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Hot-water extraction and its effect on soda pulping of aspen woodchips
b i o m a s s a n d b i o e n e r g y 3 9 ( 2 0 1 2 ) 5 e1 3
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Hot-water extraction and its effect on soda pulping of aspen woodchips Houfang Lu a,b, Ruofei Hu b,c, Al Ward d, Thomas E. Amidon b, Bin Liang a, Shijie Liu b,e,* a
College of Chemical Engineering, Sichuan University, Chengdu 610065, PR China Department of Paper and Bioprocess Engineering, State University of New York College of Environmental Science and Forestry, Syracuse, NY 13210, USA c College of Chemical Engineering and Food Science, Xiangfan University, Xiangyang 441053, PR China d Alberta Pacific Forest Industries Inc., P.O. Box 8000, Boyle, Alberta T0A 0M0, Canada e State Key Laboratory of Pulp and Paper Engineering, South China University of Technology, Guangzhou 510640, PR China b
article info
abstract
Article history:
Hot-water extraction of industrial aspen woodchips (mainly Populus tremuloides) and
Received 17 March 2010
subsequent soda pulping were carried out in an M/K digester. It was found that mass
Received in revised form
removal of woodchips was 4.74% at 160 C for 15 min with a water-to-wood ratio of 4:1, and
1 January 2011
that mass removal increased with extraction time and temperature. The mass removal
Accepted 28 January 2011
reached 21.38% at 160 C for 210 min. Less than 6% lignin (on total wood mass) from virgin
Available online 9 March 2011
woodchips was dissolved in the extraction liquor. The concentrations of xylose, acetic acid, formic acid and furfural in the extraction liquor were measured by 1H NMR, which were
Keywords:
found to increase with extraction time and temperature. Furfural increased sharply beyond
Hemicellulose
150 min of extraction at 160 C. The residual woodchips were subjected to soda pulping at
Hot-water extraction
150 C for 3 h with 20% effective alkali and a liquor-to-wood ratio of 4.5:1. Compared with
Soda pulping
the control sample, the overall pulp yield for extracted woodchips decreased little while
Kappa number
rejects decreased sharply. Kappa number of the pulps decreased with increasing extraction
Viscosity
time and temperature. Viscosity of the pulp increased with increasing extraction
Yield
temperature and duration for the first 90 min. Beyond 90 min of extraction, the resulting pulp viscosity decreased with increasing duration of extraction. The water-to-wood ratio had little effect on the entire process. Considering bioconversion of extracted hemicellulose to fuel/chemicals and the resultant pulping characteristics, hot-water extraction at 165 C for 90 min with a water-to-water ratio of 5:1 for aspen woodchips seems to be optimum. ª 2011 Elsevier Ltd. All rights reserved.
1.
Introduction
Owing to environment concerns and resources depletion, more and more attention is being paid to renewable materials, such as wood [1,2]. Wood is the principal source of cellulosic fibers for
pulp and paper manufacture. It is composed of four main components: cellulose (35e50%), hemicellulose (20e35%), lignin (10e25%), and extractives (2e8%)[3]. In the paper industry, fibers are obtained through a pulping process. During a typical chemical pulping process, 80% of the lignin and 50% of the
* Corresponding author. Department of Paper and Bioprocess Engineering, State University of New York College of Environmental Science and Forestry, Syracuse, NY 13210, USA. Tel.: þ1 315 470 6885; fax: þ1 315 470 6945. E-mail address: [email protected] (S. Liu). 0961-9534/$ e see front matter ª 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.biombioe.2011.01.054
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hemicellulose are removed. The degraded hemicelluloses (mostly aldonic acids) in the waste pulping liquor are then combusted during the chemical recovery process together with the lignin dissolved in the cooking liquor. However, the heating value of wood hemicelullose, about 13.6 MJ/kg, is only about half of that of lignin [4]. It is thus not effective to use hemicellulose as a fuel. It is currently underutilized by burning. Therefore, a more economical use of the hemicellulose would be to extract them prior to pulping, and then convert them to a higher value-added product such as ethanol, itaconic acid, or other chemicals [2,5]. Current research reports indicate that hot-water extraction of wood before pulping is a viable process [6e8] as water is maintained in liquid state at elevated temperatures under pressure and no losses to heat of vaporization occur. It is friendly to environment as no acid, base or solvents other than water are added in the system. It is desirable to reduce the cost of extraction in comparison with prehydrolysis of woodchips catalyzed by dilute acid or alkali [9,10]. Hot-water extraction is an auto-catalytic process. During hot-water extraction process, the dissolution of acidic components in water causes the liquor pH to decrease and effectively generate protons (acids) to catalyze the extraction. As a result, most of the amorphous hemicellulose polymer and part of the lignin could be dissolved in water; the cellulose crystallinity could be decreased; and there is an increase in the porosity of the residual cellulose [11]. These effects might be hypothesized to enhance a subsequent pulping process. Therefore, combining conversion of hemicellulose to fuel and/or chemicals and wood pulping is a potential way to be cost effective for a pulp mill. In this new approach, conversion of extracted hemicellulose or extracted wood components to bio-ethanol or other chemicals has received considerable attention [12e14]. Comprehensive research about the effect of hot-water extraction on pulping is less available and urgently needed. In this work, aspen woodchips were subjected to hot-water extraction followed by soda pulping. Factors including extraction temperature, duration and water-to-wood ratio were chosen to investigate for their effects on the pulping process and pulp chemical properties. The objective was to obtain a deep understanding of hot-water extraction and its impact on the pulping process, Kappa number and viscosity of the resulting pulp.
2.
Experimental
2.1.
Hot-water extraction and pulping
Industrial woodchips of debarked Northern Alberta Boreal Aspen (mainly Populus tremuloides) wood were obtained from Alberta Pacific Forest Industries Inc., Alberta, Canada. The chips were screened in the lab at SUNY ESF to a length of 3e4 cm and a width of 2e3 cm. A homogeneous sample of extracted aspen woodchips was obtained using a 6 L M/K re-circulation digester. The charge of woodchips was 500 g oven-dry (OD) per digester load. The extraction condition was depicted in Table 1. At least duplicate experiment trials were conducted. All the results reported were of average values. The moisture in the wood was
Table 1 e Condition of hot-water extraction of aspen woodchips. Sample No.
Temperature, C
Time, min
Water-to-wood ratio
145 150 155 160 165 160 160 160 160 160 160 160 160 160 160 160
90 90 90 90 90 90 90 90 90 15 30 60 120 150 180 210
4.0 4.0 4.0 4.0 4.0 4.5 5.0 5.5 6.0 4.0 4.0 4.0 4.0 4.0 4.0 4.0
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
included in the water-to-wood ratio calculation. The vessel was heated from room temperature to set temperature in about 28 min. The extracted woodchips were then thoroughly washed with water to remove dissolved substances from the woodchips after desired extraction before the subsequent soda pulping. The soda pulping on fully washed extracted woodchips (300 g oven-dry weight based on extracted wood) was also performed in an M/K circulation digester with a temperature of 150 C, effective alkali of 20%, liquor-to-wood ratio of 4.5:1 and time at temperature (150 C) for 3 h. Control cooks were also performed on the virgin woodchips with the pulping conditions as describing in Table 2.
2.2.
Analysis
Sampling and preparation of wood for analysis were according to TAPPI standards T257 cm-02 and T264 cm-07. Screened pulp was obtained according to T278 sp-04. All oven-dry samples were obtained at 105 3 C. Dissolved solids in extraction liquor were also obtained by drying at 105 3 C. Xylose formed during the hot-water extraction of aspen woodchips as well as sugar degradation products (furfural, acetic acid and formic acid) were analyzed with 1H NMR [15]. 0.1 mol l1 glucosamine (GLcN) was used as an internal standard. One ml clear centrifuged extraction liquor and 100 ml GLcN was mixed in the NMR tube. All spectra were obtained with a Bruker AVANCE 600 spectrometer. Acid-insoluble lignin in extracted or unextracted woodchips was tested using T222 om-06. Acid-soluble lignin was
Table 2 e Pulping conditions of the control samples. Control sample Sample I Sample II Sample III
Temperature, Time, Liquor-to-wood Effective C h ratio alkali, % 150 150 160
3 4 3
4.5 4.5 4.5
20 20 20
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measured using a spectrophotometric method (UM 250, 1985 TAPPI). The UV was performed at 205 nm. Kappa number and viscosity of pulp were obtained according to T236 om-06 and T230 om-08, respectively.
3.
Fig. 1 shows the mass removal based on the original woodchip dry mass as a function of temperature, duration and water-to-wood ratio. As the extraction temperature increased, mass removal increased. At a given temperature, the mass removal increased with the duration of extraction. One can also observe that the mass removal from the hotwater extraction increased sharply at higher temperatures. More macromolecules become soluble at higher temperature, which is on top of the simple Arrhenius effect of fast reaction at higher temperature [6]. The mass removal increased more sharply with extraction time prior to 150 min. Increasing extraction duration longer than 150 min, one sees the mass removal increasing at a much slower rate. The mass removal increased markedly with increasing liquor-to-wood ratio prior to 4.5:1, and remained nearly unchanged when further increasing the liquor-to-wood ratio. The little differences in mass removal at the same temperature and duration indicated that the water-to-wood ratio had little influence on the mass removal during hot-water extraction beyond a ratio of 4.5:1. Time and temperature both play a major role in the process. Extraction at 160 C for 210 min with a water-to-wood ratio of 4:1, produced a mass removal of 21.4%. As the wood and water were heated, an increasing portion of hemicellulose, some lignin/aromatic materials, and a majority of the volatile extractives were dissolved in the water, now referred to as the extraction liquor or extract. In hardwoods the major hemicellulose component is xylan, within the limits of
Results and discussion
3.1. Effects of temperature, duration and ratio of waterto-wood on hot-water extraction Hot-water extraction of aspen woodchips was conducted at different temperatures, durations and water-to-wood ratios. The extraction temperature varied between 145 C and 165 C. The duration of extraction lasted from 15 min to 210 min. The ratio of water-to-wood differed from 4.0 to 6.0. The appearance of woodchips changed after extraction. The color of the extracted woodchips was darker than that of virgin woodchips. Generally the higher the temperature was and the longer the duration was, the darker the resulting woodchip color became. The pH of the extraction liquor varied between 3.50 and 4.11, decreasing with increasing temperature and duration. During the hot-water extraction, weight loss of woodchips occurred. The mass removal was calculated by
Mass remval, %
a
loss of OD weight 100% original OD weight
24 22 20 18 16 14 12 10 8 6 4 2 0 140
(1)
b
Mass removal, %
mass removal ; % ¼
145
150 155 160 Temperature, C
Mass removal, %
c
165
170
24 22 20 18 16 14 12 10 8 6 4 2 0 0
30
60
90 120 150 Time, min
180
210
240
24 22 20 18 16 14 12 10 8 6 4 2 0 3.5
4.0
4.5 5.0 5.5 Water-to-wood ratio
6.0
6.5
Fig. 1 e Effect of extraction conditions on mass removal. a) Variation of mass removal with temperature for 90 min duration and a water-to-wood ratio of 4:1. b) Variation of mass removal with duration at 160 C with a water-to-wood ratio of 4:1. c) Variation of mass removal with water-to-wood ratio at 160 C for 90 min.
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15e30 percent of the dry wood [16]. The contents of the extraction liquor are shown in Figs. 2e4. During the process of hot-water extraction, pressure keeps water in the liquid state. In the initial stage of extraction, hydronium ions generated by water autoionization catalyze the reaction. Subsequently the production of acetic and formic acids from dissolved hemicellulose lowed the pH of liquor. Then the hydronium ions from acetic and formic acids took over as dissociation of water is very limited. Although xylan dissolves mainly as oligomers as evidenced by the bonded acetyl in the extraction liquor, the acids catalyzed hydrolysis of a portion of the oligomers to xylose. It should be pointed out that the ratio between acetic acid and bonded acetyl is not a direct measure of the monosaccharide to polysaccharide ratio. Both kinetics and thermodynamic equilibrium play roles in the distributions. The same holds true for the distributions of extractable substances among original wood, extracted wood, and the extraction liquor. Xylose degradation to furfural also occurred in the extraction liquor. The absence of measurable glucose in the extraction liquor indicated that cellulose didn’t dissolve in measurable amounts in the extraction liquor. HMF (hydroxylmethylfurfural) was not found in measurable quantities either; the lack of measurable quantities of 6-carbon sugars in the extraction liquor which degrade to HMF, is the likely explanation for not finding HMF.
a
Basically, the concentration of xylose and other sugar degradation products increased with temperature and duration. One can observe that the increase of xylose was nearly linear with extraction time under 150 min. Beyond 180 min the concentration of xylose increased more sharply. It reached 6.6 g l1 at 210 min, 160 C. An overly high water-to-wood ratio could dilute the extraction liquor and cause the concentration of xylose to decrease. To produce bio-ethanol by hydrolysis and fermentation, a higher concentration of xylose can yield more ethanol. However, furfural is undesirable in the bio-ethanol process because it may hinder yeast growth in the fermentation step [17,18]. One can observe that furfural increased sharply beginning from 180 min at 160 C. It reached 3.17 g l1 at 210 min at 160 C. Temperature and the water-to-wood ratio had little impact on the concentration of furfural. Therefore, hot-water extraction conducted for less than 180 min duration at 160 C is recommended for bioconversion utilization of hemicellulose. Lignin remaining in the woodchips and the lignin loss ratio based on the total lignin in the woodchips from extracted woodchips were shown in Figs. 5 and 6. Lignin in original woodchips was 21.68% consisting of 18.62% acid-insoluble (Klason) lignin and 3.06% acid-soluble lignin. Less than 6% lignin based on the woodchip mass (or 27.5% of the total lignin) was removed from woodchips during the extractions. The loss of lignin was calculated by
b
7
6
6
5
Xylose, g l
5 Xylose, g l
7
4 3
4 3 2
2 1
1 0
0 140
0 145
150 155 160 Temperature, C
c
165
30
170
60
90 120 150 Time, min
180
210
240
7 6
Xylose, g l
5 4 3 2 1 0 3.5
4.0
4.5 5.0 5.5 water-to-wood ratio
6.0
6.5
Fig. 2 e Effect of extraction condition on xylose content in extraction liquor. a) Variation of xylose content with temperature for an extraction time of 90 min and a water-to-wood ratio of 4:1. b) Variation of xylose content with duration of extraction at 160 C with a water-to-wood ratio of 4.1. c) Variation of xylose content with water-to-wood ratio at 160 C for 90 min.
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a
b
HAc, free bonded total
200 180
180
-1
140 Acetyl, mmol l
-1
140 120 100 80
120 100 80 60
60
40
40
20
20
0
0 140
HAc,free bonded total
160
160 Acetyl, mmol l
200
0
145
150 155 160 o Temperature, C
c
165
30
60
90
170
200
120 150 Time, min
180
210
240
HAc,free bonded total
180 160
Acetyl, mmol l
-1
140 120 100 80 60 40 20 0 3.5
4.0
4.5
5.0
5.5
6.0
6.5
Water-to-wood ratio
Fig. 3 e Effect of extraction condition on acetyl production (HAc equivalent) in the extraction liquor. a) Variation of various types of acetyl groups (HAc equivalent) in extraction liquor with temperature for an extraction time of 90 min and a waterto-wood ratio of 4:1. b) Variation of various types of acetyl groups (HAc equivalent)in extraction liquor with duration at 160 C with a water-to-wood ratio of 4:1. c) Variation of various types of acetyl groups (HAc equivalent) in extraction liquor with water-to-wood ratio at 160 C for 90 min.
lignin loss ratio ¼
loss of lignin original lignin
(2)
The lignin loss ratio increased with the extraction temperature and time. Temperature had a predominant effect. Beyond 160 C, the loss of lignin increased sharply with increasing temperature. One can observe that the loss ratio reached 27.4% at 165 C for an extraction duration of 90 min. Prior to 120 min duration, the loss of lignin increased with the duration of extraction. After that the content of lignin in the extracted woodchips increased a small amount. This perhaps resulted from the redeposition of lignin from the extraction liquor. The water-to-wood ratio had less influence on lignin loss between 4:1 and 6:1.
3.2.
Effect of hot-water extraction on pulping
Fig. 7 shows overall pulp yield and reject ratio of the pulps from extracted woodchips. Fig. 8 shows kappa number and viscosity of pulp from extracted woodchips. The results of control samples were listed in Table 3. The overall pulp yield and reject ratio calculations are based on the oven-dry weight of original woodchips (500 g). Rejects were debris separated from the pulp slurry using screening device with size of 0.10 þ 0.005/0.01 mm.
pulp yield; % ¼
reject ratio; % ¼
OD mass of pulp 100% OD mass of original wood chips OD mass of rejects 100% OD mass of original wood chips
From Table 3 and Fig. 7 one can observe that the pulping yield of control sample I was little higher than those of other samples at the same pulping conditions. Part of the reasons was that some hemicellulose and lignin had been extracted prior to pulping. But the control pulp rejects were much higher. For hot-water water extracted samples, the pulping yield and rejects decreased with an increase of extraction time and temperature. That means the process has been improved after hot-extraction because returning rejects to pulping process may reduce the production capacity of pulp mill. Kappa number (K) measures the residual lignin in pulp and estimates the degree of delignification of pulp. Kappa number has a linear relationship with lignin content for pulps with an overall yield below 70%. The percentage of Klason lignin approximately equals K 0.15 [3]. From Fig. 8 one can observe that K decreased with increasing extraction time and temperature. This is mainly because the structural modification caused an increased reactivity of woodchips after hot-water extraction [9]. One can observe that the loss of lignin during
a 3.5
b
formic acid furfural
3.0 2.5
Concentration, g l
2.5 Concentration, g l
3.5
formic acid furfural
3.0
2.0 1.5 1.0
2.0 1.5 1.0
0.5
0.5 0.0
0.0 140
145
150 155 160 Temperatuer, C
165
0
170
30
60
90
120
150
180
210
240
Time, min
c
3.5
formic acid furfural
3.0
Concentration, g l
2.5 2.0 1.5 1.0 0.5 0.0 3.5
4.0
4.5
5.0
5.5
6.0
6.5
Water- to-wood ratio
Fig. 4 e Effect of extraction condition on formic acid and furfural in extraction liquor. a) Variation of formic acid and furfural with temperature for an extraction time of 90 min and a water-to-wood ratio of 4:1. b) Variation of formic acid and furfural with duration at 160 C with a water-to-wood ratio of 4:1. c) Variation of formic acid and furfural with water-to-wood ratio at 160 C for 90 min.
b
total lignin,% acid-insoluble lignin,% acid-soluble lignin,%
25
25
20
20
15
15
Lignin, %
Lignin, %
a
10
10 5
5 0 140
total lignin,% acid-insoluble lignin,% acid-soluble lignin,%
0 145
150
155
160
165
0
170
30
60
c
25
120
150
180
210
240
total lignin acid-insoluble lignin acid-soluble lignin
20
Lignin, %
90
Time, min
o
Temperature, C
15 10 5 0 3.5
4.0
4.5
5.0
5.5
6.0
6.5
Water-to wood ratio
Fig. 5 e Effect of extraction conditions on lignin remaining in the extracted wood. a) Variation of lignin remaining in the wood with temperature for an extraction time of 90 min and a water-to-wood ratio of 4:1. b) Variation of lignin remaining in the wood with duration of extraction at 160 C with a water-to-wood ratio of 4:1. c) Variation of lignin remaining in the wood with water-to-wood ratio at 160 C for 90 min.
b
30
30
25
25
20
20
Loss of lignin, %
Loss of lignin, %
a
15 10
15 10 5
5 0 140
0 145
150
155
160
165
0
170
30
60
90
o
Temperature, C
c
120 150 Time, min
180
210
240
30
Loss of lignin, %
25 20 15 10 5 0 3.5
4.0
4.5
5.0
5.5
6.0
6.5
Water-to-wood ratio
Fig. 6 e Effect of extraction conditions on the loss of lignin from the extracted wood. a) Variation of lignin loss with temperature for an extraction time of 90 min and a water-to-wood ratio of 4:1. b) Variation of lignin loss with duration of extraction at 160 C with a water-to-wood ratio of 4:1. c) Variation of lignin loss with water-to-wood ratio at 160 C for 90 min.
a
70
b
pulp yield reject ratio
60
70 60 50
40
40 %
%
50
30
30
20
20
10
10
0 140
0
145
150
155
160
pulp yield reject ratio
165
170
0
30
60
90 120 150 Time, min
o
Temperature, C
c 70
180
210
240
pulp yield reject ratio
60 50
%
40 30 20 10 0 3.5
4.0
4.5 5.0 5.5 Water-to-wood ratio
6.0
6.5
Fig. 7 e Effect of extraction condition on overall pulp yield and reject ratio. a) Variation of overall pulp yield and reject ratio with temperature for an extraction time of 90 min and a water-to-wood ratio of 4:1. b) Variation of overall pulp yield and reject ratio with duration of extraction at 160 C with a water-to-wood ratio of 4:1. c) Variation of overall pulp yield and reject ratio with water-to-wood ratio at 160 C for 90 min.
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a 70
b
Kappa number viscosity, mPa.s
60
70
50
50
40
40
30
30
20
20
10
10
0 140
Kappa number viscosity, mPa.s
60
0
145
150
155
160
165
170
0
30
60
o
c
90
120
150
180
210
240
Time, min
Temperature, C 70 Kappa number viscosity, mPa.s
60 50 40 30 20 10 0 3.5
4.0
4.5 5.0 5.5 Water-to-wood ratio
6.0
6.5
Fig. 8 e Effect of extraction condition on Kappa number and viscosity of pulp. a) Variation of Kappa number and viscosity with temperature for an extraction time of 90 min and a water-to-wood ratio of 4:1. b) Variation of Kappa number and viscosity with duration of extraction at 160 C with a water-to-wood ratio of 4:1. c) Variation of Kappa number and viscosity with water-to-wood ratio at 160 C for 90 min.
hot-water extraction only occurred during the first 120 min (Figs. 5 and 6). The increased available surface area and higher pore volume caused by delignification and hemicellulose degradation in hot-water extraction apparently improved susceptibility towards delignification with alkaline solutions. Severe pulping conditions can also cause higher lignin removal (Fig. 7). Kappa number of control sample II (4 h at 150 C) can be comparable to the pulps from hot-water extracted woodchips at 145 C for 90 min. Kappa number of control sample III (3 h at 160 C) is comparable to the pulps from hot-water extracted woodchips at 160 C for 20 min. Thus, after hot-water extraction, the woodchips are well conditioned for pulping. One can observe that viscosity of resulting pulp increased with increasing extraction temperature (Fig. 8). The viscosity of the pulp increased more sharply at shorter extraction times and after extraction at 160 C for 90 min, the woodchips were
Table 3 e The results of pulp from control samples. Control samples
Pulp yield, %
Reject ratio, %
Kappa number
Viscosity, mPas
Sample I Sample II Sample III
61.81 60.52 54.18
16.85 7.22 0.10
61.12 46.72 13.76
17.02 22.35 21.84
pulped to the maximum observed viscosity of 47.32 mPas. The viscosity decreased slightly with increasing duration of woodchip extraction until 150 min. When the woodchips were subjected to an extraction longer than 180 min at 160 C, the resultant pulps showed a sharper decrease in viscosity. The water-to-wood ratio in woodchip hot-water extraction had little influence on the viscosity of the resultant pulps. A pulp viscosity increase indicates a higher average degree of polymerization of the polysaccharides primarily cellulose. After hot-water extraction, most hemicellulose with short chains is dissolved in liquor. Therefore, the increasing ratio of cellulose with longer chains in the pulp would result in higher viscosity. The more the hemicellulose was lost, the higher the viscosity expected and this was observed for mild extraction conditions. However, under harsher extraction conditions, cellulose might be degraded and the ratio of carbohydrate with short chains increased thereafter. Thus, the viscosity began to decrease at harsh extraction conditions. Considering that pulp strength drops dramatically if the viscosity falls too low, extensive extraction time or higher temperature is not recommended for hot-water extraction if pulps were to be produced from the extracted woodchips. Extraction also enables much shorter pulping times to equal kappa numbers and the reduced exposure to caustic conditions at high temperatures will also produce less damaged cellulose and higher viscosity pulp.
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4.
Conclusion
The effect of hot-water extraction on soda pulping of aspen woodchips has been studied. During the hot-water extraction process, mass removal increased with increasing extraction temperature and duration. It has been observed that the mass loss was 4.7% at 160 C for 15 min and 21.4% at 160 C for 210 min. Part of the hemicellose and lignin are dissolved in the extraction liquor. The lignin dissolution increased with extraction temperature and duration. However, the concentration of furfural in the extraction liquor increased sharply beyond 150 min at 160 C, which is undesirable for microorganisms in bioconversion of the dissolved hemicellulose. Extracted woodchips were more susceptible to delignification. Lower kappa numbers were obtained after hot-water extraction. Viscosity of pulp increased with an increase of extraction temperature and time prior to 90 min. It began to fall noticeably beyond 180 min. Water-to-wood ratio of the hot-water extraction had less impact on the hot-water extraction and pulping. Considering the need for bioconversion of hemicellulose to chemicals/liquid fuel and the resulting pulp properties, hot-water extraction at 165 C, 90 min and a waterto-wood ratio of 5:1 on aspen woodchips intended to be pulped is recommended.
Acknowledgments The authors are indebted to HYPERLINK "gs1:" \o "gs1:"Alberta Pacific Forest Industries, Inc. and HYPERLINK "gs2:" \o "gs2:"US DOE for financial support to the project. The authors are grateful to China Scholarship Council for the scholarships provided to Dr. H. Lu and Dr. R. Hu during their stay at SUNY ESF. Special thanks also goes to Mr. Dave Kimle for NMR analysis, and many others in the Department of Paper and Bioprocess Engineering, SUNY ESF for their assistance.
references
[1] Mao H, Genco JM, Yoon S-H, van Heiningen A, Pendse H. Technical economic evaluation of a hardwood biorefinery using the "near-neutral" hemicellulose pre-extraction process. J Biobased Mater Bioenergy 2008;2:177e85.
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[2] Amidon TE, Wood CD, Shupe AM, Wang Y, Graves M, Liu S. Biorefinery: conversion of woody biomass to chemicals, energy and materials. J Biobased Mater Bioenergy 2008;2:100e20. [3] Smook GA. Handbook for pulp and paper technologists. Vancouver: Angus Wilde Publications Inc.; 2002. [4] Amidon TE, Liu S. Water-based woody biorefinery. Biotechnol Adv 2009;27:542e50. [5] Schacht C, Zetzl C, Brunner G. From plant materials to ethanol by means of supercritical fluid technology. J Supercrit Fluids 2008;46:299e321. [6] Liu S. A kinetic model on autocatalytic reactions in woody biomass hydrolysis. J Biobased Mater Bioenergy 2008;2:135e47. [7] Sattler C, Labbe´ N, Harper D, Elder T, Rials T. Effects of hot water extraction on physical and chemical characteristics of Oriented Strand Board (OSB) wood flakes. Clean-Soil Air Water 2008;36:674e81. [8] Liu S, Amidon TE, Francis RC, Ramarao BV, Lai Y-Z, Scott GM. From forest biomass to chemicals and energy biorefinery initiative in New York State. Ind Biotechnol 2006;2:113e20. [9] Garrote G, Domı´nguez H, Parajo´ JC. Hydrothermal processing of lignocellulosic materials. Holz als Roh- und Werkstoff 1999;57:191e202. [10] Garrote G, Eugenio ME, Dı´az MJ, Ariza J, Lo´pez F. Hydrothermal and pulp processing of Eucalyptus. Bioresour Technol 2003;88:61e8. [11] Mosier N, Wyman C, Dale B, Elander R, Lee YY, Holtzapple M, et al. Features of promising technologies for pretreatment of lignocellulosic biomass. Bioresour Technol 2005;96:673e86. [12] Larsson S, Palmqvist E, Hahn-Ha¨gerdal B, Tengborg C, Stenberg K, Zacchi G, et al. The generation of fermentation inhibitors during dilute acid hydrolysis of softwood. Enzyme Microb Technol 1999;24:151e9. [13] Saha BC. Hemicellulose bioconversion. L J Ind Microbiol Biotechnol 2003;30:279e91. ´ J, Cardona CA. Trends in biotechnological [14] Sa´nchez O production of fuel ethanol from different feedstocks. Bioresour Technol 2008;99:5270e95. [15] Mittal A, Scott GM, Amidon TE, Kiemle DJ, Stipanovic AJ. Quantitative analysis of sugars in wood hydrolyzates with 1 H NMR during the autohydrolysis of hardwoods. Bioresour Technol 2009;100:6398e406. [16] Sjo¨stro¨m E. Wood chemistry: fundamentals and applications. 2nd ed. San Diego: Academic Press; 1993. [17] Martinez A, Rodriguez ME, York SW, Preston JF, Ingram LO. Effects of Ca(OH)2 treatments (“overliming”) on the composition and toxicity of bagasse hemicellulose hydrolysates. Biotechnol Bioeng 2000;69:526e636. [18] Larsson S, Reimann A, Vebrant N-ON, Jo¨nsson LJ. comparison of different methods for the detoxification of lignocellulose hydrolyzates of spruce. Appl Biochem Biotechnol 1999;77-79:91e103.
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Hot-water extraction and its effect on soda pulping of aspen woodchips Houfang Lu a,b, Ruofei Hu b,c, Al Ward d, Thomas E. Amidon b, Bin Liang a, Shijie Liu b,e,* a
College of Chemical Engineering, Sichuan University, Chengdu 610065, PR China Department of Paper and Bioprocess Engineering, State University of New York College of Environmental Science and Forestry, Syracuse, NY 13210, USA c College of Chemical Engineering and Food Science, Xiangfan University, Xiangyang 441053, PR China d Alberta Pacific Forest Industries Inc., P.O. Box 8000, Boyle, Alberta T0A 0M0, Canada e State Key Laboratory of Pulp and Paper Engineering, South China University of Technology, Guangzhou 510640, PR China b
article info
abstract
Article history:
Hot-water extraction of industrial aspen woodchips (mainly Populus tremuloides) and
Received 17 March 2010
subsequent soda pulping were carried out in an M/K digester. It was found that mass
Received in revised form
removal of woodchips was 4.74% at 160 C for 15 min with a water-to-wood ratio of 4:1, and
1 January 2011
that mass removal increased with extraction time and temperature. The mass removal
Accepted 28 January 2011
reached 21.38% at 160 C for 210 min. Less than 6% lignin (on total wood mass) from virgin
Available online 9 March 2011
woodchips was dissolved in the extraction liquor. The concentrations of xylose, acetic acid, formic acid and furfural in the extraction liquor were measured by 1H NMR, which were
Keywords:
found to increase with extraction time and temperature. Furfural increased sharply beyond
Hemicellulose
150 min of extraction at 160 C. The residual woodchips were subjected to soda pulping at
Hot-water extraction
150 C for 3 h with 20% effective alkali and a liquor-to-wood ratio of 4.5:1. Compared with
Soda pulping
the control sample, the overall pulp yield for extracted woodchips decreased little while
Kappa number
rejects decreased sharply. Kappa number of the pulps decreased with increasing extraction
Viscosity
time and temperature. Viscosity of the pulp increased with increasing extraction
Yield
temperature and duration for the first 90 min. Beyond 90 min of extraction, the resulting pulp viscosity decreased with increasing duration of extraction. The water-to-wood ratio had little effect on the entire process. Considering bioconversion of extracted hemicellulose to fuel/chemicals and the resultant pulping characteristics, hot-water extraction at 165 C for 90 min with a water-to-water ratio of 5:1 for aspen woodchips seems to be optimum. ª 2011 Elsevier Ltd. All rights reserved.
1.
Introduction
Owing to environment concerns and resources depletion, more and more attention is being paid to renewable materials, such as wood [1,2]. Wood is the principal source of cellulosic fibers for
pulp and paper manufacture. It is composed of four main components: cellulose (35e50%), hemicellulose (20e35%), lignin (10e25%), and extractives (2e8%)[3]. In the paper industry, fibers are obtained through a pulping process. During a typical chemical pulping process, 80% of the lignin and 50% of the
* Corresponding author. Department of Paper and Bioprocess Engineering, State University of New York College of Environmental Science and Forestry, Syracuse, NY 13210, USA. Tel.: þ1 315 470 6885; fax: þ1 315 470 6945. E-mail address: [email protected] (S. Liu). 0961-9534/$ e see front matter ª 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.biombioe.2011.01.054
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hemicellulose are removed. The degraded hemicelluloses (mostly aldonic acids) in the waste pulping liquor are then combusted during the chemical recovery process together with the lignin dissolved in the cooking liquor. However, the heating value of wood hemicelullose, about 13.6 MJ/kg, is only about half of that of lignin [4]. It is thus not effective to use hemicellulose as a fuel. It is currently underutilized by burning. Therefore, a more economical use of the hemicellulose would be to extract them prior to pulping, and then convert them to a higher value-added product such as ethanol, itaconic acid, or other chemicals [2,5]. Current research reports indicate that hot-water extraction of wood before pulping is a viable process [6e8] as water is maintained in liquid state at elevated temperatures under pressure and no losses to heat of vaporization occur. It is friendly to environment as no acid, base or solvents other than water are added in the system. It is desirable to reduce the cost of extraction in comparison with prehydrolysis of woodchips catalyzed by dilute acid or alkali [9,10]. Hot-water extraction is an auto-catalytic process. During hot-water extraction process, the dissolution of acidic components in water causes the liquor pH to decrease and effectively generate protons (acids) to catalyze the extraction. As a result, most of the amorphous hemicellulose polymer and part of the lignin could be dissolved in water; the cellulose crystallinity could be decreased; and there is an increase in the porosity of the residual cellulose [11]. These effects might be hypothesized to enhance a subsequent pulping process. Therefore, combining conversion of hemicellulose to fuel and/or chemicals and wood pulping is a potential way to be cost effective for a pulp mill. In this new approach, conversion of extracted hemicellulose or extracted wood components to bio-ethanol or other chemicals has received considerable attention [12e14]. Comprehensive research about the effect of hot-water extraction on pulping is less available and urgently needed. In this work, aspen woodchips were subjected to hot-water extraction followed by soda pulping. Factors including extraction temperature, duration and water-to-wood ratio were chosen to investigate for their effects on the pulping process and pulp chemical properties. The objective was to obtain a deep understanding of hot-water extraction and its impact on the pulping process, Kappa number and viscosity of the resulting pulp.
2.
Experimental
2.1.
Hot-water extraction and pulping
Industrial woodchips of debarked Northern Alberta Boreal Aspen (mainly Populus tremuloides) wood were obtained from Alberta Pacific Forest Industries Inc., Alberta, Canada. The chips were screened in the lab at SUNY ESF to a length of 3e4 cm and a width of 2e3 cm. A homogeneous sample of extracted aspen woodchips was obtained using a 6 L M/K re-circulation digester. The charge of woodchips was 500 g oven-dry (OD) per digester load. The extraction condition was depicted in Table 1. At least duplicate experiment trials were conducted. All the results reported were of average values. The moisture in the wood was
Table 1 e Condition of hot-water extraction of aspen woodchips. Sample No.
Temperature, C
Time, min
Water-to-wood ratio
145 150 155 160 165 160 160 160 160 160 160 160 160 160 160 160
90 90 90 90 90 90 90 90 90 15 30 60 120 150 180 210
4.0 4.0 4.0 4.0 4.0 4.5 5.0 5.5 6.0 4.0 4.0 4.0 4.0 4.0 4.0 4.0
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
included in the water-to-wood ratio calculation. The vessel was heated from room temperature to set temperature in about 28 min. The extracted woodchips were then thoroughly washed with water to remove dissolved substances from the woodchips after desired extraction before the subsequent soda pulping. The soda pulping on fully washed extracted woodchips (300 g oven-dry weight based on extracted wood) was also performed in an M/K circulation digester with a temperature of 150 C, effective alkali of 20%, liquor-to-wood ratio of 4.5:1 and time at temperature (150 C) for 3 h. Control cooks were also performed on the virgin woodchips with the pulping conditions as describing in Table 2.
2.2.
Analysis
Sampling and preparation of wood for analysis were according to TAPPI standards T257 cm-02 and T264 cm-07. Screened pulp was obtained according to T278 sp-04. All oven-dry samples were obtained at 105 3 C. Dissolved solids in extraction liquor were also obtained by drying at 105 3 C. Xylose formed during the hot-water extraction of aspen woodchips as well as sugar degradation products (furfural, acetic acid and formic acid) were analyzed with 1H NMR [15]. 0.1 mol l1 glucosamine (GLcN) was used as an internal standard. One ml clear centrifuged extraction liquor and 100 ml GLcN was mixed in the NMR tube. All spectra were obtained with a Bruker AVANCE 600 spectrometer. Acid-insoluble lignin in extracted or unextracted woodchips was tested using T222 om-06. Acid-soluble lignin was
Table 2 e Pulping conditions of the control samples. Control sample Sample I Sample II Sample III
Temperature, Time, Liquor-to-wood Effective C h ratio alkali, % 150 150 160
3 4 3
4.5 4.5 4.5
20 20 20
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measured using a spectrophotometric method (UM 250, 1985 TAPPI). The UV was performed at 205 nm. Kappa number and viscosity of pulp were obtained according to T236 om-06 and T230 om-08, respectively.
3.
Fig. 1 shows the mass removal based on the original woodchip dry mass as a function of temperature, duration and water-to-wood ratio. As the extraction temperature increased, mass removal increased. At a given temperature, the mass removal increased with the duration of extraction. One can also observe that the mass removal from the hotwater extraction increased sharply at higher temperatures. More macromolecules become soluble at higher temperature, which is on top of the simple Arrhenius effect of fast reaction at higher temperature [6]. The mass removal increased more sharply with extraction time prior to 150 min. Increasing extraction duration longer than 150 min, one sees the mass removal increasing at a much slower rate. The mass removal increased markedly with increasing liquor-to-wood ratio prior to 4.5:1, and remained nearly unchanged when further increasing the liquor-to-wood ratio. The little differences in mass removal at the same temperature and duration indicated that the water-to-wood ratio had little influence on the mass removal during hot-water extraction beyond a ratio of 4.5:1. Time and temperature both play a major role in the process. Extraction at 160 C for 210 min with a water-to-wood ratio of 4:1, produced a mass removal of 21.4%. As the wood and water were heated, an increasing portion of hemicellulose, some lignin/aromatic materials, and a majority of the volatile extractives were dissolved in the water, now referred to as the extraction liquor or extract. In hardwoods the major hemicellulose component is xylan, within the limits of
Results and discussion
3.1. Effects of temperature, duration and ratio of waterto-wood on hot-water extraction Hot-water extraction of aspen woodchips was conducted at different temperatures, durations and water-to-wood ratios. The extraction temperature varied between 145 C and 165 C. The duration of extraction lasted from 15 min to 210 min. The ratio of water-to-wood differed from 4.0 to 6.0. The appearance of woodchips changed after extraction. The color of the extracted woodchips was darker than that of virgin woodchips. Generally the higher the temperature was and the longer the duration was, the darker the resulting woodchip color became. The pH of the extraction liquor varied between 3.50 and 4.11, decreasing with increasing temperature and duration. During the hot-water extraction, weight loss of woodchips occurred. The mass removal was calculated by
Mass remval, %
a
loss of OD weight 100% original OD weight
24 22 20 18 16 14 12 10 8 6 4 2 0 140
(1)
b
Mass removal, %
mass removal ; % ¼
145
150 155 160 Temperature, C
Mass removal, %
c
165
170
24 22 20 18 16 14 12 10 8 6 4 2 0 0
30
60
90 120 150 Time, min
180
210
240
24 22 20 18 16 14 12 10 8 6 4 2 0 3.5
4.0
4.5 5.0 5.5 Water-to-wood ratio
6.0
6.5
Fig. 1 e Effect of extraction conditions on mass removal. a) Variation of mass removal with temperature for 90 min duration and a water-to-wood ratio of 4:1. b) Variation of mass removal with duration at 160 C with a water-to-wood ratio of 4:1. c) Variation of mass removal with water-to-wood ratio at 160 C for 90 min.
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15e30 percent of the dry wood [16]. The contents of the extraction liquor are shown in Figs. 2e4. During the process of hot-water extraction, pressure keeps water in the liquid state. In the initial stage of extraction, hydronium ions generated by water autoionization catalyze the reaction. Subsequently the production of acetic and formic acids from dissolved hemicellulose lowed the pH of liquor. Then the hydronium ions from acetic and formic acids took over as dissociation of water is very limited. Although xylan dissolves mainly as oligomers as evidenced by the bonded acetyl in the extraction liquor, the acids catalyzed hydrolysis of a portion of the oligomers to xylose. It should be pointed out that the ratio between acetic acid and bonded acetyl is not a direct measure of the monosaccharide to polysaccharide ratio. Both kinetics and thermodynamic equilibrium play roles in the distributions. The same holds true for the distributions of extractable substances among original wood, extracted wood, and the extraction liquor. Xylose degradation to furfural also occurred in the extraction liquor. The absence of measurable glucose in the extraction liquor indicated that cellulose didn’t dissolve in measurable amounts in the extraction liquor. HMF (hydroxylmethylfurfural) was not found in measurable quantities either; the lack of measurable quantities of 6-carbon sugars in the extraction liquor which degrade to HMF, is the likely explanation for not finding HMF.
a
Basically, the concentration of xylose and other sugar degradation products increased with temperature and duration. One can observe that the increase of xylose was nearly linear with extraction time under 150 min. Beyond 180 min the concentration of xylose increased more sharply. It reached 6.6 g l1 at 210 min, 160 C. An overly high water-to-wood ratio could dilute the extraction liquor and cause the concentration of xylose to decrease. To produce bio-ethanol by hydrolysis and fermentation, a higher concentration of xylose can yield more ethanol. However, furfural is undesirable in the bio-ethanol process because it may hinder yeast growth in the fermentation step [17,18]. One can observe that furfural increased sharply beginning from 180 min at 160 C. It reached 3.17 g l1 at 210 min at 160 C. Temperature and the water-to-wood ratio had little impact on the concentration of furfural. Therefore, hot-water extraction conducted for less than 180 min duration at 160 C is recommended for bioconversion utilization of hemicellulose. Lignin remaining in the woodchips and the lignin loss ratio based on the total lignin in the woodchips from extracted woodchips were shown in Figs. 5 and 6. Lignin in original woodchips was 21.68% consisting of 18.62% acid-insoluble (Klason) lignin and 3.06% acid-soluble lignin. Less than 6% lignin based on the woodchip mass (or 27.5% of the total lignin) was removed from woodchips during the extractions. The loss of lignin was calculated by
b
7
6
6
5
Xylose, g l
5 Xylose, g l
7
4 3
4 3 2
2 1
1 0
0 140
0 145
150 155 160 Temperature, C
c
165
30
170
60
90 120 150 Time, min
180
210
240
7 6
Xylose, g l
5 4 3 2 1 0 3.5
4.0
4.5 5.0 5.5 water-to-wood ratio
6.0
6.5
Fig. 2 e Effect of extraction condition on xylose content in extraction liquor. a) Variation of xylose content with temperature for an extraction time of 90 min and a water-to-wood ratio of 4:1. b) Variation of xylose content with duration of extraction at 160 C with a water-to-wood ratio of 4.1. c) Variation of xylose content with water-to-wood ratio at 160 C for 90 min.
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a
b
HAc, free bonded total
200 180
180
-1
140 Acetyl, mmol l
-1
140 120 100 80
120 100 80 60
60
40
40
20
20
0
0 140
HAc,free bonded total
160
160 Acetyl, mmol l
200
0
145
150 155 160 o Temperature, C
c
165
30
60
90
170
200
120 150 Time, min
180
210
240
HAc,free bonded total
180 160
Acetyl, mmol l
-1
140 120 100 80 60 40 20 0 3.5
4.0
4.5
5.0
5.5
6.0
6.5
Water-to-wood ratio
Fig. 3 e Effect of extraction condition on acetyl production (HAc equivalent) in the extraction liquor. a) Variation of various types of acetyl groups (HAc equivalent) in extraction liquor with temperature for an extraction time of 90 min and a waterto-wood ratio of 4:1. b) Variation of various types of acetyl groups (HAc equivalent)in extraction liquor with duration at 160 C with a water-to-wood ratio of 4:1. c) Variation of various types of acetyl groups (HAc equivalent) in extraction liquor with water-to-wood ratio at 160 C for 90 min.
lignin loss ratio ¼
loss of lignin original lignin
(2)
The lignin loss ratio increased with the extraction temperature and time. Temperature had a predominant effect. Beyond 160 C, the loss of lignin increased sharply with increasing temperature. One can observe that the loss ratio reached 27.4% at 165 C for an extraction duration of 90 min. Prior to 120 min duration, the loss of lignin increased with the duration of extraction. After that the content of lignin in the extracted woodchips increased a small amount. This perhaps resulted from the redeposition of lignin from the extraction liquor. The water-to-wood ratio had less influence on lignin loss between 4:1 and 6:1.
3.2.
Effect of hot-water extraction on pulping
Fig. 7 shows overall pulp yield and reject ratio of the pulps from extracted woodchips. Fig. 8 shows kappa number and viscosity of pulp from extracted woodchips. The results of control samples were listed in Table 3. The overall pulp yield and reject ratio calculations are based on the oven-dry weight of original woodchips (500 g). Rejects were debris separated from the pulp slurry using screening device with size of 0.10 þ 0.005/0.01 mm.
pulp yield; % ¼
reject ratio; % ¼
OD mass of pulp 100% OD mass of original wood chips OD mass of rejects 100% OD mass of original wood chips
From Table 3 and Fig. 7 one can observe that the pulping yield of control sample I was little higher than those of other samples at the same pulping conditions. Part of the reasons was that some hemicellulose and lignin had been extracted prior to pulping. But the control pulp rejects were much higher. For hot-water water extracted samples, the pulping yield and rejects decreased with an increase of extraction time and temperature. That means the process has been improved after hot-extraction because returning rejects to pulping process may reduce the production capacity of pulp mill. Kappa number (K) measures the residual lignin in pulp and estimates the degree of delignification of pulp. Kappa number has a linear relationship with lignin content for pulps with an overall yield below 70%. The percentage of Klason lignin approximately equals K 0.15 [3]. From Fig. 8 one can observe that K decreased with increasing extraction time and temperature. This is mainly because the structural modification caused an increased reactivity of woodchips after hot-water extraction [9]. One can observe that the loss of lignin during
a 3.5
b
formic acid furfural
3.0 2.5
Concentration, g l
2.5 Concentration, g l
3.5
formic acid furfural
3.0
2.0 1.5 1.0
2.0 1.5 1.0
0.5
0.5 0.0
0.0 140
145
150 155 160 Temperatuer, C
165
0
170
30
60
90
120
150
180
210
240
Time, min
c
3.5
formic acid furfural
3.0
Concentration, g l
2.5 2.0 1.5 1.0 0.5 0.0 3.5
4.0
4.5
5.0
5.5
6.0
6.5
Water- to-wood ratio
Fig. 4 e Effect of extraction condition on formic acid and furfural in extraction liquor. a) Variation of formic acid and furfural with temperature for an extraction time of 90 min and a water-to-wood ratio of 4:1. b) Variation of formic acid and furfural with duration at 160 C with a water-to-wood ratio of 4:1. c) Variation of formic acid and furfural with water-to-wood ratio at 160 C for 90 min.
b
total lignin,% acid-insoluble lignin,% acid-soluble lignin,%
25
25
20
20
15
15
Lignin, %
Lignin, %
a
10
10 5
5 0 140
total lignin,% acid-insoluble lignin,% acid-soluble lignin,%
0 145
150
155
160
165
0
170
30
60
c
25
120
150
180
210
240
total lignin acid-insoluble lignin acid-soluble lignin
20
Lignin, %
90
Time, min
o
Temperature, C
15 10 5 0 3.5
4.0
4.5
5.0
5.5
6.0
6.5
Water-to wood ratio
Fig. 5 e Effect of extraction conditions on lignin remaining in the extracted wood. a) Variation of lignin remaining in the wood with temperature for an extraction time of 90 min and a water-to-wood ratio of 4:1. b) Variation of lignin remaining in the wood with duration of extraction at 160 C with a water-to-wood ratio of 4:1. c) Variation of lignin remaining in the wood with water-to-wood ratio at 160 C for 90 min.
b
30
30
25
25
20
20
Loss of lignin, %
Loss of lignin, %
a
15 10
15 10 5
5 0 140
0 145
150
155
160
165
0
170
30
60
90
o
Temperature, C
c
120 150 Time, min
180
210
240
30
Loss of lignin, %
25 20 15 10 5 0 3.5
4.0
4.5
5.0
5.5
6.0
6.5
Water-to-wood ratio
Fig. 6 e Effect of extraction conditions on the loss of lignin from the extracted wood. a) Variation of lignin loss with temperature for an extraction time of 90 min and a water-to-wood ratio of 4:1. b) Variation of lignin loss with duration of extraction at 160 C with a water-to-wood ratio of 4:1. c) Variation of lignin loss with water-to-wood ratio at 160 C for 90 min.
a
70
b
pulp yield reject ratio
60
70 60 50
40
40 %
%
50
30
30
20
20
10
10
0 140
0
145
150
155
160
pulp yield reject ratio
165
170
0
30
60
90 120 150 Time, min
o
Temperature, C
c 70
180
210
240
pulp yield reject ratio
60 50
%
40 30 20 10 0 3.5
4.0
4.5 5.0 5.5 Water-to-wood ratio
6.0
6.5
Fig. 7 e Effect of extraction condition on overall pulp yield and reject ratio. a) Variation of overall pulp yield and reject ratio with temperature for an extraction time of 90 min and a water-to-wood ratio of 4:1. b) Variation of overall pulp yield and reject ratio with duration of extraction at 160 C with a water-to-wood ratio of 4:1. c) Variation of overall pulp yield and reject ratio with water-to-wood ratio at 160 C for 90 min.
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a 70
b
Kappa number viscosity, mPa.s
60
70
50
50
40
40
30
30
20
20
10
10
0 140
Kappa number viscosity, mPa.s
60
0
145
150
155
160
165
170
0
30
60
o
c
90
120
150
180
210
240
Time, min
Temperature, C 70 Kappa number viscosity, mPa.s
60 50 40 30 20 10 0 3.5
4.0
4.5 5.0 5.5 Water-to-wood ratio
6.0
6.5
Fig. 8 e Effect of extraction condition on Kappa number and viscosity of pulp. a) Variation of Kappa number and viscosity with temperature for an extraction time of 90 min and a water-to-wood ratio of 4:1. b) Variation of Kappa number and viscosity with duration of extraction at 160 C with a water-to-wood ratio of 4:1. c) Variation of Kappa number and viscosity with water-to-wood ratio at 160 C for 90 min.
hot-water extraction only occurred during the first 120 min (Figs. 5 and 6). The increased available surface area and higher pore volume caused by delignification and hemicellulose degradation in hot-water extraction apparently improved susceptibility towards delignification with alkaline solutions. Severe pulping conditions can also cause higher lignin removal (Fig. 7). Kappa number of control sample II (4 h at 150 C) can be comparable to the pulps from hot-water extracted woodchips at 145 C for 90 min. Kappa number of control sample III (3 h at 160 C) is comparable to the pulps from hot-water extracted woodchips at 160 C for 20 min. Thus, after hot-water extraction, the woodchips are well conditioned for pulping. One can observe that viscosity of resulting pulp increased with increasing extraction temperature (Fig. 8). The viscosity of the pulp increased more sharply at shorter extraction times and after extraction at 160 C for 90 min, the woodchips were
Table 3 e The results of pulp from control samples. Control samples
Pulp yield, %
Reject ratio, %
Kappa number
Viscosity, mPas
Sample I Sample II Sample III
61.81 60.52 54.18
16.85 7.22 0.10
61.12 46.72 13.76
17.02 22.35 21.84
pulped to the maximum observed viscosity of 47.32 mPas. The viscosity decreased slightly with increasing duration of woodchip extraction until 150 min. When the woodchips were subjected to an extraction longer than 180 min at 160 C, the resultant pulps showed a sharper decrease in viscosity. The water-to-wood ratio in woodchip hot-water extraction had little influence on the viscosity of the resultant pulps. A pulp viscosity increase indicates a higher average degree of polymerization of the polysaccharides primarily cellulose. After hot-water extraction, most hemicellulose with short chains is dissolved in liquor. Therefore, the increasing ratio of cellulose with longer chains in the pulp would result in higher viscosity. The more the hemicellulose was lost, the higher the viscosity expected and this was observed for mild extraction conditions. However, under harsher extraction conditions, cellulose might be degraded and the ratio of carbohydrate with short chains increased thereafter. Thus, the viscosity began to decrease at harsh extraction conditions. Considering that pulp strength drops dramatically if the viscosity falls too low, extensive extraction time or higher temperature is not recommended for hot-water extraction if pulps were to be produced from the extracted woodchips. Extraction also enables much shorter pulping times to equal kappa numbers and the reduced exposure to caustic conditions at high temperatures will also produce less damaged cellulose and higher viscosity pulp.
b i o m a s s a n d b i o e n e r g y 3 9 ( 2 0 1 2 ) 5 e1 3
4.
Conclusion
The effect of hot-water extraction on soda pulping of aspen woodchips has been studied. During the hot-water extraction process, mass removal increased with increasing extraction temperature and duration. It has been observed that the mass loss was 4.7% at 160 C for 15 min and 21.4% at 160 C for 210 min. Part of the hemicellose and lignin are dissolved in the extraction liquor. The lignin dissolution increased with extraction temperature and duration. However, the concentration of furfural in the extraction liquor increased sharply beyond 150 min at 160 C, which is undesirable for microorganisms in bioconversion of the dissolved hemicellulose. Extracted woodchips were more susceptible to delignification. Lower kappa numbers were obtained after hot-water extraction. Viscosity of pulp increased with an increase of extraction temperature and time prior to 90 min. It began to fall noticeably beyond 180 min. Water-to-wood ratio of the hot-water extraction had less impact on the hot-water extraction and pulping. Considering the need for bioconversion of hemicellulose to chemicals/liquid fuel and the resulting pulp properties, hot-water extraction at 165 C, 90 min and a waterto-wood ratio of 5:1 on aspen woodchips intended to be pulped is recommended.
Acknowledgments The authors are indebted to HYPERLINK "gs1:" \o "gs1:"Alberta Pacific Forest Industries, Inc. and HYPERLINK "gs2:" \o "gs2:"US DOE for financial support to the project. The authors are grateful to China Scholarship Council for the scholarships provided to Dr. H. Lu and Dr. R. Hu during their stay at SUNY ESF. Special thanks also goes to Mr. Dave Kimle for NMR analysis, and many others in the Department of Paper and Bioprocess Engineering, SUNY ESF for their assistance.
references
[1] Mao H, Genco JM, Yoon S-H, van Heiningen A, Pendse H. Technical economic evaluation of a hardwood biorefinery using the "near-neutral" hemicellulose pre-extraction process. J Biobased Mater Bioenergy 2008;2:177e85.
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[2] Amidon TE, Wood CD, Shupe AM, Wang Y, Graves M, Liu S. Biorefinery: conversion of woody biomass to chemicals, energy and materials. J Biobased Mater Bioenergy 2008;2:100e20. [3] Smook GA. Handbook for pulp and paper technologists. Vancouver: Angus Wilde Publications Inc.; 2002. [4] Amidon TE, Liu S. Water-based woody biorefinery. Biotechnol Adv 2009;27:542e50. [5] Schacht C, Zetzl C, Brunner G. From plant materials to ethanol by means of supercritical fluid technology. J Supercrit Fluids 2008;46:299e321. [6] Liu S. A kinetic model on autocatalytic reactions in woody biomass hydrolysis. J Biobased Mater Bioenergy 2008;2:135e47. [7] Sattler C, Labbe´ N, Harper D, Elder T, Rials T. Effects of hot water extraction on physical and chemical characteristics of Oriented Strand Board (OSB) wood flakes. Clean-Soil Air Water 2008;36:674e81. [8] Liu S, Amidon TE, Francis RC, Ramarao BV, Lai Y-Z, Scott GM. From forest biomass to chemicals and energy biorefinery initiative in New York State. Ind Biotechnol 2006;2:113e20. [9] Garrote G, Domı´nguez H, Parajo´ JC. Hydrothermal processing of lignocellulosic materials. Holz als Roh- und Werkstoff 1999;57:191e202. [10] Garrote G, Eugenio ME, Dı´az MJ, Ariza J, Lo´pez F. Hydrothermal and pulp processing of Eucalyptus. Bioresour Technol 2003;88:61e8. [11] Mosier N, Wyman C, Dale B, Elander R, Lee YY, Holtzapple M, et al. Features of promising technologies for pretreatment of lignocellulosic biomass. Bioresour Technol 2005;96:673e86. [12] Larsson S, Palmqvist E, Hahn-Ha¨gerdal B, Tengborg C, Stenberg K, Zacchi G, et al. The generation of fermentation inhibitors during dilute acid hydrolysis of softwood. Enzyme Microb Technol 1999;24:151e9. [13] Saha BC. Hemicellulose bioconversion. L J Ind Microbiol Biotechnol 2003;30:279e91. ´ J, Cardona CA. Trends in biotechnological [14] Sa´nchez O production of fuel ethanol from different feedstocks. Bioresour Technol 2008;99:5270e95. [15] Mittal A, Scott GM, Amidon TE, Kiemle DJ, Stipanovic AJ. Quantitative analysis of sugars in wood hydrolyzates with 1 H NMR during the autohydrolysis of hardwoods. Bioresour Technol 2009;100:6398e406. [16] Sjo¨stro¨m E. Wood chemistry: fundamentals and applications. 2nd ed. San Diego: Academic Press; 1993. [17] Martinez A, Rodriguez ME, York SW, Preston JF, Ingram LO. Effects of Ca(OH)2 treatments (“overliming”) on the composition and toxicity of bagasse hemicellulose hydrolysates. Biotechnol Bioeng 2000;69:526e636. [18] Larsson S, Reimann A, Vebrant N-ON, Jo¨nsson LJ. comparison of different methods for the detoxification of lignocellulose hydrolyzates of spruce. Appl Biochem Biotechnol 1999;77-79:91e103.