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Braz. J. Chem. Eng. vol. 14 no. 3 São Paulo Sept. 1997
ACID HYDROLYSIS OF HEMICELLULOSE FROM SUGARCANE BAGASSE
A. PESSOA JR.1* , I.M. MANCILHA1 and S. SATO2
1 Department of Biotechnology /FAENQUIL, 12600-000 Lorena/SP - Brazil.
Phone: (55)125533422 Fax:(55)125533165, E.mail: firstname.lastname@example.org
2 Biochemical and Pharmaceutical Dept./FCF/USP - P.O.Box 66083- São Paulo/SP, Brazil
(Received: March 24, 1997; Accepted: August 15, 1997)
Abstract: Hydrolysis of the hemicellulosic fraction of sugarcane bagasse by sulphuric acid was performed in laboratory (25 mL) and semi-pilot (25 L) reactors under different conditions of temperature, time and acid concentration. On the laboratory scale, the three highest recovery yields were obtained at: 140ºC for 10 min with 100 mgacid/gdm (yield=73.4%); 140ºC for 20 min with 100 mgacid/gdm (yield=73.9%) and 150ºC for 20 min with 70 mgacid/gdm (yield=71.8%). These conditions were also used for hydrolysis in a semi-pilot reactor, and the highest xylose recovery yield (83.3%) was obtained at 140ºC for 20 min with 100 mgacid/gdm.
Keywords: Acid hydrolysis, hemicellulose, sugarcane bagasse.
In the Brazilian economic context an effective utilization of sugarcane bagasse for bioproduction is very important. More than 60,000,000 tons of bagasse containing 50% moisture can be produced annually during the ethanol production season (Orlando Filho et al. 1994; Molina Junior et al. 1995). This waste has been used as a raw material to produce hydroxymethyl furfural, paper pulp, acoustic boards, pressed woods and agricultural mulch (Dominguez et al. 1996). About 70% of the dry mass in lignocellulosic biomass consists of cellulose and hemicellulose. If these two carbohydrates were utilized in an efficient hydrolysis process, the hemicellulose would be completly hydrolyzed to D-xylose (50-70% w/w) and L-arabinose (5-15% w/w), and the cellulose would be converted to glucose (Ladish 1989; Cao et al. 1995). Enzymatic hydrolysis of biomass hemicellulose does not produce toxic products, nevertheless the process is not effective in producing a suitable fermentable substrate stream because of the low levels of monosaccharides and high levels of oligosaccharides produced. The main advantages of the dilute acid treatment of biomass include the production of a soluble pentose stream that can be physically separated from the particulate residue. Secondly, a substantially increased rate of enzymatic hydrolysis of the residual cellulose portion results, in large part, to the acid-induced fiber porosity. On the other hand, acid treatment produces furfural that is toxic to many micro-organisms and the residual acid must be neutralized (Hespell et al. 1997). The nonglucose carbohydrate portion of the lignocellulosic biomass is much more sensitive to acid hydrolysis than the glucose portion. Although reasonable amounts (over 70%) of hemicellulosic sugars, specially xylose, can be easily obtained by acid hydrolysis the treatments are usually performed at temperatures between 100 and 160oC (Wayman 1986). The hydrolysis of hemicellulose is accelerated at elevated temperatures owing to a relatively high activation energy in the solid-liquid phase reaction. At high temperatures part of the xylose released from hemicellulose can be degraded rapidly, and cellulose in the amorphous region can yield glucose (Banerjee 1989).
Acid hydrolysis has been investigated as a possible process for treating lignocellulosic materials such as wood chips (Silva 1996), rice straw (Almeida 1991), sugar beet pulp (Chamy et. al. 1994) and wheat straw (Fanta et al. 1984). According to Parisi (1989), the mineral acids act simply and rapidly as reaction catalyzers of polysacharide fractions. Sugarcane bagasse can be hydrolyzed using dilute acid to obtain a mixture of sugars with xylose as the major component. However, in the hydrolyzate some by-products generated in the hydrolysis, such as acetic acid, furfural, phenolic compounds, or lignin degradation products, can be present. These are potential inhibitors of a microbiological utilization of this hydrolyzate (Dominguez et al. 1996).
Processes such as two-stage acid hydrolysis can be employed to produce xylose and glucose (Beck 1986). Treatment with dilute sulphuric acid at moderate temperatures (the first stage of acid hydrolysis) has proven to be an efficient means of producing xylose from hemicellulose (Roberto et. al. 1994; Silva 1996). In the second stage more drastic reaction conditions are employed and glucose can be produced from cellulose hydrolysis (Gregg and Saddler 1995).
In general, acid treatment is effective in solubilizing the hemicellulosic component of biomass. Proper combinations of pH, temperature, and reaction time can result in high yields of sugars, primarily xylose from hemicellulose (Elander and Hsu 1995). Sulphuric acid is a catalyst for this reaction and, in this work, was used to study the hydrolysis of sugarcane bagasse hemicellulose. The effects of temperature, acid concentration and reaction time were studied, and the effectiveness of the hydrolysis was evaluated in terms of hemicellulose solubilization.
MATERIALS AND METHODS
Hydrolysis of Sugarcane Bagasse
Sugar cane bagasse was obtained from Usina Nova América S/A (Tarumã/SP-Brazil). The schematic design of the reactor system used for acid hydrolysis is shown in Figure 1. The sugarcane bagasse samples were weighed, introduced into a 25 mL laboratory reactor and indirectly heated with saturated steam to 140, 150 and 160ºC for 10 and 20 min. The final concentrations of sulfuric acid in the hydrolysis suspension were 70 and 100 mgacid/gdm and the solid-liquid ratio was 1:10. The steam valve was closed and the reactor was kept completely closed until cooled to room temperature. Next, the reactor was opened and the material inside was weighed. Three of the best conditions used for hydrolysis in the laboratory reactor (140oC for 10 min with 100 mgacid/gdm, 140oC for 20 min with 100 mgacid/gdm and 150oC for 20 min with 70 mgacid/gdm) were also used for hydrolysis in a 25 L semi-pilot reactor utilizing 1,000g of sugarcane bagasse (Figure 2).
Sugar and Lignin Determination
Fermentable reducing sugars (FRS) and nonfermentable reducing sugars (NRS) were determined by the method of Saeman et al. (1945). Total carbohydrate contents in the bagasse were evaluated after hydrolysis with 72% sulfuric acid using Dunnings method (1949). In this work, potential FRS and NRS are closely related to the contents of cellulose and hemicellulose, respectively. Hemicellulose recovery in water extract was measured by determining the content of NRS before and after hydrolysis with 4% H2SO4 (121ºC for 15 min). The objective of this acid hydrolysis was to convert the oligosaccharides to monosaccharides. The lignin was gravimetrically estimated from the insoluble residue by of Moores method (1967).
Figure 1: Schematic design of the laboratory reactor system used for acid hydrolysis.
Figure 2: Schematic design of the semi-pilot reactor system used for acid hydrolysis.
Hemicellulose Hydrolyzate Characterization
Total reducing sugars (TRS) were determined as glucose using the Nelson method (1944). Glucose, xylose and acetic acid concentrations were determined by HPLC (an HPX-87H Bio-Rad column with a RI 16X detector). Aliquots of 20 µL were analyzed at 45ºC with 0.01N sulfuric acid as the eluent (flow rate of 0.6 mL min-1). Furfural and hydroxymethyl-furfural were analyzed by HPLC (20 µL of sample injected) under the following conditions: an RP18HP column, an acetic acid:acetonitrile:water solution (1:10:80 volume ratio) as the eluent with a flow rate of 0.8 mL min-1, a temperature of 25ºC, and a UV detector.
RESULTS AND DISCUSSION
The sugarcane bagasse composition is presented in Table 1. The amount of ash is lower than that mentioned by Pate (1982), owing perhaps to the influence of different factors on sugarcane cultivation and processing. Table 1 also shows that 95.8% of NRF is composed of xylose and 91.9% of FRS is composed of glucose. This difference could be ascribed to the presence in the hemicellulose of arabinose, mannose and oligomers like celotriose, originating in incomplete molecule hydrolysis. It could also be ascribed to the presence of compounds like galactose, 4-o-methyglucuronic acid and aldobiuronic acid, which were not detected by HPLC. On the other hand, the use of two different techniques (the colorimetric method and HPLC) also interferes with the sugar concentrations (Fox et al. 1984; Morjanoff and Gray 1987). Similar differences were also found by Roberto et al. (1994).
The hydrolysis of the hemicellulose fraction during acid pretreatment involves solubilization and partial destruction of the reducing sugar produced. As a consequence, the amount of reducing sugar recovered from the bagasse depends on treatment time, temperature and acid concentration. The time intervals necessary for heating and cooling have not been considered here, since they were shorter than 30 s. According to Morjanoff and Gray (1987), the experimental error caused by the omission of such time intervals is not important since the reaction velocity is lower than that under the treatment temperature (the reaction velocity decreases by 50% for every 10oC). The results correlating TRS, NRS and FRS as a function of temperature, time of reaction and sulphuric acid concentration are shown in Figure 3. In comparison with the 10-minute reactions, the 20-minute reactions resulted in higher TRS recovery yields for both acid concentrations. With 100 mgacid/gdm the TRS recovery yield decreased for a 20-minute reaction at 160ºC, while the NRS recovery yield decreased for all 20-min and 10-min reactions at higher temperatures. These decreases are probably due to the degradation of xylose and glucose, which results in furfural and hydroxymethyl-furfural, respectively (Ackerson et al. 1981). The highest TRS recovery yields occurred at 150ºC for most of the time reactions and acid concentrations. At 140ºC the hydrolysis of material was incomplete, while at 160ºC the sugars were degraded. This degradation was also observed for reactions performed with a 100 mgacid/gdm concentration. The increase in recovery yield when the temperature varied from 140ºC to 150ºC was owing to the cellulose hydrolysis (Ackerson et al. 1981).
The semi-pilot hydrolysis was performed after choosing, from the laboratory scale experiments, the three hydrolysis conditions that provided the highest NRS recovery from the hemicellulosic fraction. As can be seen in Table 2, the results obtained from hydrolysis at 150ºC for 20min with 70 mgacid/gdm and at 140ºC for 10 min with 100 mgacid/gdm were similar to those obtained from hydrolysis in the laboratory reactor. However, the xylose recovery yield for the 20-minute reaction at 140ºC with 100 mgacid/gdm in the semi-pilot reactor (83.3%) was higher than that obtained on the laboratory reactor (73.9% NRS recovery). This increase in yield was due to agitation (150 min-1) during reaction in the semi-pilot reactor. The sugarcane hemicellulose hydrolyzate obtained under the best reaction conditions (140ºC for 20 min at 100 mgacid/gdm) was then characterized. As can be seen in Table 3, xylose and glucose represent 85.2% of the total reducing sugar from sugarcane hemicellulose hydrolyzate. So that, this hydrolyzate, which is similar to eucalyptus and rice straw hydrolyzates, can also be utilized in fermentation processes. The hydrolysis conditions here employed provided higher xylose and glucose concentrations than that obtained by Domingues et al (1996). Furfural and hydroxymethyl furfural, which are by-products of the acid hydrolysis of lignocellulosic materials, are normally omitted from the literature. However, acetic acid is often cited owing to its importance for the utilization of the hydrolyzate in fermentation processes (Roberto et al. 1994; Dominguez et al. 1996). As mentioned previously furfural originates from xylose degradation and hydroxymethyl furfural from glucose degradation. This degradation normally occurs in acid hydrolysis using sulphuric acid and its velocity depends on temperature and acid concentration (Gonzáles et. al. 1986). Under high temperatures the hemicellulose fraction is converted to monosacharides and then transformed into subproducts. Degradation occurs when acid homogenization in the reactor is inadequate, creating regions with a high acidity. The acetic acid concentration depends on the lignocellulosic material and is one of the principal components of the hemicellulose hydrolyzates (Grohman et al. 1985).
|*1TRS - Total Reducing Sugars |
*2FRS - Fermentable Reducing Sugars
*3NRS - Nonfermentable Reducing Sugars
*4% w/w of the wet matter
Sugarcane hemicellulose is a very important and promising source of xylose which could be utilized in the production of fuels and chemicals such as ethanol, xylitol and single cell protein. Acid hydrolysis has been shown to be an alternative for recovering this monosacharide from the hemicelluloses, provided that a careful hydrolysis reaction is conducted. As demonstrated here, the xylose yield in the hydrolyzate can reach 83.3%, with 0.08 g/L of hydroxymethyl furfural, 2.0 g/L of furfural and 3.7 g/L of acetic acid after hydrolysis performed on a semi-pilot scale at 140ºC for 20 min with 100 mgac/gdm.
|Temperature (ºC)||Time (min)||Acid Concentration |
*1 from this work (conditions of hydrolysis: 140oC for 20 min/100 mgacid)
*2 from Dominguez et al. (1996).
*3 from Silva, et al. (1995)
*4 from Roberto et al. (1994).
Figure 3: TRS (a), NRS (b) and FRS (c) soluble in the sugarcane hemicellulose hydrolyzate as a function of temperature, time reaction and sulphuric acid concentration. The conditions of hydrolysis were 10 min/70 mgacid/gdm (¾ u ¾ ); 20 min/70 mgacid/gdm (¾ n ¾ ); 10 min/100 mgacid/gdm (¾ D ¾ ); 20 min/100 mgacid/gdm (¾ x¾ ).
A. Pessoa Junior acknowledges the financial support of CAPES/PICDT/Brazil in the form of a Master of Science fellowship and the assistance of Maria Eunice Machado Coelho in revising this text and of Fernando Camargo in drawing the figures.
TRS Total Reducing Sugar
FRS Fermentable Reducing Sugar
NRS Nonfermentable Reducing Sugar
%w/w Percentage weight/weight
gdm Weight of dry matter in gram
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