Kinetic Studies on Catalytic Decomposition of Methane to Hydrogen

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Ind. Eng. Chem. Res. 2004, 43, 4864-4870

Kinetic Studies on Catalytic Decomposition of Methane to Hydrogen and Carbon over Ni/TiO2 Catalyst Sharif Hussein Sharif Zein, Abdul Rahman Mohamed,* and P. Sesha Talpa Sai School of Chemical Engineering, Engineering Campus, Universiti Sains Malaysia, 14300 Nibong Tebal, S.P.S, Pulau Pinang, Malaysia

Experiments were conducted in a fixed-bed catalytic reactor to study the decomposition of methane to hydrogen and carbon over 13 wt % Ni/TiO2-based catalyst. The variables include the volumetric flow rate of reactant (relative partial pressures of methane and argon of 0.5 each) (100 < v0 < 450 mL/min) and reaction temperature (823 < T < 1173 K). The experimental data of conversion versus time were subjected to the integral method of analysis, and the rate law of the decomposition reaction was found to be first-order. An activation energy of 60 kJ/mol was obtained. A possible reaction sequence for methane decomposition was validated against experimental data. An analysis of the rate equations indicated that the adsorption of methane on the surface is the rate-controlling step. This observation was same as that obtained from an analysis of data by both integral and differential methods. Using the design equation for a packed plugflow reactor, predictions were made to obtain the conversion profiles at various operating conditions. Good agreement was obtained upon comparison of the experimental and simulated data. Introduction Hydrogen is predicted to become an efficient source of clean fuel and has been described as a long-term replacement for natural gas. With increasing interest in fuel cells and progress in hydrogen storage technologies, the use of hydrogen as a source of clean fuel is likely to increase in the future.1 There is substantial interest in developing novel methods for the production of hydrogen, and one such process is the catalytic decomposition of methane.2-8 Many experimental studies have been conducted to gain a better understanding of the kinetics of methane decomposition. The majority of the research has been focused on determination of the reaction mechanism, identification of the primary decomposition products, and estimation of the activation energy for methane decomposition. The activation energy for methane decomposition using a tubular reactor without catalyst was found 370 kJ/mol over the temperature range 1770-2270 K.9 Steinberg10 reported an activation energy of 131 kJ/mol for methane decomposition over the temperature range 973-1173 K, which is considerably lower than the value for methane decomposition without catalyst. However, he performed the reaction at pressures of 28-56 atm. Dahl et al.11 reported an activation energy of 208kJ/mol for methane decomposition using a fluid-wall graphite aerosol flow reactor over the temperature range 1533 < T < 2144 K. The activation energy of the decomposition of methane with a mixture of hydrogen over nickel catalysts was found to be 90 kJ/mol over the temperature range 723-863 K12 and 97 kJ/mol over the temperature range 803-863 K.13 Muradov14 studied the decomposition of methane over carbon catalysts, reporting activation energies of 236 kJ/mol (valid for a temperature range of 1023-1223 K) and 201 kJ/mol (valid for a temperature range of 873-893 K) for a carbon black catalyst and an activated carbon catalyst, respectively. * To whom correspondence should be addressed. E-mail: [email protected]

In this paper, kinetic studies were carried out using 13 wt % Ni/TiO2-based catalyst for the catalytic decomposition of methane for the production of hydrogen. The rate equation was obtained and the activation energy was estimated. A model, which uses the obtained rate equation for the decomposition of methane to hydrogen, was simulated to predict the methane conversion profiles. Experimental Section Methane decomposition was studied using 13 wt % Ni catalyst doped onto different supports, viz., TiO2, Al2O3, MgO, and SiO2. The catalysts were prepared using the impregnation method. Desired amounts of the transition metal oxides were dissolved in deionized water. The dopant concentrations are actually relative to the molar quantity of the support. The resulting paste was then dried in an oven and subsequently calcined in a ceramic crucible at 900 °C. The catalysts were then sieved to a size of 400-500 µm. The experiments were carried out at atmospheric pressure in a stainless steel fixed-bed reactor system. A schematic diagram of the reactor system is shown in Figure 1. The reactor was fabricated from a stainless steel tube (o.d. ) 12.7 mm, i.d. ) 10.92 mm, and length ) 600 mm). A thermocouple of type K in an inconel tube, 3 mm in diameter and 600 mm long, was used to measure the temperature of the catalyst bed in the reactor. The catalyst layer was situated in the center of the reactor. The free space before and after the catalyst layer was filled with quartz particles (RDH) to minimize the reactor dead volume. The furnace used was a single zone (model Carbolite VST 11) with a temperature controller and was supplied by Carbolite, Hope Valley, U.K. A pressure gauge (Ashcroft, Stratford, CT) located just above the reactor was used to read the inlet pressure. Methane (supplied by Malaysian Oxygen Sdn. Bhd.) with a purity of 99.999% and argon (supplied by Sitt Tatt Industrial Gases Sdn Bhd.) with a purity of 99.999% were mixed in ratio of 1:1 before entering the

10.1021/ie034208f CCC: $27.50 © 2004 American Chemical Society Published on Web 07/08/2004

Ind. Eng. Chem. Res., Vol. 43, No. 16, 2004 4865

Figure 1. Schematic diagram of the reactor system.

reactor, i.e., a mole fraction of 0.5 for methane was used for all of the experiments. Argon was used as a diluent as nitrogen might react with the hydrogen at high temperatures. The flow of methane was regulated using a mass flow controller (MKS), and the argon flow was regulated with Brooks mass flow controllers (model 5850E). The outlet gas flow was monitored by a gas flow meter (Alexander Wright DM3 B). For the kinetics study, a total of 64 experiments were conducted with fresh catalyst in each run. One gram of catalyst was packed in the reactor. The furnace was switched on, and pure argon was allowed into the reactor to create an inert atmosphere in the reactor. To activate the fresh catalyst, it was subjected to methane for 5 min at 998 K and 2700 h-1 gas hourly space velocity (GHSV). When the furnace had attained the required temperature, methane of known concentration was admitted into the reactor. The concentrations of the product gases were analyzed, and the experiment was stopped when the concentration of the product gases attained a steady value. The experiment was repeated for different volumetric flow rates (100-450 mL/min) and temperatures (823-1173 K). The product gases were analyzed using an on-line gas chromatograph (GC) (Hewlett-Packard Series 6890, Hewlett-Packard, Wilmington, DE). The GC was controlled on-line using HP ChemStation Rev. A. 06.01.[403] software. Porapaq N and molecular sieve 5 Å (1/ 8-in. diameter, 6-ft length) stainless steel columns, situated in series with the Porapaq N column located in front were used. The Porapaq N column was used to separate carbon dioxide, ethane, ethylene, and propylene and the molecular sieve 5 Å column for hydrogen, oxygen, carbon monoxide, nitrogen, and methane. Because higher hydrocarbons and carbon dioxide can ruin the molecular sieve 5 Å column, two valves operated at temperature 333 K were used to control the outlet gas from the Porapaq N column to the detectors to avoid passing it through the molecular sieve 5 Å column. Valve 1 functioned as a sampling mechanism, and valve 2 was used to control the flow of the product through the molecular sieve 5 Å column. When the valve is off, gas is allowed to flow through the molecular sieve 5 Å

column. The gas chromatograph injector temperature was set at 313 K. The initial and the final temperatures of the oven were set to 313 and 473 K, respectively. A heating rate of 5 K/min was used. The detector temperature was kept at 473 K. Pure argon gas (99.999%) was used as a carrier gas. The total analysis time was 25 min for each injection. Standard gas was injected into the gas chromatograph, and the area of each of the components in the standard gas was determined. The standard gas mixture was supplied by BOC Gases, Guilford, U.K. Chromatograms of hydrocarbons such as methane, ethylene, ethane, and propylene were obtained using a flame ionization detector (FID), whereas hydrogen, oxygen, carbon monoxid,e and carbon dioxide were detected using a thermal conductivity detector (TCD). A carbon balance of 100 ( 2% was obtained for every run over the catalyst. Pore volume and surface area measurements of the different samples were determined via nitrogen adsorption/desorption isotherms at liquid nitrogen temperature (77 K) using an automated gas sorption system (Autosorb I, QuantaChrome Corporation, Boynton Beach, FL). All samples were degassed at a temperature of 573 K for 3 h prior to the measurements. Computer programs (Micropore, version 2.46) allowed for rapid numerical results for the surface area and pore texture from the adsorption-desorption isotherm. Spent catalysts were analyzed using a transmission electron microscope (Philips TEM CM12). In preparation for TEM experiments, a few samples of the spent catalyst were dispersed in distilled water, and then a drop of the dispersion was deposited on a coated copper grid. Results and Discussion A blank reaction (i.e., decomposition of methane without catalyst) study was carried out in the stainless steel reactor filled with quartz sand with a feed consisting of methane and argon in a ratio of 1:1 at a total flow rate of 80 mL/min and in the temperature range of 673-1173 K (the range used in the present study). The conversion of methane was less than 1% in all of

4866 Ind. Eng. Chem. Res., Vol. 43, No. 16, 2004 Table 1. Physical Properties of the Fresh Ni Supported on Different Supports catalyst

surface area (m2/g)

total pore volume (Vp) (cm3/g)

Ni on SiO2 Ni on Al2O3 Ni on TiO2 Ni on MgO

11.45 5.28 8.05 7.53

0.003 0.010 0.005 0.007

Table 2. Details of Conversions Obtained in the Present Study with Different Supports at 998 K and 2700 h-1 GHSV conversion (%) catalyst

2 min

5 min

60 min

120 min

13 wt % Ni on SiO2 13 wt % Ni on Al2O3 13 wt % Ni on TiO2 13 wt % Ni on MgO

97.41 99.54 99.18 99.23

74.12 72.98 64.57 52.23

48.67 PB 61.34 15.84

PBa PB 62.44 8.05

a

PB ) pressure buildup.

these experiments, indicating that the decomposition of methane without catalyst in the temperature range of 673-1173 K was negligible. Thus, the material stainless steel of type 316 used in this study is a suitable material for the construction of a reactor for the decomposition of methane. Selection of Support Material. Table 1 shows the physical properties of 13 wt % Ni/TiO2 doped onto different supports, viz., SiO2, Al2O3, TiO2, and MgO. The surface areas of Ni on SiO2, Al2O3, TiO2, and MgO were 11.45, 5.28, 8.05, and 7.53 m2/g, respectively. The total pore volumes show the same trend as the surface areas with values of 0.003, 0.010, 0.005, and 0.007 cm3/g for Ni on SiO2, Al2O3, TiO2, and MgO, respectively. This can be attributed to the change of the structure or electronic state of the metal species due to the interaction with the supports.15 To select a suitable support material, experiments were conducted for methane dissociation into hydrogen and carbon with 13 wt % NiO catalyst supported on different supports, viz., TiO2, Al2O3, MgO and SiO2, at 998 K and 2700 h-1 gas hourly space velocity (GHSV). The results obtained are listed in Table 2. After 2 min on stream, methane conversion of ∼100, 100, 100, and 97% and COx yield of ∼60, 62, 65, and 62% were obtained using 13 wt % Ni/SiO2, 13 wt % Ni /Al2O3, 13 wt % Ni/TiO2, and 13 wt % Ni/MgO, respectively. The rest of the yield was hydrogen. No higher hydrocarbons were detected. The COx production probably occurs via the reaction of chemisorbed carbon and hydrogen, produced by the decomposition of methane with the lattice of metal oxides.4 Methane reactions were conducted over catalysts without any added oxygen. Therefore, any methane oxidation in these experiments must be derived from oxygen originating from the catalyst. The surface oxygen present on the catalyst was responsible for the formation of COx as no oxygen was present in the feed gas. The reaction takes place between gasphase or weakly adsorbed methane and strongly adsorbed oxygen or lattice oxygen. Surface OH groups and/ or oxygen from the support and/or promoter and/or the active metal, in this case NiO, might have participated in the conversion of methane to COx. Previously, it was concluded that the surface OH groups were involved in the conversion of methane to CO.16 On the basis of Table 2, it was estimated that 2-4 min is sufficient for reducing the fresh catalyst. As the time increased, the number of methane pulses over the catalyst increased,

Table 3. Repeatability and Analytical Reproducibility of the Catalytic Data Obtained in the Present Study at 998 K for 60 Min on Stream Using 13 wt % Ni on TiO2 trial

conversion (%)

1 2 3

61.34 60.08 62.11

and only hydrogen was detected because the catalyst was reduced further. After 5 min on stream, hydrogen and methane were the only components detected in the product stream for the all types of catalysts. The ratio of methane conversion and hydrogen formation was found to be 1:2. In fact, hydrogen was the only gas detected after 5 min on stream. The methane decomposition obtained within the first 5 min of reaction decreased in the order SiO2 > Al2O3 > TiO2 > MgO support. After 60 min on stream, the order of activity changed to TiO2 > SiO2 > MgO support, and a pressure buildup (PB) was observed on the Al2O3 support. At 120 min on stream, the TiO2 system maintained its activity, and the MgO decreased from 16 to 8%, whereas a pressure buildup occurred in the SiO2-supported catalyst system. Thus, it can be concluded that TiO2 is the best support. This is in line with the observations of Takenaka et al.,17,18 who concluded that TiO2 is the best support for either repeated cycles of methane decomposition or high catalytic activity of Ni/TiO2 for repeated reactions. The results of the present study also confirm that deactivation is not occurring up to 120 min on stream over TiO2-supported Ni catalyst. Because fresh catalyst was used each time, the deactivation can be neglected in the present study. Carbon samples obtained from Ni catalysts loaded onto various supports were further studied using TEM. The results obtained showed that the introduction of the support markedly influenced the carbon morphology. The TEM micrographs of carbon synthesized on Ni supported on SiO2, Al2O3, TiO2, and MgO support are shown in parts a-d, respectively, of Figure 2. The TEM image of the carbon formed on Ni on SiO2 and Al2O3 supports seems to be undeveloped carbon nanotubes, whereas the carbon formed on Ni on MgO looks like small tubes connected to each other. On the other hand, the TEM image of the carbon formed using Ni on the TiO2 support (Figure 2c) was the best among the supported catalysts. Thus, the catalytic activity depended strongly the kind of support. To test the repeatability and analytical reproducibility of the raw catalytic data, activity tests for the newly prepared 13 wt % Ni on TiO2 catalyst were conducted three times at 998 K for 60 min on stream using new catalyst each time, and the results are reported in Table 3. The ratio of methane conversion to hydrogen formation was found to be 1:2. In fact, hydrogen was the only gas detected. The results indicate that the reproducibility of the data is satisfactory. Kinetics of the Reaction. Among the catalysts studied in this work, 13 wt % Ni on TiO2 support was found to be the most effective catalyst for the production of hydrogen and carbon through the decomposition of methane. Kinetic studies were carried out using this catalyst. The experimental program for obtaining the kinetics of the reaction consisted of runs at different mole ratios and temperatures with a constant weight of the 13 wt % Ni/TiO2, and the results are presented in Table 4. The conversion of methane versus time data were subjected to the integral method for the derivation of a suitable rate law.

Ind. Eng. Chem. Res., Vol. 43, No. 16, 2004 4867

Figure 2. Transmission electron microscope images of the carbon produced on Ni supported on the different supports (a) SiO2, (b) Al2O3, (c) TiO2, and (d) MgO at 998 K and GHSV of 2700 h-1. Table 4. Details of Methane Conversions (%) Obtained in the Present Study at Various Operating Conditionsa temperature (K)

100

823 873 923 973 1023 1073 1123 1173

15.82 27.41 41.56 53.99 59.07 70.54 82.49 92.03

a

volumetric flow rate (mL/min) 150 200 250 350 11.42 20.65 33.81 45.10 49.66 59.63 69.08 76.88

7.04 16.55 27.43 37.21 39.48 49.02 54.32 65.87

6.52 14.03 24.87 31.58 33.24 35.10 48.77 53.46

7.03 16.21 24.99 26.04 24.43 37.67 47.01

450 9.87 17.88 21.97 21.07 24.33 34.68

Catalyst weight ) 1 g.

For a first-order reaction in a packed-bed reactor, the relation between conversion and weight time is as follows19

(

k′τ ) (1 + ) ln

)

1 - AXout 1 - Xout

Figure 3. Plot of conversion as a function of weight time at different temperatures.

(1)

For the present reaction, the above equation reduces to

(

k′τ ) 1.5 ln

)

1 - 0.5XCH4 1 - XCH4

(2)

From the plots of the function on the right-hand side of eq 2 versus τ (w/v0), shown typically in Figure 3 for a 1-g sample of catalyst, it follows that the data fall on a reasonably straight line through the origin. Hence, the decomposition reaction is of first order. From the values of the specific reaction rate constant, k′, calculated from the slopes at different reaction temperatures, the activation energy obtained in the present study is 60 kJ/mol following the Arrhenius equation. The plot is shown in Figure 4. Reported activation energies for methane decomposition without

Figure 4. Plot of values of logarithmic specific reaction rate constant against the reciprocal of the reaction temperature.

catalyst range from 356 to 402 kJ /mol,9,11 and those with catalyst range from 90 to 236 kJ/mol.12-14 Thus, the activation energy obtained in this study using 13 wt % Ni/TiO2-based catalyst is considerably the lower than others.9-14 Reaction Mechanism. Experimental observations of the present study indicate that the reaction is elementary. However, the reaction mechanism and the

4868 Ind. Eng. Chem. Res., Vol. 43, No. 16, 2004

decomposes on the front surface of certain active sites of the Ni/TiO2-based catalyst, and the carbon thus formed diffuses through the metal and precipitates at the rear surface. The driving force that pushes the carbon diffusion was suggested to originate from the concentration gradient of dissolved carbon between the two interfaces, i.e., the metal-gas interface to the metal-nanocarbon interface. Therefore, for high values of diffusivity, the concentration of carbon dissolved in nickel is almost uniform and small (therefore, CNi ≈ 0). When the adsorption of methane is the rate-limiting step, the expression relating the rate of reaction and the concentrations of various species is

Figure 5. Transmission electron microscope image of carbon formation on the catalyst particle.

identification of rate-controlling steps were attempted in the present study following the mechanism proposed by Snoeck et al.20 They proposed the following mechanism for methane decomposition k+1

CH4 + S {\ } CH4‚S k -1

k+2

} CH3‚S + H‚S CH4‚S +S {\ k -2

k+3

CH3‚S + S {\ } CH2‚S + H‚S k -3

k+4

CH2‚S + S {\ } CH‚S + H‚S k -4

k+5

CH‚S + S {\ } C‚S + H‚S k -5

k+6

} CNi + S C‚S {\ k -6

k+7

} H2 + 2S 2H‚S {\ k -7

(3) (4)

-rCH4 )

CTK1PCH4 1 + K71/2PH21/2

(10)

Initially, the concentration of hydrogen is zero, so eq 10 becomes

-rCH4,0 ) CTK1PCH4

(11)

(7)

If the methane decomposition is an adsorption-limited reaction, then the initial rate will be linear with the initial partial pressure of methane. This is what was observed experimentally in the present study. However, the individual rate laws and individual initial rate plots with the other steps in the mechanism as rate-limiting step are also derived below and checked to confirm whether the methane decomposition reaction is really adsorption limited.

(8)

When the removal of the first hydrogen atom from molecularly adsorbed methane is the rate-limiting step, it follows that

(5) (6)

(9)

Whereas eq 3 represents the adsorption of methane on the surface of the catalyst, eqs 4-7 represent the surface reaction to form adsorbed methyl radicals and adsorbed hydrogen atoms on the catalyst surface. Equations 8 and 9 represent the deposition of carbon on the catalyst surface and the desorption of hydrogen from the catalyst surface, respectively. For the mechanism shown in eqs 3-9, the ratelimiting step is identified as follows: It is assumed that one of the equations is the rate-limiting step, and then the rate law is formulated in terms of the partial pressures of the methane and hydrogen. From the equation obtained, the variation of the initial reaction rate with the initial partial pressure of methane is checked. If the theoretical rate varies with pressure in the same way as observed experimentally, it will be assumed that the mechanism and the rate-limiting step are correct. A mechanistic interpretation of the growth of carbon on the catalyst has been proposed in the literature.21,22 Figure 5 supports the proposed mechanism for the growth of carbon nanotubes on the catalyst. Methane

-rCH4‚S )

CTK1k+2PCH4 (1 + K1PCH4 + K71/2PH21/2)2

(12)

Initially, the concentration of hydrogen is zero, and hence eq 12 becomes

-rCH4‚S,0 )

CT2K1k+2PCH4 (1 + K1PCH4)2

(13)

If the removal of the first hydrogen atom from molecularly adsorbed methane limits the methane decomposition, the rate will increase at low initial partial pressures of methane and then decrease at high initial partial pressures of methane. This conclusion is not consistent with the experimental observations of the present study. When the removal of second, third, and fourth hydrogen atoms from molecularly adsorbed methane is the rate-limiting step, the expressions relating the rate

Ind. Eng. Chem. Res., Vol. 43, No. 16, 2004 4869

of reaction and concentrations of various species are as follows

-rCH3‚S )

(

C2TK1K2k+3PCH4 K71/2PH21/2

)/(

1/2

-rCH2‚S )

(

PH2

)

2

K7PH2

K71/2PH21/2

(

+

+ 1/2

K71/2PH2

)

2

K1K2K3PCH4

1/2

+ K7 PH2

K7PH2

1/2

)/(

(15)

1 + K1PCH4 +

K73/2PH23/2

K1K2PCH4

(14)

1 + K1PCH4 +

CT2K1K2K3K4k+5PCH4

-rCH‚S )

+ K71/2PH21/2

1/2

)/(

CT2K1K2K3k+4PCH4

K1K2PCH4

-rH‚S ) 0

1 + K1PCH4 +

K1K2PCH4 K7

K1K2K3PCH4 K1K2K3K4PCH4 + + K7PH2 K73/2PH23/2

)

2

1/2

K7

PH2

1/2

(16)

Initially, the concentration of hydrogen is zero, and the numerators and denominators of eqs 14-16 approach infinity. Therefore, the limit of these expressions approaches zero. Because the reaction rate cannot be zero, the removal of the second, third, or fourth hydrogen atom from molecularly adsorbed methane cannot be the rate-limiting step. When the deposition of carbon on the catalyst is ratelimiting step, one obtains

-rC‚S )

(

)/(

CTK1K2K3K4K5k+6PCH4 K72PH22

K1K2PCH4 1/2

K7 PH2

1/2

+

Initially, the concentration of hydrogen is zero, and eq 19 becomes

K1K2K3PCH4

+

K7PH2

1 + K1PCH4 +

K1K2K3K4PCH4

K1K2K3K4K5PCH4 K72PH22

K73/2PH23/2

+

)

+ K71/2PH21/2 (17)

Initially, the concentration of hydrogen is zero, and the numerator and denominator of eq 17 approach infinity. Therefore, the limit of this expression gives

-rC‚S ) CTk+6

(18)

Because, the reaction rate cannot be CTk+6, the deposition of carbon on the catalyst cannot be rate-limiting step. When desorption of hydrogen from the surface is the rate-limiting step, one finds

Because the reaction rate cannot be zero, desorption of hydrogen from the surface cannot be the rate-limiting step. An analysis of the above rate equations indicates that the rate equation with the adsorption of methane on the surface as the rate-controlling step exhibited a linear relationship with the concentration of methane. This observation is same as that obtained from the analysis of data by the integral method. Hence, the assumed mechanism with the adsorption of methane ratecontrolling is consistent with the experimental observations. This can be expected, as the surface reaction is unlikely to control the rate because the reaction occurs at relatively high temperatures. Simulation. Conversion profiles along the length of the reactor were obtained following the design equation for packed-bed reactor. Isothermal and plug-flow conditions (the ratio of bed diameter to catalyst particle diameter is around 25) were assumed in the reactor. Because the flow rates were sufficiently high, heat- and mass-transfer limitations were assumed to be negligible, and the reaction was assumed to be kinetically controlled. Experiments were conducted with different flow rates keeping all of the other variables constant, and the conversion (H2 yield) was found to be almost same in all of these experiments (there was a slight increase at lower flow rates). The packed-bed reactor is assumed to have no radial gradients in concentration, temperature or reaction rate. When the pressure drop through the reactor and catalyst decay are neglected, the relation between conversion and weight time is

(

-rH‚S ) -

(1 + K1PCH4)2

(19)

)

1 k′W ) 1.5 ln - 0.5 XCH4 ν 1 - XCH4

(21)

The above equation can be used to determine the amount of catalyst needed to obtain a specified conversion at a given temperature and volumetric flow rate. Similarly, it can also be used to obtain the conversion for a given amount of catalyst at a particular temperature and volumetric flow rate. The above equation is valid for 823 < T < 1173 K and 100 < v0 < 450 mL/ min, as these were ranges in which the experiments were performed in the present study. The model predictions for the conversion of methane under conditions of variable volumetric flow rates (100 < v0 < 450 mL/min) and variable temperatures (823 < T < 1173 K) agree well with the experimental observations. The predictions of the model for methane conversion are compared to the experimental values at all temperatures and volumetric flow rates. Figure 6 shows a satisfactory comparison between the experimental and predicted (using eq 21) conversions. The standard deviation (SD) of (0.13 was obtained using the equation

2

CT k-7PH2

(20)

SD )

x

N

∑ i)1

(Exp - Cal) Exp N-1

) (0.13

(22)

4870 Ind. Eng. Chem. Res., Vol. 43, No. 16, 2004

Literature Cited

Figure 6. Comparison of theoretically and experimentally determined conversions of methane.

As consequence, the experimental values are well fitted by the model at low temperature. Conclusions For the decomposition of methane to hydrogen and carbon using 13 wt % Ni/TiO2 catalyst, it was found in the present study that the decomposition reaction is of first order with an activation energy of 60 kJ/mol. This activation energy is the lowest activation energy reported in the literature for this reaction. It was also found that the adsorption of methane on the surface is the rate-controlling step. The model developed indicates the volumetric flow rate, catalyst weight, and temperature to be decisive parameters in scale-up considerations and in the extrapolation of laboratory-scale experimental results. The model predictions for the conversion agree well with the experimental observations. Acknowledgment The authors acknowledge for the financial support provided by Ministry of Education, Malaysia and Universiti Sains Malaysia under Fundamental Research Grant Scheme (Project: A/C No: 6070014). Nomenclature CA ) concentration of substance A (mol/mL) CA0 ) initial concentration of substance A (mol/mL) CCH4 ) concentration of methane (mol/mL) CNi ) concentration of carbon dissolved in nickel CT ) total concentration of sites CV ) concentration of vacant sites k+1, k-1, k+2, k-2, k+3, k-3, k+4, k-4, k+5, k-5, k+6, k-6, k+7, k-7 ) rate coefficients of the forward (+) and the reverse (-) reactions k′ ) specific reaction rate constant [mL/(gcat s)] K1, K2, K3, K4, K5, K6, K7 ) equilibrium coefficients N ) number of experiments PCH4 ) partial pressure of methane -rA ) rate of species A decomposition [mol(gcat s)] -rCH4 ) rate of methane decomposition [mol/(gcat s)] S ) surface active site T ) Temperature (K) v0 ) entering volumetric flow rate Vp ) total pore volume (cm3/g) W ) Weight of catalyst XA ) conversion of species A XCH4 ) conversion of methane Xout ) conversion at the outlet Greek Symbols  ) fractional change in volume per mole of A reacted resulting from the change in total number of moles τ ) weight time (g s/mL)

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Received for review October 25, 2003 Revised manuscript received March 25, 2004 Accepted May 4, 2004 IE034208F