Scholarly article on topic 'Alkaline Delignification of Oil Palm Empty Fruit Bunch Using Black Liquor from Pretreatment'

Alkaline Delignification of Oil Palm Empty Fruit Bunch Using Black Liquor from Pretreatment Academic research paper on "Chemical sciences"

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{"black liquor" / EFB / "enzymatic hydrolysis" / delignification / pretreatment / recycle / sugar}

Abstract of research paper on Chemical sciences, author of scientific article — Muryanto, Eka Triwahyuni, Haznan Abimayu, Agung Cahyono, Effendi Tri Cahyono, et al.

Abstract Bioethanol is providing one window of potential alternative energy. Lignocellulosic from oil palm empty fruit bunch (EFB) could be a promising bioethanol raw material because of abundant resources and it will not interfere food supply. Pretreatment is a one of the steps in the bioconversion of lignocellulosic material. Pretreatment also contributes the largest cost in the bioethanol production and produces black liquor as a wastewater that provides environmental impacts. Black liquor recycling is expected to increase economic efficiency by reducing the cost of pretreatment and the amount of wastewater. This experiment used black liquor and its mixture with NaOH solution as pretreatment solution. The pretreatment process was conducted in a 5-liter reactor at 150oC. The result using only black liquor gave the lowest value compared to other solutions. The addition of NaOH solution into black liquor caused increasing the cellulose content and delignification EFB by 57.18% and 51.62%, respectively. The maximum glucose concentration in the variation of pretreatment solutions: NaOH 2.5 M, black liquor, and black liquor with NaOH solution addition were 93.80 g l-1, 52.55 g l-1 and 87.05 g l-1, respectively, at a pretreated biomass loading of approximately 15% (w/v) with an enzyme dosage of 30 FPU g/dry biomass.

Academic research paper on topic "Alkaline Delignification of Oil Palm Empty Fruit Bunch Using Black Liquor from Pretreatment"

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Procedia Chemistry 16 (2015) 99 - 105

Alkaline Delignification of Oil Palm Empty Fruit Bunch using Black Liquor from Pretreatment

Muryantoa*, Eka Triwahyunia, Haznan Abimayua, Agung Cahyonob, Effendi Tri Cahyonob, Yanni Sudiyania

aResearch Centerfor Chemistry, Indonesian Institute of Sciences, Kawasan Puspiptek Serpong, Tangerang Selatan 15314, Indonesia hAgricultural Engineering Department, University of Brawijaya, Jl. Veteran Malang, Indonesia

Abstract

Bioethanol is providing one window of potential alternative energy. Lignocellulosic from oil palm empty fruit bunch (EFB) could be a promising bioethanol raw material because of abundant resources and it will not interfere food supply. Pretreatment is a one of the steps in the bioconversion of lignocellulosic material. Pretreatment also contributes the largest cost in the bioethanol production and produces black liquor as a wastewater that provides environmental impacts. Black liquor recycling is expected to increase economic efficiency by reducing the cost of pretreatment and the amount of wastewater. This experiment used black liquor and its mixture with NaOH solution as pretreatment solution. The pretreatment process was conducted in a 5-liter reactor at 150°C. The result using only black liquor gave the lowest value compared to other solutions. The addition of NaOH solution into black liquor caused increasing the cellulose content and delignification EFB by 57.18% and 51.62%, respectively. The maximum glucose concentration in the variation of pretreatment solutions: NaOH 2.5M, black liquor, and black liquor with NaOH solution addition were 93.80 g l"1, 52.55g l"1 and 87.05g l"1, respectively, at a pretreated biomass loading of approximately 15% (w/v) with an enzyme dosage of 30 FPU g/dry biomass.

©2015 The Authors.Publishedby Elsevier B.V. Thisis an open access article under the CC BY-NC-ND license (http://creativecommons.Org/licenses/by-nc-nd/4.0/).

Peer-review under responsibility of Research Center for Chemistry, Indonesian Institute of Sciences Keywords:black liquor; EFB, enzymatic hydrolysis, delignification; pretreatment, recycle, sugar

* Corresponding author. Tel.: +6221-7560929; fax: +6221-7560549 E-mail address: moer_yanto@yahoo.com

1876-6196 © 2015 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.Org/licenses/by-nc-nd/4.0/).

Peer-review under responsibility of Research Center for Chemistry, Indonesian Institute of Sciences doi: 10.1016/j .proche.2015.12.032

1. Introduction

Ethanol from renewable resources as an alternative fuel or oxygenated additive to the current fossil fuels has been interesting issues in recent decades. Lignocellulosic materials, which are relatively cheap and plentiful, are considered the main source of feedstock for bioethanol production. Biofuel based on lignocellulose was considered as critical issued to solve the conflict between food and energy resources for fuel production1.

Oil palm empty fruit bunch (EFB) is one of the abundant lignocellulosic waste materials in Indonesia. In 2012, Indonesia's palm oil plantation area was around 9.01 million hectare with a total of 23.5 million ton CPO production. Each ton production of crude palm oil generally produces 1.1 ton EFB2. In terms of chemical composition, the EFB predominantly contains cellulose, hemicelluloses, and lignin. Cellulose, the main important material for bioethanol, is a polysaccharide composed of D-glucose subunits, linked by P-l,4glycosidic bonds3. Meanwhile, hemicellulose and lignin protect cellulose in the outside. Lignin is further linked to both hemicellulose and cellulose forming a physical seal around the latter two components that are an impenetrable barrier preventing penetration of solution and enzymes. Delignification is processed that must be done to destroying the lignin structure, in order to give an optimum result in bioconversion lignocelluloses to fermentable sugar4.

Alkaline pretreatment is one of the chemical pretreatments. Some alkaline can be used for pretreatment lignocellulosic materials5'6. The mechanism of alkaline pretreatment is saponification of intermolecular ester bonds crosslinking xylan, hemicelluloses, and other components7. The goal of any pretreatment process is to alter or remove structural and compositional obstacles of hydrolysis, to improve the rate of enzyme digestibility, and to increase the yield of fermentable sugars from substrates8.

Although the yields were improved, alkaline pretreatment used a large amount of NaOH and produced an intermediate product such as black liquor. The black liquor contains lignocellulosic material, polyphenolic compounds, aliphatic acids, acid greases, and resinous compounds. The colouring agent in black liquor is comprised of wood extractives, tannin resins, lignin and its degradation products. The lignin content in black liquor involves a strong increase in the chemical oxygen demand (COD) and biological oxygen demand (BOD)9'10'11. The black liquor produced from alkaline pretreatment is usually discharge and not used again. Moreover, black liquor contains valuable chemical compounds, such as lignin, which could be recovered12. Black liquor also contains high NaOH concentration. As a consequence, the black liquor is a serious pollutant that must be treated, and high treatment costs can make the production process uncompetitive.

The high content of NaOH in black liquor has the potential to be used as a solvent in the pretreatment process. Rocha et al.13 reused black liquor in the pretreatment process for sugarcane bagasse. Xu et al.14 also used black liquor in corn stover pretreatment process. The current study aims to use the black liquor for EFB's pretreatment process to minimize the wastewater and reduce cost production of bioethanol so the efficiency of the process would increase.

2. Materials and Methods

2.1. Material

Oil palm empty fruit bunch (EFB) used in this experiments was obtained from a Palm Oil Mill in Riau, Indonesia. EFB was still big shape dried in the open air. The first step was physical pretreatment through milling and sieved to a typical length below 5 mm and then stored in a plastic container at room temperature. The chemical composition of untreated oil palm empty fruit bunch in this study were 32.80 % cellulose, 13.74% hemicelluloses, 30.12% lignin and 2.23 % ash. The commercial enzymes Cellic® Ctec2 and Cellic® Htec2 from Novozyme were applied to the saccharification process. The vendor reported that the activity of CTec2 was 128 FPU (Filter Paper Unit)/ml enzyme, whereas HTec2 was 240 CBU (cellobiose unit)/ml. All reagents (except sodium hydroxide using industrial grade) used in this study were of analytical grade.

2.2. ChemicalPretreatment

Chemical pretreatment was performed by heating 500 g of EFB (10 % moisture content) with 2500 ml alkali

solution in stirred reactor. The variation of pretreatment solvent in this study was 100% black liquor (BL100), and 50% black liquor with 50% NaOH 2.5 M addition (BL50). The NaOH 2.5M solution was used as a control in this process. The pressure was controlled at 4 bars; the reactor temperature was set at 150oC with reaction time 30 and 60 minutes. After pretreatment, the solid fraction was washed and neutralized with tap water then dried at 50-60°C overnight, the dry pretreated EFB stored at room temperature prior to saccharification process.

Characterization of EFB was measured before and after pretreatment. Cellulose, hemicellulose and lignin content were analyzed based on a method from National Renewable Energy Laboratory (NREL)15. Oil Palm EFB (300 mg, dry weight) was subjected to acid hydrolysis for lignin, cellulose, and hemicellulose content analysis. After hydrolysis, acid insoluble lignin (AIL) was weighed using Sartorius BS224S and acid soluble lignin (ASL) was measured using Spectrophotometry UV/Vis Spectrophotometer Optizen 2120 UV at 205 nm. Total lignin was obtained from the sum of AIL and ASL. On the other hand, after hydrolysis, cellulose and hemicellulose were measured by HPLC Waters e2695.

2.3. Saccharification

Saccharification process was conducted in 250 ml Erlenmeyer with 15% (w/v) or 15 g dry weight basis as a substrate, then 80 ml of 0.05 M citrate buffer pH 4.8 was added. Citrate buffer and substrate were sterilized at a temperature of 121°C for 15 minutes using an autoclave. The enzyme and buffer citrate addition was added in the sample until the total volume is 100 ml. Two kinds of enzymes, CTec2, and HTec2 were added with the ratio 5:1. The CTec2 loading was 30FPU/g pretreated EFB. The saccharification was conducted in a shaker incubator for 72 hr at temperature 50oC, and 150 rpm agitation. The temperature was adjusted based on the optimum condition of an enzyme.

2.4. Analysis

The sample (1 ml) was withdrawn from saccharification and fermentation medium every 24 h. Glucose and xylose were analyzed on an HPLC Waters e2695 using Aminex HPX-87H column (Bio-Rad, Richmond, CA, USA) at 650C with 0.6 ml min"1 eluent of 5mM sulfuric acid along 25 min retention time.Prior to HPLC injection, all samples filtered through 0.2^m syringe filters.

2.5. Interfacial morphology analysis (SEM)

Fracture surface of fibers before and after pre-treatment preparation were observed with a scanning electron microscope (SEM) using SEM HITACHI SU3500.

3. Result and Discussion

3.1. Effect of Pretreatment Solvent on Chemical Composition

In the alkaline pretreatment process, for every 500 g EFB (10 % moisture content or 450 g dry weight) process with 2500 ml of NaOH 2.5 M will be generated 1800-2000 ml of black liquor. The color of the black liquor is black and creates a pungent smell. After pretreatment, the NaOH content in the black liquor is around 7.6 % together with dissolving lignin. The pH of black liquor about 13, indicating the NaOH content in the black liquor is still high, and the black liquor can be reused as pretreatment solvent.

The pretreatment process was conducted with two variations of solvent (BL 100 and BL 50). After pretreatment and followed by neutralization, the mass was measured to calculate weight loss. The effect of pretreatment solution on the biomass recovery and weight loss is show in Table 1.

Table 1 . Weight loss and biomass recovery in pretreatment process

Pretreatment Solvent, time After Pretreatment (g) Weight Loss (g) Biomass Recovery (%)

NaOH 2.5 M, 30 min 157.68 292.32 35.04

BL 100, 30 min 252.23 197.77 56.05

BL 100, 60 min 236.67 213.33 52.59

BL 50, 30 min 202.69 247.31 45.04

BL 50, 60 min 194.63 255.37 43.25

After pretreatment, the EFB mass was reduced due to the reduction of lignin and other materials. The kind of pretreatment solvent gave the influence to the pretreatment process. The pretreatment with pure black liquor (BL100) gave the lower weight loss than pretreatment using NaOH solution and black liquor with NaOH addition (BL50). The weight loss result from pretreatment using BL 100, BL 50 and NaOH 2.5M solution were 197.77 g, 247.31g, and 255.37 g, respectively. Similar to pretreatment solvent, the pretreatment process time also affects the weight loss. Weight loss after chemical pretreatment increased with increasing time of the process. Only 252.23 g or approximately 56.05 % was recovered from 450g (dry weight) EFB after pretreatment process with black liquor in 30 minutes process time. This biomass recovery was reduced to 52.59 % when the processing time increased to 60 minutes. The highest weight loss was observed in pretreatment using black liquor with NaOH addition (60 minutes) that is 54.56 %, but this value still higher than the process with NaOH 2.5M solution. Pretreatment using alkali solutions such as NaOH, Ca(OH)2 or ammonia can remove lignin and a part of hemicelluloses that caused weight loss8'16.

Pretreatment of lignocellulosic materials is necessary to reduce some lignin by the breaking-down lignin structure and disorder the crystalline cellulose regions. Thus, the enzyme could easily attack the substrate. The effect of the pretreatment solution would be analyzed by comparing the component content. The composition of pretreated EFB with different pretreatment solution was listed in Table 2.

Table 2. Chemical Composition of Pretreated EFB.

Pretreatment Solvent, time Cellulose (%) Hemicellulose (%) Lignin (%) Ash (%)

NaOH 2.5 M, 30 min 63.82 12.14 12.56 1.24

BL 100, 30 min 43.86 24.94 17.39 0.59

BL 100, 60 min 44.73 18.01 16.56 0.99

BL 50, 30 min 57.18 13.52 14.59 0.67

BL 50, 60 min 55.89 14.35 14.12 0.80

Pretreatment changed the EFB composition. Table 2. shows the cellulose content after pretreatment was increased both of using NaOH solution and black liquor. Alkaline pretreatment using NaOH solution can increase the cellulose content from 30.16% (untreated EFB) to 63.82%. Reuse black liquor as the pretreatment solution also increased the cellulose content but is still lower than using the NaOH solution. The addition NaOH in BL 50 increased the cellulose content closer to NaOH solution. The cellulose content using BL 100 and BL 50 were 43.86 % and 57.18 %, respectively. The previous study in Pilot Plant Bioethanol G2 by using 10% NaOH showed the higher cellulose content as 68.86 %17. However, BL 50 process showed a similar result with pretreated EFB using 10 % NaOH with H2O2 addition in temperature 130oC, 20 min was conduct by Tan et al.ls. Furthermore, an increase the processing time resulted in a slight increase of cellulose content in pretreated EFB. The right dose of solvent pretreatment could reduce utilization of NaOH in chemical pretreatment.

The alkaline pretreatment was increasing cellulose content and decreasing hemicelluloses and lignin contents in all cases. One of the purpose alkaline pretreatment is to reduce lignin content called delignification. Delignification can improve the ability of cellulase enzymes in the saccharification process. Millet et al.19 reported that the performance of the enzyme increased by 14% to 55% on the hardwood when the lignin content was reduced from 24-55% to 20%. Lignin content at both of pretreated EFB was lower than untreated EFB. Lignin content due to pretreatment was performed in Table 2, that the pretreatment was successful to decrease the lignin content in EFB.

Using NaOH 2.5 M solution, the lignin concentration can be reduced from 30.15 % to 15.37 %. The reuse black liquor for pretreatment solvent also gave the reduction of lignin, but the value is lower than using NaOH solution.

The delignification in the variety of pretreatment condition was shown in Figure 1. Reuse of black liquor in the pretreatment process, not yet produce the same pretreated biomass result with the use of NaOH solution. Delignification with NaOH solution reached 58.36 % while the use of black liquor only reached 42.34 %. However, by the addition of NaOH solution into black liquor could increase delignification to be 51.62%, this value is slightly different with using NaOH solution. Increasing the processing time can improve the delignification, but the value is not too different. The use of black liquor by the addition of 50% NaOH solution as the solvent can reduce the use of NaOH up to 50% as well as reducing the amount of waste that is discarded. Although the results were not as good as using NaOH 2.5 M, it could minimize produced waste. The advantage of reusing black liquor was to reduce the use of NaOH in chemical pretreatment13'14'20.

3.2. Effect of Pretreatment Solvent on Enzymatic Saccharification

After alkaline pretreatment, 15 % (w/v) pretreated EFB in the variation of the pretreatment process then hydrolyzed using commercial enzymes. In this research, two enzymes were used, CTec2 and HTec2. The dose of enzyme in enzymatic hydrolysis was 30 FPU. This dose was according to Kim et al.21 who reported that an effective concentration of enzyme for cellulose hydrolysis has been determined to be 10 to 60 FPU per gram of dry cellulose or glucose. Enzymatic hydrolysis of the cellulose consists of three phases, namely the adsorption of cellulase enzymes to the cellulose surface, biodegradable cellulose into sugars that can be fermented and desorption of cellulase enzyme22.

Figure 1. Delignification of EFB using variation pretreatment solution.

Figure 2. Glucose Concentration after Saccharification.

The result of maximum glucose concentration after 72 hours saccharification process could be seen in Figure. 2. Alkaline explosion pretreatment of biomass increased the cellulose accessibility and its hydrolysis by enzymes. The highest glucose content was reached 93.80 g l"1 by pretreated EFB using NaOH solution. The glucose content by recycle black liquor at pretreatment process time 30 minutes and 60 minutes only reached 52.55 g l"1 and 69.68 g l"1 respectively. This preteated EFB would not accessible for enzymes to produce sugar because the lignin content is still high. Lignin was one of the components that restrict the access of hydrolytic enzymes to attach cellulose23. But glucose concentration in saccharification pretreated EFB using BL 50 was closer to preteated EFB using NaOH solution reached 87.05 g l"1. The NaOH addition can increase delignification and swelling structure in EFB, and then the pretreated EFB easier to hydrolysis by enzymes24. Xylose content after 72 hours hydrolysis is approximately same in both of pretreatment process with range 20-32 g l"1 (data not shown).

By using data in Table 2, when saccharification was conducted in pretreated EFB using NaOH, BL 100, and BL 50 the maximum theoretical glucose yield were 106.35 g l"1, 72.36 g l"1 and 94.35 g l"1 respectively. The efficiency

of saccharification preteated EFB was 80-90 % that is calculated by comparing the theoretical and actual condition. It was good enough for saccharification because almost all of the cellulose convert into glucose, except using BL 100 that only reached 70%.

3.3. Morphological Structure of EFB

Morphological changes in EFB resulting from chemical pretreatment showed in Figure 3. The EFB particles disintegrated into small, highly disperse fiber, porous, more homogenous, and pulp consistency that caused by delignification. Figure 3. showed the different of untreated EFB with pretreated EFB using BL 50. The untreated sample had the typical regular and compact surface structure, with highly fibrous and intact morphology. In the pretreated EFB, some stomata on the surface were also recognized. Most of the particle change from fibrous to spherical shape had steps and holes after chemical pretreatment25.

Figure 3. Morphology of Untreated EFB (A), Pretreated EFB using BL 50 (5000X) (B)

4. Conclusion

The delignification EFB using NaOH solution, BL 100 and BL 50 were 58.36%, 42.34 %, and 51.62 %, respectively. Black liquor with 50% NaOH addition (BL 50) was possible for pretreatment solvent. The addition NaOH solution into black liquor can increase the delignification and glucose concentration of EFB. Applying the right dose pretreatment solvent could reduce the operational cost in ethanol production.

Acknowledgement

The authors thank to Hendris Hendarsyah and Irni Fitria for invaluable assistances in this work. This research was funded by Unggulan Project of Indonesian Institute of Sciences (LIPI) of the fiscal year 2015.

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