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Kinetics of furfural production from pre-hydrolysis liquor (PHL) of a kraft-based hardwood dissolving pulp production process
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Kinetics of furfural production from pre-hydrolysis liquor (PHL) of a kraft-based hardwood dissolving pulp production process Haitang Liu a,b,*, Huiren Hu a, Mir Mojtaba Baktash b, M. Sarwar Jahan b,c, Laboni Ahsan b, Yonghao Ni a,b,* a
Tianjin Key Laboratory of Pulp and Paper, Tianjin University of Science and Technology, Tianjin 300457, PR China Limerick Pulp and Paper Centre, Department of Chemical Engineering, University of New Brunswick, Fredericton, NB, Canada E3B 5A3 c Pulp and Paper Research Division, BCSIR Laboratories, Dhaka, Dhaka 1205, Bangladesh b
article info
abstract
Article history:
The kinetics of C-5 sugars (xylose/xylan) conversion to furfural in an industrial pre-
Received 20 November 2013
hydrolysis liquor (PHL) in both acetic acid (HAc)-catalyzed system and sulfuric acid
Received in revised form
(H2SO4)-catalyzed system, were determined in a temperature range of 150e190 C. The
28 January 2014
main reactions involved during the process included: 1) C-5 sugars consumption for
Accepted 4 February 2014
furfural formation and side reactions; 2) furfural degradation. It was found that these re-
Available online xxx
actions followed first order kinetics. A consecutive reaction model (from xylose to furfural,
Keywords:
while a consecutive/parallel model suited for the HAc-catalyzed system due to side re-
Kinetics
actions, which also consumed C-5 sugars. The activation energy for C-5 sugar disappear-
Furfural formation
ance, and the furfural degradation, was 151 kJ mol1 and 115 kJ mol1, respectively, in the
Pre-hydrolysis liquor
HAc-catalyzed system.
then to degradation products) fitted into the data obtained in the H2SO4-catalyzed system;
C-5 sugar consumption
ª 2014 Elsevier Ltd. All rights reserved.
Furfural destruction
1.
Introduction
The forest biorefinery concept is to integrate the overall biomass conversion processes to produce fuels, power, heat, and value-added chemicals. It is analogous to today’s petroleum refinery, which produces multiple fuels and products from petroleum. In early publications, it has been shown that the extraction of hemicelluloses before pulping, especially the pre-hydrolysis kraft-based dissolving pulp production process
fits well into the overall biorefinery concept [1e6]. Furfural is a significant product of the prehydrolysis step and it comes from the degradation of xylose through intermediates. This reaction would occur without mineral acids addition due to the liberation of acetic acid from the acetyl groups of the hemicelluloses. There are examples of furfural production from Eucalyptus globulus or xylose subjected to autohydrolysis [7]. In an early publication [8], it was shown that furfural can be a potential product from the C-5 sugars (almost exclusively xylose/xylan) present in the industrial
* Corresponding authors. Limerick Pulp and Paper Centre, Department of Chemical Engineering, University of New Brunswick, Fredericton, NB, Canada E3B 5A3. Tel.: þ1 506 4516857; fax: þ1 506 4534767. E-mail addresses: [email protected], [email protected] (H. Liu), [email protected] (Y. Ni). http://dx.doi.org/10.1016/j.biombioe.2014.02.003 0961-9534/ª 2014 Elsevier Ltd. All rights reserved.
Please cite this article in press as: Liu H, et al., Kinetics of furfural production from pre-hydrolysis liquor (PHL) of a kraft-based hardwood dissolving pulp production process, Biomass and Bioenergy (2014), http://dx.doi.org/10.1016/j.biombioe.2014.02.003
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prehydrolysis liquor (PHL). Furfural is commercially produced from agricultural lignocellulosic materials under the acidic conditions involving acid hydrolysis and dehydration reaction. Due to several side reactions the furfural yield/selectivity is decreased, and these side reactions are more extensive under the conditions of higher acid concentration and temperature. There are several side reactions in the course of producing furfural, for example, decomposition of furfural to formic acid [9], it is called fragmentation side reaction; condensations of furfural itself [10], those of furfural with xylose [9,11], and those of furfural with lignin-related phenolic compounds [8,11], these are resinification side reactions. The conversion of C-5 sugars to furfural will proceed in the following steps: 1) hydrolysis of xylan to xylose, an increase in temperature and/or acid concentration will speed up the hydrolysis; 2) dehydration of xylose to furfural. It is well known that D-Xylose can be converted quantitatively into furfural under acidic conditions [11], but there are many aspects of the reaction that are not totally clear yet, especially concerning the reaction kinetics of both furfural formation and destruction. Quantitative data on the stability of furfural under aqueous and acidic conditions is needed. Many literatures are available on the kinetics studies of the conversion of xylose (model compound) with different catalysts, such as, sulfuric or hydrochloric acid catalyst [12e14] heterogeneous catalysts [15], high temperature water medium [16,17], maleic acid [18]. On the other hand, only few are available in the literatures related to the conversion of xylose/xylan from industrial waste streams, such as prehydrolysis liquor (PHL). Xing et al. [19] studied on the furfural production along with co-products like acetic acid and formic acid from two types of feedstocks including a hot water extract and a green liquor extract derived from Northeastern hardwood trees. A maximum furfural yield of 90% was achieved from the hot water extract containing a mass fraction of 10.7% xylose. The objective of this study is to establish the reaction kinetics on the furfural formation process from the industrial PHL using acetic acid as the catalyst in the system, and figure out the factors which influence the furfural yield in the xylose conversion process. The kinetics models are based on the literature [16] using xylose as the model compound in high temperature water, namely non-catalyzed decomposition of xylose. However, due to different systems and raw materials, the side reactions are different in our HAc-catalyzed PHL system. Hemicelluloses of PHL contain not only monomeric sugars but also oligomeric sugars which are subjected to high temperature to be carbonized [20], and there are in-situ acetic acid and lignin in PHL [8]. By means of introducing sulfuric acid-catalyzed system in our work, all of the hemicelluloses are assumed to be converted to monomeric sugars for the followed step of dehydration, because sulfuric acid was supposed to hydrolyze all the oligomeric sugars to monomeric sugars [21,22]. The use of activated carbon aimed at removing lignin in the PHL, and the application of anion exchange resin was with the purpose of removing/recovering acetic acid. Model A is based on the consecutive reaction of xylose to furfural and furfural to degradation products, while model B considers furfural production and side reactions including
xylose carbonization/humins formation and furfural degradation. As the experimental data was known, the postulated results by Model A and B were plotted and compared with experimental data in the legend. The comparison between Model A and B are made for the in-depth and better understanding. The results will be of particular interest to those who produce hemicelluloses in the waste streams, and are considering producing furfural as a potential by-product. The kinetic model can therefore be used for the reactor design and operation strategy optimization.
2.
Experimental
2.1.
Materials and methods
The industrially produced PHL of the kraft-based dissolving pulp production process was collected from a mill located in Eastern Canada. In this mill, the pre-hydrolysis of wood chips (a mass fraction of 70% maple, 20% poplar, and 10% birch) was carried out through steaming at 170 C for 30 min. The detailed chemical composition of the PHL is given in Table 1. To remove large particles and impurities, the PHL was filtered using Whatman qualitative filter papers (GE Healthcare UK Limited, UK). The AC-resin pretreated PHL was obtained through Activated Carbon (AC) adsorption and sequential resin adsorption. The AC sample used in this study is a woodbased powdered activated carbon (CR325W-Ultra). The ion exchange resin sample (Purolite A103S) was obtained from PUROLITE, which is a macroporous weak base anion exchange resin having tertiary amine functionality. AC was added to the flask containing PHL, and the weight ratio was 1:30. The mixture was shaken at 2.5 Hz at room temperature for 1 h then the mixture was filtered though Nylon 66 membrane filters to collect the filtrates, namely, the AC pretreated PHL. Ion exchange resin was added to the flask containing AC pretreated PHL, with a weight ratio of 1:10, and the mixture was shaken at 3.3 Hz at room temperature for 1 h, and then followed by filtration. Here, the filtrate is referred to as AC-resin pretreated PHL, which was a result of the sequential steps of activated carbon adsorption (remove lignin) and ion exchange resin (remove acetic acid) treatment. The Parr reactor (4843, Parr Instrument Company) with a liquid bleed valve was used as an autoclave for furfural formation, the heating was set at high heat. The PHL (without additional acid added)/AC-resin pretreated PHL (sulfuric acid
Table 1 e The mass fraction of sugar (total sugar, pentose and hexose), lignin, acetic acid and furfural in PHL/ACresin treated PHL. Total sugar, % (mass fraction) Pentose, % (mass fraction) Hexose, % (mass fraction) Lignin, % (mass fraction) Acetic acid, % (mass fraction) Furfural, % (mass fraction)
PHL
AC-resin treated PHL
4.91 3.52 1.39 0.87 1.04 0.15
4.14 2.91 1.23 0.13 0.46 0.06
Please cite this article in press as: Liu H, et al., Kinetics of furfural production from pre-hydrolysis liquor (PHL) of a kraft-based hardwood dissolving pulp production process, Biomass and Bioenergy (2014), http://dx.doi.org/10.1016/j.biombioe.2014.02.003
b i o m a s s a n d b i o e n e r g y x x x ( 2 0 1 4 ) 1 e8
was added) with weight of 300 g was put into the autoclave. One set of trials was done for PHL (without additional acid added) at 150 C, 170 C and 190 C and another trail for ACresin pretreated PHL with a mass fraction of 0.5% sulfuric acid addition was carried out at 170 C. For the kinetic data, every sample was collected at 10 min intervals (in the case of 190 C, some samples were taken every 2 min) through a liquid bleed valve from the reactor containing PHL/AC-resin pretreated PHL that was undergoing the reaction.
2.2.
Sugar analysis
In order to measure the oligomeric sugar of original PHL, the oligomers in the PHL needed to be hydrolyzed into monomeric sugars. A vial containing 1 mL of the PHL and 5 mL a mass fraction of 4% sulfuric acid was sealed in an autoclave before being put in an oil bath (Neslab Instruments, Inc., Portsmouth, N.H., USA) [23] The autoclave was kept in the oil bath at 121 C for 1 h. This post acid hydrolyzate was used to determine the total saccharides content in the pre-hydrolysis liquor. The monomeric sugar content in the post acid hydrolyzate stood for the total saccharides in the prehydrolysis liquor. The sugar contents in oligomeric form in the pre-hydrolysis liquor were calculated from the difference of the monomeric sugar contents with and without the post acid hydrolysis. The monomeric sugar content in the pre-hydrolysis liquor and the acid hydrolyzate were determined by using an Ion Chromatography with a Pulse Amperometric Detector and CarboPacTM PA1 column (Dionex-300, Dionex Corporation, Canada). De-ionized water was used as eluant with a flow rate of 1 mL min1, 0.2 mol L1 NaOH was used as the regeneration agent with 1 mL min1 flow rate, and 0.5 mol L1 NaOH was used as the supporting electrolyte with 1 mL min1 flow rate. The samples were filtered and diluted prior to analysis. The C-5 sugar conversion (Cc) was calculated as below Cc ð%Þ ¼ ðC1 C2 Þ=C1 100%
(1)
C1: Total amount of C-5 sugars; C2: Residual amount of C-5 sugars.
2.3.
Acetic acid and furfural analysis
The contents of acetic acid and furfural in aqueous phase were determined by following a nuclear magnetic resonance (NMR) method [24]. The furfural yield was calculated based on the following formula, Fy ð%Þ ¼ Wf =Wtheoretical 100%
(2)
Fy: Furfural yield; Wf: The furfural weight that is generated during the reaction; Wtheoretical: The theoretical amount of furfural produced.
2.4.
Lignin analysis
The lignin content of the PHL/AC-resin pretreated PHL was measured based on the UV/Vis spectrometric method at a wavelength of 205 nm (TAPPI UM 250).
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2.5. Xylose hydrolysis modeling and parameter estimation Xylose hydrolysis to furfural by acetic acid or sulfuric acid was on the hypothesis of pseudo-homogeneous irreversible firstorder reactions. Therefore, the model can be generalized as k2 k1 follows, model A: xylose/furfural/degradation productk1 and k2 can be obtained based on the integration method because the xylose/furfural concentration as a function of time was known (detailed method was described in results and discussion 3.3). Furthermore, considering the side reactions of xylose, another comprehensive model was also included in this study, it was model B:
The overall xylose consumption rate, r ¼ (r1 þ r3), was obtained from the plots of Ln(xylose concentration) versus time from the xylose disappearance experiments data. Similarly, the furfural consumption rate (r2) was obtained from the plots of Ln(furfural concentration) versus time from the results. The kinetic constant r1 was postulated based on nonlinear leastsquares fitting method. r3 was obtained by the difference of r and r1(more details was shown in 3.3).
3.
Results and discussion
3.1.
Characterization of PHL
It is well known that the PHL from hardwood kraft-based dissolving pulp production process contains substantial amount of hemicelluloses, in particular, xylose/xylan [2e6,8]. The presence of acetic acid was significant (about 10 g L1) in the PHL. Acetic acid is generated from the labile acetyl groups present in the hemicelluloses. It was reported that the amount of free acetic acid in the hydrolyzate depends on the amounts of xylan in the wood and xylan converted to xylose and xylooligo-saccharide [25]. Acetic acid presents in the PHL can be used as the acid sources for the dehydration of pentose. Activated carbon (AC) and resin treatment retained a mass fraction of 84% total sugars, while removed a mass fraction of 86% lignin and 56% acetic acid from the PHL (Table 1).
3.2. C-5 sugars conversion and furfural yield in HAccatalyzed system and H2SO4-catalyzed system It has been postulated [4,6,8] that furfural may be a potential valuable product from xylose/xylan present in the PHL. The in-situ acetic acid (HAc) or mineral acid (such as sulfuric acid) can be used as the catalyst for the conversion of sugars [5,8]. As shown in Fig. 1, the C-5 sugars conversion is much dependant on temperature in the HAc-catalyzed system.
Please cite this article in press as: Liu H, et al., Kinetics of furfural production from pre-hydrolysis liquor (PHL) of a kraft-based hardwood dissolving pulp production process, Biomass and Bioenergy (2014), http://dx.doi.org/10.1016/j.biombioe.2014.02.003
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24.4% at 405 min at 150 C, and the yield of 27.3% at 110 min at 170 C. At 190 C, the yield of 18.0% was not as high as that at 150 C and 170 C. It was interesting to note that the highest yield was 53.6% in the H2SO4-catalyzed system, which was higher than that in the HAc-catalyzed system.
3.3. Kinetic model for C-5 sugars disappearance and furfural destruction Furfural is formed from the dehydration of C-5 sugars, in the present system, it is almost exclusively from xylose. Furfural can undergo further reactions, involving degradation, for example, to humin or other by-products (D). Fig. 1 e Conversion of C-5 sugars in HAc-catalyzed and H2SO4-catalyzed systems. There was a rapid increase in C-5 sugars conversion with increasing reaction time: a near complete conversion was achieved at 35 min at 190 C, while it would need 130 min at 170 C; At 150 C, the conversion was very slow and reached 97% conversion at 525 min. The furfural results from the PHL are given in Fig. 2. It can be found that there was a maximum yield under each condition, indicating that furfural is generated, and then consumed during the course of the reaction. A number of reactions can account for the furfural consumption: 1) furfural destruction to smaller molecules like formic acid at high temperature or under acid environment [9,26]; 2)furfural can react with sugar/ sugar intermediates or phenol forming furfural-xylose or furfural-phenol [9,11,27]; 3) furfural reacts with itself under the acidic environment [10] may form black resin [26]. During the course of furfural formation from PHL, more acetic acid was generated from the acetyl groups bound to hemicelluloses, and the acetic acid concentration reached to almost 1.8% (data is not shown) in the HAc-catalyzed system. The amount of acetic acid was critical in the HAc-catalyzed system for the conversion of xylose/xylan to furfural [8]. The optimum furfural yield varied in the HAc-catalyzed system from 150 C to 190 C. At low temperatures, it took longer time for the system to reach the highest furfural yield, the yield of
Fig. 2 e Furfural yield from C-5 sugars in HAc-catalyzed and H2SO4-catalyzed systems.
It was reported in Ref. [13] that the dehydration of xylose to furfural (F) is a pseudo first order with respect to xylose. The decomposition of furfural (F) to humin or other by-products (D), was also first order with respect to furfural [13,16,28]. Thus the kinetics may be as: For xyloseðXÞ : dCX =dt ¼ k1 CX
(3)
For furfuralðFÞ : dCF =dt ¼ k1 CX k2 CF
(4)
where Cx and CF are the mole concentration of xylose and furfural, respectively. The analytical solutions of Eqs. (3) and (4) can be: 1)k1 can be readily obtained based on the integration method, because the xylose concentration (Cx) as a function of time was known. 2)At the completion of xylose (i. e., no xylose would be available) k1 ¼ 0, thus Eq. (4) can be simplified as: dCF/ dt ¼ k2CF again, k2 can be obtained by the integration method. The integration method for k1 was plotted in Fig. 3 (based on the results in Fig. 1). The rate constants were given in Table 2 for the conditions studied. It can be seen that with the
Fig. 3 e Ln (concentration of C-5 sugars) vs time (min) in HAc-catalyzed and H2SO4-catalyzed systems at different temperature.
Please cite this article in press as: Liu H, et al., Kinetics of furfural production from pre-hydrolysis liquor (PHL) of a kraft-based hardwood dissolving pulp production process, Biomass and Bioenergy (2014), http://dx.doi.org/10.1016/j.biombioe.2014.02.003
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Table 2 e Estimated kinetic parameters of Model A for C-5 sugars disappearance and furfural destruction. System
150 170 190 170
C C C C
C-5 sugars disappearance
Furfural destruction
k1*102 (min1)
R
E (kJ mol1)
k2*102 (min1)
R
E (kJ mol1)
0.64 3.60 26.27 0.96
0.9922 0.9906 0.9691 0.9917
151.0
0.11 0.95 1.82 0.49
0.9969 0.9603 0.9873 0.9915
115.1
(HAc) (HAc) (HAc) (H2SO4)
increase of temperature from 150 C to 190 C, the xylose consumption rate increased drastically. In addition, it is noted that at 170 C, the xylose disappearance rate in the HAccatalyzed system was 3.75 times higher than that in the H2SO4-catalyzed system, it indicates that a large amount of sugars are consumed suddenly (Fig. 1) to other compounds while instead of furfural (Fig. 2) in the HAc-catalyzed system than that in the H2SO4-catalyzed system. The rate constant k1 of C-5 sugars disappearance at different temperature was applied to the Arrhenius plot. The energy of activation (Ea) of C-5 sugars disappearance was 151 kJ mol1 for C-5 sugars (Table 2), which was in agreement with that of 143.5 kJ mol1 reported in the literature [29] in maleic acid-catalyzed system. For the furfural decomposition, a first-order reaction was assumed. The kinetic parameter k2 was determined based on the data of furfural concentration when the xylose concentration reached zero (in the case of HAc-catalyzed system at 150 C, the last few data points were used, where the residual xylose concentration was less than 10% of the initial concentration; the same was applied in H2SO4 system at 170 C, where the residual xylose concentration was less than 30% of the initial concentration). The integral plotting was given in Fig. 4, and the rate constants were listed in Table 2. At 150 C, the furfural destruction was very slow. Increasing temperature from 150 C to 170 C or 190 C，the furfural destruction rate increased (Table 2). The Arrhenius plot for the furfural destruction was carried out (not shown) and the activation energy (Ea) was 115 kJ mol1. This was compared to the Ea of 92.4 kJ mol1 [13] for the sulfuric acid system, and 48.1 kJ mol1 [30] for the hydrochloric acid system. Marcotullio et al. [31] studied on the furfural destruction kinetics in aqueous acidic environment, using sulfuric acid as catalyst, in the temperature range 150e200 C, acid concentration range 0.036e0.146 mol L1 and furfural initial concentration between 0.06 and 0.073 mol L1 and Ea was estimated 125.1 kJ mol1, which was close to our result. Based on the data in Table 2, it can be concluded that in the acetic acid process, the furfural degradation occurred more extensively than that in the sulfuric acid system at the same temperature (170 C). With the rate constants k1 and k2 known, the concentrations of xylose and furfural can be obtained:
The results are shown in Fig. 5. Model A predicted well the C-5 sugars concentration results. However, there are big differences between the predicted furfural concentration from Model A and the actual furfural concentrations, as Model A over-estimated the furfural concentration, especially at higher temperatures. On the contrary, the prediction of furfural of Model A in the H2SO4-catalyzed system was in good agreement with the experimental data. In summary, Model A gave a good prediction of C-5 sugars concentration for both the HAc-catalyzed system and H2SO4catalyzed system; However, for the furfural data, Model A is not satisfactory for the HAc-catalyzed system. During the experiments, some insoluble dark substances were found in the HAc-catalyzed system, while the same was not found in the H2SO4-catalyzed system. It was reported that some polysaccharides in the water extraction may be carbonized under high temperature. Usually, sulfuric acid is effective in hydrolyzing hemicelluloses to monomeric sugars [21,22]. In the HAc-catalyzed system, not all of the C-5 sugars, were hydrolyzed to the monomeric sugars, and some were carbonized at high temperatures, also the presence of lignin causes some side reactions [8].
For xyloseðXÞ : CX ¼ CX0 ek1 t
(5)
CF ¼ CX0
For furfuralðFÞ : CF ¼ CX0 ek1 t ek2 t k1 =ðk2 k1 Þ þ CF0 ek2 t
(6)
where Cx and CF refer to the mole concentration of the C-5 sugars and furfural respectively, and Cx0 and CF0 are the concentration of C-5 sugars and furfural at time zero.
To account for the sugar side reactions in the HAccatalyzed system, a third reaction, as shown in Model B, was introduced, so that C-5 sugars were consumed in parallel reactions: furfural formation and sugar side reactions: Their kinetic equations are: For xyloseðXÞ : dCX =dt ¼ r1 CX r3 CX
(7)
For furfuralðFÞ : dCF =dt ¼ r1 CX r2 CF
(8)
With the analytical solution are: CX ¼ CX0 ert r1 ert er2 t þ CF0 er2 t r2 r
(9) (10)
where Cx and CF refer to the mole concentration of the C-5 sugars and furfural respectively, Cx0 and CF0 mean the concentration of C-5 sugars and furfural at time zero. The overall xylose consumption rate, r ¼ (r1 þ r3), was obtained from the plots of Ln(Cx) versus t from the xylose
Please cite this article in press as: Liu H, et al., Kinetics of furfural production from pre-hydrolysis liquor (PHL) of a kraft-based hardwood dissolving pulp production process, Biomass and Bioenergy (2014), http://dx.doi.org/10.1016/j.biombioe.2014.02.003
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disappearance experiments data as shown in Fig. 3. Similarly, the furfural consumption rate (r2) was obtained from the plots of Ln(CF) versus time from the results in Fig. 4. Nonlinear leastsquares fitting method was used to fit the kinetic constant r1 in Eq (10). r3 was obtained by the difference of r and r1. With the kinetic data in Table 3, Model B can much better predict the furfural results in the HAc-catalyzed system than Model A, as shown in Fig. 5. It may draw the conclusion that the side reaction of decomposition of C-5 sugars to other carbonized products can play an important role in the furfural production process in HAc-catalyzed system.
Fig. 4 e Ln (concentration of furfural) vs time (min) in HAccatalyzed and H2SO4-catalyzed systems at different temperature.
4.
Conclusions
This study is related to the furfural formation and its kinetics from C-5 sugars present in the pre-hydrolysis liquor (PHL) in a temperature range of 150e190 C. The reactions followed first
Fig. 5 e The experimental data and Model A (B) data of C-5 sugars and furfural concentration vs time (min) in HAc-catalyzed and H2SO4-catalyzed systems at different temperature: (a) 150 C(HAc); (b) 170 C(HAc); (c)190 C(HAc); (d) 170 C(H2SO4). Please cite this article in press as: Liu H, et al., Kinetics of furfural production from pre-hydrolysis liquor (PHL) of a kraft-based hardwood dissolving pulp production process, Biomass and Bioenergy (2014), http://dx.doi.org/10.1016/j.biombioe.2014.02.003
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Table 3 e Estimated kinetic parameters of Model B for C-5 sugars consumption and furfural destruction. System
150 170 190 170
C C C C
C-5 sugars disappearance r*102 (min1)
R
0.64 3.60 26.27 0.96
0.9922 0.9906 0.9691 0.9917
(HAc) (HAc) (HAc) (H2SO4)
Furfural destruction
E (kJ mol1) r2*102 (min1) 151.0
0.11 0.95 1.82 0.49
order kinetics. At a temperature of 170 C, C-5 sugars disappearance rate in the HAc-catalyzed system was 3.75 times higher than that in the H2SO4-catalyzed system. The side reactions were proposed to account for these differences, so that two reactions took place at the same time for the C-5 sugar disappearance were included in the HAc-catalyzed system. The parallel/consecutive kinetic model (Model B) was shown to predict well with the experimental results. The activation energy (Ea) of C-5 sugars disappearance and furfural degradation in the HAc-catalyzed system was 150 kJ mol1 and 115 kJ mol1, respectively.
Acknowledgments This project was supported by NSERC CRD, Canada Research Chairs, and Atlantic Innovation Fund programs of the Government of Canada, and from the Tianjin Municipal Science and Technology Commission (Grant No. 12ZCZDGX01100).
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R 0.9969 0.9603 0.9873 0.9915
C-5sugars consumption for furfural formation
E (kJ mol1) r1*102 (min1) 115.1
0.15 1.48 6.96 1.01
R
E (kJ mol1)
0.0236 0.0155 0.1989 0.1105
156.6
[8] Liu H, Hu H, Jahan MS, Ni Y. Furfural formation from the pre-hydrolysis liquor of a hardwood kraft-based dissolving pulp production process. Bioresour Technol 2013;131:315e20. [9] Antal Jr MJ, Leesomboon T, Mok WS, Richards GN. Mechanism of formation of 2-furaldehyde from D-xylose. Carbohydr Res 1991;217:71e85. [10] Zhou C, Zhang T. Studies on the reaction mechanism of polymerization of furaldehyde by torsional braid analysis. Chem Res Chin Univ 1985;3:22. [11] Zeitsch KJ. The chemistry and technology of furfural and its many by-products. 13th ed. Elsevier; 2000. [12] Marcotullio G, De Jong W. Chloride ions enhance furfural formation from D-xylose in dilute aqueous acidic solutions. Green Chem 2010;12(10):1739e46. [13] Root DF. Kinetics of the acid catalyzed conversion of xylose to furfural. University of Wisconsin-Madison; 1956. [14] Morinelly JE, Jensen JR, Browne M, Co TB, Shonnard DR. Kinetic characterization of xylose monomer and oligomer concentrations during dilute acid pretreatment of lignocellulosic biomass from forests and switchgrass. Ind Eng Chem Res 2009;48(22):9877e84. [15] Dias AS, Pillinger M, Valente AA. Liquid phase dehydration of D-xylose in the presence of Keggin-type heteropolyacids. Appl Catal A Gen 2005;285(1):126e31. ¨ X. Kinetics of non-catalyzed decomposition of D[16] Jing Q, LU xylose in high temperature liquid water. Chin J Chem Eng 2007;15(5):666e9. [17] Neill R O, Ahmad MN, Vanoye L, Aiouache F. Kinetics of aqueous phase dehydration of xylose into furfural catalyzed by ZSM-5 zeolite. Ind Eng Chem Res 2009;48(9):4300e6. [18] Lu Y, Mosier NS. Biomimetic catalysis for hemicellulose hydrolysis in corn stover. Biotechnol Prog 2007;23(1):116e23. [19] Xing R, Qi W, Huber GW. Production of furfural and carboxylic acids from waste aqueous hemicellulose solutions from the pulp and paper and cellulosic ethanol industries. Energy Environ Sci 2011;4(6):2193e205. [20] Qiao X, Chen H, Ma R. Preparation of furfural from water extraction of steam-exploded corn stalk. J Beijing Univ Chem Technol (Nat Sci) 2009;36:87e91. [21] Jin Q, Zhang H, Yan L, Qu L, Huang H. Kinetic characterization for hemicellulose hydrolysis of corn stover in a dilute acid cycle spray flow-through reactor at moderate conditions. Biomass Bioenergy 2011;35(10):4158e64. [22] Rafiqul I, Mimi Sakinah AM. Kinetic studies on acid hydrolysis of Meranti wood sawdust for xylose production. Chem Eng Sci 2012;71:431e7. [23] Yang G, Jahan MS, Liu H, Ni Y. Acid hydrolysis of prehydrolysis liquor produced from the kraft-based dissolving pulp production process. Ind Eng Chem Res 2012;51(43):13902e7. [24] Ni Y, Kang GJ, Van Heiningen ARP. Are hydroxyl radicals responsible for degradation of carbohydrates during ozone bleaching of chemical pulp? J Pulp Pap Sci 1996;22(2):53e7.
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[25] Garrote G, Domı´nguez H, Parajo´ JC. Production of substituted oligosaccharides by hydrolytic processing of barley husks. Ind Eng Chem Res 2004;43(7):1608e14. [26] Sain B, Chaudhuri A, Borgohain JN, Baruah BP, Ghose JL. Furfural and furfural-based industrial chemicals. J Sci Ind Res 1982;41:431e8. [27] Niederl JB, Maurmeyer RK. Sugar-phenol condensations. The condensation of d-Glucose with phenol. J Am Chem Soc 1939;61(5):1005e10. [28] Garrett ER, Dvorchik BH. Kinetics and mechanisms of the acid degradation of the aldopentoses to furfural. J Pharm SciUs 1969;58(7):813e20.
[29] Lu Y, Mosier NS. Kinetic modeling analysis of maleic acidcatalyzed hemicellulose hydrolysis in corn stover. Biotechnol Bioeng 2008;101(6):1170e81. [30] Rose IC, Epstein N, Watkinson AP. Acid-catalyzed 2furaldehyde (furfural) decomposition kinetics. Ind Eng Chem Res 2000;39(3):843e5. [31] Marcotullio G, Cardoso MAT, De Jong W, Verkooijen AH. Bioenergy II: furfural destruction kinetics during sulphuric acid-catalyzed production from biomass. Int J Chem React Eng 2009;7(1).
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Available online at www.sciencedirect.com
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Kinetics of furfural production from pre-hydrolysis liquor (PHL) of a kraft-based hardwood dissolving pulp production process Haitang Liu a,b,*, Huiren Hu a, Mir Mojtaba Baktash b, M. Sarwar Jahan b,c, Laboni Ahsan b, Yonghao Ni a,b,* a
Tianjin Key Laboratory of Pulp and Paper, Tianjin University of Science and Technology, Tianjin 300457, PR China Limerick Pulp and Paper Centre, Department of Chemical Engineering, University of New Brunswick, Fredericton, NB, Canada E3B 5A3 c Pulp and Paper Research Division, BCSIR Laboratories, Dhaka, Dhaka 1205, Bangladesh b
article info
abstract
Article history:
The kinetics of C-5 sugars (xylose/xylan) conversion to furfural in an industrial pre-
Received 20 November 2013
hydrolysis liquor (PHL) in both acetic acid (HAc)-catalyzed system and sulfuric acid
Received in revised form
(H2SO4)-catalyzed system, were determined in a temperature range of 150e190 C. The
28 January 2014
main reactions involved during the process included: 1) C-5 sugars consumption for
Accepted 4 February 2014
furfural formation and side reactions; 2) furfural degradation. It was found that these re-
Available online xxx
actions followed first order kinetics. A consecutive reaction model (from xylose to furfural,
Keywords:
while a consecutive/parallel model suited for the HAc-catalyzed system due to side re-
Kinetics
actions, which also consumed C-5 sugars. The activation energy for C-5 sugar disappear-
Furfural formation
ance, and the furfural degradation, was 151 kJ mol1 and 115 kJ mol1, respectively, in the
Pre-hydrolysis liquor
HAc-catalyzed system.
then to degradation products) fitted into the data obtained in the H2SO4-catalyzed system;
C-5 sugar consumption
ª 2014 Elsevier Ltd. All rights reserved.
Furfural destruction
1.
Introduction
The forest biorefinery concept is to integrate the overall biomass conversion processes to produce fuels, power, heat, and value-added chemicals. It is analogous to today’s petroleum refinery, which produces multiple fuels and products from petroleum. In early publications, it has been shown that the extraction of hemicelluloses before pulping, especially the pre-hydrolysis kraft-based dissolving pulp production process
fits well into the overall biorefinery concept [1e6]. Furfural is a significant product of the prehydrolysis step and it comes from the degradation of xylose through intermediates. This reaction would occur without mineral acids addition due to the liberation of acetic acid from the acetyl groups of the hemicelluloses. There are examples of furfural production from Eucalyptus globulus or xylose subjected to autohydrolysis [7]. In an early publication [8], it was shown that furfural can be a potential product from the C-5 sugars (almost exclusively xylose/xylan) present in the industrial
* Corresponding authors. Limerick Pulp and Paper Centre, Department of Chemical Engineering, University of New Brunswick, Fredericton, NB, Canada E3B 5A3. Tel.: þ1 506 4516857; fax: þ1 506 4534767. E-mail addresses: [email protected], [email protected] (H. Liu), [email protected] (Y. Ni). http://dx.doi.org/10.1016/j.biombioe.2014.02.003 0961-9534/ª 2014 Elsevier Ltd. All rights reserved.
Please cite this article in press as: Liu H, et al., Kinetics of furfural production from pre-hydrolysis liquor (PHL) of a kraft-based hardwood dissolving pulp production process, Biomass and Bioenergy (2014), http://dx.doi.org/10.1016/j.biombioe.2014.02.003
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prehydrolysis liquor (PHL). Furfural is commercially produced from agricultural lignocellulosic materials under the acidic conditions involving acid hydrolysis and dehydration reaction. Due to several side reactions the furfural yield/selectivity is decreased, and these side reactions are more extensive under the conditions of higher acid concentration and temperature. There are several side reactions in the course of producing furfural, for example, decomposition of furfural to formic acid [9], it is called fragmentation side reaction; condensations of furfural itself [10], those of furfural with xylose [9,11], and those of furfural with lignin-related phenolic compounds [8,11], these are resinification side reactions. The conversion of C-5 sugars to furfural will proceed in the following steps: 1) hydrolysis of xylan to xylose, an increase in temperature and/or acid concentration will speed up the hydrolysis; 2) dehydration of xylose to furfural. It is well known that D-Xylose can be converted quantitatively into furfural under acidic conditions [11], but there are many aspects of the reaction that are not totally clear yet, especially concerning the reaction kinetics of both furfural formation and destruction. Quantitative data on the stability of furfural under aqueous and acidic conditions is needed. Many literatures are available on the kinetics studies of the conversion of xylose (model compound) with different catalysts, such as, sulfuric or hydrochloric acid catalyst [12e14] heterogeneous catalysts [15], high temperature water medium [16,17], maleic acid [18]. On the other hand, only few are available in the literatures related to the conversion of xylose/xylan from industrial waste streams, such as prehydrolysis liquor (PHL). Xing et al. [19] studied on the furfural production along with co-products like acetic acid and formic acid from two types of feedstocks including a hot water extract and a green liquor extract derived from Northeastern hardwood trees. A maximum furfural yield of 90% was achieved from the hot water extract containing a mass fraction of 10.7% xylose. The objective of this study is to establish the reaction kinetics on the furfural formation process from the industrial PHL using acetic acid as the catalyst in the system, and figure out the factors which influence the furfural yield in the xylose conversion process. The kinetics models are based on the literature [16] using xylose as the model compound in high temperature water, namely non-catalyzed decomposition of xylose. However, due to different systems and raw materials, the side reactions are different in our HAc-catalyzed PHL system. Hemicelluloses of PHL contain not only monomeric sugars but also oligomeric sugars which are subjected to high temperature to be carbonized [20], and there are in-situ acetic acid and lignin in PHL [8]. By means of introducing sulfuric acid-catalyzed system in our work, all of the hemicelluloses are assumed to be converted to monomeric sugars for the followed step of dehydration, because sulfuric acid was supposed to hydrolyze all the oligomeric sugars to monomeric sugars [21,22]. The use of activated carbon aimed at removing lignin in the PHL, and the application of anion exchange resin was with the purpose of removing/recovering acetic acid. Model A is based on the consecutive reaction of xylose to furfural and furfural to degradation products, while model B considers furfural production and side reactions including
xylose carbonization/humins formation and furfural degradation. As the experimental data was known, the postulated results by Model A and B were plotted and compared with experimental data in the legend. The comparison between Model A and B are made for the in-depth and better understanding. The results will be of particular interest to those who produce hemicelluloses in the waste streams, and are considering producing furfural as a potential by-product. The kinetic model can therefore be used for the reactor design and operation strategy optimization.
2.
Experimental
2.1.
Materials and methods
The industrially produced PHL of the kraft-based dissolving pulp production process was collected from a mill located in Eastern Canada. In this mill, the pre-hydrolysis of wood chips (a mass fraction of 70% maple, 20% poplar, and 10% birch) was carried out through steaming at 170 C for 30 min. The detailed chemical composition of the PHL is given in Table 1. To remove large particles and impurities, the PHL was filtered using Whatman qualitative filter papers (GE Healthcare UK Limited, UK). The AC-resin pretreated PHL was obtained through Activated Carbon (AC) adsorption and sequential resin adsorption. The AC sample used in this study is a woodbased powdered activated carbon (CR325W-Ultra). The ion exchange resin sample (Purolite A103S) was obtained from PUROLITE, which is a macroporous weak base anion exchange resin having tertiary amine functionality. AC was added to the flask containing PHL, and the weight ratio was 1:30. The mixture was shaken at 2.5 Hz at room temperature for 1 h then the mixture was filtered though Nylon 66 membrane filters to collect the filtrates, namely, the AC pretreated PHL. Ion exchange resin was added to the flask containing AC pretreated PHL, with a weight ratio of 1:10, and the mixture was shaken at 3.3 Hz at room temperature for 1 h, and then followed by filtration. Here, the filtrate is referred to as AC-resin pretreated PHL, which was a result of the sequential steps of activated carbon adsorption (remove lignin) and ion exchange resin (remove acetic acid) treatment. The Parr reactor (4843, Parr Instrument Company) with a liquid bleed valve was used as an autoclave for furfural formation, the heating was set at high heat. The PHL (without additional acid added)/AC-resin pretreated PHL (sulfuric acid
Table 1 e The mass fraction of sugar (total sugar, pentose and hexose), lignin, acetic acid and furfural in PHL/ACresin treated PHL. Total sugar, % (mass fraction) Pentose, % (mass fraction) Hexose, % (mass fraction) Lignin, % (mass fraction) Acetic acid, % (mass fraction) Furfural, % (mass fraction)
PHL
AC-resin treated PHL
4.91 3.52 1.39 0.87 1.04 0.15
4.14 2.91 1.23 0.13 0.46 0.06
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b i o m a s s a n d b i o e n e r g y x x x ( 2 0 1 4 ) 1 e8
was added) with weight of 300 g was put into the autoclave. One set of trials was done for PHL (without additional acid added) at 150 C, 170 C and 190 C and another trail for ACresin pretreated PHL with a mass fraction of 0.5% sulfuric acid addition was carried out at 170 C. For the kinetic data, every sample was collected at 10 min intervals (in the case of 190 C, some samples were taken every 2 min) through a liquid bleed valve from the reactor containing PHL/AC-resin pretreated PHL that was undergoing the reaction.
2.2.
Sugar analysis
In order to measure the oligomeric sugar of original PHL, the oligomers in the PHL needed to be hydrolyzed into monomeric sugars. A vial containing 1 mL of the PHL and 5 mL a mass fraction of 4% sulfuric acid was sealed in an autoclave before being put in an oil bath (Neslab Instruments, Inc., Portsmouth, N.H., USA) [23] The autoclave was kept in the oil bath at 121 C for 1 h. This post acid hydrolyzate was used to determine the total saccharides content in the pre-hydrolysis liquor. The monomeric sugar content in the post acid hydrolyzate stood for the total saccharides in the prehydrolysis liquor. The sugar contents in oligomeric form in the pre-hydrolysis liquor were calculated from the difference of the monomeric sugar contents with and without the post acid hydrolysis. The monomeric sugar content in the pre-hydrolysis liquor and the acid hydrolyzate were determined by using an Ion Chromatography with a Pulse Amperometric Detector and CarboPacTM PA1 column (Dionex-300, Dionex Corporation, Canada). De-ionized water was used as eluant with a flow rate of 1 mL min1, 0.2 mol L1 NaOH was used as the regeneration agent with 1 mL min1 flow rate, and 0.5 mol L1 NaOH was used as the supporting electrolyte with 1 mL min1 flow rate. The samples were filtered and diluted prior to analysis. The C-5 sugar conversion (Cc) was calculated as below Cc ð%Þ ¼ ðC1 C2 Þ=C1 100%
(1)
C1: Total amount of C-5 sugars; C2: Residual amount of C-5 sugars.
2.3.
Acetic acid and furfural analysis
The contents of acetic acid and furfural in aqueous phase were determined by following a nuclear magnetic resonance (NMR) method [24]. The furfural yield was calculated based on the following formula, Fy ð%Þ ¼ Wf =Wtheoretical 100%
(2)
Fy: Furfural yield; Wf: The furfural weight that is generated during the reaction; Wtheoretical: The theoretical amount of furfural produced.
2.4.
Lignin analysis
The lignin content of the PHL/AC-resin pretreated PHL was measured based on the UV/Vis spectrometric method at a wavelength of 205 nm (TAPPI UM 250).
3
2.5. Xylose hydrolysis modeling and parameter estimation Xylose hydrolysis to furfural by acetic acid or sulfuric acid was on the hypothesis of pseudo-homogeneous irreversible firstorder reactions. Therefore, the model can be generalized as k2 k1 follows, model A: xylose/furfural/degradation productk1 and k2 can be obtained based on the integration method because the xylose/furfural concentration as a function of time was known (detailed method was described in results and discussion 3.3). Furthermore, considering the side reactions of xylose, another comprehensive model was also included in this study, it was model B:
The overall xylose consumption rate, r ¼ (r1 þ r3), was obtained from the plots of Ln(xylose concentration) versus time from the xylose disappearance experiments data. Similarly, the furfural consumption rate (r2) was obtained from the plots of Ln(furfural concentration) versus time from the results. The kinetic constant r1 was postulated based on nonlinear leastsquares fitting method. r3 was obtained by the difference of r and r1(more details was shown in 3.3).
3.
Results and discussion
3.1.
Characterization of PHL
It is well known that the PHL from hardwood kraft-based dissolving pulp production process contains substantial amount of hemicelluloses, in particular, xylose/xylan [2e6,8]. The presence of acetic acid was significant (about 10 g L1) in the PHL. Acetic acid is generated from the labile acetyl groups present in the hemicelluloses. It was reported that the amount of free acetic acid in the hydrolyzate depends on the amounts of xylan in the wood and xylan converted to xylose and xylooligo-saccharide [25]. Acetic acid presents in the PHL can be used as the acid sources for the dehydration of pentose. Activated carbon (AC) and resin treatment retained a mass fraction of 84% total sugars, while removed a mass fraction of 86% lignin and 56% acetic acid from the PHL (Table 1).
3.2. C-5 sugars conversion and furfural yield in HAccatalyzed system and H2SO4-catalyzed system It has been postulated [4,6,8] that furfural may be a potential valuable product from xylose/xylan present in the PHL. The in-situ acetic acid (HAc) or mineral acid (such as sulfuric acid) can be used as the catalyst for the conversion of sugars [5,8]. As shown in Fig. 1, the C-5 sugars conversion is much dependant on temperature in the HAc-catalyzed system.
Please cite this article in press as: Liu H, et al., Kinetics of furfural production from pre-hydrolysis liquor (PHL) of a kraft-based hardwood dissolving pulp production process, Biomass and Bioenergy (2014), http://dx.doi.org/10.1016/j.biombioe.2014.02.003
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24.4% at 405 min at 150 C, and the yield of 27.3% at 110 min at 170 C. At 190 C, the yield of 18.0% was not as high as that at 150 C and 170 C. It was interesting to note that the highest yield was 53.6% in the H2SO4-catalyzed system, which was higher than that in the HAc-catalyzed system.
3.3. Kinetic model for C-5 sugars disappearance and furfural destruction Furfural is formed from the dehydration of C-5 sugars, in the present system, it is almost exclusively from xylose. Furfural can undergo further reactions, involving degradation, for example, to humin or other by-products (D). Fig. 1 e Conversion of C-5 sugars in HAc-catalyzed and H2SO4-catalyzed systems. There was a rapid increase in C-5 sugars conversion with increasing reaction time: a near complete conversion was achieved at 35 min at 190 C, while it would need 130 min at 170 C; At 150 C, the conversion was very slow and reached 97% conversion at 525 min. The furfural results from the PHL are given in Fig. 2. It can be found that there was a maximum yield under each condition, indicating that furfural is generated, and then consumed during the course of the reaction. A number of reactions can account for the furfural consumption: 1) furfural destruction to smaller molecules like formic acid at high temperature or under acid environment [9,26]; 2)furfural can react with sugar/ sugar intermediates or phenol forming furfural-xylose or furfural-phenol [9,11,27]; 3) furfural reacts with itself under the acidic environment [10] may form black resin [26]. During the course of furfural formation from PHL, more acetic acid was generated from the acetyl groups bound to hemicelluloses, and the acetic acid concentration reached to almost 1.8% (data is not shown) in the HAc-catalyzed system. The amount of acetic acid was critical in the HAc-catalyzed system for the conversion of xylose/xylan to furfural [8]. The optimum furfural yield varied in the HAc-catalyzed system from 150 C to 190 C. At low temperatures, it took longer time for the system to reach the highest furfural yield, the yield of
Fig. 2 e Furfural yield from C-5 sugars in HAc-catalyzed and H2SO4-catalyzed systems.
It was reported in Ref. [13] that the dehydration of xylose to furfural (F) is a pseudo first order with respect to xylose. The decomposition of furfural (F) to humin or other by-products (D), was also first order with respect to furfural [13,16,28]. Thus the kinetics may be as: For xyloseðXÞ : dCX =dt ¼ k1 CX
(3)
For furfuralðFÞ : dCF =dt ¼ k1 CX k2 CF
(4)
where Cx and CF are the mole concentration of xylose and furfural, respectively. The analytical solutions of Eqs. (3) and (4) can be: 1)k1 can be readily obtained based on the integration method, because the xylose concentration (Cx) as a function of time was known. 2)At the completion of xylose (i. e., no xylose would be available) k1 ¼ 0, thus Eq. (4) can be simplified as: dCF/ dt ¼ k2CF again, k2 can be obtained by the integration method. The integration method for k1 was plotted in Fig. 3 (based on the results in Fig. 1). The rate constants were given in Table 2 for the conditions studied. It can be seen that with the
Fig. 3 e Ln (concentration of C-5 sugars) vs time (min) in HAc-catalyzed and H2SO4-catalyzed systems at different temperature.
Please cite this article in press as: Liu H, et al., Kinetics of furfural production from pre-hydrolysis liquor (PHL) of a kraft-based hardwood dissolving pulp production process, Biomass and Bioenergy (2014), http://dx.doi.org/10.1016/j.biombioe.2014.02.003
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Table 2 e Estimated kinetic parameters of Model A for C-5 sugars disappearance and furfural destruction. System
150 170 190 170
C C C C
C-5 sugars disappearance
Furfural destruction
k1*102 (min1)
R
E (kJ mol1)
k2*102 (min1)
R
E (kJ mol1)
0.64 3.60 26.27 0.96
0.9922 0.9906 0.9691 0.9917
151.0
0.11 0.95 1.82 0.49
0.9969 0.9603 0.9873 0.9915
115.1
(HAc) (HAc) (HAc) (H2SO4)
increase of temperature from 150 C to 190 C, the xylose consumption rate increased drastically. In addition, it is noted that at 170 C, the xylose disappearance rate in the HAccatalyzed system was 3.75 times higher than that in the H2SO4-catalyzed system, it indicates that a large amount of sugars are consumed suddenly (Fig. 1) to other compounds while instead of furfural (Fig. 2) in the HAc-catalyzed system than that in the H2SO4-catalyzed system. The rate constant k1 of C-5 sugars disappearance at different temperature was applied to the Arrhenius plot. The energy of activation (Ea) of C-5 sugars disappearance was 151 kJ mol1 for C-5 sugars (Table 2), which was in agreement with that of 143.5 kJ mol1 reported in the literature [29] in maleic acid-catalyzed system. For the furfural decomposition, a first-order reaction was assumed. The kinetic parameter k2 was determined based on the data of furfural concentration when the xylose concentration reached zero (in the case of HAc-catalyzed system at 150 C, the last few data points were used, where the residual xylose concentration was less than 10% of the initial concentration; the same was applied in H2SO4 system at 170 C, where the residual xylose concentration was less than 30% of the initial concentration). The integral plotting was given in Fig. 4, and the rate constants were listed in Table 2. At 150 C, the furfural destruction was very slow. Increasing temperature from 150 C to 170 C or 190 C，the furfural destruction rate increased (Table 2). The Arrhenius plot for the furfural destruction was carried out (not shown) and the activation energy (Ea) was 115 kJ mol1. This was compared to the Ea of 92.4 kJ mol1 [13] for the sulfuric acid system, and 48.1 kJ mol1 [30] for the hydrochloric acid system. Marcotullio et al. [31] studied on the furfural destruction kinetics in aqueous acidic environment, using sulfuric acid as catalyst, in the temperature range 150e200 C, acid concentration range 0.036e0.146 mol L1 and furfural initial concentration between 0.06 and 0.073 mol L1 and Ea was estimated 125.1 kJ mol1, which was close to our result. Based on the data in Table 2, it can be concluded that in the acetic acid process, the furfural degradation occurred more extensively than that in the sulfuric acid system at the same temperature (170 C). With the rate constants k1 and k2 known, the concentrations of xylose and furfural can be obtained:
The results are shown in Fig. 5. Model A predicted well the C-5 sugars concentration results. However, there are big differences between the predicted furfural concentration from Model A and the actual furfural concentrations, as Model A over-estimated the furfural concentration, especially at higher temperatures. On the contrary, the prediction of furfural of Model A in the H2SO4-catalyzed system was in good agreement with the experimental data. In summary, Model A gave a good prediction of C-5 sugars concentration for both the HAc-catalyzed system and H2SO4catalyzed system; However, for the furfural data, Model A is not satisfactory for the HAc-catalyzed system. During the experiments, some insoluble dark substances were found in the HAc-catalyzed system, while the same was not found in the H2SO4-catalyzed system. It was reported that some polysaccharides in the water extraction may be carbonized under high temperature. Usually, sulfuric acid is effective in hydrolyzing hemicelluloses to monomeric sugars [21,22]. In the HAc-catalyzed system, not all of the C-5 sugars, were hydrolyzed to the monomeric sugars, and some were carbonized at high temperatures, also the presence of lignin causes some side reactions [8].
For xyloseðXÞ : CX ¼ CX0 ek1 t
(5)
CF ¼ CX0
For furfuralðFÞ : CF ¼ CX0 ek1 t ek2 t k1 =ðk2 k1 Þ þ CF0 ek2 t
(6)
where Cx and CF refer to the mole concentration of the C-5 sugars and furfural respectively, and Cx0 and CF0 are the concentration of C-5 sugars and furfural at time zero.
To account for the sugar side reactions in the HAccatalyzed system, a third reaction, as shown in Model B, was introduced, so that C-5 sugars were consumed in parallel reactions: furfural formation and sugar side reactions: Their kinetic equations are: For xyloseðXÞ : dCX =dt ¼ r1 CX r3 CX
(7)
For furfuralðFÞ : dCF =dt ¼ r1 CX r2 CF
(8)
With the analytical solution are: CX ¼ CX0 ert r1 ert er2 t þ CF0 er2 t r2 r
(9) (10)
where Cx and CF refer to the mole concentration of the C-5 sugars and furfural respectively, Cx0 and CF0 mean the concentration of C-5 sugars and furfural at time zero. The overall xylose consumption rate, r ¼ (r1 þ r3), was obtained from the plots of Ln(Cx) versus t from the xylose
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disappearance experiments data as shown in Fig. 3. Similarly, the furfural consumption rate (r2) was obtained from the plots of Ln(CF) versus time from the results in Fig. 4. Nonlinear leastsquares fitting method was used to fit the kinetic constant r1 in Eq (10). r3 was obtained by the difference of r and r1. With the kinetic data in Table 3, Model B can much better predict the furfural results in the HAc-catalyzed system than Model A, as shown in Fig. 5. It may draw the conclusion that the side reaction of decomposition of C-5 sugars to other carbonized products can play an important role in the furfural production process in HAc-catalyzed system.
Fig. 4 e Ln (concentration of furfural) vs time (min) in HAccatalyzed and H2SO4-catalyzed systems at different temperature.
4.
Conclusions
This study is related to the furfural formation and its kinetics from C-5 sugars present in the pre-hydrolysis liquor (PHL) in a temperature range of 150e190 C. The reactions followed first
Fig. 5 e The experimental data and Model A (B) data of C-5 sugars and furfural concentration vs time (min) in HAc-catalyzed and H2SO4-catalyzed systems at different temperature: (a) 150 C(HAc); (b) 170 C(HAc); (c)190 C(HAc); (d) 170 C(H2SO4). Please cite this article in press as: Liu H, et al., Kinetics of furfural production from pre-hydrolysis liquor (PHL) of a kraft-based hardwood dissolving pulp production process, Biomass and Bioenergy (2014), http://dx.doi.org/10.1016/j.biombioe.2014.02.003
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Table 3 e Estimated kinetic parameters of Model B for C-5 sugars consumption and furfural destruction. System
150 170 190 170
C C C C
C-5 sugars disappearance r*102 (min1)
R
0.64 3.60 26.27 0.96
0.9922 0.9906 0.9691 0.9917
(HAc) (HAc) (HAc) (H2SO4)
Furfural destruction
E (kJ mol1) r2*102 (min1) 151.0
0.11 0.95 1.82 0.49
order kinetics. At a temperature of 170 C, C-5 sugars disappearance rate in the HAc-catalyzed system was 3.75 times higher than that in the H2SO4-catalyzed system. The side reactions were proposed to account for these differences, so that two reactions took place at the same time for the C-5 sugar disappearance were included in the HAc-catalyzed system. The parallel/consecutive kinetic model (Model B) was shown to predict well with the experimental results. The activation energy (Ea) of C-5 sugars disappearance and furfural degradation in the HAc-catalyzed system was 150 kJ mol1 and 115 kJ mol1, respectively.
Acknowledgments This project was supported by NSERC CRD, Canada Research Chairs, and Atlantic Innovation Fund programs of the Government of Canada, and from the Tianjin Municipal Science and Technology Commission (Grant No. 12ZCZDGX01100).
references
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E (kJ mol1) r1*102 (min1) 115.1
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Please cite this article in press as: Liu H, et al., Kinetics of furfural production from pre-hydrolysis liquor (PHL) of a kraft-based hardwood dissolving pulp production process, Biomass and Bioenergy (2014), http://dx.doi.org/10.1016/j.biombioe.2014.02.003