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Catalytic partial oxidation of methane to syngas in a fixed-bed

reactor with an O2-distributor: The axial temperature profile

and species profile study

Shuhong Liua,b, Wenzhao Lia, Yuzhong Wanga, Hengyong Xua,⁎

aDalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, ChinabGraduate University of Chinese Academy of Sciences, Beijing 100049, China

A R T I C L E I N F O A B S T R A C T

Article history:

Received 31 March 2008

Received in revised form 4 June 2008

Accepted 6 June 2008

Catalytic partial oxidation of methane (CPOM) to syngas has been investigated in a fixed-bed

reactor with an O2-distributor (FR-OD). The axial temperature profile and species profile

along the Ni or Rh-based catalyst bed have been measured at different conditions. As the O 2

was distributed radially into the catalyst bed through several rows of holes arranged at the

special zone of the OD, a microenvironment maintaining a low O2 /CH4 ratio (0.10–0.22) was

provided in the catalyst bed. The hotspot phenomena appeared at the entrance of the

catalyst bed have been effectively controlled. A more uniform temperature profile along the

catalyst bed has been given, which is beneficial to the stability of catalyst and the safety of

reactor operation.

© 2008 Elsevier B.V. All rights reserved.

Keywords:

Catalytic partial oxidation

of methane

O2-distributor

Temperature profile

Species profile

1. Introduction

The conversion of methane to syngas will play an important

role in the 21st century for both large-scale GTL plants and

small unit providing H2 for fuel cell [1,2]. In industry, the main

route for syngas production is steam reforming of methane

(SRM),

CH4 þ H2O→COþ 3H2 ΔH-298 ¼ þ206kJ=mol ð1Þ

which is highly efficient but capital intensive because it in-

volves a big exchange of energy between the steam reformer and heat recovery unit [3]. Recently catalytic partial oxidation

of methane (CPOM)

CH4 þ 0:5O2→COþ 2H2 ΔH-298 ¼ −36kJ=mol ð2Þ

has been studied extensively by both academia and industry

[4,5]. CPOM is excelled at consumed energy and products

composition [6]. However, CPOM process has not yet been

used commercially because it involves premixing of CH4 and

O2 which can be flammable or even explosive under elevated

pressure and temperature. Meanwhile the hotspot formation

near the beginning of the catalyst bed due to highly exo-

thermic complete oxidation reaction

CH4 þ 2O2→CO2 þ 2H2O ΔH-298 ¼ −802kJ=mol ð3Þ

may create severe problems of heat management, safety of

system and stability of catalyst [7,8].

Some novel reactor configurations with the idea of manage-

ment safety of the system and hotspot problem have recently

been reported [9–12]. In the counter-current heat-exchangereactor (CHXR) [9], they fed the hydrocarbon and the oxygen

separately and mixed the two feed gas streams until a few

centimeters in front of the catalyst zone through static mixers.

So thedanger of explosionminimized.In thenew ATRproposed

by Institute of Applied Energy of Japan [10], the oxygen was

separated into three shares or more, meanwhile two different

catalysts, one for oxidation and the other for reforming were

packed with alternating layers for three or more cycles. With

F U E L P R O C E S S I N G T E C H N O L O G Y 8 9 ( 2 0 0 8 ) 1 3 4 5 – 1 3 5 0

⁎ Corresponding author. Tel./fax: +86 411 84581234.E-mail address: [email protected] (H. Xu).

0378-3820/$ – see front matter © 2008 Elsevier B.V. All rights reserved.doi:10.1016/j.fuproc.2008.06.004

a v a i l a b l e a t w w w . s c i e n c e d i r e c t . c o m

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mailto:[email protected]

http://dx.doi.org/10.1016/j.fuproc.2008.06.004

http://dx.doi.org/10.1016/j.fuproc.2008.06.004

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this design, it was possible to increase the safety of the system.

Topsoe [11] provided an improved CPOM process comprising a

plurality of sequential, separated steps, whereby in each step

only a small fraction of the stoichiometric amount of oxygen

was added and reacted with the hydrocarbon feed gas on a

catalyst bed. It could increase the safety of the system and

decrease the hotspot. The heat integrated wall reactor (HIWAR)

[12] comprised a ceramic tube on the inner and outer surface of which a metal catalyst film was deposited. The combustion

reaction (3) was taking place on the inner catalyst film and the

reforming reaction (1) on the outer one, absorbing simulta-

neously the heat of combustion. Thus, the HIWAR offered the

possibility of reducing the magnitude of hotspot.

In our previous work, an O2-distributor (OD) was adopted in

a fixed-bed reactor. Part of O2 was distributed radially into the

catalyst bed through several rows of holes arranged at the O2

distribution zone of the OD to avoid premixing with CH4.

The gaseous mixture presented near theentranceof thefixed-

bed reactor with an O2-distributor (FR-OD) was kept at a lower

O2 /CH4 ratio than 0.5, whichdecreased thedanger of explosion

often caused by mixing oxygen and methane under higher

temperature and pressure. In the present study, we further

explore the axial temperature profile and species profile along

the Ni or Rh-based catalyst bed in FR-OD during the CPOM

reaction. With the opinions of economy and safety, route

based on air as oxygen source eliminating the cryogenic air

separation plant has been suggested [13]. So air is chosen as

oxygen source in this work. Small addition of water (H2O/CH4

mole ratio=1) was introduced into the feed to prevent carbon

formation, especially in the case of lower O2 /CH4 ratio which

carbon formation becomes thermodynamics favorable.

2. Experimental

The sketch of FR-OD is shown in Fig. 1. The reactor tube is a

stainless steel tube of dimensions 26 mm o.d., 20 mm i.d. and

600 mm length. The OD is made of a stainless steel tube of

dimensions 8 mm o.d.. Thedistancefrom the first row of holes

to the last one on the OD is 30 mm. The whole holes compose

the O2 distribution zone on the OD. The O2 distribution zone is

imbedded in the catalyst bed.

As shownin Fig.1, air can befed intothe catalystbed in two

ways. We define the air injected into the system through the

OD as Air D and the air premixed with CH4 and H2O prior to thereactor as Air M, respectively. We propose that the parameter D

is used in the present study,

D ¼ FAir D

FAir D þ FAir M 100k

in which FAir Dis the flow rate of the air through the OD and

FAir M is the flowrateof the air premixedwith CH4 andH2O prior

to the reactor in ml/min. D value is in the range of 0%–100%.

When D =100%, total air is distributed into the catalyst bed

through whole holes on the OD without premixing with CH4

and H2O. When D =0%, total air enters the catalyst bed

premixing with CH4 and H2O prior to the reactor.

0.76 wt.%Rh/α-Al2O3 catalyst was prepared by impregnation

method, and 2 wt.%La2O3–7.6 wt.%Ni/MgAl2O4–α-Al2O3 catalyst

was prepared by multistep impregnation method. Experiments

wereperformedusing18gofcatalyst(10–16 mesh) atbed depths

of55 mm. The first row ofholes ontheOD werelocatedat 5 mm

of the catalyst bed. The distance from the first row of holes to

the last one on the OD is 30 mm. The last row of holes on the

OD were located at 35 mm of the catalyst bed. The O2 distribu-

tionzonecomposed of the wholeholeson the ODwas located at

5–35 mm of the catalyst bed. The catalyst was reduced under

hydrogen flow at 780 °C for 1 h before testing.

Methane and air were controlled by mass flow controllers.

H2O was injected into catalyst bed by a pulseless pump. The

feed was CH4:air (O2):H2O= 1:2.4 (0.5):1. The GHSV was

13,200 h−1. The reactor was placed inside a furnace. An on-

line gas chromatography (GC) was used for the analysis of

reactants and products. The reactants, i.e. CH4, O2 and N2 were

Fig. 1 – The sketch of FR-OD.

1346 F U E L P R O C E S S I N G T E C H N O L O G Y 8 9 ( 2 0 0 8 ) 1 3 4 5 – 1 3 5 0



analyzed in a 5 A molecular sieve column with He as a carrier

gas while the products, such as CO, CO2, unreacted CH4 and N2

were analyzed in a C molecular sieve column with He as a

carrier gas. H2O was condensed and removed in a condenser

located downstream of the reactor before GC analysis.

In order to measure the axial temperature profile of the

catalyst bed, a ϕ3 thermocouple well was placed in the middle

part between the wall of the reactor and the ektexine of theOD. A ϕ1 thermocouple can be moved upwards and down-

wards in it to measure the temperature. A ϕ2 sampling probe

was inserted into the catalyst bed. By moving the sampling

probe we can get the species profile along the catalyst bed.

3. Results and discussions

3.1. Effect of D values on the catalytic activity

When we investigated the effect of D values on the catalytic

activity, two series of experiments have been performed —

one series was performed at the same outlet temperature as

800 °C and the other was performed at the furnace tempera-

ture maintained at 600 °C. Each series experiments have been

performed at 1 atm and 8 atm on both Ni-based catalyst and

Rh-based one. In this paper, only representational data

obtained on the Ni-based catalyst are listed below. Similar

results have also been obtained on the Rh-based catalyst (not

shown in this paper). No carbon was decomposed over the Ni

or Rh-based catalyst during the CPOM reaction activity test for

the presence of water.

According to Table 1, similar conversion of CH4 and

selectivity of CO have been obtained under the same outlet

temperature as 800 °C whether D =80% or D =0%. As can be

seen in Table 2, when the furnace temperature was main-

tained at 600 °C, a little higher outlet temperature was

obtained in the case of D =80%, and the conversion of CH4

and selectivity of CO increase accordingly.

Both data shown in Tables 1 and 2 agreed with thermo-

dynamic predictions based on the catalyst bed exit tempera-

tures. It should be noted that the compositions of product are

still determined by the catalyst bed exit temperature, in spite

of changing the ways of air feeding by using the OD.

3.2. Effect of D values on the axial temperature profile

along the catalyst bed

The axial temperature profiles of the Rh or Ni-based catalyst

bed with various D values when furnace temperature was

maintained at 600 °C are shown in Fig. 2.

When D =0%, the mixture of total air, CH4 and H2O were

premixed, preheated to 600 °C and delivered to the region in

which the catalyst was sited. An obvious temperature rise

caused by the oxidation reaction of CH4 was observed at the

very entrance (within 1 mm) of both the Rh-based and the Ni-

based catalyst. Maximum temperature 764 °C and 811 °C were

detected over the Rh and Ni-based catalyst, respectively.

Within 1–15 mm of the catalyst bed, the temperatures of the

Ni-based catalyst descended more rapidly than that of the Rh-

based one, which could be ascribed to more endothermic

Table 1 – The effect of D values on the catalyticperformance at outlet temperature 800 °C

D Conv. CH4

(%)Sel. CO

(%)Per cent of product (%)

H2 N2 CO CH4 CO2 H2O

80% 90.45 66.33 35.23 33.16 10.48 1.67 5 .32 13.95

0% 90.30 66.14 35.54 33.18 10.43 1.69 5.34 13.82

Reaction conditions: Ni-based cat., W cat.=18 g, Hcat.=55 mm,

GHSV=13,200 h−1

, CH4:air:H2O=1:2.4:1, 8 atm.

Table 2 – The effect of D values on the catalyticperformance as furnace temperature was maintained at 600 °C

D T out(°C)

Conv.CH4

(%)

Sel.CO(%)

Per cent of product (%)

H2 N2 CO CH4 CO2 H2O

80% 644 85.30 51.08 35.25 33.89 7.77 2.62 7.44 13.02

0% 630 84.77 48.84 35.33 33.96 7.40 2.72 7.75 12.85


GHSV=13,200 h−1, CH4:air:H2O=1:2.4:1, P =1 atm.

Fig. 2 – Axial temperature profile along the catalyst bed at

various D values. W cat.=18 g, Hcat.=55 mm, CH4 :air:

H2O= 1:2.4:1, GHSV= 13,200 h−1, P =1 atm, T furnace=600 °C.

(a) Ni-based catalyst; (b) Rh-based catalyst; ( □ ) D=100%; ( )

D=80%; ( ) D =60%; ( ) D=40%; ( ) D=20%; ( ■ ) D=0%.

1347F U E L P R O C E S S I N G T E C H N O L O G Y 8 9 ( 2 0 0 8 ) 1 3 4 5 – 1 3 5 0



steam reforming reaction (1) of the remaining unreacted CH4

on the Ni-based catalyst than that on the Rh-based one.

When D =100%,for theNi-basedcatalyst, it is observed that

the inlet temperature decreased to 700 °C. The temperatures

within 5–35 mm of the catalyst bed (i.e. the location of the O 2

distribution zone) were maintained at much higher values

resulted from the exothermic oxidation reactions (2) and (3),

which are quite different with a sharp decrease of tempera-turescaused by the endothermic reformingreactions (1) in the

case of D =0%. In the region of 35–55 mm of the catalyst bed

(i.e. away from the O2 distribution zone), as the O2 has been

completely exhausted, there occurred only the endothermic

reforming reactions (1) which resulted in a further decreasing

of the bed temperature. A similar temperature profile could be

found over the Rh-based catalyst when D =100%.

When 0%bDb100%, the inlet temperature of the catalyst

decreased with increasing of D value, the temperatures within

5–35 mm of the catalyst bed increased with increasing of D

value. When D =60% and 80%, more uniform temperature

profiles have been obtained. That is to say, by using the OD,

hotspot often appeared at the beginning of the catalyst bed

has been controlled effectively.

3.3. O2 profile along the catalyst bed

It is hard to detect the O2 existence along the catalyst bed

during the test period, because once entered the catalyst bed,

oxygen can be exhausted rapidly. It is lucky that the oxygen

source used in this paper is air, so we can deduce the mole

number of distributed O2 by measuring the mole number of

distributed and unreactive N2.

Similar distribution states of O2 and O2 /CH4 ratio along the

catalyst bed have been obtained over both the Ni-based

catalyst and the Rh-based one. Namely the distribution states

of O2 and O2 /CH4 ratio along the catalyst bed are independence

of the types of the catalysts used in the test when the

arrangement of holes on the OD, the O2 velocity and the

reaction pressure are fixed. Only the data gained over the Ni-

based catalyst were listed in Table 3.

The O2 distribution zone is located in 5–35 mm of catalyst

bed from inlet. As can be seen from Table 3, when D =80% —

the 80% of O2 was divided into approximately 4 equivalent

shares (the data in italic) from 4 rows of holes on the OD to

enter the catalyst bed. Meanwhile, the O2 /CH4 ratios along the

catalyst bed were kept at low values of 0.1–0.22. Usually, the

carbon decomposition is thermodynamics favorable under the

circumstances of such a lower O2 /CH4 ratio. It should benoticed that due to the CH4 feed gas mixed with some water

(H2O/CH4 molar ratio=1) in this work, as a result, no carbon

deposition was observed over either catalyst (Ni or Rh-based

catalyst) or around the hole on the O2-distributor after

catalytic performance test, while plenty of carbon was formed

over the Ni-based catalyst if the water was absent in the feed.

3.4. Axial species profiles when D = 0%

Axial species profiles along the Ni or Rh-based catalyst bed

when D = 0% are shown in Fig. 3.

From Fig. 3, when D = 0%, all of the O2 and 75% of CH4 were

converted, 70% of H2 and CO product were generated, no

Table 3 – Mole number of distributed O2 and O2 /CH4 ratioalong the catalyst beds at D=80% and D= 0%

z (mm)

−1 5 10 25 35 4 5 55

D = 80% O2 (mol) 0.1 0.1 0.13 0.11 0.06 0 0

O2 /CH4 ratio 0.1 0.145 0.224 0.21 0.21 0 0

D = 0% O2 (mol) 0.5 0 0 0 0 0 0

O2 /CH4 ratio 0.5 0 0 0 0 0 0


GHSV=13,200 h−1

, CH4:air:H2O=1:2.4:1, P =1 atm.

Fig. 3 – Measured axial species profile along the Ni or

Rh-based catalyst bed at D=0% (part 1)a. (solid line): Ni-based

catalyst; (dash line): Rh-based catalyst; (a): O2; (b): CH4 ;

(c): H2O; (d): H2; (e): CO. Reaction conditions: W cat.=18 g, Hcat.=

55 mm, CH4 :air:H2O= 1:2.4:1, GHSV= 13,200 h−1, P =1 atm,

T out =750 °C.




additional H2O formation was detected within the first 1 mm

of the Rh catalyst bed.

Over the Ni catalyst, only 67% of O2 and 25% of CH4

conversions were detected withinthe first1 mm of thecatalyst

bed. Meanwhile, an additional 0.24 mol H2O was produced

which gave an evidence of the occurrence of complete

oxidation reaction (3). Only 10% of H2 and 20% of CO had

been detected simultaneously. In the following 3–55 mm of the Ni-based catalyst bed, the steam reforming of methane

played an important role — 55% of CH4 was converted and

74% of H2 and 77% of CO had been produced.

Based on the above data, it could be concluded that for the

Rh-based catalyst most of syngas is formed by direct partial

oxidation or extra fast combined combustion-reforming,

while most of syngas has to be formed through a slow steam

reforming reaction on the Ni-based catalyst.

3.5. Axial species profiles when D =80%

Axial species profiles along the Ni-based catalyst bed when

D =80% and D =0% are shown in Fig. 4.We can see from Fig. 4 c, no addition of H2O formation has

been detected at the entrance of the Ni-based catalyst when

D =80%. It seems that the complete oxidation of methane

usually happened at the very entrance of the catalyst when

O2 /CH4 ratio is 0.5 have been hindered triumphantly due to a

low O2 /CH4 ratio (0.1) was provided by using the OD.

In the range of 5–35 mm of the catalyst bed, remained

80% of oxygen was distributed into approximately 4 equal

shares in this zone through the OD and immediately

exhausted (Fig. 4 a). In the same zone, more than 50% of

CH4 converted, about 10% of H2O converted (Fig. 4 b and c),

which means most of methane was reacted with oxygen.

Associated with Fig. 4 d and e, more than 50% of H2 and CO

have been gradually produced along the Ni-based catalyst

bed. Products reached thermodynamic equilibrium value at

the outlet of the reactor.

4. Conclusions

The following conclusions can be derived from the results of

the present study:

(1) The thermodynamics equilibrium of CPOM reaction sys-

tem is notchanged whether or notchangingthe ways of

air feeding by using the O2-distributor.

(2) Due to the O2 /CH4 ratio that was kept at lower values

(0.10–0.22) along the catalyst bed, the hotspot often ap-

pearedat theNi catalyst duringthe CPOM reaction could

be controlled by using the O2-distributor.

(3) Combining the measurements of temperature profile

and species profile, we can speculate that the reaction

path of the CPOM reaction might have some changes at

lower O2 /CH4 ratio when using the O2-distributor.

Acknowledgement

Financial support by the National Basic Research Program of

China, No.2005cb221401.

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Fig. 4 – Measured axial species profile along the Ni-based

catalyst bed at D=80% and D=0% (part 1)a. (solid line): D=0%;

(dash line): D=80%; (a): O2; (b): CH4 ; (c): H2O; (d): H2; (e): CO.

Reaction conditions: W cat.=18 g, Hcat.=55 mm, CH4 :air:

H2O =1:2.4:1, GHSV= 13,200 h−1, P =1 atm, T out =750 °C.

1349F U E L P R O C E S S I N G T E C H N O L O G Y 8 9 ( 2 0 0 8 ) 1 3 4 5 – 1 3 5 0



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