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Green liquor extraction of hemicelluloses from southern pine in an Integrated Forest Biorefinery
Journal of Industrial and Engineering Chemistry 16 (2010) 74–80
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Green liquor extraction of hemicelluloses from southern pine in an Integrated Forest Biorefinery Sung-Hoon Yoon a,*, Adriaan van Heiningen b a b
Auburn University, Department of Chemical Engineering, 212 Ross Hall, Auburn, AL 36849, USA University of Maine, Department of Chemical and Biological Engineering, 5737 Jenness Hall, Orono, ME 04469, USA
A R T I C L E I N F O
A B S T R A C T
Article history: Received 25 April 2009 Accepted 24 June 2009
Loblolly Pine chips were extracted with fresh green liquor solutions containing sufficient alkalinity to approximately neutralize the acids released upon treatment of the chip-liquor mass at elevated temperatures of 170, 180 and 190 8C for various times. The components of green liquor extracts were then quantified using high pressure anion exchange chromatography and UV spectrophotometer. The time–temperature effect on the extracted yields including the wood weight loss of wood chips after extraction could be well described by the H-factor approach. Sugar components were all found in their polymeric forms in the liquor. About 2.5–3% of wood mass was extracted as total sugars at an H-factor of about 4000. In general, mannose showed lower extraction than xylose. With increasing chemical charge, arabinoglucuronoxylan and galactoglucomannan extracts decreased, whereas dissolved lignin increased. Cellulose showed a minimum dependence on H-factor and chemical charge. The total sugar showed an inverse proportionality to the final liquor pH that rapidly dropped from alkali to the neutral in the very early stages of extraction and leveled off slowly reaching the acid region. Significant difference between total organic yield and the weight loss provides a possible evidence for the presence of significant amount of low molecular weight dissolved organics that were not accounted for. ß 2010 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.
Keywords: Hemicelluloses Loblolly Pine HPAEC Green liquor Extraction H-factor
1. Introduction Global developments such as limited oil supply, increased concern about greenhouse gas emissions and decreasing competitiveness of traditional pulp and paper producers has increased the opportunities and urgency for pulp mills in temperate climates to increase revenue. This may be done by transforming a chemical pulp mill into an Integrated Forest Biorefinery which produces higher value-added products such as ethanol, polymers, carbon fibers and diesel fuel besides paper pulp [1]. Considering the present process economics, these new products should be derived from mainly hemicelluloses and not from cellulose. The development of an Integrated Forest Bio-refinery (IFBR) which accomplishes this goal represents a great opportunity for the industry. In the kraft pulping process a significant percentage of the wood weight in the form of hemicelluloses and lignin dissolve in the waste pulping liquor to produce nearly undegraded cellulose fibers [2]. Since cellulose in fiber form provides unique properties to products such as paper and tissue, its use for these commodities is most cost effective. However, the pulp yield is only about 50% or
* Corresponding author. Tel.: +1 334 844 2407; fax: +1 334 844 2063. E-mail address: [email protected] (S.-H. Yoon).
less because most of the hemicelluloses and almost all the lignin end up in the black liquor. The dissolved materials including hemicelluloses and lignin in the black liquor are simultaneously combusted for steam and electricity generation, while dissolved inorganic cooking chemicals are recovered and recycled for pulping. Since lignin has a relatively high heating value of about 26.9 MJ/kg (compared to 13.6 MJ/kg for hemicelluloses), it is most cost effective to simply recoup the heating value of the dissolved lignin by combustion. However, the simultaneous combustion of the dissolved carbohydrates is not the optimal economic use, considering its low heating value and regular chemical composition. Therefore, in the IFBR concept a significant amount of the hemicelluloses is extracted from the residual wood chips prior to pulping. The relatively pure extract of hemicelluloses is then used for the production of polymers and chemicals as well as to improve the yield and quality of the bleached pulp. Numerous wood pretreatment studies have been conducted mostly in the form of prehydrolysis of wood since the 1931 pioneering work of Richter [3], who is generally credited with introducing prehydrolysis as a separate stage in the kraft cooking process. The effect of prehydrolysis on structural changes occurring in wood has been shown in a study [4]. Most of the effort among the more notable initial advances have been confined to establishing optimum conditions for prehydrolysis and kraft
1226-086X/$ – see front matter ß 2010 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved. doi:10.1016/j.jiec.2010.01.018
S.-H. Yoon, A. van Heiningen / Journal of Industrial and Engineering Chemistry 16 (2010) 74–80
cooking on a wide variety of raw materials which would lead to a desirable pulp product [5–11]. Although pretreatment or preextraction of wood plays an important role in some commercial pulping operations, detailed studies on the quantitative analysis of this process do not appear to have been made other than qualitative analysis of the prehydrolyzates for monosaccharides [12]. Recently, the pure water pre-extraction of hemicelluloses from Loblolly Pine followed by kraft pulping was quantified in our previous publications [13,14]. Extraction with pure water caused acidic conditions due to release of acetic acid from hemicelluloses. However, the acidity could possibly lead to serious degradation of cellulose by acid hydrolysis at high temperature resulting in a significant loss in pulp yield and paper strength [15]. Mild alkaline pre-extraction of hemicelluloses from hardwoods has been reported in a separate publication [16]. The current study was undertaken to investigate the effectiveness of the mild alkaline pre-extraction of Loblolly Pine and quantify the hemicellulose extraction yields. For the practical implementation of the nearneutral pre-extraction process, the experiment employed the green liquor solution as a possible source of alkali. The total sugar contents in the extract were determined using high pressure anion exchange chromatography (HPAEC).
75
and 90 min to study the time-dependent extraction of hemicelluloses. In the extraction procedure, a liquor-to-wood ratio of 4.5, 2, 4 and 6% green liquor (GL, 30% sulfidity) charge on the ovendry wood charge were used. All the chemical charges were expressed as Na2O. Each bomb was quenched in a cold water bath immediately after the completion of the extraction. Pre-extracted wood chips obtained from each bomb digestion were thoroughly washed on a 200-mesh screen with warm (tap) water and then oven-dried to measure the wood weight loss (WWL). Preextraction liquor samples were collected from each bomb digester to be analyzed for solids content (by freeze drying), pH, and lignin and sugar contents. The polymeric sugar content was determined using HPACE (high pressure anion exchange chromatography) after 2nd hydrolysis of the samples with 4% sulfuric acid for 1 h at 121 8C in an autoclave. The lignin content was determined by UV absorbance at 280 nm wave length [17]. The chemical contents in extracts were determined in mg/ml and then converted into percent of original oven dry wood weight by multiplying with 0.45, i.e. the liquor-to-wood ratio (4.5 L/kg) divided by 10. The overall experimental procedure for the pre-extraction of softwood chips was described in detail in our previous publication [13]. 3. Results and discussion
2. Experimental 3.1. Wood weight loss Pre-extraction experiments were conducted using eight rocking bomb digesters (cylindrical high pressure vessels mounted in a rocking device) in an oil bath. The volume of the bomb digester is 300 ml. The rocking device has an 1808 rocking cycle per 30 s. Thirty grams (oven-dried wood) of air-dried southern pine (Loblolly Pine) wood chips in each bomb digester were subjected to alkaline extraction in which the wood chips were extracted with solutions prepared. Cooking temperature was ramped from a room temperature of about 20 8C to a desired terminal temperature at a rate of 2.13 8C per min. Cooking times at the preset maximum temperature (Tmax) of 170, 180 or 190 8C were varied with 15, 45
As wood chips are subjected to the mild alkaline pre-extraction, their weights (or yields) can be expected to be decreased due to components dissolved and diffused from the wood to the extraction media. Since the wood weight loss (WWL) of wood chips measured after pre-extraction could represent the overall effectiveness of the pre-extraction process used, it can be considered as one of the most important process control parameters which would be closely related to the IFBR operations. The WL and freeze dry solid (FDS) data obtained from the green liquor pre-extraction of the Loblolly Pine chips are summarized in Table 1.
Table 1 Wood weight loss (WWL) and freeze dry solids (FDS) of extracts data after green liquor pre-extraction of Loblolly Pine. Green liquor charge (% as Na2O)
Tmax (8C)
Time at Tmax (min)
H-factor (h)
Wood weight loss, WWL (%)
Freeze dry solids, FDS (%)
1 2 3 4 5 6 7 8 9
2
170
15 45 90 15 45 90 15 45 90
184 794 1487 973 2153 3441 1777 4048 7250
4.24 6.13 7.15 6.52 9.25 11.43 8.24 12.65 14.58
9.00 9.63 10.22 10.01 11.93 12.51 10.53 13.59 12.33
10 11 12 13 14 15 16 17 18
4
15 45 90 15 45 90 15 45 90
184 794 1487 973 2153 3441 1777 4048 7250
6.05 7.98 8.59 8.22 10.43 12.29 10.55 12.47 15.19
14.13 15.48 15.71 15.14 16.18 17.19 17.01 17.96 17.82
19 20 21 22 23 24 25 26 27
6
15 45 90 15 45 90 15 45 90
184 794 1487 973 2153 3441 1777 4048 7250
7.45 10.27 10.14 10.16 12.31 13.10 – – 15.8
20.21 21.51 21.74 21.48 22.10 22.77 – – 24.08
Run
180
190
170
180
190
170
180
190
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76
Fig. 1. Wood weight loss versus extraction time for 2% green liquor pre-extraction of Loblolly Pine at three different maximum temperatures of 170, 180 and 190 8C.
As can be seen from Table 1, WWL largely depends on the extraction temperature and time. The WWL increases from almost 4.2% at the mildest extraction condition (15 min at 170 8C) to 15.8% at the most severe condition (90 min at 190 8C). The WWL in the 2% green liquor (GL) pre-extraction was plotted against total extraction time (includes heating-up period) as shown in Fig. 1. It can be seen that the WWL increases both with increasing extraction time and final temperature. Similar results can also be expected with 4% and 6% GL pre-extraction. In Fig. 2, the WWL data are plotted against H-factor. H-factor is a means of expressing reaction time and temperature as a single variable [18]: H-factor ¼
Z
t 0
16; 113 dt exp 43:2 T
(1)
where T indicates the cooking temperature in Kelvin scale and t is the cooking time elapsed. It can be seen in Fig. 2 that regardless of pre-extraction temperature, a single relationship between the
Fig. 2. Wood weight loss versus H-factor for green liquor pre-extraction of Loblolly Pine chips.
WWL and H-factor is obtained at a given green liquor charge. This indicates that the activation energy of the kinetics of wood dissolution during GL pre-extraction is possibly close to that of kraft cooking, i.e. 134 kJ/mol. The results described in Fig. 2 also show that higher WWL can be obtained with higher GL charge at a given pre-extraction H-factor. The WWL increases rapidly to 7, 8 and 10% at an H-factor of about 1000 h for 2, 4 and 6% GL charge, respectively, and then the increases level off at higher H-factors. Around the H-factor hours of 7200 in the GL pre-extraction of Loblolly Pine chips, the WWL reaches about 15% that is about 62% of that (25%) of water pre-extraction reported in our previous publication [14]. The solid contents of green liquor extracts determined by freeze drying (FDS) show almost twice higher values than the WWL as can be seen in Table 1. The excess values of FDS are clearly due to the fact that they include the weight of extraneous inorganic compounds dissolved in the green liquor extracts such as sodium carbonate and sodium sulfide. Therefore, the weights of dissolved inorganic materials that were already known should be subtracted from the values of FDS to determine the amounts of dissolved organic materials (DOM) comparable to WWL. As indicated in Fig. 3 for 6% green liquor pre-extraction, WWL and the DOM determined with subtracting the chemical charge from the FDS appear to be in a reasonable agreement supporting the internal consistency of the mass balance data. Similar results may have also been obtained in 2 and 4% GL pre-extractions (figures not shown). 3.2. Sugar extracts Wood hemicelluloses are relatively easily hydrolyzed by acid to their monomeric components mainly consisting of D-glucose, Dmannose, D-galactose, D-xylose and L-arabinose. These wood sugar contents in the green liquor extracts were determined using HPAEC (high pressure anion exchange chromatography). The HPAEC analysis was conducted both before and after hydrolysis of the extracts with 4% H2SO4 at 121 8C for 1 h in an autoclave. The yield of sugars in polymeric form in the extract can be calculated from the increase in monomeric sugar content caused by the acid hydrolysis. The concentrations of arabinose, galactose, glucose, xylose and mannose after acid hydrolysis expressed in percentage based on oven dry wood are shown in Table 2. As reported in our previous publication [14,15], approximately 4–5% monomeric sugars based on the oven dry weight of wood chips were found in the pure water extraction around the final pH
Fig. 3. Plots of WWL, FDS and DOM as a function of pre-extraction H-factor.
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Table 2 HPAEC sugar analysis of green liquor extract of Loblolly Pine chips. Green liquor charge (% as Na2O)
Tmax (8C)
Time at Tmax (min)
Arabinose
Galactose
Glucose
Xylose
Mannose
2
170
15 45 90 15 45 90 15 45 90
0.17 0.18 0.27 0.22 0.30 0.19 0.22 0.17 –
0.15 0.24 0.24 0.29 0.30 0.44 0.20 0.33 0.41
0.12 0.09 0.13 0.12 0.16 0.23 0.13 0.28 0.32
0.29 0.55 1.06 0.82 1.49 1.25 1.04 1.43 –
0.17 0.16 0.18 0.16 0.26 0.60 0.27 0.59 0.66
15 45 90 15 45 90 15 45 90
0.11 0.13 0.20 0.15 0.22 0.27 0.21 0.27 0.27
0.17 0.21 0.32 0.21 0.28 0.25 0.30 0.23 0.29
0.08 0.07 0.08 0.07 0.08 0.11 0.10 0.08 0.11
0.25 0.33 0.49 0.47 0.75 0.91 0.77 1.08 1.30
0.07 0.07 0.07 0.07 0.07 0.10 0.10 0.11 0.13
15 45 90 15 45 90 15 45 90
0.10 0.17 0.18 0.14 0.20 0.24 – – 0.29
0.18 0.23 0.32 0.22 0.24 0.22 – – 0.33
0.08 0.10 0.08 0.07 0.09 0.08 – – 0.09
0.21 0.28 0.41 0.39 0.58 0.70 – – 1.12
0.07 0.06 0.06 0.05 0.06 0.06 – – 0.08
180
190
4
170
180
190
6
170
180
190
of 4. In the GL extraction, however, no trace of monomeric sugars in the fresh extract (i.e. before acid hydrolysis) was detected. This indicates that all the sugars extracted with GL solution are in their polymeric forms. Sugar extracts appeared to be decreased as GL charge increases from 2% to 6%. When the sugars extracted with 2% GL solution are plotted against H-factor variable as shown in Fig. 4, higher sugar extraction yield was obtained in xylose than mannose. Xylose extraction reaches the maximum about 1.5% whereas others remain below about 0.5%. Compared to the maximum sugar yields of the pure water extracts (2.7% and 6.9% in xylose and mannose, respectively), much lower sugar yields were observed in GL extraction, particularly in mannose.
Monosaccharides (%, based on oven dry wood)
3.3. Hemicellulose components The mannose in extracts can be considered as a major building component of O-acetyl-galactoglucomanna (AcGGM) which is the principal hemicelluloses in softwoods at about 20% (w/w) [5]. The backbone of AcGGM is a linear chain built up by (1 ! 4)-linked bD-glucopyranose and b-D-mannopyranose units. The a-D-galactopyranose residue is linked as a single-unit side chain to the framework by (1 ! 6)-bonds. In the galactose-rich fraction of AcGGM, it is known that the ratio galactose:glucose:mannose is known to be 1:1:3 [19]. The C-2 and C-3 positions in backbone units are also partially substituted by acetyl groups (O-acetyl), on the average of one group per 3–4 hexose units, that indicates the average ratio of O-acetyl:mannose is 1:3. The xylose in extracts can also be considered as the major sugar component of arabino-4-Omethylglucuronoxylan (MeAGX) that is the second abundant hemicelluloses in softwoods at 5–10% (w/w). The MeAGX is composed of a frame work containing (1 ! 4)-linked b-Dxylopyranose units that are partially substituted by 4-O-methyla-D-glucuronic acid groups, on the average two residues per ten xylose units. In addition, the backbone contains a-L-arabinofuranose units, on the average 1.3 residues per ten xylose units. These generally known monomeric ratios in AcGGM and MeAGX probably make it possible to simply approximate the AcGGM and MeAGX from any given mannose and xylose contents determined in the GL extracts, respectively, i.e., AcGGM ð%Þ ¼ Manð%Þ
1½M AcO þ 1½M Gal þ 1½M Glu þ 3½M Man 3½M Man
(2)
and
Fig. 4. Sugar extracted versus H-factor for 2% green liquor pre-extraction of Loblolly Pine chips.
MeAGX ð%Þ ¼ Xylð%Þ
2½MMeGUA þ 1:3½M Ara þ 10 M Xyl 10½M Xyl
(3)
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isons of AcGGM and MeAGX, both extraction yields are relatively low in the green liquor extraction, reaching the maximum of only about 2% in MeAGM and about 1% of AcGGM which is the most abundant hemicelluloses in softwoods. 3.4. Total organics The major organic components in the extracts are cellulose, hemicelluloses and lignin. The lignin content was determined by UV absorbance at 280 nm wave length. The total amount of cellulose, C, and hemicelluloses, H, in the extracts were calculated from the content of monosugars (glucose (Glu), mannose (Man), arabinose (Ara), xylose (Xyl) and galactose (Gal)) determined by HPAEC analysis, using the following equations: 162 Man 162 C ¼ Glu (4) 180 b 180
Fig. 5. Plots of AcGGM and MeAGX in green liquor extracts versus H-factor (the solid line indicates the MeAGX and the dotted line AcGGM contents).
where Man(%) and Xyl(%) are concentrations of mannose and xylose monosugars in extracts, respectively, and MAcO, MGal, MGlu, MAra, MMan and MXyl and MMeGUA are molecular weights of O-acetyl group, galactose, glucose, arabinose, manose and xylose units and 4-O-methyl-a-D-glucuronic acid group, respectively. The approximated AcGGM and MeAGX are plotted against H-factor as shown in Fig. 5. AcGGM and MeAGX contents increased with increasing Hfactor. In addition, as green liquor charge increases from 2% to 6% in the extraction of Loblolly Pine chips, both AcGGM and MeAGX extractions are significantly decreased. This indicates that both AcGGM and MeAGX extractions are largely dependent upon the acid-catalyzed hydrolysis rather than alkaline cleavage of glycosidic bonds or endwise degradation mechanism. In the compar-
with b = 4.15 (average value for number of mannose units per glucose unit in hemicellulose of Pine/Spruce wood, based on Janson [20]). 132 162 þ ðGal þ Glu þ ManÞ H ¼ ðAra þ XylÞ 150 180 C ffi AcGGM þ MeAGX
(5)
The hemicellulose contents can also be calculated by the summation of AcGGM and MeAGX expressed in Eqs. (2) and (3). The major organic compounds in GL pre-extracts including hemicelluloses, celluloses, total sugar, lignin and total organics and final liquor pH are summarized in Table 3. There appear to be a very good agreement between the hemicellulose contents calculated from Eqs. (2) and (3) and those from Eqs. (4) and (5). This indicates that the approximation made to determine both AcGGM and MeAGX is valid. The wood compounds including hemicelluloses, cellulose and lignin in green liquor extracts are plotted against H-factor in Fig. 6. It can be seen
Table 3 Components in green liquor extract of Loblolly Pine chips in % on OD wood. Green liquor charge (% as Na2O)
Tmax (8C)
2
170
180
190
4
170
180
190
6
170
180
190
Time at Tmax (min)
Hemicelluloses (%)
Cellulose, C (%)
Total sugar (H + C) (%)
Lignin, L (%)
Total organics (H + C + L) (%)
Final liquor pH
0.70 1.04 1.78 1.41 2.51 2.79 1.91 3.02 1.18
0.07 0.05 0.08 0.07 0.09 0.08 0.06 0.13 0.14
0.81 1.08 1.66 1.43 2.22 2.40 1.64 2.49 1.25
1.12 1.28 2.15 1.85 1.78 1.67 1.50 2.14 2.42
1.93 2.36 3.81 3.28 4.00 4.27 3.14 4.63 3.67
7.50 6.38 5.93 6.64 5.53 4.66 6.12 4.65 4.35
0.54 0.67 0.97 0.81 1.19 1.37 1.23 1.51 1.78
0.47 0.58 0.80 0.77 1.16 1.43 1.24 1.68 2.02
0.05 0.05 0.06 0.05 0.06 0.08 0.07 0.05 0.07
0.59 0.72 1.03 0.86 1.25 1.45 1.30 1.56 1.85
1.72 2.01 2.26 2.46 2.77 2.48 2.78 2.87 2.36
2.31 2.73 3.29 3.32 4.02 3.93 4.08 4.43 4.21
7.95 7.11 7.65 7.45 6.81 6.46 7.35 6.61 5.82
0.51 0.67 0.88 0.72 0.98 1.09 – – 1.63
0.41 0.49 0.67 0.63 0.91 1.07 – – 1.68
0.05 0.07 0.06 0.05 0.07 0.06 – – 0.07
0.56 0.74 0.94 0.77 1.05 1.15 – – 1.70
2.38 2.88 2.95 3.57 3.78 3.97 – – 3.01
2.94 3.62 3.89 4.34 4.83 5.12 – – 4.71
9.07 7.87 8.07 7.73 7.33 7.07 – – 6.91
H
AcGGM + MeAGX
15 45 90 15 45 90 15 45 90
0.74 1.03 1.58 1.36 2.13 2.32 1.58 2.36 1.11
15 45 90 15 45 90 15 45 90 15 45 90 15 45 90 15 45 90
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Fig. 6. Extracted wood components versus H-factor. Fig. 7. Final pH versus pre-extraction H-factor.
that the major compound data obtained at different temperatures and times are well described by a single curve. The extracted amount of lignin, hemicelluloses and cellulose increased approximately linearly with H-factor in the GL pre-extraction process. Lignin shows higher yield compared to polysaccharides. Cellulose showed the highest resistance against alkaline dissolution in the GL pre-extraction, maintaining the minimum levels of extracts between 0.05% and 0.15% over the entire H-factor range and green liquor charges. Whereas hemicelluloses significantly decreased with increasing green liquor treatments, lignin extracts significantly increased. Since dissolved lignin will negatively affect the ethanol fermentation or bioconversion of bio-refinery processes, the alkaline pre-extraction conducted at lower charge of green liquor is recommendable. 3.5. pH of GL extracts Extraction of Loblolly Pine chips with neutral water causes acidic conditions due to release of acetic acid from hemicelluloses [13,14]. The acidity may accelerate the degradation of cellulose through the acidic auto-hydrolysis at high temperatures leading to a serious loss of pulp yields and strength in paper products. At alkaline conditions glucomannan is rapidly degraded by the peeling reaction, while xylan is dissolved in oligomeric form [21]. This hot alkaline condition, therefore, is not so much effective in hemicellulose extraction for softwoods as hardwoods. At mild alkaline conditions (initial pH 13) such as GL pre-extraction, however, it can be expected that the acidic auto-hydrolysis of cellulose can be minimized due to the early neutralization of the acetic acid released from the wood and the extraction process pH can be maintained neutral or slight acidic conditions to facilitate the extraction. The final liquor pH was plotted against extraction H-factor as shown in Fig. 7. The liquor pH rapidly dropped from the alkaline to the neutral in the very early stages of extraction up to an H-factor of about 500. The rapid pH drop in the initial preextraction is supposed to be caused by acetic acid formed by hydrolysis of acetyl groups in the hemicelluloses. In case of 2% green liquor extraction, the liquor pH reached the lowest pH level up to around 4.5 at an H-factor of around 4000, whereas in 4% and 6% green liquor extractions, their liquor pH could never reached below 6 even at the significantly prolonged extractions. The total sugar yields were plotted against final liquor pH of the GL extracts in Fig. 8. It can be seen that the total sugar yield is
Fig. 8. Total sugar yield versus final liquor pH.
highly pH-dependent. The extraction of total sugar increases linearly as the pH drops from the mild alkaline to acidic level around 4.5 showing an inverse proportionality relationship. About 2.5% of maximum sugar yield could be obtained at an acidic region near pH 4 in the 2% green liquor pre-extraction. The results obtained from the study indicates that a lowered GL charge is preferable for higher sugar extraction yield since it would provide more acidic conditions during the GL pre-extraction. 3.6. Extraction material balance A mass balance analysis on the total organics was performed to test the internal consistency of the data. The total organics were calculated from the sum of the major wood components in the green liquor extracts including cellulose, hemicelluloses and lignin as presented in Table 3. The total organics were plotted against WWL at 170, 180 and 190 8C as shown in Fig. 9. It can be seen that the total organics obtained with GL initially increased with increasing WWL and leveled off after passing about 1% WWL. The dotted arrow indicates the deviation of total organics data from the
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ylan and galactoglucomannan were about 2 and 1%, respectively. The hemicellulose yield and the dissolved lignin appeared to be at the similar level with 2% green liquor charge. With increasing green liquor charge, however, dissolved lignin significantly increased and hemicellulose yields significantly decreased. Cellulose showed a minimum dependence on green liquor charge and degree of extraction expressed as H-factor. The total sugar extracts were inversely proportional to the final liquor pH. Final liquor pH showed a rapid drop from the alkaline to the neutral in the very early stages of extraction and leveled off reaching to the acidic around pH 4.5 in case 2% green liquor extraction and to the near neutral in the 4% and 6% GL extractions. There was a systematic deviation between total organics and total weight loss possibly due to the presence of significant amount of low molecular weight dissolved organics that were not accounted for. Acknowledgements
Fig. 9. Mass balance analysis on green liquor extraction Loblolly Pine chips.
ideal material balance line. Significantly lower total organic yields were observed compared to WWL of wood chips. Around 11% of WWL, only a 4% total organics could be obtained from the GL extracts. This low total organic yield compared to WWL should be partly caused by the AcGGM’s high stability in the green liquor extraction environment and partly by some dissolved organic components. The dissolved organic components should comprise such low molecular organic substances as acetic acid, methanol, 4O-methyl glucoronic acid, as well as sugar degradation products such as furfural and others are not accounted for. 4. Conclusions Green liquor extraction effectiveness of Loblolly Pine chips has been investigated by studying total weight loss, sugar yields, total organic compounds and final liquor pH. The sugar and lignin components of the extracts were quantified using high pressure anion exchange chromatography and UV spectroscopy, respectively. The effect of time and temperature during green liquor extraction on total weight loss and wood component removal could be well described by a single control parameter, the H-factor. Total weight loss of wood chips largely depends on the extraction temperature and time. It increases from almost 4.2% at the mildest extraction condition to 15.8% at the most severe condition. Sugar components were all found in their polymeric forms in the green liquor extraction. Higher sugar extraction yield was obtained in xylose than mannose. The maximum yields of arabinoglucurox-
This research was conducted in the University of Maine, Department of Chemical and Biological Engineering, 5737 Jenness Hall, Orono, ME 04468, USA, supported by the United State Department of Energy (Grant No. DE-FC36-04GO14306) and International Paper. The authors would like to acknowledge Dr. Kimberley MacEwan, InterChem Inc. 1502 Fort Worth St., Liberty, TX77, USA, for helping with the chemical analysis. References [1] A.R.P. van Heiningen, Pulp Paper Canada 107 (6) (2006) 38. [2] J. Gullichsen, C.-J. Fogelholm, in: J. Gullichsen, H. Paulapuro (Eds.), Chemical Pulping, Book 6A of Papermaking Science and Technology, Fapet Oy, Helsinki, 1999, p. A28. [3] G.A. Richter, U.S. pats. 1,787,953 (1931); 1,816,343 (1931); 1,819,002 (1931); 1,880,043 (1932); 2,036,606 (1936). [4] V. Jacopian, S. Casperson, Faserforsch. Textiltech. 17 (1966) 267. [5] S.A. Rydholm, Pulping Processes, Interscience, New York, 1965, pp. 576, 666. [6] (a) G. Annergren, S. Rydholm, J. Richter, A. Backlund, Paper Trade J. 149 (10) (1965) 42; (b) B.U. Cho, J.Y. Ryu, B.K. Song, J. Ind. Eng. Chem. 15 (2009) 119; (c) B.U. Cho, J.Y. Ryu, B.K. Song, J. Ind. Eng. Chem. 15 (2009) 196. [7] F.A. Simmonds, Tappi J. 46 (1) (1963) 145A. [8] D.J. Brasch, K.W. Free, Tappi J. 47 (4) (1964) 186. [9] S. Takahashi, K. Nishiyama, Y. Fuhuda, Nippon Nogeikagaku Kaishi 34 (1960) 766. [10] J.M. Lawther, R. Sun, W.B. Banks, J. Appl. Polym. Sci. 60 (1996) 1827. [11] L. Rigal, S. N’Diaye, Bioresource Technol. 75 (2000) 13. [12] J.H. Lora, M. Wayman, Tappi J 64 (6) (1978) 47. [13] S.-H. Yoon, K. MacEwan, A. van Heiningen, TAPPI Engineering, Pulping and Environmental Conference Proceedings, Marriott Marquis, Atlanta, GA, November 2006. [14] S.-H. Yoon, K. MacEwan, A. van Heiningen, Tappi J. (June) (2008) 27. [15] S.-H. Yoon, A. van Heiningen, Tappi J. (July) (2008) 27. [16] S.-H. Yoon, A. van Heiningen, G. Krishnagopalan, TAPPI Engineering, Pulping and Environmental Conference Proceedings, Portland, Oregon, August, 2008. [17] E. Sjostrom, R. Alen, Analytical Methods in Wood Chemistry, Pulping, and Papermaking, Springer-Verlag, Berlin, 1999, p. 96. [18] K.E. Vroom, Pulp Paper Mag. Canada 58 (3) (1957) 228. [19] J.P. Casey, 3rd ed., Pulp and Paper Chemistry and Chemical Technology, vol. 1, Interscience Publishers, New York, 1979, p. 33. [20] J. Janson, Faserforsch. Textiltech. 25 (9) (1974) 375. [21] E. Sjostrom, Wood Chemistry—Fundamentals and Applications, Academic Press, New York, 1981, p. 60.
Contents lists available at ScienceDirect
Journal of Industrial and Engineering Chemistry journal homepage: www.elsevier.com/locate/jiec
Green liquor extraction of hemicelluloses from southern pine in an Integrated Forest Biorefinery Sung-Hoon Yoon a,*, Adriaan van Heiningen b a b
Auburn University, Department of Chemical Engineering, 212 Ross Hall, Auburn, AL 36849, USA University of Maine, Department of Chemical and Biological Engineering, 5737 Jenness Hall, Orono, ME 04469, USA
A R T I C L E I N F O
A B S T R A C T
Article history: Received 25 April 2009 Accepted 24 June 2009
Loblolly Pine chips were extracted with fresh green liquor solutions containing sufficient alkalinity to approximately neutralize the acids released upon treatment of the chip-liquor mass at elevated temperatures of 170, 180 and 190 8C for various times. The components of green liquor extracts were then quantified using high pressure anion exchange chromatography and UV spectrophotometer. The time–temperature effect on the extracted yields including the wood weight loss of wood chips after extraction could be well described by the H-factor approach. Sugar components were all found in their polymeric forms in the liquor. About 2.5–3% of wood mass was extracted as total sugars at an H-factor of about 4000. In general, mannose showed lower extraction than xylose. With increasing chemical charge, arabinoglucuronoxylan and galactoglucomannan extracts decreased, whereas dissolved lignin increased. Cellulose showed a minimum dependence on H-factor and chemical charge. The total sugar showed an inverse proportionality to the final liquor pH that rapidly dropped from alkali to the neutral in the very early stages of extraction and leveled off slowly reaching the acid region. Significant difference between total organic yield and the weight loss provides a possible evidence for the presence of significant amount of low molecular weight dissolved organics that were not accounted for. ß 2010 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.
Keywords: Hemicelluloses Loblolly Pine HPAEC Green liquor Extraction H-factor
1. Introduction Global developments such as limited oil supply, increased concern about greenhouse gas emissions and decreasing competitiveness of traditional pulp and paper producers has increased the opportunities and urgency for pulp mills in temperate climates to increase revenue. This may be done by transforming a chemical pulp mill into an Integrated Forest Biorefinery which produces higher value-added products such as ethanol, polymers, carbon fibers and diesel fuel besides paper pulp [1]. Considering the present process economics, these new products should be derived from mainly hemicelluloses and not from cellulose. The development of an Integrated Forest Bio-refinery (IFBR) which accomplishes this goal represents a great opportunity for the industry. In the kraft pulping process a significant percentage of the wood weight in the form of hemicelluloses and lignin dissolve in the waste pulping liquor to produce nearly undegraded cellulose fibers [2]. Since cellulose in fiber form provides unique properties to products such as paper and tissue, its use for these commodities is most cost effective. However, the pulp yield is only about 50% or
* Corresponding author. Tel.: +1 334 844 2407; fax: +1 334 844 2063. E-mail address: [email protected] (S.-H. Yoon).
less because most of the hemicelluloses and almost all the lignin end up in the black liquor. The dissolved materials including hemicelluloses and lignin in the black liquor are simultaneously combusted for steam and electricity generation, while dissolved inorganic cooking chemicals are recovered and recycled for pulping. Since lignin has a relatively high heating value of about 26.9 MJ/kg (compared to 13.6 MJ/kg for hemicelluloses), it is most cost effective to simply recoup the heating value of the dissolved lignin by combustion. However, the simultaneous combustion of the dissolved carbohydrates is not the optimal economic use, considering its low heating value and regular chemical composition. Therefore, in the IFBR concept a significant amount of the hemicelluloses is extracted from the residual wood chips prior to pulping. The relatively pure extract of hemicelluloses is then used for the production of polymers and chemicals as well as to improve the yield and quality of the bleached pulp. Numerous wood pretreatment studies have been conducted mostly in the form of prehydrolysis of wood since the 1931 pioneering work of Richter [3], who is generally credited with introducing prehydrolysis as a separate stage in the kraft cooking process. The effect of prehydrolysis on structural changes occurring in wood has been shown in a study [4]. Most of the effort among the more notable initial advances have been confined to establishing optimum conditions for prehydrolysis and kraft
1226-086X/$ – see front matter ß 2010 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved. doi:10.1016/j.jiec.2010.01.018
S.-H. Yoon, A. van Heiningen / Journal of Industrial and Engineering Chemistry 16 (2010) 74–80
cooking on a wide variety of raw materials which would lead to a desirable pulp product [5–11]. Although pretreatment or preextraction of wood plays an important role in some commercial pulping operations, detailed studies on the quantitative analysis of this process do not appear to have been made other than qualitative analysis of the prehydrolyzates for monosaccharides [12]. Recently, the pure water pre-extraction of hemicelluloses from Loblolly Pine followed by kraft pulping was quantified in our previous publications [13,14]. Extraction with pure water caused acidic conditions due to release of acetic acid from hemicelluloses. However, the acidity could possibly lead to serious degradation of cellulose by acid hydrolysis at high temperature resulting in a significant loss in pulp yield and paper strength [15]. Mild alkaline pre-extraction of hemicelluloses from hardwoods has been reported in a separate publication [16]. The current study was undertaken to investigate the effectiveness of the mild alkaline pre-extraction of Loblolly Pine and quantify the hemicellulose extraction yields. For the practical implementation of the nearneutral pre-extraction process, the experiment employed the green liquor solution as a possible source of alkali. The total sugar contents in the extract were determined using high pressure anion exchange chromatography (HPAEC).
75
and 90 min to study the time-dependent extraction of hemicelluloses. In the extraction procedure, a liquor-to-wood ratio of 4.5, 2, 4 and 6% green liquor (GL, 30% sulfidity) charge on the ovendry wood charge were used. All the chemical charges were expressed as Na2O. Each bomb was quenched in a cold water bath immediately after the completion of the extraction. Pre-extracted wood chips obtained from each bomb digestion were thoroughly washed on a 200-mesh screen with warm (tap) water and then oven-dried to measure the wood weight loss (WWL). Preextraction liquor samples were collected from each bomb digester to be analyzed for solids content (by freeze drying), pH, and lignin and sugar contents. The polymeric sugar content was determined using HPACE (high pressure anion exchange chromatography) after 2nd hydrolysis of the samples with 4% sulfuric acid for 1 h at 121 8C in an autoclave. The lignin content was determined by UV absorbance at 280 nm wave length [17]. The chemical contents in extracts were determined in mg/ml and then converted into percent of original oven dry wood weight by multiplying with 0.45, i.e. the liquor-to-wood ratio (4.5 L/kg) divided by 10. The overall experimental procedure for the pre-extraction of softwood chips was described in detail in our previous publication [13]. 3. Results and discussion
2. Experimental 3.1. Wood weight loss Pre-extraction experiments were conducted using eight rocking bomb digesters (cylindrical high pressure vessels mounted in a rocking device) in an oil bath. The volume of the bomb digester is 300 ml. The rocking device has an 1808 rocking cycle per 30 s. Thirty grams (oven-dried wood) of air-dried southern pine (Loblolly Pine) wood chips in each bomb digester were subjected to alkaline extraction in which the wood chips were extracted with solutions prepared. Cooking temperature was ramped from a room temperature of about 20 8C to a desired terminal temperature at a rate of 2.13 8C per min. Cooking times at the preset maximum temperature (Tmax) of 170, 180 or 190 8C were varied with 15, 45
As wood chips are subjected to the mild alkaline pre-extraction, their weights (or yields) can be expected to be decreased due to components dissolved and diffused from the wood to the extraction media. Since the wood weight loss (WWL) of wood chips measured after pre-extraction could represent the overall effectiveness of the pre-extraction process used, it can be considered as one of the most important process control parameters which would be closely related to the IFBR operations. The WL and freeze dry solid (FDS) data obtained from the green liquor pre-extraction of the Loblolly Pine chips are summarized in Table 1.
Table 1 Wood weight loss (WWL) and freeze dry solids (FDS) of extracts data after green liquor pre-extraction of Loblolly Pine. Green liquor charge (% as Na2O)
Tmax (8C)
Time at Tmax (min)
H-factor (h)
Wood weight loss, WWL (%)
Freeze dry solids, FDS (%)
1 2 3 4 5 6 7 8 9
2
170
15 45 90 15 45 90 15 45 90
184 794 1487 973 2153 3441 1777 4048 7250
4.24 6.13 7.15 6.52 9.25 11.43 8.24 12.65 14.58
9.00 9.63 10.22 10.01 11.93 12.51 10.53 13.59 12.33
10 11 12 13 14 15 16 17 18
4
15 45 90 15 45 90 15 45 90
184 794 1487 973 2153 3441 1777 4048 7250
6.05 7.98 8.59 8.22 10.43 12.29 10.55 12.47 15.19
14.13 15.48 15.71 15.14 16.18 17.19 17.01 17.96 17.82
19 20 21 22 23 24 25 26 27
6
15 45 90 15 45 90 15 45 90
184 794 1487 973 2153 3441 1777 4048 7250
7.45 10.27 10.14 10.16 12.31 13.10 – – 15.8
20.21 21.51 21.74 21.48 22.10 22.77 – – 24.08
Run
180
190
170
180
190
170
180
190
S.-H. Yoon, A. van Heiningen / Journal of Industrial and Engineering Chemistry 16 (2010) 74–80
76
Fig. 1. Wood weight loss versus extraction time for 2% green liquor pre-extraction of Loblolly Pine at three different maximum temperatures of 170, 180 and 190 8C.
As can be seen from Table 1, WWL largely depends on the extraction temperature and time. The WWL increases from almost 4.2% at the mildest extraction condition (15 min at 170 8C) to 15.8% at the most severe condition (90 min at 190 8C). The WWL in the 2% green liquor (GL) pre-extraction was plotted against total extraction time (includes heating-up period) as shown in Fig. 1. It can be seen that the WWL increases both with increasing extraction time and final temperature. Similar results can also be expected with 4% and 6% GL pre-extraction. In Fig. 2, the WWL data are plotted against H-factor. H-factor is a means of expressing reaction time and temperature as a single variable [18]: H-factor ¼
Z
t 0
16; 113 dt exp 43:2 T
(1)
where T indicates the cooking temperature in Kelvin scale and t is the cooking time elapsed. It can be seen in Fig. 2 that regardless of pre-extraction temperature, a single relationship between the
Fig. 2. Wood weight loss versus H-factor for green liquor pre-extraction of Loblolly Pine chips.
WWL and H-factor is obtained at a given green liquor charge. This indicates that the activation energy of the kinetics of wood dissolution during GL pre-extraction is possibly close to that of kraft cooking, i.e. 134 kJ/mol. The results described in Fig. 2 also show that higher WWL can be obtained with higher GL charge at a given pre-extraction H-factor. The WWL increases rapidly to 7, 8 and 10% at an H-factor of about 1000 h for 2, 4 and 6% GL charge, respectively, and then the increases level off at higher H-factors. Around the H-factor hours of 7200 in the GL pre-extraction of Loblolly Pine chips, the WWL reaches about 15% that is about 62% of that (25%) of water pre-extraction reported in our previous publication [14]. The solid contents of green liquor extracts determined by freeze drying (FDS) show almost twice higher values than the WWL as can be seen in Table 1. The excess values of FDS are clearly due to the fact that they include the weight of extraneous inorganic compounds dissolved in the green liquor extracts such as sodium carbonate and sodium sulfide. Therefore, the weights of dissolved inorganic materials that were already known should be subtracted from the values of FDS to determine the amounts of dissolved organic materials (DOM) comparable to WWL. As indicated in Fig. 3 for 6% green liquor pre-extraction, WWL and the DOM determined with subtracting the chemical charge from the FDS appear to be in a reasonable agreement supporting the internal consistency of the mass balance data. Similar results may have also been obtained in 2 and 4% GL pre-extractions (figures not shown). 3.2. Sugar extracts Wood hemicelluloses are relatively easily hydrolyzed by acid to their monomeric components mainly consisting of D-glucose, Dmannose, D-galactose, D-xylose and L-arabinose. These wood sugar contents in the green liquor extracts were determined using HPAEC (high pressure anion exchange chromatography). The HPAEC analysis was conducted both before and after hydrolysis of the extracts with 4% H2SO4 at 121 8C for 1 h in an autoclave. The yield of sugars in polymeric form in the extract can be calculated from the increase in monomeric sugar content caused by the acid hydrolysis. The concentrations of arabinose, galactose, glucose, xylose and mannose after acid hydrolysis expressed in percentage based on oven dry wood are shown in Table 2. As reported in our previous publication [14,15], approximately 4–5% monomeric sugars based on the oven dry weight of wood chips were found in the pure water extraction around the final pH
Fig. 3. Plots of WWL, FDS and DOM as a function of pre-extraction H-factor.
S.-H. Yoon, A. van Heiningen / Journal of Industrial and Engineering Chemistry 16 (2010) 74–80
77
Table 2 HPAEC sugar analysis of green liquor extract of Loblolly Pine chips. Green liquor charge (% as Na2O)
Tmax (8C)
Time at Tmax (min)
Arabinose
Galactose
Glucose
Xylose
Mannose
2
170
15 45 90 15 45 90 15 45 90
0.17 0.18 0.27 0.22 0.30 0.19 0.22 0.17 –
0.15 0.24 0.24 0.29 0.30 0.44 0.20 0.33 0.41
0.12 0.09 0.13 0.12 0.16 0.23 0.13 0.28 0.32
0.29 0.55 1.06 0.82 1.49 1.25 1.04 1.43 –
0.17 0.16 0.18 0.16 0.26 0.60 0.27 0.59 0.66
15 45 90 15 45 90 15 45 90
0.11 0.13 0.20 0.15 0.22 0.27 0.21 0.27 0.27
0.17 0.21 0.32 0.21 0.28 0.25 0.30 0.23 0.29
0.08 0.07 0.08 0.07 0.08 0.11 0.10 0.08 0.11
0.25 0.33 0.49 0.47 0.75 0.91 0.77 1.08 1.30
0.07 0.07 0.07 0.07 0.07 0.10 0.10 0.11 0.13
15 45 90 15 45 90 15 45 90
0.10 0.17 0.18 0.14 0.20 0.24 – – 0.29
0.18 0.23 0.32 0.22 0.24 0.22 – – 0.33
0.08 0.10 0.08 0.07 0.09 0.08 – – 0.09
0.21 0.28 0.41 0.39 0.58 0.70 – – 1.12
0.07 0.06 0.06 0.05 0.06 0.06 – – 0.08
180
190
4
170
180
190
6
170
180
190
of 4. In the GL extraction, however, no trace of monomeric sugars in the fresh extract (i.e. before acid hydrolysis) was detected. This indicates that all the sugars extracted with GL solution are in their polymeric forms. Sugar extracts appeared to be decreased as GL charge increases from 2% to 6%. When the sugars extracted with 2% GL solution are plotted against H-factor variable as shown in Fig. 4, higher sugar extraction yield was obtained in xylose than mannose. Xylose extraction reaches the maximum about 1.5% whereas others remain below about 0.5%. Compared to the maximum sugar yields of the pure water extracts (2.7% and 6.9% in xylose and mannose, respectively), much lower sugar yields were observed in GL extraction, particularly in mannose.
Monosaccharides (%, based on oven dry wood)
3.3. Hemicellulose components The mannose in extracts can be considered as a major building component of O-acetyl-galactoglucomanna (AcGGM) which is the principal hemicelluloses in softwoods at about 20% (w/w) [5]. The backbone of AcGGM is a linear chain built up by (1 ! 4)-linked bD-glucopyranose and b-D-mannopyranose units. The a-D-galactopyranose residue is linked as a single-unit side chain to the framework by (1 ! 6)-bonds. In the galactose-rich fraction of AcGGM, it is known that the ratio galactose:glucose:mannose is known to be 1:1:3 [19]. The C-2 and C-3 positions in backbone units are also partially substituted by acetyl groups (O-acetyl), on the average of one group per 3–4 hexose units, that indicates the average ratio of O-acetyl:mannose is 1:3. The xylose in extracts can also be considered as the major sugar component of arabino-4-Omethylglucuronoxylan (MeAGX) that is the second abundant hemicelluloses in softwoods at 5–10% (w/w). The MeAGX is composed of a frame work containing (1 ! 4)-linked b-Dxylopyranose units that are partially substituted by 4-O-methyla-D-glucuronic acid groups, on the average two residues per ten xylose units. In addition, the backbone contains a-L-arabinofuranose units, on the average 1.3 residues per ten xylose units. These generally known monomeric ratios in AcGGM and MeAGX probably make it possible to simply approximate the AcGGM and MeAGX from any given mannose and xylose contents determined in the GL extracts, respectively, i.e., AcGGM ð%Þ ¼ Manð%Þ
1½M AcO þ 1½M Gal þ 1½M Glu þ 3½M Man 3½M Man
(2)
and
Fig. 4. Sugar extracted versus H-factor for 2% green liquor pre-extraction of Loblolly Pine chips.
MeAGX ð%Þ ¼ Xylð%Þ
2½MMeGUA þ 1:3½M Ara þ 10 M Xyl 10½M Xyl
(3)
S.-H. Yoon, A. van Heiningen / Journal of Industrial and Engineering Chemistry 16 (2010) 74–80
78
isons of AcGGM and MeAGX, both extraction yields are relatively low in the green liquor extraction, reaching the maximum of only about 2% in MeAGM and about 1% of AcGGM which is the most abundant hemicelluloses in softwoods. 3.4. Total organics The major organic components in the extracts are cellulose, hemicelluloses and lignin. The lignin content was determined by UV absorbance at 280 nm wave length. The total amount of cellulose, C, and hemicelluloses, H, in the extracts were calculated from the content of monosugars (glucose (Glu), mannose (Man), arabinose (Ara), xylose (Xyl) and galactose (Gal)) determined by HPAEC analysis, using the following equations: 162 Man 162 C ¼ Glu (4) 180 b 180
Fig. 5. Plots of AcGGM and MeAGX in green liquor extracts versus H-factor (the solid line indicates the MeAGX and the dotted line AcGGM contents).
where Man(%) and Xyl(%) are concentrations of mannose and xylose monosugars in extracts, respectively, and MAcO, MGal, MGlu, MAra, MMan and MXyl and MMeGUA are molecular weights of O-acetyl group, galactose, glucose, arabinose, manose and xylose units and 4-O-methyl-a-D-glucuronic acid group, respectively. The approximated AcGGM and MeAGX are plotted against H-factor as shown in Fig. 5. AcGGM and MeAGX contents increased with increasing Hfactor. In addition, as green liquor charge increases from 2% to 6% in the extraction of Loblolly Pine chips, both AcGGM and MeAGX extractions are significantly decreased. This indicates that both AcGGM and MeAGX extractions are largely dependent upon the acid-catalyzed hydrolysis rather than alkaline cleavage of glycosidic bonds or endwise degradation mechanism. In the compar-
with b = 4.15 (average value for number of mannose units per glucose unit in hemicellulose of Pine/Spruce wood, based on Janson [20]). 132 162 þ ðGal þ Glu þ ManÞ H ¼ ðAra þ XylÞ 150 180 C ffi AcGGM þ MeAGX
(5)
The hemicellulose contents can also be calculated by the summation of AcGGM and MeAGX expressed in Eqs. (2) and (3). The major organic compounds in GL pre-extracts including hemicelluloses, celluloses, total sugar, lignin and total organics and final liquor pH are summarized in Table 3. There appear to be a very good agreement between the hemicellulose contents calculated from Eqs. (2) and (3) and those from Eqs. (4) and (5). This indicates that the approximation made to determine both AcGGM and MeAGX is valid. The wood compounds including hemicelluloses, cellulose and lignin in green liquor extracts are plotted against H-factor in Fig. 6. It can be seen
Table 3 Components in green liquor extract of Loblolly Pine chips in % on OD wood. Green liquor charge (% as Na2O)
Tmax (8C)
2
170
180
190
4
170
180
190
6
170
180
190
Time at Tmax (min)
Hemicelluloses (%)
Cellulose, C (%)
Total sugar (H + C) (%)
Lignin, L (%)
Total organics (H + C + L) (%)
Final liquor pH
0.70 1.04 1.78 1.41 2.51 2.79 1.91 3.02 1.18
0.07 0.05 0.08 0.07 0.09 0.08 0.06 0.13 0.14
0.81 1.08 1.66 1.43 2.22 2.40 1.64 2.49 1.25
1.12 1.28 2.15 1.85 1.78 1.67 1.50 2.14 2.42
1.93 2.36 3.81 3.28 4.00 4.27 3.14 4.63 3.67
7.50 6.38 5.93 6.64 5.53 4.66 6.12 4.65 4.35
0.54 0.67 0.97 0.81 1.19 1.37 1.23 1.51 1.78
0.47 0.58 0.80 0.77 1.16 1.43 1.24 1.68 2.02
0.05 0.05 0.06 0.05 0.06 0.08 0.07 0.05 0.07
0.59 0.72 1.03 0.86 1.25 1.45 1.30 1.56 1.85
1.72 2.01 2.26 2.46 2.77 2.48 2.78 2.87 2.36
2.31 2.73 3.29 3.32 4.02 3.93 4.08 4.43 4.21
7.95 7.11 7.65 7.45 6.81 6.46 7.35 6.61 5.82
0.51 0.67 0.88 0.72 0.98 1.09 – – 1.63
0.41 0.49 0.67 0.63 0.91 1.07 – – 1.68
0.05 0.07 0.06 0.05 0.07 0.06 – – 0.07
0.56 0.74 0.94 0.77 1.05 1.15 – – 1.70
2.38 2.88 2.95 3.57 3.78 3.97 – – 3.01
2.94 3.62 3.89 4.34 4.83 5.12 – – 4.71
9.07 7.87 8.07 7.73 7.33 7.07 – – 6.91
H
AcGGM + MeAGX
15 45 90 15 45 90 15 45 90
0.74 1.03 1.58 1.36 2.13 2.32 1.58 2.36 1.11
15 45 90 15 45 90 15 45 90 15 45 90 15 45 90 15 45 90
S.-H. Yoon, A. van Heiningen / Journal of Industrial and Engineering Chemistry 16 (2010) 74–80
79
Fig. 6. Extracted wood components versus H-factor. Fig. 7. Final pH versus pre-extraction H-factor.
that the major compound data obtained at different temperatures and times are well described by a single curve. The extracted amount of lignin, hemicelluloses and cellulose increased approximately linearly with H-factor in the GL pre-extraction process. Lignin shows higher yield compared to polysaccharides. Cellulose showed the highest resistance against alkaline dissolution in the GL pre-extraction, maintaining the minimum levels of extracts between 0.05% and 0.15% over the entire H-factor range and green liquor charges. Whereas hemicelluloses significantly decreased with increasing green liquor treatments, lignin extracts significantly increased. Since dissolved lignin will negatively affect the ethanol fermentation or bioconversion of bio-refinery processes, the alkaline pre-extraction conducted at lower charge of green liquor is recommendable. 3.5. pH of GL extracts Extraction of Loblolly Pine chips with neutral water causes acidic conditions due to release of acetic acid from hemicelluloses [13,14]. The acidity may accelerate the degradation of cellulose through the acidic auto-hydrolysis at high temperatures leading to a serious loss of pulp yields and strength in paper products. At alkaline conditions glucomannan is rapidly degraded by the peeling reaction, while xylan is dissolved in oligomeric form [21]. This hot alkaline condition, therefore, is not so much effective in hemicellulose extraction for softwoods as hardwoods. At mild alkaline conditions (initial pH 13) such as GL pre-extraction, however, it can be expected that the acidic auto-hydrolysis of cellulose can be minimized due to the early neutralization of the acetic acid released from the wood and the extraction process pH can be maintained neutral or slight acidic conditions to facilitate the extraction. The final liquor pH was plotted against extraction H-factor as shown in Fig. 7. The liquor pH rapidly dropped from the alkaline to the neutral in the very early stages of extraction up to an H-factor of about 500. The rapid pH drop in the initial preextraction is supposed to be caused by acetic acid formed by hydrolysis of acetyl groups in the hemicelluloses. In case of 2% green liquor extraction, the liquor pH reached the lowest pH level up to around 4.5 at an H-factor of around 4000, whereas in 4% and 6% green liquor extractions, their liquor pH could never reached below 6 even at the significantly prolonged extractions. The total sugar yields were plotted against final liquor pH of the GL extracts in Fig. 8. It can be seen that the total sugar yield is
Fig. 8. Total sugar yield versus final liquor pH.
highly pH-dependent. The extraction of total sugar increases linearly as the pH drops from the mild alkaline to acidic level around 4.5 showing an inverse proportionality relationship. About 2.5% of maximum sugar yield could be obtained at an acidic region near pH 4 in the 2% green liquor pre-extraction. The results obtained from the study indicates that a lowered GL charge is preferable for higher sugar extraction yield since it would provide more acidic conditions during the GL pre-extraction. 3.6. Extraction material balance A mass balance analysis on the total organics was performed to test the internal consistency of the data. The total organics were calculated from the sum of the major wood components in the green liquor extracts including cellulose, hemicelluloses and lignin as presented in Table 3. The total organics were plotted against WWL at 170, 180 and 190 8C as shown in Fig. 9. It can be seen that the total organics obtained with GL initially increased with increasing WWL and leveled off after passing about 1% WWL. The dotted arrow indicates the deviation of total organics data from the
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ylan and galactoglucomannan were about 2 and 1%, respectively. The hemicellulose yield and the dissolved lignin appeared to be at the similar level with 2% green liquor charge. With increasing green liquor charge, however, dissolved lignin significantly increased and hemicellulose yields significantly decreased. Cellulose showed a minimum dependence on green liquor charge and degree of extraction expressed as H-factor. The total sugar extracts were inversely proportional to the final liquor pH. Final liquor pH showed a rapid drop from the alkaline to the neutral in the very early stages of extraction and leveled off reaching to the acidic around pH 4.5 in case 2% green liquor extraction and to the near neutral in the 4% and 6% GL extractions. There was a systematic deviation between total organics and total weight loss possibly due to the presence of significant amount of low molecular weight dissolved organics that were not accounted for. Acknowledgements
Fig. 9. Mass balance analysis on green liquor extraction Loblolly Pine chips.
ideal material balance line. Significantly lower total organic yields were observed compared to WWL of wood chips. Around 11% of WWL, only a 4% total organics could be obtained from the GL extracts. This low total organic yield compared to WWL should be partly caused by the AcGGM’s high stability in the green liquor extraction environment and partly by some dissolved organic components. The dissolved organic components should comprise such low molecular organic substances as acetic acid, methanol, 4O-methyl glucoronic acid, as well as sugar degradation products such as furfural and others are not accounted for. 4. Conclusions Green liquor extraction effectiveness of Loblolly Pine chips has been investigated by studying total weight loss, sugar yields, total organic compounds and final liquor pH. The sugar and lignin components of the extracts were quantified using high pressure anion exchange chromatography and UV spectroscopy, respectively. The effect of time and temperature during green liquor extraction on total weight loss and wood component removal could be well described by a single control parameter, the H-factor. Total weight loss of wood chips largely depends on the extraction temperature and time. It increases from almost 4.2% at the mildest extraction condition to 15.8% at the most severe condition. Sugar components were all found in their polymeric forms in the green liquor extraction. Higher sugar extraction yield was obtained in xylose than mannose. The maximum yields of arabinoglucurox-
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