Chapter 16
Integrated Forest Biorefinery − Sulfite Process Downloaded by STANFORD UNIV GREEN LIBR on June 15, 2012 | http://pubs.acs.org Publication Date (Web): July 11, 2011 | doi: 10.1021/bk-2011-1067.ch016
Pedram Fatehi and Yonghao Ni* Chemical Engineering Department and Limerick Pulp and Paper Centre, University of New Brunswick, Fredericton, New Brunswick, Canada E3B 5A3 *Email:
[email protected], Tel: 506-453-4547; Fax: 506-453-4767
The majority of lignin and a part of the hemicelluloses in wood chips are dissolved during the sulfite pulping process. The spent liquor of sulfite pulping process contains a significant amount of lignocellulosic materials, which can be converted to various value-added products. Such a process can fit into the forest biorefinery concept very well. In this chapter, the processes configurations of biorefinery commercially relevant to various sulfite pulping processes are discussed. Additionally, the production of various value-added products and the compatibility of these processes to the existing facilities and configuration of sulfite pulping processes are included.
Introduction Sulfite pulping process has been practiced for the pulp production for more than a century (1, 2). In this process, lignin and hemicelluloses are dissolved in the sulfite spent liquor (SSL) and removed from the pulp (solid residue). The SSL contains a significant amount of dissolved organics, which can be converted to several value-added products. The utilization of the lignocellulosic-rich SSL for the production of various value-added products has been practiced commercially. However, the research and development activities are still on-going due to the complexity involved in recovering and converting the dissolved lignocellulosic materials of the SSL to these desired products. Today, the sulfite pulping process is used for the production of paper- or dissolving-grade pulps. Approximately, 60% of dissolving pulps are produced via the sulfite pulping process (3). In the production of dissolving pulp, it is crucial to design various process steps so that the maximum efficiency in removing © 2011 American Chemical Society In Sustainable Production of Fuels, Chemicals, and Fibers from Forest Biomass; Zhu, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2011.
Downloaded by STANFORD UNIV GREEN LIBR on June 15, 2012 | http://pubs.acs.org Publication Date (Web): July 11, 2011 | doi: 10.1021/bk-2011-1067.ch016
the lignin and hemicelluloses are achieved (2). Similarly, the acidic environment of the sulfite pulping process for paper-grade pulp production results in a major dissolution of hemicelluloses and lignin during pulping. The hardwood SSL of paper-grade pulping contains 15-22% sugars (4, 5), and 50-65% lignosulfonate (based on the total solids) (6). Similarly, the chemical pretreatment of wood chips in the production of chemimechanical pulp, e.g. CTMP or semichemical pulp, such as NSSC, causes lignin and hemicelluloses dissolution in the spent liquor. Due to the more severe pulping conditions, the SSL from the dissolving pulpgrade sulfite pulping is richer in ligocellulosic materials than that from the papergrade sulfite pulping. These lignocellulosic materials in the SSLs can be employed in the production of various value-added products. In this case, instead of burning the organics for the energy production or treating them in the costly wastewater treatment system of mills, various value-added products can be extracted from the SSL. Alternatively, the sulfite treatment, together with mechanical treatment, can be the basis for a new process called the SPORL process (7–9). In such a process, ethanol is the major product, and pulp is no longer a product. In this chapter, the biorefinery units commercialized or proposed for various sulfite pulping processes are comprehensively described; and the process characteristics, commercial relevance and other aspects related to the forest biorefinery are discussed.
Biorefinery Possibilities for Dissolving Pulp- and Paper-Grade Sulfite Pulping Process Integrated Forest Biorefinery for Ammonium-Based Sulfite Pulping Process for Producing Dissolving Pulps The spent sulfite liquor (SSL) of the dissolving pulp production can be utilized to produce various value-added products. Figure 1 shows a commercialized integrated forest biorefinery unit for ammonium-based sulfite process for dissolving pulp production (10, 11). As can be seen, the main cellulosic product of this process is dissolving pulp. The SSL of this process is concentrated from 12% (wt.) to 22% prior to fermentation via evaporator-I. This evaporation not only increases the concentration of sugars for the fermentation, but also reduces the amount of potentially toxic volatile sulfur compound (SO2). The volatile sulphur compounds are carried off in the evaporators for subsequent sulphur recovery. Ethanol is produced in a continuous-flow cascade fermentor with a retention time of 24 h using S. cerevisiae. After the fermentation, the residual liquor is concentrated in Evaporator-II to 50% wt. A part of the concentrated solution can be marketed as liquid lignin, which can be used as animal feed, binders or highway dust control agent. Another portion can be spray-dried and marketed as lignosulfonates used for other value-added applications. The remainder is sent to the recovery process (11). Additionally, a part of effluent of the mills can be sent to the anaerobic effluent treatment system to generate biogas. The generated biogas is first scrubbed to purify methane, and then sent to the recovery boiler to supply a part 410 In Sustainable Production of Fuels, Chemicals, and Fibers from Forest Biomass; Zhu, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2011.
of the fuel requirement of the mill. The scrubber operating unit may include Claus type reactor, which will convert the produced H2S to sulphur, another by-product of the plant (10).
Downloaded by STANFORD UNIV GREEN LIBR on June 15, 2012 | http://pubs.acs.org Publication Date (Web): July 11, 2011 | doi: 10.1021/bk-2011-1067.ch016
Integrated Forest Biorefinery for Magnesium-Based Sulfite Pulping Process for Producing Paper-Grade Pulps The hemicelluloses are essential for providing a good hydrogen bonding capacity of the pulp, thus the strength for paper-sheets. Therefore, the sulfite pulping process for the production of paper-grade pulps is usually performed in such ways that a significant amount of hemicelluloses is still present in cellulose fibers. Magnefite or magnesium-based sulfite pulping process was designed for the production of paper-grade pulps.
Figure 1. Process diagram of a biorefinery unit based on ammonia-based sulfite dissolving pulp production process. (Reproduced with permission from reference (11). Copyright 1993 Springer).
411 In Sustainable Production of Fuels, Chemicals, and Fibers from Forest Biomass; Zhu, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2011.
Downloaded by STANFORD UNIV GREEN LIBR on June 15, 2012 | http://pubs.acs.org Publication Date (Web): July 11, 2011 | doi: 10.1021/bk-2011-1067.ch016
Figure 2. Stream 1: the magnesium-based sulfite process for the production of paper grade pulp, Stream 2: proposed process diagram for a magnesium-based biorefinery unit. (Reproduced with permission from reference (13). Copyright 1980 John Wiley & Sons). The SSL of magnesium-based sulfite pulping process can be used to produce value-added products. In the case of the hardwood SSL, the SSL contains a significant amount of xylan (50-70% of the total sugars in SSL), which could potentially be converted to ethanol, but the inhibitors should be eliminated prior to ethanol production (5, 12). Therefore, extra purification (detoxification) steps may be required. In the case of softwood SSL, the SSL may contain a significant amount of lignin (4). This lignin can be separated and sold as a product. The high amount of hexose sugars present in the SSL would facilitate the ethanol production via several hexose-fermenting yeasts used in industry. Figure 2 shows the magnesium-based sulfite process for the production of paper-grade pulp (Stream 1) (13), and proposed process diagram for a magnesium-based biorefinery unit (Stream 2). 412 In Sustainable Production of Fuels, Chemicals, and Fibers from Forest Biomass; Zhu, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2011.
Downloaded by STANFORD UNIV GREEN LIBR on June 15, 2012 | http://pubs.acs.org Publication Date (Web): July 11, 2011 | doi: 10.1021/bk-2011-1067.ch016
It is noted that the SSL is separated from the pulp and then Mg(OH)2 is added to the SSL (Stream 2), which is used in the existing configuration of magnesium-based sulfite pulping process. This liming process (via adding Mg(OH)2) would adjust the pH of the SSL for the downstream biorefinery stages to produce ethanol, for example. The precipitation of some organics from the pH adjustment process would also serve for decreasing the inhibitor concentration of the SSL. The precipitates are sent to the recovery boiler of the mill, in which the lignosulfonates supply a part of the heat/electricity requirement of the mill. A part of SSL may be sent via evaporation to the boiler in stream 1(open line in Figure 2). The subsequent stages of this biorefinery unit resemble the stages existing in the magnesium-based sulfite process. The treated SSL can be further utilized for value-added production.
Composition of the SSL Produced in Various Sulfite Pulping Processes SSL of Dissolving Pulp-Grade Process The compositions of the SSL produced from hardwood and softwood in the ammonia-based dissolving pulp production is listed in Table I. No arabinose was found in any of the SSL samples listed in Table I. Evidently, the SSL of hardwood contained a significant amount of xylose, whereas the SSL of softwood contained a considerable amount of mannose. This is a basic feature of the wood chemistry of the two species. It is therefore important that the biorefinery design reflects the porposed wood supply, which is intended to be used in the mill. The conversion process of hemicelluloses to ethanol, for example, must be optimized for either wood species. The concentration of acetic acid was much higher in the hardwood SSL than in the softwood SSL (11). The acetic acid can be recovered, especially from the hardwood SSL, as a by-product of this process. In another laboratory study, beech and spruce wood species were cooked for the dissolving pulp production. The compositions of the SSL produced are listed in Table II (2). It is noted that xylan and mannose were the major hemicelluloses of hardwood and softwood SSL, respectively. Under strong acidic conditions of sulfite pulping, some other sugar related products, such as furfural, xylonic acid, mannonic acid, are also present in the SSL (14).
SSL of Paper-Grade Process The compositions of the SSL from magnesium sulfite pulping of beech wood for paper-grade pulp production are listed in Table III (2).
413 In Sustainable Production of Fuels, Chemicals, and Fibers from Forest Biomass; Zhu, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2011.
Downloaded by STANFORD UNIV GREEN LIBR on June 15, 2012 | http://pubs.acs.org Publication Date (Web): July 11, 2011 | doi: 10.1021/bk-2011-1067.ch016
Table I. Composition of hardwood and softwood SSL obtained from an Eastern Canada mill at a concentration of 20% (wt.). (Reproduced with permission from reference (11). Copyright 1993 Springer) Xylose, g/l
Mannose, g/l
Glucose, g/l
Galactose, g/l
Reducing sugars, g/l
Acetic acid, g/l
Hardwood SSL, No 1
20.8
7.1
1.8
-
29.7
9.3
Hardwood SSL, No 2
24
6.5
3
-
33.5
7.6
Softwood SSL
8.3
17.1
6.3
3.7
35.4
2.7
Table II. The SSL compositions of beech and spruce for the production of dissolving pulp at an intrinsic viscosity of 700 ml/g. (Reproduced with permission from reference (2). Copyright 2006 WILEY-VCH) Component
Beech
Spruce
Xylose, g/kg1
100.9
18.4
Mannose, g/kg
7
58.1
Glucose, g/kg
8.4
19.8
Galactose, g/kg
4.8
n.d.2
Arabinose, g/kg
1.6
0.6
Rhamnose, g/kg
2.8
