- Email: [email protected]
Importance of phase equilibria for understanding supercritical fluid environments
Fluid Phase Equilibria 158–160 Ž1999. 673–684
Importance of phase equilibria for understanding supercritical fluid environments Kunio Arai ) , Tadafumi Adschiri Tohoku UniÕersity, Department of Chemical Engineering, Aoba-ku Aramaki Aza Aoba-07, Sendai 980-8579, Japan Received 25 May 1998; accepted 18 December 1998
Abstract In this work, several applications are reviewed and the importance of phase equilibria is discussed. In the reaction of cellulose in subcritical and supercritical water for producing chemicals, cellulose decomposes to form lower carbon number sugars, oligomers, and various acids. As conditions change from a subcritical reaction environment to a supercritical reaction environment, we observed a drastic change of the kinetic rate constant, which is thought to be the result of the cellulose–water system changing from a heterogeneous state to a homogeneous state. Diamond anvil cell measurements ŽDAC. are used to provide visualization of phase changes in the cellulose–water system. In the reaction of polyethylene terephthalate ŽPET. in supercritical water for recycling, PET decomposes to form oligomers, ethylene glycol, and the monomer, terephthalic acid. The reaction kinetics for this polymer–water system is much slower and requires 10 min for 100% conversion vs. 50 ms for 100% conversion of cellulose. The reason is thought to be due to the number and type of reaction phases present in the system, that is, solid–fluid or liquid–fluid phases. In the hydrogenation of heavy oils for energy conversion applications, the reaction kinetics can be very slow. Supercritical water provides a homogeneous reaction environment for the hydrogenq heavy oil system. However, for this case, we have found that an excellent hydrogenating atmosphere can be made by addition of oxygen, which apparently provides active hydrogen from CO formed, through the water–gas shift reaction. In catalytic reactions of heavy oils, in-situ extraction of coke precursors is provided by the supercritical water environment. Further studies are reported on the oxidation of hydrocarbons. In the treatment of high-level liquid wastes from the nuclear industries, it is found that many metals can be removed from solution by raising the temperature and pressure of the aqueous solutions. These results are discussed in view of recent supercritical water metal oxide solubility and supercritical water density measurements. q 1999 Elsevier Science B.V. All rights reserved. Keywords: Supercritical water; Cellulose; Hydrogenation; Energy conversion; Nuclear wastes
)
Corresponding author. Tel.: q81-22-217-7247; Fax: q81-22-217-7246; E-mail: [email protected]
0378-3812r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 8 - 3 8 1 2 Ž 9 9 . 0 0 1 1 6 - 8
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K. Arai, T. Adschirir Fluid Phase Equilibria 158–160 (1999) 673–684
1. Introduction Physical properties, chemical and phase equilibria are fundamental to understanding and interpreting new reaction phenomena observed in supercritical fluids w1–4x. Supercritical water has great potential as a generic technology for waste treatment, material production, chemical processing and a number of new applications w5x. In this work, some areas in which we have performed research will be discussed and places where the chemical and phase equilibria play an important role in the reaction environment will be shown.
2. Biomass Biomass is an important future raw material for energy and chemical applications. However, its practical conversion to chemical products, has not been realized yet on an industrial scale. About 5 = 10 9 tonsryear of cellulose occurs in the form of waste, mainly as agricultural and municipal wastes, in addition to the large amount of wood harvested. A thorough comparison of plant biomass with alternate energy resources has been made by Bobleter w6x. In this section, our interest was to examine conversion of cellulose and its oligomers in supercritical water. Cellulose is a linear polymer in which anhydrocellobiose units are bound by bŽ1–4. glycosidic bonds. Due to the presence of intra- and inter-molecular hydrogen bonding, cellulose aggregates in elementary fibrils and micro fibrils that have alternating high crystallinity and less ordered regions w7x. Cellulose is insoluble in most solvents due to these regions that limit solution accessibility. Therefore, cellulose is usually studied in the form of an aqueous slurry. Fig. 1 shows an apparatus that we developed to study such mixtures. Here, pure water is pressurized with pump 1 and preheated to the supercritical state and then mixed with cellulose via a specially designed high pressure slurry pump. Following the reactor, the mixture is brought to ambient temperature by direct injection of cooling water and a heat exchanger. After particulate matter or solid product is removed with filters, pressure is reduced with a back-pressure regulator. Residence time can be varied, typically between 10 ms and 2 s, by changing the heated length of the reactor or by changing the flow rate. The detailed kinetics of cellulose, cellulosic oligomers, glucose, and a number of related reaction products have been reported in the literature w8x. Fig. 2 shows HPLC chromatograms for 100% conversion of cellulose in subcritical and supercritical water. It is apparent from the figures that higher yields of hydrolysis products are achieved in supercritical water than in subcritical water. That is, for 100% conversion of cellulose, more pyrolysis products are produced in subcritical water. Fig. 3 shows results for cellulose and model compounds cellobiose and glucose decomposition in supercritical water studied under flow conditions, where apparent rate constants are plotted vs. inverse temperature. The results show that the kinetics of cellulose hydrolysis undergo a jump to unprecedented higher hydrolysis rates as the conditions change from sub to supercritical water. The products are mainly water soluble oligomers, glucose, fructose and decomposition products of glucose. For temperatures less than the critical temperature of water, the glucose decomposition rate constant is greater than that for cellulose. However, at temperatures greater than Tc , the glucose rate constant is less than that of cellulose. This is why high yields of hydrolysis products were obtained and we believe that this phenomenon is due to the phase behavior of the celluloseq water system.
K. Arai, T. Adschirir Fluid Phase Equilibria 158–160 (1999) 673–684
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Fig. 1. Flow apparatus for studying reaction of slurries with supercritical water.
We have performed some visual studies on the decomposition of cellulose in supercritical water with a diamond anvil cell ŽDAC. . In the DAC technique, a sample is placed in a 250–500 mm diameter hole of a metal gasket and then the gasket is placed between two diamond anvils. Pressure is
Fig. 2. HPLC analyses for 100% conversion of cellulose in water at 25 MPa at subcritical and supercritical temperatures. Residence times are shown beneath each chromatogram.
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K. Arai, T. Adschirir Fluid Phase Equilibria 158–160 (1999) 673–684
Fig. 3. Arrhenius plot for cellulosic compound decomposition in subcritical and supercritical water. Key: Ža. cellulose; Žb. cellobiose; Žc. glucose.
Fig. 4. Diamond anvil cell study of cellulose in water at 60 MPa. Conditions: Ža. 295 K; Žb. 553 K; Žc. – Žf. photos at approximately 1 s intervals with heating from 553 K at a rate of 10 Krs. Diameter of Re gasket hole is 500 mm with a thickness of 250 mm. Ruby is for pressure measurement.
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measured by determining the peak shift of a ruby with Raman spectroscopy. Temperature is controlled by heaters external to the anvils. Typical results are shown in Fig. 4. In Fig. 4a, the loading and conditions are those of room temperature and 60 MPa. The solution was slowly heated to 553 K and held at that temperature. Only slight changes can be observed. Then, the system was rapidly heated at approximately 10 Krs heating rate as shown in Fig. 4b–c. The cellulose gradually disappears over a few seconds and seemingly melts away in the solution. The phenomenon is probably one of dissolution and reactive hydrolysis, however, the phase behavior probably accounts, to some extent, for the drastic change of the kinetics shown in Fig. 2.
3. Polymers Because many of the polymers cannot be readily put into slurry form, we have studied their reaction in subcritical and supercritical water using either semi-batch or batch type apparatus. For semi-batch experiments, a reactor is loaded with polymer and reaction solvent at the desired temperature is flowed through the reactor. For batch experiments, a stainless steel Ž SS 316. tube bomb reactor Ž6 cm3 . equipped with an internal thermocouple was used. Results discussed next are those from batch experiments. 4. Polyethylene terephthalate ( PET) Results for the decomposition of PET in supercritical water are shown in Fig. 5. The reaction produced primarily its monomer, terephthalic acid and ethylene glycol. Formation of other products such as benzoic acid, benzene, acetic acid and carbon dioxide due to the further decomposition or carboxylation were negligible under the present conditions. Consequently, little or no gaseous products were formed in these reactions.
Fig. 5. PET conversion in supercritical water determined from batch reactor experiments.
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K. Arai, T. Adschirir Fluid Phase Equilibria 158–160 (1999) 673–684
Comparing the PET results with those of the cellulose experiments, one can see that the reaction times are much longer for that of PET. This is generally true for other polymers that we have studied including nylon, polyurethane, and other condensation polymers. The functional group reactivity and chemistry are the most important factors but the phase equilibria plays an important role in the kinetics, especially whether one is dealing with a homogeneous or a heterogeneous reaction environment. For the PET system, we are also performing DAC experiments to gain additional insight on the reactive system phase equilibria.
5. Polyethylene For polymers derived from addition polymerization, such as polyethylene or polypropylene, reaction is slow in supercritical water. For these polymers, thermal cracking is a well known conversion technique to recover oils w9,10x. Control of product distribution of the pyrolyzed oils is considered to be an important technical factor for practical recycle. Pyrolysis in the presence of a solvent may be one method for controlling pyrolysis product distribution as reported in studies of heavy oil cracking or its model compounds w11,12x. Reaction temperature is frequently above 673 K, which is well above the critical temperature of most applicable solvents. Therefore, we employed supercritical water as a reaction solvent for the chemical cracking of these plastics expecting control of the product distribution. 5.1. Materials and experimental conditions Samples used were with powder form Žnominally 50 mm. low density polyethylene ŽMw s 68,000. and n-hexadecane Ž99 q %.. Experiments were conducted under batch conditions with water loading densities ranging from 0.1 to 0.42 grcm3 in bomb reactors as described above. Pressure in the reactor for experiments using water were estimated from temperature and density data along steam table data w13x. Experiments were also conducted without water and in the presence of an argon atmosphere. Experiments were conducted at 30 min reaction time. In the polyethylene experiments, samples were first fractionated to THF soluble and insoluble Ž THFI. products.
6. n-Hexadecane Since n-hexadecane is miscible with supercritical water w14x, the reaction phase is homogeneous. As alcohols, carboxylic acids, and carbon dioxide were not detected in the products, the main reaction for the n-alkane decomposition under the present experimental conditions is considered to be pyrolysis even in supercritical water. The product distribution and 1-alkenern-alkane ratios obtained for each case were practically the same. The n-hexadecane first order rate constant obtained in the presence of supercritical water was not significantly different from that obtain for pyrolysis in an argon atmosphere. In general, the role of supercritical water in a reaction w15x can be Ž 1. cage effect, Ž2. water attack on species or thermolysis, Ž 3. rate or equilibrium change due to variation of dielectric
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Fig. 6. Product distribution for the pyrolysis of polyethylene at 963 K and 30 min reaction time. Symbols: Žfilled triangles. in argon Ž0.1 MPa.; Žfilled circles. in supercritical water, r s 0.13 grcm3; Žopen circles. in supercritical water, r s 0.42 grcm3.
constant, and Ž4. change of phase behavior. However, under the present condition, the effect of SCW on the n-hexadecane decomposition was found to be insignificant w16x. 6.1. Polyethylene product distribution Pyrolysis of polyethylene is considered to proceed mainly in a molten polymer phase, and thus the reaction environment is very different from the case of n-hexadecane pyrolysis. Fig. 6 shows product
Fig. 7. Product distribution for pyrolysis of polyethylene at 963 K and 30 min reaction time. Symbols: Žfilled triangles. in argon Ž0.1 MPa.; Žfilled circles. in supercritical water, r s 0.13 grcm3; Žopen circles. in supercritical water, r s 0.42 grcm3.
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distribution for lighter product gas products plus THF soluble products. Yields of shorter chain hydrocarbons increased with increasing water density, which showed that polyethylene pyrolysis was enhanced in supercritical water. As shown in Fig. 7, the 1-alkenern-alkane ratio for the lighter products increased with increasing water density. Thus, polyethylene pyrolysis in supercritical water was clearly different from that in argon, even though differences could not be observed for n-hexadecane pyrolysis. This seemed to be due to the difference in the reaction phase, that is, a homogeneous reaction phase for the case of n-hexadecane and a heterogeneous phase for the case of polyethylene. In the argon atmosphere, pyrolysis of polyethylene occurs mainly in the molten polymer phase. On the other hand, supercritical water can dissolve some lighter hydrocarbons produced from pyrolysis. Therefore, pyrolysis of these hydrocarbons in the supercritical phase affects the product distribution. Also, some water is considered to dissolve into the molten polymer phase, which must dilute the phase and influence the pyrolysis.
7. Catalytic hydrogenating desulfurization and denitrogenation In previous work, we reported on advantageous features of catalytic reactions in supercritical fluids w17,18x. Hydrogenating desulfurization of heavy oil with solid catalyst in SCW is attractive for industrial processing. The conventional method for nitrogen removal from coal tar pitch is to perform the catalytic hydrotreatment with H 2 gas. Problems involved in this process are severe coke deposition on the catalyst and slow reaction due to diffusion of H 2 into the molten pitch on the catalyst surface. In previous work, we demonstrated that these two problems can be solved by using SCF as a reaction media. Coke deposition can be suppressed due to the in-situ extraction of coke precursor during reaction and faster reaction was achieved because of the homogeneous reaction atmosphere for H 2-SCF-pitch. Similar specific features of solid catalyzed reactions in SCF have been reported for the other reactions. Tiltscher et al. w19x proposed using SCF to remove catalyst inhibitors in situ and demonstrated the process with studies of solid catalyzed reactions. Yokota et al. w20x have intensively studied Fischer–Tropsch synthesis in supercritical n-hexane. The problem of the liquid phase reaction is the slow reaction due to the limited mass transfer rate. On the other hand, the problem of the gas phase reaction is the deactivation of catalyst by the deposition of wax formed. They reported that both fast reaction and in-situ extraction of wax were performed in supercritical n-hexane. Saim and Subramaniam w21x studied the catalytic hydrogenation of heavy oil and also reported the effective in-situ extraction of coke precursor from the catalyst surface. While similar effects can be expected for the catalytic desulfurization from heavy oil by using SCF, through experiment we found another specific feature for the reaction in SCW, which is that an excellent hydrogenating atmosphere can be supplied through partial oxidation of hydrocarbons. We conducted experiments w22,23x of dibenzothiophene Ž DBT. hydrogenation with NiMorAl 2 O 3 at 673 K and 30 MPa, in various atmospheres Ž H 2-SCW, CO-SCW, CO 2 –H 2-SCW, and HCOOH-SCW. , using tube bomb reactors. Higher conversion of DBT was obtained in CO-SCW than in H 2-SCW. This result is probably due to the hydrogen forming reaction through the water–gas shift reaction ŽCO q H 2 O s CO 2 q H 2 .. Next, CO 2 was added to H 2-SCW. For this case, higher conversion of DBT was obtained than in H 2-SCW. In the products, trace amounts of CO were observed which is
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strong evidence of the reverse water–gas shift reaction. This means that hydrogenating species probably forms through the H 2 –H 2 O exchange in water–gas shift reaction. Researchers have suggested w24,25x that the water–gas shift reaction in SCW proceeds via HCOOH. For examining this possibility, we used HCOOH as a reactant. Similarly high conversion of DBT was obtained, which is further evidence that the water–gas shift reaction in SCW produced the species which could hydrogenate DBT more effectively than H 2 gas. Next, we conducted another experiment for partial oxidation of DBT–hexylbenzene solution in SCW. Even in the presence of oxygen, effective hydrogenation of DBT took place. This result is probably because CO forms through the partial oxidation of hexylbenzene and converts to the hydrogenating species through water–gas shift reaction. From these results, catalytic desulfurization of heavy oil in SCW seems to be a promising new technology, since even by introducing oxygen instead of costly hydrogen, excellent hydrogenating atmosphere can be supplied, and the reaction phase for the oil–water gases can be homogeneous.
8. Inorganics Our group has a fair number of applications under development regarding hydrothermal syntheses in supercritical water w26,27x. These include materials processing, new material production Ž BaO 6Fe 2 O 3 , phosphor, TiO 2 , etc.. and metal recovery from waste water. 8.1. Wastewater In this work, an application concerning treatment of high level liquid waste ŽHLLW. from the nuclear power industry will be discussed. As is well documented, approximately 30% of the total electrical power generated in Japan, 10 q % of all energy, is derived from nuclear sources w28x. As a result, treatment of HLLW has become important for future use. HLLW can be stabilized by a solidification process known as vitrification w29x and then stored for 30 to 50 years above ground before it can be buried in deep geological formations w30x. The storage represent a major problem since in Japan, such space is limited. Heat and reactivity of the HLLW is dominated by the elements 90 Sr and 137Cs, which have half-lives of 27.7 and 30.0 years. This heat creates excessive cracking and can greatly accelerate corrosion of the glass. Our objective was to examine the separation of metals from simulated HLLW mixtures through addition of thermal energy. Hydrothermal techniques have the advantage over extraction or adsorption techniques in that only thermal energy is required and no additional materials are contaminated. Complete details can be found in Ref. w31x. We considered simulated wastes dissolved in 1.6 M nitric acid, which is the typical acid concentration used in processing these streams, and used batch experiments in a similar fashion as for the plastics. Results are shown in Fig. 8, where it can be seen that it is possible to achieve selective separation of various metals by only adding thermal energy. For the system studied, almost 40% of the metals in solution could be precipitated or crystallized from solution. The phase behavior of this system is very complex and probably involves three Ž VLS. or four phase equilibria ŽVLLS. . Much is still unknown about the phase equilibrium of oxideq
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Fig. 8. Separation of metals from simulated HLLW through a reactive crystallization. Total metal content is around 95,000 ppm and nitric acid concentration is 1.6 M. Pressure approximately 30 MPa. Ref. w27x.
supercritical water systems and even more importantly, about reactive metal nitrateq supercritical water systems. 8.2. Particle morphology For understanding chemical reaction equilibria, a fundamental study for metal oxide solubility was conducted. For explaining the results, a model based on the Gibb’s free energy change by temperature, solvent effects and ion–ion interactions was employed. Our estimation model is represented as follows w32x: DG o ln K s y
RT
sAq
B T
q C ln
T
D 1 q
Tr
T
ž
where the terms in Eq. Ž 1. are given by: o D Sro q DCp,r Asy R o Tr DCp,r y D Hro Bs R DCp,r Cs R ji zi D s 83,549 Ý ri i log 10g i s yA D z i2 Ž 'I r Ž 1 q 'I . y 0.2 I . A D s 1.82 = 10 6 Ž ´ T . I s 1r2 Ý z i Ci i
y3r2
´
1 y
´0
/
q ln Pg iÕi
Ž1.
Ž2. Ž3. Ž4. Ž5. Ž6. Ž7. Ž8.
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and Tr refers to a reference temperature Ž 298.15 K.. The g i in the final term can be evaluated by the Debye–Huckel law. The model expressed the solvation effect with decreasing dielectric constant around the critical point by Born’s equation and for ion–ion interaction by the Debye–Huckel equation. The equations are applied in a recent study w28x. Around the critical point, the morphology of boehmite ŽAlOOH. particles varied strongly with the reaction temperature, pressure and concentration of aqueous aluminum nitrate solution. Particle morphology obtained experimentally could be related to the chemical species in solution. By using this model, the distribution of chemical species for the AlOOH system Ž Al 3q, AlŽOH. 2q, AlŽ OH.q 2, y. Ž . Ž . AlŽOH. 3 , AlŽOH.y , NO in subcritical 3508C, 30 MPa and supercritical water 4008C, 30 MPa 4 3 was calculated. Particle morphology seemed to be determined by selective adsorption of positive w x charged species, AlŽOH. q 2 , on the surface of AlOOH crystal 28 . 9. Conclusions Some recent research on reacting systems in supercritical water has been reviewed in this work. For each system, the chemical and phase equilibria has been shown to be important for interpreting the results and for understanding the fluid environment. Knowledge of the phase equilibria will allow the development of more theoretical techniques that can be widely applied. 10. Nomenclature A, B, C, D Ci G H K k Cp R z r Greek ´ g r n
Constants in Eq. Ž1. Concentration Gibb’s free energy Enthalpy Equilibrium constant Kinetic rate constant Heat capacity Gas constant Ionic charge Born radius Dielectric constant Activity coefficient Density Stoichiometric coefficient
Acknowledgements The authors wish to acknowledge support of the following: Proposal-Based Advanced Industrial Technology Development Organization Ž NEDO. of Japan; Grant-in-Aid for Scientific Research on Priority Areas Ž06214202, 06214242.; Grant-in-Aid for Scientific Research on Priority Areas
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Ž05453105, 07242207.; Grant-in-Aid for Scientific Research Ž 07405936, 07455433. ; Grant-in-Aid for Developmental Scientific Research Ž 06555230, 07555232. .
References w1x S. Saito, J. Supercritical Fluids 8 Ž1995. 177. w2x E. Kiran, J.M.H. Levelt Sengers, Supercritical Fluids, Fundamentals for Application, Kluwer Academic Publishers, Dordrecht, 1994. w3x M.A. Abraham, A.K. Sunol ŽEds.., Supercritical fluids—extraction and pollution prevention, ACS Symposium Series 670, ACS, Washington, DC, 1997. w4x M. Perrut, P. Subra ŽEds.., Materials and natural products processing, Proceedings of the 5th Meeting on Supercritical Fluids, Nice, France, March 23–25, 1998. w5x K. Arai, Simulation, material conversion, and chemistry of supercritical water systems, 4th International Symposium on Supercritical Fluids, Sendai, Japan, May 11–14, 1997. w6x O. Bobleter, J. Anal. Appl. Pyrol. 17 Ž1994. 319. w7x D. Fengel, G. Wegener ŽEds.., Wood: Chemistry, Ultrastructure, Reactions, Walter de Gruyter Publishing, New York, 1989. w8x M. Sasaki, B. Kabyemela, R. Malaluan, S. Hirose, N. Takeda, T. Adschiri, K. Arai Ž1998. in press. w9x W. Kaminsky, H. Rossler, CHEMTECH 2 Ž1992. 108. w10x H. Kastner, W. Kaminsky, Hydrocarbon Process. 5 Ž1995. 109. w11x F. Khorasheh, M.R. Gray, Ind. Eng. Chem. Res. 32 Ž1993. 1864. w12x F. Khorasheh, M.R. Gray, Energy and Fuels 7 Ž1993. 960. w13x L. Haar, J.S. Gallagher, G.S. Kell, NBCrNRC Steam Tables, McGraw-Hill, New York, 1984. w14x E. Brunner, Fluid mixtures at high pressures, J. Chem. Thermodyn. 22 Ž1990. 335. w15x P.E. Savage, S. Gopalan, T.I. Mizan, C.J. Martino, E.E. Brock, AIChE J. 41 Ž1995. 1723. w16x M. Watanabe, H. Hirakoso, S. Sawamoto, T. Adschiri, K. Arai, J. Supercrit, Fluids Ž1998. in press. w17x T. Adschiri, T. Suzuki, K. Arai, Fuel 70 Ž12. Ž1991. 1483–1484. w18x T. Adschiri, K. Nakata, S. Ogasawara, K. Arai, Kagaku Kogaku Ronbunshu 20 Ž1994. 965–970. w19x H. Tiltscher, H. Wolf, J. Schelchshorn, Ber. Bunsenges. Phys. Chem. 88 Ž1984. 897–900. w20x K. Yokota, Y. Hanakata, K. Fujimoto, Fuel 70 Ž1991. 989–994. w21x S. Saim, B. Subramaniam, J. Catal. 131 Ž2. Ž1991. 445–456. w22x T. Adschiri, T. Sato, R. Shibata, K. Arai, Catalytic hydrogenating desulfurization of heavy oil through partial oxidation in supercritical water, 9th International Conference on Coal Science, 1997, pp. 1505–1508. w23x T. Adschiri, R. Shibata, T. Sato, M. Watanabe, K. Arai, I&EC Res. Ž1998. in press. w24x Th. Hirth, E.U. Franck, Ber. Bunsenges. Phys. Chem. 9 Ž1993. 1091–1098. w25x C.F. Melius, N.E. Bergan, J.E. Shepherd, 23rd Sympos. ŽInternational. on Combustion, The Combustion Institute, 1990, pp. 217–223. w26x T. Adschiri, K. Kanazawa, K. Arai, J. Am. Ceram. Soc. 75 Ž1992. 1019–1022. w27x T. Adschiri, K. Kanazawa, K. Arai, J. Am. Ceram. Soc. 75 Ž1992. 2615–2618. w28x Y. Hidehiko, New nuclear energy toward the 21st century, J. Jpn. Trade Ind. 5 Ž1994. 15. w29x W. Lutze, B. Grambow, Radiochim. Acta 58r59 Ž1992. 3. w30x P.W. Cains, K.C. Yewer, S. Waring, Radiochim. Acta 56 Ž1992. 99. w31x R.L. Smith Jr., P. Atmaji, Y. Hakuta, M. Kawaguchi, T. Adschiri, K. Arai, J. Supercritical Fluids 11 Ž1997. 103–114. w32x Y. Hakuta, T. Adschiri, H. Hirakoso, K. Arai, Poster presentation at 8th International Conference on Properties and Phase Equilibria for Product and Process Design, April 26–May 1, 1998.
Importance of phase equilibria for understanding supercritical fluid environments Kunio Arai ) , Tadafumi Adschiri Tohoku UniÕersity, Department of Chemical Engineering, Aoba-ku Aramaki Aza Aoba-07, Sendai 980-8579, Japan Received 25 May 1998; accepted 18 December 1998
Abstract In this work, several applications are reviewed and the importance of phase equilibria is discussed. In the reaction of cellulose in subcritical and supercritical water for producing chemicals, cellulose decomposes to form lower carbon number sugars, oligomers, and various acids. As conditions change from a subcritical reaction environment to a supercritical reaction environment, we observed a drastic change of the kinetic rate constant, which is thought to be the result of the cellulose–water system changing from a heterogeneous state to a homogeneous state. Diamond anvil cell measurements ŽDAC. are used to provide visualization of phase changes in the cellulose–water system. In the reaction of polyethylene terephthalate ŽPET. in supercritical water for recycling, PET decomposes to form oligomers, ethylene glycol, and the monomer, terephthalic acid. The reaction kinetics for this polymer–water system is much slower and requires 10 min for 100% conversion vs. 50 ms for 100% conversion of cellulose. The reason is thought to be due to the number and type of reaction phases present in the system, that is, solid–fluid or liquid–fluid phases. In the hydrogenation of heavy oils for energy conversion applications, the reaction kinetics can be very slow. Supercritical water provides a homogeneous reaction environment for the hydrogenq heavy oil system. However, for this case, we have found that an excellent hydrogenating atmosphere can be made by addition of oxygen, which apparently provides active hydrogen from CO formed, through the water–gas shift reaction. In catalytic reactions of heavy oils, in-situ extraction of coke precursors is provided by the supercritical water environment. Further studies are reported on the oxidation of hydrocarbons. In the treatment of high-level liquid wastes from the nuclear industries, it is found that many metals can be removed from solution by raising the temperature and pressure of the aqueous solutions. These results are discussed in view of recent supercritical water metal oxide solubility and supercritical water density measurements. q 1999 Elsevier Science B.V. All rights reserved. Keywords: Supercritical water; Cellulose; Hydrogenation; Energy conversion; Nuclear wastes
)
Corresponding author. Tel.: q81-22-217-7247; Fax: q81-22-217-7246; E-mail: [email protected]
0378-3812r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 8 - 3 8 1 2 Ž 9 9 . 0 0 1 1 6 - 8
674
K. Arai, T. Adschirir Fluid Phase Equilibria 158–160 (1999) 673–684
1. Introduction Physical properties, chemical and phase equilibria are fundamental to understanding and interpreting new reaction phenomena observed in supercritical fluids w1–4x. Supercritical water has great potential as a generic technology for waste treatment, material production, chemical processing and a number of new applications w5x. In this work, some areas in which we have performed research will be discussed and places where the chemical and phase equilibria play an important role in the reaction environment will be shown.
2. Biomass Biomass is an important future raw material for energy and chemical applications. However, its practical conversion to chemical products, has not been realized yet on an industrial scale. About 5 = 10 9 tonsryear of cellulose occurs in the form of waste, mainly as agricultural and municipal wastes, in addition to the large amount of wood harvested. A thorough comparison of plant biomass with alternate energy resources has been made by Bobleter w6x. In this section, our interest was to examine conversion of cellulose and its oligomers in supercritical water. Cellulose is a linear polymer in which anhydrocellobiose units are bound by bŽ1–4. glycosidic bonds. Due to the presence of intra- and inter-molecular hydrogen bonding, cellulose aggregates in elementary fibrils and micro fibrils that have alternating high crystallinity and less ordered regions w7x. Cellulose is insoluble in most solvents due to these regions that limit solution accessibility. Therefore, cellulose is usually studied in the form of an aqueous slurry. Fig. 1 shows an apparatus that we developed to study such mixtures. Here, pure water is pressurized with pump 1 and preheated to the supercritical state and then mixed with cellulose via a specially designed high pressure slurry pump. Following the reactor, the mixture is brought to ambient temperature by direct injection of cooling water and a heat exchanger. After particulate matter or solid product is removed with filters, pressure is reduced with a back-pressure regulator. Residence time can be varied, typically between 10 ms and 2 s, by changing the heated length of the reactor or by changing the flow rate. The detailed kinetics of cellulose, cellulosic oligomers, glucose, and a number of related reaction products have been reported in the literature w8x. Fig. 2 shows HPLC chromatograms for 100% conversion of cellulose in subcritical and supercritical water. It is apparent from the figures that higher yields of hydrolysis products are achieved in supercritical water than in subcritical water. That is, for 100% conversion of cellulose, more pyrolysis products are produced in subcritical water. Fig. 3 shows results for cellulose and model compounds cellobiose and glucose decomposition in supercritical water studied under flow conditions, where apparent rate constants are plotted vs. inverse temperature. The results show that the kinetics of cellulose hydrolysis undergo a jump to unprecedented higher hydrolysis rates as the conditions change from sub to supercritical water. The products are mainly water soluble oligomers, glucose, fructose and decomposition products of glucose. For temperatures less than the critical temperature of water, the glucose decomposition rate constant is greater than that for cellulose. However, at temperatures greater than Tc , the glucose rate constant is less than that of cellulose. This is why high yields of hydrolysis products were obtained and we believe that this phenomenon is due to the phase behavior of the celluloseq water system.
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Fig. 1. Flow apparatus for studying reaction of slurries with supercritical water.
We have performed some visual studies on the decomposition of cellulose in supercritical water with a diamond anvil cell ŽDAC. . In the DAC technique, a sample is placed in a 250–500 mm diameter hole of a metal gasket and then the gasket is placed between two diamond anvils. Pressure is
Fig. 2. HPLC analyses for 100% conversion of cellulose in water at 25 MPa at subcritical and supercritical temperatures. Residence times are shown beneath each chromatogram.
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Fig. 3. Arrhenius plot for cellulosic compound decomposition in subcritical and supercritical water. Key: Ža. cellulose; Žb. cellobiose; Žc. glucose.
Fig. 4. Diamond anvil cell study of cellulose in water at 60 MPa. Conditions: Ža. 295 K; Žb. 553 K; Žc. – Žf. photos at approximately 1 s intervals with heating from 553 K at a rate of 10 Krs. Diameter of Re gasket hole is 500 mm with a thickness of 250 mm. Ruby is for pressure measurement.
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measured by determining the peak shift of a ruby with Raman spectroscopy. Temperature is controlled by heaters external to the anvils. Typical results are shown in Fig. 4. In Fig. 4a, the loading and conditions are those of room temperature and 60 MPa. The solution was slowly heated to 553 K and held at that temperature. Only slight changes can be observed. Then, the system was rapidly heated at approximately 10 Krs heating rate as shown in Fig. 4b–c. The cellulose gradually disappears over a few seconds and seemingly melts away in the solution. The phenomenon is probably one of dissolution and reactive hydrolysis, however, the phase behavior probably accounts, to some extent, for the drastic change of the kinetics shown in Fig. 2.
3. Polymers Because many of the polymers cannot be readily put into slurry form, we have studied their reaction in subcritical and supercritical water using either semi-batch or batch type apparatus. For semi-batch experiments, a reactor is loaded with polymer and reaction solvent at the desired temperature is flowed through the reactor. For batch experiments, a stainless steel Ž SS 316. tube bomb reactor Ž6 cm3 . equipped with an internal thermocouple was used. Results discussed next are those from batch experiments. 4. Polyethylene terephthalate ( PET) Results for the decomposition of PET in supercritical water are shown in Fig. 5. The reaction produced primarily its monomer, terephthalic acid and ethylene glycol. Formation of other products such as benzoic acid, benzene, acetic acid and carbon dioxide due to the further decomposition or carboxylation were negligible under the present conditions. Consequently, little or no gaseous products were formed in these reactions.
Fig. 5. PET conversion in supercritical water determined from batch reactor experiments.
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Comparing the PET results with those of the cellulose experiments, one can see that the reaction times are much longer for that of PET. This is generally true for other polymers that we have studied including nylon, polyurethane, and other condensation polymers. The functional group reactivity and chemistry are the most important factors but the phase equilibria plays an important role in the kinetics, especially whether one is dealing with a homogeneous or a heterogeneous reaction environment. For the PET system, we are also performing DAC experiments to gain additional insight on the reactive system phase equilibria.
5. Polyethylene For polymers derived from addition polymerization, such as polyethylene or polypropylene, reaction is slow in supercritical water. For these polymers, thermal cracking is a well known conversion technique to recover oils w9,10x. Control of product distribution of the pyrolyzed oils is considered to be an important technical factor for practical recycle. Pyrolysis in the presence of a solvent may be one method for controlling pyrolysis product distribution as reported in studies of heavy oil cracking or its model compounds w11,12x. Reaction temperature is frequently above 673 K, which is well above the critical temperature of most applicable solvents. Therefore, we employed supercritical water as a reaction solvent for the chemical cracking of these plastics expecting control of the product distribution. 5.1. Materials and experimental conditions Samples used were with powder form Žnominally 50 mm. low density polyethylene ŽMw s 68,000. and n-hexadecane Ž99 q %.. Experiments were conducted under batch conditions with water loading densities ranging from 0.1 to 0.42 grcm3 in bomb reactors as described above. Pressure in the reactor for experiments using water were estimated from temperature and density data along steam table data w13x. Experiments were also conducted without water and in the presence of an argon atmosphere. Experiments were conducted at 30 min reaction time. In the polyethylene experiments, samples were first fractionated to THF soluble and insoluble Ž THFI. products.
6. n-Hexadecane Since n-hexadecane is miscible with supercritical water w14x, the reaction phase is homogeneous. As alcohols, carboxylic acids, and carbon dioxide were not detected in the products, the main reaction for the n-alkane decomposition under the present experimental conditions is considered to be pyrolysis even in supercritical water. The product distribution and 1-alkenern-alkane ratios obtained for each case were practically the same. The n-hexadecane first order rate constant obtained in the presence of supercritical water was not significantly different from that obtain for pyrolysis in an argon atmosphere. In general, the role of supercritical water in a reaction w15x can be Ž 1. cage effect, Ž2. water attack on species or thermolysis, Ž 3. rate or equilibrium change due to variation of dielectric
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Fig. 6. Product distribution for the pyrolysis of polyethylene at 963 K and 30 min reaction time. Symbols: Žfilled triangles. in argon Ž0.1 MPa.; Žfilled circles. in supercritical water, r s 0.13 grcm3; Žopen circles. in supercritical water, r s 0.42 grcm3.
constant, and Ž4. change of phase behavior. However, under the present condition, the effect of SCW on the n-hexadecane decomposition was found to be insignificant w16x. 6.1. Polyethylene product distribution Pyrolysis of polyethylene is considered to proceed mainly in a molten polymer phase, and thus the reaction environment is very different from the case of n-hexadecane pyrolysis. Fig. 6 shows product
Fig. 7. Product distribution for pyrolysis of polyethylene at 963 K and 30 min reaction time. Symbols: Žfilled triangles. in argon Ž0.1 MPa.; Žfilled circles. in supercritical water, r s 0.13 grcm3; Žopen circles. in supercritical water, r s 0.42 grcm3.
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distribution for lighter product gas products plus THF soluble products. Yields of shorter chain hydrocarbons increased with increasing water density, which showed that polyethylene pyrolysis was enhanced in supercritical water. As shown in Fig. 7, the 1-alkenern-alkane ratio for the lighter products increased with increasing water density. Thus, polyethylene pyrolysis in supercritical water was clearly different from that in argon, even though differences could not be observed for n-hexadecane pyrolysis. This seemed to be due to the difference in the reaction phase, that is, a homogeneous reaction phase for the case of n-hexadecane and a heterogeneous phase for the case of polyethylene. In the argon atmosphere, pyrolysis of polyethylene occurs mainly in the molten polymer phase. On the other hand, supercritical water can dissolve some lighter hydrocarbons produced from pyrolysis. Therefore, pyrolysis of these hydrocarbons in the supercritical phase affects the product distribution. Also, some water is considered to dissolve into the molten polymer phase, which must dilute the phase and influence the pyrolysis.
7. Catalytic hydrogenating desulfurization and denitrogenation In previous work, we reported on advantageous features of catalytic reactions in supercritical fluids w17,18x. Hydrogenating desulfurization of heavy oil with solid catalyst in SCW is attractive for industrial processing. The conventional method for nitrogen removal from coal tar pitch is to perform the catalytic hydrotreatment with H 2 gas. Problems involved in this process are severe coke deposition on the catalyst and slow reaction due to diffusion of H 2 into the molten pitch on the catalyst surface. In previous work, we demonstrated that these two problems can be solved by using SCF as a reaction media. Coke deposition can be suppressed due to the in-situ extraction of coke precursor during reaction and faster reaction was achieved because of the homogeneous reaction atmosphere for H 2-SCF-pitch. Similar specific features of solid catalyzed reactions in SCF have been reported for the other reactions. Tiltscher et al. w19x proposed using SCF to remove catalyst inhibitors in situ and demonstrated the process with studies of solid catalyzed reactions. Yokota et al. w20x have intensively studied Fischer–Tropsch synthesis in supercritical n-hexane. The problem of the liquid phase reaction is the slow reaction due to the limited mass transfer rate. On the other hand, the problem of the gas phase reaction is the deactivation of catalyst by the deposition of wax formed. They reported that both fast reaction and in-situ extraction of wax were performed in supercritical n-hexane. Saim and Subramaniam w21x studied the catalytic hydrogenation of heavy oil and also reported the effective in-situ extraction of coke precursor from the catalyst surface. While similar effects can be expected for the catalytic desulfurization from heavy oil by using SCF, through experiment we found another specific feature for the reaction in SCW, which is that an excellent hydrogenating atmosphere can be supplied through partial oxidation of hydrocarbons. We conducted experiments w22,23x of dibenzothiophene Ž DBT. hydrogenation with NiMorAl 2 O 3 at 673 K and 30 MPa, in various atmospheres Ž H 2-SCW, CO-SCW, CO 2 –H 2-SCW, and HCOOH-SCW. , using tube bomb reactors. Higher conversion of DBT was obtained in CO-SCW than in H 2-SCW. This result is probably due to the hydrogen forming reaction through the water–gas shift reaction ŽCO q H 2 O s CO 2 q H 2 .. Next, CO 2 was added to H 2-SCW. For this case, higher conversion of DBT was obtained than in H 2-SCW. In the products, trace amounts of CO were observed which is
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strong evidence of the reverse water–gas shift reaction. This means that hydrogenating species probably forms through the H 2 –H 2 O exchange in water–gas shift reaction. Researchers have suggested w24,25x that the water–gas shift reaction in SCW proceeds via HCOOH. For examining this possibility, we used HCOOH as a reactant. Similarly high conversion of DBT was obtained, which is further evidence that the water–gas shift reaction in SCW produced the species which could hydrogenate DBT more effectively than H 2 gas. Next, we conducted another experiment for partial oxidation of DBT–hexylbenzene solution in SCW. Even in the presence of oxygen, effective hydrogenation of DBT took place. This result is probably because CO forms through the partial oxidation of hexylbenzene and converts to the hydrogenating species through water–gas shift reaction. From these results, catalytic desulfurization of heavy oil in SCW seems to be a promising new technology, since even by introducing oxygen instead of costly hydrogen, excellent hydrogenating atmosphere can be supplied, and the reaction phase for the oil–water gases can be homogeneous.
8. Inorganics Our group has a fair number of applications under development regarding hydrothermal syntheses in supercritical water w26,27x. These include materials processing, new material production Ž BaO 6Fe 2 O 3 , phosphor, TiO 2 , etc.. and metal recovery from waste water. 8.1. Wastewater In this work, an application concerning treatment of high level liquid waste ŽHLLW. from the nuclear power industry will be discussed. As is well documented, approximately 30% of the total electrical power generated in Japan, 10 q % of all energy, is derived from nuclear sources w28x. As a result, treatment of HLLW has become important for future use. HLLW can be stabilized by a solidification process known as vitrification w29x and then stored for 30 to 50 years above ground before it can be buried in deep geological formations w30x. The storage represent a major problem since in Japan, such space is limited. Heat and reactivity of the HLLW is dominated by the elements 90 Sr and 137Cs, which have half-lives of 27.7 and 30.0 years. This heat creates excessive cracking and can greatly accelerate corrosion of the glass. Our objective was to examine the separation of metals from simulated HLLW mixtures through addition of thermal energy. Hydrothermal techniques have the advantage over extraction or adsorption techniques in that only thermal energy is required and no additional materials are contaminated. Complete details can be found in Ref. w31x. We considered simulated wastes dissolved in 1.6 M nitric acid, which is the typical acid concentration used in processing these streams, and used batch experiments in a similar fashion as for the plastics. Results are shown in Fig. 8, where it can be seen that it is possible to achieve selective separation of various metals by only adding thermal energy. For the system studied, almost 40% of the metals in solution could be precipitated or crystallized from solution. The phase behavior of this system is very complex and probably involves three Ž VLS. or four phase equilibria ŽVLLS. . Much is still unknown about the phase equilibrium of oxideq
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Fig. 8. Separation of metals from simulated HLLW through a reactive crystallization. Total metal content is around 95,000 ppm and nitric acid concentration is 1.6 M. Pressure approximately 30 MPa. Ref. w27x.
supercritical water systems and even more importantly, about reactive metal nitrateq supercritical water systems. 8.2. Particle morphology For understanding chemical reaction equilibria, a fundamental study for metal oxide solubility was conducted. For explaining the results, a model based on the Gibb’s free energy change by temperature, solvent effects and ion–ion interactions was employed. Our estimation model is represented as follows w32x: DG o ln K s y
RT
sAq
B T
q C ln
T
D 1 q
Tr
T
ž
where the terms in Eq. Ž 1. are given by: o D Sro q DCp,r Asy R o Tr DCp,r y D Hro Bs R DCp,r Cs R ji zi D s 83,549 Ý ri i log 10g i s yA D z i2 Ž 'I r Ž 1 q 'I . y 0.2 I . A D s 1.82 = 10 6 Ž ´ T . I s 1r2 Ý z i Ci i
y3r2
´
1 y
´0
/
q ln Pg iÕi
Ž1.
Ž2. Ž3. Ž4. Ž5. Ž6. Ž7. Ž8.
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and Tr refers to a reference temperature Ž 298.15 K.. The g i in the final term can be evaluated by the Debye–Huckel law. The model expressed the solvation effect with decreasing dielectric constant around the critical point by Born’s equation and for ion–ion interaction by the Debye–Huckel equation. The equations are applied in a recent study w28x. Around the critical point, the morphology of boehmite ŽAlOOH. particles varied strongly with the reaction temperature, pressure and concentration of aqueous aluminum nitrate solution. Particle morphology obtained experimentally could be related to the chemical species in solution. By using this model, the distribution of chemical species for the AlOOH system Ž Al 3q, AlŽOH. 2q, AlŽ OH.q 2, y. Ž . Ž . AlŽOH. 3 , AlŽOH.y , NO in subcritical 3508C, 30 MPa and supercritical water 4008C, 30 MPa 4 3 was calculated. Particle morphology seemed to be determined by selective adsorption of positive w x charged species, AlŽOH. q 2 , on the surface of AlOOH crystal 28 . 9. Conclusions Some recent research on reacting systems in supercritical water has been reviewed in this work. For each system, the chemical and phase equilibria has been shown to be important for interpreting the results and for understanding the fluid environment. Knowledge of the phase equilibria will allow the development of more theoretical techniques that can be widely applied. 10. Nomenclature A, B, C, D Ci G H K k Cp R z r Greek ´ g r n
Constants in Eq. Ž1. Concentration Gibb’s free energy Enthalpy Equilibrium constant Kinetic rate constant Heat capacity Gas constant Ionic charge Born radius Dielectric constant Activity coefficient Density Stoichiometric coefficient
Acknowledgements The authors wish to acknowledge support of the following: Proposal-Based Advanced Industrial Technology Development Organization Ž NEDO. of Japan; Grant-in-Aid for Scientific Research on Priority Areas Ž06214202, 06214242.; Grant-in-Aid for Scientific Research on Priority Areas
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Ž05453105, 07242207.; Grant-in-Aid for Scientific Research Ž 07405936, 07455433. ; Grant-in-Aid for Developmental Scientific Research Ž 06555230, 07555232. .
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