Ind. Eng. Chem. Res. 1999, 38, 1183-1191
1183
APPLIED CHEMISTRY Hydrothermal Reaction of Saturated and Unsaturated Nitriles: Reactivity and Reaction Pathway Analysis Bill Izzo and Michael T. Klein* Department of Chemical Engineering, University of Delaware, Newark, Delaware 19716
Concetta LaMarca and Noel C. Scrivner E.I. du Pont de Nemours & Company, Wilmington, Delaware 19898
The reactions of acrylonitrile, crotonitrile, propionitrile, and a small set of related reaction products were investigated in high-temperature water. Hydrolysis of the cyano group yielded the corresponding amides, which underwent similar hydrolysis to yield the carboxylic acids. The conversion of acrylonitrile was more rapid than that of its saturated analogue propionitrile, as the unsaturated site of acrylonitrile provided a facile pathway for the formation of alcohols, amines, and ethers. Carbon-carbon bond cleavage pathways involving the hydration of the olefin moiety were also observed for acrylonitrile. Introduction Nitriles are a commercially vital class of reactive intermediates and solvents, finding applications in a wide range of industries including petrochemical, polymers and plastics, pharmaceutical, and pesticides. One of the most important compounds of this class is acrylonitrile, which has a worldwide production of nearly 10 billion lbs/year (Shahani et al., 1996). The production volume and subsequent industrial application of acrylonitrile and other cyano-containing compounds not withstanding, these materials can pose several environmental- and health-related concerns. Nitriles show a high level of toxicity, with prolonged exposure resulting in metabolic cyanide formation. One important concern arises from the water solubility of these materials, which increases their potential for environmental impact. Acetonitrile is completely miscible with water at room temperature, and the solubility of acrylonitrile at ambient conditions is 8 wt % (Hashimoto et al., 1983, 1993). As water is produced via acrylonitrile manufacture, aqueous waste streams containing significant amounts of acrylonitrile and its major byproducts acetonitrile and hydrogen cyanide are an inherent part of the process. These factors call attention to the release of nitrilecontaining waste streams into the environment. Deepwell injection is an economically feasible disposal method for nitrile-containing waste streams. Initially, the method was employed for disposal of waste brines from chemical manufacturing; however, its applicability subsequently expanded to waste acid solutions, inorganic and organic chemicals, and heavy metals (Warner, 1989). Recognizing the potential for contamination of groundwaters, the U.S. EPA designed strict regulations governing the operation and monitoring of such wells in 1980. More * To whom correspondence should be addressed. Present address: Rutgers University, College of Engineering, 98 Brett Road, Piscataway, NJ 08854. E-mail:
[email protected].
stringent regulations were adopted in 1988 including containment for 10 000 years or until the waste becomes nonhazardous (LaMarca et al., 1996; Stalzer and Thornton, 1996). The goal of this work was to determine the chemical reaction pathways followed by nitriles in a deep-well environment through experiments at elevated temperatures. The efficacy of extrapolation to deep-well conditions clearly being dependent on the quality of the resolved information, special emphasis was placed on the procurement of mechanistic information. The hope is that such information can be of value in the determination of the fate of chemicals released into the environment and the establishment of associated regulations. Most previous studies of nitrile-containing compounds have been in the area of wet air oxidation and supercritical water oxidation. The attention, therefore, has been largely focused on the overall destruction efficiencies, with less analysis of the underlying reaction pathways and quantitative kinetics. This motivated the present investigation into the nonoxidative hydrothermal reaction of both saturated (acetonitrile, propionitrile, etc.) and unsaturated (acrylonitrile, methacrylonitrile, etc.) nitriles and their associated reaction products. More specifically, the goals include assessing the overall reactivity of these materials, identifying of reaction products, postulating mechanisms for their formation, and developing hydrothermal reaction networks. Experimental Methods Reactor System. All reactions were performed in the constant-volume batch reactors consisting of a reactor body and fastening cap, both constructed of 316 stainless steel by Trico Machine Products, Inc., based on design specifications by Autoclave Engineers. These reactors had an internal volume of 4.0 cm3 and a rated burst pressure of 171 MPa (25 000 psi). Most experimental pressures were in the range 40-120 bar. Prior to
10.1021/ie9803218 CCC: $18.00 © 1999 American Chemical Society Published on Web 02/10/1999
1184 Ind. Eng. Chem. Res., Vol. 38, No. 4, 1999
collection of experimental data, the reactors were conditioned with pure water at 300 °C for several hours, typically in excess of 4 h. On the basis of previous findings, this procedure served to passivate the reactor walls (Klein et al., 1992). A more detailed investigation of wall effects likewise proved such effects to be negligible for heterolytic reaction chemistries (Harrell and Klein, 1998). Experimental Procedure. A typical experimental procedure began with the preparation of aqueous reaction solutions at room temperature using argon-purged distilled water. All reactants, standards, and solvents were commercially available from Aldrich or Sigma Chemical Co. in purities of 98% or higher and were used as received. Water loadings were selected to ensure that the entire reactor volume was occupied by compressed liquid water. Saturated or compressed liquid densities at the desired reaction temperature were obtained from steam tables (Haar, 1984). Water was both a solvent and a reactant for many of the reactions investigated. For typical initial reactant loadings (0.4-4 mmol), the change in water concentration with reactant conversion was small, amounting to less than 5%. Thus, the reaction environment was assumed to be isobaric and isopicnic over the course of a reaction. The reaction solution was loaded into the reactor on a mass basis and verified with a Mettler AE200 balance ((0.1 mg). No further attempt was made to purge the reactor of air, as the reaction solution occupied a majority of the reactor volume. A 1-mm-thick titanium gasket was placed between the base and the top of the reactor to create a leak-free seal. The reactor was sealed in a benchtop vise using a torque wrench (120 ft‚lb) to achieve a consistent sealing pressure. The sealed reactor was subsequently placed into a steel basket and lowered into a preheated Techne SBL2D fluidized sandbath maintained at the desired reaction temperature, (2 °C, by an CS-6001PI Omega temperature controller connected to an Omega RTP probe. Reactors were submersed approximately 6 in. below the surface of the sand. Previous investigations have shown the nonisothermal reaction period for this reactor setup (Townsend and Klein, 1985) to be on the order of 3 min. This limited the application of this reactor system to reactions occurring on the time scale of tens of minutes to hours. All reactions reported here were for a time in excess of 100% of the maximum heating time. After the desired reaction time had elapsed (8 min to 12 h), the reactors were removed from the sandbath and immersed in an ice/water bath to quench the reaction. After cooling, the reactors were opened and the soluble contents were then recovered in a single phase in HPLCgrade solvent, either acetone or methanol. Reactors were triple rinsed with the solvent (∼12 mL). Analytical Methods. Two methods were employed in the ex situ identification of reaction products. First, a structural determination was made by mass spectrometry using a HP-5890 gas chromatograph equipped with an HP-5970 mass-selective detector. The separation of the reaction mixture was accomplished using a 50 m HP-FFAP acid-modified poly(ethylene glycol) capillary column with a column diameter of 0.2 mm and a film thickness of 0.33 µm. This column was not damaged by water present in the reaction samples and offered an efficient separation of low molecular weight, polar compounds (nitriles, amides, alcohols, and car-
Table 1. Reactants and Experimental Reaction Conditions Investigated
reactant H2CdCHCtN acrylonitrile CH3CH2CtN propionitrile
temp (°C)
pressure (bar)
water density (g/cm3)
reaction time (min)
250 250 250 300 300
40 120 40 87 120
0.799 0.808 0.799 0.713 0.720
10-300 10-45 90-300 30-300 180
250
120
0.808
20-50
250
40
0.799
60-180
250
40
0.799
10-180
250
40
0.799
10-180
boxylic acids). The mass spectra were matched to Wiley or NIST mass spectra libraries. Contingent upon the commercial availability of standards, these structural assignments were confirmed by co-injection and retention time matching. Quantitative analysis was completed via gas chromatography using an HP-5880A gas chromatograph equipped with a flame ionization detector. Response factors were determined from analysis of prepared standard mixtures which closely reproduced the concentrations of the reactant and products including the reactor water loadings. Biphenyl was added to the reaction mixture after extraction from the reactor and served as a calibration standard. An HP-5890 gas chromatograph equipped with a Porpak Q packed column with a thermal conductivity detector was used in the qualitative identification of nonflammable reaction components such as ammonia. Estimation of Experimental Uncertainty. An estimate of experimental error for measured values of molar yields, yi, defined as the moles of species per mole initial reactant, or calculated rate parameters, ki, is provided wherever possible. The error for experimental measurements was evaluated by performing multiple runs, typically 3-5 runs, at identical reaction conditions. Values of molar yields are reported as the mean value, with the error bounds being 1 standard deviation. This method of error estimation encompasses all stages of the reaction procedure including reaction solution preparation, reactor loading and unloading, and error associated with analytical techniques. Standard propagation of error methodology was employed to arrive at error estimates for calculated values of rate parameters. Experimental Results Reactants and Reaction Conditions Investigated. Table 1 summarizes the reactants, their chemical structures, and the reaction conditions studied. Acrylonitrile and propionitrile were studied in detail.
Ind. Eng. Chem. Res., Vol. 38, No. 4, 1999 1185 Table 2. Product Spectrum for Hydrothermal Reaction of Propionitrile and Acrylonitrile
Figure 1. Hydrothermal reaction kinetics of acrylonitrile and propionitrile.
In combination these materials cover two classes of nitriles. Propionitrile is a aliphatic or saturated nitrile, while acrylonitrile is an olefinic or unsaturated nitrile. The reaction chemistry of these reactants defines major reaction pathways and kinetics that can be generalized to their respective classes, although additional reaction pathways may arise with increasing structural differences. Supplemental experiments were conducted with crotonitrile, 3-hydroxypropionitrile, propionamide, butyramide, acrylamide, propionic acid, and butyric acid. These experiments provided additional information concerning the reaction networks of the former nitriles. Collectively, these experiments provided information about the global kinetics, the product spectra, and the quantitative yields. In a discussion of the results, a comparative analysis of the global kinetics for these nitriles first sets the basis for subsequent scrutiny of product yields. This allows inference of the controlling reaction pathways and mechanisms. Hydrothermal Reactivity of Nitriles. Figure 1 shows the kinetics of conversion for acrylonitrile and propionitrile at 250 °C and 40 bar as well as for propionitrile at 300 °C and 87 bar. Nearly complete conversion of acrylonitrile was obtained in 4 h. The reactivity of the propionitrile was significantly lower than that of the unsaturated acrylonitrile, as a conversion of only 32% was achieved after 5 h of reaction time. The lower reactivity for propionitrile allowed study of its kinetics at more severe conditions. At 300 °C, a propionitrile conversion comparable to that for acrylonitrile at 250 °C was observed at long reaction times. At short times, however, the initial reaction rate for acrylonitrile was still significantly higher than that for the saturated propionitrile. This difference in short-time reactivity is directly related to the chemical structure of these reactants and will be discussed further in the following sections. Product Spectrum of Acrylonitrile and Propionitrile. Table 2 lists, for acrylonitrile and propionitrile, the observed reaction products and their corresponding structures. Ten products were observed for the reaction of acrylonitrile. Of these, the major products were 3-hydroxypropionitrile, acrylamide, acrylic acid, acetic acid, and ammonia. No evidence of the secondary alcohol, 2-hydroxypropionitrile, was observed in the product spectrum. The minor products included 2-cya-
Figure 2. Time dependence of the product spectrum for the reaction of acrylonitrile at 250 °C and 40 bar.
noethyl ether, acetonitrile, acetamide, 3,3′-iminodipropionitrile, and cis/trans-dicyanocyclobutane. The cis/ trans-dicyanocyclobutane was observed in only small quantities and was unstable at room temperature. It was observed only when analysis was performed immediately following quenching of the reaction. The product spectra of propionitrile consisted solely of propionamide, propionic acid, and ammonia. No lower molecular weight products were present in the product spectrum of propionitrile. Temporal Variation of Molar Yields. Figure 2 shows the temporal variations of the molar yields for the major products from reaction of acrylonitrile in water at 250 °C. Note that the molar yield is defined
1186 Ind. Eng. Chem. Res., Vol. 38, No. 4, 1999
Figure 3. Time dependence of the product spectrum for the reaction of propionitrile at 300 °C and 87 bar.
here as the ratio of the species concentration to the initial reactant concentration. For reactants the term molar recovery is used. The minor products from the reaction of acrylonitrile (2-cyanoethyl ether, acetonitrile, acetamide, and 3,3′-iminodipropionitrile) appeared in less than 4% yield and are omitted from Figure 2. Clearly, 3-hydroxypropionitrile was the major primary product, as its initial rate of appearance matched the initial disappearance of acrylonitrile. A small amount of acrylamide also formed as a primary product. The ratio of the yields of 3-hydroxypropionitrile to those of acrylamide was approximately 50 at 75 min of reaction time. The shape of Figure 2 also shows that 3-hydroxypropionitrile underwent appreciable secondary reaction, possibly dehydration to regenerate the reactant, acrylonitrile. The stable end products of the reaction of acrylonitrile were acrylic acid and acetic acid. Figure 3 shows the temporal variation of the molar recovery of propionitrile and the yields of its reaction products at 300 °C and 87 bar. Reaction times of less than 50 min provided slightly higher yields of propionamide than propionic acid although this trend is reversed at longer residence times with the yield of propionic acid. At the longest residence time investigated, both products appear to approach constant values of molar yield, suggesting a possible thermodynamic equilibrium. The ratio of the propionic acid yield to the propionamide yield was 3 at 240 min of reaction time. This apparent equilibrium will be addressed in reaction studies with the amides as reactants along with the long-term stability of the carboxylic acid product. Hydrothermal Reaction of Primary and Secondary Products Reaction of Amides. The reactions of the nitriles were probed further through experiments conducted with the amides as the starting reactants. The results are shown in Figure 4 in the form of reactant recovery versus time for acrylamide and propionamide at 250 °C and 40 bar. A higher reactivity for both amides relative to their corresponding nitriles was observed. The molar recovery of acrylamide dropped to 2% after 60 min of reaction time, corresponding to 98% conversion. The reactivity of propionamide was significantly higher than that observed for propionitrile at the same reaction conditions. The molar recovery dropped to 30% in 90 min of reaction time. Recall that minimal reaction of
Figure 4. Time dependence of the reactant recovery for the reaction of propionamide and acrylamide at 250 °C and 40 bar. Table 3. Product Spectrum for Hydrothermal Reaction of Acrylamide
propionitrile was observed over the same reaction time. In addition, the molar recovery of propionamide achieved a constant value at long reaction times, confirming the existence of a thermodynamic equilibrium for the amide reaction. The disparity between the product spectra for propionitrile and acrylonitrile was likewise observed for the reactions of their product amides. The only products from reaction of propionamide were propionic acid and ammonia. No dehydration of the propionamide to propionitrile was observed. Acrylamide produced not only acrylic acid but several additional products as well. Its products, namely, acrylic acid, acetic acid, acetamide, 3-hydroxypropionamide, and 3-hydroxypropionic acid, are summarized in Table 3. Recall that reaction of acrylonitrile under identical conditions yielded detect-
Ind. Eng. Chem. Res., Vol. 38, No. 4, 1999 1187
able amounts of 3-hydroxypropionitrile but not the hydroxylated amide and acid products. The absence of these products in the acrylonitrile product spectrum is likely due to the low concentration of precursor amide and acid molecules. In addition to the above reaction products for acrylamide, small amounts of substituted pyridines and N-methylamides were also observed but not quantified. These reactions are considered less significant in the reaction of acrylonitrile based on their bimolecular nature and the low concentration of observed acrylamide in this reaction. A separate experiment with an equimolar mixture of propionic acid and acrylamide resulted in the formation of propionamide and subsequently confirmed the reversibility of this reaction. The reversibility of the amide reaction was probed further by reacting a mixture of propionamide and butyric acid at 250 °C and 40 bar. The initial concentration of both reactants was 0.4 M. A reaction time of 150 min was selected for these runs. Figure 4 indicates that this is a sufficient residence time to establish the equilibrium between the amide and acid. Eighty-three percent of the propionamide was converted to propionic acid and ammonia. More importantly, 8% of the butyric acid was converted to butyramide. Thus, the reaction of amide to carboxylic acid and ammonia is truly reversible. Reaction of Carboxylic Acids. Investigation of carboxylic acid stability under hydrothermal reaction conditions was probed through long-times studies of propionic acid. No decarboxylation of the propionic acid was observed after 5 h of reaction time at 300 °C, and nearly complete recovery of the starting material was achieved. Thus, aliphatic acids produced during the hydrothermal reaction of their parent nitriles are the stable end product. These results are in agreement with similar studies by Meyer and co-workers (Meyer et al., 1995). These previous studies have demonstrated the stability of acetic acids at reaction temperatures of 200400 °C. At higher reaction temperatures (∼600 °C), 35% hydrothermal conversion of acetic acid to methane and carbon dioxide was reported. With the addition of oxygen, 100% conversion of the acid was obtained at 600 °C. The results described above indicate that carboxylic acids are stable under neat hydrothermal conditions; however, the actual form of the carboxylic acid, neutral molecule versus anion, and the relative stability of these chemical species in the nitrile reaction system becomes an issue. Reaction of the intermediate amide produces both carboxylic acid and ammonia in equimolar proportions. The final form of these species in aqueous solutions is governed by the thermodynamics of the system. The carboxylic acids of interest in the present investigation are weak acids with less than 20% of the acid dissociating to the anionic form at ambient conditions. Rigorous analysis of the acid-base thermodynamics of carboxylic acid/ammonia systems confirmed that the majority of the acid remains in molecular form at hydrothermal conditions (Izzo et al., 1997). In addition, previous studies of the hydrothermal reactions of formic acid/sodium formate (Merchant, 1992) and malonic acid/ monosodium malonate (Maiella and Brill, 1996) systems indicate that the reaction of the anion is slower than the reaction of the neutral form of the molecule. Thus, the results of the neat hydrothermal reaction of propi-
Figure 5. Hydrothermal reaction of 3-hydroxypropionitrile at 250 °C and 120 bar.
Figure 6. First-rank Delplot for the reaction of acrylonitrile at 250 °C and 40 bar.
onic acid establish the stability of aliphatic acids over the reaction times of interest. Reaction of 3-Hydroxypropionitrile. The concentration profile for 3-hydroxypropionitrile given in Figure 2 showed appreciable secondary reaction of this species. The independent studies with 3-hydroxypropionitrile at 250 °C and 120 bar shown in Figure 5 verified these findings. The reaction of 3-hydroxypropionitrile was slow in comparison to the reaction of acrylonitrile with only 14% conversion in 30 min of reaction time. The major product formed from reaction of 3-hydroxypropionitrile was acrylonitrile. Thus, the reaction of acrylonitrile to 3-hydroxypropionitrile was reversible, with the forward reaction being more dominant. Small yields of acrylamide (500 °C). Literature Cited Abraham, M. A.; Klein, M. T. Pyrolysis of Benzylphenylamine Neat and with Tetralin, Methanol, and Water Solvents. Ind. Eng. Chem. Prod. Res. Dev. 1985, 24, 300-306. Abraham, M. A.; Klein, M. T. Solvent Effects During the Reaction of Coal Model Compounds. In Supercritical Fluids: Chemical and Engineering Principles and Applications; Squires, T. G., Paulaitis, M. E., Eds.; American Chemical Society: Washington, DC, 1987; Chapter 6. Abraham, M. A.; Wu, B. C.; Paspek, S. C.; Klein, M. T. Reactions of Dibenzylamine Neat and in Supercritical Fluid Solvents. Fuel Sci. Technol. Int. 1988, 6 (5), 557-568. Bhore, N. A.; Klein, M. T.; Bischoff, K. B. The Delplot Technique: A New Method for Reaction Pathway Analysis. Ind. Eng. Chem. Res. 1990a, 29, 313-316. Bhore, N. A.; Klein, M. T.; Bischoff, K. B. Species Rank in Reaction Pathways: Application of Delplot Analysis. Chem. Eng. Sci. 1990b, 45 (8), 2109-2116. Bjerre, A. B.; Sørensen, E. Thermal Decomposition of Dilute Aqueous Formic Acid Solutions. Ind. Eng. Chem. Res. 1992, 31, 1574-1577. Butskus, P. F. Cyclization Reactions Based on Acrylonitrile. Russ. Chem. Rev. 1962, 31 (5), 283-290. Fyfe, C. A. The Chemistry of the Hydroxyl Group; Interscience Publishers: London, 1970. Haar, L. NBS/NRC steam tables: thermodynamic and transport properties and computer programs for vapor and liquid states of water in SI units; Hemisphere Publishing Corp.: Washington, DC, 1984. Harrell, C. L.; Klein, M. T. Wall Effects for Reaction of Nitriles in High-Temperature Water. Unpublished results, 1998. Harrell, C. L.; Aschiri, T.; Klein, M. T. Hydrothermal Kinetics of Nitriles: Mechanism Elucidation. Adv. Environ. Res. 1997, 1 (3), 373-383. Hashimoto, K.; Morimoto, K.; Dobson, S. Acrylonitrile health and safety guide Environmental Health Criteria 28: Acrylonitrile; World Health Organization: Geneva, Switzerland, 1983.
Ind. Eng. Chem. Res., Vol. 38, No. 4, 1999 1191 Hashimoto, K.; Morimoto, K.; Dobson, S. IPCS International Program on Chemical Safety Environmental Health Criteria 154: Acetonitrile; World Health Organization: Geneva, Switzerland, 1993. Izzo, B. Ph.D. Thesis, University of Delaware, Newark, DE, 1998. Izzo, B.; Harrell, C.; Klein, M. T. AIChE J. 1997, 43, 2048-2058. Klein, M. T.; Torry, L. A.; Wu, B. C.; Townsend, S. H.; Paspek, S. C. Hydrolysis in Supercritical Water: Solvent Effects as a Probe of the Reaction Mechanism. J. Supercrit. Fluids 1990, 3, 222227. Klein, M. T.; Mentha, Y. G.; Torry, L. A. Decoupling Substituent and Solvent Effects during Hydrolysis of Substituted Anisoles in Supercritical Water. Ind. Eng. Chem. Res. 1992, 31, 182187. LaMarca, C.; Scrivner, N. C.; Gron, L. U.; Izzo, B. J.; Chu, A.; Klein, M. T. Hydrolysis of Nitriles: Experiments and Modeling. In Deep Injection Disposal of Hazardous and Industrial Waste: Scientific and Engineering Aspects; Apps, J. A., Tsang, C., Eds.; Academic Press: San Diego, CA, 1996. Lawson, J. R.; Klein, M. T. Influence of Water on Guaiacol Pyrolysis. Ind. Eng. Chem. Fundam. 1985, 24, 203-208. Maiella, P. G.; Brill, T. B. Spectroscopy of Hydrothermal Reactions. 5. Decarboxylation Kinetics of Malonic Acid and Mono Sodium Malonate. J. Phys. Chem. 1996, 100, 14352-14355. March, J. Advanced Organic Chemistry: Reactions, Mechanisms, and Structure, 4th ed.; John Wiley & Sons: New York, 1992. McMurry, J. Organic Chemistry; Brooks/Cole Publishing Co.: Monterey, CA, 1984. Merchant, K. P. Studies in Heterogeneous Reactions; University of Bombay: Bombay, India, 1992. Meyer, J. C.; Marrone, P. A.; Tester, J. W. Acetic Acid Oxidation and Hydrolysis in Supercritical Water. AIChE J. 1995, 41 (9), 2108-2121. Mok, W. S.; Antal, M. J. Formation of Acrylic Acid from Lactic Acid in Supercritical Water. J. Org. Chem. 1989, 54, 45964602. Rondestvedt, C. S.; Rowley, M. E. The Base-catalyzed Cleavage of β-Hydroxy Acids. J. Am. Chem. Soc. 1956, 78 (15), 38043811.
Shahani, G. H.; Gunardson, H. H.; Easterbrook, N. C. Consider Oxygen for Hydrocarbon Oxidation. Chem. Eng. Prog. 1996, 92 (11), 66-71. Stalzer, R. B.; Thornton, T. D. Reactivity and Fate of Acrylonitrile Process Wastewater on Deep-Well Injection. In Deep Injection Disposal of Hazardous and Inustrial Waste: Scientific and Engineering Aspects; Apps, J. A., Tsang, C., Eds.; Academic Press: San Diego, 1996. Thornton, T. D.; Savage, P. E. Phenol Oxidation in Supercritical Water. J. Supercrit. Fluids 1990, 3, 240-248. Thornton, T. D.; Savage, P. E. Phenol Oxidation Pathways in Supercritical Water. Ind. Eng. Chem. Res. 1992a, 31, 2451. Thornton, T. D.; Savage, P. E. Kinetics of Phenol Oxidation in Supercritical Water. AIChE J. 1992b, 38, 321. Townsend, S. H.; Klein, M. T. Dibenzyl ether as a probe into the supercritical fluid solvent extraction of volatiles from coal with water. Fuel 1985, 64 (5), 635. Townsend, S. H.; Abraham, M. A.; Huppert, G. L.; Klein, M. T.; Paspek, S. C. Solvent Effects during Reactions in Supercritical Water. Ind. Eng. Chem. Res. 1988, 27, 143-149. Tsao, C. C.; Zhou, Y.; Liu, X.; Houser, T. J. Reactions of Supercritical Water with Benzaldehyde, Benzylidenebenzylamide, Benzyl Alcohol, and Benzoic Acid. J. Supercrit. Fluids 1992, 5, 107-113. Warner, D. L. Land Storage and Disposal: Subsurface Injection of Liquid Hazardous Wastes. In Standard Handbook of Hazardous Waste Treatment and Disposal; Freeman, H. M., Ed.; McGraw-Hill Book Company: New York, 1989; pp 10.41-10.53.
Received for review May 22, 1998 Revised manuscript received December 4, 1998 Accepted December 18, 1998 IE9803218
