 2000). The L penetration increased with increasing AH intensities. For example, the widths of the impregnation zone in the R distance of 0 mm are 10.5, 6.0 and 6.0 mm for WCHF74, WCHF11 and Wcontr (Figure 5a). The CNa/Ce,Na ratio of WCHF74 is much higher than that for WCHF11 and Wcontr. Taking the position at an L distance of 9 mm, shown in Figure 5a as an example, for the inside layer, the CNa/Ce,Na ratios of Wcontr and the WCHF11 and WCHF74 are 0.127, 0.161 and 0.468, respectively.

Figure 5: The penetration in the longitudinal direction increased with increasing the intensity of autohydrolysis.Distribution of Na+ concentration ratio inside poplar sapwood (sW) chips in the longitudinal (L) direction: (a) R distance of 0 mm; (b) R distance of 5 mm; and (c) R distance of 10 mm.
Figure 5:

The penetration in the longitudinal direction increased with increasing the intensity of autohydrolysis.

Distribution of Na+ concentration ratio inside poplar sapwood (sW) chips in the longitudinal (L) direction: (a) R distance of 0 mm; (b) R distance of 5 mm; and (c) R distance of 10 mm.

In the R distances 5 mm (Figure 5b) and 10 mm (Figure 5c), the CNa/Ce,Na ratios for the three slices were 0.145, 0.209, 0.585 and 0.407, 0.578, 0.797, respectively. Clearly, AH pretreatment facilitates the subsequent alkali impregnation in the L direction. There is no remarkable CNa/Ce,Na ratio difference between WCHF11 and Wcontr, demonstrating that 60 min alkali impregnation time is not enough in cases of low AH severity with CHF 11 to obtain a satisfactory effect.

Distribution of Na+ in the R direction after AH

As is visible in Figure 6, the impregnation zone of WCHF74, WCHF11 and Wcontr are 7.5, 4.5 and 4.5 mm, respectively. Not surprisingly, the diffusion extent in the R direction (Figure 6a and b) is much smaller (4 mm in the middle layers, with CHF 11) than that in the L direction (6 mm), (Figure 5a and b). The situation is not changed essentially with WCHF74 (Figures 6c and 5c). This means, the AH does not improve essentially the R diffusion. As Figure 6c shows, in R distances ≤9.0 mm, all of the CNa/Ce,Na ratios on the outside layers (L distance 10 mm) are much higher than those on the middle and inside layers (Figure 6b and a).

Figure 6: The penetration in the radial direction also increased with increasing the intensity of autohydrolysis.Distribution of Na+ concentration ratio inside poplar sapwood (sW) chips in the radial (R) direction: (a) L distance of 0 mm; (b) L distance of 5 mm; and (c) L distance of 10 mm.
Figure 6:

The penetration in the radial direction also increased with increasing the intensity of autohydrolysis.

Distribution of Na+ concentration ratio inside poplar sapwood (sW) chips in the radial (R) direction: (a) L distance of 0 mm; (b) L distance of 5 mm; and (c) L distance of 10 mm.

Distribution pattern of Na+

The distribution profile of Na+ concentration ratio in the alkali impregnated WAHs is plotted in Figure 7. The diffusion extent of NaOH solution in the L is always stronger than that in the R direction. In the case of CNa/Ce,Na 0.55 of WCHF11, for example, see Figure 7b, the distance from the chip edge is 6 mm in the L direction and 2 mm in the R direction. Figure 7 also shows that the diffusion depth of WAH>Wcontr both in the L and R directions. The higher the CHF, the deeper is the NaOH diffusion as a result of tyloses removal and pit membrane damage.

Figure 7: The diffusion depth of autohydrolyzed poplar sapwood chips was higher than thatin the original chips both in the L and R directions.Distribution profile of the equal concentration ratio of Na+ inside poplar sapwood (sW) chips at impregnation time of 60 min: (a) Wcontr; (b) WCHF11; and (c) WCHF74.
Figure 7:

The diffusion depth of autohydrolyzed poplar sapwood chips was higher than thatin the original chips both in the L and R directions.

Distribution profile of the equal concentration ratio of Na+ inside poplar sapwood (sW) chips at impregnation time of 60 min: (a) Wcontr; (b) WCHF11; and (c) WCHF74.

Conclusions

AH pretreatment cleans up tyloses in the vessels, dissolves and/or disrupts some pit membranes of poplar sW chips, while the fiber cell walls are not affected. In the subsequent alkali impregnation process, autohydrolyzed chips are more penetrable both in the L and R directions. The higher the severity of AH (the CHF), the deeper is the alkali diffusion layer. AH had a positive influence on the diffusion and uniform distribution of NaOH solution in the autohydrolyzed samples. As expected, the diffusion extent of NaOH solution in the L direction is stronger than that in the R direction even after AH pretreatment.

Acknowledgments

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

  1. Author contributions: All the authors have accepted responsibility for the entire content of this submitted manuscript and approved submission.

  2. Research funding: None declared.

  3. Employment or leadership: None declared.

  4. Honorarium: None declared.

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Received: 2017-8-2
Accepted: 2017-12-15
Published Online: 2018-1-24
Published in Print: 2018-4-25

©2018 Walter de Gruyter GmbH, Berlin/Boston

Articles in the same Issue

  1. Frontmatter
  2. Original Articles
  3. Improved permeability of autohydrolyzed poplar sapwood against sodium hydroxide for CMP production
  4. Demethylation and other modifications of industrial softwood kraft lignin by laccase-mediators
  5. Activated carbon fiber from liquefied wood and polyvinyl butyral as an additive for production of flexible all-carbon yarn supercapacitors
  6. Short Note
  7. Fire retardancy of graphene oxide/wood composite (GOW) prepared by a vacuum-pulse dipping technique
  8. Original Articles
  9. Comparison of physical and thermal properties of various wood-inorganic composites (WICs) derived by the sol-gel process
  10. Radiata pine wood treatment with a dispersion of aqueous styrene/acrylic acid copolymer
  11. Density profiles of novel kraft pulp and TMP based foam formed thermal insulation materials observed by X-ray tomography and densitometry
  12. Influence of timber moisture content on wave time-of-flight and longitudinal natural frequency in coniferous species for different instruments
  13. Dimensional changes of cell wall and cell lumens upon water sorption revisited. Literature review and mathematical considerations based on the cylindrical model
  14. Dynamic mechanical analysis (DMA) of waterlogged archaeological wood at room temperature
  15. Obituary
  16. Professor Dr. Mikhail Yakovlevich Zarubin
https://doi.org/10.1515/hf-2017-0124
Keywords for this article
alkali impregnation; autohydrolysis (AH); Na+ concentration and distribution; permeability of poplar; poplar sapwood (sW); scanning electron microscopy and energy dispersive X-ray analysis (SEM-EDXA); severity of autohydrolysis

Articles in the same Issue

  1. Frontmatter
  2. Original Articles
  3. Improved permeability of autohydrolyzed poplar sapwood against sodium hydroxide for CMP production
  4. Demethylation and other modifications of industrial softwood kraft lignin by laccase-mediators
  5. Activated carbon fiber from liquefied wood and polyvinyl butyral as an additive for production of flexible all-carbon yarn supercapacitors
  6. Short Note
  7. Fire retardancy of graphene oxide/wood composite (GOW) prepared by a vacuum-pulse dipping technique
  8. Original Articles
  9. Comparison of physical and thermal properties of various wood-inorganic composites (WICs) derived by the sol-gel process
  10. Radiata pine wood treatment with a dispersion of aqueous styrene/acrylic acid copolymer
  11. Density profiles of novel kraft pulp and TMP based foam formed thermal insulation materials observed by X-ray tomography and densitometry
  12. Influence of timber moisture content on wave time-of-flight and longitudinal natural frequency in coniferous species for different instruments
  13. Dimensional changes of cell wall and cell lumens upon water sorption revisited. Literature review and mathematical considerations based on the cylindrical model
  14. Dynamic mechanical analysis (DMA) of waterlogged archaeological wood at room temperature
  15. Obituary
  16. Professor Dr. Mikhail Yakovlevich Zarubin
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