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Potassium hydroxide pulping of rice straw in biorefinery initiatives
Bioresource Technology 219 (2016) 445–450
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Potassium hydroxide pulping of rice straw in biorefinery initiatives M. Sarwar Jahan a,⇑, Fahmida Haris a,b, M. Mostafizur Rahman a, Purabi Rani Samaddar b, Shrikanta Sutradhar a a b
Pulp and Paper Research Division, BCSIR Laboratories, Dhaka, Dr. Qudrat-i-Khuda Road, Dhaka 1205, Bangladesh Department of Chemistry, Eden Girls College, Dhaka, Bangladesh
h i g h l i g h t s KOH pulping of rice straw and wheat straw were carried out. Pulp was bleached and papermaking properties were evaluated. Pulp yield and bleachability were better than the corresponding soda pulp. Silica and lignin were separated from the black liquor of KOH black liquor. Silica, lignin and hemicelluloses separated waste water can be used in irrigation.
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Article history: Received 11 June 2016 Received in revised form 2 August 2016 Accepted 3 August 2016 Available online 5 August 2016 Keywords: Potassium hydroxide Pulp yield Kappa number Papermaking properties Bleaching Silica Lignin
a b s t r a c t Rice straw is supposed to be one of the most important lignocellulosic raw materials for pulp mill in Asian countries. The major problem in rice straw pulping is silica. The present research is focused on the separation of silica from the black liquor of rice straw pulping by potassium hydroxide (KOH) and pulp evaluation. Optimum KOH pulping conditions of rice straw were alkali charge 12% as NaOH, cooking temperature 150 °C for 2 h and material to liquor ratio, 1:6. At this condition pulp yield was 42.4% with kappa number 10.3. KOH pulp bleached to 85% brightness by D0EpD1 bleaching sequences with ClO2 consumption of 25 kg/ton of pulp. Silica and lignin were separated from the black liquor of KOH pulping. The amount of recovered silica, lignin and hemicelluloses were 10.4%, 8.4% and 13.0%. The papermaking properties of KOH pulp from rice straw were slightly better than those of corresponding NaOH pulp. Ó 2016 Elsevier Ltd. All rights reserved.
1. Introduction Bangladesh is very small country with high population density. But GDP growth rate in Bangladesh is above 6 since last 10 years except 2009–10, consequently living standard of people is increasing. Global production of paper and paperboard in 2014 was 400.2 million tones (FAO, 2016). The per capita paper and board consumption in Bangladesh is about 3.5–4 kg, which is much lower than the advanced countries (300 kg/capita), the world average (ffi50 kg/capita) and the Asian average ((ffi50 kg/capita) (Quader, 2011). To reach the paper and board consumption to Asia level, our consumption will increase to 10 times of current consumption. So Bangladesh needs alternative fibrous raw materials as forest
⇑ Corresponding author. E-mail address: [email protected] (M.S. Jahan). http://dx.doi.org/10.1016/j.biortech.2016.08.008 0960-8524/Ó 2016 Elsevier Ltd. All rights reserved.
resources is very limited. Rice straw and wheat straw are the most abundant agriculture residues those are produced in large quantity in Asian countries. Non-wood materials, particularly wheat straw, rice straw, bagasse, bamboo are exploited as the main raw material for papermaking in China and India. The main problem that hinders the intensive use of non-wood raw materials in papermaking industry is the environmental pollution caused by black liquor. The major agricultural waste in Bangladesh is rice straw, which produced around 34.4 million tons of rice in FY 2013-14 (FAO, 2016). Every kilogram of grain harvested is accompanied by production of 1–1.5 kg of the straw (Maiorella, 1985). As a result 34–52 4 million tons of rice straw is produced in Bangladesh. Like many other lignocellulosic biomass, rice straw is mainly composed of cellulose (32–47% by dry weight), hemicellulose (19–27%), and lignin (15–24%) (Binod et al., 2010). The abundance of carbohydrates in straw renders it as a potential source for biofuel and biochemicals in addition to pulp production. Silica is a problem
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specific to straw pulp production when using the soda or kraft process. In the pulp production from straw, the recovery of chemicals and energy from the spent cooking liquor is very much difficult due to the presence of silica in the liquor, which causes fouling of heat transfer surfaces and high viscosity when the liquor is evaporated. By lowering the liquor pH, the silica can be precipitated and separated. To overcome these problems of straw pulping many new technologies have been studied. In the organic acid pulping, the major portion of silica remains on the fiber, therefore, spent liquor recovery problem would be solved (Jahan et al., 2006; Seisto and Poppius, 1997). Silica retained on the pulp fiber can be dissolved by alkaline extraction; subsequently silica can be separated from the alkaline extracted liquor by reducing pH to 7 and used for industrial purpose (Jahan et al., 2015). Huang et al. (2006, 2007a, b) explored wheat straw pulping without generating black liquor waste. The cooking liquor was aqueous ammonia mixed with a small amount of potassium hydroxide (KOH), which enriched the black liquor with nutrients such as potassium and nitrogen as a fertilizer. Rodríguez et al. (2008) studied rice straw pulping in different process and obtained highest pulp yield in KOH process. Pulping of corn stover using KOH and NH4OH was investigated by Sun et al. (2012). The combined alkaline system could effectively remove lignin during pulping. Approximately 90% delignification was achieved at the temperature of 150 °C for over 30 min. But those studies did not mention alkalinity of the black liquor. The liquor with pH above 8 cannot be used in soil. In this paper new technology has been proposed in biorefinery concept. During alkaline pulping process lignin, part of hemicellulose and silica were dissolved. Silica can be precipitated from the alkaline liquor by reducing pH to 7 (Jahan et al., 2015; Minu et al., 2012). Ultrafiltration (UF) and nanofiltration can separate and recover lignin and hemicellulosic sugars from the black liquor (Ahsan et al., 2014; Toledano et al., 2010). After the separation of silica and dissolved biomass, potassium rich liquor can be used in irrigation purposes. The main aim of this work was to produce pulp from rice straw and wheat straw by KOH process, which comprises i) characterization of rice straw and wheat straw, ii) pulping of rice straw and wheat straw by KOH and soda (NaOH) processes with varying alkali charge, iii) bleaching of the produced pulps D0EpD1 bleaching sequences, iv) evaluation of papermaking properties, v) characterization of spent liquor and recovery of silica and lignin from the spent liquor by reducing pH, specifically.
2.3. Pulping Pulping of rice straw and wheat straw was carried out by KOH and NaOH process in a electrically heated 5 lit capacity digester. Active alkali charge was varied from 12, 14, and 16% on od raw materials. The following parameters were kept constant: i) Liquor to fiber ratio: 6:1, ii) Temperature: 150 °C, iii) Cooking time: 120 min at 150 °C. After desire time of cooking, pulp was filtered and black liquor was collected for subsequent experiment. Pulp was washed with tap water till the removal of all chemicals. The yield of the pulp was determined gravimetrically from the ovendried weight of raw material. The kappa number of the resulting pulp was determined in accordance with Tappi Test Methods (T 236 om-99). 2.4. Evaluation of pulps The produced pulps at the optimum conditions from rice straw and wheat straw were beaten in a PFI mill in different revolution and handsheets of about 60 g/m2 were made in a Rapid Kӧthen Sheet Making Machine. The papermaking properties were according to TAPPI Standard Test Methods. The sheets were tested for tensile (T 494 om-96), burst (T 403 om-97) and tear strength (T 414 om-98) according to TAPPI Standard Test Methods. 2.5. Silica and lignin separation from the black liquor The pH of the black liquor was reduced to pH 7 by adding 4N sulfuric acid or carbon dioxide purging with constant stirring by magnetic bar. After adjusting the pH to the desired value, the conical flask was kept undisturbed for settling of the flocs. After complete precipitation, the content in the flask was centrifuged. After silica separation, the pH of the black liquor was again reduced to pH 2 by 4N sulfuric acid to precipitate lignin. The silica and lignin precipitate were then air-dried overnight followed by oven drying at 105 °C overnight to obtain constant weight. The solid content and ash of silica and lignin free black liquor were measured by TAPPI Standard Test Methods. 2.6. D0EpD1 bleaching
Rice and wheat straw were collected and cut to 2–3 cm in length. After determination of the moisture content of air dried raw materials equivalent to 300 g o.d. (oven dried) was weighed separately in a polyethylene bag for subsequent cooking experiments.
Pulps were bleached by D0EpD1 bleaching sequences (where D represents chlorine dioxide and Ep represents peroxide reinforced alkaline extraction). In the first stage (D0) of D0EpD1 bleaching sequences ClO2 was 2%. The temperature was 70 °C in Do stage for 45 min. Pulp consistency was 10%. The pH was adjusted to 2.5 by adding dilute H2SO4. In the alkaline extraction stage, temperature was 70 °C for 60 min in a water solution of 2% NaOH and 0.5% H2O2 (on od pulp). Pulp consistency was 10%. The end pH in the D1 stage was adjusted to 4 adding dilute NaOH. The ClO2 charge in the D1 was 0.5%. The brightness (T525 om 92), for tensile (T 494 om-96), burst (T 403 om-97) and tear strength (T 414 om-98) were determined in accordance with Tappi Test Methods.
2.2. Chemical analysis
3. Results and discussion
The chemical compositions of rice straw and wheat straw were carried out by following Tappi Test Methods: the extractive (T204 om88), water solubility (T207 cm99), Klason lignin (T211 om83). The Holocellulose was prepared by treating extractive free bagasse meal with NaClO2 solution (Browining, 1967). The pH of the solution was maintained at 4 by adding CH3COOH-CH3COONa buffer and a-cellulose was determined by treating holocellulose with 17.5% NaOH (T203 om 93).
3.1. Characteristics of rice straw and wheat straw
2. Materials and methods 2.1. Raw materials
Table 1 shows chemical characteristics of rice straw and wheat straw and compared with previously reported data. The content in acetone extracts of rice straw and wheat straw is lower than other agricultural residues and alternative raw materials (Jiménez et al., 1990). Some substances including resins, wax, fat and acetone extractables can precipitate upon pulping and leave stains in the
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50
Table 1 Chemical characteristics of rice straw and wheat straw.
Lignin (%) Pentosan (%) Extractive (%) Ash (%)
Rice straw (Rodríguez et al., 2008)
Wheat straw
Wheat straw (Tutus and Eroglu, 2003)
65.4 38.5 12.7 19.1 1.1 17.2
60.7 41.2 21.9 – 0.56 9.2
65.6 40.1 15.0 21.9 0.9 9.7
64.1 33.7⁄ 17.1 23.6 5.4 8.0
45 Pulp yield (%)
Holocellulose (%)
a-Cellulose (%)
Rice straw
40
Rice straw (NaOH) Rice straw (KOH) wheat straw (NaOH)
35
Wheat straw (KOH) 30
resulting paper sheets. The low extracts of rice straw (1.1%) and wheat straw (0.9%) is suggestive of the presence of a low proportion of these compounds in the raw material. The ash content of rice straw is higher than wheat straw (17.2% vs 9.7%). As compare to other alternative agricultural wastes, these amounts are much higher (Jiménez et al., 1990; Rodríguez et al., 2008). Ash content in rice and wheat straw is higher than the previously studied by Jiménez et al. (1990). Such high ash content deriving from the silica content of rice straw and wheat straw can cause problems during recovery of the cooking liquor. This is why present study is focused on pulp and other valuable chemicals production without recovery system. The holocelluose content in rice straw and wheat straw was similar (65%), which was lower than bagasse (Jahan et al., 2009)an important non-wood resource for pulping in Bangladesh. The holocelluose content in wheat straw is lower than the previously reported result (Jiménez et al., 1990). The a-cellulose content in rice straw and wheat straw was 38.5% and 40.1%, respectively. A higher proportion of alfacellulose content in wheat straw can be expected to higher pulp yield with good mechanical properties. This a-cellulose content was lower than those bagasse and hardwood (Jahan et al., 2009, 2011). These results were close to previously studied data on rice straw and wheat straw (Jiménez et al., 1990; Rodríguez et al., 2008). Lignin is undesirable polymer in pulp production and lignin removal requires energy and chemicals. Lower lignin content of raw materials makes them suitable for delignification at milder pulping conditions. As shown in Table 1, lignin content in rice straw and wheat straw was 12.7% and 15.0%. This lignin content of rice straw and wheat straw was lower than the previously reported result (Tutus and Eroglu, 2003; Rodríguez et al., 2008). This difference can be explained by ash content in lignin, which was not corrected in those experiments. 3.2. Pulping Rice straw and wheat straw pulping was carried out by potassium hydroxide (KOH) and compared with the results of pulping by sodium hydroxide (NaOH) (Table 2). It was found that only 12% alkali charge (as NaOH) at 150 °C for 120 min is enough to get bleachable pulp for both raw materials. For rice straw, kappa
5
7
9
11
13
15
17
19
Kappa number Fig. 1. Pulp yield and kappa relationship of rice straw and wheat straw pulps.
number decreased from 10.3 to 6.2 with increasing KOH charge from 12% to 14%. Further increase of KOH charge did not decrease kappa number. Similar change was observed for NaOH charge. At 14% alkali charge, wheat straw produced similar pulp yield with kappa number 15.3 for NaOH and 14.8 for KOH. As compared to wheat straw, rice straw was easier to dignify due to the lower lignin (Table 1). The pulp yield from agricultural wastes is generally lower than for wood fibres (50–60%). The pulp yield in this study is shown in Fig. 1 as a function of kappa number. As shown in Fig. 1, pulp yield of KOH process was higher at any kappa number. At kappa number 6.5, pulp yield in KOH process was 0.5% higher than the NaOH process for rice straw, while at kappa number 15, pulp yield in KOH process was 0.6% higher than the NaOH process for wheat straw. Rodríguez et al. (2008) studied rice straw pulping in different process and obtained highest pulp yield in KOH process. Pulp yield from wheat straw was higher than rice straw due to higher a-cellulose content (Table 1). 3.3. Bleaching and papermaking properties Table 3 shows the papermaking properties of bleached and unbleached pulps produced from rice straw and wheat straw by NaOH and KOH pulping at °SR 40 (by extrapolation). Figs. 2–4 show the tensile, burst and tear indexes of rice straw and wheat straw pulps by NaOH and KOH process with beating degree. It is seen that there was no significant difference of papermaking properties between NaOH and KOH pulping. Pulp from NaOH process showed slightly better tensile and burst indexes at the higher °SR value in both raw materials. Wheat straw pulp showed better tensile, burst and tear indexes than rice straw pulp (tensile index: 70–71 N.m/g vs 40–42 N.m/g; burst index: 6 kPa.m2/g vs 3 kPa.m2/g; tear index: 9–10 mN.m2/g vs 9 mN.m2/g). Bleaching reduced tensile index about 7% for rice straw pulp and 11% for wheat straw pulp. Tear index value of rice straw pulp in KOH pulping decreased from 8.9 mN.m2/g to 6.8 mN.m2/g on bleaching, while for wheat straw pulp it decreased from 9.3 mN.m2/g to 7.0 mN.m2/g.
Table 2 Pulping of rice straw and wheat straw. Raw material
NaOH charge (% as NaOH)
Pulp yield (%)
Kappa number
KOH charge (% as NaOH)
Pulp yield (%)
Kappa number
Rice straw
12 14 16
41.1 40.2 39.7
10.8 6.3 6.1
12.8 14.2 15.6
42.4 40.7 36.1
10.3 6.2 6.1
Wheat straw
12 14 16
48.2 45.6 44.9
16.9 15.3 14.9
12.8 14.2 15.6
48.0 45.5 44.0
16.1 14.8 12.0
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Table 3 Papermaking properties of rice straw and wheat straw pulp at °SR 40. Burst index (kPa.m2/g)
Tensile index (N.m/g)
Tear index (mN.m2/g)
Raw material
Process
Unbleached
Bleached
Unbleached
Bleached
Unbleached
Bleached
Rice straw
NaOH KOH
41.8 40.0
37.0 38.4
3.2 3.3
3.0 2.9
8.7 8.9
6.9 6.8
Wheat straw
NaOH KOH
71.4 69.8
62.8 62.0
6.4 6.2
5.2 5.3
9.5 9.3
7.3 7.0
Tensile index (N.m/g)
80
Table 4 Brightness and viscosity of rice straw and wheat straw pulp.
Rice straw (NaOH) Rice straw (KOH) Wheat straw (NaOH) Wheat straw (KOH)
70 60
Raw material
Rice straw
50
Wheat straw
40
Process
NaOH KOH NaOH KOH
Brightness (%)
Viscosity (mPa.s)
Unbleached
Bleached
Unbleached
Bleached
36.5 35.7 39.1 40.0
80.3 81.0 83.3 84.0
18.9 16.6 24.3 24.4
16.4 15.0 20.5 20.0
30 20
10
20
30
40
50
60
Drainage resistance (oSR)
of rice straw pulp. With the consumption of 25 kg ClO2/ton of pulp, brightness reached to 83–84% for wheat straw and 80–81% for rice straw pulp. There was no significant difference of bleachability on NaOH and KOH pulping.
Fig. 2. Tensile index and °SR relationship of rice straw and wheat straw pulps.
3.4. Black liquor
Burst index (kPa.m2/g)
7 6 Rice straw (NaOH) Rice straw (KOH) Wheat straw (NaOH) Wheat straw (KOH)
5 4 3 2
20
30
40
50
60
Drainage resistance (oSR) Fig. 3. Burst index and °SR relationship of rice straw and wheat straw pulps.
Tear index (mN.m2/g)
10
9
8
Rice straw (NaOH) Rice straw (KOH) Wheat straw (NaOH) Wheat straw (KOH)
7
6
20
30
40
50
60
Drainage resistance (oSR) Fig. 4. Tear index and °SR relationship of rice straw and wheat straw pulps.
Unbleached brightness is an indication of easier bleachability of pulp. As shown in Table 4, unbleached pulp brightness of wheat straw pulp was better than rice straw pulp, consequently final brightness of bleached wheat straw pulp was 3% higher than that
Chemical recovery is an important step in chemical pulping process, which makes the process economically and environmentally friendly. The chemical recovery process is based on black liquor evaporation-incineration-lixiviation-causticization operations; but it is difficult to apply this process in pulp mills with straw as the raw material. Black liquor of straw pulping has the following disadvantages: (1) less total solid content than the wood; (2) higher silica concentration resulting in scaling; (3) lower heat value. Therefore, many efforts have been made on black liquor treatment. During pulping process silica and part of biomass are dissolved, those can be exploited into biobased products. Since silica and ash content in rice straw was much higher than wheat straw, silica separation experiment was carried out only for rice straw in this study. Silica was separated from the black liquor of rice straw pulping by reducing pH to 7 through dilute sulfuric acid (H2SO4) addition and carbon dioxide bubbling. Silica separated from the black liquor of KOH pulping was 10.4% by H2SO4 and 8.6% by CO2 process based of starting raw material (Table 5). In the NaOH pulping slightly lower silica was separated from the black liquor. Similarly Minu et al. (2012) observed that decreasing the pH value of the black liquor from pH 12 to 13, there was no substantial precipitation up to pH 8. As the pH value reached to pH 7–8, a gradual increase in the weight of the precipitate. Pekarovic et al. (2005) studied on silica-leaching from wheat straw by sodium carbonate impregnation. With controlled time and temperature desilication degree can reach 70–75%. Pekarovic et al. (2006) also studied on wheat straw and rice straw desilication and found lower desilication rate of rice straw (32.7–51.0%) as compared to wheat straw (90.8%). Lignin can be the starting material for high value-added applications in renewable polymeric materials development (Wang et al., 2010). Lignin was separated from the silica separated black liquor by reducing pH to 2–3 through H2SO4. The lignin yield was 7.9% and 7.5% in the NaOH pulping and 8.4% and 8.2 in KOH pulping for the H2SO4 and CO2 addition, respectively. The separated lignin was more than 50% of original lignin. In addition to lignin black liquor contains considerable amounts of sugars in mono and oligomeric form. If we want use this sugars in fermentation
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M.S. Jahan et al. / Bioresource Technology 219 (2016) 445–450 Table 5 Recovery of silica and dissolved biomass from the black liquor of rice straw pulping. Silica (%)
Lignin (%)
Silica & lignin tree liquor (%)
A
B
Solid content C
Ash D
Pulp yield (%)
Recovered organics plus silica
Organics O=CD
E
A+B+O+E
NaOH
H2SO4 CO2
8.7 7.0
7.9 7.5
29.3 29.9
11.6 11.9
17.7 18.0
41.1 41.1
75.3 73.6
KOH
H2SO4 CO2
10.4 8.6
8.4 8.2
26.8 29.6
13.8 12.4
13.0 17.2
42.4 42.4
74.2 76.4
Fig. 5. Flow diagram of KOH pulping of rice straw in biorefinery concept.
industry, lignin degraded phenolics need to be removed in the downstream fermentation process. As shown in Table 5, solid content and ash content in the silica and lignin free black liquor were 27–30% and 12–14%, respectively. Ash in the silica and lignin free black liquor presented inorganics, which includes NaOH and KOH used in the pulping. Certainly, subtraction of ash content from solid content represented organics dissolved in the pulping process, which represented mostly hemicellulosic sugars and degraded cellulose. The sugars in the biorefinery process can be enzymatically and chemically transformed into chemicals and fuel (Chundawat et al., 2011). 3.5. Proposed biorefinery concept in KOH pulping of rice straw A biorefinery initiative on KOH pulping for paper grade pulp has been proposed in Fig. 5. During pulping process lignin and part of hemicelluloses are dissolved. Pulp yield after KOH pulping was 42.4%. The silica present in the rice straw was dissolved in the black liquor that was precipitated by reducing pH to 7 by adding dilute sulfuric acid. The amount of recovered silica, lignin and hemicelluloses were 10.4%, 8.4% and 13.0%. The lignin and hemicellulosic sugars from the black liquor can separate by ultrafiltration (UF) and nanofiltration. For example, the separation of different lignin fractions with specific molecular weights from the black liquor was studied based on the UF membrane technology (Toledano et al., 2010). Nanofiltration (NF) has been used to separate and concentrate saccharides in the fermentation broth (Goulas et al., 2002; Qi et al., 2011; Morao et al., 2006). NF concentrated sugars from pre-hydrolysis liquor of a hardwood kraft-based dissolving pulp production process (Ahsan et al., 2014). In our proposed concept of KOH based biorefinery process, lignin can be removed from the silica separated black liquor by UF and hemicellulosic sugars can be concentrated by nanofiltration (Ahsan et al., 2014), which is fermented by Pitichia stipitis for ethanol production. The hemicelluloses can also be used for furfural production (Liu et al., 2013, 2014). The studies on lignin and hemicelluloses separation from the silica separated black liquor and potassium rich waste water can
be used in irrigation purposes as shown in Fig. 5. The result obtained in this study can be utilized with other non-wood raw materials. 4. Conclusions In this study potassium hydroxide pulping of rice straw and wheat straw was carried out and dissolved biomass and silica was separated from to black liquor to produce biobased products. It was found that only 12% alkali charge (as NaOH) at 150 °C for 120 min is enough to get bleachable pulp for both raw materials. There was no significant difference of bleachability and papermaking properties on NaOH and KOH pulping. Silica, lignin and hemicelluloses were separated from the black liquor of KOH pulping of rice straw. The amount of recovered silica, lignin and hemicelluloses were 10.4%, 8.4% and 13.0%. Acknowledgement Authors wish to thanks Bangladesh Council of Scientific and Industrial Research for providing necessary fund to carry out this research. References Ahsan, L., Jahan, M.S., Ni, Y., 2014. Recovering/concentrating of hemicellulosic sugars and acetic acid by nanofiltration and reverse osmosis from prehydrolysis liquor of kraft based hardwood dissolving pulp process. Bioresource. Technol. 155, 111–115. Binod, P., Sindhu, R., Singhania, R.R., Vikram, S., Devi, L., Nagalakshmi, S., Satya, N., Kurien, Noble., Rajeev, K.S., Pandey, A., 2010. Bioethanol production from rice straw: an overview. Bioresour. Technol 101 (13), 4767–4774. Browining, B.L., 1967. Methods in Wood Chemistry. J. Wiley and Sons Interscience, New York. Goulas, A.K., Kapasakalidis, P.G., Sinclair, H.R., Rastall, R.A., Grandison, A.S., 2002. Purification of oligosaccharides by nanofiltration. J. Membr. Sci. 209, 321–335. FAOSTAT (2016) http://faostat3.fao.org/download/F/FO/E. Huang, G., Shi, J.X., Langrish, T.A., 2007a. NH4OH–KOH pulping mechanisms and kinetics of rice straw. Bioresour. Technol 98, 1218–1223. Huang, G., Shi, J.X., Langrish, T.A., 2007b. A new pulping process for wheat straw to reduce problems with the discharge of black liquor. Bioresour. Technol 98, 2829–2835.
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Potassium hydroxide pulping of rice straw in biorefinery initiatives M. Sarwar Jahan a,⇑, Fahmida Haris a,b, M. Mostafizur Rahman a, Purabi Rani Samaddar b, Shrikanta Sutradhar a a b
Pulp and Paper Research Division, BCSIR Laboratories, Dhaka, Dr. Qudrat-i-Khuda Road, Dhaka 1205, Bangladesh Department of Chemistry, Eden Girls College, Dhaka, Bangladesh
h i g h l i g h t s KOH pulping of rice straw and wheat straw were carried out. Pulp was bleached and papermaking properties were evaluated. Pulp yield and bleachability were better than the corresponding soda pulp. Silica and lignin were separated from the black liquor of KOH black liquor. Silica, lignin and hemicelluloses separated waste water can be used in irrigation.
a r t i c l e
i n f o
Article history: Received 11 June 2016 Received in revised form 2 August 2016 Accepted 3 August 2016 Available online 5 August 2016 Keywords: Potassium hydroxide Pulp yield Kappa number Papermaking properties Bleaching Silica Lignin
a b s t r a c t Rice straw is supposed to be one of the most important lignocellulosic raw materials for pulp mill in Asian countries. The major problem in rice straw pulping is silica. The present research is focused on the separation of silica from the black liquor of rice straw pulping by potassium hydroxide (KOH) and pulp evaluation. Optimum KOH pulping conditions of rice straw were alkali charge 12% as NaOH, cooking temperature 150 °C for 2 h and material to liquor ratio, 1:6. At this condition pulp yield was 42.4% with kappa number 10.3. KOH pulp bleached to 85% brightness by D0EpD1 bleaching sequences with ClO2 consumption of 25 kg/ton of pulp. Silica and lignin were separated from the black liquor of KOH pulping. The amount of recovered silica, lignin and hemicelluloses were 10.4%, 8.4% and 13.0%. The papermaking properties of KOH pulp from rice straw were slightly better than those of corresponding NaOH pulp. Ó 2016 Elsevier Ltd. All rights reserved.
1. Introduction Bangladesh is very small country with high population density. But GDP growth rate in Bangladesh is above 6 since last 10 years except 2009–10, consequently living standard of people is increasing. Global production of paper and paperboard in 2014 was 400.2 million tones (FAO, 2016). The per capita paper and board consumption in Bangladesh is about 3.5–4 kg, which is much lower than the advanced countries (300 kg/capita), the world average (ffi50 kg/capita) and the Asian average ((ffi50 kg/capita) (Quader, 2011). To reach the paper and board consumption to Asia level, our consumption will increase to 10 times of current consumption. So Bangladesh needs alternative fibrous raw materials as forest
⇑ Corresponding author. E-mail address: [email protected] (M.S. Jahan). http://dx.doi.org/10.1016/j.biortech.2016.08.008 0960-8524/Ó 2016 Elsevier Ltd. All rights reserved.
resources is very limited. Rice straw and wheat straw are the most abundant agriculture residues those are produced in large quantity in Asian countries. Non-wood materials, particularly wheat straw, rice straw, bagasse, bamboo are exploited as the main raw material for papermaking in China and India. The main problem that hinders the intensive use of non-wood raw materials in papermaking industry is the environmental pollution caused by black liquor. The major agricultural waste in Bangladesh is rice straw, which produced around 34.4 million tons of rice in FY 2013-14 (FAO, 2016). Every kilogram of grain harvested is accompanied by production of 1–1.5 kg of the straw (Maiorella, 1985). As a result 34–52 4 million tons of rice straw is produced in Bangladesh. Like many other lignocellulosic biomass, rice straw is mainly composed of cellulose (32–47% by dry weight), hemicellulose (19–27%), and lignin (15–24%) (Binod et al., 2010). The abundance of carbohydrates in straw renders it as a potential source for biofuel and biochemicals in addition to pulp production. Silica is a problem
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specific to straw pulp production when using the soda or kraft process. In the pulp production from straw, the recovery of chemicals and energy from the spent cooking liquor is very much difficult due to the presence of silica in the liquor, which causes fouling of heat transfer surfaces and high viscosity when the liquor is evaporated. By lowering the liquor pH, the silica can be precipitated and separated. To overcome these problems of straw pulping many new technologies have been studied. In the organic acid pulping, the major portion of silica remains on the fiber, therefore, spent liquor recovery problem would be solved (Jahan et al., 2006; Seisto and Poppius, 1997). Silica retained on the pulp fiber can be dissolved by alkaline extraction; subsequently silica can be separated from the alkaline extracted liquor by reducing pH to 7 and used for industrial purpose (Jahan et al., 2015). Huang et al. (2006, 2007a, b) explored wheat straw pulping without generating black liquor waste. The cooking liquor was aqueous ammonia mixed with a small amount of potassium hydroxide (KOH), which enriched the black liquor with nutrients such as potassium and nitrogen as a fertilizer. Rodríguez et al. (2008) studied rice straw pulping in different process and obtained highest pulp yield in KOH process. Pulping of corn stover using KOH and NH4OH was investigated by Sun et al. (2012). The combined alkaline system could effectively remove lignin during pulping. Approximately 90% delignification was achieved at the temperature of 150 °C for over 30 min. But those studies did not mention alkalinity of the black liquor. The liquor with pH above 8 cannot be used in soil. In this paper new technology has been proposed in biorefinery concept. During alkaline pulping process lignin, part of hemicellulose and silica were dissolved. Silica can be precipitated from the alkaline liquor by reducing pH to 7 (Jahan et al., 2015; Minu et al., 2012). Ultrafiltration (UF) and nanofiltration can separate and recover lignin and hemicellulosic sugars from the black liquor (Ahsan et al., 2014; Toledano et al., 2010). After the separation of silica and dissolved biomass, potassium rich liquor can be used in irrigation purposes. The main aim of this work was to produce pulp from rice straw and wheat straw by KOH process, which comprises i) characterization of rice straw and wheat straw, ii) pulping of rice straw and wheat straw by KOH and soda (NaOH) processes with varying alkali charge, iii) bleaching of the produced pulps D0EpD1 bleaching sequences, iv) evaluation of papermaking properties, v) characterization of spent liquor and recovery of silica and lignin from the spent liquor by reducing pH, specifically.
2.3. Pulping Pulping of rice straw and wheat straw was carried out by KOH and NaOH process in a electrically heated 5 lit capacity digester. Active alkali charge was varied from 12, 14, and 16% on od raw materials. The following parameters were kept constant: i) Liquor to fiber ratio: 6:1, ii) Temperature: 150 °C, iii) Cooking time: 120 min at 150 °C. After desire time of cooking, pulp was filtered and black liquor was collected for subsequent experiment. Pulp was washed with tap water till the removal of all chemicals. The yield of the pulp was determined gravimetrically from the ovendried weight of raw material. The kappa number of the resulting pulp was determined in accordance with Tappi Test Methods (T 236 om-99). 2.4. Evaluation of pulps The produced pulps at the optimum conditions from rice straw and wheat straw were beaten in a PFI mill in different revolution and handsheets of about 60 g/m2 were made in a Rapid Kӧthen Sheet Making Machine. The papermaking properties were according to TAPPI Standard Test Methods. The sheets were tested for tensile (T 494 om-96), burst (T 403 om-97) and tear strength (T 414 om-98) according to TAPPI Standard Test Methods. 2.5. Silica and lignin separation from the black liquor The pH of the black liquor was reduced to pH 7 by adding 4N sulfuric acid or carbon dioxide purging with constant stirring by magnetic bar. After adjusting the pH to the desired value, the conical flask was kept undisturbed for settling of the flocs. After complete precipitation, the content in the flask was centrifuged. After silica separation, the pH of the black liquor was again reduced to pH 2 by 4N sulfuric acid to precipitate lignin. The silica and lignin precipitate were then air-dried overnight followed by oven drying at 105 °C overnight to obtain constant weight. The solid content and ash of silica and lignin free black liquor were measured by TAPPI Standard Test Methods. 2.6. D0EpD1 bleaching
Rice and wheat straw were collected and cut to 2–3 cm in length. After determination of the moisture content of air dried raw materials equivalent to 300 g o.d. (oven dried) was weighed separately in a polyethylene bag for subsequent cooking experiments.
Pulps were bleached by D0EpD1 bleaching sequences (where D represents chlorine dioxide and Ep represents peroxide reinforced alkaline extraction). In the first stage (D0) of D0EpD1 bleaching sequences ClO2 was 2%. The temperature was 70 °C in Do stage for 45 min. Pulp consistency was 10%. The pH was adjusted to 2.5 by adding dilute H2SO4. In the alkaline extraction stage, temperature was 70 °C for 60 min in a water solution of 2% NaOH and 0.5% H2O2 (on od pulp). Pulp consistency was 10%. The end pH in the D1 stage was adjusted to 4 adding dilute NaOH. The ClO2 charge in the D1 was 0.5%. The brightness (T525 om 92), for tensile (T 494 om-96), burst (T 403 om-97) and tear strength (T 414 om-98) were determined in accordance with Tappi Test Methods.
2.2. Chemical analysis
3. Results and discussion
The chemical compositions of rice straw and wheat straw were carried out by following Tappi Test Methods: the extractive (T204 om88), water solubility (T207 cm99), Klason lignin (T211 om83). The Holocellulose was prepared by treating extractive free bagasse meal with NaClO2 solution (Browining, 1967). The pH of the solution was maintained at 4 by adding CH3COOH-CH3COONa buffer and a-cellulose was determined by treating holocellulose with 17.5% NaOH (T203 om 93).
3.1. Characteristics of rice straw and wheat straw
2. Materials and methods 2.1. Raw materials
Table 1 shows chemical characteristics of rice straw and wheat straw and compared with previously reported data. The content in acetone extracts of rice straw and wheat straw is lower than other agricultural residues and alternative raw materials (Jiménez et al., 1990). Some substances including resins, wax, fat and acetone extractables can precipitate upon pulping and leave stains in the
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50
Table 1 Chemical characteristics of rice straw and wheat straw.
Lignin (%) Pentosan (%) Extractive (%) Ash (%)
Rice straw (Rodríguez et al., 2008)
Wheat straw
Wheat straw (Tutus and Eroglu, 2003)
65.4 38.5 12.7 19.1 1.1 17.2
60.7 41.2 21.9 – 0.56 9.2
65.6 40.1 15.0 21.9 0.9 9.7
64.1 33.7⁄ 17.1 23.6 5.4 8.0
45 Pulp yield (%)
Holocellulose (%)
a-Cellulose (%)
Rice straw
40
Rice straw (NaOH) Rice straw (KOH) wheat straw (NaOH)
35
Wheat straw (KOH) 30
resulting paper sheets. The low extracts of rice straw (1.1%) and wheat straw (0.9%) is suggestive of the presence of a low proportion of these compounds in the raw material. The ash content of rice straw is higher than wheat straw (17.2% vs 9.7%). As compare to other alternative agricultural wastes, these amounts are much higher (Jiménez et al., 1990; Rodríguez et al., 2008). Ash content in rice and wheat straw is higher than the previously studied by Jiménez et al. (1990). Such high ash content deriving from the silica content of rice straw and wheat straw can cause problems during recovery of the cooking liquor. This is why present study is focused on pulp and other valuable chemicals production without recovery system. The holocelluose content in rice straw and wheat straw was similar (65%), which was lower than bagasse (Jahan et al., 2009)an important non-wood resource for pulping in Bangladesh. The holocelluose content in wheat straw is lower than the previously reported result (Jiménez et al., 1990). The a-cellulose content in rice straw and wheat straw was 38.5% and 40.1%, respectively. A higher proportion of alfacellulose content in wheat straw can be expected to higher pulp yield with good mechanical properties. This a-cellulose content was lower than those bagasse and hardwood (Jahan et al., 2009, 2011). These results were close to previously studied data on rice straw and wheat straw (Jiménez et al., 1990; Rodríguez et al., 2008). Lignin is undesirable polymer in pulp production and lignin removal requires energy and chemicals. Lower lignin content of raw materials makes them suitable for delignification at milder pulping conditions. As shown in Table 1, lignin content in rice straw and wheat straw was 12.7% and 15.0%. This lignin content of rice straw and wheat straw was lower than the previously reported result (Tutus and Eroglu, 2003; Rodríguez et al., 2008). This difference can be explained by ash content in lignin, which was not corrected in those experiments. 3.2. Pulping Rice straw and wheat straw pulping was carried out by potassium hydroxide (KOH) and compared with the results of pulping by sodium hydroxide (NaOH) (Table 2). It was found that only 12% alkali charge (as NaOH) at 150 °C for 120 min is enough to get bleachable pulp for both raw materials. For rice straw, kappa
5
7
9
11
13
15
17
19
Kappa number Fig. 1. Pulp yield and kappa relationship of rice straw and wheat straw pulps.
number decreased from 10.3 to 6.2 with increasing KOH charge from 12% to 14%. Further increase of KOH charge did not decrease kappa number. Similar change was observed for NaOH charge. At 14% alkali charge, wheat straw produced similar pulp yield with kappa number 15.3 for NaOH and 14.8 for KOH. As compared to wheat straw, rice straw was easier to dignify due to the lower lignin (Table 1). The pulp yield from agricultural wastes is generally lower than for wood fibres (50–60%). The pulp yield in this study is shown in Fig. 1 as a function of kappa number. As shown in Fig. 1, pulp yield of KOH process was higher at any kappa number. At kappa number 6.5, pulp yield in KOH process was 0.5% higher than the NaOH process for rice straw, while at kappa number 15, pulp yield in KOH process was 0.6% higher than the NaOH process for wheat straw. Rodríguez et al. (2008) studied rice straw pulping in different process and obtained highest pulp yield in KOH process. Pulp yield from wheat straw was higher than rice straw due to higher a-cellulose content (Table 1). 3.3. Bleaching and papermaking properties Table 3 shows the papermaking properties of bleached and unbleached pulps produced from rice straw and wheat straw by NaOH and KOH pulping at °SR 40 (by extrapolation). Figs. 2–4 show the tensile, burst and tear indexes of rice straw and wheat straw pulps by NaOH and KOH process with beating degree. It is seen that there was no significant difference of papermaking properties between NaOH and KOH pulping. Pulp from NaOH process showed slightly better tensile and burst indexes at the higher °SR value in both raw materials. Wheat straw pulp showed better tensile, burst and tear indexes than rice straw pulp (tensile index: 70–71 N.m/g vs 40–42 N.m/g; burst index: 6 kPa.m2/g vs 3 kPa.m2/g; tear index: 9–10 mN.m2/g vs 9 mN.m2/g). Bleaching reduced tensile index about 7% for rice straw pulp and 11% for wheat straw pulp. Tear index value of rice straw pulp in KOH pulping decreased from 8.9 mN.m2/g to 6.8 mN.m2/g on bleaching, while for wheat straw pulp it decreased from 9.3 mN.m2/g to 7.0 mN.m2/g.
Table 2 Pulping of rice straw and wheat straw. Raw material
NaOH charge (% as NaOH)
Pulp yield (%)
Kappa number
KOH charge (% as NaOH)
Pulp yield (%)
Kappa number
Rice straw
12 14 16
41.1 40.2 39.7
10.8 6.3 6.1
12.8 14.2 15.6
42.4 40.7 36.1
10.3 6.2 6.1
Wheat straw
12 14 16
48.2 45.6 44.9
16.9 15.3 14.9
12.8 14.2 15.6
48.0 45.5 44.0
16.1 14.8 12.0
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Table 3 Papermaking properties of rice straw and wheat straw pulp at °SR 40. Burst index (kPa.m2/g)
Tensile index (N.m/g)
Tear index (mN.m2/g)
Raw material
Process
Unbleached
Bleached
Unbleached
Bleached
Unbleached
Bleached
Rice straw
NaOH KOH
41.8 40.0
37.0 38.4
3.2 3.3
3.0 2.9
8.7 8.9
6.9 6.8
Wheat straw
NaOH KOH
71.4 69.8
62.8 62.0
6.4 6.2
5.2 5.3
9.5 9.3
7.3 7.0
Tensile index (N.m/g)
80
Table 4 Brightness and viscosity of rice straw and wheat straw pulp.
Rice straw (NaOH) Rice straw (KOH) Wheat straw (NaOH) Wheat straw (KOH)
70 60
Raw material
Rice straw
50
Wheat straw
40
Process
NaOH KOH NaOH KOH
Brightness (%)
Viscosity (mPa.s)
Unbleached
Bleached
Unbleached
Bleached
36.5 35.7 39.1 40.0
80.3 81.0 83.3 84.0
18.9 16.6 24.3 24.4
16.4 15.0 20.5 20.0
30 20
10
20
30
40
50
60
Drainage resistance (oSR)
of rice straw pulp. With the consumption of 25 kg ClO2/ton of pulp, brightness reached to 83–84% for wheat straw and 80–81% for rice straw pulp. There was no significant difference of bleachability on NaOH and KOH pulping.
Fig. 2. Tensile index and °SR relationship of rice straw and wheat straw pulps.
3.4. Black liquor
Burst index (kPa.m2/g)
7 6 Rice straw (NaOH) Rice straw (KOH) Wheat straw (NaOH) Wheat straw (KOH)
5 4 3 2
20
30
40
50
60
Drainage resistance (oSR) Fig. 3. Burst index and °SR relationship of rice straw and wheat straw pulps.
Tear index (mN.m2/g)
10
9
8
Rice straw (NaOH) Rice straw (KOH) Wheat straw (NaOH) Wheat straw (KOH)
7
6
20
30
40
50
60
Drainage resistance (oSR) Fig. 4. Tear index and °SR relationship of rice straw and wheat straw pulps.
Unbleached brightness is an indication of easier bleachability of pulp. As shown in Table 4, unbleached pulp brightness of wheat straw pulp was better than rice straw pulp, consequently final brightness of bleached wheat straw pulp was 3% higher than that
Chemical recovery is an important step in chemical pulping process, which makes the process economically and environmentally friendly. The chemical recovery process is based on black liquor evaporation-incineration-lixiviation-causticization operations; but it is difficult to apply this process in pulp mills with straw as the raw material. Black liquor of straw pulping has the following disadvantages: (1) less total solid content than the wood; (2) higher silica concentration resulting in scaling; (3) lower heat value. Therefore, many efforts have been made on black liquor treatment. During pulping process silica and part of biomass are dissolved, those can be exploited into biobased products. Since silica and ash content in rice straw was much higher than wheat straw, silica separation experiment was carried out only for rice straw in this study. Silica was separated from the black liquor of rice straw pulping by reducing pH to 7 through dilute sulfuric acid (H2SO4) addition and carbon dioxide bubbling. Silica separated from the black liquor of KOH pulping was 10.4% by H2SO4 and 8.6% by CO2 process based of starting raw material (Table 5). In the NaOH pulping slightly lower silica was separated from the black liquor. Similarly Minu et al. (2012) observed that decreasing the pH value of the black liquor from pH 12 to 13, there was no substantial precipitation up to pH 8. As the pH value reached to pH 7–8, a gradual increase in the weight of the precipitate. Pekarovic et al. (2005) studied on silica-leaching from wheat straw by sodium carbonate impregnation. With controlled time and temperature desilication degree can reach 70–75%. Pekarovic et al. (2006) also studied on wheat straw and rice straw desilication and found lower desilication rate of rice straw (32.7–51.0%) as compared to wheat straw (90.8%). Lignin can be the starting material for high value-added applications in renewable polymeric materials development (Wang et al., 2010). Lignin was separated from the silica separated black liquor by reducing pH to 2–3 through H2SO4. The lignin yield was 7.9% and 7.5% in the NaOH pulping and 8.4% and 8.2 in KOH pulping for the H2SO4 and CO2 addition, respectively. The separated lignin was more than 50% of original lignin. In addition to lignin black liquor contains considerable amounts of sugars in mono and oligomeric form. If we want use this sugars in fermentation
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M.S. Jahan et al. / Bioresource Technology 219 (2016) 445–450 Table 5 Recovery of silica and dissolved biomass from the black liquor of rice straw pulping. Silica (%)
Lignin (%)
Silica & lignin tree liquor (%)
A
B
Solid content C
Ash D
Pulp yield (%)
Recovered organics plus silica
Organics O=CD
E
A+B+O+E
NaOH
H2SO4 CO2
8.7 7.0
7.9 7.5
29.3 29.9
11.6 11.9
17.7 18.0
41.1 41.1
75.3 73.6
KOH
H2SO4 CO2
10.4 8.6
8.4 8.2
26.8 29.6
13.8 12.4
13.0 17.2
42.4 42.4
74.2 76.4
Fig. 5. Flow diagram of KOH pulping of rice straw in biorefinery concept.
industry, lignin degraded phenolics need to be removed in the downstream fermentation process. As shown in Table 5, solid content and ash content in the silica and lignin free black liquor were 27–30% and 12–14%, respectively. Ash in the silica and lignin free black liquor presented inorganics, which includes NaOH and KOH used in the pulping. Certainly, subtraction of ash content from solid content represented organics dissolved in the pulping process, which represented mostly hemicellulosic sugars and degraded cellulose. The sugars in the biorefinery process can be enzymatically and chemically transformed into chemicals and fuel (Chundawat et al., 2011). 3.5. Proposed biorefinery concept in KOH pulping of rice straw A biorefinery initiative on KOH pulping for paper grade pulp has been proposed in Fig. 5. During pulping process lignin and part of hemicelluloses are dissolved. Pulp yield after KOH pulping was 42.4%. The silica present in the rice straw was dissolved in the black liquor that was precipitated by reducing pH to 7 by adding dilute sulfuric acid. The amount of recovered silica, lignin and hemicelluloses were 10.4%, 8.4% and 13.0%. The lignin and hemicellulosic sugars from the black liquor can separate by ultrafiltration (UF) and nanofiltration. For example, the separation of different lignin fractions with specific molecular weights from the black liquor was studied based on the UF membrane technology (Toledano et al., 2010). Nanofiltration (NF) has been used to separate and concentrate saccharides in the fermentation broth (Goulas et al., 2002; Qi et al., 2011; Morao et al., 2006). NF concentrated sugars from pre-hydrolysis liquor of a hardwood kraft-based dissolving pulp production process (Ahsan et al., 2014). In our proposed concept of KOH based biorefinery process, lignin can be removed from the silica separated black liquor by UF and hemicellulosic sugars can be concentrated by nanofiltration (Ahsan et al., 2014), which is fermented by Pitichia stipitis for ethanol production. The hemicelluloses can also be used for furfural production (Liu et al., 2013, 2014). The studies on lignin and hemicelluloses separation from the silica separated black liquor and potassium rich waste water can
be used in irrigation purposes as shown in Fig. 5. The result obtained in this study can be utilized with other non-wood raw materials. 4. Conclusions In this study potassium hydroxide pulping of rice straw and wheat straw was carried out and dissolved biomass and silica was separated from to black liquor to produce biobased products. It was found that only 12% alkali charge (as NaOH) at 150 °C for 120 min is enough to get bleachable pulp for both raw materials. There was no significant difference of bleachability and papermaking properties on NaOH and KOH pulping. Silica, lignin and hemicelluloses were separated from the black liquor of KOH pulping of rice straw. The amount of recovered silica, lignin and hemicelluloses were 10.4%, 8.4% and 13.0%. Acknowledgement Authors wish to thanks Bangladesh Council of Scientific and Industrial Research for providing necessary fund to carry out this research. References Ahsan, L., Jahan, M.S., Ni, Y., 2014. Recovering/concentrating of hemicellulosic sugars and acetic acid by nanofiltration and reverse osmosis from prehydrolysis liquor of kraft based hardwood dissolving pulp process. Bioresource. Technol. 155, 111–115. Binod, P., Sindhu, R., Singhania, R.R., Vikram, S., Devi, L., Nagalakshmi, S., Satya, N., Kurien, Noble., Rajeev, K.S., Pandey, A., 2010. Bioethanol production from rice straw: an overview. Bioresour. Technol 101 (13), 4767–4774. Browining, B.L., 1967. Methods in Wood Chemistry. J. Wiley and Sons Interscience, New York. Goulas, A.K., Kapasakalidis, P.G., Sinclair, H.R., Rastall, R.A., Grandison, A.S., 2002. Purification of oligosaccharides by nanofiltration. J. Membr. Sci. 209, 321–335. FAOSTAT (2016) http://faostat3.fao.org/download/F/FO/E. Huang, G., Shi, J.X., Langrish, T.A., 2007a. NH4OH–KOH pulping mechanisms and kinetics of rice straw. Bioresour. Technol 98, 1218–1223. Huang, G., Shi, J.X., Langrish, T.A., 2007b. A new pulping process for wheat straw to reduce problems with the discharge of black liquor. Bioresour. Technol 98, 2829–2835.
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