Kinetic Modeling of Hemicellulose Hydrolysis from Triticale Straw in a

Ind. Eng. Chem. Res. 2010, 49, 6367–6375

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Kinetic Modeling of Hemicellulose Hydrolysis from Triticale Straw in a Pressurized Low Polarity Water Flow-Through Reactor Carl Pronyk and Giuseppe Mazza* Pacific Agri-Food Research Centre, Agriculture and Agri-Food Canada, 4200 Highway 97, Summerland, British Columbia, Canada, V0H 1Z0

Two classic and one modified kinetic models were used to study the hydrolysis of triticale straw using pressurized low-polarity water (PLPW) in a flow-through reactor. Results indicated that for these experiments hemicellulose did not follow a biphasic reaction pathway. High yields of 72% were achieved at 170 °C, but these decreased to 60% at 150 °C and only 13% at 130 °C. The kinetics were controlled more by increases in temperature than flow rate in the reactor. Increases in flow rate reduced the overall hemicellulose yields but increased the portion extracted as oligomers. The kinetic rate constants when plotted as an Arrheniustype temperature relationship displayed a dependency with flow rate. Curvature in the Arrhenius plots of the kinetic rate constants was due to differences in acetic acid formation with temperature. A modified monophasic kinetic model which incorporated reactor geometry and fluid flow was successful at modeling the yield of xylo-oligomers and monomers in PLPW. 1. Introduction The biorefinery concept aims to shift our current production system for energy and chemicals from fossil carbon sources (coal, petroleum, natural gas) to sustainable and renewable sources such as biomass to create a wide range of bioproducts and bioenergy. Lignocellulosic biomass is an attractive feedstock for the production of bioproducts and second-generation biofuels. Sources of lignocellulosic biomass include agricultural residues, woody biomass, or dedicated energy crops. Problems arise when these sources compete with land for food production. Triticale, a hybrid of wheat and rye, is a promising source of lignocellulosic biomass that produces more straw for a comparable yield of grain when compared to other crops.1 Thus, there is an increase in the raw material for production of lignocellulosic biofuels, while maintaining an equivalent quantity of grain for food, without an increase in land use. Lignocellulosics are composed of hemicellulose (15-35%), cellulose (30-50%), lignin (10-30%), and lesser amounts of ash, oils, waxes, and other minor components.2-4 Cellulose and hemicellulose derived from lignocellulosic biomass are potential raw materials for the production of ethanol and have been identified as a potential source for many chemicals and chemical feedstocks.2,5,6 To make biofuels economically feasible, the biorefinery concept dictates that a wide variety of coproducts be produced. The hemicellulose, which is a branched polysaccharide consisting of the pentoses (D-xylose and L-arabinose) and hexoses (D-galactose, D-glucose, and D-mannose) with uronic acid units, is an excellent raw material for the production of a wide range of bioproducts. In agricultural residues the hemicellulose is composed mainly of xylan, made up of xylose units. In addition to fermentation into ethanol, hemicellulose may be used to produce furfural, furfuryl alcohol, and furanbased bioplastics.2 Xylo-oligomers have applications as food ingredients and pharmaceuticals.5 Xylo-oligosaccharides are prebiotics7 related to their ability to stimulate the growth of beneficial bacteria (bifidobacteria).8 Xylose monomers from the hemicellulose are intermediates for the production of xylitol, a * To whom correspondence should be addressed. Tel.: +1 250 494 6376. Fax: +1 250 494 0755. E-mail: [email protected]. 10.1021/ie1003625

sugar alcohol, which is a sugar substitute, and has been used in food applications.9 The fractionation of lignocellulosics into their constituent components (cellulose, hemicellulose, and lignin) can be accomplished in a variety of ways, including direct enzymatic conversion of the raw material, acid hydrolysis, or through autohydrolysis (hydrothermal treatment). The autohydrolysis treatment occurs in an aqueous media, and the hydrolysis reaction is the result of the catalytic action of hydronium ions. It is thought that initially the hydronium ions are the result of water ionization due to processing conditions, and later it is supplemented and exceeded by the production of in situ acids (such as acetic acid generated from acetyl groups).10,11 Autohydrolysis is commonly accomplished by steam explosion12,13 or pressurized low-polarity water (PLPW).14,15 Pressurized lowpolarity water (also known as superheated water, subcritical water, pressurized hot water) is a green technology that has shown promise in the fractionation and extraction of bioproducts from a wide variety of lignocellulosic feedstock.16,17 Understanding the kinetics of hemicellulose hydrolysis is extremely important for the design of equipment and process development. Hemicellulose hydrolysis has been extensively modeled, and this has led to the improved understanding of the complex reaction and for predicting hemicellulose recovery.18 The basis for most hemicellulose hydrolysis model is from the two-step consecutive reaction of cellulose undergoing acid hydrolysis first proposed by Saeman,19 with a first-order dependence on reactant concentration and an Arrhenius temperature relationship for the rate constants. This model was later adapted to hemicellulose acid hydrolysis and concluded that the hemicellulose displayed a biphasic tendency with one portion of the hemicellulose hydrolyzing quickly and the other at a much slower rate.20 These models assume that any oligomers formed during hydrolysis break down to monomers faster than they are formed.18 Increasingly it has been observed that oligomer formation plays a crucial role in the hydrolysis kinetics of biomass, especially for autohydrolysis and flow-through systems. As such, these models have been modified to include the formation of oligomer intermediates during batch hydrolysis with dilute acid,18,21,22 with batch PLPW,10,23-25 and flowthrough systems.18,26

Published 2010 by the American Chemical Society Published on Web 06/28/2010

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Ind. Eng. Chem. Res., Vol. 49, No. 14, 2010

1) during the autohydrolysis of hemicellulose in PLPW. The variation of each component may be theoretically determined by the following set of differential equations: dHf ) -k1f[Hf] dt

(8)

dHs ) -k1s[Hs] dt

(9)

Figure 1. Hemicellulose reaction pathways during hydrolysis: Model I, monophasic hydrolysis of hemicellulose; Model II, biphasic hydrolysis of hemicellulose.

dO ) k1f[Hf] + k1s[Hs]-k2[O] dt

(10)

Still, there is concern about these kinetic models’ ability to accurately and uniformly describe the hemicellulose hydrolysis reaction for all system configurations or operating conditions.25,27-29 Kinetic models often produce rate constants which change from study to study, thus indicating that the models may fit the experimental data very well but at the expense of accurately describing the true mechanism of hemicellulose hydrolysis.25 To alleviate these concerns, researchers have developed more complex models that incorporate some aspects of system configuration and include mass transfer parameters.28,30-32 In this study, classic kinetic models (monophasic and biphasic hemicellulose hydrolysis) and a modified kinetic model which includes aspects of the reactor configuration and flow parameters were applied to the experimental data. The basic kinetics were determined and used to explain the hydrolysis of triticale straw in PLPW using a flow-through reactor.

dX ) k2[O]-k3[X] dt

(11)

dD ) k3[X] dt

(12)

2. Hemicellulose Hydrolysis Models 2.1. Model I (Monophasic Hemicellulose Hydrolysis Model). A monophasic hydrolysis model based on Saeman19 was modified to incorporate the production of oligomers (Figure 1) during the autohydrolysis of hemicellulose in PLPW. The variation of each component may be theoretically determined by the following set of differential equations: dH ) -k1[H] dt

(1)

dO ) k1[H]-k2[O] dt

(2)

dX ) k2[O]-k3[X] dt

(3)

dD ) k3[X] dt

(4)

Solving the differential equations with the initial conditions at t ) 0, O ) 0, X ) 0, the analytical solutions for eqs 1-3 are

O ) H0

H ) H0e-k1t

(5)

k1 [e-k1t - e-k2t] k2 - k1

(6)

Solving the differential equations with the initial conditions at t ) 0, Hf ) F1H0, Hs ) F2H0, Hf + Hs ) H0, O ) 0, and X ) 0, the analytical solutions for eqs 8-11 are

O ) F1H0

Hf ) F1H0e-k1ft

(13)

Hs ) F2H0e-k1st

(14)

k1f k1s [e-k1ft - e-k2t] + F2H0 × (k2 - k1f) (k2 - k1s) [e-k1st - e-k2] (15)

k1fk2 [e-k1ft - e-k3t] + (k2 - k1f)(k3 - k1f) k1sk2 k2 [e-k1st - e-k3t] × F2H0 (k2 - k1s)(k3 - k1s) k3 - k2 k1s k1f [e-k2t - e-k3t] (16) + F2H0 F1H0 (k2 - k1f) (k2 - k1s)

X ) F1H0

[

]

[

]

2.3. Model III (Flow-Through Reactor). The kinetic modeling of a flow-through reactor utilizing reactor geometry and processing conditions has been developed by Lee’s group at Auburn University (Auburn, AL).30-32 The following assumptions are made for the proposed model: (1) The kinetics follow a series of consecutive first-order reactions; (2) internal and external diffusion is negligible; (3) reaction and physical conditions (temperature, superficial velocity, bed porosity, bed depth) are uniform and constant during the reaction. The material balance for monophasic hemicellulose hydrolysis over an incremental height (∆x) in the reactor (Figure 2) on the oligomer content is thus ∂O ∂O ) (k1H + k2O) - u ∂t ∂x

( )

(17)

where x

H ) H0e[-k1(t-( u ))],t > (x/u)

(18)

k1k2 [e-k1t - e-k3t] X ) H0 (k2 - k1)(k3 - k1) k1k2 [e-k2t - e-k3t] (7) H0 (k2 - k1)(k3 - k2)

In dimensionless form the partial differential equation becomes

2.2. Model II (Biphasic Hemicellulose Hydrolysis Model). A biphasic hydrolysis model based on Kobayashi and Sakai20 was modified to incorporate the production of oligomers (Figure

with the initial and boundary conditions t ) 0, SO ) 0, and z ) 0.

( )

∂SO ∂SO ) βe[-β(τ-z)] - RβSO ∂τ ∂z

(19)


Figure 2. Diagram of a flow-through reactor showing a finite volume element of the particle bed.

By the Laplace transform method the solution to eq 19 is SO )

[ R1 ][1 - e

-Rβz

][e-β(τ-z)]

(20)

The oligomer yield is then calculated by integrating eq 20 at the reactor exit point (z ) 1) over the range of operating time: YO )

∫

τ+1

1

(SO)z)1dτ )

[

]

1-e-Rβ [ 1-e-βτ] Rβ

(21)

The corresponding material balance for xylose follows the same procedure as for oligomers, and the resulting equation for xylose yield is YX )

[

]

e-β(1+γ)(τ+1) - e-β(1+γ) 1 β e-β - e-β(τ+1) e + + γ β β(1 + γ) e-βτ - 1 1 -βγ 1 - e-β(1+γ)τ e + + γ β(1 + γ) β eβ(1-γ) - eβ(1-R) e-β - e-β(τ+1) (22) γ-R β

[

[

][

]

]

3. Methods and Materials 3.1. Materials. Triticale straw (cv. AC Ultima), from the 2008 crop year, grown near Indian Head, Saskatchewan, was obtained from the Semiarid Prairie Agricultural Research Centre of Agriculture and Agri-Food Canada in Swift Current, Saskatchewan. Samples were coarsely ground in a Retsch mill (model SM 2000, Retsch GmbH, Haan, Germany) to pass through an 8 mm discharge screen. The ground sample was then sieved for 5 min in a tapping sieve shaker (model AS200Tap, Retsch GmbH, Haan, Germany) equipped with a No. 10 sieve (2 mm size) to separate out the long pieces of straw. This process segregated the particles according to the smallest dimensions which passed through the sieve. The resulting sample, which passed through the 2 mm sieve, contained some fine particles as well as some pieces of straw up to 10 mm in length. The particle distribution should not affect the hydrolysis because the wall thickness of the straw is on the order of 0.2 mm, which should be the controlling dimension for the process, and is largely independent of overall particle size.33 The ground sample was bagged and stored in a freezer at -20 °C prior to use. 3.2. PLPW Flow-Through Reactor. The PLPW flowthrough reactor system used for the kinetic study was constructed

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Figure 3. Schematic of the flow-through pressurized low-polarity water (PLPW) reactor.

in house and based on the design of Mazza and Cacace34 (Figure 3). All connections, fittings, tubing, valves, and vessels were constructed of stainless steel to resist corrosion. The pressure in the system was maintained at 11 MPa (1600 psi) for all experiments by adjusting the back-pressure regulator (Tescom, Elk River, MN). Distilled water was pressurized and pumped at a constant flow rate using a metering pump (model P300, Wanner Engineering Inc., Minneapolis, MN) with a pulsation dampener (Wanner Engineering Inc., Minneapolis, MN) installed after the pump to ensure steady flow in the system. A tube-in-tube heat exchanger (Exergy LLC, Garden City, NY) performed two duties within the system: first, the heat exchanger cooled the solvent after the reactor before exhausting to the collection vessel; second, the heat removed from the exhaust solvent was transferred to the incoming solvent before entering the immersion heater (ASB Heating Elements Ltd., Bethridge, ON). In this way the heat exchanger preheats the solvent and reduces the energy requirements of the system. A sample of 126 g (approximately 120 g of dry matter) of coarsely ground triticale straw was uniformly packed into a stainless steel flanged reaction column of 5 cm i.d. × 50 cm length (MODcol, Mandel Scientific Company Inc., Guelph, ON) to a bed depth of 40 cm. To keep the sample inside of the reaction vessel and help promote dispersion of the solvent, both ends were packed with 5 cm of stainless steel wool and capped with a 20 and 100 µm stainless steel frit at the inlet and outlet, respectively. The reactor was equipped with two band heaters (ASB Heating Elements Ltd., Bethridge, ON) to allow for independent temperature control for the top and bottom of the reaction column. The hydrolysis reaction procedure was initiated by first flooding the reactor with a small amount of solvent (PLPW) and then warming the system to the experimental temperature (130, 150, or 170 °C) and holding it there for 1 h to allow the temperature of the sample to equilibrate within the column before commencing flow through the reactor. There was no indication of significant reaction occurring during the preheat before the initiation of flow within the reactor due to the limited availability of PLPW.35 Upon commencement of flow through the reactor (66, 100, 150, 200, or 234 mL/min corresponding to a superficial velocity in the reactor of 0.034, 0.051, 0.077, 0.102, or 0.119 m/min, respectively) the first portion of solution, which contained no analyte (corresponding to the dead volume in the system from the top of the reaction vessel to the collection vessel), was discarded and the predetermined volume of solution based on the chosen solvent-to-solid ratio was collected. The same solvent-to-solid ratio (60 mL of solvent per 1 g of starting

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material) was maintained for all experiments so that differences in yield due to flow rate could be discerned because the same volume of extract was collected. After each experiment, the system was flushed with PLPW for 1 h to remove any residue. Extracts collected from each experiment were stored at -20 °C, and the solid residues were removed from the extraction vessel, freeze dried, and stored at -20 °C until they were analyzed. 3.3. Compositional Analysis. 3.3.1. Solid Residues. Solid residues were analyzed for structural carbohydrates according to NREL (National Renewable Energy Laboratory) standard analytical procedures.36 Residues were hydrolyzed with 72% sulfuric acid for 1 h at 30 °C in a water bath and then diluted to 4% sulfuric acid and autoclaved at 121 °C for 1 h in sealed glass pressure tubes. The total xylose content was determined quantitatively from the hydrolysate by HPLC using an Aminex HPX-87P column (Bio-Rad, Hercules, CA) with a deashing guard cartridge (Bio-Rad, Hercules, CA) at 75 °C and a refractive index detector at 55 °C. The HPLC system consisted of a G1329A autosampler and G1312A delivery system, which were controlled by Agilent Chemstation Plus software (Agilent Technologies, Palo Alto, CA). HPLC-grade filtered water (MilliQ) was used as the mobile phase at a flow rate of 0.5 mL/min, and for each sample, 50 µL of prefiltered aliquot was injected automatically. The sugar concentrations were determined by comparison against a set of known sugar standards and the application of a sugar recovery factor according to NREL standard procedures.36 3.3.2. Liquid Extracts. Liquid extracts from the kinetic experiments were analyzed for structural carbohydrates according to NREL standard analytical procedures.37 Total xylose in the liquid was determined by hydrolyzing the liquid extracts with 4% sulfuric acid and autoclaving at 121 °C for 1 h in sealed glass pressure tubes. Samples were neutralized with calcium carbonate, filtered through a 0.20 µm syringe filter, and analyzed with the same equipment as the solid residues. The total xylose content for the samples included both the xylose monomers and the hydrolyzed xylo-oligomers. A subsample of each liquid extract was taken and neutralized with calcium carbonate, filtered through a 0.20 µm syringe filter, and used for direct HPLC determination of xylose monomers. The degradation product furfural was determined from the same sample using a diode array detector (DAD) and the same equipment used for the solid residues. The concentration of xylooligomers was then calculated by taking the difference between the hydrolyzed xylose content and the nonhydrolyzed xylose monomer content. 3.4. Parameter Estimation. Data were analyzed using the NLIN procedure of SAS38 (v9.2, SAS Institute Inc., Cary, NC) to estimate the rate constants in the nonlinear kinetic models [Model I (monophasic), Model II (biphasic), and Model III (flow-through)] for hemicellulose hydrolysis in PLPW. The Marquardt method was the optimization algorithm used to solve the nonlinear least-squares problem to minimize the residual sum of squares.38 4. Results and Discussion 4.1. Native Triticale Straw Composition. Before conducting the extraction experiments, the triticale straw was first analyzed for its major components: cellulose, hemicellulose, and lignin. Values were determined for the whole native straw and not the extractives-free material recommanded by the NREL laboratory procedure,36 because industry would use the material as received, which still contains the extractives. The extractives

Figure 4. Xylo-oligomer yield (extracted/potential × 100%) during the hydrolysis of triticale straw recovered from a flow-through reactor: (blue () 150 °C, 66 mL/min; (pink 9) 150 °C, 150 mL/min; (black 2) 150 °C, 234 mL/min.

Figure 5. Xylose monomer yield (extracted/potential × 100%) during the hydrolysis of triticale straw recovered from a flow-through reactor: (blue () 150 °C, 66 mL/min; (pink 9) 150 °C, 150 mL/min; (black 2) 150 °C, 234 mL/min.

include nonstructural sugars, organic acids, inorganic material, nitrogenous material, chlorophyll, waxes, and other minor components.4,39 Extractive material in triticale straw can comprise 20% of the total mass.4 Removal of the extractives has a significant effect on the measured lignin and ash contents, resulting in an overestimation of the lignin and ash content of the sample, but there is no effect of extractives on the glucan and xylan content of triticale straw.4 As the focus of this work was on the extraction of hemicellulose, there should be no issues with the compositional analysis using the native straw. The composition of the native triticale straw was determined to be 24.8% hemicellulose, 36.3% cellulose, and 19.9% lignin, with the balance comprised of protein, ash, and other minor components. Characterization of the native triticale straw allowed for calculation of the yields achieved from each PLPW fractionation run. Yields were calculated as the quantity of component collected in the liquid fractions divided by the potential amount of the component in the native triticale straw and reported as a percentage. 4.2. Hydrolysis Experimental Results. 4.2.1. Degradation Products. The main products from the autohydrolysis of hemicellulose (xylan) with PLPW were xylo-oligomers and lesser amounts of xylose monomers (Figures 4-7). The degradation product furfural only accumulated to significant levels in the samples processed at 170 °C and only during the early stages of autohydrolysis. The total yield of furfural was only 4% of the potential xylose available for hydrolysis. However, hemicellulose is also comprised of small amounts of


Figure 6. Xylo-oligomer yield (extracted/potential × 100%) during the hydrolysis of triticale straw recovered from a flow-through reactor: (blue () 130 °C, 100 mL/min; (pink 9) 130 °C, 200 mL/min; (red 2) 170 °C, 100 mL/min; (black b) 170 °C, 200 mL/min.

Figure 7. Xylose monomer yield (extracted/potential × 100%) during the hydrolysis of triticale straw recovered from a flow-through reactor: (blue () 130 °C, 100 mL/min; (pink 9) 130 °C, 200 mL/min; (red 2) 170 °C, 100 mL/min; (black b) 170 °C, 200 mL/min.

the pentose sugar arabinose. The structure of arabinan makes it more susceptible to hydrolysis,23 and in turn it may contribute significantly toward the production of furfural, especially early in the hydrolysis. Concentrations of furfural remained low for all experiments and reached a maximum of 0.16 mg/mL at 170 °C. As such, the degradation products were neglected in the proposed kinetic models, and only the oligomers and monomers of xylose were modeled. This is believed to be due to the fact that in flow-through reactors the hydrolysis products are constantly removed from the reactor. There is often not enough time for degradation to occur before the hydrolysis extracts exit and are cooled, which is also evident in the large portion of xylose recovered in oligomer form.26 Graphs of xylose yield (extracted/potential × 100%) show that overall xylose oligomer and monomer yields decreased with increasing flow rate through the reactor for an equivalent solvent consumption (Figures 4-7). However, the rate of extraction was higher with an increase in flow rate, although this effect was much smaller at flow rates of 150 and 234 mL/min at 150 °C (Figure 4). 4.2.2. Hemicellulose. In batch systems, PLPW has been successful in removing up to 60% of the available hemicellulose; in flow-through reactors, PLPW can yield more than 90% of the hemicellulose.26 Particles in batch reactors are in contact with the solvent, which has uniform properties and contains an equivalent concentration of dissolved material at all times. In a flow-through reactor there is an axial concentration gradient of solute from inlet to outlet. In addition, particles near the inlet are in contact with only PLPW and those further into the reactor are exposed to dilute acids from in situ production from the biomass within the reactor. However, there is some debate as

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to the importance of the role these acids have on the hydrolysis process.26 There is a finite quantity of fluid in a batch reactor and it can become saturated, limiting product yields. In a flowthrough reactor, the fluid is continuously replenished and the concentration gradient between the solid and solvent is greater, resulting in larger possible yields. However, this is at the expense of concentration, which is lower in flow-through reactors and has been reported to vary between 0.6 and 5.8 mg/mL.17 Over the first 900 mL fraction, the concentration of hemicellulose was at a maximum of 17.0 mg/mL at 170 °C and 100 mL/min flow rate. The concentration of hemicellulose in the first fraction at 150 °C was between 8.0 and 10.1 mg/mL, depending on flow rate, dropping to only 1.7 mg/mL at 130 °C. Over the course of the hydrolysis and extraction the concentration of hemicellulose within the liquid extracts continued to fall. A total of 7.2 L of extracts was collected for each condition. Final concentrations of hemicellulose in those liquid extracts were 3.1-4.6 mg/ mL at temperatures of 150 and 170 °C. At 130 °C the hemicellulose concentration was very dilute and only reached 0.8 mg/mL. Overall, there was a decrease in hemicellulose concentration within the liquid extracts with an increase in the flow rate of the system. Increased flow rates may be responsible for the creation or exacerbation of channelling of fluid through the bed.15 Material around the channels would experience greater extraction at the expense of material not receiving flow and contact with the PLPW. The decrease in hemicellulose concentration with increasing flow rate may also be due to the reduced residence time of the acids produced within the reactor before they have time to act upon the sample. Total yield of xylose oligomers and monomers was 72% at 170 °C and fell to 60% at 150 °C. The conditions at 130 °C were not favorable to the hydrolysis of the hemicellulose, and only 13% yield was achieved. However, along with the increase in yield with temperature, there was an increase in the proportion of monomers produced. At 130 °C the hydrolysis yielded 4.5% of the total extracted xylose as monomers. This proportion of xylose monomers increased slightly to 5% at 150 °C and drastically increased to 12% at 170 °C, in addition to the increase in furfural production reported earlier. 4.3. Parameter Estimation (Model I and Model II). 4.3.1. Determination of the Kinetic Constants. The experimentally obtained data for hemicellulose hydrolysis in a PLPW flow-through reactor was first fitted to the monophasic (Model I) and biphasic models (Model II) described earlier. This was done to ascertain whether hemicellulose hydrolysis for triticale straw in a PLPW flow-through reactor followed a monophasic or biphasic hydrolysis reaction pathway. Some have suggested that the biphasic nature of the substrates is negligible40 or that the different rates of hemicellulose hydrolysis may not be due to any biphasic nature but could be due to transport limitations (diffusion), nonhomogeneous reactions at the hemicellulose/ water interface, or accessibility (some hemicellulose may be more tightly bound to the lignin).18 The estimated parameters for the monophasic and biphasic hydrolysis models are presented in Table 1. To estimate the kinetic constants for the biphasic hydrolysis model, the fast (F1) and slow (F2) fractions of hemicellulose (eqs 13-16) were taken to be 0.65 and 0.35 respectively, which is typical for most hemicellulose.18 The nonlinear regression of the biphasic hydrolysis model suggested that there were problems with its application to the experimental data. The regression for the biphasic model often resulted in a Hessian matrix that was singular. This is an indication that the model is over parametrized and not all of the parameters are justified by the data.

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Table 1. Estimated Kinetic Constants for Monophasic (Model I) and Biphasic (Model II) Hydrolysis Models monophasic

biphasic model R2

model R2

temperature (°C)

flow rate

k1 (min-1)

k2 (min-1)

k3 (min-1)

oligomers

monomers

k1f (min-1)

K1s (min-1)

k2 (min-1)

k3 (min-1)

oligomers

monomers

130

100 200 66 150 234 100 200

0.0017 0.0028 0.0119 0.0245 0.0261 0.0418 0.0672

0.0014 0.0044 0.0016 0.0026 0.0074 0.0087 0.0207

0.0003 0.0361 0.0035 0.0010 0.0430 0.0634 0.1618

0.95 0.97 0.97 0.96 0.96 0.99 0.99

0.62 0.15 0.82 0.76 0.78 0.23 0.25

0.0017 0.0033 0.0119 0.0245 0.0160 0.0775 0.0412

0.0017 0.0033 0.0119 0.0245 0.0503 0.0041 0.1437

0.0014 0.0138 0.0016 0.0026 0.0066 0.0024 0.0163

0.0003 0.2346 0.0035 0.0000 0.0274 0.0003 0.1301

0.95 0.98 0.98 0.96 0.96 0.99 0.99

0.62 0.62 0.82 0.76 0.77 0.22 0.28

150 170

This is displayed in the results where k1f and k1s are equal when the sums of squares are minimized in the nonlinear regression analysis (Table 1). The kinetic constants for k2 and k3 were determined to be equal to those in the monophasic model for all instances where the kinetic rate constants k1f and k1s were equal. This suggests that the hydrolysis of hemicellulose in a PLPW reactor under the conditions tested in these experiments is not well described by a biphasic hydrolysis reaction (Model II, Figure 1). Instead of a biphasic pathway for hemicellulose hydrolysis, other researchers have postulated that a portion of the hemicellulose is not susceptible to hydrolysis and that the difference in the reactivity (fast or slow) is not due to differences in chemical structure but to physical inaccessibility of a portion of the hemicellulose to hydrolysis under the given processing conditions.10,23,41 Conner and Lorenz41 reported that the susceptible portion of hemicellulose increased with increasing temperature (increased severity), and Carvalheiro et al.23 found that the proportion of ‘susceptible’ hemicellulose in biomass was 0.708-0.886 of the total xylan in the sample. Although a portion of the hemicellulose in these experiments may be not susceptible at the tested hydrolysis conditions, the hydrolysis of triticale hemicellulose in a PLPW flow-through reactor is best explained by Model I as a consecutive monophasic hydrolysis reaction. 4.3.2. Evaluation of the Kinetic Constants. For the monpohasic model, all kinetic constants increased with an increase in flow rate and temperature. This is consistent with the literature, which has shown that increases in flow rate result in the acceleration of hemicellulose hydrolysis in biomass.26,42 The problem is that the influence of flow rate in flow-through reactors on the hydrolysis of hemicellulose is not consistent with first-order kinetic models. These models assume that the rate constants depend only on acid concentration and temperature and follow an Arrhenius-type equation of the form k ) Ae

( ) -

Ea

RT

(23)

In dilute acid hydrolysis, the pre-exponential factor A is dependent on acid concentration. This deviation in first-order kinetics may be due to the hydrophobic nature of lignin, protein, and lipids,26 which may serve to shield the hemicellulose from the action of the PLPW, and the solubility of hemicellulose oligomers, which increases with decreasing degree of polymerization (DP), with high solubility at a DP less than 6.43 Liu and Wyman26 hypothesize that these high DP xylo-oligomers could diffuse to and then build up on the solid surface and resist the actions of the PLPW due to their hydrophobic nature. The flow of liquid in a flow-through reactor could enhance the removal of the less soluble oligomers by constantly replenishing the solvent, thereby increasing the concentration gradient, in addition to disrupting any stagnant boundary layer that may form

around the biomass particles in the reactor. For the monophasic model the kinetic rate constant k1 was higher than k2 except at 130 °C at a flow rate of 200 mL/min. The differences in kinetic rate constants show that the experimental conditions favored hemicellulose (xylan) hydrolysis over xylo-oligomer hydrolysis to xylose monomers. The monophasic model successfully predicted the yield of xylo-oligomers (Figures 4 and 6), but the model consistently underpredicted the yield of xylose monomers during the beginning of the hydrolysis (Figures 5 and 7), as indicated by the low R2 values (0.15-0.82) for the monomers. The rate of increase for the kinetic constants due to flow rate is much greater for k2 than for k1. However, the rate of increase for the kinetic constants is greater for k1 than for k2 due to increasing temperature. This increase is much greater for temperature than flow rate, suggesting that temperature has much more effect on the hydrolysis process than does flow rate. At equivalent flow rates, the rate of increase in the kinetic constant for oligomers (k2) is slightly less at higher flow rates, suggesting that removal of the hydrolyzed oligomers does reduce the further hydrolysis to monomers and degradation products. The Arrhenius plots of the kinetic rate constants are shown in Figure 8. The kinetic rate constant for xylose degradation (k3) did not produce a reasonable Arrhenius plot due to the poor fit of the xylose monomer curves. Lu and Mosier29 also found that kinetic rate constants for xylose degradation could not be expanded to produce a reasonable plot, suggesting that the rate constant from the monophasic model does not accurately describe the true mechanism of xylose degradation. It was found that xylose degredation did not always adhere to the first-order kinetics assumption. Deviations from first-order kinetics may be a product of a secondary reaction which takes place in the form of an additional consecutive reaction. Garrote et al.10 proposed a secondary consecutive reaction to produce decomposition products directly from low DP xylo-oligomers to improve the accuracy of the model, which predicted lower levels of furfural than were evident from the mass balance. The hydrolysis of hemicellulose is not understood as well as that for cellulose and even less so when the intermediate products are xylo-oligomers. Deviations from first-order kinetics are reported when xylan oligomers are the main products of the hydrolysis reaction, as they are here with the PLPW autohydrolysis.42,44 This may be due to the influence of oligomer solubility resulting in mass transfer limitations.42 When oligomers are the product of hemicellulose hydrolysis the resulting xylo-oligomers initially have a high degree of polymerization (DP). Over the course of batch hydrolysis there is a resulting shift in xylo-oligomers to lower degrees of polymerization. Xylose is then produced from these lower DP oligomers. It has been reported that different xylo-oligomer species decompose at different rates, further indicating a deviation from first-order kinetics. Yang et al.42 reported that the decomposition rate constants for xylopentaose and xylotetraose were similar and


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Figure 8. Arrhenius plot of rate constants from monophasic hydrolysis of hemicellulose (xylan) hydrolysis (Model I).

that the rate constants were one-third lower for xylotriose and xylobiose. For this study in a flow-through reactor it would be expected that much of the oligomers would be removed before they had a chance to degrade to a lower DP and then to xylose monomers. The kinetic rate constants (k1 and k2, Figure 8) are proportional to one another at constant flow rates. Even though current first-order kinetic models would dictate that the rate constants depend only on acid concentration and temperature, it is evident that they are also dependent on the fluid velocity in the reactor. However, there is curvature in the relationship of the rate constants with 1/T. This may be a consequence of the autohydrolysis process, which is the result of the catalytic action of hydronium ions from the production of in situ acids, such as acetic acid. The amount of acetic acid produced is dependent on the duration and severity of the processing conditions.29 In these experiments, early fractions at 170 °C had a pH of 3.8 when measured at room temperature, at 150 °C this value increased to 4.5, and at 130 °C the pH increased to 5.2. This indicates that acetic acid production increased at the higher temperatures, which would aid in the hydrolysis process. The result is the Arrhenius plots of the rate constants are not at isoacidic conditions, and this is potentially responsible for the curvature in Figure 8. Acetic acid released from the biomass during the hydrolysis does seem to play a larger role in the kinetics of the reaction than other researchers may have thought.42 Still this effect needs to be examined more closely to determine its exact role on the hydrolysis and reaction kinetics. 4.4. Parameter Estimation (Model III). Hemicellulose was determined to follow a monophasic reaction pathway, and the kinetic rate constants were dependent on the temperature and solvent flow rate through the reactor. As such, a modified kinetic model (Model III) was selected to incorporate the effects of flow to attempt to better model and explain the reaction kinetics. The model assumed a monophasic reaction for hemicellulose hydrolysis, and the estimated parameters for the model are presented in Table 2. In this case the kinetic rate constant k2 was higher than k1 at all processing conditions, which was opposite the result for the classic monophasic model (Table 1). This increase in the kinetic rate constant k2 was responsible for the improved prediction of the yield of xylose monomers during the beginning of the hydrolysis (Figures 4 and 6). The R2 values for the model of monomers also reflected the improvements over the classic monophasic model. This was accomplished without sacrificing the accuracy of the xylo-oligomer yields predicted by Model III (Figures 5 and 7). As with the monophasic model, the rate of increase due to flow rate is much greater for k2 than for k1 and the rate of increase for k1 due to

Table 2. Estimated Kinetic Constants for the Flow-Through Reactor Hydrolysis Model (Model III) flow-through model R2 k2 k3 temperature flow rate k1 (°C) (mL/min) (min-1) (min-1) (min-1) oligomers monomers 130 150

170

100 200 66 150 234 100 200

0.0024 0.0183 0.0134 0.0267 0.0316 0.0816 0.1386

0.1074 1.4059 0.0242 0.0321 0.1332 0.1187 0.2944

2.1675 26.7809 0.3892 1.2532 2.1466 0.0858 2.2869

0.95 0.98 0.97 0.96 0.96 0.99 0.99

0.97 0.84 0.95 0.95 0.98 0.96 0.91

temperature is much greater. The kinetic rate constant for hemicellulose hydrolysis (k1) is similar in Models I and III at 150 °C but is higher in Model III for the other processing conditions. Model III better predicts yields at longer reaction times as Model I starts to predict a reduction in yield, as happens in a batch reactor but would not happen in a flow-through reactor where the product is constantly removed. Overall, Model III is able to better predict product yields from the PLPW hydrolysis of triticale straw with the incorporation of solvent flow into the kinetic model. 5. Conclusions The autohydrolysis of triticale straw by PLPW in a flowthrough reactor was assessed by use of kinetic models based on consecutive first-order reactions of the hemicellulose. Regression of the kinetic equations showed that at most of the tested conditions the hemicellulose did not display a biphasic reaction of fast and slow portions of hemicellulose. This was probably due to the autohydrolysis process itself, where the hemicellulose would be better described as being ‘susceptible’ to hydrolysis at the processing conditions. Both the monophasic and flow-through reactor models were successful at predicting the production of xylo-oligomers. However, the monophasic model underpredicted the production of xylose monomers from the xylo-oligomers, which the flow-through reactor model was able to do with much better accuracy. Temperature played a larger role in the hydrolysis process, although flow rate did influence the proportions of oligomers to monomers produced. The kinetic rate constants are proportional to each other at constant flow rates, suggesting that they are also dependent on the fluid velocity in the reactor and not just acid concentration and temperature as described by previous studies. However, differences in acetic acid concentration during the autohydrolysis process prevent a full understanding of the mechanisms of

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hemicellulose hydrolysis in a flow-through reactor but provide a background for better understanding and future research in the area. Acknowledgment The authors are grateful to the Agricultural Bioproducts Innovation Program (ABIP) CBioNet and CTBI networks for financial support of this work. The triticale straw was kindly provided by Agriculture and Agri-Food Canada SPARC research station in Swift Current, SK, Canada. We thank Lana Fukumoto, David Godfrey, and Yukihiro Tamaki for technical assistance and Michael Parker and Maggie Lai for their help. Nomenclature A ) pre-exponential constant D ) degradation products (mg) Ea ) activation energy (kJ/mol) F1 and F2 ) fast and slow fractions of hemicellulose, respectively (decimal) H ) hemicellulose (xylose equivalents) (mg) Hf ) fast hemicellulose (xylose equivalents) (mg) Hs ) slow hemicellulose (xylose equivalents) (mg) H0 ) total potential hemicellulose (xylose equivalents) at t ) 0 (mg) k1, k2, k3, k1f, k1s ) reaction rate constants (min-1) L ) bed depth (m) O ) xylo-oligomers (xylose equivalents) (mg) R ) universal gas constant (8.3143 × 10-3 kJ mol-1 K-1) SO ) dimensionless xylo-oligomer concentration (-) t ) extraction time (min) T ) absolute temperature (K) x ) axial position along the bed (m) X ) xylose monomers (mg) u ) superficial velocity (m min-1) YO ) yield of xylo-oligomers (decimal) YX ) yield of xylose monomers (decimal) z ) x/L Greek Symbols R ) k2/k1 β ) (k1L)/u γ ) k3/k1 τ ) (tu)/L

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ReceiVed for reView February 16, 2010 ReVised manuscript receiVed May 4, 2010 Accepted May 25, 2010 IE1003625

Kinetic Modeling of Hemicellulose Hydrolysis from Triticale Straw in a

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