Kinetics of the Liquid-Phase Oxidation of Ethanol by Oxygen over a

524

Ind. Eng. Chem. Process Des. Dev., Vol. 17, No. 4, 1978

Kinetics of the Liquid-Phase Oxidation of Ethanol by Oxygen over a PaIladium- Alumina CataIy st Shlh-Hau Hsu and John A. Ruether" Department of Chemical Engineering, University of Ottawa, Ottawa K 7N 6N5, Canada

Downloaded by FLORIDA STATE UNIV on September 10, 2015 | http://pubs.acs.org Publication Date: October 1, 1978 | doi: 10.1021/i260068a023

Ethanol is oxidized by gaseous oxygen to acetate ion in an aqueous solution containing sodium carbonate over a palladium-alumina catalyst at 25,35,and 45 OC and ambient pressure in a stirred tank reactor. Reactions with slurry catalyst establish the intrinsic kinetics. Reactions with whole pellets establish that the tortuosity factor for the support is 2.9. The reaction is a model for the study of gas-liquid-solid systems with fast reaction.

The use of fixed bed catalytic reactors with cocurrent gas and liquid flows is well established in the petroleum refining and chemical processing industries. When this type reactor is operated a t relatively high gas and liquid feed rates the interaction among the three phases is intense, resulting in a large capacity for interphase heat and mass transfer. It has been suggested that with use of a catalyst having the active component deposited as a thin layer a t the surface of the support, this kind of reactor is competitive with slurry reactors for conducting fast, heterogeneously catalyzed gas-liquid reactions (Lemay et al., 1975). Research concerning mass transfer (Charpentier, 1976) and heat transfer (Hashimoto et al., 1976) in fixed beds with rapid cocurrent flows is continuing. In order to test reactor design procedures it is useful to employ a model reaction capable of operation a t a Damkohler number (ratio of rate of reaction to rate of diffusional transport) similar to that encountered with the reaction of commercial interest. Desirable attributes of a model reaction would include a fast rate at temperatures and pressures near ambient, inexpensive reactants and non-hazardous reactants and products, a well defined stoichiometry, ease of chemical analysis, and a stable catalyst. A reaction that meets these criteria is the oxidation of ethanol in alkaline aqueous solution by oxygen over a supported palladium-alumina catalyst. Sato et al. have actually employed the reaction in a study of fixed bed reactors with fast gas and liquid flow (Sato et al., 1972). They did not do a kinetic study, however, operating instead at a single temperature and a single concentration of liquid phase reactants. Furthermore, they used catalyst only in the form of 3.8-mm spheres, so it was not possible to determine the extent of internal diffusion resistance. It was the purpose of this work to determine the effects of oxygen, ethanol, and sodium carbonate concentrations on the intrinsic, or kinetic, rate of oxidation of ethanol to acetate ion. We also wished to determine the effectiveness factor of a pelleted catalyst a t typical reaction conditions. In the course of the investigation the tortuosity factor for the catalyst was also determined. Previous Work Klassen and Kirk (1955) reviewed the early work of Mueller and Schwabe (1930) on the use of platinum-group metals for catalyzing the oxidation of ethanol. Mueller and Schwabe found that any platinum-group metal would work, but only with palladium was an incubation period avoided. These workers also studied the effect of the pH Pittsburgh Energy Research Center, U.S. Department of Energy, 4800 Forbes Avenue, Pittsburgh, Pa. 15213. 0019-7882/78/1117-0524$01.00/0

Table I. Physical Properties of 0.5% Pd-Al,O, Cylindrical Pellets average pellet dimensions diameter, cm height, cm volume fraction of support containing Pd surface area, m'/g pore volume, cm3/g bulk density, g/cm3 average pore radius, A porosity a

Measured value.

~

0.322a 0.346a 0.513" 152.2a 0.272" 1.556" 18.0' 0.423'

Calculated value.

of the aqueous ethanol solution on the reaction. They found that in a neutral solution no reaction occurred; in acid solution, the ethanol was oxidized to acetaldehyde; and in basic solution, acetic acid was formed. Klassen and Kirk tried a number of alkaline liquid phase reagents and settled on the use of sodium carbonate, since it was nonvolatile and did not attack the alumina catalyst support. They established that in the presence of sodium carbonate concentrations on the order of 0.1 M the reaction went cleanly as follows.

Pd-Al203

C2H50H + O2+ Na2C03 NaOOCCH3 + H 2 0 + NaHC03 (1) They conducted their reactions with catalyst in the form of irregularly shaped 3-6-mm particles in a trickle bed reactor in the temperature range 23-45 O C . Although these authors considered external mass transfer resistance, their conclusions about the kinetics of the reaction are rendered suspect by the fact that they used reaction rate data to determine simultaneously mass transfer and kinetic constants. They did not consider internal diffusion resistance. They presented a kinetic rate expression of the Langmuir-Hinshelwood type in which the rate was first order in ethanol and carbonate ion concentrations, and of order less than unity in oxygen partial pressure. As mentioned, Sat0 et al. measured the rate at 30 "C for a single liquid phase composition with 3.8-mm catalyst particles. They determined that the rate was first order in oxygen over an unspecified range of oxygen partial pressure. They presented an apparent first-order rate constant valid for their particular reaction conditions. Experimental Section The catalyst used was 0.5% Pd-alumina cylindrical pellets 3.46 mm long by 3.22 mm diameter supplied by US. Engelhard Co. The palladium was deposited in the external portion of the support particle, the interior portion of the pellet being palladium free. The thickness of the palladium layer was determined by light microscope. The 0 1978 American Chemical Society

Ind. Eng. Chem. Process Des. Dev., Vol. 17, No. 4, 1978 525 Table 11. Range of Experimental Variables


reactor temperature, O C oxygen partial pressure, atm dissolved oxygen concentration, mg/L ethanol concentration, g m o l / L sodium carbonate concentration, g m o l / L catalyst loading, g for slurry runs for pellet runs

25-45 0.08-0.6 3.5-18.5 0.45-1.80 0.01-0.40 3.5-1 0.0 35.0

effective thickness of the Pd-containing shell, defined as the ratio of shell volume to outside pellet surface area, was 0.282 mm. Physical properties of the catalyst are listed in Table I. Rates reported here in terms of catalyst volume refer to volume of catalyst containing Pd. Reaction rate data were taken with the catalyst in two forms, whole pellets and powder. The powder catalyst was prepared by grinding pellets and passing them through a sieve with 88-km openings. All reactions were conducted in a jacketed, 1-gal stirred autoclave containing 2600 mL of solution. The reactor was fitted with two four-bladed, 6-cm diameter impellers, one placed one-third of the distance from the bottom to the gas-liquid interface, the other placed at the gas-liquid interface. The impeller speed was 203 rpm. Gas could be fed to the reactor either by a sparger located beneath the lower impeller, or via a nozzle whose tip was 1.5 cm above the gas-liquid interface. The latter means of gas injection was used to sweep the gas head space prior to measurements of the gas absorption coefficient. Oxygen, nitrogen, and helium could be fed to the reactor. The oxygen partial pressure in the reactor during reactions was controlled by initially introducing a predetermined amount of nitrogen into the reactor. Oxygen was then added to reach the desired total pressure. During the reaction pure oxygen was added continuously such that the total pressure remained constant. Reactor pressure was 33-37 Torr above atmospheric, and was measured with a mercury manometer. The rate of oxygen consumption was measured with a soap bubble meter with a kathetometer. The reactor had four symmetrically placed baffles constructed of stainless steel mesh. They contained 5-mm glass spheres, and when appropriate, catalyst pellets. The pellets were placed a t the level of the lower agitator to maximize the particle--liquid mass transfer coefficient. A dissolved oxygen electrode was attached to one of the baffles with its tip also at the level of the lower impeller. Temperature in the reactor was controlled by circulating water to the jacket from a water bath having a nominal 0.01 "C precision. Reactor temperature was read to 0.01 "C with a precision mercury thermometer and kathetometer. Reactions were conducted at 25,35, and 45 "C. In determining reaction rates, about 80 min was allowed for steady state to establish. Following this data were collected over 17 min. Rate was determined from a plot of oxygen volume absorbed vs. time. Liquid-phase concentrations of ethanol and sodium carbonate changed on the order of loe4 g-mol/L during a run and could be considered constant. The stoichiometry of eq 1 was verified in two ways. Some special reactions of 30 h length were tested for the presence of aldehydes or ketones with the reagent 2,4-dinitrophenylhydrazine (Pasto and Johnson, 1969). The t,est was negative. A reaction of the same length was analyzed by titration with acetic acid. The consumption of oxygen thus calculated agreed within 2.1 % with that calculated from volumetric oxygen data. The range of operating conditions for reactions is shown in Table 11. In the reaction system under study, the kinetics can be disguised by three kinds of mass transfer resistance: gas

absorption, particle-liquid, and intraparticle. To estimate the importance of mass transfer resistance, gas absorption and particle-liquid coefficients were measured in independent experiments. Gas absorption coefficients a t 25 and 45 "C were determined by measuring the rate of unsteady-state physical absorption of oxygen. The volumetric absorption coefficient, kLa, was obtained using the integrated form of the mass balance equation for oxygen -In

( K2*) 1-

~

= kLat

The volume of absorbed oxygen A V as a function of time was determined with the bubble meter, and the saturated oxygen concentration C, was read from the oxygen analyzer. Average values of the volumetric absorption coefficient at the two temperatures were 0.0183 and 0.0258 cm/s, respectively. For a reaction with pellet catalyst, gas absorption resistance represented typically about 0.5 % of the total oxygen concentration driving force. Particle-liquid mass transfer coefficients were determined by coating pellets with a thin layer of benzoic acid containing a fluorescent dye and following the unsteady-state concentration of dye in the liquid with a fluorometer (Lemay et al., 1975). The governing equation is

(3) The particle mass-transfer coefficients thus determined are corrected for use with oxygen by assuming they depend on the molecular diffusivity to the z / 3 power. At 25 "C the particle coefficient for oxygen transfer was 0.060 sd, giving rise to a resistance for particle-liquid mass transfer of about 1% of the total oxygen concentration driving force for a typical reaction rate. With regard to particle-liquid mass transfer resistance in the powder catalyst system, it has been shown that except for special cases external diffusion resistance is negligible when internal diffusion resistance is negligible (Petersen, 1965; Ruether and Puri, 1973). Recently it has been shown that for a first-order reaction with a slurry catalyst, external resistance can be neglected when the internal effectiveness factor is greater than about 0.8 (Hsu and Ruether, 1978). In the present study internal effectiveness factors for reactions with slurry catalyst were calculated to be in excess of 0.95 always. Complete experimental details are given by Hsu (1977). Results I. Slurry Catalyst. The dependence of rate on oxygen concentration at fixed concentrations of ethanol and sodium carbonate is shown in Figure l. The plots are nearly linear, and when the data are plotted on a log-log graph, the indicated reaction orders at 25,35, and 45 "C are 0.92, 0.97, and 0.94, respectively. This essentially first-order dependence on oxygen is in agreement with the results of Sat0 et al. (1972). Oxygen solubility in the reaction solution decreased with increasing ethanol or sodium carbonate concentrations. Given the essentially first-order dependence of rate on oxygen, it is convenient to show the effects of liquid-phase reactant concentrations on rate by plotting rate divided by oxygen Concentration. This has been done in Figures 2 and 3. Figure 2 shows that the rate increases monotonically with ethanol concentration over the range studied. Figure 3 indicates a more complicated dependence of rate on sodium carbonate concentration. The rate exhibits a maximum in the neighborhood of a sodium carbonate

526

Ind. Eng. Chem. Process Des. Dev., Vol. 17, No. 4, 1978

m

0-

35°C

A 45'G 8-

- 1 3 00

2

10

I0

14

C, x IO', g - mol ~ c m '

-

1

1

I

,

1

I

1

2 3 4 C , x IO', g - m G l l C m 3

I

0

1

Figure 4. Effect of ethanol concentration on oxidation rate with pellet catalyst at 35 "C: C3 = 0.1 g-mol/L. I

I ,

Figure 1. Effect of dissolved oxygen concentration on oxidation rates with slurry catalyst; Cz = 0.6 g-mol/L, C3 = 0.1 g-mol/L.

2

::I

2

261

Q x

+-


2

L

1

2

I

I

,

2

I

3

I

4

c,x104, g-mol/cm3

Figure 5. Effect of sodium carbonate concentration on oxidation rate with pellet catalyst at 35 O C : C2 = 0.6 g-mol/L.

d 1 'd k

'

'

I

4

I l 0 '

10

C, x

lo4,q.

ma1 / c m 3

Figure 2. Effect of ethanol concentration on oxidation rate with slurry catalyst at 35 'C: C3 = 0.1 g-mol/L.

, n

I 37-

18-

.lo-

2 87 -

0-

2

P

2 9 -

2 7 -

P x

I

3 00 -t 3 N.

2

2

I

5

1716-

,

- -C o l c u l o l e d

14-

I 3

,

from Eq

,

,

c,

x

104,

,\

2

9 .mol /cm3

Figure 3. Effect of sodium carbonate concentration on oxidation rate with slurry catalyst at 35 O C : Cz = 0.6 g-mol/L.

concentration of 0.1 g-mol/L. The data should not be extrapolated to a carbonate concentration of zero (Mueller and Schwabe, 1930). The dependence of rate on oxygen and ethanol concentrations can be interpreted in terms of LangmuirHinshelwood kinetics as being due to adsorbed ethanol reacting with oxygen from the liquid phase. This model yields a rate expression first order in oxygen. Interpretation of the rate dependence on sodium carbonate concentration is less straightforward. In constructing an analytical rate equation an empirical expression was used for the sodium carbonate dependence. An adsorption term for oxygen was also added to account for the observed oxygen order of slightly less than unity. The form of the rate equation is

At 35 "C the constants in eq 2 have the following values, as determined by a nonlinear least-squares fit: k = 6226

cm3/ g-mol-s, K1 = 3.212 X lo5 cm3/g-mol, K 2 = 4409 cm3/g-mol, ul = 0.2217, u2 = 752.8, u3, = 3.339 X lo4, u4 = 1.0 X g-mol/cm3. Equation 2 was used to generate the lines in Figures 1 and 2. It reproduces all the data at 35 "C (14 points) with an average relative error of 1.0%. Rate data obtained with slurry catalyst yield intrinsic kinetics only if the effectiveness factor is essentially unity. In calculating possible internal diffusion resistance, we considered the reaction to be pseudo one-component (oxygen), after the manner of Satterfield et al. (1968). A conservative estimate of internal diffusion resistance can be made using the largest particle size present, in this case, 88 pm. Calculation of the Weisz parameter, a,, and use of a graph of 11 vs. for a first-order reaction shows that 11 was essentially unity at 25 and 35 "C, assuming d, = 88 pm (Satterfield, 1970). Use of d, = 88 pm with the data at 45 "C indicated significant internal diffusion resistance. The calculation was redone more precisely using the catalyst particle size distribution. This procedure, and the method of estimating the diffusion coefficient for oxygen, has been reported elsewhere (Hsu and Ruether, 1978). Reactions at 45 "C with liquid phase reactant concentrations of C2 = 0.6 g-mol/L and C3 = 0.1 g-mol/L were computed by the more precise procedure to have an average effectiveness factor of 0.97. The activation energy for the rate constant k can be estimated. Sufficient data were not collected to determine the temperature dependence of the several parameters in eq 4, However, if it is assumed that a t constant reactant concentrations the variation in rate with temperature is due solely to change in k , an apparent activation energy of 7000 cal/g-mol (29.3 MJ/kg-mol) is calculated. 11. Pellet Catalyst. The dependence of rate on reactant concentrations for the catalyst in the form of pellets was similar to that for slurry catalyst. The apparent order with respect to oxygen at 25,35, and 45 "C was 0.91, 0.93, and 0.96, respectively. The dependence of rate on ethanol and sodium carbonate concentrations at 35 "C is shown in Figures 4 and 5. The rate was strongly affected by internal diffusion resistance at all three temperatures. For liquid phase

Ind. Eng. Chem. Process Des. Dev., Vol. 17, No. 4, 1978 527

Table 111. Specific Surface Area, Tortuosity Factor, and Internal Void Fraction for Alumina Catalyst Support area, m2/g 7 9 authors 0.585 Satterfield (1970) 197 3.9 0.56 Kawakami et al. (1976) 100 2.8 0.54 Kenney and Sedriks (1972) 265 1.6 6.4 2.8 0.389 Satterfield and Cadle (1968) 152 2.9 0.423 This work


~~~

~

~

concentrations Cz = 0.6 g-mol/L and C3 = 0.1 g-mol/L the effectiveness factors a t 25, 35, and 45 " C for the pellets were 0.168, 0.158, 0.142, respectively. Recall that the calculations are based on the fraction of the catalyst support that contains palladium. The tortuosity factor was evaluated a t all three temperatures in the conventional manner (Kawakami et al., 1976) assuming the reaction to be first order in oxygen. The values obtained a t 25, 35, and 45 O C were 2.90, 2.88, and 2.92, respectively. The consistency of the values is gratifying, and comparison of previously reported values for similar catalyst supports in Table I11 indicates the values are reasonable. The lines in Figures 4 and 5 were generated using eq 2 and the expression for the effectiveness factor in an infinite plate with first-order reaction, with 7 = 2.90. The average relative error of the computed values for the data in Figures 4 and 5 is 1.8%.

Summary The oxidation of ethanol by gaseous oxygen in aqueous solution containing sodium carbonate and catalyzed by Pd-alumina catalyst represents a model reaction for studying fast gas-liquid-solid reactions. The rate is essentially first order in oxygen, increases with ethanol concentration in the range 0.45-1.80 g-mol/L, and exhibits a maximum with respect to sodium carbonate concentration. At 35 " C the maximum occurs a t C3 N 0.1 gmol/L. The tortuosity factor for the alumina catalyst support employed is 2.9. Nomenclature al, a2, a3, a4 = constants in eq 4 a = external surface of particles per unit liquid volume, cm-' = oxygen concentration, g-mol/cm3 Cz = ethanol concentration, g-mol/cm3

6

C3 = sodium carbonate concentration, g-mol/cm3 C, = saturation concentration of dissolved oxygen, g-mol/cm3 C, = benzoic acid concentration, g/g of solution C,, = saturation concentration of benzoic acid, g/g of solution k = kinetic rate constant, cm3/g-mol-s KLa = volumetric gas absorption coefficient, s-l K1 = adsorption coefficient for oxygen, cm3 g mol K 2 = adsorption coefficient for ethanol, cm /g-mol Po = oxygen pressure in bubble meter, atm r = rate of oxygen consumption, g-mol/s-cm3 of catalyst R = universal gas constant VL = volume of liquid, cm3 AV = volume of oxygen absorbed in time t , cm3 t = time, s T = temperature, K Greek Letters 7 = internal effectiveness factor, unitless 6' = internal void fraction, unitless 9, = Weisz reaction factor for a sphere, unitless T = tortuosity factor, unitless Literature Cited

/-

Charpentier. J. C., Chem. Eng. J., 11, 161 (1976). Hashimoto, K., Muroyama, K., Fujiyoshi, K., Nagata, S., Int. Chem. Eng., 16, 720 (1976). Hsu, S-H., M.A. Sc. Thesis, University of Ottawa, 1977. Hsu, S-H., Ruether, J. A., Can. J. Chem. Eng., in press, 1978. Kawakami, K., Ura, S., Kusunoki, K., J . Chem. Eng. Jpn., 9, 392 (1976). Kenney. C. N., Sedriks, W.. Chem. Eng. Sci., 27, 2029 (1972). Klassen, J., Kirk, R . S., AIChE J., 1, 488 (1955). Lemay, Y., Pineault, G., Ruether, J. A., Ind. Eng. Chem. Process Des. D e v . , 14,280 (1975). Mueller, E., Schwabe, K., KollOid-Z., 52, 163 (1930). Pasto, D. J., Johnson, C. R., "Organic Structure DeterminaMn," p 39, P r e n t i i l , Englewood Cliffs, N.J., 1969. Petersen, E. E., "Chemical Reaction Analysis," Prentice-Hall, Engiewood Cliffs, N.J., 1965. Ruether, J. A., Puri, P. S., Can. J . Chem. Eng.. 51,345 (1973). Sato, Y., Hirose, T., Takahashi, F., Proceedings of the First Pacific Chemical Engineering Conference, Kyoto, Japan, paper 8-3, p 187, 1972. Satterfield, C. N., "Mass Transfer in Heterogeneous Catalysis," MIT Press, Cambridge, Mass., 1970. Satterfield, C. N., Cadle, P. J., Ind. Eng. Chem. Fundam., 7, 202 (1968). Satterfield, C. N., Ma, Y. H., Sherwood, T. K., I. Chem. E . Symp. Ser. No. 28, 28 (1968).

Received f o r review January 3, 1978 Accepted June 7, 1978

Financial support was provided by the National Research Council of Canada. Dr. Sid S. Pollack of the Pittsburgh Energy Research Center determined catalyst surface area and pore volume.

Kinetics of the Liquid-Phase Oxidation of Ethanol by Oxygen over a

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