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Ind. Eng. Chem. Res. 2009, 48, 4–6
Diffusional Effects in Nickel Oxide Reduction Kinetics Peter Erri and Arvind Varma* School of Chemical Engineering, Purdue UniVersity, West Lafayette, Indiana 47907
Nickel is widely used as an industrial catalyst and has recently been investigated as an oxygen carrier in chemical looping combustion (CLC), which is a novel approach for power generation that offers inherent CO2 capture. However, prior studies have reported a wide range of activation energy values for nickel formation from the reduction of NiO by hydrogen. In the present work, this reaction was investigated using the Friedman isoconversional method for a series of thermogravimetric curves, which were obtained at varying heating rates. The intrinsic activation energy for the reduction of NiO was observed to be in the range of 91.8-94.5 kJ/mol, whereas values of 42.6 kJ/mol and ∼0 kJ/mol were found at intermediate and high heating rates, respectively, which correspond to intermediate and high temperatures. This demonstrates the presence of internal and external mass-transfer limitations during the reaction, and it provides a likely explanation for the varying activation energy values reported in prior work. Therefore, these findings emphasize the importance of assessing diffusional effects in the determination of reaction kinetics. 1. Introduction
2. Kinetic Analysis
Nickel is an important industrial catalyst and is typically formed in situ via the reduction of NiO.1 It also exhibits potential as an oxygen carrier in chemical looping combustion (CLC), which is a novel approach for power generation that offers inherent CO2 capture.2,3 In this process, NiO is alternatingly reduced and oxidized as it circulates between two reactors. In the first reactor, the oxygen carrier is oxidized by air, providing thermal energy for power generation while it is reduced by fuel (methane or coal gas) in the second reactor. In this manner, upon water condensation, a pure CO2 stream is released from the fuel reactor, despite the use of air as an oxidant during power generation. In CLC and the activation of industrial nickel catalysts, intrinsic kinetics of the oxide reduction are important for the selection of process conditions and reactor size. Many studies have been reported in this area, including those of Richardson et al.4,5 who performed hot-stage X-ray diffraction (XRD) analyses of NiO and NiO/Al2O3 reduction by excess hydrogen, in the absence of water. They proposed a three-step reaction process, where, first, nickel-metal clusters are formed during an induction period. Next, the reaction rate accelerates as the cluster size increases, followed by a pseudo-first-order reaction, with respect to nickel. The authors obtained an activation energy of Ea ) 85.6 kJ/mol for the reduction of NiO, in the absence of water vapor, and they also summarized previous results, which range from 17 kJ/mol to 98 kJ/mol. Similarly, recent investigations have reported differing values, as Hossain and de Lasa6 reported a value of Ea ) 53.5 kJ/mol for NiO supported on Al2O3, whereas Jankovic et al.7 obtained a value of Ea ) 96.4 kJ/mol. Richardson et al. hypothesized that the broad range of values was due to mass-transfer effects, but they did not elaborate on this issue. In the present work, diffusional effects on the reduction of NiO and NiO/NiAl2O4 are analyzed. The reaction activation energy is evaluated using the Friedman isoconversional method8 for a series of thermogravimetric curves obtained at varying heating rates. The obtained values are compared, and the diffusion limitations are discussed.
In this section, the Friedman method, which is used to extract the reaction kinetics, is described briefly, followed by a discussion of the activation energy falsification, which is due to diffusional effects. 2.1. Friedman Method. The activation energy was determined using the differential isoconversional Friedman method,7,8 which enables the use of nonisothermal thermogravimetric analysis (TGA) data. This technique takes advantage of the changes in the reaction process that are caused by varying the heating rates. Specifically, for low heating rates, the oxide spends more time at low temperature while for faster heating, it is quickly exposed to elevated temperatures, where the reaction rate are faster. Therefore, a comparison of the resulting reaction rates enables estimation of the activation energy of the process. Consider the reaction rate expression, which is generally given as
r
( )
-Ea m dx ) k′0 exp y (1 - x)n dT RgT
(1)
where n k′0 ) k0CHm2,bulkCNiO,i
Introducing a heating rate β, which is defined as β ) dT/dt, eq 1 becomes
β
( )
-Ea m dx ) k′0 exp y (1 - x)n dT RgT
(2)
which, for a constant value of x, leads to
[ ( dTdx )] ) - E
d ln β
d(1 ⁄ T)
* Author to whom correspondence should be addressed. Tel.: 765494-4075. Fax: 765-494-0805. E-mail address:
[email protected]. 10.1021/ie071588m CCC: $40.75 2009 American Chemical Society Published on Web 06/21/2008
a
Rg
(3)
Ind. Eng. Chem. Res., Vol. 48, No. 1, 2009 5
Thus, eq 3 enables evaluation of the reaction activation energy (Ea) by plotting the reaction rate versus 1/T, at fixed conversion for varying heating rates, yielding an isoconversional Arrhenius plot. 2.2. Falsification of Activation Energy. To investigate the effect of mass-transfer limitations in the Friedman method, Weisz-Prater analysis is performed, as described previously in detail for the diffusion reaction in catalyst pellets.9,10 For this, an effectiveness factor (η) is introduced, being the ratio of the observed reaction rate to that of the maximum possible one, i.e., if all the NiO were exposed to the bulk-phase hydrogen concentration. The observed reaction rate can then be expressed as follows: robs ) β
( )
Ea dx ) k′0 exp (1 - x)nη dT RgT
(4)
Proceeding as shown above now yields
( dTdx )
ln β
obs
) ln(k′0) + n ln(1 - x) -
Figure 1. Thermogravimetric plots for NiO reduction at varying heating rates.
Ea + ln(η) RgT
(5)
so that
{ ( dTdx ) } ) - E + d{ln(η)} d{ln(φ)}
d ln β
obs
d(1 ⁄ T)
a
Rg
d{ln(φ)} d(1 ⁄ T)
(6)
where φ is the Thiele modulus. From its definition, with an assumed constant diffusion coefficient, Ea d{ln(φ)} )d(1 ⁄ T) 2Rg
(7)
which leads to -
Ea,obs Ea 1 d{ln(η)} )- 1+ Rg Rg 2 d{ln(φ)}
[ ()
]
(8)
Equation 8 illustrates the possibility of falsification of the activation energy in isoconversional analysis. Here, external and internal mass-transfer limitations can produce misleading results through the dependence of η on σ. As discussed by Rossberg and Wicke11 and Ishida and Wen,12 the features of the noncatalytic and catalytic reactions, in this regard, are similar. Thus, for small values of φ (i.e., kinetic control),9,10 d{ln(η)}/ d{ln(φ)} f 0, implying that Ea,obs ) Ea, which is the intrinsic value. For large values of φ, d{ln(η)}/d{ln(φ)} f -1 or -2, for internal or external mass-transfer control, which, from eq 8, leads to Ea,obs ) Ea/2 or Ea,obs ) 0, respectively. Given the previously described analysis, it seems that the broad range of activation energies reported for nickel reduction, as described in section 1, could be attributed to diffusional effects. This issue is explored by the experimental work that is discussed next. 3. Experimental Section NiO and NiO/NiAl2O4 were synthesized using the solution combustion technique, as described previously.3 Briefly, the metal nitrates (oxidizer) were mixed with glycine (fuel) in water and heated. When water was evaporated, the resulting mixture was ignited, forming a fine powder. The unsupported cubic oxide was calcined at 700 °C (to ensure full oxidation) and ball-milled for 24 h. The spinel supported oxide (weight ratio of 40% NiO: 60% NiAl2O4) was pressed, calcined at 1300 °C, and crushed to a particle size of 125-150 µm. A relatively high calcination
Figure 2. Isoconversional Arrhenius plot for the reduction of NiO at a heating rate of 1-45 °C/min, for a conversion of x ) 0.2.
temperature was applied to enable the formation of high-strength materials with potential application in CLC. The reduction experiments were conducted in a TGA apparatus (TGA-DSC Q600, TA Instruments) under 20% H2 (with the balance being argon), at 100 sccm and heating rates of 1-45 °C/min. The samples (2.5 mg) were placed on a flat alumina plate, to eliminate possible diffusional effects that may arise from the sample holder geometry. 4. Results and Discussion Figure 1 displays the thermogravimetric curves for NiO reduction at selected heating rates, where complete conversion of NiO to elemental nickel results in a final weight that is 78.6% of the initial value. Note that, as reported previously,7 the onset of reduction occurs at progressively higher temperatures when increasing heating rates are applied. This feature enables the use of the Friedman method as described previously, yielding isoconversional Arrhenius plots for NiO reduction. These were generated for heating rates of 1-45 °C/min and a conversion range of 20%-40%, which previously has been shown to yield reliable results.7 An example is shown in Figure 2 for x ) 0.2. Note that three regions are present, with increasing slopes at lower temperatures (T) (which correspond to lower heating rates). At high temperature (ramp ) 30-45 °C/min), the slope is zero, which indicates that the reaction is limited by external
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Ind. Eng. Chem. Res., Vol. 48, No. 1, 2009
Table 1. Intrinsic Activation Energies for Unsupported NiO Reduction at Varying Conversions conversion (%) 20 30 40
Ea (kJ/mol)
R2
91.8 94.5 92.8
0.926 0.929 0.925
mass transfer. In the intermediate region (ramp ) 5-25 °C/ min), the plot yields a value of Ea ) 42.6 kJ/mol, whereas, for the low-temperature region (ramp 1-3 °C/min), Ea ) 91.8 kJ/ mol. These findings illustrate the transitions from the intrinsic kinetic regime to internal diffusion control and finally to the external mass-transfer limitation as higher heating rates are applied. The fact that internal diffusion control was observed, even at the relatively moderate ramp of 5 °C/min, could explain the differing activation energies reported previously. The obtained intrinsic Ea values for the conversion range of 20%-40%, along with the corresponding coefficients of determination (R2), are listed in Table 1. Note that the activation energies are independent of conversion x, and are in good agreement with the Ea value of 90.8 kJ/mol that was reported by Jankovic et al.7 Reduction of the spinel-supported NiO was studied in a similar manner, using heating rates of 1-25 °C/min. As shown in Figure 3, for x ) 0.3, the reduction does not seem to be limited by diffusion effects and yields a value of Ea ) 96.4 kJ/mol. The absence of mass-transfer limitations, even at higher heating rates for NiO/NiAl2O4, is contrary to the behavior of pure NiO, which indicates that the supported oxide exhibits a lower reaction rate because of its higher calcination temperature. To ensure that the obtained activation energy indeed was the intrinsic one, additional experiments were conducted with oxide particles 225-425 µm in size, obtaining a value of Ea ) 94.3 kJ/mol for x ) 0.3, which is similar to the value observed for the smaller particles. Thus, for the range of heating rates investigated, diffusion limitations do not affect the obtained data for the spinel supported oxide. 5. Concluding Remarks Nickel oxide reduction by hydrogen was investigated, by applying the isoconversional Friedman method8 to thermogravimetric experimental data. Falsification of activation energy due to diffusional limitations was observed for the reduction of unsupported NiO. Although the intrinsic activation energy (Ea)
Figure 3. Isoconversional Arrhenius plot for NiO/NiAl2O4 reduction at a heating rate of 1-25 °C/min, for x ) 0.3.
was determined to be in the range of 91.8-94.5 kJ/mol, values of Ea ) 42.6 and ∼0 kJ/mol were generated at intermediate and high heating rates, respectively. This demonstrates the presence of internal and external mass-transfer limitations during the isoconversional kinetic analysis, and it provides a likely explanation for the varying activation energy values that have been reported previously. Therefore, these findings emphasize the importance of Weisz-Prater analysis9 for reaction studies, to ensure that the obtained data indeed represent the intrinsic kinetics. Notation j H2,bulk ) bulk phase hydrogen concentration (mol/m3) C CNiO,i ) initial nickel oxide concentration (mol/m3) De ) effective H2 diffusivity in NiO (m2/min) Ea ) activation energy (kJ/mol) Ea,obs ) apparent activation energy (kJ/mol) k0′ ) pre-exponential factor (min-1) m, n ) reaction orders r ) reaction rate (min-1) Rg ) universal gas constant (J mol-1 K-1) Rp ) particle radius (m) T ) temperature (K) x ) nickel oxide conversion y ) dimensionless hydrogen concentration z ) radial position (m) Greek Letters β ) heating rate (°C/min) n 1/2 φ ) Thiele modulus; φ ) Rp2(k(T)CHm-1 2,bulkCNiO,i/De) η ) effectiveness factor; η ) {∫0Rp k0′ exp[(-Ea/(RgT)]ym(1 - x)n dz}/{∫0Rp k0′ exp[(-Ea/(RgT)](1 - x)n dz}
Literature Cited (1) Twigg, M. V. Catalyst Handbook, 2nd Edition; Manson Publishing, Ltd.: London, 1996. (2) Mattisson, T.; Johansson, M.; Lyngfelt, A. The Use of NiO as an Oxygen Carrier in Chemical-Looping Combustion. Fuel 2006, 85, 736. (3) Erri, P.; Varma, A. Solution Combustion Synthesized Oxygen Carriers for Chemical Looping Combustion. Chem. Eng. Sci. 2007, 62, 5682. (4) Richardson, J. T.; Scates, R.; Twigg, M. V. X-ray Diffraction Study of Nickel Oxide Reduction by Hydrogen. Appl. Catal., A 2003, 246, 137. (5) Richardson, J. T.; Scates, R. M.; Twigg, M. V. X-ray Diffraction Study of the Hydrogen Reduction of NiO/R-Al2O3 Steam Reforming Catalysts. Appl. Catal., A 2004, 267, 35. (6) Hossain, M. M.; de Lasa, H. Reactivity and Stability of Co-Ni/Al2O3 Oxygen Carrier in Multicycle CLC. AIChE J. 2007, 53, 1817. (7) Jankovic, B.; Adnadevic, B.; Mentus, S. The Kinetic Analysis of Non-Isothermal Nickel Oxide Reduction in Hydrogen Atmosphere using the Invariant Kinetic Parameters Method. Thermochim. Acta 2007, 456, 48. (8) Friedman, H. L. Kinetics of Thermal Degradation of Char-Forming Plastics from Thermogravimetry. J. Polym. Sci., Part C 1964, 6, 183. (9) Weisz, P. B.; Prater, C. D. Interpretation of Measurements in Experimental Catalysis; Academic Press: New York, 1954; Vol. VI. (10) Aris, R. The Mathematical Theory of Diffusion and Reaction in Permeable Catalysts; Clarendon Press: Oxford, U.K., 1975; Vol. I. (11) Rossberg, M.; Wicke, E. Transportvorgange und Oberflachenreaktionen bei der Verbrennung Graphitischen Kohlenstoffs. Chem. Ing. Tech. 1956, 3, 181. (12) Ishida, M.; Wen, C. Y. Comparison of Zone-Reaction Model and the Unreacted-Core Shrinking Model in Solid-Gas Reactions;I: Isothermal Analysis. Chem. Eng. Sci. 1971, 26, 1031.
ReceiVed for reView November 22, 2007 ReVised manuscript receiVed April 8, 2008 Accepted April 15, 2008 IE071588M
