Ind. Eng. Chem. Res. 1992,31, 107-119 state. Proceedings of the Ninth International Congress on Catalysis, Calgary 1988;The Chemical Institute of Canada: Ottawa, 1988; VOl. 1, p 74. Majchrowisz, B. B.; Yperman, J.; Reggers, G.; Frangois, J. P.; Gelan, J.; Martens, H. L.; Mullens, J.; van Poucke, L. C. Characterization of organic sulfur functional groups in coal by means of temperature programmed reduction. Fuel Process. Technol. 1987,15,363. Mangnus, P. J.; Bos, A.; van Langeveld, A. D.; Moulijn, J. A. A reduction study of sulfided carbon, silica and alumina supported Mo and CoMo hydrotreating catalysts. Submitted for publication, 1991. Massoth, F. E. Characterization of coke on coal catalysts by an oxidation technique. Fuel Process. Technol. 1981,4 , 63. Moulton, D. S.Processing and utilization of high sulfur coals; Attia, Y.A., Ed.; Coal Science and Technology 9; Elsevier: Amsterdam, 1985;p 729. Parera, J. M.; Figoli, N. S.; Traffano, E. M. Catalytic action of platinum on coke burning. J. Catal. 1983,79,481. Pazos, J. M.; Andreu, P. Hydrodesulphurization mechanism of thiophene and tetrathiophene on a cobalt molybdenum catalyst. Can. J. Chem. 1980,58,479. Pollack, S. S.; Sanders, J. V.; Tischer, R. E. High-reflectance and single layer MoS2: two new forms. Appl. Catal. 1983,8, 383. Sanders, J. V.;Pratt, K. C. The relationship of structure and activity of NiMo sulfides to composition of precursor oxides. J. Catal. 1981,67,331. Scheffer. B.: Dekker. N. J. J.: Manenus. P. J.: Mouliin. J. A. A temperature-programmed reduction study of sulfidei Co-Mo/Al2OB HDS catalysts. J. Catal. 1990,121,31. Speight, J. G. The Desulfurization of Heavy Oils and Residue; Chemical Industries 4;Marcel Dekker: New York, 1981.
107
Ternan, M.; Furimsky, E.; Parsons, B. I. Coke formation on HDS catalysts. Fuel Process. Technol. 1979,2,45. Walker, P., Jr.; Rusinko, F.; Austin, L. G. Gas reactions of carbon. Adv. Catal. 1959,11,133. van Doom, J.; Moulijn, J. A. Extraction of spent hydrotreating catalysts studied by Fourier Transform Infra Red Spectroscopy. Fuel Process. Technol. 1990,26,39. van Doorn, J.; Verheul, R. C. S.; Singoredjo, L.; Moulijn, J. A. Characterization of carbon deposits on alumina-supported cobalt and nickel catalysta by temperatureprogrammed gasification with 02,COz and Hz. Fuel 1986,65,1383. van Doorn, J.; Bosch, J. L.; Bakkum, R. J.; Moulijn, J. A. TPO as an analysis technique for deactivated catalysts. Studies in Surface Sciences and Catalysis, Catalyst Deactivation 1987,Delmon, B., Froment, G. F., Eds.; Elsevier: Amsterdam, 1987;p 391. van Doom, J.;Staugaard, P.; Moulijn, J. A.; de Beer, V. H. J. A novel type of carbon-supported catalysts. Part I Preparation and characterization. Appl. Catal. 1988,48,253. van Doom, J.; Moulijn, J. A.; DjBga-Mariadaesou,G. High Resolution Electron Microscopy of spent Ni-Mo/A120zcatalysts. Appl Catal. 1990,63,77. Yoshimwa, Y.; Furimsky, E. Removal of carbonaceous deposita from the surface of cobalt-molybdate catalyst via oxidative regeneration. Fuel 1986,65,1388. Yoshimura, Y.; Shimada, H.; Sato, T.; Kubota, M.; Nishiyama, A. Initial catalyst deactivation in the hydrotreatment of coal liquid over Ni-Mo and Co-Mo alumina catalysts. Appl. Catal. 1987,29, 125.
Received for review April 30, 1991 Accepted August 26, 1991
Kinetics and Reaction Network in Propane Ammoxidation to Acrylonitrile on V-Sb-A1 Based Mixed Oxides Roberto Catani, Gabriele Centi,* and Ferruccio Trifirb Department of Industrial Chemistry and Materials, University of Bologna, Viale Risorgimento 4, 40136 Bologna, Italy
Robert K. Grassellit Department of Chemistry, Georgetown University, Washington, D.C. 20057
The kinetics of propane ammoxidation to acrylonitrile on a V-Sb-A1 based mixed oxide catalyst are described by a Langmuir-Hinshelwood approach, six parallel reactions of formation (propylene, acrylonitrile, carbon oxides, acetonitrile, HCN, and C2hydrocarbons), three reactions of consecutive transformation of the intermediate propylene to acrylonitrile, acetonitrile and HCN, five decomposition reactions of intermediate products to carbon oxides, and one reaction of decomposition of ammonia. The kinetics were studied in a quartz tubular flow reactor operating both in differential and integral conditions, using propane, oxygen, and ammonia concentrations in the 0-20% range and reaction temperatures in the 680-810 K range. Acrylonitrile forms principally from the intermediate propylene, and the limiting factor in the formation of acrylonitrile is the relative slowness of this step compared to the others and the higher rate of propylene oxidation to carbon oxides as compared to that of acrylonitrile to CO,. The maximum yields and selectivities to acrylonitrile obtained were in the 35-40 and 50430% ranges, respectively, for propane conversions in the 60-80 and 40-50% ranges, respectively. The relevance of the kinetic information in determining the best reaction conditions and performance using different possible feedstocks is also discussed.
Introduction Direct propane ammoxidation to acrylonitrile is one of the most promising new chemical oxidation processes using alkanes as feedstocks instead of functionalized hydrocarbons (olefins, aromatics, etc.). Indeed, due to the considerable price difference between propane and propylene and to the already good selectivities/yields to 'Present address: Mobil Central Research, P.O. Box 1025, Princeton, N J 08540.
acrylonitrile from propane reported in the patent literature (Guttmann et al., 1988a,b; Glaeser et al., 1988a,b), the economics of the process look very promising for new plants and have stimulated applied industrial research as shown by the rapidly growing number of patents on this subject. Despite this interest, relatively few fundamental studies have been published on the nature of the catalyst and on the reaction mechanism (Centi et al., 1987 and references therein) and, in particular, on the reaction kinetics. Studies on this latter aspect should provide more quan-
0888-5885/92/ 2631-0107$03.00/0 0 1992 American Chemical Society
108 Ind. Eng. Chem. Res., Vol. 31, No. 1, 1992
titative data for the analysis of the reaction network and the economics of the reaction. A survey of the patent literature indicates that two main classes of catalysts give the best results, in terms of selectivity or yield to acrylonitrile,from propane: (i) systems based on an antimonate rutile structure and containing V as the key element (Bartek and Guttmann, 1989; Brazdil et al., 1989; Glaeser et al., 1988a,b; Guttmann et al., 1988a,b) and (ii) systems based on a Bi-Mo-V scheelite structure (Glaeser et al., 1988~; Hatano and Kayo, 1987). Better results are reported, especially in terms of maximum yields, for the first class of catalysts. Rutile-type based catalysts, especially Fe-antimonate containing an excess of Sb-oxide and Sn02-Sb20,, are well-known systems for propylene ammoxidation (Centi and Trifirb, 1986), but they show poor activitylselectivity for propane ammoxidation (Centi et al., 1987). Better performances are obtained using V-antimonate systems which also contain an excess of Sb-oxide (Centi et al., 1987), but the results can be improved considerablyby the introduction of Al-oxide into the system (Centi et al., 1990a,b). Such catalysts result in propane ammoxidation comparable to that of the patented systems. It has also been suggested that the active phase is an amorphous VSb04-Sb204system epitaxially intergrown with AlSb04 rutile whose formation is catalyzed by the presence of V; this nonstoichiometric mixed metal oxide is supported on y-A1203(Centi et al., 1990a,b). However, no definitive evidence regarding the active phase is reported, even though it is shown that the role of aluminum oxide is not that of a simple support for the dispersion of the V-antimonate phase. There exists a strong active phase support interaction. Ga-antimonate (Osipova and Sokolovskii, 1979a,b) and Ag-doped bismuth vanadomolybdate (Kim et al., 1990) have also been reported to be active in propane ammoxidation, but in the first case the data are unclear and in the second the prevailing contribution of homogeneous gas-phase reactions in the propane to propylene activation step (due to high propane and NH, concentrations) considerably limits the validity of the data. V-silicalite and V-aluminum phosphate (Miyamoto et al., 1989) and vanadyl pyrophosphate (Centi et al., 1987) catalysts have also been tested for propane conversion to acrylonitrile, but poor performances were obtained, probably due to the high rate of parallel ammonia combustion (formation of NO, which further reacts with ammonia itself giving N2 as the final product). This parallel reaction, which causes loss of ammonia, is a detrimental factor in the economics of the reaction due to the higher cost of ammonia compared to that of propane. In conclusion, both patent and primary literature data indicate that V-Sb-A1 based systems represent currently the most promising candidate catalysts for the development of a process of direct acrylonitrile synthesis from propane as a feedstock. In this paper we report a kinetic analysis of this reaction, including the rate of the parallel ammonia conversion and of all byproducts of reaction, using a V-Sb-Al based catalyst promoted with W whose composition and method of preparation are analogous to those reported for the best patented catalysts (Guttmann et al., 1988a,b) and are also found to be optimal by us (Centi et al., 1990a,b). The aim of this work is to contribute to a better understanding of the reaction network, to single out the experimental conditions in which the selectivity and yield to acrylonitrile are maximized, and to provide a reaction model to evaluate the optimal reaction conditions and feed composition. The kinetic analysis
was extended to a wide range of propane, oxygen, and ammonia concentrations in order to evaluate reaction rates using feeds both inside and outside the lower and upper explosion limits of the ternary mixture.
Experimental Section Catalyst Preparation and Characterization. The V-Sb-Al based mixed oxide was prepared by a slurry method. NH4V03(1.295 g), Sb203(8.673 g), and Al(OH)3 (42.82 g) were added to 0.5 L of a hot aqueous solution of ammonia (1 N NH3). The slurry was maintained with continuous stirring under reflux for more than 24 h and then (NH4)5W1204-5H20 (2.891 g) was added, while the slurry was maintained under stirring and reflux for an additional 2 h. The water was then removed by evaporation in a rotavapor and the recovered solid dried overnight at 423 K. The solid was then calcined at 900 K for 3 h with intermediate steps a t 623 K (24 h) and 773 K (3 h). The heating rate was 50 K/h. The final composition of the catalyst was 30 wt % active component (assumed to be VSb04 + 2Sb205+ WO,) and 70 wt % y-A1203. The V:Sb:W atomic ratio was 1:5:1. Hereafter the catalyst will be referred to as V:Sb:W(1:5:1)-Al2O3 [70%]. The surface area after calcination, determined by the BET method using N2 adsorption at 77 K, was 117 m2/g and did not change after the catalytic tests. For catalytic measurements the catalyst powder was granulated in particles with dimensions in the 0.2-0.4mm range; the apparent density of the granulated catalyst was 1.1 g/cm3. X-ray powder diffraction analysis of the catalyst indicated that the catalyst is amorphous both before and after catalytic tests; only weak and broad lines suggesting the presence of cr-SbzO4 and 7-A1203were observed. At higher calcination temperatures (lo00 K) than those used for the preparation of the sample for the kinetic study, the diffraction lines corresponding to antimony oxide disappear, but stronger diffraction lines due to a rutile-like phase with cell parameters intermediate between those of VSb04and A1SbO4 appear. Apparatus. The experimental investigations were conducted using a quartz tubular fixed bed integral down-flow reactor of 10 cm3 (0.8-cm i.d.). The choice of the material of construction (quartz versus stainless steel) and of the dead space (the dead space should be minimized as much as possible) is very important in order to minimize homogeneous or wall-catalyzed reactions and especially to minimize the rate of parallel ammonia combustion. A series of preliminary steps using empty or quartz-sphere filled reactors were carried out in order to optimize the reactor configuration. Inside the reactor, a thermocouple sliding in a 0.4-cm0.d. quartz tube immersed in the catalyst bed allows control of the axial temperature profile. The reactor is heated by external cylindrical electrical resistances immersed in a cylindrical copper block (suitably isolated in the external part) which provides a good heat exchange with the reactor and a uniform axial temperature profile. However, notwithstanding (i) the relatively low rate of propane depletion as compared to the rate of propylene or aromatics ammoxidation and (ii) the use of an already diluted catalyst (about 30 wt % catalyst constitutes the active phase), a dilution of the catalyst particles with quartz particles (generally in the ratio 1:3) was necessary in some cases in order to ensure a uniform temperature profile, especially for the higher rates of hydrocarbon depletion. The catalyst charges used were in the 0.2-7-g range, with the total flow at STP conditions usually higher than 6 L/h
Ind. Eng. Chem. Res., Vol. 31, No. 1,1992 109 in order to avoid interphase diffusional limitations and minimize the rate of ammonia combustion. The catalyst, with particle dimensions in the 0.2-0.4-mm range, was placed on a quartz porous plate between two layers of glass wool, and the gas flow was fed from the top. The pressure drop was below 0.1 atm, and the reactor operated at atmospheric pressure. The reaction feed was prepared by mixing calibrated amounts of the various reagents (propane, ammonia, oxygen, helium), stored under pressure in cylinders in already calibrated mixtures of each component with helium, in order to have a more precise control of the composition. The feed was preheated at 650 K before being sent to the reactor. The preheated feed also could be sent to the analytical section of the apparatus for analysis of the feedstock composition. The exit stream from the reactor led into a furnace heated at 503 K, where it could be sampled and sent directly into the oven of a gas chromatograph (flame ionization detector; carrier gas, nitrogen) for analysis of the organic products. The column was Porapak QS (4 m long). The oven temperature was programmed as follows: 8 min isothermal at 333 K and then increased at 10 K/min up to 393 K, 1 min isothermal at 393 K and then increased at 25 K/min up to 453 K, 10 min isothermal at 453 K and then increased at 25 K/min up to 498 K, and 15 min isothermal at 498 K. This temperature program allowed good separation of the various organic products (methane, ethane, ethylene, propylene, propane, acetonitrile, acrylonitrile). The gas exiting from the furnace could be alternatively bubbled into a suitable reactant for the analysis of NH3 and HCN or sampled after it had passed through a cooled condenser for the analysis of noncondensable N2, CO, COP,and possibly NO,) using a second gases (02, gas chromatograph (thermal conductivity detector; carrier gas, helium). A Carbosieve-II,100-200-meshcolumn was used. The oven temperature was programmed from room temperature to 498 K at a rate of 20 K/min, after an initial 9 min in isothermal conditions. Both gas chromatographs were interfaced with a data analysis system and process computer, to evaluate experimental results and verify the material (C and N) balances in each test. The analysis of NH3was performed by titration with 0.1 N NaOH after absorption of the total gas flow in 50 mL of 0.1 N HC1 for a calibrated time, usually 5-10 min. The analysis of HCN was performed by titration with 0.1 N AgN03 of the solution obtained after absorption of the total gas flow in 20 mL of 0.1 N NaOH for a calibrated time (usually 30-40 min), subsequent dilution up to 75 mL, and addition of 6 mL of 6 N NH3 and of 2 mL of an aqueous solution containing 10 w t % KI. Experimental Setup. By assessing the influence of mass and heat transport with the appropriate criteria (Carberry, 1976,1987; Madon and Boudart, 1982; Mears, 1971; Turner, 1984), it was ensured that no hidden factors due to transport limitations were influencing the kinetic parameters under the present experimental conditions. In addition, experimental verification of the absence of interphase diffusion phenomena by varying the volume of the catalyst and the gas flow rate was made. The role of interphase diffusion phenomena was also experimentally verified to be negligible in the range of the parameters used in this study. Special care was also devoted to achieve a nearly isothermal axial temperature profile, as discussed above. Repeated tests were carried out to c o n f i i that both the activity and the selectivity of the catalyst were not altered during the kinetic experiments. Each new catalyst was
conditioned in situ with the reactant mixture (propaneammonia-oxygen-helium) at the higher reaction temperature (815 K) for at least 4 h before the catalytic teats. The catalytic behavior was then analyzed up to the steady-state conditions. In selected experimental points, independent tests were repeated in order to estimate the internal error variance. Evaluation of Kinetic Data. The parameters were obtained in a step strategy. The starting values for estimation of the parameters were assessed from plots of the experimental values in linear form according to some suitable transformation. The data utilized in this step were obtained under differential conditions (conversionslower than approximately 5-10%) in order to evaluate directly the rate of formation of all products. The optimization of the parameters was then performed with a robust specific optimization routine for multiresponse nonlinear regression analysis, based on several combined direct search procedures (Buzzi-Ferraris, 1970). The parameter estimates were obtained by minimizing the sum of the squara of the weighted residuals over all responses m
r
where r is the number of responses, m the number of experiments, yi; the experimental rates, and j$ the model responses. The weight (wij)in eq 1was set equal to the inverse of the square of the experimental rate in order to improve the conditioning of the objective function assuming the error percentages in the various experiments are constant (Himmelblau, 1968, 1970; Froment, 1976; Froment and Bishoff, 1976). The parameters evaluated by this procedure (parameters of the rates of direct propane conversion; r1-r6,Scheme I) were utilized to evaluate the parameters of the rates of consecutive reactions (r7-rl4, Scheme I), using data obtained in integral conditions. In this case, the experimental dependent variables were the measured outlet mole fractions of the reaction product observed (yi;), while the model responses were the yields of the same species calculated from integration of the respective rate expressions. An isothermal monodimensional pseudohomogeneous plug flow reactor model was utilized for the numerical integration coupled with atomic balances for C, H, 0, and N. A final optimization and statistical evaluation was then followed using all experimental data and using a suitable reparametrization (in particular, Arrhenius dependence on the reaction temperature) in order to improve the conditioning of the objective function (Himmelblau, 1968, 1970).
Results Analysis of the Reaction Network. Reported in Figure 1is the dependence of the selectivity to propylene (C3=) and to acrylonitrile (ACN) on the propane conversion for various reaction temperatures. At low propane conversion the main product is propylene, whereas with an increase in the conversion the selectivity to propylene decreases and the selectivity to acrylonitrilepasses through a maximum. For the higher propane conversions, carbon oxides are the main products of reaction. These data suggest that acrylonitrile formation occurs mainly by a consecutive reaction of the intermediate propylene, whereas its direct formation from propane is less relevant. Both propylene and acrylonitrile, furthermore, are consecutively oxidized to carbon oxides. The relative rate of ACN formation and consecutive transformation to CO, depends on the reaction temperature (Figure 1);this effect can be explained by taking into account the rate of non-
material balances and reagent and products profiles along the catalytic bed: direct formation of products from propane
C3Hs + f/zO2
4
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(2)
C3H8 + 202 + NH3 C3H3N + 4H20 (3) C3HB + 402 2CO + C02 + 4H20 (4) C3He + 302 + NH3 C2H3N + 4H2O + C02 (5) C3H8 + 7 2 0 2 + 3NH3 3HCN + 7H20 (6) 4
4
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(7)
+ 7202 + NH3 C3H3N + 3H20 (8) C3H6 + 7 . 0 2 + NH3 CzH3N + 3H20 + CO2 (9) C3H6 + 3 0 2 + 3NH3 3HCN + 6H20 (10) C3H6 7 2 0 2 2 c 0 cot + 3H20 (11) C3H3N + 7 2 0 2 + NH3 2CO + C02 + 3H20 + N2 C3H6
-+
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(12) CzH3N + 302 NH3 CO C02 3H20 + N2 (13) HCN 202 NHS C02 3H20 + N2 (14) C2H4
7202
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selective ammonia conversion to N2 The amount of NH3 nonselectively converted to N2 instead of that to nitriles depends on the contact time and is thus higher at the lower reaction temperatures, which require longer contact times to achieve the same conversion of propane as that of tests performed at higher reaction temperatures. The parallel nonselective conversion of ammonia is thus of critical importance in the development of a reaction model for propane ammoxidation. Diagrams similar to that of Figure 1, obtained for the various other reaction products, acetonitrile (AcCN), HCN, ethane and ethylene (CZ), and carbon oxides (CO,), suggest that the general reaction networks for propane ammoxidation may be written as given in Scheme I. Acetonitrile and HCN were found to form both from propane and from intermediate propylene, but mainly from the former, whereas C2 products form only from propane. Reported in Scheme I is also the reaction pattern in ammonia conversion. Between brackets is suggested the intermediate formation of NO,, which, however, is never detected due to its very high rate of reaction with NH3 to give N2. The reaction stoichiometries for the reactions of the Scheme I were considered as follows for the calculation of
served parallel reaction of ammonia conversion which gives N2 as the only product observed, as confirmed also in preliminary testa to evaluate the direct rate of ammonia conversion in the absence of propane in the reactant feed. In order to simplify the reaction network the formation of carbon oxides was lumped into a single reaction product, with a CO:CO2 ratio of about 2 as usually found in the catalytic tests. For this reason, in eqs 4, 5,9, and 11-15 the formation of CO and COz was tentatively written with a formal stoichiometry utilized for the material balances. However, it should be pointed out that this stoichiometry of the relative formation of CO and C 0 2 in the various equations does not correspond to any direct experimental evidence, with only the determination of the cumulative formation of carbon oxides from all reactions being possible. Kinetic Study. Experimental Design. The actual experiments for the determination of the kinetic parameters were performed according to a modified design, where the temperature and propane, oxygen, and NH3 concentrations were chosen as the independent variables. The strategy for the experimentation was to investigate a representative experimental grid and to explore the dependence of the rates on the single variables in order to study better the form of the reaction rates. The lower and upper explosion libnits of the ternary air, ammonia, and propane mixture depend on pressure, temperature, and composition. As a general indication, for an NH3 concentration of 10% and pressure of 1 atm, the air and propane composition at the lower and upper explosion limit are 0.9% propane and 89.1% air (lower limit) and 9.5% propane and 80.5%air (upper limit). This suggests that the productivity will be very low in a process operating with a propane concentration lower than the lower explosion limit and a technical processing solution using higher propane concentrations will be probably necessary. For this reason, the kinetic study was extended to include
Ind. Eng. Chem. Res., Vol. 31, No. 1,1992 111 h
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Figure 2. Rate of propylene formation as a function of propane (a), oxygen (b), and ammonia (c) concentrations for different reaction temperatures: symbols,experimental points; linea, calculated values. Experimental conditions: (a) oxygen, 4.33 x mol/L, ammonia, 3.04 X mol/L; (b) propane, 2.47 X mol/L, ammonia, 4.22 X mol/L; (c) propane, 3.12 X mol/L, oxygen, 4.92 X mol/L. Data obtained in differential conditions (C2,02,and NH3 conversions lower than 510%). Key: propylene, C3=; acrylonitrile, ACN; acetonitrile, AcCN, cyanidric acid, HCN; carbon oxides, CO,; propane, C3; oxygen, 02; ammonia, ”3; yield, Y; selectivity, S; conversion, C. Calculated values from the kinetic model versus experimental rates of propylene formation (d).
a wide range of propane, oxygen, and NH3 concentrations with composition both inside and outside the region between the upper and lower explosion limits. Results of Differential Conditions. The dependence of the rates of formation of the single products on the propane, oxygen, and ammonia concentrations (varying one reactant and maintaining constant the other two) and on the reaction temperature was studied at low conversion (lower than 5-10%) for all reactants, so that the initial concentrations along the catalytic bed (differentialreactor) could be assumed constant. Moreover, due to the short contact time and low concentrations of the reaction products, their reactions of consecutive transformation (reactions 7-14 in Scheme I) were neglected. The nonselective conversion of ammonia (reaction d, Scheme I) was also considered negligible. The range of reagent concentrations investigated was 610.5% propane, 0-21.5% oxygen, and 0-16.0% ammonia. Reported in Figures 2-4 are the experimental values of the rates of propylene, acrylonitrile, and CO, formation as a function of the propane (a), oxygen (b), and ammonia (c) concentrations at different reaction temperatures. Similar results were obtained for acetonitrile, HCN, and c2. The rates of propylene and acrylonitrile formation have a similar dependence on the reagent concentration, but, in particular, propylene formation shows an accentuated
maximum in the plot against ammonia concentration, contrary to that found for acrylonitrile. However, in all cases a Langmuir-Hinshelwood (Carberry, 1976) dependence on reactant concentration is observed. The higher rates of propylene formation compared to acrylonitrile also should be noted, in agreement with the higher initial selectivity to the olefin. The behavior of the acetonitrile and HCN formation is similar to that of propylene and ACN. Very different, on the contrary, is the dependence of the CO, formation (Figure 4) on reactant concentration. The * C2 dependence is similar to that of COX. Results of Integral Conditions. In order to study the reactions of consecutive transformation of the products formed by direct propane ammoxidation, catalytic experiments were also carried out in integral conditions, varying the spacevelocity for reaction temperatures in the 683-793 K range and using different feed compositions, with propane concentrations in the 618% range and propane to ammonia or to oxygen ratios in the 1-5 range. Reported in Figures 5 and 6 is the effect of the spacevelocity on the yield of the various reaction products and on the propane, oxygen, and ammonia conversion for two different reaction temperatures. In particular, the yields to propylene and to acrylonitrile pass through a maximum, in agreement with their consecutive transformation to carbon oxides and to ACN (for propylene). Acrylonitrile is the main product, and yields up to 3635% can be ob-
112 Ind. Eng. Chem. Res., Vol. 31, No. 1, 1992 120
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tained. The other reaction products (AcCN, HCN, C2) are formed in small amounts, particularly the latter. The effect of the propane conversion and of the reaction temperature on the selectivity to the various products is shown in Figure 7, which compares results obtained at 753 K (Figure 7a) and 793 K (Figure 7b). The selectivity to acrylonitrile passes through a maximum for conversions in the 3040% range, and as the reaction temperature increases, the maximum selectivity to ACN increases slightly from nearly 50 to nearly 60%. This is related mainly to the reduced relative importance of the rate of parallel ammonia combustion (reaction d, Scheme I) compared to reactions of selective ammoxidation (reactionsa-c, Scheme I). The selectivity to acetonitrile is generally low, about 5% or lower; slightly higher selectivities are found for HCN, which however are normally lower than 10%. The selectivity to these products does not depend greatly on the propane conversion. It should be noted that compared to the results reported in Figures 5 and 6, the results reported in Figure 7 were obtained with a higher propane concentration and with oxygen to propane and ammonia to propane ratios in the 2.0-2.5 range, which correspond to the optimal conditions, in agreement also with feed concentrationsreported in the patent literature (Guttmann et al., 1988a,b). The effect of reaction temperature on yield and selectivity to propylene and acrylonitrile is shown in Figure 8, for two propane concentrations, around 6% (Figure 8a) and around 16% (Figure 8b), and fixed oxygen and am-
monia concentrations. With increasing propane concentration, the limiting reagents become oxygen and ammonia. Lower yields to acrylonitrile are obtained, but the selectivity to ACN increases considerably. This observation suggests future commercial operations to be performed at high propane partial pressures, relatively low oxygen partial pressure, and unreacted propane recycle. As previously observed, the reaction temperature has only a limited effect on the selectivity, but generally better results are obtained in the 770-800 K range. Mathematical Workup of Kinetic Data. Reaction Model. The rate equation to fit the experimental data was derived using the usual Langmuir-Hinshelwood approach (Froment, 1987). However, as is well recognized in the literature (Carberry, 1976; Froment, 1987; Froment and Bishoff, 1976), in spite of the fact that the LangmuirHinshelwood formalism for developing the rate equations explicitly accounts for the interaction of the reacting species with the catalyst, the approach is oversimplified with respect to the true reaction mechanism. The rate equations derived from the Langmuir-Hinshelwood approach allow a better description of the experimental data with respect to empirical approaches (for example, power law or even firsborder rate equations), especially in a wide range of operating conditions and in the case of complex catalytic reactions, as in the present work, but the goodness of fit of the experimental results for one reaction model does not imply that the inherent kinetic model represents the true reaction mechanism, when detailed information
Ind. Eng. Chem. Res., Vol. 31, No. 1,1992 113 Cox. u
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Cone. “3,MDbrll(‘€-3)
Figure 4. Rate of carbon oxide formation as a function of propane (a), oxygen (b), and ammonia (c) concentrations for different reaction temperatures: symbols, experimental points, lines, calculated values. Experimental conditions as in Figure 2. Key as in Figure 2. Calculated values from the kinetic model versus experimental rates of propylene formation (d).
on the reaction mechanism and on the nature of the active sites is not available. Therefore, the Langmuir-Hinshelwood approach and some physicochemical constraints on the parameters (Froment, 1987) were used to develop a reasonable kinetic model that fit the experimental results in a wide range of operating conditions, but mechanistic deductions based on the goodness of the fitting are not corrected. For the same reason, the dependence of the rates of product formation from the concentration of the various reagents (tests in differential conditions) was also analyzed in order to have direct evidence on the structure of the rate equation and on the consequent kinetic model to be adopted for fitting the experimental results; for example, the competition between ammonia and oxygen was assumed in deriving a rate equation that can fit the selfinhibition effect shown in the propylene formation. Other alternative models, however, for the reasons outlined above, where not evaluated, because the aim of this work is not to have evidences on the reaction mechanism but to develop a kinetic model of the reaction of propane ammoxidation that can be used to evaluate optimal reaction conditions. The tests performed under differential conditions of reagents conversion (Figures 2-4) evidence that the rate of formation of propylene, ACN, and COXdepend on the concentration of the three reactants (propane, oxygen, and ammonia) and an effect of saturation of active sites against the concentration of all reactants is observed. Similar results were obtained for acetonitrile, HCN, and C2. In some cases, e.g., the dependence of the rate of propylene
formation on ammonia concentration (Figure 2 4 , a selfinhibition effect also is observed. According to a Langmuir-Hinshelwood approach, the observed trends of the rates of formation of products from the concentration of propane, oxygen, and ammonia can be described on the basis of a hypothesis on the rate-determining step, on the adsorption of reagents on the active sites of an idealized surface (for example, the adsorption heat must be independent from the surface coverage), and on the surface reaction between adsorbed reagents. In the development of a rate equation according to the Langmuir-Hinshelwood approach which describes the observed trend, it is reasonable to assume different sites for propane adsorption and for NH3 or O2adsorption, due to the different electronic natures of these reactants, and in order to explain the self-inhibition effect shown by propylene formation and the negative order for COXformation from ammonia, a competition between O2and NH3 for the same site is assumed. On the basis of these hypotheses and on the participation of adsorbed propane, oxygen, and ammonia in the rate-determining steps for propylene, ACN, AcCN, and HCN and of only propane and oxygen in the rate-determining steps for CO, and C2, the following rate equations can be derived. for propylene, ACN, AcCN, and HCN (rl, r2,r,, and r5, Scheme I)
r=
kSPK&NpON (1 + KpP)(1 + KO0
+ K“)*
(17)
114 Ind. Eng. Chem. Res., Vol. 31, No. 1, 1992
for COXand C2 (r3 and r6, Scheme I)
kCKPKCP0 r= (1 + KpP)(l + KoO+ K")
Yield (Y), %
7 1
35
(18)
where P, 0, and N are the concentrations of propane, oxygen, and nitrogen, respectively, k , is a kinetic constant, and Kp, KO, and KN are the adsorption constants for propane, oxygen, and nitrogen, respectively. For the rate of propylene formation, a dependence on the concentration of ammonia was assumed in order to fit the experimental data. This may be justified if a role for ammonia in the stage of selective propane activation is assumed. Similar rate equations were adopted for the rates of the consecutive reactions of the various products (rates 7-14 in Scheme I):
20 L
Y-ACN Y-AcCN
5
0
0
1000 1500 2000 W/F, g.h/Moles C3
500
Yield (Y). %
60
where I indicates the concentration of the relative intermediate (propylene in r7-10, ACN in rll, AcCN in r12,HCN in r13, and C2 in r14). Furthermore, in order to reduce the number of parameters, it was assumed that the values of the K p ,KO,and KN adsorption constants were the same as that found for the rate of propylene formation, since this is the primary reaction product in the propane transformation. The K p adsorption constant, however, was corrected by a factor N (equal to 3 for propylene, 2 for ACN, 10 for AcCN, 5 for HCN, and 15 for C2) in order to take into account the differences in the concentration of the various intermediates. This procediwe allows a drastic reduction of the number of parameters, and taking into account the high correlation of the kinetic parameters due to the intrinsic properties of the Langmuir-Hinshelwood approach does not markedly affect the quality of the fit obtained. A factor n = 3 also was adopted for the rate of HCN conversion to COX(rlJ due to stoichiometric reasons; n = 1 was used in all other reactions. Parameter Estimation. A step strategy as described in the Experimental Section was used to estimate the kinetic parameters. The number of parameters for the rates of direct formation from propane (rates rl-r6) are 8 for each product (preexponential factor and activation energy for the kinetic constant, k,, and for the three adsorption constants, K p ,KO,and KN), and a total number of 64 experimental points obtained in differential conditions (direct evaluation of reaction rates) for each product (56 degrees of freedom) were utilized for the nonlinear regression analysis. An Arrhenius dependence of the kinetic parameters on the reaction temperature was reparametrized in order to reduce parameter correlation (Himmelblau, 1970)) according to the following expression: ki = Ai*e-Ei/Rp where, Ai* = Aie--G/R(730)and 1 / p = (1/T) - (1/730), 730 K being the mean reaction temperature in the range investigated. In the regression analysis,physical constraints on the rate coefficients and adsorption equilibrium constants (Froment, 1976) were assumed. The parameter estimates for rates 1-6 of Scheme I together with their 95% individual confidence limits are reported in Table I. The goodness of the fit is shown in Figures 2-4 from the comparison of the experimental values (symbols) and calculated values (solid lines) and from the graphs of experimental versus calculated values (Figurea 2d, 3d, and 4d). The curve fit taken together with the analysis of residuals and the statistical indexes indicates that the kinetic model provides a good representation of the data over the region investigated. The large intervals of the individual confidence limits observed are
2500
3000
Conversion (C), % ,100
1
180
I
40t 30
,
I
0
500
--
Y-cox
0
c-c3
1000 1500 2000 2500 3000 W/F, g.h/Moles C3
Figure 5. Yield to propylene, acrylonitrile,acetonitrile, HCN, and carbon oxides and conversions of propane, oxygen, and ammonia aa a function of space-velocity (grama of catalyst divided by moles per hour of propane feed): symbols, experimental points; lines, calculated values. Experimental conditions: propane, 6.40 X lo-' mol/L (1.62%);oxygen, 3.16 X mol/L (8.0%);ammonia, 3.16 X mol/L (8.0%); reaction temperature, 753 K; 7 g of VSbW(1:B:l)A1203[70%] catalyst. Key aa in Figure 2.
related to the high correlation of the parameters in the rate equations. Parameters associated with consecutive reactions (preexponential factors and activation energies of kinetic constanta of rates r7-r14,according to that diElcussed above) were determined using the data obtained in integral conditions. A total number of 56 experimental values (yields) for each reaction product (propylene,ACN, COX, AcCN, HCN, and C2) and for each reagent conversion (propane, oxygen, and ammonia) was used for the multivariable nonlinear regression method coupled with a standard routine of numerical integration of reagent and product variation along the catalytic bed. Reported in Table I1 are the values obtained together with their individual 95% confidence limits. The goodneas of the fit is shown in Figures 5-8 from the comparison of the experimental values (symbols) and calculated values (solid lines) for both product yields and reagent conversions and in Figure 9 from the comparison of experimental versus calculated yields of propylene and of acrylonitrile. A rate equation of the type
(20) where N is the ammonia concentration and g the amount of catalyst considered in the integration step, was used to describe the rate of the parallel reaction of ammonia nonselective conversion to N2 (reaction d, Scheme I), as determined from the fitting of the difference between the calculated ammonia conversion according to rates r 1 ~ 1 4 -rd
= 0.356ge-3340/TN
Ind. Eng. Chem. Res., Vol. 31, No.1,1992 115 Yield (Y), % 40 i
20+
80
[
a
\
/ c
Y-AcCN
'51 10
Selectivity (S), %
I/
Y-HCN
V
5 0
0
500
Yield (Y), % 80 I
1000 1500 2000 W/F, g.h/Moles C3
2500
3000
0
Conversion (C). %
1
'
0
10
30 40 50 Conversion C3. %
20
70
60
Selectivity (S),%
0
b
IC
Y-cox
0
c-c3
A
c-02
S-ACN
~
C-"3
-0
500 1000 1500 2000 2500 3000 W/F, g.NMoles C3
Figure. 6. Yield to the various products and reagent conversions as a function of space-velocity: symbols, experimental points; lines, calculated values. Experimental conditions as in Figure 5, but a reaction temperature of 773 K. Key as in Figure 2.
and experimental values of ammonia conversion. The corresponding consumption of Oz,according to eq 16, was taken into account.
Discussion Reaction Network. The results of the kinetic analysis clearly show that the main route to acrylonitrile passes through the intermediate formation of propylene, as indicated by the analysis of the reaction rates. The rate of propylene to acrylonitrile transformation is relatively slow compared to the rate of propane to propylene transformation and its consecutive oxidation to carbon oxides. The ratio of the rates depends on the ammonia concentration. It is thus very important to avoid the parallel reaction of nonselective ammonia combustion; a proper choice of catalyst(s) and reactor configuration is critical from this point of view. Furthermore, acrylonitrile is relatively more stable against consecutive oxidation to carbon oxides than propylene, and thus an increase in the rate of propylene to acrylonitrile transformation would strongly affect the ultimate yield of acrylonitrile. This could be achieved by the use of a cocatalyst capable of efficient transformation of propylene to acrylonitrile. The cocatalyst can be added to the paraffin activation catalyst (V-Sb-AI based oxide) as a physical mixture, or the V-Sb-A1 based oxide could be modified chemically in the synthesis of the catalyst to improve the olefin to acrylonitrile conversion function. It should be emphasized, in fact, that the initial cumulative selectivity (propylene plus acrylonitrile) of the catalyst at low conversion is higher than 80%, as shown in Figures 1, 5, and 6 and in the tests under differential reaction conditions (Figures 2-4).
*
S-AcCN
v
S-HCN
--0
20
40
60
80
Conversion C3, %
Figure 7. Selectivity to propylene, acrylonitrile, acetonitrile, and HCN as a function of propane conversion: symbols,experimental points;lines, calculated values. Experimental conditions: propane, 2.22 X lo-' mol/L (5.62%); oxygen, 4.66 X mol/L (11.8%); mol/L (11.8%); reaction temperature, 753 K ammonia, 4.66 X (a) and 793 K (h); 7 g of VSh:W(1:51)-Al2O3 [70%] catalyst; total flow rate (at STP conditions), 7.2 L/h. Key as in Figure 2.
Analysis of Optimal Reaction Conditions. The best acrylonitrile yields using the V-Sb-AI based oxide catalyst were found in the 35-40% range, at conversions in the 6040% range, in good agreement with patented results (Guttmann et al., 1988a,b). The maximum selectivity to acrylonitrile is around 60%, obtained at lower propane conversions, in the 40-50% range. This derives mainly from the competition between propylene conversion to ACN or to COXand of ACN conversion to COX;the latter reaction is much slower compared to propylene conversion to carbon oxides, due to the greater stability of acrylonitrile. The oxygen to propane and the ammonia to propane ratios have a considerable effect on these results. Better results were found using OZ:C3 and NH3:C3ratios in the 2-3 range and propane concentrations in the 4-6% range, due to the inhibition effect of ammonia on propylene formation (Figure 2c) and on the increase in the rate of COXformation at the higher O2concentrations. Too low oxygen and ammonia concentrations, on the other hand, limit the productivity to acrylonitrile. Optimal reaction temperatures were found in the 750-790 K range. Application of the Kinetic Reaction Model. On the basis of the kinetic reaction model the maximum yields to acrylonitrile (maximum yield obtained in all ranges of feed conversion) can be calculated. Reported in Figure 10 is the mnnimum yield calculated at T = 753 K as a function of oxygen and ammonia concentrations for three propane concentrations (1, 5, and lo%, corresponding to 3.95 X loa, 1.9 X and 3.95 X mol/L, respectively). It
116 Ind. Eng. Chem. Res., Vol. 31, No. 1, 1992 Selectivity
Yield (V), %
40
(S),%
~
I 35 r
v----
c3:1%
1
~.
-
30 25v==/
40 30
20 i I
* 20
15 1 10
Y-C3=
0
Y-ACN
a
s43-
V
S-ACN
_____
t
750
7
20 %
-10
765 780 Temperature, K
795
5%
Selectivity (S), % I _ _ $60
3o Yield (Y), %
~
20 %
I
20
7
lot
,
, 10
- ' 5
0 L750
S-ACN
4
10%
--A0 765
780
795
Temperature, K
Figure 8. Selectivity and yield to propylene and to acrylonitrile as a function of reaction temperature: symbols, experimental points; lines, calculated values. Experimental conditions: propane, 2.38 X mol/L (6.04%) (a) and 6.18 X mol/L (15.65%) (b); oxygen, 8.65 X mol/L (21.9%); ammonia, 7.59 X 10" mol/L (19.22%); 7 g of VSbW(1:51)-A1203 [70%] catalyst; total flow rate (at STP conditions), 7.2 L/h. Key as In Figure 2. c310 r-
20 %
Figure 10. Values calculated on the basis of the kinetic model of maximum yield (Ym=)of acrylonitrile from propane at 753 K, ae a function of oxygen and ammonia concentrations, for different propane concentrations in the inlet feed. See also test. Key as in Figure
7 -Calculated Yield
2.
lo 20
i
0
c3-
a
ACN
Experimental Yield. % Figure 9. Experimental versus calculated (on the basis of the kinetic model) yields of propylene and acrylonitrile in propane ammoxidation on VSb:W(1:51)-A1203 [70%] catalyst. Key as in Figure 2.
is shown that ACN formation passes through a maximum with respect to oxygen concentration; the maximum shifts to higher O2 concentrations with increasing ammonia concentration. The yield to ACN increases as the ammonia concentration increases, with a trend similar to that expected for the saturation of active sites. The higher yields of ACN were obtained for the higher ammonia concentrations, the use of which, however, will ultimately be limited by the cost of NH3 when high ammonia concen-
trations are used. At the lower NH3 concentrations, ammonia is the limiting reagent and low maximum yields of ACN are obtained, which are further decreased at the higher O2concentrations, due to the increases in the rates of the parallel ammonia combustion and propylene and ACN combustion to carbon oxides. The reaction model can also be used to compare the performances obtained by processes using different feedstocks. In order to compare possible realistic situations, two possible types of feeds were evaluated one which uses a propane concentration,and O& and NH3:C3 ratios similar to those reported in the patents (Guttmann et al., 1988a,b) and found good also by us; another is based on a higher propane concentration. As is usually found in many oxidation processes, a certain amount of water is added to the feedstock in order to help the heat transfer, and thus we have considered a 13% water content in the feed. We have also assumed the use of air as the oxidizing agent. On the basis of these considerations, the oxygen and ammonia concentrations are lower in the second case. Reported in Figure 11 are the yield and selectivity to acrylonitrile as a function of the propane conversion and of the reaction temperature, for the two types of feeds
Ind. Eng. Chem. Res., Vol. 31, No. 1,1992 117 Table I. Parameter Estimates and 95% Individual Confidence Limits for Rates 1-6 (Scheme I) of Product Formation by Direct Propane Depletion individual confidence intervals reaction parameter optimal estimate lower limit upper limit 3.89069 X 106 -8.89475 X lo6 A*, mol/(g.h) 9.67289 X lo6 1.31170 X lo4 -5.09619 X lo3 3.13302 X lo4 EIR, K A*, L/mol 1.01002 x 10 -3.61354 X 10' 3.81554 X 10' -2.93788 X lo3 -3.10900 X lo' 2.52142 X lo4 EIR, K A*, L/mol 7.94937 x 10 -1.80027 X lo4 1.81617 X l@ -3.89682 X lo3 -2.07805 X lo5 2.00011 x 105 EIR, K A*, L/mol 7.43090 X 10 -1.64078 X lo4 1.65564 X lo' -3.49243 X lo3 -2.03567 X lo5 1.96583 X lo5 EIR, K 2.73141 X lo8 2 A*, mol/(g.h) -2.56617 X 10" 2.62080 X 1O'O 1.84960 X lo' -5.56029 X lo4 9.25950 X lo4 EIR, K A*, L/mol 4.03946 X lo2 -1.56155 X lo4 1.64234 X lo' 1.27582 X 10' -3.02697 X lo4 3.05249 X lo4 EIR, K A*, L/mol 1.60852 X 10' -1.07960 X lo4 1.11177 X lo4 1.40666 X lo2 -5.19646 X lo4 5.22469 X lo' EIR, K A*, L/mol 4.46378 X lo-' -3.518 78 3.608 06 -5.44484 X lo3 -6.68667 X lo4 5.59699 X lo' EIR, K A*, mol/(g.h) 8.02542 X 10s -1.73486 X 10" 3 1.75091 X 1O1l 1.94436 X lo4 -1.47990 X l o 5 1.86878 X lo5 EIR, K A*, L/mol 1.62636 X lo-' -1.80805 X 10 1.84058 X 10 -9.08011 X lo4 -4.55090 X lo3 8.16993 X lo4 EIR, K A*, L/mol 7.42725 X 10 -1.19585 X lo4 1.21070 X lo' -1.24371 X lo5 -5.06594 X 10' 1.23357 X lo5 EIR, K 5.05492 X lo-' A*, L/mol -1.25261 X 10' 1.26272 X lo2 -1.93390 X lo5 -3.60260 X lo3 1.86185 X lo5 EIR, K 4 2.47330 X lo5 -2.11895 X lo7 A*, mol/(g.h) 2.16842 X 10' -5.44905 X lo4 8.201 29 X lo4 1.37612 X lo4 EIR, K A*, L/mol 3.928 82 -1.45826 X 10' 1.53684 X 10' -3.59924 X lo3 -3.28598 X lo4 2.56614 X lo' EIR, K 1.132 28 A*, L/mol -8.61769 X 10 8.84414 X 10 -3.27932 X lo3 -6.23949 X lo4 5.58363 X l o 4 EIR, K 1.76892 X -1.108 15 A*, L/mol 1.14353 -5.60395 X lo4 -6.07697 x 103 4.38855 X lo4 EIR, K 3.07175 X 10' -3.50927 X lo9 5 A*, mol/(g.h) 3.57071 X lo9 1.49341 X lo4 -7.68134 X lo4 1.06682 X lo5 EIR, K -5.74403 X l o 3 A*, L/mol 7.53101 X 10 5.89565 X lo3 -5.94563 x 104 -2.08015 X 10' 5.90403 X lo4 EIR, K 3.969 88 A*, L/mol -3.54773 x 102 3.62713 X 10' -2.05264 X lo3 -7.14767 X lo4 6.737 14 X lo4 EIR, K A*, L/mol 6.37657 X loa -7.28949 X 8.56480 X -8.29790 X lo3 -2.36506 X lo4 7.05474 X lo3 EIR, K 1.73767 X lo6 6 A*, mol/(g.h) -2.05050 X 10' 2.08526 X lo8 -7.51321 X lo4 1.64049 X lo' 1.07942 X l o 5 EIR, K A*, L/mol 2.08285 X lo-' -2.13832 X 10 2.17998 X 10 -8.38645 X lo4 -4.31966 X lo3 7.52252 X lo4 EIR, K A*, L/mol 2.90740 X 10' -5.31707 X lo' 5.37522 X l o 4 -1.39411 X lo5 1.08016 X lo3 1.41572 X lo5 EIR, K A*, L/mol 5.08969 X 10 -6.65043 X lo3 6.75222 X lo3 -2.61997 X 10' -1.00282 X lo5 9.97582 x 104 EIR, K Table 11. Parameter Estimates and 9.5% Individual Confidence Limits for Rates 7-14 (Scheme I) of Consecutive Transformation of Products individual confidence intervals parameter optimal estimate lower limit upper limit reaction 7 kC A*,mol/(g.h) 6.634 34 X105 -3.69452 X IO6 5.02139 X lo6 EIR, K 1.18952 X lo' 7.00078 x 103 1.67897 X lo4 8 k C A*,mol/(g.h) 3.97062 X10' -6.70075 x 104 6.78017 X lo' EIR, K 1.00626 x 104 -1.20917 X lo5 1.41042 X lo5 9 kC A*,mol/(gh) 3.56581 X lo8 -5.29847 X lo5 5.36984 X lo5 EIR, K 1.17299 X lo4 -1.02544 X lo5 1.26004 X lo5 10 k C A*,mol/(g.h) 1.012 03 -1.44419 X 10' 1.46443 X 10' EIR, K 5.08453 X lo3 -9.59856 X lo4 1.061 55 X lo5 11 kC A*,mol/(g.h) 9.76201 X 10' -1.30655 X lo4 1.501 78 X lo' 8.71782 X lo3 -2.221 64 x 103 1.96573 X lo4 EIR, K 12 kc A*,mol/(g.h) 3.02893 X lo3 -1.19824 X lo5 1.25882 X lo5 -2.17430 X lo4 4.04952 X lo4 EIR, K 9.37608 X lo3 -8.73744 X lo5 9.07540 X lo5 13 k C A*,mol/(g.h) 1.68980 X lo4 -2.86542 X lo4 5.27887 X lo4 EIR, K 1.20672 X lo' -5.78067 X lo7 6.86088 X lo7 14 kC A*,mol/(gh) 5.40105 X lo6 2.36178 X lo4 1.46979 X lo4 5.77795 x 103 E/& K
discussed above. The yield of ACN increases continuously with increasing conversion up to a limiting propane conversion due to total oxygen consumption. An increase in
the limiting C3 conversion increases the reaction temperature and is due to the increase in the relative importance of the principal reactions with respect to the non-
118 Ind. Eng. Chem. Res., Vol. 31, No. 1, 1992
530
520 530
conv.X
n .
Figure 11. Yield and selectivity to acrylonitrile as a function of propane concentration and reaction temperature for two inlet feeds: (a) mol/L (14%); nitrogen, 2.09 X (53%); water, 5.13 X mol/L (13%); mol/L (6%), oxygen and ammonia, 5.53 X propane, 2.37 X mol/L (12%); nitrogen, 1.78 X (45%); water, 5.13 X mol/L mol/L (la%), oxygen and ammonia, 4.74 X (b) propane, 7.11 X (13%). See also text. Key as in Figure 2.
selective combustion of ammonia, which decreases the availability of both ammonia and oxygen. The maximum yield of ACN, which is found in these cases at the higher propane conversions that can be obtained, is lower for a propane concentration of 18%, because it is limited by the total consumption of oxygen. It can be noted, that the productivity in the two cases is similar. An increase in the productivity to ACN using high propane concentrations thus requires the use of pure oxygen instead of air, with unreacted propane recycle, and complete conversion of ammonia and oxygen. Ammonia concentration can best be lowered to an acrylonitrile commercial use level by catalyst modification (use of dopants to control NH3 combustion reaction conditions without impairing the propane activation properties of the catalysts).
Conclusions The kinetic analysis of propane ammoxidation to acrylonitrile of a V-Sb-AI based mixed oxide catalyst was carried out in a quartz tubular flow reactor operating both under differential and integral conditions, using propane, oxygen, and ammonia concentrations in the 0-2070 range and reaction temperatures in the 680-810 K range. The reaction network can be modeled using a Langmuir-Hinshelwood approach, six parallel formation reactions (propylene, acrylonitrile,carbon oxides, acetonitrile, HCN, and Cz hydrocarbons), three reactions of consecutive transformation of the intermediate propylene to acrylonitrile, acetonitrile, and HCN, five consecutive reactions of consecutive decomposition to carbon oxides of the other products, and one reaction of the decomposition of ammonia. The main route of acrylonitrile formation passes through the intermediate formation of propylene, and the limiting factor in the formation of acrylonitrile is the relative slowness of this step compared to the others. Maximum yields and selectivities of acrylonitrile obtained were in the 3540% and 5040% range, respectively. The kinetic reaction model was used to determine the best
reaction conditions using this VSb-AI based oxide catalyst and to compare the performance obtained using different possible feedstocks. Our kinetic analysis suggests future work in the area of cocatalysts to enhance the propylene to ACN conversion and to the use of higher propane:(O2 NH,)ratios with propane recycle and oxygen rather than air as the source of feed oxygen.
+
Acknowledgment We thank Prof. G. Buzzi-Ferraris for providing his nonlinear regression program. Registry No. VSb04, 12311-81-8;Sbz05,1314-60-9;WOs, 1314-35-8;7-Al203,1344-28-1; AcCN, 75-05-8;HCN, 74-90-8; Cz, 74-85-1;COX, 12795-06-1;ethane, 74-84-0;propane, 74-98-6; propylene, 115-07-1;acrylonitrile, 107-13-1.
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Received for review May 3, 1991 Revised manuscript received August 3, 1991 Accepted September 20,1991
Reduction of Sulfur Dioxide over Alumina-Supported Molybdenum Sulfide Catalysts David J. Mulligan and Dimitrios Berk* Department of Chemical Engineering, McCill University, Montreal, Quebec, Canada H3A 2A7
An experimental investigation of the reduction of SO2with CHI using molybdenum sulfide supported on alumina as a catalyst was carried out. Three molybdenum loadings of 5,10, and 15% were used. In addition, a catalyst which contained cobalt ( 5 % Co-15% Mo/A1203) was evaluated. The evaluations were based on the activity as well as the yields of sulfur and carbon dioxide. Experiments were carried out at temperatures ranging from 650 to 725 OC and inlet molar feed ratios of SO2 to CH4 of 1.0 and 2.0. The 5 and 10% molybdenum loadings showed similar activities and yields to each other. The catalyst containing 15% molybdenum had the highest activity and yields (above 77% for both sulfur and COS). All catalysts tested were more effective than alumina itself. The activity of the 15% Mo/A1203catalyst was 1.5-2 times that of alumina. This catalyst was stable under all reaction conditions. The addition of cobalt reduced the activity by 20%. In order to minimize the production of undesired by-products, the reaction temperature should be less than 700 'C.
Introduction The reduction of sulfur dioxide with methane is an important reaction because of its possible use in a process for the treatment of SOz which is removed from the waste gas streams produced by the roasting of sulfide ores. The primary reaction between SO2 and CHI is 2S02 + CHI 2Hz0 + 2[S]+ COZ (1) where [SIrepresents various sulfur species in the gas phase. Along with the primary reaction products, a number of undesired by-products are also possible. These include HzS, COS, CO, and elemental carbon. Therefore, an effective catalyst for this reaction system is one that has a high selectivity for elemental sulfur as well as carbon dioxide. In the past, the reduction of SO2 was implemented in an industrial process which used alumina as the catalyst (Hunter, 1972). Mulligan and Berk (1989) examined the
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use of pure crystalline MoS2,WSz, and FeS as catalysts for the same reaction. MoSz was found to be not only better than either of the other two metal sulfides, but also better than the commonly used alumina catalyst as MoS2 had a higher selectivity for the production of sulfur, high C02 yields, and comparable activity. Although pure crystalline MoS2is a promising catalyst, there are two problems which would have to be solved if the catalyst were to be used in a large-scale industrial process. First, the pure MoSz pellets used in the above study had a specific surface area of about 4 m2/g, which was 1125th that of alumina. This implies that a relatively large mass of MoSz would be required to obtain conversions found with much smaller amounts of alumina. The second consideration is cost, as pure MoSz is prohibitively expensive. Both of these problems can be addressed by using a catalyst support for MoS2 thus providing the required high surface area and a cost more in line with the
0888-5885/92/2631-0119$03.00/00 1992 American Chemical Society
