Kinetics of Carbon Dioxide Reforming of Natural Gas Using Different Catalysts

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Kinetics of Carbon Dioxide Reforming of Natural Gas Using Different CatalystsS. A. El-Temtamy a; S. A. Ghoneim a; A. K. El-Morsy a; A. Y. El-Naggar a; R. A. El-Salamouny a

a Egyptian Petroleum Research Institute (EPRI), Nasr City, Cairo, Egypt

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Petroleum Science and Technology, 27:1661–1673, 2009

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ISSN: 1091-6466 print/1532-2459 online

DOI: 10.1080/10916460802455897

Kinetics of Carbon Dioxide Reforming of

Natural Gas Using Different Catalysts

S. A. El-Temtamy,1 S. A. Ghoneim,1 A. K. El-Morsy,1

A. Y. El-Naggar,1 and R. A. El-Salamouny1

1Egyptian Petroleum Research Institute (EPRI), Nasr City, Cairo, Egypt

Abstract: The Kinetics of CO2 reforming of natural gas to produce synthesis gas

(CO C H2) has been investigated using 2 g of 0.5% wt of each of the catalysts;

rhodium, ruthenium and iridium supported on -alumina. The experiments were

carried out in a tubular reactor at three temperature levels namely 600, 700, 800ıC and

four gas weight hourly space velocities; 18000, 36000, 45000, and 60000 ml g�1 h�1.

The reaction was found to obey first-order kinetics for the depletion of both of the

reacting components; CH4 and CO2 on all the investigated catalysts. At the same

temperature, CO2 had a higher reaction rate constant, k0, as compared to CH4 for all

the catalysts. This was more pronounced for Rh/ -alumina catalyst, which occupied

the highest reaction rates. Activation Energies were calculated from the Arrhenius

relation.

Keywords: CO2 reforming, iridium supported catalyst, natural gas, reforming kinet-

ics, rhodium supported catalyst, ruthenium supported catalyst and synthesis gas

1. INTRODUCTION

The Catalytic reforming of CH4 with CO2 to synthesis gas (reaction 1)

has received increasing attention in the past few years (Kroll, Swaan, and

Mirodatos, 1996; Bhat and Sachtler, 1997; Wang and Au, 1997; Stagg et al.,

1998; Bitter, Seshan, and Lercher, 1998; Ruckenstein and Wang, 2000; Yokota,

Okumura, and Niwa, 2002; Wei and Iglesia, 2004; Li et al., 2005; and Mo

et al., 2005) because it provides a low H2/CO ratio (�1) that is suitable for the

Fischer-Tropsch synthesis to olefins. Spiewak, Tyner, and Langnickel (1992)

decided that the high reaction enthalpies associated with methane reforming

by carbon dioxide and its reverse reaction, make this process one of the most

suitable for applications in the storage of renewable energy sources, examples

Address correspondence to Salwa A. Ghoneim, Egyptian Petroleum Research

Institute, Nasr City, 11727, Cairo, Egypt. E-mail: [email protected]

1661

Downloaded By: [Ghoneim, S. A.] At: 20:29 31 August 2009

1662 S. A. El-Temtamy et al.

are thermochemical heat pipe applications and chemical energy transmission

systems where CO2, a green house gas, is consumed in a useful manner.

CH4 C CO2 ��! 2COC 2H2 : : : �H298ı D 247 kg=mol {1}

Gadalla and Bower (1998) indicated that the main reaction is accompanied

by several secondary reactions [Eq. (2–5)], of which the reverse water-gas

shift reaction appears to be the most important, because of its dependence on

the product hydrogen of reaction {1}.

CO2 C H2��! �� COC H2O : : : : : : reverse water-gas shift reaction {2}

CO2 C 4H2��! �� CH4 C 2H2O : : : : methanation {3}

2CO ��! �� CC CO2: : : : : : : : Boudouard reaction {4}

CH4��! �� CC 2H2 : : : : : : : : Cracking {5}

Depending on the reaction conditions and on the composition of the gas

mixture, carbon build up due to the Boudouard reaction and methane crack-

ing becomes thermodynamically favorable. Erdöhelyi, Cserényi, Papp, and

Solymosi (1994) noted that reproducible results were obtained only at stoicho-

metric composition or with excess of carbon dioxide. Solymosi, Erdöhelyi,

and Cserényi (1992) noted the formation of three different carbon species

depending on temperatures and various noble metals. Most of the group VIII

metals (e.g., Ni, Co, Ru, Rh, Pd, Pt, and Ir) are more or less catalytically

active toward the reforming reaction. However, being strongly endothermic,

the CO2 reforming of CH4 requires high temperatures and this stimulates de-

activation caused by carbon deposition and/or metal centering. Generally, the

noble-metal based catalysts exhibit less carbon deposition. Rostrup-Nielsen

and Bak-Hansen (1993) compared the catalyst based on nickel, ruthenium,

rhodium, palladium, iridium and platinum, they found that rhodium and

ruthenium provided high selectivity with carbon free operation and high

activities.

The kinetics of the dry reforming reaction of CH4 by carbon dioxide has

been studied by several authors. Richardson and Paripatyadar (1990) used 45–

60 mesh particles of Rh/Al2O3 catalyst at three different temperature levels;

600, 650, and 700ıC and a range of CO2/CH4 ratios from 1.0 to 5.0. They

recommended a rate equation of the Langmuir-Hinshelwood formula. Bhat

and Sachtler (1997) and Wei and Iglesia (2004) also used Rh/Al2O3 catalyst

and concluded that the reforming reaction is first order with respect to CH4

and zero order with respect to CO2. Mark and Maier (1996) used 0.5 wt%

of Rh, Ru, Ir Pd & Pt/Al2O3 catalyst in the temperature range of 550–850ıC

with catalyst particle size in the range of 20 to 200 �m. Despite the high

reaction temperature, which would favor transport limitation, they proved that


Kinetics of Carbon Dioxide 1663

the reaction was chemically controlled for the catalyst particle size studied.

The rate of reaction expressed per unit metal surface area was constant over

a wide dispersion range. The observed activity sequence was Rh > Ru >

Ir >> Pd, Pt.

The present work investigates the kinetics of CO2 and natural gas (CH4)

utilization during reforming of natural gas to produce synthesis gas (CO C

H2) using the catalysts rhodium, ruthenium and iridium supported on -

alumina of �200 �m particle size. The experiments were carried out in a flow

reactor at three different temperature levels; 600, 700, 800ıC and four gas

weight hourly space velocities; 18000, 36000, 45000, and 60000 ml g�1 h�1.

2. THEORETICAL

For many reactions, the rate expression can be written as a product of a

temperature dependent term and a composition dependent term, or

ri D f1.temperature/ � f2.composition/D k � f2.composition/ (1)

For such reactions the temperature dependent term, the reaction rate constant,

k, has been found in practically all cases to be well represented by the

Arrhenius’ law;

k D koe�E=RT (2)

Where ko is the frequency or pre-exponential factor and E is called the

activation energy of the reaction.

The temperature dependency of reaction is determined by the activation

energy and temperature level of the reaction.

The reaction rate constant itself can be found by studying the kinetics of

a given reaction under different operating conditions to determine the form of

the concentration dependent term in Eq. (1) as will be given in the following

section.

2.1. Testing for First-Order Kinetics

At a given temperature the reaction rate is usually expressed as a function of

reactant concentration in the form:

�r D kCn (3)

Where n is the order of the reaction.

If the reaction volume changes during reaction, like for example in case

of gas reactions then the final volume of the reactant C products for a reactant



A is given by:

V D Vo.1C �CAXA/ or XA DV � Vo

Vo"A

(4)

Where Vo D initial volume of reactants.

XA D conversion.�CA D change in the system volume between no conversion and com-

plete conversion and is called, expansion factor, thus;

�CA DVXD1 � VXD0

VXD0

(5)

Noting that moles of A left unreacted

NA D NAo.1 � XA/ (6)

And the concentration of

A D CA DNA

VD

NAo.1 �XA/

Vo.1C "AXA/(7)

CAo D.1 �XA/

.1C "AXA/(8)

Thus

CA

CAo

D.1 � XA/

.1C "AXA/(9)

Or, the conversion of

A; XA D1 � CA=CAo

1C "ACA=CAo

(10)

The performance equation can be deduced as follows, for a plug flow reactor

containing porous catalyst particle, take a thin slice of the PFR see Figure 1.

At steady state a material balance for a reactant A gives:

Input D outputC accumulationŒmolA�=h: (11)

Figure 1. A thin slice of solid catalyzed plug flow reactor.



In symbols;

FAo � FAoXin D FAo � FAoXout C .�r0

A/�W (12)

in differential form:

FAodXA D .�r0

A/�W (13)

Integrating over the whole reactor gives:

W

FAo

D

Z XAout

0

dXA

�r 0

A

(14)

Let

W CAo

FAo

D �kg � h

l.weight � time term) (15)

For a first order reaction,

�r0D k0CA (16)

Substituting from (7) for CA in (16) and integrating (14) taking into account

the definition given in (15). The performance equation for a first order reaction

in a plug flow reactor would be as given by Levenspiel (1999) as follows:

k0� 0D .1C �CA/ ln

1

1 �Xout

� �CAXout (17)

A plot of � 0 versus the r.h.s of Eq. (17) should yield a straight line passing

through the origin if the reaction obeys first-order kinetics. The slope of the

straight line is equal to k0, the reaction rate constant.

Reforming of methane using CO2 can be expressed as follows:

CH4 C CO2 D 2COC 2H2

To calculate the expansion factor �C, one must consider the inert gas intro-

duced with the reactants (Levenspiel, 1999). The reactant feed gas were CH4,

CO2 and N2 in the ratio 1:1:4 respectively. The six volumes of entering gases

will produce eight volumes of leaving product gases then the expansion factor

would be calculated as;

�C D .8 � 6/=6 D 1=3

Equation (17) was investigated for each of the reacting species, namely CH4

and CO2 for the three types of catalysts prepared; Rh/ -Al2O3, Ru/ -Al2O3

and Ir/ -Al2O3. The conversion X is calculated from the mole% (Dvol%)

and partial pressure of the reacting species.



3. EXPERIMENTAL

3.1. Catalysts

Three catalysts were prepared using incipient wetness impregnation tech-

niques of -alumina (Puralox, Condea) was used as a support. Impregnation

was carried out with aqueous solution of the following precursor salts: RhCl3 �

2H2O (Alfa Aesar), RuCl3 � 3H2O (Merck Chemie) and IrCl3 � 3H2O (Merck

Chemie) with 0.5 wt% metal loading. Then, catalyst was dried overnight at

110ıC and calcined in air at 500ıC for 3 h. The catalyst powder was sieved

to �200 �m to ensure the absence of mass transfer limitations (Mark and

Maier, 1996). Details about the preparation of these catalysts have been given

elsewhere (Ferreira-Aparicio, Rodriguez-Ramos, and Gurrero-Ruiz, 1997).

Before reaction, the catalyst was submitted to a standard reduction pretreat-

ment by heating in pure hydrogen with rate of 20 ml/min at 500ıC for

7 h.

3.2. Reaction Apparatus

The flow reactor (Figure 2) was a quartz tube (13 mm in diameter, 800 mm

overall length) filled with 2 g of catalyst between two layers of ceramic fibers

are placed in a ventilated oven, with a K-type thermocouple located inside

the catalytic bed (to control the reaction temperature). The reaction mixture

of CH4:CO2:N2 in proportions (1:1:4) regulated by mass flow controllers

was adjusted to give gas hourly space velocities: 18000, 36000, 45000,

60000 ml g�1 h�1 respectively. Reaction temperatures of 600–800ıC were

used at atmospheric pressure. An ice-cold trap was set between the reactor exit

and gas sampling to remove the water formed during reaction. After shifting

from one temperature to another and the new temperature was reached, the

catalyst bed was left for at least 15 min. to be sure that the temperature were

almost constant through the catalyst bed. After another 30 min. products were

received in gas samplers.

3.3. Gas Chromatographic Analysis

The used natural gas components (N2, CO2 and C1–C7) and the produced

gases (H2 and CO) were analyzed using Agilent 6890 plus, HP, gas chromato-

graph, equipped with thermal conductivity (TCD) and flame ionization (FID)

detectors. Elution of the studied gas mixtures was achieved with temperature

programming from 60 to 200ıC at a rate 10ıC min�1. Nitrogen (oxygen-free)

was used as a carrier gas for the analysis of natural gas, while helium for the

detection of CO and H2.



Figure 2. Experimental Setup.

4. RESULTS AND DISCUSSIONS

The volumetric analysis (percentage remained) of the reacting species in

Table 1 and the reaction conditions, were used to calculate the conversion

and hence the r.h.s of Eq. (17) as well as the weight—time term of the L.h.s

of the same equation.

Least squares regression analysis was used to fit the data according to

Eq. (17), k0 is calculated as the slope of the fitted straight lines. Figure 3

shows a typical case corresponding to CO2 component over Rh/ -Al2O3.

The assumption of first-order kinetics with the reactants CH4 and CO2 was

found to be obeyed in all the cases studied as indicated by the value of the

correlation coefficient being >0.9 in most cases. The calculated k0 values

together with the correlation coefficients are given in Table 2 for all the

experimental runs.



Table 1. Volume% remained of CO2 and CH4

600ıC 700ıC 800ıC

Catalyst

Temperature

space

velocity

ccg�1� h�1 CH4 CO2 CH4 CO2 CH4 CO2

Rh/ -Al2O3 18000 41.53 28.38 32.97 22.37 5.17 6.56

36000 66.04 58.03 53.38 40.51 41.06 33.99

45000 73.99 56.59 69.59 48.95 63.02 43.2

60000 89.21 94.85 83.29 74.94 74.44 62.26

Ru/ -Al2O3 18000 88.26 91.04 87.44 81.62 85 69.91

36000 78.5 79.7 65.54 68.46 52.58 49.05

45000 78.95 73.69 73.38 70.95 67.83 59.35

60000 91.17 90.83 88.72 76.01 82.9 66.92

Ir/ -Al2O3 18000 89.26 91.43 89.04 90.99 87.02 87.48

36000 89.05 89.22 86.44 81.51 75.37 67.01

45000 79.03 77.25 75.44 72.02 69.89 57.12

60000 98.63 97.43 99.46 94.22 95.03 92.89

The k0 values thus calculated at different temperatures, are fitted to Ar-

rhenius’ equation, for each catalyst type. The activation energy was calculated

by plotting ln k0 versus 1/T, a typical plot is shown in Figure 4. The results

of the least squares regression analysis for ko (pre-exponential factor), the

intercept and –E/R T, and the slope are given in Table 3.

Figure 3. Test for first-order reaction kinetics of CO2 over Rh/Al2O3 catalyst.



Figure 4. Arrhenius plot for Rh/Al2O3:

Discussion for each catalyst type is as follows:

4.1. Rh/ -Al2O3

Figure 3 is a typical test for the first order assumption for methane and CO2

reaction over Rh/ -Al2O3 catalyst. It was shown that the first-order reaction

kinetics assumption is valid for both reactants under almost all operating

conditions. However, higher deviation is observed at the highest temperature

investigated for the longest reaction time. This point was therefore excluded

from the correlation. A correlation coefficient (>0.9) shown in Table 2 reflects

the validity of the first order kinetics. The deviation from first order equation

observed for both reactants at the highest temperature and the longest reaction

time may indicate a change in the reforming reaction mechanism or the onset

of other reactions.

As shown in Table 2 the reaction rate constant for CO2 is found to be

approximately double that calculated for CH4 at the same temperature, thus

strongly suggesting a route other than reforming for the reaction of CO2 for

example the RWGS reaction. This result is in confirmation with yields of CO

that was higher than H2 despite the equimolal volumes of the reacting gases

as given elsewhere (El-Salamouny, 2005). Working with Rh (0.5 wt%) over

different type of supports (Wang and Ruckenstein, 2000) similarly reported

that CO2 conversion was always higher than that of CH4 and that CO yield

was always higher than H2. H2/CO ratio was always below one. This result

was attributed to that the reverse water gas shift reaction was taking place

simultaneously with the reforming reaction.



Table 2. Reaction rate constant as determined by the least squares regression

analysis

CO2 CH4

Catalyst Temperature k0 c.c k0 c.c

Rh/ -Al2O3 600 5.74E4 0.96 3.80E4 0.981

700 13.10E4 0.957 5.74E4 0.979

800 19.1E4 0.99 8.13E4 0.911

Ru/ -Al2O3 600 2.28E4 0.945 2.1E4 0.964

700 3.66E4 0.952 3.48E4 0.848

800 7.22E4 0.881 5.44E4 0.761

Ir/ -Al2O3 600 1.66E4 0.897 1.64E4 0.936

700 2.57E4 0.933 2.18E4 0.938

800 5.20E4 0.918 3.49E4 0.968

Arrhenius’ plot for this catalyst and Table 3 reveal that the activation

energy for CO2 reaction is 47.06 kJ/mol, which is more than that for CH4

(29.51 kJ/mol), that is, CO2 is more temperature sensitive than CH4. Similar

trends though different values have been reported by (Ferreira, Guerrero-Ruiz,

Rodriguez-Ramos, 1998) for the same catalyst.

4.2. Ru/ -Al2O3 and Ir/ -Al2O3 Catalysts

Table 2 shows that the first-order reaction kinetics assumption is valid for both

reactants on the two catalysts; Ru/ -Al2O3 and Ir/ -Al2O3 catalysts under

almost all the investigated operating conditions. However, higher deviation is

observed at the longest reaction time. These points were therefore excluded

from the correlation. The correlation coefficient is �0.9 in most of the cases

thus reflecting the validity of the first-order kinetics. The deviation from first

order equation observed for both reactants at the longest reaction time may

indicate a change in the reforming reaction mechanism or deactivation of

both catalysts Ir and Ru/ -Al2O3 with time, contrary to Rh/ -Al2O3, which

showed increased activation at the highest temperature with time.

Table 3. Activation energy and pre-exponential factor from Arrhenius’ equation

CO2 CH4

Catalyst ko �E/R E, J/mol c.c ko �E/R E, J/mol c.c

Rh/ -Al2O3 39.8E6 �5.66E3 47.06E3 �0.98 2.21E6 �3.55E3 29.51E3 �0.999

Ru/ -Al2O3 10.2E6 �5.38E3 44.65E3 �0.988 3.30E6 �4.42E3 36.24E3 �0.999Ir/ -Al2O3 6.58E5 �5.27E3 43.81E3 �0.981 8.38E5 �3.47E3 28.85E3 �0.980



As shown in Table 2 the reaction rate constants for CO2 were found to

be approximately equal to or slightly higher than those calculated for CH4

at the same temperature, thus as discussed above in the case of Rh/ -Al2O3

suggesting a route other than reforming for the reaction of CO2, for example

the RWGS reaction. However, Rh/ -Al2O3 catalyst is the most active catalyst

among the investigated three; this is manifested through its higher reaction

rate constants reported in Table 2 as compared to the other two. Ru/ -Al2O3

come next and finally come Ir/ -Al2O3. Mark and Maier (1996) observed the

same sequence of activity. The Arrhenius plots for these catalysts reveal that

the activation energies for CO2 depletion reaction are more than that for CH4,

that is, CO2 is more temperature-sensitive than CH4. The activation energy for

CH4 dissociation is 28.85 kJ/mol while that for CO2 is 43.81 kJ/mol for Ir/ -

Al2O3 catalyst and 36.24 kJ/mol for CH4 dissociation and 44.65 kJ/mol for

CO2 for Ru/ -Al2O3 catalyst. Ferreira et al. (1998) reported approximately

equal values of activation energies for the reactants CO2 and CH4 over Ru

and Ir catalysts. These values are approximately double those reported in the

present work.

5. CONCLUSIONS

� The first-order kinetics assumption is valid for both CH4 and CO2 reactants

under the operating conditions investigated.� The reaction rate constant for CO2 was higher than that of CH4; this was

especially true for Rh/ -Al2O3 catalyst. For example at 800ıC reaction

rate constants were equal to 19.1E4 and 8.13E4 Lit/h kg.cat/ for CO2 and

CH4, respectively.� The observed activity sequence as reflected from the values of the reaction

rate constants is Rh > Ru > Ir.� The activation energy was calculated from Arrhenius’ equation for each

catalyst, average values are: 45.17E3 J/mol. and 31.53E3 J/mol. for CO2

and CH4 respectively.

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