Ind. E n g . C h e m . Res. 1988,27, 2050-2055
2050
British Petroleum, Br. Patent 930 457, 1963a. British Petroleum, Belg. Patent 625 486, 1963b. British Petroleum, Br. Patent 625 269, 1963c. British Petroleum, Belg. Patent 621 204, 1963d. British Petroleum, Fr. Patent 1376 095, 1964. British Petroleum, Neth. Patent Appl. 506 019, 1965a. British Petroleum, Belg. Patent 659 679, 196513. British Petroleum, Ger. Offen 2 108276, 1971. Coenen, J. W. E.; Linsen, B. G. Physical and Chemical Aspects of Adsorbents and Catalysts; Academic: London, 1970. Corma, A.; Miranda, M. A.; Perez-Pariente, J. React. Kinet. Catal. Lett. 1983,23, 153. Corma, A.; FornBs, V.; Mifsud, A.; PBrez-Pariente, J. Clay Miner. 1984, 19, 673. Corma, A.; Mifsud, A.; Perez-Pariente, J. Clay Miner. 1986,21, 69. Dandy, A. J.; Nadiye-Tabbiruka, M. S.Clays Clay Miner. 1975,23, 428. Feitknecht, W.; Berger, A. Helu. Chim. Acta 1942, 25, 1543. Fernindez Alvarez, T. Proc. Znt. Clay Conf. 1972, 571. Fernindez Hemindez, M. N.; Fernhdez Alvarez, T. An. Quim. 1982, 79, 342. Fuentes, S.; Figueras, F. J. Catal. 1978,54, 397. Fuentes, S.; Figueras, F.; GBmez, R. J. Catal. 1981, 68, 419.
Ioka, M.; Wakabayashi, M.; Oguchi, Y. Japan Kokai 7830996,1978. Nagata, S.; Shimoda, S.; Sudo, T. Clays Clay Miner. 1974,22, 285. Occelli, M. L. Znd. Eng. Chem. Process Res. Deu. 1983, 22, 553. Perez-Pariente, J. Ph.D. Thesis, Institute of Catalisis, Madrid, Spain, 1984. Pupko, S.Fr. Patent 1383 960, 1965. Rautureau, M. Ph.D. Thesis, Orleans, France, 1974. Rautureau, M.; Mifsud, A. C. R. Hebd. Seances. Acad. Sci., Ser. C 1975,281, 1071. Rautureau, M.; Mifsud, A. Clay Miner. 1977,22, 285. Pare!&, H.; Ganesan, P.; Russell; S. N.; Davis, B. H. Reucroft, P. 0.; Appl. Catal. 1982,3, 79. Serna, C.; Fernindez Alvarez, T. An. Quim. 1975, 71, 371. Serna, C . ; Van Scoyoc, G. E. Proc. Znt. Clay Conf. 1978, 197. Shil, L. N.; Bhatla, S. Znd. Eng. Chem. Prod. Res. Dev. 1986,25,530. Singer, A.; Galan, E. Palygorskite-Sepiolite. Ocurrences, Genesis and Uses; Elsevier: Amsterdam, 1984. Spenadel, L.; Boudart, M. J. Phys. Chem. 1960, 64, 205. Suzuki, N.; Tsutsumi, K.; Takahashi, H. Zeolites 1982,2, 51, 185.
Received for review October 14, 1987 Revised manuscript received June 27, 1988 Accepted July 7, 1988
Dehydrogenation of Butan-8-01on Zinc Oxide Catalyst: A Continuous Stirred Tank Reactor Study Sankaran Ravi and T. S. Raghunathan* Department of Chemical Engineering, Indian Institute of Technology, Bombay 400 076, India
The kinetics of dehydrogenation of butan-2-01 on a zinc oxide catalyst was studied in a specially fabricated continuous stirred tank reactor using annular ring-shaped disks of the catalyst. Experimental data were collected in the temperature range 400-490 "C and a t W/FA,,'sranging from 5X to 48 X lo4 g of catalyst/mol of butan-2-ol/s. An interesting feature concerning the rates of reaction is the occurrence of a maximum in conversion at constant temperature. A dual-similar-site mechanism with surface reaction as the rate-controlling step together with negligible K R and K s is shown to account for all the observed results. Above 475 "C, however, a dual-dissimilar-site mechanism with hydrogen desorption as the rate-controlling step also explains the data. An evaluation of the internal diffusion effects on the basis of Weisz-Hicks criterion indicates that the effectiveness factors were probably close to unity under most of the experimental conditions. Thus, the rate ~ , be said to describe the true kinetics of this equation proposed, viz., r = k p A / ( l + K A ~ A )could reaction. Dehydrogenation of butan-2-01 (sec-butyl alcohol) is an industrially important reaction in that it yields methyl ethyl ketone or butan-2-one, an industrial solvent, as the chief product. This reaction is usually conducted in the temperature range 325-425 "C on a catalyst such as zinc oxide activated with Na2C03,Cu-Ni on Kieselguhr, Raney copper etc. (Stephenson, 1966). A literature survey revealed that there are no published kinetic data on zinc oxide catalyst. Miller and Wu (19721, however, report that the activation energies of the reaction on a zinc oxide catalyst with and without an oxygen pretreatment me 1.8 and 28.9 kcal/mol, respectively. No attempt was made by Miller and Wu to present a rate equation for this reaction on the zinc oxide catalyst. The present study was undertaken with the aim of obtaining a rate equation for the dehydrogenation of butan-2-01 on a zinc oxide catalyst. It is well-known that the reactor system that yields direct rate information on any reaction is the well-mixed continuous stirred tank reactor or the CSTR. This type of reactor is characterized by uniformity of concentration of the species in the reaction together with constancy of temperature in the entire reactor space. An additional
* To whom all correspondence
should be addressed.
point is that the measurable exit conditions of concentrations and temperature correspond to those inside the reactor. The rate of a reaction represented by the stoichiometric equation
A=R+S may readily be obtained from the equation
(1)
where X A is the exit fractional conversion of A in a CSTR operating with a mass of catalyst equal to W and molar feed rate of A equal to F A @ When (1)occurs on a solid surface and the reaction mixture itself is in the gas (or vapor) phase, it is necessary that any kinetic data collected be such that it is possible to eliminate or take into account the external physical transport resistances. In this respect, CSTR affords certain advantages over a fixed bed reactor in that the agitation control can be achieved independently even at low feed rates of the reactants. An experimenter is thus at a greater advantage with a CSTR for kinetic studies. The experimental reactor system employed in the present study amounted (Shah, 1982) to a well-mixed re-
0888-5885/88/2627-2050$01.50/0 0 1988 American Chemical Society
Ind. Eng. Chem. Res., Vol. 27, No. 11, 1988 2051
REACTOR
@ GUN
METAL BUSH BEARING SCALE -1 1 ( All dimension are in m m )
Figure 2.
'L - - - U I ---l A- Feed system
G - Motor
B - Proheater
H-Thermocouple position in Reactor
-
C Furnace
S-Soap Filmmeter
D-Reactor
Th -Thermocouple
E-Stutting boa
Ms-Motor starter
F- Condensers
Figure 1. Schematic diagram of the setup.
actor where annular disks of zinc oxide were rotated at such speeds that external mass-transfer resistances were essentially absent. The details of the experimental system and procedure are discussed below.
Experimental Section A schematic diagram of the experimental setup comprising the feed system, the reactor, and the condenser is shown in Figure 1. A brief description of the reactor is as follows. Reactor Details. The reactor body was made of stainless steel by drilling out of a 50- X 70-mm cylindrical rod a cylindrical space 33 mm in diameter and 50 mm in depth, the last 5 mm rounded off to avoid stagnant regions. A 100-mm-diameter, 12-mm-thick flange having six 9-mm holes at a PCD of 91 mm was provided on the top part of the reactor body to be mated with an identical closure flange carrying the stirrer assembly and the stuffing box. The stirrer assembly itself consisted of a 6-mm cylindrical rod fitted with propeller blades and catalyst arms on which annular disks of the catalyst could be mounted. A section of the reactor, stirrer, and the stuffing box is shown in Figure 2. Catalyst disks of 13-mm 0.d. and 4-mm i.d. of differing thicknesses in the range 0.9-2.5 mm were prepared in a hand-operated press at a load of about 2 tons. Freshly prepared catalyst powder was used to form these disks without any addition of binders, die lubricants, etc. Plate 1 shows a view of the catalyst disk and the stirrer assembly. The entire reactor containing the catalyst disks was housed in a detachable two-piece furnace provided with a proportional temperature controller to maintain the temperature in the reactor at any level in the range investigated. The temperature measurement in the reactor
was effected with the aid of a chromel-alumel thermocouple entering through the exit at the bottom of the reactor and positioned at a distance of about 1 mm below the tip of the stirrer shaft. Traces of temperature-time recordings during the reactor operation showed that the temperatures remained within f1.5 "C.
Experimental Procedure Preliminary blank experiments showed that there was no catalytic activity of the metal parts of the reactor with which the vapors of butan-2-01 came into contact. The general procedure of any run was to send a constant liquid alcohol flow (f3%) to the evaporator-preheater, and the resulting hot vapors were sent into the catalytic reactor. The exit vapors were cooled and condensed to obtain alcohol- and ketone-free exit gases. The liquid collected was analyzed for the constituents on a gas-liquid chromatograph on a column filled with 5% Carbowax 20M on Chromsorb W with the aid of a hydrogen flame ionization detector. The peaks obtained were very sharp and the peak heights were taken to characterize the composition. The liquid reaction mixture when analyzed indicated the presence of methyl ethyl ketone and butan-2-01 only. The ratio of the ketone to alcohol in any experimental run was compared with a calibration curve of this ratio plotted against the molar composition of the ketone in synthetically prepared samples. The fractional conversion of the alcohol, XA,was thus obtained. Traces of dehydration product (but-2-ene) were observed on virgin catalyst, and this ceased within 30 min of operation. Catalyst Preparation. Several batches of zinc oxide were prepared starting from either basic zinc carbonate or zinc nitrate hexahydrate (supplied by Robertson Johnson Co.). The various catalysts prepared were pelleted in the form of annular disks (13 X 4 mm) for use in the reactor. Some of the catalyst preparations did not yield mechanically strong disks, and hence only such preparations that resulted in mechanically strong disks were tested in the kinetic study. Of the various catalysts prepared, the zinc oxide obtained by the decomposition of the basic zinc carbonate at 400 "C for 2 h followed by kneading with distilled water and drying at 110 "C for 2 h gave the highest conversion coupled with good mechanical stability of the disks. All the kinetic results reported in this work were obtained on this preparation. The details of the performance of the other preparations tested are given elsewhere (Ravi, 1980). In any given run, upto four disks of the catalyst could be charged in the reactor, and this indeed was one of the
2052 Ind. Eng. Chem. Res., Vol. 27, No. 11, 1988 Table I. Exuerimental Data at 425 "C
at 450 "C
r
X A
r
X A
r
X A
r
2.79 3.64 9.14 12.51 14.42 18.21
0.06 0.12 0.26 0.31 0.82 0.40
0.022 0.033 0.028 0.025 0.057 0.022
0.09 0.18 0.37 0.41 0.895 0.49
0.032 0.049 0.040 0.033 0.062 0.027
0.16 0.26 0.47 0.52 0.93 0.60
0.056 0.071 0.051 0.042 0.064 0.033
0.20 0.31 0.52 0.59 0.93 0.64
0.072 0.085 0.057 0.047 0.064 0.035
Results and Discussion Table I lists the experimental data with respect to the rates of reaction at various conversion at 425 "C. The complete set of data used in this study is available in Ravi (1980). Attempts were made to fit the data into equations derived on the basis of assumed mechanisms, and the various equations tested are shown in Table 11. At the outset, it may be remarked that none of these expressions fit the data in the entire range of conditions, meaning the data analysis revealed one or more negative constants of the 14 equations shown in Table 11. However at 475 and 490 "C,it was found that the dual-dissimilar-site mechanism with hydrogen desorption as the rate-controlling step described the data well. The rate equation under these conditions is given by r=
~~(PA/PR)
1 - XA P A = -
PR=PS=-
XA
(4)
at a total pressure of 1 atm, corresponding to a pure alcohol feed to the reactor. The estimates obtained for k and Ks in ( 3 ) are shown in Table 111. Dual-Similar-Site Mechanism with Negligible Adsorption Equilibrium Constants of Ketone and Hydrogen. In an attempt to find a single rate equation that would describe the kinetics of the reaction in the entire range of experimental conditions, the rates of reaction were plotted against X , at various temperatures (Figure 3). It is seen that these curves exhibit a maximum, and this led to the necessity of examining such rate equations which display this characteristic. One such rate equation is r =
o
009
(3)
1 + K$(s@A/PR)
The equilibrium constants in the reaction, K,, are reported by Kolb and Burwell (1945). The partial pressures of the three components in the reaction are related to XA; thus,
kPA
(5)
(1 + KAPA)'
By differentiation of ( 5 ) with respect to XA,one observes that
+ + ( 1 - KA)XA)' +
= ( - 2 k [ x ~ ( ( K 1~)
(1 - X A ) ' ] [ (-~KA') + ( 1 - K A ) ' X A ] ) / ( [ ( K+A1 ) + ( 1 - K A ) X A ] (6) ~)
Equating the right-hand side of (6)to zero, one finds that 1-KA KA-1 &)T = 0 at X A , , = -= (7) KA+1 KA+1
(
a t 490 " C
X A
major limitations employed in the present study. The only way to increase the mass of the catalyst was to increase the thickness of the pellets which might bring in significant pore diffusion effects.
(&IT
at 475 " C
WIF
~
iL
0
l
0.1
l
l
0.2
l
l
l
0.3
l
l
0.4 Convorsion X A
l
i
05
l
l
06
l
0.7
Figure 3. Rate of reaction versus conversion with temperature as a parameter.
A second differentiation of (6) may be carried out to obtain the second derivative of r with respect to X A which can then be evaluated at XA,, given by (7); thus, 2 k [ ( l - KA)' - ( 1
[
(1
+ KA)']
+ KA)' - ( 1 - KA)' l + K A
I
Thus, the sign of (d2r/dXA2)Tis the same as that of the numerator of the right-hand side of (€3)) viz., 2k(-4K~), Le., negative ( 1) in order that physically meaningful situations
may prevail with respect to XA,m. The estimates of KA of the present study comply with this condition. It may also be remarked that at least two earlier workers reported the validity of the rate equation of the form shown in (5).
2054 Ind. Eng. Chem. Res., Vol. 27, No. 11, 1988 Table 111. Constants k and K, in Equation 3 temp, “C w 5 k , mol/((a cat) s) 475 490
8.52 6.57
KR 0.027 0.008
Table IV. k,KA, and the Rate Constant of the Surface Reaction in Equation 5 temp, “C 425 450 475 490
W k , mol
A/((g cat) s atm)
1.72 1.70 1.66 1.61
KA,
atm-* 4.41 3.52 2.64 2.11
106k, = k / K A 3.90 4.83 6.29 7.63
Thus, Nondek and Kraus (1975) report that the initial rates of dehydrogenation of several C3-C7 secondary alcohols on various chromia catalysts are expressible by an equation identical in form with that of (5). The same reaction on a brass catalyst was studied by Thaller and Thodos (1960) and was shown to have an identical rate equation. It is useful to point out certain implications of (7) and (9) with respect to the variation of KA with increasing temperature. Thus, if KA decreases as the temperature is increased (as is the case in the present study), (7) dictates that XA,, should decrease with a temperature increase. According to (9), the value of rm should increase at the same time considering that k varies only slightly (Table IV), the overall variation being about 6% decrease. Thus, an anomalous situation is observed in the present study in that the results conform with the latter implication concerning rm but not the former. In other words, both r , and XA,m increase with an increase of temperature. Internal Diffusion Effects. Attempts were made to assess the possible involvement of significant pore diffusion effects in the kinetic experiments of the study. At the outset, it may be pointed out that the effectiveness factors in situations represented by kinetic expressions such as (5) are decided by at least two parameters: (a) the modulus combining the rate constant, effective diffusivity, shape and size of pellet, etc.; (b) the adsorption coefficients such as KA together with the partial pressure levels on the catalyst surface. In fact, in (l), there is bound to be a third factor in the form of an “expansion factor” accounting for the density decrease of the reaction mixture due to the stoichiometry of the reaction. A rigorous approach would involve the solution of a nonlinear differential equation, and it is a two-point boundary-value problem that needs to be solved numerically. A simplified approach, however, was resorted to: in the modulus of Weisz and Hicks according to Satterfield (1971), it was assumed that, under conditions when this modulus is quite small, the porediffusion effects are unimportant and hence the effectiveness factor wouldbe essentially unity. In order to obtain estimates of this modulus, it is necessary to have an estimate of De, the effective diffusivity of A in the reaction mixture. Based on the effective diffusivity data reported for the ethanol-hydrogen system (Satterfield, 1971), measurements of Defor H2-N2 in zinc oxide pellets (Raghunathan, 1977) assumed that D e= 0.1 cm2/s for purposes of the calculation of the modulus. Calculations of the modulus, (Vp/S,,)2[r,,,/(oeC,)], at the various operating conditions showed that this modulus was much less than one except in the case of the runs with W / F = 14.42 g of catalyst/mol of A/h. It may therefore be inferred that the pore diffusion effects are probably significant with respect to the experimental data obtained in these experiments. A correct approach would be to evaluate the effectiveness factors of the catalyst under
Table V. Asymptotic Effectiveness Factors at Various Temperatures: Experiments with W/FA, = 14.42 g of catalyst/mol of A/h temp, XAfrom the “C ?asymptotic X~srptlltaaymptotic soh of (11) 425 1.762 0.465 0.482 450 1.364 0.651 0.620 475 1.222 0.757 0.704 490 1.200 0.771 0.733
these conditions but would involve the solution of the nonlinear differential equation relating the concentration of A at any position inside the catalyst disk and the size of the disk together with the other parameters such as p ~ , KA, etc. It must be remembered that this equation is formulated to include the effects of (1)the peculiarities of the kinetics, i.e., of (5), and (2) decreasing density of the reaction mixture, owing to the stoichiometry of the reaction. No detailed numerical solution was attempted at this stage, but an asymptotic approach to the effectiveness factors was taken, and this leads to the following expression: VaaVmDtotic
--
The results of the evaluation of 7 at the four temperatures indicated are shown in Table V lists the calculated values of XA,ax,,fl/?)wymputic, which could be looked upon as being proportional to the “pore-diffusion effects-corrected” rate of reaction at W / F = 14.42 g of catalyst/mol of A/h. The last column of Table V provides the solution of the cubic equation obtained from (2) together with (4) and (5), i.e., x~~+ bXA2 CXA d = 0 (12)
?%f%.
+
+
where b=
C=
2(1 - KA2) + k ( W / F ) (1- K A ) ~
(1 + K.J2 (1- K A ) ~
d=
k(W/F)
(13)
(1- K A ) ~
Equation 1 2 relates the conversion (XA)to the operating conditions in the CSTR, viz., temperature (which decides k and KA) and WIF. The above equation was solved with the parameters k and KA as given in Table IV together with W / F = 14.42 at the four temperatures shown. The good agreement clearly indicates the validity of the proposed rate equation in explaining the experimental data of the present study. Thus, it may be concluded that eq 5 describes the kinetics of the dehydrogenation of butan-2-01 on the zinc oxide studied. The constants k and KA which are tabulated in Table IV show the following temperature dependencies:
It may be added here that (14)implies an activation energy of about 10.8 kcal/mol for the catalyst studied.
Ind. Eng. Chem. Res., Vol. 27, No. 11, 1988 2055 Dual-Dissimilar-Site Mechanism. It was remarked earlier that the data obtained at 475 and 495 "Ccould be described by (3) as well. That such a form of the rate equation describes the kinetics of this reaction on various zinc oxides has been established by Spolnicki (1964)) Nirdosh (1974),and Raghumthan (1977).An explanation for the validity of both mechanisms a t the two higher temperaturea in the preaent study may roughly be sketched as follows. Under these reaction conditions, the rate of hydrogen evolution due to reaction was higher, and this could possibly m u l t in a surface reduction of the zinc oxide which in turn could be the precursor to the formation of a second type of active site (Zn atom). Further, it is postulated that hydrogen molecules tend to get adsorbed strongly on these active sites. It is then probable that a switch in mechanism from a dual similar site (surface reaction controlling) to a dual dissimilar site (hydrogen desorption controlling) occurs under these conditions. That two different types of active centers are indeed produced on zinc oxide during hydrogen treatment is reported by Kesavulu and Taylor (1960). Nomenclature A = butan-2-01 Cs = concentration of A at the catalyst surface corresponding to p M , = p M / R T , mol/cm3 De = effective diffusivity E = activation energy k = rate constant in (51, mol of alcohol/(s (g of catalyst) atm) k l = rate constant in (3), mol of alcohol/(s (g of catalyst)) k, = k/KA KA, KR, KS = adsorption equilibrium constants of A, R, and S, atm-' K , = equilibrium constant of (3)
pA,pR,p s = partial pressures of A, R, and S, atm r = rate of reaction, mol/(s (g of catalyst))
robs= rate of reaction per unit volume of catalyst, mol/(s (cm3 of catalyst)) Sex= external surface area of catalyst pellet, cm2 T = temperature, "C or K V , = volume of catalyst pellet, cm3 XA= fractional conversion of A Greek Symbols
= effectiveness factor pellet density, g of catalyst/cm3 R, S = methyl ethyl ketone and hydrogen, respectively Registry No. Butan-2-01,78-92-2;butan-2-one, 78-93-3;zinc
TJ
pp =
oxide, 1314-13-2.
Literature Cited Kesavulu, S.; Taylor, H. S. J . Phys. Chem. 1960,64, 1124. Kolb, H. J.; Burwell, R. L. J. Am. Chem. SOC.1945, 67, 1084. Miller, K. J.; Wu, J. L. J. Catal. 1972, 27, 60. Nirdosh, I. Ph.D. Thesis, The University of Birmingham, 1974. Nondek, L.; Kraus, M. J. Catal. 1975,40,40. Raghunathan, T. S. Ph.D. Thesis, The University of Birmingham, 1977. &vi, S. M.Tech. Dieaertation, Indian Institute of Technology, Bombay, 1980. Satterfield, C. N. Mass Transfer in Heterogeneous Catalysis, M.I.T. Press: Cambridge, 1971. Shah, J. M. M.Tech. Dissertation, Indian Institute of Technology, __ Bombay, 1982. SDohicki. J. Ph.D. Thesis. The Universitv of Birmineham. 1964. Stephenson, R. M. Introduction to the Chemical ProcesHZnd&ries; Van Nostrand: Reinhold, NY, 1966; p 269. Thaller, L. H.; Thodos, G . AZChE J. 1960, 6, 369.
Received for review April 24, 1987 Accepted March 31, 1988
