Bioresource Technology 149 (2013) 216–224

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Bioresource Technology

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Kinetics of levulinic acid and furfural production fromMiscanthus � giganteus

0960-8524/$ - see front matter � 2013 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.biortech.2013.09.006

⇑ Corresponding author. Tel.: +353 61 202631.E-mail address: Michael.H.Hayes@ul.ie (M.H.B. Hayes).

K. Dussan a, B. Girisuta b, D. Haverty a, J.J. Leahy a, M.H.B. Hayes a,⇑a Carbolea Research Group, Department of Chemical and Environmental Sciences, University of Limerick, Limerick, Irelandb Institute of Chemical and Engineering Sciences, 1 Pesek Road, Jurong Island, 627833, Singapore

h i g h l i g h t s

� Experiments with Miscanthus at 150, 175 and 200 �C; and 0.10, 0.21 and 0.53 M H2SO4.� Low temperatures improve levulinic acid selectivity but increase the reaction time.� High temperatures favour the formation of furfural (27%mol at 185 �C, 0.5 M H2SO4).� A two-stage process was proposed to maximise furfural and optimise levulinic acid.� Yields between 58–72%-mol levulinic acid are obtained at 160–200 �C.

a r t i c l e i n f o

Article history:Received 30 July 2013Received in revised form 28 August 2013Accepted 1 September 2013Available online 19 September 2013

Keywords:Levulinic acidAcid hydrolysisKinetic modellingMiscanthusFurfural

a b s t r a c t

This study investigated the kinetics of acid hydrolysis of the cellulose and hemicellulose in Miscanthus toproduce levulinic acid and furfural under mild temperature and high acid concentration. Experimentswere carried out in an 8 L-batch reactor with 9%-wt. biomass loading, acid concentrations between0.10 and 0.53 M H2SO4, and at temperatures between 150 and 200 �C. The concentrations of xylose,glucose, furfural, 5-hydroxymethylfurfural and levulinic acid were used in two mechanistic kinetic mod-els for the prediction of the performance of ideal continuous reactors for the optimisation of levulinic acidand the concurrent production of furfural. A two-stage arrangement was found to maximise furfural inthe first reactor (PFR – 185 �C, 0.5 M H2SO4, 27.3%-mol). A second stage leads to levulinic acid yieldsbetween 58% and 72%-mol at temperatures between 160 and 200 �C.

� 2013 Elsevier Ltd. All rights reserved.

1. Introduction

Lignocellulosic biomass is recognised as a realistic alternativeresource for the production of fuels and chemicals that arecurrently derived from non-renewable sources. Miscan-thus � giganteus (Miscanthus), a perennial grass, is currentlyutilised in Europe as a commercial energy crop. Due to its highhexose content (40–48%), it is considered as a promising feedstockfor biorefining in the production of carbohydrate-based fuels andchemicals (Girisuta et al., 2012; Hayes, 2013). The acid hydrolysisis one of the commonly used thermochemical approaches for theconversion of lignocellulosic biomass. This process is carried outin an aqueous phase using acid catalysts (e.g., mineral acids,inorganic salts, solid Lewis/Brønsted acid catalysts) to release thesugars and separate the lignin (Kang et al., 2013; Marcotullioet al., 2011; Rong et al., 2012; Weingarten et al., 2011).

The cellulose fraction in biomass is a highly crystalline polymerthat can be hydrolysed to release small oligosaccharides andglucose. These are further dehydrated to 5-hydroxymethylfurfural(HMF), which is rapidly hydrated in acidic aqueous media atelevated temperatures (>150 �C) to equimolar amounts of LA andformic acid (FA). LA has been identified as a platform chemicalfor the production of biofuels (Bozell and Petersen, 2010) and asa versatile precursor for the production of polymers (Cora et al.,2011). Previous kinetics studies on the formation of LA fromvarious lignocellulosic biomasses are summarised in Table 1.Under the severity of these studies (high temperatures and acidconcentrations), the depolymerisation of cellulose is consideredto occur directly to glucose and the formation of LA to occur viathe intermediate HMF (Chang et al., 2009; Girisuta et al., 2013,2007; Tarabanko et al., 2002). Some of these previous studies alsoincluded the parasitic pathway reactions from cellulose tounidentified insoluble products (Girisuta et al., 2007). Glucose isnot, however, completely dehydrated to HMF, but is also decom-posed to the undesired polymeric products that some researchers

Tabl

e1

Kin

etic

mod

els

for

the

acid

cata

lyse

dhy

drol

ysis

tole

vulin

icac

idof

ligno

cellu

losi

cbi

omas

s.

Mec

han

ism

Aci

dTe

mpe

ratu

reSu

bstr

ate

Kin

etic

mod

elR

efs.

0.1–

1.0

MH

2SO

415

0–20

0�C

1.7–

14.0

%-w

t.ce

llu

lose

r ij¼ð�Þk

ijC

n i i;

i¼C;G;H

;j¼

1;2

k ij¼

Ao;

ij½Hþ�m

ijex

pð�

E A;ij=

RTÞ

E A;1

C¼

151:

5kJ=

mol

E A;2

G¼

164:

7kJ=

mol

E A;2

C¼

174:

7kJ=

mol

E A;1

H¼

110:

5kJ=

mol

E A;1

G¼

152:

2kJ=

mol

E A;2

H¼

111:

3kJ=

mol

Gir

isu

taet

al.(

2007

,200

6)

0.25

–1.0

MH

2SO

498

–181

�C0.

1–1.

7M

HM

F

1–5%

-wt.

H2SO

419

0–23

0�C

5.9%

-wt.

wh

eat

stra

wr i¼ð�Þk

iCj;

i¼1;

2;..

.;5

k i¼

Ao;

i½H2SO

4%�m

iex

pð�

E A;i=

RTÞ

k su

gar¼

k 2þ

k 4k H

MF¼

k 3þ

k 5E A

;1¼

78:6

6kJ=m

olE A

;HM

F¼

51:3

7kJ=

mol

E A;s

uga

r¼

61:0

6kJ=m

olE A

;3¼

56:4

7kJ=

mol

E A;2¼

54:5

1kJ=

mol

Ch

ang

etal

.(20

09)

1–5%

-wt.

H2SO

415

0–20

0�C

10%

-wt.

suga

rcan

eba

gass

er i¼ð�Þk

iCj;

i¼G

LN;G

LC1;

GLC

2;H

MF

k i¼

Ao;

i½H2SO

4�m

iex

pð�

E A;i=

RTÞ

E A;G

LN¼

144:

8kJ=m

olE A

;GLC

1¼

152:

1kJ=m

olE A

;GLC

2¼

161:

4kJ=m

olE A

;HM

F¼

101:

6kJ=

mol

Gir

isu

taet

al.(

2013

)

K. Dussan et al. / Bioresource Technology 149 (2013) 216–224 217

label as humins (Chang et al., 2009; Girisuta et al., 2007;Weingarten et al., 2012). A dedicated study of the formation of hu-mins from HMF has been carried out by Girisuta et al. (2006), andthey have shown that humins formation can be suppressed at lowHMF concentrations.

The second fraction in biomass, hemicellulose, is characterisedby an amorphous polymer of pentoses (xylose, arabinose), andsome hexoses (glucose, galactose, and mannose), as well as glucu-ronic acid units. Hemicellulose is easily hydrolysed under mildtemperatures and with low acid catalyst loadings to form pentosesthat are subsequently converted into furfural (Lavarack et al.,2002; Morinelly et al., 2009; Nabarlatz et al., 2004). Furfural iswidely used for various industrial applications, such as in refiningof lubricating oils (Coto et al., 2006) and as a precursor for poly-meric binding agents, such as furfuryl alcohol (Zeitsch, 2000).Attention is being given also to the production of fuel-misciblecompounds from furfural, such as ethyl furfuryl ether, 2-methylfu-ran, 2-methyltetrahydrofuran and C10–C15 alkanes (Lange et al.,2012; Sitthisa et al., 2011). Some kinetic studies of the acid cata-lysed hydrolysis of hemicellulosic feedstocks are summarised inTable 2. In these mechanistic studies, xylose was considered todehydrate to form furfural and undesired humins. Unlike LA, furfu-ral undergoes a variety of reactions, such as resinification withother furfural molecules (Gandini and Belgacem, 1997), or conden-sation with xylose (Cai et al., 2013). The kinetics of LA and furfuralformation from the cellulose and hemicellulose fractions ofMiscanthus was investigated in the present study. A series of exper-iments in a batch reactor system were carried out at 150, 175 and200 �C using sulphuric acid concentrations of 0.10, 0.21 and0.53 M. The concentrations of glucose, xylose, HMF, furfural andLA were determined at different reaction times to develop the ki-netic models that describe the acid-catalysed hydrolysis reactionsof the cellulose and hemicellulose fractions. Finally, these kineticmodels were applied to maximise the production of LA and furfuralin two ideal continuous reactors, i.e., a plug flow reactor and a con-tinuous stirred tank reactor. A further optimisation study was alsocarried out by assessing various multi-reactors configurations toobtain the maximum yields of LA and furfural.

2. Methods

2.1. Biomass feedstock and chemicals

Miscanthus was supplied by JHM Crops Ltd. (http://www.jhmcrops.ie), in Adare, Co. Limerick, Ireland. It was received aschipped cane with particle size less than 2 cm that were used di-rectly for kinetic experiments without any further treatment. Allchemicals used in this study were of analytical grade and usedwithout purification.

2.2. Experimental procedure

2.2.1. Characterisation of biomassMiscanthus was analysed for its carbohydrate and lignin contents

following the standard method developed by the NationalRenewable Energy Laboratory (Sluiter et al., 2008; Hyman et al.,2008). Before characterisation, Miscanthus was milled to a particlesize between 180 and 850 lm (FOSS Cyclotec 1093 Mill, RetschAS200 sieve shaker). Once milled and sieved, the material was ex-tracted using ethanol (95%-wt.) in a Dionex Accelerated SolventExtractor 200 operating at 100 �C and 103 psi. Once extracted, thesamples were hydrolysed with sulphuric acid, first for 60 min in72%-wt. sulphuric acid at 30 �C, and after, for 60 min in 4%-wt.sulphuric acid at 120 �C. The product mixture of this hydrolysis stepwas filtered to separate the solid residue (Klason lignin) and the

Tabl

e2

Var

ious

kine

tic

mod

els

for

the

acid

-cat

alys

edhy

drol

ysis

ofhe

mic

ellu

lose

sto

furf

ural

.

Mec

han

ism

Aci

dTe

mpe

ratu

reSu

bstr

ate

Kin

etic

mod

elR

efs.

0.6–

1.2%

-wt.

H2SO

4

140–

180

�C10

%-w

t.C

orn

stov

err i¼ð�Þk

iCj;

i¼f;

s;2

k i¼

Ao;

i½H2SO

4�m

iex

pð�

E A;i=

RTÞ

E A;f¼

129:

8kJ=m

olE A

;s¼

167:

6kJ=

mol

E A;2¼

98:4

kJ=

mol

Este

ghla

lian

etal

.(1

997)

0.25

–4w

t.%

H2SO

4

90–1

60�C

Suga

rcan

eba

gass

er i¼ð�Þk

iCj;

i¼1e;1

h;2;

3;4

k i¼

Ao;

i½H2SO

4�m

iex

pð�

E A;i=

RTÞ

E A;1

e¼

E A;1

h¼

83:8

kJ=

mol

E A;2¼

150:

5kJ=

mol

E A;3¼

115:

0kJ=

mol

E A;4¼

60:3

kJ=

mol

Lava

rack

etal

.(2

002)

Au

to-h

ydro

lysi

s15

0–19

0�C

11%

-wt.

Cor

nco

bsr i¼ð�Þk

iCj;

i¼1;

2;..

.;8

k i¼

Ao;

iex

pð�

E A;i=

RTÞ

E A;1¼

127:

3kJ=m

olE A

;5¼

132:

0kJ=

mol

E A;2¼

251:

7kJ=m

olE A

;6¼

106:

2kJ=

mol

E A;3¼

119:

0kJ=

mol

E A;7¼

125:

2kJ=

mol

E A;4¼

122:

5kJ=m

olE A

;8¼

65:1

kJ=

mol

Nab

arla

tzet

al.

(200

4)

0.25

–0.7

5%-w

t.H

2SO

4

150–

175

�C10

%-w

t.A

spen

woo

dr i¼ð�Þk

iCj;

i¼1;

2;3;

4k i¼

Ao;

i½H2SO

4%�m

iex

pð�

E A;i=R

TÞE A

;1¼

97kJ=m

olE A

;3¼

132

kJ=

mol

E A;2¼

69kJ=

mol

E A;4¼

106

kJ=

mol

Mor

inel

lyet

al.

(200

9)

218 K. Dussan et al. / Bioresource Technology 149 (2013) 216–224

liquid hydrolysate. The clear hydrolysate was analysed to esti-mate the amount of acid-soluble lignin (Agilent HP 8452A DiodeArray Spectrophotometer). Subsequently, the composition ofthe carbohydrates in the hydrolysate was measured using anion chromatograph (described in Section 2.3). The Miscanthusused in this study was composed by 40.7% glucan, 19.6% xylan,2.2% arabinan, 1.0% galactomannan, 23.8% total lignin, 1.8% etha-nol extractives, and 3.9% ash.

2.2.2. Kinetic experiments of acid-catalysed hydrolysis of MiscanthusThe hydrolysis reactions were carried out in a stainless steel

Parr Series 4550 floor stand reactor (2 US gallon) comprising amagnetic-drive stirrer (1200 rpm max.), a temperature monitoringsystem with modified injection and sampling ports. A glass linerfitted to the reactor was used in all experiments to prevent anymetal leaching into the reaction mixture. The total amount ofmaterial in every experiment was 3000 g. For the kinetics experi-ments, the biomass intake was set at 9%-wt. on a dry basis. Atthe beginning of each experiment, water was poured into the reac-tor vessel according to the biomass intake and the desired sulphu-ric acid concentration following which the Miscanthus was addedand mixed at a constant speed. The reactor was sealed and thenheated with an external heating jacket until the desired tempera-ture was reached. The time required to reach the set temperaturevaried between 1 and 1.5 h, depending on the temperature. Oncethe temperature inside the reactor was steady at the set point,the catalyst injector was pressurised with high pressure nitrogen(at least 5 bar higher than the pressure of the reactor vessel), andthe corresponding amount of concentrated sulphuric acid (96%-wt.) was injected into the vessel. The time when the sulphuric acidwas injected into the reactor was defined as the time zero for thekinetics experiments. Before the acid injection, one sample was ta-ken to measure the amounts of sugars (pentoses or hexoses) re-leased and of any products that had formed during the heatingperiod. However, in most of the experiments negligible amountsof glucose, HMF and LA were found prior to the introduction ofthe acid. On average, 13 samples were taken at different time inter-vals. Subsequently, the samples were diluted with deionised waterand filtered through a 0.20 lm filter. The composition of mono-meric sugars (pentoses and hexoses), of furan compounds (furfuraland HMF) and of LA were determined using the chromatographicsystems described in Section 2.3.

2.3. Analytical methods

The quantification of monomeric sugars, furan compounds andorganic acids was carried out in a HPLC system (ICS-3000, DionexCorp., Sunnyvale, CA) comprised of an AS-50 auto-sampler (10 lLsample-loop injection), a dual pump, a column oven, an integratedelectrochemical flow cell and a diode array detector. Monomericsugars (arabinose, galactose, glucose, xylose, mannose) werequantified with the DionexCarboPac�PA1 guard (4 � 50 mm) andanalytical (4 � 250 mm) columns connected in series, operatingat 18 �C and isocratic elution with deionised water (1.1 mL/min).For the measurement of the concentration of furan compounds(HMF and furfural) and LA, the Dionex Acclaim�Organic Acid guard(5 lm, 4 � 10 mm) and analytical (5 lm, 4 � 250 mm) columnswere used, operating at 30 �C and isocratic elution with 100 mMNa2SO4 (0.8 mL/min).

3. Results and discussion

3.1. Kinetic modelling of the cellulose fraction to LA

During the acid hydrolysis of biomass, it is assumed that thedifferent carbohydrate fractions in biomass (cellulose and

Table 3Kinetics parameters for the conversion to LA of the cellulose fraction of Miscanthus.

Rate constant Ao,i (min�1) EA,i (kJ/mol) mi

kg1 2.69 (±0.43) � 1021 188.9 (±28.5) 1.40 (±0.61)kg2 2.36 (±0.44) � 1017 155.5 (±26.5) 1.39 (±0.57)kg3 2.38 (±0.62) � 1020 186.2 (±43.9) 0.90 (±0.94)kg4 6.58 (±1.78) � 1014 121.3 (±32.9) 1.95 (±0.71)

Fig. 1. Concentration of glucose, 5-hydroxymethylfurfural and levulinic acid duringthe acid hydrolysis of Miscanthus under different conditions (initial glucanconcentration 0.25 M): j 150 �C, 0.102 M H2SO4; d 150 �C, 0.206 M H2SO4;N 150 �C, 0.526 M H2SO4; . 175 �C, 0.102 M H2SO4; � 175 �C, 0.206 M H2SO4;

175 �C, 0.526 M H2SO4; 200 �C, 0.102 M H2SO4; d 200 �C, 0.206 M H2SO4.

Table 4Kinetic parameters for the conversion of the hemicellulose fraction in Miscanthus tofurfural.

Rate constant Ao,i (min�1) EA,i (kJ/mol) mi

kx1 4.25 (±1.20) � 1012 107.9 (±29.4) 1.22 (±0.63)kx2 1.32 (±0.35) � 1019 167.9 (±42.2) 1.23 (±0.91)kx3 3.31 (±0.63) � 1020 179.3 (±32.0) 0.75 (±0.69)kx4 3.33 (±0.33) � 1011 105.7 (±9.4) 1.54 (±0.20)

K. Dussan et al. / Bioresource Technology 149 (2013) 216–224 219

hemicellulose) have a different reactivity and no interference witheach other. The cellulose fraction (GLN) is depolymerised toglucose (GLC), which is further dehydrated to HMF and finally con-verted to LA and formic acid. Only one parallel reaction from GLC tohumins (HUM) was considered, because the concentrations of HMFwere low throughout the experiments, and no humins were ex-pected to form from HMF. For a batch reactor system, and usinga pseudo first-order approach, the concentrations of reacted spe-cies (GLN, GLC, HMF, and LA) as a function of time are representedby the following equations:

½GLN�½GLN�o

¼ expð�kg1tÞ ð1Þ

½GLC�½GLN�o

¼ kg1

kg2 þ kg3 � kg1expð�kg1tÞ � expð�ðkg2 þ kg3ÞtÞ� �

ð2Þ

½HMF�½GLN�o

¼ kg1kg2

kg2 þ kg3 � kg1

expð�kg1tÞkg4 � kg1

� expð�ðkg2 þ kg3ÞtÞkg4 � ðkg2 þ kg3Þ

� �

þ kg1kg2 expð�kg4tÞðkg4 � kg1Þðkg4 � ðkg2 þ kg3ÞÞ

ð3Þ

½LA�½GLN�o

¼ kg1kg2kg4

kg2þ kg3�kg1

expð�kg1tÞ�1kg1ðkg1�kg4Þ

� expð�ðkg2þ kg3ÞtÞ�1ðkg2þkg3Þðkg2þkg3�kg4Þ

� �

� kg1kg2ðexpð�kg4tÞ�1Þðkg1�kg4Þðkg2þkg3�kg4Þ

ð4Þ

A total of 315 measurements of GLC, HMF, and LA concentra-tions were used to determine the reaction rate constants (i.e., kg1,kg2, kg3, kg4). The Matlab� optimisation tool fminsearch was usedto minimise the determinant of the matrix of the residuals as thecriterion for the minimisation of the difference between measuredand approximated values (Ziegel and Gorman, 1980). The reactionrate constants can be expressed through a modified Arrheniusequation (Eq. (5)), which combines the effects of temperature (T)and hydronium ion concentration ([H+]):

k ¼ A exp�EA

RT

� �½Hþ�m ð5Þ

In this expression, A corresponds to the frequency factor, EA tothe activation energy, m to the reaction order of the hydroniumion concentration and R to the ideal gas constant. In this study,the hydronium ion concentration was estimated by solving theequilibrium equations for the sulphuric acid dissociation, usingthe regressed equilibrium constants for the first and second disso-ciation, published by Que et al. (2011). Table 3 shows the kineticparameters that have been optimised to describe the acid-catalysed hydrolysis to LA of the cellulose fraction in Miscanthus.

Fig. 1 shows the profile of concentration of GLC, HMF, and LA foreach set of experimental conditions. Despite the fact that the datafor LA concentration appear to fall after a certain time followingcomplete conversion of glucose at high temperatures (200 �C),additional experiments carried out by the authors confirmed thatLA did not decompose under the conditions used in this study.The goodness of fit of the kinetic model for the cellulose conversionis reported in Fig. S-3 in the Supplementary information.

The activation energy of the reaction GLN ? GLC (EA,g1 = 188.9kJ/mol) is higher than values reported for other feedstocks, suchas sugarcane bagasse (EA = 144.8 kJ/mol) (Girisuta et al., 2013),wheat straw (EA = 78.7 kJ/mol) (Chang et al., 2009), and cellulose(EA = 151.5 kJ/mol) (Girisuta et al., 2007). This suggests that highertemperatures are required for the hydrolysis of the glucan fractionin Miscanthus in comparison with the temperature required forother feedstocks. Also this high activation energy indicates a highersensitivity to temperature for the formation of glucose in the caseof Miscanthus; hence, high temperatures promote the glucose for-mation to a greater extent.

The activation energies for the glucose conversion to HMF andHUM (155.5 and 186.2 kJ/mol, respectively) are in agreement with

Fig. 2. Concentration of xylose and furfural during the acid hydrolysis of Miscanthusunder different conditions (initial xylan concentration 0.15 M): j 150 �C, 0.102 MH2SO4; d 150 �C, 0.206 M H2SO4; N 150 �C, 0.526 M H2SO4; . 175 �C, 0.102 MH2SO4; � 175 �C, 0.206 M H2SO4; 175 �C, 0.526 M H2SO4; 200 �C, 0.102 M H2SO4;d 200 �C, 0.206 M H2SO4.

220 K. Dussan et al. / Bioresource Technology 149 (2013) 216–224

those reported in other studies (Chang et al., 2009, 2006; Girisutaet al., 2013, 2007; Weingarten et al., 2012). These values imply thathigh temperatures will favour the formation of humins relative toHMF. Although the orders of the hydronium ion concentration forkg2 and kg3 are both higher than those reported in earlier studies(Girisuta et al., 2013, 2007), they nevertheless agree that the reac-tion of GLC ? HUM has a lower order (0.90) than that of theGLC ? HMF reaction (1.39). This indicates that high acid concen-trations will promote the reaction of GLC ? HMF to a higher extentthan the reaction GLC ? HUM.

The activation energy for the reaction HMF ? LA (121.3 kJ/mol)corresponds with values reported by Girisuta et al. (2006)(110.5 kJ/mol), and Girisuta et al. (2013) (101.6 kJ/mol), and it isthe lowest of the activation energies of the reactions involved inthe cellulose conversion to LA. This relatively low activation energyindicates that high temperatures will favour the formation of HMFand will increase its maximum concentration in the reactionsolution.

3.2. Kinetic modelling of hemicellulose fraction to furfural

During the acid-catalysed hydrolysis of the hemicellulose frac-tion in Miscanthus to furfural, the xylan fraction (XLN) in the hemi-cellulose of Miscanthus is directly hydrolysed to xylose (XYL),which is simultaneously dehydrated to furfural (FUR) and decom-posed to the undesired humins (HUM). FUR is also decomposed toform resinification products (RES). All the reactions involved wereassumed to follow a first-order reaction. The concentrations of

reacted species (XLN, XYL, and FUR) as a function of time are asfollow:

½XLN�½XLN�o

¼ expð�kx1tÞ ð6Þ

½XYL�½XLN�o

¼ kx1

kx2 þ kx3 � kx1½expð�kx1tÞ � expð�ðkx2 þ kx3ÞtÞ� ð7Þ

½FUR�½XLN�o

¼ kxk1kx2

kx2 þ kx3 � kx1

expð�kx1tÞkx4 � kx1

� expð�ðkx2 þ kx3ÞtÞkx4 � ðkx2 þ kx3Þ

� �

þ kx1kx2 expð�kx4tÞðkx4 � kx1Þðkx4 � ðkx2 þ kx3ÞÞ

ð8Þ

A total of 210 experimental points (XYL and FUR concentra-tions) were used to determine the reaction rate constants (i.e.,kx1, kx2, kx3, kx4) by applying the same approach used to determinethe reaction rate constants for the cellulose fraction. Table 4 showsthe kinetic parameters for the acid-catalysed hydrolysis of thehemicelluloses fraction in Miscanthus to furfural.

Fig. 2 shows the profile of concentration of XYL and FUR for thedifferent experimental sets. The goodness of fit of the kinetic mod-el is shown in Fig. S-3 of the Supplementary information. Theyields of furfural obtained at the conditions of this work were rel-atively lower than those observed in other kinetic studies (40–60%-mol). It is believed that other fractions in biomass (lignin, othercarbohydrates) may lead to additional condensation reactions thatcontribute to the loss of furfural, thus reporting higher disappear-ance rates than in studies where xylose is the only initial reactant.

In general, the activation energy determined for the reactionXLN ? XYL (EA,x1 = 107.9 kJ/mol) is consistent with values reportedfor the saccharification of the hemicellulose from other feedstocks,such as aspen wood (EA = 97 kJ/mol) (Morinelly et al., 2009), ba-gasse (EA = 83.8 kJ/mol) (Lavarack et al., 2002), and corn stover(EA = 129–167 kJ/mol) (Esteghlalian et al., 1997). However, theslight differences imply a distinction in the reactivity of the specifichemicellulose fraction in different biomass feedstocks, as men-tioned before. Compared with the activation energy of the otherreactions, the low activation energy of the XLN ? XYL reactionindicates that this reaction does not represent a limiting step evenat low temperatures for the continuation of the xylose dehydrationto furfural.

The activation energies of kx2 and kx3 for the xylose dehydrationand condensation reactions (167.9 and 179.3 kJ/mol, respectively)are in agreement with the activation energies determined by Jingand Lu (2007), who reported activation energies of 111.5 and143.1 kJ/mol respectively, following the same mechanism utilisedin the present study. Weingarten et al. (2010) also reported a valuefor activation energy of the xylose dehydration of 123.9 kJ/mol.These values indicate that high temperatures will favour the for-mation to a greater extent of unidentified soluble and insolubleproducts from xylose than the formation of furfural. The ordersof the hydronium ion concentration for the reaction rate constantskx2 and kx3 are higher than those established in other studies(Nabarlatz et al., 2004). However, the order determined in thisstudy is in agreement with the order determined for the similarset of reactions for the glucan fraction. In a similar way, as ob-served for the cellulose, the order of the hydronium ion concentra-tion determined for the reaction of XYL ? HUM is lower than thatfor the reaction of XYL ? FUR, suggesting that the effect of the acidconcentration is greater for the dehydration of the carbohydrate.Additionally, the activation energy of the FUR ? RES reaction(105.7 kJ/mol) corresponds with the value reported by Nabarlatzet al. (2004) (132 kJ/mol) and by Morinelly et al. (2009)(106.0 kJ/mol). This activation energy is relatively low comparedto those values for the other reactions involved in the conversion

K. Dussan et al. / Bioresource Technology 149 (2013) 216–224 221

of xylose, and it indicates that the dependence on temperature ofthe rate of the loss of furfural through the formation of resinifica-tion products is smaller than for the reactions of the formation offurfural and the loss of xylose.

4. Reactor modelling and optimisation

4.1. Single reactor

The kinetic parameters determined for the hydrolysis of thehemicellulose and cellulose fractions in Miscanthus were used toassess the performance of two ideal continuous reactors, i.e., aplug-flow reactor (PFR) and a continuous stirred-tank reactor(CSTR). The following assumptions were used in assessing the per-formance of these reactors:

� The two different substrates in the biomass do not influence thereaction rates of each other. The lignin fraction does not affectthe rate of loss reactions of the carbohydrates or intermediatecompounds.� The reactors operate in constant density and isothermal

regimes.

The reactor design equations of a PFR are similar to the onealready derived for a batch reactor (Eqs. (1)–(4) and (6)–(8)) withthe substitution of the time t by the residence time s. The reactordesign equations of a CSTR are given below:

YGLN ¼1

1þ kg1sð9Þ

YGLC ¼kg1s

ð1þ ðkg2 þ kg3ÞsÞYGLN ð10Þ

Fig. 3. Modeled molar yield of compounds derived from the acid hydrolysis of the glucfunction of residence time under different conditions: (a) glucose; (b) levulinic acid; (c)

YHMF ¼kg2s

ð1þ kg4sÞYGLC ð11Þ

YLA ¼ kg4sYHMF ð12Þ

YXLN ¼1

1þ kx1sð13Þ

YXYL ¼kx1s

ð1þ ðkx2 þ kx3ÞsÞYXLN ð14Þ

YFUR ¼kx2s

ð1þ kx4sÞYXYL ð15Þ

Fig. 3a and b show the simulated yields of GLC and LA as a func-tion of residence time for the two reactor types under the two dif-ferent sets of conditions which were used for this interpretation,i.e., high acid concentration at low temperature (150 �C and0.5 M H2SO4) and low acid concentration at high temperature(200 �C and 0.1 M H2SO4). For a given residence time, higher molaryields of LA were obtained from a PFR configuration than from aCSTR. Regardless of the reactor configuration, LA formation was fa-voured at lower temperatures, but a longer residence time was re-quired to achieve the maximum LA yield. In a PFR configuration,the LA yield (80%-mol) at low temperature and high acid concen-tration (150 �C, 0.5 M H2SO4) obtained after a residence time of920 min was about 1.7 times higher than the LA yield (50%-mol)at high temperature and low acid concentration (200 �C, 0.1 MH2SO4) reached after 28 min.

As implied from the kinetic parameters, high temperature waspreferred to maximise the yield of GLC. The PFR configuration gavea higher maximum yield of GLC than a CSTR. At 200 �C and 0.1 MH2SO4, a maximum GLC yield of 39.4%-mol was obtained in aPFR, while under the same conditions and after a similar residence

an and xylan fractions of biomass in CSTR (solid lines) and PFR (dashed lines) as axylose; and (d) furfural.

Fig. 4. Maximum yields of levulinic acid and furfural from Miscanthus at various temperatures and acid concentrations: (a, c) CSTR; and (b, d) PFR.

222 K. Dussan et al. / Bioresource Technology 149 (2013) 216–224

time (s = 4.2 min) from a CSTR, the maximum GLC yield was re-duced to 26.3%-mol. In general, the residence time required toreach the complete conversion of the glucan fraction, and conse-quently the maximum LA production, is longer in a CSTR than ina PFR.

Fig. 3c and d show the predicted yields of XYL and FUR as afunction of residence time for the two reactor types at the two setsof conditions used for the simulation of the glucan fraction to GLCand to LA. The PFR configuration, which was operated at a temper-ature of 200 �C and 0.1 M H2SO4 as the catalyst, was predicted togive a maximum yield of 18.3%-mol of FUR at a residence time of5.5 min, while, the CSTR configuration under the same operatingconditions required a slightly longer residence time (s = 6.5 min)to reach the maximum FUR yield of 13.0%-mol.

For either reactor configuration, the XYL yields were signifi-cantly higher at the lower temperature and reached maximumyields of 40–60%-mol at 150 �C and 0.5 M H2SO4. No significant dif-ference in the residence time was predicted between the PFR andthe CSTR in terms of reaching the maximum xylose yield. The max-imum yields of FUR shown in Fig. 3d were obtained mostly atshorter residence times than those required to reach the maximumyields of LA shown in Fig. 3b. As a result, the conditions and reactortype that maximise the production of LA would not favour the pro-duction of furfural, and vice versa.

In order to elucidate the combined effects of temperature andacid concentration on the production of LA from Miscanthus,Fig. 4a and b show the predicted LA yields obtained in a CSTR ora PFR, respectively. The LA yields shown in Fig. 4a and b were eval-

uated at the residence time needed to reach 99% conversion of GLNunder the selected conditions. The yield of LA is predicted to reachabout 85%-mol at a temperature of 140 �C and a sulphuric acid con-centration of 0.5 M when using a CSTR configuration. A slightlylower yield of LA (84%-mol) was obtained under the same condi-tions using a PFR. The difference in the LA yield obtained for thesetwo reactors is more evident at high temperatures and low acidconcentrations. At 230 �C and 0.08 M H2SO4, a CSTR is predictedto produce about 35%-mol of LA while a PFR gives a LA yield of27.2%-mol. These results are in line with previous discussion thatLA yields were improved by decreasing the reaction temperature.However, decreasing the temperature, for example from 180 to140 �C, and using 0.5 M H2SO4, increased the residence time from40 min to 2800 min. It is evident that the balance between LAyields, residence time, and temperature is the key criterion whenoptimising the production of LA from lignocellulosic biomass.

The maximum furfural yields that can be obtained with either aCSTR or a PFR configuration are shown in Fig. 4c and d, respec-tively. The maximum furfural yields were calculated by findingthe residence time that equalises the first derivative of the Eq.(7) (PFR) and Eq. (14) (CSTR), and evaluating the furfural yield withthe respective residence time and equation (see Supplementaryinformation). The maximum yield of furfural in a CSTR was ex-pected to reach about 17–18%-mol at operating temperaturesaround 200 �C and a sulphuric acid concentration of 0.5 M. In con-trast, a much higher maximum yield of furfural (27%-mol) could beobtained with a PFR at a relatively lower temperature (185 �C) andthe same sulphuric acid concentration (0.5 M). This specific

Fig. 5. Second stage levulinic acid molar yield as a function of the residence time at different temperatures: (a) CSTR, h: XGLN = 99%, YLA = 70%, s = 109 min; and (b) PFR,s: XGLN = 99%, YLA = 67%, s = 14 min.

Table 5Comparison of the prediction of LA and FUR yields in a two reactors-process following the conditions proposed by Biofine (Fitzpatrick, 1997).

Case First type of reactor –220 �C, 5% H2SO4, 15 s

Second type of reactor –210 �C, 5% H2SO4, 20 min

LA molar yield afterfirst reactor, %-mol

LA molar yield aftersecond reactor, %-mol

FUR molar yield afterfirst reactor, %-mol

FUR molar yield aftersecond reactor, %-mol

1a,b PFR CSTR 35.07 59.01 24.59 2.422b CSTR PFR 25.45 60.15 16.48 0.003b PFR PFR 35.07 59.44 24.59 0.004b CSTR CSTR 25.45 59.37 16.48 2.41

a Biofine configuration.b Feedstock: wood flour (42% cellulose), 0.7 L/min, 10% biomass intake.

K. Dussan et al. / Bioresource Technology 149 (2013) 216–224 223

temperature for the maximal furfural formation originates in thecombined effect of the temperature on the different reactions thatlead to furfural as well as the side reactions that reduce its selectiv-ity. The loss of furfural through resinification reactions is notstrongly affected within the temperature range of the currentstudy, as mentioned in Section 3.2. Extremely high temperatureswill promote the loss of xylose through condensation products,thereby reducing the potential amount of furfural that can be pro-duced from xylose. Finally, it can be concluded that a PFR operatedat the specific temperature at which furfural production can bemaximised for the corresponding feedstock is preferable to a CSTR.

4.2. Multi reactors

Although the results presented so far provide sufficient indica-tions in order to choose a single stage reactor for the production ofLA, it should be acknowledged that furfural is not favoured bythese conditions. To the best of our knowledge, no other publica-tions have included a combined analysis of the LA and FUR produc-tion under the same process conditions. Biofine (Fitzpatrick, 1990,1997) is a recognised continuous two stage process that maximisesthe formation of LA and claims to produce furfural simultaneously.In this process, the cellulosic feedstock is continuously fed into thefirst reactor (PFR) at temperatures between 210 and 230 �C for res-idence times of about 15–25 s. The stream from the first reactor iscontinuously fed into the second unit (CSTR) at lower temperatures(195–215 �C) for longer residence times (15–30 min). The modelestablished in the present study for the hydrolysis of the cellulosicfraction is in agreement with experimental results presented byBiofine for lignocellulosic materials with similar cellulose composi-tion. However, the furfural yields reported by Biofine (Fitzpatrick,1990) were substantially higher (70–80%-mol) than those pre-dicted by the hemicellulose hydrolysis model reported here. Differ-ences in the process parameters such as preheating in theexperiments of the present study and the utilisation of steam forthe heating of the biomass in the Biofine process may explain this

discrepancy. Also, it is expected that the presence of other sub-strates in the biomass, such as lignin and other sugars, will pro-mote the loss of the furfural. Table 5 presents a comparison ofthe predicted yields of LA and FUR for different arrangements oftwo reactors in series under the conditions proposed by the Biofineprocess. Experimental data presented by Biofine indicated thatusing wood flour (42% cellulose) in a Biofine configuration at0.7 L/min (10% solids) led to a final LA molar yield of about 57.5%(T1 = 220 �C; s1 = 15.7 s; T2 = 210 �C; s2 = 20 min; 0.5 M H2SO4),while our model predicted 59.0%. Furthermore, the predicted FURyield after the second reactor is significantly lower (2.4%-mol), fall-ing from 24.6%-mol after the first reaction stage. As mentioned ear-lier from Fig. 3d, a longer reaction time is detrimental for theproduction of furfural.

The evaluation of different configurations of reactors (Table 5)shows that at the conditions proposed for Biofine, a CSTR-PFRarrangement leads in fact to a slightly higher LA yield, but the dif-ferences are not significant. More evident differences are found inregard to furfural where a single stage PFR leads to high yields offurfural. However, residence times that allow for the complete con-version of cellulose and the formation of LA are particularly detri-mental to furfural, due to the parasitic reactions of furfural andxylose.

The authors confirm the selection of a two stage reactor systemidentified by other authors (Fitzpatrick, 1990, 1997; Weingartenet al., 2012) as being optimal, with the conditions of the first stagebeing selected for the maximal production of furfural. For this firstsystem, a relatively high temperature is preferable in order to im-prove not just the formation of furfural but also to facilitate thesaccharification of the cellulose in the Miscanthus feedstock. Afterthis first stage, a separation process would be required in orderto avoid further degradation of the furfural contained in the prod-uct stream going into the next reactor. The selection of the condi-tions for the second reactor, however, should consider not justconditions that maximise the yields of LA but also other factorswhich are outside the scope of this paper, such as the size of the

224 K. Dussan et al. / Bioresource Technology 149 (2013) 216–224

reactor, corrosion, mixing mechanics of the biomass sludge, solidloadings, and the formation of humin residues.

From the analysis presented in Fig. 4c and d, the first reactorwas selected to operate at 185 �C, 0.5 M H2SO4, and with a resi-dence time of 3.11 min. This leads to a FUR production of 27.3%-mol, which in the case of Miscanthus corresponds to about4.0 kg FUR/100 kg of biomass. After the first reactor, 29.2%-mol ofthe cellulose remains unreacted in the Miscanthus and about19.0%-mol of LA is already formed. The conversion and yields fromthe glucan after the first reactor are taken as the initial values inthe equations describing the performance of the CSTR and PFRreactors mentioned in the previous section. Fig. 5 shows the pre-dicted LA molar yield after the second stage as a function of theresidence time for both a CSTR and a PFR, at different temperatures(140–200 �C) and at the same acid concentration as in the firststage. When the second stage operates at 180 �C, for instance, aCSTR leads to 70%-mol of LA in about 109 min while a PFR leadsto 67%-mol of LA in just 14 min (points indicated in Fig. 5). Themodel predicts that LA yields as high as 80%-mol are reached attemperatures as low as 140 �C. However the time required forthe completion of the reaction under these conditions impliesimpractical residence times of several days (27 h in a PFR and200 h in a CSTR). The selection of the conditions for the secondreactor must ponder over the LA yields (process throughput) andthe reactor size (residence time) in order to have an economicalbalance between the profit of the process and the capital costs re-lated to the hydrolysis process.

5. Conclusion

Kinetic parameters were determined for the conversion into LAand FUR of the cellulose and hemicellulose, respectively, of Miscan-thus in aqueous media at mild temperatures (150–200 �C) cata-lysed by sulphuric acid (0.10–0.53 M H2SO4). The kineticinformation was included in the models of ideal CST- and PF-reac-tors for determining the conditions that maximise the LA and FURproduction. A first stage reactor (PFR) at 185 �C and 0.5 M H2SO4

was found to produce a maximum of 27.3%-mol FUR. A secondstage (PFR) with temperatures between 160 and 200 �C is preferredin order to reach LA yields around 58–72%-mol.

Acknowledgements

The authors acknowledge financial support via a research Grantfrom European Community’s Seventh Framework Programme(FP7/2007–2013) No. 227248-2 (DIBANET).

Appendix A. Supplementary data

Supplementary data associated with this article can be found, inthe online version, at http://dx.doi.org/10.1016/j.biortech.2013.09.006.

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