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Effect of pre-extraction on soda-anthraquinone (AQ) pulping of rice straw
Industrial Crops and Products 37 (2012) 164–169
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Effect of pre-extraction on soda-anthraquinone (AQ) pulping of rice straw M. Sarwar Jahan a,∗ , M. Shamsuzzaman a , M. Mostafizur Rahman a , S.M. Iqbal Moeiz b , Y. Ni c a
Pulp and Paper Research Division, BCSIR Laboratories, Dhaka, Dr. Qudrat-i-Khuda Road, Dhaka 1205, Bangladesh Department of Chemistry, Dhaka College, Dhaka 1205, Bangladesh c Limerick Pulp and Paper Centre, University of New Brunswick, Fredericton, New Brunswick E3B 5A3, Canada b
a r t i c l e
i n f o
Article history: Received 19 October 2011 Received in revised form 21 November 2011 Accepted 29 November 2011 Available online 7 January 2012 Keywords: Rice straw Pre-extraction Pulping Drainage resistance Strength properties Bleaching
a b s t r a c t The effect of hot-water and alkaline pre-extraction of rice straw on soda-anthraquinone pulping was carried out. The pre-extraction with hot water at 150 ◦ C for 1 h dissolved 34.7% biomass and the preextracted liquor comprised of 16.6% sugars, 6.7% lignin, 6.6% acetic acid and other unknown products. But the pre-extraction with 1% NaOH at 100 ◦ C for 1 h dissolved 10.2% sugars, 5.1% lignin and 10% acetic acid from rice straw. Pre-extracted rice straw was cooked by soda-anthraquinone process with varying alkali charges. The pulp from pre-extracted rice straw was low in kappa number with reduced pulp yield. The drainage resistance (◦ SR) improved obviously on pre-extraction of rice straw. Pulp strength properties such as the tensile index and the burst index were found to be lower, but the tear index was higher both with hot-water and alkaline pre-extraction. After bleaching, the gaps of the overall pulp yield and strength properties between pre-extracted and non-extracted rice straw became narrower. The alkaline pre-extraction showed improved yield and properties compared with hot-water pre-extracted rice straw. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Pulp and paper sector has been facing several problems in regard to the shortage of traditional resources of fibrous raw material with the continuous increase of product consumption tendency in the last 15 years. In this context, considerable research efforts are being been carried out to find alternative fibrous resources (Jahan et al., 2000, 2002, 2007). Rice straw is one of the most important raw materials in future for the pulp and paper industry in Asian countries. The major problems in rice straw pulping are silica, which inhibits the recovery of black liquor in alkali pulping processes and high amount of fines, which causes drainage problem. Much efforts have been made to solve these problems (Jahan et al., 2006; Pan et al., 1999). Unfortunately, no attempt reached to commercialization. Recently interest is growing on the conversion of biomass feed stock into fuel and chemicals in addition to pulp (Bridgwater, 2003; Sammons et al., 2008; Wingren et al., 2008). The unused biomass fractions, including lignin and hemicelluloses solubilized during pulping, can add value through conversion to liquid transportation fuels and extract components. In the forest product biorefinery concept, pre-pulping extraction has been shown to make
∗ Corresponding author. Tel.: +880 2 7911311; fax: +880 2 8613022. E-mail address: [email protected] (M. Sarwar Jahan). 0926-6690/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.indcrop.2011.11.035
available hemicellulose components of wood while preserving both the yield and quality of the pulp production (Van Heiningen, 2006). In our earlier investigation, it was observed that hot water pre-extraction of bagasse prior to pulping improved drainage resistance without affecting papermaking properties. However, pulp yield dropped slightly (Jahan et al., 2009b). At the same time, pre-extraction isolated hemicelluloses, acetic acid, and lignin from bagasse, which could be used in producing fuels and chemicals. In this paper, hot-water and alkaline pre-extraction of rice straw was carried out in order to improve drainage resistance and recover of hemicelluloses. Subsequently, the pre-extracted rice straw was subjected to soda-anthraquinone (AQ) pulping. The bleachability, physical properties and optical properties of pre-extracted rice straw pulp were assessed and compared with non-extracted counterpart. 2. Experimental 2.1. Raw materials Rice straw was collected from the Khustia dist. in Bangladesh and cut to 2–3 cm in length. After determination of the moisture content, air dried raw material equivalent to 100 g od (oven dried) material was weighed separately in polyethylene bags for subsequent pre-extraction and cooking experiments.
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2.2. Chemical analysis The chemical compositions of rice straw were carried out according to Tappi Test Methods (Tappi, 2003–2004): extractive (T204 om88), water solubility (T207 cm99), Klason lignin (T211 om83). Holocellulose sample was prepared by treating extractive free rice straw meal with NaClO2 solution (Browining, 1967). The pH of the solution was maintained at 4 by adding CH3 COOH CH3 COONa buffer, and ␣-cellulose was determined by treating holocellulose with 17.5% NaOH (T203 om 93).
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18% on od raw materials or pre-extracted rice straw residue. The following parameters were kept constant: -
Anthraquinone charge: 0.1% on od rice straw. Fiber to liquor ratio: 1:6 g/mL. Temperature: 150 ◦ C. Cooking time: 60 min at 150 ◦ C.
The solid content in the black liquor was determined by Tappi T650 om 99 methods.
2.3. Pre-extraction
2.9. Evaluation of pulps
The pre-extraction was carried out in an electrically heated oil bath containing 4 bombs of 1.5 L capacity. The bombs were rotated at 1 rpm. Water pre-extraction was carried out at 150 ◦ C for 60 min. In the alkaline pre-extraction alkali charge was 1% on od rice straw at 100 ◦ C for 60 min. The raw material to liquor ratio was 1:6 (g/mL). The time required to raise max temperature from room temperature (30 ◦ C) was 50 min. After completing pre-extraction, bombs were cooled by dipping in cold water and the liquor was drained for analysis.
Pre-extracted and non-extracted rice straw pulps were beaten in a PFI mill at different revolutions. The handsheets of about 60 g/m2 were made in a Rapid Kothen Sheet Making Machine. The sheets were tested for tensile, burst and tear strength according to TAPPI Standard Test Methods (2003–2004).
2.4. Lignin analysis The dissolved lignin in the prehydrolysate was measured based on the UV/vis spectrometric method at wavelength 205 nm (TAPPI UM 250). 2.5. Analysis of acetic acid Varian 300 NMR-spectrometer was employed for analyzing furfural, acetic acid and methanol as described in (Roger and Virgil, 1991). Calibration curves were made with the standard solution of each component to determine the unknown concentration for each of these present in the pre-extracted liquor (PHL). Solvent suppression method was used with D2 O to water ratio of 1:4. 2.6. Solid contents The total solid content in the PHL was determined by drying 10 mL sample at 105 ◦ C till to constant weight. 2.7. Sugar analysis The sugars in the untreated PHL and in the acid hydrolyzed PHL were determined by ion chromatography unit equipped with CarboPacTM PA1 column (Dionex-3000, Dionex cooperation, Canada and pulsed amperometric detector (PAD). Acid hydrolysis of PHL with 4% sulfuric acid was carried out at 121 ◦ C for 2 h in an oil bath (Neslab Instruments, Inc., Portsmouth, NH, USA) to convert oligosaccharide to monosaccharide. The PAD settings were E1 = 0.1 V, E2 = 0.6 V and E3 = −0.8 V. De-ionized distilled water was used as eluant with a flow rate of 1 mL/min. The column regenerant and post column detector’s supporting electrolyte were 0.2 mol/L and 0.5 mol/L NaOH, respectively, with 1 mL/min flow rate. The addition of 0.5 mol/L NaOH was necessary to maintain optimum detector sensitivity and minimize baseline drift for an isocratic separation. 2.8. Pulping Pulping of pre-extracted and original (without pre-extraction) rice straw by the soda-AQ process was done in the same digester as in pre-extraction. Alkali charge was varied from 12, 14, 16 and
2.10. D0 Ep D1 bleaching Pulps were bleached by D0 Ep D1 bleaching sequences. In the first stage (D0 ) of D0 Ep D1 bleaching sequences ClO2 charge was 1%. The temperature was 70 ◦ C and the time was 60 min in D0 stage. Pulp consistency was 10%. The initial pH was adjusted to 2.5 by adding dilute H2 SO4 . In the alkaline extraction stage, NaOH and H2 O2 charge were 2% and 0.5% (on od pulp), respectively. The temperature was 70 ◦ C for 60 min and pulp consistency was 10%. In the D1 stage, the end pH 4. The ClO2 charge in the D1 was 0.5%. The brightness, opacity, tensile, burst and tear strength were determined in accordance with Tappi Test Methods (2003–2004). 3. Results and discussion 3.1. Chemical characteristics Table 1 shows that 1% NaOH solubles were 49.2%, which was lower than the other reported data of rice straw (Rodriguez et al., 2008). But due to high 1% NaOH solublities, rice straw can be expected to provide a medium–low pulp yield. 1% NaOH solubility in rice straw was higher than that of the agricultural residues like cotton stalks, corn stalks, Dhaincha (Ali et al., 2001; Jahan et al., 2009a; Mehmet et al., 2010). The hot water solubility of rice straw is higher than value in corn stalks (Timell, 1957). Hot-water soluble substances in the raw materials included starch and proteins, which could consume pulping reagents. The Klason lignin content in rice straw was 22.1%, which was almost similar to the other reported results, and higher than the corn stalks (Table 1) and similar to cotton stalks (Jimenez et al., 2006). The lower lignin indicates easier pulping. The acid soluble lignin was 4.1%, which was mainly represent sryingyl units (Sun et al., 2003). The higher sryingyl unit is Table 1 Chemical characteristics of rice straw.
1% NaOH solubility Cold water solubility (%) Hot water solubility (%) Lignin (%) Klason lignin Acid soluble lignin ␣-Cellulose (%) Pentosan (%) Ash (%)
Rice straw
Rice straw (Rodriguez et al., 2008)
Corn stalk (Timell, 1957)
49.2 14.4 19.9
57.7 – –
– – 7.3
22.1 4.1 38.2 23.5 14.6
21.9 – 41.2 – 9.2
14.0 – 46.5 27.6 4.53
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Table 2 Chemical properties of pre-extracted liquor (PHL).
3.4. Lignin in the pre-extract
Pre-extraction
Biomass yield (%)
pH
Solid content (%)
Acetic acid (%)
Lignin (%)
Hot-water Alkaline
65.3 71.2
4.1 8.0
32.9 27.05
6.60 9.96
6.69 5.12
associated with the higher the delignification rate. The ␣-cellulose in rice straw was lower than the other reported value (Rodriguez et al., 2008). The lower ␣-cellulose in rice straw in this investigation may be the basis of lower pulp yield. The pentosans content was 23.5%, which was higher than the tropical hardwoods (Jahan and Mun, 2003; Jahan et al., 2007). The ash content (14.6%) was much higher than wood and other nonwood species (1–3%) (Jahan et al., 2010; Gunter et al., 2010). Such high ash content causes problems in chemical recovery. 3.2. Alkaline and hot water extraction of rice straw Table 2 shows the results of alkaline and hot water preextraction prior to pulping of rice straw. The pre-extraction resulted in liquid solution containing dilute oligo sugars, acetic acid, lignin organic acids degradation products and minerals. It is observed that hot water extraction removed 34.7% biomass while alkaline preextraction removed 28.8% (Table 2). The sum of biomass residue after pre-extraction and solid content in the PHL was close to 98% in both processes (Table 2). This 2% loss in the mass balance due to the degradation in unknown products. This concurs with the finding of our previous results (Jahan et al., 2011) and Leschinnsky et al. (2009) and Tunc and van Heiningen (2008). During the hot water extraction process, acetylated polysaccharides in biomass release acetic acid and the pH is reduced to 4 (Table 2). This makes autohydrolysis similar to acid hydrolysis in the way that it leads to unwanted side reactions at elevated temperatures, which lowers the overall yield of hemicelluloses recovery and generates inhibitors for a potential subsequent fermentation process (Larsson et al., 1999; Palmqvist and Hahn-Ha¨gerdal, 2000). In addition, acid pre-extraction causes random chain cleavage in cellulose and hemicelluloses; the products have lower degree of polymerization (DP) and more reducing end-groups. Wafa Al-Dajani and Tschirner (2008) showed that the pre-extraction of wood chips with alkaline media gave lowers loss of biomass but improved pulp yield and properties. Alkaline pre-extraction increased the pH to 8 of the pre-extract that helped to protect carbohydrate, thus lowering biomass loss than the hot water pre-extraction (29% vs 35%). 3.3. Acetic acid in the pre-extract Acetic acid is an important by-products in the pre-extraction process. The acetic acid liberated from the hemicelluloses of rice straw by hot water pre-extraction was 6.6%, while it was 9.96% for alkaline pre-extraction (Table 2). Similarly Walton et al. (2010) showed that the pre-extracted liquor released acetic acid from hemicelluloses more with increasing alkalinity. Acetic acid can be a valuable co-product from the forest bio-refinery. For a pulp mill of 50 ton/day (15,000 ton/year), at a 40% pulp yield, it can be estimated that there will be about 2475–3735 ton acetic acid/year. This certainly represents a significant revenue source for the small pulp mill. The separation of acetic acid from the hydrolyzed solution may be done by the liquid–liquid extraction technology, which consists of a solvent recovery section for recovery of the solvent, and a distillation section for upgrading the different components removed in the extraction process (King and Othmer, 1958; Jones, 1967; Geankoplis, 2003).
Lignin in hot water pre-extraction was found to be 6.7%, while for alkaline pre-extraction it 5.1% (Table 2). This amounts to 20.3 and 18.9% of total solid content, respectively. Lignin degraded phenolics are inhibitors to the downstream fermentation process. It should be removed from the PHL before fermentation process. But this lignin can be the starting material for high value-added applications in renewable polymeric materials development (Satheesh et al., 2009). The value added applications of lignin not only helps to boost the economic viability of the biorefinery but also serves as a source of renewable materials. 3.5. Sugars in the pre-extract The remaining parts of solid content apart from lignin and acetic acid were mono and oligo sugars. For paper pulp, complete removal of hemicelluloses is unnecessary because they play an important role in formation of sheets and facilitate fiber bonding in sheets. In the present investigation, purpose of pre-extraction was to remove fines and improve pulping efficiency of straw. Table 3 shows that a good extraction of oligo sugars with minimal conversion to mono sugars was occurred in an alkaline pre-extraction at 100 ◦ C for 60 min. The total extracted sugars in hot water extract was 15.6%, while that in alkaline extract was 10.2%, comprising 47% and 37% of the total solid content, respectively. The decrease in sugar yield in alkaline extraction can be interpreted as a result of alkaline hydrolysis of acetyl groups at higher pH levels, leading to lower solubility of oligo sugars. The C-5 sugar yield in hot water and alkaline extracts was 7.4% and 4.7%, respectively. Perez et al. (2007) achieved 53% xylose yield from wheat straw with hot water pretreatment at 200 ◦ C for 0 min. Sreenath et al. (1999) obtained a liquid phase with a hemicellulose-derived sugars yield of 50% after treating alfalfa fibers at 220 ◦ C for 2 min. Both the extraction processes yielded a considerable amount of glucose (Table 3), which may be contributed by hydrolysis of starch or hydrolysis of part of amorphous cellulose (Dietrichs, 1964). The xylan content in the hot water extract was lower than that of alkaline extract (3.6% vs 2.7%). The galactose (oligo) content in pre-extract liquor was 4.2% and 2.5% for hot water and alkaline extraction. These hemicelluloses may be used as wet end additives in papermaking (Lima et al., 2003). 3.6. Soda-AQ pulping Table 4 shows the effect of pre-extraction on the soda-AQ pulping of rice straw. It is seen that the pre-extraction of rice straw resulted lower kappa number of pulp than the non pre-extraction rice straw with considerable lowering of pulp yield. Similar results were observed for hot water pre-extraction of bagasse followed by alkaline pulping (Lei et al., 2010). Hemicelluloses compared to cellulose have high accessibility to acidic or alkaline hydrolysis. This is due to their amorphous structure and much lower Table 3 Sugars in the pre-extracted liquor. Sugar (% on od rice straw)
Hot H2 O Pre-extraction
Alkaline Pre-extraction
Mono-
Mono-
Oligo-
Oligo-
Rhamnose Arabinose Galactose Glucose Xylose Mannose
0 0 0 0 0.966 0
0.966 2.741 4.242 3.241 3.612 0.791
0 0 0 0 0 0
0.271 1.753 2.517 2.532 2.688 0.44
Total sugars (%)
0.966
15.595
0
10.199
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Fig. 1. Effect of alkali charge and pre-extraction on the tensile index of pulp.
degree of polymerization (100–200). During the pre-hydrolysis, some hemicelluloses and lignin were removed and delignification was thus improved. The extracted straw had a more open structure due to the hemicelluloses and lignin removal. After pre-hydrolysis, lignin migrated to the surface of straw. Therefore the delignification was faster for pulping of extracted straw. So pulping of extracted straw yielded a lower kappa number even though with lower alkali charge. The total pulp yield and kappa number decreased with increasing alkali charge. Alkaline pre-extracted straw demonstrated improved pulp yield and kappa number compared with the hot H2 O pre-extracted. At 12% alkali charge, alkaline pre-extracted rice straw gave 2.7% point higher pulp yield, which was 9.8% unit lower than the non pre-extracted rice straw. The initial pulp drainage resistance is an important parameter for pulp process unit operations, for example, pulp (brownstock) washing efficiency. The initial drainage resistance (measured as Schopper–Riegler beating degree) of pre-extracted pulp was 24–26 ◦ SR while it was 28–31 ◦ SR for non-extracted pulp (Table 4). This decrease of drainage resistance certainly improves pulp washing efficiency. High specific area due to high amount of fines caused the poor drainability of the non-extracted pulp (Jahan et al., 2009b). Therefore, it is evident that the pre-extraction can effectively dissolve the fines producing sugars in the pre-extracted liquor.
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Fig. 2. Effect of alkali charge and pre-extraction on the burst index of pulp.
cooking conditions. At 16% alkali charge, the tensile index of alkaline pre-hydrolyzed pulp was very close to non-hydrolyzed pulp (55.5 N m/g vs 57.6 N m/g), but tensile index of hot water prehydrolyzed was 32% lower than the non-hydrolyzed pulp. Similar results were observed for burst index (Fig. 2). This may be explained by the loss of hemicelluloses during pre-extraction. Due to hemicelluloses extraction, pulp had high cellulose/hemicelluloses ratios than the non-extracted pulp. Several studies have emphasized the importance of hemicelluloses for the strength properties of pulp fibers. Molin and Teder (2002) showed that fibers with high cellulose/hemicelluloses ratio had lower tensile stiffness and tensile index. Schönberg et al. (2001) showed that the tensile index increased after sorption of xylan to the fibers, which facilitate fiber bonding. The role of xylan in the fibers was further investigated. The chemical sorption of xylan was found to significantly increase the Scott Bond-value, further supporting the significance of xylan on
3.7. Physical properties All the pulps were beaten to SR value 30 and strength properties plotted against alkali charge in Figs. 1–3. The tensile index of hot water pre-extracted rice straw pulp increased when alkali charge increased from 12 to 14%, thereafter it decreased, for alkaline pre-extracted pulp tensile index decreased with alkali charge higher than 16%. The tensile index of non-extracted pulp increased with increasing alkali charge. These results indicated that the pre-extracted pulps reached to the residual stage of delignification at lower alkali charge than the non-extracted pulp employed
Fig. 3. Effect of alkali charge and pre-extraction on the tear index of pulp.
Table 4 Effect of pre-extraction and alkali charge on the pulping of rice straw. Alkali charge (%)
Hot H2 O Pre-extraction
→
12
14
16
18
12
14
16
18
12
14
16
18
Total pulp yield%) Kappa number pH of black liquor Solid content in BL Initial ◦ SR
44.0 10.5 11.1 8.85 25
43.5 9.5 11.3 9.9 26
39.8 8.3 11.8 9.9 24
35.5 7.8 12.1 11.1 25
46.7 11.3 12.0 8.7 26
44.2 10.7 12.5 9.3 25
42.3 9.6 12.9 9.9 26
38.2 8.7 13.3 10.5 25
56.5 12.5 11.5 8.9 28
54.2 11.3 11.9 9.6 29
51.9 9.8 12.3 10.9 31
50.4 8.9 12.8 11.1 30
Alkaline Pre-extraction
Without Pre-extraction
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Table 5 Effect of pre-extraction on the bleaching of rice straw pulp.
Bleached yield (%) Overall yield (%) ◦ SR after 1000 PFI rev Tensile index (N m/g) Tear index (mN m2 /g) Burst index (kPa m2 /g) Brightness (%) Opacity (%)
Hot H2 O Pre-extraction
Alkaline Pre-extraction
Without Pre-extraction
95.1 42.0 33
95.4 44.6 33
86.2 46.7 39
36.1
40.2
41.7
5.8
6.2
5.7
2.9
3.1
3.2
85.3 88.1
85.9 88.2
86.1 89.0
the bonding ability (Schönberg et al., 2001). Addition of 1% (w/w) of hemicelluloses to cellulosic pulp was able to increase about 30% of the mechanical properties (Lima et al., 2003). Wafa Al-Dajani and Tschirner (2008) observed that alkaline pre-extraction of aspen prior to pulping reduced 10% tensile index. The negative effect of pre-extraction on the tensile strength of kraft pulp of loblolly pine was also observed by Yoon and Van Heiningen, 2008. The tear index of pre-hydrolyzed pulp was better than that of non-hydrolyzed pulp (Fig. 3). Alkaline pre-hydrolyzed pulp retained its superiority over hot water pre-hydrolyzed pulp. At 16% alkali charge, alkaline pre-hydrolyzed pulp had 7% and 23% higher tear index compared with the water-pre-hydrolyzed and non-hydrolyzed pulps, respectively. 3.8. Bleaching Table 5 shows the bleaching results of pre-extracted and non pre-extracted rice straw. The bleaching yield of the untreated rice straw was much lower than that of extracted rice straw (86% vs 95%) due to higher hemicelluloses contents. After bleaching, overall pulp on original straw basis yield differences from pre-extracted and non pre-extracted rice straw became narrower. But still there were 2.1% and 4.7% lower pulp yield for alkaline pre-extracted and water preextracted rice straw (based on od rice straw). It is clearly seen from Table 5 that all pulps can be easily bleached to 86% brightness by D0 Ep D1 sequences with total ClO2 charge 15 kg/ton). The lower pulp drainage resistance (◦ SR) is better for papermaking. During refining, pulp drainage resistance (◦ SR) falls; but it unavoidable to increase the surface area of the pulp fibers to enhance fiber bonding during papermaking. After 1000 PFI revolution, SR value reached to 33 for pre-extracted pulp and while 39 for non pre-extracted pulp. The main reason in the latter case was due enhanced hydration with higher amount of hemicelluloses. The tensile index of alkaline pre-extracted and non pre-extracted pulps was very close (40 N m/g vs 42 N m/g) but hot water pre-extracted pulp was inferior. This can be explained by acidic condition of prehydrolysis damage the fiber to some extent. The pH value of hot water pre-extracted liquor was 4.1, while it was 8.1 for alkaline preextracted liquor. The tear index value of alkaline pre-hydrolyzed pulp was slightly better than the hot water pre-extracted and non pre-extracted pulp. 4. Conclusions Pre-extraction prior to pulping dissolved 28–34% biomass from rice straw. The dissolved components content hemicelluloses (10–17%), lignin (5–7%) and acetic acid (6–9%). Hot water pre-extraction showed drastic conditions (pH 4.1), consequently removed more biomass compared with alkaline pre-extraction. Pulp produced from pre-extracted rice straw improved drainage
resistance (◦ SR). Pulp yield from the pre-extracted rice straw was inferior but delignification was better compared with nonextracted rice straw. The pre-extraction affected tensile and burst index of the pulp produced because of removal of higher amount of hemicelluloses. Improved tear index was observed for the preextracted pulp, which indicated that pre-extraction did not degrade cellulose. Rice straw after alkaline pre-extraction showed better pulp yield and strength properties compared with the pulp from hot-water pre-extraction. Final brightness of the pulp was almost similar in both pre-extracted and non-extracted rice straw. The difference in pulp yield and strength properties was narrower on bleaching. The precise optimization of pulping of pre-extracted rice straw can provide similar pulp yield physical properties to nonextracted rice straw. So study on the delignification kinetics of pre-extracted rice straw is needed.
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M. Sarwar Jahan et al. / Industrial Crops and Products 37 (2012) 164–169 Pan, X.-J., Sano, Y., Ito, T., 1999. Atmospheric acetic acid pulping of rice straw. II: behavior of ash and silica in rice straw during atmospheric acetic acid pulping and bleaching. Holzforschung 53 (1), 49–55. Roger, C.P., Virgil, H.S., 1991. Wood sugar analysis by anion chromatography. J. Wood Chem. Technol. 11 (4), 495–501. Rodriguez, A., Moral, A., Serrano, L., Labidi, L., Jimenez, L., 2008. Rice straw pulp obtained by using various methods. Bioresour. Technol. 99, 2881–2886. Sammons, N.E., Yuan, M., Cullinan, W., Aksoy, H.B., 2008. A flexible framework for optimal biorefinery product allocation. Environ. Prog. 26, 349. Satheesh, M.N.K., Mohanty, A.K., Erickson, L., Misra, M., 2009. Lignin and its applications with polymers. J. Biobased Mater. Bioenergy 3 (1), 1–24. Schönberg, C., Oksanen, T., Suurnäkki, A., Kettunen, H., Buchert, J., 2001. The importance of xylan for the strength properties of spruce kraft pulp fibres. Holzforschung 55, 639–644. Sreenath, H.K., Koegel, R.G., Moldes, A.B., Jeffries, T.W., Straub, R.J., 1999. Enzymic saccharification of alfalfa fiber after liquid hot water pretreatment. Process Biochem. 35, 33–41. Sun, J.X., Sun, X.F., Sun, R.C., Fowler, P., Baird, M.S., 2003. Inhomogeneities in the chemical structure of sugarcane bagasse lignin. J. Agric. Food Chem. 51 (23), 6719–6725.
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Tappi, 2003–2004. CD Version of Tappi Test Methods. Tappi Press, Atlanta, GA. Timell, T.E., 1957. The cellulose component of cornstalk and wheat straw. Tappi J. 40 (9), 749. Tunc, M.S., van Heiningen, A.R.P., 2008. Hydrothermal dissolution of mixed southern hardwoods. Holzforschung 62 (5), 539–545. Van Heiningen, A.R.P., 2006. Converting a kraft pulp mill into an integrated forest biorefinery. Pulp Paper Canada 107 (6), 38–43. Wafa Al-Dajani, W., Tschirner, U., 2008. Pre-extraction of hemicelluloses and subsequent Kraft pulping. Part I. Alkaline extraction. Tappi J. 7, 3–8. Walton, S.L., Hutto, D., Genco, J.M., Walsum, J.P.V., van Heiningen, A.R.P., 2010. Pre-extraction of hemicelluloses from hardwood chips using an alkaline wood pulping solution followed by kraft pulping of the extracted wood chips. Ind. Eng. Chem. Res. 49 (24), 12638–12645. Wingren, A., Galbe, M., Zacchi, G., 2008. Energy considerations for a SSF-based softwood ethanol plant. Bioresour. Technol. 99, 2121. Yoon, S.-H., Van Heiningen, A., 2008. Kraft pulping and papermaking properties of hot-water pre-extracted loblolly pine in an integrated forest products biorefinery. Tappi J. 7, 22–27.
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Industrial Crops and Products journal homepage: www.elsevier.com/locate/indcrop
Effect of pre-extraction on soda-anthraquinone (AQ) pulping of rice straw M. Sarwar Jahan a,∗ , M. Shamsuzzaman a , M. Mostafizur Rahman a , S.M. Iqbal Moeiz b , Y. Ni c a
Pulp and Paper Research Division, BCSIR Laboratories, Dhaka, Dr. Qudrat-i-Khuda Road, Dhaka 1205, Bangladesh Department of Chemistry, Dhaka College, Dhaka 1205, Bangladesh c Limerick Pulp and Paper Centre, University of New Brunswick, Fredericton, New Brunswick E3B 5A3, Canada b
a r t i c l e
i n f o
Article history: Received 19 October 2011 Received in revised form 21 November 2011 Accepted 29 November 2011 Available online 7 January 2012 Keywords: Rice straw Pre-extraction Pulping Drainage resistance Strength properties Bleaching
a b s t r a c t The effect of hot-water and alkaline pre-extraction of rice straw on soda-anthraquinone pulping was carried out. The pre-extraction with hot water at 150 ◦ C for 1 h dissolved 34.7% biomass and the preextracted liquor comprised of 16.6% sugars, 6.7% lignin, 6.6% acetic acid and other unknown products. But the pre-extraction with 1% NaOH at 100 ◦ C for 1 h dissolved 10.2% sugars, 5.1% lignin and 10% acetic acid from rice straw. Pre-extracted rice straw was cooked by soda-anthraquinone process with varying alkali charges. The pulp from pre-extracted rice straw was low in kappa number with reduced pulp yield. The drainage resistance (◦ SR) improved obviously on pre-extraction of rice straw. Pulp strength properties such as the tensile index and the burst index were found to be lower, but the tear index was higher both with hot-water and alkaline pre-extraction. After bleaching, the gaps of the overall pulp yield and strength properties between pre-extracted and non-extracted rice straw became narrower. The alkaline pre-extraction showed improved yield and properties compared with hot-water pre-extracted rice straw. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Pulp and paper sector has been facing several problems in regard to the shortage of traditional resources of fibrous raw material with the continuous increase of product consumption tendency in the last 15 years. In this context, considerable research efforts are being been carried out to find alternative fibrous resources (Jahan et al., 2000, 2002, 2007). Rice straw is one of the most important raw materials in future for the pulp and paper industry in Asian countries. The major problems in rice straw pulping are silica, which inhibits the recovery of black liquor in alkali pulping processes and high amount of fines, which causes drainage problem. Much efforts have been made to solve these problems (Jahan et al., 2006; Pan et al., 1999). Unfortunately, no attempt reached to commercialization. Recently interest is growing on the conversion of biomass feed stock into fuel and chemicals in addition to pulp (Bridgwater, 2003; Sammons et al., 2008; Wingren et al., 2008). The unused biomass fractions, including lignin and hemicelluloses solubilized during pulping, can add value through conversion to liquid transportation fuels and extract components. In the forest product biorefinery concept, pre-pulping extraction has been shown to make
∗ Corresponding author. Tel.: +880 2 7911311; fax: +880 2 8613022. E-mail address: [email protected] (M. Sarwar Jahan). 0926-6690/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.indcrop.2011.11.035
available hemicellulose components of wood while preserving both the yield and quality of the pulp production (Van Heiningen, 2006). In our earlier investigation, it was observed that hot water pre-extraction of bagasse prior to pulping improved drainage resistance without affecting papermaking properties. However, pulp yield dropped slightly (Jahan et al., 2009b). At the same time, pre-extraction isolated hemicelluloses, acetic acid, and lignin from bagasse, which could be used in producing fuels and chemicals. In this paper, hot-water and alkaline pre-extraction of rice straw was carried out in order to improve drainage resistance and recover of hemicelluloses. Subsequently, the pre-extracted rice straw was subjected to soda-anthraquinone (AQ) pulping. The bleachability, physical properties and optical properties of pre-extracted rice straw pulp were assessed and compared with non-extracted counterpart. 2. Experimental 2.1. Raw materials Rice straw was collected from the Khustia dist. in Bangladesh and cut to 2–3 cm in length. After determination of the moisture content, air dried raw material equivalent to 100 g od (oven dried) material was weighed separately in polyethylene bags for subsequent pre-extraction and cooking experiments.
M. Sarwar Jahan et al. / Industrial Crops and Products 37 (2012) 164–169
2.2. Chemical analysis The chemical compositions of rice straw were carried out according to Tappi Test Methods (Tappi, 2003–2004): extractive (T204 om88), water solubility (T207 cm99), Klason lignin (T211 om83). Holocellulose sample was prepared by treating extractive free rice straw meal with NaClO2 solution (Browining, 1967). The pH of the solution was maintained at 4 by adding CH3 COOH CH3 COONa buffer, and ␣-cellulose was determined by treating holocellulose with 17.5% NaOH (T203 om 93).
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18% on od raw materials or pre-extracted rice straw residue. The following parameters were kept constant: -
Anthraquinone charge: 0.1% on od rice straw. Fiber to liquor ratio: 1:6 g/mL. Temperature: 150 ◦ C. Cooking time: 60 min at 150 ◦ C.
The solid content in the black liquor was determined by Tappi T650 om 99 methods.
2.3. Pre-extraction
2.9. Evaluation of pulps
The pre-extraction was carried out in an electrically heated oil bath containing 4 bombs of 1.5 L capacity. The bombs were rotated at 1 rpm. Water pre-extraction was carried out at 150 ◦ C for 60 min. In the alkaline pre-extraction alkali charge was 1% on od rice straw at 100 ◦ C for 60 min. The raw material to liquor ratio was 1:6 (g/mL). The time required to raise max temperature from room temperature (30 ◦ C) was 50 min. After completing pre-extraction, bombs were cooled by dipping in cold water and the liquor was drained for analysis.
Pre-extracted and non-extracted rice straw pulps were beaten in a PFI mill at different revolutions. The handsheets of about 60 g/m2 were made in a Rapid Kothen Sheet Making Machine. The sheets were tested for tensile, burst and tear strength according to TAPPI Standard Test Methods (2003–2004).
2.4. Lignin analysis The dissolved lignin in the prehydrolysate was measured based on the UV/vis spectrometric method at wavelength 205 nm (TAPPI UM 250). 2.5. Analysis of acetic acid Varian 300 NMR-spectrometer was employed for analyzing furfural, acetic acid and methanol as described in (Roger and Virgil, 1991). Calibration curves were made with the standard solution of each component to determine the unknown concentration for each of these present in the pre-extracted liquor (PHL). Solvent suppression method was used with D2 O to water ratio of 1:4. 2.6. Solid contents The total solid content in the PHL was determined by drying 10 mL sample at 105 ◦ C till to constant weight. 2.7. Sugar analysis The sugars in the untreated PHL and in the acid hydrolyzed PHL were determined by ion chromatography unit equipped with CarboPacTM PA1 column (Dionex-3000, Dionex cooperation, Canada and pulsed amperometric detector (PAD). Acid hydrolysis of PHL with 4% sulfuric acid was carried out at 121 ◦ C for 2 h in an oil bath (Neslab Instruments, Inc., Portsmouth, NH, USA) to convert oligosaccharide to monosaccharide. The PAD settings were E1 = 0.1 V, E2 = 0.6 V and E3 = −0.8 V. De-ionized distilled water was used as eluant with a flow rate of 1 mL/min. The column regenerant and post column detector’s supporting electrolyte were 0.2 mol/L and 0.5 mol/L NaOH, respectively, with 1 mL/min flow rate. The addition of 0.5 mol/L NaOH was necessary to maintain optimum detector sensitivity and minimize baseline drift for an isocratic separation. 2.8. Pulping Pulping of pre-extracted and original (without pre-extraction) rice straw by the soda-AQ process was done in the same digester as in pre-extraction. Alkali charge was varied from 12, 14, 16 and
2.10. D0 Ep D1 bleaching Pulps were bleached by D0 Ep D1 bleaching sequences. In the first stage (D0 ) of D0 Ep D1 bleaching sequences ClO2 charge was 1%. The temperature was 70 ◦ C and the time was 60 min in D0 stage. Pulp consistency was 10%. The initial pH was adjusted to 2.5 by adding dilute H2 SO4 . In the alkaline extraction stage, NaOH and H2 O2 charge were 2% and 0.5% (on od pulp), respectively. The temperature was 70 ◦ C for 60 min and pulp consistency was 10%. In the D1 stage, the end pH 4. The ClO2 charge in the D1 was 0.5%. The brightness, opacity, tensile, burst and tear strength were determined in accordance with Tappi Test Methods (2003–2004). 3. Results and discussion 3.1. Chemical characteristics Table 1 shows that 1% NaOH solubles were 49.2%, which was lower than the other reported data of rice straw (Rodriguez et al., 2008). But due to high 1% NaOH solublities, rice straw can be expected to provide a medium–low pulp yield. 1% NaOH solubility in rice straw was higher than that of the agricultural residues like cotton stalks, corn stalks, Dhaincha (Ali et al., 2001; Jahan et al., 2009a; Mehmet et al., 2010). The hot water solubility of rice straw is higher than value in corn stalks (Timell, 1957). Hot-water soluble substances in the raw materials included starch and proteins, which could consume pulping reagents. The Klason lignin content in rice straw was 22.1%, which was almost similar to the other reported results, and higher than the corn stalks (Table 1) and similar to cotton stalks (Jimenez et al., 2006). The lower lignin indicates easier pulping. The acid soluble lignin was 4.1%, which was mainly represent sryingyl units (Sun et al., 2003). The higher sryingyl unit is Table 1 Chemical characteristics of rice straw.
1% NaOH solubility Cold water solubility (%) Hot water solubility (%) Lignin (%) Klason lignin Acid soluble lignin ␣-Cellulose (%) Pentosan (%) Ash (%)
Rice straw
Rice straw (Rodriguez et al., 2008)
Corn stalk (Timell, 1957)
49.2 14.4 19.9
57.7 – –
– – 7.3
22.1 4.1 38.2 23.5 14.6
21.9 – 41.2 – 9.2
14.0 – 46.5 27.6 4.53
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Table 2 Chemical properties of pre-extracted liquor (PHL).
3.4. Lignin in the pre-extract
Pre-extraction
Biomass yield (%)
pH
Solid content (%)
Acetic acid (%)
Lignin (%)
Hot-water Alkaline
65.3 71.2
4.1 8.0
32.9 27.05
6.60 9.96
6.69 5.12
associated with the higher the delignification rate. The ␣-cellulose in rice straw was lower than the other reported value (Rodriguez et al., 2008). The lower ␣-cellulose in rice straw in this investigation may be the basis of lower pulp yield. The pentosans content was 23.5%, which was higher than the tropical hardwoods (Jahan and Mun, 2003; Jahan et al., 2007). The ash content (14.6%) was much higher than wood and other nonwood species (1–3%) (Jahan et al., 2010; Gunter et al., 2010). Such high ash content causes problems in chemical recovery. 3.2. Alkaline and hot water extraction of rice straw Table 2 shows the results of alkaline and hot water preextraction prior to pulping of rice straw. The pre-extraction resulted in liquid solution containing dilute oligo sugars, acetic acid, lignin organic acids degradation products and minerals. It is observed that hot water extraction removed 34.7% biomass while alkaline preextraction removed 28.8% (Table 2). The sum of biomass residue after pre-extraction and solid content in the PHL was close to 98% in both processes (Table 2). This 2% loss in the mass balance due to the degradation in unknown products. This concurs with the finding of our previous results (Jahan et al., 2011) and Leschinnsky et al. (2009) and Tunc and van Heiningen (2008). During the hot water extraction process, acetylated polysaccharides in biomass release acetic acid and the pH is reduced to 4 (Table 2). This makes autohydrolysis similar to acid hydrolysis in the way that it leads to unwanted side reactions at elevated temperatures, which lowers the overall yield of hemicelluloses recovery and generates inhibitors for a potential subsequent fermentation process (Larsson et al., 1999; Palmqvist and Hahn-Ha¨gerdal, 2000). In addition, acid pre-extraction causes random chain cleavage in cellulose and hemicelluloses; the products have lower degree of polymerization (DP) and more reducing end-groups. Wafa Al-Dajani and Tschirner (2008) showed that the pre-extraction of wood chips with alkaline media gave lowers loss of biomass but improved pulp yield and properties. Alkaline pre-extraction increased the pH to 8 of the pre-extract that helped to protect carbohydrate, thus lowering biomass loss than the hot water pre-extraction (29% vs 35%). 3.3. Acetic acid in the pre-extract Acetic acid is an important by-products in the pre-extraction process. The acetic acid liberated from the hemicelluloses of rice straw by hot water pre-extraction was 6.6%, while it was 9.96% for alkaline pre-extraction (Table 2). Similarly Walton et al. (2010) showed that the pre-extracted liquor released acetic acid from hemicelluloses more with increasing alkalinity. Acetic acid can be a valuable co-product from the forest bio-refinery. For a pulp mill of 50 ton/day (15,000 ton/year), at a 40% pulp yield, it can be estimated that there will be about 2475–3735 ton acetic acid/year. This certainly represents a significant revenue source for the small pulp mill. The separation of acetic acid from the hydrolyzed solution may be done by the liquid–liquid extraction technology, which consists of a solvent recovery section for recovery of the solvent, and a distillation section for upgrading the different components removed in the extraction process (King and Othmer, 1958; Jones, 1967; Geankoplis, 2003).
Lignin in hot water pre-extraction was found to be 6.7%, while for alkaline pre-extraction it 5.1% (Table 2). This amounts to 20.3 and 18.9% of total solid content, respectively. Lignin degraded phenolics are inhibitors to the downstream fermentation process. It should be removed from the PHL before fermentation process. But this lignin can be the starting material for high value-added applications in renewable polymeric materials development (Satheesh et al., 2009). The value added applications of lignin not only helps to boost the economic viability of the biorefinery but also serves as a source of renewable materials. 3.5. Sugars in the pre-extract The remaining parts of solid content apart from lignin and acetic acid were mono and oligo sugars. For paper pulp, complete removal of hemicelluloses is unnecessary because they play an important role in formation of sheets and facilitate fiber bonding in sheets. In the present investigation, purpose of pre-extraction was to remove fines and improve pulping efficiency of straw. Table 3 shows that a good extraction of oligo sugars with minimal conversion to mono sugars was occurred in an alkaline pre-extraction at 100 ◦ C for 60 min. The total extracted sugars in hot water extract was 15.6%, while that in alkaline extract was 10.2%, comprising 47% and 37% of the total solid content, respectively. The decrease in sugar yield in alkaline extraction can be interpreted as a result of alkaline hydrolysis of acetyl groups at higher pH levels, leading to lower solubility of oligo sugars. The C-5 sugar yield in hot water and alkaline extracts was 7.4% and 4.7%, respectively. Perez et al. (2007) achieved 53% xylose yield from wheat straw with hot water pretreatment at 200 ◦ C for 0 min. Sreenath et al. (1999) obtained a liquid phase with a hemicellulose-derived sugars yield of 50% after treating alfalfa fibers at 220 ◦ C for 2 min. Both the extraction processes yielded a considerable amount of glucose (Table 3), which may be contributed by hydrolysis of starch or hydrolysis of part of amorphous cellulose (Dietrichs, 1964). The xylan content in the hot water extract was lower than that of alkaline extract (3.6% vs 2.7%). The galactose (oligo) content in pre-extract liquor was 4.2% and 2.5% for hot water and alkaline extraction. These hemicelluloses may be used as wet end additives in papermaking (Lima et al., 2003). 3.6. Soda-AQ pulping Table 4 shows the effect of pre-extraction on the soda-AQ pulping of rice straw. It is seen that the pre-extraction of rice straw resulted lower kappa number of pulp than the non pre-extraction rice straw with considerable lowering of pulp yield. Similar results were observed for hot water pre-extraction of bagasse followed by alkaline pulping (Lei et al., 2010). Hemicelluloses compared to cellulose have high accessibility to acidic or alkaline hydrolysis. This is due to their amorphous structure and much lower Table 3 Sugars in the pre-extracted liquor. Sugar (% on od rice straw)
Hot H2 O Pre-extraction
Alkaline Pre-extraction
Mono-
Mono-
Oligo-
Oligo-
Rhamnose Arabinose Galactose Glucose Xylose Mannose
0 0 0 0 0.966 0
0.966 2.741 4.242 3.241 3.612 0.791
0 0 0 0 0 0
0.271 1.753 2.517 2.532 2.688 0.44
Total sugars (%)
0.966
15.595
0
10.199
M. Sarwar Jahan et al. / Industrial Crops and Products 37 (2012) 164–169
Fig. 1. Effect of alkali charge and pre-extraction on the tensile index of pulp.
degree of polymerization (100–200). During the pre-hydrolysis, some hemicelluloses and lignin were removed and delignification was thus improved. The extracted straw had a more open structure due to the hemicelluloses and lignin removal. After pre-hydrolysis, lignin migrated to the surface of straw. Therefore the delignification was faster for pulping of extracted straw. So pulping of extracted straw yielded a lower kappa number even though with lower alkali charge. The total pulp yield and kappa number decreased with increasing alkali charge. Alkaline pre-extracted straw demonstrated improved pulp yield and kappa number compared with the hot H2 O pre-extracted. At 12% alkali charge, alkaline pre-extracted rice straw gave 2.7% point higher pulp yield, which was 9.8% unit lower than the non pre-extracted rice straw. The initial pulp drainage resistance is an important parameter for pulp process unit operations, for example, pulp (brownstock) washing efficiency. The initial drainage resistance (measured as Schopper–Riegler beating degree) of pre-extracted pulp was 24–26 ◦ SR while it was 28–31 ◦ SR for non-extracted pulp (Table 4). This decrease of drainage resistance certainly improves pulp washing efficiency. High specific area due to high amount of fines caused the poor drainability of the non-extracted pulp (Jahan et al., 2009b). Therefore, it is evident that the pre-extraction can effectively dissolve the fines producing sugars in the pre-extracted liquor.
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Fig. 2. Effect of alkali charge and pre-extraction on the burst index of pulp.
cooking conditions. At 16% alkali charge, the tensile index of alkaline pre-hydrolyzed pulp was very close to non-hydrolyzed pulp (55.5 N m/g vs 57.6 N m/g), but tensile index of hot water prehydrolyzed was 32% lower than the non-hydrolyzed pulp. Similar results were observed for burst index (Fig. 2). This may be explained by the loss of hemicelluloses during pre-extraction. Due to hemicelluloses extraction, pulp had high cellulose/hemicelluloses ratios than the non-extracted pulp. Several studies have emphasized the importance of hemicelluloses for the strength properties of pulp fibers. Molin and Teder (2002) showed that fibers with high cellulose/hemicelluloses ratio had lower tensile stiffness and tensile index. Schönberg et al. (2001) showed that the tensile index increased after sorption of xylan to the fibers, which facilitate fiber bonding. The role of xylan in the fibers was further investigated. The chemical sorption of xylan was found to significantly increase the Scott Bond-value, further supporting the significance of xylan on
3.7. Physical properties All the pulps were beaten to SR value 30 and strength properties plotted against alkali charge in Figs. 1–3. The tensile index of hot water pre-extracted rice straw pulp increased when alkali charge increased from 12 to 14%, thereafter it decreased, for alkaline pre-extracted pulp tensile index decreased with alkali charge higher than 16%. The tensile index of non-extracted pulp increased with increasing alkali charge. These results indicated that the pre-extracted pulps reached to the residual stage of delignification at lower alkali charge than the non-extracted pulp employed
Fig. 3. Effect of alkali charge and pre-extraction on the tear index of pulp.
Table 4 Effect of pre-extraction and alkali charge on the pulping of rice straw. Alkali charge (%)
Hot H2 O Pre-extraction
→
12
14
16
18
12
14
16
18
12
14
16
18
Total pulp yield%) Kappa number pH of black liquor Solid content in BL Initial ◦ SR
44.0 10.5 11.1 8.85 25
43.5 9.5 11.3 9.9 26
39.8 8.3 11.8 9.9 24
35.5 7.8 12.1 11.1 25
46.7 11.3 12.0 8.7 26
44.2 10.7 12.5 9.3 25
42.3 9.6 12.9 9.9 26
38.2 8.7 13.3 10.5 25
56.5 12.5 11.5 8.9 28
54.2 11.3 11.9 9.6 29
51.9 9.8 12.3 10.9 31
50.4 8.9 12.8 11.1 30
Alkaline Pre-extraction
Without Pre-extraction
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Table 5 Effect of pre-extraction on the bleaching of rice straw pulp.
Bleached yield (%) Overall yield (%) ◦ SR after 1000 PFI rev Tensile index (N m/g) Tear index (mN m2 /g) Burst index (kPa m2 /g) Brightness (%) Opacity (%)
Hot H2 O Pre-extraction
Alkaline Pre-extraction
Without Pre-extraction
95.1 42.0 33
95.4 44.6 33
86.2 46.7 39
36.1
40.2
41.7
5.8
6.2
5.7
2.9
3.1
3.2
85.3 88.1
85.9 88.2
86.1 89.0
the bonding ability (Schönberg et al., 2001). Addition of 1% (w/w) of hemicelluloses to cellulosic pulp was able to increase about 30% of the mechanical properties (Lima et al., 2003). Wafa Al-Dajani and Tschirner (2008) observed that alkaline pre-extraction of aspen prior to pulping reduced 10% tensile index. The negative effect of pre-extraction on the tensile strength of kraft pulp of loblolly pine was also observed by Yoon and Van Heiningen, 2008. The tear index of pre-hydrolyzed pulp was better than that of non-hydrolyzed pulp (Fig. 3). Alkaline pre-hydrolyzed pulp retained its superiority over hot water pre-hydrolyzed pulp. At 16% alkali charge, alkaline pre-hydrolyzed pulp had 7% and 23% higher tear index compared with the water-pre-hydrolyzed and non-hydrolyzed pulps, respectively. 3.8. Bleaching Table 5 shows the bleaching results of pre-extracted and non pre-extracted rice straw. The bleaching yield of the untreated rice straw was much lower than that of extracted rice straw (86% vs 95%) due to higher hemicelluloses contents. After bleaching, overall pulp on original straw basis yield differences from pre-extracted and non pre-extracted rice straw became narrower. But still there were 2.1% and 4.7% lower pulp yield for alkaline pre-extracted and water preextracted rice straw (based on od rice straw). It is clearly seen from Table 5 that all pulps can be easily bleached to 86% brightness by D0 Ep D1 sequences with total ClO2 charge 15 kg/ton). The lower pulp drainage resistance (◦ SR) is better for papermaking. During refining, pulp drainage resistance (◦ SR) falls; but it unavoidable to increase the surface area of the pulp fibers to enhance fiber bonding during papermaking. After 1000 PFI revolution, SR value reached to 33 for pre-extracted pulp and while 39 for non pre-extracted pulp. The main reason in the latter case was due enhanced hydration with higher amount of hemicelluloses. The tensile index of alkaline pre-extracted and non pre-extracted pulps was very close (40 N m/g vs 42 N m/g) but hot water pre-extracted pulp was inferior. This can be explained by acidic condition of prehydrolysis damage the fiber to some extent. The pH value of hot water pre-extracted liquor was 4.1, while it was 8.1 for alkaline preextracted liquor. The tear index value of alkaline pre-hydrolyzed pulp was slightly better than the hot water pre-extracted and non pre-extracted pulp. 4. Conclusions Pre-extraction prior to pulping dissolved 28–34% biomass from rice straw. The dissolved components content hemicelluloses (10–17%), lignin (5–7%) and acetic acid (6–9%). Hot water pre-extraction showed drastic conditions (pH 4.1), consequently removed more biomass compared with alkaline pre-extraction. Pulp produced from pre-extracted rice straw improved drainage
resistance (◦ SR). Pulp yield from the pre-extracted rice straw was inferior but delignification was better compared with nonextracted rice straw. The pre-extraction affected tensile and burst index of the pulp produced because of removal of higher amount of hemicelluloses. Improved tear index was observed for the preextracted pulp, which indicated that pre-extraction did not degrade cellulose. Rice straw after alkaline pre-extraction showed better pulp yield and strength properties compared with the pulp from hot-water pre-extraction. Final brightness of the pulp was almost similar in both pre-extracted and non-extracted rice straw. The difference in pulp yield and strength properties was narrower on bleaching. The precise optimization of pulping of pre-extracted rice straw can provide similar pulp yield physical properties to nonextracted rice straw. So study on the delignification kinetics of pre-extracted rice straw is needed.
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