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Znd. E n g . Chem. Res. 1995,34,94-100
Liquid-Phase Hydrogenation of Acetylene in a Monolithic Catalyst Reactor Rolf K. Edvinsson, Anna M. Holmgren, and Said Irandoust* Department of Chemical Reaction Engineering, Chalmers University of Technology, S-412 96 Giiteborg, Sweden
The selective, liquid-phase hydrogenation of acetylene in the presence of excess ethylene has been studied. A new process using a monolith reactor is presented. The liquid, heptane, was used to continuously remove the green oil formed by polymerization. The liquid also absorbs most of the heat formed i n the exothermic hydrogenation reactions. The total pressure was 1.3-2.0 MPa and the temperature was 303-313 K. Several Pd catalysts supported on alumina have been tested. The best selectivity was obtained using a n a-alumina support with a n average pore diameter of 0.08 pm. This reaction is known to be structure-sensitive in the gas phase and the same was found for the liquid phase. The best catalyst was tested for a total of 113 h. The activity in the acetylene hydrogenation reaction and the selectivity toward ethylene dropped during the first 60 h of reaction (turnover number from 3 to 1 s-l and selectivity from 80 to 55%). After t h a t period, both remained approximately constant.
Introduction Ethylene is produced on a large scale by steam cracking of naphtha or some other light hydrocarbons. In this process, small amounts of acetylene are formed as a byproduct. Since even small amounts of acetylene deactivate the polymerization catalyst used in the subsequent manufacture of polyethylene, the acetylene concentration has to be reduced to a very low level (a few ppm). Acetylene can be removed by absorption, but the preferred method is t o selectively hydrogenate acetylene to ethylene, thus increasing the yield of the desired product. This step can be located directly after the cracker unit (front-end)or after the first separation step (tail-end). The reactive removal is conventionally done in a gas-phase process using a Pd catalyst supported on a-A1203.Small amounts of CO are added t o suppress the hydrogenation of ethylene to ethane, thereby increasing the selectivity. This is especially important a t low C2Hz levels. The role of CO in this reaction has been investigated by, among others, McGown et al. (1977), Al-Ammar and Webb (19791, LeViness et al. (1984), Sarkany et al. (19841, Weiss et al. (19841, Cider and Schoon (1991), and Park and Price (1991). One important, but undesirable, side reaction is the hydropolymerization of acetylene into oligomeric and polymeric substances, called green oil. It influences both selectivity and conversion of the reaction (Moses et al., 1984; Sarkany et al., 1984; Derrien, 1986; Lee, 1990; Wehrli et al., 1991). As large amounts are formed, catalyst fouling and plugging of pipes become a problem, making it necessary to shut down the reactor in order to regenerate the catalyst. It has been suggested that a certain type of adsorbed acetylene is the precursor of this polymerization (Al-Ammar and Webb, 1978; Berndt et al., 1983; Wehrli et al., 1991). Process conditions, such as H2:CzHz ratio and temperature, influence the rate of green oil formation (Bos and Westerterp, 1993). It is known that, due to the deposition of green oil, the catalyst deactivation is affected by the surface acidity of the support (Wehrli et al., 1991). Mean pore size, pore-size distribution, and the metallic dispersion also affect green oil formation (den Hartog et al., 1990).
* Author t o whom correspondence should be addressed.
Moreover, green oil has been reported to lower the ethylene selectivity (Moses et al., 1984; Sarkany et al., 1984). In the gas-phase process, temperature control poses some problems. The highly exothermic hydrogenation reactions can cause a large adiabatic temperature rise. With high levels of hydrogen and ethylene, there is even a risk of a thermal runaway. There is currently a trend toward using cracker units with short residence time. The product streams from these are richer in acetylene (Derrien, 19861, making them more difficult to process by the conventional gasphase method. Institut FranGais du Petrole (IFP) has developed a process in which acetylene is hydrogenated in the presence of an inert liquid phase (Derrien, 1986; Cosyns and Boitiaux, 1989). A trickle-bed reactor (TBR) is used. The liquid phase serves several purposes: one is to wash the catalyst, thus removing the soluble oligomers; another is to absorb the heat generated by reaction. This reduces the risk of a thermal runaway. A low temperature also improves the stability of the catalyst. The acetylene content can be brought down to low levels without any CO addition. However, in order to attain the very low acetylene levels required for polymerization-grade ethylene, a final step, using the conventional gas-phase process, is needed. A special type of fured-bed reactor that can be used for gasAiquidIsolid reactions is the monolith reactor (MR). Applications of monoliths in heterogeneous catalytic processes have been reviewed by Irandoust and Andersson (1988) and Cybulski and Moulijn (1994). A monolith support consists of a large number of narrow, parallel channels separated by thin walls (Figure 1).The walls may contain the catalytically active material, but more frequently a washcoat consisting of a thin layer of a porous oxide is deposited onto the channel wall. Owing to its porosity it has a large surface area on which the catalytically active material, in this case Pd, is fixed. Currently, this type of reactor is mainly used for gas-phase reactions, such as the cleaning of automotive exhaust gases and industrial off-gases. In these applications, where large volumetric flows must be handled, it is essential to have as low pressure drops as possible. Compared to conventional packed-bed reactors, pressure drops in MRs may be up. to several orders of magnitude lower (Satterfield and Ozel, 1977).
0888-5885/95/2634-0094$09.00/00 1995 American Chemical Society
Ind. Eng. Chem. Res., Vol. 34, No. 1, 1995 95
Figure 1. Monolithic support.
MRs also have properties that make them interesting for multi-phase reactions. The low pressure drop is an advantage compared to the TBR. In the latter, pressure drop considerations limit the size of the catalyst particles that can be used. Since diffusional distances in the washcoat are much shorter than in the particles used in the TBR, the internal transport resistance will be lower in the MR. This can affect conversion and selectivity positively. The MR can be operated in the Taylor flow regime in which the gas and the liquid form a sequence of separate plugs, each gas bubble being surrounded by a thin liquid film. The presence of gas bubbles induces a recirculation in the liquid plug which increases the mass transfer, and the liquid film around the gas bubble provides a large gas-liquid contact area. The scaling-up of MRs is easier than that of TBRs since the individual channels remain unchanged on scale-up. The objective of this investigation is to study experimentally the selective hydrogenation of acetylene in an ethylene-rich stream. A number of monolithic catalysts have been prepared using different support oxides and Pd precursors. The dependence of selectivity and activity on several factors has been studied. Asplund et al., 1994, investigated the use of the monolithic catalyst for the gas-phase hydrogenation of acetylene and made a comparison with the liquid-phase process.
Experimental Section Monolithic Support. All the monoliths used in this study have channels with square cross sections. The properties of the monoliths are summarized in Table 1. A monolithic catalyst typically had a diameter of 22 mm and a length of 40 mm. The a-alumina monolith was obtained by sintering a y-alumina substrate at 1473 K for 1 h. This y-alumina monolith as well as the one washcoated with y-alumina and the cordierite supports were obtained from Corning Glass Works. A series of monoliths with a-alumina washcoats was prepared by dipping a cordierite support into a slurry containing powdered a-alumina and a binder in an aqueous acidic solution. Catalyst Preparation. In all cases, Pd was used as the active phase. The catalysts are described in Table 1. The Pd dispersion was controlled by the method of preparation. Two different Pd precursors have been used: chloride and nitrate. Pd was deposited by impregnation. An adsorptive method using acetylacetonate as Pd precursor was also tested. This method resulted in a Pd dispersion of 20% on a-alumina. All catalysts, but one, were dried at room temperature for
16 h (see Table 1). Finally the catalysts were calcined in an air flow for 1h and then reduced in hydrogen for 2 h. Both steps were carried out at 573 K. Catalyst Characterization. The total surface area (BET)was determined by Nz adsorption. The mercurypenetration method was used to measure the pore-size distributions. The Pd content of the catalysts was measured by atom absorption spectroscopy. The dispersion of Pd was measured by CO chemisorption. For washcoated monoliths, the distribution of the washcoat was checked by scanning electron microscopy. An example of such a micrograph is given in Figure 2a. The location of the Pd deposited on a washcoated monolith was checked by electron-probe X-ray spectroscopy and it was found to be evenly distributed within the washcoat (Figure 2b). A temperature-programmed desorption (TPD) study of a used catalyst was performed in order to characterize the surface species deposited during the reaction. In this experiment, a catalyst sample of 0.41 g was heated from room temperature to 1113 K at a rate of 30 Wmin. The flow rate of inert gas (helium) was 60 nmUmin. The WC ratio of the carbonaceous residues on the catalyst surface was calculated by a temperatureprogrammed oxidation (TPO) of the spent catalyst. The catalyst sample (0.10 g) was heated from room temperature to 1100 K at a rate of 10 Wmin. The gas flow rate was 60 nml/min (1.7 vol % oxygen in nitrogen). The spent catalyst was also analyzed using a temperature-programmed hydrogenation (TPH) where 0.02 g of the catalyst sample was heated from room temperature to 620 K at a rate of 4 Wmin. The flow rate of hydrogen was 10 nmUmin. Physical Properties. The physical properties of the liquid were estimated on the basis of n-heptane and are summarized in Table 2. Density, viscosity, and the vapor pressure were taken from Gallant and Yaws (1992). The diffusivities were calculated from data of Reid et al. (1987). The solubility of hydrogen and acetylene were estimated from data given by Reid et al. (1987) and Miyano and Hayduk (19811, respectively. Experimental Setup. The experimental setup is shown in Figure 3. The reactor consists of a stainlesssteel tube (length 100 cm, inner diameter 2.54 cm). The monolithic catalyst was placed near the top of the tube. The reactor was operated in batch mode. Due to problems in operation of the mass flow controllers for acetylene/ethylene flows at a pressure exceeding 1MPa, the setup was divided into two sections. The catalyst was placed in the high-pressure section (1.3-2.0 MPa) and the gasous reactants were mixed in the lowpressure section (%0.3MPa). The latter section includes a 10 dm3 accumulation vessel in order to increase the capacity. The gas was recirculated by a double-acting, reciprocating compressor. A pressure controller was used to keep the pressure constant in the high-pressure section. As the reaction proceeded, the pressure in the low-pressure section decreased slowly. A displacement pump was used for the liquid recirculation. This automatically gave a pulsation of the flow. The liquid entered the top of the reactor tube through a liquid distributor. Temperature was controlled in three places: two on the gas and liquid inlet lines, and one on the reactor wall. Experimental Procedure. The reactor was operated in batch mode. The volume of the liquid used in a run was 0.3 dm3. The weight of the monoliths used was 4-7 g. The gas phase was fed to the system and then premixed by recirculation in the external loop for 1h. The two feed compositions used were 3%acetylene, 28%
96 Ind. Eng. Chem. Res., Vol. 34, No. 1, 1995 Table 1. Monoliths Tested in the Hydrogenations a-Al203
BET (m2/g) pore volume (cm3/g) average pore diameter him) cell density (cells/cm2) washcoat thickness (pm) Pd load (wt CTC) Pd precursor Pd dispersion (92) selectivity (%) TON" (s-l)
8
0.20 0.08 110 0.04 PdN03 PdCl2 PdC12' 8 14 40 71 78 71 4.8 4.2 2.2
washcoat 6-9 0.14-0.22 0.14 56 20- 100 0.04 PdC12 14 50b 4
y-Al203 washcoat
a-Al203
40 0.29 0.01 80
30 0.01 PdN03 7 very low 5.5
Ethylene selectivity decreased very rapidly. a At 80% conversion, fresh catalyst, reaction conditions: 303 K, 2 MPa, Hz:C2Hz = 3.7:l. Drying temperature: 120 "C.
Figure 2. Washcoated monoliths: ( a , left) washcoat distribution on the monolithic wall; tb. right) Pd distribution within thc washcoat.
(S) and turnover number (TON). The selectivity is defined as the net number of moles of ethylene produced
Table 2. Physical Properties of the Solvent
10-3~ea
303 313 a
677 670
3.68 3.31
19 18.3
7.58 20.83 1.18h 18.4 12.41 19.48 1.30h 21.1
per net moles ethylene and ethane formed: 6.99 8.02
Adjusted for temperature and pressure. In n-hexane.
ethylene, 6 or 11% H2, and N2 to balance. The acetylene content was chosen somewhat higher than that commonly reported because the new, faster cracking methods produce more acetylene. The reactions were carried out at a total pressure of 1.3 or 2.0 MPa and at a temperature of 303 or 313 K. The gas and liquid flow rates were 2.0 dm3/min at NTP (273 K and 1atm) and 0.20 dm3/min, respectively. ChemicalAnalysis. The gas-phase composition was followed by gas chromatography (packed column, Carbosphere 80/100, Alltech). The concentrations of acetylene, ethylene and ethane were determined. Some liquid samples were analyzed in a GC-MS system in order to determine the content of higher hydrocarbons considered to be green oil precursors. The formation of higher hydrocarbons, mainly C4's, was followed in a few runs by using a GC equipped with a capillary column (Chrompack, PLOT Fused Silica, Al203/KCl).
Results and Discussion The outline is as follows: first the experiments leading to the selection of a proper catalyst are reported; then the testing of this catalyst, including a test over a longer time (113 h), is described. Catalyst Selection. A number of experiments were performed in order to select an appropriate support material and metallic dispersion. The catalysts were rated on the basis of two criteria: ethylene selectivity
S=
AnC,H" AnC2H,
4
+ AnC2H,
This definition is only appropriate if the rate of formation of C4's and higher hydrocarbons is low compared to the main hydrogenation reactions. As will be seen below, this is the case here. The TON is defined as the number of acetylene molecules converted per exposed Pd atom per second. In order to select a proper monolithic support, a series of runs was carried out using different support materials. These are summarized in Table 1. All the catalysts were prepared with a Pd load of 0.04 wt%. The catalysts used to compare the different supports had a metallic dispersion of approximately 14%. The a-alumina with an average pore diameter of 0.08 pm showed the highest selectivity, while the y-alumina showed the lowest selectivity. The difference is attributed to the pore-size distributions, which are shown for the a- and y-aluminas in Figure 4. Moreover, there is a chemical difference between these two materials. The surface acidity of the support was reported to increase the deactivation rate of the catalyst (Wehrli et al., 1991). The a-alumina support was therefore selected for further investigation. For the monoliths washcoated with a-alumina, the selectivity toward ethylene dropped very fast within a few experiments. The poorer performance of these monoliths will be discussed below. Metallic dispersion can alter the electronic properties of the metal, resulting in a change of the adsorption mode and the chemisorption strength of the reactive species (Boitiaux et al., 1983; Gigola et al., 1986; Sarkany et al., 1986; Guerrero et al., 1989). The
Ind. Eng. Chem. Res., Vol. 34, No. 1, 1995 97 To Vent
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Pore diameter, d / pm Pore diameter, d / pm Figure 4. Pore-size distribution of the monolithic supports used: (a>a-alumina; (b) y-alumina (in case b, the pores are too small t o be properly measured by mercury penetration).
influence of the metallic dispersion on S and TON was studied by preparing and testing a series of catalysts with three dispersions: 8, 14 and 40%, supported on a-alumina. The results given in Table 1indicate that the selectivity is not considerably affected while the TON decreases with increasing dispersion in this range. The decrease in TON is in accordance with findings by Aduriz et al. (1990) and Gigola et al. (1986). The latter suggested that there is a strong interaction between the unsaturated hydrocarbon and the small, electrondeficient Pd particles. They also reported a drop in
selectivity with increasing dispersion, which is contrary to the findings of Aduriz et al. (1990) and Sarkany et al. (1986). Particle-size dependent deposition of carbonaceous layers on the metal surface (den Hartog et al., 1990)and local heat transfer effects in small crystallites altering the thermodynamic factors (Gigola et al., 1986) are among those factors which may explain the influence of metallic dispersion on the selectivity reported in the literature. Catalyst Performance. In the second part of the study, the most promising catalyst (0.04 wt % Pd with
Ind. Eng. Chem. Res., Vol. 34, No. 1, 1995
l , j
25/ 5
OO
0
75
50
25
100
Time I min Figure 5. Liquid-phase hydrogenation of acetylene in a monolithic catalyst reactor. Working pressure: 2 m a ; reactor temperature: 303 K. Initial gas mixture, molar composition: 3%CZHZ, 28% CzH4,11% Hz, balance N2. 100 1
I
20 t
0
0
20
40
60
80
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Conversion ( % ) Figure 6. Selectivity to ethylene as a function of the acetylene conversion. Working pressure: 2 MPa; reactor temperature: 313 K. Initial gas mixture, molar composition: 3% CZHZ,28% CZ&, 6% H2, balance Nz.
14% metallic dispersion on an a-alumina support) was used. The gas-phase concentrations of the three C2's vs time for one of the hydrogenations are illustrated in Figure 5. Here, hydrogen is in large excess. Since the basic reaction scheme is a consecutive one, it is possible, at least initially, to have a net production of ethylene. With less hydrogen present, it is possible to have a net gain over the whole concentration range. Similar behavior for the effect of hydrogen have been reported in the gas-phase process (Battiston et al., 1982, and Moses et al., 1984). The gas-phase content of C i s was analyzed in two runs and was found to be small (total content always less than 300 ppm). The major C4compounds detected were cis-2-butene, l-butene, and n-butane. The dependence of selectivity on acetylene conversion is illustrated in Figure 6. The selectivity drops more rapidly initially. A similar decrease in selectivity with conversion is observed for the gas-phase process too; see e.g. Adiiriz et al. (1990) and Leviness et al. (1984). The degree of diffusion limitation in the solid catalyst was estimated by the method of Weisz. The Weisz modulus (@ = rv L2/D,cj) was calculated to be less than 0.8 for acetylene and below 1.6 for hydrogen over the whole conversion range. The average value of the effective diffusivity used in the Weisz modulus was estimated to be 10% of the corresponding bulk diffusivity in the solvent. However, the value of the effective diffusivity is likely to decrease in the course of reaction due to the deposition in the pores and dissolution in the
JIIY
I , 100
50 Time on stream I h
" I
4 4
'
I
'
50 '
100
Time on stream / h Figure 7. Influence of aging on the (a) turnover number, (b) ethylene selectivity. Working pressure: 2 MPa; reactor temperature: 313 K. Initial gas mixture, molar composition: 3% CzHz, 28% C&, 6% Hz,balance Nz.
solvent of higher hydrocarbons. Hence, these values of the Weisz modulus indicate that the mass transfer limitation of hydrogen is more pronounced than that of acetylene. Generally, mass transfer limitation for hydrogen alone will increase the selectivity toward ethylene and decrease the activity of the acetylene hydrogenation. The opposite is expected when mass transfer of acetylene limits the process. Hence, the higher ethylene selectivity observed for monoliths of pure a-alumina compared to that of a-alumina washcoated monoliths can be explained on the basis of mass transfer limitation of hydrogen. In the case of y-alumina monoliths, one possible consequence of the presence of small pores is that mass transfer of acetylene also affects the process. This will in turn lower the selectivity toward ethylene. Also, for small catalyst pores, removal of the dissolved polymeric compounds is slower. This may explain the observed differences in the solvent color, green in the case of y-alumina monoliths and yellow in the case of a-alumina monoliths. An estimation similar t o that of Cybulski et al. (1993) indicated that external mass transfer limitations were absent. Influence of CO. Carbon monoxide is used in the gas-phase process to improve selectivity. The influence of CO in the liquid-phase process was tested in some preliminary runs. With CO levels of 400 or 1200 ppm in the initial gas mixture, it was found that the rate of acetylene hydrogenation decreased considerably (about 50% decrease at the low CO level and a complete stop a t the high CO level). The selectivity toward ethylene was not significantly affected. The latter result is in contrast to what is found for the gas-phase process, where the acetylene concentration is much lower. At high levels of acetylene, the influence of CO on the selectivity is expected t o be small. The reduction of the hydrogenation rate of acetylene might be attributed to blocking of some of the hydrogen adsorption sites by CO. In the case of the ethylene selectivity, however, the poisoning of the ethylene adsorption sites will not have any marked effect on the hydrogenation rate of ethylene a t high fractional coverages of acetylene. Catalyst Aging. In order to test the deactivation behavior, a catalyst was used for a total of 113 h. The catalyst was the previously selected one with Pd (14% dispersion) on an a-AlzO3 support. The same catalyst was used in a series of hydrogenation experiments. This study was not carried out as a single run. At a few times the catalyst was exposed to air in order to remove small samples of the catalyst for TPD or TPO studies. M e r each exposure to air, the catalyst was treated with H2 at 523 K for 1.5 h before the next experiment was started. The reaction temperature was 313 K and the initial H2:CzHz ratio was 2:l. Since the reactor was operated in batch mode, the selectivity and TON plotted vs accumulated time on stream in Figure 7 are the
Ind. Eng. Chem. Res., Vol. 34, No. 1, 1995 99
MS signal
MS signal
400
600
800
1000
400
Temperature / K
600
800
1000
Temperature / K
Figure 8. Characterization of the spent catalyst: (a) TPD analysis; (b) TPO analysis.
average values over the actual conversion range (80%). The TON and the ethylene selectivity decrease during the first 60 h and then they level out. The times a t which the H2 treatments (HYD)were done are indicated in Figure 7. The TPH analysis of the spent catalyst showed that no surface adsorbate desorbed at temperatures below 620 K. This explains why the H2 treatments were not found to change the deactivation behavior. The change in the catalyst performance might be ascribed to the formation of carbonaceous residues on the metal sites. These deposits may originate from the multiply bonded species, which require ensemble sites (van Broekhoven et al., 1985).Also, according to Moses et al. (1984) and Guczi et al. (1985)the formation of an ethane precursor, ethylidyne, requires adjacent metal sites. LeViness et al. (1984) attributed the formation of polymer species to the dissociative adsorption of acetylene on the multiple Pd sites. It is therefore reasonable to expect that affer a certain time when the ensemble sites are no longer available, the catalyst behavior stabilizes. Further, one may assume that the formation of green oil is faster than its removal by the solvent during the initial period. This will result in a decrease of the TON and ethylene selectivity. However, when the reaction rate has dropped, the rates of green oil formation and of its removal by the solvent become equal. At this point, the TON and ethylene selectivity will be stable. The used catalyst was also analyzed using TPD and TPO; see Figure 8. In the TPD analysis, some components desorbed in the range 450-700 K, though most of the species desorb in the temperature range 700850 K. We identified benzene, methyl benzene, carbon dioxide, ethene (formed by fragmentation of higher hydrocarbons), aromatics, and polyaromatics. The TPO analysis resulted in two distinct peaks, the first one around 570 K and the second around 760 K. These are commonly interpreted as carbonaceous species on the metallic and the support portion of the catalyst, respectively. The WC ratio differs markedly between the two peaks. This ratio was approximately 1.2 in the first and as low as 0.2 in the second. It is generally believed that the surface polymers are formed on the Pd sites (LeViness et al., 1984). These compounds migrate to the nearby support, where some of them will be dissolved into the solvent while some highly unsaturated ones will be adsorbed. Some of the used solvent, heptane, was analyzed using a GC-MS system in order to determine the nature of the species washed off the catalyst. There was no single component present in large concentration, but many components at low concentrations were detected. The dominating compounds were mono- and diunsaturated CS’S and CIO’S;some of them were branched.
Small amounts of unsaturated C12’s were also detected. Also some alkylbenzenes were detected in the liquid phase. LeViness et al. (1984) studied the aging of the catalyst used in the gas-phase hydrogenation of acetylene. They found that the selectivity toward ethylene decreased significantly with time (120 h). Sarkdny et al. (1984) also reported a loss of selectivity during the first 4060 h. The acetylene conversion was only slightly affected. Lee (1990) studied a titania-supported Pd catalyst and observed a notable decrease in selectivity. Weiss et al. (1984) studied aging with a y-alumina supported Pd catalyst, some of which also contained Cu. Without CO, the selectivity dropped with time (100 h), while conversion remained almost constant. With CO, the rate of acetylene hydrogenation decreased with time (200 h).
Conclusions This study represents the first step in the development of a monolithic catalyst for the liquid-phase hydrogenation of acetylene. Several catalysts, unpromoted Pd on various alumina supports, have been prepared and tested. The best catalyst tested had a metallic dispersion of 14% and was supported on pure a-alumina. The higher selectivity found is attributed to the relatively large pores and mass transfer effects. Metallic dispersion was not found to be important in this respect. In the aging test, the selectivity and activity were measured during a period of 113 h. Activity and selectivity were found to drop during the first 60 h, then both leveled out. It was also found that the liquid is capable of removing green oil from the catalyst, giving the liquid a yellowish color. The presence of unsaturated Cis, C~o)s,and C12’s in the liquid was also verified by a GC-MS analysis. The use of a monolith reactor for this type of process exhibits several attractive features such as low pressure drop, good mass transfer properties, short diffusion distances, and the relative ease of scale-up. Acknowledgment Financial support from the Swedish National Board for Industrial and Technical Development is gratefully acknowledged. The monolithic supports were kindly supplied by Corning Glass Works. The a-alumina powder was supplied by Condea Chemie and is gratefully acknowledged.The authors also wish to thank M. Sc. Martin Wannholt and M. Sc. Ann Jezak for setting up the experimental equipment and performing some preliminary experiments.
100 Ind. Eng. Chem. Res., Vol. 34, No. 1, 1995
Nomenclature c, = bulk concentration of component j , mol/m3 D(C2Hz) = bulk diffusivity of acetylene, m2/s D(H2) = bulk diffisivity of hydrogen, m2/s De = effective diffusivity, m2/s He(CZH2) = Henry's law constant for CzH2, m3 Pa/mol He(H2) = Henry's law constant for Hz,m3 Pdmol L = wall half-width, m MR = monolith reactor Anj = net number of moles of component j produced, mol p * = vapor pressure of solvent, Pa rv = reaction rate, mol/(s m3 of cat.) S = ethylene selectivity T = absolute temperature, K TBR = trickle-bed reactor TON = turnover number, l/s TPD = temperature-programmed desorption TPH = temperature-programmed hydrogenation TPO = temperature-programmed oxidation Greek Letters p = solvent viscosity, Pa s = solvent density, kg/m3 u = surface tension, N/m @ = Weisz modulus Literature Cited Aduriz, H. R.; Bodnariuk, P.; Dennehy, M.; Gigola, C. E. Activity and Selectivity of PcUa A 1 2 0 3 for Ethyne Hydrogenation in a Large Excess of Ethene and Hydrogen. Appl. Catal. 1990,58, 227. Al-Ammar, A. S.;Webb, G. Hydrogenation of Acetylene over Supported Metal Catalysts. Part 2.- [14ClTracer Study of Deactivation Phenomena. J . Chem. SOC.,Faraday Trans. 1 . 1978,74,657. Al-Ammar, A. S.; Webb, G. Hydrogenation of Acetylene over Supported Metal Catalysts. Part 3.-[l4C1Tracer Studies of the Effects of Added Ethylene and Carbon Monoxide on the Reaction Catalysed by Silica-supported Palladium, Rhodium and Iridium. J. Chem. SOC.,Faraday Trans. 1 1979, 75,1900. Asplund, S.;Fornell, C.; Holmgren, A.; Irandoust, S. Catalyst Deactivation in Liquid and Gas-Phase Hydrogenation of Acetylene using a Monolithic Catalyst reactor. To be presented at the European Symposium on Catalysis in Multiphase Reactors, Lyon-France, 7-9 Dec, 1994. Battiston, G. C.; Dalloro, L.; Tauszik, G. R. Performance and Aging of Catalysts for the Selective Hydrogenation of Acetylene: A Micropilot-Plant Study. Appl. Catal. 1982,2,1. Berndt, G. F.; Thomson, S. J.; Webb, G. Hydrogenation of Acetylene over Supported Metal Catalysts. J . Chem. SOC., Faraday Trans. 1 1983,79,195. Boitiaux, J. P.; Cosyns, J.; Vasudevan, S. Hydrogenation of Unsaturated Hydrocarbons over Highly Dispersed Palladium Catalyst. Part I: Behaviour of Small Metal Particles. Appl. Catal. 1983,6,41. Bos, A. N. R.; Westerterp, K. R. Mechanism and Kinetics of the Selective Hydrogenation of Ethyne and Ethene. Chem. Eng. Process. 1993,32,1. Cider. L.: Schoon. N.-H. Hvdrogenation of Acetvlene at Transient Conditions in the Presence oFOlefins and C a r b n Monoxide over PalladiudAlumina. Ind. Eng. Chem. Res. 1991,30,1437. Cosyns, J.;Boitiaux, J. P. European Patent EP 0 334 742 B1,1989. Cybulski, A.;Moulijn, J. A. Monoliths in Heterogeneous Catalysis. Catal. Reu.-Sci. Eng. 1994,36,179. Cybulski, A.; Edvinsson, R.; Irandoust, S.; Andersson, B. LiquidPhase Methanol Synthesis: Modelling of a Monolithic Reactor. Chem. Eng. Sci. 1993,48,3463.
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IE940169J Abstract published in Advance ACS Abstracts, November 15, 1994. @
