Oxidation reactions of solid carbonaceous and resinous substances in supercritical water

J. of Supercritical Fluids 47 (2009) 400–406

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The Journal of Supercritical Fluids journal homepage: www.elsevier.com/locate/supflu

Review

Oxidation reactions of solid carbonaceous and resinous substances in supercritical water Seiichiro Koda ∗ Department of Materials and Life Sciences, Faculty of Science and Technology, Sophia University, 7-1 Kioi-cho, Chiyoda-ku, Tokyo 102-8554, Japan

a r t i c l e

i n f o

Article history: Received 28 June 2008 Received in revised form 8 August 2008 Accepted 8 August 2008 Keywords: Shadowgraphy X-ray radiography SCWO Carbon Coal Resin

a b s t r a c t Recent kinetic studies, particularly those by means of shadowgraphy and X-ray radiography, for supercritical water oxidation of solid carbonaceous and resinous substances have revealed the importance of the O2 mass transfer process over the intrinsic surface reaction at higher temperatures. The mass transfer processes, internal and external one, should be incorporated in designing SCWO processes for solid substances and related processes such as catalytic SCWO. Some model calculation efforts of late are briefly described. Finally, fundamental information required for future development is itemed. © 2008 Elsevier B.V. All rights reserved.

Contents 1. 2. 3.

4.

5.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Methods of visualization for reactions of solid substances in supercritical water . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Direct observation of size change of solid carbonaceous and resinous substances under SCWO and relevant kinetics . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1. Carbon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1.1. Carbon particles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1.2. Packed bed of carbon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2. Coal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3. Resins . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.4. Relevant kinetics of the solid substances . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Mass transfer contribution in other reaction systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1. Heterogeneous catalysis in SCWO . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2. Catalytic SCWO of phenol with active carbon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3. Carbon particle size distribution change . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Future prospect—what are needed for design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1. Introduction Supercritical water oxidation (SCWO) is a technology benefited by the formation of a single phase of O2 , organic substances and water above the critical point. Solid substances are also potential targets in SCWO technology, e.g. for energy recovery from coal and

∗ Tel.: +81 3 3238 3315; fax: +81 3 3238 3361. E-mail address: [email protected]. 0896-8446/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.supflu.2008.08.006

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various kinds of biomass, and for decomposing solid organic wastes and hazardous materials. Concerning the utilization of low-quality coal and biomass, Bermejo et al. [1] demonstrated theoretically that SCWO power plants appear as an alternative in power generation from coal because of their higher net energetic efficiencies than other power plants and also because of their environmentally friendly nature. Concerning the treatment of ion-exchange resins which have been used for water purification in a nuclear facility, SCWO is a potential technology to reduce the volume of the spent resins [2–4]. However, relevant kinetics for SCWO of solid

S. Koda / J. of Supercritical Fluids 47 (2009) 400–406

substances are usually complicated, principally owing to the severe contribution of mass transfer processes between the bulk and solids and also inside the solid substances. For example, importance of the sample particle size in determining the kinetics was demonstrated, using biomass in a batch type reactor [5]. Moreover, if the unconsumed solid substance continues to exist, the progress of SCWO is affected in a complicated manner by various mass transfer processes, heterogeneous reactions, and homogeneous reactions in the fluid. For the kinetic analysis of the complicated system, in particular, composed of more than one phase, the direct observation of reaction progress, in addition to usual chemical component analysis, should be very helpful. In the present paper, we will mainly describe the kinetic behavior of solid carbonaceous and resinous substances studied in suband supercritical water oxidation where more than one phase are usually involved. Work done around the world will be discussed, but more detailed attention will be given to work done in our laboratory, where direct visualization of SCWO progress has been recently developed. Some important processes where mass transfer is taken into account will be also described.

2. Methods of visualization for reactions of solid substances in supercritical water Due to the very high pressure and temperature and also corrosive nature of supercritical water (SCW), direct observation of the reaction progress is formidable. In the field of geochemistry, the behavior of inorganic materials has been frequently observed using a diamond anvil cell (DAC) as exemplified by the Raman spectroscopic study of inorganic salts in high temperature and high pressure aqueous solution [6]. Similar techniques have been recently applied to observation of the phase behavior and reactions of polymeric materials such as polyethylene, nylon 6/6 and polyvinylchloride in SCW [7–9]. Various interesting phase changes and reaction progress were observed. However, the DAC studies are usually limited to a static batch system. As another trial, a direct optical observation using a flow apparatus with sapphire windows was reported for the salt precipitation from SCW [10]. For the kinetic study of SCWO, direct observation of reaction progress in flowing systems is also very desirable.

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The observation of SCWO progress of a spherical carbon particle as a model solid substance was first reported by Sugiyama et al. using shadowgraph photography [11,12]. Later, Schlieren technique [13] gave a direct observation of density field around the reacting carbon particle. More recently, X-ray radiography [14–16] joined the line-up of the direct observation techniques. The shadowgraph experimental system consisted of a hot-wall reaction cell, a flow system and optics as shown in Fig. 1(a). A carbon particle (∼4 mm in diameter) was first stored in the cold cylinder. After the flow and temperature fields of sub- and supercritical water with O2 supplied from H2 O2 decomposition were stabilized, the particle was transferred to the center of the cell to initiate the reaction. Shadowgraph was continuously observed in the flow through the sapphire windows using a light source and a video camera. In the case of Schlieren photography [13], the cell was located in the observation zone of the conventional laser-Schlieren optical system. In order to avoid the usage of sapphire windows, and also to observe simultaneously the change inside the solid substances, X-ray radiography was successfully adopted with a good time resolution of several seconds [14–16], owing to the recent extended dynamic range using a multi-color scintillation detector [17]. Fig. 1(b) shows the setup for X-ray radiography monitoring SCWO of solid carbonaceous particles and packed beds. The reaction proceeded in the vertical Inconel tubular reactor. The particle sample was supported by Pt spiral wires, while the packed bed was settled in an inner tube with a bottom mesh. The brightness of the sample on the CRT could be made proportional to the density of the sample.

3. Direct observation of size change of solid carbonaceous and resinous substances under SCWO and relevant kinetics 3.1. Carbon Solid carbon with an apparently simple chemical reaction has been used as a model for which the fundamental rate processes may be more clearly seen than more practical substances such as coal, resin and biomass. Eventually oxidation of carbon has been a target of intensive research for a long time in the field of combustion science. In this section, the shadowgraph images of carbon particles

Fig. 1. Schematic diagram of the reaction cell: (a) cell with sapphire windows for shadowgraph observation and (b) system for X-ray radiography. Reproduced with some modification from Ref. [19] with kind permission of Taylor & Francis (License No. 1994520695868). Reprinted with some modification from Ref. [14] with kind permission of John Wiley & Sons.

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Fig. 3. Reaction rate q vs reciprocal particle temperature: sample, active carbon () and synthetic graphite (). The carbon consumption rate according to reaction (1) that is controlled by O2 mass transfer, qmass , at 400 ◦ C and 30 MPa is indicated by an arrow. Reproduced with some modification from Ref. [19] with kind permission of Taylor & Francis (License No. 1994520695868).

In the atmospheric carbon oxidation, reaction at moderate temperatures usually proceeds as C + O2 → CO2

(1)

and then shifts to C + 12 O2 → CO

Fig. 2. Shadowgraphs of active carbon particle: (a) 0 s, (b) 180 s and (c) 300 s after the particle was transferred to the hot-zone of the cell at 450 ◦ C, 30 MPa, 2 cm3 min−1 of the flow rate and 3.6 mass% of O2 fraction. Reprinted with permission from Ref. [12] with some modification. Copyright (2004) American Chemical Society.

will be given, whose analysis will be used as a base for the later analyses for other solid substances.

3.1.1. Carbon particles Shadowgraph of an active carbon particle has been pursued during SCWO as shown in Fig. 2 [12]. After the insertion of the particle into the center of the cell, the size decreased, sometimes after an induction time, with keeping the shape nearly unchanged. The time evolution was considerably different for different kind of carbons. The size reduction rate of active carbon was much faster than those of synthetic graphite and highly oriented pyrolytic graphite (HOPG). HOPG showed no detectable decrease in size after a long exposure to severe SCWO conditions of 600 ◦ C, 30 MPa with 10 mass% O2 in water. The kinetic analysis was conducted for active carbon and synthetic graphite particles whose relevant properties are shown in Table 1.

Table 1 Typical properties of carbon particles Species

Density/kg m−3

BET surface area/m2 kg−1

Active carbon Synthetic graphite

7.5 × 102 1.8 × 103

1.0 × 106 7.8 × 102

(2)

with increasing the temperature [18]. The main reaction path in SCWO that is usually conducted at a temperature lower than 600 ◦ C is expected to be reaction (1) leading to CO2 . Indeed, gas analysis of SCWO of active carbon at 400 ◦ C showed that the ratio of produced CO/CO2 was ca. 0.02 [19]. The phenomenological reaction rate at a unit surface area of the outer surface, q (in the unit of kg m−2 s−1 ) was obtained from experiments as, q = p ×

 dR  dt

,

(3)

where R is the particle radius and p , the particle density. The surface temperature might be different from the surrounding fluid temperature owing to the reaction heat release. In fact, buoyancydriven flow fields due to the temperature rise were visualized by means of Schlieren photography in SCWO of an active carbon particle [13]. The temperature rise could be estimated to determine the particle temperature Tp on the basis of the film theory, assuming that the heat release from the oxidation reaction was equal to the heat loss rate from the particle at Tp . For details, consult the reference [19]. It was more than 50 degrees when the reaction rate q exceeded 5 × 10−4 kg m−2 s−1 . However, under 400 ◦ C, the temperature rise might be neglected in most cases. The experimentally obtained reaction rates q are plotted against the reciprocal particle temperature in Fig. 3. The temperature dependence of SCWO rate for the active carbon over ca. 400 ◦ C was much smaller than that below 400 ◦ C and also than that for the synthetic graphite. The large change in the temperature dependence at around 400 ◦ C was considered to correspond to the change in the rate controlling step from the surface reaction controlling to the mass transfer controlling region in the higher temperature range. The carbon consumption rate according to reaction (1) that is controlled by O2 mass transfer, qmass , at 400–500 ◦ C and 30 MPa could be estimated theoretically to be ca. 1 × 10−6 m s−1 (which

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Under lower temperatures, the reaction should be controlled by the surface reaction. The reaction rate q* at a unit surface area of the internal surface of the particle is related to the experimentally obtained q. q∗ = q ×

Sout Sin × 

(4)

Here, Sout ( = 4R2 ) is the outer surface area of the particle. Sin is the total inner surface area of the particle, which may be estimated by BET measurements. Based on the first-order dependence of the reaction rate on the concentration of O2 , the rate constant of the surface reaction at a unit internal surface area ks∗ can be calculated by ks∗ =

Fig. 4. Rate constant ks∗ for the internal surface reaction vs reciprocal particle temperature: sample, active carbon () and synthetic graphite (). The dashed line shows the Arrhenius rate expression after correcting for the estimated effectiveness factor which is not unity (see text for more detail). Reproduced with some modification from Ref. [19] with kind permission of Taylor & Francis (License No. 1994520695868).

corresponds to qmass = 7.5 × 10−4 kg m−2 s−1 ) according to the film theory and using the estimated Sherwood number by means of Ranz–Marshall equation [11,12]. The uppermost qmass value may be a little bit higher as 2 × 10−6 m s−1 (qmass = 1.5 × 10−3 kg m−2 s−1 ) due to the enhanced O2 supply by the heat driven flow as estimated by Computational Fluid Dynamics (CFD) calculation [12] which took numerically into account the buoyancy effect due to the reaction heat release.

q∗ , [O2 ]Mc

(5)

where Mc is the atomic weight of carbon.  in Eq. (4) is the effectiveness factor for the inner surface reaction. The larger the pore diffusion resistance inside the particle compared to the inner surface reaction rate, the smaller the effectiveness factor. It can be evaluated by means of Thiele modulus [20]. In the previous literature [19], the effectiveness factor of 0.8 was employed for the active carbon at 300 ◦ C under the saturated condition, and 0.7 for the synthetic graphite at 600 ◦ C and 30 MPa. Although it was not evident whether the reactivity of the inner surface of the synthetic graphite and that of the active carbon were the same, the data of active carbon and those of synthetic graphite could be combined as in Fig. 4 to obey a single Arrhenius expression, ks∗ = 1.7 × exp

 −117 kJ  RT

m s−1 .

(6)

The reason why the reaction of synthetic graphite at higher temperatures was still in the surface reaction controlling region was

Fig. 5. X-ray radiography images of a bed of carbon particles under SCWO progress: (a) 3 min, (b) 4 min, (c) 5 min, and (d) 6 min after the start of the reaction flow. The O2 containing flow reached the observation part at 3 min. Experimental conditions; 400 ◦ C, 30 MPa, 12 cm3 min−1 of the flow rate and 3.6 mass% of O2 fraction. Reproduced with some modification from Ref. [15] with kind permission of Elsevier Limited (License No. 1981670901167).

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its slower reaction rate due to much smaller surface area of the graphite particle. The comparison between the present SCWO reaction rate and the rates in ordinary combustions is the future problem. In the ordinary gas phase, the overall reaction is now becoming better understood on the basis of elementary reactions. The reaction mechanism in SCWO may be also built on the detailed elementary reactions of the surface, and then the effect of the high pressure water on relevant elementary rate processes is expected to be made clear. 3.1.2. Packed bed of carbon The SCWO progress of a packed bed of active carbon (particle diameter of 500–850 ␮m) was pursued using the system shown in Fig. 1(b) [15]. The change of the intensity being caused by the change of the size and/or the density of the target materials during the reaction was visually imaged. The decrease of the density of a packed bed of active carbon is seen in Fig. 5. Up to 3 min, the water flow contained no O2 , and there was no indication of reaction progress. At t = 3 min, the water flow containing O2 reached the bed, when the brightness started to decrease and it disappeared completely at t = 6 min. The time profile of the brightness was little dependent on the height, which implied that the bed was oxidized almost homogeneously. The initial consumption rate of carbon rini [kg s−1 ] could be evaluated as rini = −

d(Q ) (at t = 0) dt

(7)

where Q is the total amount of carbon in the bed. Then, if the reaction is described by reaction (1) and its rate is proportional to the total inner surface area of the carbon particles of the bed and also to the oxygen concentration, the first-order reaction rate constant ks∗ per unit inner surface area could be evaluated as ks∗ =

rini Mc QAS [O2 ]

(8)

From the experiments, rini was measured to be 1.8 × 10−6 kg s−1 , and then ks∗ was evaluated to be 1.7 × 10−9 m s−1 by using the given Q, AS , and [O2 ] values, which was in a reasonable agreement with that estimated from Eq. (6) at 400 ◦ C. Here, we will summarize the usefulness of the direct observation of the reaction progress of solid substances. The direct observation method can easily pursue the reaction progress continuously along with the reaction time, and thus can give reaction rate parameters without confusing assumptions. It is also easy to analyze whether the reaction mechanism is changed or not during the reaction progress. The phase behaviors are pursued in real time and thus the change in the rate controlling steps are easily noticed. The newly obtained information, such as for the flow fields and fluidization of solid particles, contributes to future design of optimum apparatus. 3.2. Coal As already mentioned, the coal SCWO process has the advantage by electric efficiency and environmental friendly nature. Coal is different from carbon in several points. Most of coal is constructed from a very complicated hydrocarbon network, and also contains large amounts of ash components that may remain on the particle surface to affect the diffusion process of relevant oxidants and products. The results for carbons may be consulted in order to understand the mechanisms and kinetics of SCWO process of coal. By using a bench-scale semi-continuous installation, Wang and Zhu [21] studied the conversion and kinetics of coal oxidation and reported that the oxidation of coal in SCW was a pseudo-first-order

Fig. 6. Reaction rate q vs reciprocal temperature: sample, active carbon (), coal (), and styrene-divinylbenzene resin (). Experimental conditions; 30 MPa, 1 cm3 min−1 of the flow rate and 3.6 mass% of O2 fraction. The carbon consumption rate according to reaction (1) that is controlled by O2 mass transfer, qmass , at 400 ◦ C and 30 MPa is indicated by an arrow. Reproduced with some modification from Ref. [23] with kind permission of Elsevier Limited (License No. 1981661373235).

process with an activation energy of 155 kJ mol−1 at 25 MPa. However, what was the dominant mechanism to determine the apparent rate seems not to have been elucidated. Very recently, Vostrikov et al. [22] studied the SCWO reaction of simple coal particles (from Kuznetsk basin) of 1–5 mm diameter, using a semi-batch reactor in the flow of H2 O/O2 supercritical fluid with an O2 share of 0–6.6% under the pressure of 30 MPa and temperature ranging from 400 to 750 ◦ C. They used chemical analysis for pursuing reaction progress. The diffusion process in the ash layer was taken into account. It was shown that under the studied conditions, the gasification caused a loss of particle mass comparable with a loss through O2 oxidation, when the mass share of O2 in the fluid was 2–3%. It was determined that for the temperature of 500–750 ◦ C, the progress of oxidation was limited by the rate of O2 mass transfer to the particle surface. Therefore, the time of particle consumption decreased only insignificantly with rising the temperature. Below 500 ◦ C, the rate of surface oxidation by O2 was described by the first-order reaction in concentration of O2 and zero-order reaction in concentration of H2 O with activation energy of 166 kJ mol−1 . Koda et al. [23] also studied the SCWO progress of Breasol coal particle in a similar way to the case of carbon particles described in Section 3.1.1. The apparent reaction rate q was obtained and was plotted against the reciprocal reaction temperature in Fig. 6. The rate almost coincided with that for active carbon above 400 ◦ C, which supported that the main reaction in the Breasol coal was described by reaction (1). The rate decreased steeply with decreasing temperature under 400 ◦ C, but with a smaller activation energy than that for active carbon. The smaller activation energy for the coal might be related to the evolution of volatile organic-materials at lower temperatures from the coal. The previous researchers seem to be in agreement with each other in the overall kinetic behavior against the temperature. However, the activation energy for the surface reaction of coal seems not to be in good agreement; one reason may be the different kinds of coal employed. 3.3. Resins Application of SCWO process to waste resins has been interested [2–4,24]. Recent interest seems to be addressed extensively to ionexchange resins wasted from a nuclear power plant for which any

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appropriate treatment technology is not established. Koda et al. [23] observed SCWO progress of styrene-divinylbenzene copolymer, whose network structure was basically the same as those of ion-exchange resins. The apparent reaction rate q was obtained similarly to the case of carbon particles and was plotted against the reciprocal reaction temperature in Fig. 6, together with those of active carbon and coal. The very similar temperature dependence of the rate against the temperature indicates a similar change in the rate controlling step from the heterogeneous reaction at lower temperatures to the external O2 mass transfer limitation above ca. 400 ◦ C. It is clearly seen, however, that the rates of the resin are much larger than that of active carbon. The O2 requirement for 1 kg active carbon obeying reaction (1) is 2.7 kg O2 . The ten-times larger mass transfer limited rate for the resin indicates that the resin of 1 kg required O2 of only ca. 0.27 kg. This very small O2 requirement for the resin decomposition is owing to the mechanism where the resin structure can be destroyed to dissolve by only ca. 10% O2 compared to reaction (1). Oxygen should attack the polymer network to produce fragmental species that are soluble in the bulk supercritical water and then oxidized. Indeed analysis by HPLC showed a large amount of various organic species remaining in the homogeneous fluid. 3.4. Relevant kinetics of the solid substances As exemplified in Sections 3.1–3.4, the detailed phenomenology and rate processes of the size and shape change of solid substances have been considerably made clear. Although the pressure effect has not been extensively studied, it does not to be significant in the range from Pc to 30 MPa. In general, the rate process is controlled by heterogeneous reaction at lower temperatures such as under 400 ◦ C. At higher temperatures, the rate is controlled by a certain external mass transfer process. The temperature for the change of mechanism seems not to be intrinsically related to the critical temperature. It more strongly depends on the internal structure such as pore size distribution through the competitive internal diffusion and inner surface reaction, and also on the amount of O2 required for the destruction of the solid structure. In the case of biomass, the chemistry should be more complicated. Indeed in the case of wood, several different kinetic steps were successively observed with the elapsed time by means of shadowgraph photography [25,26]. It is worthy to note that the direct visualization technique is very helpful to analyze these complicated targets. The above mentioned competitive and/or successive rate processes should be adequately taken into account for understanding the overall kinetics and designing the adequate process. 4. Mass transfer contribution in other reaction systems 4.1. Heterogeneous catalysis in SCWO In the previous section, the importance of mass transfer in SCWO of solid substances has been demonstrated. However, it is not limited to SCWO. One example is found in heterogeneous catalysis in SCWO. The total oxidation of organic compounds by heterogeneous catalysts in supercritical water has received much attention in recent years, expecting to mitigate the severity of the reaction conditions of SCWO. As is summarized by Savage et al. [27], phenols have been frequently chosen as a target material owing to their ubiquitous existence in waste streams and their refractory characteristics. Yu and Savage [28] and Oshima et al. [29] studied the kinetic behavior of phenol catalytic oxidation in SCWO with MnO2 catalysts. They analyzed the contribution of internal and external mass transfer resistance. The overall reaction rate seemed to be

405

appreciably influenced by internal mass transfer in the catalyst particles under usual catalytic SCWO conditions. Thus the estimation of effectiveness factor was very important. Finally they offered dual-site Langmuir–Hinshelwood rate equations for phenol disappearance, though some mutual discrepancies remained. 4.2. Catalytic SCWO of phenol with active carbon Another important example is the carbon catalysis of the phenol SCWO oxidation studied by Nunoura et al. [30]. They reported that the addition of active carbon catalyst promoted the oxidation of phenol in SCW and that production of tarry materials was remarkably suppressed at 400 ◦ C and 25 MPa. From the experiments with a packed bed flow reactor, they indicated that mass transfer limitation between the bulk fluid and the catalyst surface was negligible at 400 ◦ C and 25 MPa, whereas mass transfer within the pores of the active carbon considerably controlled the overall reaction rate. In their reaction model, they took into account homogeneous phenol oxidation, heterogeneous phenol oxidation on the catalyst surface and combustion of carbon catalyst. The heterogeneous reaction rate of phenol was expressed as the intrinsic rate multiplied by the effectiveness factor. The extent of the limitation due to internal mass transfer was location- and time-dependent in the packed bed. For the combustion reaction, a gas phase combustion model in which the combustion rate was proportional to the external surface area of the active carbon catalyst particle was adopted. The total reaction progress was solved on the basis of one-dimensional mass balance equation for a plug-flow packed bed reactor with varying concentrations of phenol and O2 along the reactor length, and satisfactory results were obtained. 4.3. Carbon particle size distribution change A mathematical model of a tank type reactor for hydrothermal oxidation of solid particles to pre-process waste or biomass was developed by Marias et al. [31]. Governing equations were composed of the mass, species and energy balances for the fluid phase. The particle size distribution of the output of the reactor was computed as a function of the incoming one and the operating parameters. The numerical predictions were compared to experimental profiles in the case of hydrothermal oxidation treatment of black carbon between 250 and 350 ◦ C with the activation energy of 136 kJ mol−1 . They claimed that a good agreement was obtained between the calculation and experiments. 5. Future prospect—what are needed for design One of the reasons for elucidating the reaction mechanism and reaction rate of solid carbonaceous and resinous substances is to discuss the optimized process for energy recovery and/or disposing waste with minimum environmental impact, and also for designing individual reactors. The subjects listed below need more future elaboration. (1) Reaction mechanism and rate: The oxidative decomposition of solid organic substances is very complicated because not only the chemistry is complicated, but also it is strongly coupled with the mass and heat transfer processes. The reaction progress thus differs dependent on the size and shape of the substances. The direct observation should be useful for separately understanding the individual steps. Many solid organic substances including various kinds of coal, biomass and resins are not yet sufficiently studied. Moreover, if any direct observation of chemical species distributions around the solid substances in the surrounding water becomes possible, e.g. by means of

406

(2)

(3)

(4)

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Raman spectroscopy, more detailed chemical kinetics based on elementary reactions may be established. Phase behavior: The real SCWO system is composed of various kinds of species; among them, H2 O, O2 and CO2 are ubiquitous. Most of the model studies up to now, however, have assumed pure water as the medium due to the lack of enough information. The reaction progress as well as other relevant processes such as salt precipitation in the multi-components system may be different from those in pure water. The phase behavior of the multi-components system should be investigated. Transport properties: In order to analyze the mass and heat transfer processes combined with heterogeneous reaction, transport properties are necessary. The diffusion constants in supercritical water have been very scarcely measured, and, indeed, the self-diffusion constant of water itself [32] has been used in model studies in place of the multi-components diffusion constant. On the other hand, the effectiveness factor is determined from the competition between the internal diffusion and inner surface reaction. Analysis of the interaction of solutes, water and surface under high pressure is necessary for understanding the physical chemistry of the pore diffusion relevant in SCWO. Model construction: For kinetics so far, global reaction model coupled with mass balance consideration has been adopted, e.g. in the case of wheat straw [5]. More detailed kinetics may be demanded. For designing, relatively simple reactor models such as plug-flow and continuous-stirred tank reactor as mentioned in the previous sections have been adopted. Because of the low kinematic viscosity of supercritical fluids, however, the flow is easy to become turbulent, and we must be careful enough to check whether the assumptions of idealized flow fields in the reactor are realized. The abrupt change in many physicochemical properties around the critical points frequently makes the idealized designing method unreliable. Computational Fluid Dynamics calculation: In order to investigate the detailed flow and temperature fields, and the interaction of solids and fluids more precisely, CFD calculation coupled with chemistry may be very useful. Such CFD calculations have been developed [12,13,33], and may be helpful to designing reactors in near future.

Acknowledgements The present author is very grateful to his collaborators for their contribution in developing the visualization technology and in kinetic analysis. The works from the laboratory of the present author were supported in part by a grant provided by the New Energy and Industrial Technology Development Organization (NEDO) via Japan Chemical Innovation Institute (JCII) based on the project “Research & Development of Environmentally Friendly Technology Using SCF” of the Industrial Science Technology Frontier Program (Ministry of Economy, Trade and Industry (METI), Japan), which is greatly appreciated. References [1] M.D. Bermejo, M.J. Cocero, F. Fermandez-Polanco, A process for generating power from the oxidation of coal in supercritical water, Fuel 83 (2004) 195–204. [2] M.A. Dubois, J.F. Dozol, C. Massiani, M. Ambrosio, Reactivities of polystyrenic polymers with supercritical water under nitrogen and air. Identification and formation of degradation compounds, Ind. Eng. Chem. Res. 35 (1996) 2743–2747. [3] Y. Akai, K. Yamada, T. Sako, Ion-exchange resin decomposition in supercritical water, High Pressure Res. 20 (2001) 515–524.

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