CO2 Conversion and Utilization - American Chemical Society

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on June 11, 2015 | http://pubs.acs.org Publication Date: January 24, 2002 | doi: 10.1021/bk-2002-0809.ch020

Chapter 20

Methane Reforming with Carbon Dioxide and Oxygen under Atmospheric and Pressurized Conditions Using Fixed- and Fluidized-Bed Reactors Keiichi Tomishige, Yuichi Matsuo, Mohammad Asadullah, Yusuke Yoshinaga, Yasushi Sekine, and Kaoru Fujimoto Department of Applied Chemistry, School of Engineering, The University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113-8656, Japan

Effect of the reactor in methane reforming with carbon dioxide and oxygen over NiO-MgO catalysts under atmospheric and pressurized conditions was investigated. Under atmospheric pressure, the stable activity was observed using the fluidized bed reactor. However, in the fixed bed reactor, the activity decreased gradually with time on stream. On the other hand, at low temperature and high space velocity, methane conversion decreased rapidly to the level of methane combustion using fluidized bed reactor. Under pressurized condition, the stable production of syngas was possible even at high space velocity since the catalyst is in more reducing atmosphere at higher pressure. The fluidized bed reactor enhanced more effective conversion of methane to syngas than the fixed bed reactor.

Dry reforming of methane (CH4 + C 0 ~> 2CO + 2H ΔΗ=247 kJ/mol) is a suitable to the production of CO-rich syngas which can be utilized to Fischer2

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© 2002 American Chemical Society In CO2 Conversion and Utilization; Song, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2002.

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304 Tropsch, methanol and dimethyl ether syntheses (7). One of the problems in C 0 reforming of methane is the heat supply because the reaction is highly endothermic. Internal heat supply by the combination of the reforming with the combustion (CH4 + 2Ο2 - » C 0 + 2H 0 AH=-%6\ kJ/mol) is one of the solutions (2-6). Some researches about this kind of methane reforming including the partial oxidation in thefixedbed reactor have been carried out (2, 3). It has been reported that methane combustion is followed by methane reforming and water-gas shift reaction. In that case, significant temperature gradient in the catalyst bed was generated. Groote et. al. simulated the temperature gradient of catalyst bed in partial oxidation of methane using the fixed bed reactor (2). They pointed out that the inlet temperature of the catalyst bed increased up to 1700 Κ though the reactor was controlled at about 1223 K. In addition, at the beginning of the catalyst bed, the catalyst was oxidized and exhibited low activity in the reforming reaction. Dissanayake et.al. have reported that the catalyst bed was divided into three parts in the partial oxidation of methane over Ni/Al 0 using fixed bed reactor (3). Thefirstpart was composed of NiAl 0 , and the second part consisted of NiO/Al 0 . It is found that oxygen reached these two parts. Thus the catalyst in these two parts contributed to combustion of methane. However in the third part, surface nickel was in the metallic state and showed high activity in methane reforming. Several researches on the reforming of methane with internal heat supply using the fluidized bed reactor have been reported (4-6). It has been insisted that the high rates of heat transfer and the stability of the operation were given by the fluidized bed reactor. Combustion proceeded in the front part of the catalyst bed and the reforming occurred in the rear part in both cases of fixed andfluidizedbed. It is thought that both combustion and reforming proceed on one catalyst particle in fluidized bed reactor. Santos et al. have reported the partial oxidation of methane over Ni/MgO and Co/MgO using fluidized bed reactor (4). It has been discussed that Ni and Co change between oxidized and reduced states, and these continuous redox cycles can affect the stability and catalytic behaviors in the long run. Pressurized syngas is more favorable because the synthesis reactions have been carried out under pressurized condition. The problem in C 0 reforming of methane is the carbon deposition, which becomes more serious under higher pressure. Therefore, it is necessary to develop the catalyst with higher resistance to carbon deposition (7). Our research group has developed NiOMgO solid solution catalysts with high resistance to carbon deposition in C 0 reforming of methane (8-16). In this article, we investigated the effect of the internal heat supply in the C 0 reforming of methane over NiO-MgO catalysts under atmospheric and pressurized conditions using the fixed and the fluidized bed reactors. 2


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In CO2 Conversion and Utilization; Song, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2002.

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Experimental Ni Mgi_ O (x=0.03, 0.10) catalysts were prepared by the coprecipitation method from aqueous solution of N i ( C H C O O ) * 4 H 0 (Kanto Chemical Co., inc. >98.0 %) and M g ( N 0 ) · 6 H 0 (Kanto Chemical Co., inc. >99.0 %) using K C 0 (Kanto Chemical Co., inc. >99.5 %) as the precipitant. After being filtered and washed with hot water, the precipitate was dried at 393 Κ for 12 h, and then pre-calcined in air at 773 Κ for 3 h. Furthermore, they were pressed into disks at 600 kg/m , and then calcined at 1423 Κ for 20 h. The catalysts were crushed and sieved to particles with 150-250 μτη diameter. Methane reforming was carried out in a fixed and a fluidized bed flow reaction systems undo atmospheric and pressurized conditions. The illustration of the fluidized bed reactors is shown in Figure 1. The fluidized bed reactor under atmospheric pressure was the quartz tube (15 mrnî.d.) with a sintered quartz mesh as a distributor (Figure 1(a)). In the fixed bed reactor, quartz wool was put on the catalyst bed so as to prevent catalysts from moving. Pretreatrnent of catalysts was H reduction at 1173 Κ for 0.5 h under atmospheric pressure. Oxygen was introduced to the reactor through the thin quartz tube, whose outlet was located just before the distributor. C 0 and CH4 were introduced outside the oxygen-feed tube. The partial pressure of the reactant gases was described in each result, and the total pressure was 0.1 MPa. Reaction temperature was monitored inside and outside the reactor. The reaction temperature was controlled by monitoring the thermocouple at the inside. The fluidized bed reactor under pressurized conditions was the quartz tube (6 mmî.d.) which was placed inside the stainless steel tube (10 mmî.d.) (Figure 1(b)). A sintered quartz mesh was used as a distributor in fluidized bed reactor. In the fixed bed reactor, quartz wool was put on catalyst bed so as to prevent catalysts from moving. Pretreatrnent of catalysts was H reduction at 1173 Κ for 0.5 h at atmospheric pressure. CH4 was introduced to the reactor through the thin quartz tube, whose outlet was located just before the distributor. C 0 and 0 were introduced into the reactor outside the CH4 feed tube. The conversion of oxygen was 100% in all reaction results. GHSV is calculated on the basis of total gas flow rate of the reactants (CHi+C0 +0 ) at room temperature and under atmospheric pressure. The effluent gas was analyzed with FID gaschromatograph (Gaskuropack 54) equipped with a methanator for CH4, CO, C 0 and T C D gaschromatograph (Molecular Sieve 13X) for H . A n ice bath was set between the reactor exit and a sampling port for G C analysis in order to remove water from the effluent gas. CH4 (99.9%), 0 (99 %), C 0 (99.9 %), and H (99 %) were purchased from Takachiho Co. Ltd., and were used without further purification. x

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Figure L Illustration of the fluidized bed reactors, (a) atmospheric pressure condition, (b) pressurized condition.

Resutls and Discussion

Reforming under Atmospheric Pressure Condition Figure 2 shows the dependence of conversion and H /CO ratio on the time on stream in the reforming of methane with C 0 and 0 over Nio.03Mgo.97O catalyst using fixed and fluidized bed reactors under atmospheric reaction condition. In fluidized bed reactor, high CH4 and C 0 conversions were maintained. On the other hand, the conversion decreased gradually with time on steam in the fixed bed reactor. The state of the catalyst in the reactors was observed after the reaction. In the fixed bed reactor, the catalyst near the inlet of the bed was green and the catalyst at the upper part was gray. The green catalyst is in the oxidized state and this indicates that oxygen can reach the green region. In contrast, the gray catalyst is in the reduced state. It has been reported that Nio.03Mgo.97O in oxidized state exhibited no reforming activity. 2

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Time on stream/min Figure 2. Comparison of CE4 ( β , Π,) and CO2 (Φ,Ο) conversion andU2/CO (A, A) betweenfluidized(Μ,Φ,Α) andfixedbed {Ο,Ο,Δ^ reactors over Nio.03Mgo.97O. Reaction conditions: reaction temperature 1123 K, total pressure OA MP a, CH/C0 /0 =35/35/30, GHSV=19000 cm/gh, 0.5 g-cat. 2

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Temperature/K Figure 3. Effect of reaction temperature on CE4 (H), CO2 (9) conversion and H2/CO ratio (A) over Nio.03Mgo.97O. Reaction conditions: total pressure 0.1 MPa, CH/CO /O =40/40/20, GHSV=19000 cm /gh, 05 g-cat, fluidized bed reactor. Dotted line represents methane conversion due to the combustion. 3

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308 It is suggested that the methane conversion decreased with the time on stream because the oxidized region became larger and larger. After the reaction the catalyst in the fluidized bed reactor was almost gray. This indicates that Nio.03Mgo.97O which is oxidized at the inlet of the catalyst bed can be reduced rapidly by produced syngas at the upper part of the catalyst bed. Figure 3 shows the reaction temperature dependence of the conversion and H / C O ratio over Nio.03Mgo.97O using fluidized bed reactor. The reforming reaction proceeded at 1123 and 1173 K , where methane conversion reached about 100%. However, combustion was the main reaction at 1073 K . In this case, the rapid deactivation of the catalyst due to the catalyst oxidation was observed in the fluidized bed reactor. This is contrastive to the fact that the deactivation in the fixed bed is not so rapid as shown in Figure 2. Under the reaction conditions where the rate of the catalyst oxidation is faster than that of the catalyst reduction, the catalyst fluidization drastically enhances the deactivation rate.


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Figure 4. Dependence ofCH (M), C0 (Φ) conversion, H /CO ratio (+), and temperature difference (A) on space velocity over Nio.03Mgo.97O. Reaction conditions: reaction temperature 1123 K, total pressure 0.1 MP a, CHVCO /O =35/35/30, 0.5 g-cat, fluidized bed reactor. Temperature difference = Τ (outside)-T(inside), and dotted line represents methane conversion due to the combustion. 4

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The difference in methane conversion between experimental results and combustion can be assigned to methane reforming and C O production. Furthermore since H / C O ratio is close to one in the results, the reforming 2


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reaction is similar to dry reforming. At low temperature, methane conversion was close to that due to combustion, and the main product containing hydrogen is water. At high temperature, methane conversion reached 100%, the selectivity of hydrogen formation can be estimated to be 75%. Figure 4 shows the dependence of the conversion, H / C O ratio, and temperature difference on space velocity over Nio.03Mgo.97O. Methane conversion was high in the space velocity range of 9000-19000 cm /gh. Temperature difference between the reactor inside and outside was almost zero at these space velocities. However, at 27000 cm /gh, methane conversion decreased to the combustion level, and the large temperature difference was observed. A t this condition, only methane combustion proceeded and thus the inside temperature was about 30 Κ higher than the outside temperature. This indicates that the catalyst amount in the oxidized state increased and exhibited no reforming activity at high space velocity. This is probably because the rate of oxidation is faster than that of the reduction of the catalyst. 2

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Figure 5. Effect of oxygen concentration on CH4 (M), C0 (%) conversion, H2/CO ratio (Φ), and temperature difference (A) over Nio.03Mgo.97O. Reaction conditions: reaction temperature 1123 K, total pressure 0.1 MP a, CH/CO /O =(50-x)/(50-x)/2x (x=5, 10, and 15), GHSV=19000 cm/gh, 0.5 gcat, fluidized bed reactor, temperature difference = Τ(outside)-T(inside). 2

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Figure 5 shows the effect of oxygen concentration in the reactant on conversion, H / C O ratio, and temperature difference. At 19000 cm /gh, the catalyst exhibited high methane conversion on all oxygen concentration. Temperature difference between the reactor inside and outside in methane 3

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310 reforming with only C 0 was rather large (about 50 K). The addition of oxygen to the reactant gases made the temperature difference smaller. This is the effect of internal heat supply by the methane combustion. For the stable operation of methane reforming with C 0 and 0 over Nio.03Mgo.97O using the fluidized bed reactor, the space velocity of the reactant gases and the reaction temperature are key factors. This is the relation to the oxidizing and reducing atmosphere in the reactor. 2


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Reforming under Pressurized Condition

Figure 6. Dependence ofCH4 (B), CO2 (Φ) conversion andH2/CO ratio (A) on the reaction pressure over Nio.03Mgo.97O. Reaction conditions: reaction temperature 1123 K, total pressure 0.1 MPa, CH4/CO /O =50/25/25 GHSV=37000 cm /gh, 0.3 g-cat,fixedbed reactor. The reactant gas was introduced without using thin tube. 3

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Figure 6 shows the dependence of conversion and H /CO ratio on the total pressure over Nio.03Mgo.97O. At atmospheric pressure, methane combustion only proceeded because of the high space velocity. In contrast, under pressurized condition, methane reforming proceeded. Methane conversion decreased with the increase of the total pressure since the equilibrium conversion of methane is lower under higher reaction pressure. In each experiment, the methane conversion was very stable as a function of time on stream. 2


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(b)>(a). In contrast, the difference in methane conversion is so small in the reaction under atmospheric pressure. This suggested that homogeneous methane oxidation proceeds in the gas phase before the reactants reach the catalyst bed. This reaction proceeds more rapidly at higher pressure. Therefore, the amount of oxygen which reaches the catalyst bed in (a) must be smaller than that in (b) and (c). The stable operation was possible in (b). However, it was not in (c). The reaction temperature was not stable and often suddenly increased like the explosion. This is probably because the oxygen concentration was too high at the outlet of the introduction tube. Therefore, we used the reactor as shown in Figure 1(b).


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GHSVcm /gh Figure 8. Dependence of CH4 conversion on space velocity usingfluidizedbed (Μ,φ) andfixed bed (Ώ,Ο) reactor over Ni .o3Mgo.970(U,0) and Nio.10Mgo.90O (Φ,Ο).Reaction conditions: reaction temperature 1073 K, total pressure 10 MP a, CH4/CO2/O2S0/20/30, 02 g-cat 0

Figure 8 shows the dependence of methane conversion on the space velocity over Nio.03Mgo.97O and Ni .ioMg .9oO. Under pressurized condition, the conversion was stable at low reaction temperature (1073 K ) . Methane conversion decreased with the increase in the space velocity when the fixed bed 0

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CO2 Conversion and Utilization - American Chemical Society

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