Investigation of synthesis gas production from methane bypartial oxidation over selected steam reforming commercial catalysts

Catalysts in Petroleum Refining and Petrochemical Industries 1995 M. Absi-Halabi et al. (Editors) 9 1996 Elsevier Science B.V. All fights reserved.

437

INVESTIGATION OF SYNTHESIS GAS PRODUCTION F R O M METHANE BY PARTIAL OXIDATION OVER SELECTED STEAM R E F O R M I N G C O M M E R C I A L CATALYSTS

H. AI-Qahtani Chemical Engineering Department, University of Bahrain, Isa town, P. O. Box 32038, State of Bahrain. I. ABSTRACT The production of synthesis gas (CO, H2) from methane by partial oxidation is investigated over commercial steam reforming catalyst at several flow rates, temperatures, and at different methane/oxygen ratios (R). Optimum synthesis gas selectivity and yield achieved are 70% and 60%, respectively at methane/oxygen ratio close to 2 and at flow rates of 500 cm3/min. An initial temperature (665 ~ is necessary to initiate the reaction and then the reaction is stabilized at 883 ~ The effect of methane/oxygen ratios and residence time are effective in determining the synthesis gas selectivity and yield. 2. INTRODUCTION Steam reforming is the principle process for carbon monoxide and hydrogen production. Steam reforming process is applied for several industrial applications to provide the necessary amount of the synthesis gas. Those industries such as oil refineries, iron and steel manufacturing, methanol and ammonia synthesis, and other several petrochemical industries. The future demand for synthesis gas utilization will increase especially when methanol is used as a combustible fuel in large scale and when compact fuel-cells is used in wider applications. One of the major alternatives methods for the production synthesis gas is the partial oxidation of fuel oil and coal gasification. However, capital costs for the partial oxidation of fuel oil and coal gasification are approximately 1.5 and 2 times higher, respectively, than that for steam reforming of natural gas [ 1]. Studies investigating the direct conversion of methane into methanol, formaldehyde, ethane, and ethylene found that these compounds could not be produced commercially due to the limitation on yield and selectivity of the desired products [2]. It is economically more viable to convert methane into synthesis gases and then to the final product [3]. A large amount of research on methane oxidative coupling has been conducted in recent years. The main setback of direct coupling is the high selectivity and yield of unfavoured products (CO2, and H20), and hence, the limited of C 2 yield [4]. Recently, active studies have been conducted investigating the possibility of oxidizing methane to synthesis gas catalytically at lower temperatures. Studies of methane to CO and H 2 over Ni/AI203 were reported. The formation of CO and H 2 rather than CO 2 and H20 were achieved at high synthesis gas selectivity (90%) and yield (95%) [5].

438 Chouddhury and co-worker[6] oxidized methane at high temperatures ranging from 300900~ over Ni/CaO. High methane conversion (90%) and high synthesis gas selectivity (92%) were found when the reaction took place over reduced Ni catalyst [6]. Schmidt et al. [7], studied the catalytic partial oxidation of CH 4 in air and pure 0 2 at atmospheric pressure over Pt and Rh coated monoliths. High selectivity for H 2 and CO (90's%) were achieved at 950~ over Rh catalyst when pure 0 2 was used; with air, the selectivity's were 70% and 40% over Rh and Pt, respectively. The production of synthesis gas from methane oxidation was also studied over Fe catalyst in fuel cell using solid electrolyte (YSZ) at 850-950~ at atmospheric pressure [8]. The anodic electrode was Fe and the cathode that was exposed to air was Pt. Reduced iron was more active than oxidized iron for synthesis gas formation. The maximum CO selectivity and yield were nearly 100% and 73%, respectively. Carbon deposition was reported at high methane to oxygen ration. The scope of the present study is the investigation of partial oxidation of methane over commercial steam reforming catalyst. Thus, the main purpose of using this type of catalyst is not to compare between the synthesis gas selectivity and yield of steam reforming to partial oxidation reactions over this type of catalyst, but to investigate the performance of partial oxidation reaction over commercial steam reforming catalyst. Satisfactory performance over the given catalyst is expected to provide information needed to develop commercial catalysts for partial oxidation. The reason for choosing this type of catalyst is due to the similarity between steam reforming and partial oxidation with respect to their operating conditions and type of species involved and produced during the reactions. 3. EXPERIMENTAL The system consisting of a tubular reactor, furnace, gas cylinders, flow meters, temperature controller, gas chromatography, and bubble meter is shown in figure 1. All flow rates measurements are monitored by the bubble meter. The reactor is a stainless steel tube with ID. = 2.0 cm and L. = 9.0 cm where 5 g of the catalyst is loaded in the tube (Figure 2). The catalyst used for this study is a commercial steam reforming type brought from the Gulf Petrochemical Industries (GPIC), the only petrochemical plant in the state of Bahrain. The catalyst consists 20% Ni and the rest is magnesium oxide mixed with a ceramic material. All the gases are premixed at room temperature, 25 ~ before entering the reactor. 4. RESULTS AND DISCUSSION Three sets of experiments have been conducted. The first set is examining the influence of methane/oxygen ratios on the performance of the catalyst; the second set is studying the effect of temperature on the synthesis gas formation; and the third set is investigating the influence of residence time on synthesis gas selectivity and yield. The experimental data are shown in tables 1 and 2. Selectivity, yield and conversion are defined according to the following: Selectivity o f H 2 = [rate of H2/2 (rate of CH 4 in - rate of CH4out)] Selectivity of CO = [rate of CO/(rate of CH 4 in - rate of CH 4 out)] Yield o f H 2 = [rate of H2/2 (rate of CH 4 in)]

(1) (2) (3)

439

2

2]

LIJ 1

(s) Figure 1. Schematic diagram of the tubular reactor system. (1: gas cylinder; 2: rotometer; 3:

reactor; 4: furnace; 5: temperature controller; 6: gas chromatograph; 7: bubble meter) in 3

out

Figure 2. Schematic diagram of the reactor. (1:furnace; 2: reactor; 3: thermocouple) Yield of CO Conversion (%X)

= =

[rate of CO / rate of CH4 in] [(rate of CH 4 in - rate of CH 4 out)/rate of CH 4 in]

(4) (5)

In the first set of experiments, the inlet flow rate is fixed at 500 cm3/min, and temperature at 883~ It is observed that the outlet flow rate is usually higher than the inlet by 100 to 150 cm3/min. As shown in table 1 and figures 3, 4, and 5, the rates o f H 2 and CO increased with the increase in the methane/oxygen ratios (R). It may be seen from the given figures that the hydrogen rate reached to a maximum at methane to oxygen ratio around 2. Therefore, most of the methane enters are converted to hydrogen and CO at that given R. At low methane to oxygen ratios (R < 2), the hydrogen yield

440 Table 1. Influence of methane/oxygen ratio on catalyst performance.

Methane/Oxygen Ratio ( 10 -3 mol/min.)

R=0.715

R=I.15

R=2.06

R=3.61

n (O2)in

11.930

9.510

6.668

4.440

n (CH4)in

8.530

10.950

13.780

16.020

n (CO)out

1.360

5.733

8.100

8.880

n (H2)out

2.730

11.739

16.380

18.325

n (CH4)out

0.191

0.730

2.730

4.095

%SH2

16.37

57.43

74.11

76.83

%Sco

16.31

56.09

74.12

74.46

%YH2

16.00

53.60

59.43

57.19

%Yco

15.94

52.35

59.43

55.43

%XCH 4

97.76

93.33

80.19

74.44

I

20

I

I

I

I

I f

15-

A

/

E "6 'o E

H2

/

/

o==

v

[]

/

10-

,-

[]

CO

5 -

0

0.5

I

I

I

I

i

1.5

2

2.5

3

3.5

ratio (CH4102)

Figure 3. Variation of H 2 and CO rates with methane to oxygen ratios at 500 cm3/min, and 883~

441 I

80

I

I

I

I

I o

CO

70-

D

60-

H2

> ..,= 0 0

50-

m

0

4030-

m

2010 0.5

I

I

I

I

I

I

1

1.5

2

2.5

3

3.5

ratio

(CH4102)

Figure 4. Variation of CO and H 2 selectivities at several methane/oxygen ratios at 500 cm3/min, and 883~

I

80

I

I

I

I

I

70-

m

H2 =,,.t

60-

m

co

50403020-

m

10 0.5

I

I

I

I

I

I

1

1.5

2

2.5

3

3.5

ratio

(CH4102)

Figure 5. Variation ofH 2 and CO yields at several methane/oxygen ratios at 500 cm3/min, and 883~

442 Table 2. Influence of inlet flow rate on catalyst performance.

Flow Rate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Q=250 (cm3/min.)

Q=500cm3/min.)

Q=750 cm3/min.

n (O2)in

3.580

7.160

10.740

n (CH4)in

6.649

13.30

19.950

n (CO)out

5.650

7.490

5.670

n (H2)out

8.664

16.130

17.980

n (CH4)out

0.001

2.087

6.957

%SH2

65.15

71.93

69.19

%Sco

84.97

66.80

43.64

%YH2

65.15

60.64

45.54

%Yco

84.97

56.32

28.42

%XCH A

99.97

84.31

65.13

(10-3 mol/min.)

reduced due to the reaction of the excess oxygen available in the system with the hydrogen and, therefore, more carbon dioxide and water are observed at lower R values. At high methane to oxygen ratios, carbon deposition and C2+ are detected. This indicates that the limitation of oxygen species caused the free carbon formation. In the second set of experiments, temperatures are varied ( 400, 500, 600, 700, 800~ at constant inlet flow rate 500 cm3/min, and at a value of R about 1.86. All the given temperatures are reported from a thermocouple attached to the catalyst inside the reactor. At low temperatures (400 to 600~ formation of synthesis gas is insignificant. However, at about 665~ pulses of explosion occurs initially and then temperature increases rapidly above 800~ and the amounts of CO, H 2 increase significantly. At 700 and 800~ no pulses of explosion are observed but the temperature increases till it is stabilized at 883~ Therefore, heating of the reaction is needed only to initiate the reaction and then reaction is sustained by the exothermic heat of reaction. The explosion behavior that occurs at temperature of about 665~ is due to the sensitivity of the reaction to the variation of the temperatures. At temperature of 665~ the interaction between 0 2 and CH 4 over the catalyst surface is more likely to follow an explosion mechanism due to the types of intermediates that are dominated at this condition. In the third set of experiments, inlet flow rates are varied and temperature is held constant at temperature 883~ and at methane to oxygen ration 1.86. As shown in table 2 and figures 6, 7, 8, and 9, CO and H 2 rates increase then decreased slightly. Also selectivity and yield decrease at high and low flow rates. Methane conversion also decreased with the increase in the flow rate. At low flow rate ( < 400 cm3/min.), carbon deposition is detected. At high flow rate, lower CO and H 2 yields are recorded. Therefore, flow rate is an important parameter controlling the selectivity of synthesis gas.

443 I

100

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9080x 7060504030 0.5

I

I

I

I

I

I

1

1.5

2

2.5

3

3.5

ratio

(CH4102)

Figure 6. Variation of methane conversion at several ratios of methane/ oxygen at 500 cm3/min, and 883 ~

I

18

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J

16-

H2

J

m

J 14-

m

J

A r o . .

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12-

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o

E 10-

m

8

-

6

-

200

m

CO m

o-"

I

I

I

I

I

300

400

500

600

700

Q (cm31mi

800

n)

Figure 7. Variation of CO and H 2 rates at several inlet flow rates at ratio = 1.9 and 883 ~

444 I

90

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8070-

>, > .e..* O O ~)

u)

f

B--"'-

""e...

-'-~

H2

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2010

i 200

300

i 400

I 500

I 600

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800

700

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Figure 8. Variation of CO and H 2 selectivities at several inlet flow rates at ratio = 1.9 and 883 ~ I

90

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-D

706050-

H2-

4030-

CO

20

i 200

300

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800

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Figure 9. Variation of CO and H 2 yields at several inlet flow rates at ratio = 1.9 and 883 ~

445 In the fourth set of experiments, different ratios and flow rates are examined in the absence of catalyst (homogenous). The rates of hydrogen and carbon monoxide are very low where their selectivity and yield are not exceeding 3% to 5%. This set of experiments indicates that the role of catalyst is significant to improve the synthesis gas production. It is believed that methane and oxygen are adsorbed dissociatively and then interact on the surface during the steam reforming and partial oxidation reactions over Ni, Ir, Pd, Re, and Pt [9-14]. The mechanism is summarized according to the following scheme : CH4(g ) + S O2(g ) + S H20(g ) + S

--> --> -->

C(ads) + 4H(ads) 20(ads) O(ads) + H2(gas)

The formation of CO, H2, carbon, H20, CO 2 may be expressed according to the above mechanism. Thus, at high ratios of R, adsorbed oxygen will be the limiting reactant and thus carbon deposition is achieved according to the following reaction: nC(ads)

+

mO(ads)

-->

mCO(ads)

+ (n-m) C(ads)

At low ratios of R, adsorbed oxygen sites are high and carbon sites on the surface are relatively low with the result that, adsorbed oxygen species may interact with adsorbed hydrogen to form water and with one carbon species adsorbed on the surface to form carbon dioxide. yC(ads) 2H(ads)

+ +

zO(ads) O(ads)

---> xCO(ads) --> H20(ads)

+

vCO2(ads )

Maximum synthesis gas selectivity and yield are about 70% and 60%, respectively, although those values are considered much lower than those achieved over Ni, Ir, Re, and others. 4. CONCLUSION Hydrogen and carbon monoxide production from partial oxidation of methane over commercial steam reforming catalyst is influenced by the methane to oxygen ratios and by the gas mixture flow rates. Both the selectivity and yield of synthesis gas are maximized at R about 2 and decrease at higher and lower ratios of methane to oxygen. H20 and CO 2 are formed at low ratios and carbon deposition is detected at high ratios. No heat is required to assist the reaction, however, initial heating is necessary to bring the reaction above the explosion temperature. Optimum selectivity and yield to synthesis gas are achieved at mixture flow rate of around 500 cm3/min, and methane to oxygen ratio of about 2.0. REFERENCE

1. T. Czuppon and J. Buridas, Hydrocarbon Process, 58 (1979) 197. 2. D. Eng and M. Stoukides, Catal. Rev.-Sci. Eng., 33 (1991) 375. 3. J. Lee and S. Oyama, Catal. Rev.-Sci., 30 (1988) 249. 4. A. Amenomiya and G. Sanger, Catal. Rev.-Sci. Eng., 32(3) (1990) 163.

446 5. D. Dissanyake, M. Rosynek, K. Kharas and J. Lunsford, J. Catal., 132 (1991) 117. 6. V. Chouddury, A. Rajput and B. Prabhakr, Catalysis Letters, 15 (1992) 363. 7. D. Hickman and L. Schmidt, J. Catal., 136 (1992) 300. 8. H. Alqhtani, D. Eng, and M. Stoukides, J. Electrochem. Soc., Vol. 140, 1993. 9. P. Munster, H. Grabe and Ber Bunseges, Phy. Chem., 84 (1980) 1068. 10. C. Cullis, T. Newell and D. Trimm, J. Chem. Soc. Faraday Trans., 68 (1972) 1406. 11.A. Frannet and G. Lienard, J. Chim. Phys. Physicochim. Biol., 68 (1971) 1526. 12. C. Coekelbergs, J. Delannois, A. Frannet and G. Lienard, J. Chim. Phys. Physicochim. Biol., (1964) 1167. 13.N. Meshenko, V. Veselov, F. Shub and M. Temldn, Kinet. Katal., 18 (1977) 962.