I n d . Eng. Chem. Res. 1991,30, 1437-1443
76SF00098 through the Pittsburgh Energy Technology Center, Pittaburgh, PA.
Literature Cited Chang, 5. G.; Littlejohn, D.; Hu, K. Y. An Intermediate to Sulfuric Acid in Acid Rain Formation. Science 1987,237, 756-758. Hofmeister, H. K.; Van Wazer, J. R. Hydrolysis of Sodium Pyrosulfate. Inorg. Chem. 1962,1,811-812. Littlejohn, D.; Hu, K. Y.; Chang, S. G. The Oxidation of HSOy by Op Ind. Eng. Chem. Res. 1988,27, 1344-1348. Millen, D. J. Vibrational Spectra of Ionic Forms of Oxides and Oxyacids of Nitrogen. Part 11. Raman Spectra of Solutions in Sulfuric Oleum. The Polysulfuric Acids. Ionisation of Nitric Acid in Oleum. J . Chem. SOC.1950, 2589-2600. Plathotnick, V. N.; Drabkina, A. Kh. Kinetia, and mechanism of the substitution of certain acid complexes of sulfur trioxide. Kinet. Katal. 1976, 17, 1356.
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Plathotnick, V. N.; Drabkina, A. Kh. Effect of ionic force on the kinetics of some substitution reactions in aqueous solutions. Kinet. Katal. 1977,1&, 227-231. Ryss, I. G.; Drabkina, A. Kh. Kinetics of alkaline hydrolysis of pyrosulfate ion. Kinet. Katal. 1973,14,242-245. Thilo, E.; von Lampe, F. Beitrage zur Chemie der Alkalidi(=pyro)sulfate. Z. Anorg. Allg. Chem. 1963,319, 387-403. Thilo, E.; von Lampe, F. Uber die katalytische Wirkung von Kationen auf die Hyrolyse von Disulfaten. Ber. Dtsch. Chem. Ges. 1964,97, 1775-1782.
Thilo, E.; von Lampe, F. Die beiden Komponenten der beschleunigenden Wirkung von Kationen auf die Hydrolyse der Disulfain wassrigen Losungen. Z . Anorg. Allg. Chem. tionen (S207%) 1967,349, 1-18.
Received for review October 24, 1990 Revised manuscript received February 20, 1991 Accepted March 12, 1991
Hydrogenation of Acetylene at Transient Conditions in the Presence of Olefins and Carbon Monoxide over Palladium/Alumina Lennart Cider* and Nils-Herman Schoon Department of Chemical Reaction Engineering, Chalmers University of Technology, S-412 96 Cothenburg, Sweden
Hydrogenation of acetylene, ethylene, and propylene at transient conditions caused by cross-desorption of carbon monoxide due to a pulse of acetylene was studied. Mathematical modeling of the transient reaction system showed that a simple model is able to explain the competition for the active sites and the difference in reactivity. A comparison was made with the two-site theory.
Introduction Our knowledge about catalytic hydrogenation of acetylene dates back to 1899, when Sabatier and Senderens (1899) made their first study in the presence of a nickel catalyst. Today, palladium has been the most studied catalyst for this reaction, and the kinetic properties of acetylene hydrogenation in the presence of palladium have been quantitatively studied in numerous works since the first study by Cremer et al. (1941). All reported kinetic studies of acetylene hydrogenation have been performed in a traditional way: either in a closed system or in an open system at stationary conditions. It is a well-known fact that this approach gives only a limited possibility to discriminate between different reaction models, as demonstrated, for example, by Prairie and Bailey (1987) in a study of ethylene hydrogenation. Recently we showed (Cider and SchBdn (1991a)) that a pulse of acetylene was able to rapidly increase the desorption of carbon monoxide from a precious-metal catalyst and thus made it possible for ethylene present in excess to hydrogenate, a phenomenon called cross-desorption. The kinetics of this cross-desorption of carbon monoxide were also studied (Cider and Schoon (1991b)). In a separate in situ FTIR study (Cider et al. (1991)) it was shown that both linear and bridge-bonded carbon monoxide were rapidly cross-desorbed by acetylene. The purpose of the present work is to study the formation and reaction of ethylene and the reaction of propylene at the transient conditions caused by a short pulse of acetylene. Propylene was added to make it possible to discriminate between the hydrogenation of the olefin (propylene) present in excess in the inflow gas and the olefin (ethylene) formed by hydrogenation of acetylene. The inflow also contained a small amount of carbon monoxide in order to depress the olefin hydrogenation. To
understand the dynamic behavior of the reaction system, the kinetics of the different steps have been mathematically modeled and the kinetic parameters have been determined by regression analysis. The study was performed in a well-mixed reactor that behaved like an ideal tank reactor, and the catalyst was an egg-shell palladium catalyst on a-Al2O3 in order to minimize the influence of the pore transport.
Experimental Section Catalyst. The catalyst, 0.044% Pd/a-alumina l/* in. X 4 mm cylinders (the carrier was delivered from Sudchemie AG Miinchen), was prepared by impregnation in 30% HN03(aq) solution of 0.5 w t % Pd(NO& After impregnation for 24 h the catalyst was dried at 120 "C for 12 h, calcined in air at 450 OC for 5 h, reduced in 20% H2/N2mixture at 450 O C for 5 h, and finally left to cool in N2at room temperature. The dispersion, measured with carbon monoxide, was 15%. Gases. Acetylene 99.6%, ethylene 99.5%, propylene 99.4%, argon 99.995% , hydrogen 99.98%, and carbon monoxide (112 ppm/Ar) were all supplied by AGA Gas AB, Sweden. Apparatus. The reactor was a stainless steel cylinder with an inner diameter of 22 mm and a length of 33 mm. A layer of pellets was placed between two layers of glass wool (see Figure 1 in Cider and Sch6Bn (1991a)). A residence time distribution test showed that the reactor behaved like an ideal tank reactor during experimental conditions. Gas flow rates were controlled by mass flow controllers (MFC) from HI-TEC. The chemical analysis was performed by means of a gas chromatograph (Varian 3300) with a flame ionization detector. A Megabore column was used for C2 + C3 sepa-
oess-5ss5/91/2630-1437$02.50/0 Q 1991 American Chemical Society
1438 Ind. Eng. Chem. Res., Vol. 30, No. 7, 1991
time run 1 run 2 Tt t run 3 ?t ?t t t T run I!, t ? t - run 5 rt ?t Figure 1. Sampling technique. The experiment is repeated and the time when the samples are collected (f) is changed from run to run.
T
t
T-
'
ration. Detected peaks were integrated on an integrator (SP4270, Spectra Physics). A multiposition valve was used for fast sampling (Valco). Outflow carbon monoxide content was monitored by means of a CO gas analyzer (Maihak). Experimental Procedure. The experiments were performed at atmospheric pressure and at 30 "C. Argon, carbon monoxide, and propylene or ethylene were continuously fed to the reactor. A pulse of acetylene was added to the inflow in both the presence and absence of hydrogen. For the latter case hydrogen was added to the inflow at a certain time after the acetylene pulse. Use of the Multiposition Valve. The multiposition valve is equipped with 12 loops where samples of the outflow can be collected every second and be stored for several hours. One C2 + C3 analysis takes 10 min and consequently it takes 2 h to analyze all the 12 samples. If more than 12 analyses are needed, it is possible to repeat the experiment and delay the start of the sampling, which is illustrated in Figure 1. This technique is, however, applicable only if the repeatability in the experiments is high. Theory Kinetics of Acetylene Hydrogenation. The initial step of mathematically modeling the kinetics of acetylene hydrogenation consisted of a consultation of the literature in order to find a starting proposal of the model. However, the literature about acetylene hydrogenation in the presence of palladium catalysts gives a rather contradictory picture of the kinetic properties. Most kinetic results are given in a power rate equation form with hydrogen and acetylene pressures as the only variables. Many authors like McGown et al. (1977) and Moses et al. (1984) found that the rate of acetylene hydrogenation is zero order in acetylene and first order in hydrogen. Other authors (e.g., Sheridan (1945) and Bond and Wells (1965)) stated that the reaction order with respect to acetylene is -0.5 or more negative. Addriz et al. (1990) found that the reaction order with respect to acetylene was dependent on the palladium dispersion. At low dispersion the reaction was zero order, whereas at high dispersion the reaction order decreased to -0.5. The negative reaction order in acetylene is said to reflect the strong adsorption of acetylene on the catalyst surface. The reaction order in hydrogen has also been a subject of much debate. Bond and Wells (1965), for example, found that first-order in hydrogen was valid for hydrogenation at 0-30 "C, whereas a reaction order of 1.4 was recorded at 125 "C. Addriz et al. (1990), however, found a reaction order of 1.3-1.6 even at 15 "C and found this reaction order to be independent of the dispersion of palladium. Many authors (e.g., Margitfalvi et al. (1980, 1981)) reported that the rate of hydrogenation of acetylene increases in the presence of even a small excess of ethylene. This
influence has not been taken into consideration by the power rate equations published. Only a few authors have reported their kinetic results in a Langmuir-Hinshelwood rate equation. Among them are Cremer et al. (1941) in an early work from 1941 and Tamaru (1950) in a work from 1950. The reason for the dominating use of the power rate equation may be the fact that the acetylene is strongly adsorbed on the catalyst. On the other hand, a power rate equation cannot describe the possibility that the rate of hydrogenation passes a maximum value, when increasing the acetylene pressure. Margitfalvi et al. (1980,1981) and LeViness et al. (1984) found a maximum value of the hydrogenation rate for an acetylene partial pressure of about 0.1 kPa at a total pressure of 101.3 kPa and in the presence of an excess of hydrogen. The maximum was found in experiments between 0 and 42 "C (LeVinesset al., 1984), and it may be seen from the data that the activation energy is higher after the rate maximum than before. The data, moreover, show a decrease in reaction order in acetylene from about first order before the maximum rate value to reaction order 4 . 5 after this maximum. The fact that the reaction rate has a clear maximum value as a function of the starting acetylene pressure has not resulted in any revised rate equation. It should be noted, in connection with this, that Moyes et al. (1989) recently found that the initial reaction rate when increasing the acetylene partial pressure did not continuously pass a maximum value but instead dropped sharply with a corresponding change of the reaction order in acetylene from a positive to a slightly negative value. The activation energy was also shown to be quite different, parallel to this sharp change of the rate of reaction. The phenomenon was supposed by the authors to be interpretable on the basis of two different modes of packing the adsorbed acetylene molecules on the catalyst surface. This conclusion allows us to establish that the adsorption of acetylene is known to give rise to different adsorbed species, but little is known about the hydrogention kinetics of these species. Moreover, acetylene has also been found to hydrogenate both on metallic active sites and on a hydrocarbon overlayer (Thomson and Webb (1976) and Berndt et al. (1983)), and very little is known about the possible difference in kinetics between these two reaction routes. The possibility that the hydrogenation of olefins can occur on metallic sites of different properties has been made use of in the mathematical modeling of, for example, the hydrogenation of propylene and isobutylene (Rogers et al. (1966), Lih (1970), Mezaki (1968), Kolboe (1972)), where the active sites were assumed to differ in their capacity of mutually adsorbing the olefin and hydrogen. It is also worth mentioning that Hussey et al. (1968), in mathematically modeling the hydrogenation of cycloalkenes, proposed two types of active sites that differ in their abfity to adsorb the cycloalkenea reversibly. No such difference between active metallic sites has been discussed in connection with the effort to understand the kinetics of acetylene hydrogenation. The possibility that different active sites are operating in hydrogenation of acetylene and in hydrogenation of ethylene has, on the contrary, been an important way to explain the selectivity properties of acetylene hydrogenation (McGown et al. (1977, 1978), Berndt et al. (1983), Weiss et al. (1984)), but this is quite another problem than to assume different active sites in, specifically, hydrogenation of acetylene. Mechanism of Acetylene Hydrogenation. If we disregard the reports that acetylene can hydrogenate both on metallic sites and on overlayer sites and that acetylene,
Ind. Eng. Chem. Res., Vol. 30, No. 7, 1991 1439 moreover, can be adsorbed giving different adsorbed species, very little has been published about the reaction mechanism of acetylene hydrogenation compared with great debate in the literature about the mechanism of ethylene hydrogenation. The mechanism hitherto discussed for the hydrogenation of acetylene is based on the classic Horiuti-Polanyi mechanism for ethylene hydrogenation (Horiuti and Polanyi (1934)) and states that adsorbed acetylene reacts stepwise with hydrogen via the formation of an adsorbed vinyl radical. The reversible formation of this half-hydrogenated intermediate follows from a study of the distribution of deuterium in deuterated ethylene formed in hydrogenation of acetylene with Dz (Bond and Wells (1966)). A simplified reaction sequence can thus be written: Hz + 2* 2H* (1) C2H2 + 2* e CzH2** (2) C2H2** + H* e C2H3** + * (3) CzH3** + H* C2H4* + 2* (4) C2H4* e CzH4 + * (5) Adsorbed H2 is regarded as a possible reactant (Prairie and Bailey (1987), Saltsburg and Mullins (19821, Westerterp et al. (1985, 1986)) in many discussions of the reaction mechanism of ethylene hydrogenation, and this has also been considered in the reaction mechanism of acetylene hydrogenation (Bond and Wells (1965)). In the present simplified mechanism the reaction is supposed to include only adsorbed hydrogen atoms. Since a test with D2added during the hydrogenation gave a rapid formation of HD in the present study, the adsorption of hydrogen is obviously reversible. The adsorption of acetylene is, moreover, assumed to be strongly displaced toward the adsorbed state and the ethylene formed is supposed to be very easily desorbed. The analysis of the mechanism must start from the experimental observations that the reaction order in hydrogen varies from first-order to about the order 1.5 and that the reaction order in acetylene most often is reported to be zero order but also values from -0.5 up to about first order are reported. The differences found in reaction orders may be explained by the quite different mixing conditions prevailing in the experiments reported in the literature. The commonly observed combination of first order in hydrogen and zero order in acetylene may be explained by the possibility that the adsorption of hydrogen is the rate-determining step. It is common practice in kinetic analysis of a sequence of consecutive reactions to simplify the analysis by assuming one step to be rate-determining. Two alternative reactions have to be considered as rate-determining. In the first, which is assumed to be valid at low acetylene pressure, the formation of C2H3** is rate-determining and the steady-state approximation of the net formation of this species is valid. In the second alternative, which is assumed to be valid for high acetylene pressure, reaction 4 is assumed to be rate-determining. The steady-state approximation with respect to the net formation of CzH3** is not applicable here, and instead reaction 3 is assumed to proceed to equilibrium. In both these approximations the rate of reaction is proportional to 8,. If we assume that hydrogen adsorption is rapid enough to proceed to equilibrium, the fractional coverage 8H is, however, approximately proportional to pH:I2, which is not in accord with the experimental observation of a first-order dependence on hydrogen pressure. The hydrogenation should therefore proceed mainly according to another mechanism. At low
-
acetylene pressure and in an excess of hydrogen, we assume that in this mechanism adsorbed acetylene reacts readily with the pair of hydrogen atoms formed at adsorption. At the present reaction conditions, the rate equation will thus be
r = k228228H2
(6)
The symbols introduced are given in the Nomenclature section. In the present study eq 6 will be the basis for simulating the rate of acetylene hydrogenation. Corresponding rate equations with OZ2 replaced by 824 and 8, will be used as a basis for simulating the hydrogenation of ethylene and propylene, respectively. It should be noted that the rate equations published for hydrogenation of these olefins are like those for acetylene hydrogenation, anything but unambiguous. The Mathematical Model Used in Simulation. The model consists of four main groups of equations. The first group expresses the differential mole fraction balances for the seven components over the well-mixed reactor: dyz/dt =
(e2- 9
2
- Fzz)/N
(7)
dy24/dt =
(fl4
-9
4
- F24)/N
(8)
dy26/dt = dy36/dt =
( f l 6 - F26) / N
(96
dY3,/dt =
-e 6 - F36) /N
(a- F,)/N
(9) (10) (11)
(Qo +
- e o - Fco)/N (13) The next group consists of two differential balances for the fractional coverages of acetylene and carbon monoxide on the surface of the catalyst: dyco/dt
d822/dt = (02 - wsnZ2)/ws
(14)
dkO/dt = ( e o - l%o)/ws
(15)
where is the turnover frequency, w the mass of catalyst, and s the molar adsorption capacity of the catalyst. In the third group, the rate of cross-desorption of carbon monoxide in the absence of hydrogen is given according to Cider and Schoon (1991b) as
l%o = wska@P22"8co@
(16)
where CY = 0.56,B = 2.0, and k,, = 2.07 X s-l Pa4-". In formulating the rate equations for adsorption and chemical reactions, we have to consider that the fractional coverage of carbon monoxide is 8co = 1 before the acetylene pulse starts. Due to the special nature of the crossdesorption process, we introduce the hypothesis that the acetylene action includes an adsorption both on the carbon monoxide layer and on the vacant sites that are not accessible to carbon monoxide for geometrical reasons. This assumption is based on the fact that adsorption-assisted desorption of carbon monoxide, which means cross-desorption of ("C)carbon monoxide with carbon monoxide, was found (Schraer and SchaGn (1991)) to be more rapid than cross-desorption of (%)carbon monoxide with acetylene. In the first case carbon monoxide has to be adsorbed on a catalyst surface that is precovered with ("C)carbon monoxide at a fractional coverage close to one. It seems probable that carbon monoxide in the gas phase can interact with the adsorbed layer of (W)carbon monoxide to some extent in a process that can be described
1440 Ind. Eng. Chem. Res., Vol. 30, No. 7, 1991
as an adsorption on the (W)carbon monoxide layer. This ~ attains values greater than means that the sum 1 9 +~ Bco 1 during a main part of the cross-desorption of carbon monoxide. The fraction of vacant sites in adsorption of acetylene is thus (1 - 022), and the rate of adsorption of acetylene will be 4 2
= wskA(1 - 0
2 2 ) ~ ~ ~
(17)
For adsorption of carbon monoxide, which does not give rise to any cross-desorption of acetylene at the low partial pressure of carbon monoxide, the fraction of vacant sites is instead (1 - 8%- dc0) as long as the constraint On + dc0 C 1 is fulfilled. For the rate of adsorption of carbon monoxide, we then have
60
wskc(1 - e22 - ~ C O ) P C O
(18)
If the constraint is violated the rate of adsorption is Pc0 = 0.
In the remaining fourth group of the mathematical model we have to consider the rate of the hydrogenation reactions. The rate of acetylene hydrogenation is given by eq 6, and in the model we denote the corresponding turnover frequency with nZ2as seen from eq 14. The fractional coverage 822 is calculated from eqs 14 and 22 since the adsorption equilibrium is strongly displaced toward the adsorbed state. Ethylene, propylene, and hydrogen are more loosely bound to the surface, and at adsorption equilibrium, which is assumed to be reached at every moment, we have (19) e24 = K24(1 - e22 - eC0)P24
"0
40
80
0
60
120
180
240 time/s
Figure 2. Outflow content of CO from the reactor with the inflow: 19 ppm CO, 3% C2H4,and 15-spulses of 3% CzH2. Catalyst 0.63 g ofPd/a-AlzOa, temperature 30 O C , pressure 1 bar, and flow rate 290 ctmol/s: (A) C2H2pulse in the presence of 12% H,; (B)C2H2pulse in the absence of H,; (C)simulation of experiment A (D)simulation of experiment B.
tative understanding of how this cross-desorption can control the hydrogenation reactions, the influence of hydrogen on this cross-desorption was first studied. Cross-Desorption of Carbon Monoxide in the Presence of Hydrogen. The result is given in Figure 2, which consists of four parts. Parts A and B give the experimental result in the presence of hydrogen (Figure 2A) (20) and in the absence of hydrogen (Figure 2B), whereas parts 038 = KM(1 - 022 - eC0)PW C and D give the corresponding calculated carbon mon= 0, and when 022 + Oc0 < 1, otherwise Bzr = oxide content. The inflow had the same composition referring to the content of carbon monoxide and ethylene OH = (1 - ae2z)(K&#/2/(1 + (kHpH,)"2) (21) in the two experiments, and the same pulse of acetylene for the fractional coverages of ethylene, propylene, and was added. The only difference was that the inflow conhydrogen, respectively. For the adsorption of hydrogen, tained 12% hydrogen in experiment A. From parts A and we assume that acetylene and hydrogen compete for the B it is seen that desorbed carbon monoxide relative to the active sites up to a certain value of the fractional coverage inflow level is the same in the two experiments, which of acetylene, and that above this value acetylene cannot means that eq 16 is also valid in the presence of hydrogen. compete. At moderate hydrogen pressure (KH~H,)'/'
