Hydrodynamic Cavitation Reactor for Efficient Pretreatment of

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Hydrodynamic Cavitation Reactor for Efficient Pretreatment of Lignocellulosic Biomass Kazunori Nakashima,*,†,# Yuuki Ebi,† Naomi Shibasaki-Kitakawa,† Hitoshi Soyama,‡ and Toshikuni Yonemoto† †

Department of Chemical Engineering, and ‡Department of Nanomechanics, Tohoku University, Aoba-ku, Sendai 980-8579, Japan ABSTRACT: In the present study, hydrodynamic (HD) cavitation was combined with sodium percarbonate (SP) for the pretreatment of lignocellulosic biomass. For HD cavitation, a circular flow system equipped with a venturi tube was employed. The combined system of HD and SP (HD-SP) was compared to a pretreatment system using ultrasonication and SP (US-SP). The efficacy of each pretreatment method was evaluated by measuring lignin removal, and glucose and xylose formation. Fourier transform infrared spectra indicated that both systems resulted in a similar degree of lignin removal; however, the HD-SP system was more efficient than the US-SP system for glucose and xylose production. Neither system generated the inhibitor furfural, while it was detected in dilute acid (DA)-pretreated biomass. Furthermore, changing the venturi tube throat diameter improved the efficacy of the HD-SP system. The HD-SP system is promising for the industrial pretreatment of lignocellulosic biomass. Cavitation generated in a flowing fluid system with a constriction, such as a venturi tube or an orifice plate, is called hydrodynamic (HD) cavitation.11−14 HD cavitation can be explained on the basis of the relationship between pressure and velocity of the fluid according to Bernoulli’s equation. When pressure at the constriction (venturi throat) falls below the vapor pressure of the liquid, HD cavities (e.g., bubbles) are generated. These bubbles subsequently collapse when the pressure increases downstream of the constriction.11,13 For industrial applications, a HD cavitation reactor could be easily scaled up for a high-throughput system, and compared to an US cavitation reactor it requires much lower energy input.12,14 HD has been studied for industrial applications such as sterilization of food and water,15,16 cell disruption,17 wastewater treatment,18 decomposition of excess sludge,19 generation of submicrometer emulsions,20,21 and biodiesel production.22,23 Recent studies also have shown that HD could be used for delignification of wheat straw and wood for pulp and paper manufacturing.24−26 Here, we have proposed a novel pretreatment technique of lignocellulosic biomass employing the combination of HD cavitation and SP (HD-SP) for an efficient degradation of polysaccharides in the biomass. The HD could have a potential effect on biomass pretreatment under milder conditions, providing localized high energy to break down crystalline cellulose and facilitate the formation of radical species to degrade lignin when combined with SP, as is shown in the USSP system.10 In the present study, we compared the efficiency of delignification and the digestibility of polysaccharides in HDSP and US-SP systems.

1. INTRODUCTION Biorefinery is a next-generation production system for fuels and chemicals from biomass and has many advantages in terms of environmental impact and sustainability.1 Lignocellulosic biomass, which is composed mainly of cellulose, hemicellulose, and lignin, is a promising material as a renewable resource for use in biorefinery. The cellulose and hemicellulose in lignocellulose are the main targets of biorefinery because they can be hydrolyzed to fermentable sugars such as glucose and xylose, which are subsequently converted to biofuels and biobased chemicals by genetically engineered microbes and chemical catalysts.2,3 Lignocellulosic biomass is usually pretreated before enzymatic hydrolysis to enhance its digestibility. Pretreatment should improve the formation of sugars, avoid excessive degradation, which can lead to loss of sugars and formation of inhibitors, and be cost-effective.4 Ultrasonication (US) in a solution causes cavitation, which is a combined phenomenon of formation, growth, and subsequent collapse of microbubbles, which produces a hot spot of localized extreme temperature and pressure (approximately 5000 °C and 500 atm).5 The high local energy from cavitation is known to disintegrate microcrystalline cellulose to cellulose nanofibers.6 Focusing on this effect, researchers have tried to apply US to the pretreatment of lignocellulosic biomass.7−9 However, the improvement in digestibility was not sufficient just by introducing US, partly because of insufficient removal of lignin. Sodium percarbonate (SP), which is composed of Na2CO3 and H2O2, is an environmentally benign oxidation reagent. Hydrogen peroxide in SP dissociates into highly reactive hydroxyl (·OH) and superoxide (O2−·) radicals, which decompose lignin by attacking its side chains. Recently, we reported a novel pretreatment technique combining US and SP (US-SP) for efficient degradation of lignocellulosic biomass corn stover under mild conditions.10 The digestibility of the biomass was greatly enhanced with the US-SP technique compared to each single pretreatment. © XXXX American Chemical Society

Received: November 19, 2015 Revised: January 29, 2016 Accepted: February 1, 2016

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DOI: 10.1021/acs.iecr.5b04375 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research

2. MATERIALS AND METHODS 2.1. HD and US Pretreatment. Corn stover was used as a lignocellulosic biomass in this study. The corn stover was crushed in a blender (Wonder Blender, Osaka Chemical Co., Ltd., Osaka, Japan) and screened to obtain a particle size of less than 0.25 mm. A schematic of the circular flow system for the HD-SP pretreatment is shown in Figure 1. The system included a 500

Table 1. Flow Characteristics of Venturi Tube

a

throat diameter (mm)

flow rate (L/min)

number of circulationa

inlet pressure (kPa)

cavitation number σb

1.8 1.4

3.2 2.4

480 360

220 340

0.44 0.29

The number of circulation is calculated as number of circulation flow rate (L/min) = × operating time (60 min) volume of holding tank (0.4 L)

b

The cavitation number σ is calculated as p − pv σ= 2 p1 − p2

where p1 is the upstream pressure; p2 is the downstream pressure; pv is the vapor pressure.

2.3. Enzymatic Saccharification of Biomass. Enzymatic saccharification of biomass was carried out using a mixture of cellulase from Trichoderma reesei and β-glucosidase from Aspergillus niger, both of which were obtained from SigmaAldrich (St. Louis, MO, USA). After the biomass suspension (20 mg/mL, 100 mL) in acetate buffer (50 mmol/L, pH 5) was preheated at 40 °C for 30 min, enzymatic hydrolysis was started by adding cellulase (0.01 g/g-biomass) supplemented with βglucosidase (0.01 g/g-biomass) to the suspension, with gentle shaking at 100 rpm and 40 °C. A preliminary test confirmed that cellobiose was completely converted to glucose by βglucosidase under these conditions. Samples (300 μL) were collected from the reaction mixture at set times and centrifuged to separate the supernatant from unreacted solid cellulose. For saccharification of hemicellulose in biomass, hemicellulase (0.2 g/g-biomass, from Aspergillus niger, SigmaAldrich) was used in addition to cellulase (0.01 g/g-biomass) and β-glucosidase (0.01 g/g-biomass). The concentration of sugars in the supernatant after enzymatic saccharification was determined by high-performance liquid chromatography (HPLC; Acquity UPLC H-Class, Waters, Milford, MA, USA) equipped with a BEH amide column (2.1 mm × 50 mm, particle size 1.7 μm) and an evaporative light-scattering detector. Acetonitrile and ultrapure water containing 0.2% (v/v) triethylamine was used as the mobile phase. Each saccharification experiment was carried out in duplicate, and the results were averaged. For the analysis of lignin degradation and furfural formation during pretreatment, a HPLC equipped with a BEH C18 column (2.1 mm × 100 mm, particle size 1.7 μm) and ultraviolet detector (λ = 280 nm) was used.

Figure 1. Schematic of the hydrodynamic cavitation system equipped with a venturi tube for pretreatment of corn stover.

mL holding tank, motor-powered diaphragm pump (Duplex II AC Demand Pump D3635E7011A, FLOJET, 30 W), and venturi tube (length, 40 mm; internal diameter φ, 3.6 mm; throat diameter φt, 1.8 mm or 1.4 mm). The venturi tube with a throat diameter of 1.8 mm was used in all experiments unless otherwise stated. The holding tank was placed in a water bath, which was used to control the reaction temperature. The biomass powder (16 g) was suspended in 400 mL of SP solution (Na2CO3, 0.4 mol/L; H2O2, 0.6 mol/L) in the holding tank. The biomass pretreatment was carried out by circulating the suspension with the pump for 1 h at 30 °C. The characteristics of these venturi tubes were shown in Table 1. After circulation, a 50 mL aliquot of the suspension was removed and washed with 50 mL of water, and then washed twice with 50 mL of acetate buffer (50 mmol/L, pH 5) to neutralize the SP solution. For US-SP pretreatment, 2.0 g of biomass in 50 mL of SP solution was treated using US as described in our previous report,10 where the biomass (2.0 g) was pretreated in 50 mL of SP solution. 2.2. FT-IR Spectroscopy Characterization of Pretreated Biomass. Chemical changes in the biomass after pretreatment were analyzed by Fourier transform infrared (FTIR) spectroscopy with attenuated total reflection (Nicolet6700, Thermo Fisher Scientific, Waltham, MA, USA). Spectra were recorded in the mid-IR region from 2000 to 1200 cm−1 at a resolution of 2 cm−1 using an average of 16 scans.

3. RESULTS AND DISCUSSION 3.1. Characterization of Pretreated Biomass. Figure 2a shows photographs of HD-SP-pretreated, US-SP-pretreated, and untreated biomass. The untreated biomass was brown, which indicated it contained the polyphenolic compound lignin. Pretreatment by HD-SP or US-SP removed this brown color, and the biomass swelled to some extent. The decolorization and swelling of the biomass could be attributed to removal of lignin and a decrease in cellulose crystallinity in the biomass, respectively. Lignin is a complex three-dimensional polymer that is composed of three types of phenylB

DOI: 10.1021/acs.iecr.5b04375 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research

Figure 3. Time courses of saccharification of the biomass by cellulolytic enzymes.

the HD-SP system was higher than that provided by the US-SP system. The comprehensive perspective including the throughput of biomass, glucose yield, and energy consumption in each pretreatment method shown in Table 2 revealed that the HDSP pretreatment exhibits 20 times greater pretreatment efficiency than US-SP pretreatment. During enzymatic saccharification, glucose disappeared at 48 h in the untreated system. This decline was probably caused by microbe contaminants in the reaction mixture. Biomass naturally contains microbes, which digest sugars such as glucose produced by enzymatic saccharification. Because pretreatment is commonly conducted at high temperature (usually >160 °C), the level of microbes in the biomass would decrease during the pretreatment process. In the HD-SP and US-SP systems, glucose did not disappear even though the pretreatments were conducted at room temperature. In this case, the SP solution probably acted as an antibacterial agent to reduce microbe levels and their digestion of glucose. 3.3. Enzymatic Saccharification of Hemicellulose in Biomass to Xylose. Lignocellulosic biomass generally contains a relatively large proportion of hemicellulose (20− 35%),30 which can be recovered as fermentable sugars such as xylose after saccharification by hemicellulase. However, conventional acid pretreatment at high temperature (160 °C) often causes excessive degradation of xylose, resulting in the formation of the microbe inhibitor furfural.31 A successful method would recover the hemicellulose fraction without causing excessive degradation or furfural formation. Here, we investigated xylose recovery from the biomass with both pretreatment systems. Figure 4 shows the HPLC results from analysis of the supernatant after HD-SP and US-SP pretreatment, and for conventional dilute acid (DA) pretreatment.32 No furfural (retention time 4 min) was detected in the HD-SP- or US-SP-pretreated biomass, but it was detected in the DA-pretreated biomass. Therefore, HD-SP and US-SP pretreatment should be used to avoid formation of furfural. Moreover, the peaks for degraded lignin (retention time 1.1 min) were much higher in the HD-SP- and US-SP-pretreated biomass than that in the DA-pretreated biomass. This indicates that the HD-SP and US-SP systems are more effective for lignin removal than DA pretreatment. When comparing HD-SP and US-SP, lignin removal was found to be 50% greater in the HDSP system than the US-SP system based on the peak area in the chromatogram. Baxi et al. reported that delignification was higher in a HD system than in a US system in the Kraft pulping process.24 This difference could be responsible for the higher

Figure 2. Photographs (a) and FTIR spectra (b) of the biomass.

propane units. The SP solution, which contained H2O2 under alkaline conditions in Na2CO3, caused dissociation of H2O2 to hydroperoxy anions (HOO−), which subsequently reacted with H2O2 to generate highly reactive ·OH and O2−· radicals. These radicals readily react with phenolic components in lignin, resulting in degradation to low-molecular-weight compounds and lignin depolymerization.27,28 The FTIR spectra were also used to analyze the biomass (Figure 2b). In the untreated biomass, there were obvious peaks at 1745 and 1606 cm−1, which were assigned to the side chain and aromatic ring of lignin, respectively. In the HD-SPpretreated and US-SP-pretreated biomass, the intensities of the peaks at 1745 and 1606 cm−1 decreased, suggesting that HD-SP and US-SP pretreatment were effective for lignin removal. The FTIR spectra were similar for the HD-SP- and US-SPpretreated biomass, suggesting they had similar lignin content. Both the HD-SP and US-SP systems generated cavities, leading to hot spots of localized high temperature (5000 °C) and high pressure (500 atm).5 This high energy at these hot spots disrupts hydrogen bonds in the cellulose microfibrils and crystals, resulting in biomass swelling. Furthermore, asymmetrical collapse of bubbles near a solid surface produces highspeed microjets (>100 m/s), which can impact the biomass surface and result in pitting.5,29 These microjets could cause the swelling of cellulose fibers in lignocellulose in both the HD-SP and US-SP systems. 3.2. Enzymatic Saccharification of Cellulose in Biomass to Glucose. Figure 3 shows the time courses of enzymatic saccharification of cellulose in HD-SP- pretreated, US-SP-pretreated, and untreated biomass. Compared to the untreated biomass, the US-SP-pretreated biomass exhibited higher digestibility. In our previous work, we reported that biomass digestibility greatly increased when US was combined with SP, and individual pretreatment (i.e., only US or SP) had a negligible effect on biomass digestibility. The HD-SP-pretreated biomass showed much higher digestibility compared with untreated biomass, and the degree of improvement provided by C

DOI: 10.1021/acs.iecr.5b04375 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research Table 2. Pretreatment Efficiency in HD-SP and US-SP Systems system

pretreatment volume (mL)

estimate of obtained glucose (g)a

energy dissipated in pretreatment (×105 J)

pretreatment efficiency (×10−5 g glucose/J)b

HD-SP US-SP

400 50

2.42 0.196

1.08 1.80

2.24 0.109

a Estimate of obtained glucose is calculated based on the assumption that all of the pretreated biomass is subjected to enzymatic saccharification and the data obtained at 48 h in Figure 3. bPretreatment efficiency = (estimate of obtained glucose)/(energy dissipated in pretreatment).

production plateaued after 24 h. This could be attributed to excess degradation and loss of the hemicellulose fraction in the DA-pretreated biomass. In the US-SP system, xylose production increased with time, and the yield was higher than that obtained with the DA system. The HD-SP system resulted in further increases in the production rate and yield of xylose. Therefore, the HD-SP pretreatment is promising for xylose recovery from lignocellulosic biomass. 3.4. Influence of Throat Diameter on Pretreatment Efficacy. The geometry of the constriction in a HD system is known to greatly affect cavitation intensity,11,18,33 which is closely related to pretreatment efficacy for biomass. We investigated the effect of the throat diameter of the venturi tube on pretreatment efficacy. Two venturi tubes with different throat diameters (1.8 mm and 1.4 mm) were used. Figure 6

Figure 4. HPLC chromatograms of the supernatant obtained after pretreatment with HD-SP (a), US-SP (b), and DA (c) systems.

digestibility obtained with the HD-SP system compared with the US-SP system. Next, we examined enzymatic saccharification of hemicellulose to xylose in the pretreated biomass by hemicellulase. Figure 5 shows the time courses of xylose production in the HD-SP, US-SP, and DA systems. In the DA system, xylose

Figure 6. Production of glucose (a) and xylose (b) by saccharification of biomass pretreated with HD-SP systems equipped with different venturi tubes. The saccharification data for the 1.8 mm system are from the results in Figure 3 and Figure 5.

shows the enzymatic saccharification of biomass pretreated with the HD systems with different diameter venturi tubes. The results for glucose formation (Figure 6a) showed that higher digestibility was obtained in the venturi tube with a throat diameter of 1.4 mm than in that with a throat diameter of 1.8 mm diameter. The narrower throat would provide a greater

Figure 5. Time courses of saccharification of the biomass by hemicellulase. D

DOI: 10.1021/acs.iecr.5b04375 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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pressure gradient and stronger cavitation, resulting in increased pretreatment efficacy compared to the wider throat. This could be one of reasons for the increase in glucose formation in the narrower venturi tube compared to the wider venturi tube. Furthermore, as mentioned in Section 3.1, bubbles collapse violently near the solid wall of the venturi tube. Because the relative total cross-sectional area near the wall of the 1.4 mm system is larger than that of 1.8 mm system, this could also contribute to effectiveness of the 1.4 mm system over the 1.8 mm system. Pandit et al.18,33 studied the effect of hole diameter of orifice plate on cavitational effects. They introduced a parameter α, which is characteristic of the orifice plate having a unique cross-sectional flow area which takes into account the increase in the area of the shear layer, and demonstrated that the plate having the smaller diameter exhibited a higher effect. Also, they showed that cavitational effects are generally higher at a lower cavitation number. These tendencies are basically consistent with the results obtained in our present study. The change in throat diameter did not affect xylose formation. This indicates that the hemicellulose fraction in the biomass was pretreated sufficiently enough for degradation even in the 1.8 mm system. Further investigation would be necessary to elucidate the relationship between the throat diameter and the efficacy of pretreatment. However, changing the constriction geometry of the HD system can improve pretreatment efficiency.

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4. CONCLUSIONS A HD-SP pretreatment technique for efficient degradation of lignocellulose was developed. This HD-SP system was more efficient than a US-SP system in terms of both glucose and xylose production. Both the HD-SP and US-SP systems generated no furfural because the pretreatment was conducted under mild conditions at 30 °C. The pretreatment efficacy with the HD-SP system could be improved by changing the geometry of the constriction, and a narrower throat was more effective for biomass pretreatment. Because of its efficacy and high throughput, the HD-SP system is promising for industrial pretreatment of lignocellulosic biomass. Important factors to acquire high pretreatment efficiency in the HD-SP system could be found by further study of pretreatment parameters such as inlet pressure, SP concentration, pretreatment temperature and time, and biomass loading.

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AUTHOR INFORMATION

Corresponding Author

*Tel./FAX: +81-22-795-7256. E-mail: [email protected]. ac.jp. Address: Department of Chemical Engineering, Tohoku University, Aoba-yama 6-6-07, Aoba-ku, Sendai 980-8579, Japan Present Address #

Division of Sustainable Resources Engineering, Faculty of Engineering, Hokkaido University, Kita 13, Nishi 8, Kita-Ku, Sapporo 060−8628, Japan Tel./FAX: + 81−11−706−6322. Email: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was partly supported by the Japan Society for the Promotion of Science KAKENHI (Grant No. 25820396) and JST A-STEP program (241FT0189). E

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DOI: 10.1021/acs.iecr.5b04375 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX