Ind. Eng. Chem. Res. 1995,34, 1588-1595
1588
Catalytic Oxidation of Ethanol and Acetaldehyde in Supercritical Carbon Dioxide Lubo Zhou and Aydin &german* Department of Chemical Engineering, Texas A&M University, College Station, Texas 77843
Catalytic oxidation of ethanol and acetaldehyde over a 4.45% Pt/Ti02 catalyst in supercritical carbon dioxide was studied in a '/2 in. fixed bed reactor. Experiments for ethanol oxidation were performed at temperatures from 423 to 573 K and at a pressure of 8.96 MPa with a 5:l molar ratio of oxygen to ethanol in the feed. Acetaldehyde oxidation was performed at temperatures from 423 to 548 K and at 8.96 MPa with a n approximate 4.7:l molar ratio of oxygen to acetaldehyde in the feed. In addition to CO2, the complete oxidation product, acetaldehyde and trace amounts of CO were generated during ethanol oxidation, while a trace amount of CO was the only partial oxidation product during acetaldehyde oxidation. A parallel and consecutive reaction mechanism was postulated for ethanol oxidation, whereas dissociative adsorption of acetaldehyde on the catalyst surface and surface reaction rate control were postulated for acetaldehyde oxidation. The kinetic parameters in the rate expressions based on the mechanisms were obtained by fitting the experimental data with the results of the model calculation. The models were used to predict the conversion and yield for ethanol oxidation and acetaldehyde oxidation.
Introduction Supercritical fluid (SCF)extraction has been receiving increasing attention for the remediation of environmental matrices contaminated with organic compounds. Among the SCFs, supercritical carbon dioxide (SCCO2) is particularly attractive for site remediation since it is nontoxic, nonflammable, and cheap and has a reasonably high solvent power for most organic components. As a result of those properties, extensive research and development work has been conducted on the removal of organic pollutants from environmental media, such as soils, using SCCO2 (Erkey et al., 1993;Akgerman et al., 1992;Dooley et al., 1987;Brady et al., 1987;Capriel et al., 1986). Supercritical extraction has an advantage over extraction by liquid solvents since the extracted species can be separated from the supercritical fluid by simple pressure reduction. On the other hand, recompression of the fluid back to the supercritical conditions is a major cost item associated with supercritical extraction. In addition, during the expansion, nozzle clogging is a frequent problem due t o the salting out of the extracted species. Organic pollutants which have been extracted from the contaminated environmental media are not desired and are ultimately converted into environmentally acceptable compounds, such as C 0 2 and water. Currently, this is accomplished by incineration after separation from the solvent SCCO2, which is costly. In order to solve the problems associated with pressure reduction and recompression in supercritical extraction and to achieve destruction of the extracted organic compounds, we proposed a process which combines SCC02 extraction with on-line catalytic destruction (Figure 1). In this process, SCCO2 is continuously circulated over the soil bed to remove pollutants. At the same time, the circulated fluid, after a small amount of oxygen addition, passes through a fixed bed reactor where the extracted organic compounds are oxidized and are converted into GO2 and water at the same pressure as the extraction unit. The circulation continues until the organic pol-
* Author t o whom correspondence
should be addressed.
0888-588519512634-1588$09.00/0
lutants are completely removed from the soil. The feasibility of such a process is governed by the extent of the oxidation and the reaction kinetics in SCCO2. We recently reported on the catalytic oxidation of aromatic hydrocarbons in SCCO2 (Zhou et al., 1995). This paper reports on the catalytic oxidation of oxygenated species, specifically ethanol and acetaldehyde. SCCO2 has been proven t o be a feasible reaction medium under some reaction conditions. Hammond et al. (1985)investigated the enzymatic oxidation of pcresol and p-chlorophenol to their corresponding obenzoquinones in SCCO2. Randolph et al. (1988)investigated the reaction of cholesterol to form cholest-4enone in SCCO2 with cholesterol oxidase. Dooley and Knopf (1987)studied the partial oxidation of toluene in SCCO2 over a COOcatalyst. Complete oxidation of an aromatic-aliphatic hydrocarbon mixture in SCCO;! over a PliAl203 catalyst was reported by Pang et al. (19911, and we reported on the oxidation of toluene and tetralin over a PliAl2O3 catalyst (Zhou et al., 1995). Complete oxidation of ethanol to C02 in air has been widely studied because of its importance in use as a blend in gasoline (Ismagilov et al., 1979,1983;McCabe and Michell, 1983,1984;Yao, 1984;Goto and Mabuchi, 1984;Gonzalez and Nagai, 1985;Nagai and Gonzalez, 1985;Oyama and Somorjai, 1990;Rajesh and Ozkan, 1993). The catalysts used were precious metals (Pt,Pd, Ag, and Rh) and base metal oxides. Among these catalysts, Pt and Pd had higher activities and higher selectivities for complete oxidation to C02. Acetaldehyde had been the common and most important partial oxidation product during the oxidation of ethanol. Therefore, studies on the oxidation of acetaldehyde have also been reported (McCabe and Michell, 1983, 1984; Rajesh and Ozkan, 1993). The mechanism for the oxidation of ethanol and acetaldehyde was not widely reported because of the complexity of the reaction system. Nagai and Gonzalez (1985)gave the results of in situ infrared studies and two schemes of ethanol oxidation, but they did not propose a kinetic model for the reactions. Ismagilov et al. (1983)proposed several models for ethanol oxidation on the supported CuO catalyst; however, they did not consider the deep oxida-
0 1995 American Chemical Society
Ind. Eng. Chem. Res., Vol. 34, No. 5, 1995 1589
illi
0 2
supplier
co,
supplier
U
Figure 1. Schematic representation of a process for supercritical extraction combined with catalytic oxidation of the extracted species. 1. oaygau yllnder
L COSylluder 3. Uquld pump 4. ayrhge pump 5. m u r flow fontroller 6. tube reactor 7. hrnlce 8. temperatun contmlkr
9. temperature monltor 10. m l u valve 11. pronure transducer 12. computer monltor 13. bacbpnuure regulator 14. on-line aampllag loop 15. gu cbromatopnpher 16. bulblr mater
Figure 2. Schematic representation of the experimental assembly
tion of acetaldehyde during the ethanol oxidation. To the best of our knowledge, studies on the oxidation of ethanol and acetaldehyde in SCCO2 have not been reported. In this paper, we report on the oxidation of ethanol and acetaldehyde over a 4.45% PfliO2 catalyst in SCCO2. The experimental results showed that acetaldehyde had been the only significant partial oxidation product during the oxidation of ethanol. Complete oxidation of ethanol and acetaldehyde to COZ can be realized by either increasing the reaction temperature or increasing the amount of catalyst with excess oxygen. A mechanism which includes parallel and consecutive reactions is proposed. Rate expressions are developed on the basis of the mechanism, and the parameters (rate constants and activation energies) are obtained from the experimental data.
Experimental Section The catalyst, obtained from Engelhard Co., was 4.45% with a nominal BET surface area of 35.74 m2/ g. The catalyst was first crushed and sieved to 100pt/Ti02
200 pm size and pretreated by being calcined in air a t 723 K for 16 h. Ethanol and acetaldehyde (HF'LC grade)
were obtained from Sigma-Aldrich. CO2 and oxygen were bought from Bob Smith Corp. in Bryan, TX, with purity of 99.5%. Experiments were carried out in the experimental setup shown in Figure 2. Ethanol or acetaldehyde was pumped by a syringe pump (ISCO,LC-2600) and was mixed with COZ,which was pumped through an ice bath by the liquid pump (LDCMilton Roy, minipump vs) at 273 K and 5.52 MPa. Oxygen was delivered by a highpressure oxygen cylinder, and its flow rate was controlled by a mass flow controller (Brooks Instrument Division, 58503). The mixture of organic compounds and C 0 2 was mixed with oxygen at a three-way valve. In order for the organic compound to be thoroughly mixed with oxygen and C02, the stream was passed over a 5 in. long column filled with 25/40 mesh glass beads. The system pressure was controlled by a back-pressure regulator (Grove Valve and Regulator Corp., S-Slxw). The reactor was operated isothermally, and the inlet fluid mixture was preheated up to the reaction temperatures. It then was passed through a l/2 in. tubular
1690 Ind. Eng. Chem. Res., Vol. 34,No. 5, 1995
fxed bed reactor containing the 4.45% Pt/TiOz catalyst which was diluted by glass beads of the same size. The preheater and the fixed bed reactor were placed in the same furnace whose temperature was controlled with fl K accuracy. The temperature of the reactor was monitored by a thermocouple which was placed in a thermowell inside the reactor. During the experiment, the temperature difference between the inlet and the outlet was less than 2 K. The concentrations of ethanol or acetaldehyde and possible partial oxidation products were analyzed by on-line sample injection to an HP-5890 gas chromatograph (GC) equipped with a FID, with a detection limitation of 5 ppm. The column material was Prorapak Type P, from Alltech Associates, Inc., and the length of the column was 1.5 m. The column was operated at 323 K constant temperature and at a carrier gas flow rate of 30 mIJmin. The feed concentrations of ethanol and acetaldehyde were measured by on-line sample injection to the HP-5890 gas chromatograph through the reactor bypass line. The fixed bed reactor was operated in an integral mode. The reactor was filled with 0.0015-0.0091 g of 4.45% Pt/TiOz catalyst which was diluted to 4 cm3 by 60/80 mesh glass beads. A total flow rate ranging from 3.51 to 6.31m o m was used with an approximate 99:l molar ratio of CO2 to 0 2 and a 5:l molar ratio of 0 2 to ethanol in the ethanol oxidation experiments and an approximate 4.7:lmolar ratio of 0 2 to acetaldehyde in the acetaldehyde oxidation experiments. The inlet concentration of ethanol was about 1500 ppm (mole fraction), and the inlet concentration of acetaldehyde was 1350 ppm. These inlet concentrations are in the typical concentration range of organic compounds in SCCO2 after extraction. The system was operated at temperatures from 423 t o 573 K and at the pressure of 8.96 MPa. The system pressures used in the experiments were in the typical pressure range of supercritical extraction by SCCO2. Only one operating pressure was used in the experiments because the results from the oxidation of hydrocarbons showed that the operating pressure has no effect on the kinetic rate constant (Zhou et al., 1995).
Results and Discussions a. Conversion, Yield, and Partial Oxidation Products. The concentrations of the feed and the reactor effluent were measured, and the conversion and yield were calculated by the following definitions:
concentrations as XA
yFAO
where XA and xp are conversion and yield, respectively, Y A I V ~is the ratio of stoichiometric coefficients of the reactant A and product p, FAand Fpare the molar flow is the inlet molar rates of A and p, respectively, and FAO flow rate of A. Because the system is about 99% C02, it is reasonable t o assume a constant overall density of the system a t constant temperature and pressure. Therefore, eqs 1 and 2 can be written in terms of
cAO
- cA
x
100%
(3)
'A0
x =-YACp
x 100%
(4)
"pcAO
Possible partial oxidation products were detected by the GC-MS method using off-line samples. The results showed that acetaldehyde and a trace amount of CO were the only partial oxidation products during the oxidation of ethanol. Other byproducts, such as acetic acid, ethyl acetate, and diethyl ether, which were detected by other researchers (McCabe and Mitchell, 1984;Nagai and Gonzalez, 1985;Rajesh and Ozkan, 1993),were not found during ethanol oxidation over the 4.45% p t p I ' i 0 2 catalyst in SCCO2. In addition, the yield of CO was very small (the maximum yield for every experimental run was less than 2%); hence, it was neglected. The detection limitation of the GC was 5 ppm; therefore, if any other partial oxidation product which was not detected by GC was present, the yield of this byproduct should be less than 0.4%. The yield of acetaldehyde passed over a maximum at constant temperature with increasing space time, indicating that it is a primary product which is further oxidized to COz. In order to determine ethanol oxidation kinetics, acetaldehyde oxidation over the same 4.45% Pt/TiO, catalyst is also investigated at the same temperature range as ethanol oxidation but at higher space times. In acetaldehyde oxidation, the only partial oxidation compound detected was a trace amount of carbon monoxide. b. Mechanism for the Oxidation of Ethanol and Acetaldehyde in SCCO2. (i) Oxidation of Ethanol. Based on the products identified, the following reactions take place during the oxidation of ethanol on 4.45% Pt/ Ti02 in SCCO2. C,H,OH
+ 30,
-
2C0,
+ 3H@
+ '/,O, - CH,CHO + H,O C,H,OH + 20, - 2CO + 3H,O CH,CHO + ,/,O, - 2CO + 2H,O CH3CH0 + ,/,O, - 2C0, + 2H,O 2 c o + 0, - 2 c 0 ,
C,H,OH
(5)
(6) (7) (8)
(9) (10)
These reactions can be combined in terms of a simple parallellconsecutive mechanism. C,H,OH
xp = yAFp x 100%
=
-
CH,CHO
-
- CO,
CO,
(11)
C,H,OH CO, CO, (12) Nagai and Gonzalez (1985)showed that an adsorbed ethoxy intermediate existed on the catalyst surface and that the dehydrogenation of ethoxy is the rate-determining step in ethanol oxidation to acetaldehyde. They assumed that the dehydrogenation step is irreversible. We used Nagai and Gonzalez's model and further assumed that the rate of oxidation of ethanol to acetaldehyde is independent of oxygen concentration. This oxygen independence was reported by Ismagilov et al. (1979)before. Our experimental results indirectly verify
Ind. Eng. Chem. Res., Vol. 34, No. 5, 1995 1591 ethanol is Ethanol conversion 80.00
rr
70.00
Experimental data
__
Prodlcted data Predlcted data
c
.o
8
40.00
.
‘.
20.00
0.001
1
1
4
3
,
I
1
5
1
1
6
6
1
7
I
Since the system contains about 99% COZ,the concentrations of acetaldehyde, ethanol, 0 2 , CO, and HzO are very small compared to C02. Therefore, if the constants Kl,,KY, Ky, and KIO.are not very large, the rate expression can be reduced to
8
(14)
xOz/xE
Figure 3. Effect of the oxygen molar ratio of the feed on ethanol conversion and acetaldehyde formation a t 573 K and 8.96 MPa.
this assumption. In Figure 3, ethanol conversion and acetaldehyde yield are shown as a function of the molar ratio of oxygen t o ethanol in the feed. During the experiment, the inlet mole fraction of ethanol was kept constant at 2250 ppm while the inlet mole fraction of oxygen was varied. Figure 3 shows that ethanol conversion increases and acetaldehyde yield decreases with an increase in oxygen concentration, indicating that ethanol conversion to acetaldehyde does not increase with the amount of oxygen present. The oxygen dependence of the acetaldehyde conversion is addressed later. The predicted lines are from rate expressions developed in this study (eqs 13 and 17). The direct oxidation of ethanol to COz is assumed to obey a LangmuirHinshelwood type mechanism (Ismagilov et al., 1983) with dissociative adsorption of oxygen and with the surface reaction between adsorbed oxygen and ethanol to be the rate-controlling step. Based on these assumptions, the mechanism for the oxidation of ethanol to acetaldehyde and C02 can be expressed by the following reactions:
-
+ 2s 20s C,H,OH + S == C,H,OHS co, + s * C0,S C,H,OHS - C,H,OS + H C,H,OS - CH,CHOS + H 2H + OS - H20S CH,CHOS = CH,CHO + S 0,
C,H,OHS C2H,0,S
+ OS - C,H,O,S + S
+ 50s
-
+ 3H,OS + S H20+ S
2C0,S
( 1’)
(2’) (3’)
(4‘) (5’)
with
(15)
(16) The mechanism results in a rate expression that is first order in ethanol for the conversion to both acetaldehyde and C02. McCabe and Mitchell (1984) reported the first-order parallel reactions for the oxidation of ethanol over a platinum catalyst with a volume ratio of oxygen to ethanol of 10 in the feed as well. (ii) Oxidation of Acetaldehyde. Acetaldehyde generated by the oxidation of ethanol further reacts with oxygen to produce CO and C02. Prior t o a mechanism for acetaldehyde oxidation in SCCOz being postulated, different kinds of kinetic models (such as Mars-van Krevelen, Langmuir-Hinshelwood, and power law) were used t o fit the experimental data. It was observed that the best fit was the power law model with a onehalf order for oxygen and a (0.43 & 0.11) order for acetaldehyde. On the basis of these results, it is assumed that 0 2 and acetaldehyde are dissociatively adsorbed on the catalyst surface. Dissociative adsorption of oxygen in oxidation reactions has been widely reported (Satterfield, 1980; Golodets, 1983; Chuang et al., 1992; Chang and Weng, 1993; Zhou et al., 1995). Dissociative adsorption of organic compounds or the scission of the carbon-carbon bond on the noble metal catalyst surface has been found in some reactions (Yagasaki and Masel, 1990; Brucker and Rhodin, 1977; Barteau et al., 1984; Kohler et al., 1987; Zhou et al., 1994). The mechanism of oxidation of acetaldehyde can then be expressed as
0,
(6‘) CH,CHO
+ 2s
(9’)
H,OS (10’) Reaction 9’ is a series of very fast surface reactions that is expressed as a single reaction. Based on the above mechanism, the rate expression for the oxidation of
20s
(1”)
+ 2s * SCH(H0)CS= SCH, + SCHO
(7’) (8’)
-
co, + s
-
(2”)
C0,S
+ OS - SCH,O + S SCHO + OS SCO + OHS SCH,O + OHS + 20s SCO, + 2H,OS + S sco + os sco, + s SCH,
-
+
(3”)
(4”) (5”)
(6) (7“)
1692 Ind. Eng. Chem. Res., Vol. 34, No. 5, 1995
or
sco = co + s H20S
-
H20
(8”)
+S
(9”) It is further assumed that reactions 4“ and 5“ are the rate-controlling steps and that reactions l”,2”, 3“, 8”, and 9” are in dynamic equilibrium. Reaction 6” represents deep oxidation of acetaldehyde to C02 and water and is assumed to occur very fast and is not kinetically significant. It is also postulated that reaction 4“ is irreversible. On the basis of those assumptions, the following Langmuir-Hinshelwood type rate expression is obtained:
+ k~)C,,~(~l,,c02)1’2(~2,,cA)1’2]/ [[I + (K11,c02)1’2 + 2(K2McA)1’2+ K3”cC02+
rA = [(kq
represents acetaldehyde. Since the system contains 99%COB,the volume changes of the mixture during the reaction a t constant temperature and pressure are negligible. If XI is the conversion of ethanol to COSand x2 is the conversion of ethanol to acetaldehyde, the molar concentrations of ethanol, acetaldehyde, and 0 2 can be calculated as
CE = cEO(1 - x1 - x2)
(24)
co2= Co2o(l- 3&c, - 0.5&2)
(25)
= cE$2 (26) where 4 is the ratio of ethanol concentration to oxygen concentration in the feed. Substituting eqs 24-26 into eqs 22 and 23, leads to cA
KgnCHz0+ KvC~o121(17) As explained previously, since the system contains about 99%C02, the above rate expression can be reduced t o
rA = k3(c0,)1’2(cA)1’2
(18)
with
k, =
(kq + k5,)C,2Kl,,1’2K2,,1/2 (1
+ Kycco2)2
(19)
k3CE:’5C02:’5x~’5(1 - 3&1 - 0.5&c,)0.5](28) with initial conditions
The reaction orders in both acetaldehyde and oxygen are one-half. The one-half order in acetaldehyde is due to dissociation adsorption of acetaldehyde on the catalyst surface. CO is generated during the oxidation of acetaldehyde due to the scission of the carbon-carbon bond. Therefore, the mechanism can explain the CO generation during acetaldehyde oxidation as well. c. Parameter Estimation for Kinetic Rate Expressions. For a given reaction system, the following mass balance equation can be written for any species i in a differential reactor element:
atW=O x1=x2=0 (29) The Runge-Kutta method was used to solve the system of ODES (eqs 27 and 28). However, the rate constant k3 in the above equation was obtained from the acetaldehyde oxidation. The integral method was used for parameter estimation for acetaldehyde oxidation. The rate constant k3 for acetaldehyde oxidation was obtained by fitting the experimental space time data with those calculated by
(30)
dF,= r, dW
(20) Substituting eq 1 or into the above equation, one obtains
Conversion to CO being neglected since it was observed only in trace amounts, the chemical reactions can be written as
+
-
+ 3H2O C,H,OH + 1/20, - CH3CH0 + H,O CH,CHO + ,i202- 2C0, + 2H20 C2H50H 302
2C0,
(5)
(6)
(9) When the rate expressions of eqs 14 and 18 are substituted into eq 21 for ethanol and acetaldehyde, two differential equations are obtained:
where subscript E represents ethanol and subscript A
using the rate expression given by eq 18. A nonlinear least squares algorithm, which minimized the following objective function, was used. Rate constants k1 and k2
(31) were obtained by fitting the experimental conversion and yield data with those calculated by eqs 27 and 28 using a nonlinear least squares algorithm which minimized the following objective function:
f = x[l(XE)pred
+ I(X2)pred
(32) where XE is the conversion of ethanol, which can be calculated from - (XE)expl
- (X2)expl12
xE=Xl+X2 (33) The experiments for ethanol and acetaldehyde oxidation were carried out at four different temperatures and seven different space times. Ethanol conversion at four different temperatures of 423,448,473, and 498 K and at different space times is presented in Figure 4. The yield of acetaldehyde in ethanol oxidation is presented in Figure 5. Figure 6 shows the oxidation of acetaldehyde as a function of temperature and space time. Data in Figures 4-6 were used to fit the kinetic model. To
Ind. Eng. Chem. Res., Vol. 34,No. 5, 1995 1593 8.00
-
8.00
-
4.00
-
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I
I
I
1
I
I
+ I n +k
-
70.00 n
5
60.00
6
50.00
'C>
s=
Y C
4
40.00 50.00
moo 10.00 0.00 0.0 0.1
0.2
0.4
0.3
W/Fm
0.5
0.6
(g cat. hr/mol
0.7
0.8 0.9
Figure 4. Ethanol conversion as a function of space time (oxidation of ethanol in SCCO2 a t 8.96 MPa). 50.00
0.0021
20.00 n
E, ism I!
10.00
1 /T
0.0025
0.0024
(K-1)
Figure 7. Arrhenius plot of the rate constants for the parallel.' consecutive oxidation mechanism of ethanol.
2;)
k, = 1.409 x lo7 exp(- -
5.00
(L1'5/gof cat. h
0.00 0.0 0.1
0.2
0.3
0.6
0.5
0.4
W/Fw (g cat hr/mol
0.7
0.8 0.9
k, = 6.528 x lo6 exp(-
- -o-
80.00
--
70.00
-
~ ~ 4 8 8 --t T=473 T-448 --C T-423
0.0 0.2
( '",",""i
k 2 = 1.746 x 10' exp - -
C2H6O)
Figure 5. Acetaldehyde yield as a function of space time (oxidation of ethanol in SCCOz at 8.96 MPa).
8o.w
0.0022
were obtained, and the Arrhenius fits are given in Figure 7. For direct oxidation of ethanol to C 0 2 , the activation energy is 8.71 kcdmol, which is close to the reported data of 10.2-12.1 kcaVmol for the oxidation of ethanol to C 0 2 on silica-supported platinum catalyst (Nagal and Gonzalez, 1985). The activation energy for the oxidation of ethanol to acetaldehyde is 12.38 kcaV mol. For the oxidation of acetaldehyde to C 0 2 , the activation energy is 10.60 kcdmol. The rate constants are given by
25.00
e
0.0020
C2H6O)
0.4
0.6
W/FM
0.8
1.0
1.2
(g cat. hr/mol
1.4
1.6
1.8 2.0
C2H40)
Figure 6. Acetaldehyde conversion as a function of space time (oxidation of acetaldehyde in SCCOz at 8.96 MPa).
verify the assumption that K1, and Ky are not very large, the experimental data of ethanol oxidation a t 498 K were used to fit eq 13. If the adsorption of water and acetaldehyde were negligible, it was found that the best fit gave Kr = 4.289 f 2.011, KZ.= 0.284 & 1.22, and &, = 1.543 f 0.855. The equilibrium constants are very close to each other, but the concentration of C 0 2 is much higher than that of ethanol and oxygen. Therefore, the simplified rate expressions can be used. By fitting the experimental data to the rate expressions, the rate constants kl, k2, and ks a t four different temperatures
w) RT
(34)
(Ugofcat. h) (35)
(Ugof cat. h) (36)
The kinetic model was used to calculate the conversion of ethanol and yield of acetaldehyde at temperatures from 423 to 573 K and at space times of 0.655 and 0.814 g of cat. Wmol for ethanol oxidation. The predicted and experimental data are presented in Figure 8. Both the experimental data and the predictions show that ethanol conversion increases with the temperature as expected, while the yield of acetaldehyde has a maximum value. The predictions agree well with the experimental data. The equations are also used for predicting the change in conversion and yield with the oxygen molar ratio in the feed (Figure 3). It should be noted that data in Figure 3 are not used in regression of the kinetic parameters. The predictions, again, are very good. Figure 9 shows the predicted and experimental conversion of acetaldehyde as a function of temperature during acetaldehyde oxidation. A very good agreement is also obtained for the acetaldehyde conversion. Figures 10 and 11 are the parity plots for ethanol and acetaldehyde conversion and for ethanol and acetaldehyde oxidation, respectively. In these two figures, the largest deviation between the predicted and experimental data is less than 6%.
Conclusions Oxidation of ethanol over the 4.45% I " i 0 2 catalyst in SCCO2 produces partial oxidation product acetalde-
1694 Ind. Eng. Chem. Res., Vol. 34,No. 5,1995
80.00
-
80.00
-
110.00 70.00
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-
60.00
Yield W/F~o-O.855 (pfed) Conversion W/F~o-O.856 (pred) Yield w/FAO-o.814 (pred) Conversion W / F A O - O ~4 ~(pred)
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