Supercritical Water Possibilities

J. of Supercritical Fluids 47 (2009) 373381

Contents lists available at ScienceDirect

The Journal of Supercritical Fluids

journa l homepage: www.e lsev ier .com

Review

Near c roproces

G. BrunnInstitute for Th

a r t i c l

Article history:Received 21 JuReceived in revAccepted 9 Sep

Keywords:Supercritical wHydrolysisHydrothermalBiomassGasication

with operating conditions have so far limited the large scale implementations. 2008 Elsevier B.V. All rights reserved.

Contents

1. Gener2. Prope3. Sub-c4. Water

4.1.

4.2.5. Hydro6. Concl

Refere

1. General

Interestwater, start

Tel.: +49 4E-mail add

0896-8446/$ doi:10.1016/j.sal introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 373rties of water . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 374ritical water for extraction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 375for hydrolysis. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 375Water in bio-fuel processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3754.1.1. Biomass. Total liquefaction or gasication . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3754.1.2. Biomass compounds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3764.1.3. Starch . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3764.1.4. Cellulose . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3764.1.5. Sugars, glucose . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3774.1.6. Lignin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3774.1.7. Hydrolysis and fermentation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 377Proteins, amino acids . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 377lysis and hydrothermal reactions in sub- and supercritical water (no oxidative reagent) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 378usions and future development. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 380nces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 380

introduction

in the application of hot, pressurized and supercriticaled in the late 70s of last century, when it was spurred

0428783240.ress: [email protected].

by the rst oil crisis, by environmental concerns, and the investiga-tion of supercritical uids. Before that, interest was concentratedon the properties of high-pressure steam for power plant cycles,on physicalchemical properties of water, and on hydrothermalreactions. Renewed interest was concentrated on alternative fuels,on coal and biomass conversion, and waste disposal. The uniqueproperties of hot and supercritical water led to the investigation ofspecic reactions for production of chemicals, of the formation of

see front matter 2008 Elsevier B.V. All rights reserved.upu.2008.09.002er

ermal and Separation Processes, Hamburg University of Technology, Eissendorfer Str. 38, D-21073 Hamburg, Germany

e i n f o

ly 2008ised form 9 September 2008tember 2008

ater

a b s t r a c t

The potential of hot and supercritical water in applications to produce useful products, or to processunwanted compounds into environmentally compatible materials is reviewed. The potential of hot andsupercritical water is high. Water changes its character from a solvent for ionic species at ambient condi-tions to a solvent for non-ionic species at supercritical conditions. Water at temperatures higher thanambient boiling temperature can be applied for extraction. At modest temperatures, ionic and polarspecies will be extracted. At higher temperatures, in particular approaching the critical temperature,nonpolar substances are readily dissolved and extracted. Hot pressurized water has a high reactivity. Thereactions are commonly summarized as hydrolysis reactions which are catalyzed by acids, or may arisefrom simply hydrothermal transformations. Since CO2, dissolved in water increases the availability ofprotons, the addition of CO2 to liquid water catalyses hydrolysis reactions. Hydrolysis of natural plantmaterials provides a route to obtain fuel from non-food plant material. However, difculties associated/ locate /supf lu

lytic and hydrothermalritical and supercritical water. Part I. Hydses

374 G. Brunner / J. of Supercritical Fluids 47 (2009) 373381

particles, and more. The potential of hot and supercritical wateris high and led to extensive research. Operating conditions andproperties of water and species involved cause problems whichare difcult to manage in process equipment which so far led toonly limited practical applications. In the following, after a shortintroduction to the properties of water, a review of research effortsas reected in the publications of the Journal of Supercritical Flu-ids (JSF), complemented with the personal view of the author ispresented.

2. Properties of water

The specic properties of water for hydrolysis, hydrothermal,and oxidative processes were pointed out convincingly by M. Mod-ell on several occasions from the 70s of last century on. Water fromambient to supercritical conditions changes its character from asolvent for ionic species to a solvent for non-ionic species. Electro-chemical properties, e.g. dipole moment decreases from the highvalue at ambient conditions, but water in the critical region isstill as polar as acetone. The pH-value decreases by 3 units, pro-viding much more hydronium ions for acid catalyzed reactions.Just below critical temperature, the ionic product changes tremen-dously, rendering near critical and supercritical water a much lesspolar compound than ambient water. Reactivity of water increasesin the neighcatalyst.

It is useSiO2 in waested in hydin Fig. 1. SiOchanges treature. At relsolubility ddrawbacksstream willalso be seesolubility isnot been ap

Amongnear criticaerties of waand gases. Pgas; density

strongly with slight changes in pressure and temperature. Viscos-ity is of the order of a normal gas and the diffusion coefcient is atleast one order of magnitude higher than that of a liquid. Solubilityof water for gases is high in the critical region. At near critical andsupercritical conditions water and gases like O2, N2, NH3, CO, CO2,are completely miscible. Solvent power of water decreases for inor-ganic compounds in the critical region. It is drastically reduced inthe region of about 450 C. Organic compounds, on the other hand,are readily dissolved by water in the near critical and supercriticalregionup to totalmiscibility. But itmustbeborne inmind that thesestatements hold for binary or quasi-binary systems. Phase equilib-rium of ternary and multi-component systems of water, organiccompounds, inorganic salts and common gases may deviate frombinary behavior. Properties of water have been investigated inten-sively for a long time. In our context it may be of interest to lookat phase equilibrium of water and salt components. A small reviewhas been presented at the 3rd International Symposium on Super-critical Fluids in Strasbourg 1994 [2]. Phase behavior and criticalphenomena for binary mixtures and ternary mixtures of hydro-carbons with water in the critical region have been measured inparticular by Brunner et al. [3], conrming essentially the com-plete miscibility of water and hydrocarbons in the critical region,but exhibiting an interesting phase behavior in detail (Fig. 2).

Thepropertiesofpurewater canbemodelledwithhighaccuracymodynamicmodels derived fromthevirial equationof state.omprise quite a number of constants and may be impracti-many purposes. Therefore, work has been published on theling of such properties with simpler equations of state suchAnderko-Pitzer EoS modied for easier access to parame-calculate thermodynamic properties of systems involved in

Left):watersupeborhood of the critical point without as well as with a

ful to look at experimental results for the solubility ofter, as determined by mineralogists, who were inter-rothermal reactions. This solubility behavior is shown2 is dissolved by water to some extent. This solubility

mendously in the neighborhood of the critical temper-atively low pressures, with increasing temperature, therops to practically zero. This effect is one of the majorof processing waste, since salts dissolved in the feedprecipitate and eventually block the reactor. But it cann, that at relatively moderate and high pressures, theconstant and is even increasing. So far, this aspect hasplied to waste treatment.the basic knowledge necessary for a process applyingl or supercritical water is the physicalchemical prop-ter and its solvent power for organic substances, salts,, v, T and transport properties are that of a supercriticalis relatively high, up to liquid-like densities, but varies

Fig. 1. Solubility of SiO2 in water [1].

by therThey ccal formodelas theters, to

Fig. 2. (rials) insoil with[14].Solubility of a real contamination (extracted from various soil mate-. (Right): Course of an extraction of hydrocarbon contaminants fromrcritical water. The full line is obtained by modelling the extraction

G. Brunner / J. of Supercritical Fluids 47 (2009) 373381 375

the supercritical water oxidation process [4]. Molecular dynamicssimulations with a simple exible point-charge water model wasapplied to model the critical point and the coexistence curve [5].The local strtigated usinclustering otal conditioof the localhood of thewas investithe methanwater cageconductancmined withcomplete diation to mois availablenear criticaprove tedioof the IAPWWater and S

For invewater, speciinstallationpotentiometions [9], adepositionnear the wachannel-tee[11], and a rformation i

3. Sub-crit

Water ature can bepolar specielar approacbe readily dto be necesstrateby intIn such a wapplied to cmetals [14]

Cleaningeffectivelyin differentextraction atime needethe semi-coextraction.98% degreemust be keping to aboutAlternativelor a multipof heavy metemperaturtion, supercas discussedissue of thefrom plantfor the extrerties of the

[15]. Sub-critical and supercritical water has been also successfullyapplied to cleaning of bone materials from organic compounds.Bones contain lipid and protein compounds, which have to be

d froit cand ps, nedati

ter fo

ater i

presnlywhi

n:

H O

reaces thter caterto o

woodysis rrmalis thns ths onistinhermd hyhat a

iomaass

availte. Fpou

rt, coundsotaled mprodllulopresspartgated9]. ABesidrized].theriomaen rtelys, cotrati(100, correspond-1wt.% of soil material in the aqueous feed suspension.y, a longer tubular reactor (increased residence time)le treatment could be applied. Mixed contaminationstals and hydrocarbons could be cleaned with water ates of 250350 C at a pressure of 25MPa. For destruc-ritical water oxidation proved to be a good alternative,in Part II of the present review appearing also in this

journal. Polar compounds can be extracted effectivelymaterial with hot pressurized water. One application isaction of anthraquinones, where anti-oxidative prop-extract were highest compared to organic extractants

removebeforelipids aproteina degra

4. Wa

4.1. W

Hotcommotions inreactio

A B +The

increasuid waplant ma routestraw,Hydrolby thegreaterreactioReportdom dhydrotysis ansomew

4.1.1. BBiom

vastlyas wascal comThe pacompois the tsecludwastelignoceby hotsolvedinvestiESA [1waste.pressuoil [20

AnoWet bhydrogcomplerich gaconcenuct gasbe neeinorgangasicathan ccentratm the hydroxyapatite, the structural material of bones,n be used for implants. Using hot pressurized water,roteins could be extracted. For total extraction of thear critical and supercritical water in combination withon of the proteins proved necessary [16].

r hydrolysis

n bio-fuel processes

surized water has a high reactivity. The reactions aresummarized as hydrolysis reactions. These are reac-ch a compound is split bywater according to the formal

H A H + B OH. (1)tion is catalyzed by acids. Since CO2, dissolved in water,e availability of protons, the addition of CO2 to liq-

atalyses the hydrolysis reactions. Hydrolysis of naturalials has been intensively investigated, since it providesbtain ethanol from non-food plant materials such asand bagasse, as summarized by the author in [17,18].

eactions occurring in hot water are often accompaniedreactions (pyrolysis). The higher the temperature, thee extent of thermal reactions. These are hydrothermalat are discussed below and also in part II of this review.investigations of the conversion of bio-polymers sel-guish between hydrolytic reactions in hot water andal reactions. Therefore, the classication, into hydrol-drothermal reactions, as chosen here in this article, isrbitrary.

ss. Total liquefaction or gasication, consisting of plant material and animal products, isable and either considered as a valuable resource orrom the part considered as valuable resource, chemi-nds or materials for energy conversion are produced.nsidered as waste can be transformed in to disposableor also in to useful chemical compounds. One examplerecycling of organic materials as needed for long-timeissions, e.g. travel to planet Mars. Most of the biologicaluced can be easily treated by microorganisms, but thesic components cause problems, which can be solvedurized water and eventually, for a small residual undis-, by application of oxidation in supercritical water, asin a project sponsored by the European Space Agency

nother example is the useful transformation of woode just burning it, it is possible to liquefy wood with hotwater and produce a combination of fuel gas and fuel

route is the gasication of biomass and organic wastes.ss and organic wastes can be gasied to produce aich fuel gas. At 600 C and 250bar all compounds aregasied by addition of KOH or K2CO3, forming a H2ntaining CO2 as the main carbon compound, with lowons of CO, CH4 and C2C4 hydrocarbons in the prod-3 and


Fig. 3. Producglucose yield (

a processinto the non-ticular withis a route, woil is theneffective gawith the wcan be prod

4.1.2. BiomaPlant bio

processed bhemi-cellulsugar-polymstarch, monisms or bywhich is of

uation to food. Cellulose and hemi-cellulose, being available fromagricultural sources without competition to food, can be trans-formed to mono-sugars, but so far scientic and technologicaldevelopment has not led to a commercial process, which has beeninstalled. Nevertheless, this will be the future for bio-ethanol pro-duction.

4.1.3. StarchHydrolysis of starch to mono-sugars is an old industrial pro-

cess. The hydrolysis reaction was catalyzed with mineral acids, andis now replaced by a bio-transformation. Microorganisms for thisreaction are selective but slow. Hydrolysis using CO2, dissolved inhot pressurized water, without mineral acids was investigated toevaluate the feasibility of such a process [24]. From corn starch atconcentrations from0.2up to10wt.% in theaqueous feed slurry andpressures of 60240bar, temperatures of 170380 C in a tubularreactor with residence times of about 180 s, high yields of glucose

obtayst. Tkepthydre yie

ellululosmprriousylfurpositresidt thein to

orts,t yields at 230 C and 240bar (left); inuence of carbon dioxide onright). CO2 concentrations are in percentage of saturation.

g facility, the technological problems are multiple duecontrollable composition of the biomass feed, in par-

can bea catalcan beof theGlucos

4.1.4. CCell

hot coand vaymethdecomgases,[26]. Buof 30mall represpect to inorganic contaminations. More promisinghere the biomass is liqueed to an oil-product, the

transported to a central processing facility, where ansication to synthesis gas is carried out, from which,ell known FischerTropsch-process, fuel hydrocarbonsuced [23].

ss compoundsmass consists of a number of compounds which can bey pressurized hot water, like sugars, starch, cellulose,ose, and lignin. Starch, cellulose, and hemi-cellulose areers and can be transformed to sugar-monomers. From

o-sugars are produced straightforward by microorgan-hydrolysis and can be further transformed to ethanol,some concern nowadays because of the competitive sit-

cose and olaqueous proformation oalyst catalyas intermedmethane iswhile the pregion.Asually accompresidence tported by oand residenof gas specigasicationof hot press

Fig. 4. Degree of liquefaction f versus residence timined with dissolved and dissociated carbon dioxide ashere are only few by-products, and their concentrationlow by short residence time. Fig. 3 shows the results

olysis of starch at 230 C and 240bar, for an example.ld is increased from5% to 60% by adding carbon dioxide.

osee, the major component of plant biomass reacts inessed water to oligomer sugars, monomer sugars,degradation products as pyruvaldehyde and hydrox-

fural (HMF), as e.g. reported by Sasaki et al. [25]. Theion of cellulose can lead to a reaction mixture of oil,ue and an aqueous phase, as reported by Minova et al.batch reactor used and the residence times in the orderseveral hours are no basis for a process. According to

hydrolysis can play an important role in forming glu-igomers, which can decompose quickly to non-glucoseducts, oil, char and gases. An alkali catalyst, inhibits thef char from oil and stabilizes the oil, while a nickel cat-zes the steam reforming reaction of aqueous productsiates and the reaction to methane. The formation ofthe preferred reaction pathway at lower temperatures,roduction of hydrogen proceeds in the supercritical

bstantial conversion togaseous species is, however, usu-lished at much higher temperatures (400500 C) andimes in the order of hours. This observation was sup-ur own measurements at temperatures of 350375 Cce times from 20 to 40 s, which yielded a contributiones to the carbon balance of only about 1wt.%. A markedof cellulose can therefore be excluded at the conditionsurized water hydrolysis [19].

e at P=250bar.


The overall degree of conversion (liquefaction) of micro-crystalline cellulose was determined in a plug-ow type reactor.The operating pressure had a negligible inuence on the degreeof liquefaction, but temperature markedly affects the rate of reac-tion as illustrated in Fig. 4. The degree of liquefaction f is the ratioof dissolved carbon in the efuent to the total carbon of the feedsuspension.

ln(1 f ) = k (2)where k is the reaction rate constant of the cellulose hydrolysis.

The rate of liquefaction increases with temperature, leading to acomplete conversion to soluble products in less than 30 s at 310 C.A further increase in temperature results in an even more rapiddegradation, with a complete conversion within seconds.

The addition of carbon dioxide and the resulting decrease inpH leads to an increased rate of liquefaction at lower tempera-ture (250 C) compared to pure water. Glucose yield during thecourse of the reaction is shown in Fig. 5. Rate constants for for-mation of glucose are higher for starch than for cellulose. Glucoseis not stable at reaction conditions and is degraded by consecutivereactions to different products, including pyruvaldehyde, levoglu-cosan, and hydroxymethylfurfural. Reaction kinetics are discussedin a separat

The optiin the rangecompared tyield of glutimes. Arouthan an ordthe glucosecose yield rresidence ti

4.1.5. SugarReaction

tions resultein high-tem5-hydroxymfurfural. DeHMF were400 C [27]water yield(40MPa) coand pyruva

Fig. 5. Glucoseidence times a

enhanced retro-aldol reactions and water related reactions such ashydrolysis and dehydration. Comparison between d-fructose andd-glucose experiments showed that higher yields for 5-HMF wereobtained stof furfuralfructose [28

4.1.6. LigninLignin is

available thapplicationfaction of biby hydrolystreatedbybincreased toobserved dcompoundsreacted witperoxide coOnly aboutphase, with

4.1.7. Hydronhanmbintiond, coin woliges. H% xyentximarabethyatalyiduad itant l

otein

teinshot

ers arolyn atog, fohydrcarbo

rolyprotow rtheepenC. Pon the paper [4].mum glucose yields are obtained with residence timesof seconds to minutes but remain relatively low (e.g.

o the values for hydrolysis of starch). The maximumcose increases with temperature, at shorter residencend the critical point, the hydrolysis rate jumps to moreer of magnitude higher level and becomes faster thanor oligomer decomposition rate [25]. The reported glu-eached a maximum of 50% at 400 C and 25MPa, at ame of 0.00025 s.

s, glucoseof glucose and fructose in water at supercritical condi-d in a number of compounds. Dehydration ofd-glucoseperature water at pressures up to 80MPa yieldedethylfurfural (5-HMF), 1,2,4-benzenetriol (BTO) andhydration reaction to 5-HMF and hydrolysis of 5-both enhanced by the increase in water density at. An analogue study of reactions of d-fructose ined at high-temperature (400 C) and moderate pressurenditions products as glyceraldehyde, dihydroxyacetoneldehyde. High-temperature, high-pressure conditions

yield from sub-critical water hydrolysis of cellulose at different res-nd temperatures; and the effect of dissolved CO2.

To ethe comentadefatte5% lignmainlyenzymand 35treatmthe maa compboxymas biocThe reshigh ansignic

4.2. Pr

Protion ofoligomthe hydnitrogebondinstep, ato the

Hydsklero-plug-and inacids dat 310effectarting from d-fructose than d-glucose. Higher yieldswere obtained starting from d-glucose than from d-].

also a major component of plant biomass. To makee elemental chemical compounds of lignin for furtheris still a task for research. During studies on the lique-omass, it was found that the biomass could be liqueedis up to 7080% [19]. The efuents were subsequentlyiological degradation.Overall efciencyof CODremoval9095%. No toxic effects on the microorganisms were

ue to the prior hydrolytic treatment. The remainingwere attributed to lignin-derivates. They could not beh water. Oxidation in near critical water by hydrogennverted all solid material, mostly to gaseous products.10% of the initial carbon load remained in the aqueousthe main product being acetic acid [19].

lysis and fermentationce yield in mono-sugars for the treatment of biomass,ation of hot pressurized water hydrolysis and fer-was considered. As an example, rice bran, milled andnsisting of starch, 27% cellulose, 37% hemi-cellulose,as selected [17]. After hydrolysis, the reaction products,omer sugars, are easily transformed to mono-sugars byot water hydrolysis at 200 C resulted in 5% glucoselose. Hot water hydrolysis with subsequent enzymaticresulted in 70% glucose and 70% xylose, relative toum obtainable content in the feed. Results leading inle direction, were obtained on the hydrolysis of car-lcellulose. Cellulase, immobilized on silica gel was usedst for hydrolysis in a high-pressure system of CO2/H2O.l activity of the immobilized cellulase at 110 C was stillcould be reused for more than 20 times without anyoss of activity [29].

s, amino acids

are the other type of important bio-polymers. The reac-pressurized water can be of interest for producing

nd amino acids as the building blocks of the proteins. Insis reaction for proteins, rst a proton is attached to themof the peptide bonding. This leads to a splitting of therming a carbo-cation and an amino group. In the nextoxide ion, from a dissociated water molecule, attachesn-cation, forming a carboxy group.

sis of a model protein (BSA, bovine serum albumin) andeins like feathers and hair, carried out in a continuouseactor, resulted in a total liquefaction of the proteinsformation of amino acids [30]. Production of aminods mainly on reaction temperature, with an optimumressure in the range of 1527MPa had no signicante reaction. At 250 C the amino acid yield increases


up to a residence time of 300 s and then decreases due to decom-position reactions. Considerable quantities of glycine and alaninewere produced from decomposition of complex amino acids. Otheramino acidide resultedbonds. At 2BSA) was obto approximout the adDFK) at 90to the shorsub-criticalering aminoas hairs and

Hydrolyproteins (ptive reactiohydrolyticamino acidsproteins arelinkages inresulting mproductswup to about[18].

5. Hydrolysupercritic

Quite ahydrothermmal reactiohydrothermcontext of stion from h

Hydrolypressure deconstant, anreaction merated carboundergo pawater. Selewater denstration, thehydrolysis rreaction rattionmechanincreases inhydrolysis rthe rate obsolution physis conditiThose publreviewed b

Behaviopoint is ofmthe neutralical water uexamples. Fsociation coand thus, thaddition of

The groics of the awater. In th

sulfuric acid, ethanol undergoes rapid and selective dehydrationto ethene in supercritical water. The kinetics of this reaction areconsistent with an acid-catalyzed E2 mechanism. In the group of

[36] oriticaent

convexpelizatnd forbon

dendydroce zsulft traompwasMPa,tratiainehyddehyn-Arepenes torefefree

nd hesterin th0 Cthecusseed ats. Inthe cect nleadrolymatiowithtedaaterum rsupeed s1:17ctionningy 0.3ts arer deversreacchemrepot.% h

and all amed wzeroion onics were only found in traces. Addition of carbon diox-in higher yields due to acid hydrolysis of the peptide

50 C and 25MPa an amino acid yield of 150.3mg/(gtained by sub-critical water being saturated with CO2ately 90%. The experiments with duck feathers (with-

dition of CO2) led to amino acid yields (122.0mg/(g0 s) higher than for BSA without addition of CO2 dueter chain length of the sklero-keratin molecule. Thus,water hydrolysis can be an efcient process for recov-acids from organic protein-rich waste-materials, suchfeathers [31].

sis kinetics of starch, cellulose (polysaccharides), andolypeptides) could be modelled by a single consecu-n following rst order kinetics. Rate constants of theconversion to the resulting monomers (glucose and), strongly dependon the type of bond. Peptide bonds inmuch more stable than the-1,4- and-1,6-glycosidiccellulose and starch, respectively. The stability of theonomers and their conversion to further degradationere determined. The addition of carbon dioxide towater250 C resulted in a signicant increase of reaction rates

sis and hydrothermal reactions in sub- andal water (no oxidative reagent)

number of investigations have concentrated onal and hydrolysis reactions. Hydrolytic and hydrother-n are not and cannot always be separated. What is aal reaction? The term hydrothermal was used in theynthesis of minerals in natural deposits by crystalliza-ot pressurized aqueous solutions.sis in supercritical water is determined by the extremependence of the solvent propertiesdensity, dielectricd solubility parameter. Klein et al. [32] investigated thechanism in that region. Molecules containing a satu-n attached to a heteroatom-containing leaving grouprallel pyrolysis and hydrolysis reactions in supercriticalctivity and rate constant to hydrolysis increased withity and with the addition of salts. At low salt concen-addition of salts to the reaction mixture increased theate, while having no effect on a background pyrolysise [33]. This is consistent with a polar hydrolysis reac-ismwherein the rate constantwouldbe increasedwiththe solvent polarity. At higher salt concentrations, theate reached a maximum then decreased, approachingserved in the absence of salts, which corresponds toase behavior. Behavior of various compounds at hydrol-ons and at hydrothermal conditions havebeen reported.ished in the Journal of Supercritical Fluids are shortlyelow.r of acids and bases in the neighborhood of the criticalajor interest. JohnstonandChlistunoff [34] investigated

ization of acids and bases in sub-critical and supercrit-sing KOHacetic acid or NH3acetic acid systems asrom 25 C to the critical temperature of water, the dis-nstant for HCl decreases by 13 orders of magnitude,e basicity of Cl becomes signicant. Consequently, theNaCl to HCl raises the pH.up of Antal [35] investigated mechanism and kinet-cid-catalyzed dehydration of ethanol in supercriticale presence of a low concentration (


generated by self-dissociation of water. At higher salt content themechanism is rationalized with Lewis acid/base behavior of Na+and Cl ions in solution. The rate of hydrolysis is proportional tothe square rreaction me

For conv[43] that smtic acid dradehydrationical waterdehydrationicalmethanwith 79% seacid 5-acetotivity at 98%d-fructosetures. The aHMF and M

Hydroly[45,46]. Thetemperatur128 to 2600product spenia; gas formodel incoted the datato investigano effect onas the use owaswithinconstant (6.rate constan

Productride hydrol[47]. Reacti25600 C atem. The prcarbon montrace amoubreakdownHCl, followand subseqresidence tionly producof total carband modell

Propertias reactionand Dinjushot pressursynthesis retert-butylbtion modelreactions. Sably. The reby a factorin an inertmolecules rmechanismmining kinereactions arrst-orderalgorithm,similar resuconvenient

Formaldehyde (HCHO) reaction in supercritical water wasstudied with batch experiments to determine the water densitydependence [53]. At higher water densities, CH3OH yields were

t lowctionaphtgatedunds. At lermof t

es innd tundsrothles dh rapduct

ractio2O at carbion.decolaniminocouldreaca bahe mter dPa,

ble,ons,watethaneffecf ali8]. Tn fortes wociatodigpercrriticagradaticalts iden, mioxarothnvesshowentacid

ey deTrim

C, bC. Tationts at% aftfor 1namin wpurpoot of the salt concentration in the supercritical waterdium.ersion of carbohydrates it was found by Bicker et al.all amounts of a salt can increase the yield of lac-

stically. The group of Vogel [44] also investigated theof d-fructose to hydroxymethylfurfural. In supercrit-

only unsatisfying yields of HMF are achieved by theof d-fructose. But in the reaction in sub- and supercrit-

ol the resulting 5-methoxymethylfurfuralwas obtainedlectivity at 99% conversion and with sub-critical aceticxymethylfurfural (MMF) was obtained with 38% selec-conversion, similar to previously obtained results for

to HMF in sub- and supercritical acetone/water mix-uthors propose a continuous production process forMF.sis of nitriles was investigated by the group of Klein

reactions of butyronitrile were investigated in high-e water at 330 C and various pressures ranging frombar [45]. Residence times ranged from5 to 180min. Thectrum included butanamide, butyric acid and ammo-mation was negligible. A four-step autocatalytic raterporating the product acid as the catalytic species t-well. The kinetics of benzonitrile hydrolysis were usedte reactor wall effects [46]. Repeated reactor use hadeither the rate constant or product selectivity as wellf stainless steel and titanium reactors. Wall inuencethe experimental uncertainty for the homogeneous rate1%). Fromthat is concluded that the laboratory-deducedts can be applied to kinetics at commercial conditions.

distributionandreactionpathways formethylenechlo-ysis and oxidation was investigated by Testers groupons were carried out with dilute feeds at 24.6MPa,t residence times of 7 to 23 s in a tubular reactor sys-oducts detected were formaldehyde, hydrochloric acid,oxide, hydrogen, methanol, and carbon dioxide, withnts of other compounds. The main route for CH2Cl2was via sub-critical hydrolysis to formaldehyde and

ed by decomposition of formaldehyde to CO and H2,uent CO conversion to CO2 and H2. At 600 C and 6 sme under oxidation conditions, CO2 and HCl were thets at complete destruction of all compounds (>99.99%on). An analysis of hydrolysis kinetics, phase equilibria,ing has been reported also [48].es and synthesis reactions in hot compressed watermedium and reactant have been reviewed by Kruse[4051]. Macroscopic and microscopic properties ofized water are described, and a summary of publishedactions is presented. For the thermal degradation ofenzene in hot pressurized water, a high-pressure reac-was developed. The model consists of 171 elementaryimulation and experimental results correlated reason-action in hot pressurizedwater is strongly slowed downof 1000 in comparison to the reaction at low pressureenvironment. It is assumed that a cage effect of watereduces the reactivity of these species. Detailed reactions are presented. For application, methods for deter-tic parameters for simplied hydrothermal oxidatione of use [52]. Three different methods, namely pseudo-kinetics, multiple linear regression and RungeKuttawere used to determine the kinetic parameters, withlts. The RungeKutta algorithm was found to be morefor the kinetic parameter determination.

high, aRea

and ninvesticomporeactorwere thylationactivitiwas foucompo

HydmolecutorwitThe proHPLC fCSH6Nsuggesdensat

Theacids awere awhich

Theied in[57]. TFor wato 25Mnegligiconditihigherthe me

Thesition oet al. [5positiocorrelain diss

Thiand susuperccol desub-criproduchydrogacid, th

Hydbeen istable,treatmboxylicbut th350 C.at 350at 250degradamounwas 73250 C

Cyareactedfor thewater densities, CO yields increased.mechanisms for the decomposition of phenanthrenehalene under hydrothermal conditions have beenby Onwudili and Williams [54]. Polycyclic aromatichave been oxidized in a hydrothermal oxidation batch

ower temperatures the polycyclic aromatic compoundsally cracked but as the temperature increased, hydrox-he aromatic moiety occurred leading to a series ofcluding ring-opening and rearrangement reactions. Ithat up to 99wt.% destruction of the polycyclic aromaticoccurred at supercritical conditions.ermal synthesis of low-molecular uorescent organicerived from an amino acid, glycine, using a ow reac-id expansion cooling is reported by Futamura et al. [55].mixture solution had emission fromblue to ultraviolet.ns of the products also had uorescence and included

nd its methyl derivative. The uorescent compoundsoncarbon bond formation aswell as dehydration con-

mposition in sub- and supercritical water of the aminone and glycine was studied [56]. The main reactions

acid decarboxylation and amino acid deamination,be modelled by simple global rate laws.

tivity of methylamine in supercritical water was stud-tch reactor at temperatures between 386 and 500 Cajor products measured are ammonia and methanol.ensities less than 0.28g/cm3 and pressures less thanthe effect of water on the reaction rate appears to beand there is little evidence of hydrolysis. Under thesethe reaction seems to be governed by pyrolysis. Atr densities, hydrolysis becomes more important andol yield increases with water density.t ofwaterdensity on the rate constant for thedecompo-phatic nitrocompounds, was investigated by Anikeevhe density dependence of the rate constant of decom-each nitromethane, nitroethane, and 1-nitropropane,ith the density dependence of the H30+ concentration

ed supercritical water.lycol [(HOC2H4)2S] hydrolysis and oxidation in sub-,itical water was studied by Testers group [59]. Underl conditions (T=400525 C and P=25MPa), thiodigly-tion occurred rapidly with and without oxidant. Attemperatures from 100 to 360 C, hydrolysis reactionentied included carbon monoxide, carbon dioxide,ethane, ethylene, acetaldehyde, acetic acid, formic

ne, sulfuric acid, hydrogen sulde, and elemental sulfur.ermal stability of six aromatic carboxylic acids hastigated by Savage [60]. Benzoic acid was the mosting negligible degradation after 1h of hydrothermal

at 350 C. Terephthalic acid, 2,6-naphthalene dicar-and isophthalic acid were stable after 1h at 300 C,

carboxylated to form monoacids in 1015% yields atellitic anhydride decomposed completely after 30minut showed no appreciable decomposition after 30minerephthalic acid and isophthalic acid were the mainproducts, but o-phthalic acid was also formed in small350 C. The o-phthalic acid conversion to benzoic acider 60min at 300 C, but the diacid remained stable ath.ide, dicyandiamide and related cyclic azines wereater at 100300 C in a sealed 316 SS tube (27.5MPa)ose of characterizing the hydrothermolysis chemistry


of cyanamide [61]. The conversion of cyanamide to dicyandi-amide dominates at 100175 C. At 175250 C, for reaction timesshorter than 15min, the major pathway is hydrolysis of thecyanamide225 C, hydoccurs. At 3is complete

Of majorpolymers inchloride plChlorine inful chlorinagas fractiontechnique pweight aromfractions. Fwaterprodualiphatic alcombustion

Polyethyup to 173Mrange of PEbecame hoslowly, the hing point (2underwentneous tempwere rapiding, the homThe homogfound to be

In a simsupercriticato 670 C hPE) mixture110 to 690phase. Theas a heterogsitions studOnly after Pabove abou

At moreof polyethybatch reacttime of 30In supercribons, higheobtained thcritical watatmosphere

6. Conclus

Propertiin combinaThe knowleprocesses ucan be usedtransformain special csurized waof productof by-produmolecules l

obtain special compounds at high concentrations. But, some inves-tigations have shown, that with optimized conditions, it may bepossible. A combination of hydrolytic and bio-catalytic processes

to bel seesion,ses t

nces

runnids ant, New: G.C

653)irus, Gercritt systids, Mp. 195runneaviorrocar. Bermhe caercritMizan, simpugimoe aroubukurrogenaturesical cite Jr.,ional Cse, Nue, F.ercrit. Hurith, Opctroscercritizawaervatier, J. SBremhctor der-crironhoractionsoil crunnesupe

tz (Edslin, 20ongnaractioer: anDonchlantacriticarnatioineerrunnnolog

e Proc7.ogalinubcritssure27, 20issensseur, AerativFibrobWillnff und74.chmiri, M.ercritdicyandiamide mixture to CO2 and NH3. Above aboutrolysis of these cyclic azines to aqueous NH3 and CO200 C the conversion of all compounds to CO2 and NH3in 10min.interest for hydrolysis and hydrothermal reactions areparticular for the treatment of waste. Waste polyvinylastics were treated at hydrothermal conditions [62].PVC dissolved in water as hydrochloric acid. No harm-ted organic compoundswere observed in the liquid ands after treatment at 300 C. Between250and350 C, thisroduced polyene as a residual solid, and low-molecularatic and aliphatic compounds in the liquid and gas

urther decomposition at over 350 C in supercriticalcedacetone, phenol, benzene, benzenederivatives, andkane and alkene in the liquid and gas fractions. Theenthalpy of the residual solid was 9270kcal/kg.lene terephthalate was reacted in water at pressuresPa and temperatures up to 490 C [63]. Over the

T concentrations studied (1259wt.%), most systemsmogeneous. For the case where samples were heatedomogenization temperature was around the PET melt-41 C). For PET samples that were rapidly heated anda solidliquid transition upon heating, the homoge-erature was between 297 and 318 C. For samples thatly heated and underwent crystallization during heat-ogenization temperaturewas between 360 and 396 C.

enization temperature for terephthalic acid +water wasaround 356 C.ilar way, polyethylene phase behavior and reaction inl water at pressures up to 2.6GPa and temperatures upave been investigated [64]. When PE+water (1230%s were rapidly heated at initial pressures ranging fromMPa, PE rst melted and formed a liquid spherule PEresults of this study show that PE and water remaineneous system over the polymer (1230% PE) compo-ied during heating and reaction in supercritical water.E decomposes to lowermolecularweight hydrocarbons,t 565 C, homogeneous reaction conditions result.accessible conditions, an investigation on pyrolysislene and n-hexadecane [65] was carried out in aor at temperatures from 400 to 450 C, a reactionmin, and water density between 0 and 0.42g/cm3.tical water, higher yields of shorter chain hydrocar-r l-alkene/n-alkane ratio, and higher conversion werean from pyrolysis in Ar. Pyrolysis rate of nC16 in super-er was almost the same as that in 0.1MPa argon (Ar).

ions and future development

es of sub- and supercritical water are well known, alsotion with gases, salts, and many organic compounds.dge of these properties is a prerequisite for carrying outnder these conditions. The special properties of waterfor extraction of polar compounds, for the hydrolytic

tionofmonomers andpolymers. Catalystsmaybeusefulases. In particular, CO2, dissolved in liquid hot pres-ter may be of advantage for producing higher yieldscomponents at milder conditions, avoiding formationcts. Hydrolysis and hydrothermal reactions of organicead to a bunch of compounds, and it proves difcult to

seemswewilconverable ca

Refere

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Near critical and supercritical water. Part I. Hydrolytic and hydrothermal processesGeneral introductionProperties of waterSub-critical water for extractionWater for hydrolysisWater in bio-fuel processesBiomass. Total liquefaction or gasificationBiomass compoundsStarchCelluloseSugars, glucoseLigninHydrolysis and fermentation

Proteins, amino acids

Hydrolysis and hydrothermal reactions in sub- and supercritical water (no oxidative reagent)Conclusions and future developmentReferences

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Supercritical Water Possibilities