Oxidation kinetics of carbon monoxide in supercritical water - Energy

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Energy &Fuels 1987,1, 417-423

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Oxidation Kinetics of Carbon Monoxide in Supercritical Water Richard K. Helling and Jefferson W. Tester* Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139 Received March 2, 1987. Revised Manuscript Received June 22, 1987

Supercritical water is an ideal environment in which to oxidize and destroy dilute, aqueous, hazardous organic materials, but fundamental reaction kinetics in this medium are not known. This work determined the oxidative reaction kinetics of dilute carbon monoxide in supercritical water. Conditions in the isothermal, plug-flow reactor included temperatures between 400 and 540 "C, pressures around 24.6 MPa (3550 psig), and 6-13 s residence times. Empirical Arrhenius parameters and orders of reaction were determined in a global expression for the oxidation of carbon monoxide in supercritical water. The reaction was first order in carbon monoxide and zero order in oxygen, with an activation energy of 120 f 7.7 kJ/mol and a preexponential of 107.25*0.53. Hydrogen was a major product during the oxidation reactions and was produced by the water gas shift reaction. The fraction of carbon dioxide produced during oxidation experiments by the water gas shift decreased from 75% to 20% with increasing temperature. The water gas shift was about one-half order in carbon monoxide and less activated than the global oxidation, with an activation energy of 63 f 8.6 kJ/mol. The direct oxidation of carbon monoxide in supercritical water was much slower than predicted by existing models, either global expressions or a system of elementary reactions. The water gas shift was much faster than predicted. These results are qualitatively consistent with a reaction model where the solvent (water) forms a "cage" about the reactants.

Introduction Supercritical fluids are used as solvent extraction media' and also as an environment for chemical reactions, due to the unique solvent characteristics of these fluids. Supercritical conditions exist for water at temperature above 374 "C (705 O F ) and pressures above 22.1 MPa (3200 psia). Supercritical water properties are much different from those of room-temperature water and correlate strongly with the density of the water. Under supercritical conditions, water behaves like a dense gas with a high solubility of organics: complete miscibility in all proportions with ~ x y g e nhigh , ~ diffusivities,4r5low viscosity,6 and low solubility and dissociation of inorganics, particularly ionic salts.'q8 The solvation properties make supercritical water an excellent medium for oxidation of wastes, since organics and oxygen can be intimately mixed in a single, homogeneous phase and inorganics can be readily removed from solutions by precipitation. The oxidation of a variety of organics in supercritical water has been demonstrated by several authors.*12 All these investigators used flow reactors with residence times of 4 min or less and temperatures greater than 400 OC. In her experiments, Price12was able to eliminate 88-93% of the liquid TOC (total organic carbon), although the depressurized gas phase contained up to 11% carbon monoxide. In their experiments, Modell et al.l0 oxidized several toxic chlorinated hydrocarbons in supercritical water, destroying a t least 99.99% of the organic chlorides and 99.97% of the TOC (the maximum destruction efficiencies were limited by the resolution of the analytical techniques). Cunningham et al." used this process to completely destroy biopharmaceutical wastes. These studies proved the diverse applicability of the supercritical water oxidation process and identified that carbon monoxide oxidized relatively slowly, but the experiments and apparatus were

* Author t o whom correspondence should be addressed. 0887-0624/87/2501-0417$01.50/0

not designed to provide isothermal kinetic data. Oxidative reaction kinetics of supercritical water were also investigated by Wightman for phenol,13 a common component in industrial waste, and acetic acid, a characteristic byproduct of conventional wet oxidation. The activation energies for the oxidations were 63 and 230 kJ/mol(l5 and 55 kcal/mol) for phenol and acetic acid, respectively. The high activation energy for acetic acid is consistent with its slow rate of reaction under wet oxidation conditions. Pyrolysis reactions in supercritical water have been more extensively studied. The unique solvent properties of the fluid and the possibility of promoting hydrolysis reactions have led to the use of supercritical water in the pyrolysis of wood, coal, and model compounds related to these. (1)Paulaitis, M. E.;Krukonis, V. J.; Kurnik, R. T.; Reid, R. C. Rev. Chem. Eng. 1983,1(2),181. (2)Connolly, J. F. J. Chem. Eng. Data 1966,11, 13. (3) Pray, C.M.; Schweickert, C. E.; Minnich, B. H. Ind. Eng. Chem. 1952,44(5),1146. (4) Franck, E. U. Pure Appl. Chem. 1970,24,13. (5)Flarsheim, W.; Tsou, Y. M.; Trachtenberg, I.; Johnston, K. P.; Bard, A. J. J . Phys. Chem. 1986,90,3857. (6) Todheide, K. Water: A ComDrehensive Treatise; Plenum: New York, 1972;Vol: 1 p 482. (7)Marshall, W. L. High Temperature,High Pressure Electrochemistry in Aqueous Solutions, January 7-12, 1973. The University of Surry, England; National Association of Corrosion Engineers: Houston, TX, 1976;p 117. (8) Martynova, 0. I. High Temperature, High Pressure Electrochemistry in Aqueous Solutions, January 7-12, 1973. The University of Surry, England; National Association of Corrosion Engineers: Houston, TX, 1976;p 131. (9)Timberlake, S.H.; Hong, G. T.; Simson, M.; Modell, M. SAE Tech. Pap. Ser. 1982,No. 820872. (10)Modell, M.; Gaudet, G. C.; Simson, M.; Hong, G. T.; Bieman, K. Solid Wastes Management 1982,August. (11)Cunningham, V. L.;Burk, P. L.; Johnston, J. B.; Hannah, R. E. Presented at the AIChE Summer National Meeting, Boston, MA, August 26, 1986;paper 45c. (12)Price, C. M. S.M. Thesis, Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, MA, February 1981. (13)Wightman, T. J. M.S. Thesis, Department of Chemical Engineering, University of California, Berkeley, March 1981.

0 1987 American Chemical Society

Helling and Tester

418 Energy &Fuels, Vol. 1, No. 5, 1987

Initial work demonstrated the profound influence of supercritical water on pyrolysis products: no char formed from pyrolysis of glucose in supercritical water, and gasification increased over that possible in subcritical water.14-16 Recent work has confirmed the lack of char formation and provided evidence of carbonium ion, free radical, and hydrolysis reactions from the products of pyrolysis of glycerol," guaiaco1,18 and benzylphenylamine.lg The fundamental pyrolysis investigations have highlighted specific mechanisms for the influence of supercritical water on chemical reactions, which could aid the interpretation of oxidation data. The objective of this work was to determine the oxidative reaction kinetics of carbon monoxide in supercritical water under well-defined conditions. Carbon monoxide was selected as it is the simplest reactive carbon compound and has been well studied in uncatalyzed and catalyzed gas-phase reactions. It also oxidiza more slowly than other materials and limits the overall conversion of organic carbon from complex molecules to carbon dioxide. Experimental Section A tubular reactor system was designed to produce well-char-

acterized data for the quantitative determination of reaction kinetics in supercriticalwater and is described in detail elsewhere.2o Analysis of kinetics was simplified by the reactor both being isothermal and having radially well-mixed,one-dimensionalplug flow. The reactor was 4.24 m of 0.635 cm 0.d. X 0.211 cm i.d. (1/4 X 0.083 in.) Inconel 625 tubing, which was selected for its high strength and corrosion resistance, and the reactor was immersed in a fluidized-bed sand bath for temperature control. Dilute inlet concentrations of carbon monoxide and oxygen were used in this system, prepared by dissolving reagents in room-temperature water in 1-L, agitated tanks. The two feed streams were heated separately in narrow diameter tubing (0.108 cm i.d.) to the reaction temperature, with both radiative preheaters and the sand bath used. Although the total heating time was about 15 s, the time within 20 O C of the reaction temperature was typically only 10-15% of the reactor residence time. The calculated thermal history of the streams was included in the data analysis. The reactor effluent was cooled quickly in a heat exchanger (by at least 200 "C in the first 0.5 s), depressurized, and separated into measured gas and liquid flows. The composition of the gas phase was determined by gas chromatography. A two-column system was used to separate carbon dioxide from the other permanent gases. A thermal conductivity detector was used to determine the species concentrations. The instrument was calibrated regularly for all gases, and the volume of gas samples were selected to keep all the responses within linear calibration ranges. Fifty-nine experiments were performed either at conditions to oxidize carbon monoxide in supercritical water containing dissolved oxygen (38) or to react the carbon monoxide with the supercritical water to produce carbon dioxide and hydrogen by the water gas shift reaction (21). Reactor conditions for these experiments ranged in temperature from 400 to 540 "C at a nominally constant pressure of 24.6 MPa (3550 psig) and a constant mass flow rate (1.67 X lo4 kg/s or 10 g/min). Under these conditions, Reynolds numbers in the reactor ranged from 2700 (14) Amin, S.;Reid, R. C.; Modell, M. Am. SOC. Mech. Eng., [Pap.] 1975, 75-ENAS-21. (15) Woerner, G . A. S.M. Thesis, Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, MA, August

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(16) Whitlock, D. R., S.M. Thesis, Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, MA, May

1978. (17) Antal, M. J., Jr.; Mok, W. S.L.; Roy, J. C.; T.-Raissi, A.; Anderson, D. G . M. J. Anal. Appl. Pyrolysis 1986, 8, 291. (18) Lawson, J. R.; Klein, M. T. Ind. Chem. Eng. Fundam. 1985,24, 203.

(19) Abraham, M. A.; Klein, M. T. Ind. Eng. Chem. Prod. Res. Deu. 1985, 24, 300.

(20) Helling, R. I