Environ. Sci. Techno/. 1995, 29, 2748-2753
Catatytic Supercritical Water Oxidation: Phenol Conversion and Product Selectivity ZHONG-YI DING, S U D H I R N. V. K. AKI, A N D M A R T I N A. A B R A H A M * Department of Chemical Engineering, The University of Tulsa, Tulsa, Oklahoma 74104
~~~
~~~~
Catalytic supercritical water oxidation (SCWO) has been demonstrated as an effective method of destroying organic compounds contained within an aqueous waste stream. Whereas SCWO is effective in the destruction of the original organic compound, incomplete conversion to low molecular weight partial oxidation products may be achieved. In all cases, enhanced phenol conversion and COn yield were obtained relative to the homogeneous case. Under selected operating conditions (temperature ~ 4 5 0"C, 500% excess oxygen), the addition of MnOdCeO or V205 catalysts can enhance the conversion to Con such that essentially quantitative conversion is obtained. The MnOdCeO catalyst is stable in the harsh reaction environment, at least to the limits of detection of the analytical instruments. A simple kinetic model, based on parallel reaction pathways, was used to evaluate the experimental data. Rate constants were obtained that adequately modeled the experimental results. The values of the rate constants were higher in the catalytic experiments compared with the homogeneous runs, further indicating that the addition of the heterogeneous catalyst promoted the oxidation pathways a t the expense of the oligimerization pathway.
Introduction and Background For many years, wet oxidation has been successfully used as a destructive treatment method for many organic species. Increased destruction efficiency has been obtained by increasing the reaction temperature to that above the critical temperature of water or through the addition of a catalyst. During supercritical water oxidation (SCWO), advantage is gained in that the organic components and the oxygen are all present in a single, dense, fluid phase, minimizing mass transfer resistance and providing relatively rapid reaction rates. The oxidation of phenol is of practical interest since it is a common pollutant in industrial waste streams and is also believed to be an intermediate product in the oxidation pathway of higher molecular weight aromatic hydrocarbons. Catalytic wet oxidation of phenol over a CuO catalyst has been previously investigated (1,2). The reaction kinetics Corresponding author: telephone: 918-631-2974: fax: 918-6313268: e-mail address:
[email protected].
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ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 29, NO. 11, 1995
were determined and were thought to proceed through a homogeneous-heterogeneous mechanism. The homogeneous reaction pathway caused an induction period during which free radicals were accumulated in the aqueous phase. In addition, numerous reaction intermediates, including benzoquinones, acetic acid, and other low molecular weight organic acids, have been identified (3). There have been many recent reports on supercritical water oxidation of specific compounds as models of species that might be present in a wastewater stream [seethe review article of Savage et al. ( 4 ) ] . Of particular relevance is the work that has been reported on supercritical water oxidation of phenol (5) between 300 and 500 "C. In addition to developing a rate expression describing the kinetics of phenol conversion, these authors have obtained a kinetic model that describes the formation of COS (6). They identified over 50 partial oxidation products that are formed during supercritical water oxidation of phenol and postulated two primary oxidation pathways, one leading to the formation of dimers, dibenzofurans, and dibenzodioxins and the other leading to substituted phenols and ring opening compounds (7). Some of the products identified from supercritical water oxidation of phenol represent compounds that are more hazardous than the starting organic component. In order to minimize the formation of these partial oxidation products and to promote complete conversion, it is feasible to either increase the reaction temperature or add a catalyst. As described by Li et al. (8),supercritical water oxidation involves parallel reaction paths, one leading to desired products and the other leading to undesired products. Thus, the use of a catalyst, which promotes the desired reaction pathway relative to the undesired pathway, represents a logical course of investigation. One such study has recently been reported (9) using a proprietary heterogeneous catalyst containing copper oxide and zinc oxide. The authors did not analyze for partial oxidation products, but concluded that high selectivity to COS was obtained based on the similar values for COS production and total organic carbon reduction. Frisch (10) catalyzed the oxidation of acetic acid in supercritical water with the addition of CeOzlMnOz. High oxygen concentrations were used to maintain the catalyst in the oxide form. An increase in the destruction of acetic acid at 400 "C and a residence time of 5 min from less than 40% to greater than 95%was observed through the addition of the catalyst. No information regarding the products of the reaction are reported. Within the current paper, a more detailed analysis of the use of a heterogeneous catalyst to promote the oxidation of phenol is provided. Several partial oxidation products have been identified, and a reaction model involving a sequence of parallel and series reaction pathways has been developed. The performance of several heterogeneous catalysts has been compared.
Experimental Section The packed bed reactor used for this research is described in Figure 1. The reactor consisted of separate inlet systems for gas and liquid feeds, the main reactor unit along with pressure and temperature control equipment, and product separation and analytical equipment.
0013-936W95/0929-2748%09.00/0
D 1995 American Chemical Society
FIGURE 1. Schematic diagram of experimental reaction system.
Feed gas was compressed from an air cylinder with a regulated pressure of 1000 psia to the desired operating pressure using a Haskel AG-62 air-driven gas booster pump, with a maximum outlet pressure of approximately 5000 psia. The gas feed was heated externally using an electric furnace and Thermolyne heating tapes, controlled with a temperature controller, and monitored at the inlet to the reactor. Air was fed to the reactor through a tube inserted into the reactor, the exit of which was placed immediately above the catalyst layer. This diminished the impact of homogeneous oxidation that could occur if the reactant stream was mixed with the oxygen prior to the catalyst. The flow rate of the gas stream was measured at the reactor exit. Solvent feed, containingdissolved phenol,was controlled with an LDC Analytical 2396-89 high-pressure metering pump, which had previously been calibrated by direct liquid flow measurement. The solvent feed was heated using an electric furnace and Thermolyneheat tapes. Temperature was monitored at both the inlet and the exit of the reactor. The reactor was an HiP MS-17microreactor, constructed of Hastelloy C-276, with an internal diameter of 1.25 cm and a length of 25 cm. The reactor was enclosed in a hightemperature furnace, which maintained the reaction temperature at the isothermal set point. Inert 20-60 mesh a-alumina particles, crushed to a diameter of 0.25-0.83 mm, were used as the packing material. The inert packing was present in all experiments; the homogeneous data reported below refer to experiments in which the inert a-A1203particles were used throughout the reactor without any catalyst material being present. The size of the particles was chosen to assure plug flow performance and good flow distribution, and to allow isothermal operation in a temperature-controlled environment (11). Based on the average particle size (dp= 0.05 cm), a Reynolds number of approximately 6.93 was estimated this is well within the range of particle Reynolds numbers that are normally encountered in packed beds. Further calculation based on available mass transfer correlations indicates that there were no mass transfer limitations within the reactor (12). Catalyst was placed in a small layer within the bed just below the exit of the oxygen feed line. The particles were supported using a sintered Hastelloy disk. The catalyst properties are summarized in Table 1. All of the catalysts were received as 118 x 3/16 in. pellets and were crushed to 20-60 mesh prior to use.
The effluent exited the reactor and passed through a tube-in-tube heat exchanger and a high-pressure gas-liquid separator. The gas then flowed through a back pressure regulator (GO Inc. BP-66) and a high-precision flow rotameter. The rotameterwas calibrated periodically using a soap film bubble flowmeter. The liquid drained through the high-pressure separator and flowed out continuously via a high-pressure needle valve. The flow rate of liquid product was confirmedby collecting liquid in the separator for a given time and then draining all of the liquid from the separator. Liquid samples were collected for analysis in 20-mL sample cylinders, operated in parallel with the main separator. Flow was diverted into the samplevessel through means of a valving system. After collection, the sample vessel was isolated and the pressure was released slowly. Analysis of the gas effluent was accomplished using a Hewlett Packard 5890A gas chromatograph. The sample was analyzed on-line using a six-way gas sampling valve and an Alltech Heliflex AT-Q 30 m x 0.53 mm i.d. capillary column. A temperature ramp was used; holding at 50 "C for 2 min, then ramping at the rate of 30 "Clmin to 150 "C, and holding at that temperature until all products had eluted. Athermal conductivity detectorwas used. Air, CO, C02,and other light hydrocarbonscould be detected during this analysis. Quantification was accomplished using area fraction, which had been previously calibrated with standards containing known mixtures. The recovered liquid was also analyzed using gas chromatography. The HP 5890A GC with an HP-17 10 m x 0.53 mm i.d. capillary column was used in temperatureprogrammed mode. An initial temperature of 50 "C was held for 4 min, followed by a temperature ramp of 16 "C/ min to 220 "C and extended operation at 220 "C. A flame ionization detector was used. Quantification was obtained by comparingpeak areas with those from samples of known composition. The reactant and partial oxidation products could be detected during this analysis. Identification of unknown products was accomplished in a two-step process. First, qualitative identification of products was made using an Hewlett Packard 5890GC coupled with a HP 5970B mass selective detector. Based on the product description, pure component samples of the suspected products were injected into the GC, and the retention times were compared with the retention time of the product peaks. Then an increase in the peak area at the suspected retention time brought about about through the addition of a small amount of the suspected product, provided compelling evidence that the suspected product was indeed a reaction product. These pure component standardswere tested at several times throughout the course of the experiments to correct for shifts in retention time caused by changes in flow rates and/or temperature programming. In all cases, a minimum of three independent measurements of phenol conversionand CO2 were recorded. Good reproducibility between the various samples was obtained. For phenol conversion, the average error was estimated as +lo%, whereas the error on Conyield was less than &5%. The quality of the experimental measurement is indicated in the figures presented within the followingsection. Where practical, all data points have been included on the graph. Where greater deviation between the points was observed, an average value is indicated along with an error bar, the range of which was set at one standard deviation. In some cases, only a single data point and no error bar is shown; VOL. 29, NO. 11, 1995 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
2749
TABLE 1
Summary of Catalyst Properties vzo5
MnO2
Cr203
manufacturer concn of components
Strem Chemical Co.
Strem Chemical Co.
surface area (m2/g) pore vol (cm3/g) bulk density (g/cm3) catalyst loading (g)
Carus Chemical Co. 60-75 wt % M n 0 2 5-10 wt % Ce02 balance: A1203
'150 0.38 0.99 9.18
62 0.30 0.99 8.76
150 0.38 0.99 17.8
10 wt % v205
balance:
AI203
Results
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ENVIRONMENTAL SCIENCE &TECHNOLOGY / VOL. 29, NO. 11,1995
balance:
A1203
1
these represent points in which the error associated with the measurement was less than the size of the marker used in the figure.
Reaction experiments were completed over a range of temperature (390-410 "C), oxygen concentration (100500% excess oxygen), and phenol concentration (0.0020.007 mol/L). Under all reaction conditions, phenol conversion was greater than 0.85. The phenol conversion was increased by the addition of a heterogeneous catalyst. For example, at 200% excess oxygen,the phenol conversion from homogeneous SCWO at 390 "C and 14.3 s residence time was 0.92 compared with a conversion of 0.9985 for SCWO with MnOdCeO catalyst and a conversion of 0.9997 for SCWO with Vz05 catalyst, both at a residence time of 5.4 s. Many products were identified for all experiments. These products can be classified as low molecular organic acids (formic acid, acetic acid), partially oxygenated aromatic species (benzyl alcohol, benzoic acid), condensation products (dibenzofuran,xanthone), and dimerization products (bibenzyl,dibenzyl ether, biphenyl). The large number of products detected is consistent with previous reports in the literature (3,which revealed over 50 specific partial oxidation products for the homogeneous supercriticalwater oxidation of phenol. A similar product spectrum was observed in the case where a catalyst was present, although the peak area for all products was generally smaller in the catalyzed case. No difficultywasencountered in obtaining quantitative data for the organic acids by direct injection of these species into the GC. One measure for the effectiveness of the catalyst is the ability to close a carbon balance, defined as the fraction of carbon collected in the products relative to the amount of carbon injected in the feed. In the homogeneous case, the carbon balance ranged from 0.3to 0.5,whereas the addition of a catalyst increased the carbon balance to between 0.5 and 0.98. The low carbon balance is attributed primarily to the large number of specific components within the complex product spectrum that were not identified as well as solid condensation products that were not quantified in any way. The higher carbon balance in the catalytic case indicates that more of the products were identified as either COz or low molecular weight organic compounds. Because of the high phenol conversions that were obtained in all cases, it is not especially useful to compare the performance of the catalysts against homogeneous oxidation usingphenol conversion. The use of two alternate measures [namely, (I) the conversion to COz, indicating oxidation to fully mineralized species, and (2) the production of partial oxidation products, indicating incomplete
12 wt % Cr203
Homogeneous
m
i
0.8 9 0.6
f
N
00
0.4
0.2
0
100
300 400 ExcessOxygen (%)
200
500
FIGURE 2. Comparison of the effect of oxygen concentration on the yield of COZ during homogeneous (&,OH = 6.0 mM, t = 13.6 s)and catalytic (&OH = 4.8 mM, t = 5.4 s)supercriticel water oxidation at 390 "C.
conversion of the phenol] provides more information regarding the role of the catalyst in the oxidation reaction. The comparisons are reported below as a function of oxygen and phenol concentrations. Complete Conversion to C02. Because a waste treatment process aims at convertingthe organic carbon species to C02, the formation of this product is a key variable in determiningthe effectiveness of a proposed waste treatment technology. The effect of oxygen concentration (measured as amount of excess oxygen relative to stoichiometry) on the yield of COZ is indicated in Figure 2 at a reaction temperature of 390 "C. Here, yield is defined on a carbon basis; that is
where Fi refers to the molar flow rate of component i. For homogeneous SCWO of phenol at a residence time of 13.6 s, C02yield never exceeded 0.3, even at 500%excess oxygen. This result indicates that although high phenol conversion could be obtained through this process, low destruction of the organic carbon in the aqueous stream was obtained. The addition of a heterogeneous catalyst, either VzO5/A.l2O3 or Mn021Ce0, more than doubled the yield of COz despite a shorter residence time of 5.4 s. In the case of the V205 catalyst, the COZyield increased from approximately 0.4 at 100% excess oxygen to approximately 0.7 at 500% excess oxygen. The MnOdCeO had a similar COZyield at the low oxygen concentration but increased to a COz yield greater than 0.9 at 500% excess oxygen. This latter condition indicates that nearly 100% selectivity (COz yield/phenol conversion) could be obtained by carrying out SCWO of phenol in the presence of the MnOdCeO catalyst.
1
I
v)
. I -
o
3
U
E
m" 3 0.15
Homogeneous
8 0.4 O I
I
V205 II I
MnOZlCeO I
X
4,000
6,000
I I
Y
-
I 1' ,
8,000
Phenol Concentration (pprnw) FIGURE 3. Comparison of the effect of phenol concentration on the = 0.090 M, t = 13.6 s) and yield of COz during homogeneous (6, catalytic (C,= 0.193 Wfor V ~ 0 5and 0.256 M for MnOb t = 5.4 s) supercritical water oxidation at 390 "C.
Figure 3 compares the performance of the catalysts against homogeneous SCWO of phenol as a function of the phenol concentration for a fixed oxygen concentration and residence time. In the homogeneous case, the COZyield decreases from a high of approximately 0.2 to approximately 0.1 as the phenol concentration increases from 1000 to 3000 ppmw. Results are consistent with those reported in Figure 2 for the high excess oxygen condition; roughly 0.8 yield of C02 using VZO~ as catalyst and greater than 0.9 yield of C 0 2 for the MnOZ/CeOcatalyst. For Mn02/Ce0, there was no effect of phenol concentration on the yield of C02. However, for the VZO, catalyst, a slight decrease in C 0 2 yield was observed as the phenol concentration increased. This is the expected result based on Figure 2, since the amount of excess oxygen decreases as the phenol concentration increases. Formation of Partial Oxidation Products. Consistent with previous reports, SCWO of phenol leads to the formation of numerous partial oxidation, condensation, and dimerization products which for the purposes of further discussion will be described simply as byproducts. Analysis of the liquid phase products from the homogeneous oxidation revealed low molecular weight oxygenated products (methanol, ethanol, formic acid, and acetic acid), oxygenated aromatic compounds (benzyl alcohol, benzoic acid, benzaldehyde, and hydroquinone), and condensation and dimerization products (bibenzyl, dibenzyl ether, biphenyl, dibenzofuran, and xanthone). Acetic acid, benzoic acid, and xanthone were produced in the greatest yield, although no single product accounted for more than 2%of the total yield. The yield of byproducts was suppressed through the use of a catalyst, but not entirely eliminated. No additional byproducts were detected with the use of the catalyst. Figure 4 describes the effect of oxygen concentration on the formation of total byproducts. Homogeneous SCWO at 200% excess oxygen gave a yield of byproducts of 0.1; increasing to 400% excess oxygen led to a decrease in the byproduct yield to 0.05. Using the MnOz/CeO catalyst enhanced the formation of byproducts at low oxygen concentration, but led to a decrease in the amount of byproduct formation compared to homogeneous SCWO when the oxygen was greater than 400% excess. However, the V2O5 catalyst was always very selective in not producing byproducts. The maximum yield of byproducts withV205was approximately 0.03 at 100%excess oxygen; increasing the excess oxygen decreased the yield
(e,
r. -
MnOSlCeO
~
I
2,000
0.25
V205
-
g 0.2 1
-I
0.21
Homogeneous
0.3 -
0.8
-
L
'
100
'
$0 300 400 Excess Oxygen (%) '
'
590
6
'
FIGURE 4. Comparison of the effect of oxygen concentration on the yield of partial oxidation products during homogeneous (&,OH = 6.0 mM, t = 13.6 s)and catalytic (&OH = 4.8 mM, t = 5.4 s)supercritical water oxidation at 390 "C.
fn
ri I
c
0
0.1
- --
[
I
T'
,
J
7
Homogeneous
1
I
V205
I
II
9 0.04 a,
F
0.02
OO
,
I
4,000 6,000 Phenol Concentration (pprnw)
8,000
-
,
?
-
2,000
FIGURE 5. Comparison of the effect of phenol concentration on the yield of partial oxidation products during homogeneous (C,= 0.090 M, t = 13.6 s)and catalytic (C,= 0.193 M for Vz05 and 0.256 M for MnOz, t = 5.4 s) supercritical water oxidation at 390 "C.
of byproducts to negligible amounts. The influence of phenol concentration on the yield of byproducts, at fixed oxygen concentration, is indicated in Figure 5 . Under these conditions, homogeneous SCWO always gave the highest yield of byproducts, with a yield near0.1. For theV205catalyst, the yield ofbyproducts was always very low, with a maximum value of approximately 0.01 at the highest phenol concentration of 7000 ppmw. The MnOz/CeO gave the unexpected result of decreasing byproduct yield with increasing phenol concentration (and thus decreasing excess oxygen). With this catalyst,the yield of byproducts was similar to the yield of byproducts from homogeneous SCWO at low phenol concentration and similar to the yield of byproducts from SCWO over VzO, at high phenol concentration. Note that Mn02/Ce0also gave very high yield of COZ at all phenol concentrations (see Figure 3). Because of the high yield of C02, the carbon balance (proportion of carbon atoms in the product to that in the reactant) was nearly 100% for many of the experiments with the MnOz/CeO catalyst.
Discussion Within this section, the effectiveness of the heterogeneous catalysts in promoting the conversion of phenol and the formation of complete oxidation products is described by direct comparison of the results. Also, a kinetic model is developed that further emphasizes the effectiveness of the catalyst. VOL. 29, NO. 11, 1995 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
2751
TABLE 2
TABLE 3
Comparison of Catalyst Performance at 380 "C homogeneous VZOS MnOdCeO CrzOdAIzOa
Metal Ion Effluent Concentration during Reaction of Phenol
300% Excess Oxygen phenol concn (mol/L) residence time (SI conversion CO2 yield yield of partial oxidation products
0.0061 13.7 0.94 0.276 0.090
0.0066 5.4 0.995 0.631 0.006
4Mw0 Excess Oxygen phenol concn (mol/L) 0.0023 0.0066 residence time (s) 15.3 5.3 conversion 0.98 0.995 COz yield 0.17 0.677 yield of partial 0.090 0.004
0.0048 5.4 0.995 0.640 0.013
0.082 11.1 0.995 0.66 0.064
vzo5 MnO$CeO Cr~03/Al203 a
0.0066 5.3 0.995 0.786 0.051
0.0024 15.9 0.986 0.366 0.352
oxidation products
Table 2 summarizes the results from several individual runs that serve as a basis for comparison across the several catalysts investigated in the present study. Included in this comparison are a set of runs that were completed with Cr203/Al~03as the catalyst. The results indicated in Table 2 again point out the effectiveness of the catalyst in promoting the conversion of phenol. In all cases, a higher conversionwas obtained during the catalyzed reaction than in the homogeneous SCWO, usuallywith a shorter residence time. Comparison of the COZyield indicates that all of the catalysts were effective in enhancing the conversion to the complete oxidation product, although with varying degrees of success. At 400% excess oxygen, it is clear that Cr2O3/ A 1 2 0 3 was less efficient for the formation of COZthan V2O5, which was less effective than MnOZ/CeO. At the same time, Cr203/Al203 gave the highest yield of partial oxidation products of all of the catalysts and at 400% excess oxygen enhanced the yield of byproducts to levels in excess of that obtained from homogeneous SCWO under essentially identical conditions. It is necessary to note that for none of the experiments indicated in Table 2 was a carbon balance closed. This result is in agreement with the observation of a small amount of insoluble organic product, which was extracted from the sample using an organic solvent. It is reasonable to assume, based on our identification of dimerization products and previous reports of the formation of high molecular weight products (3, that these insoluble organic products were high molecular weight oligomers that were produced through a condensation reaction pathway. The amount of material recovered in this form was not quantified, but it is presumed that this accounts for a large percentage of the unrecovered carbon product. One additional point worth noting in the evaluation of the catalysts tested in this study is the stability of the catalyst under the severe reaction conditions. The stability was tested by measuring the concentration of the metal ion in the effluent using flame atomic absorption spectrophotometry [see Aki and Abraham (131, for more information on the analytical procedures]. The results from these tests are reported in Table 3. Whereas Cr203/Al203 yielded substantial amount of chromate ion in the effluent, MnOz/ CeO produced no detectable manganese ion. Since this process is proposed as a wastewater treatment process, any amount of metal ion in the effluent presents a significant potential drawback to the commercialization of this technology. Fortunately,the most effective catalyst (MnOz/ 2752
catalyst
ENVIRONMENTAL SCIENCE &TECHNOLOGY I VOL. 29. NO. 11, 1995
phenol
metal ion concn
temp ("C)
conversion
in effluent (ppm)
400 410
0.9998 0.9991 0.995
10.71 NDa 5.3
390
ND, detected.
CeO) was also the most stable catalyst, thus indicating the potential of this method. Additional calculations have been completed based on the loss of metal ion reported in Table 3. These calculations reveal that greater than 50%of the original catalyst material will remain within the reactor even under the worst conditions. This information is reported elsewhere in a companion paper (13). Without proof, but based on the current results and previous reports in the literature (81, it is possible to speculate that the oxidation of phenol occurs through a sequence of parallel and series reactions, describedin Figure 6. Pathway 2 leads to the formation of partial oxidation products, which can then react further to give the desired product, COZ. The goal of the addition of the catalyst is to promote pathway 1relative to pathway 3 or to enhance the formation of COZ through the enhancement of pathways 2 and 4. V2O5 gave almost exclusively COZ,suggestingthat this catalyst was effective in promoting the direct oxidation pathway. On the other hand, C T Z O ~ / Agave ~~O a high ~ yield of partial oxidation products with a small enhancement in COZformation, suggesting that this catalyst promoted the indirect oxidation pathway (2 and 41, with the partial oxidation products being relatively stable under these reaction conditions. Mn02/Ce0produced large yields of byproducts at low oxygen concentration but mostly COZat higher oxygen loading, with almost no production of oligimerizationproduct. Apparently,this catalystpromoted pathway 1 or 2, depending upon the oxygen loading, but these pathways were always enhanced relative to the undesirable oligimerization pathway. The available data are amenable to reaction pathway analysis using the pathways described in Figure 6. In order to carry out this task, it is necessary to assume appropriate reaction kinetics. For lack of information, it is reasonable to assume that each of the reactions proceeds through firstorder kinetics, providing a set of rate equations as
(4)
In developing these rate expressions, it was assumed that the oxidation reactions require oxygen, whereas the dimerization pathway may proceed independent of oxygen, and for the purposes of stoichiometry, the principal partial oxidation product is acetic acid. The constants within the rate expressions may be evaluated by comparing the model with the experimental
High molecular wieght oligimerization products
1
Phenol
c02
~~~~~~~~
2
'\oxygenates Small organic (e.g. acetic
/6
acid)
FIGURE 6. Postulated reaction pathways for the oxidation of phenol in supercritical water. TABLE 4
Best-Fit Estimates of Kinetic Rate Constants with Model of Figure 8 ki
(Umol-l
catalyst none
s-1)
0.39 5.52 7.16
MnOJCeO v205
kz k3 ( U m o l - ' ~ - ~ ) (Umol-'s-O 0.067 0.27 0.23
0.38 4.00 2.33
ks
(Us) 0.19 0.54 0.76
1
I 1
to the experimental data. However, the addition of a fifth adjustable parameter could not be justified for the number of experimental measurements. Evidence of the effectiveness of adding a heterogeneous catalyst can be found in the values of the rate constants obtainedusing this simple kinetic model. The rate constant kl describes the direct oxidation pathway and is enhanced by an order of magnitude through the use of either catalyst. The constants kz and k4, which describe the pathway to COzthrough partial oxidation products, were also enhanced by more than an order of magnitude through the addition of the catalyst. It should be noted that the rate constant k3, which represents the pathway leading to oligomeric products, was also increased through the addition of the catalyst,although the increase was of the order of 2-4 times rather than the order of magnitude increases observed for the rate constants from the oxidation pathways.
Homogeneous 1
Acknowledgments This research was supported by Grant CTS-8909940 from the National Science Foundation. Support for Z.-Y.D. was provided by Grant NAG8-1005 from NASA under the JOVE program. Additional support was received from The University of Tulsa Office of Research and the Department of Chemical Engineering.
Literature Cited
"0
0.2
0.4
0.6
0.8
1
Experimental C02 Yield FIGURE 7. Comparison of experimental COZ yield with model predictions of pathways of Figure 6 and rate constants from Table 4 (all experimental conditions).
data at 390 "C, the only temperature for which substantial kinetic data was obtained. A sequential simplex minimization routine was completed, which minimized the sum of the squares error between the predicted and experimental values of COzyield,yield of partial oxidation products, and phenol conversion. The best fit values are summarized for the two catalysts and the homogeneous case in Table 4. In order to maintain the unit the same for the two catalytic cases and the homogeneous test, it was necessary to define the reaction rate on the basis of the reactor volume. It is possible to convert the rate constants for the catalytic experiments by dividing by the bulk density of the catalyst; which is 0.59 g/cm3 for V Z O and ~ 0.56 g/cm3 for MnOz. The experimentally measured COz yield is compared with the model predictions in Figure 7 for both catalysts and the homogeneous case. In the homogeneous case, the model predictions match exceedingly well with the experimental measurements. Some deviation between the model and the experimental values is observed for the MnOz/CeO and the VzO5 catalyst, particularly as the COz yield approached 1. This under prediction of the model may be explained by notingthat the model does not provide a pathway for the oxidation of the oligomeric products, which would be expected to occur at high conversion. Indeed, calculations that included an oxidation pathway for the oligomeric products revealed a much improved fit
(1) Sadana, A.; Katzer, J. R. Catalytic Oxidation of Phenol in Aqueous Solution over Copper Oxide. Ind. Eng. Chem. Fundam. 1974,13, 127-134. (2) Pintar, A,; Levec, J. Catalytic Oxidation of Organics in Aqueous Solutions. J. Catal. 1992, 135, 345-347. (3) Devlin, H. R.; Harris, I. J. Mechanism of the Oxidation ofAqueous Phenol with Dissolved Oxygen. Ind. Eng. Chem. Fundam. 1984, 23, 387-392. (4) Savage, P. E.; Gopalan, S.; Mizan, T. I.; Martino, C. J.; Brock, E. E. Reactions at Supercritical Conditions: Applications and Fundamentals. AIChE J. 1995, 41, 1723-1778. (5) Thornton, T. D.; Savage, P. E. Phenol Oxidation in Supercritical Water. J. Supercritical Fluids 1990, 3, 240-248. (6) Thornton, T. D.; Savage, P. E. Kinetics of Phenol Oxidation in Supercritical Water. AIChE J. 1992, 38, 321-327. (7) Li, R.; Thornton, T. D.; Savage, P. E. 2-Chlorophenol Oxidation in Supercritical Water: Global Kinetics and Reaction Products. AIChE J. 1993, 39, 178-187. (8) Li, L.; Chen, P.; Gloyna, E. F. Kinetic Model for Wet Oxidation of Organic Compounds in Subcritical and Supercritical Water. In Supercritical Fluid Engineering Science: Fundamentals and Applications;Kiran, E., Brennecke, J., Eds.; ACS Symposium Series 514; American Chemical Society: Washington, DC, 1993;p 305. (9) Krajnc, M.; Levec, J. Catalytic oxidation of toxic organics in supercritica1water.Appl. Catal. B: Enuiron. 1994,3, L101-L107. (10) Frisch, M. A. Supercritical Water Oxidation of Acetic Acid Catalyzed by Ce02/Mn02. M.S. Thesis, University of TexasAustin, 1992. (11) Race, H. F. Fixed Bed Reactor Design and Diagnostics; Butterworths: Boston, MA, 1990; p 149. (12) Ding, Z. Y. Catalytic Supercritical Water Oxidation o f k o m a t i c Compounds on Transition Metal Oxides. Ph.D. Dissertation, The University of Tulsa, Tulsa, OK, 1995. (13) Aki, S. N. V. K.; Abraham, M. A. Catalytic Supercritical Water Oxidation: Catalyst Dissolution and Stability. AIChe 1. Submitted for publication.
Received f o r review December 13, 1994. Revised manuscript received June 12, 1995. Accepted July 14, 1995.@
ES940752R Abstract published in AdvanceACSAbstracts,September 1, 1995.
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