Environ. Sci. Technol. 1992, 26, 2388-2395
Kinetics of C02 Formation from the Oxidation of Phenols in Supercritical Water Ruokang LI, Thomas D. Thornton,+ and Phlllip E. Savage"
University of Michigan, Department of Chemical Engineering, Ann Arbor, Michigan 48109-2136 Global power-law rate expressions were determined for the formation of C 0 2 from the oxidation of aqueous solutions of phenol and 2-chlorophenol. The oxidation experiments were accomplished in a plug-flow reactor at temperatures between 300 and 420 "C and pressures from 185 to 278 atm. These conditions included oxidations in both near-critical and supercritical water. Reactor residence times ranged from 1.2 to 109 s. The initial reactant and 2.1 X M, concentrations were between 8.0 X and the initial oxygen concentrations ranged from 5.5 X to 4.8 X M. The phenolic compound was always the limiting reactant, and the excess oxygen was between 180% and 1360%. Nonlinear regression analysis of the kinetics data for the formation of C 0 2 from phenol oxidation revealed that the reaction was 0.82 f 0.20 order in total organic carbon and 0.71 f 0.26 order in oxygen. The activation energy was 6.2 f 2.6 kcal/mol. The rate law for COz formation from 2-chlorophenol was 0.51 f 0.08 order in total organic carbon and 0.80 f 0.13 order in oxygen. The activation energy for this rate law was 9.0 f 5.8 kcal/mol. The uncertainties given here represent 95% confidence intervals. There was no statistically significant dependence of the rate of COz formation on the water concentration for either compound. The low activation energies suggest that the major pathways leading to COz do not include carbon monoxide or acetic acid as refractory intermediates. For both phenolic compounds, the conversion of organic carbon to COz was always less than the conversion of the phenol, indicating that products of incomplete oxidation were present.
Introduction Organic compounds present in aqueous waste streams can be destroyed through reactions with oxygen at elevated temperatures and pressures. Such oxidative destruction forms the basis for the well-established technology of wet-air oxidation (WAO). Under typical WAO reaction conditions of 150-325 "C and 20-200 atm, residence times of 15-60 min are required to achieve high (e.g., 99%) destruction and removal efficiencies (DREs) for many compounds (1). Even under these conditions, however, products of incomplete oxidation such as low molecular weight carboxylic acids remain in the liquid effluent. Thus, this effluent would need to be treated, by biological oxidation for instance, prior to direct discharge or reuse. Recently, there has been considerable interest in conducting aqueous-phase oxidation reactions at conditions more severe than those typically employed in WAO applications. When the reaction conditions exceed the critical temperature and pressure of water (57,= 374 "C, P, = 218 atm), several advantages manifest themselves (2). First, a single fluid phase is present at supercritical conditions so that the rate of oxidation is not limited by interphase oxygen transport as it can be in WAO. Second, the higher reaction temperatures promote higher intrinsic reaction rates so that very high DREs can be achieved on the order of several seconds. Third, the oxidation reactions are 'Present address: BP America, Warrensville Research and Environmental Science Center, Cleveland, OH 44128. 2388
Envlron. Scl. Technol,, Vol. 26, No. 12, 1992
sufficiently rapid that they go essentially to completion and there is no need for secondary treatment of the reactor effluent. The technology that has been developed to exploit these advantages has been termed supercritical water oxidation (SCWO) (2-5). Previous fundamental research into the reaction kinetics, pathways, and mechanisms operative during SWCO has typically focused on the disappearance of a single compound. Tester and co-workers, for instance, have reported global rate expressions for CO (6), NH3 (7,8), CzH60H(7, 8), CHSOH (91, CHI (IO), and H2 (11). In addition, Rofer and Streit (12,13)studied CH4 and CH30H oxidation in SCW. Wightman (14) reported limited kinetics data for the oxidation of model organic pollutants such as acetic acid and phenol. Yang and Eckert (15) examined the kinetics of 4-chlorophenol oxidation, and we have recently reported the kinetics for the oxidation of phenol (16,17) and 2-chlorophenol (18) in near-critical and supercritical water. Although the disappearance kinetics for a large number of compounds have recently been reported, it is important to keep in mind that the objective of SCWO is not simply the conversion of an organic compound into a large number of different organic products. Rather, the goal is the complete conversion of organic carbon to COz. Accordingly, it is important that the kinetics of COz formation during SCWO also be investigated and understood. In this paper we provide the results of a kinetics study for C 0 2 formation during the SCWO of two representative organic pollutants, phenol and 2-chlorophenol (2CP).
Experimental Section Phenol and 2-chlorophenol were oxidized at temperatures between 300 and 420 "C and pressures from 185 to 278 atm. Reactor residence times ranged from 1.2 to 109 s. This range of reaction conditions was selected because it provided reactant conversions that ranged from 2.8% to 99.7% on an experimentally convenient time scale. Examination of this complete range of conversions allowed us to probe regions of the parameter space where the oxidation reaction was incomplete. Experiments under such conditions are central to achieving one of our long-term goals, the development of complete reaction networks for the SCWO of representative organic pollutants. The initial organic concentrations were between 8.0 X and 2.1 X M at reaction conditions. The initial oxygen concentrations ranged from 180% excess (Le., 2.8 times the precise stoichiometric amount needed to convert the organic carbon to CO,) to 1360% excess, which resulted and 4.8 X in oxygen concentrations between 5.5 X M at reaction conditions. These organic and oxygen concentrations were calculated by assuming that the density of the reaction mixture was the same as the density of water. This assumption is a good one because the reaction mixture was greater than 99.5% water. Use of low reactant concentrations also ensured that a single phase existed at all reaction conditions and that the maximum adiabatic temperature rise would be limited to less than 1 "C. Oxidation experiments were conducted in an isothermal, isobaric, plug-flow reactor fashioned from lI8-in.-o.d.
0013-936X/92/0926-2388$03.00/0
0 1992 American Chemical Society
Hastelloy (2-276 tubing. The reactor feed streams were prepared by dissolving oxygen into deionized water in one feed tank and loading an aqueous solution of the phenol into a second feed tank. Helium blanketed the headspace of the phenol-water feed tank so that no oxygen entered the reactor through this feed stream. High-pressure metering pumps were used to pressurize the two feed streams. The two streams were than separately preheated by flowing through coiled 1/16-in.-o.d.Hastelloy tubing immersed in a preheated, temperature-controlled, fluidized-sand bath. The preheated feed streams were mixed a t the reactor inlet using a specially machined Hastelloy mixing tee, and the reaction temperature was measured via a thermocouple placed within this mixing tee. After passing through the reactor, which was also coiled and immersed in the sand bath, the mixture was cooled rapidly and the system pressure was reduced. The product stream was then separated into a liquid and a vapor phase. The flow rate of the vapor phase was measured, and the vapor was then directed to a 10-port Valco valve, which injected a 0.5-mL sample into a Hewlett-Packard (HP) Model 5890 series I1 gas chromatograph (GC) equipped with a H P Model 3392A integrator. A 10 f t X l/s-in. 0.d. stainless steel column packed with 100-120-mesh Carboseive S-I1 (Supelco) separated the sample constituents, which were monitored with a thermal conductivity detector. Helium flowing at 20 mL/min served as the carrier gas. The GC oven temperature was held at 35 "C for 7 min and then increased to 225 OC at a rate of 16 OC/min. Detedor response factors for CO and COz were determined experimentally using calibration gas mixtures from Scott Specialty Gases. The flow rate of the liquid phase was also measured, and the concentration of phenol or 2-chlorophenol in the reactor effluent was determined by reverse-phase, highperformance liquid chromatography. The concentrations of CO and COz dissolved in this aqueous liquid phase were calculated by assuming that the vapor and liquid phases were in equilibrium in the phase separator and then using tabulated solubility data (19). Additional details about the reactor, experimental procedure, and analytical chemistry have been reported previously (17, 20-22).
Kinetics of C02 Formation The experiments and analyses outlined above permitted calculation of the conversions of the phenolic reactant, X, = 1- [organic]/[organi~]~, and the molar yields of CO and COz. These molar yields were calculated as the molar flow rate of CO or COBin the reactor effluent divided by the molar flow rate of organic carbon in the reactor feed stream. Figure 1provides representative results for a set of 2chlorophenol oxidation experiments at 380 OC and 278 atm and different reactor residence times. The yield of COz always exceeded the yield of CO. The COzyield increased steadily throughout the reaction whereas the increase in the CO yield was much more modest. At 59.1 s, the C02 yield was 72.7%, but the CO yield was only 6.4%. Clearly, COZ was the dominant carbon oxide formed during the SCWO of these phenols. Yang and Eckert (15)also found that COz was the most abundant gaseous product from their SCW oxidation experiments with 4-chlorophenol. A complete set of results from the 73 phenol oxidation experiments are listed in Table I, and the results from the 62 2-chlorophenoloxidation experiments are summarized in Table 11. The objectives of our COz formation global kinetics analysis were to determine the Arrhenius parameters (A,
Carbon Dioxide
-
0.6
0.4,
Carbon Monoxide
0
10
20
30
40
50
60
Residence Time (s) Flgure 1. Molar yields of CO and COPfrom 2-chlorophenol oxIdation in SCW. Nomlnal reaction conditions: 380 O C , 278 atm. [2CPl0 = 3.9 X lo-' M, and [O2I0 = 3.0 X lo-* M.
E,) for the reaction rate constant (k) and the reaction orders (a, b, c ) for total organic carbon (TOC), oxygen, and water for the power-law rate expression in eq 1. Power-law rate of COz formation = k[TOC]a[O~]b[HzO]C(1) rate expreasions have been used frequently to correlate the global kinetics of reactant disappearance during SCWO. Thus, using the same type of model in this analysis facilitates comparison of the rates of COz formation and reactant disappearance. These simple rate laws typically capture the general trends in the experimental data (6-13, 16-18), but they cannot be expected to model the details of the oxidation chemistry, which is undoubtedly complex and involves a large number of individual elementary reaction steps. TOC is defined here as the molar concentration of carbon contained in all compounds that can be further oxidized to COz. Thus, the rate of TOC disappearance is equivalent to the rate of COz formation. The TOC concentration can be written in terms of its initial concentration and the conversion of organic carbon to C02 (X,) as [TOC] = [TOC]o(1- X,)
(2)
Note that X, is also equivalent to the molar yield of COz, which can be calculated from the experimental measurements. By combining the design equation (23) for the isobaric, isothermal, plug-flow reactor with the power-law rate expression of eq 1and the relationship for [TOC] in eq 2 we obtain eq 3.
-ddrx-c - k[TOC]o"-l(l - X,)a[0,]b[H20]c
(3)
The quantity [HZO]' can be taken to be conversion invariant because water was the major component in the reaction mixture and its concentration did not change appreciablyduring the reaction. Additionally, the quantity [OZlbcan be taken to be conversion invariant and approximately equal to [Oz]obbecause all of the data used in this kinetic analysis were obtained from experiments that were conducted with at least 180% excess oxygen. Thus, the concentration of oxygen could be approximated as being roughly equal to its initial concentration. Note that working with excess oxygen is required in this kinetics Environ. Scl. Technol., Voi. 28, No. 12, 1992
2389
Table I. Summary of Phenol Oxidation Experiments
T,“ C
P , atm
300 300 300 300 300 300 299 300 299 300 300 300 300 300 300 340 340 340 340 340 340 380 380 380 377 380 380 380 380 380 380 380 380 380 380 380 380 380 380 378 380 380 380 380 380 380 380 380 380 380 380 380 380 380 396 400 400 400 400 400 400 420 420 420 420 420 420 420 416 419 420 420 420
278 278 278 278 278 278 279 278 279 278 278 278 278 278 278 278 279 278 278 278 279 188 188 188 218 218 218 279 278 279 279 278 278 279 279 278 278 278 278 278 278 278 278 278 278 278 277 278 278 278 278 278 279 278 278 278 278 277 278 278 278 278 278 278 278 278 278 278 278 278 278 278 278
7,8
20.8 31.9 48.2 60.9 79.3 104.0 20.8 26.5 33.2 46.9 67.5 77.9 89.1 100.9 108.8 16.6 33.4 48.1 68.9 83.3 101.1 2.9 9.5 17.4 1.2 2.8 5.7 16.6 24.8 31.9 60.3 65.7 82.3 16.3 24.9 32.6 49.7 66.0 74.2 13.2 25.1 33.5 3.5 4.1 6.1 8.3 9.9 13.4 63.4 73.3 96.1 41.2 69.1 78.1 8.1 7.0 10.3 15.6 24.1 34.6 53.4 4.3 8.2
12.0 16.1 25.7 40.8 7.8 8.2 11.7 16.1 20.0 24.3
[phenol], M 7.15 X lo4 7.95 x 10”’ 7.47 x lo-’ 7.87 x 10-4 8.27 x 10-4 8.66 X 1.86 x 10-3 1.92 x 10-3 1.92 x 10-3 1.94 x 10-3 1.73 x 10-3 1.83 x 10-3 1.84 x 10-3 2.15 x 10-3 1.91 x 10-3 6.99 x 10-4 6.92 x 10-4 6.78 x 10-4 6.78 x 10-4 7.70 X 10”’ 6.99 x 10-4 3.00 x 10-4 3.05 x 10-4 3.00 x 10-4 5.01 x m . 4 4.49 x 10-4 4.30 x 10-4 2.67 x 10-4 2.72 x 10-4 2.67 x 10-4 2.50 x 10-4 2.67 x 10-4 2.67 x 10-4 2.67 x 10-4 2.67 X lo4 2.50 X 10”’ 2.61 X 10”’ 2.61 X lo-‘ 2.78 X 10”’ 2.65 X 10“‘ 2.67 X 10”’ 2.72 X lo-’ 5.33 x 10-4 5.22 X lo-’ 5.11 X lo4 5.11 X 10”’ 5.33 x 10“‘ 5.66 X 10”’ 5.44 x 10-4 5.55 x 10-4 5.39 x 10-4 1.32 x 10-3 1.32 x 10-3 1.34 x 10-3 8.52 x 10-4 2-67 x 10-4 2.83 x 10-4 2.75 x 10-4 2.64 x 10-4 2.75 x 10-4 2.75 x 10-4 1.76 x 10-4 1.87 x 10-4 1.66 X lo-‘ 1.76 X lo4 1.69 X lo-’ 1.66 X 10”’ 4.35 x lo4 4.80 x 10-4 4.35 x 10-4 4.19 x 10-4 4.68 x 10-4 4.37 x 10-4
[0219
investigation because the formation of numerous intermediate reaction products (21,22) makes it impossible to 2390
Envlron. Sci. Technol., Vol. 26, No. 12, 1992
M
5.37 x 10-2 4.93 x 10-2 5.00 X 4.89 X 4.63 X 4.52 X 5.56 X 5.30 x 5.10 X 5.21 X loT2 5.25 X 5.35 x 10-2 5.30 X 4.88 X 5.01 X 4.55 x 10-2 4.41 X 4.80 X 4.45 x 10-2 4.42 X 4.37 x 10-2 6.81 x 10-3 6.76 x 10-3 6.83 x 10-3 1.10 x 10-2 8.80 x 10-3 9.14 x 10-3 8.75 x 10-3 8.07 x 10-3 8.77 x 10-3 9.20 x 10-3 8.72 x 10-3 8.85 x 10-3 1.48 X 1.41 X 1.48 X 1.50 X 1.47 X 1.44 X 2.71 X low2 2.14 X 2.12 x 10-2 4.15 X 3.97 x 10-2 3.88 X lo-’ 3.76 X 3.82 X 3.55 x lo-* 3.58 X 3.29 X 3.57 x 10-2 3.50 X 3.56 X 3.52 X 2.21 x 10-2 1.80 X 1.87 X 1.77 X 1.79 X 1.77 X 1.75 X 1.09 x 10-2 9.84 x 10-3 1.13 X 9.48 x 10-3 1.05 X 1.12 x 10-2 1.20 x 10-2 1.19 x 10-2 1.14 X 1.11 x 10-2 1.06 X lo-’ 1.13 X
[HzOI, M
x,,?h
C 0 2yield, %
CO yield, %
41.51 41.51 41.51 41.51 41.51 41.51 41.51 41.51 41.51 41.51 41.51 41.51 41.51 41.51 41.51 36.88 36.88 36.88 36.88 36.88 36.88 5.96 5.96 5.96 10.54 9.09 9.09 28.42 28.42 28.42 28.42 28.42 28.42 28.42 28.42 28.42 28.42 28.42 28.42 29.41 28.42 28.42 28.42 28.42 28.42 28.42 28.42 28.42 28.42 28.42 28.42 28.42 28.42 28.42 18.31 14.50 14.50 14.50 14.50 14.50 14.50 9.40 9.40 9.40 9.40 9.40 9.40 9.40 9.99 9.43 9.40 9.40 9.40
17.8 38.7 53.1 64.8 73.7 83.4 26.2 20.6 27.5 40.4 78.2 82.0 78.8 95.6 95.2 38.7 54.2 78.6 93.7 94.7 95.7 2.8 6.9 9.6 4.8 4.3 6.9 37.6 38.9 55.0 72.3 73.2 81.8 41.4 50.9 63.8 75.7 85.4 83.6 42.9 58.6 67.3 15.5 17.1 20.8 16.2 27.0 48.6 94.6 95.4 97.3 68.6 94.5 93.1 21.6 26.0 37.6 52.2 46.5 73.7 74.8 16.0 26.0 29.2 40.2 47.2 74.8 19.7 6.5 21.5 22.3 34.0 36.1
4.9 8.8 17.9 34.0 30.6 33.2 3.3 4.0 4.9 15.4 39.0 41.3 34.2 43.7 46.9 9.3 23.9 35.1 54.9 50.5 37.3 0.7 2.2 3.7 0.1 0.3 1.1 5.7 7.6 11.8 27.7 10.2 17.5 8.3 12.1 19.3 27.8 29.6 33.7 17.8 45.9 23.2 0.8 1.7 3.0 7.0 8.1 4.2 51.5 47.3 44.6 26.1 38.6 47.4 1.0 1.5 7.7 12.9 16.0 28.2 24.5 3.6 7.5 10.0 21.6 27.9 24.9 4.7 1.0 2.6 3.4 3.7 6.4
0.0 0.3 0.8 1.3 1.8 2.0 0.1 0.2 0.3 1.3 3.5 3.5 2.4 3.3 3.3 0.3 1.4
2.2 3.1 3.1 1.8 0.0 0.0 0.1 0.0 0.0 0.0 0.3 0.4 0.6 2.5 0.7 0.9 0.5 0.8 1.3 1.7 2.0 2.3 1.2 3.6 1.8 0.0 0.0 0.0 0.0 0.5 0.3 5.0 4.2 4.3 2.7 3.9 5.0 0.1 0.0 0.4 0.8 1.3 2.2 2.1 0.2 0.4 0.7 1.6 2.4 2.5 0.4 0.2 0.4 0.5 0.6 1.0
determine a priori a stoichiometric relationship between TOC disappearance and oxygen consumption.
Table 11. Summary of 2-Chlorophenol Oxidation Experiments
T,O C
P,atm
7,8
300 330 360 380 380 380 380 380 380 380 380 380 380 380 380 380 380 380 380 380 380 380 380 380 380 380 380 380 380 380 380 380 380 380 380 380 380 380 380 380 380 380 380 380 380 380 380 380 380 380 380 380 380 380 380 380 390 400 410 420 420 420
278 278 278 185 205 219 225 225 232 246 260 278 278 278 278 278 278 278 278 278 278 278 278 278 278 278 278 278 278 278 278 278 278 278 278 278 278 278 278 278 278 278 278 278 278 278 278 278 278 278 278 278 278 278 278 294 278 278 278 278 278 278
14.6 13.6 11.8 13.2 17.7 19.0 22.9 26.8 34.5 24.1 27.1 20.4 35.1 49.2 69.8 5.9 10.2 21.1 32.8 44.4 59.1 19.9 34.0 47.3 59.6 7.2 29.6 40.2 50.1 58.6 69.5 3.6 5.2 7.3 10.1 13.9 18.6 24.7 29.5 33.9 40.0 49.5 57.4 67.5 29.4 39.7 41.0 27.9 40.5 38.4 40.9 11.4 5.8 40.5 14.7 29.9 45.2 28.1 21.6 10.0 16.9 24.0
PCPI, M 5.99 x 5.51 x 4.85 x 8.03 X 1.04 x 1.32 x 1.56 x 1.54 x 2.28 x 3.42 x 3.77 x 3.19 x 3.28 X 3.70 x 3.26 x 3.86 X 4.00 x 4.02 x 3-76 x 3.84 x 3.95 x 9.90 x 1.03 x 1.04 x 1.04 x 1.41 x 1.55 x 1.60 x 1.48 x 1.57 x 1.64 x 1.95 x 1.98 x 2.05 x 2.03 x 2.04 x 2-13 x 2.07 x 2.04 x 2.15 x 2.09 x 1.97 x 2.12 x 2.05 x 1.82 x 1.98 x 2.32 X 2.52 x 3-02 x 4.41 X 5.79 x 6.27 x 6.46 x 8.17 x 1.13 x 4.06 x 3.15 x 1.98 x 1.50 x 5.04 x 5.08 X 5.43 x
10-4 10-4 10-4 10-4 10-4 10-4 10-4 10-4 10-4 10-4 10-4 10"' 10-4 10-4
lo4
10-4 10-4 10-4 10-4 10-4 10-4 10-3 10-3 10-3 10-3 10-3 10-3 10-3 10-3 10-3 10-3 10-3 10-3 10-3 10-3 10-3 10-3 10-3 10-3 10-3 10-3 10-3 10-3 10-4 10-4 10"' 10-4 10-4
lo4
10-4 10-4 10-4 10-4 10-3 10-4 10-4 10-4 10-4 10-4 lo4 10-4
[Ozl, M 4.12 X 3.79 x 3.33 x 5.49 x 7.12 x 8.99 x 1.06 X 1.05 X 1.55 X 2.57 X 2.89 X 1.07 X 9.63 X 1.01 x 9.76 x 2.97 X 2.87 X 3.02 X 3.22 X 3.02 X 3.08 X 3.76 X 3.50 X 3.47 x 3.65 X 3.60 X 3.64 X 3.58 X 3.74 x 3.61 X 3.53 x 4.09 X 4.29 X 4.19 X 3.97 x 4.20 X 3.80 X 3.88 X 4.07 X 3.77 x 3.99 x 4.14 X 3.95 x 4.03 X 1.94 X 1.47 X 1.21 x 1.42 X 7.01 x 4.44 x 3.63 X 3.47 x 3.41 X 2.21 x 3.73 x 3.19 X 2.58 X 1.62 X 1.22 x 1.27 X 1.26 X 1.22 x
lo+ 10-2 10-3 10-3 10-3
lo-'
lo-' 10-2
10-3 lo-' lo-' lo-*
lo-' 10-2 lo-' lo-'
10-2 lo-' 10-2 lo-' lo-'
lo-' 10-2
lo-' 10-2 lo-' lo-' 10-2
10-3 10-2
10-2
lo+
10-2
10-2
With the oxygen and water concentrations taken to be conversion invariant, eq 3 becomes a separable differential equation that can be solved analytically with the initial condition X, = 0 at r = 0 to give (1- XC)lQ= 1 + (a - l)k[TOC]ou-1[02]0b[H~O]C~ (4) for a # 1 and to give In (1- X,)= -k[02]ob[H20]cr
(5) for a = 1. In these equations k = A exp(-E,/RT) is the rate constant written in Arrhenius form.
[HZOI,M 41.51 38.20 33.61 5.71 7.40 9.28 10.97 10.97 15.97 24.22 26.96 28.42 28.42 28.42 28.42 28.42 28.42 28.42 28.42 28.42 28.42 28.42 28.42 28.42 28.42 28.42 28.42 28.42 28.42 28.42 28.42 28.42 28.42 28.42 28.42 28.42 28.42 28.42 28.42 28.42 28.42 28.42 28.42 28.42 28.42 28.42 28.42 28.42 28.42 28.42 28.42 28.42 28.42 28.42 28.42 29.37 23.09 14.50 10.96 9.40 9.40 9.40
xp,
5%
24.3 29.5 31.0 8.6 16.7 23.6 41.1 60.6 72.1 65.2 67.5 45.7 62.0 76.0 85.2 17.2 23.4 59.0 76.4 88.9 98.6 57.5 74.1 93.2 98.5 19.1 68.9 82.4 93.6 99.0 98.6 9.2 11.4 13.2 18.9 34.7 46.7 54.7 67.2 69.1 78.7 82.1 97.1 99.7 75.4 79.5 70.9 61.0 58.3 92.2 83.5 28.4 20.7 66.4 30.4 76.0 89.1 69.7 54.4 25.4 40.7 58.1
C02 yield, %
CO yield, %
16.6 18.8 12.8 5.3 10.1 13.5 23.0 28.0 49.3 36.4 31.7 14.8 23.2 33.3 44.8 3.1 6.1 25.0 40.1 52.8 72.7 19.4 34.5 54.1 66.0 2.6 27.1 34.5 29.7 54.4 48.3 1.7 2.3 2.8 5.5 11.7 15.1 20.3 27.7 26.1 43.0 38.9 41.4 50.0 41.3 40.3 33.5 26.1 21.6 64.7 46.9 9.1 2.7 29.1 8.8 40.3 70.6 53.1 37.0 6.5 12.6 20.2
0.4 3.2 5.8 1.5 1.2 4.2 3.3 4.6 7.7 11.5 10.1 1.1 2.0 2.5 4.7 0.2 2.4 1.8 8.0 4.8 6.4 4.7 6.4 8.5 10.2 2.1 3.6 6.9 8.3 9.9 11.3 0.2 0.1 0.3 0.7 1.2 1.5 2.3 3.5 3.3 5.2 6.1 7.4 8.1 4.5 3.4 3.4 2.1 1.9 6.7 8.3 4.0 0.3 3.5 2.4 10.2 12.8 12.3 9.0 1.1 3.4 6.0
Optimized values for the parameters a, b, c, A, and E, were determined by applying nonlinear regression techniques. The objective function that was minimized was the sum of the squares of the differences between the values of X,calculated from eq 4 and those determined experimentally. These experimental values, determined a t different temperatures, residence times, and reactant concentrations for the oxidation of both phenol and 3chlorophenol are listed in Tables I and 11. The resulting parameter estimates and their 95% confidence intervals are summarized in Table 111. No value of c, the water Environ. Sci. Technol., Vol. 26, No. 12, 1992
2391
Table 111. Parameters in C 0 2 Formation Rate L a w
a
b E, A
phenol
2-chlorophenol
0.82 f 0.20 0.71 f 0.26 6.2 k 2.6 kcal/mol
0.51 k 0.08 0.80 0.13 9.0 f 5.8 kcal/mol
100.6
1.3 M4.63 g-l
101.1
A
d
u
w E
e
2.0 M4.31 g-l
v-
'c1
-Bma
-m 0
0 v 0
x
0.0
0.2
0.4
0.6
0.8
1.0
0.8
1.0
X, (experimental)
I
I
!
-6.0
0.0014
0.001 6
0.0018
3
u
W
E
e
1 / l (l/K)
c
Figure 2. Arrhenius plot for TOC conversion during phenol oxidation in SCW at 278 atm.
reaction order, is given because there was no statistically signifcant correlation between the water concentration and the TOC conversion. The reaction orders in Table I11 show that the rate of conversion of TOC for both compounds depended more strongly on oxygen and less strongly on the organic carbon concentration than did the corresponding rate (17,18) of reactant disappearance. The Arrhenius parameters in Table I11 for both phenol and 2-chlorophenol are similar, and they exhibit large uncertainties at the 95% level. To examine this aspect of the kinetics further, we calculated the rate constant for TOC conversion during phenol oxidation at five different temperatures and at 278 atm. Figure 2 presents the results of these calculations as an Arrhenius plot, and the 95% confidence interval is displayed for each of the rate constants. The line in Figure 2 was calculated from the optimized Arrhenius parameters in Table 111, and it is clear that the rate constants, within their uncertainties, do fall on this line. The large uncertainty in the rate constants, however, leads to the large uncertainties in the estimated activation energy and preexponential factor. Nevertheless, the global power-law rate expression with the parameters in Table I11 does appear to provide a reasonable representation of the experimental data for phenol in Figure 2. This assertion is further supported by Figure 3, which compares the experimental and calculated C02 molar yields from both phenol and 2-chlorophenol. Comparison with Disappearance Kinetics The kinetics data most frequently reported for SCWO experiments with pure compounds are the reactant conversions. As noted earlier, however, the ultimate objective of SCWO processes is the complete conversion of organic carbon to COz,not simply its conversion to intermediate species, Thus, it is instructive to compare the COZ yields with the measured reactant conversions. Figure 4 provides a comparison of the COz yields (X,) and the reactant conversions (X,)for a set of 2-chloro2392
Environ. Sci. Technol., Vol. 26, No. 12, 1992
'0
-A! 0
-m
3
0 0
v
0
x
0.0
0.2
X
0.6
0.4
(experimental)
Figure 3. Comparison of experimental and calculated TOC conversions: (a, top) phenol oxidation; (b, bottom) 2-chiorophenoi oxidation.
0
x
0.0
0.2
0.4
0.6
0.8
1.0
XP Figure 4. TOC conversions achieved at dlfferent 2-chlorophenol conversions. Nominal reaction conditions: 380 OC, 278 atm, [2CPl0 = 3.9 X M, and [O2lO= 3.0 X IO-' M.
phenol SCWO experiments conducted at 380 "C and 278 atm with initial organic, oxygen, and water concentrations 3.0 X and 28.4 M, reof approximately 3.9 X
spectively. It is evident that X, is always lower than X,, indicating that 2-chlorophenol oxidation in SWC leads to carbon-containing products other than COz. A similar conclusion can be drawn from the data for phenol in Table I. The formation of these products, which include low molecular weight carboxylic acids, dihydroxybenzenes, phenoxyphenols, biphenols, and dibenzofurans, has been discussed elsewhere (18,21,22,24). This comparison of the conversion of organic carbon with the conversion of 2-chlorophenolleads to the observation that COz formation lags reactant disappearance. That is, a high reactant conversion does not necessarily imply an equally high TOC conversion and COz yield. A more quantitative comparison of the reactant conversion and the TOC conversion would be useful in assessing the efficacy of SCWO proceeaes. The present study of COz formation kinetics can be combined with the global reaction rate laws for the destruction of the phenolic reactanta, shown generally in eq 6, to derive an expression that relates the disappearance of the phenol to the appearance of COz. rate of reactant disappearance = A ' exp(-E,'/RT) [organic]"'[ O2lob'[HzOIc' (6) The first step in this derivation is to write the differential equation that is analogous to eq 3 but that describes the variation of the 2-chlorophenol or phenol conversion with reactor residence time.
-dXP - - kJorgani~]~"'-~(l - X,)a'[Oz]~[HzO]c'
(7)
d7
--
dXp
The curve appearing in Figure 4 was calculated from eq 14, and it shows that the equation does a good job of capturing the key features of the data and correlating the two conversions. One potential application for eqs 10 and 11 is the prediction of TOC destruction, or alternatively COz formation, from a knowledge of the conversion of the phenols. For example, eq 14 predicts that a 2-chlorophenol conversion of 99% at the reaction conditions of Figure 4 would provide a TOC conversion of 87%. Presumably, the balance of the carbon would reside primarily in organic compounds other than 2-chlorophenol. It is tempting to extrapolate to higher chlorophenol conversions, but such extrapolating must be done with caution because calculated values of X, exceed X, when X, closely approaches unity. Such a result is physically impossible because the definition of X, requires it to always be less than or equal to X,. The root of this apparent conflict between the mathematics and the chemistry can be ascertained via inspection of the analog of eq 4 for 2-chlorophenol disappearance, which is (1- X,)'-'' = 1
+ (a'-
~)~J~CP]O"'-~[OZ]~[H~O]"T
x, = 1 - (1 + P7)1/(1"')
(I - X,)"
where y is independent of the conversions and given by
Equation 8 can be separated and integrated and the resulting expression rearranged to obtain an explicit analytical expression for X,.
fora # 1 a n d a ' Z l a n d
X, = 1- [I + y(1 - a) In (I - X,)]l/(l-")
(11)
for a # 1 and a ' = 1. Equations 10 and 11show that the precise correlation between X, and X, is a complex function of temperature and the concentrations of organic, oxygen, and water. For a series of experiments where each of these parameters is held constant, however, a simpler correlation emerges. For instance, for the specific case of 2-chlorophenoloxidation, one can use the previously determined (18) rate law given as rate of 2CP disappearance = 102.0exp(-1 1.0/ RT) [2CP]o~88[Oz]oo~41 [HzO]0.34 (12) the parameters in Table 111, and the relation [TOCIo = 6[2CPl0to show that eq 9 becomes y = 0.052 exp(1000/T)
(14)
~ O lrearc' Defining p' = (a' - l ) k J 2 C P ] o " ' ~ 1 [ 0 ~ 1 ~ [ Hand ranging, one can express X, as
(1- X,)' -7
x, = 1 - [i + i.47((1- X,)OJZ- 1)12.04
(15)
Dividing eq 3 by eq 7 leads to dX,
Now, the data in Figure 4 were obtained from 2chlorophenol oxidation in SCW under conditions where y = 0.36. Furthermore, a' = 0.88 for 2-chlorophenol oxidation and a = 0.51. Therefore, under these specific reaction conditions, the relationship between COz formation and 2-chlorophenol disappearance given by eq 10 becomes
[Oz100.39 [2CP]oo.37 [HzO]0.34
(13)
(16)
Inspection of eq 16 reveals that X, does not approach 1 as 7 approaches infiiity, as is required chemically. Rather, X, = 1at 7max= -l/fl, and at higher residence times, eq 16 ceases to be meaningful. This type of behavior renders extrapolation to residence times greater than ,T meaningless. Consequently, the behavior of eq 16, and eqs 10 and 11, which are similar mathematically, prevents their use in extrapolating to very high TOC conversions. Thus, the reason that eq 14 predicts that X, can exceed X, lies with the form of the integrated reactor design equation and power-law rate expression and not with the chemistry of the SCWO process. The limitation is in the mathematics, not in the chemistry. Implications to SCWO Reaction Pathways Previous SCWO research has led to the suggestion that the oxidation of refractory intermediates can limit the rate of complete conversion of organic carbon to COz. The distinguishing feature of a refractory intermediate is that ita rate of oxidation is slower than the rate of disappearance of the original reactant. Previous SCWO kinetics studies (6,14) showed that acetic acid and carbon monoxide oxidize relatively slowly, and these have been suggested as refractory intermediates (6,25). Calculations using published kinetics data (6,14,17,18) confirm that both CO and acetic acid oxidize more slowly at 400 OC than the phenols used in the present investigation. Thus, the present work provides insight into the relative proportion of C02 formed through these refractory intermediates during the oxidation of phenols in SCW. Li et al. (25) recently proposed a generalized kinetic model for wet oxidation based on the existence of two Envlron. Sci. Technol., Vol. 26, No. 12, 1992 2303
parallel pathways for the conversion of organic carbon to C02. The first pathway is the direct oxidation of organic carbon to C02,and the second involves partial degradation of the reactant to acetic acid (the refractory intermediate), which is then oxidized to COP Li et al. defied a selectivity a as the ratio of the rate of formation of acetic acid to the rate of direct formation of COz. Values of a were given as 0.15 and 0.37-0.96 for the wet oxidation of phenol and 2-chlorophenol, respectively. These high values for 2chlorophenolindicate that a substantial portion of organic carbon was converted to C02 via the acetic acid intermediate. The present results for 2-chlorophenol oxidation in SCW differ, however. For example, the activation energy for TOC disappearance was 9.0 kcal/mol, which is much lower than the 41 kcal/mol activation energy for acetic acid oxidation in SCW Li et al. (25) calculated from Wightman’s data (14).This difference suggests that a must be small for 2-chlorophenol oxidation in SCW. In fact, fitting a subset of the data in Table I1 to the kinetic model of Li et al. (25) provided a value of a equal to 0.05. Thus, a for the oxidation of 2-chlorophenol under the conditions of our experiments is markedly lower than a for the WAO data analyzed by Li et al. (25). Note, however, that the highest TOC conversion appearing in Tables I and I1 is 73%. Therefore, our data do not exclude the possibility of a pathway involving a refractory intermediate being important at higher TOC conversions. Helling and Tester (6)suggested that the oxidation of CO to COS could limit the rate of complete conversion of organic carbon to COZ during SCW oxidation. The present data provide information about the amount of C02 that could be formed through a CO intermediate from the oxidation of phenols. If the major pathway to COzproceeded via a slowly oxidizing CO intermediate, then the CO yield should exceed the C02yield at some point in time. In none of our 135 experiments with phenol and 2-chlorophenol did we observe this behavior. Rather, the CO yield was always lower than the COz yield, even at the lowest reactant conversions studied. Moreover, the low activation energies for COz formation determined in this study are not consistent with a Iarge portion of the COz being formed from CO oxidation, which has a higher activation energy (6). We note again, however, that these results do not exclude the possibility of a minor pathway involving CO as an intermediate. In fact, our data for 2-chlorophenol oxidation in Figure 1would be consistent with about 10% of the TOC being eventually converted to COz via a pathway involving a CO intermediate. The foregoing discussion suggests that the conversion of the majority of the TOC to COz during the SCW oxidation of phenols does not proceed via a refractory CO or acetic acid intermediate a t the reaction conditions and TOC conversions reported in this work. A minor pathway involving a refractory intermediate, in which case the influence of this intermediate on COz formation kinetics would be observed only at high TOC conversions, remains a possibility. An alternate explanation would be that the pure compound oxidation kinetics for acetic acid and CO are different from their kinetics in the presence of reactive centers derived from the phenols. SCW oxidation experiments a t higher TOC conversions and with mixtures of phenols and CO or acetic acid would be required to resolve these issues.
Summary and Conclusions The formation of COz (or disappearance of TOC) from phenol oxidation in near-critical and supercritical water proceeds with a global rate law given as 2394
Environ. Scl. Technol., Vol. 26, No. 12, 1992
rate = 2.6/R13 [TOC]0.82*0.20[OZIO0.71*0.26 (17) The global kinetics of COz formation from 2-chlorophenol oxidation can be described by a rate law given by rate = 10’.’*2.0 exp(-9.0 f 5.8/RT)[TOC]0.51*0.0s [OZIO0.80*0~13 (18) In both rate laws, the reaction rate has units of mol/L.s, all concentrations are in mol/L, and the activation energy is in kcal/mol. The uncertainties represent 95% confidence intervals. There was no statistically significant correlation between the TOC conversion and the water concentration. The yield of C02 formed from the oxidation of phenol and 2-chlorophenol was always less than the conversion of the corresponding phenol. A quantitative relationship between the conversion of organic carbon to COz and the disappearance of the phenolic reactant was given in eqs 10 and 11. The activation energies for C02 formation from the phenols used in this study were much lower than the reported activation energies for the SCW oxidation of CO and acetic acid, compounds that have been suggested to be rate-limiting intermediates in the complete conversion of organic carbon to C02. This observation suggests that the oxidation of these phenols under the conditions of the present investigation does not proceed via a major pathway involving a refractory CO or acetic acid intermediate. This conclusion, in turn, requires either that CO and acetic acid appear exclusively in minor pathways to COz formation or that their oxidation kinetics in the presence of reactive centers derived from the phenols differ from their oxidation kinetics as pure compounds. 100.6*1.3exp(-6.2
f
Acknowledgments We thank David Szmukler and Doug LaDue for experimental assistance.
Glossary A , A’ Arrhenius preexponential factors a, a’ reaction orders for TOC and organic, respectively b, b’ reaction orders for oxygen c , c’ reaction orders for water E,, E,‘ activation energies k , k’ global reaction rate constants PC critical pressure R gas constant T absolute temperature TC critical temperature conversion of TOC to C 0 2 or COZ molar yield xc XP conversion of phenolic reactant Greek Letters selectivity parameter used in eq 16 p’ parameter defined in eq 9 Y 7 residence time Registry No. PhOH, 108-95-2; C1C6H4-o-OH,95-57-8; Cog, CY
124-38-9.
Literature Cited (1) Copa, W. M.; Gitchel, W. B. In Standard Handbook of Hazardous Waste Treatment and Disposal; Freeman, H. M., Ed.; McGraw-Hill: New York, 1989; pp 8.77-8.90. (2) Modell, M. In Standard Handbook of Hazardous Waste Treatment and Disposal; Freeman, H. M., Ed.; McGrawHill: New York, 1989; pp 8.153-8.168.
Envlron. Sci. Technol. 1992, 26, 2395-2408
(16) Thornton, T. D.; Savage, P. E. Proc. Int. Conf. Supercrit. Fluids, 2nd 1991, 421. (17) Thornton, T. D.; Savage, P. E. AIChE J. 1992,38, 321. (18) Li, R.; Savage, P. E.; Szmukler, D. AIChE J., in press. (19) Dean, J. A., Ed. Lunge's Handbook of Chemistry, 12th ed.; McGraw-Hik New York, 1979. (20) Thornton, T. D. Ph.D. Diseertation,Universityof Michigan, 1991. (21) Thornton, T. D.; Savage,P. E. J . Supercrit. Fluids 1990, 3, 240. (22) Thornton, T. D.; LaDue, D. E., 111; Savage, P. E. Environ. Sci. Technol. 1991, 25, 1507. (23) Fogler, H. S. Elements of Chemical Reaction Engineering, 2nd ed.; Prentice-Hall: Englewood Cliffs, NJ, 1992. (24) Thornton, T. D.; Savage, P. E. Ind. Eng. Chem. Res., in press. (25) Li, L.; Chen, P.; Gloyna, E. F. AIChE. J 1991,37, 1687.
Thomason, T. B.; Modell, M. Hazard. Waste 1984,1,453. Modell, M. U.S. Patent No. 4,338,199, July 6, 1982. Modell, M. U.S.Patent No. 4,543,190, Sept 24, 1985. Helling, R. K.; Tester, J. W. Energy Fuels 1987, 1, 417. Helling, R. K.; Tester, J. W. Environ. Sci. Technol. 1988, 22, 1319. Webley, P. A.; Tegter, J. W.; Holgate, H. R. Ind. Eng. Chem. Res. 1991, 30, 1745. Webley, P. A.; Tester, J. W. In Supercritical Fluid Science and Technology;Johnston, K. P., Penninger,J. M. L., Eds.; ACS Symposium Series 406; American Chemical Society: Washington, DC 1989; pp 259-275. Webley, P. A.; Tester, J. W. Energy Fuels 1991, 5 , 411. Holgate, H. R.; Tester, J. W. Proc. Int. Conf. Supercrit. Fluids, 2nd 1991, 177. Rofer, C. K.; Streit, G. E. Los Alamos National Laboratory Report, LA-11439-MS, DOE/HWP-64 1988. Rofer, C. K.; Streit, G. E. Los Alamos National Laboratory Report, LA-11700-MS,DOE/HWP-90, 1989. Wightman, T. J. M.S. Thesis, University of California at Berkeley, 1981. Yang, H. H.; Eckert, C. A. Ind. Eng. Chem. Res. 1988,27, 2009.
Received for review March 23,1992. Revised manuscript received July 30, 1992. Accepted August 3, 1992. This project was supported by the National Science Foundation (CTS-8906860 and CTS-9015738).
Respeciation of Organic Gas Emissions and the Detection of Excess Unburned Gasoline in the Atmosphere Robert A. Harley, Mlchael P. Hannlgan, and Glen R. Cam"
Environmental Engineering Science Department, Callfornia Institute of Technology, Pasadena, Callfornia 91 125 The development of a set of organic gas composition profiles for key source categories is described. This information is used to recompute the organic gas emission inventory for the Los Angelea area. Comparisons are made between the revised emission inventory and ambient concentration measurements in southern California. Respeciation of the organic gas emissions results in large changes in the basinwide emissions estimates for many individual organic species, including l,&butadiene, ethylene glycol, methanol, and cyclohexane. Significant changes are observed in the reactivity of the chemical composition profiles for individual source categories, especially for surface-coating activities and associated thinning solvent use. Receptor-modelingmethods are used to identify the relative importance of major sources that contribute to atmospheric organic gas concentrations in southern California. The receptor modeling results indicate a key discrepancy between the emisaion inventory and ambient data: there is much more unburned gasoline in the atmosphere than is indicated in the emission inventory. These excess unburned gasoline emissions may be coming from a combinationof sources including tailpipe emissions, hot-soak evaporative emissions, and fuel spillage. ~~
1 . Introduction Preparation of accurate speciated organic gas emission inventories is necessary for photochemical modeling calculations and for the design of ozone abatement strategies. Knowledge of organic gas emissions is also required if the concentrations of toxic air contaminants (e.g., formaldehyde, benzene, and 1,3-butadiene) are to be controlled in a systematic fashion. Current emission inventories in use in the Los Angeles area specify temporally and spatially resolved organic gas emissions for over 800 source Categories. These inventories have been used in the formulation of pollutant abatement 0013-936X/92/0926-2395$03.00/0
strategies (I). Because the Los Angeles ozone control problem plays a critical role in establishing California and nationwide emission control policies, a correct understanding of the Los Angeles area organic gas emission inventory is very important. An organic gas emission inventory combines source activity factors with pollutant emission factors and speciation profiles. For example, detailed vehicle exhaust emissions are calculated as the product of an activity factor (vehicle miles traveled), emission factors (total organic gas, oxides of nitrogen, and carbon monoxide mass emission rates per mile traveled), and speciation factors for organic gases and oxides of nitrogen (percent by weight of individual compounds). Many of the same chemical composition profiles for organic gas emissions used in the California emission inventory have been incorporated into the United States Environmental Protection Agency (EPA) volatile organic compound speciation data system (2). In light of recent studies of motor vehicle emissions on the road, a great deal of attention has been focused on mobile source emission factors (3-5). Measurements made in a roadway tunnel in the Los Angeles area suggest that organic gas emissions from vehicles in actual use on the road are up to 3 times higher than those predicted by the EMFAC 7E mobile source emissions model (6). The EMFAC model is a central part of all mobile source emission calculations for regulatory planning purposes in California A comparison of emission inventory and ambient concentration ratios of carbon monoxide, oxides of nitrogen, and non-methane organic gases (NMOG) also suggests that on-road vehicle emissions of CO and NMOG are understated by the EMFAC model (7).Resolving the emission factor questions is an important step, but both source activity and speciation profile components of the emission calculation also must be considered. In the present study, existing speciation profiles for organic gas emissions will be reviewed, and new or updated
0 1992 American Chemical Society
Environ. Scl. Technol., Vol. 26, No. 12, 1992
2395
