- Email: [email protected]
Effect of autohydrolysis on the wettability, absorbility and further alkali impregnation of poplar wood chips
Accepted Manuscript Effect of autohydrolysis on the wettability, absorbility and further alkali impregnation of poplar wood chips Ningpan Xu, Wei Liu, Qingxi Hou, Peiyun Wang, Zhirong Yao PII: DOI: Reference:
S0960-8524(16)30737-4 http://dx.doi.org/10.1016/j.biortech.2016.05.096 BITE 16593
To appear in:
Bioresource Technology
Received Date: Revised Date: Accepted Date:
30 March 2016 6 May 2016 7 May 2016
Please cite this article as: Xu, N., Liu, W., Hou, Q., Wang, P., Yao, Z., Effect of autohydrolysis on the wettability, absorbility and further alkali impregnation of poplar wood chips, Bioresource Technology (2016), doi: http:// dx.doi.org/10.1016/j.biortech.2016.05.096
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Effect of autohydrolysis on the wettability, absorbility and further alkali impregnation of poplar wood chips Ningpan Xu, Wei Liu, Qingxi Hou∗, Peiyun Wang, Zhirong Yao Tianjin Key Laboratory of Pulp & Paper, Tianjin University of Science & Technology, Tianjin 300457, P.R. China
ABSTRACT: Autohydrolysis with different severity factors was performed on poplar wood chips prior to pulping, and the wettability, absorbility and the following impregnation of NaOH solution for the poplar wood chips were then investigated. The results showed that after autohydrolysis pretreatment the porosity, shrinkage and fiber saturation point (FSP) of the poplar wood chips were increased, while the surface contact angle decreased as the severity factor was increased. The autohydrolyzed chips absorbed more NaOH in impregnation that resulted in a low NaOH concentration in the bulk impregnation liquor (i.e., the impregnation liquor outside wood chips), while the concentration in the entrapped liquor (i.e., the impregnation liquor inside wood chips) was increased. Autohydrolysis substantially improved the effectiveness of alkali impregnation. Keywords: Autohydrolysis; Impregnation; Poplar wood chips; Absorption 1. Introduction Biorefinery aimed at fractionation for efficient utilization of biomass received a lot of attention in the last decade. Biomass can be converted to renewable energy, chemicals and materials (Liu et al., 2012; Lu et al., 2012;
∗ Corresponding author. Tel.: +86 22 60601293; fax: +86 22 60600300;
Email address: [email protected] (Q.X. Hou) 1
Liu et al., 2015). Various biomass fractionation methods have been reported, such as dilute acid extraction, alkaline extraction, dilute acid−steam explosion and autohydrolysis (Grondahl et al., 2004; Hasanjanzadeh et al., 2014). Among those methods, autohydrolysis has good selectivity in separating hemicelluloses (Yu et al., 2010; Hou et al., 2014a). So far, many studies have reported the applications of autohydrolysis pretreatment in pulp and paper industry, which focus on the woody biomass pretreatment before chemi-thermomechanical pulping or chemical pulping and enzymatic hydrolysis (Mosier et al., 2005; Zhang et al., 2015). Impregnation, however, often used as a preprocessing step in traditional pulping, can change the properties of wood chips through improved pulping chemical penetration into the chips (Kazi, 1996). As a result, it can affect the subsequent pulping process performance in terms of mass transfer and pulping uniformity (Malkov et al., 2003a; Kolavali, 2013). The impregnation of wood chips involves penetration and diffusion, both are affected by the chip moisture content (MC), structure of the chips, liquor temperature and etc (Malkov et al., 2001b; Malkov et al., 2003b; Stone and Forderreuther, 1956). In wood pulping process, both a decrease in the thickness of wood chips and an increase in the liquor temperature can improve the mass transfer of chemicals into the chips and enhance the rate of chemical reactions. Not all chemical such as alkali transferred into wood chips can be consumed in the reactions and can accumulate in the entrapped liquor (EL, i.e., the impregnation liquor inside wood chips) (Egas et al., 2002). When wood chips are presteamed, the air entrapped in wood chip pores will be replaced by steam to result in improved chemical penetration efficiency (Malkov et al., 2
2001b). However, the wetting behavior and swelling of wood are related to the wood chemical composition, especially extractives (George et al., 1995; Hernandez, 2007; Wang et al., 2015). The porous structure of the wood cell wall can be considered as a matrix of polymers cross-linked mainly by non-covalent forces, and acts as the swellable hydrogel (Williams and Hodge, 2014). Due to the partial removal of extractives, hemicelluloses, and lignin by autohydrolysis (Mosier et al., 2005; Liu et al., 2012), the structure, swelling, and porosity of the wood cell wall and the accessibility to chemicals can be enhanced (Duarte et al., 2011), which should promote chemical impregnation. In this study, the poplar wood chips were selected and autohydrolyzed under different conditions before being impregnated using NaOH solution. Key physical properties of the autohydrolyzed and non-autohydrolyzed chips were examined, including the volume porosity, wettability, wood absorbency, shrinkage and fiber saturation point (FSP). In the impregnation of the autohydrolyzed chips in NaOH solution, concentrations of spent NaOH liquor and dissolved substances including lignin were also determined to explore the effects of autohydrolysis on the subsequent NaOH impregnation and chemical reactions for the poplar wood chips. The study is aimed at improving the effect of alkaline treatment in the high-yield pulping process using the autohydrolyzed wood chips as raw materials, and therefore wood chips can be utilized fully and efficiently. 2. Materials and Methods 2.1. Materials A 2.0 m length poplar stem wood with a diameter of 34 cm and MC of 3
approximately 96% was obtained from Hebei Province, China. After debarking, the stem wood was first cut into wood boards with a dimension of 2000 mm (length) × 10 mm (thickness) with an electric saw, and then the boards were further cut into laths with a dimension of 2000 mm (length) × 30 mm (width) × 10 mm (thickness), as shown in Fig. 1. After separating heartwood and sapwood fractions by hand in terms of the clear border between the two sections, the sapwood laths were selected and then cut into chips with a length of 30 mm (Fig. 1). All four vertical surfaces of the sapwood chips were carefully planned to peel off the damaged surface layer using a hand plane. After being picked out the knots, the chips were washed with deionized water and then air-dried. The air-dried chips equivalent to 4.8 kg oven-dry weight were divided into six groups. One group was randomly selected as the control, and other five groups (namely A1, A2, A3, A4 and A5) were used for autohydrolysis pretreatment. The MC and basic density of the chosen sapwood chips were determined according to ISO 3130-1975 and ISO 3131-1975, respectively. And the wood substance density and volume porosity of the sapwood chips were determined according to the literature methods (Poul and David, 2006). All chemicals used in this study were analytical grade. 2.2. Autohydrolysis pretreatment of poplar wood chips The chips of the five groups were autohydrolyzed, respectively. The 4
autohydrolysis pretreatment of an equivalent of 400 g oven-dried poplar wood chips was performed in a 6 L M/K digester (M/K Systems Inc., USA) each run. The condition of the autohydrolysis pretreatment is listed in Table 1, and other operation procedures were the same as our previous study (Hou et al., 2014b). In this study, we used the severity factor (expressed as logR0), which was proposed by Overend et al. (1987), to quantify the intensity of the autohydrolysis treatment of woody biomass by the following expression: R0 = t × exp ((T - 100)/14.75)
(1)
where t is the hydrolysis time in minutes, and T is the hydrolysis temperature in ºC. After being thoroughly washed with deinioned water, the autohydrolyzed chips were air-dried and packed in a polyethylene bag to equilibrate the MC at room temperature. The yield of the autohydrolyzed chips and the solids content of the autohydrolysis liquid (AHL) were determined. In addition, the basic density, wood substance density, and volume porosity of the autohydrolyzed chips were also determined. Finally, as for the rest chips in each group, 100 pieces were selected and used for determining the FSP and shrinkage, 5 pieces for measurement of the contact angle, and the chips remained for impregnation of NaOH solution. 2.3. Determination of the FSP and shrinkage of poplar wood chips A total of 600 pieces of the chips from the six groups (i.e., the control, A1, A2, A3, A4 and A5), that is, every 100 pieces of chips (i.e., 10 subgroups) came from each group, were equilibrated in a conditioning room at 20 °C and 5
50% RH until their MCs reached about 10%. Then weighing about 100 g (about 30 pieces of the chips) test samples of each group every time and immersing them with 1000 mL distilled water in a 2000 mL conical flask at room temperature for 3 days. During that period, vacuum (up to -0.1 MPa) and atmospheric pressure cycles were applied to ensure full saturation (Naderi and Hernandez, 1996), of which vacuum and atmospheric pressure were lasted for 1 and 7 h, respectively, for each cycle. After finishing immersion, the masses and dimensions in all main directions of each chip were then determined to the nearest 0.001 g and 0.001 mm, respectively. After finishing the measurements above, the chips of one subgroup that was randomly selected from each group were oven-dried to a constant weight at 105 °C. The chips of nine subgroups remained were then air-dried until their MCs were close to 3.0%, 6.5%, 10.0%, 13.5%, 19.0%, 33.0%, 50.0%, 100.0% and 150.0% at 20 °C, respectively. The first five subgroups out of nine ones were equilibrated their MCs according to the operation procedure described in the literature (Wang et al., 2010). And the last four subgroups were equilibrated their MCs in the polyethylene bags at 4 °C for 3 days. When the chips reached the equilibrium moisture content (EMC), their masses and dimensions were determined once again with a digital balance and digital micrometer, respectively. Finally, all of the determined chips of these nine subgroups were oven-dried to constant weight at 105 °C to calculate their actual MCs. The volumetric shrinkage (βv) of the chips was calculated according to the 6
following expression (ISO 4858, 1982): βv＝(Vmax-V)/ Vmax
(2)
where Vmax is the maximum volume achieved by the saturated chips, and V is the volume of the chips with a certain MC. The FSP was defined as the MC at which the cell walls are saturated with bound water (i.e., no free water) in the cell cavities. In the present study, the MC of the wood chips at which the extended straight linear portion of the shrinkage–EMC curve intersects the line of zero shrinkage is considered as the FSP, as shown in Fig. 2. Only volumetric shrinkage values obtained between 3% and 19% EMC were used to estimate the FSP because of the non-linearity of the shrinkage-EMC curve at low MCs (Hernandez and Bizon, 1994), and the hysteresis at saturation can affect the shrinkage at high MCs (Kelsey, 1956). 2.4. Measurement of the time-dependent contact angle A total 30 pieces of the chips from six groups (i.e., 5 pieces for each group) were chosen and balanced their MCs in a conditioning room at 50% RH and 20 ºC. The contact angle was then measured on the surface of the section paralleled to the axis of the stem wood by the sessile drop method on a dynamic contact angle analyzer (PGX, Fibro System. AB, Sweden). Three sites on each specimen were randomly chosen to collect the testing data. The contact angle analyzer dispensed a droplet of about 4 µL deionized water on the specimen with an auto-micro syringe, and the contact angle data were 7
collected within 3.0 seconds. 2.5. Impregnation of poplar wood chips in NaOH solution The control and autohydrolyzed poplar wood chips remained in each group were pre-impregnated with deionized water to a MC of 100% (i.e., a relative MC of 50%). The impregnation agent was a 20 g/kg (i.e., 0.5 mol/L) NaOH solution. The experiments were carried out in polyethylene bags placed in a thermostatic water bath at 80 ºC. Both the test chips equivalent to 50 g oven-dry weight and a 200 g NaOH solution were separately conditioned to the same temperature in a polyethylene bag using a similar bath. As the temperatures of the chips and NaOH solution all reached 80 ºC, the NaOH solution was poured into the chips’ bag straight away. After the chips and the NaOH solution were well mixed by rocking the bag over and over, the bag was moved in the thermostatic water bath, and the impregnation time varied from 10 to 300 min. Once finishing impregnation, the bag was rapidly taken out of the bath, and the free liquor (FL, i.e., the impregnation liquor outside wood chips) was separated from the impregnated chips and poured into a 250 mL flask fully filled with nitrogen and then placed in an ice bath for rapid cooling. The excess liquor at the surface of the impregnated chips was carefully wiped off with a piece of quantitative filter paper (Φ11.0 cm, Hangzhou, China) quickly. The obtained impregnated chips were all placed on a stainless steel plate with a diameter of 20 cm and then put into a liquid nitrogen tank. After being cooled 8
for 3 min, the impregnated chips were taken out of the tank and measured the weight using a digital balance to calculate the amount of the absorbed liquor. The chips were pressed to 350 bar for 2-3 min to release the EL as much as possible, which was then collected in a flask and cooled down in a similar ice bath. 2.6. Chemical analysis Both NaOH concentrations and total dissolved substances in the FL and EL were analyzed according to TAPPI T625-cm-99. The concentration of the dissolved lignin in all test liquid samples was also determined using a UV spectrophotometry (UV-2550, Shimadzu, Japan) according to the following method: about a 300 µL of every test liquor samples was taken and diluted in 0.01 mol/L NaOH and the absorbance was then measured at 280 nm (Santos et al., 1997). 3. Results and discussion 3.1. Properties of the AHL and autohydrolyzed poplar wood chips The properties of the AHL and autohydrolyzed poplar wood chips were shown in Table 2. It can be seen that the solids content in the AHL increased, but pH of the AHL and the yield of the poplar wood chips decreased as severity factor increased. When the severity factor was increased from 0 to 3.54, the volumetric porosity and the saturated MC increased from 78.34% and 230.16% to 80.97% and 286.05%, respectively. As well known, autohydrolysis can dissolve hemicelluloses along with acetyl groups (Li et al., 2014). The 9
depolymerization of hemicelluloses could be catalyzed by hydronium ions and dissolved into the AHL. The dissolution of hemicelluloses, extractives and some acid soluble lignin increased the wood cell wall volumetric porosity, and resulted in an improved fluid capillary effect and therefore wood chips capacity of absorbing water. 3.2. Change in contact angle of poplar wood chips The contact angles of the poplar wood chips from the six groups were determined, as shown in Fig. 3. It can be found that both the volume of water droplet (Fig. 3a) and contact angle (Fig. 3b) decreased with increasing the severity factor and/or wetting time. The change of contact angle is mainly attributed to the interaction between water and wood surface, which is affected by the surface free energy and surface chemical functional groups (Bao et al., 2004). The decrease in contact angles demonstrated that after autohydrolysis the surface free energy of the poplar wood chips was declined and thus the wettability of the wood surface was enhanced. Under the condition of the same wetting time, the higher the severity factor, the smaller the contact angle. For the same test sample, the longer the wetting time, the smaller the contact angle became. The volume of water droplet above the wood surface decreased mainly due to the water sinking into the porous structure of wood chip during wetting (Wang et al., 2015). The changes of the water droplet volume above the wood surface with various actors were similar to those of the contact angle. 10
3.3. FSP and relationship between volumetric shrinkage and EMC The FSP was estimated using the volumetric shrinkage intersection method, as shown in Fig. 2. The FSP of the poplar wood chips increased from 28.62% to 40.21% when the severity factor reached 3.54 (Table 1), indicating that autohydrolysis promoted water absorbility of the poplar wood chips. The shrinkage of the poplar wood chips increased with decreasing the EMC and/or raising the severity factor (Fig. 4). Four kinds of poplar wood chips, i.e., Control, A2, A3, and A5, showed similar shrinking behavior. At the same EMC, the extent of volumetric shrinkages of these four kinds of the poplar wood chips was sequenced in the following order: A5 > A3 > A2 > Control, indicating that the higher the severity factor, the higher the volumetric shrinkage. When the EMC exceeded 80%, the volumetric shrinkage was almost zero. Moreover, a slight reduction in the EMC would result in a greater volumetric shrinkage at a low EMC (< 40%) than that at a high EMC (exceeding 40%). This is because any moisture loss of the chips would come from only the bound water in the cell walls if the EMC was below the FSP, leading to an obvious volumetric shrinkage of the poplar wood chips. 3.4. Absorption of poplar wood chips in NaOH solution The amount of the absorbed NaOH solution in the autohydrolyzed chips was larger than that in the unautohydrolyzed ones as impregnation time and/or the severity factor increased (Fig. 5a), indicating that autohydrolysis could promote the impregnation of the poplar wood chips in NaOH solution. From 11
this point of view, autohydrolysis is favorable for chemical pretreatment in chemical pulping or chemi-mechanical pulping using the autohydrolyzed poplar wood chips as raw materials. Furthermore, it can also be found in Fig. 5a that the absorption of NaOH by the autohydrolyzed poplar wood chips in NaOH solution increased rapidly in the first 60 min of impregnation and then began to increase slowly, in agreement with the result reported in the literature (Malkov et al., 2001a; Malkov et al., 2003a). Taking A5 as an example, when the impregnation time reached 60 min, the amount of the absorbed NaOH solution in the autohydrolyzed poplar wood chips was 1.601 g/g, a 20% increase compared to the control sample; when extending to 300 min, the amount of the absorbed NaOH solution was just 1.831 g/g, only a 22% increase along with much long impregnation time. When the poplar wood chips were impregnated in a 20 g/kg NaOH solution, the profile of NaOH concentrations in the FL and EL is illustrated in Fig. 5b. It can be clearly seen in Fig. 5b that after autohydrolyzed pretreatment the NaOH concentration in the EL was obviously increased. For example, at the impregnation time of 30 min, the NaOH concentration of A5 was 2.54 mg/g, a 56.8% and 98.4% increase, compared with that of A2 and the control, respectively. Accordingly, the NaOH concentration in the FL had a remarkable decline. It can be noted in Fig. 5b that the NaOH concentration in the EL had a slight descent as the impregnation time exceeded a certain time, e.g., 90 and 120 12
min for A5 and A2, respectively, revealing that some chemical reactions between NaOH and components of the auothydrolyzed poplar wood chips took place. 3.5. Concentrations of dissolved substances of autohydrolyzed poplar wood chips in FL and EL Since some chemical reactions occurred in the NaOH impregnation treatment, the dissolved substances including lignin of the autohydrolyzed poplar wood chips in the FL and EL were explored, as shown in Fig. 6. When the impregnation time and/or severity factor was increased, the concentration of the dissolved substances (Fig. 6a) including lignin (Fig. 6b) in the FL went up obviously, indicating that the autohydrolysis pretreatment promoted the dissolution of components of the poplar wood chips. In addition, the mass transfer of chemicals between the inside and outside of the poplar wood chips was also improved, and most of the dissolved substances were alkali lignin, implying that lignin was dissolved easily in the impregnation liquor. When the impregnation time was extended to 120 min, the concentration of the dissolved substances of A5 in the FL reached 28.31 mg/g (Fig. 6a), a 59.0% higher than that of the control sample, meanwhile a 55% of the dissolved substances in the FL were alkali lignin. It can also be found in Fig. 6 that the concentration of the dissolved substances including lignin in the EL for A5 began to decline as the impregnation time exceeded 90 min. This can be attributed to the decrease in 13
the concentration of NaOH in the EL (Fig. 5b) and the improvement in the diffusion rate of the dissolved substances from the EL to the FL (Fig. 6a). 4. Conclusions The autohydrolysis improved the volumetric porosity, wettability, FSP, volumetric shrinkage, and capacity of fluid absorption of the poplar wood chips, in which their trends became evident as the severity factor was increased. Moreover, after the autohydrolysis, the absorbed amount of NaOH solution in the poplar wood chips was increased obviously, and the impregnation effect of NaOH solution was improved. Meanwhile, some components of the poplar wood chips became relatively easy to be dissolved in the impregnation liquor, especially lignin. Acknowledgments This work was financially supported by the Natural Science Foundation of China (Grants 31270630, 31300491 and 31570574). The authors would like to express their appreciation to Dr. J. Y. Zhu (USDA Forest Products Lab, Madison, Wisconsin, USA) for conducting a thorough review of this paper. References 1. Bao, F.C., Wang, Z., Guo, W.J., 2004. Study on the surface properties of poplar and Chinese fir wood. Scientia Silvae Sinicae 40, 131–136. 2. Duarte, G.V., Ramarao, B.V., Amidon, T.E., Ferreira, P.T., 2011. Effect of hot water extraction on hardwood kraft pulp fibers (acer saccharum, sugar maple). Ind. Eng. Chem. Res. 50, 9949–9959. 14
3. Egas, A.P.V., Simao, J.P.E., Costa, I.M.M., Francisco, S.C.P., Castro, J.A.A.M., 2002. Experimental methodology for heterogeneous studies in pulping of wood. Ind. Eng. Chem. Res. 41, 2529–2534. 4. George, I.M., Raymond, A.Y., Roger, M.R., 1995. Swelling of wood part III: effect of temperature and extractives on rate and maximum swelling. Holzforschung 49, 239–248. 5. Grondahl, M., Eriksson, L., Gatenholm, P., 2004. Material properties of plasticized hardwood xylans for potential application as oxygen barrier films. Biomacromolecules 5, 1528–1535. 6. Hasanjanzadeh, H., Hedjazi, S., Ashori, A., Mahdavi, S., Yousefi, H., 2014. Effects of hemicellulose pre-extraction and cellulose nanofibers on the properties of rice straw soda-anthraqinone pulp. Int. J. Biol. Macromol. 68, 198–204. 7. Hernandez, R.E., Bizon, M., 1994. Changes in shrinkage and tangential compression strength of sugar maple below and above the fiber saturation point. Wood Fiber Sci. 26, 360–369. 8. Hernandez, R.E., 2007. Swelling properties of hardwoods as affected by their extraneous substances, wood density, and interlocked grain. Wood Fiber Sci. 39, 146–158. 9. Hou, Q.X., Liu, L.H., Liu, W., Wang, Y., Xu, N.P., Liang, Q., 2014a. Achieving refining energy savings and pulp properties for poplar chemi-thermomechanical pulp improvement through optimized 15
autohydrolysis pretreatment. Ind. Eng. Chem. Res. 53, 17843–17848. 10. Hou, Q.X., Wang, Y., Liu, W., Liu, L.H., Xu, N.P., Li Y., 2014b. An application study of autohydrolysis pretreatment prior to poplar chemi-thermomechanical pulping. Bioresour. Technol. 169, 155–161. 12. ISO 4858, 1982. Wood–Determination of volumetric shrinkage. 13. Kazi, K.M.F., 1996. Impregnation: a key step of biomass conversion processes. Ph. D. thesis, University of Sherbrooke, Quebec, Canada. 14. Kelsey, K.E., 1956. The shrinkage intersection point – its significance and the method of its determination. Forest Prod. J. 6, 411–416. 15. Kolavali, R., 2013. Diffusion of ions in wood: Local concentration profiles of Li+ ion in a single wood piece of Norway spruce. Ph. D. thesis, Chalmers University of Technology, Goteborg, Sweden. 16. Li, Y., Liu, W., Hou, Q.X., Han, S., Wang, Y., Zhou, D.D., 2014. 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. 53, 8366–8371. 17. Liu, S.J., Lu, H.F., Hu, R.F., Shupe A., Lin, L., Liang, B., 2012. A sustainable woody biomass biorefinery. Biotechnol. Adv. 30, 785–810. 18. Lu, H.F., Hu, R.F., Ward, A., Amidon, T.E., Liang, B., Liu, S.J., 2012. Hot-water extraction and its effect on soda pulping of aspen woodchips. Biomass Bioenerg. 39, 5−13. 19. Liu, L.H., Liu W., Hou, Q.X., Chen, J.W., Xu, N.P., 2015. Understanding of 16
pH value and its effect on autohydrolysis pretreatment prior to poplar chemi-thermomechanical pulping. Bioresour. Technol. 196, 662–667. 20. Malkov, S., Tikka, P., Gullichsen, J., 2001a. Towards complete impregnation of wood chips with aqueous solutions – part 2. Pap. Puu – Pap. Tim. 83, 468-473. 21. Malkov, S., Tikka, P., Gullichsen, J., 2001b. Towards complete impregnation of wood chips with aqueous solutions – part 3. Pap. Puu – Pap. Tim. 83, 605–609. 22. Malkov, S., Tikka, P., Gustafson, R., Nuopponen, M., Vuorinen, T., 2003a. Towards complete impregnation of wood chips with aqueous solutions – part 5. Pap. Puu – Pap. Tim. 85, 215–220. 23. Malkov, S., Tikka, P., Gullichsen, J., 2003b. Towards complete impregnation of wood chips with aqueous solutions – part 1. Pap. Puu – Pap. Tim. 85, 460-466. 24. Mosier, N., Wyman, C., Dale, B., Elander, R., Lee, Y.Y., Holtzapple, M., Ladisch, M., 2005. Features of promising technologies for pretreatment of lignocellulosic biomass. Bioresour. Technol. 96, 673–686. 25. Naderi, N., Hernandez, R.E., 1996. Effect of re-wetting treatment on the dimensional changes of sugar maple wood. Wood Fiber Sci. 29, 340–344. 26. Overend, R.P., Chornet, E., Gascoigne, J.A., 1987. Fractionation of lignocellulosics by steam-aqueous pretreatments. Philos. T. R. Soc. A 321, 523–536. 17
27. Poul, J., David J. G.,2006. Selected physical parameters to characterize the state of preservation of waterlogged archaeological wood: a practical guide for their determination. J. Archaeol. Sci., 33, 551-559. 28. Santos, A., Rodriguez, F., Gilarranz, M.A., Moreno, D., Garcia-Ochoa, F., 1997. Kinetic modeling of kraft delignification of eucalyptus globulus. Ind. Eng. Chem. Res. 36, 4114–4125. 29. Stone, J.E.; Forderreuther, C., 1956. Studies of penetration and diffusion into wood. Tappi J. 39, 679–683. 30. Wang, H.K., Yu, Y.S., Yu, Y., Sun, F.B., 2010. Variation of the fiber saturation point of bamboo with age. J. Cent. S. Univ. 30, 112–115. 31. Wang, W., Zhu, Y., Cao, J.Z., Sun, W.J., 2015. Correlation between dynamic wetting behavior and chemical components of thermally modified wood. Appl. Surf. Sci. 324, 332–338. 32. Williams, D.L., Hodge, D.B., 2014. Impacts of delignification and hot water pretreatment on the water induced cell wall swelling behavior of grasses and its relation to cellulolytic enzyme hydrolysis and binding. Cellulose 21, 221–235. 33. Yu, G., Yano, S., Inoue, H., Inoue, S., Endo, T., Sawayama, S., 2010. Pretreatment of rice straw by a hot-compressed water process for enzymatic hydrolysis. Appl. Biochem. Biotech. 160, 539. 34. Zhang, J.P., Liu, W., Hou, Q.X., Chen, J.W., Xu, N.P., Ji, F.Z., 2015. Effects of different pre-extractions combining with chemi-thermomechanical 18
treatments on the enzymatic hydrolysis of wheat straw. Bioresour. Technol. 175, 75–81.
19
Captions of Figures Fig. 1. Preparation of poplar wood chips from the poplar stem wood. Fig. 2. Relationship between the volumetric shrinkage and EMC for poplar wood chips at 25 ºC. Fig. 3. Pictures of a water droplet on the surface of the poplar wood chips and effect of water wetting time on the contact angle: (a) pictures of water droplet on the surface; and (b) effect of water wetting time on the contact angle. Fig. 4. Relationship between the EMC and volumetric shrinkage of the poplar wood chips. Fig. 5. Amount of the absorbed NaOH solution in the autohydrolyzed poplar wood chips and NaOH concentrations in the free liquor (FL) and entrapped liquor (EL): (a) absorbed NaOH solution in the chips; and (b) NaOH concentrations in the FL and EL. Fig. 6. Concentrations of the dissolved substances including lignin in the free liquor (FL) and entrapped liquor (EL): (a) dissolved substances; and (b) dissolved lignin.
Captions of Tables Table 1. The conditions of the autohydrolysis pretreatment and relevant fiber saturation points (FSPs) for the poplar wood chips. Table 2. The properties of the AHL and autohydrolyzed poplar wood chips. 20
Fig. 1
Heartwood
L
Length of stem wood (2 m)
Diameter of stem wood (34 cm)
L
T W
W Sapwood
Poplar stem wood
T
L×W×T= 30 mm × 30 mm × 10 mm
L (Length) × W (Width) × T (Thickness) = 2000 mm × 30 mm × 10 mm Poplar wood chip
21
Fig. 2
12 Shrinkage (Control of poplar wood chips )
Volumetric shinkage (%)
10
FSP (Control of poplar wood chips) 8 y = -0.3794x + 10.859 R² = 0.9998
6 4 2 0
FSP，28.62% -2 0
40
80
120
EMC (%)
22
160
200
240
Fig. 3
(a)
(a) Control, 0.0 s
(b) Control, 0.4 s
(c) Control, 2.0 s
(d) A2, 0.0 s
(e) A2, 0.4 s
(f) A2, 2.0 s
(g) A5, 0.0 s
(h) A5, 0.4 s
(i) A5, 2.0 s
(b) 135 Control
Contact angle (°)
115
A2
A5
95 75 55 35 15 0.0
0.6
1.2 1.8 Wetting time (s)
23
2.4
3.0
Fig. 4
18
Volumetric shrinkage (%)
16 14
Control
A2
12
A3
A5
10 8 6 4 2 0 -2 0
20
40
60
80
100 120 140 160 180 200
EMC (%)
24
Fig. 5
Amount of absorbed NaOH solution (g/g o.d. chips)
(a)
1.9 1.7 1.5 1.3 Control A2 A4
1.1
A1 A3 A5
0.9
(b)
21
Concentration of NaOH in FL or EL (mg/g)
0
18
60
120 180 240 Impregnating time (min)
15
Control FL
Control EL
A2 FL
A2 EL
A5 FL
A5 EL
300
12 9 6 3 0 0
60
120
180
240
Impregnating time (min)
25
300
Concentration of dissolved substances in FL or EL (mg/g)
(a)
56 48 40 32 24 16 Control FL A2 FL A3 FL A5 FL
8 0 0
60
Control EL A2 EL A3 EL A5 EL
(b)
32
Concentration of dissolved lignin in FL or EL (mg/g)
Fig. 6
28 24 20 16 12 8 Control FL A2 FL A3 FL A5 FL
4 0
120 180 240 300
0
Impregnating time (min)
26
Control EL A2 EL A3 EL A5 EL
60 120 180 240 300 Impregented time (min)
Table 1
The ratio of liquid to solid (g/g) Hydrolysis temperature (ºC) Hydrolysis time (min) Severity factor, logR0 FSP (MC, %)
Control 28.62
27
A1 8:1 120 40 2.19 29.53
A2 8:1 140 40 2.78 31.43
A3 8:1 160 20 3.07 34.94
A4 8:1 160 40 3.37 37.90
A5 8:1 160 60 3.54 40.21
Table 2
Control -
Hydrolysis temp. (ºC) Hydrolysis time (min) Severity factor, logR0 pH of the AHL
-
Solids content (mg/g)
-
Yield of autohydrolyzed chips (%) 3
Oven-dry density (g/cm ) Wood substance density 3 (g/cm ) Volume porosity (%) Saturated MC (%)
100.00 0.330± 0.001 1.523± 0.012 78.34± 0.11 230.16± 2.12
A1 120 40 2.19 4.81± 0.06 0.64± 0.02 98.71± 0.32 0.330± 0.002 1.523± 0.011 78.34± 0.09 234.27± 1.24
A2 140 40 2.78 4.22± 0.03 1.14± 0.05 97.56± 0.64 0.329± 0.02 1.524± 0.008 78.39± 0.12 242.39± 2.47
28
A3 160 20 3.07 3.91± 0.05 2.66± 0.04 96.48± 0.72 0.318± 0.06 1.524± 0.009 79.13± 0.20 258.26± 3.65
A4 160 40 3.37 3.64± 0.05 5.28± 0.10 92.28± 0.83 0.305± 0.003 1.524± 0.012 79.98± 0.14 276.88± 4.01
A5 160 60 3.54 3.54± 0.04 8.88± 0.16 88.79± 0.82 0.290± 0.001 1.524± 0.010 80.97± 0.10 286.05± 3.26
Highlights (Revised)
(1) Autohydrolysis with different severity factors for poplar wood chips was done. (2) Absorbed amount of NaOH solution in the autohydrlyzed chips were increased. (3) Wettability and absorbility of the autohydrlyzed poplar wood chips were improved. (4) Autohydrolysis can promote NaOH impregnation of the poplar wood chips.
29
S0960-8524(16)30737-4 http://dx.doi.org/10.1016/j.biortech.2016.05.096 BITE 16593
To appear in:
Bioresource Technology
Received Date: Revised Date: Accepted Date:
30 March 2016 6 May 2016 7 May 2016
Please cite this article as: Xu, N., Liu, W., Hou, Q., Wang, P., Yao, Z., Effect of autohydrolysis on the wettability, absorbility and further alkali impregnation of poplar wood chips, Bioresource Technology (2016), doi: http:// dx.doi.org/10.1016/j.biortech.2016.05.096
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Effect of autohydrolysis on the wettability, absorbility and further alkali impregnation of poplar wood chips Ningpan Xu, Wei Liu, Qingxi Hou∗, Peiyun Wang, Zhirong Yao Tianjin Key Laboratory of Pulp & Paper, Tianjin University of Science & Technology, Tianjin 300457, P.R. China
ABSTRACT: Autohydrolysis with different severity factors was performed on poplar wood chips prior to pulping, and the wettability, absorbility and the following impregnation of NaOH solution for the poplar wood chips were then investigated. The results showed that after autohydrolysis pretreatment the porosity, shrinkage and fiber saturation point (FSP) of the poplar wood chips were increased, while the surface contact angle decreased as the severity factor was increased. The autohydrolyzed chips absorbed more NaOH in impregnation that resulted in a low NaOH concentration in the bulk impregnation liquor (i.e., the impregnation liquor outside wood chips), while the concentration in the entrapped liquor (i.e., the impregnation liquor inside wood chips) was increased. Autohydrolysis substantially improved the effectiveness of alkali impregnation. Keywords: Autohydrolysis; Impregnation; Poplar wood chips; Absorption 1. Introduction Biorefinery aimed at fractionation for efficient utilization of biomass received a lot of attention in the last decade. Biomass can be converted to renewable energy, chemicals and materials (Liu et al., 2012; Lu et al., 2012;
∗ Corresponding author. Tel.: +86 22 60601293; fax: +86 22 60600300;
Email address: [email protected] (Q.X. Hou) 1
Liu et al., 2015). Various biomass fractionation methods have been reported, such as dilute acid extraction, alkaline extraction, dilute acid−steam explosion and autohydrolysis (Grondahl et al., 2004; Hasanjanzadeh et al., 2014). Among those methods, autohydrolysis has good selectivity in separating hemicelluloses (Yu et al., 2010; Hou et al., 2014a). So far, many studies have reported the applications of autohydrolysis pretreatment in pulp and paper industry, which focus on the woody biomass pretreatment before chemi-thermomechanical pulping or chemical pulping and enzymatic hydrolysis (Mosier et al., 2005; Zhang et al., 2015). Impregnation, however, often used as a preprocessing step in traditional pulping, can change the properties of wood chips through improved pulping chemical penetration into the chips (Kazi, 1996). As a result, it can affect the subsequent pulping process performance in terms of mass transfer and pulping uniformity (Malkov et al., 2003a; Kolavali, 2013). The impregnation of wood chips involves penetration and diffusion, both are affected by the chip moisture content (MC), structure of the chips, liquor temperature and etc (Malkov et al., 2001b; Malkov et al., 2003b; Stone and Forderreuther, 1956). In wood pulping process, both a decrease in the thickness of wood chips and an increase in the liquor temperature can improve the mass transfer of chemicals into the chips and enhance the rate of chemical reactions. Not all chemical such as alkali transferred into wood chips can be consumed in the reactions and can accumulate in the entrapped liquor (EL, i.e., the impregnation liquor inside wood chips) (Egas et al., 2002). When wood chips are presteamed, the air entrapped in wood chip pores will be replaced by steam to result in improved chemical penetration efficiency (Malkov et al., 2
2001b). However, the wetting behavior and swelling of wood are related to the wood chemical composition, especially extractives (George et al., 1995; Hernandez, 2007; Wang et al., 2015). The porous structure of the wood cell wall can be considered as a matrix of polymers cross-linked mainly by non-covalent forces, and acts as the swellable hydrogel (Williams and Hodge, 2014). Due to the partial removal of extractives, hemicelluloses, and lignin by autohydrolysis (Mosier et al., 2005; Liu et al., 2012), the structure, swelling, and porosity of the wood cell wall and the accessibility to chemicals can be enhanced (Duarte et al., 2011), which should promote chemical impregnation. In this study, the poplar wood chips were selected and autohydrolyzed under different conditions before being impregnated using NaOH solution. Key physical properties of the autohydrolyzed and non-autohydrolyzed chips were examined, including the volume porosity, wettability, wood absorbency, shrinkage and fiber saturation point (FSP). In the impregnation of the autohydrolyzed chips in NaOH solution, concentrations of spent NaOH liquor and dissolved substances including lignin were also determined to explore the effects of autohydrolysis on the subsequent NaOH impregnation and chemical reactions for the poplar wood chips. The study is aimed at improving the effect of alkaline treatment in the high-yield pulping process using the autohydrolyzed wood chips as raw materials, and therefore wood chips can be utilized fully and efficiently. 2. Materials and Methods 2.1. Materials A 2.0 m length poplar stem wood with a diameter of 34 cm and MC of 3
approximately 96% was obtained from Hebei Province, China. After debarking, the stem wood was first cut into wood boards with a dimension of 2000 mm (length) × 10 mm (thickness) with an electric saw, and then the boards were further cut into laths with a dimension of 2000 mm (length) × 30 mm (width) × 10 mm (thickness), as shown in Fig. 1. After separating heartwood and sapwood fractions by hand in terms of the clear border between the two sections, the sapwood laths were selected and then cut into chips with a length of 30 mm (Fig. 1). All four vertical surfaces of the sapwood chips were carefully planned to peel off the damaged surface layer using a hand plane. After being picked out the knots, the chips were washed with deionized water and then air-dried. The air-dried chips equivalent to 4.8 kg oven-dry weight were divided into six groups. One group was randomly selected as the control, and other five groups (namely A1, A2, A3, A4 and A5) were used for autohydrolysis pretreatment. The MC and basic density of the chosen sapwood chips were determined according to ISO 3130-1975 and ISO 3131-1975, respectively. And the wood substance density and volume porosity of the sapwood chips were determined according to the literature methods (Poul and David, 2006). All chemicals used in this study were analytical grade. 2.2. Autohydrolysis pretreatment of poplar wood chips The chips of the five groups were autohydrolyzed, respectively. The 4
autohydrolysis pretreatment of an equivalent of 400 g oven-dried poplar wood chips was performed in a 6 L M/K digester (M/K Systems Inc., USA) each run. The condition of the autohydrolysis pretreatment is listed in Table 1, and other operation procedures were the same as our previous study (Hou et al., 2014b). In this study, we used the severity factor (expressed as logR0), which was proposed by Overend et al. (1987), to quantify the intensity of the autohydrolysis treatment of woody biomass by the following expression: R0 = t × exp ((T - 100)/14.75)
(1)
where t is the hydrolysis time in minutes, and T is the hydrolysis temperature in ºC. After being thoroughly washed with deinioned water, the autohydrolyzed chips were air-dried and packed in a polyethylene bag to equilibrate the MC at room temperature. The yield of the autohydrolyzed chips and the solids content of the autohydrolysis liquid (AHL) were determined. In addition, the basic density, wood substance density, and volume porosity of the autohydrolyzed chips were also determined. Finally, as for the rest chips in each group, 100 pieces were selected and used for determining the FSP and shrinkage, 5 pieces for measurement of the contact angle, and the chips remained for impregnation of NaOH solution. 2.3. Determination of the FSP and shrinkage of poplar wood chips A total of 600 pieces of the chips from the six groups (i.e., the control, A1, A2, A3, A4 and A5), that is, every 100 pieces of chips (i.e., 10 subgroups) came from each group, were equilibrated in a conditioning room at 20 °C and 5
50% RH until their MCs reached about 10%. Then weighing about 100 g (about 30 pieces of the chips) test samples of each group every time and immersing them with 1000 mL distilled water in a 2000 mL conical flask at room temperature for 3 days. During that period, vacuum (up to -0.1 MPa) and atmospheric pressure cycles were applied to ensure full saturation (Naderi and Hernandez, 1996), of which vacuum and atmospheric pressure were lasted for 1 and 7 h, respectively, for each cycle. After finishing immersion, the masses and dimensions in all main directions of each chip were then determined to the nearest 0.001 g and 0.001 mm, respectively. After finishing the measurements above, the chips of one subgroup that was randomly selected from each group were oven-dried to a constant weight at 105 °C. The chips of nine subgroups remained were then air-dried until their MCs were close to 3.0%, 6.5%, 10.0%, 13.5%, 19.0%, 33.0%, 50.0%, 100.0% and 150.0% at 20 °C, respectively. The first five subgroups out of nine ones were equilibrated their MCs according to the operation procedure described in the literature (Wang et al., 2010). And the last four subgroups were equilibrated their MCs in the polyethylene bags at 4 °C for 3 days. When the chips reached the equilibrium moisture content (EMC), their masses and dimensions were determined once again with a digital balance and digital micrometer, respectively. Finally, all of the determined chips of these nine subgroups were oven-dried to constant weight at 105 °C to calculate their actual MCs. The volumetric shrinkage (βv) of the chips was calculated according to the 6
following expression (ISO 4858, 1982): βv＝(Vmax-V)/ Vmax
(2)
where Vmax is the maximum volume achieved by the saturated chips, and V is the volume of the chips with a certain MC. The FSP was defined as the MC at which the cell walls are saturated with bound water (i.e., no free water) in the cell cavities. In the present study, the MC of the wood chips at which the extended straight linear portion of the shrinkage–EMC curve intersects the line of zero shrinkage is considered as the FSP, as shown in Fig. 2. Only volumetric shrinkage values obtained between 3% and 19% EMC were used to estimate the FSP because of the non-linearity of the shrinkage-EMC curve at low MCs (Hernandez and Bizon, 1994), and the hysteresis at saturation can affect the shrinkage at high MCs (Kelsey, 1956). 2.4. Measurement of the time-dependent contact angle A total 30 pieces of the chips from six groups (i.e., 5 pieces for each group) were chosen and balanced their MCs in a conditioning room at 50% RH and 20 ºC. The contact angle was then measured on the surface of the section paralleled to the axis of the stem wood by the sessile drop method on a dynamic contact angle analyzer (PGX, Fibro System. AB, Sweden). Three sites on each specimen were randomly chosen to collect the testing data. The contact angle analyzer dispensed a droplet of about 4 µL deionized water on the specimen with an auto-micro syringe, and the contact angle data were 7
collected within 3.0 seconds. 2.5. Impregnation of poplar wood chips in NaOH solution The control and autohydrolyzed poplar wood chips remained in each group were pre-impregnated with deionized water to a MC of 100% (i.e., a relative MC of 50%). The impregnation agent was a 20 g/kg (i.e., 0.5 mol/L) NaOH solution. The experiments were carried out in polyethylene bags placed in a thermostatic water bath at 80 ºC. Both the test chips equivalent to 50 g oven-dry weight and a 200 g NaOH solution were separately conditioned to the same temperature in a polyethylene bag using a similar bath. As the temperatures of the chips and NaOH solution all reached 80 ºC, the NaOH solution was poured into the chips’ bag straight away. After the chips and the NaOH solution were well mixed by rocking the bag over and over, the bag was moved in the thermostatic water bath, and the impregnation time varied from 10 to 300 min. Once finishing impregnation, the bag was rapidly taken out of the bath, and the free liquor (FL, i.e., the impregnation liquor outside wood chips) was separated from the impregnated chips and poured into a 250 mL flask fully filled with nitrogen and then placed in an ice bath for rapid cooling. The excess liquor at the surface of the impregnated chips was carefully wiped off with a piece of quantitative filter paper (Φ11.0 cm, Hangzhou, China) quickly. The obtained impregnated chips were all placed on a stainless steel plate with a diameter of 20 cm and then put into a liquid nitrogen tank. After being cooled 8
for 3 min, the impregnated chips were taken out of the tank and measured the weight using a digital balance to calculate the amount of the absorbed liquor. The chips were pressed to 350 bar for 2-3 min to release the EL as much as possible, which was then collected in a flask and cooled down in a similar ice bath. 2.6. Chemical analysis Both NaOH concentrations and total dissolved substances in the FL and EL were analyzed according to TAPPI T625-cm-99. The concentration of the dissolved lignin in all test liquid samples was also determined using a UV spectrophotometry (UV-2550, Shimadzu, Japan) according to the following method: about a 300 µL of every test liquor samples was taken and diluted in 0.01 mol/L NaOH and the absorbance was then measured at 280 nm (Santos et al., 1997). 3. Results and discussion 3.1. Properties of the AHL and autohydrolyzed poplar wood chips The properties of the AHL and autohydrolyzed poplar wood chips were shown in Table 2. It can be seen that the solids content in the AHL increased, but pH of the AHL and the yield of the poplar wood chips decreased as severity factor increased. When the severity factor was increased from 0 to 3.54, the volumetric porosity and the saturated MC increased from 78.34% and 230.16% to 80.97% and 286.05%, respectively. As well known, autohydrolysis can dissolve hemicelluloses along with acetyl groups (Li et al., 2014). The 9
depolymerization of hemicelluloses could be catalyzed by hydronium ions and dissolved into the AHL. The dissolution of hemicelluloses, extractives and some acid soluble lignin increased the wood cell wall volumetric porosity, and resulted in an improved fluid capillary effect and therefore wood chips capacity of absorbing water. 3.2. Change in contact angle of poplar wood chips The contact angles of the poplar wood chips from the six groups were determined, as shown in Fig. 3. It can be found that both the volume of water droplet (Fig. 3a) and contact angle (Fig. 3b) decreased with increasing the severity factor and/or wetting time. The change of contact angle is mainly attributed to the interaction between water and wood surface, which is affected by the surface free energy and surface chemical functional groups (Bao et al., 2004). The decrease in contact angles demonstrated that after autohydrolysis the surface free energy of the poplar wood chips was declined and thus the wettability of the wood surface was enhanced. Under the condition of the same wetting time, the higher the severity factor, the smaller the contact angle. For the same test sample, the longer the wetting time, the smaller the contact angle became. The volume of water droplet above the wood surface decreased mainly due to the water sinking into the porous structure of wood chip during wetting (Wang et al., 2015). The changes of the water droplet volume above the wood surface with various actors were similar to those of the contact angle. 10
3.3. FSP and relationship between volumetric shrinkage and EMC The FSP was estimated using the volumetric shrinkage intersection method, as shown in Fig. 2. The FSP of the poplar wood chips increased from 28.62% to 40.21% when the severity factor reached 3.54 (Table 1), indicating that autohydrolysis promoted water absorbility of the poplar wood chips. The shrinkage of the poplar wood chips increased with decreasing the EMC and/or raising the severity factor (Fig. 4). Four kinds of poplar wood chips, i.e., Control, A2, A3, and A5, showed similar shrinking behavior. At the same EMC, the extent of volumetric shrinkages of these four kinds of the poplar wood chips was sequenced in the following order: A5 > A3 > A2 > Control, indicating that the higher the severity factor, the higher the volumetric shrinkage. When the EMC exceeded 80%, the volumetric shrinkage was almost zero. Moreover, a slight reduction in the EMC would result in a greater volumetric shrinkage at a low EMC (< 40%) than that at a high EMC (exceeding 40%). This is because any moisture loss of the chips would come from only the bound water in the cell walls if the EMC was below the FSP, leading to an obvious volumetric shrinkage of the poplar wood chips. 3.4. Absorption of poplar wood chips in NaOH solution The amount of the absorbed NaOH solution in the autohydrolyzed chips was larger than that in the unautohydrolyzed ones as impregnation time and/or the severity factor increased (Fig. 5a), indicating that autohydrolysis could promote the impregnation of the poplar wood chips in NaOH solution. From 11
this point of view, autohydrolysis is favorable for chemical pretreatment in chemical pulping or chemi-mechanical pulping using the autohydrolyzed poplar wood chips as raw materials. Furthermore, it can also be found in Fig. 5a that the absorption of NaOH by the autohydrolyzed poplar wood chips in NaOH solution increased rapidly in the first 60 min of impregnation and then began to increase slowly, in agreement with the result reported in the literature (Malkov et al., 2001a; Malkov et al., 2003a). Taking A5 as an example, when the impregnation time reached 60 min, the amount of the absorbed NaOH solution in the autohydrolyzed poplar wood chips was 1.601 g/g, a 20% increase compared to the control sample; when extending to 300 min, the amount of the absorbed NaOH solution was just 1.831 g/g, only a 22% increase along with much long impregnation time. When the poplar wood chips were impregnated in a 20 g/kg NaOH solution, the profile of NaOH concentrations in the FL and EL is illustrated in Fig. 5b. It can be clearly seen in Fig. 5b that after autohydrolyzed pretreatment the NaOH concentration in the EL was obviously increased. For example, at the impregnation time of 30 min, the NaOH concentration of A5 was 2.54 mg/g, a 56.8% and 98.4% increase, compared with that of A2 and the control, respectively. Accordingly, the NaOH concentration in the FL had a remarkable decline. It can be noted in Fig. 5b that the NaOH concentration in the EL had a slight descent as the impregnation time exceeded a certain time, e.g., 90 and 120 12
min for A5 and A2, respectively, revealing that some chemical reactions between NaOH and components of the auothydrolyzed poplar wood chips took place. 3.5. Concentrations of dissolved substances of autohydrolyzed poplar wood chips in FL and EL Since some chemical reactions occurred in the NaOH impregnation treatment, the dissolved substances including lignin of the autohydrolyzed poplar wood chips in the FL and EL were explored, as shown in Fig. 6. When the impregnation time and/or severity factor was increased, the concentration of the dissolved substances (Fig. 6a) including lignin (Fig. 6b) in the FL went up obviously, indicating that the autohydrolysis pretreatment promoted the dissolution of components of the poplar wood chips. In addition, the mass transfer of chemicals between the inside and outside of the poplar wood chips was also improved, and most of the dissolved substances were alkali lignin, implying that lignin was dissolved easily in the impregnation liquor. When the impregnation time was extended to 120 min, the concentration of the dissolved substances of A5 in the FL reached 28.31 mg/g (Fig. 6a), a 59.0% higher than that of the control sample, meanwhile a 55% of the dissolved substances in the FL were alkali lignin. It can also be found in Fig. 6 that the concentration of the dissolved substances including lignin in the EL for A5 began to decline as the impregnation time exceeded 90 min. This can be attributed to the decrease in 13
the concentration of NaOH in the EL (Fig. 5b) and the improvement in the diffusion rate of the dissolved substances from the EL to the FL (Fig. 6a). 4. Conclusions The autohydrolysis improved the volumetric porosity, wettability, FSP, volumetric shrinkage, and capacity of fluid absorption of the poplar wood chips, in which their trends became evident as the severity factor was increased. Moreover, after the autohydrolysis, the absorbed amount of NaOH solution in the poplar wood chips was increased obviously, and the impregnation effect of NaOH solution was improved. Meanwhile, some components of the poplar wood chips became relatively easy to be dissolved in the impregnation liquor, especially lignin. Acknowledgments This work was financially supported by the Natural Science Foundation of China (Grants 31270630, 31300491 and 31570574). The authors would like to express their appreciation to Dr. J. Y. Zhu (USDA Forest Products Lab, Madison, Wisconsin, USA) for conducting a thorough review of this paper. References 1. Bao, F.C., Wang, Z., Guo, W.J., 2004. Study on the surface properties of poplar and Chinese fir wood. Scientia Silvae Sinicae 40, 131–136. 2. Duarte, G.V., Ramarao, B.V., Amidon, T.E., Ferreira, P.T., 2011. Effect of hot water extraction on hardwood kraft pulp fibers (acer saccharum, sugar maple). Ind. Eng. Chem. Res. 50, 9949–9959. 14
3. Egas, A.P.V., Simao, J.P.E., Costa, I.M.M., Francisco, S.C.P., Castro, J.A.A.M., 2002. Experimental methodology for heterogeneous studies in pulping of wood. Ind. Eng. Chem. Res. 41, 2529–2534. 4. George, I.M., Raymond, A.Y., Roger, M.R., 1995. Swelling of wood part III: effect of temperature and extractives on rate and maximum swelling. Holzforschung 49, 239–248. 5. Grondahl, M., Eriksson, L., Gatenholm, P., 2004. Material properties of plasticized hardwood xylans for potential application as oxygen barrier films. Biomacromolecules 5, 1528–1535. 6. Hasanjanzadeh, H., Hedjazi, S., Ashori, A., Mahdavi, S., Yousefi, H., 2014. Effects of hemicellulose pre-extraction and cellulose nanofibers on the properties of rice straw soda-anthraqinone pulp. Int. J. Biol. Macromol. 68, 198–204. 7. Hernandez, R.E., Bizon, M., 1994. Changes in shrinkage and tangential compression strength of sugar maple below and above the fiber saturation point. Wood Fiber Sci. 26, 360–369. 8. Hernandez, R.E., 2007. Swelling properties of hardwoods as affected by their extraneous substances, wood density, and interlocked grain. Wood Fiber Sci. 39, 146–158. 9. Hou, Q.X., Liu, L.H., Liu, W., Wang, Y., Xu, N.P., Liang, Q., 2014a. Achieving refining energy savings and pulp properties for poplar chemi-thermomechanical pulp improvement through optimized 15
autohydrolysis pretreatment. Ind. Eng. Chem. Res. 53, 17843–17848. 10. Hou, Q.X., Wang, Y., Liu, W., Liu, L.H., Xu, N.P., Li Y., 2014b. An application study of autohydrolysis pretreatment prior to poplar chemi-thermomechanical pulping. Bioresour. Technol. 169, 155–161. 12. ISO 4858, 1982. Wood–Determination of volumetric shrinkage. 13. Kazi, K.M.F., 1996. Impregnation: a key step of biomass conversion processes. Ph. D. thesis, University of Sherbrooke, Quebec, Canada. 14. Kelsey, K.E., 1956. The shrinkage intersection point – its significance and the method of its determination. Forest Prod. J. 6, 411–416. 15. Kolavali, R., 2013. Diffusion of ions in wood: Local concentration profiles of Li+ ion in a single wood piece of Norway spruce. Ph. D. thesis, Chalmers University of Technology, Goteborg, Sweden. 16. Li, Y., Liu, W., Hou, Q.X., Han, S., Wang, Y., Zhou, D.D., 2014. 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. 53, 8366–8371. 17. Liu, S.J., Lu, H.F., Hu, R.F., Shupe A., Lin, L., Liang, B., 2012. A sustainable woody biomass biorefinery. Biotechnol. Adv. 30, 785–810. 18. Lu, H.F., Hu, R.F., Ward, A., Amidon, T.E., Liang, B., Liu, S.J., 2012. Hot-water extraction and its effect on soda pulping of aspen woodchips. Biomass Bioenerg. 39, 5−13. 19. Liu, L.H., Liu W., Hou, Q.X., Chen, J.W., Xu, N.P., 2015. Understanding of 16
pH value and its effect on autohydrolysis pretreatment prior to poplar chemi-thermomechanical pulping. Bioresour. Technol. 196, 662–667. 20. Malkov, S., Tikka, P., Gullichsen, J., 2001a. Towards complete impregnation of wood chips with aqueous solutions – part 2. Pap. Puu – Pap. Tim. 83, 468-473. 21. Malkov, S., Tikka, P., Gullichsen, J., 2001b. Towards complete impregnation of wood chips with aqueous solutions – part 3. Pap. Puu – Pap. Tim. 83, 605–609. 22. Malkov, S., Tikka, P., Gustafson, R., Nuopponen, M., Vuorinen, T., 2003a. Towards complete impregnation of wood chips with aqueous solutions – part 5. Pap. Puu – Pap. Tim. 85, 215–220. 23. Malkov, S., Tikka, P., Gullichsen, J., 2003b. Towards complete impregnation of wood chips with aqueous solutions – part 1. Pap. Puu – Pap. Tim. 85, 460-466. 24. Mosier, N., Wyman, C., Dale, B., Elander, R., Lee, Y.Y., Holtzapple, M., Ladisch, M., 2005. Features of promising technologies for pretreatment of lignocellulosic biomass. Bioresour. Technol. 96, 673–686. 25. Naderi, N., Hernandez, R.E., 1996. Effect of re-wetting treatment on the dimensional changes of sugar maple wood. Wood Fiber Sci. 29, 340–344. 26. Overend, R.P., Chornet, E., Gascoigne, J.A., 1987. Fractionation of lignocellulosics by steam-aqueous pretreatments. Philos. T. R. Soc. A 321, 523–536. 17
27. Poul, J., David J. G.,2006. Selected physical parameters to characterize the state of preservation of waterlogged archaeological wood: a practical guide for their determination. J. Archaeol. Sci., 33, 551-559. 28. Santos, A., Rodriguez, F., Gilarranz, M.A., Moreno, D., Garcia-Ochoa, F., 1997. Kinetic modeling of kraft delignification of eucalyptus globulus. Ind. Eng. Chem. Res. 36, 4114–4125. 29. Stone, J.E.; Forderreuther, C., 1956. Studies of penetration and diffusion into wood. Tappi J. 39, 679–683. 30. Wang, H.K., Yu, Y.S., Yu, Y., Sun, F.B., 2010. Variation of the fiber saturation point of bamboo with age. J. Cent. S. Univ. 30, 112–115. 31. Wang, W., Zhu, Y., Cao, J.Z., Sun, W.J., 2015. Correlation between dynamic wetting behavior and chemical components of thermally modified wood. Appl. Surf. Sci. 324, 332–338. 32. Williams, D.L., Hodge, D.B., 2014. Impacts of delignification and hot water pretreatment on the water induced cell wall swelling behavior of grasses and its relation to cellulolytic enzyme hydrolysis and binding. Cellulose 21, 221–235. 33. Yu, G., Yano, S., Inoue, H., Inoue, S., Endo, T., Sawayama, S., 2010. Pretreatment of rice straw by a hot-compressed water process for enzymatic hydrolysis. Appl. Biochem. Biotech. 160, 539. 34. Zhang, J.P., Liu, W., Hou, Q.X., Chen, J.W., Xu, N.P., Ji, F.Z., 2015. Effects of different pre-extractions combining with chemi-thermomechanical 18
treatments on the enzymatic hydrolysis of wheat straw. Bioresour. Technol. 175, 75–81.
19
Captions of Figures Fig. 1. Preparation of poplar wood chips from the poplar stem wood. Fig. 2. Relationship between the volumetric shrinkage and EMC for poplar wood chips at 25 ºC. Fig. 3. Pictures of a water droplet on the surface of the poplar wood chips and effect of water wetting time on the contact angle: (a) pictures of water droplet on the surface; and (b) effect of water wetting time on the contact angle. Fig. 4. Relationship between the EMC and volumetric shrinkage of the poplar wood chips. Fig. 5. Amount of the absorbed NaOH solution in the autohydrolyzed poplar wood chips and NaOH concentrations in the free liquor (FL) and entrapped liquor (EL): (a) absorbed NaOH solution in the chips; and (b) NaOH concentrations in the FL and EL. Fig. 6. Concentrations of the dissolved substances including lignin in the free liquor (FL) and entrapped liquor (EL): (a) dissolved substances; and (b) dissolved lignin.
Captions of Tables Table 1. The conditions of the autohydrolysis pretreatment and relevant fiber saturation points (FSPs) for the poplar wood chips. Table 2. The properties of the AHL and autohydrolyzed poplar wood chips. 20
Fig. 1
Heartwood
L
Length of stem wood (2 m)
Diameter of stem wood (34 cm)
L
T W
W Sapwood
Poplar stem wood
T
L×W×T= 30 mm × 30 mm × 10 mm
L (Length) × W (Width) × T (Thickness) = 2000 mm × 30 mm × 10 mm Poplar wood chip
21
Fig. 2
12 Shrinkage (Control of poplar wood chips )
Volumetric shinkage (%)
10
FSP (Control of poplar wood chips) 8 y = -0.3794x + 10.859 R² = 0.9998
6 4 2 0
FSP，28.62% -2 0
40
80
120
EMC (%)
22
160
200
240
Fig. 3
(a)
(a) Control, 0.0 s
(b) Control, 0.4 s
(c) Control, 2.0 s
(d) A2, 0.0 s
(e) A2, 0.4 s
(f) A2, 2.0 s
(g) A5, 0.0 s
(h) A5, 0.4 s
(i) A5, 2.0 s
(b) 135 Control
Contact angle (°)
115
A2
A5
95 75 55 35 15 0.0
0.6
1.2 1.8 Wetting time (s)
23
2.4
3.0
Fig. 4
18
Volumetric shrinkage (%)
16 14
Control
A2
12
A3
A5
10 8 6 4 2 0 -2 0
20
40
60
80
100 120 140 160 180 200
EMC (%)
24
Fig. 5
Amount of absorbed NaOH solution (g/g o.d. chips)
(a)
1.9 1.7 1.5 1.3 Control A2 A4
1.1
A1 A3 A5
0.9
(b)
21
Concentration of NaOH in FL or EL (mg/g)
0
18
60
120 180 240 Impregnating time (min)
15
Control FL
Control EL
A2 FL
A2 EL
A5 FL
A5 EL
300
12 9 6 3 0 0
60
120
180
240
Impregnating time (min)
25
300
Concentration of dissolved substances in FL or EL (mg/g)
(a)
56 48 40 32 24 16 Control FL A2 FL A3 FL A5 FL
8 0 0
60
Control EL A2 EL A3 EL A5 EL
(b)
32
Concentration of dissolved lignin in FL or EL (mg/g)
Fig. 6
28 24 20 16 12 8 Control FL A2 FL A3 FL A5 FL
4 0
120 180 240 300
0
Impregnating time (min)
26
Control EL A2 EL A3 EL A5 EL
60 120 180 240 300 Impregented time (min)
Table 1
The ratio of liquid to solid (g/g) Hydrolysis temperature (ºC) Hydrolysis time (min) Severity factor, logR0 FSP (MC, %)
Control 28.62
27
A1 8:1 120 40 2.19 29.53
A2 8:1 140 40 2.78 31.43
A3 8:1 160 20 3.07 34.94
A4 8:1 160 40 3.37 37.90
A5 8:1 160 60 3.54 40.21
Table 2
Control -
Hydrolysis temp. (ºC) Hydrolysis time (min) Severity factor, logR0 pH of the AHL
-
Solids content (mg/g)
-
Yield of autohydrolyzed chips (%) 3
Oven-dry density (g/cm ) Wood substance density 3 (g/cm ) Volume porosity (%) Saturated MC (%)
100.00 0.330± 0.001 1.523± 0.012 78.34± 0.11 230.16± 2.12
A1 120 40 2.19 4.81± 0.06 0.64± 0.02 98.71± 0.32 0.330± 0.002 1.523± 0.011 78.34± 0.09 234.27± 1.24
A2 140 40 2.78 4.22± 0.03 1.14± 0.05 97.56± 0.64 0.329± 0.02 1.524± 0.008 78.39± 0.12 242.39± 2.47
28
A3 160 20 3.07 3.91± 0.05 2.66± 0.04 96.48± 0.72 0.318± 0.06 1.524± 0.009 79.13± 0.20 258.26± 3.65
A4 160 40 3.37 3.64± 0.05 5.28± 0.10 92.28± 0.83 0.305± 0.003 1.524± 0.012 79.98± 0.14 276.88± 4.01
A5 160 60 3.54 3.54± 0.04 8.88± 0.16 88.79± 0.82 0.290± 0.001 1.524± 0.010 80.97± 0.10 286.05± 3.26
Highlights (Revised)
(1) Autohydrolysis with different severity factors for poplar wood chips was done. (2) Absorbed amount of NaOH solution in the autohydrlyzed chips were increased. (3) Wettability and absorbility of the autohydrlyzed poplar wood chips were improved. (4) Autohydrolysis can promote NaOH impregnation of the poplar wood chips.
29