~ APPLIED CATALYSIS

A: GENERAL

ELSEVIER Applied Catalysis A: General 138 (1996) 245-264

Simulation of the catalytic partial oxidation of methane to synthesis gas

Ann M. De Groote, Gilbert F. Froment *

Laboratorium voor Petrochemische Techniek, Universiteit Gent, Krijgslaan 281, B-9000 Ghent, Belgium

Received 1 June 1995; revised 16 October 1995; accepted 20 October 1995

Abstract

The modeling and simulation of reactors for the catalytic partial oxidation of natural gas to synthesis gas is complex and requires detailed kinetics if it is to be representative and reliable. Adiabatic fixed bed reactors with a catalytic combustion zone fed with me thane /oxygen or me thane / a i r mixtures were simulated based upon the kinetics of total combustion, steam reforming and water-gas shift on a Ni catalyst. The steam reforming reactions and water-gas shift reaction are parallel or more or less consecutive to the total combustion, depending upon the degree of reduction of the catalyst, which is determined by the temperature and the gas phase composition. The calculation of the net rates of coke formation was included in the simulation. The influence of carbon dioxide and steam was also investigated.

Keywords: Oxidation (partial); Synthesis gas production; Methane

I. Introduction

In recent years, natural gas has received increased attention as a feedstock for the chemical industry.

The first step in natural gas conversion is often the production of synthesis gas (CO + H2). The synthesis gas can then be used for the production of methanol, for oxo-synthesis and Fischer-Tropsch synthesis. After removal of CO it also provides the hydrogen for ammonia synthesis and hydrogenations.

Syngas is mainly produced by steam reforming in large, gas fired furnaces, containing a large number of parallel reactor tubes. The CO content of the synthesis gas obtained by steam reforming is too low for methanol synthesis, for

* Corresponding author. Tel. ( + 32) 92644516, fax. (+ 32) 92644999, e-mail gf@elptrs3.rug.ac.be.

0926-860X/96/$15.00 © 1996 Elsevier Science B.V. All rights reserved SSDI 0 9 2 6 - 8 6 0 X ( 9 5 ) 0 0 2 9 9 - 5

246 A.M. De Groote, G.F. Froment /Appl ied Catalysis A: General 138 (1996) 245-264

which a ratio H 2 / ( C O + CO 2) of 2 is required. To achieve the desired H2 /CO ratio, recycling CO 2 is necessary. This may cause carbon deposition and deactivation of the catalyst.

An alternative technique for producing syngas with the desired C O / H 2 ratio is partial oxidation with oxygen or air. For the non-catalytic production of synthesis gas by partial oxidation of hydrocarbons, two industrial processes are in use: the Texaco process, developed in 1954 for the production of syngas from natural gas and heavy hydrocarbons, and the Shell gasification process, devel- oped in 1956. Non-catalytic partial oxidation is well suited for the production of syngas for ammonia production. The CO is then converted in a catalytic shift reactor to CO 2, which is removed by means of absorption and reaction.

One of the advantages of partial oxidation, catalytic or not, over steam reforming is the possibility of operating at high pressure and much higher temperatures. Indeed, the overall exothermicity of the process permits adiabatic operation, in refractory lined vessels, thus avoiding metallurgical problems.

The partial oxidation can also be carried out on a bifunctional combustion/steam reforming catalyst. In that case, the gas is fed at a much lower temperature and pure oxygen can be used. This process combines the exothermic combustion of a fraction of the natural gas:

CH 4 + 202 ~--- CO 2 + 2H20 (1)

with the endothermic steam reforming reactions:

CH 4 + H20 ~ CO + 3H 2 (2)

CH 4 + 2H20 ~ C O 2 -1- 4H 2 (3)

the accompanying water-gas shift:

CO + H20 # C O 2 "]- H 2 (4)

and the CO2-reforming reaction:

CH 4 + CO 2 # 2CO + 2H 2 (2')

to achieve a syngas with the desired composition [1-3] [4-9]. Care has to be taken to avoid carbon deposition according to the Boudouard

reaction [5]:

2CO .~ C + CO 2

and to methane cracking:

CH 4 ~ C + 2 H 2

A fraction of the carbon can be gasified by steam or oxygen:

C + H 2 0 ~- C O + H 2

C + 02 ~ CO 2

(5)

(6)

(7) (8)

A.M. De Groote, G.F. Froment /Applied Catalysis A: General 138 (1996)245-264 247

The H 2 / C O ratio of the process gas can be adjusted by controlling the O2 /C H 4 ratio of the feed. Feeding only 02 and CH 4 leads to a syngas with low H 2 /CO ratio, but coke deposition is difficult to avoid. Air can be used instead, but the separation of N 2 from the synthesis gas is not easy. Air is required when the syngas is intended for ammonia synthesis.

The present paper deals with the simulation of the catalytic partial oxidation of methane on a Ni catalyst by means of oxygen (or air) sometimes mixed with steam (in case of autothermal reforming), hydrogen a n d / o r carbon dioxide.

2. Rate equations

In the modeling of the partial oxidation of methane to syngas on a Ni catalyst, it is necessary to combine a reaction mechanism for the combustion (1) with a reaction mechanism for the reforming reactions [(2), (3) and (2')], WGS reaction (4) and coking and gasification reactions [(5)-(8)]. Kinetic equations for some of the steps of the catalytic partial oxidation process were reported by Hickmann and Schmidt [I0], but for Pt and Rh catalysts, not for a Ni catalyst.

Trimm and Lam [11] have published a kinetic equation for the complete combustion of methane to CO 2 and H 2 0 on a Pt /A1203 catalyst, under the assumption that the rate determining step is the surface reaction between adsorbed methane and both diatomically adsorbed oxygen and oxygen from the gas phase. The equation is given in Table 1.

Xu and Froment [12,13] described the steam reforming of methane on a Ni /MgOA1203 catalyst by the following reactions:

+ H20

-3 H2

CH4

- H 2 0 -2 H20

+3 H 2 +4 H2

H 2 0 + H2 CO ~ ~ CO 2

+ H 2 0 - H 2

+2 H20

-4 H2

III

248 A.M. De Groote. G.F. Froment/Applied Catalysis A: General 138 (1996) 245-264

The overall reaction scheme includes the CO 2 reforming (2') and WGS reaction (4) as well as the steam reforming reactions (2) and (3). Indeed, CO 2 reforming is a linear combination of reactions (II) and (III) of the above scheme. The corresponding rate equations are given in Table 1.

Another problem encountered in the simulation of the catalytic partial oxida- tion of methane is the degree of reduction of the Ni catalyst required for the steam reforming. In this process, the catalyst is operating in the reduced state, at least as soon as hydrogen is formed. It is not clear whether or not a partially reduced catalyst is active for steam reforming; if so then to a lesser degree. Will steam reforming only occur in the catalytic partial oxidation process after all the oxygen has been used? In other words: is steam reforming completely or partially consecutive or completely parallel to the catalytic total combustion of natural gas? According to Dissanayake et al. [14], three different zones exist in a reactor for the catalytic partial oxidation of methane over a Ni /AI203 catalyst. In the first zone, there is a NiA1204 phase with moderate activity for the total combustion to CO 2 and H20. In the second zone, the catalyst consists of NiO and Al203 and shows high activity for the total combustion. Finally, in the last zone, metallic Ni is present and CO and H 2 are produced. These observations would mean that steam reforming is consecutive to complete combustion, so that the state of reduction of the catalyst has to be incorporated in the kinetic model. This has been accounted for in the present simulations by multiplying the rate of

Table 1 Kinetic equations

Reaction Kinetic equation

(l)

(2)

(3)

(4)

(5)

(6)

(7)

(8)

k.[C.,][O2] k~[CH,][O2]-~ = +

r~ (l+r'[C"4]+K2[O:]): 1+r,[C.,]+X2[O2]

r 2

( l + K c o P c o + K n ~ p " 2+Kcg4pc" +K,2oPn20/PH2) 2

r 3

(1 + KcoPc 0 + Kn2Pn 2 + Ken,Pen ,+ Kn20Pn20/p ,2) 2

k4/Pn2( PcoPn:o - P"2 Pc02/K4) r4

(1 + KcoPc 0 + K n : p . : + Kcn4Pc.4 + K.20prl20/p .~) 2

k6 Pco - k7Pc02/Pco rs ( 1 + goPco 2 / P c o )

3 1

k8 Pc.4/P~2 - kgP~2

r~ ( I+K"~P"~) ~

kto PH20 / P~l: r7

( l + krt2PH2 + Kwpn20/Pn2) 2

rs = f(Po:)

A.M. De Groote, G.F. Froment // Applied Catalysis A: General 138 (1996) 245-264 249

T, (K) (%) 1800 100

red 8O

6O

40

20 8O0

60O 0 o o.~ 1 1.s 2 2.5

z (m)

Fig. 1. Reduction profile of the catalyst for a simulation using the VDR-model.

1600

1400

1200

1000

the steam reforming reactions and the WGS reaction with a reduction factor, which is a power function of the fractional oxygen conversion. The power in this reduction factor (Xo2)~2 was determined by trial and error with respect to the maximum temperature in the partial oxidation of a C H 4 / O 2 feed. The same reduction factor is used for feeds containing CO 2 and H20, since, no informa- tion regarding the maximum temperature was available for those cases. An example of the reduction profile of the catalyst is shown in Fig. 1 for a feed consisting of CH 4 and O 2 only. It is clear from the figure that the catalyst is completely reduced as soon as all oxygen is consumed.

In previous work, the C-formation zones were based upon thermodynamic calculations. Wagner and Froment [15] predicted the zones in which C-forma- tion is possible through methane cracking and through the Boudouard reaction by means of experimentally determined 'threshold constants'. To go beyond this and to predict the amounts of carbon that can be formed on the catalyst, requires kinetic equations. These were also derived by Wagner and Froment [16] from experiments in a differentially operated electrobalance reactor.

For the Boudouard reaction (5), methane cracking (6) and carbon gasification by steam (7), the reaction rates determined by Wagner and Froment [16] are applied. The rate equations are presented in Table 1.

3. R e a c t o r m o d e l

For the simulation of the partial oxidation of methane to synthesis gas on a Ni catalyst, a one dimensional heterogeneous model is used [17]. Since, the reactor

250 A.M. De Groote, G.F. Froment /Applied Catalysis A: General 138 (1996) 245-264

is adiabatic, concentration and temperature gradients only occur in the axial direction. The only transport mechanism operating in axial direction is the overall flow itself and this is considered to be of the plug flow type. Intraparticle diffusion limitations are expressed here for the sake of brevity in terms of effectiveness factors. Thus, the simulation of the partial oxidation reaction is based on the following set of differential equations:

Continuity equations:

d XCH ~ Pb ff2 d----~ = F ~ ( 7 / l r l d- 172r 2 + "q3r3 + Y/6r6)

C H 4

dxo 2 Pb~ d'---~ = E "----6-(2rllrl + r/8r8)

02

d Xco Pb 0 dz = F, 0 (T/2r2 - 7/4r4 - 2r/sr5 + r / 7 r7 )

CH4

d Xco 2 Pb 0 d------~ = F ----6--(~11rl + "r/3r3 Jr "r/4r 4 a t- "r/5r 5 + "08r8)

CH4

Energy equation:

8 d r Pb E r l i r i ( - A H ) i d z u s pgCp i= 1

Pressure drop equation:

dpt fpg u 2

d z dp

4. Simulation of an adiabatic fixed bed reactor

In an industrial autothermal reformer, operating at 25 bar, with a feed consisting of methane, steam and oxygen, H 2 0 / C H 4 ratios of 2 are common. Ratios as low as 1.4 have been applied, thereby decreasing the H 2 / C O product ratio from 3.27 to 2.80. The O2/CH 4 molar feed ratio is 0.598 and the reactor outlet temperature amounts to 950°C. Based on this case, simulations were performed for an adiabatic fixed bed reactor. The effectiveness factors used in these simulations were obtained from the integration of the second order differential equations describing the evolution of the various species inside the Ni catalyst pellet. To avoid having to integrate the continuity equations inside the pellets in each increment used in the integration of the fluid field equations,

A.M. De Groote, G.F. Froment /Appl ied Catalysis A: General 138 (1996) 245-264 251

an average value based upon a number of off-line pellet simulations was selected for the various effectiveness factors. The values of the effectiveness factors are:

"O~ = 0.05 for "02 = 0.07 for "O,s = 0.06 for "O4 = 0.70 for

the total combustion of CH 4 to CO 2 and H 20 CO production by steam reforming CO 2 production by steam reforming the WGS-reaction

The rates of carbon formation and gasification are also affected by the concentration profiles resulting from the four main reactions and this is also accounted for

"O5 = 0.05 for "O6 = 0.05 for r/7 = 0.05 for "O8 = 0.05 for

Notice the ([12,13]).

by the use of appropriate averaged effectiveness factors:

the Boudouard reaction methane cracking C-gasification by steam C-gasification by oxygen.

very low values of these "O's, except that for the WGS reaction

4.1. Discussion o f simulation results

An industrial autothermal reformer for syngas production is simulated using two models for the state of the Ni catalyst. In the first the catalyst has a varying

Table 2 Comparison of the simulation results with the industrially observed values for an autothermal reformer

Industrial VDR-model BV-model

Feed 1: CH4, 02, H20 F°H4 (Nm3/h) 3483 3483 3483 0 2 / C H 4 0.598 0.598 0.598 H 2 0 / C H 4 1.4 1.4 1.4 pt ° (bar) 25 25 25 Tma x (K) 1444 1210

Product yields Yo2 0 0.00026 0 YCH a 0.008 0.0055 0.010 YH ~ 0.456 0.458 0.456 YCO 0.160 0.156 0.153 YCO 2 0.07 0.06 0.068 YH20 0.306 0.318 0.312

Outlet conditions Tou t (K) 1223 1247 1210 FH 2 (Nm3/h) 6976 6925 6872 Fco (Nm3/h) 2444 2355 2307

252 A.M. De Groote, G.F. Froment/Applied Catalysis A: General 138 (1996) 245-264

Table 3 Simulation results for the catalytic partial oxidation of methane with oxygen using the VDR-model. Influence of the addition of steam and CO 2 to the feed mixture

Feed 1 2 3 4

XcH, (%) 97.61 98.37 92.85 93.01 go2 (%) 99.81 100 99.30 100 x co (%) 67.31 80.97 73.28 92.96 Xco 2 (%) 27.38 5.25 16.28 - 5.19 Tma x (K) 1444 1782 1305 1540 ax max (m) 1.3963 0.5224 1.7731 0.7344 Tou t (K) 1247 1412 1159 1274 P~out (bar) 25.1 40 25.221 25.112 25.207 H 2//CO 2.9407 2.0825 2.3513 1.6030 Fco (Nma /h ) 2355 2829 2576 3254 Fn 2 (Nm 3/h) 6925 5891 6057 5216 C-bal. 97.07 87.85 97.81 95.55

degree of reduction (VDR-model) and the steam reforming is mainly consecu- tive to the catalytic combustion. In the second model the catalyst is bivalent (BV-model) and the catalytic combustion and steam reforming operate purely in parallel. The simulation results are compared with the industrial values in Table 2, for an inlet temperature of 808 K. It is clear from Table 2 that the results at the exit are in reasonable agreement with those measured in industry. This indicates at least that the equilibrium values calculated in the simulation program are correct. Intermediate values, however, are also important, e.g., the maximum temperature, so as to garantee that the Ni catalyst is not damaged by excessive temperatures. It also follows from Table 2 that the main difference between the simulations with the BV-model and the VDR-model lies in the

Table 4 Simulation results for the catalytic partial oxidation of methane with oxygen using the BV-model. Influence of the addition of steam and CO 2 to the feed mixture

Feed 1 2 3 4

Xcn, (%) 95.51 97.24 91.84 Xo2 (%) I00 100 99.87 Xco (%) 65.94 86.90 74.55 Xco 2 (%) 29.51 4.76 17.22 Tma x (K) 1210 1393 1153 aXma x (m) 5.9335 1.2747 6 Tou t (K) 1210 1393 1153 Pt,out (bar) 24.992 25.106 24.908 H 2//CO 2.9793 1.9714 2.3240 Fco (Nm3//h) 2307 3036 2609 Fn 2 (Nm3/h) 6872 5985 6064 C-bal. 99.94 94.42 99.95

96.29 100 95.03

- 6.57 1319

1.9228 1318

25.077 1.6300

3326 5422

93.35

A.M. De Groote, G.F. Froment /Applied Catalysis A: General 138 (1996) 245-264

Table 5 Reaction dimensions and feed conditions

253

Feed 1 2 3 4 5

Ft ° (Nm3/h) 10490.40 5583.78 11661.36 6215.76 6184.26 pt ° (bar) 25.331 25.331 25.331 25.331 25.331 T O (K) 808 808 808 808 808 0 2 / C H 4 0.598 0.598 0.598 0.598 0.598 H 2 0 / C H 4 1.4 0.0 1.4 0.0 0.0 yO ~ 0.19945 0.37419 0.17950 0.33676 0.33680 Y~n 4 0.33352 0.62571 0.30017 0.56314 0.56320 y° 2 0.00010 0.00010 0.00010 0.00010 0.10000 Y~o2 0 0 0.10000 0.100130 0 yO 20 0.46693 0 0.42023 0 0 d r (m) 1.20 1.20 1.20 1.20 1.20 l (m) 3 or 6 3 or 6 3 o r6 3 o r6 3 dp (m) 0.0050 0.0050 0.0050 0.0050 0.0050

maximum temperature, which is much higher when total combustion and steam reforming occur consecutively.

4.1.1. Influence of carbon dioxide and steam in the feed mixture The influence of CO 2 and H 2 ° in the feed mixture is investigated at 25 bar

and constant F°H,, using the BV-model and the VDR-model. The results are listed in Tables 3 and 4. Table 5 gives the composition of the corresponding feeds, feed 1-4, and the reactor dimensions. Since, the partial pressure of hydrogen is a multiplying factor in the denominator of the kinetic expressions for the steam reforming reactions, 0.01% H 2 is added to the feed mixture in all simulations, to avoid infinite reaction rates at the inlet.

A reactor length of 3 m is used in the simulations with the VDR-model and of 6 m in the simulations with the BV-model. It is clear from the Tables that addition of CO 2 and H20 to the feed is mainly reflected in the H 2 / C O product ratio. Adding CO 2 to the feed decreases the conversion of methane, but increases the conversion towards CO, thus leading to a syngas with lower H 2 / C O ratio. On the other hand, the H 2 / C O ratio in the effluent becomes higher when steam is added. By adding carbon dioxide or steam to the C H 4 / O 2 mixture, a syngas with the desired H 2 / C O ratio can be produced. A given H J C O ratio in the effluent can also be obtained by changing the C H 4 / O 2 feed ratio. The low carbon balances result either from a high rate of carbon deposition or from its insufficient gasification.

The main difference between the BV-model and the VDR-model lies in the shape of the temperature profiles, shown in Figs. 2 and 3. Because of the partially consecutive character of total combustion and reforming reactions, the temperature profile predicted by the VDR-model shows a peak. The top of this peak corresponds to the take off of the endothermic reforming reactions. Since

254 A.M. De Groote, G.F. Froment/Applied Catalysis A: General 138 (1996) 245-264

T, (K) 1600

1400 2

f : _, 1200 3

1000 t

o

800 41CH 4. Oa: CO 2' C02

6oo i i i i i

0 1 2 3 4 5 z (m)

Fig, 2. Temperature profiles through the catalyst bed for the partial oxidation of methane with oxygen at 25 bar. BV-model. Influence of the addition of steam and carbon dioxide to the feed.

all reactions occur simultaneously in the BV-model, a plateau is observed in the predicted temperature profile, instead of a peak. This difference between the temperature profiles is also reflected in the net coking rate, shown in Figs. 4 and 5, as will be discussed later in this section.

T s (K) 2000

IBO0

1800

1400

12O0

1000

Feed: 1: CH 4, 0 2, H20 2: CH 4, 02 3: CH 4, 0 2, H20, C02

02, C02 I I ,-c_,..

. o o '% 0 .5 1 1.5 2 2.5

z (rn)

Fig. 3. Temperature profiles through the catalyst bed for the partial oxidation of methane with oxygen at 25 bar. VDR-model. Influence of the addition of steam and carbon dioxide to the feed.

A.M. De Groote, G.F. Froment /Applied Catalysis A: General 138 (1996) 245-264 255

r c (mol /kgca t s) 0 . 0 3

0.02

0.01 \ 0

0

Feed: 1 12: CH 4, 0 2 4: CH 4, 02, CO 2

i r

0.2 0 .4 0.6 0.8 z (m)

Fig. 4. Effective net coking rate through the first part of the catalyst bed for the partial oxidation of methane with oxygen at 25 bar. BV-model. Influence of the addition of steam and carbon dioxide to the feed.

Adding CO 2 and/or H20 to the feed mixture shifts the temperature peak further downstream in the reactor and lowers the maximum temperature. The temperature peak also becomes broader. Indeed, the simulations are performed for constant methane flow rate and constant pressure, so that addition of CO 2 and H20 also increases the total flow rate.

r c (mol/kgca t s) 0 .04

2

0.03

0 .02

0.01

0 i i

0 0 .2 0 .4

z (m)

Feed: 02j 1: CH 4, 02 , H20 2: CH 4, 02 3: CH 4, 02 , H20, C 4: CH 4, 02, CO 2

i i

0 .6 0 .8

Fig. 5. Effective net coking rate through the first part of the catalyst bed for the partial oxidation of methane with oxygen at 25 bar. VDR-model. Influence of the addition of steam and carbon dioxide to the feed.

256 A.M. De Groote, G.F. Froment/Applied Catalysis A: General 138 (1996) 245-264

i IIItn s) r 2 ~rno.-= cat

2

1.5

0.5

-0.5

Feed: 1: CH4, 02, H20 2: CH 4, 02 3: CH 4, 02, H20, CO 2 4: CH 4, 02. CO 2

CH 4 + H20 --=~CO + 3 H 2

0,5 1 1.5 2 2.5

z (m)

Fig. 6. Effective rate of carbon monoxide production by steam reforming through the catalyst bed for the partial oxidation of methane with oxygen at 25 bar. VDR-model. Influence of the addition of s eam and carbon dioxide to the feed.

When steam is added to the feed, steam reforming starts sooner in the reactor (Fig. 6), so that the maximum temperature is lower. This leads in turn to lower reforming rates (Fig. 6) and lower combustion rates (Fig. 7), since, both the temperature and the partial pressure of methane are lower. Because of the presence of steam in the feed, the equilibrium in the WGS reaction (Fig. 8)

r I (mol/kgca t s) 0.5

0.4

0.3

0.2

0.1

4

0.5

Feed: 1: CH 4, 02 , HzO 2: CH 4, 02 3: CH 4, 02, H20, CO 2 4: CH 4, 02, CO 2

CH 4 + 202 ~ C O 2 + 2H20

1

1 1.5 Z (m)

2 2.5 3

Fig. 7. Effective total combustion rate of methane to carbon dioxide and steam through the catalyst bed for the partial oxidation of methane with oxygen at 25 bar. VDR-model. Influence of the addition of steam and carbon dioxide to the feed.

A.M. De Groote, G.F. Froment /Applied Catalysis A: General 138 (1996) 245-264 257

r4 (mol/kgcat s) 0.2

0.1

0

-0.1

-0.2

-0.3

-0.4

-0.5

1

. _ _

3

CO + H 2 0 ~ C O 2 + H 2

Feed: 1: CH 4, 02 , H20 2: CH 4, 02 3: CH 4 , 02 , H20, CO 2 4: CH 4, 02, CO 2

2 4

0.5 1 1.5 2 2.5

Z (m)

Fig. 8. Effective rate of the water-gas shift reaction through the catalyst bed for the partial oxidation of methane with oxygen at 25 bar. VDR-model. Influence of the addition of steam and carbon dioxide to the feed.

shifts towards CO 2 and H 2. This explains the higher conversion to CO 2 (Table 3) and the higher H 2/CO product ratio. When both CO 2 and steam are added to the feed, the effective reaction rate, i.e., the rate including diffusion limitations of the WGS is first positive, exhibits a dip and then becomes positive again (Fig. 8, curve 3). This also explains the small discontinuity in curve 3 in Fig. 9.

Xco=(%) 30

Feed: ~ 1 1: CH 4, 02, H20 2: CH 4, 0 z

20 3: CH 4, 02 , H20, CO 2 4: CH 4, 02, CO 2

2

0

4

-10 ~ 0.5 1 1.5 2 2.5 3

Z (m)

Fig. 9. Conversion of methane to carbon dioxide through the catalyst bed for the partial oxidation of methane with oxygen at 25 bar. VDR-model. Influence of the addition of steam and carbon dioxide to the feed.

258 A.M. De Groote, G.F. Froment /Appl ied Catalysis A: General 138 (1996) 245-264

When the feed does not contain any steam, a peak is observed in the conversion to CO 2 (Fig. 9, curves 2 and 4). The rising part of the peak results from the higher effective combustion rate due to the higher temperature and the lower effective reforming rates resulting from the lower partial pressure of steam. The decreasing part of the peak can be explained by the WGS reaction. Indeed, when a considerable amount of CO 2 is formed, the equilibrium of the reaction shifts towards H 2 ° and CO. This in turn leads to a higher conversion to CO and to a syngas with lower H2 /CO product ratio (Table 3).

The effective net coking rate predicted by the BV-model and by the VDR- model is presented in Figs. 4 and 5. A peak is observed in the net coking rate near the reactor inlet. This is due to the endothermic methane cracking and also explains the small temperature decrease in Figs. 2 and 3. Although the reactor inlet temperature is relatively low and methane cracking is enhanced at high temperatures, the reaction still proceeds because of the high partial pressure of methane. It is also clear from Fig. 5 (curves 1 and 3) that for feeds containing steam there is almost no coke deposition. In the literature, it is found that addition of CO 2 to the partial oxidation feed decreases the amount of carbon deposited on the catalyst surface, because of the equilibrium in the Boudouard reaction. Checking the carbon balances in Table 4 for simulations using the BV-model shows that the opposite would be true. When the total combustion reaction and the reforming reactions are considered in series (VDR-model), on the other hand, addition of carbon dioxide to the feed decreases the amount of carbon considerably (Table 3, feed 2 and 4), although the zone of the catalyst bed covered with coke is slightly larger (Fig. 5, curves 2 and 4). The methane

r (mol/kgca t s) 0.3

A: 2 CO --~C + CO 2 B B: CH 4 "--~'C + 2 H 2 I C:C+ H20"-~CO+ H2

0.2 0.1 1D:C+O2--~CO2 /

-o.1 ~ A

-0.2 I I

0 0.2 0.4 z (m)

Feed 2: CH 4, 02

0.6 0 8

Fig. 10. Effective rate of the coking reactions for feed 2 through the first part of the catalyst bed for the partial oxidation of methane with oxygen at 25 bar. VDR-model. Influence of the addition of steam and carbon dioxide to the feed.

A.M. De Groote, G.F. Froment / Applied Catalysis A: General 138 (1996) 245-264 259

(mol/kg cat s)

0.03

0.02

0.01

-0.01

-0.02

A: 2 CO --~-C + CO 2 B:CH 4 - -~C + 2 H 2 C: C + H20 --=-CO + H 2 D : C + O2 - -~ CO 2

Feed 4: CH 4, 02, CO 2 B

C

o o12 o:, o16 0:8 z (m)

Fig. ] 1. Effective rate of the coking reactions for feed 4 through the first part o f the catalyst bed for the partial oxidation of methane with oxygen at 25 bar. VDR-model. Influence of the addition of steam and carbon dioxide to the feed.

cracking is slower because of the lower temperature, but the reverse Boudouard reaction is slower too, so that the zone of the reactor in which coke is deposited is extended. This follows from Figs. 10 and 11, in which the effective rates of the individual coking and gasification reactions are shown for feed 2 and 4, in

(mol/kgca t s)

0.04

0.03

0.02

Reaction I A: 2 CO--="C + CO 2 ~B: CH 4 --P-C + 2 H 2

~ ~ 2B 0.01

o

-o.o12

-o,o2

I Feed: 2: CH 4, O z 4: CH 4, 02 , CO 2

4B

4A

Fig. 12. Effective rate of the coking reactions through the first part of the catalyst bed for the partial oxidation of methane with oxygen at 25 bar. BV-model. Influence of the addition of steam and carbon dioxide to the feed.

0 0.2 0.4 0.6 0.8 z (m)

260 A.M. De Groote, G.F. Froment /Appl ied Catalysis A: General 138 (1996)245-264

Table 6 Simulation results for the catalytic partial oxidation of methane with oxygen using the VDR-model. Influence of the addition of hydrogen to the feed mixture

Feed 2 5

pt ° (bar) 25.33125 25.33125 XcH, (%) 98.37 95.61 Xo2 (%) 100 100 Xco (%) 80.97 84.36 Xco 2 (%) 5.25 5.03 Tma x (K) 1782 1776 aXma x (m) 0.5224 0.3397 Tou t (K) 1412 1361 Pt.out (bar) 25.221 25.265 H 2/CO 2.0825 2.1787 Fco (Nm3/h) 2829 2938 Fn 2 (Nm 3/h) 5891 6401 C-bal. 87.85 93.78

that part of the reactor in which net coke formation is observed. For parallel combustion and reforming reactions, the reverse Boudouard reaction is much slower when CO 2 is present in the feed mixture (Fig. 12), so that the net coking rate is increased.

4.1.2. Influence o f H 2 in the feed In the catalytic partial oxidation of methane to synthesis gas on a Ni catalyst

in a fixed bed reactor, carbon is mainly deposited by methane cracking. Therefore, a simulation is performed considering a catalyst with varying reduc- tion state (VDR-model), whereby 10% hydrogen is added to the CH4/O 2 feed mixture. The reactor dimensions and feed conditions are listed in Table 5. The results are compared with those obtained for a CH 4 /02 feed mixture in Table 6. When hydrogen is present in the feed mixture, the percentage carbon deficiency is lowered by about 6%, but the conversion of CH 4 is about 3% lower, too.

For the C H a / O 2 / H 2 feed mixture, the maximum temperature in the catalyst bed is slightly lower. In the presence of hydrogen, there is no temperature decrease near the reactor inlet and the temperature peak is shifted upstream. This can be explained by referring to Fig. 13, which presents the effective net coking rate in the first part of the reactor and to Fig. 14, which shows the rates of the individual coking and gasification reactions. It is clear from Figs. 13 and 14 that when hydrogen is added to the feed mixture there is no carbon formation due to the endothermic methane cracking near the reactor inlet. Because of this there is no temperature decrease and the combustion reaction starts further upstream in the reactor. The fraction of the catalyst bed covered with carbon is significantly smaller for a feed mixture containing hydrogen, but the net coking rate is higher than for a hydrogen-lean feed, so that this part of the catalyst bed is subject to

A.M. De Groote, G.F. Froment / Applied Catalysis A: General 138 (1996) 245-264 261

r C (mo l / kgca t s)

o.05

Feed: 2 2: CH 4 , 0 2 5

0.04 5: CH 4 , O 2, H

0.03

0.02

0.01

o . J , o o'., o.2 ;.3

z (m)

2

0'.4 o.5 0 .s

Fig. 13. Effective net coking rate through the first part of the catalyst bed for the partial oxidation of methane with oxygen at 25 bar. VDR-model. Influence of the addition of hydrogen to the feed.

severe deactivation. It may be concluded that adding hydrogen to the feed mixture only prevents methane cracking in the first part of the reactor and is not efficient for avoiding carbon deposition.

r (mo l / kgca t s)

0.3

A: 2 C O ~ , C + CO 2 tSB / 2B B: CH 4 C + 2 H 2 / [

o. / / Ol

-o.1 2: CH 4, 0 2 5: CH 4, 0 2 , H 2 5A 2A

-0.2 o o., o.2 o12 oi, o15 06

Z (m)

Fig. 14. Effective rate of the individual coking and gasification reactions for feeds 2 and 5 through the first part of the catalyst bed for the partial oxidation of methane with oxygen at 25 bar. VDR-model. Influence of the addition of hydrogen to the feed.

262 A.M. De Groote, G.F. Froment /Applied Catalysis A: General 138 (1996) 245-264

Table 7 Simulation results for the catalytic partial oxidation of methane with air using the VDR-model

Feed A B

pt ° (bar) 25.33125 61.423 XcH, (%) 98.17 89.09 Xo2 (%) 100 100 Xco (%) 70.05 72.77 Xco ~ (%) 7.67 8.00 Tma x (K) 1292 1461 aXma x (m) 5.0017 1.4116 Tou t (K) 1291 1286 Pt.out (bar) 24.553 61.096 H z /CO 2.3150 2.0255 Fco (Nm3/h) 2440 2535 FH 2 (Nm3/h) 5648 5134 C-bal. 79.55 91.68

4.1.3. Methane-air mixtures Because of the high temperatures encountered in the partial oxidation of

methane with oxygen only, some simulations were performed, using the VDR- model, for a CHa/a i r mixture. In the first simulation, the inlet flow rate of methane, FOR,, the total pressure and the O2//CH 4 feed ratio are kept constant. The reactor length has been increased to 6 m. The results are given in Table 7 (feed A). The maximum temperature in the reactor is considerably lower and is acceptable. The temperature peak is shifted further downstream in the reactor, because of the dilution. The conversion of 0 2 and CH 4 and the conversion to CO are much lower. Indeed, the effective combustion rate and the effective rates of the reforming reactions are significantly slower, because of the lower temperature and partial pressures of methane and oxygen. It should also be noted that the amount of deposited carbon is much higher compared to the corresponding case with oxygen only (feed 2, Table 3).

A second simulation was performed, whereby the partial pressures of the components in the feed were set equal to the ones in the corresponding case for catalytic partial oxidation with oxygen only (Table 3, Feed 2). This is done by increasing the total pressure to 61.4 bar. The results are shown in Table 7 (Feed B). The above conclusions are confirmed, except that the amount of carbon deposited on the catalyst is lower when air is used instead of oxygen. The maximum temperature can rise to 1200°C for feed mixtures without steam and carbon dioxide, which is within the acceptable range for the Ni catalyst.

5. Conclusions

From the simulations, it appears that the temperatures during the catalytic partial oxidation of methane to synthesis gas are within acceptable bounds when

A.M. De Groote, G.F. Froment /Applied Catalysis A: General 138 (1996) 245-264 263

air is used or when steam or carbon dioxide are added to the feed. In the catalytic partial oxidation of methane with oxygen, a maximum temperature of about 1500°C was predicted by the VDR-model. This peak temperature is detrimental for the Ni catalyst, because of plastification.

When steam is added, the amount of coke becomes negligible. CO 2 also reduces the amount of carbon formation, but increases the area in which coke is deposited. Adding H 2 to the feed is not an efficient way of reducing coke formation.

In the catalytic partial oxidation of methane with air instead of oxygen, the maximum temperatures are considerably lower and less coke is formed. The separation of N 2 from the synthesis gas is a drawback of this mode of operation.

6. Symbols

[A]

a x max

Cp

dp dr f F °, Ft ° FA 0

FA Ft KA

ki

K i l PA P t ' pO r I

/'2

/'3 r 4

r 5

/'6

/ '7

/'8 T Tmax Tout Us

mol-% of component A (%). axial position where the maximum temperature occurs (m). specific heat of fluid ( k J / k g K). particle diameter (mp). reactor diameter (m). friction factor in Fanning equation. total molar feed rate (kmol /h) . molar feed rate of reactant A (kmol /h) . flow rate of component A (Nm3/h) . total molar flow rate (kmol /m~ s). adsorption constant of species A. reaction rate constant. adsorption and equilibrium constants. length of reactor (m). partial pressure of component A (bar). total pressure (bar). reaction rate of total combustion (kmol/kgca t s). rate of the steam reforming reaction (kmol/kgca t s). rate of CO 2 production by steam reforming (kmol /kgca t s). rate of WGS reaction (kmol /kgca t s). rate of Boudouard reaction ( k m o l / k g ~ t s). rate of methane cracking (kmol/kg~a t s). rate of carbon gasification by steam (kmol/kgc~ t s). rate of carbon gasification by oxygen (kmol/kgca t s). temperature (K). maximum temperature (K). outlet temperature (K). superficial velocity 3 2 ( m g / m r s).

264 A.M. De Groote, G.F. Froment /Appl ied Catalysis A: General 138 (1996) 245-264

XCH 4

XCO 2

XO 2 yO YA Z

conversion of methane (%). conversion of methane to CO 2 (%). conversion of oxygen (%). mol fraction of species A in the feed. mol fraction of species A. axial position (mr).

6.1. Greek symbols

r/1 effectiveness factor for r/2 effectiveness factor for 7/3 effectiveness factor for r/4 effectiveness factor for r/5 effectiveness factor for r/6 effectiveness factor for r/7 effectiveness factor for r/s effectiveness factor for

Pb Pg 0

total combustion. the steam reforming reaction. the CO 2 production by steam reforming. the WGS reaction. Boudouard reaction. methane cracking. carbon gasification by steam. carbon gasification by oxygen.

catalyst bulk density (kg cat/m3r). gas density (kg/m3r). cross section of reactor (m2).

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