Ni

Article pubs.acs.org/IECR

Steam Methane Reforming over Ni-based Pellet-type and Pt/Ni/Al2O3 Structured Plate-type Catalyst: Intrinsic Kinetics Study Ana Obradović,† Blaž Likozar,† and Janez Levec*,†,‡ †

Laboratory of Catalysis and Chemical Reaction Engineering, National Institute of Chemistry, Hajdrihova 19, 1000 Ljubljana, Slovenia ‡ Faculty of Chemistry and Chemical Technology, University of Ljubljana, Aškerčeva cesta 5, 1000 Ljubljana, Slovenia ABSTRACT: Intrinsic kinetics study of steam methane reforming (SMR) was performed on two different systemscommercial Ni-based pellet catalyst and Pt/Ni/Al2O3 structured plate catalyst. Experiments were carried out in the absence of external mass transfer resistance, and temperature 500−575 °C, pressure 2.5−7.5 bar, and H2O/CH4 reactant ratio range of 3−5 mol/mol. Reactors operated in the integral mode, and it was shown that both systems could be described by the same kinetics, based on Langmuir−Hinshelwood mechanism. In the case of plate catalyst, the regeneration treatment after deactivation led to platinum particle redispersion, which further influenced the values of the pre-exponential factors, whereas the activation energy values remained unchanged. Comparison of the two catalyst systems was made based on the active metal content, and it was shown that the catalytic activity of the Pt/Ni/Al2O3 plate catalyst after the second regeneration treatment was 8 times of that observed for the Ni-based pellet catalyst.

1. INTRODUCTION According to world energy experts, fuel cell and hydrogen energy technologies will play an important role in future energy economics, particularly in transport sector, which is today marked by an extreme dependency on oil. Hydrogen needs to be produced cost-effectively and with zero or near-zero CO2 emissions.1 Steam methane reforming (SMR) is the most widely used method for producing hydrogen nowadays.1−3 In order to produce pure hydrogen by SMR, sorption-enhanced steam methane reforming (SE−SMR) presents itself as one of the most promising methods for the SMR performance improvement. SE−SMR involves the addition of a CO2 acceptor to the SMR reaction system, and consequently in situ CO2 separation by the reactions of the targeted molecule with a chemical acceptor. In that way, the SMR reaction system is shifted beyond its conventional thermodynamic limits, according to the Le Chatelier’s principle, which consequently leads to a lower temperature of operation and results in many other benefits as well.3,4 The SE−SMR process can be performed in both unsteady and steady state with respect to the sorbent. In the unsteady SE−SMR processes, pellet-type catalyst is commonly applied, as described by Balasubramanian et al.,5 who used a laboratory fixed bed reactor, packed with a mixture of commercial pellettype reforming catalyst (NiO/Al2O3) and calcium-based CO2 acceptor (CaO). The process discontinuity (with respect to the sorbent) can be overcome by the circulating fluidized bed concept, where the regenerated and spent sorbent are transported between the H2 production and sorbent regeneration reactors.4,6 Recently, Obradović et al.7 proposed a novel nickel plate-type catalyst for the SE−SMR process, enhanced by thermally annealed platinum and alumina coatings, presented in Figure 1 and denoted as Pt/Ni/Al2O3 plate-type catalyst in this work, where the catalyst serves as a gas-phase radial mixer and solid-phase distributor, as well. Such a design © 2013 American Chemical Society

Figure 1. Structured Pt/Ni/Al2O3 plate catalyst element employed in the study.

of catalyst would enable continuous operation of SE−SMR with respect to the sorbent that would flow downward through the packing of the structured plate-catalyst. In order to describe and optimize the SE−SMR process, the intrinsic SMR kinetic study on an appropriate catalyst has to be carried out in the first place. Wei and Iglesia8 investigated the mechanisms of the reactions of CH4 with CO2 and H2O on Ni-, Ru-, Pt-, Rh-, and Ir-based catalysts. They found that Pt surfaces are the most reactive among the metal clusters examined and that the reaction rates are proportional to the partial pressure of CH4 and independent of the partial pressures of the coreactants, leading to the conclusion that only C−H bond activation steps are kinetically relevant; nonetheless, this applies to the reaction rates determined far from chemical equilibrium and upon utilizing excessive amounts of steam.8 Akers and Camp9 came to a similar conclusion with Ni-based catalysts. Received: Revised: Accepted: Published: 13597

May 15, 2013 August 22, 2013 August 22, 2013 August 22, 2013 dx.doi.org/10.1021/ie401551m | Ind. Eng. Chem. Res. 2013, 52, 13597−13606

Industrial & Engineering Chemistry Research

Article

Table 1. Experimental Conditions Used in Intrinsic Kinetics Study over Ni-Based Pellet-type Catalyst pressure (bar) 3.8 4.5 5.5 6.5 7.5 reference conditions 3.8

temperature (°C)

total volumetric flow rate (LSTP min−1)

H2O/CH4 (mol/mol)

H2/CH4 (mol/mol)

N2/CH4 (mol/mol)

4.00 4.00 3.00, 4.00, 5.00 4.00 4.00

1.00 1.00 1.00 1.00 1.00

0.05 0.05 0.05 0.05 0.05

500 500, 500, 500, 500,

575 575 575 575 575

1.0, 1.0, 1.0, 1.0, 1.0,

4.00

1.00

0.05

500, 525, 550, 575

1.0

525, 525, 525, 525, 525,

550, 550, 550, 550, 550,

1.1, 1.1, 1.1, 1.1, 1.1,

1.2 1.2 1.2 1.2 1.2

Figure 2. Flow scheme of the equipment used for plate catalyst kinetic study: (1) gas cylinders, (2) pressure reducers, (3) mass flow controllers, (4) water reservoir, (5) pump, (6) water preheating section, (7) reactor, (8) quartz wool, (9) heating jacket, (10) structured plate catalyst element, (11) condenser, (12) needle valve, (13) gas liquid separator, (14) back pressure regulator, (15) electronic timer.

However, the model proposed by Xu and Froment10 continuously proves to be the one which is the most extensively used.2−4 Xu and Froment10 investigated a large number of detailed mechanisms and established the reaction kinetics for SMR on Ni/MgAl2O4 catalyst, based on the Langmuir− Hinshelwood mechanism. They indicated that the reactions that produce the adsorbed forms of CO and CO2 in the proposed mechanism are rate-determining steps, suggesting that the concentration of the oxygen-containing species in a large extent contributes to the reaction kinetics (especially in the cases when conversion is close to the equilibrium value), contradictory to the above-mentioned findings. Allen et al.11 proposed the reaction kinetics based on the Hougen−Waston formulations, but with CO and CO2 desorption as the ratedetermining steps. Xu and Froment10 introduced a triangular reaction scheme, consisting of three global equilibrium-limited reactions 1−3: CH4 + H 2O ↔ CO + 3H 2

CH4 + 2H 2O ↔ CO2 + 4H 2

(3)

The first and third reactions are strongly endothermic, while the second one is moderately exothermic and is popularly called the water−gas shift (WGS) reaction. The rate equations were derived for the following reaction conditions: 500−575 °C, 3−15 bar, and an initial molar ratio of water to methane of 3−5. The rate expressions proposed by Xu and Froment10 were used integrated in the SE−SMR processes, where they proved to be suitable in the presence of an adsorbent, as well.4 This research work deals with the comparison study of the SMR reaction kinetics on two types of catalysts, namely a commercial Ni-based pellet-type catalyst, and the abovementioned structured plate-type catalyst that can be potentially used in both the unsteady and steady state processes of SE− SMR.

ΔH = 205.8 kJ mol−1

2. EXPERIMENTAL SECTION 2.1. Ni-Based Pellet-type Catalyst Experiments. The original 10-hole 3 mm × 3 mm tablet-type reforming catalyst, made of nickel oxide, dispersed on the mixture of aluminum oxide and calcium aluminate−cement as carrier (Süd-Chemie

(1)

CO + H 2O ↔ CO2 + H 2

ΔH = 164.6 kJ mol−1

ΔH = − 41.2 kJ mol−1 (2) 13598

dx.doi.org/10.1021/ie401551m | Ind. Eng. Chem. Res. 2013, 52, 13597−13606


Article

Table 2. Experimental Conditions Used in Intrinsic Kinetics Study over Pt/Ni/Al2O3 Plate Catalyst pressure (bar)

H2O/CH4 (mol/mol)

total volumetric flow rate (LSTP min−1)

N2/CH4 (mol/mol)

0.15 0.15 0.15 0.15

0.85 0.85 0.85 0.85

500, 500, 500, 500,

575 575 575 575

3 1, 2, 3, 4, 5 3 3

0.15

0.85

500, 525, 550, 575

3

2.5 4.00 4.0 3.00, 3.50, 4.00, 4.50, 5.00 6.5 4.00 7.5 4.00 reference conditions 1.0 4.00 a

temperaturea (°C)

H2/CH4 (mol/mol)

525, 525, 525, 525,

550, 550, 550, 550,

The temperatures given are the nominal values.

used for the feed flow rate control of different gases (15 vol % H2/N2, N2, air, and CH4), whereas the water was metered by a Beckman 114 M Solvent Delivery Module (USA) pump, after which it was electrically preheated and vaporized in 1/8 in. stainless steel tubing. The reactor effluent stream after steam condensation was diverted into two separate streams; the first one was analyzed by the same GC system as described above, while the other one was monitored by a nondispersive infrared detector (Binos 1001, Rosemount, USA) for CO2 and CO. Analogously to the Ni-based pellet-type catalyst, the 40 h stabilization procedure was needed. (Reaction conditions: T = 500 °C, P = 1.0 bar. Reaction mixture: H2/CH4 = 0.15 mol/ mol, H2O/CH4 = 4.0 mol/mol, N2/CH4 = 0.85 mol/mol. Q = 3 LSTP min−1.) The tests at reference conditions (Table 2) were carried out every 7 h. Kinetic experiments were performed at the temperatures of 500 and 525 °C, after which the regeneration treatment was executed (7.0 h in air stream, T = 860 °C, heating rate = 10 °C min−1, then 6.5 h in 15 vol % H2 in N2 stream, T = 600 °C, heating rate = 10 °C min−1). The kinetic experiments, performed at the temperature of 550 °C, were also followed by the regeneration treatment, before the last set of experiments (at the temperature of 575 °C) was to be carried out. 2.3. Catalyst Characterization. The average particle size and particle size distribution of the sieved catalyst powder were measured by laser diffraction technique using particle size analyzer (Microtrac S3500, Microtrac Inc., USA). The specific surface area of fresh catalyst was measured by the standard Brunauer−Emmett−Teller (BET) technique with automated catalyst characterization system (AutoChem II 2920, Micromeritics, USA). The surface morphology of the plate catalyst was analyzed by a field emission scanning electron microscope (FEG−SEM, Zeiss Supra 35 VP, Carl Zeiss AG, Germany), using a secondary electron detector and low acceleration voltages (∼1 keV). SEM images were further analyzed by image processing program ImageJ (National Institutes of Health, USA). The surface topography of the plate catalyst was examined by 3D optical interferometer (Bruker, ContourGT-K, Germany) with measurements based on white light interference. The scanning range was ∼1 mm × 1 mm (X−Y: area), and 0.05 mm (Z: vertical), with lateral and vertical resolution, 40 and 0.1 nm, respectively. The total carbon analysis for the spent Ni-based pellet-type catalyst was done using a Rosemount-Dohrmann DC 190 TC analizator (San Francisco, USA).

AG, Germany), was crushed into the particles of diameter 700 °C in oxygen, while the process in hydrogen never

⎛ pH 3 pCO ⎞ dX1 k1 ⎜ 2 ⎟ /DEN2 r1 = = − p p d(W /FCH4,0) K1 ⎟⎠ pH 2.5 ⎜⎝ CH4 H2O 2

(mol gcat r2 = Figure 5. Redispersion phenomenon of Pt particles observed by SEM technique. Plate-type catalyst surface before (a) and after (b) applied regeneration treatment.

−1 −1

h )

(7)

pH pCO ⎞ d(X1X 2) k ⎛ 2 2 = 2 ⎜pCO pH O − ⎟ /DEN2 2 d(W /FCH4,0) pH ⎝ K2 ⎠ 2

(mol gcat−1 h−1) 13601

(8)



Article

Table 3. Calculated Parameter Values for Ni-Based Pellet-type Catalyst reaction rate constant

units

k1 = (1.5 ± 0.3) × 10 exp[− (240266 ± 25040) J mol /(RT )]

mol bar1/2 gcat−1 h−1

k 3 = (3.0 ± 0.2) × 1012exp[− (209424 ± 7467) J mol−1/(RT )]

mol bar1/2 gcat−1 h−1

−1

14

adsorption constant

units

−3

bar−1

−1

K CH4 = (1.1 ± 0.4) × 10 exp[(34628 ± 19045) J mol /(RT )] K H2O = (1.04 ± 0.05) × 106exp[− (98847 ± 7806) J mol−1/(RT )]

Table 4. Calculated Parameter Values for Pt/Ni/Al2O3 Plate-type Catalyst reaction rate constant

units

k1 = (1.17 ± 0.03) × 10 exp[− (247406 ± 3235) J mol /(RT )]

mol bar1/2 mcat−2 h−1

k 3 = (2.80 ± 0.07) × 1016exp[− (265177 ± 2105) J mol−1/(RT )]

mol bar1/2 mcat−2 h−1

−1

16

adsorption constant

units

−4

bar−1

−1

K CH4 = (2.8 ± 0.2) × 10 exp[(43930 ± 12770) J mol /(RT )] K H2O = (1.23 ± 0.01) × 106exp[− (99878 ± 4590) J mol−1/(RT )] redispersion constant (after first regeneration)

units

c1 = 6.2362 ± 0.0004 redispersion constant (after second regeneration)

units

c2 = 8.999 ± 0.005

r3 =

⎛ pH 4 pCO ⎞ dX3 k3 ⎜ 2 2 2⎟ p p = − d(W /FCH4,0) K3 ⎠⎟ pH 3.5 ⎝⎜ CH4 H2O

⎛ 26830K ⎞ ⎟ K1 = 1.198 × 1013exp⎜ − ⎝ T ⎠

2

2

/DEN

(mol gcat

−1 −1

h )

⎛ 4400K ⎞ ⎟ K 2 = 1.767 × 10−2exp⎜ ⎝ T ⎠

(9)

The denominator in the above equations is the same, since it is assumed that all three reactions take place on the same active site. DEN = 1 + K COpCO + K H2pH + K CH4pCH 2

+ K H2OpH O /pH 2

2

⎛ 22430K ⎞ ⎟ K3 = 2.117 × 1011exp⎜ − ⎝ T ⎠

X1 = X 2 = X3 = 0

(10)

(11)

Constants ki, where i = 1, 2, 3, and Kj, where j = H2, CO, H2O, CH4, in eqs 7−9 are reaction rate and adsorption constants, respectively, and are expressed as follows: ⎛ −Ea, i ⎞ ki = Ai exp⎜ ⎟ ⎝ RT ⎠

(12)

⎛ −ΔHa, j ⎞ Kj = K ◦j exp⎜ ⎟ ⎝ RT ⎠

(13)

(bar 0) (bar 2)

(14)

(15)

(16)

3.3. Kinetic Parameter Estimation. SMR kinetic rate expressions (eqs 7−9) contain three reaction rate constants, k1, k2, and k3, and four adsorption equilibrium constants, KH2, KCO, KH2O, and KCH4. The WGS reaction is very close to equilibrium at the reforming conditions, and consequently, its kinetic constants cannot be determined significantly at the experimental conditions used. No statistically significant values can be obtained for the adsorption equilibrium constants KCO and KH2 at the reforming conditions, as well, because of the low CO partial pressures and high temperatures, respectively.10 Therefore, it was assumed in our work that the WGS reaction was always at equilibrium. This assumption was checked by some authors8,17 by a coefficient that describes the approach to equilibrium

4

No statistically significant values were obtained for KCO2 in the original work,10 which is the reason why there is no term containing KCO2 in the denominator of the above equations (eqs 10). It should be noted that the left side of eq 8 is derived for the case there is no CO present in the feed mixture, but CO is produced by the first reaction (eq 1) only. Partial pressures pi (bar) in eqs 7−9 are dependent on X1, X2, and X3, and boundary conditions are given as follows: W /FCH4,0 = 0

(bar 2)

βWGS =

pCO pH 1 2 2 pCO pH O K 2 2

(17)

However, this coefficient is uncertain due to the low pCO values, a fact that was also observed by Jakobsen et al.17 The constant values for KCO and KH2 were adopted from the work of Xu and Froment.10 Therefore, eight parameters in total were determined simultaneously during the optimization procedure, since both types of constants have the Arrhenius form (eqs 12 and 13). In order to keep the parameters balanced, it is suitable to

The constants K1, K2, and K3 are the equilibrium constants for the reactions represented by eqs 1−3, respectively, and were taken from the work of Hou and Hughes16 13602



Article

Figure 7. Parity plots for XCH4 (a) and XCO2 (b) for Ni-based pellet-type catalyst.

reparameterize the kinetic and adsorption constants of Arrhenius form in the following way ⎛ ⎛ −E ⎞ ⎛1 ⎞⎞ A ·exp⎜ a ⎟ = exp⎜b1 + C1b2⎜ + C2⎟⎟ ⎝T ⎠⎠ ⎝ RT ⎠ ⎝

procedure, the OFs that introduce the yield of hydrogen, XH2 (or yH2 at the reactor outlet), as shown in eq 21, contribute to the refined kinetic parameter estimation procedure. Carbon monoxide was not taken into account during our optimization procedure, since the molar fractions on a dry-basis of CO were very low at the reactor outlet, which further introduced larger experimental errors than those met for the other components. A more detailed approach (OF depicted by eq 21) was used for the parameter estimation, because of the new catalyst system employed. Although these kinds of objective functions, in which the sums of the absolute values of the residuals are minimized, are not useful for the statistical methods based on variances, they have the advantage of the same contribution of all the elements in a sum, which further results in a more correct estimation of parameters. The approximate 95% parameter confidence intervals given in Tables 3 and 4 were calculated by varying one parameter at a time while holding all the other parameters at their optimal values and observing the change in the results.The independent variable (eqs 7−9) in the case of the plate-type catalyst was Splate/FCH4,0, where Splate was the geometrical surface area of the catalyst. For both catalytic systems, the Levenberg−Marquardt optimization procedure was used, coupled with the Runge−Kutta method for the integration of the system of differential equations (eqs 7−9), since both reactors were operated in the integral mode. The calculations were performed using Matlab R2011a software (Mathworks, Natick, Massachusetts, USA). It was assumed that the redispersion phenomenon, described in section 3.1, influenced only the number of the collisions for the reactions, i.e. frequency factor, whereas the activation energy of the reactions remained unchanged. This is wellargued by the fact that the values of the platinum particle size (Figures 5 and 6) after the redispersion treatment were still large enough so that the characteristics of the bulk metal were retained. Namely, smaller size particles (smaller than those met in our study), ranging from roughly 1−50 nm exhibit physical and chemical properties that are between those of the smallest element from which they can be composed (such as a metal atom) and those of the bulk material. It further influences the catalyst performance, because the surface structure and electronic properties can change greatly in the abovementioned size range.19 Therefore, after the performed regeneration treatment, the pre-exponential factors in the reaction rate constants were multiplied by the constants c1 and c2, given in Table 4, with the subscripts 1 and 2 corresponding

(18)

Constants C1 and C2 in eq 18 are in the form: C1 =

1 1 Texp

and

+ C2

C2 = −

1 Texp ̅ (19)

∞

where Texp denotes experimental temperature (discrete value), and T̅ exp is the mean experimental temperature. b1 and b2 represent new parameters related to pre-exponential factor, A, and activation energy, Ea. It should be noted that with this kind of reparametrization the parameters are well balanced, since they are both in exponent. Also, constant C1 further improves the balance between the two parameters, all of which consequently leads to reducing the problem in the process of objective function minimum search.18 The calculated values of the parameters (Table 3 and 4) were obtained by minimizing the objective functions (OF) for all the temperatures simultaneously including conversions for Nibased pellet-type catalyst N

OFpellet =

⎛

∑ ⎜⎜ i=1

X̂CH4 − XCH4 XCH4

⎝

+ i

⎞ X̂CO2 − XCO2 ⎟ ⎟ XCO2 i⎠ (20)

and molar fractions of CH4, CO2, and H2 on a dry-basis in the reactor effluent stream for the plate-type Pt/Ni/Al2O3 catalyst ⎛ y ̂ − yCH ⎜ CH 4 4 = ∑⎜ ⎜ y i=1 ⎝ CH4 N

OFplate

+

yĤ − yH 2

yH

2

2

⎞ ⎟ ⎟⎟ i⎠

+ i

yCO ̂ − yCO 2

2

yCO

2

i

(21)

where N is the number of experimental points, X and y denote the conversion and the molar fraction of a certain component, respectively, and superscript ̂ denotes calculated values. It should be noted that although the SMR catalytic systems are well-described by using just XCH4 and XCO2 (or yCH4 and yCO2 at the reactor outlet) during the parameter optimization 13603



Article

Figure 8. Parity plots for volume fraction (on dry basis) of CO2 (a), CH4 (b), and H2 (c) in reactor effluent stream obtained for Pt/Ni/Al2O3 platetype catalyst.

to the number of the subsequent regeneration treatments. The good agreement of the calculated and measured data upon redispersion implied that the correction of pre-exponential factors might be considered valid. The parity plots for the conversion of CH4 and the conversion of CH4 into CO2 for Ni-based pellet-type catalyst are shown in Figure 7. The average relative errors for XCH4 and XCO2 were 0.11 and 0.12, respectively. For the plate-type Pt/Ni/Al2O3 catalyst, the parity plots for the component volume fractions on a dry basis are shown in Figure 8. The average relative errors for the volume fractions on the dry basis for CO2, H2, and CH4 were 0.10, 0.09, and 0.07, respectively. The mass balances for C, O, H, and N were within 0.07 of relative error for both catalyst systems. Some disparity was found for the experiments performed at the highest temperatures and pressures in the experimental range. This is in agreement with the results of the external mass transfer resistance test performed in our previous study, which showed that at the reaction conditions, T = 540 °C, P = 5.0 bar, no mass transfer resistance was observed for the volumetric flow rates higher than 2 LSTP min−1.7 Larger discrepancy between the calculated and experimental values for CO2 was found at the lower temperatures, because of the low values of CO2 molar fractions (especially at the flow rates of 5 LSTP min−1) and, therefore, larger experimental errors. It should also be noted that the equations with the calculated parameters presented in Table 4 did not hold for the experiments at the total pressure of 1 bar and held with an average error up to 30% for all the components for the experiments performed at the total pressure of 2.5 bar (presented in Figure 8 outside the range of ±20%). The reason for this most probably lies in the fact that the original mechanism proposed by Xu and Froment10 was derived for the experiments in the range of total pressure 3−15 bar. The graph showing the conversion of methane vs contact time, Splate/FCH4,0, is presented in Figure 9. The experimental points were not obtained at exactly the same temperatures as indicated in the graph, but in the temperature ranges (see Figure 9 caption). The estimated kinetic parameters should obey some of the physicochemical constraints, as follows: 1. Ea > 0 and −ΔHa > 0, which holds for both types of catalysts (Table 3 and 4). The values of −ΔHa,H2O do not satisfy the van’t Hoff equation, which was also observed in the original model work.10 2. The pre-exponential term in adsorption constants Kj can ° /R), where ΔSa,j ° = Ss,j ° − be expressed as Kj° = exp(ΔSa,j ° . The rule ΔSa,j ° < 0, i.e. Kj° < 1, should be satisfied. It Sg,j

Figure 9. Conversion of methane vs contact time, P = 4.0 bar, H2O/ CH4 = 4.00 mol/mol, N2/CH4 = 0.85 mol/mol, H2/CH4 = 0.15 mol/ mol. Temperature at the graph denotes the calculation temperature. Temperature ranges of the experimental points: 493.0−498.0, 518.0− 524.0, 545.0−549.5.0, 568.5−570.5 °C.

can be seen from Tables 3 and 4 that this rule holds for CH4. 3. The rule −ΔS°a < S°g should also be satisfied.20 For CH4, the S°g value at 298 K is 186.1 J mol−1 K−1. The −ΔS°a,CH4 obtained for Ni-based pellet-type catalyst and Pt/Ni/ Al2O3 plate-type catalyst were 56.7 and 68 J mol−1 K−1, respectively. 4. The fourth thermodynamic criterion is given by the Everett’s compensation effect,20 expressed linearly as ΔSa° = −12.2 + 0.0014ΔHa, where ΔSa° and ΔHa are in gcal gmol−1 K−1, and gcal gmol−1, respectively. Relation was not found to be generally applicable for all values of ΔS°a and ΔHa obtained from the catalyst data, which mostly fell below this straight line.20 Therefore, this criterion can be given as ln(K°) ≤ (12.2 × 4.184 − 0.0014ΔHa)/R, with ΔHa expressed in J mol−1. The results for CH4 for both catalyst systems employed in our study were in accordance with this rule (Table 5). Table 5. Globalized Everett’s Compensation Rule Applied to Calculated Parameter Values Ni-based pellet-type catalyst

13604

Pt/Ni/Al2O3 plate-type catalyst

comp

ln(K°)

(12.2 × 4.184 − 0.0014ΔHa)/R

comp

ln(K°)

(12.2 × 4.184 − 0.0014ΔHa)/R

CH4

−6.8

0.3

CH4

−8.2

−1.3



Article

5. The fifth thermodynamic rule is given by the theorem that the negative value of ΔS°a must be larger than about ten entropy units (gcal gmol−1 K−1)20 or −ΔS°a > 42 J mol−1 K−1. The values for −ΔSa° satisfy this rule, as well. 3.4. Comparison of the Catalyst Systems. The calculated parameters for both catalysts are compared to the values obtained by Xu and Froment10 in Table 6. It could be

that the Pt/Ni/Al2O3 plate catalyst is 8 and 1.8 times more active than the Ni-based pellet-type catalyst and Ni/MgAl2O4 catalyst employed in the study of Xu and Froment,10 respectively. This kind of comparison cannot be done for the Rh/CeαZr1−αO2 catalyst due to the fact that ceria-zirconia support exhibits the catalytic activity for SMR, as well. Therefore, Figure 10b was provided in order to compare the catalyst systems based on the total mass of catalyst. It could be seen from Figure 10b that the Rh/CeαZr1−αO2 catalyst was 1.6 times more active for SMR than the Pt/Ni/Al2O3 plate catalyst (after the second regeneration treatment) at the simulated reaction conditions.

Table 6. Comparison of Calculated Parameter Values Obtained in the Present Study and Those Obtained by Xu and Froment on a Commercial Ni/MgAl2O4 Catalyst

reaction rate constant

A/AXu−Froment

■

Pt/Ni/Al2O3 plate-type catalyst

Ni-based pellet-type catalyst Ea/

Ea,Xu−Froment

CONCLUSIONS The SMR intrinsic kinetics study presented in this work showed that for both types of catalysts employed, specifically Ni-based pellet-type and Pt/Ni/Al2O3 plate-type catalyst, the well-established model of Xu and Froment10 satisfactorily fitted the experimental data. Platinum particle redispersion phenomenon was observed after the regeneration treatment of the plate-type catalyst. The comparison between the two catalytic systems based on the active metal content showed that the Pt/ Ni/Al2O3 plate-type catalyst after the second regeneration treatment was 8 times more active than the Ni-based pellet catalyst employed in this study. However, the simulated data showed that the Rh/CeαZr1−αO2 catalyst was the most active among the catalyst systems observed.

Ea/

A/AXu−Froment

Ea,Xu−Froment

4.17 × 10−1 4.13

1.03 1.09

k1 k3 adsorption constant

3.53 × 10−2 1.00 2.93 × 10−3 0.86 K°/ Ha/ K°Xu−Froment ΔHa,Xu−Froment

K°/ K°Xu−Froment

KCH4

1.64

0.91

4.24 × 10−1

1.15

KH2O

5.85

1.11

6.94

1.13

ΔHa/ ΔHa,Xu−Froment

noticed that the activation energies and adsorption enthalpies did not differ severely from the original model. The differences in the pre-exponential factors for the Ni-based plate catalyst were to be expected, since the catalyst had the lower specific surface area compared to the catalyst employed in the study of Xu and Froment.10 For the plate-type Pt/Ni/Al2O3 catalyst, the comparison of the pre-exponential factors was based on the sum of the Ni, Pt, and Al2O3 loadings explained in the section 2.3. This sum multiplied by the geometrical surface area of the catalyst was assumed to be the total mass of catalyst. The catalyst systems in this study were compared based on the mass of the active metal, and the total mass of the catalyst, as well. Figure 10 depicts the simulated data for the conversion of methane for the catalyst systems in this study, along with the simulated data from the study of Xu and Froment10 and Halabi et al.21 As one can see, the original Pt/Ni/Al2O3 plate catalyst is slightly less active than the Ni-based pellet catalyst. However, the activity of the plate catalyst increased after the first and even more after the second regeneration treatment. The calculations presented with the solid lines in Figure 10a (based on the plate catalyst activity after the second regeneration treatment) show

■

AUTHOR INFORMATION

Corresponding Author

*Fax: +386 1 476 0280. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The authors would like to thank Slovenian Research Agency (ARRS) (Program P2-0152) for financing this work, SüdChemie AG, Germany, for providing the commercial pellettype catalyst, and Sulzer Chemtech Ltd, Switzerland, for their cooperation and provision of the structured element.

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NOMENCLATURE Ai = pre-exponential factor of rate coefficient, ki ci = redispersion constant

Figure 10. Conversion of methane for different catalyst systems based on the active metal mass (a) and the total catalyst mass (b): T = 500 °C, P = 5 bar, H2O/CH4 = 3 mol/mol, H2/CH4 = 1.25 mol/mol. 13605



Article

Ea,i = activation energy of reactions 1, 2, and 3, J mol−1 FCH4,0 = molar feed rate of CH4, mol h−1 ΔH = enthalpy change of reaction or adsorption, J mol−1 k1, k3 = rate coefficient of reaction 1 and 3, mol bar1/2 gcat−1 h−1 k2 = rate coefficient of reaction 2, mol gcat−1 h−1 bar−1 K1, K3 = equilibrium constant of reactions 1 and 3, bar2 K2 = equilibrium constant of reaction 2 KCH4, KCO, KH2 = adsorption equilibrium constant for CH4, CO, and H2, bar−1 KH2O = dissociative adsorption constant of H2O KCH ° 4, KCO ° , KH° 2 = pre-exponential factor of adsorption constant for CH4, CO, and H2, bar−1 K°H2O = pre-exponential factor of adsorption constant for H2O N = number of experiments pj = partial pressure, bar P = total pressure, bar Q = total volume flow, mLSTP min−1 r1, r2, r3 = rates of reaction 1, 2, and 3, mol gcat−1 h−1 R = gas constant (8.314), J mol−1 K−1 ΔS = entropy change, J mol−1 K−1 Sg = standard entropy of gas component, J mol−1 K−1 Splate = geometrical surface area of plate catalyst, m2 t = time, h T = temperature, K W = mass of catalyst, gcat XCH4 = conversion of methane, molCH4 h−1 (molCH4 h−1 fed)−1 XCO2 = conversion of methane into CO2, molCO2 h−1 (molCH4 h−1 fed)−1 X1, X3 = conversion of methane per reaction, molCH4 h−1 (molCH4 h−1 fed)−1 X2 = conversion of carbon monoxide per reaction, molCO h−1 (molCO h−1 fed (obtained from reaction 1))−1 yj = component molar fraction

(6) Johnsen, K.; Ryu, H. J.; Grace, J. R.; Lim, C. J. SorptionEnhanced Steam Reforming of Methane in a Fluidized Bed Reactor with Dolomite as CO2-Acceptor. Chem. Eng. Sci. 2006, 61, 1195. (7) Obradović, A.; Likozar, B.; Levec, J. Catalytic Surface Development of Novel Nickel Plate Catalyst with Combined Thermally Annealed Platinum and Alumina Coatings for Steam Methane Reforming. Int. J. Hydrogen Energy 2013, 38 (3), 1419. (8) Wei, J.; Iglesia, E. Mechanism and Site Requirements for Activation and Chemical Conversion of Methane on Supported Pt Clusters and Turnover Rate Comparisons among Noble Metals. J. Phys. Chem. B 2004, 108, 4094. (9) Akers, W. W.; Camp, D. P. Kinetics of the Methane−Steam Reaction. AIChE J. 2004, 1 (4), 471. (10) Xu, J.; Froment, G. F. Methane Steam Reforming, Methanation and Water-Gas Shift: I. Intrinsic Kinetics. AIChE J. 1989, 35 (1), 88. (11) Allen, D. W.; Gerhard, E. R.; Likins, M. R. Kinetics of the Methane−Steam Reaction. Ind. Eng. Chem., Process Des. Dev. 1975, 14 (3), 256. (12) Fiedorow, R. M. J; Wanke, S. E. The Sintering of Supported Metal Catalysts: I. Redispersion of Supported Platinum in Oxygen. J. Catal. 1976, 43 (1), 34. (13) Fiedorow, R. M. J.; Chahar, B. S.; Wanke, S. E. The Sintering of Supported Metal Catalysts: II. Comparison of Sintering Rates of Supported Pt, Ir, and Rh Catalysts in Hydrogen and Oxygen. J. Catal. 1978, 51 (2), 193. (14) Hassan, S. A.; Khalil, F. H.; El-Gamal, F. G. The Effect of Different Atmospheres on the Sintering of Pt/Al2O3 Reforming Catalysts. J. Catal. 1976, 44 (1), 5. (15) Ruckenstein, E.; Chu, Y. F. Redispersion of Platinum Crystallites Supported on Alumina−Role of Wetting. J. Catal. 1979, 59 (1), 109. (16) Hou, K.; Hughes, R. The Kinetics of Methane Steam Reforming over a Ni/α-Al2O3 Catalyst. Chem. Eng. J. 2001, 82 (1), 311. (17) Jakobsen, J. G.; Jørgensen, T. L.; Chorkendorff, I.; Sehested, J. Steam and CO2 Reforming of Methane over a Ru/ZrO2 Catalyst. Appl. Catal. A: Gen. 2010, 377 (1), 158. (18) Buzzi-Ferraris, G.; Manenti, F. Kinetic Models Analysis. Chem. Eng. Sci. 2009, 64 (5), 1061. (19) Bell, A. T. The Impact of Nanoscience on Heterogeneous Catalysis. Science 2003, 299 (5613), 1688. (20) Boudart, M.; Mears, D. E.; Vannice, M. A. Kinetics of Heterogeneous Catalytic Reactions. Congres Int. Chim. Ind., C.-R. 36, Ind. Chim. Belge 1967, 32 (1), 281. (21) Halabi, M. H.; De Croon, M. H. J. M.; Van der Schaaf, J.; Cobden, P. D.; Schouten, J. C. Intrinsic Kinetics of Low Temperature Catalytic Methane−Steam Reforming and Water−Gas Shift over Rh/ CeαZr1−αO2 Catalyst. Appl. Catal. A: Gen. 2010, 389 (1), 80.

Superscripts

̂ = calculated values ° = standard conditions Subscripts

a = adsorption calc = calculated values cor = corrected values exp = experimental values g = gaseous component i = chemical reaction i (i = 1−3) j = component j

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REFERENCES

(1) Neef, H.-J. International Overview of Hydrogen and Fuel Cell Research. Energy 2009, 34, 327. (2) Van Beurden, P. On the Catalytic Aspects of Steam-Methane Reforming, A Literature Survey; ECN: The Netherlands, 2004; ECN-I04-003. (3) Harrison, D. P. Sorption-Enhanced Hydrogen Production: A Review. Ind. Eng. Chem. Res. 2008, 47, 6486. (4) Barelli, L.; Bidini, G.; Gallorini, F.; Servili, S. Hydrogen Production through Sorption-Enhanced Steam Methane Reforming and Membrane Technology: A Review. Energy 2008, 33 (4), 554. (5) Balasubramanian, B.; Ortiz, A. L.; Kaytakoglu, S.; Harrison, D. P. Hydrogen from Methane in a Single-Step Process. Chem. Eng. Sci. 1999, 54, 3543. 13606


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