Mechanical pretreatment of biomass – Part I: Acoustic and hydrodynamic cavitation

Biomass and Bioenergy 98 (2017) 135e141

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Research paper

Mechanical pretreatment of biomass e Part I: Acoustic and hydrodynamic cavitation Maxine Jones Madison a, Guillermo Coward-Kelly b, Chao Liang c, *, M. Nazmul Karim c, Matthew Falls c, Mark T. Holtzapple c a b c

ConocoPhillips, 600 N. Dairy Ashford St., Houston, TX 77079, USA Novozymes North America Inc., 77 Perry Chapel Church Rd, P.O. Box 576, Franklinton, NC 27525, USA Texas A&M University, College Station, TX 77843, USA

a r t i c l e i n f o

a b s t r a c t

Article history: Received 29 August 2016 Received in revised form 29 November 2016 Accepted 4 January 2017

Acoustic and hydrodynamic cavitation were examined as suitable mechanical pretreatments for lignocellulosic biomass. Microcrystalline cellulose and lime-treated sugarcane bagasse were subjected to acoustic cavitation, whereas raw and lime-treated sugarcane bagasse were subjected to hydrodynamic cavitation. Acoustic cavitation successfully increased microcrystalline cellulose enzymatic digestibility by 37% compared to no acoustic cavitation treatment; however, there was no significant effect on limetreated sugarcane bagasse. Hydrodynamic cavitation increased the enzymatic digestibility of both raw and lime-treated sugarcane bagasse. Best results were obtained using cavitation treatment of bagasse followed by lime treatment; the 3-d enzymatic digestion increased by 46% when compared to lime treatment only. © 2017 Elsevier Ltd. All rights reserved.

Keywords: Hydrodynamic Acoustic Cavitation Crystallinity Pretreatment

1. Introduction Increasing interest in alternative energy sources is occurring for multiple reasons: (1) concern over dependence on foreign petroleum; (2) rising oil prices; (3) increasing public interest in environmental preservation; and (4) global warming from the buildup of greenhouse gases, primarily carbon dioxide [1]. Biological conversion of lignocellulosic materials to liquid biofuels addresses each of these concerns. Processes that convert lignocellulose into usable products have been studied for many years [2e6]. These processes are especially attractive because they can convert a variety of feedstocks (e.g., crop wastes, municipal solid waste, sewage sludge, energy crops, woody biomass) into liquid fuels. Additionally, the combustion of these alternative liquid fuels will not contribute to global warming because there is no net addition of carbon dioxide into the atmosphere. Lignocellulosic biomass is the world's most abundant biological material and is composed primarily of cellulose, hemicellulose, and lignin. Cellulose is a linear, unbranched polymer of b-glucose that

* Corresponding author. E-mail address: [email protected] (C. Liang). http://dx.doi.org/10.1016/j.biombioe.2017.01.007 0961-9534/© 2017 Elsevier Ltd. All rights reserved.

provides structure to plants [7,8]. There are two configurations of cellulose: crystalline and amorphous. Amorphous sections are more disordered and allow water to penetrate, thereby increasing susceptibility to enzymatic hydrolysis. In contrast, crystalline sections have hydrogen bonds between the polymers that make it more resistant to enzymatic hydrolysis [9]. Hemicellulose is more readily hydrolyzed than cellulose because of its amorphous structure. It is primarily composed of pentoses (xylose and arabinose) and hexoses (glucose, galactose, and mannose). Hemicellulose polymers are shorter than cellulose. Hemicellulose has a degree of polymerization (DP) of 50e200, whereas cellulose has a DP of 500e15,000. Lignin is a phenyl-propane polymer that acts as a glue to hold hemicellulose and cellulose together. Its monomers are p-coumaryl alcohol, coniferyl alcohol, and sinapyl alcohol. In plant cell walls, cellulose provides the cell structure and strength and is enclosed in a hemicellulose matrix surrounded by lignin, which holds the entire framework together [7]. The crystalline structure of cellulose hinders enzymatic hydrolysis by limiting the number of enzyme adsorption sites [10e12]. Successful pretreatments should open the biomass structure to make it more accessible to enzymes [13]. To increase the extent of biomass digestion, Chang and Holtzapple [14] found the two main

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contributors are the lignin content and the degree of crystallinity. Crystallinity measures the relative amounts of the amorphous and crystalline regions of cellulose [11]. It is often described by the crystallinity index (CrI); a higher CrI denotes a more crystalline material. Segal et al. [15] developed the following crystallinity index:

CrI ¼

I002  Iam  100 I002

(a)

P1

P2 Venturi Cavitator

Mixer

[1]

where, I002 ¼ intensity at 2q of 22.5 Iam ¼ intensity at 2q of ~18.7 Crystallinity is measured using an X-ray diffractometer (XRD). High lignin content and high CrI reduce enzymatic digestibility [14,16]; therefore, the most effective pretreatment is one that both delignifies and decrystallizes the biomass. Lime (Ca(OH)2) pretreatment is an effective, inexpensive, and safe delignification method. Long-term lime pretreatment involves mixing the biomass and calcium hydroxide into a large pile at low temperatures (25e70  C). Then, for about one month, air is purged through the pile while the pile is wetted with water. This pretreatment removes sufficient lignin from the biomass to enhance enzymatic digestion [17]. Short-term lime pretreatment treats biomass for a short time (1e6 h) at elevated temperatures (100e160  C) with oxygen [18,19] or without oxygen [20,21]. Cellulose decrystallization is typically performed using physical pretreatment methods, such as ball-milling or two-roll milling [22,23]. In this work, cavitation is used as a physical pretreatment to determine if it enhances the enzymatic digestibility of lignocellulose. Cavitation is the formation, growth, and rapid collapse of gas- or vapor-filled bubbles. There are two main types of cavitation: acoustic and hydrodynamic. Acoustic cavitation is caused by pressure variations from ultrasonic waves passing through a fluid. Acoustic waves create microcavities where gas bubbles grow and then collapse. The collapse generates “shock waves” that cause mechanical effects, such as particle erosion [24]. This high-power, low-frequency ultrasound is usually used to create a permanent chemical or physical change in a substance [25]. High-power acoustic cavitation is used for cleaning and welding [26]. For this study, a laboratory-scale sonicator was used to decrystallize both microcrystalline cellulose and limetreated biomass via acoustic cavitation. Acoustic cavitation reduces particle size, likely resulting from erosion caused by collapsing gas bubbles. Additionally, acoustic cavitation lowers crystallinity. Preliminary studies have shown an increase in biomass enzymatic digestibility when cavitation is used as a pretreatment step [27,28]. Hydrodynamic cavitation occurs when a moving fluid encounters a sudden change in velocity that results in a localized pressure drop. For example, this can occur in a venturi (Fig. 1). Cavities form at the throat when the pressure falls below the fluid vapor pressure. The bubbles collapse when pressure is recovered. Relative to cavitation, there are two main features of bubble dynamics: (1) the maximum bubble size and (2) the distance traveled by the bubble before collapse, i.e., bubble life. The maximum bubble size defines the cavitation intensity. Bubbles grow at low pressure or high temperature [15]. Larger bubbles implode with a higher intensity and can cause greater effects on a substance than smaller bubbles. For example, a large number of exploding bubbles can alter the structure of biomass. Bubble life is a measure of the active volume where cavitation effects are observed. Generally, bubble life decreases as the region of

(b) Biomass particle

Collapsing bubble

Bubble formation Cavitated biomass particle Fig. 1. Scheme of cavitation in a venturi cavitator showing (a) cavitation system including the mixing vessel, centrifugal pump, cavitator, valves, and pressure gauges; and (b) biomass particles inside the cavitator.

active cavitation decreases and vice versa. Fig. 1 shows hydrodynamic cavitation of biomass in a venturi. Many factors affect cavitation. To create cavitation, highly viscous fluids require higher energy input. A fluid with a low vapor pressure requires more energy to produce cavitation. Also, the size and geometry of the cavitator affects the efficiency [24]. The cavitation numberCv measures the resistance to cavitation. It is a dimensionless parameter and is given by the following equation:

Cv ¼

Pf  Pv 0:5rU 2

[2]

where Pf is the downstream pressure, Pv is fluid vapor pressure, r is the fluid density, and U is the average velocity near the orifice. A high cavitation number indicates cavitation will not likely occur and vice versa. If cavitation is already occurring, lowering the cavitation number by decreasing the pressure or by increasing the flow rate will increase the cavitation intensity. Raising the cavitation number may stop cavitation altogether [24]. At low cavitation numbers, bubbles may combine to form larger bubbles or bubble clusters, which are carried away with the liquid and thereby reduce cavitation effectiveness [29]. Current hydrodynamic cavitation applications include water treatment [30], biogas production [31], cell disruption [32], refining of wood pulp, and creating agitation in chemical reactors [33]. Hydrodynamic cavitation is easier and more economical to scale-up than acoustic cavitation [28,34]. In addition, the numerous local

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2. Materials and methods

force the biomass slurry through the venturi. Experiments were conducted with 55 L of water and 550 g of sugarcane bagasse (1%). Tests were run using 10-mesh raw bagasse for up to 2 h at a time. The temperature was controlled to 22  C by adding ice to the water. Approximately 20 kg of ice were required to maintain the temperature at 22  C for the duration of the 120-min treatment. Samples were taken at 0, 20, 40, 60, and 120 min. The solids were then separated from the liquid and allowed to air dry. Afterwards, enzymatic hydrolysis and XRD analysis were performed on the samples.

2.1. Bagasse

2.6. Crystallinity

Sugarcane bagasse was obtained from the W. R. Cowley Sugar House, a sugar mill in Santa Rosa TX. To maintain its freshness, the sugarcane bagasse used throughout this work was stored in the freezer. On average, bagasse was stored in the freezer for six months before it was used for any experiments. Before use, the bagasse was thawed under a hood for 1e2 days. Then the moisture content was determined using the NREL standard procedure [37].

After each pretreatment, samples were air-dried and ground before testing the crystallinity. To achieve a homogenous sample, samples were ground for approximately 1 min using a 12-cup coffee grinder (Mr. Coffee Precision Coffee Grinder with Chamber Maid Cleaning System, Model #IDS77) on the fine setting. Grinding the samples improved the accuracy and repeatability of the crystallinity analysis. There was no significant effect of grinding on crystallinity. Table 1 shows the bagasse composition used in this work. (Note: Not reported are other components such as ash and extractives.) To determine the effect of each pretreatment, the crystallinity of the samples was measured before and after pretreatment. A Brukar D8 Powder X-ray Diffractometer Long Arm was used to measure the crystallinity of each sample. The samples were filled flush to the top of an aluminum sample holder and were scanned at 1 /min from 2q ¼ 10 to 26 with a step size of 0.04 .

low-intensity temperature and pressure pulses produced in hydrodynamic cavitation make it ideal for reactions that require moderately mild temperatures and pressures [35]. This study investigates biomass pretreatment using sonication and hydrodynamic cavitation. The hypothesis is that the implosion of the bubbles created during sonication or hydrodynamic cavitation will alter the cellulose structure and thereby improve enzyme effectiveness during enzyme hydrolysis [36].

2.2. Lime pretreatment A short-term lime pretreatment was used whereby biomass was treated with calcium hydroxide (lime) in the presence of water and boiled at 100  C for 2 h. The ground biomass and calcium hydroxide (0.1 g/g dry biomass) were placed in a metal tray and thoroughly mixed. Distilled water (10 mL/g dry biomass) was added to the dry mixture. The tray was then covered with aluminum foil and heated to 100  C for 2 h. After boiling, the pretreated biomass was neutralized by purging with CO2 until the pH of the mixture was 7. 2.3. Drying After each pretreatment, the samples were air-dried. Before starting the drying procedure, excess water was carefully pressed out of the samples by hand. Then the biomass was spread evenly in long rectangular stainless steel pans that were placed beneath a hood with an air velocity of 0.51 m/s. Once per day, the biomass was turned to ensure even drying. Each sample was considered dry when the moisture content was 10% or less, measured using the NREL standard procedure [37], which usually occurred after a minimum of 2 days. 2.4. Acoustic cavitation Acoustic cavitation (sonication) was performed using a 300-W Fisher Ultrasonic Dismembrator, Model 300 at 60% power as recommended by Coward-Kelly [38]. Pure microcrystalline cellulose (Avicel PH-101), 10-mesh raw bagasse, and 10-mesh lime-treated bagasse were analyzed. The sonication procedure consisted of adding 2.5 dry g of bagasse or Avicel microcrystalline cellulose to 30 mL of water in a 50-mL centrifuge tube. The sample was then placed in an ice bath for 10 min. Finally, the sample was sonicated in the ice bath from 0 to 120 min. The sonication occurred in 15-min increments to allow periodic cooling of the sonicator. 2.5. Hydrodynamic cavitation A modified venturi meter (cavitator) was used to mechanically pretreat the biomass via hydrodynamic cavitation. Hydrodynamic cavitation experiment samples were mixed in an open 200-L jacketed tank with a mixer powered by a variable-speed motor. A low mixing rate of approximately 30 rpm was required to keep the biomass suspended in the water. A centrifugal pump was used to

2.7. Enzymatic hydrolysis To determine enzymatic digestibility from each pretreatment, enzymatic hydrolysis was performed on 2.5 g biomass samples by adding Genecor Spezyme CP cellulase (loading ~5 FPU/g dry biomass), cellobiase (loading ~30 CBU/g dry biomass), citrate buffer (pH of 4.8), and sodium azide (0.01 g/mL), which acted as a microbial inhibitor. The substrate concentration was 50 g/L. To maintain optimal reaction temperature and to ensure thorough mixing, the samples were incubated for 3 days at 50  C inside a shaking incubator. Samples were taken at time 0, 6, and 72 h to determine the concentration of glucose in the sample, the initial digestion rate, and the extent of digestion, respectively. After the incubation period, the samples were boiled to denature the enzymes and stop the enzymatic hydrolysis. Then, the dinitrosalicylic (DNS) assay [39] was performed to determine the equivalent glucose concentration. Each DNS assay was performed in triplicate. 3. Results and discussion To determine the effect of storing the bagasse in the freezer, crystallinity samples were run in duplicate. Results showed that there was a negligible change in crystallinity from storing the bagasse in the freezer for 6 months. 3.1. Acoustic cavitation results Fig. 2 shows the results of acoustic cavitation pretreatment for Table 1 Composition of bagasse samples. Bagasse

Lignin (%)

Glucan (%)

Xylan (%)

Raw Lime-treated

25 16

55 39

18 16

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microcrystalline cellulose and lime-treated bagasse. The microcrystalline cellulose results indicate an increase in the 3d enzymatic digestibilities after acoustic cavitation pretreatment. Compared to untreated microcrystalline cellulose, the 3d enzymatic hydrolysis of the 120-min treatment showed a 37% increase in enzymatic digestibility. Under the same conditions used for the microcrystalline cellulose, acoustic cavitation had no significant effect on the enzymatic hydrolysis of 10-mesh lime-treated sugarcane bagasse within an error band of ±2 standard deviations (approximately the 95% confidence interval). A higher energy input or a higher frequency might be necessary for the acoustic cavitation to affect lignocellulose structure. Lignin and hemicellulose in biomass may absorb part of the shock waves produced by the collapsing bubbles thereby reducing the effect of acoustic cavitation. The presence of lignin and hemicellulose in the structure may also decrease the ability of cellulose to swell, therefore reducing the effect of sonication. For Avicel microcrystalline cellulose, the highest sugar yield is 689 mg sugar/g biomass (120 min @ 60% power).

Energy consumption ¼ ¼

Actual power ðWÞ  Time ðsÞ Sugar produced ðgÞ 300 W  0:6  7200 s 0:689 g sugar 2:5 g biomass  g biomass

¼ 752 kJ=g sugar Assuming electricity costs $0.08/kWh, the energy cost is $16.70/ kg sugar. For bagasse, the highest sugar yield is 535 mg glucose/g biomass (105 min @ 60% power). A similar calculation reveals that the energy cost is $18.80/kg sugar. By over two orders of magnitude, the energy cost of sonication is too high, so other options must be considered. 3.2. Hydrodynamic cavitation results

3-d equivalent glucose yield (mg sugar/g biomass)

All hydrodynamic cavitation tests were performed with 10mesh bagasse. To determine if the centrifugal pump alone mechanically pretreats the bagasse, tests were performed without the

Angle 7º

t

a

b

c

d

Fig. 3. Diagram of cavitator. Dimensions for each cavitator are given in Table 2.

cavitator using raw bagasse. Therefore, a portion of straight 3.81cm-diameter Plexiglass pipe (the same length as the cavitator and designated as Cavitator A) was installed in the experimental apparatus instead of the cavitator. Statistically, there was no significant change in crystallinity or enzymatic hydrolysis for Cavitator A based on a 95% confidence interval (±2 standard deviations); therefore, any reduction in crystallinity was attributed to cavitation and not the centrifugal pump. A diagram of the cavitator is given in Fig. 3 and the dimensions for each are shown in Table 2. Experiments were performed using conditions that showed bubble generation in the cavitator. Table 3 presents the experimental conditions for the hydrodynamic cavitation tests. An outlet pressure of 198 kPa was chosen for Cavitator B because it was the maximum pressure that could be obtained by closing the valve downstream of the cavitator while preventing cavitation in the downstream piping (as could be observed by the returning liquid into the tank). Likewise, the maximum inlet pressure that could be obtained for Cavitator B was 370 kPa. Experimental conditions for Cavitators C and D were also chosen in the same manner and are given in Table 3. All tests were run in duplicate. Figs. 4e6 give the effect of hydrodynamic cavitation on raw and lime-treated bagasse. In Cavitators B and C, the 3-d enzymatic digestibility for bagasse showed an increase of 22% and 27%, respectively. However, at a 95% confidence interval (±2 standard deviations), these results are not significant. In addition, hydrodynamic cavitation was not able to reduce the crystallinity of raw bagasse or lime-treated bagasse in Cavitators B and C.

700 600 500 400 300 200 100 0

0

15

30

45

60

75

90

105

120

Treatment Time (min) Bagasse

Cellulose

Fig. 2. Effect of acoustic cavitation treatment time on microcrystalline cellulose and lime-treated bagasse for 3-d enzymatic digestibility (error bars ¼ ±2 standard deviations, n ¼ 3).

M.J. Madison et al. / Biomass and Bioenergy 98 (2017) 135e141 Table 2 Dimensions of cavitators. Cavitator

Opening (%)

Throat Diameter, t (cm)

a (cm)

b (cm)

c (cm)

d (cm)

B C D

15 20 19

0.572 0.762 0.714

1 1 6.85

3.85 3.85 1.7

1 1 1

13 13 13

For Cavitator B, the bagasse crystallinity remained centered at 52% even though the treatment time increased from 0 to 120 min

139

(Fig. 4(c)). Likewise, for Cavitator C (Fig. 4(d)), the crystallinity of bagasse was centered on 49% as the treatment time increased from 0 to 120 min. For Cavitator B, Fig. 4(e) shows the effect of cavitation on crystallinity for lime-treated bagasse. Although the crystallinity varied over a slightly larger range than for the raw bagasse, the greatest effect on the crystallinity produced an increase from 61% (no treatment) to 64% (20-min treatment), which was not statistically significant within an error band of ±2 standard deviations (approximately the 95% confidence interval). For Cavitator C,

Table 3 Hydrodynamic cavitation experimental conditions. Cavitator

Opening (%)

Cavitator Throat Diameter (cm)

Inlet Pressure (kPa)a

Outlet Pressure (kPa)a

Average Flow (L/s)

Average Temperature ( C)

A B C D

100 15 20 19

3.81 0.572 0.762 0.714

101 370 384 446

101 198 198 170

0.97 1.13 1.25 1.31

22 22 22 22

a

Absolute pressure.

Fig. 4. Effect of hydrodynamic cavitation on 3-d enzymatic hydrolysis for raw bagasse using (a) Cavitator B, (b) Cavitator C. Effect of hydrodynamic cavitation on crystallinity for raw bagasse using (c) Cavitator B, (d) Cavitator C. Effect of hydrodynamic cavitation on crystallinity for lime-treated bagasse using (e) Cavitator B, (f) Cavitator C (error bars ¼ ±2 standard deviations, n ¼ 2).

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M.J. Madison et al. / Biomass and Bioenergy 98 (2017) 135e141

Fig. 5. Effect of hydrodynamic cavitation on 3-d enzymatic hydrolysis for (a) raw, (b) lime-pretreated bagasse using Cavitator D (error bars ¼ ±2 standard deviations, n ¼ 2).

results were similar for lime-treated bagasse. There was no significant change in crystallinity as hydrodynamic cavitation treatment time increased from 0 to 120 min; the crystallinity remained centered at 65%.

Because of the poor results obtained with Cavitators B and C using old bagasse that had been stored for well over a year, fresh bagasse (less than 6 months old) was used for experiments performed with Cavitator D. Figs. 5 and 6 give the enzymatic digestibility results for Cavitator D. This cavitator generated the lowest standard deviations between experiments, which may have resulted because experiments run in Cavitator D used fresh bagasse. In Cavitator D, hydrodynamic cavitation increased the enzymatic digestibility of bagasse and lime-treated bagasse. Best results were obtained using a cavitation treatment of bagasse followed by lime treatment; this increased the 3-d digestion by 46% when compared to lime treatment only. The collapsing bubbles during hydrodynamic cavitation generated “shock waves”; this energy was further transformed into sound and heat. During the 2-h experiment when ice was not added to control the temperature, the temperature of the biomass slurry increased 10e20  C. Further, a loud sound was generated constantly. When low inlet pressures were used, large bubbles were generated but little or no sound was generated, implying that bubbles collapsed with little intensity, thereby not affecting biomass enzymatic digestibility. Low temperatures and high inlet and outlet pressures generated bubbles that collapsed with high enough intensity to affect the lignocellulose structure and give a higher enzymatic digestibility for raw and lime-treated bagasse. Because of the high cost of hydrodynamic cavitation (see the following section), the hydrodynamic experiments were discontinued and therefore, crystallinity measurements were not performed on the biomass used in Cavitator D. For hydrodynamic cavitation pretreatment, the theoretical energy consumption (J/g glucose) was calculated according to Eq. (3) where the pressure difference is measured across the venturi and the pump is assumed to be 100% efficient. Table 4 summarizes the energy consumption and energy cost of sugarcane bagasse (this

Fig. 6. Effect of hydrodynamic cavitation on 3-d enzymatic hydrolysis for multiple conditions using Cavitator D (error bars ¼ ±2 standard deviations, n ¼ 2).

Table 4 Summary of energy consumption and energy cost during hydrodynamic cavitation pretreatment. System

Biomass

Power (W)

Reaction time (h)

Energy consumptiond (kJ/g sugar)

Energy coste ($/kg glucose)

References

Cavitator D þ Lime HC-NaOHa,c HC-SPb,c

Sugarcane bagasse Reed Corn Stover

362 18.2 6.3

2 0.685 1

12.8 9.1 9.4

0.28 0.20 0.21

This study [40] [41]

a b c d e

HC-NaOH: Hydrodynamic cavitation-assisted NaOH pretreatment. HC-SP: Hydrodynamic cavitation-assisted sodium percarbonate pretreatment. Assumes outlet pressure is 101 kPa. Assumes pump is 100% efficient. $0.08/kWh.

M.J. Madison et al. / Biomass and Bioenergy 98 (2017) 135e141

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Energy consumption ðJ=g glucoseÞ ¼

Pressure difference ðPaÞ  Flow rate ðm3 =sÞ  Time ðsÞ Glucose produced ðgÞ [3]

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