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J. of Supercritical Fluids 46 (2008) 299–321

Review

From plant materials to ethanol by means ofsupercritical fluid technology

Christian Schacht, Carsten Zetzl, Gerd Brunner ∗Institute for Thermal and Separation Processes, Hamburg University of Technology,

Eissendorfer Strasse 38, D-21073 Hamburg, Germany

Received 14 September 2007; received in revised form 9 January 2008; accepted 10 January 2008

bstract

Plant and waste material from agriculture or food industry represents on of the the worlds largest resources of ligno-cellulose and thereforeermentable sugars. Conversion of these sugars to ethanol is one way to take optimized profit of the solar energy incorporated in the plant growth.

For the target product of ethanol of >99.8 wt.%, there are several plant material sources available. The carbohydrate compounds of theseaterials can be pretreated and partly hydrolyzed by nearcritical water. CO2 dissolved in water may be used as catalyst. Hydrolysis is favorably

ccomplished by enzymatic catalysis. The product streams from the hydrolytic treatment are fermented. The resulting diluted ethanol solution is

rocessed by multistage counter-current supercritical carbon dioxide extraction to ethanol of 99.8 wt.% concentration. Non-fermentable residuesay be subjected to a second hydrolysis or transferred to a biogas production. Solid residues of the biogas reactor, in particular lignin containing

ractions, can be oxidized with near and supercritical water to mainly gas and a smaller fraction of mainly short chain fatty acids, which can beeintroduced to the biogas reactor.

2008 Elsevier B.V. All rights reserved.

eywords: Bio-ethanol; Renewable resources; Ligno-cellulose; Hydrolysis; Supercritical; Hot water

ontents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3002. The starting material (ligno-cellulosic material) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3003. The goal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3014. State of the art: conventional processes of the 1st and 2nd generation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 301

4.1. Pre-treatment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3014.2. Hydrolysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 302

4.2.1. Kinetic modeling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3024.2.2. Enzymatic saccharification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 302

4.3. Fermentation of C5 and C6 sugars to ethanol . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3034.4. Separation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 303

5. Supercritical/near critical fluid technology contributions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3035.1. Pre-treatment with CO2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3045.2. Hydrolysis of starch and ligno-cellulose compounds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 304

5.2.1. Type of reactor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3055.2.2. Influence of solids concentration in the feed . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 305

5.2.3. Influence of linear velocity . . . . . . . . . . . . . . . . . . . . . . . .5.2.4. Influence of pressure . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5.2.5. Inhibiting degradation products . . . . . . . . . . . . . . . . . . .5.2.6. Carbonic acid addition. . . . . . . . . . . . . . . . . . . . . . . . . . .

∗ Corresponding author. Tel.: +49 40428783040; fax: +49 40428784072.E-mail address: brunner@tu-harburg.de (G. Brunner).

896-8446/$ – see front matter © 2008 Elsevier B.V. All rights reserved.oi:10.1016/j.supflu.2008.01.018

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 305. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 305

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 305

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 305

300 C. Schacht et al. / J. of Supercritical Fluids 46 (2008) 299–321

5.3. Contributions of TUHH laboratory. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3055.3.1. Hydrolysis of starch . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3065.3.2. Liquefaction kinetics and conversion of cellulose and starch . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3065.3.3. Ligno-cellulose and lignin conversion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 309

6. Application: hydrolysis of rice bran . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3116.1. Influence of hot water treatment on pH-value . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3116.2. Formation of sugar monomers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3116.3. Concentration of oligomers after treatment with hot pressurized water . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3126.4. Monomers and oligomers: kinetic modelling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3126.5. Formation of degradation products . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3146.6. Enzymatic hydrolysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 315

7. Fermentation to ethanol and separation with CO2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3167.1. Estimation of the necessary number of theoretical equilibrium plates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3177.2. Experimental reliability tests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 317

7.2.1. Counter current SFE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3177.2.2. Hydrodynamics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3187.2.3. Mixer settler SFE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 318

7.3. Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3188. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 319

Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 319. . . . .

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References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. Introduction

The ongoing worldwide discussion about fuels from renew-ble resources has several products in mind, which shallontribute to the replacement of fossil fuels. One of them isio-ethanol, i.e. ethanol from plant material with a concentra-ion of >99.8 wt.%. Ethanol is only one product option of manyhich can be produced by different pathways.Biomass can be converted for fuel purposes to heat by

ombustion, to a product mix consisting of char, oil, and gasy pyrolysis, to oil by chemical processes (reduction), to gassynthesis gas) by gasification, to methane by anaerobic diges-ion, and to ethanol by disintegration and fermentation [1].or motor fuels, gasification to synthesis gas with subsequentischer–Tropsch reaction to liquid products, and fermentationf sugars to bio-ethanol seem to be the most favored productionines. This contribution will be restricted to bio-ethanol and tospects of high-pressure technology only.

The efficiency of plants to convert radiation energy from theun into useful biomass is not overwhelming. C4-plants (namedrom the photosynthetic process, like wheat, rice, sugar beet,otato) have an efficiency between 1.6% and 2.9% (5.7% in the-ry) and C3-plants (corn, sugar cane) have an efficiency between.5% and 0.9% (3.2% in theory) in percentage of the total amountf incident radiation [1]. But the available amount of biomass isnormous. The harvestable dry matter for sugar cane is between5 t ha−1 year−1 and 90 t ha−1 year−1, for sugar beet betweenand 18, and for temperate grass between 7 t ha−1 year−1 and

5 t ha−1 year−1, [1].The major disadvantage for the time being is the higher

roduction cost for bio-ethanol as compared to gasoline. Bio-

thanol production costs are (to be looked at with care, as alleneral cost data): 0.78 D /L (24 D /GJ) from sugar, 0.72 D /L22 D /GJ) from starch, 0.31 D /L (10 D /GJ) from sugar caneBrazil), and from ligno-cellulose 0.98 D /L (30 D /GJ). For com-

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. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 319

arison: Bio-diesel 0.69 D /L (19 D /GJ) (from rape seed), biogas.74 D /L (21 D /GJ), BtL-fuel 1.03 D /L (30 D /GJ) [2]. Fossiluels have a heating value of about 42.7 GJ/t, the lower heatingalue of Ethanol at ambient temperature is about 26.7 GJ/t.

. The starting material (ligno-cellulosic material)

Ligno-cellulosic material, or plant biomass, is the world-ide greatest resource of sugars and thus, for the productionf ethanol. The ligno-cellulosic material contains mainly cellu-ose (35–50%), hemi-cellulose (20–35%), and lignin (10–25%)3]. The composition varies in dependence on the type of sub-trate and the processing. In addition, the material may containtarch (which usually is used separately), ash (mineral matter),roteins (e.g. gluten), oils, and other minor compounds.

Physico-chemical behavior of ligno-cellulose shall not beixed up with starch. Starch is an alpha-linked polysaccha-

ide. It is composed of two components with different moleculareights: 20–30% of linear amylose and 70–80% of branched

mylopectin.The structure of cellulose and ligno-cellulose as well as its

iological degradations can be reviewed at [4–9].In plants, ligno-cellulose is forming a complex crystalline

tructure that is resistant to enzymatic attack and insoluble inater. It consists of cellulose, hemicellulose and lignin. Theolecules are held together by covalent bonding, various inter-olecular bridges, and van-der-Waals forces.Cellulose is a linear polymer consisting of �-1,4-glycosidic

ound glucose molecules. Each glucose unit is turned around by80 degrees relatively to the neighboring molecules. Therefore,he repeating units are cellobiose, the double sugar of glucose.

he chain length of cellulose ranges from about 100 to 14000nits [5].

Hemi-cellulose consists of pentoses (xylose, arabinose),exoses (mannose, glucose, galactose) and sugar acids, with dif-

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ering concentrations depending on the type of substrate. Mostly,4-bound xylose units form a basic structure with different sidehains. These hetero-xylanes are bound together and onto theignin molecules by ferula acid bonding. They form a network,n which the cellulose micro-fibrils are inserted [8].

Lignin is a complex, aromatic polymer compound consist-ng of p-hydroxyphenylpropanoid units, which are connected by–C and C–O–C bridges. Lignin mostly is integrated into the cellalls. It stabilizes them and makes them resistant against water.tructure and composition of the lignin molecules vary with theubstrate. Lignin encloses the micro-fibrils and strengthens theber [10].

. The goal

In order to convert ligno-cellulosic material into ethanol, aydrolysis of the polymers to the mono-sugars must be carriedut first. These mono-sugars can then be converted to ethanol infermentation process by means of micro-organisms or yeast.n efficient process should also make use of the pentose-sugarseing bound within the hemicellulose, (xylose and arabinose)ince the yield of ethanol may be doubled by their conversion.he ethanol is then separated from the fermentation broth, wheremaximum concentration of about 10% ethanol can be achieved

11]. Higher concentrations of ethanol may inhibit the fermen-ation. Ethanol for use as motor fuel must then be concentratedo >99.8%, beyond the azeotrope (95.57 wt.% at 1 bar, 78.2 ◦C).

The process, nevertheless, comprises some steps, which haveo be improved for the efficient use of ligno-cellulosic material,.g. pre-treatment, hydrolysis, fermentation, and the concen-ration step to absolute alcohol. This will be discussed in theollowing. More questions are still open for the use of lignin,hich mostly is used for energy supply, for the waste water, and

or the solid residues.

. State of the art: conventional processes of the 1st andnd generation

The conversion of biomass to ethanol generally includes threeteps: The hydrolysis of the cellulose and hemi-cellulose intoonomer sugars, the fermentation of the sugars to ethanol asell as the purification of ethanol. Although different ways ofydrolysis have been studied, enzymatic treatment provides thereatest potential to lead the technology towards a successfulompetition with conventional fuel technologies [12–14].

Unlike 2nd generation processes, 1st generation processesasically use starch, contained in grains or seeds (e.g. corn,heat, rye, etc.) as a starting material for ethanol production.nd generation processes try to make use of the whole plant,.g. corn stover, rye stover, bagasse or grass. Processes, whichirectly use sugar solutions, e.g. from sugar cane or sugar beet,tart with their process line after the saccharification. 1st gen-ration processes can be subdivided into dry milling and wet

illing processes [15].Wet-milling plants, which comprise about 25% of the capac-

ty in the USA, produce ethanol together with a variety ofo-products. In contrast, dry grind plants are designed for the

Dahi

l Fluids 46 (2008) 299–321 301

roduction of ethanol and produce animal feed as co-product.apital cost are lower than for wet milling plants.

Ligno-cellulosic biomass can be pre-treated and enzymati-ally hydrolyzed to yield a mixture of sugars including glucose,alactose, arabinose, and xylose [16].

So far, no commercial plant is operating on the basis of cellu-osic material. There is a demonstration plant, operated by Iogenorporate, Ottawa, Canada. The process is basically the sames for starch-based plants with appropriate steps for the pre-reatment of the biomass. Technological innovations claimed byogen include [17]: Pre-treatment by a modified steam explosionrocess, new, highly potent and efficient cellulase enzyme sys-ems, separate hydrolysis and fermentation using a multistageydrolysis process, advanced micro-organisms and fermenta-ion systems that convert both C6 and C5 sugars into ethanol,nergy efficient heat integration, water recycling and co-productroduction.

.1. Pre-treatment

An effective pre-treatment includes the decrease of the cel-ulose crystallinity by the removal of hemi-cellulose and lignins well as the increase of the cellulose porosity [18,19]. Fur-hermore, the formation of decomposition products, that inhibiturther process steps, have to be avoided, size reduction of theeed stocks has to be obviated and energy demands as well asosts have to be minimized.

During the last 25 years plenty of pre-treatment-methods haveeen developed, which can be reviewed at other places [12–14].hereas dilute acid pre-treatments are already commercially

vailable, treatment of biomass with liquid hot water (LHW)s still being worked at on laboratory scale [11,20]. However,ue to its simplicity, low generation of inhibiting by-productsnd projected yields of up to 98% LHW pre-treatment exhibitsgreat potential, and Hamelinck [11] points out the need to

erform further research on this pre-treatment method.Lignin hinders water-molecules to enter the cellulosic micro-

brils and thus inhibits the action of enzymes. In addition, crystaltructure of cellulose reduces the surface available for contactith the enzymes [13].Pre-treatment of ligno-cellulosic biomass should conse-

uently eliminate steric hindrance and enlarge porosity of theubstrate. Hereby, reaction of mono-sugars to degradation prod-cts must be avoided since it reduces yields and producesnhibiting compounds.

Dilute acid pre-treatment was a long-time preferred pro-edure and has been reviewed repeatedly [12,18,21–23]. Theubstrate is contacted with dilute sulphuric acid (0.5–1.5 wt.%),r other acids like nitric acid or hypo-chloric acid, at tempera-ures of 160–220 ◦C and reaction times ranging from seconds toeveral minutes [11,24]. Hemi-cellulose can be fully dissolvedxylose yields of 75–90% are reported) and the downstreamnzymatic hydrolysis of cellulose reaches values up to 90% [13].

isadvantageous are the corrosive medium, the neutralization to

djust the pH for the following process steps, (e.g. with calciumydroxide, which produces gypsum [25]), and the formation ofnhibiting degradation products [13].

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Steam pre-treatment was first used in the Masonite-processor fibreboard [26]. The advantage that no toxic or inhibitingubstances are added, is obvious. Biomass is contacted with sat-rated steam at elevated pressures (7–50 bar) and temperatures160–260 ◦C) for several minutes and then suddenly depres-urized. Thermal, chemical, and mechanical effects change thetructure:

. Hydrolysis reactions produce small amounts of acids (i.e.acetic acid), which themselves act then catalytically. The lowpH of water at the elevated temperatures [27] also enhanceshydrolysis.

. The sudden evaporation of water after pressure reductiononly loosens the structure of the substrate [28].

Enhanced enzymatic conversion of cellulose (up to 90%) isttributed to the removal of hemi-cellulose [13]. The methods particularly suitable for leaf-wood. Disadvantages are theimited decomposition of the ligno-cellulosic structure, the for-

ation of xylose degradation products and the lower yield inentose monomers of about 65% [29].

.2. Hydrolysis

Hydrolysis of ligno-cellulosic material can be carried outs acid catalyzed or as enzymatically catalyzed process. Acidatalysis encounters the disadvantages of adding a substance,hich is corrosive and must be recovered, enzymatic catalysis

eads to long residence times and to additional biomass, whichas to be disposed of. The relatively small pore size within theignocellulose (about 51 A, [30]) is the reason for the long reac-ion time during enzyme catalysis [28], while in acid catalysishe active species, protons of 4 A diameter, can easily dif-use through the substrate. Pre-treatment of the ligno-cellulosicaterial, as already discussed, leads to much shorter reaction

imes for enzymatic decomposition. Reaction conditions muste chosen in a well defined interval, since hemi-cellulose decom-oses rapidly into mono-sugars, while cellulose needs a mucharsher treatment for substantial yield of mono-sugars [12,13].t has been shown experimentally that the mono-sugars of hemi-ellulose will subsequently convert to degradation products atlevated process temperature resp. residence time in the appa-atus.

The molecular mechanism of acid catalyzed hydrolysisncludes three steps [31]. First, a conjugated acid is formed byhe addition of a proton onto the oxygen atom connecting bothugar-units. Then, the C–O-linkage between the glycosidic oxy-en atom is split and a cyclic carbonium ion in shape of a chairs formed. This step is rate determining [32]. By a fast additionf water, this ion is transferred into glucose in the last step ofhe reaction mechanism.

The reaction rate is very high at first and after a certain

eaction time and degree of conversion, decreases. The initiallyigh reaction rates are attributed to the amorphous and surfacearts of the ligno-cellulose, the lower rates are explained by therystalline structure of the remaining substrate.

twoa

l Fluids 46 (2008) 299–321

.2.1. Kinetic modelingKinetic modeling of the decomposition of ligno-cellulose

s complicated by the structure of the substrate, where ligninnhibits the path of the protons. Accessibility of the glycosidicinkages is changing with reaction place and reaction progress.everal types of linkages (e.g. between xylose, the acetyl groups,

inkages in lignin) are stronger than the glycosidic linkages.The number and variety of modeling approaches is enormous

nd may vary according the specific compounds inside the ligno-ellulose system. An overview can be found by Jacobsen andyman [33].Sasaki et al. [34] studied the kinetics of cellulose hydrolysis at

levated temperatures (above 300 ◦C) and applied the shrinkingore model for the numerical description.

A common model, initially designed for Douglas fir wood,s based on two first order reactions [35].

ellulosek1−→glucose

k2−→degradation products.

This model could also be applied with success to the decom-osition of hemi-cellulose. It can be generalized to

olymersk1−→monomers

k2−→degradation products

herein k1 and k2 are the reaction rate constants. Integration ofhe differential equations leads to an equation for the amount of

onomers [36,37]

= M0 e−k2t + P0k1

k2 − k1(e−k1t − e−k2t) (1)

ith M for the monomer concentration, and P for the polymeroncentration, and t as the reaction time. The index 0 indicateshe initial values.

The model may be broadened by assuming a reactive type ofolymers as a fraction α of the total amount of substrate [38],nd a non-reactive type, thus incorporating the decompositionehavior of hemi-cellulose and cellulose.

= M0 e−k2t + αP0k1

k2 − k1(e−k1t − e−k2t). (2)

The combined influence of temperature and residence timean be shown by introducing a severity coefficient, which makesossible to consider both influences at the same time. This coef-cient R0 was introduced by Overend [39]

0 = t exp

(T − 100

14.75

), (3)

ith t as reaction time (min) and T as temperature (◦C).

.2.2. Enzymatic saccharificationDue to the disadvantages of acid catalyzed hydrolysis,

nzymatic hydrolysis is nowadays preferred. The reaction mech-nism of the enzymatic hydrolysis is discussed in the literature[5,10]), but not fully understood, yet. To enable the reac-

ion, the enzyme (cellulase) must be coupled to the substrate,hich requires the substrate to exhibit a certain size and typef the three-dimensional structure. Different types of cellulasesre known. Exogluconases act at the end of the chains, while

critical Fluids 46 (2008) 299–321 303

esc

deumt

4

ts2macah

4

dtrfsaudpEavpmr

5c

wsch

wloagL

(ifwT1TohaafssSa

otacdifl

tilfthtr[

r

C. Schacht et al. / J. of Super

ndogluconases split the chains. Both actions lead to wateroluble oligomers which are transformed by �-glucosidase orellobiase to glucose [40,41].

The costs for the enzymes are a substantial part of pro-uction costs for bio-ethanol. For recycling the enzymes, thenzymes must be separated from the product flow, for repeatedse, enzymes must remain in the reactor and the product flowust be removed. A detailed discussion is beyond the scope of

his contribution.

.3. Fermentation of C5 and C6 sugars to ethanol

Saccharomyces cerevisiae (bakers yeast) produces adenosin-riphosphate (ATP) during anaerobic fermentation via glycoly-is. Pyruvate is formed and is transformed to ethanol. In total,mol ethanol and 2 mol CO2 are formed per mol of glucose. Toake use of most of the ligno-cellulosic material, pentose sug-

rs must also be transformed to ethanol. Genetically modified S.erevisiae is developed in order to transform glucose and xyloset the same time [42–44]. So far, glucose is transferred at muchigher rates than xylose.

.4. Separation

The common process uses for separation of the ethanol a firstistillation column, called beer column. Remaining solids andhe major part of water is separated in this column. Ethanol isemoved, together with CO2 which was produced during theermentation, over top. CO2 is washed out with water and thistream is recycled to the bottom. The bottom flow containsll the solids which are separated with a centrifuge. The liq-id flow in the centrifuge is concentrated with heat from theistillation. Condensed water is returned into the process. Theroduct flow from this column contains about 37% ethanol.thanol is enriched in another distillation column up to thezeotropic concentration of 95%. To produce absolute alcohol,arious distillation methods have been developed and employed,artially with compounds like benzene or tri-chlor-ethylene. Aore recent and more environmentally friendly process is the

emoval of water by molecular sieve adsorption [25].

. Supercritical/near critical fluid technologyontributions

Recently, research has been carried out to compare hot liquidater (LHW) pre-treatment to the treatment of biomass with

team. The concept is a treatment of ligno-cellulose by sub-ritical pressurized water, eventually assisted by CO2-enhancedydrolysis.

Utilization of a steam treatment could significantly lower theater usage. However, due to the advantage of significantly

ower hydrolysate inhibition, the main attention is being paid

n the development of LHW processes, which promises alsohigher conversion. Higher xylan recovery suggesting lower

eneration of degradation products could be recognized for theHW treatment [45,46].

w9

w

Fig. 1. Treatment of ligno-cellulosic material with liquid hot water.

Generally, an aqueous suspension of ligno-cellulose10–20%) is treated at elevated pressures, to keep the watern the liquid phase (Fig. 1). For an industrial application theollowing scenario was developed for a LHW pre-treatment, inhich the products are already separated into different streams.he biomass is contacted with water at temperatures from70–230 ◦C and at pressures of 25–100 bar (compare Fig. 1).he effluent is separated in a cyclone into dissolved monomers,ligomers and a first residual suspension. This suspension iseated to 210–250 ◦C and partially hydrolyzed. The effluent isgain separated by a cyclone into a liquid with dissolved mono-nd oligomers, now mostly of cellulose. The fully liquid streamsrom the cyclones are transferred to an enzymatic hydrolysis foraccharification and further to ethanol fermentation. The suspen-ion from the second cyclone is again heated to 300 ◦C or higher.ome oxygen is added (e.g. by in-line high-pressure electrolysis)nd the effluent streams are processed further.

Generally no particle size reduction is required for this kindf pre-treatment [13,19,47]. The pioneer work regarding pre-reatments with hot liquid water has been performed by Bobleternd co-workers, who found out that hemi-cellulose could beompletely separated from the ligno-cellulose and enzymaticigestibility of cellulose can be significantly increased by treat-ng the ligno-cellulosic material with hot compressed water inow through systems [48,49].

The contact time is one of the main variables determininghe efficiency of the pre-treatment in which the hemi-celluloses removed from the biomass and the structure of the cellu-ose is changed enabling accelerated enzymatic hydrolysis. Theeasibility of breaking chemical bonds with water are facili-ated by the fact that water develops acidic characteristics atigh temperatures. The ion product Kw of water increases withhe temperature up to a maximum of 6.34 × 10−12 at 250 ◦Cesulting in a pH of 5.5 for water at a temperature of 220 ◦C27].

By percolating biomass at temperatures of 200–230 ◦C andesidence times between 0 min and 15 min, Antal and co-workers

ere able to achieve a total decomposition of hemi-cellulose and0% yield of pentose sugars [50,51].

In Table 1 an overview shows work carried out with hot liquidater for pre-treatment of biomass [21,29,45,46,49–59]. The

304 C. Schacht et al. / J. of Supercritical Fluids 46 (2008) 299–321

Table 1Bibliography on liquid hot water pretreatment

Author Year Parameters Flow (mL/min) Operation Substrate Yield (% of the theoretical maximum)

Temperature (◦C) Res. time (min) Xylose Glucose

Liquef. Recovery

S.G. Allen 1996 220 2 – FT Bagasse >80a N/A N/AS.G. Allen 2001 215 2 – B Corn Fiber 82a N/A N/AH. Ando 2000 180 rising 60 10 FT Hardwood N/A N/A N/AT.H. Kim 2006 190 30 5 FT Corn Stover 86a 90a 94b

M. Laser 2002 220 2 – B Bagasse <83a N/A 96b

W. Mok 1992 230 2 – FT Bagasse 99a 99d N/AN. Mosier 2005 190 15 – B Corn Stover N/A 82b 90b

M. Sasaki 2003 200 rising 10 FT Bagasse N/A N/A N/AH.K. Srenath 1999 220 2 – FT Alfalfa 70a 75d 91a

G.P. Walsum 2004 180 32 – B Corn Stover 64a N/A N/AS.E. Jacobsen 2002 200 10 – B Bagasse 86a N/A N/AC. Liu 2003 220 16 10 FT Corn Stover 97a 99d N/AC. Liu 2004 200 24 10 PFP Corn Stover 89a 95d 90c

C. Liu 2005 200 24 10 FT Corn Stover 96a 99d 95c

a Recovery from fluid phase after pretreatment FT flowthrough.

mttvCo

ihlofaaacy

cfu

5

mcarcpaCy

aoaaposc

spihcp

5

nfoar

adcAk

b After enzymatic hydrolysis; B: batch.c Referring to the cellulosis content in feed PFP: particle flowthrough.d Sum of xylanes in the fluid phase and in solid residue.

ost important parameters influencing the quality of the pre-reated product stream are temperature, residence time, and theype of the reactor. Pressure is adjusted to be higher than theapor pressure of water, since its influence is small [60], unlessO2 is dissolved in water, the concentration of which dependsn pressure.

Generally, the amount of dissolved hemi-cellulose rises withncreasing temperature and time of the treatment. However, theydrolyzed hemi-cellulose monomers (or some liquefied cel-ulose) can further react to hydroxy-methyl-furfural (HMF) orther by-products, most of which are inhibitory for one of theollowing process steps of the ethanol generation. Addition-lly, any kind of by-product reduces the final ethanol yields it is produced by the degradation of the fermentable sug-rs. Thus, an optimum set of time and temperature parametersan be found for each substrate, maximizing the final ethanolield.

A systematic investigation of the decomposition of ligno-ellulosic material according to reaction kinetics could not beound in the literature, yet. It would greatly contribute to thenderstanding of the design of reactors.

.1. Pre-treatment with CO2

Disadvantages emerging from the pretreatment includingineral acids could be avoided by the substitution of the acidic

ompounds with CO2. While keeping the advantage of ancid catalyzed process, corrosiveness is therefore significantlyeduced. At the same time CO2 is a green process component,an be easily removed by depressurization and creates no waste

roducts. If pre-treatment of lignocellulose with CO2 could bepplied at lower temperatures than a similar process withoutO2 addition, xylose degradation could be avoided [61] andield enhanced. Pure supercritical CO2 did not enhance yield

u

sa

t conditions of 214–275 bar, 112–165 ◦C and reaction timesf 10–60 min [19]. With water added, about 70% saturation,substantial increase of the yield in mono-sugars could be

chieved with leaf-wood: 85% of the theoretical value, as com-ared to 16% for the untreated sample. The efficient applicationf water and carbon dioxide at higher temperatures and pres-ures seems to be very promising. Sun [12] even expects lowerost.

Results in the literature differ on the effect of CO2. Van Wal-um [29,62] found for corn stover that CO2 in the pre-treatmentrocess enhanced the xylan- and the furan-concentration, andncreased the pH-value. Therefore, it was suggested that CO2inders the formation of organic acids. All experimental resultsited have been obtained in batch-reactors, which renders themroblematic for transfer to other types of reactors.

.2. Hydrolysis of starch and ligno-cellulose compounds

The first question which arises is to which extent the compo-ents of ligno-cellulosic material undergo hydrolysis and yieldermentable sugars or other products. In chapter 5.3 the influencef LHW-treatment on the hydrolyses of starch, cellulose, ligninnd a lignocellulosic material (rice bran) will be investigated,espectively.

Studies on the hydrolysis of glucose have been reported inrange of temperature from 150 ◦C to 300 ◦C [63–65] and a

etailed kinetic investigation of the hydrolytic reactions of glu-ose near the critical point was published by Kabyemela [66].lso studies on hydrolysis of cellulose were available, but to ournowledge the hydrolysis of starch had not been investigated

ntil end of the 90s.

Sakaki et al. [67] described the saccharification of cellulose,tarting from temperatures at 180 ◦C, increasing to values tobove 280 ◦C.

critica

5

cbwartbm

actaaitosgutcsp

5

cdiRe[tadcwbf

5

tboIt[

crpwt

rmaum

5

ctt

5

pmwhsMamotc

5

fwotItdccpCw

cAtaaf

5

has been established, exhibiting the advantage of a process

C. Schacht et al. / J. of Super

.2.1. Type of reactorFor pre-treatment with hot liquid water three types of reactors

an be used: Batch, semi-continuous, and continuous reactors. Inatch-reactors the solid substrate (ligno-cellulosic biomass) asell as the reaction medium (water) is reacted together without

dding or removing products during reaction time. In semi-batcheactors a fixed bed of biomass is contacted by a flow of the reac-ion medium, which is constantly added and removed, and maye additionally recycled. Data from semi-continuous reactorsay be transferred to industrial scale reactors.Continuous processes can be operated in a co-current and

counter-current mode. Ligno-cellulosic biomass is mostlyontacted co-currently. A slurry of biomass and water is con-inuously pumped first through a heat-exchanger and then keptt constant reaction conditions in a flow-through reactor, ideallyt plug-flow conditions. Short residence times are easily real-zed, but longer residence times can only be achieved with longubular reactors or a high recycle ratio. The problems for thisperation mode are connected with the continuous handling ofolid material, especially if higher pressures are involved. Butround biomass (about 1 mm) can be pumped into the reactorsnder any condition. High solids concentrations are also not easyo achieve. Most experimental data are obtained therefore at lowoncentrations and are not relevant for industrial application. Aolids concentration of at least 10 wt.% must be achieved forractical and economic purposes.

.2.2. Influence of solids concentration in the feedSince most experiments have been carried out at low con-

entration, the concentration dependence of the ligno-celluloseecomposition is not clear. Most results indicate that anncreased biomass concentration results in a lower pH-value.esults on xylan yield are differing. In a batch reactor no influ-nce on yield of xylans could be found by Jacobsen and Wyman56], while Laser et al. [46] found an increased concentra-ion of xylanes (beside higher concentration of furfural, andn increased ethanol-yield after fermentation). Allen et al. [45]etermined a decreased decomposition with increasing biomassoncentration. All investigations agree on the decrease of pHith increasing concentration of ligno-cellulose, which maye explained by the increasing amount of organic acids beingormed.

.2.3. Influence of linear velocityAn increased flow rate in the reactor enhances degrada-

ion of the ligno-cellulosic substrate. This result was confirmedy several groups [50,54,57] and may be considered as resultf the higher mass transfer rates at higher Reynolds-numbers.ncreasing (e.g.) the linear velocity from 2.8 to 10.7 cm/min,he resulting xylans conversion increased from 60% to 82%58].

To avoid enhanced water consumption and decreased con-entration of biomass, Liu and Wyman [59] used a fixed-bed

eactor with periodically changing fluid flow velocity. After aeriod of zero flow rate, the fixed bed was contacted with a hotater flow, which was shut down after the pre-designed reac-

ion time and followed by another batch-reaction phase. The

wrsy

l Fluids 46 (2008) 299–321 305

esult is in context with a normal extraction curve from solidaterial. For the reduction of the specific solvent consumption

nd increase of product concentration, it is to be preferred tose several fixed beds which are operated in a counter-currentode.

.2.4. Influence of pressureThe influence of the pressure on hydrolysis of biomass is basi-

ally limited to keeping the water liquid according to the processemperature. Liu [60] could show that the pressure influence onhe monomer concentration after pre-treatment can be neglected.

.2.5. Inhibiting degradation productsDuring the decomposition process of biomass, degradation

roducts are formed, which inhibit the normal action of theicro-organisms. Such degradation products are acetic acid,hich stems from the decomposition of hemi-cellulose [68],ydroxy-methyl-furfural (HMF) and furfural. For these sub-tances a strong inhibiting influence on yeast could be shown.icro-organisms, active in ethanol fermentation from pentoses,

re more inhibited than those which ferment hexoses [69]. Theechanism is explained by Palmquist [70,71]. Decomposition

f lignin produces phenolic compounds, which strongly inhibithe action of yeast [69] and the enzymatic hydrolysis of ligno-ellulose.

.2.6. Carbonic acid additionDilute acid pre-treatment processes are applied on large scale

or biomass to ethanol processes [11]. Hemi-cellulose yields asell as glucose yields after enzymatic treatment reach up to 90%f the possible maximum (in [57]). The main disadvantages ofhis process can be found in the application of the acid itself.t results in a higher corrosion and the neutralization leads tohe formation of waste particles which have to be separated andisposed. These drawbacks could be overcome by introducingarbon dioxide to the HLW process instead of acid, because CO2an be neutralized by a pressure reduction. Van Walsum [62]resented an increase of xylan hydrolysis by adding pressurizedO2 to the HLW treatment. These results could not be replicatedith aspen wood being the feedstock in 2002 [72].However, utilising corn stover an increase of xylose and furan

oncentration was demonstrated by van Walsum and Shi [29].lso, an increase of the final pH was noticed in the LHW

reatment to which CO2 was introduced. These results are ingreement with the measurements from 2002, implicating thessumption that the presence of carbonic acid represses theormation of side products.

.3. Contributions of TUHH laboratory

In this part a continuous process for the treatment of biomass

ithout interruptions and frequent start-ups. The experimentalesults are presented in this paper. Furthermore the effect ofimultaneous CO2 supply has been investigated regarding xylanields as well as cellulose digestibility.

3 critical Fluids 46 (2008) 299–321

5

eoooaa

4os(b

1a3biqwIF2

f

cfsyai

5s

tmm

Fa

Fd

ti

(a2ta

Tvsetreactor made of high temperature resistant steel (o.d. = 6.35 mm,i.d. = 3.05 mm).

Cellulose degradation experiments using pure micro-

06 C. Schacht et al. / J. of Super

.3.1. Hydrolysis of starchThe hydrolysis of starch with water and carbon dioxide at

levated pressure and temperature was carried out with the aimf optimizing the reaction conditions to obtain a maximum yieldf sugars, such as maltose, glucose and fructose. The hydrolysisf starch is an acid catalyzed hydrolytic decomposition whereC–O–C linkage is cracked between two glycopyranose units

nd a water molecule is inserted.Experiments on starch were carried out in a tubular reactor of

.02 m length an inner diameter of 6 mm and an outer diameterf 10 mm. The used Inconel 600® tubes were made of a corro-ion resistant and high temperature resistant nickel-based alloy2.4816). A detailed description of the experimental setup cane found elsewhere [57].

Corn starch with concentrations varying from 0.2 up to0 wt.% with residence times from 0.4 min to 20 min werepplied. Temperature was kept in the range from 170 ◦C to00 ◦C and the pressure varied between 60 bar and 240 bar. Car-on dioxide influence is expressed in degree of saturation, whichs the ratio of the added carbon dioxide to the maximum solubleuantity of carbon dioxide in water at given conditions. Wateras saturated with nitrogen to exclude the influence of oxygen.

nitial starch concentration was 0.2 wt.%, if not stated otherwise.ig. 2 shows the results of the hydrolysis of starch at 230 ◦C and40 bar.

The influence of the concentration of carbon dioxide on theormation of glucose is shown in Fig. 3.

Glucose yield is significantly higher than without the use ofarbon dioxide. With log(R0) = 4.5 the yield can be increasedrom 5% to 60% by the addition of CO2. Increasing the initialtarch concentration up to 10 wt.% did not affect the glucoseield significantly (Fig. 4). Therefore, the results obtained withlow initial concentration, seem to be transferable to higher

nitial starch concentrations.

.3.2. Liquefaction kinetics and conversion of cellulose andtarch

The overall degree of conversion of the biopolymer as well ashe yield and selectivity of the main reaction products are deter-

ined experimentally in a flow through type reactor. Additionaleans like the acidification by carbon dioxide [60] to lower

ig. 2. Product yields for conversion of starch under influence of carbon dioxides a function of residence time at T = 230 ◦C.

c

ig. 3. Glucose yields with different CO2 concentrations as a function of resi-ence time.

he pH and promote acid catalysed hydrolysis steps are beingnvestigated.

Micro-crystalline cellulose as purchased from MerckAvicel®) was used. Real biomass samples were obtained frommesophilic biogas reactor run at a hydraulic residence time of0 days, the biomass being the non-biodegradable residues ofhe process. The solid residues were ground in a rotary cutter toparticle size of less than 250 �m prior to the experiments.

The experimental set-up is schematically illustrated in Fig. 5.he main building blocks of the apparatus are the feed supplyessel, the feed pump, the tubular reactor and the down-tream processing units, which consist of a double-pipe heatxchanger, an expansion valve and the effluent collection sys-em. The high-pressure reaction unit is designed as a tubular

rystalline cellulose were conducted to gain information about

Fig. 4. Glucose yields at different initial starch concentrations.

C. Schacht et al. / J. of Supercritica

Fh

tu

ttiitfd

oc

Ft

ktb

l

wh

otr

k

ai

k

E

k

E

sddts

ig. 5. Flow scheme of experimental set-up for continuous hot pressurized waterydrolysis.

he rate of liquefaction and the yield of main degradation prod-cts for this most abundant plant constituent.

In this context, the degree of liquefaction was calculated ashe ratio of dissolved organic carbon (DOC) in the effluent to theotal carbon of the influent suspension. The results reveal a strongncrease of the rate of liquefaction with increasing temperaturen the range of 250–310 ◦C, leading to a complete conversiono soluble products in less than half a minute at 310 ◦C. Aurther increase in temperature results in an even more rapidegradation, yielding a complete conversion within seconds.

This behavior is illustrated in Fig. 6, which depicts the degreef liquefaction on a carbon balance versus process severity atonstant pressure.

ig. 6. Cellulose: degree of liquefaction f at different temperatures and residenceimes at P = 250 bar.

bw

h

F

l Fluids 46 (2008) 299–321 307

The calculated curves were derived assuming a first orderinetic for the cellulose decomposition for each appliedemperature. For this approach, the degree of conversion cane described by Eq. (4):

n(1 − f ) = −kτ, (4)

here k denotes the reaction rate constant of the biomassydrolysis.

A common approach to express the temperature dependencef the reaction rate constant is the Arrhenius law with kS,j,0 beinghe pre-exponential factor and EA,j the activation energy of theeaction.

j(T ) = kj,0 exp

(−EA,j

RT

). (5)

The reaction rate constant for the degradation of cellulosend of corn starch (obtained in an analogous manner) are statedn Fig. 7.

For cellulose conversion, one may obtain

C,0 = 7.7 × 1013 s−1

A = 163.9 × 103 J/mol.

For corn starch conversion, one may obtain

S,0 = 5, 3 × 1012 s−1

A = 147.9 × 103 J/mol.

Regarding the temperature dependence, the reaction rate con-tant at 250 ◦C and short residence times reveals a significanteviation from the linear relationship derived from the otherata points. This behavior of decreased values might be con-ributed to some limitation, which is due to the crystallinetructure.

It can be inferred from Fig. 6 that the simulation curve derived

y linear regression reflect the respective data points reasonablyell, thus justifying the application of a first order approach.The influence of acidification by carbon dioxide on both the

ydrolysis kinetics and the product formation was investigated

ig. 7. Reaction rate constant of starch and cellulose in function of temperature.

308 C. Schacht et al. / J. of Supercritical Fluids 46 (2008) 299–321

Fs

ata

tAdairiam

wp

td

C

tktcawit

tdc

Fr

f

c

ga

smbby consecutive reactions. Different secondary degradation andisomerization products were detected in this work, includingpyruvaldehyde, levoglucosan and HMF (from glucose) as wellas several carboxylic acids (from amino acids). However, a

ig. 8. Liquefaction of cellulose at 240 ◦C, 260 ◦C and 280 ◦C in pure and CO2-aturated water.

nd compared to the hydrolysis in pure water. For that reason,he density and solubility of carbon dioxide under the conditionspplied were taken from literature data [8].

Fig. 8 shows the effect of carbon dioxide on the liquefac-ion of cellulose exemplarily for 240 ◦C, 260 ◦C, and 280 ◦C.

marked rate enhancement of cellulose hydrolysis by carbonioxide addition could be determined at temperatures of 240 ◦Cnd 250 ◦C (results not shown). The catalytic effect dimin-shes with increasing temperature, as can be inferred from theespective degrees of liquefaction at 260 ◦C and 280 ◦C. Theres still a slightly increased rate of hydrolysis at 260 ◦C, whilet 280 ◦C no differences in hydrolysis kinetics could be deter-ined.The kinetic modeling of cellulose liquefaction in the system

ater/CO2 was done in accordance with the method applied forure water.

Thus, CO2 was believed to promote the acid-catalyzed reac-ion pathway of biopolymer hydrolysis by the formation andissociation of carbonic acid according to Eq. (6).

O2 + H2O ⇔ H2CO3 ⇔ H+ + HCO3− ⇔ 2H+ + CO3

2−.

(6)

Besides the kinetics of substrate hydrolysis, the investiga-ion of product formation was of special interest. A detailednowledge of product formation is important to optimize reac-ion conditions with respect to the selective production of desiredompounds, e.g. glucose or amino acids. Their product yieldsre strongly affected by the choice of the reaction temperature asell as by the residence time. For all temperatures a maximum

n yield is observable, which shifts to lower residence times ashe temperature increases.

The reaction rate constants of formation and decomposi-

ion can be determined by fitting Eq. (7) to the experimentallyetermined yields. With ci,0 denoting the initial substrate con-entration, the concentration of glucose can be calculated as

Fa

ig. 9. Glucose yields after subcritical water hydrolysis of cellulose at differentesidence times and temperatures at P = 25 MPa; effect of dissolved CO2.

ollows (Fig. 9):

i = kfi

kdi − k

fi

ci,0[exp(−kfi t) − exp(−kd

i t)]. (7)

The rate constants for the formation and decomposition oflucose (both from starch and cellulose) were obtained in annalogous manner and are also stated in Fig. 10.

The constants for glucose confirm that its formation fromtarch passes on faster than from cellulose due to the aboveentioned reasons. Furthermore, glucose is obviously insta-

le under the conditions applied and accordingly consumed

ig. 10. Rate constants of formation and decomposition for glucose (from starchnd cellulose).

critical Fluids 46 (2008) 299–321 309

ctbtt

ddyse4as

f

5

uam[aIssc

iwa

msip

i

Table 2Composition of model waste and respective suppliers

Component Portion(wt.%)

Source Dry mass content(wt.%)

Wheat straw 23.3 Local farmer 94.5Cabbage 23.3 Market 9.7SAF

ataybOeocuia

mrb

clbpp(trhcu

toorpta

loptc6

C. Schacht et al. / J. of Super

loser inspection of these degradation products goes beyondhe scope of this work. The same applies for the differenty-products (mainly maltose, and oligomers and oligopep-ides) formed by parallel reactions of the substrate accordingo Fig. 10.

The optimum glucose yields, however, are obtained with resi-ence times in the range of a few seconds up to several minutes,epending on the reaction temperature. The maximal glucoseield is reached at increased temperatures in combination withhort residence times. This finding is in accordance with lit-rature results on cellulose hydrolyses at temperatures up to00 ◦C. The reported glucose yields amounted to maximal 50%t 400 ◦C and 25 MPa. This value was, however, stated for a veryhort residence time of 0.00025 s [5].

Based on the results of Fig. 10, the kinetic parameters for theormation and decomposition of glucose can be evaluated.

For glucose yield conversion from starch, one may obtain:

kG,S(formation) = 1.1 × 1011 s−1

kG,S(decomposition) = 1.8 × 105 s−1

EAG,S(formation) = 134.4 × 103 J/mol

EAG,S(decomposition) = 75.5 × 103 J/mol.

For glucose conversion from cellulose, one may obtain:

kG,C(formation) = 1.7 × 1013 s−1

kG,S(decomposition) = 4.6 × 1014 s−1

EAG,S(formation) = 160.7 × 103 J/mol

EAG,S(decomposition) = 168.7 × 103 J/mol.

.3.3. Ligno-cellulose and lignin conversionHydrothermal treatment of ligno-cellulose becomes partic-

larly attractive, when implemented into a complete cycle ofll components, as might be achieved in future including,icro-organisms and higher plant based life support systems

73]. In such applications, biomass offers the potential of anutonomous system allowing for almost complete recycling.t enables us to harvest its products, followed by a sub- orupercritical water treatment of the wastes generated and theubsequent, renewed build-up of plant biomass, thus closing theycle.

Hydrothermal conversion of ligno-cellulosic biomass wasnvestigated on the compounds cellulose, lignin, the complexaste specified by European Space Agency (ESA) (wheat straw

nd indigestible residues from a methane reactor) and rice bran.Lignin was chosen as a model compound, since as one of the

ain components in plant biomass, it proved to be the most per-istent component. Beside the conversion of lignin by hydrolysis

n near-critical water, the oxidative treatment using hydrogeneroxide was also investigated (Table 2).

Treatment of ligno-cellulose was carried out continuouslyn a plug flow reactor at pressures from 15 MPa to 25 MPa

2tpc

oya 23.3 Oil-mill 91.1lgae 10 BlueBioTechGmbH 95.5aecal material 20 – 27.4

nd at temperatures from 240 ◦C to 500 ◦C with residenceimes ranging from a few seconds to 3 min. Pure ligninnd ligno-cellulosic biomass could be liquefied by hydrol-sis up to 70–80%. Effluents were subsequently treated byiological degradation. Overall efficiency of DOC (Dissolvedrganic Carbon) removal increased to 90–95%. No toxic

ffects on the micro-organisms were observed. The oxidationf ligno-cellulose in near-critical water by hydrogen peroxideonverted all carbonaceous material to mainly gaseous prod-cts. Only about 10% of the initial carbon load remainedn the aqueous phase, with the main product being aceticcid.

In contrast to the hydrolysis of pure cellulose, the hydrother-al degradation of the biomass residues from the methane

eactor yielded an incomplete liquefaction, which can probablye attributed to the lignin present in the samples.

As a result, the model waste specified by ESA can be readilyonverted by hydro-thermolysis without the addition of a cata-yst or an oxidant. Degrees of liquefaction up to 90–95% coulde obtained on a carbon basis. Carbon in form of gaseous com-ounds had a minor contribution and amounted to less than threeercent. Regarding the nitrogen balance, even higher degreesTable 2) of liquefaction up to 100% could be achieved. Bothhe carbon and the nitrogen balance could be closed satisfacto-ily. Based on these findings, the conclusion is drawn that theydrothermal conversion of complex biomass is a suitable pro-ess for the production of soluble hydrolysates, which can betilized by subsequent biological treatment.

In this context, the degree of liquefaction was calculated ashe ratio of dissolved carbon in the effluent to the total carbonf the influent suspension. The results reveal a strong increasef the rate of liquefaction with increasing temperature in theange of 250–310 ◦C, leading to a complete conversion to solubleroducts in less than half a minute at 310 ◦C. A further increase inemperature results in an even more rapid degradation, yieldingcomplete conversion within seconds (Fig. 11).

As in the case of cellulose, the addition of carbon dioxideeads to distinctly increased rate of reaction. For the continu-us water oxidation of ligno-cellulosic biomass, the set-up wasartially modified. Due to the corrosive atmosphere of high-emperature water in the presence of an oxidant, a new reactionoil made of corrosion-resistant nickel alloy (Inconel®, Alloy00, o.d. 6,35 mm, i.d. 2,13 mm) with an internal volume of

0 mL was installed downstream of the mixing point. The reac-or was placed in a 4 kW oven (Heraeus RO 7/75), which inrinciple allows temperatures up to 1000 ◦C. Under operatingonditions, reactor outlet temperatures up to 500 ◦C could be

310 C. Schacht et al. / J. of Supercritica

Fo

adtp

cplptTmcb

so

Fe

cu

wacn

tbnwbtTuwpirvad

ob

dmpcu

s

ig. 11. Experimental results on model waste specified by ESA, computationf carbon balance, P = 25 MPa, initial solid concentration: 1 wt.%.

ccomplished. Hydrogen peroxide was used as an oxidant. It wasirectly introduced into the feed vessel and delivered to the reac-or along with the biomass in order to facilitate the experimentalrocedure.

Water-soluble alkali lignin was used in these studies, since theontinuous processing of insoluble organosolv lignin was ham-ered by the formation of lignin coagulates in the feed vessel andignin deposition in the reactor. As for the Cellulose, hydrogeneroxide was used as the oxidant. It was directly introduced tohe feed vessel and delivered to the reactor together with lignin.ime in the reaction zone was between 5 to 17 s for all experi-ents. The conversion progress and the deduced rate constant,

onsidering a 1st order gasification of alkali lignin on a carbonasis is shown in Fig. 12.

It can be concluded that the stoichiometric and over-toichiometric oxidant supply leads to an almost completexidation of lignin in less than 20 s. About 10% of the influent

ig. 12. Conversion of alkali lignin to gaseous species on a carbon basis, influ-nce of amount of oxidant and temperature, initial lignin concentration: 1 wt.%.

hlbtalpe

Fh

l Fluids 46 (2008) 299–321

arbon remains in the liquid effluents as DOC at temperaturesp to 390 ◦C.

Experiments on the hydrolysis of lignin were conductedith organosolv lignin, which is essentially water insoluble at

mbient conditions. The results show that organosolv ligninompletely dissolves in near-critical water and undergoes sig-ificant chemical modification.

The results discussed in literature and own experiments pointo the conclusion that a complete liquefaction of isolated ligniny non-catalyzed hydrolysis in pure high-temperature water isot feasible. At lower temperatures in the well-subcritical region,here the cleavage of the most widespread linkage, the �-O-4-ond, by hydrolysis should yield a high degree of degradation,he portion of insoluble residues is still in the range of 30–40%.he formation of insoluble residues cannot be prevented evennder optimized conditions. Furthermore, the treatment in pureater does not lead to the selective production of valuable com-ounds but to a broad product spectrum. Treatment of ligninn high temperature water led to the build-up of carbon-richesidues, which eventually resulted in reactor clogging and pre-ented the attempted continuous processing. Summing up thesespects, the non-catalyzed hydrolysis of lignin in pure wateroes not appear to be a promising approach.

This finding was derived from mass spectroscopy analysesf soluble reaction products, which are illustrated exemplarilyy the total ion chromatogram in Fig. 13.

The chromatogram shows the existence of numerous degra-ation products in considerable concentrations. Although someajor classes of degradation products could be identified, e.g.

henols, phenol derivatives and other aromatic substances, aomplete identification and quantification of all reaction prod-cts is virtually impossible.

Comparing the results of ESA biomass with those from thetudies on lignin (and wheat straw), it can be seen that theydrolysis of the model waste yields much higher degrees ofiquefaction than in case of lignin and wheat straw, which cane attributed to the much lower content of ligno-cellulose inhe model waste. Some ingredients, e.g. algae, do not haveny ligno-cellulosic structures at all. This repeatedly shows that

igno-cellulose is the most persistent fraction of biomass andoints towards the potential of algae as a feedstock for bio-thanol production.

ig. 13. Total ion chromatogram of products from Lignin degradation afterydrolysis, T = 417 ◦C, P = 25 MPa, t = 29 s.

C. Schacht et al. / J. of Supercritica

Table 3Composition of model waste and respective suppliers

Fraction (mass%) Fraction (mass%)

Glucosea 28.2 Sugara 39.9Xylosea 5.5 Lignina 13Arabinosea 4.7 Ashb 18Galactosea 1.1 Proteinb 16Mannosea 0.3 Waterc 5.2Ramnosea 0.1 Sum 92.1

6

w3otm

beeps1aa

isb

6

lov([r

d

lwai

6

tt

aimct

topftocp

tisa

aipssdt

aaTrdegradation of arabinose in comparison with xylose/galactose.And yet, a decrease of the maximal arabinose monomer concen-tration at higher temperatures cannot be detected. This indicatessignificantly slower (degradation) reactions. This assumption is

a Analysis of the Bundesforschungsanstalt furForst-und Holzwirtschaft.b Analysis report Euryza GmbH (10.06.2004).c Inhouse analysis.

. Application: hydrolysis of rice bran

Due to its low lignin content compared to other biologicalaste products, rice bran, milled and defatted (27% cellulose,7% hemi-cellulose, 5% lignin), is an interesting representativef the group of ligno-cellulosic biomass [20]. Rice bran produc-ion is in the range of 40 megatonnes/year. Defatted rice bran is

ainly used for cattle feed.Rice bran was obtained from a rice mill (Euryza Ham-

urg GmbH). Non-polar substances like fat and vitamins wherextracted in a supercritical fluid extraction with CO2 [74]. Tonsure delivery of a water rice bran suspension by a membraneump (LEWA EL1), the residue was ground to a particle sizemaller than 500 �m (Ika MF10 basic) and passed through a80 �m sieve. Finally, the composition was determined (Table 3)nd the rice bran was stored at −18 ◦C until usage. The mannosend ramnose contents are neglected in the following.

Hot pressurized water treatment was investigated for rice brann order to evaluate the formation of fermentable mono-sugars orugar-oligomers which are easily transformed to mono-sugarsy enzymes.

.1. Influence of hot water treatment on pH-value

During hydrolysis of hemi-cellulose and cellulose, as well asignin, organic acid may be formed. This influences the pH-valuef the treated product solution or suspension. In general, the pH-alue decreases with increasing temperature and residence timeFig. 14), in accordance with results published by van Walsum29], and can be explained by the progress of the hydrolysiseactions.

The dissolved carbon dioxide reduces the pH-value due to itsissociation and in this way catalyses the hydrolysis reaction.

In addition, it can be noticed that increasing residence timesead to smaller deviations between the pH of the products treatedith pure water and the products treated with a mixture of water

nd CO2. This indicates that the influence of CO2 decreases withncreasing process severity.

.2. Formation of sugar monomers

Sugar monomers, directly fermentable to ethanol seem to behe main goal of hydrolysis. The higher the monomer concen-rations after the pre-treatment, the more fermentable sugars are

Fro

l Fluids 46 (2008) 299–321 311

lready present in the suspension. Hereby, it should be takennto account that in a real substrate part of the carbohydrates

ay be already monomer sugars. For rice bran, 225 mg/L glu-ose, 290 mg/L xylose and 135 mg/L arabinose are already inhe feed solution.

During the pre-treatment the dissolved oligomers will reacto monomeric sugars, which can further degrade dependingn temperature and residence time. Thus, the sugars can berotected from decomposition by keeping them in oligomericorm. It should therefore be the target to minimize the reactionowards the monomeric sugars and maximize the dissolution ofligomeric sugars. Taking a closer look at the sugar monomeroncentration will give an idea of the influence of the processarameters on the degradation behavior.

If a fermentative saccharification is carried out after the pre-reatment, oligomers, are a target product as they readily reactn the presence of enzymes. Even the changes in structure of theubstrate, enabling enzymes to access the polymer molecules,re advantageous.

In the course of the treatment, the xylose/galactose as wells the glucose monomer fractions decrease rapidly with increas-ng pre-treatment time. Increasing temperature accelerates thisrocess. Obviously, these monomeric sugars undergo decompo-ition reactions, consuming the present amount of monomericugars. A concentration raise can not be seen, indicating that theecomposition reaction is very fast compared to the reactionsowards the monomeric sugars.

In contrary, the arabinose monomer concentrations increases a function of the severity parameter R0. Arabinose is prefer-bly found in the side chains of the hemi-cellulose [10].herefore, the accessibility of arabinose towards hydrolysation

eactions is increased and one would expect fast release and

ig. 14. Dependency of pH-value of hydrolysis product on temperature andesidence time (a) P = 75 bar and (b) liquid solution saturated with CO2 (P seeriginal table).

3 critical Fluids 46 (2008) 299–321

cdttcp

fcima

cbp

6p

mootdd

t(1iw

6

co

rcuNwa

brTmro

a

n

F(

tdegradation products. In order to produce oligomers, which canbe treated by enzymes, the reaction rate constants should be keptlow. The lowest values are obtained at 200 ◦C. The addition ofCO2 significantly enhances the ratio of k1/k2 . Secondly, Fig. 22

Fig. 16. Glucose concentration during hydrolysis; influence of carbon dioxide;P = 75 bar without CO2, P: saturation pressure (see original table).

12 C. Schacht et al. / J. of Super

onfirmed by Garrett and Dvorchik [75], who found that the acidegradation of xylose proceeds 5.4 times faster than the degrada-ion of arabinose. This observation adds significant importanceowards the recovery of arabinose during pre-treatment pro-esses and further utilization of arabinose after the pre-treatmentrocesses.

The effect of CO2 is definitely smaller for arabinose thanor xylose and glucose. Arabinose is bound mainly in the side-hains, from where it can be easily contacted and transformednto the monomer. Xylose and glucose are mainly fixed in the

ain chain of the polymer und are therefore more difficult toccess and react slower.

By plotting the monomer concentration (xylose) versus theoefficient log(R0), the experimental data can be representedy one linear line, which proves once more R0 to be a usefularameter.

.3. Concentration of oligomers after treatment with hotressurized water

A substantial part of the polymers is not transformed toonomers but to oligomers. According to literature, up to 70%

f dissolved sugars may be oligomers [51]. The quantity ofligomers was determined by difference from the amount ofotal sugars and monomer sugars, which could be determinedirectly. Due to that procedure, the values are uncertain andefinitely somewhat too high.

From the data can be concluded that there is an optimum forhe thermal hydrolysis, which is in the range of log(R0) = 4.0–4.5230 ◦C, 2–5 min) for xylose and arabinose, and at 170–200 ◦C,–2 min for the glucose (due to the original content of starchn the rice bran). The influence of CO2 is in the range of 10%,hich is non-significant for these type of experiments.

.4. Monomers and oligomers: kinetic modelling

The reactions are modeled in adaptation of the proposition inhapter 4.2.1 as a sequential reaction, assuming two reactionsf first order according Eqs. (1) and (2).

For the definition of the parameters k1 and k2, a non-linearegression has been executed. The sum of deviations of thealculated concentration results from five experimental val-es for each temperature program has been minimized via theewton-Approach. The model according Eq. (1) was insufficienthen focusing on Glucose conversions, therefore the extension

ccording Eq. (2) has been chosen.As the substrate type was identical, the adapting factor a must

e kept constant. Due to this, the average value of calculatedesults for the fast-reacting fraction has been defined and fixed.hen, the regression has been repeated for obtaining the opti-ized values k1 and k2. Secondly, the error R2 of the linear

egression for the logarithmic approach of the reaction constantn the inverse temperature scale has been minimized.

The result of the modeling is presented in the following tablesnd figures (Figs. 15–21).

The reaction rate of the degradation is about one order of mag-itude higher than the rate for the formation of monomers. From

FP

ig. 15. Xylose concentration during hydrolysis; influence of carbon dioxidea) P = 75 bar without CO2 and (b) P: saturation pressure (see original table).

hat can be concluded that mono-sugars are rapidly reacting to

ig. 17. Arabinose concentration during hydrolysis; influence of carbon dioxide;= 75 bar without CO2, P: saturation pressure (see original table).

C. Schacht et al. / J. of Supercritical Fluids 46 (2008) 299–321 313

sw

hsmot

atct

Fig. 18. Concentration of xylose in dependency of the coefficient R0.

hows that the addition of CO2 increases k2 only marginally,hile k1 is enhanced.The concentration dependence of the oligomers shows the

ydrolysis of hemi-cellulose to soluble oligomers and the

equential reaction to xylose monomers. The first reaction step isuch faster than the formation of monomers. The reaction from

ligomers to monomers is the rate determining step. A reac-ion temperature of 230 ◦C seems to be the optimum. If CO2 is

Fig. 19. Xylose monomers and oligomers: total concentration.

Fig. 20. Glucose: monomers and oligomers: total concentration .

c

cα

T

et

dttiot(i

Fx

Fig. 21. Arabinose: monomers and oligomers: total concentration .

dded, the reaction rates seem to be slowed down, contradictinghe findings for the monomers. This can be explained by the con-entration dependence of the reaction rate on CO2 which makeshe model not applicable. Therefore, the rate constants for bothases can not be compared.

In the following, the same modeling is carried out for glu-ose. The fraction α for the fast reacting part was determined to= 0.588. Results are shown in the diagrams of Figs. 24 and 25.hey are basically similar to that obtained for xylose.

The significant information which can be concluded from thexperiments mentioned above is the optimum temperature forhe controlled hemicellulose and cellulose conversion.

Hemicellulose converts faster to xylose oligomers than theegradation of the oligomers to monomers. The degradation ofhe monomers, however, is considerably faster than its produc-ion. Consequently, the second oligomer hydrolysis reaction steps defining the overall reaction rate. In order to optimize the yield

f the total process, it is considered useful to avoid this hydrolysisowards the monomers. In consequence moderate temperaturesbelow 260 ◦C) shall be applied. The existing oligomers remainn solution, and can be easily converted to monomeric sugars

ig. 22. Reaction rates k1 for modeling the reaction of xylose oligomers toylose monomer and k2 for the degradation of xylose.

314 C. Schacht et al. / J. of Supercritical Fluids 46 (2008) 299–321

Fx

id

mt

lv(oqf

copp

6

src4

oo

mli(

trit

F(

dii

The conclusion is straightforward: Hot pressurized waterhydrolysis for itself – even when catalyzed with dissolved carbondioxide – is not sufficient to produce mono-sugars for ethanol

Fig. 25. Glucose: reaction rate constants for the reaction of cellulose to glucoseoligomers (k1) and the reaction of glucose oligomers to glucose monomer (k2).

ig. 23. Reaction rate constants k1 and k2 for the reaction of hemicellulose toylose oligomers (k1) and the hydrolytic reaction to xylose (k2).

n a subsequent enzymatic hydrolysis step without additionalegradation losses.

On the other hand, the conversion of hemicellulose to xyloseonomer shall be enhanced, this will lead to an optimum reac-

ion temperature of about 230 ◦C.The addition of carbon dioxide for the treatment of hemicel-

ulose (Fig. 23) will lead to a decrease of both process reactionelocities. This finding contradicts to the observation of Fig. 22xylose oligomer conversion). One may deduce a direct influencef the carbon dioxide saturation on the reaction rate. Conse-uently, the reaction rate constants are not directly comparableor the application with and without carbon dioxide saturation.

Analogous observations can be found for the hydrolysis ofellulose. In the same way, a non-desired – slow-conversion ofligomers to monomers and direct – fast-degradation to sideroducts shall be avoided by the application of moderate tem-eratures.

.5. Formation of degradation products

The hydrolysis of hemi-cellulose and cellulose to mono-ugars may yield degradation products, which are at leasteducing the yield of fermentable sugars. HMF and furfuralould not be detected in the product flow (limit of detection:0 mg/L).

Temperature has a dominant influence on the formation ofrganic acids. The addition of CO2 inhibits the formation ofrganic acids, as has also been reported by Walsum [29].

For an ethanol process, the total concentration of sugars is ofajor important, since an enzymatic saccharification will fol-

ow the thermal hydrolysis. The total concentration of sugarss shown in the following diagrams, including the role of CO2Figs. 24 and 25).

The picture of degradation products changes if residence

ime is increased substantially. Rice bran was treated in a batcheactor from 20 min to 60 min. The degree of liquefaction isncreasing even at low temperatures (160–180 ◦C), but in addi-ion to the formation and degradation of mono-sugars, several

Fi

ig. 24. Glucose: reaction rate constants for the reaction of oligomers to glucosek1) and the reaction of glucose to degradation products (k2).

egradation products in a relatively high concentration, includ-ng HMF and furfural, were detected after the reaction as shownn Figs. 26 and 27 [20].

ig. 26. Formation of acetic acid during treatment of rice bran with hot pressur-zed water.

C. Schacht et al. / J. of Supercritical Fluids 46 (2008) 299–321 315

Fs

fyfatspssb

6

rie

icgc

c4tot(

ea[eswT(p

F2

1xCO2, glucose concentrations remain constant in the range of1000 mg/L and 1500 mg/L. The reason could be found in is thereduced temperature dependence, when CO2 is used, at highervalues for the temperature.

ig. 27. Formation of formic acid during treatment of rice bran with hot pres-urized water.

ermentation, since the formation of by-products reduces theield. On the other hand, for hemi-cellulose and in particularor cellulose, no really satisfying biological methods are avail-ble which are really competitive to a hot water treatment. Fromhat follows, that only a combination of both methods will beuccessful. In concurrence, what is already carried out, is are-treatment with steam at relatively low temperatures and pres-ures, which does not lead to an optimum use of the enzymes foraccharification. For the combination, the best conditions muste found.

.6. Enzymatic hydrolysis

Preliminary experiments have been carried out and have beeneported by Baig et al. [20]. Further work is going on involv-ng experts from microbiology, who involve hyper-thermopilenzymes for saccharification.

The table in Fig. 29 confirms also the fact that thesolated hydrolysis is efficient for xylose conversion from hemi-ellulose, but glucose yield is still quite poor. The pre-treatmentives favorable conditions for the subsequent enzyme-catalyzedonversion of cellulose (Figs. 28–32).

The addition of enzymes (commercial, Depol 692, max.ellulase-activity of 800 U/g at 68 ◦C and pH 5; at a pH-value ofor 6, activity is reduced to 10% of the maximum value) to a non

reated rice bran suspension resulted in a glucose concentrationf 730 mg/L. In pre-treated suspensions of rice bran the enzymereatment resulted in glucose concentrations up to 2100 mg/LFig. 30).

Glucose concentration after combined pre-treatment andnzyme-catalyzed hydrolysis decreases with increasing temper-ture during pre-treatment, as has been reported by other groups27]. This may be due to reduced enzyme activity caused bynhanced lignin concentrations in solution. The amount of dis-olved lignin increases with temperature and residence time,

hich leads to reduced yield of the enzymatic hydrolysis [53,57].he maximum concentration of 2100 mg/L is obtained at log

R0) of 3.5 to 4, which corresponds to a temperature for there-treatment of 200 ◦C and 5 min residence time, or 230 ◦C at

Fy

ig. 28. (a) Hot water rice bran liquefaction in a piston reactor at a pressure of00 bar and (b) production of side products in function of time.

to 2 min residence time. This is about the same values as forylose and arabinose. If the feed suspension is saturated with

ig. 29. Influence of hot water hydrolysis on the liquefaction of rice bran andield of xylose and glucose.

316 C. Schacht et al. / J. of Supercritica

7

twtcmtsh

cdt

oo

tiVaovecittCb

der

Fig. 30. Glucose: concentration after enzymatic hydrolysis.

. Fermentation to ethanol and separation with CO2

Fermentation of the resulting mono-sugars to ethanol is aechnique which is known for a long time and usually carried outith strains of bakers yeast (Saccharomyces cerevisiae). In order

o obtain ethanol from the sugar solution of about 100–200 g/L,ontaining C5- and C6-sugars, still a satisfying consortium of

icro-organisms has to be developed. This is outside of the

opic covered here, therefore, we assume that an aqueous ethanololution of 7 to 10% is available after fermentation, which nowas to be concentrated to ≥99.8 wt.% ethanol. Sakaki et al. [76]

mfaa

Fig. 31. The system EtOH–H2O–CO2 at (a) 100 bar,

l Fluids 46 (2008) 299–321

ontributed to the description of the fermentability of celluloseecomposition products resulting from a near-critical hydrolysisreatment.

SFE-technology can provide an alternative for the separationf CO2 solution, reducing the number of columns and applyingnly separating agents inherent to the process [77,78].

In the past decade, numerous studies have been performedo describe the separation efficiency of the system carbon diox-de/ethanol/water at elevated pressures and temperature [79].apor–liquid equilibrium (VLE) data of CO2 + ethanol + waternd its binary mixtures have been published in a wide rangef temperature and pressure. From an economic point ofiew, gas extraction of ethanol + water mixtures should yieldthanol of high purity to compete with conventional pro-esses. However, in order to get a high solubility of ethanoln the vapor phase, many studies were carried out at condi-ions of complete miscibility of ethanol and CO2 [80–82]. Athese conditions, the phase behavior of the ternary mixtureO2 + ethanol + water is of type I and anhydrous ethanol cannote produced.

One of the first studies reporting the possibility to pro-uce anhydrous ethanol by means of CO2 without adding anyntrainer was by Nagahama et al. [83]. Experiments were car-ied out at conditions of type II phase behavior. Further VLE

easurements of the mixture CO2 + ethanol + water were per-

ormed at Kobe Steel Ltd. in Japan (e.g. Furuta et al., [84,85])t 10.1 MPa and 313 K, 323 K and 333 K. The separation factort 100 bar, 60 ◦C, decreased from around 30 at infinite dilution

60 ◦C (type II) and (b) 140 bar, 60 ◦C (type I) .

critica

ow

a6hopisei

attawp

rea

7e

cre

Pr

FE9

iartm

s3

cs5fs

tfarsu

pasd

7

7

C. Schacht et al. / J. of Super

f ethanol in water to approximately 1.25 at infinite dilution ofater in ethanol.Data have been concluded and transformed to Fig. 31.The Ponchon-Savarit diagrams Fig. 31 can be interpreted

s follows: at a pressure of 100 bar and a temperature of0 ◦C, the phase equilibrium between the light phase and theeavy liquid phase exists up to absolute purity. The con-des which are connecting the referring liquid and gaseoushase show a high slope. This indicates immediately that anmportant number of equilibrium plates in a counter-currentupercritical fluid extraction column is necessary when the purethanol extract shall be produced from low concentrated feednput.

At a pressure of 140 bar, the phase equilibrium line closes atmaximum ethanol concentration of about 90%, which means

hat this pressure is not adapted for the absolute ethanol produc-ion. However, the difference in extract and raffinate densityllows to establish a stripping system, which separates pureater from the feed solution with a low number of equilibriumlates.

Consequently, process design may be optimized when car-ying out the supercritical extraction in two pressure steps: thenrichment section (7% → 100% EtOH) at 100 bar, 60 ◦C, andstripping section (7% → 0% EtOH) at 160 bar and 60 ◦C.

.1. Estimation of the necessary number of theoreticalquilibrium plates

Numerical modeling tools or graphical methods allow to cal-ulate the number of equilibrium plates for a required product oraffinate purity, provided a reflux ratio and the number minimum

quilibrium plates has been defined.

The number of theoretical stages were calculated by theonchon-Savarit method [88]. Fig. 32 illustrates calculatedesults that are based on a feed mixture of 10 wt.% ethanol that

ig. 32. Calculation of the number of theoretical plates for separation oftOH–H2O: CC-SFE: 100 bar, 60 ◦C, EtOH concentrations: feed 7%, extract9.9%, raffinate 0.1%.

btod

l Fluids 46 (2008) 299–321 317

s separated into an ethanol-rich extract (99.9 wt.% ethanol) andwater-rich raffinate (0.1 wt.% ethanol). The minimum reflux

atio for this separation task was calculated to be 10, equalo a minimum solvent-to-feed ratio of around 2.5, while the

inimum number of stages is 35–40.Ethanol separation from a feed mixture with 10 wt.% ethanol

hould be carried out at solvent-to-feed ratios between 20 and0.

Solvent-to-feed ratios are relatively small compared to otherounter-current gas extraction processes. This is due to largeeparation factors and a solubility of pure ethanol in CO2 ofwt.% at the conditions investigated. Instead of the solvent-to-

eed ratio, the ratio of solvent flow rate to product flow ratehould be used as an indicator for operating costs.

If the extract is the only product, as in ethanol purification,he solvent-to-extract ratio becomes 300 kg/kg at a solvent-to-eed ratio of just 30 kg/kg. The influence of feed compositionnd the number of theoretical stages on the solvent-to-extractatio reveals that the production of pure ethanol by means ofupercritical CO2 needs the highest solvent-to-extract ratio whensing a feed with less than 20 wt.% ethanol [74].

Ethanol solubility below 0.2 wt.% can only be achieved atressures close to the vapor pressure of liquid CO2. Largemounts of CO2 dissolve in the ethanol-rich phase. Therefore,eparation of ethanol and CO2 has to be optimized by solventistillation.

.2. Experimental reliability tests

.2.1. Counter current SFELab scale experiments on ethanol/water purification have

een performed to confirm the numerical estimation and to provehe feasibility of enriching ethanol above the azeotrope point. Inur work, the experimental unit and the procedure is given inetail by Budich et al. [77] (Fig. 33).

Fig. 33. Draft of the lab scale CC-SFE unit.

318 C. Schacht et al. / J. of Supercritical Fluids 46 (2008) 299–321

Table 4Experimental results for purification of ethanol from aqueous solutions by countercurrent extraction. Extractor: temperature 60 ◦C; pressure: 100 bar; separator:temperature: 50–60 ◦C; pressure: 50–60 bar

Expt. no. Wt.% ethanol in feed Feed flow rate (kg/h) CO2 flow rate (kg/h) Solvent to feed ratio Wt.% ethanol in extract Wt.% ethanol in raff.

1. 70 0.36 2 5.5 99.18 69.82234

o2S

eaF

cl

7

pe(9Hcta

gpfwab

FP

tCbpdosmgsr

7

SpPtcASe

. 80 0.36 3.6

. 80 0.36 4.32

. 80 0.36 5.76

Counter-current multistage extraction was carried out in lab-ratory extraction columns of 6 m total height (17.5 mm and5 mm i.d., equipped with 6 m and 2 m × 2 m, respectively ofulzer EX packing) [75].

Loaded solvent was withdrawn from the top. Solvent andxtract were separated by pressure reduction down to 5 MPa. Inddition, solvent distillation was established as proposed by Deilippi and Vivian [86] and reported by Ikawa et al. [87].

Experimental results for a purification of ethanol with aounter-current extraction column are summarized in the fol-owing table (Table 4).

.2.2. HydrodynamicsMass transfer is of some concern in the process of ethanol

urification. During experiments with feed mixtures of lowthanol content, the height equivalent to one theoretical stageHETS) was found to be in the range of 1 m. When a feed with4 wt.% ethanol was used, HETS was in the range of 0.33 m.ETS values are reported in the literature [77]. From these data

an be concluded that there are two different regimes of massransfer, the one in the mostly aqueous region with high HETS,nd the one in the ethanol region with low HETS.

Determination of the capacity of a counter-currently operatedravity driven column requires information about the floodingoint. Budich et al. [77] have measured fluid dynamic capacities

or the system ethanol–water–CO2 in a counter-current columnith Sulzer EX packing. Data of flooding point measurements

re shown in the Fig. 34. A general correlation has been reportedy Stockfleth and Brunner [89] (Fig. 34).

ig. 34. Flooding point diagram of the system EtOH–water–CO2 for Sulzer EXackings, parameter: concentration of EtOH in water.

7

c1ita

TSu

FSERRPM

PM

P

10 99.62 74.0412 98.65 70.2216 99.68 59.75

Maximum cross-section capacity was found to be a func-ion of the ethanol content of the solvent-free liquid phase.hanges in flooding behavior at high ethanol concentrationsetween 100 wt.% and 70 wt.% are mainly due to the influence ofhase composition on the density of the phases. With a furtherecrease in ethanol content of the liquid phase, the influencef viscosity and surface tension of the liquid phase becomesignificant. Therefore, for the removal of ethanol from the fer-entation broth down to low concentrations, the counter-current

ravity column seems not to be sufficient. Instead, a mixer-ettler was experimentally applied for that task, with excellentesults.

.2.3. Mixer settler SFEThe mixer settler unit (Fig. 35) has been described by

chaffner et al. [90]. Experiments in the high-pressure pilotlant have been performed with a feed input of 5% EtOH.rocess conditions were 140 bar and 60 ◦C. Although simula-

ion suggested the necessity of only 2 mixing plates, due toonstructive reasons, the apparatus was operating with 5 cells.

pure water raffinate output (99.6% water) was achieved at/F = 74, mixer-settler extract contains consequently 20–30%thanol.

.3. Design

With the information given above, it may directly be con-luded that a “conventional” counter-current SFE, operating at

00 bar and 60 ◦C may lead to unrealistic diameter design inndustrial scale (Table 5). Even a CC-SFE which operates withwo pressure steps will still require considerable investment inutoclave volume:

able 5cale up of the ethanol purification process with a single countercurrent SFEnit

eed 1 t/holvent 3 t/hxtract 70 kg/h 99.95% wt EtOHeflux 700 kg/haffinate 930 kg/h 0.5% wt EtOHrocess conditions stripping section 160 bar, 60 ◦Cin diameter stripping section D = 1.6 m, h = 1 m,

vol = 2 m3

rocess conditions enriching section 100 bar, 60 ◦Cin diameter enriching section D = 0.35 m h = 22 m,

vol = 2.2 m3

acking: Sulzer EX

C. Schacht et al. / J. of Supercritical Fluids 46 (2008) 299–321 319

ab sca

fC

(

(

(

mst2c

eas

mie

8

ctbseh

bcfmfi

A

b

oD(

R

Fig. 35. Draft of the l

Alternatively the production of high purity ethanol (≥99.5%)rom fermentation broth (7–10% ethanol) by using supercriticalO2 is suggested in the following steps:

1) Feed of 7–10% ethanol is fed to a counter current mixer set-tler unit with 2–3 stages, operating at 150 bar, 60 ◦C. Ethanolis enriched to about 75–80%.

2) The extract from the mixer-settler unit is fed to counter cur-rent packed column operating at 100 bar and 60 ◦C. In thiscolumn, ethanol is concentrated to ≥99.5% purity.

3) The raffinate from this packed column contains 60–70%ethanol. It is fed to the mixer settler unit as reflux.

The design of the process plant was performed for the treat-ent of 4000 t/year (resp. 1 t/h) of a feed input of a 7% ethanol

olution. Operating time is 4000 h/year, since for this design aypical sugar cane processing plant in India with a capacity of5,000 t/year was used as basis. Hence the distilleries operate atomparable capacities.

Investment cost for the CC-SFE and mixer settler can bestimated according delValle et al. [91]. Extraction column hasn inner diameter of 0.3 m, the autoclave volume of the mixerettler is negligible.

Side product carbon dioxide from fermentation (approxi-ately 34 kg/h) is equivalent to about 0.5% of the CO2 cycle,

t may be used to replace the mass losses of the solvent at thextract and raffinate outlet.

. Conclusion

Fuel ethanol production from ligno-cellulosic material is notommercial so far. There is still a wide range of improvementso be achieved before such a process is competitive to mineral oil

ased fuels. SFE technology can contribute at several processteps advantageously. Several aspects could not be discussed,.g. the role of SFE in lignin processing or the role of SCWO inandling the residues. But it became obvious that only a com-

[

le mixer-settler unit.

ination of technologies will be able to achieve the goal. In thisase it is biotechnology and high-pressure technology. There-ore, the cooperation of experts in these fields seems to be theost efficient way to overcome certain difficulties within thiseld.

cknowledgements

Numerous co-workers and students contributed to the dataase of this work, as can be seen from the list of references.

In addition, several organisations and companies contributedver the years: Deutsche Forschungsgemeinschaft (DFG),eutsche Bundesstiftung Umwelt (DBU), DSM Vitamins Ltd.

former F. Hoffmann-La Roche AG).

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