Bioresource Technology 101 (2010) 4889–4894

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Bioresource Technology

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Separation of furans and carboxylic acids from sugars in dilute acid rice strawhydrolyzates by nanofiltration

Yu-Hsiang Weng a,b,*, Hwa-Jou Wei a,c, Tsung-Yen Tsai a, Ting-Hsiang Lin a, Tsong-Yang Wei a,Gia-Luen Guo a, Chin-Pao Huang b

a Institute of Nuclear Energy Research, Atomic Energy Council, Executive Yuan, 1000 Wenhua Rd., Jiaan Village, Longtan Township, Taoyuan County 325, Taiwan, ROCb Department of Civil and Environmental Engineering, University of Delaware, Newark, DE 19716, USAc Department of Chemical and Materials Engineering, National Central University, Chungli 320, Taiwan, ROC

a r t i c l e i n f o a b s t r a c t

Article history:Received 7 August 2009Received in revised form 20 November 2009Accepted 20 November 2009

Keywords:Rice straw hydrolyzateXyloseAcetic acidNanofiltration (NF)Negative retention

0960-8524/$ - see front matter � 2009 Elsevier Ltd. Adoi:10.1016/j.biortech.2009.11.090

* Corresponding author. Address: Institute of NuclEnergy Council, Executive Yuan, 1000 Wenhua Rd., JiaaTaoyuan County 325, Taiwan, ROC. Tel.: +886 3 4711

E-mail addresses: yhweng@iner.gov.tw, f88541105

This work studied the concentration of hydrolyzates obtained from dilute acid hydrolysis of rice strawusing nanofiltration (NF). In order to minimize the Donnan exclusion effect of the membrane, the hydro-lyzate solution was controlled at low pH value. Negative retentions of both furans and carboxylic acidswere observed. The maximum separation factor of acetic acid over xylose was 49, while the maximumseparation factor of acetic acid over arabinose was 52, when the system was operated at pH 2.9 andan applied pressure of 24.5–34.3 bar. The separation factors of inhibitors over glucose became infinitydue to the complete retention of glucose. The separation performance decreased when the operatingtemperature was increased from 25 to 40 �C. The flux deterioration was recovered by flushing with0.01 N of NaOH and water.

� 2009 Elsevier Ltd. All rights reserved.

1. Introduction

Cellulosic ethanol is a biofuel produced from lignocellulosicmaterials. Recently, considerable attention has been paid to cellu-losic ethanol as lignocellulosic materials are abundant and renew-able energy sources. Sanchez and Cardona (2008) summarized theprospective lignocellulosic biomasses for bioethanol production.These include crop residues (e.g., rice straw), hardwood (e.g., aspenand poplar), softwood (e.g., pine and spruce), cellulose wastes (e.g.,newsprint and waste office paper), herbaceous biomass (e.g.,switchgrass) and municipal solid wastes. In 2006–2009, more than627 million metric tons of rice was produced worldwide (420 mil-lion metric tons on milled basis), of which more than 90% wasgrown in Asia (Foreign Agricultural Service-USDA, 2009).

One of the biggest challenges in commercializing cellulosic eth-anol technology is that the ethanol concentration is low after fer-mentation, resulting in high energy consumption for subsequentpurification steps. Generally, 10–40 g L�1 of ethanol can be ob-tained from an initial xylose concentration of 20–80 g L�1 giventhe complete conversion of xylose into ethanol. To increase the su-gar content before the fermentation step, current research focuses

ll rights reserved.

ear Energy Research, Atomicn Village, Longtan Township,

400; fax: +886 3 4711064.@ntu.edu.tw (Y.-H. Weng).

on increasing the solid/liquid ratio in the pretreatment step. Thedrawback of this strategy is that the workability and transportabil-ity of high viscosity biomass are low, potentially resulting in lowethanol yield. As an alternative, increasing sugar concentrationby removing solvent (mainly water) has been considered. Methodssuch as heating, vacuum evaporation and membrane separationhave been used for hydrolyzate concentration (Huang et al.,2008; Mussatto and Roberto, 2004; Parajo et al., 1998). Althoughvacuum evaporation and heating have been successful in manyapplications, there is a phase change in the process, resulting inhigh energy cost for operation.

Nanofiltration (NF), with molecular weight cutoffs (MWCO) be-tween ultrafiltration (UF) and reverse osmosis (RO), has drawngreat interest in biorefineries due to its low energy consumptionand unique separation properties (Huang et al., 2008). Han andCheryan (1996) investigated the recycling of glucose and process-ing of acetate from a fermentation broth by NF. They found that theseparation factor of acetate over glucose in calcium magnesiumacetate broth was ca. 40 by using a NTR 729 NF membrane. In addi-tion, based on the economic analysis, NF was a desirable optionwhen combined with microfiltration (MF) and evaporation for ace-tate production. Murthy et al. (2005) reported an increase in theconcentration of xylose from 2% to 10% from reaction liquor pro-duced by dilute acid hydrolysis of rice husk using NF. Operationalcosts were reduced by greater than 75%. Vegas et al. (2008) appliedseveral UF and NF membranes to oligosaccharide fractionation and

4890 Y.-H. Weng et al. / Bioresource Technology 101 (2010) 4889–4894

purification. They found that the 4 kDa polymeric membrane(ESP04) rejected 93% and 10% of oligosaccharides and monosaccha-rides, respectively, giving the best fractionation results. Sjomanet al. (2008) investigated the separation of xylose from glucose inhemicellulose hydrolyzate by using Desal-5 DK, Desal-5 DL andNF270 membranes. They reported that separation factors of xyloseover glucose ranged from 1.5 to 3.8 despite the small molecularmass differences between xylose (150 g mol�1) and glucose(180 g mol�1). More recently, Liu et al. (2008) applied NF to con-centrate and purify the sugars from hot-water wood extracts.

In our previous study, separation of acetic acid from xylose inmodel hydrolyzates by NF was examined at various concentra-tions, pH values, applied pressures and crossflow velocities (Wenget al., 2009). The maximum separation factor of acetic acid over xy-lose was 5.4 at pH �3 and 24.5 bar. In the present study, thehydrolyzates obtained from dilute acid hydrolysis of rice straw ina pilot scale plant were filtered with a Desal-5 DK membrane underpreviously optimized conditions. Experimental variables such asapplied pressure and temperature were studied.

2. Methods

2.1. Preparation of hydrolyzates

Rice straw used in this study was obtained from the field nearLongtan (Taoyuan County, Taiwan). After receipt, it was air-dried,cut into small pieces and stored dry until use. Dilute acid hydroly-sis was conducted in a pilot scale twin-screw conveyer connectedto a high-pressure hydrolysis reactor. Operational conditions wereas follows: 3% (w/w) H2SO4 was added at 12–14 kg h�1, retentiontime of the reactor was 3–4 h, raw material feed rate was 6–7 kg h�1, screwing rate was 40 rpm and the reaction temperaturewas 120 �C. Each time, 20-kg of rice straw debris was fed intothe conveyer. The resulting product was 60 kg of mud-like materialwith a water content of 70%. The acid hydrolysis materials werethen subjected to high temperature steam (130–145 �C) washingfor 10–30 min followed by filter press separation. The obtainedhydrolyzate was stored in a refrigerator (4 �C) for future use.

2.2. Membrane and filtration protocol

A NF membrane (Desal-5 DK) with molecular weight cutoff of150–300 Da was purchased from GE-Osmonic. The equivalent poresize of the membrane was 0.8–0.9 nm (Bowen et al., 2002; Wenget al., 2009). The isoelectric point of membrane was �4 (Tanninenet al., 2004; Weng et al., 2009). The membrane module was spiralwound with 0.3 m in length and 0.047 m in diameter. The filtrationarea was 0.2 m2. Results of our previous study showed that thismembrane was able to separate acetic acid from xylose at pH �3(Weng et al., 2009).

Before being pumped to NF for concentration, the hydrolyzatewas adjusted to pH �3 and pre-filtered with a 0.22 lm membrane(Millipore Express TM Plus) to remove particulates. The membranewas flushed with DI water before each experiment to measure thepure water flux. Five liters of hydrolyzate were used for eachexperiment. Hydrolyzate was circulated between the feed tankand the reactor until the solution temperature reached the set va-lue. In order to concentrate sugars in the hydrolyzate, the systemwas operated in ‘‘batch filtration with full recycle of retentate”mode, i.e., only the concentrate stream was recycled back to thefeed tank. This type of operation is commonly used for laboratoryand pilot scale studies (Cheryan, 1998). Volume concentration ratio(VCR) was used as an indicator of the level of concentration. In Eq.(1), V0 is the initial feed volume (L) and VR is the retentate volume(L). During filtration, VCR increased continuously due to the de-crease in volume in the tank.

VCR ¼ V0

VRð1Þ

After each filtration experiment, the membrane was washedwith DI water followed by 0.01 N of NaOH to restore the flux.Thereafter pure water flux was measured again. The membranemodule was stored in 0.01 N NaOH to prevent bacterial growth.

2.3. Chemical analyses

Samples collected during the experiments were analyzed byhigh performance liquid chromatography (HPLC) to determinethe amount of ethanol, sugars, such as glucose, xylose and arabi-nose, and inhibitors, such as acetic acid, formic acid, 5-hydroxy-methyl-furfural (HMF), and furfural. The chromatography system(Agilent 1200 LC) used in this study consisted of a pump with deg-asser, auto-sampler, chromatography column, column thermalcontroller and refractive index (RI) detector. The chromatographyanalysis was performed on an 87H analysis column (TransgenomicInc., USA) at 65 �C. 4 mM of H2SO4 solution was used as the mobilephase, flow rate was 0.8 mL min�1, and the detection cell temper-ature in the RI detector was set at 45 �C. For each analysis, 20 lL ofsample solution was injected into the system.

The observed retention (Robi ) was calculated from Eq. (2), where

Ci;t and Ci;p are the sugar, furan or carboxylic acid concentration(g L�1) in the tank and permeate, respectively. During the experi-ments, samples from the tank and permeate stream were takenand concentrations were determined.

Robi ¼ 1� Ci;p

Ci;t

� �� 100% ð2Þ

Alternatively, the separation factor (ainhi=sug) can be used to eval-uate the performance of the membrane. In Eq. (3) where Cinhi;p andCsug;p are the concentrations of inhibitor and sugar in the permeateand Cinhi;t and Csug;t are the concentrations of inhibitor and sugar inthe tank.

ainhi=sug ¼Cinhi;p=Csug;p

Cinhi;t=Csug;t¼ 1� Rob

inhi

1� Robsug

ð3Þ

3. Results and discussion

3.1. Physical properties of the sugars, furans and carboxylic acids

Table 1 shows the molecular formula, molecular weight, disso-ciation constant, diffusion coefficient and Stokes diameter of typi-cal sugars, furans and carboxylic acids found in the hydrolyzate.Glucose has the largest molecular weight of 180.156 g mol�1 fol-lowed by xylose and arabinose, HMF, furfural, acetic acid and for-mic acid. Since the Desal-5 DK has a MWCO of �150 Da, it wasexpected that NF could achieve the concentration of the sugars.On the other hand, all the inhibitors have a molecular weight smal-ler than the cutoff value of the membrane. While the membraneMWCO is defined as 90% rejection of the calibration compound(Mulder, 1996), the Desal-5 DK membrane cannot expel theseinhibitors when charge effect is neglected. When the charge exclu-sion dominates, the retention of acetic acid can even exceed that ofxylose with the same membrane (Weng et al., 2009).

The Stokes diameter of the molecule (ds(m)) was calculated bythe Stokes–Einstein equation (Hiemenz and Rajagopalan, 1997)according to Eq. (4), in which kb is the Boltzmann constant (J K�1),T is temperature (K), and D is the diffusion coefficient (m2 s�1).

ds ¼kbT

3plDð4Þ

Table 1Physical properties of the sugars, furans and carboxylic acids in the hydrolyzate.

Xylose Glucose Arabinose Acetic acid Furfural HMF Formic acid

Molecular formula C5H10O5 C6H12O6 C5H10O5 CH3COOH C5H4O2 C6H6O3 HCOOHMolecular structure O

OH

OH

OH

OHO

OH

OH

OH

OH

OHO

HO

OH

OH

OH COHH3C

O O O O O

OH COHH

O

Molecular weight(g mol�1)

150.13 180.156 150.13 60.05 96.08 126.11 46.0254

Diffusion coefficient(10�6 cm2 s�1)

7.69a 6.76a 7.73a 11.9b 11.2b 10.6c 15.2b

Stokes diameter (nm) 0.638 0.726 0.635 0.412 0.438 0.463 0.323Dissociation constant

(pKa)12.15d 12.28d 12.34d 4.756d �e � e 3.751d

a From Mogi et al. (2007).b From Yaws (2003).c From Wilke and Chang (1955).d From Gokel (2004).e Not available.

Y.-H. Weng et al. / Bioresource Technology 101 (2010) 4889–4894 4891

The diffusion coefficients were obtained from the literature(Mogi et al., 2007; Yaws, 2003). For the sake of the experimentaldata, the Wilke–Chang correlation (Wilke and Chang, 1955) wasused to estimate the diffusion coefficient of HMF. Although theStokes–Einstein equation treats molecules as spheres and maynot reflect the real geometry of molecules in the aqueous solution,it provides the molecular size information which is valuable forunderstanding the separation performance as well as for estimat-ing membrane pore size, especially for the uncharged molecules(Sharma et al., 2003; Van der Bruggen et al., 1999). ComparingStokes diameters of the solutes (see Table 1), the molecular sizesfollowed the order: sugars > furans > carboxylic acids. Note thatthe reported average pore diameters of the DK membrane rangedfrom 0.8 to 1.3 nm in the literature (Bouchoux et al., 2005; Bowenet al., 2002; Weng et al., 2009). Although these values are slightlylarger than the sizes of sugars, high retentions of sugars (>80%)were observed (Sjoman et al., 2008). The results suggested thatthe separation of inhibitors from sugars could be achieved giventhat the charge repulsion was insignificant.

It is worth mentioning that in addition to the sieving mechanism,NF offers an attractive opportunity to separate molecules by charge,i.e., Donnan exclusion. This is why the retention of charged moleculesis greater than that of uncharged molecules during NF operation. Forexample, it has been shown that retention of small molecular weightcompounds, such as acetic acid, by NF increased dramatically due tothe dissociation of the proton at high pH even though the moleculesare smaller than the pore size of the membrane (Han and Cheryan,1995; Weng et al., 2009). Therefore, in order to successfully separatethe inhibitors from sugars, it is essential to eliminate the exclusion orretention of inhibitors by the Donnan effect.

Table 1 also shows the dissociation constants (pKa) of the mol-ecules. Since the pKa values of sugars are greater than 12 (Gokel,2004), xylose, arabinose and glucose are in the uncharged stateat pH 3. In contrast, acetic acid and formic acid have dissociationconstants of 4.756 and 3.751 (Gokel, 2004), respectively, meaningthat they are negatively charged at neutral pH. Note that at pH 3,only 1.72% of acetic acid and 15.1% of formic acid are deprotonated,indicating that the sieving mechanism would dominate during NF.It is noted that the pKa values of furfural and HMF are not available.However, it can be inferred that their pKa values are greater than12 based on their chemical structures.

3.2. Concentration of the hydrolyzate by NF

The typical hydrolyzate contained 24 g L�1 of xylose, represent-ing more than 70% of the hydrolyzed sugars in solution. Glucoseand arabinose concentrations were 3 and 5 g L�1, respectively. In

general, the concentrations of sugars were similar to the results re-ported by Guo et al. (2008). Fig. 1 shows the concentration of thehydrolyzate over permeate volume by NF. Xylose concentrationin the tank increased from 24.6 to 31.8 g L�1 after a permeate vol-ume of 5.5 L m�2 at 25 bar. In addition, the concentrations of glu-cose and arabinose increased from 3.6 and 5.5 g L�1 to 4.5 and7.2 g L�1, respectively, at the same time. Note that the systemwas operated in batch filtration with full recycle of retentate, i.e.only retentate was recycled back to the feed tank. Therefore, thesugar concentration increased over the course of NF. At this stage,xylose concentration increased by ca. 29% in the feed tank and theVCR was 1.28, meaning that the loss of sugars was insignificant.The concentrations of xylose and arabinose in the permeate were0.73 and 0.14 g L�1 until reaching a permeate volume of 2.5L m�2, and then increased to 1.62 and 0.29 g L�1, respectively.The increase in xylose and arabinose concentrations in the perme-ate was attributed to the build-up of the concentration polarization(CP) layer near the membrane surface. The CP layer is a conse-quence of convection, back diffusion and permeation of the solute(Mulder, 1996). Under the present experimental conditions, thecontinuous elevation of feed concentration resulted in an increasein the CP layer over time.

The CP layer was interrupted when the applied pressure wasraised to 34.3 bar. This was demonstrated by the temporary de-crease in the concentrations of xylose and arabinose in the perme-ate upon the elevation of the applied pressure. However, the CPlayer eventually built up again as the NF operation proceeded. Atthe end of the experiment, concentrations of xylose, glucose andarabinose in the tank were 45.3, 6.5 and 10.3 g L�1, respectively.In addition, the incremental rate of xylose concentration in the per-meate was higher at 34.3 bar than that at 24.5 bar, indicating astronger convective transport toward the membrane at high oper-ational pressure. The glucose concentration in the permeate wasnot detectable during the course of NF. This could be explainedby the sieving effect because the Stokes diameter of glucose is lar-ger than that of xylose or arabinose (see Table 1). Overall, the con-centrations of xylose, glucose and arabinose in the tank increasedby 82%, 84% and 88%, respectively. This is similar to a VCR of1.88 after a permeate volume of 11.75 L m�2.

In contrast, both acetic acid and HMF concentrations in the tankdid not increase significantly as NF proceeded (see Fig. 1b). The ini-tial acetic acid in the tank was 2.5 g L�1 and then decreased to2.2 g L�1 at a permeate volume of 5.5 L m�2. Further increase in ap-plied pressure from 24.5 to 34.3 bar decreased the acetic acid con-centration to 2.0 g L�1 in the end of the experiment. HMFconcentration in the tank increased from 0.048 to 0.074 g L�1 atpermeation volume of 11.75 L m�2. In fact, both acetic acid and

Permeate volume (L m-2)0.0 2.0 4.0 6.0 8.0 10.0 12.0

Con

cent

ratio

n in

tank

(g L

-1)

0

10

20

30

40

50

Con

cent

ratio

n in

per

mea

te (g

L-1

)

0

1

2

3

Xylose in tankXylose in permeateGlucose in tank

Glucose in permeateArabinose in tankArabinose in permeate

24.5 bar 34.3 bar

Permeate volume (L m-2)0.0 2.0 4.0 6.0 8.0 10.0 12.0

Con

cent

ratio

n (g

L-1

)

0.0

2.0

3.0

Acetic acid in tankAcetic acid in permeateHMF in tankHMF in permeate

24.5 bar 34.3 bar

0.1

Permeate volume (L m-2)0.0 2.0 4.0 6.0 8.0 10.0 12.0

Ret

entio

n (%

)

-40

-20

0

2080

90

100

GlucoseXyloseArabinoseAcetic acidHMF

24.5 bar 34.3 bar

(a)

(b)

(c)

Fig. 1. The concentrations of (a) sugars and (b) furans and carboxylic acids in thefeed tank and the permeate and the retentions of (c) sugars, furans and carboxylicacids as a function of permeate volume during NF. Experimental conditions: appliedpressure = 24.5 and 34.3 bar, temperature = 25 �C and pH 2.9.

4892 Y.-H. Weng et al. / Bioresource Technology 101 (2010) 4889–4894

HMF concentrations in the tank were even lower than those in thepermeate at the end of the experiment.

Fig. 1c clearly shows that all sugars were concentrated by NF.The retention of the sugars was greater than 94%. Notably, 100%retention of glucose could be achieved. Additionally, negativeretentions of acetic acid and HMF were observed. The retentionsof acetic acid ranged from �19.0% to �14.9% at 24.5 bar and from�31.8% to �27.7% at 34.3 bar, while those of HMF were from�20.0% to 8.5% at 24.5 bar and from �24.6% to �5.0% at 34.3 bar.Although Weng et al. (2009) have shown a similar phenomenon

during NF of a model solution containing xylose and acetic acid,the acetic acid retentions were from �6.8% to 0.87% in the modelsolution. The negative retention of acetic acid was more pro-nounced in the real hydrolyzate than that in the model solution.At the present pH value of 2.9, all the molecules were presumablyneutral based on their dissociation constants in aqueous solution(see Table 1). Since the solution pH was close to the isoelectricpoint of the membrane, the Donnan exclusion would be minimal.The enhanced transport of uncharged molecules through the mem-brane was attributed to the interactions between the CP layer andinhibitors. Note that in the absence of xylose, the acetic acid reten-tion was positive (Weng et al., 2009). In addition, the negativeretention was observed only in the presence of sugars, such as xy-lose, indicating the importance of the CP layer. This explained afurther decrease in the acetic acid retention when the CP layerwas more significant at 34.3 bar (see Fig. 1b). However, HMF reten-tion tended to increase with permeate volume, indicating a differ-ent interaction between HMF and the CP layer. Nevertheless, theresults in Fig. 1 clearly demonstrated that the hydrolyzate wassimultaneously concentrated and detoxified to some extent bythe NF membrane.

3.3. Effect of temperature on concentration performance

In the dilute acid hydrolysis process, the hydrolyzate is ob-tained from washing hydrolysis materials at high temperature(e.g., 130–145 �C) followed by solid–liquid separation. In order tobetter understand the influence of temperature on NF separationproperties, the temperature of hydrolyzate was raised to 40 �C.

Fig. 2 shows the NF of hydrolyzate at 35 bar and 40 �C. The con-centrations of xylose, arabinose and glucose increased by 23% at apermeate volume of 5 L m�2. The sugar concentrations in the per-meate were 6.1, 0.9 and 1.5 g L�1 for xylose, glucose and arabinose,respectively. Furthermore, the averaged retentions of xylose, glu-cose and arabinose were 85.7%, 95.2% and 86.1%, respectively.The decrease in observed retention of sugars at 40 �C in compari-son to that at 25 �C was primarily attributed to the increase inmembrane pore diameter and MWCO, and to a lesser extent, theincreased diffusion of solute through the membrane (Ben Amaret al., 2007; Sharma et al., 2003). Again, the concentrations of aceticacid, formic acid, furfural and HMF in the permeate were higherthan those in the feed, indicating negative retentions of theseinhibitors (see Fig. 2c). The retentions of acetic acid, HMF, furfuraland formic acid were from �41.0% to �31.6%, �56.1% to �38.8%,�53.8% to �29.5% and �9.5% to 22.3%, respectively. In addition,the decrease in retentions of acetic acid, furfural, HMF and formicacid with permeate volume suggested that such facilitated trans-port was highly related to the CP layer which also increased as con-centration proceeded. A further increase in retentions of furfuraland HMF might be due to their lower diffusion as compared to thatof acetic acid and formic acid. A similar negative retention of for-mic acid was observed by Liu et al. (2008), when they used a tigh-ter NF membrane (MWCO = 100 Da) to concentrate hot-waterwood extracts.

Fig. 3 shows the separation performance of the NF membrane.Note that the glucose retention was 100% at 25 �C (see Fig. 1a),leading the separation factor to infinity (not shown). Moreover,all the data showed that the separation factors decreased overthe course of filtration. Since both the sugar and inhibitor reten-tions decreased with time during NF, the decrease in separationfactor was due to the decrease in the retentions of sugar (see Eq.(3)). Overall, the maximum separation factors of acetic acid andHMF over xylose were 49 and 43, respectively, at 25 �C. Theseparation performance was reduced when the hydrolyzatetemperature was increased to 40 �C. The maximum separation fac-tors of acetic acid, HMF, furfural, and formic acid over xylose were

Permeate volume (L m-2)

0.0 1.0 2.0 3.0 4.0 5.0

Con

cent

ratio

n (g

L-1

)

0

5

10

15

20

25

30

35

Xylose in tankXylose in permeateGlucose in tank

Glucose in permeateArabinose in tankArabinose in permeate

Permeate volume (L m-2)

0.0 1.0 2.0 3.0 4.0 5.0

Con

cent

ratio

n (g

L-1

)

0

1

2

3

4

5

6

Acetic acid in tankAcetic acid in permeateHMF in tankHMF in permeate

Furfural in tankFurfural in permeateFormic acid in tankFormic acid in permeate

Permeate volume (L m-2)

0.0 1.0 2.0 3.0 4.0 5.0

Ret

entio

n (%

)

-60

-40

-20

0

20

40

60

80

100

GlucoseXyloseArabinoseAcetic acidHMFFormic acidFurfural

(a)

(b)

(c)

Fig. 2. The concentrations of (a) sugars and (b) furans and carboxylic acids in thefeed tank and the permeate and the retentions of (c) sugars, furans and carboxylicacids. Experimental conditions: applied pressure = 34.3 bar, temperature = 40 �Cand pH 2.7.

Permeate volume (L m-2)

0.0 2.0 4.0 6.0 8.0 10.0 12.0

Sepa

ratio

n fa

ctor

0

10

20

30

40

50

Acetic acid over xyloseHMF over xylose

24.5 bar 34.3 bar

Permeate volume (L m-2)

0.0 1.0 2.0 3.0 4.0 5.0

Sepa

ratio

n fa

ctor

0

20

40

60

Acetic acid over glucoseHMF over glucoseFurfural over glucoseFormic acid over glucoseAcetic acid over xyloseHMF over xyloseFurfural over xyloseFormic acid over xylose

(a)

(b)

Fig. 3. The separation factors of furans and carboxylic acids over xylose andglucose. Experimental conditions: (a) applied pressure = 24.5 and 34.3 bar, tem-perature = 25 �C and pH 2.9 (b) applied pressure = 34.3 bar, temperature = 40 �C andpH 2.7.

Y.-H. Weng et al. / Bioresource Technology 101 (2010) 4889–4894 4893

18, 19, 18 and 11, respectively. Additionally, the maximum separa-tion factors of acetic acid, HMF, furfural, and formic acid over glu-cose were 52, 55, 51, and 31 at 40 �C. The lower separation factorsin Fig. 3b in comparison to those in Fig. 3a were due to the substan-tial decrease in the retentions of xylose and glucose, from an aver-age of 96.4% and 100% at 25 �C to 85.7% and 95.2% at 40 �C. Thedecrease in separation performance was also observed at elevated

temperature when NF was applied to fractionate xylose from glu-cose (Sjoman et al., 2008). The separation factor of inhibitors overarabinose was slightly higher than that over xylose due to lowarabinose content in the hydrolyzate. The maximum separationfactors of acetic acid and HMF over arabinose were 52 and 43,respectively.

3.4. Change of flux during NF

Although NF offers a means to separate the inhibitors from sug-ars in the hydrolyzate, it is inevitable to encounter membrane foul-ing which causes flux decline with time and, therefore, reduces theseparation performance. Fig. 4 shows the flux decline during NF ofhydrolyzates at various conditions. The pure water flux of the vir-gin membrane was 165.6 L h�1 m�2 at 24.5 bar, corresponding to apermeability of 6.8 L bar�1 h�1 m�2. When the NF was operated at25 �C and pH 2.9, the flux reductions were 73% and 86% of its initialvalue at 24.5 and 34.3 bar, respectively, after a permeation volumeof 5 L m�2. Similarly, there was a 64% flux reduction at a perme-ation volume of 5 L m�2 at 34.3 bar and 40 �C. Flux decline canbe due to many factors, such as concentration polarization, adsorp-tion, gel layer formation and plugging of the membrane pores(Cheryan, 1998; Mulder, 1996). While the concentrations of sugarin the feed tank increased with permeate volume, it was expectedthat the flux would decrease as the CP layer developed. The in-crease in the CP layer decreased the effective trans-membranepressure. This is consistent with the study by Murthy et al.(2005) in which they showed that there was an 86.5% reductionin flux during NF of hydrolyzate solution. Vegas et al. (2008) also

P1-B

Pure

wat

er fl

ux (L

m-2

h-1

)

0

20

40

60

80

100

120

140

160

180

25 oC and pH = 2.9 40 oC and pH = 2.7

P1-A P2-B P2-A

Perm

eabi

lity

(L m

-2 h

-1 b

ar-1

)

0

2

4

6

8Pure water permeability

Fig. 5. Changes in pure water flux and permeability of the membrane beforeexperiment (B) and after cleaning (A) P1 = 9.8 bar and P2 = 19.6 bar.

Permeate volume (L m-2)0.0 2.0 4.0 6.0 8.0 10.0 12.0

Perm

eate

flux

(L m

-2 h

-1)

0

2

4

6

8

10

12

14

16

18

25 oC, pH = 2.9, and 24.5-34.3 bar40 oC, pH = 2.7, and 34.3 bar

24.5 bar 34.3 bar

Fig. 4. Changes in permeate flux with permeate volume.

4894 Y.-H. Weng et al. / Bioresource Technology 101 (2010) 4889–4894

observed significant flux decline during concentration of raw auto-hydrolysis liquors containing xylo-oligosaccharides.

Fig. 5 shows the pure water flux before experiment and aftercleaning for each experimental run. The averaged pure water fluxesbefore experiment were 73.8 and 144.9 L h�1 m�2 at 9.8 and19.6 bar, respectively. The averaged pure water permeability was7.3 L bar�1 h�1 m�2 which was higher than the permeability of vir-gin membrane. Other researchers also reported that alkaline clean-ing and extensive uses of membrane tended to increase waterpermeability (Nilsson et al., 2008). Generally, the cleaning proce-dures adopted in the present study with the combination of DI waterand 0.01 N NaOH cleanings were effective to recover the flux, indi-cating low fouling irreversibility in the short term experiment.

4. Conclusions

The results showed that the Desal-5 DK NF membrane can effec-tively concentrate sugars in the hydrolyzate obtained from diluteacid hydrolysis of rice straw. Increasing the operating temperaturetended to increase the pore size of the membrane, causing a de-crease in retention of sugars. Negative retentions of furans and car-boxylic acids were observed during concentration. The separationmechanisms during NF of hydrolyzate were mainly due to the siev-ing effect and the high affinity of furans and carboxylic acids to theCP layer. Low irreversible fouling was found in this study.

Acknowledgements

The author (Y.-H. Weng) thanks the National Science Council ofTaiwan, Republic of China, for support to the visiting program

under the contract: NSC-096-2917-I-564-126. Dr. Wen-Hua Chen,Dr. Wei-His Chen and Mr. Mou-Shian Tsai are thankful to theirtechnical support with preparation of the hydrolyzates and sampleanalysis. Dr. Maureen Dolan is acknowledged for her editorialassistance.

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