Accepted Manuscript A novel diffusion-biphasic hydrolysis coupled kinetic model for dilute sulfuric acid pretreatment of corn stover Longjian Chen, Haiyan Zhang, Junbao Li, Minsheng Lu, Xiaomiao Guo, Lujia Han PII: DOI: Reference:

S0960-8524(14)01662-9 http://dx.doi.org/10.1016/j.biortech.2014.11.060 BITE 14266

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Bioresource Technology

Received Date: Revised Date: Accepted Date:

13 September 2014 12 November 2014 13 November 2014

Please cite this article as: Chen, L., Zhang, H., Li, J., Lu, M., Guo, X., Han, L., A novel diffusion-biphasic hydrolysis coupled kinetic model for dilute sulfuric acid pretreatment of corn stover, Bioresource Technology (2014), doi: http://dx.doi.org/10.1016/j.biortech.2014.11.060

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A novel diffusion-biphasic hydrolysis coupled kinetic model for dilute sulfuric acid pretreatment of corn stover Longjian Chen a, Haiyan Zhang a, Junbao Li a, Minsheng Lu a, Xiaomiao Guo a, and Lujia Han a, * a

China Agricultural University (East campus), 17 Qing-Hua-Dong-Lu, Hai-Dian

District, Beijing 100083, P. R. China *

Corresponding author: Prof. Lujia Han, P.O. Box 191, College of Engineering, China Agricultural University (East campus), 17 Qing-Hua-Dong-Lu, Haidian district, Beijing 100083, P. R. China. Telephone: 86-10-62736313, Fax: 86-10-62736778 Email: [email protected], [email protected]

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Abstract Kinetic experiments on the dilute sulfuric acid pretreatment of corn stover were performed. A high xylan removal and a low inhibitor concentration were achieved by acid pretreatment. A novel diffusion-hydrolysis coupled kinetic model was proposed. The contribution to the xylose yield was analyzed by the kinetic model. Compared with the inhibitor furfural negatively affecting xylose yield, the fast and slow-hydrolyzing xylan significantly contributed to the xylose yield, however, their dominant roles were dependent on reaction temperature and time. The impact of particle size and acid concentration on the xylose yield were also investigated. The diffusion process may significantly influence the hydrolysis of large particles. Increasing the acid concentration from 0.15 M to 0.30 M significantly improved the xylose yield, whereas the extent of improvement decreased to near-quantitative when further increasing acid loading. These findings shed some light on the mechanism for dilute sulfuric acid hydrolysis of corn stover.

Keywords: diffusion; dilute acid; hydrolysis; kinetic model; lignocellulose.

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1. Introduction The conversion of lignocellulose substrate to biofuel by using biological enzymes has attracted tremendous interest because of its potential for addressing problems such as climate change, energy security, and rural economic development (Himmel, 2007). Corn (Zea mays L.) stover is an inexpensive, abundant and renewable lignocellulose biomass; its cell wall is primarily composed of cellulose, hemicellulose, and lignin. Similar to other lignocellulosic substrates, the physiochemical, structural and compositional features of corn stover cell wall create a protective barrier and prevent enzymatic depolymerization of structural polysaccharides (Sanderson, 2011). To destroy the compact lignin–hemicellulose–cellulose network structure so as to improve the enzyme hydrolysis conversion efficiency, a number of pretreatment methods, such as dilute acid, alkaline, ionic liquid, organosolv, ammonia fiber explosion, and biological pretreatments, have been explored (Avci et al., 2013; Wang et al., 2013). Pretreatment with dilute sulfuric acid, which is a strong acid, can effectively degrade hemicellulose in a cell wall network and has been extensively investigated using experimental and theoretical approaches. Experimental studies of dilute sulfuric acid pretreatment have been performed using wheat straw (Rajan and Carrier, 2014), rice straw (Hsu et al., 2010), sorghum bagasse (Dogaris et al., 2012), rapeseed straw (Choi et al., 2013), sugarcane bagasse (Chen et al., 2011), sunflower stalks (Ruiz et al., 2013), and switchgrass (Shi et al., 2011). Because pretreatment experiments are frequently time-consuming, labor-intensive, and environmentally unfavorable, some efforts have also been dedicated to modeling dilute sulfuric acid hydrolysis kinetics of 3

lignocellulose substrate. The simplest kinetic model of hemicellulose hydrolysis was based on Saeman’s study of cellulose hydrolysis (Saeman, 1945). It is a two-step pseudo-first-order irreversible reaction where the xylan is hydrolyzed directly to xylose which is subsequently degraded to furfural. Based on the observation that the reaction rate decreased significantly at a subsequent stage of hydrolysis, Kobayashi and Sakai (1956) proposed a biphasic hydrolysis model where the xylan was divided into fast-reacting part and slow-reacting part (Kobayashi and Sakai, 1956). Although the Saeman model and the biphasic hydrolysis model have been widely applied to predict hemicellulose hydrolysis kinetics in the sulfuric acid pretreatment of lignocellulose substrates (Guerra-Rodriguez et al., 2012; Lu and Mosier, 2008; Yat et al., 2008), they only consider the hydrolysis as the rate-limiting step and assume that acid impregnation into porous lignocellulose materials occurs at a rapid rate. Acid impregnation is a diffusion process that represents the rate-limiting step of the prehydrolysis stage, especially for a lignocellulose substrate with a large size, low porosity and high pore tortuosity (Kim and Lee, 2002). Some previous studies have investigated diffusion process within porous lignocellulose particles. Kim and Lee (2002) proposed an unsteady-state diffusion model of sulfuric acid within the biomass matrix and assessed the effect of acid diffusion in dilute acid pretreatment. Their study mainly focuses on acid diffusion process and does not consider the subsequent hydrolysis process of hemicellulose. Additionally, the diffusion model does not take into account the porous geometrical structure of biomass particle. Based on the porous geometrical structure of sugar maple and aspen wood chips, Mittal et al. (2009)

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presented a mathematical model to describe xylan solubilization during autohydrolysis pretreatment (Mittal et al., 2009), where the reaction kinetics of hemicellulose hydrolysis and diffusion transport of soluble sugars from the wood pores to the bulk liquid were incorporated. Due to that their model emphasized on hemicellulose hydrolysis kinetics during autohydrolysis pretreatment, it is not applicable for dilute acid pretreatment. This study firstly executed the kinetic experiments of the dilute sulfuric acid pretreatment of corn stover. Based on the porous geometrical structure and composition of corn stover particle (Figure 1), a novel general model that combines diffusion and biphasic hydrolysis was proposed to characterize the kinetics of dilute sulfuric acid pretreatment. The refined model was employed to quantitatively analyze the contributions of fast-hydrolyzing xylan, slow-hydrolyzing xylan and the inhibitor furfural to xylose yield. The impact of particle size and acid concentration on the xylose yield was also investigated. 2. Materials and Methods 2.1 Samples and chemicals In 2013, corn stover feedstock was collected from the Shangzhuang experimental station of the China Agricultural University in Beijing, China (Latitude 40º02 N, Longitude 116º20 E). The corn stover was air dried and ground in a Knifetec 1095 sample mill fitted with a 40-mesh screen (Foss Tecator, Hoganas, Sweden). The samples were stored in a sealed plastic bag at 4 °C before use in all experiments. Xylose and furfural were purchased from Sigma-Aldrich (St. Louis, MO, USA). 5

Sulfuric acid was purchased from Beijing Chemical Works (Haidian district, Beijing, China). 2.2 Dilute acid pretreatment Glass vials with a pressure-release crimp seal (C4020-10 and C4020-32AP, Thermo Fisher Scientific Inc., Rockford, IL USA) were employed as the reactors for the hemicellulose hydrolysis kinetics study. Corn stover with a dry mass of 0.6 g and 6 mL sulfuric acid solution with concentration of 0.15 M was loaded into the reactor vials. The reactor vials was then lowered into a silicon oil bath (DKU-30, Jinghong Laboratory, Instrument Co. Ltd., Shanghai, China) which was preheated to target reaction temperatures (110 °C, 120 °C, and 130 °C). The bath temperature was kept within ± 1 °C of the target temperature. Due to the little size of reactors (10 mL glass vial), the reactants can be quickly brought up to the desired temperature and remain at that temperature. The reactions were stopped by immersing the reactor vials into cool water after 2, 5, 10, 20, 30, 40, 50, and 60 min. Triplicates were run for each reaction condition. Based on the triplicate experiments, standard errors on each data point were computed to indicate 95% confidence intervals for each point. 2.3 Chemical analyses The carbohydrates and lignin concentrations of the raw corn stover were determined by the laboratory analytical procedures proposed by the National Renewable Energy Laboratory, USA (NREL/TP-510-42618). The hydrolysate was diluted two times with deionized water. The liquor was filtered through a 0.22 µm filter and analyzed for sugar and by-product concentration with a HPLC system

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(Hitachi L-7200 with refractive index detector L-2490, Hitachi Ltd., Tokyo, Japan). The concentrations of xylose and furfural were obtained by the HPLC system, which was equipped with a BP-800 H+ carbohydrate column (Benson Polymeric, Reno, NV, USA). The mobile phase consisted of 5 mM sulfuric acid in ultrapure water (Milli-Q, Millipore, Billerica, MA). The operating conditions for the HPLC column were 55 °C with a mobile phase flow rate of 0.6 mL/min. For the sample analysis, 20 µL of sample was injected with an elution time per injection of 50 min. 2.4 Diffusion-biphasic hydrolysis coupled kinetic model The geometrical structure of corn stover particles was modeled as porous cylinders of infinite length (Figure 1) based on Luterbacher et al.’s study (Luterbacher et al., 2013). These authors accounted for the heterogeneity of biomass particle and proposed a pore-hindered diffusion and kinetic model for enzymatic hydrolysis of biomass. Some important concepts such as porosity, tortuosity and density in their model were also adopted in current model for dilute acid hydrolysis. Although the acid diffusion model of Kim and Lee was also referred (Kim and Lee, 2002), two limitations were improved in current model. On one hand, current acid diffusion model considered the porous geometrical structure by relating to porosity and tortuosity properties of biomass particles. On the other hand, in order to improve model predictive power the effective diffusivity was determined by closed-form equation not by fitting to experimental data. Detailed derivation of the diffusion-biphasic hydrolysis coupled kinetic model was described as follows. Considering that acid diffusion process is precondition of subsequent hydrolysis

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process, this study only considered the acid diffusion but not the diffusion of hydrolysis products. The acid diffusion model involved an acid balance in the solid particles and the bulk liquid. The acid balance in the liquid phase was expressed by the following equation VL

ε (t ) D dC AS (r , t ) dC AL (t ) = −Ss ∂t τ dr

(1)

r = R1

where t is the reaction time, VL is the total volume of the liquid phase, CAL(t) is the concentration of sulfuric acid in the liquid phase, CAS(r,t) is the concentration of sulfuric acid in a solid particle, SS is the total surface area of the solid particles, ε(t) is the porosity of a solid particle, τ is the tortuosity of a solid particle, R1 is the radius of a particle, r is the diffusion distance, and D is the diffusion coefficient of sulfuric acid in water, which is dependent on temperature. Umino and Newman (1997) proposed the following model (Umino and Newman, 1997)  1 1  −4 D = exp c0  −  × c1 × 10   T 298.15 

(2)

where T is the reaction temperature and c0 and c1 are related to the applied concentration of sulfuric acid (C0) as follows: 0.5

1.5

2

1.5

2

c0 = exp{7.699+ 0.2352[1- exp(-12C0 )] - 0.2977C0 + 0.04164C0 + 0.02023C0 } (3) 0.5

c1 = exp{-10.56− 0.3666[1- exp(-12C0 )] - 0.1107C0 + 0.2888C0 − 0.00915C0 } (4)

Diffusion into the corn stover particles was described by the following partial difference equation (PDE): ∂C AS (r , t ) ε (t ) D 1 ∂  ∂C AS (r , t )  = r  ∂t τ r ∂r  ∂r 

(5)

The sulfuric acid that permeated into the solid particles subsequently degraded the

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hemicellulose. The hydrolysis reaction of the hemicellulose was described by the following biphasic model

dCHf (t )

dt

= −k1 f (t ) C Hf (t )

(6)

dCHs (t ) = −k1s (t ) C Hs (t ) dt dC X (t ) = k1 f (t ) CHf (t ) + k1s (t ) CHs (t ) − k2 (t ) C X (t ) dt dCP (t ) = k2 (t ) C X (t ) dt

(7) (8) (9)

where CHf(t), CHs(t), CX(t) and CP(t) are the concentrations of the fast-hydrolyzing xylan, slow-hydrolyzing xylan, the xylose and furfural, respectively; k1f(t), k1s(t) and k2(t) are the reaction rate constants for the xylose formation from the fast-hydrolyzing xylan, the xylose formation from the slow-hydrolyzing xylan and the xylose degradation, respectively. The reaction rate constants were assumed to follow an Arrhenius-type expansion equation  E n k1 f (t ) = A1 f [C AS (t )] 1 f exp − a1 f  RT

  

(10)

 E  k1s (t ) = A1s [C AS (t )]n1s exp − a1s   RT 

(11)

 E  k 2 (t ) = A2 [C AL (t )]n2 exp − a 2   RT 

(12)

where Ai, Eai and ni are the Arrhenius pre-exponential factors, the activation energies and acid concentration exponent for each reaction, respectively; R is the gas constant, and C AS (t) is the average acid concentration in a solid particle, which can be calculated by the following equation:

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C AS

∫ (t ) =

R

0

C AS (r , t ) × 2πrdr

(13)

πR12

Owing to the continuous degradation of xylan, the porosities of the solid particles also change and subsequently affect the acid diffusion and xylan hydrolysis. The porosities of the solid particles evolved as a function of the xylan volume fraction

VFX(t) according to the following relationship

ε (t ) = 1 − VFG − VFX (t ) − VFL − VFO

(14)

where VFG, VFL and VFO are the volume fractions of glucan, lignin and other compositions, such as ash in the solid particles, respectively. Based on the assumption that the dilute acid pretreatment primarily hydrolyzes xylan, VFG, VFL and VFO as constants can be related to the mass concentrations in raw material and the specific volume per mass of raw material (Vt) VFG =

MG ρ CVt

Vt = VP +

MG

ρG

VFL =

ML ρ LVt

MX

ML

+

ρX

+

ρL

VFO =

+

MO ρOVt

MO

ρO

(15)

(16)

where MG, MX, ML, and MO and ρG, ρX, ρL, and ρO are the mass percentages on a dry basis and the densities of glucan, xylan, lignin and other compositions in raw materials, respectively, and VP is the pore volume per mass of raw material. The xylan volume fraction (VFX) can be related to the concentrations of fast-hydrolyzing xylan and slow-hydrolyzing xylan as

VFX (t ) =

[C Hf (t ) + C Hs (t )] × VL

ρ X × VS

(17)

where Vs is the total volume of solid particles. By substituting Eq. (17), Eq. (14) can be

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expressed as

ε (t ) = 1 − VFG −

[C Hf (t ) + CHs (t )] × VL

ρ H × VS

− VFL − VFO

(18)

The boundary and initial conditions were listed as t>0

∂C AS ( r , t ) ∂r

t =0

C AL (t ) t=0 = C0 C AS (r , t ) t =0 = 0 CHf (t ) t =0 = ϕCH 0  CHs (t ) t =0 = (1 − ϕ )CH 0 C X (t ) t =0 = 0 CP (t ) t =0 = 0 ε (t ) t =0 = ε 0

r =0

= 0 C AS ( r , t ) r =R = C AL

(19) (20)

where CH0 is the initial concentration of xylan in a solid particle, ε0 is the initial porosity of a solid particle, φ is the mass fraction of fast-hydrolyzing xylan to the total xylan in raw material. The diffusion-hydrolysis coupled kinetic model contained seven dependent variables (CAL(t), CAS(r,t), CHf(t), CHs(t), CX(t), CP(t), and ε(t)) and can be numerically solved by combining Eqs. (1), (5), (6), (7), (8), (9), and (18). The predicted xylose concentrations were compared with the observed values and were used to estimate model parameters. The model parameters (A1f, A1s, A2, Ea1f, Ea1s, Ea2, n1f, n1s, n2, and φ) were simultaneously fitted to all experimental data using a custom written program in MATLAB software (Mathworks, Natick, MA, USA). Table 1 provides a description of these symbols. 3. Results and Discussion

3.1. Xylan hydrolysis The composition of the corn stover was as follows: 32.92 ± 0.51% (Mean ± SD) glucan, 16.53 ± 0.24% xylan, and 17.36 ± 0.28% lignin on a dry-weight basis. This range of values is similar to the ranges of values reported by other researchers for 11

lignocellulose materials (Jin et al., 2011). Xylose was the main product of the sulfuric acid-catalyzed xylan hydrolysis reactions. If xylan polymer is assumed to be completely converted to xylose without additional degradation, the maximum potential xylose concentration (CXM) from these hydrolysis experiments was calculated by the following equation C XM =

150 M X × 132 LSR

(21)

where LSR is the ratio of liquid volume to solid mass on a dry basis (10 mL g-1) and 150/132 is the stoichiometric factor for the interrelationship between xylose and xylan. Applying Eq. (21), a maximum potential xylose concentration of 18.78 g L-1 was obtained. Figure 2 shows the xylose concentration profiles. The data indicated that higher xylose concentrations are achieved at higher reaction temperatures. This effect is pronounced with the observed maximum xylose concentrations in the range of ca. 45% (at 110 °C) to ca. 80% (at 130 °C) of the potential concentration (CXM). The results also suggested that dilute sulfuric acid pretreatment at a moderate temperature allows a significant amount of xylan to be removed (ca. 70% at 120 °C for 1 h). For all experimental conditions, the time courses of xylose concentration were divided into two stages: the initial fast-hydrolyzing stage (≤ 5 min) and the subsequent slow-hydrolyzing stage (5-60 min). In the initial fast-hydrolyzing stage, the xylose concentration rapidly increased with reaction time, whereas the xylose concentration moderately increased and trended toward a maximum value in the subsequent slow-hydrolyzing stage. This finding may be attributed to the biphasic pattern of xylan.

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The xylose in the initial stage is predominantly produced from fast-hydrolyzing xylan, whereas the xylose in the subsequent stage is produced from the slow-hydrolyzing xylan. Note that the xylose concentration always increased with reaction time even if the xylan removal ratio approached ca. 80%. This observation differed from previously reported results regarding high-temperature dilute acid pretreatment, in which the xylose yield frequently decreased in the subsequent hydrolysis stage because of a long reaction time and a high reaction temperature, which rapidly caused the degradation of xylose to furfural (Morinelly et al., 2009). 3.2. Decomposition of xylose During dilute acid hydrolysis, furfural is usually generated by the degradation of pentoses in parallel to the formation of sugars. However, the furfural yield is dependent on the pretreatment severity (pretreatment temperature, reaction time, and acid concentration). In this study, the furfural concentrations were maintained at minimal levels and were only detectable at 30, 40, 50 and 60 min points at 130 °C (Figure 3). Using the stoichiometric factor between furfural and xylan (96/132), a maximum potential furfural concentration of 12.02 g L-1 was obtained. The maximum value of furfural concentrations was 0.23 g L-1 in the experiments performed at 130 °C for 60 min, which corresponded to 1.91% of the calculated potential furfural concentration. The results also corresponded with the increasing xylose concentration during the pretreatment time because a small amount of xylose was degraded into furfural. As a byproduct, furfural has an inhibiting effect on the efficiency of the saccharification of cellulose and the fermentation of sugars for bioethanol production.

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Thus, the dilute acid hydrolysis process with a low furfural concentration is preferable. Dilute sulfuric acid pretreatment at a moderate temperature may provide a route for achieving the desired hydrolysis process with a high xylan removal rate and a low inhibitor concentration. 3.3. Diffusion-biphasic hydrolysis coupled kinetic model The diffusion-hydrolysis coupled kinetic model was simultaneously fitted to all experimental data for the xylose concentrations. Figure 2 shows the experimental and predicted data for the xylose concentrations. Table 2 lists the kinetic and statistical parameters of the fitting. The statistical parameters R2 (= 0.97) and root mean squared error (RMSE = 0.14 g L-1) indicated that the diffusion-hydrolysis coupled kinetic model achieved a suitable fit. The diffusion-hydrolysis coupled kinetic model was also employed to predict the furfural concentration (Figure 3). The predicted furfural concentration corresponded with the observed data. These positive outcomes demonstrated the robust predictability of the proposed kinetic model. The mass fraction ratio of the fast-hydrolyzing xylan to the total xylan in the raw material φ was estimated to be 0.30, which indicated that the slow-hydrolyzing fraction accounts for a large percentage of xylan. The reported values of φ varied with the substrates and pretreatment conditions. Guerra-Rodriguez et al. (2012) reported values of φ in the range of 0.18-0.63 with a mean value of 0.31 for wheat straw pretreated by sulfuric acid at 130 °C (Guerra-Rodriguez et al., 2012). For sweet sorghum bagasse pretreated by sulfuric acid below 150 °C, the reported values of φ ranged from 0.14-0.33 (Liu et al., 2012). For corn stover pretreated by dilute nitric acid above 150

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°C, Zhang et al. (2010) obtained values of φ in the range of 0.43-0.88 (Zhang et al., 2011). Aguilar et al. (2002) reported values of φ in the range of 0.55-0.83 for sugarcane bagasse pretreated by more than 2% sulfuric acid (Aguilar et al., 2002). Table 3 summarizes the comparison of the kinetic parameters obtained in this study with the parameters derived from the literature for different substrate materials. A direct comparison was difficult because of the differences in substrates, reaction conditions, and kinetic models. All kinetic parameters in this study fell in the range reported in the literature for lignocellulose materials. Applying these fitting kinetic parameters into Eqs.(10)-(12), the rate constants for three reactions (k1f(t), k1s(t), and k2(t)) were obtained. The selectivity factor (S(t) = k1(t)/k2(t), the ratio of the xylan hydrolysis rate to the xylose degradation rate) was employed to evaluate the efficiency of the catalytic reaction condition. To calculate the total k1(t) value for the biphasic hydrolysis model, the ratio of fast-hydrolyzing xylan to the total xylan in raw material (φ = 0.30) was employed; the total k1(t) = 0.30k1f(t)+0.70k1s(t). The k1f(t), k1s(t), k1(t), k2(t) and S(t) profiles for three different temperatures are shown in Figure 4. The k1f(t), k1s(t), k1(t) and S(t) profiles rapidly increased to steady-state values, which may have contributed to the acid diffusion process. The k1f(t) values were almost two orders of magnitude higher than k1s(t) values. Among all experimentally examined conditions, the k1(t) values were higher than the k2(t) values by three orders of magnitude, which subsequently resulted in high steady-state selectivity factors (> 2000) and implied that all examined conditions favored xylan hydrolysis over xylose degradation. Two trends for the estimated kinetic constants were observed: the increase in reaction temperature

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resulted in high k1f(t), k1s(t), k1(t) and k2(t) values, whereas S(t) values decreased as the reaction temperature increased. The first observation was attributed to the finding that the relationships among k1f(t), k1s(t), k1(t) and k2(t) and temperature obeyed the Arrhenius law, in which the rate constant is proportional to the reaction temperature. The reduction in the S(t) values and the temperature was attributed to the notion that k2(t) was more sensitive to temperature than k1(t). When the reaction temperatures varied from 110 °C to 130 °C, the steady-state values of k1(t) and k2(t) ranged from 0.23 min-1 to 0.84 min-1 and from 2.92×10-5 min-1 to 3.50×10-4 min-1, respectively. The values of k2(t) increased by ten times, which implied an increase of xylose degradation at higher temperatures. This result was also consistent with the observed data on the furfural concentrations, which were only detected at 130 °C. 3.4. Contribution analysis of xylose yield Xylose yield is one of most important indicators for successful acid hydrolysis. The majority of previous studies focused on the optimization of the xylose yield; however, few studies include a contribution analysis of xylose yield. Based on the previously mentioned kinetic parameters, the diffusion-hydrolysis coupled kinetic model with suitable predictability was employed to perform a contribution analysis of xylose yield. According to Eq.(8), three items such as the fast-hydrolyzing xylan, slow-hydrolyzing xylan and inhibitor furfural contribute to xylose yield. The contributions of these three items at time t can be calculated by the following equations t

CPHf (t ) =

∫k 0

1f

(t ) C Hf (t ) dt C X (t )

× 100%

(22)

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t

CPHs

∫k (t ) = 0

1s

(t ) C Hs (t ) dt C X (t )

× 100%

(23)

t

CPX (t ) =

− ∫ k2 (t ) C X (t )dt 0

C X (t )

× 100%

(24)

where CPHf(t), CPHs(t) and CPX(t) are the contribution percentages of fast-hydrolyzing xylan, slow-hydrolyzing xylan and inhibitor furfural, respectively, for the xylose yield. The contributions of the three items with reaction time for three different temperature conditions are shown in Figure 5. Among all experimentally examined conditions, the xylose yield from the fast-hydrolyzing xylan was considerably higher than the xylose yield from the slow-hydrolyzing xylan in the initial hydrolysis stage (< 10 min). These results suggested that the fast-hydrolyzing xylan exhibited a higher reaction rate than the slow-hydrolyzing xylan. The contribution of every item was dependent on the reaction temperature. For the 110 °C pretreatment, the fast-hydrolyzing xylan dominated the hydrolysis with CPHf(t) > 50% during the entire reaction process. For the 120 °C pretreatment, the fast-hydrolyzing xylan dominated the hydrolysis process for approximately 32 min and the subsequent hydrolysis was led by the slow-hydrolyzing xylan with CPHs(t) > 50%. When the reaction temperature continued to rise, the fast-hydrolyzing xylan was induced to a more rapidly releasing xylose. The xylose yield from the slow-hydrolyzing xylan continued to increase. The fast-hydrolyzing xylan had a shorter dominant time (approximately 17 min at 130 °C) and the slow-hydrolyzing xylan contributed to approximately 70% of the xylose yield at the end of the reaction (60 min). The furfural negatively contributed to the xylose yield. Although the contribution of furfural also increased as the reaction temperature

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increased (approximately -0.13% at 110 °C and -1.61% at 130 °C at the end of the reaction), it was limited compared with the contributions of the fast-hydrolyzing xylan and slow-hydrolyzing xylan. The low and negative contribution of the furfural to the xylose yield causes less degradation of xylose. These results also explained why a high xylose yield can be achieved at moderate temperatures. 3.5. Analysis of particle size and acid concentration Particle size and acid concentration were two key parameters that impacted the efficiency of acid hydrolysis. The analysis was performed to evaluate the influence of two parameters on xylose yield. The particle size and acid concentration were set to 0.42 mm (40-mesh), 2 mm (10-mesh), 4 mm (5-mesh) and 0.15 M, 0.30 M, 0.45 M, 0.60 M, respectively. The reaction temperature was set to 120 °C and other model parameters were similar to the model parameters at 120 °C. The effect of particle size on xylose yield is shown in Figure 6a. Larger particle size resulted in a significant reduction in xylose concentration in the initial hydrolysis stage. For a hydrolysis time of 3 min, the xylose concentration with a particle size of 0.42 mm was approximately two-fold higher than the xylose concentration for 4 mm. However, the difference in the xylose yield between 0.42 mm and 4 mm was not significant after 10 min. This finding may be attributed to the diffusion process, which is the rate-limiting step of the prehydrolysis stage. When acid fully saturated the lignocellulose substrate and the diffusion process attained a steady-state, the diffusion will most likely not affect the total process. These findings may be important for the optimization of pretreatment conditions. For a pretreatment process with more than 10 min of reaction, as described

18

in the previous example, lignocellulose substrates with particle sizes of 4 mm are preferable based on the economy of milling energy. If the pretreatment process required less than 3 min of reaction time, the total process will be substantially affected by acid diffusion. The reaction may only occur in the outer surface region of the large particles. Therefore, the particle with 0.42 mm may be preferable from the xylose yield standpoint. Previous studies suggested that a higher acid loading should produce a higher xylose yield (Sun and Cheng, 2005). The influence of acid loading on xylose yield was also investigated in this study (Figure 6b). At a reaction temperature of 120 °C, increasing the acid loading to 0.30 M, 0.45 M and 0.60 M would result in a xylose yield of 84.34%, 87.11% and 87.21% of the maximum potential xylose concentration in contrast to 65.01% at 0.15 M acid loading. The theoretical values suggested that the xylose yield improved by about 20% when the acid loading increased from 0.15 M to 0.30 M; however, an additional increase in acid loading did not result in a significant improvement in xylose yield. Because the catalyst cost will be closely associated with the economics of final ethanol production, the application of a reasonable acid loading is desirable. 4. Conclusions

A novel kinetic model of hydrolysis was proposed by considering not only hydrolysis but also the diffusion process. The predicted results from the proposed model correspond with the observed data for both xylose (R2 = 0.97 and RMSE = 0.14 g L-1) and furfural concentrations. The validated model was employed to quantitatively 19

analyze the contributions of the fast-hydrolyzing xylan, slow-hydrolyzing xylan and the inhibitor furfural to the xylose yield. The impact of particle size and acid concentration on the xylose yield were also investigated. These findings shed some light on the mechanism for dilute sulfuric acid hydrolysis of corn stover. Acknowledgments

This study was supported by the Program for New Century Excellent Talents in University (Project No. NCET-11-0477), the Beijing Nova Program (Project No. Z131105000413056), the Beijing Excellent Talents Cultivation Program (Project No. 2013D009007000001), and the Beijing Youth Talent Plan Program in University (Project No. YETP0317).

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References

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15. Kim, I., Rehman, M.S.U., Han, J.I., 2014. Enhanced glucose yield and structural characterization of corn stover by sodium carbonate pretreatment. Bioresour. Technol. 152, 316-320. 16. Kim, S.B., Lee, Y.Y., 2002. Diffusion of sulfuric acid within lignocellulosic biomass particles and its impact on dilute-acid pretreatment. Bioresour. Technol. 83, 165-171. 17. Kobayashi, T., Sakai, Y., 1956. Hydrolysis rate of pentosan of hardwood in dilute sulfuric acid. B. Agr. Chem. Soc. Japan 20, 1-7. 18. Lavarack, B.P., Griffin, G.J., Rodman, D., 2002. The acid hydrolysis of sugarcane bagasse hemicellulose to produce xylose, arabinose, glucose and other products. Biomass Bioenerg. 23, 367-380. 19. Liu, X.J., Lu, M.Z., Ai, N., Yu, F.W., Ji, J.B., 2012. Kinetic model analysis of dilute sulfuric acid-catalyzed hemicellulose hydrolysis in sweet sorghum bagasse for xylose production. Ind. Crop. Prod. 38, 81-86. 20. Lu, Y.L., Mosier, N.S., 2008. Kinetic modeling analysis of maleic acid-catalyzed hemicellulose hydrolysis in corn stover. Biotechnol. Bioeng. 101, 1170-1181. 21. Luterbacher, J.S., Parlange, J.Y., Walker, L.P., 2013. A pore-hindered diffusion and reaction model can help explain the importance of pore size distribution in enzymatic hydrolysis of biomass. Biotechnol. Bioeng. 110, 127-136. 22. Mittal, A., Chatterjee, S.G., Scott, G.M., Amidon, T.E., 2009. Modeling xylan solubilization during autohydrolysis of sugar maple and aspen wood chips: Reaction kinetics and mass transfer. Chem. Eng. Sci. 64, 3031-3041. 23. Morinelly, J.E., Jensen, J.R., Browne, M., Co, T.B., Shonnard, D.R., 2009. Kinetic characterization of xylose monomer and oligomer concentrations during dilute acid pretreatment of lignocellulosic biomass from forests and switchgrass. Ind. Eng. Chem. Res. 48, 9877-9884. 24. Rajan, K., Carrier, D.J., 2014. Effect of dilute acid pretreatment conditions and washing on the production of inhibitors and on recovery of sugars during wheat straw enzymatic hydrolysis. Biomass Bioenerg. 62, 222-227. 25. Ranganathan, S., Macdonald, D.G., Bakhshi, N.N., 1985. Kinetic-studies of wheat straw hydrolysis using sulfuric-acid. Can. J. Chem. Eng. 63, 840-844. 26. Richard, T.L., Veeken, A.H.M., de Wilde, V., Hamelers, H.V.M., 2004. Air-filled porosity and permeability relationships during solid-state fermentation. Biotechnol. Progr. 20, 1372-1381. 27. Ruiz, E., Romero, I., Moya, M., Cara, C., Vidal, J.D., Castro, E., 2013. Dilute sulfuric acid pretreatment of sunflower stalks for sugar production. Bioresour. Technol. 140, 292-298.

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28. Saeman, J.F., 1945. Kinetics of wood saccharification - Hydrolysis of cellulose and decomposition of sugars in dilute acid at high temperature. Ind. Eng. Chem. 37, 43-52. 29. Sanderson, K., 2011. A chewy problem. Nature 474, S12-S14. 30. Shi, J., Ebrik, M.A., Wyman, C.E., 2011. Sugar yields from dilute sulfuric acid and sulfur dioxide pretreatments and subsequent enzymatic hydrolysis of switchgrass. Bioresour. Technol. 102, 8930-8938. 31. Sun, Y., Cheng, J.J., 2005. Dilute acid pretreatment of rye straw and bermudagrass for ethanol production. Bioresour. Technol. 96, 1599-1606. 32. Umino, S., Newman, J., 1997. Temperature dependence of the diffusion coefficient of sulfuric acid in water. J. Electrochem. Soc. 144, 1302-1307. 33. Wang, F.Q., Xie, H., Chen, W., Wang, E.T., Du, F.G., Song, A.D., 2013. Biological pretreatment of corn stover with ligninolytic enzyme for high efficient enzymatic hydrolysis. Bioresour. Technol. 144, 572-578. 34. Yat, S.C., Berger, A., Shonnard, D.R., 2008. Kinetic characterization for dilute sulfuric acid hydrolysis of timber varieties and switchgrass. Bioresour. Technol. 99, 3855-3863. 35. Zhang, R., Lu, X.B., Sun, Y.S., Wang, X.Y., Zhang, S.T., 2011. Modeling and optimization of dilute nitric acid hydrolysis on corn stover. J. Chem. Technol. Biotechnol. 86, 306-314.

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Figure captions

Figure 1. The schematic diagram of diffusion coupled with biphasic hydrolysis for

xylan hydrolysis process

Figure 2. The observed and predicted xylose concentrations for different pretreatment

temperatures (110 °C, 120 °C, and 130 °C)

Figure 3. The observed and predicted furfural concentrations for pretreatment at

130 °C

Figure 4. The k1f(t), k1s(t), k1(t), k2(t) and S(t) profiles for different pretreatment

temperatures (110 °C, 120 °C, and 130 °C)

Figure 5. The contribution percentages for fast-hydrolyzing xylan, slow-hydrolyzing

xylan and furfural to the xylose yield for different pretreatment temperatures (110 °C, 120 °C, and 130 °C)

Figure 6. The influence of (a) particle size and (b) acid concentration on xylose yield

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Tables Table 1. The model symbols Parameters VL CAL(t) CAS(r,t) t SS ε(t) τ R1 r D T c0 c1 C0 CHf(t) CHs(t) CX(t) CP(t) k1f(t) k1s(t) k2(t) Ai Eai ni R C AS (t )

VFG VFL VFO

Description Total volume of liquid phase Concentration of sulfuric acid in the liquid phase Concentration of sulfuric acid in the solid particle Diffusion time Total surface area of solid particles Porosity of solid particle Tortuosity of solid particle Radius of particle Diffusion distance Diffusion coefficient of sulfuric acid in water Reaction temperature Coefficient in diffusion coefficient of sulfuric acid in water Coefficient in diffusion coefficient of sulfuric acid in water Applied concentration of sulfuric acid Concentration of fast-hydrolyzing xylan Concentration of slow-hydrolyzing xylan Concentration of xylose Concentration of furfural Rate constant of xylose formation from the fast-hydrolyzing xylan Rate constant of xylose formation from the slow-hydrolyzing xylan Xylose degraded rate constant Arrhenius pre-exponential factor Activation energy Acid concentration exponent Gas constant Average acid concentration in solid particle Glucan volume fraction in raw material Lignin volume fraction in raw material Volume fraction of other compositions, such as ash in raw material

Units m3 mol L-1 mol L-1 min m2 m m m2 s-1 K mol L-1 g L-1 g L-1 g L-1 g L-1 min-1 min-1 min-1 min-1 kJ mol-1 kJ mol-1 K-1 g L-1 -

Value 6×10-6 Dep. var. Dep. var. Indep. var. Indep. var. Dep. var. 2.9 0.42×10-3 Indep. var. Indep. var. 383.15-403.15 Indep. var. Indep. var. 0.15 Dep. var. Dep. var. Dep. var. Dep. var. Indep. var. Indep. var. Indep. var. Indep. var. Indep. var. Indep. var. 8.314×10-3 Dep. var. Indep. var. Indep. var. Indep. var.

Sources This study (Luterbacher et al., 2013) This study This study -

25

Vt VFX(t) MG MX ML MO ΡG ΡX ρL ρO VP Vs CH0 ε0 φ

Specific volume per mass of raw material Xylan volume fraction Mass percentage of glucan in raw material Mass percentage of xylan in raw material Mass percentage of lignin in raw material Mass percentage of other compositions, such as ash in raw material Glucan density Xylan density Lignin density Density of other compositions, such as ash Pore volume per mass of raw material Total volume of solid particles Initial concentration of xylan in solid particle Initial porosity of solid particle Mass fraction of fast-hydrolyzing xylan to total xylan in raw material

% % % % g cm-3 g cm-3 g cm-3 g cm-3 cm3 g-1 m3 g L-1 -

Indep. var. Indep. var. 32.92 16.53 17.36 33.19 1.52 1.56 1.39 2.50 1.10 Indep. var. Indep. var. Indep. var. Indep. var.

This study This study This study This study (Ehrnrooth, 1984) (Ehrnrooth, 1984) (Ehrnrooth, 1984) (Richard et al., 2004) (Kim et al., 2014) -

26

Table 2. The kinetic and statistical parameters of the model fitting

Parameters φ A1f (min-1) Ea2f (kJ mol-1) n1f A1s(s-1) Ea1s (J mol-1) n1s A2 (min-1) Ea2 (kJ mol-1) n2 R2 RMSE (g L-1)

Value 0.30 6.06×108 83.24 1.25 1.40×107 83.39 1.57 9.17×1013 159.60 0.33 0.97 0.14

27

Table 3. Comparison of the kinetic parameters in this study with the kinetic parameters from the literature Raw material

Pretreatment conditions

Wheat straw

Switch grass Polar Corn stover

Sulfuric acid (0.5-1.5%, 100-210 °C) Sulfuric acid (0.44-1.90%, 120-150 °C) Sulfuric acid (0.6-1.2%, 140-180 °C) Sulfuric acid (0.6-1.2%, 140-180 °C) Sulfuric acid (0.6-1.2%, 140-180 °C)

Corn cob

Sulfuric acid (0.1-0.3 M, 98-130 °C)

Corn cobs/stover

Sugarcane bagasse Bagasse Bagasse Corn stover Aspen Balsam Basswood Red Maple Switch grass Corn stover Hazelnut shell Corn stover

Sulfuric acid (2-6%, 100-128 °C) Sulfuric acid (0.25-8%, 80-200 °C) Hydrochloric acid (0.25-8%, 80-200 °C) Maleic acid (0.05-0.2 M, 150-170 °C) Sulfuric acid (0.25-1.0%, 160-190 °C) Sulfuric acid (0.25-1.0%, 160-190 °C) Sulfuric acid (0.25-1.0%, 160-190 °C) Sulfuric acid (0.25-1.0%, 160-190 °C) Sulfuric acid (0.25-1.0%, 160-190 °C) Sulfuric acid (1-3%, 90-100 °C) Sulfuric acid (0.3-0.5 M, 100-120 °C) Sulfuric acid (0.15 M, 110-130 °C)

Hydrolysis of xylan A (min-1) E (kJ mol-1)

n

2.25×1020 1.05×1014

167.0 27.50

1.55 1.20

Degradation of xylose Literature A (min-1) E (kJ n mol-1) 15 1.52×10 141.0 2.00 (Ranganathan et al., 1985) 8.99×1011 28.20 1.00 (Chen et al., 1996)

1.90×1021 3.30×1021 6.70×1016 1.49×1010-2.00×1010

169.0 176.7 129.8 80.34-85.67

0.40 0.40 1.50 1.21

3.80×1010 8.50×1010 3.70×1010 6.34×1014

99.5 102.0 98.4 133.70

1.45 0.55 0.50 0.78

2.15×1013

109.09

0.73

-

-

-

8.76×107-5.86×109 1.38×108

73.50-88.10 74.50

0.79 0.93

1.76×1013 111.20 4.09×1013 114.80

0.79 (Lavarack et al., 2002) 0.93 (Lavarack et al., 2002)

2.37×1010 1.53×1011-2.65×1017 7.53×104-2.78×1017 4.46×1011-2.63×1020 5.77×109-1.40×1017 1.89×107-1.03×1019 1.40×1014 5.36×105 1.40×107-6.06×108

83.30 97.18-151.85 48.72-151.52 102.67-179.13 88.65-149.45 65.94-167.48 111.60 52.74 83.24-83.29

1.51 1.75 1.75 1.75 1.75 1.75 0.68 0.90 1.25-1.57

2.18×1015 6.51×1016 7.59×1015 2.52×1013 6.83×1013 3.73×1017 3.30×1010 2.90×104 9.17×1013

0.29 1.0 0.9 1.2 1.0 1.4 0.40 0.56 0.33

143.50 155.36 147.56 126.89 129.64 165.59 95.70 46.58 159.60

(Esteghlalian et al., 1997) (Esteghlalian et al., 1997) (Esteghlalian et al., 1997) (Eken-Saracoglu et al., 1998) (Aguilar et al., 2002)

(Lu and Mosier, 2008) (Yat et al., 2008) (Yat et al., 2008) (Yat et al., 2008) (Yat et al., 2008) (Yat et al., 2008) (Jin et al., 2011) (Arslan et al., 2012) This study

28

Figure 1.

29

Figure 2.

30

Figure 3.

31

Figure 4.

32

Figure 5.

33

Figure 6.

34

35

Highlights

(1) Dilute acid pretreatment of corn stover at moderate temperatures was investigated. (2) A novel diffusion-biphasic hydrolysis coupled kinetic model was proposed. (3) The contributions to xylose yield were quantitatively analyzed. (4) The impact of particle size and acid concentration on xylose yield was investigated.

36