4150
Ind. Eng. Chem. Res. 2006, 45, 4150-4155
Kinetic Study of Ammonia Synthesis on a Promoted Ru/C Catalyst Ilenia Rossetti,† Nicola Pernicone,‡ Francesco Ferrero,§ and Lucio Forni*,† Dipartimento Chimica Fisica ed Elettrochimica, UniVersita` di Milano, Via C. Golgi, 19 I-20133 Milano, Italy, Via Pansa, 7 I-28100 NoVara, Italy, and Via Turbigo, 28 I-28100 Pernate (NO), Italy
A set of kinetic tests on a promoted Ru/C catalyst for ammonia synthesis has been carried out under industrially relevant reaction conditions (T ) 370-460 °C, P ) 50-100 bar). The results have been elaborated by using some kinetic models either derived from literature or here-developed. The best results, as for fitting and consistency of the optimized parameters, were obtained by simply modifying the Temkin equation with the addition of H2 and NH3 adsorption terms, to take into account their inhibiting effect on catalyst performance. Such an equation can be advantageously applied for better design and operation of ammonia synthesis reactors using Ru/C catalysts. The activation energy for the synthesis reaction was 23 kcal/mol, much lower than that previously found for Fe-based catalysts. From a practical point of view, it has been shown that the working pressure can be decreased, with the same plant productivity, by 40-50% when using the present Ru/C catalyst. Likewise, the ammonia concentration in the exit gas can be increased by about the same value if the working pressure is not decreased. Introduction Ru-based catalysts for ammonia synthesis are gaining growing attention, due to a significant increase of productivity with respect to the traditional Fe-based catalyst.1-4 This is mainly due to the kinetic inhibition of the Fe catalyst by ammonia,1,5 which levels reactant conversion to values far below with respect to thermodynamic equilibrium conversion. A commercial catalyst was then developed, in which Ru was supported on specially modified active carbon and promoted with alkali and alkali-earth compounds.6,7 The kinetics and mechanism of ammonia synthesis on Ru catalysts are still far from being completely understood, although a deeper comprehension of the nature of the active sites, accounting for the structure sensitivity of the reaction, has been reached during the past few years.8-13 The availability of reliable kinetic equations is of utmost importance for both design and operation of catalytic reactors. However, to the best of our knowledge, a few papers only, dealing with the main reaction steps, have been published till now,14-19 mainly based on Ru/MgO catalytic systems, hardly suitable for industrial exploitation. These studies are supported by data collected through different techniques, such as isotopic exchange reaction (IER), temperature programmed adsorptiondesorption (TPA-TPD), temperature programmed surface reaction (TPSR), or microkinetic analysis,9,14,20 aiming at better defining the rate-determining step, i.e., dissociative N2 adsorption, through the determination of the rate constants of the single reaction steps. Kinetic modeling on those catalysts was based on a Langmuir-Hinshelwood-Hougen-Watson (LHHW) approach, either as such or by imposing restrictive hypotheses on the rate determining step.16,20 The already accepted1 conclusion that N2 dissociative adsorption is by far slower than every other step could be simply confirmed by calculating the kinetic constants of every reaction step from the kinetic parameters there reported.16 Moreover, empirical equations, such as the power rate law,1,17,21 can be applied in practice, although it is preferable * To whom correspondence should be addressed. Fax: +39-0250314300. E-mail:
[email protected]. † Universita ` di Milano. ‡ Novara. § Pernate.
to base reactor design on a physically meaningful proven theoretical equation. As for Ru/C catalytic systems, Kowalczyk et al.18 compared the kinetic performance of their sample with that of Fe-based commercial samples under some different reaction conditions. The reaction rate experimentally determined, derived under differential conditions, confirmed that the activity of the Ru catalyst is much less sensitive to NH3 partial pressure, besides being much higher than that of promoted Fe. However, no detailed kinetic equation was proposed. In the present work, we undertook an extensive kinetic investigation on one of our promoted Ru/C catalysts, to compare different kinetic models and to find out a reliable kinetic equation, suitable for reactor design. Kinetic data have been collected by varying independently and systematically temperature (370-460 °C), pressure (50-100 bar), and H2/N2 feeding ratio (1.5 and 3 vol/vol). Data elaboration was accomplished by nonlinear regression and Runge-Kutta numerical integration of the model rate equation. At last, the present results have been compared with those obtained by using the most recent commercial catalysts based on Fe. Experimental Section Catalyst Preparation. A graphitised carbon and as supplied Aldrich and Acros “pro-analysi” reagents were used. Details about the preparation route can be found elsewhere.22 Promoters were added by impregnation from aqueous solutions of hydroxides (K and Cs) or nitrates (Ba), in the optimal amount determined in a previous work:23 Ba/Ru ) 0.6 (mol/mol), Cs/ Ru ) 1 (mol/mol), and K/Ru ) 3.5 (mol/mol). The Ru content, referred to the final catalyst weight, was 3.2%. The Ru dispersion, determined by oxygen chemisorption,24 was 11%. Activity Testing Apparatus and Procedure. Kinetic data have been collected by means of a mini plant, based on a vertical, downflow tubular reactor (Incoloy 800), 12.7 × 9.0 mm in diameter and 40 cm in length, fitted with an axial thermowell of 1.6 mm external diameter. Heating of the reactor was accomplished by an electric furnace, through a massive metal block surrounding the reactor. Temperature was controlled by an Eurotherm 904 TRC through a 425S solid-state-contactor power unit. The reactant gas mixture (Sapio H2 and N2, purity g99.9995 vol. %) was further carefully purified from oxygenates
10.1021/ie051398g CCC: $33.50 © 2006 American Chemical Society Published on Web 05/16/2006
Ind. Eng. Chem. Res., Vol. 45, No. 12, 2006 4151
Pseudo-saturation models such as
Table 1. Kinetic Tests Program test no.
pressure (bar)
temperature (°C)
H2/N2 vol.
1 2 3 4 7 8 10 11 13 15 16 17 18 19 20 21 22 23 24
100 100 85 85 70 70 85 85 100 100 100 70 50 100 70 50 100 70 50
430 370 370 430 370 430 370 430 370 430 430 430 430 400 400 400 460 460 460
1.5 1.5 1.5 1.5 3 3 3 3 3 1.5 3 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5
[
a b + + 1 + 100τ (1 + 100τ)2 d c + (3) (1 + 100τ)3 (1 + 100τ)4
]
have also supplied very good regression results. The data here presented were obtained by means of the e1 saturation model. All of the parameters in the regression equation have been determined with average errors not exceeding 3.5%. The parameters in the rate equation, i.e., kinetic constant and equilibrium adsorption constants, have been optimized by an iteration procedure. The following mathematical equations, symbols, and conventions have been used in the present kinetic analysis. H2/N2 molar ratio in the feed: q ) 3 or q ) 1.5; Time factor (τ) defined as
by passing through a trap packed with a proper amount of frequently regenerated, reduced Fe-based commercial ammonia synthesis catalyst. Gases flow rate was regulated by means of Brooks (mod. 5850S) mass flow meters and the reaction pressure was set by means of a homemade PTFE-membrane relief valve, pressurized with nitrogen at the desired value. A high-precision manometer, a bursting-disk safety device, a drechsel gas absorber, and a volumetric wet-test meter completed the apparatus. Approximately 0.1 g of catalyst was loaded into the reactor after dilution with quartz powder of the same particle size (0.15-0.25 mm). The catalyst/quartz volumetric ratio was 1/32, to minimize the temperature gradient due to reaction exothermicity. The bed was kept at the mid length of the reactor by flocks of quartz wool, and the void space over and below the catalyst bed was filled with 0.25-0.85 mm quartz beads. Catalyst activation was achieved by flowing the reacting mixture (H2/N2 ) 1.5 vol/vol) at 30 bar total pressure and GHSV ) 20 000 h-1. The temperature was increased by 1 °C/min from r.t. up to 450 °C, kept for 5 h, and then decreased by 1 °C/min down to the selected reaction temperature. Every activity test was performed by varying the gas mixture feeding rate from GHSV ) 50,000 to 400,000 h-1. Effluent gas analysis was done by absorption in diluted H2SO4 and subsequent titration of the residual acid with NaOH. Temperature, pressure, and H2/N2 feeding ratio were varied according to the kinetic test program summarized in Table 1. Kinetic Analysis. The fit of the experimental data of conversion vs time factor has been performed by means of Runge-Kutta numeric integration of the various differential rate equations. The integration initial conditions, η0 and τ0, have been set to values greater than zero to avoid singularity error message due to the possible presence of aNH3 ) 0 at the denominator of the rate equation. Suitable values of η0 and τ0 have been calculated by means of non linear regression of the fractional conversions vs. time factor (τ), making use of saturation models such as:
τa+b‚τ+c‚τ
τ0.5 + τ
τ)
22.414 litres of catalyst ) GHSV‚xreadif of either N2 (q ) 3) or mol h H2 (q ) 1.5) fed to the reactor
where xreadif ) 0.25 if q ) 3, 0.60 if q ) 1.5; Fractional conversion of either N2 (q ) 3) or H2 (q ) 1.5) (ηs): ηs ) a‚(x/1+x) where x ) mole fraction of NH3 measured in the exit gas, a ) 2 if q ) 3 and a ) 2.5 if q ) 1.5. The equilibrium constant for ammonia synthesis in terms of activities was calculated according to Gillespie and Beattie25,26
log10 Ka ) -2.691122‚log10 T - 5.519265‚10-5‚T + 2001.6 + 2.6899 (4) 1.848863‚10-7‚T2 + T The fugacity coefficients were calculated through the NewtonCooper equations25
γN2 ) 0.93431737 + 0.3101804 × 10-3T + 0.295896 × 10-3P - 0.2707279 × 10-6T2 + 0.4775207 × 10-6P2 (5)
[
γH2 ) exp exp(-3.8402T0.125 + 0.541)P exp(-0.1263T0.5 - 15.98)P2... + P 300 exp(-0.011901T - 5.941) exp -1 300
( (
γNH3 ) 0.1438996 + 0.2028538 × 10-2T - 0.4487672 × 10-3P - 0.1142945 × 10-5T2 + 0.2761216 × 10-6P2 (7) Finally, the partial activities of reactants and product were expressed as
a N2 )
1 - ηβ(q) Pγ 1 + q - 2ηβ(q) N2
(8)
a H2 )
q - 3ηβ(q) Pγ 1 + q - 2ηβ(q) H2
(9)
aNH3 )
2ηβ(q) Pγ 1 + q - 2ηβ(q) NH3
(10)
2
a+b‚τ+c‚τ2
d+τ
(1)
) )] (6)
where β(q) ) 1 if q ) 3, β(q) ) 0.5 if q ) 1.5, and P is the total pressure (atm).
or
τea+b‚τ+c‚τ
2
d + τea+b‚τ+c‚τ
2
with a, b, c, and d being the parameters to be optimized.
(2)
Results and Discussion A preliminary check of the possible effect of mass and heat transfer limitations under the selected experimental conditions
4152
Ind. Eng. Chem. Res., Vol. 45, No. 12, 2006
showed that the adopted catalyst particle size allowed any intraparticle diffusional effect to be virtually eliminated, without increasing unacceptably the pressure drop along the catalyst bed. Furthermore, tests carried out at the same GHSV values with different linear velocity, e.g., by varying the amount of catalyst loaded (with catalyst/quatz dilution ratio ) 1/22), showed the same ammonia content in the exit gas. Hence, it was concluded that the effect of external diffusion was negligible under the reaction conditions adopted. The latter test also allowed any possible by-passing effect to be excluded. As for this point, we referred to the recommendations by Moulijn et al.27 Indeed, during kinetic analysis, a 1/32 catalyst to quartz dilution ratio was used, corresponding to b ) 0.969 of the notations of Moulijn. On this basis, a decrease of conversion (∆) of 0.0073 should be expected, caused by the high dilution ratio at the highest conversion value (mean particle size ) 0.2 mm and bed height ) 60 mm).27 This value can be considered clearly negligible, mainly due to the low conversion degree. The experimental data of the whole set of tests are reported in Table 2, as ammonia vol % in the outlet gas. The average temperature along the bed, determined by moving the thermocouple along the axial thermowell is also reported for every test. As expected, the highest temperature excursion was noticed for the highest value of GHSV (400 000 h-1), despite the lower conversion, due to higher productivity. However, due to the high dilution ratio with quartz powder, the ∆T along the bed was always kept below 3.5 °C. It is well-known that Ru-based catalysts are inhibited by h2.1 Hence, a nonstoichiometric feeding mixture is commonly preferable. This was confirmed by tests no. 7, 8, 10, 11, 13, and 16 (Tables 1 and 2), performed with H2/N2 ) 3 (vol/vol), corresponding to the stoichiometric feeding ratio, with respect to tests 1, 2, 3, 4, 15, and 17, carried out with H2/N2 ) 1.5 (vol/vol). A comparison of our results with equilibrium conversion is reported in the last column of Table 2. These data allow us to conclude that equilibrium conversion can be attained at 460 °C, with H2/N2 ) 1.5 vol/vol, for every reaction pressure. Conversely, the ratio of experimental over equilibrium ammonia concentration in the outlet gas attained 80-85% at 430 °C, whereas it dropped to ca. 40% at 400 °C and to ca. 16% at 370 °C. H2 inhibition of the catalyst was confirmed by a similar comparison based on the tests carried out under stoichiometric feeding conditions. Indeed, lower values of experimental over equilibrium conversion ratio were obtained with H2/N2 ) 3 vol/ vol, for every reaction pressure (ca. 60% at 430 °C and ca. 10% at 370 °C). Kinetic Modeling. Temkin Equation. A first attempt of kinetic modeling was carried out by using the well-known Temkin equation,1,5,28 by far the most used to represent Febased catalysts performance. The main assumptions for this model are (i) N2 dissociative adsorption as rate determining step and (ii) negligible influence of H2 and NH3 on N2 adsorption. These assumptions lead to the following rate equation:
( [ ] [ ])
(aH2)3 dη ) krλ(q) Ka2aN2 dτ (aNH3)2
R
-
(aNH3)2 (aH2)3
1-R
(11)
where kr is the kinetic constant of the reverse reaction (ammonia decomposition), Ka is the reaction equilibrium constant, dη/dτ expresses the rate of consumption of the defective reactant in mol/h‚Lcat, ai represent the activities of reactants and product, and λ(q) was set to 1 or 3 when the H2/N2 feeding ratio was 3
or 1.5, respectively. R is a function of the catalyst nature, to be kept at a constant value (usually 0.75) for comparison purposes. As expected, fitting of experimental data of our Ru-based catalyst by this rate equation was not satisfactory. In fact, though the rate determining step for both catalytic systems is the same, i.e. N2 dissociative adsorption, the Fe-based catalyst is unfavored at high conversion, due to the inhibiting effect of ammonia, whereas Ru is rather inhibited by H2. Further attempts were carried out by varying the R value in order to obtain a better data fitting, but the model did not show satisfactory results even with R as low as 0.15 and therefore it was abandoned. LHHW Equation. A LHHW approach was then tried, to better highlight the role of single reaction steps on the overall kinetics. Starting from the separation of the reaction into elementary steps, this model expresses the reaction rate as the velocity of the slowest step, the other ones virtually attaining equilibrium. For example
N2 + σ f 2 N-σ
(r1)
H2 + σ T 2 H-σ
(r2)
N-σ + H-σ T NH-σ + σ
(r3)
NH-σ + H-σ T NH2-σ + σ
(r4)
NH2-σ + H-σ T NH3-σ + σ
(r5)
NH3-σ T NH3 + σ
(r6)
the symbol σ representing an active site on the catalyst surface. Following the LHHW approach, Buzzi Ferraris and coworkers29 considered 23 possible kinetic models, among which the following one gave the best fit. Hence, it was applied also in the present investigation
[ ]
2 1 (aNH3) aN2(aH2) (Ka)2 (aH2) 2
dη ) λ(q) dτ (aNH3)2 2 c1(aN2) + c2 + c3aNH3aH2 aH 2
(12)
where c1, c2, and c3 are parameters to be optimized. The second term in the denominator was negligible with respect to the other two. c1 and c3 include both the kinetic constant kf and the adsorption constants, leading to problems when attempting to interpret their dependence on temperature. Moreover, despite the generally acceptable fitting of the data, some discrepancies were observed between the constants obtained at the same temperature with different feeding ratios. Hence, the model showed unsatisfactory results, very likely due to low accuracy in describing H2 inhibition. Indeed, although linear regression of ln c3 vs 1/T gave a positive slope with a correlation coefficient of 0.999, the same regression on c1 was not satisfactory, giving a positive slope with a low correlation coefficient (0.945). In conclusion, despite the relatively good fitting of this model, its main limitations depend on the lack of physical meaning of the optimized parameters. Modified Temkin Equation. The above-reported results showed, as mentioned, that no completely satisfactory interpretation of kinetic data could be obtained by using either the pure LHHW model, with explicit adsorption terms and restrictive hypotheses on the rate-determining step, or the Temkin equation, which was developed for a specific, different catalytic
Ind. Eng. Chem. Res., Vol. 45, No. 12, 2006 4153 Table 2. Kinetic Tests Experimental Data Expressed as Vol. % NH3 in the Reactor Exit Gas
d
test no.
50 000a
60 000a
90 000a
120 000a
200 000a
1 2 3 4 7 8 10 11 13 15 16 17 18 19 20 21 22 23 24
13.20 3.55 3.41 11.82 2.19 8.21 2.24 9.45 3.13 14.16 12.53 9.49 8.21 7.28 6.62 5.33 13.23 10.07 8.79
12.62 3.32 3.08 11.33 1.91 7.75 2.00 8.42 2.90 12.50 10.00 9.03 7.34 6.64 5.66 4.86 13.17 9.92 7.72
11.66 2.62 2.43 9.53 1.43 6.78 1.62 6.94 2.19 11.13 8.29 7.82 6.49 5.16 4.61 4.10 12.11 9.35 7.16
10.05 2.24 2.00 8.25 1.14 5.80 1.28 5.64 1.62 9.06 6.99 6.20 5.12 4.32 4.11 3.32 11.28 8.74 6.56
7.62 1.48 1.43 5.46 0.60 4.15 0.60 4.16 1.04 6.91 5.24 4.83 3.98 3.51 2.59 2.48 8.84 7.04 5.58
300 000a
400 000a
4.87 0.89 n.d.d 4.17 n.d.d 2.81 n.d.d 2.72 n.d.d 4.83 3.42 3.31 3.18 2.25 1.88 2.23 6.95 5.55 4.29
3.60 0.60 n.d.d 3.30 n.d.d 2.24 n.d.d 2.10 n.d.d 3.90 2.81 2.85 2.25 1.88 1.56 2.09 5.87 4.57 3.69
TAveb (°C)
Exp/Eqc
431.1 366.9 366.9 431.2 366.6 430.7 366.6 430.7 366.9 431.7 431.7 431.7 431.4 400.9 400.3 400.2 461.3 461.2 460.6
0.80 0.17 0.16 0.80 0.10 0.59 0.10 0.59 0.12 0.86 0.69 0.76 0.85 0.38 0.42 0.43 1.00 1.01 1.09
a GHSV values expressed in h-1. b Average reaction temperature measured by means of the axial thermocouple. c Experimental/equilibrium conversion. n.d. ) not determined. NH3 < 0.5% data considered not reliable.
system. Moreover, changing the parameters of the Temkin equation by modulation of the R parameter was not sufficient to account for a catalytic behavior substantially different from that of the Fe-based catalyst. Hence, we tried to introduce terms accounting for the competitive adsorption properties of the Rubased catalyst, by modifying the classical Temkin equation. A general form of the proposed model is the following: dη ) kfλ(q) dτ (aN2)
n
[ ]
R
(aH2)3n
(aNH3)2n
[ ]
2n 1 (aNH3) (Ka)2n (aH2)3n
2nw3
1-R
(aNH3) 1 1 + KN2 + KH2(aH2)3n‚w2 + KNH3(aNH3)2nw3 2nw3 (Ka) (aH2)3nw2 (13) A preliminary application of this model allowed to define the following values of the optimized parameters: n ) 0.5, R ) 0.25 and w2 ) w3 ) 0.2. This resulted in the following equation:
(aN2)
[ ] [ ]
0.5
(aH2)0.375
0.75 1 (aNH3) Ka (a )1.125 H2
(aNH3)0.25 dη ) kfλ(q) dτ 1 + KH2(aH2)0.3 + KNH3(aNH3)0.2
(14)
The adsorption term relative to N2 showed a negligible influence, and hence, only three parameters, kf, KH2, and KNH3, needed to be optimized. Data fitting showed a good agreement for every test run, giving an error mostly lower than 5% and well represented the overall catalytic behavior for the whole set of experimental tests. An example of the results obtained is reported in Figure 1, relative to tests no. 11 and 22 (Tables 1 and 2). The consistency of the optimized parameters was checked again through the Arrhenius (Figure 2a) or Van’t Hoff equations (Figure 2b,c), which allowed to obtain reliable kinetic and thermodynamic parameters (Table 3), with excellent correlation coefficients. The values of ∆Hads derived from this model confirm H2 inhibition. The NH3 adsorption term gives however a significant contribution (Table 3).
Figure 1. Example of data fitting with the proposed modified Temkin equation. (a) test no. 22, H2/N2 ) 1.5; (b) test no. 11, H2/N2 ) 3 (Tables 1,2). O: experimental points, full line: integration curves. Table 3. Kinetic and Thermodynamic Parameters of Ru/C Catalyst (Modified Temkin Equation)a Ea
A
CC (A)
23.0 9.02 × 108 0.9998
CC CC ∆Hads,H2 ∆Sads,H2 (VH) ∆Hads,NH3 ∆Sads,NH3 (VH) -9.0
-13.6 0.9956
-7.0
-8.3
0.9998
a
Ea and ∆Hads are expressed as kcal/mol, and ∆Sads is expressed as cal/ mol K.
Comparison with Commercial Fe-Based Catalysts. From a practical point of view, lowering the operating pressure to a value nearer to that of synthesis gas preparation would lead to a substantial energy saving.30 Hence, it is interesting to compare, under different working conditions, the performance of our catalyst with a commercial Fe-based catalyst, tested under its usual reaction conditions, i.e., at 100 bar, 430 °C, and H2/N2 ) 3 (vol/vol). The present results show that with our Ru-based catalyst it is possible to obtain a conversion comparable to that of the Fe-based sample, but at considerably lower pressure. Indeed, when fixing, for example, operating conditions such as τ ) 0.0010 h L mol-1, H2/N2 ) 3 (vol/vol) and T ) 460 °C, the same ammonia content in the exit gas obtainable with the Fe-magnetite catalyst at 100 bar (6.6%) can be obtained at 57 bar with the Ru/C catalyst (Figure 3). From the same figure it can be also inferred that the advantage of using the Ru/C catalyst increases at the higher contact times typical of the industrial conditions. Hence our Ru-based catalyst allows the working pressure to decrease by 40-50% with respect to a commercial magnetite-derived catalyst. Similar considerations can be extended to calculate the increase of NH3 concentration in the
4154
Ind. Eng. Chem. Res., Vol. 45, No. 12, 2006
Figure 4. Comparison between the Arrhenius plots of kf for the present Ru-based catalyst and for Fe-based commercial catalysts from magnetite and wustite. Table 4. Comparison between Kinetic Parameters of Ru/C Catalyst (Modified Temkin Equation) and of Fe-Based Catalysts from Magnetite and Wustite (Temkin Equation)a Ru/C Ea 23.0
Fe (magnetite) A
9.02 ×
Ea 108
35.0
A 3.93 ×
1010
Fe (wustite) Ea
A
32.4
1.04 × 1010
a Data referred to the kinetic constant of the synthesis reaction. E is a expressed as kcal/mol.
Figure 2. Arrhenius (a) and Van’t Hoff (b and c) plots of the kinetic parameters for the modified Temkin equation (Ru/C catalyst).
with bed volumes -34% and -54% at 430 and 460 °C, respectively. The value of the activation energy obtained with the present analysis for our Ru-based catalyst was Ea ) 23 kcal/mol (Table 3). In a previous paper,31 we compared the activity of a commercial Fe-based catalyst from magnetite with that of a catalyst prepared from wustite. The activation energy determined on the kinetic constant of the reverse reaction (kr) for those samples by using the classical Temkin equation was found to be 47.5 kcal/mol for the former and 44.9 kcal/mol for the latter. Calculating the kinetic constant kf for the synthesis reaction from kr and Ka, the activation energy, referred to the direct synthesis reaction, was found to be 35 kcal/mol for the magnetite-based catalyst and 32.5 kcal/mol for the wustite-based one. The Arrhenius plots of the three catalysts (Ru/C, Fe-magnetite, and Fe-wustite) are shown in Figure 4. It follows that Ru strongly decreases the energy required for the dissociation of the dinitrogen molecule (Table 4). Conclusions
Figure 3. Performance of Fe-based catalysts and of Ru/C catalysts at 460 °C, H2/N2 ) 3: (a) Fe at 100 atm; (b) Ru/C at 57 atm; (c) Ru/C at 100 atm.
outlet gas at the fixed pressure of 100 bar. Such an increase is +28% and +42% at 430 and 460 °C, respectively. If preferred, it is also possible to decrease the bed volume of Ru-based catalyst. In this case, the same conversion of the magnetitebased catalyst is attained, under identical operating conditions,
The present results show that with our Ru/C catalyst the equilibrium conversion can be attained at 460 °C by feeding under-stoichiometric H2/N2 mixture. The simple addition of adsorption terms for H2 and NH3 in the Temkin equation allowed to obtain a reliable kinetic model, useful for both design and operation of industrial reactors. An activation energy value as low as 23 kcal/mol for the ammonia synthesis reaction was obtained, showing that the activation barrier for N2 dissociation on Ru is by far lower than for the traditional Fe-based catalyst, prepared from either magnetite or wustite precursor. When comparing our Ru/C catalyst with the best commercial Fe catalysts, a substantial reduction of the reaction pressure can be obtained with the same plant productivity.
Ind. Eng. Chem. Res., Vol. 45, No. 12, 2006 4155
List of Symbols A ) preexponential factor ai ) activity of the indicated species (atm) c1, c2, and c3 ) parameters to be optimized CC ) correlation coefficient ∆Hads ) adsorption enthalpy change (kcal/mol) Ea ) activation energy (kcal/mol) Ka ) equilibrium constant of the reaction, in terms of activities Ki ) equilibrium constants for the adsorption of the indicated species (kcal/mol) Kr ) kinetic constant of the reverse reaction (ammonia decomposition) kf ) kinetic constant of the direct reaction q ) H2/N2 molar ratio in the feeding gas mixture r ) reaction rate (mol/h Lcat) Greek Symbols γi ) fugacity coefficient of the species i η ) fractional conversion dη/dτ ) rate of consumption of the reactant (mol/h Lcat) λ(q) ) stoichiometric parameter σ ) active site on catalyst surface τ ) time factor (h L/mol) Literature Cited (1) Tennison, S. R. In Catalytic Ammonia Synthesis; Jennings, J. R., Ed.; Plenum Press: New York, 1991; p 303. (2) Kowalczyk, Z.; Sentek, J.; Jodzis, S.; Mizera, E.; Goralski, J.; Paryjczak, T.; Diduszko, R. Catal. Lett. 1997, 45, 65. (3) Forni, L.; Molinari, D.; Rossetti, I.; Pernicone, N. Appl. Catal. A: General 1999, 185, 269. (4) Liang, C.; Wei, Z.; Xin, Q.; Li, C. Appl. Catal. A: General 2001, 208, 193. (5) Gramatica, G.; Pernicone, N. In Catalytic Ammonia Synthesis; Jennings, J. R., Ed.; Plenum Press: New York, 1991; p 211. (6) U. S. Patent 4,163,775, Aug. 7, 1979, to BP Co. (7) U. S. Patent 4,568,532, Feb. 4, 1986, to Kellogg, M. W. Co. (8) McClaine, B. C.; Davis, R. J. J. Catal. 2002, 210, 387. (9) Siporin, S. E.; Davis, R. J.; Rarog-Pilecka, W.; Szmigiel, D.; Kowalczyk, Z. Catal. Lett. 2004, 93 (1-2) 61. (10) Dahl, S.; Logadottir, A.; Egeberg, R. C.; Larsen, J. H.; Chorkendorff, I.; Tornqvist, E.; Norskov, J. K. Phys. ReV. Lett. 1999, 83, 1814.
(11) Dahl, S.; Tornqvist, E.; Chorkendorff, I. J. Catal. 2000, 192, 381. (12) Zambelli, J.; Trost, J.; Wintterlin, J.; Ertl, G. Phys. ReV. Lett. 1996, 76, 795. (13) Jacobsen, C. J. H.; Dahl, S.; Hansen, P. L.; Tornqvist, E.; Jensen, L.; Topsoe, H.; Prip, D. V.; Moenshaug, P. B.; Chorkendorff, I. J. Mol. Catal. A 2000, 163, 19. (14) Hinrichsen, O.; Rosowski, F.; Hornung, A.; Muhler, M.; Ertl, G. J. Catal. 1997, 165, 33. (15) Hinrichsen, O. Catal. Today 1999, 53, 177. (16) Hinrichsen, O.; Rosowski, F.; Muhler, M.; Ertl, G. Chem. Eng. Sci. 1996, 51, 1683. (17) Rosowski, F.; Hornung, A.; Hinrichsen, O.; Herein, D.; Muhler, M.; Ertl, G. Appl. Catal. A: General 1997, 151, 443. (18) Kowalczyk, Z.; Jodzis, S.; Sentek, J. Appl. Catal. A: General 1996, 138, 83. (19) Kowalczyk, Z.; Sentek, J.; Jodzis, S.; Muhler, M.; Hinrichsen, O. J. Catal. 1997, 169, 407. (20) Siporin, S. E.; Davis, R. J. J. Catal. 2004, 225, 359. (21) Aika, K.; Kumasaka, M.; Oma, T.; Kato, O.; Matsuda, H.; Watanabe, N.; Yamazaki, K.; Ozaki, A.; Onishi, T. Appl. Catal. A: General 1986, 28, 57. (22) Rossetti, I.; Pernicone, N.; Forni, L. Catal. Today 2005, 102103, 219. (23) Rossetti, I.; Pernicone, N.; Forni, L. Appl. Catal. A: General 2001, 208, 271. (24) Rossetti, I.; Pernicone, N.; Forni, L. Appl. Catal. A: General 2003, 248, 97. (25) Dyson, D. C.; Simon, J. M. Ind. Eng. Chem., Fundam. 1968, 7 (4), 605. (26) Gillespie, L. J.; Beattie, J. A. Phys. ReV. 1930, 36, 743. (27) Berger, R. J.; Perez-Ramı`rez, J.; Kapteijn, F.; Moulijn, J. A. Chem. Eng. Sci. 2002, 57, 4921. (28) Temkin, M. I.; Pyzhev, V. Acta Physicochim. U.S.S.R. 1940, 12, 327. (29) Buzzi Ferraris, G.; Donati, G.; Rejna, F.; Carra`, S. Chem. Eng. Sci. 1974, 29, 1621. (30) Ullmann’s Encyclopedia of Industrial Chemistry, 5th ed.; Gerhartz, W., Campbell, F. T., Pfefferkorn, R., Ronnsaville, J. F., Eds.; VCH: Weinheim, Germany, 1985; Vol. A2, p 143. (31) Pernicone, N.; Ferrero, F.; Rossetti, I.; Forni, L.; Canton, P.; Riello, P.; Fagherazzi, G.; Signoretto, M.; Pinna, F. Appl. Catal. A: General 2003, 251 (1), 121.
ReceiVed for reView December 15, 2005 ReVised manuscript receiVed March 23, 2006 Accepted April 19, 2006 IE051398G
