Modeling xylan solubilization during autohydrolysis of sugar maple and aspen wood chips: Reaction kinetics and mass transfer

Chemical Engineering Science 64 (2009) 3031 -- 3041

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Modeling xylan solubilization during autohydrolysis of sugar maple and aspen wood chips: Reaction kinetics and mass transfer Ashutosh Mittal ∗ , Siddharth G. Chatterjee, Gary M. Scott, Thomas E. Amidon Department of Paper and Bioprocess Engineering, SUNY College of Environmental Science and Forestry, 1 Forestry Drive, Syracuse, NY 13210, USA

A R T I C L E

I N F O

Article history: Received 1 December 2008 Received in revised form 2 March 2009 Accepted 11 March 2009 Available online 21 March 2009 Keywords: Autohydrolysis Hemicellulose Hot-water extraction Hydrothermal processing Kinetics Mass transfer Modeling Sugar maple Xylan Xylose Xylooligomers Wood chips

A B S T R A C T

A mathematical description is presented for xylan solubilization during hydrothermal pretreatment of sugar maple and aspen wood chips. It assumes first-order kinetics with Arrhenius-type temperature dependence of the reaction rate constants and includes limitations on the transport of reaction products formed in the wood pores to the bulk liquor. The model is calibrated with experimentally measured yields of residual xylan, xylooligomers, and xylose obtained in the autohydrolysis of sugar maple wood chips under pretreatment conditions of 152−175 ◦ C, reaction times of 0−6 h, and liquor to solid ratio of 6:1 in a batch digester. It is validated against experimental data obtained from the autohydrolysis of aspen wood chips at 160 ◦ C under similar experimental conditions as those used for sugar maple. The model can also satisfactorily correlate data of deacetylation of xylan and formation of products (free acetyl groups and acetyl groups attached to xylooligomers in the hydrolyzate) in the autohydrolysis of sugar maple and aspen wood chips with fair accuracy. © 2009 Elsevier Ltd. All rights reserved.

1. Introduction With declining reserves and increasing price of fossil fuels, lignocellulosic materials (LCM) are drawing attention as an economical and a renewable source of energy. Lignocellulosic biomass, a heterogeneous polymeric material, is composed of carbohydrate-based cellulose and hemicellulose, and polyphenol-based lignin (Rubio et al., 1998). The Scandinavian countries, United States, and Canada, which have forest reserves, are paying increased attention to LCM with the hope of becoming self-sufficient in the production of organic fuels and chemicals (Rubio et al., 1998; Sun and Cheng, 2002). In LCM, the two carbohydrate components, i.e., cellulose and hemicellulose, can be decomposed into sugars for further fermentation to ethanol—a fuel additive. According to Garrote et al. (1999), there are economic and environmental benefits in using hot-water extraction for fractionating LCM into its constituents.

∗ Corresponding author at: NREL, 1617 Cole Blvd., Golden, CO 80401, USA. Tel.: +1 303 384 6136; fax: +1 303 384 7752. E-mail addresses: [email protected], [email protected] (A. Mittal). Abbreviations: DI, de-ionized; LCM, lignocellulosic material or materials; NMR, nuclear magnetic resonance; OD, oven dry; ODW, oven-dry untreated wood; RM, raw material; WC, wood chips; WM, wood meal 0009-2509/$ - see front matter © 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.ces.2009.03.011

In earlier works (Garrote et al., 1999, 2002a; Garrote and Parajo, 2002; Jacobsen and Wyman, 2002; Mittal et al., 2009) on hot-water extraction (autohydrolysis) of LCM, no diffusion limitation was assumed for a particle size < 8 mm, and the kinetics of hemicellulose extraction were satisfactorily modeled by first-order reactions with Arrhenius-type temperature dependence of the rate constants. However, to accurately describe the kinetics of xylan solubilization and transport of soluble sugars from the wood pores to the bulk liquor in autohydrolysis of wood chips with a bigger particle size, it is necessary to account for mass-transfer effects. The importance of masstransfer limitations can be clearly observed in Fig. 1, which show experimental data obtained in our laboratory with sugar maple wood meal (WM) and wood chips (WC) at 160 and 175 ◦ C, respectively. More xylan is solubilized with wood meal (Fig. 1-a1) compared to wood chips (Fig. 1-b1) at both autohydrolysis temperatures. Since wood meal has a smaller particle size and a larger specific surface area than wood chips, more glycosidic bonds are accessible for cleavage by hydronium ions resulting in higher xylan solubilization. In wood chips, hydronium ions, which have a typical size of 0.4 nm (Converse et al., 1989), have to penetrate into the wood pores to hydrolyze the glycosidic bonds of xylan molecules. Because of this limited accessibility, autohydrolysis of wood chips gave lower xylan solubilization than wood meal. Figs. 1-a2, b2, a3, and b3 reveal that at both 160 and 175 ◦ C, more xylooligomers and xylose were present

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15 Wood meal Wood chips

12

Xylan (g/100g of ODW)

Xylan (g/100g of ODW)

15

9 6 3 0 2

4

6 3

6

9

6

3

0 0

2

4

6

Xylooligomers (g/100g of ODW)

Xylooligomers (g/100g of ODW)

9

0 0

8

0

0.5

1

1.5

2

0

0.5

1

1.5

2

0

0.5

1

1.5

2

9

6

3

0 8

Xylose (g/100g of ODW)

Xylose (g/100g of ODW)

Wood meal Wood chips

12

6 4 2 0

6 4 2 0

0

2

4 Time (h)

6

Time (h)

Fig. 1. The effect of mass transfer on sugar yields obtained during autohydrolysis of sugar maple wood at 160 ◦ C: (a1) residual xylan in extracted sugar maple wood, (a2) xylooligomers, and (a3) xylose; at 175 ◦ C (b1) residual xylan in extracted sugar maple wood, (b2) xylooligomers, and (b3) xylose.

in the bulk liquor in wood-meal experiments compared to those which used wood chips. The lower concentration of reaction products in the bulk liquor in the case of wood chips can be attributed to restrictions on the transport of xylooligomers and xylose formed inside the wood pores to the bulk liquor. From the above it is evident that during autohydrolysis of wood chips, transport limitations give lower concentrations of xylose and xylooligomers in the hydrolyzate compared to autohydrolysis of wood meal. Thus, to satisfactorily predict the yield of reaction products during autohydrolysis of wood chips, it is essential to account for mass-transfer effects on reaction kinetics. To the best of our knowledge, systematic theoretical and experimental work that includes the effect of mass transfer in the autohydrolysis of wood chips is lacking in the open literature. In this context, Jacobsen and Wyman (2002) noted that in hemicellulose hydrolysis of sugarcane bagasse, increased solids concentration affected the yield of xylose (in oligomeric and monomeric forms) and lowered the pH of the hydrolyzate. Brennan and Wyman (2004) observed that masstransfer considerations had not been incorporated into hemicellulose hydrolysis models. They found that pure mass-transfer models (biphasic and branched-pore leaching models) could also predict the performance of batch and flow systems for hemicelluose hydrolysis as well as, if not better than, reaction-only models. In this paper,

mass-transfer effects are incorporated into the kinetic model presented earlier (Mittal et al., 2009) in order to predict the concentration of xylan hydrolysis products (xylooligomers, xylose, and furfural) in the bulk liquor during autohydrolysis of wood chips. The resulting model is calibrated and validated with experimental data obtained in hot-water extraction of sugar maple and aspen wood chips performed under various conditions. 2. Model development Schematics of a typical wood chip and of pores within it are shown in Figs. 2a and b, respectively. The schematic of xylan solubilization in a wood pore in Fig. 2c shows that xylooligomers, xylose, and furfural formed in the wood pore are transported to the bulk liquor (by diffusion within the pore to the surface of the chip and by convective mass transfer outside the chip) where they undergo further transformation. The following chief assumptions are made in the model: (1) The kinetics of xylan solubilization and the formation of xylooligomers and xylose follow the reaction scheme proposed earlier (Mittal et al., 2009). According to this scheme, xylose monomers are formed directly from wood xylan as well as from

A. Mittal et al. / Chemical Engineering Science 64 (2009) 3031 -- 3041

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2.2. Formation and degradation of xylooligomers Xylooligomers are formed in the wood pores due to xylan solubilization and one portion of them transfers out to the bulk liquor by diffusion within the pores of the chips and convection outside the chips. The degradation of xylooligomers in the pore and bulk liquors occurs via further hydrolysis to xylose. The formation or appearance and degradation of xylooligomers in the pore and bulk liquors are presented below as `reaction in pore liquor' and `reaction in bulk liquor', respectively.

Xylan

2.2.1. Reaction in pore liquor The mass balance describing the formation and degradation of xylooligomers in the wood pores can be written as

k2 Xylose

k1

Furfural

Wood chip

k3

Xylooligomers

kFD

kFC

kFB

Bulk liquor

k3 Xylooligomers

k4

k4 Xylose

Furfural

Fig. 2. Schematic of xylan solubilization in a wood chip during autohydrolysis (a) wood chip; (b) pores formation; (c) formation and transport of products from the pores to the bulk liquor.

oligomers formed due to xylan hydrolysis. The xylose monomers then undergo a dehydration reaction to yield furfural. (2) The void volume in the wood chips (i.e., volume of wood pores) at time t and temperature T is equal to the volume of hydrolyzate present in the wood pores. (3) The densities of the liquor in the wood pores and the bulk liquor are the same.

2.1. Xylan solubilization The concentration of xylan in the wood chips undergoing autohydrolysis at any time t can be expressed as d (MRM XA ) = −(k1 + k2 ){XA − (1 − )XARM }MRM dt

(1)

where k1 and k2 are first-order rate constants for xylan hydrolysis,

 is the susceptible fraction of xylan, MRM is the initial mass of

oven-dry raw wood (ODW) in grams present at the beginning of autohydrolysis (t = 0), XARM is the initial concentration of xylan in grams of potential xylose per gram of ODW at t = 0, and XA is the concentration of xylan at any time t in grams of potential xylose per gram of ODW. Eq. (1) can be simplified to d (MA ) = −(k1 + k2 ){MA − (1 − )MARM } dt

(2)

where MA is the concentration of xylan in grams of potential xylose per 100 g of ODW at time t and MARM is the initial concentration of xylan in grams of potential xylose per 100 g of ODW at the beginning of autohydrolysis. Upon integration of Eq. (2), the following expression for MA is obtained: MA = MARM {(1 − ) + e−(k1 +k2 )t }

(3)

d (XB,pore p Vchips ) = k1 {MA − (1 − )MARM } − k3 XB,pore p Vchips dt   1 + p A(XB,pore − XB,bulk ) − kFB 2

(4)

where XB,pore and XB,bulk are the concentrations of xylooligomers in the pore and bulk liquors, respectively (expressed as g potential xylose/cm3 of pore volume). Vchips is the initial volume of wood chips (cm3 /100 g of ODW) while p is their water-accessible porosity. A is the mass-fraction weighted average surface area of wood chips (cm2 /100 g of ODW), k3 is a first-order rate constant for xylooligomer hydrolysis, and kFB is an overall or apparent mass-transfer coefficient for xylooligomers (cm h−1 ), which is based on an average of the external surface area of the chips (A) and their internal area (p A) that is available to the diffusing xylooligomer molecules within the chips. The linear driving force or mass-transfer coefficient concept embodied in the formulation of the mass-transport term on the righthand-side of Eq. (4) has found widespread use in different areas like absorption of gases in liquids, distillation, adsorption (Tien, 1994), leaching (Plawsky, 2001), and evaporative cooling (van der Smaan, 2003; Mittal et al., 2006). According to Cussler (1997), in contrast to the more fundamental diffusion model (which can predict concentration distributions), the mass-transfer coefficient approach is more approximate and simpler, and is useful when only average concentrations are involved and experimental resources are limited. However, the advantage of the mass-transfer approach taken in this work is that it can be used for modeling the autohydrolysis of biomass with irregular or ill-defined geometry. In such cases, the surface area term for mass transfer can be incorporated into the overall masstransfer coefficient. Also, one of the findings of the present work, as will be seen later, is that the mass-transfer coefficient approach is able to correlate experimental data even in a system where chemical reactions occur. Eq. (4) can be rewritten as d (XB,pore Vporespace ) = k1 {MA − (1 − )MARM } − k3 XB,pore Vporespace dt   1 + p A(XB,pore − XB,bulk ) − kFB (5) 2 where Vporespace is the total volume of pore space in the wood chips that is accessible to water (cm3 /100 g of ODW), which is given by Vporespace (t) = p (t)Vchips

(6)

Converse and co-workers (1989) have shown that as mass from wood chips is removed during autohydrolysis, the pore volume of the chips (Vporespace ) increases with time. This was estimated in this work from the measured mass of hydrolyzate retained in the chips obtained at the end of each autohydrolysis experiment as follows: Vporespace =

Mliq

liq

(7)

Hydrolyzate retained in treated chips (g/100 of ODW)

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A. Mittal et al. / Chemical Engineering Science 64 (2009) 3031 -- 3041

25

140 152 °C (◊, ―) 160 °C (○, ― ―) 167 °C (□, ― - ―) 175 °C (∆, ― - - ―) 160 °C (*, ― ―) Aspen

120

152 °C (◊, ―) 160 °C (○, ― ―) 167 °C (□,― - ―) 175 °C (∆, ― - - ―) 160 °C (*, ------) Aspen

20

1/ (m-meq)

15

100

10

80 5

60 0 1

0

2

3

40

4

6

5

Time (h)

5

0

10

15

20

25

30

Mass removed (%) Fig. 3. Correlation between the mass of hydrolyzate retained in extracted wood chips and the mass removed during autohydrolysis of sugar maple and aspen wood chips. Points are experimental data and lines show trends predicted by Eq. (8).

Fig. 4. Plot of 1/(m − meq ) vs. time t during autohydrolysis of sugar maple and aspen wood chips. Points are experimental data and lines show trends predicted by Eq. (12).

Table 2 Coefficient of determination (r2 ) and rate constant K of Eq. (12) for mass solubilization of sugar maple and aspen wood chips. Table 1 Coefficient of determination (r2 ), slope, and intercept of Eq. (8) for sugar maple and aspen wood chips. T (◦ C)

Slope `m1 '

Intercept `c1 '

r2

152 160 160a 167 175

1.32 1.21 2.14 1.23 1.16

64.35 61.60 81.77 55.90 54.34

0.937 0.973 0.873 0.962 0.972

a

Aspen wood chips.

(8)

where m1 is the slope of the line and c1 is its intercept (see Table 1). Mremoved is given by Mremoved = M

RM

−M

(10)

Here K is a rate constant and Meq is the total mass of wood chips remaining after a long extraction time. Integrating Eq. (10) gives 1 1 = RM + Kt M − Meq M − Meq

r2

152 160 160a 167 175

0.015 0.032 0.021 0.047 0.108

0.985 0.980 0.981 0.994 0.992

a

Aspen wood chips.

1 1 = + (MRM K)t m − meq 1 − meq

(12)

Here m(=M/MRM ) is the mass fraction of extracted wood chips remaining at time t while meq is their mass fraction after a sufficiently long extraction time. [In our experiments, meq = 0.665 for large t (∼8 h).] Plotting experimental values of 1/(m − meq ) (at a particular autohydrolysis temperature) versus t will yield the value of K (see Fig. 4 and Table 2). Solving Eq. (11) for M gives M=

MRM + K(MRM − Meq )Meq t 1 + K(MRM − Meq )t

(13)

From Eqs. (7)−(9) and Eq. (13) it may be shown that m1 Vporespace =

K(MRM − Meq )2 t + c1 1 + K(MRM − Meq )t

liq

(14)

(9)

where M is the OD mass of wood chips remaining after an extraction time of t, whose time evolution could be fitted very well with the second-order model given by dM = −K(M − Meq )2 dt

K (h−1 )

which can also be written as

where Mliq is the mass of hydrolyzate retained in the chips (g/100 g of ODW) and liq is the density of hydrolyzate (assumed to be the same as that of water) expressed in g/cm3 . The weight loss that occurred after oven drying 100 g of extracted chips at 105 ◦ C for 16 h after an autohydrolysis experiment provided a measure of Mliq . Since Mliq is the amount of hydrolyzate retained in pores of the wood chips and Vporespace increases with mass removal (Stone and Scallan, 1967, 1968), it is expected that Mliq should also increase with increased mass removal, viz., Mliq (at any time t) can be expressed as a linear function of Mremoved (see Fig. 3), i.e., Mliq = m1 Mremoved + c1

T (◦ C)

(11)

2.2.2. Reaction in bulk liquor The formation and degradation of xylooligomers in the bulk liquor can be written as   1 + p d (XB,bulk Vbulk ) = kFB A(XB,pore − XB,bulk ) dt 2 (15) − k3 XB,bulk Vbulk where Vbulk is the volume of bulk liquor (cm3 /100 g of ODW). Since the total volume of liquor VT (cm3 /100 g of ODW) remains constant, Vbulk can be expressed as Vbulk = VT − Vporespace

(16)

A. Mittal et al. / Chemical Engineering Science 64 (2009) 3031 -- 3041

Algebraic manipulation of Eqs. (3), (5), and (15) yields    dMB,bulk VT + k3 + kFB A MB,bulk dt Vporespace Vbulk   1 + p kFB AC2 2 = (e−k3 t − e−(k1 +k2 )t ) Vporespace

i.e., furfural, were calculated by solving Eqs. (17), (21), and (22) numerically by the fourth-order Runge–Kutta method with initial conditions of MB,bulk = MC,bulk = MD,bulk = 0 at t = 0.

(17)

where MB,bulk is the mass of xylooligomers (g potential xylose/100 g of ODW) in the bulk liquor, which is given by MB,bulk = XB,bulk Vbulk

(18)

C1 = k1 MARM

(19)

C1 C2 = − {k3 − (k1 + k2 )}

(20)

2.3. Formation and degradation of xylose and formation of furfural In the wood pores, xylose is formed from the hydrolysis of xylooligomers while its depletion occurs by diffusion to the surface of the wood chips and dehydration into furfural. In the bulk liquor, xylose appears due to convective mass transfer from the surface of the chips and hydrolysis of xylooligomers present in the bulk liquor, where it also degrades into furfural. In the wood pores, furfural is formed due to dehydration of xylose, whereas in the bulk liquor, it appears due to convective mass transfer from the surface of the chips and from the dehydration of xylose. The equations describing the time variation of the mass of the xylose (MC,bulk ) and furfural (MD,bulk ) in the bulk liquor can be obtained in a similar manner as that done for xylooligomers; they are shown by Eqs. (21) and (22), respectively.      dMC,bulk 1 + p VT MC,bulk + k4 + kFC A dt 2 Vporespace Vbulk   1 + p A kFC C3 2 e−(k1 +k2 )t = k3 MB,bulk + Vporespace {k4 − (k1 + k2 )}  k 3 C2 e−k3 t + C4 e−k4 t (21) + (k4 − k3 )     dMD,bulk 1 + p VT MD,bulk + kFD A dt 2 Vporespace Vbulk   1 + p A kFD 2 = k4 MC,bulk + (C6 e−(k1 +k2 )t + C7 e−k3 t Vporespace (22)

Here, MC,bulk and MD,bulk are expressed in g/100 g of ODW while the constants C3 −C7 are given by C3 = (k2 MARM − k3 C2 ) C4 = − C5 =

2.4. Relation between overall and individual mass-transfer coefficients in xylan hydrolysis The expression for the rate of transfer Rtrans,j of species j (j = B for xylooligomers, C for xylose, and D for furfural) from the pore space of the wood to the bulk liquor is given by  Rtrans,j = kFj

and

− C4 e−k4 t + C5 )

C3 k3 C2 − (k4 − k3 ) {k4 − (k1 + k2 )}

k4 C3 k3 k 4 C2 + + C4 k3 (k4 − k3 ) (k1 + k2 ){k4 − (k1 + k2 )}

3035

(23) (24) (25)

C6 = −

k 4 C3 (k1 + k2 ){k4 − (k1 + k2 )}

(26)

C7 = −

k3 k 4 C2 k3 (k4 − k3 )

(27)

The transient concentrations of the recovered soluble saccharides, i.e., xylooligomers and xylose, and the sugar degradation product,

1 + p 2

 A(Xj,pore − Xj,bulk )

(28)

where kFj is an overall mass-transfer coefficient for transport of the species from the pore space to the bulk liquid while all other terms have their usual meaning. A mass balance at the solid–liquid interface can be expressed as Rtrans,j = kpj p A(Xj,pore − Xj,surface ) = kfj A(Xj,surface − Xj,bulk ) =

A(Xj,pore − Xj,bulk ) 1/(kpj p ) + 1/kfj

(29)

where kpj and kfj are the (individual) pore-space and liquid-phase mass-transfer coefficients. From Eqs. (28) and (29) 1 1 1 = + kFj (1 + p )/2 kpj p kfj

(30)

which is the law of addition of resistances. 3. Experimental procedures 3.1. Raw material The autohydrolysis experiments were conducted on two wood species, i.e., sugar maple (Acer saccharum) and aspen (Populus tremoloides). Sugar maple wood logs, obtained from ESF Forest Properties, were debarked and chipped in a Carthage chipper located in the Department of Paper and Bioprocess Engineering at SUNY-ESF, and were air dried to a moisture content of 9–12%. The air-dried chips were screened using a vibratory screen to a size normally used in industry (2.5×2.0×0.5 cm3 ). The screened chips were mixed in the same ratio as that obtained after screening, and were stored at room temperature in barrels in a single lot to avoid variation in the experiments. The aspen chips, also obtained from the Department of Paper and Bioprocess Engineering, were screened and stored following the same procedures as that for the sugar maple chips. The chemical composition of oven dried sugar maple and aspen wood is given in Table 3. The mass-fraction weighted average specific Table 3 Composition (% dry basis) of raw materials (sugar maple and aspen). Composition

Sugar maple

Aspen

Glucose Xylose Mannose Galactose Arabinose Rhamnose Acetate Furfural HMF Klason lignin Acid sol. lignin Extractives

45.5 ± 0.7 15.1 ± 0.5 2.4 ± 0.2 2.1 ± 0.9 0.6 ± 0.2 0.8 ± 0.2 3.8 ± 0.1 0.6 ± 0.2 0.5 ± 0.3 22.3 3.8 3.2

47.5 ± 1.8 14.7 ± 0.6 2.6 ± 0.2 1.2 ± 0.5 1.3 ± 0.1 0.7 ± 0.2 3.7 ± 0.3 0.1 ± 0.2 < 0.05 20.0 2.9 nda

a

Not determined.

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A. Mittal et al. / Chemical Engineering Science 64 (2009) 3031 -- 3041

P

T

P

RV1

for sugars and lignin. Sugar analysis of the raw wood and extracted wood samples obtained for each autohydrolysis experiment was performed by 1 H-NMR spectroscopy with a Bruker ADVANCE 600 MHz NMR system using a method described earlier (Mittal et al., 2009). Klason lignin and acid soluble lignin were determined by standard TAPPI methods T 222 om-88 and UM 250, respectively.

T

RV2 3.4. Analysis of hydrolyzate from hydrothermal treatment

P1

HC V

CW HE

P1: Pump P: Pressure gauge RV: Reaction vessel T: Temperature gauge V: Valve CW: Cooling water HE: Heat exchanger HC: Heating coil and liquor recirculation line

Hydrolyzate

Fig. 5. Schematic of the M/K digester used in the autohydrolysis experiments with wood chips.

surface areas of the sugar maple and aspen chips were 18.13 and 18.0 cm2 /g, respectively.

The pH of the hydrolyzate samples after autohydrolysis was measured at room temperature and varied from 6 to 3. About 25 g of a sample were used to determine the total solids content at each extraction condition (time and temperature). The rest of the samples were stored in plastic (Nalgene) bottles in a cold room maintained at 4 ◦ C for later analysis of sugars and sugar degradation products in the hydrolyzate. Sugar analysis of hydrolyzate samples was performed by 1 H-NMR spectroscopy as described earlier (Mittal et al., 2009). To summarize, the determination of monosaccharides, acetyl groups, furfural, and HMF was performed by 1 H-NMR analysis of an aliquot of the hydrolyzate. Another aliquot of the hydrolyzate (5 g) was mixed with 96% H2 SO4 to obtain a 4% (by wt) H2 SO4 , solution, which was then autoclaved at 121 ◦ C for 45 min to convert the oligomers into their monomers. The acid-digested sample was filtered through #1 Whatman filter paper and, upon addition of 0.1 ml of 72% deuterated sulfuric acid to 1 g of the sample, was tested for quantification of the increased monosaccharide concentration with 1 H-NMR. The difference in the monosaccharide concentration of the two samples provided a measure of the concentration of the xylooligomers. In both samples, TMA (trimethylamine hydrochloride) was used as an internal standard (IS).

3.2. Autohydrolysis experiments with wood chips 4. Results and discussion 4.1. Mass removal Fig. 3 shows that the mass of hydrolyzate retained in the wood chips Mliq and the mass removed Mremoved show a high degree of correlation. According to Table 1, the coefficient of determination (r2 ) is between 94% and 97% at all four autohydrolysis temperatures (152–175 ◦ C), which points to the validity of Eq. (8) whose

40

30 Mass removed (%)

Wood chips were treated in a 4.7-liter M/K digester equipped with a centrifugal pump for liquor circulation and a PID temperature controller; a schematic of the experimental arrangement is shown in Fig. 5. At the end of the desired extraction time, the reaction was terminated by switching off the digester and discharging the liquor through a heat exchanger. The normal heating-up time to heat the wood chips to the desired temperature in the digester was about 25–30 min during which period about 5–6% of the wood mass was solubilized. Since this was significant, which could have an important effect on the modeling of the kinetics of hemicellulose solubilization, it was decided to reduce the heating-up period in the experiment. This was achieved by introducing pre-heated dilution water to the digester instead of using regular dilution water [de-ionized (DI) water at an ambient temperature of 24 ◦ C]. The pre-heated dilution water was obtained by heating DI water in another digester, which was then introduced into the M/K digester preloaded with wood chips through a tubing as shown in Fig. 5. By using the preheated dilution water, the heating-up duration of the wood chips was reduced to about 3–5 min. Since this period was very short, any mass solubilization occurring during it was neglected and the beginning of the experiment was assumed to be the beginning of the isothermal stage.

20 152 °C (◊, ―) 160 °C (○, ― ―) 167 °C (□, ― - ―) 175 °C (∆, ------) 160 °C (*, ― ―) Aspen

10 3.3. Sample handling and analysis of wood After an autohydrolysis experiment, the solid residue obtained was weighed immediately and an aliquot was oven dried at 105 ◦ C for 16 h for moisture determination. The treated wood chips were dried and then milled to a particle size that could pass through a 30mesh screen. The milled wood samples obtained after autohydrolysis under different time and temperature conditions were stored in sealed polythene bags at room temperature (24 ◦ C) for later testing

0 0

1

2

3 Time (h)

4

5

6

Fig. 6. Mass removed from sugar maple wood chips at autohydrolysis temperatures of 152−175 ◦ C for sugar maple and 160 ◦ C for aspen wood chips. Points are experimental data and lines show theoretical predictions from Eqs. (9) and (13).

A. Mittal et al. / Chemical Engineering Science 64 (2009) 3031 -- 3041

16 Yield (g/100 g of ODW)

Yield (g/100 g of ODW)

16 12 8 4 0

12 8 4 0

2

0

4

6

0

16

1

2

3

4

16 Yield (g/100 g of ODW)

Yield (g/100 g of ODW)

3037

12 8 4 0

Xylan (◊, ―) Xylooligomers (□, — - ―) Xylose (∆, – –) Furfural (*, -----)

12 8 4 0

1

0

2

3

Time (h)

0.0

0.5

1.0 Time (h)

1.5

2.0

Fig. 7. Experimental and theoretical time courses of residual xylan and xylan solubilization products at (a) 152 ◦ C, (b) 160 ◦ C, (c) 167 ◦ C, and (d) 175 ◦ C with sugar maple wood chips. Points with error bars are average values from triplicate experiments with the standard deviation (95% confidence interval) shown for the 160 ◦ C series. Lines show values calculated with the model using the parameters given in Table 4.

16 Xylan (◊, ―)

Yield (g/100 g of ODW)

Xylooligomers (□, — - ―) Xylose (∆, – –)

12

Furfural (*, -----)

8

4

0 0

2

1

3

Time (h) Fig. 8. Experimental and theoretical time course of residual xylan and xylan solubilization products at 160 ◦ C with aspen wood chips. Lines show values calculated with the model using the parameters given in Table 4.

parameters (m1 and c1 ) are provided in the table. Fig. 6 shows the good fit of the second-order model [Eq. (13)] with experimental mass removal data at four different autohydrolysis temperatures for sugar maple and at 160 ◦ C for aspen. Table 2 reports values of r2 and the rate constant K of mass removal, which increases with temperature. 4.2. Xylan solubilization and formation of products The experimental data and theoretical predictions of xylan solubilization and formation of xylooligomers, xylose, and furfural during autohydrolysis are presented in Fig. 7 (sugar maple) and Fig. 8

(aspen), which indicate that under the experimental conditions considered in this study, sugar maple xylan was solubilized steadily with a maximum removal of about 75% of the initial amount achieved at 175 ◦ C after 2 h during which period only about 30% of the initial xylan was removed at 152 ◦ C. The yield of soluble saccharides (xylooligomers and xylose) in the liquor increased progressively with time at all autohydrolysis temperatures, reached a maximum, and then declined due to the hydrolysis of xylooligomers to xylose and further dehydration of xylose into furfural. The maximum yield of xylooligomers and xylose in the liquor obtained in the autohydrolysis experiments varied from 51% at 152 ◦ C after 4.5 h to 73% at 160 ◦ C after 3 h. To validate the proposed mass-transfer and kinetic model, autohydrolysis experiments were conducted with aspen chips at 160 ◦ C under similar experimental conditions as those used for the sugar maple chips. The model was fitted to experimental data obtained with aspen chips by initially using the same values of the regression parameters as those obtained for sugar maple at 160 ◦ C, which are reported in Table 4. The optimum parameters for correlating the aspen data were then obtained by making an effort to minimize the sum of squares of the deviation between prediction and experiment. Fig. 8 shows the good agreement between theory and experiments conducted with aspen chips, which demonstrates the capability of the model for correlating time courses of xylan degradation and the yield of xylooligomers and xylose during autohydrolysis of wood chips of different species. Comparing Fig. 7b with Fig. 8, we note that for the same amount of xylan solubilization at 160 ◦ C, the yields of xylooligomers and xylose in the hydrolyzate were lower in the case of aspen than for sugar maple. One possible reason for the low yield of soluble sugars in the case of aspen could be the degradation of xylose into furfural. Since the amounts of furfural recovered in the hydrolyzates of both sugar maple and aspen were about the same, the low yields of xylooligomers and xylose in the autohydrolysis of aspen can be attributed to the formation of other intermediate compounds during the dehydration of xylose to furfural under the acidic conditions prevailing in autohydrolysis (Root et al., 1959).

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A. Mittal et al. / Chemical Engineering Science 64 (2009) 3031 -- 3041

Table 4 Values of model parameters in the autohydrolysis of sugar maple and aspen wood chips. T (◦ C)

Model parameters Rate constants (h−1 ) and 

152 160 160a 167 175 a

Mass-transfer coefficients (cm h−1 )

r2 of variables

k1

k2

k3

k4



kFB

kFC

kFD

MA

MB,bulk

MC,bulk

MD,bulk

0.28 0.54 0.43 0.69 1.56

0.024 0.032 0.099 0.060 0.091

0.14 0.16 0.17 0.35 0.69

0.033 0.058 0.032 0.14 0.31

0.805 0.822 0.822 0.810 0.810

0.076 0.34 0.49 0.78 2.46

0.051 0.27 0.72 1.02 3.06

0.66 1.02 1.44 2.11 3.31

0.979 0.978 0.987 0.986 0.996

0.923 0.989 0.917 0.973 0.999

0.969 0.987 0.990 0.988 0.947

0.993 0.969 0.952 0.987 0.994

Aspen wood chips.

1.5 ln(k1) ln(k2)

0.5

ln(k3)

Table 5 Xylan-hydrolysis Arrhenius parameters of sugar maple wood chips [ko i , preexponential factor (h−1 ); Ea i , activation energy (kJ mol−1 )]; ki (T) = ko i exp[−Ea i /(RT)]. Rate constants

ln(ko i )

Ea i (kJ mol−1 )

k1 k2 k3 k4

30.6 23.3 30.6 41.4

112.6 95.8 115.9 158.8

ln (ki), ki in h-1

ln(k4)

-0.5 -1.5 -2.5

Table 6 Values of overall and individual mass-transfer coefficients in xylan hydrolysis.

-3.5 -4.5 0.00222

T (◦ C)

0.00225

0.00228

0.00231

0.00234

0.00237

1/T, K-1 Fig. 9. Arrhenius plots showing the temperature dependence of the kinetic parameters k1 −k4 .

152 160 160a 167 175 a

Table 4 shows that at 160 ◦ C the kinetic parameters for sugar maple and aspen are different and two observations can be made. First, the rate constants k1 (xylan hydrolysis) and k3 (formation of xylose monomers from the hydrolysis of xylooligomers) are of comparable magnitude for both species, whereas the rate constant k2 (formation of xylose monomer directly from wood xylan) is higher for aspen. This implies that during autohydrolysis, in spite of obtaining a similar amount of xylan solubilization, more xylose monomers were generated from the direct hydrolysis of xylan in aspen than in sugar maple. Second, the overall mass-transfer coefficients of xylooligomers, xylose, and furfural are much higher for aspen than for sugar maple. This suggests that these species diffused quickly from the pore liquor to the bulk liquor in aspen compared to sugar maple, which is also reflected in the fact that extracted aspen chips obtained after autohydrolysis were more porous than extracted sugar maple chips as can be seen in Fig. 3. 4.3. Calibration of mass-transfer and kinetic model with xylan-hydrolysis data The proposed mass-transfer and kinetic model was fitted to the experimental data by minimizing the sum of squares of the deviation between prediction and experiment. The regression parameters were derived sequentially, i.e., k1 and k2 were obtained from xylan data; kFB and k3 from xylooligomer data; kFC and k4 from xylose data; and kFD from furfural data. The values of the regression parameters are reported in Table 4. These values were used to establish the temperature dependence of the kinetic coefficients k1 −k4 using the Arrhenius relationship, which is shown in Fig. 9 and Table 5.

Mass-transfer coefficients (cm h−1 ) kFB

kpB

kfB

kFC

kpC

kfC

kFD

kpD

kfD

0.076 0.34 0.49 0.78 2.46

0.65 2.84 2.52 7.36 25.28

0.07 0.30 0.52 0.66 2.04

0.051 0.27 0.72 1.02 3.06

0.44 2.25 3.70 9.62 31.44

0.04 0.23 0.76 0.87 2.54

0.66 1.02 1.44 2.11 3.31

5.66 8.59 7.41 19.80 33.91

0.58 0.90 1.53 1.78 2.74

Aspen wood chips.

The values obtained for  for sugar maple wood chips lie in the range 0.80–0.82 and the average value of 0.81 is close to the value of 0.83 reported earlier for sugar maple wood meal (Mittal et al., 2009), which is also close to the value of 0.84 reported by Garrote et al. (1999) for Eucalyptus globulus. The activation energy of xylan hydrolysis to xylooligomers (k1 ) obtained for sugar maple wood chips in this work is 112 kJ mol−1 , which is close to 117 kJ mol−1 for sugar maple wood meal (Mittal et al., 2009), and is within the range 103-154 kJ mol−1 reported for xylan hydrolysis in the case of hardwoods (Garrote et al., 2002b). The activation energies for xylan and xylooligomer hydrolysis to form xylose (k2 and k3 ) during the autohydrolysis of sugar maple wood chips obtained in this study are 96 and 116 kJ mol−1 , respectively. The activation energies for the same reactions in the autohydrolysis of sugar maple wood meal (Mittal et al., 2009) are 114 and 93.5 kJ mol−1 , respectively. The lower activation energy in the formation of xylose via xylan hydrolysis of wood chips compared to wood meal indicates that the pathway of xylose formation directly from wood xylan is more dominant in wood chips than in wood meal. Furthermore, we note that the reaction rate constants for xylan hydrolysis and formation of xylan hydrolysis products (xylooligomers and xylose) obtained for sugar maple wood meal (Mittal et al., 2009) and wood chips (this work) are not the same. This indicates that the reaction rate constants obtained in this work are not `true' rate constants. However, the wood-meal rate constants can be used as initial guesses for obtaining the rate constants for wood chips. The values of the individual mass-transfer coefficients (kpj and kfj ) obtained from Eq. (30) are reported in Table 6.

A. Mittal et al. / Chemical Engineering Science 64 (2009) 3031 -- 3041

We note that the pore-space mass-transfer coefficients (kpj ) are an order of magnitude higher than the liquid-phase mass-transfer coefficients (kfj ). This shows that once the hydrolysis products are formed in the interior of the wood pores, they quickly diffuse to the surface of the chips with the major resistance to mass transfer to the bulk liquor lying in the liquid phase. Fig. 10 shows predicted time courses of the ratio of solubilized xylose and xylooligomers in the bulk liquor to the total solubilized xylose and xylooligomers present in the bulk liquor and pore volume of the wood chips. It can be observed that this ratio increases slowly at 152 ◦ C but rapidly at all the other temperatures and reaches a plateau of about 0.75–0.82 within less than an hour of extraction time. This indicates that about a fifth of the total solubilized xylose and xylooligomers may be present in the pore volume of wood chips after autohydrolysis.

4.4. Deacetylation

0.6

0.4 152°C 160°C 160°C Aspen 167°C 175°C

0.0 0

1

2

3

4

5

6

Time (h) Fig. 10. Ratio of the mass of solubilized xylose (xylooligomers and xylose) in the bulk liquor to the mass of the total solubilized xylose in the pore and bulk liquors predicted during the autohydrolysis of sugar maple and aspen wood chips.

152°C (◊, ―) 160°C (○, – –) 167°C (□, — - ―) 175°C (∆, -----) 160°C (*, – –) Aspen

3 Acetyl groups in hydrolyzate

Acetyl groups in extracted wood (g/100 g of ODW)

4

2

1

0 0

4

2 Time (h)

6

Monomers (g/100 g of ODW)

MBulk / MTotal

0.8

Oligomers (g/100 g of ODW)

To understand the kinetics of deacetylation and formation of acetic acid, the same kinetic scheme, proposed for xylan solubilization, was used for making theoretical predictions for the amount of acetyl groups remaining in the extracted wood chips and the yield of acetyl groups attached to xylooligomers and those present in the hydrolyzate as free acetic acid. According to this scheme, a fraction of the acetyl groups cleaved from the main xylan chain remains attached to xylooligomers whereas another fraction yields acetic acid, which further dissociates to yield hydronium ions that catalyze the hydrolysis reaction of hemicellulose. Eqs. (3), (17), and (21) were used to predict the amount of acetyl groups remaining in the extracted wood chips, those attached to xylooligomers, and those present in the hydrolyzate as free acetic acid, respectively; the experimental data and theoretical predictions are shown in Fig. 11. We see in Fig. 11a that the acetyl group content of the treated residue decreased steadily and followed similar kinetic trends to those observed in xylan hydrolysis. It can be further noted that more than 85% of the initial acetyl groups were removed during the autohydrolysis of sugar maple wood, which is consistent with the deacetylation value reported for eucalyptus wood (Garrote et al., 2001). It can also be postulated that the hydrolysis of acetyl groups from the xylan chain follows two reaction pathways: direct hydrolysis of acetyl groups substituted on the solid xylan chain and the hydrolysis of acetyl groups substituted on the xylooligomers present in the hydrolyzate. The concentrations of acetyl groups bound to xylooligomers and those of free acetyl groups present in the hydrolyzate increase with time at a particular temperature, and have higher values at higher temperature for the same reaction time (Figs. 11b and c). This shows the temperature dependence of deacetylation in autohydrolysis. From post hydrolysis of the hydrolyzate samples it was revealed that only about 30–40% of the total acetyl groups cleaved from the xylan backbone remained attached to xylooligomers in the hydrolyzate while about 60–75% of acetyl groups contained in the raw material were obtained as acetic acid.

1.0

0.2

3039

1.0 0.8 0.6 0.4 0.2 0.0 0

2

0

2

4

6

4

6

3

2

1

0 Time (h)

Fig. 11. Experimental and theoretical time courses of (a) residual acetyl groups in extracted wood chips, (b) acetyl groups attached to xylooligomers, and (c) free acetyl groups in the hydrolyzate obtained during autohydrolysis of sugar maple and aspen wood chips. Lines show values calculated with the model using the parameters given in Table 7.

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A. Mittal et al. / Chemical Engineering Science 64 (2009) 3031 -- 3041

Table 7 Values of model parameters in the deacetylation of sugar maple and aspen wood chips (B = acetyl groups attached to oligomers and C = free acetyl groups in the liquor). T (◦ C)

Model parameters Rate constants (h−1 ) and 

152 160 160a 167 175 a

Mass-transfer coefficients (cm h−1 )

r2 of variables

k1

k2

k3

k4



kFB

kFC

MA

MB,bulk

MC,bulk

0.28 0.53 0.43 0.69 1.56

0.02 0.03 0.03 0.04 0.09

0.56 0.81 0.81 1.18 2.59

0.56 1.89 1.31 2.30 3.95

0.81 0.82 0.82 0.81 0.81

0.93 1.31 0.99 1.57 1.87

1.93 2.17 0.84 2.42 3.06

0.974 0.972 0.998 0.979 0.993

0.962 0.890 0.915 0.927 0.936

0.979 0.992 0.992 0.976 0.983

Aspen wood chips.

Table 8 Deacetylation Arrhenius parameters of sugar maple wood chips [ko i , pre-exponential factor (h−1 ); Ea i , activation energy (kJ mol−1 )]; ki (T) = ko i exp[−Ea i /(RT)]. Rate constants

ln(ko i )

Ea i (kJ mol−1 )

k1 k2 k3 k4

31.9 26.2 23.1 5.4

117.1 104.4 82.6 32.0

c1 C1 , C2 C3 –C7 Eai ki

kfj The values of the mass-transfer and kinetic coefficients for deacetylation of sugar maple and aspen wood chips are given in Table 7 while Table 8 reports the deacetylation Arrhenius parameters for sugar maple. The activation energy of deacetylation to form acetyl groups attached to xylooligomers obtained for sugar maple wood chips in this work is 117 kJ mol−1 , which is close to 121 kJ mol−1 determined for hardwoods as reported by Garrote et al. (2002b).

kpj

koi kFB

5. Conclusions

kFC

Hydrothermal treatment of sugar maple wood selectively solubilized hemicellulose. A maximum xylan solubilization of about 75% of the initial amount was achieved at 175 ◦ C after 2 h whereas only about 30% of the initial xylan was removed in the same duration at 152 ◦ C. The yield of soluble saccharides as xylooligomers and xylose in the liquor increased progressively at all the autohydrolysis temperatures used, reached a maximum, and then declined because of the hydrolysis of xylooligomers to xylose and dehydration of xylose to furfural. The maximum yield of xylooligomers and xylose in the liquor obtained in the autohydrolysis experiments varied from 51% at 152 ◦ C after 4.5 h to 73% at 160 ◦ C after 3 h. To predict the yields of residual xylan, xylooligomers, and xylose obtained during autohydrolysis, a mass-transfer model, which assumed first-order reaction kinetics, was developed. The model was calibrated with experimental data obtained in the autohydrolysis of sugar maple wood chips performed at temperatures of 152–175 ◦ C, reaction times of 0–6 h, and liquor to solid ratio of 6 g/g in a batch digester. The experimental concentrations of the products at different temperatures were well described by the model. The model was further validated against experimental data obtained with aspen chips at 160 ◦ C under similar reaction conditions as those used for sugar maple. The ability of the model to correlate the empirically observed data of xylan hydrolysis and deacetylation during autohydrolysis of sugar maple and aspen wood chips demonstrates the applicability of the underlying theoretical framework for investigating hemicellulose hydrolysis of other LCM.

kFD

Notation % A

wt% based on oven-dried untreated wood (ODW) mass-fraction weighted average surface area of wood chips, cm2 /100 g of ODW

K m meq m1 M MRM MA MARM MB,bulk MC,bulk MD,bulk Meq Mliq Mremoved r2 R Rtrans,j

t T Vbulk Vchips Vporespace VT XA

constant in Eq. (8) constants defined by Eqs. (19) and (20), respectively constants defined by Eqs. (23)–(27), respectively activation energy in the Arrhenius expression for ki , kJ mol−1 first-order rate constant for the hydrolysis of xylan, xylooligomers, xylose, and furfural (i = 1, 2, 3, and 4), respectively, h−1 liquid-phase mass-transfer coefficient for species j (j = B for xylooligomers, C for xylose, and D for furfural, respectively), cm h−1 pore-space mass-transfer coefficient for species j (j = B for xylooligomers, C for xylose, and D for furfural, respectively), cm h−1 pre-exponential factor of ki , h−1 overall mass-transfer coefficient for xylooligomer transport from the wood pores to the bulk liquor, cm h−1 overall mass-transfer coefficient for xylose transport from the wood pores to the bulk liquor, cm h−1 overall mass-transfer coefficient for furfural transport from the wood pores to the bulk liquor, cm h−1 rate constant for mass solubilization of wood chips, h−1 M/MRM Meq /MRM constant in Eq. (8) OD mass of wood chips in the digester at any time t, g OD mass of raw wood at the beginning of autohydrolysis, g concentration of xylan in the solid phase at time t, % concentration of xylan in the raw material, % mass of xylooligomers present in the bulk liquor, % mass of xylose present in the bulk liquor, % mass of furfural present in the bulk liquor, % OD mass of wood chips in the digester after a long extraction time, g mass of hydrolyzate retained in the wood chips, % mass removed (MRM − M), g coefficient of determination universal gas constant, 8.3144×10−3 kJ (mol K)−1 rate of transfer of species j (j = B for xylooligomers, C for xylose, and D for furfural, respectively) from the pore space of wood to the bulk liquor, g h−1 time, h temperature, K volume of the bulk liquor, cm3 /100 g of ODW initial volume of the wood chips, cm3 /100 g of ODW pore volume of the wood chips, cm3 /100 g of ODW total volume of liquor (Vbulk + Vporespace ), cm3 /100 g of ODW concentration of xylan in the solid phase, g potential xylose/g of ODW

A. Mittal et al. / Chemical Engineering Science 64 (2009) 3031 -- 3041

XARM XB,bulk XB,pore XC,bulk XC,pore XD,bulk XD,pore Xj,bulk Xj,pore Xj,surface

concentration of xylan in the raw wood at the beginning of autohydrolysis, g potential xylose/g of ODW concentration of xylooligomers in the bulk liquor, g potential xylose/cm3 concentration of xylooligomers in the wood-pore liquor, g potential xylose/cm3 concentration of xylose in the bulk liquor, g cm−3 concentration of xylose in the wood-pore liquor, g cm−3 concentration of furfural in the bulk liquor, g cm−3 concentration of furfural in the wood-pore liquor, g cm−3 concentration of species j in the bulk liquor, g cm−3 concentration of species j in the wood-pore liquor, g cm−3 liquid-phase concentration of species j at the surface of the wood chips, g cm−3

Greek letters

 p liq

susceptible or accessible fraction of xylan in wood porosity of wood chips density of hydrolyzate, g cm−3

Acknowledgement The authors thank Dr. A. J. Stipanovic and Mr. D. J. Kiemle for help with 1 H-NMR testing. References Brennan, M.A., Wyman, C.E., 2004. Initial evaluation of simple mass transfer models to describe hemicellulose hydrolysis in corn stover. Appl. Biochem. Biotechnol. 115–116, 965–976. Converse, A.O., Kwarteng, I.K., Grethlein, H.E., Ooshima, O., 1989. Kinetics of thermochemical pretreatment of lignocellulosic materials. Appl. Biochem. Biotechnol. 20–21, 63–78.

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