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Chemical Engineering Journal 166 (2011) 868–872

Contents lists available at ScienceDirect

Chemical Engineering Journal

journa l homepage: www.e lsev ier .com/ locate /ce j

ombined supercritical and subcritical conversion of cellulose for fermentableexose production in a flow reaction system

an Zhaoa, Hong-Tao Wanga,∗, Wen-Jing Lua, Hao Wangb

Department of Environmental Science and Engineering, Tsinghua University, Beijing 100084, ChinaCollege of Environmental Science and Engineering, Beijing Forestry University, Beijing 100083, China

r t i c l e i n f o

rticle history:eceived 25 September 2010eceived in revised form3 November 2010ccepted 16 November 2010

eywords:

a b s t r a c t

Using research on a batch system as basis, a flow reactor was designed and applied in the combinedsupercritical and subcritical hydrolysis of cellulose for fermentable hexose production. The results showthat when the supercritical parameters were maintained, the hexose yield first increased with the risein subcritical temperature, and then decreased after the maximum yield was obtained. This maximumyield of fermentable hexoses from cellulose was 31.5% ± 1.4%, which was obtained under the followingconditions: cellulose concentration of 3.53 ± 0.24 g L−1, supercritical temperature of 380 ◦C, supercrit-

◦

ombined supercritical and subcriticalydrolysislow reaction systemelluloseermentable hexose production

ical reaction time of 9.70 ± 0.66 s, subcritical temperature of 240 C, and subcritical reaction time of48.49 ± 3.31 s. The appropriate ranges of cellulose concentration (around 3.5 g L−1) and reaction time(9–10 s for supercritical process and 45–50 s for subcritical process), which depended on the flows ofwater and material sludge, were also crucial in obtaining a high hexose yield. Compared with the batchsystem, the flow reaction system can yield a reasonable amount of hexose from cellulose hydrolysis and

promligno

ydrothermal technology proved to be considerablysubcritical technology on

. Introduction

The current pressure to adopt more efficient energy consump-ion methods stems from the high price of fossil fuel, energyecurity, and environmental concerns. Renewable energy is ancknowledged solution to this problem [1]. Biomass has elicitedncreasing attention because it is renewable, inexpensive, and read-ly available worldwide, thereby guaranteeing a high level of energyecurity. It is also a carbon-neutral resource and does not cause aet increase in green-house gases [2]. Therefore, because of theolymeric and crystalline structure of lignocellulose, many pre-reatment, hydrolysis, and fermentation technologies have beennvestigated and developed to convert lignocellulose into energyr fuel, including bioethanol [3–6].

Hydrothermal technologies have proven promising in ligno-ellulose conversion because of their high efficiency in dissolvingnd hydrolyzing cellulose [7–9]. Supercritical water technology hasbvious advantages in lignin separation and cellulose hydrolysis,

hich is attributed to its high dissolution and catalyzing capac-

ty [10,11]. However, considering the high decomposition rate ofydrolyzates in supercritical water, subcritical water was intro-uced for the hydrolysis of dissolved cellulose [12]. Combined

∗ Corresponding author. Tel.: +86 10 6277 3438.E-mail address: htwang@mail.tsinghua.edu.cn (H.-T. Wang).

385-8947/$ – see front matter © 2010 Elsevier B.V. All rights reserved.oi:10.1016/j.cej.2010.11.058

ising for practical applications, especially for combined supercritical andcellulosic resources.

© 2010 Elsevier B.V. All rights reserved.

supercritical and subcritical technology has been suggested andproven efficient for hexose production from lignocelluloses. In thiscombined approach, cellulose in biomass is first dissolved andhydrolyzed in supercritical water to produce oligosaccharides, towhich subcritical water is then applied for hydrolysis into fer-mentable hexoses [13,14].

In our previous work on a batch reaction system, the feasibilityand reaction mechanism of the combined supercritical and subcrit-ical hydrolysis of cellulose and lignocellulosic waste were studiedand demonstrated [13,15,16]. The batch system cannot be used forpractical purposes considering its non-continuity and high energycosts. Therefore, in this study, a flow reaction system was designedand investigated. The combined supercritical and subcriticalhydrolysis of cellulose using the flow reaction system was exam-ined, along with the effects of subcritical temperature, celluloseconcentration, and reaction time on the final hexose production.The relatively optimal parameters obtained can be valuable for theconversion of lignocellulosic waste, such as in the conversion ofcorn stalks into fermentable hexoses, using the flow system.

2. Materials and methods

2.1. Reagents and analysis methods

Microcrystalline cellulose powder, the substrate used for thecombined supercritical and subcritical hydrolysis, was obtained

Y. Zhao et al. / Chemical Engineering Journal 166 (2011) 868–872 869

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Preheating subsystem Supercritical reaction subsystem

Feed-in subsystem

Subcritical reaction subsystem

Temp. control subsystem

F s. (1)( ctor, (m

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ig. 1. Flow reaction system for the combined supercritical and subcritical proces5) preheater, (6) supercritical reactor, (7) primary water cooler, (8) subcritical rea

anometer, (13) thermoelement, (14) reducing valve, and (15) safety valve.

rom Beijing Fengli Jingqiu Commerce and Trade Co., Ltd.Beijing, China). The liquid hydrolysis products were analyzedy high performance liquid chromatography (HPLC, Shimadzu,C-10ADvp, RID-10A, Japan) using a sugar column (Shodex, SugarS-801, Japan). The products were analyzed under the followingonditions: 50 ◦C, 1.0 mL min−1, and 3.0 MPa. The standard sub-tances for HPLC analysis, such as cellopentaose, cellotetraose,ellotriose, cellobiose, glucose, xylose, fructose, erythrose, glyc-raldehyde, 1,6-anhydroglucose, dihydroxyacetone, and 5-HMF,ere from Sigma–Aldrich Inc. (Missouri, USA).

.2. Flow reaction system for the combined supercritical andubcritical hydrolysis

The structure of the flow reaction system is shown in Fig. 1. Theain body of the flow system is made of stainless steel 316 and

as five subsystems, namely, the feed-in, preheating, supercriticaleaction, subcritical reaction, and temperature control subsystems.he feed-in subsystem comprises a water tank and pump, as wells a material sludge tank and pump for storing and feeding waternd material sludge through the flow pipes into the respectivereheating and supercritical reaction subsystems. The preheatingubsystem is composed of a preheater with a coil pipe inside and aalt bath outside. The salt bath is filled with NaNO3 and KNO3 (1:1,/w), and it can heat the water in the coil pipe to a temperature

anging from 260 to 500 ◦C. The supercritical reaction subsystemncludes a supercritical reactor (� = 4 mm, l = 800 mm, V = 10 mL)

ith a salt bath outside similar to that of the preheating subsys-em, and a primary water cooler for cooling the products fromhe supercritical reaction. The subcritical reaction system includes

subcritical reactor (� = 7 mm, l = 1300 mm, V =50 mL) with anlectric heater outside (providing a temperature of 100–350 ◦C),nd a final water cooler for cooling the products from subcriticaleaction. The temperature controlling subsystem can monitor andontrol the temperatures in the preheater, supercritical reactor,

ater cooler, and subcritical reactor through four thermoelements.oreover, the supercritical and subcritical reactors can be modi-

ed into reactors of different lengths to provide different volumes.he reaction time is considered the residence time in which theixture flows through the supercritical or subcritical reactor.

Water tank, (2) material sludge tank, (3) water pump, (4) material sludge pump,9) final water cooler, (10) product collector, (11) temperature control system, (12)

However, due to the heating time (less than 1 s) of the mixturefrom the temperature after preheating or primary cooling to thechosen supercritical or subcritical temperature, the real reactiontime is actually slightly shorter than the residence time. The reac-tion pressures inside the reactors are measured using manometersand adjusted by the reducing valves.

2.3. Experimental design

In the combined supercritical and subcritical hydrolysis exper-iments on cellulose, deionized water was pumped from the watertank into the preheater maintained at 370 ◦C. Prepared materialsludge with a cellulose concentration of 10 g L−1 was pumped fromthe material sludge tank into the supercritical reactor immediatelyafter being mixed with the preheated water. The products from thesupercritical reactor were cooled by the primary water cooler tostop supercritical reaction and then transferred into the subcriticalreactor to undergo further hydrolysis under subcritical conditions.Finally, the products were collected by a product collector. Con-sidering the interaction of the operational parameters includingtemperature, pressure, and flow, the reaction system was deemedto achieve steady state only when the measured temperatures forthe preheating, supercritical and subcritical reactions reached thechosen temperatures (with an accuracy of ±1 ◦C), and at the sametime, the measured pressures for supercritical and subcritical reac-tions reached the chosen pressures (with an accuracy of ±1 MPa).The stable experimental period was defined as the period duringwhich the system maintained steady state, and all the operationalparameters and samples mentioned in this paper were obtainedduring the stable experimental period. The flows of the two pumpswere measured based on the water and material sludge consump-tion during the stable experimental period, and adjusted to providedifferent levels of cellulose concentration. In accordance with ourprevious work on the supercritical hydrolysis of cellulose [15], atemperature of 380 ◦C was applied for supercritical reaction, and

the cellulose concentration of the mixture was adjusted in the rangeof 3.5–4.5 g L−1. Four subcritical reaction temperatures, i.e., 210,240, 270, and 300 ◦C, were investigated. The supercritical and sub-critical reaction times were calculated according to Eq. (1). Threeparallel samples were collected for HPLC analysis at equal intervals

870 Y. Zhao et al. / Chemical Engineering Journal 166 (2011) 868–872

Table 1Oligosaccharide yields from the supercritical hydrolysis of cellulose at 380 ◦C.

No. Celluloseconcentration(g L−1)

Pressure(MPa)

Reactiontime (s)

Oligosaccharideyield (%)

1 3.29 25 ± 1 8.45 35.92 3.64 24 ± 1 8.44 31.53 3.66 25 ± 1 7.34 28.94 5.47 25 ± 1 6.56 24.35 5.32 25 ± 1 6.44 17.9

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e.g., 5, 10, and 15 min) during each stable experimental period,hich lasted for at least 15 min. The yield of each component was

he average value of those in the parallel samples with an error bar.

= V

Q(1)

here Q stands for the flow of the mixture (mL s−1), and V standsor the volumes of the supercritical or subcritical reactor, which are0 and 50 mL, respectively.

. Results and discussion

.1. Oligosaccharide production from the supercritical hydrolysisf cellulose at 380 ◦C

The hermetic structure of the flow reaction system makeshe collection of intermediate products from the supercriticaleactor difficult when performing combined experiments. There-ore, considering that a temperature marginally higher than theritical point is more suitable for accumulating oligosaccharidesnd enhancing the stability of oligosaccharide production in super-ritical reactions (as determined in our previous work on theupercritical hydrolysis of cellulose) [15], 380 ◦C was chosen as theptimal and fixed supercritical temperature. Furthermore, whenhe cellulose concentrations were in the range of 3–4.5 g L−1 andhe reaction times in the range of 7–10 s, the oligosaccharide yieldsn the supercritical reaction can normally reach over 30% to around0%, which are relatively high compared with those obtained underther conditions. The results are shown in Table 1. A yield of5.9 ± 1.5% oligosaccharide was accumulated when the celluloseoncentration was at 3.29 g L−1 and the reaction time was 8.45 s.hese results served as an important guide for supercritical param-ter control in the combined experiments conducted in this study,lthough the intermediate products obtained after the supercriticaleaction were not analyzed because of the flow-type structure.

.2. Effect of temperature on subcritical reaction and hexoseroduction

Four temperature levels (210, 240, 270, and 300 ◦C) were inves-igated to reveal the effect of temperature on the subcriticaleaction for the combined hydrolysis of cellulose. Under the samearameters as those in the supercritical reaction, subcritical reac-ion, especially hexose production, can be influenced to a greatxtent by the reaction temperature. Fig. 2 presents the oligosaccha-ide and hexose yields at different subcritical temperatures underhe following operational parameters: cellulose concentration of

−1 ◦

.53 ± 0.24 g L , supercritical temperature of 379 ± 1 C, supercrit-cal reaction time of 9.70 ± 0.66 s, and subcritical reaction time of8.49 ± 3.31 s.

Fig. 2 shows that after a subcritical reaction lasting around 48 st 210 ◦C, approximately 13.1% of the oligosaccharides produced

Fig. 2. Effect of subcritical temperature on the oligosaccharide, glucose, and fructoseyields.

in the supercritical reaction remained in the final liquid prod-uct, whereas the glucose and fructose yields reached 21.6% and2.2%, respectively. With the increment of subcritical temperature,the hydrolysis rate of the oligosaccharides increased accordingly,inducing the higher consumption of oligosaccharides during sub-critical reactions. However, the yields of hexoses, including glucoseand fructose, increased at first and then decreased after a maxi-mum yield was obtained. This is because the hexoses were furtherdecomposed as they were being produced from oligosaccharides.Moreover, the hexoses decomposed more rapidly at higher temper-atures, a finding that has been proven and analyzed in our previouswork on the batch system. The hexose yields were 23.8%, 31.5%,26.1%, and 22.6% at 210, 240, 270, and 300 ◦C, respectively. There-fore, 240 ◦C was determined as the optimal subcritical temperatureof the combined process for cellulose conversion.

3.3. Influence of cellulose concentration and reaction time onhexose yields

Due to the structural integrity of the flow reaction system, thevariety of cellulose concentrations and reaction times influencedboth supercritical and subcritical reactions in the combined experi-ments. In fact, the reaction time depended on the flows of water andmaterial sludge when the reactor volumes were fixed. Therefore,four levels of cellulose concentrations in the range of 3.5–4.5 g L−1

were adjusted by water and material sludge flows and investigatedto reveal their effects on hexose production. The corresponding fourlevels of operational parameters (groups A–D in terms of celluloseconcentration), including information on water and material sludgeflow, are shown in Table 2.

Fig. 3 represents the final yields of hexoses at different sub-critical temperatures for each experimental group. The initialincrease and subsequent decrease in the yields of hexoses after amaximum yield was reached, along with an increase in the sub-critical temperature, can be observed in each experimental group.Furthermore, the cellulose concentration and reaction time con-siderably influenced hexose production. For example, when thesubcritical temperature was 240 ◦C, the hexose yield obtained

at a cellulose concentration of 3.53 ± 0.24 g L−1 and supercriticalreaction time of 9.70 ± 0.66 s (group A) was 31.5% ± 1.4%, whichwas larger than those obtained at higher cellulose concentrations,including 25.1% ± 1.1% obtained at a cellulose concentration of

Y. Zhao et al. / Chemical Engineering Journal 166 (2011) 868–872 871

Table 2Operational parameters for the investigation of the effects of cellulose concentration and reaction time on combined hydrolysis.

Experimental group A B C D

Water flow (mL s−1) 0.67 ± 0.06 0.90 ± 0.04 0.75 ± 0.04 0.56 ± 0.04Material sludge flow (mL s−1) 0.36 ± 0.03 0.60 ± 0.05 0.55 ± 0.03 0.43 ± 0.05Cellulose concentration in mixture (g L−1) 3.53 ± 0.24 4.00 ± 0.24 4.22 ± 0.17 4.38 ± 0.35Supercritical temperature (◦C) 379 ± 1 380 ± 0 379 ± 1 379 ± 1Supercritical pressure (MPa) 24.0 ± 0.5 23.5 ± 0.5 23.5 ± 0.5 23.5 ± 0.5Supercritical reaction time (s) 9.70 ± 0.66 6.65 ± 0.33 7.72 ± 0.31 10.11 ± 0.59Subcritical pressure (MPa) 9.0 ± 0.0 9.0 ± 1.0 9.0 ± 0.0 8.5 ± 0.5Subcritical reaction time (s) 48.49 ± 3.31 33.24 ± 1.64 38.60 ± 1.54 50.55 ± 2.94

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Table 3Maximum yields of hexoses and the corresponding operational parameters in thebatch and flow systems.

Parameter Batch reactionsystem

Flow reactionsystem

Cellulose concentration (g·L−1) 24 3.53 ± 0.24Supercritical temperature (◦C) 380 379 ± 1Supercritical pressure (MPa) 25 24.0 ± 0.5Supercritical reaction time (s) 16 9.70 ± 0.66Subcritical temperature (◦C) 280 240Subcritical pressure (MPa) 10 9.0 ± 0.0

ig. 3. Hexose yields produced from cellulose at different subcritical temperaturesor each experimental group.

.00 ± 0.24 g L−1, 22.0% ± 1.1% at 4.22 ± 0.17 g L−1, and 19.4% ± 0.5%t 4.38 ± 0.35 g L−1. This reveals that in the 3.5–4.5 g L−1 range, aelatively low cellulose concentration can result in high yields ofexoses when the reaction times are in reasonable ranges, such as–10 s for supercritical reaction and 40–50 s for subcritical reaction.onversely, an excessively short or long reaction time may cause

nsufficient hydrolysis or immoderate decomposition, yielding rel-tively low amounts of hexoses. In this research, the operationalonditions, including a cellulose concentration around 3.5 g L−1,upercritical temperature of 380 ◦C, supercritical reaction time of–10 s, subcritical temperature of 240 ◦C, and subcritical reactionime of 45–50 s, were thus determined as the optimal parametersor the combined supercritical and subcritical process of celluloseonversion.

.4. Comparison of hexose production in the batch reaction andow reaction systems

The results obtained using the flow reaction system were com-ared with those obtained with the batch reaction system carriedut in our previous work. The batch system was composed of twoarallel reactors (5 mL, stainless steel 316), two salt baths for theupercritical and subcritical reactions respectively (providing tem-eratures of 260–500 ◦C), an ice–water cooler, and a temperatureontrol subsystem. During the batch experiments, 60 mg cellulosend 2.5 mL deionized water were mixed and dispensed into eacharallel reactor, and then hydrolyzed at 380 ◦C, 16 s and 280 ◦C,4 s for the supercritical and subcritical reactions respectively,hich had been determined as the optimum conditions for cel-

ulose hydrolysis [13]. The corresponding operational parametersnd hexose yields are presented in Table 3. The maximum hexoseield obtained with the flow reaction system is comparable to butlightly lower than that obtained with the batch system. However,he maximum oligosaccharide yield from the supercritical hydrol-

Subcritical reaction time (s) 44 48.49 ± 3.31Maximum hexose yield (%) 39.5 31.5% ± 1.4%

ysis of cellulose in the flow system is almost equivalent to that inthe batch system. This probably stems from the fact that the oper-ational parameters in the flow reaction system, such as reactiontime, cannot be adjusted separately for supercritical and subcriti-cal reactions because of the fixed ratio of the reactor volumes. Thispresents difficulties in the optimization of the combined process. Incontrast, supercritical and subcritical reactions can be performedseparately in the batch system, so that the reaction times can beeasily controlled as the combined process is optimized. Further-more, the cellulose concentration in the flow system is much lowerthan that in the batch system because of the problem of fluidity.

Despite the considerable contributions of the batch system totheoretical research on the hydrothermal conversion of biomassand the relatively higher amount of hexose it yields through thecombined hydrolysis of cellulose, it cannot be used in practicalapplications because of the low efficiency of batch operation. Theflow reaction system is much more promising for hydrothermalbiomass conversion because of the high efficiency generated byits continuous flow structure. It is especially suitable for com-bined supercritical and subcritical technology on lignocellulosicresources.

4. Conclusions

This paper examined the combined supercritical and subcriticalhydrolysis of cellulose in the flow reaction system. On the basis ofthe optimal supercritical parameters obtained in a previous study,the effects of subcritical temperature, cellulose concentration,and reaction time on final hexose production were investigated.When all other parameters were maintained, a maximum hexoseyield was obtained during the increase in subcritical tempera-ture. Appropriate ranges of cellulose concentration and reactiontime, which depended on the flows of water and material sludge,were also crucial in obtaining high hexose yields. The flow reac-tion system can yield reasonable amounts of hexose from cellulose

hydrolysis compared with the batch system, and it has higherpotential for use in biomass conversion. Experiments on combinedsupercritical and subcritical processes for the conversion of ligno-cellulosic waste, such as corn stalks, are currently underway.

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cknowledgements

This work was supported by the National High-Tech Researchnd Development Program of China (No. 2006AA10Z422), thehina Postdoctoral Science Foundation (No. 20100470018), andhe Science and Technology Innovation Program of Beijing Forestryniversity (No. BLYX200911).

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