Energy & Fuels 1994,8, 1123-1125 1123

Carbon Dioxide Reforming with Methane to CO and H2 in a Hot Wire Thermal Diffusion Column (TDC) Reactor

H. D. Gesser,* N. R. Hunter, A. N. Shigapov,? and Verina Januati;

Department of Chemistry, University of Manitoba, Winnipeg, MB, Canada, R3T 2N2

Received January 27, 1994. Revised Manuscript Received May 27, 1994@

In a water-cooled tubular Pyrex reactor of 2.0 cm i.d. and 85 cm long with a tungsten wire of 0.25 mm diameter and 70 cm long, it was possible to obtain 93% conversion with an input ratio COdCH4 = 1 at a wire temperature of 1030 "C and a linear upward flow rate of 32 cdmin. The reaction is CHI + COZ - 2CO + 2H2, AHO = 247 kJ/mol (reaction 1). Lower temperatures required lower flow rates to maintain high conversions. When the flow rates are reduced or the ratio COdCH4 > 1 then the reaction COZ + HZ - CO + HzO, AH" = 41.2 kJ/mol (reaction 21, becomes important resulting in a product ratio of CO/H2 reaching values as high as 30 with water formation. The thermal electrical efficiency for reaction 1 is, however low, being about 3% based on the electrical energy used. Some formation of oxides of tungsten is observed on the cold walls.

Introduction

The furation of COZ has become an important aspect of the solution to the earth's greenhouse prob1em.l In parallel with such activity, the useless release of CO2 into the atmosphere is to be avoided. Such release is common when landfill gas consisting of 50% CH4 and 50% COZ is utilized for heat or methane conversion. The conversion of such gas mixtures to synthesis gas (CO + H2) has recently been the subject of several papersZv3 and patent^.^ Some natural gas fields in Indonesia have up to 70% C02 and the venting of this C02 is most undesirable. Hence this study was initiated to deter- mine the feasibility of reforming COdCH4 mixtures into syn gas. The TDC reactor was initially described by Hirota5 for the conversion of natural gas to liquid hydrocarbons. This work has been recently repeated by Yamaguchi in Japan6 and Gesser in Canada.7 Yamaguchi has also shown that the products formed are very much dependent on the direction of flow. When the natural gas flows upwards the main products are hydrogen and liquid aromatic hydrocarbons. In the downward flow the main products are ethane, ethylene, and hydrogen.

One of the main problems associated with the reform- ing reaction

CH4 + C 0 2 - 2CO + 2H2 AHo = 247 kJ/mol, AGO = 170.7 kJ/mol, ASo = 256.5 J/(K mol) (1)

is the formation of coke which can be reduced or eliminated by the use of noble metal catalysts. This

+ Institute of Chemistry of Natural Organic Materials, Krasnoyarsk,

t Research Grants Program, Faculty of Engineering, University of

@ Abstract published in Advance ACS Abstracts, July 15, 1994. (1) Arakawa, H. Now & Future, Jpn. Ind. Technol. Assoc. 1992, 7 ,

Russia.

Indonesia, Jakarta, Indonesia.

10. ~~

(2) Ashcroft, A.T.; et al. Nature 1991,352,225. ( 3 ) Inui, T.; et. al. Energy Convers. Manage. 1992, 33, 513. (4) Gustafson, B. L.; Walden, J. V. US. Patent No. 5,068,057, Issued

Nov. 26, 1991. ( 5 ) Hirota, K. Bull. Chem. SOC. Jpn. 1941, 12, 274. (6)Yamaguchi, T. Bull. Chem. SOC. Jpn. 1988, 61, 2. (7) Gesser, H. D.; Morton, L. A. Catal. Lett. 1991, 11 , 357.

work describes the application of the TDC reactor to the COdCH4 reforming reaction.

Experimental Section

Several reactors were used, all of which had a water-cooled jacket. The length and inside diameter of the reactor, the length of the tungsten wire, and the temperature of the wire were varied.

The two gases were usually mixed from dual feed lines though some experiments were also performed using premixed gases. Nitrogen was added as an internal standard and permitted conversion and material balance calculations. The gases were analyzed by two gas chromatographs with thermal conductivity detectors. The first GC (GCI) was an Aerograph Model 1200 fitted with a Servomex micro TC detector, cell volume 2.6 pL. The gases were separated on a 10 m x 1/8 in. 0.d. column packed with Carbosieve Si1 (100/120 mesh) using He as a carrier gas. Temperature programming was from 50 to 200 "C at 15 deg/min with a He flow of 30 mumin. The second GC (GCII) was a Chrom-analyzer 100 using argon as the carrier gas with 8 m x 3/16 in. column packed with molecular sieve 5A (60/80 mesh) a t 90 "C. The GCI separated CH4, COz, Nz, CO, and Hz but the sensitivity of the Hz was poor. The GCII separated Hz, CH4, Nz, and CO with good sensitivity for Hz though the COz peak could not be detected.

A detailed description of the apparatus has been described.8 The tungsten wire, 0.025 cm diameter, was positioned in

the center of the TDC reactor and held taut by a spring (by- passed by a shorting conducting wire) a t the lower end.

The reactor system was first flushed with Nz and then with the reaction mixture with the W wire cold until the inlet and outlet gas showed identical composition. The temperature of the wire was controlled by a variable autotransformer. The ac current and voltage was measured, and an optical pyrom- eter was used to determine the average temperature of the wire. The temperature difference along the wire was ap- proximately 100 "C. In some experiments a small drying tube filled with Drierite was used to remove water from the exit line of the reactor. Material balance was determined by the equality of the carbodnitrogen ratio

(8) Januatic, Verina M.Sc. Thesis, Faculty of Engineering, Univer- sity of Indonesia, Jakarta, Indonesia, 1993.

0887-0624/94/2508-1123$04.50/0 0 1994 American Chemical Society

1124 Energy & Fuels, Vol. 8, No. 5, 1994 Gesser et al.

Table 1. Reforming of CH4/CH2 Mixtures in a TDC ReactoP % mass balance % outlet concentration (mol %) expt V Z power temp

no. (VI (A) (W) ("C) H2 CO C& CO2 N2 C2H4 C2H6 con@ C 0 H 21 40 6.0 240 966 30.3 31.0 19.3 17.7 1.6 0.08 0.07 50 86 85 86.6 22 45 6.2 280 996 43.5 44.9 5.5 4.8 1.2 0.02 0.01 81 93 93 91.7 23 50 6.5 325 1027 46.5 48.3 2.1 2.0 1.1 0.10 - 92 96.3 97.6 92.5

Tungsten wire 70 cm long, 0.25 mm diameter. Composition of reaction mixture 48.2% CHI; 48.7% C02; 2.0% Nz; 1.1% C2H6; 0.07% C3Hs. Feed flow rate = 1.68 mLis upwards. Outputhnput.

Table 2. Reforming CH4/C02 Mixture in a TDC Reactor 25 nun i.d.'

expt temp Z V W flow rate tb mol % mol % product % conv %field no. ("C) (A) (VI (W) mumin cdmin (min) CH4 C02 N2 CH4 COZ Nz CO Hz HzO" C& CO Hz 24 910 6.5 30 195 120 24.4 2.0 44.6 54.7 0.7 31.6 39.5 0.6 13.5 13.2 1.6 17.7 13.6 14.8 25 910 6.5 30 195 95 19.4 2.6 46.0 53.3 0.7 27.1 32.6 0.6 18.1 16.6 5.1 28.9 18.2 18.0 26 910 6.5 30 195 69 14 3.6 43.6 55.7 0.7 16.5 23.1 0.6 25.3 21.0 13.6 50.7 25.5 24.0 27 910 6.5 30 195 43 8.7 5.7 45.1 54.1 0.7 7.9 13.3 0.5 32.5 27.8 18.0 75.6 32.7 30.8 28 910 6.5 30 195 20 4.1 12 47.4 51.9 0.7 3.9 7.9 0.5 39.9 36.1 11.8 87.3 40.1 38.1 29 882 5.9 25 148 95 19.4 2.6 48.8 49.3 1.9 46.6 46.0 1.8 1.5 1.5 2.6 1.9 1.5 1.6 30 905 6.4 30 192 95 19.4 2.6 49.6 48.5 1.9 38.9 40.2 1.8 7.7 7.4 3.9 17.5 7.9 7.5 31 921 7.1 35 240 95 19.4 2.6 51.8 46.4 1.8 22.6 20.2 1.4 19.4 22.7 13.7 44.9 19.8 21.9 32 1026 7.5 40 300 95 19.4 2.6 51.0 47.2 1.8 13.6 12.9 1.3 25.2 29.2 17.8 63.5 25.7 28.7 33d 893 6 25 150 70 14.2 3.5 52.5 45.8 1.7 46.1 44.0 1.8 2.4 3.0 2.8 13.6 2.5 2.8 34d 915 6.5 30 195 70 14.2 3.5 51.3 46.6 2.2 35.2 32.5 1.8 12.2 18.3 0 16.7 12.5 17.9 35d 971 7.2 35 252 70 14.2 3.5 50.9 47.3 1.8 11.6 9.6 1.2 27.3 35.4 14.9 64.7 27.8 34.8 36d 1048 7.5 38 285 70 14.2 3.5 50.7 47.5 1.8 5.0 3.4 1.2 32.5 37.6 20.4 84.4 33.1 37.1 37d 1150 7.7 40 308 70 14.2 3.5 53.3 44.9 1.7 0.9 0.5 1.2 38.3 38.2 20.9 97.4 38.9 35.8

Length of W wire = 65 Tungsten wire, 50 cm long, 0.025 cm diameter. 5 = wire 1engtMinear flow velocity. Calculated values. cm.

50

40

1 30

1 2 0

s b

10

0

I * n r * m - c.mXIu0Ra.d. + M h W 0 c,mw..m ,a E- * E t M ~

Figure 1. Effect of wire temperature on product gas composi- tion at flow rate of 100 mumin.

CH, + CO, + CO c CH4 + co =c in N , W out N2(out)

Results and Discussion Preliminary results using laboratory gas (pipeline

gas) are shown in Table 1 and plotted in Figure 1. The presence of CZ + C3 in the methane complicate the material balance. However, the results show high conversion and over 95% material balance (output/ input) for carbon and oxygen but only 92.5% for hydro- gen for the highest temperature. Traces of water was detected as well as ethylene. Coke formation on the tungsten wire was not observed.

The results of a more detailed study are listed in Table 2 and plotted in Figures 2, 3, and 4. Significant amounts of water was formed which require the intro- duction of a drying tube after the reactor. Water formation was calculated from the imbalance of oxygen and hydrogen.

Figure 2. Effect of flow rate of CHdCO2 mixture on product gas composition with tungsten wire at 910 "C.

Water formation is probably due to a longer contact time with the wire resulting in further reaction of hydrogen by (2) which becomes an important feature of

AH0 = 41.2 kJ/mol (2) CO, + H, - CO + H,O

the reaction system. This was confirmed when the reaction was conducted wiBh the input ratio COdCH4 = 1.5. The yield of HZ approached zero and water formation was prominent in the reactor. A yellow-white deposit on the cold reactor walls was identified as an oxide of tungsten probably formed by the reaction

w + xco , - wo, + x c o

The approach t o thermodynamic equilibrium is de- termined by the contact time, z, at any temperature, i.e. (wire 1ength)flinear flow velocity. This will also be a function of the reactor diameter.

Carbon Dioxide Reforming Energy & Fuels, Vol. 8, No. 5, 1994 1125

I 1 -- :. -

I c --

15

- 10

850 900 950 1000 1050 1100 1150 Temperature (deg Celriurl

Figure 3. Effect of wire temperature on product gas composi- tion with CHdC02 mixture flow rate of 70 mumin.

Calculations show that thermodynamic equilibrium should give the following conversion for CH4, about 84% at 1150 K, 94% at 1265 K, and 98% at 1390 K. The values obtained seem to approach such conversion.

Water formation can be reduced by operating at shorter contact time, narrower reactors, and higher wire temperatures. Since significant gas fields in Indonesia have 70% C02, it is essential that the reaction 2 be prevented from occurring. This may be possible by using a palladium membrane wall reactorg which will

(9) Uemiya, S.; et. al. Ind. Eng. Chem. Res. 1991, 30, 585.

ea0 900 920 940 960 980 1000 1020 1040 Temperature ldeg C e l s ~ r l

I+- i m - + - - O m +-- 1

Figure 4. Effect of wire temperature on product gas composi- tion with C W C 0 2 mixture flow rate of 95 mumin.

allow the H2 to be separated as it forms. The thermal electrical efficiency for reaction 1 was calculated to be about 3%. This efficiency could be increased by using a lower wire temperature with a membrane TCD reactor.

Acknowledgment. Grateful acknowledgment is made to the Department of Energy, Mines and Re- sources, the Natural Sciences and Engineering Research Council of Canada, and the Research Grants Program of the University of Indonesia for their support of this work.