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J. Phys. Chem. 1996, 100, 15834-15842
Kinetics and Mechanism of Methanol Oxidation in Supercritical Water Eric E. Brock, Yoshito Oshima,† Phillip E. Savage,* and John R. Barker‡ Department of Chemical Engineering, UniVersity of Michigan, Ann Arbor, Michigan 48109-2136 ReceiVed: May 17, 1996; In Final Form: August 5, 1996X
We oxidized methanol in supercritical water at 246 atm and temperatures between 500 and 589 °C. Pseudofirst-order rate constants calculated from the data led to Arrhenius parameters of A ) 1021.3(5.3 s-1 and Ea ) 78 ( 20 kcal/mol. The induction time for methanol oxidation decreased from 0.54 s at 525 °C to 0.093 s at 585 °C and the reaction products were formaldehyde, CO, and CO2. Formaldehyde was a primary product, while CO and CO2 were secondary products. Formaldehyde was more reactive than methanol and its yield was always less than 24%. The temporal variation of the CO yield exhibited a maximum, whereas the CO2 yield increased monotonically. The experimental data were consistent with a set of consecutive reactions (CH3OH f CH2O f CO f CO2) with pseudo-first-order global kinetics. The experimental data were also used to validate a detailed chemical kinetics model for methanol oxidation in supercritical water. With no adjustments, this elementary reaction model quantitatively predicts the product distribution as a function of the methanol conversion, and it accurately predicts that this distribution is nearly independent of temperature. A sensitivity analysis revealed that only eight elementary reaction steps most strongly influenced the calculated species’ concentrations. A reaction path analysis showed that the fastest reactions that consumed methanol involved OH attack and the resulting radicals produced formaldehyde, which was attacked by OH to form, eventually, CO. The CO was then oxidized to CO2 via reaction with OH. This work shows that the chemistry for methanol oxidation in supercritical water at temperatures around 500-600 °C is quantitatively analogous to combustion chemistry within the same temperature range.
Introduction Conducting chemical reactions in fluids at temperatures and pressures that exceed their critical values is of both technological and scientific interest. A fluid above its critical point can have physical and transport properties that are intermediate between those of the same fluid as a gas or a liquid. Thus, a better understanding of chemical reactions in supercritical fluid phases could serve as a bridge that connects gas- and liquid-phase reaction dynamics. Current technological interest arises largely from the trend toward pollution prevention in the chemical processing industries. Replacing hazardous organic solvents with environmentally benign solvents such as supercritical CO2 and H2O can be attractive process alternatives. The broad field of reactions at supercritical conditions has recently been the subject of a comprehensive review.1 One of the promising applications of a supercritical reaction medium is the waste treatment technology termed supercritical water oxidation (SCWO). Water above its critical point (Tc ) 374 °C, Pc ) 218 atm) is completely miscible with both organic compounds and oxygen. The presence of this single phase and the accompanying elevated temperatures cause rapid and complete oxidation of organic compounds to CO2 and H2O. SCWO technology has been shown to be effective for destroying a large variety of industrial and high-risk wastes.2 Although the efficacy of SCWO technology has been proven, much remains unknown about SCWO reaction kinetics and the governing chemistry. One working hypothesis is that SCWO chemistry at temperatures of about 600 °C is analogous to the chemistry that occurs at similar temperatures under gas-phase * Corresponding author. † Department of Chemical System Engineering, Faculty of Engineering, The University of Tokyo, Hongo 7-3-1, Bunkyo-ku, Tokyo 113, Japan. ‡ Departments of Chemistry and of Atmospheric, Oceanic, and Space Sciences, University of Michigan, Ann Arbor, MI 48109-2143. X Abstract published in AdVance ACS Abstracts, September 15, 1996.
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combustion conditions. If this hypothesis is correct, then the elementary reaction steps and the rate consants used in combustion work can be used to develop detailed chemical kinetics models (DCKM) for SCWO reaction kinetics. The pioneering work done to test this hypothesis suggested that combustion models perform poorly in describing SCWO of simple compounds, such as CO and methane.3,4 In retrospect, this poor performance may have been due to inadequate treatment of unimolecular reactions and chemically activated association reactions. Indeed, the more recent work in this field,5-10 which has properly accounted for pressure effects, has strengthened the analogy between combustion chemistry and SCWO chemistry. The objective of the present study was to test more thoroughly the applicability of combustion mechanisms at SCWO conditions. Although there have been several recent modeling studies of SCWO reactions, few experiments have been carried out with the specific aim of validating a DCKM. The experimental data required for model validation should possess several key features. One feature is that the data be for oxidation of a simple hydrocarbon (e.g., methanol) for which the combustion chemistry is reasonably well established. A second requirement is that the data provide concentration profiles at several different temperatures for both the reactant and the oxidation products. These temporal variations of species’ concentrations are important because they reveal the induction times that typically accompany free-radical chain reactions. A final desired feature is that the data be obtained at temperatures well above the critical temperature, because thermodynamic nonidealities in the fluid phase diminish above ∼550 °C. Fluid-phase nonidealities might be important in implementing DCKMs because fugacity coefficients are required to calculate chemical equilibrium constants, which in turn are used to relate forward and reverse reaction rate constants.5 Thus, the fugacity coefficients can influence the values of the rate constants used in the model, but fugacity © 1996 American Chemical Society
Methanol Oxidation in Supercritical Water coefficients for free radicals cannot be calculated with any real accuracy. It is noteworthy that at temperatures above 550 °C supercritical water behaves as a nearly ideal gas. Consequently, using high-temperature data to validate a DCKM allows one to focus exclusively on the reaction mechanism and kinetics without having the confounding influence of thermodynamic nonidealities. We are not aware of any published data that simultaneously fulfill all three of the requirements outlined above (simple hydrocarbon, concentration profiles at different temperatures, high temperatures). Previous experimental studies of methane4,11,12 and methanol10,13 SCWO were designed and performed for purposes other than testing a DCKM. Most previous experiments were performed at temperatures where fluid-phase nonidealities could influence the reaction rates. Moreover, the data obtained from high-temperature experiments consist of concentrations at isolated times, rather than complete concentration profiles. This lack of suitable data led us to initiate the present work. We oxidized methanol in supercritical water at temperatures up to 589 °C, and we measured the temporal variations of the reactant and products’ concentrations. This paper presents these new experimental results and a complementary DCKM. Experimental Section Experimental Procedure. We conducted methanol oxidation experiments in supercritical water at temperatures between 500 and 589 °C and at 246 atm in an isothermal, isobaric, tubular plug-flow reactor. The reactor was a Hastelloy C-276 tube, and we used three different inner diameter-length combinations (0.108 cm × 33 cm, 0.108 cm × 100 cm, and 0.140 cm × 100 cm) to explore a wide range of residence times. All water was distilled, deionized, and degassed prior to use. Aqueous solutions of methanol (Fisher, 99.9% purity) and of oxidant were preheated to the reaction temperature separately in two 0.108 cm i.d. × 2 m long Hastelloy C-276 preheat lines immersed in an isothermal fluidized sand bath. The two solutions were mixed at the reactor entrance and the reactor temperature was measured using a thermocouple. In all experiments, molecular oxygen (O2) was the oxidant present at the reactor entrance. In most of the experiments, the O2 was produced from the decomposition of H2O2 (Fisher, 99.9%) in the preheater line. Complete conversion of H2O2 to O2 and water in the preheater line was verified experimentally. In other experiments, high-pressure, gas-phase O2 was dissolved in water to produce the oxidant stream. In all experiments, methanol oxidation occurred as the reactants traveled through the tubular reactor. In a given experiment, the residence time in the reactor was fixed by controlling the total flow rate of the reactants while maintaining a nearly constant molar ratio of methanol to O2 at the reactor conditions (temperature and density). Immediately upon leaving the reactor, the effluent stream was cooled in two consecutive water-cooled heat exchangers. The pressure was then reduced to ambient conditions, and the gaseous and liquid phases were separated. Martino et al.14 provide additional details about the reactor system and the operating procedure. Analytical Techniques. The amounts of CO and CO2 in the gas phase were determined by a gas chromatograph (GC) with a thermal conductivity detector (TCD). The GC housed a 10 ft stainless steel column packed with 100/120 mesh Supelco Carboseive S-II. The liquid phase was collected and then analyzed by a GC with a flame ionization detector (FID) to measure the amount of methanol remaining. This GC housed a 6 ft glass column packed with 80/100 mesh HayeSep P. The analysis was done isothermally at 150 °C. Savage et al.12
J. Phys. Chem., Vol. 100, No. 39, 1996 15835 provide additional details about the analytical protocol. This analytical protocol for the liquid phase provided good separation of methanol and formaldehyde (a likely oxidation product), but the formaldehyde peaks in the reactor effluent samples were too small to allow reliable quantitative analysis. Therefore, we estimated the yield (Y is the moles of product formed per mole of methanol fed to the reactor) of formaldehyde by using the measured yields of methanol, CO, and CO2 with the assumption that the carbon mass balance depends only on CO, CO2, methanol, and formaldehyde as shown by eq 1.
YF ) 1 - YM - YCO - YCO2
(1)
Experimental Results Table 1 displays the experimental conditions investigated and the results obtained. Methanol conversions from less than 10% to essentially 100% were obtained by changing the residence time at each temperature. CO and CO2 were the major gasphase products, and the yield of CO almost always exceeded that of CO2. Exceptions occurred for experiments where the gas yield was low (around 1%) and thus subject to an uncertainty that was comparable to the yield itself. Exceptions also occurred at very high methanol conversions where presumably much of the CO had been further oxidized to CO2. Formaldehyde was present as an aqueous-phase product in some of the experiments, but its yield was almost always less than 20%. Global Kinetics of Methanol Disappearance The global kinetics of methanol disappearance can be conveniently examined and compared with previous work by assuming that the global rate of this reaction is proportional to the methanol concentration and independent of the O2 concentration and the water density. This type of pseudo-first-order analysis has been employed in previous SCWO studies. Moreover, there is a growing interest in computational fluid dynamics studies of SCWO reactors,15 and reliable but simple kinetics models are required. For first-order kinetics, a plot of ln(1 - X), where X is the methanol conversion, against the residence time should give a straight line at each temperature. Figure 1 shows that the data can be fit by a straight line. The line does not pass through the origin, but rather intersects the x-axis at a positive residence time. The x-intercepts for each line provide estimates of the length of the induction period, and the slopes provide estimates of the rate constants. The kinetics parameters that result from this analysis appear in Table 2. The uncertainties in Table 2 and elsewhere in this paper are the 95% confidence intervals. The data in Table 2 show that the length of the induction period decreases as the temperature increases. This trend is consistent with recent results from Rice et al.,10 but if we extrapolate their results to our higher temperatures, the “predicted” induction times are much shorter than those we determined experimentally. We note that Rice et al. used much higher methanol concentrations in their experiments, and they found that the kinetics were not truly first order at lower temperatures (
