Ind. Eng. Chem. Res. 1994,33, 2554-2562
2554
Experimental Kinetics and Mechanistic Modeling of the Oxidation of Simple Mixtures in Near-Critical Water Lori Torry Boock and Michael T. Klein* Department of Chemical Engineering, University of Delaware, Newark, Delaware 19716
A novel kinetics lumping strategy is assessed through the confrontation of experimental kinetics for the hydrothermal oxidation of mixtures of simple alcohols and acetic acid with the predictions of a mechanistic model. According to this lumping strategy, each of the elementary steps in the reaction model was lumped into one of eight reaction families. Each reaction family, in turn, was assigned a n Arrhenius A factor, a Polanyi relation slope a = 0.5, and a Polanyi parameter Eo* determined via optimization to previous pure component experimental data only. Quantitative prediction of the kinetics of mixtures of these components was achieved by adjusting only the A factor for the H-abstraction reaction family to the value loglo A (L/mol-s)) = 8.3, characteristic of H-abstraction for secondary alcohols. In short, the 167 rate constants of the mechanistic model were predicted by the eight reaction family parameter vectors [A:a:E,*]such that a n excellent correlation (r2= 0.987) existed between experimental (YE) and predicted (Yp) yields.
Introduction The oxidation of organics in supercritical aqueous waste streams is an appealing waste treatment complement to the current technologies of incineration, land application, and deep-well injection (Dickenson, 1981, 1986; Modell et al., 1982; Thomason and Modell, 1984). The potential for complete oxidation of dilute aqueous streams without the need to evaporate the water, the increased solubility of oxygen and organics in supercritical water (Connolly, 1966;Japas and Franck, 19851, and the relative ease of separation of insoluble salts and inorganics (Khaibullin and Borisov, 1965; Marshall and Franck, 1981; Pitzer et al., 1987) have motivated much research in this area. Many of the early studies on supercritical oxidation focused on demonstrating the technology (Freeman, 1985; Modell, 1982; Modell et al., 1982; Thomason and Modell, 1984). However, the complexity of the waste streams and the need for optimized reaction conditions have directed some research programs to the study of reaction chemistry and kinetics. Quantitative reaction models, consisting of controlling pathways, mechanisms, and rate parameters, provide a basis for understanding, manipulating, and designing novel processes or process improvements. The traditional chemical engineering approach to the kinetics analysis of complex mixtures has involved lumped reaction models. Model lumps or pseudospecies are generally defined by an analytical chemistry protocol that aggregates groups of molecules into boiling point or solubility classes, for example. Devoid of any other property, the lump reacts as a pseudospecies in the reaction model. The challenge in using the lumped model approach is that model parameters of reaction order and Arrhenius parameters, as well as the interactions of pseudospecies, can vary with conversion, temperature, and mixture composition. This has led to the development of molecular, mechanistic chemistry-based models for reaction mixtures. Wet air oxidation of complex waste streams presents special challenges to the development and use of mechanistic models. Frequently a large number of different compounds and associated unique chemical structures and oxidation rates will be present in the multicomponent waste. Both rate-enhancing and -inhibiting kinetic 0888-5885/94/2633-2554$04.5~/0
coupling effects can be anticipated, the ultimate resolution being dependent on the details of the mixture compositions and reaction conditions. Clearly, then, it is important to understand both the characteristics of the oxidation of individual compounds and their interactions in a mixture. The foregoing motivated the present analysis of the oxidation of comparatively tractable mixtures of small alcohols and acetic acid. The pure component experimental kinetics and mechanistic modeling of these molecules have been reported earlier (Boock and Klein, 1993). The rate of oxidation and the product spectra were dependent on the structure of the reactant and the reaction conditions. It was thus anticipated that the oxidation of even simple mixtures of these compounds would not be a “linear combination” of the pure component data. Developing the tools for modeling the oxidation of binary and multicomponent mixtures, with special attention devoted to the kinetic interactions between compounds, was undertaken in support of the overall goal of modeling complex waste streams. In short, the underlying thesis was that the mechanistic richness of elementary step modeling would provide kinetic parameters derived from only pure component data that could be used in the analysis of mixtures. The essential challenge introduced by the goal of the mechanistic modeling of the oxidation reactions of complex mixtures is the combinatorial explosion of reactions and associated rate parameters as the number of components in the mixture increases. The resolution of this conflict advanced in the present approach is to capitalize on the statistical nature of this explosion. That is, the number of elementary reaction types is finite, being of order 8-10, and the near-infinite explosion in reactions is due to similar but not identical reactants, products, and intermediates participating in these reactions. Our earlier work (Boock and Klein, 1993) sought to reduce this complexity by “lumping” all of the elementary steps of oxidation into one of eight controlling reaction families. This lumping defined a reaction family t o comprise members whose rate constants differed only because of electronic effects on the reactive center. These electronic effects were estimated using a Polanyi structure-reactivity correlation for each 1994 American Chemical Society
Ind. Eng. Chem. Res., Vol. 33, No. 11, 1994 2555 Table 1. Oxidation of Mixtures: Reaction Conditions and Product Spectra ~
~
concn, mol&
temp, "C
ethanouacetic acid methanoyl-propanol
0.06 acetic acid: 0.0-0.08 ethanol 0.115 methanol; 0.06 l-propanol
380 300
methanoyl -propanoV 2-propanol/l-butanol
0.15-0.29 total
300
reaction mixture
reaction family. Experimental data for each pure component alcohol and acetic acid were well fit by a model with Arrhenius A factors and Polanyi a slopes assigned a priori, and one fitted value for the Polanyi E,* for each reaction family. The present work tests the thesis that these purecomponent-derived reaction families can be used to predict the observed experimental kinetics of mixtures. This would be the first step toward laying the foundation for the prediction of complex mixture behavior with models using 0(102-104) rate constants calculated from a small number 0(101) of reaction family structurereactivity correlations, the parameters of which being derived from only pure component data. This report therefore chronicles the experimental kinetics and quantitative mechanistic modeling of twoand four-component mixtures of methanol, ethanol, 1and 2-propanol, l-butanol, and acetic acid. The oxidation reactions were at the temperatures and initial concentrations listed in Table 1. The mechanistic model contains the distinctive feature of structure-reactivity correlations as the basis for lumping the kinetics of homologous elementary steps.
Experimental Procedures The oxidation experiments were conducted in batch reactors. They consisted of a 12 cm3316 stainless steel base connected by 1/4 in. 316 stainless steel tubing to a Whitey ball valve and a Swagelok full flow quickconnect stem. All reactors, tubing, valves, and fittings were cleaned for oxygen service prior to use. An explosion-proofbarricade, equipped with a view window, was used for all tasks involving the reactor under oxygen pressure. Our experience with this system is that only neat pyrolysis experiments are subject to wall effects, and only then in reactors used for the first time. Results for reaction in high-temperature water, such as those reported here, are not sensitive to the reactor type or history, or other hardware issues. A typical experimental protocol was as follows. The desired volumes and concentrations of the reactant solutions (in distilled water) were loaded into the batch reactors. Solution loadings were determined gravimetrically with a Sartorius L2200S balance (&lo mg). The reactors were sealed using titanium gaskets. The reactor was then connected to the oxygen cylinder via a quick connect, which was located behind the barricade. The reactor was cycle-purged three times with oxygen, which attained a final loading of 500 psia. The valve was closed and the reactor was then disconnected from the quick-connect body. The oxygen concentrations were then determined gravimetrically. The reactor was connected to a pulley system, also located behind the explosion-proof barrier. It was lowered, at time zero, into a Techne fluidized sand bath heated to a predetermined constant temperature. The reactor was lowered until the valve was approximately 2 in. above the surface of the sand. The reactors reached the designated temperature in less than 4 min. After the desired reaction time had elapsed, the reactor
liquid products water, acetic acid, methanol water, acetic acid, methanol, ethanol, 2-propanol water, acetic acid, methanol, ethanol, 2-propanol
~
_
gaseous products COz, CO, methane COZ. -, CO. methane I
COz, CO, methane
5.0
4.0
A
A
A 0.00
0.02
0.04
0.06
0.08
0.10
Ethanol Concentration (molll)
Figure 1. Initial rates for the oxidation of acetic acid and ethanol in SC water.
was removed from the sand bath, using the pulley system, and the reaction was quenched by lowering the reactor into a cold water bath. Gaseous products were collected for analysis in a gas sampling bag connected to the quick-connect stem by another quick-connect body. The reactors were then opened, and an external standard (either ethylene glycol or propionic acid) was added to the liquid products, which were then collected in distilled water for analysis. Liquid products were analyzed on a HP 5710 gas chromatograph (GC),equipped with a Porapak Q packed column or a 30 m, 0.53 mm i.d. Nukol capillary column (acetic acid) and a flame ionization detector (FID), and a HP 5880 GC, equipped with a DB-5 silica capillary column (alcohols)and a FID detector. Response factors were estimated from analysis of standard mixtures of all liquid reactants, products, and the external standard. Gaseous products were analyzed on a HP 5890A GC equipped with 100/120 Carbosieve S-I1 packed column and a thermal conductivity detector (TCD). Literature values of response factors were utilized for gaseous products.
Experimental Results Three systems were oxidized under the conditions summarized in Table 1. The acetic acid-ethanol system was studied at 380 "C with a constant acetic acid concentration of 0.06 M and varying initial ethanol concentrations of 0 < 0.08. These experiments revealed the effect of the presence and concentration of an easily oxidized compound on a more refractory compound. The methanol and l-propanol system was oxidized at 300 "C at the constant methanol concentration C M ~ O=H0.115 and propanol concentrations of 0.06 < C1p < 1.0 M. Finally, the four-component system of methanol, 2-propanol, l-propanol, and l-butanol was oxidized at 300 "C and varying initial total organic concentrations t o determine the coupling effects of multicomponent mixtures. The ethanol-acetic acid cooxidation reaction kinetics are summarized in Figure 1 as a plot of the instantaneous oxidation rate of each component at t = 15 min
_
_
2556 Ind. Eng. Chem. Res., Vol. 33, No. 11, 1994 1 .o
0
2
f
I
0
.
o’6L A A
Methanol Methanol (1-cornp)
0 1-Propanol
0.4
0 1-Propanol (1-comp)
0
0.0 0.2
20
10
0
40
30
50
T h e (rnln)
Figure 2. Oxidation of methanol and 1-propanol in near-critical water.
0.05
7 t A
0.04
s
0.03
U
c
Table 2. Summary of Multicomponent Oxidation Results init concn, init rate x 105, k(first) x lo4,” compound mom m0LKL.s) S-1 Two Component, Total m o m = 0.09 methanol 0.06 5.14 f 0.4 6.88 f 0.8 16.4 f 0.8 1-propanol 0.03 6.18 f 0.4 Four Component, Total mol/L = 0.29 10.9 f 0.8 methanol 0.115 12.6 f 0.4 16.6 f 0.8 1-propanol 0.06 10.1 f 0.4 2-propanol 0.06 8.19 f 0.4 13.3 f 0.8 1-butanol 0.05 8.23 f 0.4 16.7 f 0.8 Four Component, Total m o m = 0.22 8.27 i 0.8 0.115 9.60 f 0.4 methanol 3.00 f 0.4 11.8f 0.8 0.025 1-propanol 6.73 f 0.4 11.0 f 0.8 0.06 2-propanol 1-butanol 0.02 3.09 i 0.4 14.9 i 0.8 Four Component, Total mol/L = 0.15 methanol 0.06 3.95 f 0.4 12.1 f 0.8 1-propanol 0.03 4.80 f 0.4 15.4 f 0.8 12.1 i 0.8 2-propanol 0.03 2.87 i 0.4 1-butanol 0.025 4.00 f 0.4 15.9 f 0.8
0.02 0.01
A
A
t t
A
A A
a Based on initial rate and initial concentration of each component.
0 2-Propnnol
t
0
Acetlc Acid Ethanol
I 10
20
30
40
50
Time (mln)
Figure 3. Oxidation of methanol and 1-propanol: product yields.
versus the initial ethanol concentration. The rates were comparable in overall magnitude at about lo5 mol L-l s-l. Note that the oxidation rates of both acetic acid and ethanol increased with ethanol concentration. Clearly, the presence of ethanol accelerated the rate of oxidation of acetic acid. However, comparison with the pure component data (Boock and Klein, 1993) reveals that acetic acid suppressed the rate of ethanol oxidation. The methanol- 1-propanol cooxidation kinetics at CIP = 0.06 M are summarized in Figure 2 as a plot of conversion(1 - Ni/Nio) versus time. Results from pure component experiments (Boock and Klein, 1993) are included to facilitate comparison. Clearly the kinetics of methanol conversion were enhanced by the presence of 1-propanol. In contrast, the conversion of 1-propanol was slightly lower in the mixture. Quantitatively, the suppression of the l-propanol oxidation by the methanol was smaller than the enhancement of the methanol oxidation rate by the 1-propanol. The observable liquid-phase products of cooxidation were acetic acid, 2-propanol, and ethanol, the same products as those observed from the pure component oxidation of 1-propanol. Clearly, any methanol produced from the 1-propanol could not be distinguished from the reactant methanol. The time dependence of the yields of these products is shown in Figure 3. Acetic acid was the major liquid product, reaching a maximum yield of 0.04 after 10 min. The yields of the gas-phase products methane and CO decreased with time while that for COz increased. The four-component mixture of methanol, 2-propanol, 1-butanol, and 1-propanol comprised components spanning a wide range of oxidation rates and product spectra. Methanol was the most refractory pure component alcohol whereas 1-propanol was most easily
oxidized. The initial concentrations were selected to probe various technological and scientific issues. In one case the initial concentration of the individual alcohols was the same as in the pure component studies reported earlier, which resulted in an increase in the total organic concentration. In the second case, the concentrations of the individual alcohols were reduced so that the total organic concentration approximated the values used in the pure component experiments. This revealed kinetic coupling effects without an increase in total organics. The third mixture included methanol and 2-propanol, the more refractory alcohols, in the concentration used in their pure component experiments, whereas the concentrations of 1-propanol and 1-butanol were reduced. This highlighted the effect of both the addition of components and an increase in total organics. Table 2 summarizes this composition information and reactant disappearance results in terms of the initial rates and associated pseudo-first-order rate constants for the oxidation of each alcohol in the four-component mixture. Figures 4 and 5 provide a three-dimensional graphical comparison of the initial oxidation rates and first-order rate constants, respectively, for each component in the single-component,two-component, and fourcomponent alcohol systems at 300 “C. The information in Table 2 and Figures 4 and 5 reveals evidence of kinetic coupling. In general, the oxidation of the slower-oxidizingalcohols, methanol and 2-propanol, was enhanced by the presence of the other alcohols. At the same time, the rate of disappearance of the faster-oxidizing alcohols, 1-propanol and l-butanol, was suppressed by the presence of the sloweroxidizing alcohols. The magnitude of the enhancement was larger than that of the suppression. These concepts are clearly apparent in Figure 5, where a component’s pseudo-first-order rate constant, based on its initial concentration, shows a variation with total organic concentration. It is revealing that this variation in rate constant does not follow a simple monotonic dependence on total organic concentration. For example, the rate constant for methanol oxidation is largest in the lowest concentration (0.15 total mol/L) system, whereas the rate constants for the other alcohols are largest in the highest concentration system (0.29 total mom). The intermediate concentration
Ind. Eng. Chem. Res., Vol. 33,No. 11,1994 2557 Initialion 1-
n
'
Propagation 2-
..
RH+Ol+R'+HC(
R'+O,+RO;
3
RO; + RH + ROOH
4
RO;+
5
R'
-
HOOR'
+ R" + C=RH
Chain Transfer
6
&pm@mhh
ZRO;
+ 0,+ 2RO'
Branching ROOH + RO' + HO'
7
R
. .
R& t RO;
+ Pmducls
Figure 6. Elementary steps and reaction families for oxidation in supercritical water. 4
Figure 4. Initial rates of oxidation in multicomponent mixtures.
Hz x
i
Figure 5. Pseudo-firsborder rate constants for oxidation in multicomponent mixtures.
system (0.22 mom) is characterized by the smallest rate constant for all four alcohols. These results suggest that the numerous kinetic interactions in the multicomponent system can lead to either rate enhancement or suppression. Clearly optimization of the oxidation process conditions would benefit from mechanistic understanding and modeling of these interactions.
Mechanistic Modeling Oxidation in dense high-temperature water shows features characteristic of both gas-phase combustion and liquid-phase oxidation mechanisms. The elementary steps involve the creation and loss of free-radical intermediates that consume the reactant in free-radical propagation cycles. As noted earlier, previous models (Boock and Klein, 1993) for the reactions of pure components using the eight reaction families illustrated
in Figure 6 reproduced the experimentally observed kinetics well. The eight reaction families in Figure 6 are initiation (H-abstraction by Od, addition to oxygen, H-abstraction by a radical, isomerization, ,!?-scission, chain-transfer, Le., nonterminating radical recombinatioddisproportionation, decomposition, and terminating radical recombination. According to this model of free-radical oxidation, the total radical concentration is influenced by initiation (11, branching (71, and termination (8) reaction families. Radicals are consumed in the termination steps of radical recombination and/or disproportionation. The propagation steps are balanced in the consumption and generation of free radicals. Bimolecular radical addition to 0 2 (2) forms peroxy radicals. The peroxy radical can subsequently undergo hydrogen abstraction (3)to form hydroperoxides and reactantderived free radicals. Radical isomerization reactions (4) are always possible. Chain transfer reactions (6), which involve nonterminating collisions of radicals, result in the conversion of peroxy radicals into RO' radicals which can also propagate the reaction chain. B-scission (5)reactions lead to the ultimate degradation of the reactant. Applications of these reaction families in the mechanistic modeling of the mixture is conceptually straightforward the same steps are operative. However, in mixtures, the cross reactions, which involve interaction of radicals from compound i with all species (j = i and j t i) and their derived radicals must be included. This leads to a combinatorial explosion of reactions and associated rate constants. This suggests that insights leading to a reduction in the number of model parameters should be developed. Figure 7 illustrates the explosion of terms resulting from exhaustive application of the eight reaction families to each component in the four-component mixture, including cross reactions. Careful examination shows that much of the complexity of Figure 7 is statistical. That is, the magnitude of Figure 7 is due to the repeated application of a handful of reaction operations to reaction centers differing in, essentially, only substituents. This, in turn, suggests that the classic Polanyi (Boudart, 1968) or Hammett (1935)structure/property relationships could be useful in organizing or constraining the rate constants of reactions within each of the eight reaction families. The chemical basis for the Polanyi structure-reactivity correlations is the similarity of
2558 Ind. Eng. Chem. Res., Vol. 33, No. 11, 1994
-
Reaction
log A
Initiation CH30H t 0 2 4 CH2'0H t H@* CH3CHOHCH3 t 02CH3C'OHCH3 + H02' CH3CHzCH'OH t H02* CH3CH2CH20H t 02C H ~ C H Z C H ~ C H ~tO02-H CH~CH~CHZCH'OH + H02' Okygen Addition CH2'OH t 0 2 CH2OO'M CH3C'OHCH=j + 0 2 ___) CH3COO0OHCH3 'CH3 + 02CH300' CH3CH2CH'OH t 0 2 CH3CH2CHOO'OH CH3CH2' t 02CH3CH200. t CH~CH~CHZCHOO'OH CH~CHZCH~CH'OH Isomerization CH200'0H 4'CHOOHOH CH3COO'(OH)CH3 CH3COOH(O0)CH3 CH3CO(O') C'H2CqOH) CH300' d CX2OOH CH3CHzCHOO'OH CH3CH2C'OOHOH CH2=CO'OH C'H2C=O(OH) CH3CH2CHO'OH CH3CH2C'(OH)2 CH2=C(OH)24C H 3 C S H CH~CHZCH~C'OOHOH CH3CH2CH2CHOO0OH CH3CH2CH2CHOO0OH CH3C?lCH2CHOOHOH CH3C0HCH2CHOOHOHd C H 3 C H ( O H ) C H 2 C H ( O H ) o . CH3CH2CH2CHO'OH I C H 3 C H 2 C H 2 C ' ( O H ) 2 p-Scission 'CHOOHOH HO' t M H O H CH2O'OH H 2 C d + HO' CH3CO.OHCH3 4'CH3 t CH3COOH 'CH3 t CH3CO(OOH) CH3CO'(OOH)CH3 C'H2OOH H 2 C 4 t HO' 'CH3 t CHz=COOHOH CH3CH2C'OOHOH CH~CH~C'(OH)Z 'CH3t CH2=C(OH)2 CH3CH2' + HOCH=O CH3CH2CHO'OH CH3CH20' 'CH3 t H2C=O CH3CH2CH2C.OOHOH 'CH2CH3 + CH2COOHOH CH3CH(OH)CH2CH(OH)O' CH3CH(OH)CH2 + HC=O(OH) CH~CHZCH~C'(OH)~ 'CH2CH-j t CH2=C(OH)2 H-Abstraction CH2OO.OH + CH30H CHz'OH t CH2OOHOH 'CHOOHOH t CH3OH CHz*OH t C H W H O H HOOH t CH2'0H HO2' + CH30H HO' t CH30H H20 t CH2'OH CH3COO.OHCH3 t CH3CHOHCH3 CH3COOHOHCH3 t CH3C'OHCH3 HO2' t CH3CHOHCH3 HOOH t CH3C'OHCH3 HO' t CH3CHOHCH3 H20 + CH3C'OHCH3 CH3OO' t CH3CHOHCH3_i) CH3OOH + CH3C'OHCH3 CH3COOH t CH3C'OHCH3 CH3CqO') t CH3CHOHCH3 CH30' t CH3CHOHCH3 CH30H + CH-jC'OHCH3 CHq + CH3C'OHCH3 'CH3 t CH3CHOHCH3 t CH3C0OHCH3 C'H2CO(OH) t CH3CHOHCH3,CH3COOH C'H2OOH t CH3CHOHCH3 CH3OOH t CH3C'OHCH3 CH3CH2CHOO'OHt C H 3 C H 2 C H 2 0 H d CH3CH2CHOOHOH t CH3CH2CH'OH CH3CHzCHOOHOH t CH3CH2C'OOHOH t CH~CHZCH~OHCH3CH2CH'OH HOOH + CH3CH2CH'OH Hop' t C H ~ C H Z C H ~ O H CH300H + CH3CH2CH'OH CH300' t C H ~ C H Z C H ~ O H CH30H t CH3CHzCH'OH CH30' + CH3CHzCH20H
-
+
-
*
*
+
--
/I"-'mol *-"rnin-
k callmol
12.5
23.5 16.3 19.1 18.9
12.5
12.5 12.5
10.5 10.3 10.3 10.3 10.3 10.3
-17.8 -7.8 -24.8 -12.4
13.1 13.1 13.1 13.1 13.1 13.1 13.1 13.1 13.1 13.1 13.1 13.1
-54.9 10.0 -14.4 -37.4 -47.1 -0.7 -26.0 -18.0 -48.9
13.8 13.8 13.8 13.8 13.8 13.8 13.8 13.8 13.8 13.8 13.8 13.8
6.0 25.4
10.5 61.5 -16.6 32.0
8.3 8.3 8.3 8.3 8.3
-14.2 40.7 -5.5 -30.1 -20.9
8.3 8.3 8.3 8.3 8.3 8.3 8.3 8.3 8.3
-12.6 -37.4 -13.6 -30.8
8.3
30.4
8.3 8.3 8.3
-9.8 -9.7 -18.7
-17.6 -12.4
-14.9 -36.3 -27.1
-6.7 -8.0 2.9 74.9 39.6 -13.0
-21.6
-22.1 -16.5 23.8 -16.7
(Figure 7 continued on next page)
--
Ind. Eng. Chem. Res., Vol. 33, No. 11, 1994 2669
'CH3 t CH~CH~CHZOH C'H2COOH t CH3CH2CH20H-
CH4 t CH3CHzCH'OH CH3COOH t CH3CH2CH0OH CH3CH2OOH t CH3CH2CH.OH CH3CHzOO. t CH3CH2CH20H CH3CH20H t CH3CH2CH.W CH3CH20' t CH3CH2CH2OH
8.3
-19.2
8.3 8.3 8.3
-13.5 -17.9 -20.3
H20 + CH~CHZCH'OH HO' t CH3CH2CH20H CH3CH3 t CH3CHzCH'OH *CH2CH3+ CH3CHzCH20H CH3OOH t CH3CH2CH'OH C'H2OOH t CH3CH2CH20H 'CH2COOH t CH3CH2CH2CH20H CH3COOH t CH~CH~CHZCH'OH HOOH t CH~CH~CHZCH'OH H02' t CH3CH2CH2CH20H CH3CH20' t CH~CH~CHZCH~OH CH3CH2OOH t CH~CHZCHZCH'OH CH3CH20'+CH3CH2CH2CH2OH+CH3CH20Ht CH-jCH2CH2CH.OH %H2CH3tCH3CH2CH2CH20H+CH3CH3 t CH3CH2CH2CH'OH HO' t C H ~ C H Z C H ~ C H ~ O H H20 t CH~CHZCH~CH'OH CH3OO0tCH3CH2CH2CH20H CH300H t CH~CHZCH~CH'OH
8.3 8.3 8.3
-34.5 -10.8 26.6
8.3
-14.9
a .3 a .3
-10.0
8.3
-20.6
8.3 8.3 8.3 8.3 8.3 8.3 8.3
-11.1 -34.7 -11.0
CH~CH~CHZCHOOHOH t CH3CH2CH2CHoOH CH-jCH2CH2C*OOHOHtCH-jCH2CH2CH2OHCH3CH2CH2CHOOHOH t CH~CHZCH~CH'OH
8.3
30.5
CH3C'HCH2CHOOHOHtCH3CH2CH2CH2OH-
8.3
-3.5
8.3
-12.4
8.3
-18.6
8.3 8.3 8.3
-18.5 -21.3 36.4
8.3 8.3 8.3 8.3
36.3 33.6 -13.7 -18.1
8.3
-18.2
CH300. t CH30H CH3CO(O')t CH30H
CH2*OH t CH3OOH CH2'OH t CH3COOH CH3COOH t CH3CH2CH'OH CH3CqO') t CH3CH2CH20H
8.3 8.3 8.3
-6.4 -23.6 -28.0
CH3CO(O')tCH3CH2CH2CH20H-CH3COOHt CH3CH2CH2CH'OH CH2'OH t CH30H CH30' t CH30H CH3* t CH30H d CH2*OH t CH4 %H2COOH t CH30H CH2'OH t CH3COOH CH2'0H t CH3COH 'CH2OOH t CH30H CH3CH2CHOO'OH + CH30H CH2'OH t CH3CH2CHOOHOH C H3 C H 2C HOO 'OH t CH3C HOHCH 3 j C H 3 C H 2 C H O O H O H t
8.3
-28.1
8.3 8.3 8.3 8.3 8.3 8.3
-14.4 -29.2 -9.3 31.0 -12.4 -19.6
8.3
-16.9
8.3
8.3
34.1 27.5
8.3
30.2
a .3
-13.6 -20.8 -16.0 -23.2
CH30H t CH~CHZCH~CH'OH CH3O' + C H ~ C H ~ C H Z C H ~ O H 'CH3 t C H ~ C H ~ C H Z C H ~ O H CH4 t CH3CH2CH2CH0OH C'H200HtCH3CH2CH2CH2OH-CH3OOH + CH3CH2CH2CH0OH
CH3CH2CH2CH00'0HtCH3CH2CH2CH20H~
CH~CH~CHZCHOOHOH t CH3CH2CH2CH0OH CH3CH(OH)C'H2 t CH3CH2CH2CH20H j CH3CH(OH)CH3 + CH3CH2CH2CH'OH CH200'0HtCH3CH2CH2CH20H-CH2OOHOHt CH3CH2CH2CH'OH CH2OO'OH t CH3CHzCHzOH d CH2OOHOH t CH3CH2CH'OH CH200HOH t CH3CbOHCH3 CH200'0H t CH3CHOHCH3 *CHOOHOHtCH~CH~CH~CH~OH~CH@OHOHt CH3CH2CH2CH'OH CH2OOHOH t CH3CH2CH'OH 'CHOOHOH t CH3CH2CH20H 'CHOOHOH t CH3CHOHCH3 CH2OOHOH t CH3C'OHCH3 CH3COO.OHCH3 t CH30H CH2'OH t CH3COOHOHCH-j CH3CO0'0HCH3 t CH3CH2CH20H CH3COOHOHCH3 t CH3CH2CH0OH CH3COOHOHCH3t CH3CO0'0HCH3 t CH3CH2CH2CH20H CH3CH2CH2CH'OH
--
CH3C'OHCH3 CH3CH2CHOO'OHt CH3CH2CH2CH20H CH3CH2CHOOHOHt CH3CH2CH2CH'OH CH2'OH t CH3CHzCHOOHOH CH3CH2C'OOHOH t CH30H CH3CH2C'OOHOH t CH3CHOHCH3 -CH3CH2CHOOHOHt CH3C'OHCH3 CH3CH2C'OOHOHt CH3CH2CH2CH2OH CH3CH2CHOOHOHt CH~CH~CHZCH'OH CH2'OH + CH3CH2OOH CH3CH2OO' t CH30H CH3CH200' t CH3CHOHCH3 +CH3CH200Ht CH3C*OHCH3 CH2'OH t CH3CHzOH CH~CHZO't CH30H CH3CH20' t CH3CHOHCH3 r C H 3 C H 2 0 H t CH3C*OHCH3
8.3 8.3 8.3
-18.2
-19.0 -19.5 26.4 -18.4
(Figure 7 continued on next page)
-
2560 Ind. Eng. Chem. Res., Vol. 33, No.11, 1994 'CH2CH3 t CH30H CH2'OH t CH3CH3 'CH2CH3 t CH3CHOHCH3-CH3CH3+ CH3CbOHCH3 CH3CH2CH2CHOO'OHtCH3OH--)C"0H+
8.3 8.3 8.3
-6.5 -13.7 -13.9
CH3CH2CH2CHOOHOH CH3CH2CH2CHOO*OHtCH3CHOHCH3~CH3CH2CH2CHOOHOH
8.3
-21.1
8.3
-18.3
+ CH3C'OHCH3 CH3CH2CH2CHOO'OHtCH3CH2CH20HI CH~CH~CHZCHOOHOH + CH3CHzCH'OH CH3CH2CH~C*OOHOHtCH3OH~CH2*OHt
8.3
35.1
CH3CH2CH2CHOOHOH CH3CH2CH2C'OOHOHtCH3CHOHCH3-CH3CH2CH2CHOOHOH + CH-jC'OHCH3
8.3
27.9
CH3CH2CH2C'OOHOH+CH3CH2CH2OH-
8.3
30.8
CH3CH2CH2CHOOHOH + CH3CH2CH'OH CH3C'HCH2CHOOHOH+CH3OH-CH2"c
8.3
1 .o
8.3
-6.2
8.3
-3.4
CH3CH2CH2CHOOHOH
-
CH3C'HCH2CHOOHOHtCH3CHOHCH3~CH3CH2CH2CH00HOH+ CH3C'OHCH3 CH3C'HCH2CHOOHOH+CH3CH2CH2OH+ CH3CH2CH2CHOOHOHt CH3CH2CH'OH CH3CH(OH)C'H2 t CH30H CH2'OH t CH3CHOHCH3 CH3CH(OH)C%2+CH3CHOHCH3+CH3CHOHCH3+ CH3C0OHCH3 CH3CH(OH)C'H2tCH3CH2CH2OH*CH3CHOHCH3+ CH3CH2CH'OH CH2'OH t CH3CHOHCH3 CH3C'OHCH3 t CH30H CH2'OH + CH3CH2CH20H CH3CH2C'HOH t CH30H CH2'OH + CH3CH2CH2CH20H CH3CH2CH2CXOH + CH30H CH2'0H + CH3CHOHCH3 ----.)CH3C'OHCH3t CH30H CH3CH2C'HOH+CH3CHOHCH3-CH3C*OHCH3+ CH3CH2CH20H
-7.8
CH3CH2CH2C'HOHtCH3CHOHCH3---)C"OHCH3C*OHCH3+
8.3 8.3 8.3 8.3 8.3 8.3 8.3 8.3 8.3
CH3CH2CH2CH20H CH30H CH2'OH t CH3CH2CH20H ----.)CH3CH2CoHOH+ CH3C'OHCH3 tCH3CH2CH20H--)CH3CH2CDHOH+ CH3CHOHCH3 CH3CH2CH2C'HOH +CH~CH~CHZOH-CH~CH~C*HOH~
8.3 8.3 8.3
-4.4 2.8 0.1
CH3CH2CH2CH20H CH2'OH + C H ~ C H Z C H ~ C H ~+CH3CH2CH2C0HOH+ OH CH3C'OHCH3t CH3CH2CH2CH20H -CH3CH2CH2C0HOHt
8.3 8.3
-4.5 2.7
8.3
-0.1
8.3 8.3 8.3
8.0 0.8 3.5
CH30H
CH3CHOHCH3 OH CH3CH2C'HOH + C H ~ C H ~ C H ~ C H Z-CH~CH~CHZC*HOH~ CH3CH2CH20H CH2=C(OH)O*+ CH30H CH2'0H + CH2C(OH)2 CH2-COHO't CH3CHOHCH3 +CH3CgOHCH3t CHzC(OH):! CH2=COHO't CH3CH2CH2CH20H -CH3CH2CH2C'HOH+ CH2C(OH)2 Decomposition CH2OOHOH
-
CH2OOH + HO' HOOH 2HO' CH3OOH CH30. + HO' CH3CO(OOH) CH3CO(O') t HO' CH3C0'0HCH3 t HO' CH3COOHOHCH3 CH3CHzCHOOHOH CH3CH2CHO'OH t NO' C H 2 = C O O H O H d CH2=CO'OH HO' CH3CH200H CH3CH20' + HO' CH3CH2CH2CHOOHOH CH3CH2CH2CHO*OH+€10' Non-Terminating Radical Recombination CH200'0H t CH2OO'OH % t 2CH20'OH H02' t HO2' 0 2 t 2HO' t 2CH3O' CH300' + CH300' __$02 2CH3CH2CHO'OH t 0 2 CH3CH2CHOO'OH + CH3CH2CHOO'OH CH3CH200' t CH3CH200' d 2CH3CH20'+ 0 2 CH3CH2CH2CHOO'OHtCH3CH2CH2CH00*OH-O2t 2CH3CH2CH2CHO0OH CH3COO'OHCH3' CH3C00'0HCH3 40 2 + 2CH3C0'0HCH3 t CH2O'OH t HO' CH200'0H H02O-02 0 2 t CH20'0H +CH30* CH200'0H t CH300'C H ~ O O * O H t C H ~ C H ~ C H O O * O H ~ O ~ + C HCH3CHzCHO'OH ~O*OHt
-15.0 -12.2 7.2 4.4 4.5 -7.2 -2.8 -2.7
17.3 17.3 17.3 17.3 17.3 17.3 17.3 17.3 17.3
29.1 39.8 21.7 33.9 32.6 29.0 0.02 29.0 29.5
10.3 10.3 10.3 10.3 10.3 10.3
-27.9 11.0 -27.4 -24.6 -27.0 -26.6
10.3 10.3 10.3 10.3
-20.0 -8.4 -27.6 -26.2
(Figure 7 continued on next page)
Ind. Eng. Chem. Res., Vol. 33,No. 11, 1994 2661 CH2OO'OH t CH3CH200' r
Q t CH20'0H t CH3CH20' CH200'OH+CH3CH2CH2CHOo*OH~tCH20'0Ht
10.3 10.3
-27.4 -27.2
10.3 10.3 10.3 10.3 10.3 10.3 10.3 10.3 10.3
-23.9 -8.2 -6.9 -8.0 -27.0 -4.5 -26.0 -21.2 -1.9
10.3 10.3 10.3
-23.7 -25.8 -26.8
10.3
-22.3
10.3
-26.8
CH3CH2CH2CHO'OH
CH2OO'OHtCH3C00*OHCH3---)02 He'
+
CH300'+02
t
t CH20'0H t
CH3CO0OHCH3
HO' t CH3O'
-
HO2.t CH3CH2CH00'OHCH3CH2CHO'OH t H o t 0 2 H02' CH3CH2OO' H o t CH3CH20. t 0 2 0 2 t CH~CHZCH~CHO'OH t HO' HO2' t CH3CH2CH2CHOO.OH@'t CH3C00'0HCH3 0 2 t CH3C0'0HCH3 t HO' CH300.t CH3CH2CHOO'OH CH3CH2CHO'OH t CH3O' t 02 CH300.t CH3CH200'CH3O' t CH3CH20.t 0 2 CH300't CH3CH2CH2CHOOo0H+ 0 2 t CH3CH2CH2CHO0OHt CH30' CH300' t CH3C00'0HCH3 0 2 t CH3CO'OHCH3 t CH3O' CH3CH2CHOO'OHtCH-jCH200*~CH3CH2CHO*OHtCH3CH2O' t 0 2 CH3CH2CHOO'OHtCH3CH2CH2CHOO*OH+ 02t CH3CH2CH2CHO'OH t CH3CH2CH2CHO'OH CH3CH2CHOO'OH t CH3C00'0HCH3 0 2 t CH3CO'OHCH3 t CH3CH2CH2CHO'OH CH3CH200'tCH3CH2CH2CHOO'OH+O2t CH3CH2CH2CHO'OH t CH3CHzO' CH3CH200'tCH3C00'OHCH3~02tCH3CO*OHCH3 t CH3CH2O'
+
CH3CH2CH2CHOO'OHtCH3COO'OHCH3 + CH~CH~CHZCHO'OH Termination
CH200'OH t CH200'OH H Q ' + H02' 'IP
-02
t
CH3CO0OHCH3
-
+'IP
CH300' CH300' 'IP CH3CH2CHOO0OHt CH3CH2CHOO'OH CH3CH200' t C H 3 C H 2 0 0 ' 7 TP
TF'
CH~CHZCH~CHOO'OH t CH3CH2CH2CHOO'OH CH3COO'OHCH3t CH3C00'0HCH3 'IP CH200'OH HO2'TP CH2OO'OH + CH300' TP CH2OO'OH t CH3CHzCHOO'OH 4TP CH200'0H t CH3CH200' __$ TP CH200'0H02 t CH~CHZCH~CHOO'OH CH200'0H t CH3C00'0HCH3 H@* CH300''I7
'IP
TP
-23.5
10.3
-23.3
8.8 8.8 8.8 8.8 8.8 8.8 8.8 8.8 8.8 8.8 8.8 8.8 8.8 8.8 8.8 8.8 8.8 8.8 8.8 8.8 8.8 8.8 8.8 8.8 8.8 8.8 8.8 8.8
d TP
-
10.3
H@'+ CH3CH2CHOO'OH'IP H02' CH3CH200''IP H02' t CH~CHZCH~CHOO'OH'IP H@*t CH3C00'0HCH3 'IP CH300.t CH3CH2CHOO'OH 'IP CH300.t CH3CH200''IP CH300.t CH3CH2CH2CHOO0OH'IP CH300' t CH3C00'0HCH3 TF' CH3CH2CHOO'OH t CH3CH200' TP CH3CHzCHOO'OH t CH3CH2CH2CHOO0OH CH3CH2CHOO'OH t CH3COO.OHCH3 TF' CH3CH2OO' t CH3CH2CH2CHOO'OH 'Ip
+TF'
CH3CH200' + CH3C00'0HCH3 'I7 CH3CH2CH2CHOO0OHt CH3C00'0HCH3 d 'IP
Figure 7. Detailed mechanism for the oxidation of the four-component mixture.
transition states for members of a reaction family (Dewar, 1969). The power of this is that the ultimate number of structure-reactivity relationship parameters would be far fewer than the number of rate constants thusly correlated. The present application of these ideas is to define a reaction family as a set of reactions with identical activation entropies, i.e., A(AS*)i-o = 0 for all members i in a reaction family. Thus, changes in the reactivities of family members are modeled as due to changes in
activation enthalpies, which are in turn proportional to changes in the overall reaction enthalpy change. The approach is forgiving because even if A(AS*)i-o is not strictly zero, often it is proportional t o A ( m l i - 0 . The net effect is that E$ = Eoy* + a i A H R for members i in reaction familyj. In other words, each reaction family will have a single Arrhenius A factor and a structurereactivity correlation for E*. Thus the ultimate mixture mechanistic model parameters were the Polanyi parameters a and E,u* for each
2562 Ind. Eng. Chem. Res., Vol. 33, No. 11, 1994 Table 3. Polanyi Parameters (E,*) for the Oxidation of Alcohols and Acetic Acid in Near-Critical WateP reaction family H-abstraction by oxygen H-abstraction by radical isomerization p-scission nonterminating radical recombination/ disproportionation
Polanyi parameter, Eo* (kcal/mol) 1" alcohol 2" alcohol acetic acid
25 21 38 40 53
38 26 38 40.0 53
19 46 38 50 70
a a, Eo* = 0 for oxygen addition and termination, a = 0, E,* = 40 for dissociation, a = 0.5 for all other reaction families. 0.08
,
i
A A
Methanol ?-Propanol
0 1-Propanol
0
0.00
0.02
0.04
0.06
addition to oxygen, hydrogen abstraction by a radical, isomerization, B-scission, nonterminating radical recombinatioddisproportionatian, hydroperoxide decomposition, and termination. Structure-reactivity relationships were utilized to constrain the rate constants for each of the elementary steps in the oxidation mechanisms. In these relationships, the preexponential factor was set at a constant value for each of the reaction families and the activation energy was related to the heat of reaction by a Polanyi equation. This allowed a drastic reduction in the number of fitted parameters required, while imparting additional chemical significance to the value of the rate constants. Parameters optimized to the pure component data provided an excellent prediction of the mixture reaction kinetics when the H-abstraction A factor for secondary alcohols was used. The parity between experimental and model yields was excellent.
Literature Cited
1-Butanol
0.08
Experimental Concentratlon mol/l
Figure 8. Oxidation of a four-component mixture: parity plot for assessment of the reaction family. Structure/reactivity correlations lumping strategy.
reaction family. These parameters were estimated from the pure component oxidation experiments described earlier (Boock and Klein, 1993). These parameters, shown in Table 3, represent a priori input to the mixture model. The four component mixture model of Figure 7 contains 167 rate constants. These arose from the pure component elementary steps and the cross H-abstraction, nonterminating, and terminating radical recombination steps. Clearly the 15 Polanyi parameters of Table 3, obtained from pure component data only, represent a significant reduction in the number of model parameters. Quantitative optimization of the model to the experimental data in Figure 8 required adjustment of only a single A factor. Certainly the reduction from potentially 167 to a single adjustable parameter provides increased confidence in the predictions of the model. Figure 8 summarizes the quantitative match between model prediction and experimental data obtained by optimizing only the A factor for H-abstraction. The optimized value of log&VL mol-l s-l) = 8.3 provided the excellent correlation (r2= 0.987) shown. Note that loglo A = 8.3 is the value for the 2-propanol oxidation H-abstraction by a radical reaction family. This suggests that H-abstraction reactions involving secondary alcohols (including cross reactions) are some of the ratelimiting steps in the oxidation mechanism.
Boock, Lori Torry; Klein, Michael T. A Lumping Strategy for Modeling the Oxidation of C I - C ~Alcohols and Acetic Acid in High-Temperature Water. Ind. Eng. Chem. Res. 1993, 32, 2464-2473. Boudart, M. Kinetics of Chemical Processes; Prentice Hall: Englewood Cliffs, NJ, 1968. Connolly, J . Solubility of Hydrocarbons in Water Near the Critical Solution Temperature. J . Chem. Eng. Data 1966,11, 13-16. Dewar, M. J. S. The Molecular Orbital Theory of Organic Chemistry; Series in Advanced Chemistry; McGraw-Hill: New York, 1969. Dickenson, N. L. Pollutant-Free Low Temperature Combustion Processes Utilizing the Supercritical State. US Patent 4292953, 1981. Dickenson, N. L. Combination of Supercritical Wet Combustion and Compressed Air Energy Storage. US Patent 4593202,1986. Freeman, Harry. Innovative Thermal Hazardous Waste Treatment Processes; Pollution Technology Review 125; Noyes Publications: Park Ridge, NJ, 1985. Hammett, Louis P. Some Relations Between Reaction Rates and Equilibrium Constants. Chem. Rev. 1935,17, 125-136. Japas, M.L.; Franck, E. U. High Pressure Phase Equilibria and PVT Data of the Water-Oxygen System Including Water-Air to 673 K and 250 MPa. Ber. Bunsen-Ges. Phys. Chem. 1985,89, 1268-1275. Khaibullin, I. Kh.; Borisov, N. M. Phase Equilibria in the Sodium Chloride-Water System at High Temperatures. Russ. J . Phys. Chem. 1965,39,361-364. Marshall, William L.; Franck, E. U. Ion Product of Water Substance, 0-1000 "C, 1-10,000 Bars. New International Formulation and Its Background. J . Phys. Chem. Ref. Data 1981,10,295-304. Modell, Michael. Processing Methods for the Oxidation of Organics in Supercritical Water. Modar, Inc. U.S. Patent 4,338,199,1982. Modell, Michael; Gaudet, Gary G.; Simson, Morris; Hong, Glenn T.; Biemann, Klaus. Supercritical Water: Testing Reveals New Process Holds Promise. Solid Wastes Manage. 1982,Aug, 2630. Pitzer, Kenneth S.; Bishoff, James L.; Rosenbauer, Robert J . Critical Behavior of Dilute NaCl in HzO. Chem. Phys. Lett. 1987, 134, 60-63. Thomason, Terry B.; Modell, Michael. Supercritical Water Destruction of Aqueous Wastes. Hazard. Wastes 1984, l, 453467.
Summary and Conclusions Detailed mechanistic models were developed for the oxidation of mixtures of simple alcohols and acetic acid. These models organize all elementary steps into one of eight reaction families: hydrogen abstraction by oxygen,
Received for review March 17, 1994 Accepted July 25, 1994 @
Abstract published in Advance A C S Abstracts, October 1, 1994. @
